Abstract
Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018–2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.
Keywords: organic photovoltaics, organic solar cells, NFA, small molecule acceptors, ladder type
1. Introduction
Photovoltaics is a major pillar in tackling climate change, one of the biggest current threats to mankind. Aiming at a resource- and cost-efficient production combined with scalability and a low carbon footprint, organic solar cells (OSCs) are a third-generation photovoltaic technology, which could well meet these targets. OSCs have been the objective of intensive research for several decades, and thanks to continuous advancements in the properties of the absorber materials and in particular due to the introduction of non-fullerene acceptors (NFAs), power conversion efficiencies (PCEs) have very recently surpassed 18%,1 thus being in terms of efficiency already highly competitive with other established and emerging thin film technologies.2,3
In addition to these very promising power conversion efficiencies, organic solar cells possess unique properties making them attractive for a variety of appliations.4 The absorber in organic solar cells consists of a very thin layer comprising at least two different organic semiconductors, e.g., a conjugated polymer and a small molecule, with high absorption coefficients. Thus, lightweight and flexible solar cells can be realized (Figure 1A).5,6 In addition, OSCs can be processed from solution via coating and printing techniques without the need for high temperature treatments,7 and the usage of flexible substrates makes large area, high throughput roll-to-roll processing highly feasible (Figure 1B).8−10 The possibility to tune the absorption range of the active layer components by modifying their chemical structure allows the realization of colored and semitransparent devices (Figure 1C).11−13 These properties enable interesting new applications, such as their integration into glass facades and windows of buildings in an urban environment, into greenhouses in the agricultural sector,14 or into wearables and self-powered devices, where they can be used also for indoor light energy recycling.15−18 However, even though impressive PCEs are obtained, the long-term stability of OSCs is still a challenging factor in terms of their usage in a broad range of applications.19,20 Recent reviews address the status-quo and the current challenges and progress regarding the stability.21−23
Figure 1.
(A) Example of a flexible organic solar cell and (B) roll-to-roll printing of the active layer. Reprinted with permission from ref (24). Copyright 2013 Elsevier. (C) Example of a semitransparent organic solar cell integrated into self-powered sunglasses. Reprinted with permission from ref (25). Copyright 2017 Wiley-VCH.
As already briefly mentioned above, the significant increase of PCEs observed in the past years has been made possible to a great extent by the concerted effort of the whole research community working in this field leading to continuous improvement as well as the introduction of new materials. In the first organic solar cells reported by Tang et al. in 1986, a perylene-based acceptor was used.26 Only a few years after, Sariciftci et al. published a seminal paper on photoinduced electron transfer from a conducting polymer to fullerenes.27 The outstanding electronic properties of fullerenes and their derivatives soon established them as dominant acceptor materials in the first two decades of OSC research. Thus, a lot of our current understanding of how organic solar cells function is coming from this era.28 While with organic solar cells based on polymer/PCBM absorber layers maximum PCEs slightly above 11% have been reported,29−32 these values can be exceeded with modern n-type small molecular acceptors, also referred to as non-fullerene acceptors.1,33 In the middle of the last decade, first very efficient NFAs (e.g., ITIC, IDIC, O-IDTBR, IDT-2BR, or IEIC)34−39 revealing high performance in organic solar cells competitive or higher compared to similar devices based on fullerenes as acceptors were found. Up to now, several hundred new NFA structures have been reported and applied in organic solar cells in combination with a large variety of donor materials.
1.1. Device Configuration and Working Principle
The absorber layer of OSCs typically consists of a combination of at least two organic semiconductors, a so-called donor (an electron-donating semiconductor, mainly a conjugated polymer) and an acceptor (electron-accepting semiconductor—another polymer, fullerene derivatives, or non-fullerene acceptors) which are arranged either in a bilayer heterojunction or in a bulk heterojunction, in which both form an interpenetrating, bicontinuous network (Figure 2).40 This absorber layer is typically embedded between one transparent (e.g., indium tin oxide - ITO) and one metal (e.g., Ag, Ca/Al) electrode. In addition, selective electron and hole transport layers are used between the electrodes and the active layer in order to facilitate charge extraction.41,42 Often used electron transport layers are metal oxides such as ZnO or organic polyelectrolytes (e.g., PFN-Br, poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-(2,7-(9,9-dioctylfluorene))dibromide), while typically applied hole transport layers are PEDOT:PSS or MoO3. For a comprehensive summary on interlayers, a recent review and references therein are suggested.43 Moreover, organic solar cells can be prepared either in the conventional or the inverted device architecture, which are illustrated schematically in Figure 2. In the conventional architecture, a glass/ITO substrate is typically coated with a hole transport layer followed by the absorber layer, an electron transport layer, and a metal electrode. In the inverted architecture, the layers are stacked in the following sequence on the glass/ITO substrates: electron transport layer (e.g., ZnO)/absorber layer/interlayer (e.g., MoO3)/metal electrode, leading to an inverted flow of the charge carriers in the device compared to the conventional architecture.
Figure 2.
Device architecture and working mechanism of non-fullerene organic solar cells.
Figure 2 schematically outlines the charge generation mechanism in the absorber layer of OSCs. First, absorption of an incident photon excites an electron, forming a bound electron–hole pair, the exciton (1). Next, the exciton diffuses to the donor–acceptor interface (2). Due to a higher electron affinity of the acceptor, exciton dissociation occurs (3), leading to free electrons in the lowest unoccupied molecular orbital (LUMO) of the acceptor and free holes in the highest occupied molecular orbital (HOMO) of the donor. Once these free charge carriers are generated, they are transported to the respective electrodes (4), where they are collected (5). In Figure 2, the exciton formation, diffusion, and dissociation are shown based on light absorption in the donor; however, in non-fullerene organic solar cells, also in the acceptor phase, efficient light absorption takes place, leading to an exciton formation in the non-fullerene acceptor phase, followed by a diffusion to the interface to the donor, where the exciton dissociates into free charge carriers.
To achieve high PCE values, it is essential that each of these steps starting from light absorption and the exciton formation to the charge collection at the electrodes takes place efficiently.44 The knowledge generated in more than two decades of OSC research allows breaking down important parameters for each of the successive steps.
First, the optoelectronic properties of the active material, e.g., the HOMO–LUMO levels of both the donor and acceptor, and their relative difference in energy are important parameters. The HOMO–LUMO gap has a direct impact on the absorption, and thus, the photoresponse of the solar cell can be maximized by selecting active layer materials with complementary band gaps (typically small band gap NFAs and medium/wide band gap conjugated polymers are used in the most efficient solar cells). On the other hand, for the realization of semitransparent solar cells, material combinations which allow the passing of certain wavelengths in the visible light spectrum can be chosen.45 Regarding the exciton dissociation at the donor–acceptor interface, the donor material needs to have higher HOMO and LUMO energies than the acceptor in order to enable this process, wherein typically first a charge transfer (CT) state is formed at the interface, which is subsequently converted into the free charge carriers, an electron, and a hole. Thereby, the energy difference between the LUMO of the donor and the LUMO of the acceptor has to be as high as the energy needed in order to overcome the exciton binding energy (which originates from Coulomb interactions of an electron and a hole). Regarding the hole transfer to the donor when the exciton is formed in the acceptor, similar considerations apply for the energy difference of the HOMO levels of the donor and the acceptor. Additionally, the difference between the HOMO level of the donor and the LUMO of the acceptor correlates to the maximal open circuit voltage (VOC) the device can theoretically deliver.
The second important issue is the spatial distribution of the donor and acceptor phase within the active layer, i.e., the phase separation also called phase morphology. As stated before, there are two basic concepts, the bilayer heterojunction and the bulk heterojunction approach. Whereas the first has a well-defined interface, the latter is comprised of a mixture of both with phase separation on the nanoscale (few tens of nanometers) in order to enhance the charge separation.46,47 This phase morphology, i.e., the domain sizes of the donor and the acceptor phase as well as their local distribution and purity, is one of the crucial parameters to obtain efficient OSCs, as it affects the transport related phenomena in the absorber layer such as exciton diffusion and therefore also their probability for dissociation as well as the transport of the separated charges to the respective electrodes.48,49 Since the diffusion length of an exciton in organic materials is limited within the range of a few ten nanometers,50 the donor–acceptor interface should be located within this distance from the place the exciton was generated in order to be dissociated into free charge carriers. Furthermore, also trapping of excitons by defects in the films or at interfaces can result in exciton quenching (non-productive recombination) and thereby a reduction of the charge generation yield. In addition, free charge carriers are prone to recombination.51 This happens mainly at the interfaces between the donor and the acceptor domains or due to traps within both phases. However, the distances free charge carriers can travel are significantly larger than those of excitons, due to the electric field applied to the device. A certain amount of traps is an intrinsic feature of organic semiconductors, introduced, e.g., during the film formation or by outer forces (such as oxygen or UV-light), while additional traps can also result from interactions with other phases. A detailed analysis of various traps is described by Haneef et al.51 Regarding the charge transport, balanced charge carrier mobilities in the donor and the acceptor phase are beneficial. Moreover, pristine layers of donor and acceptor would be ideal as realized in the bilayer concept, but here the yield of excitons reaching the interface is limited. In contrast to the bilayer concept, the bulk heterojunction is a compromise between a high interface for charge separation and domain sizes, which are large enough to provide good pathways for charge transport.52−55 Therefore, morphology control of the active layer has attracted much research attention.47,56,57
At present, the most often used active layers contain one donor and one acceptor in a bulk heterojunction. However, a single component active layer is also possible. In this case, the donor and acceptor moieties are a part of the same molecule (or oligomer, or polymer). The efficiencies of these solar cells currently reach values up to around 11%.58−60 At the same time, if a third component is added to the active layer, ternary solar cells are obtained.61,62 The combination of three materials allows for a more efficient harvesting of the solar light and can also have advantages regarding reduced trap states and device performance and stability;63 however, the morphological control becomes more challenging. Another way to broaden the solar light absorption is by use of two (tandem)64−66 or multiple junction cells.67
1.2. The Scope of This Work
The imaginativeness of chemists has created a variety of new non-fullerene acceptors, and the amount of new literature focusing solely on NFA design is overwhelming. In light of the large amount of published data, it is desirable to summarize and to find ways to organize and generalize the latest findings. It is not always necessary to strictly separate the donors from acceptors, as often common improvement strategies (such as chlorination68 and fluorination69,70), design strategies (ladder-type compounds71), or substance classes (such as diketopyrrolopyrroles72) are used for both. Also, computational methods are very useful to guide the development of both substance classes or even the entire OSC.73,74
The fast moving field of non-fullerene organic solar cells has been the objective of several recent perspectives and reviews covering small molecule NFAs and the corresponding solar cells in general (e.g., refs (33, 49, and 75−77)) or specific compound classes, for example, rylene dyes,78−81 Y-type82−84 and IDIC/ITIC-type acceptors,85 or polymeric acceptors.86,87 Also, design strategies have been reviewed, such as A–D–A-structure-type88,89 and fused-ring molecules90,91 as well as isomeric,92 star shaped,93 and asymmetric compounds.94
In this Review, we aim at giving an overview of recently introduced NFA structures based mainly on ladder-type fused-ring systems investigated in the years 2018–2020. Due to the large number of studies reporting on the synthesis and characterization of NFAs and their application in solar cells, we only focus on data of solar cells containing one polymer donor and one small molecule acceptor material in the absorber layer and ternary solar cell data are not included in our discussion to be able to give a better comparability between the photovoltaic performances of the different NFAs. The following chapters will give general ideas on the acceptor design of fused-ring systems. As a basis for categorizing, we have chosen the size of the central fused-ring core. The most efficient solar cells to date are based on seven-ring structures; thus, they will be covered first, followed by NFAs containing five, six, eight, nine, and more fused-ring cores. Moreover, we analyzed and compared the photovoltaic properties of these over 700 NFA structures reported within the last three years and discuss obtained correlations at the end of this Review.
2. Non-Fullerene Acceptor Design
The earliest (but still most common) design strategy for non-fullerene acceptors is the combination of a weak electron-donating core (D) and two strong electron-withdrawing groups (A) as peripheral units, also referred to as the acceptor–donor–acceptor (A–D–A) structure.95,96 This framework profits from π-electron push–pull effects, which is not only good for light absorption but also good for charge transfer. A prominent example of this structural motif is ITIC (7-1), which was first reported in 2015.34 Since then, ITIC and its derivatives are among the most popular non-fullerene acceptors. It consists of a planar, rigid, ladder-type core unit containing a fused aromatic ring system, namely, indacenodithieno[3,2-b]thiophene (IDTT or IT) referred to as the donor unit. This unit is decorated with strong electron-withdrawing 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN or IC) end groups on each side of the central donor unit; thus, they are referred to as the acceptor units (see Figure 3, left).34,97
Figure 3.
Possible design strategies for non-fullerene acceptors. Left, illustration of an A–D–A-type acceptor (ITIC, 7-1); right, example of an A-π-D-π-A-type acceptor based on ITIC.
An important modification of the A–D–A structural motif was the introduction of an electron-poor ring into the central donor unit leading to the A–DA′D–A structure type. The most prominent examples are the Y-series acceptors, which currently also hold the record PCEs.98 Another strategy is to include π-spacers, whereby compounds with the general structure A−π–D−π–A (Figure 3, right) are obtained.99 By selecting the appropriate type of π-spacers, the quinoid character of the conjugated backbone can be increased.100 Good results can also be achieved if asymmetric molecules are designed, for example, A–D–A′ or A–D−π–A (as well as combinations thereof).94 Such molecules often have larger dipole moments and improve packing in the solid state, which can reduce energy loss and improve the fill factor (FF) in solar cells.
An overall coplanar structure is advantageous for light absorption and charge mobility in the solid state. In the ladder-type central core structures, this is achieved by a covalent ring locking of neighboring heterocycles. In order to keep the π-spacers and end groups coplanar to the central core, non-covalent interactions, i.e., O···S, N···S, H···S, and X···S (X = Cl, Br, F) are utilized. Nevertheless, too much planarity can cause poor solubility if no side chains of sufficient length are used. Side chains not only ensure solution processability but can also have an influence on the optical band gap and the energy levels of the molecule despite not being a part of the electronic conjugation (π-system). Due to their steric bulkiness, they can also prevent too strong self-aggregation in film. Usually, side chains are attached on the central core and/or the π-spacer. This allows the end groups to form intermolecular interactions, crucial to the electron transport in the solid state. Since the end groups are a part of the conjugated π-system, already small modifications can lead to relatively large changes of the optical band gaps and the energy levels of the entire molecule. A very common way to modify the end groups is the introduction of halogens. For example, fluorine atoms lead to downshifted energy levels and reduced optical band gaps.101,102
All of the above-mentioned design principles can be found among the large set of new seven-ring acceptors published over the last three years. For this central core size, we have elucidated them in detail. However, a more exhaustive analysis of π-spacers is outlined in the five-ring chapter. This is the logical consequence of the smaller conjugation length of the five-ring central core, which leads to higher optical band gaps. Using π-spacers can shift the band gap values to energies which are more similar to those of the seven-ring central cores. Asymmetric central cores are most often found in six-ring and larger central cores.
Figure 4 contains a summary of all side chains and their abbreviations as well as selected donor materials used in the most efficient non-fullerene organic solar cells to date. For a complete summary of state-of-the-art donor materials, the reader is referred to recent reviews and references therein.103−105
Figure 4.
Summary of all side chains used in this Review with their respective abbreviations and selected donor molecules.
3. Seven Fused Aromatic Ring Systems
3.1. Impact of the Acceptor Units
IDTT, the core unit in ITIC (7-1), is the most frequently used central donor unit for NFAs in the years 2018–2020, which is why we decided to discuss the influence of different acceptor units based on this donor core. The discussed structures are depicted in Figure 5, and the material and photovoltaic properties are summarized in Table 1. The ITIC acceptor has an INCN unit as an end group and possesses a band gap of 1.59 eV as well as strong and broad absorption in the region 500–750 nm.34 Being the first highly efficient NFA, many research groups investigated ITIC itself and its behavior in OSCs with different polymer donors, e.g., PBDB-T,106 PTB7-Th,107 PM6,108 etc. Before 2020, Bin et al. achieved the highest PCE (11.4%) of ITIC-based single-junction OSCs with J71 as a donor.109 In 2020, this PCE was topped by Li et al. with 13.5%, where the new polymer donor PBTA-PS-F was used. The device showed in comparison a higher electron mobility of 3.80 × 10–4 cm2 V–1 s–1, VOC of 0.97 V, JSC of 18.5 mA cm–2, and FF of 75%.110
Figure 5.
Structures of non-fullerene acceptors with the same core unit bearing hexylphenyl side chains and different acceptor units.
Table 1. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-1–7-47.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μec(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-1 | ITIC | –5.50 | –3.89 | PBDS-T | 1.2:1 | 0.98 | 18.2 | 60 | 10.7 | -/3.6 × 10–4 | (135) | |
| ITIC | –5.50 | –3.85 | 1.61 | PM6 | 1.01 | 15.6 | 66 | 10.3 | 1.7 × 10–4/1.1 × 10–4 | (108) | ||
| ITIC | –5.68 | –4.03 | 1.58 | PBDB-T | 0.89 | 16.5 | 71 | 10.4 | 6.4 × 10–4/3.1 × 10–4 | (106) | ||
| ITIC | PBTA-PS-F | 1:1 | 0.97 | 18.5 | 75 | 13.5 | -/3.8 × 10–4 | (110) | ||||
| 7-2 | IT-2F | –5.63 | –4.06b | PM6 | 0.92 | 19.3 | 70 | 12.7 | 8.4 × 10–5/- | (111) | ||
| 7-3 | IT-3F | –5.67 | –4.09b | PM6 | 0.90 | 20.0 | 73 | 13.8 | 1.0 × 10–4/- | (111) | ||
| 7-4 | IT-4F | –5.68 | –4.14b | PM6 | 0.86 | 20.8 | 74 | 13.6 | 9.1 × 10–5/- | (111) | ||
| IT-4F | –5.66 | –4.14 | D18 | 1:1.6 | 0.86 | 22.3 | 75 | 14.9 | (136) | |||
| 7-5 | a-IT-2F | –5.67 | –4.07 | 1.56 | PBDB-T | 1:1 | 0.78 | 19.1 | 69 | 10.3 | -/4.0 × 10–5 | (114) |
| 7-6 | ITIC-2Cl | –5.68 | –3.99 | 1.55 | PM6 | 1:1 | 0.92 | 19.1 | 75 | 13.2 | (115) | |
| 7-7 | ITIC-4Cl | –5.75 | –4.09 | 1.48 | PM6 | 1:1 | 0.79 | 22.7 | 75 | 13.5 | (115) | |
| 7-8 | ITIC-γCl-2F | –5.52 | –3.88 | PM6 | 1:1.2 | 0.85 | 19.6 | 72 | 12.0 | -/1.2 × 10–4 | (116) | |
| 7-9 | ITIC-2Cl-β | –5.30 | –3.71 | PM6 | 1:1 | 0.94 | 18.5 | 65 | 11.2 | -/1.1 × 10–4 | (117) | |
| 7-10 | α-ITIC-2Cl | –5.29 | –3.77 | PM6 | 1:1 | 0.88 | 18.9 | 74 | 12.2 | -/2.9 × 10–4 | (117) | |
| 7-11 | ITIC-2Br-m | –5.53 | –3.90 | 1.53 | PM6 | 1:1 | 0.87 | 18.0 | 70 | 10.9 | -/7.6 × 10–4 | (118) |
| 7-12 | ITIC-2Br-γ | –5.54 | –3.90 | 1.53 | PM6 | 1:1 | 0.89 | 19.0 | 71 | 12.1 | -/8.3 × 10–4 | (118) |
| 7-13 | IT-M | –5.58 | –3.98 | PDTF-TZNT | 1:1 | 0.80 | 17.3 | 73 | 10.1 | -/2.2 × 10–4 | (119) | |
| IT-M | –5.51 | –3.80 | PBDFP-Bz | 1:1 | 1.02 | 18.3 | 69 | 12.9 | (137) | |||
| 7-14 | IT-CF3 | –5.71 | –3.97 | 1.49 | PM6 | 1:1 | 0.84 | 20.9 | 76 | 13.3 | 4.7 × 10–4/2.8 × 10–4 | (122) |
| 7-15 | ITEN | –5.63 | –3.90 | 1.58 | PM6 | 0.99 | 16.5 | 67 | 10.9 | 8.8 × 10–4/3.6 × 10–4 | (108) | |
| 7-16 | IT-DM | –5.58 | –3.82 | 1.63 | J71 | 1:1 | 1.02 | 16.7 | 71 | 12.1 | 7.0 × 10–4/4.1 × 10–4 | (112) |
| 7-17 | ITCF | –5.59 | –3.95 | 1.57 | J71 | 1:1 | 0.91 | 18.5 | 79 | 13.3 | 7.4 × 10–4/5.3 × 10–4 | (112) |
| 7-18 | a-IT-2OM | –5.61 | –3.92 | 1.63 | PBDB-T | 1:1 | 0.93 | 18.1 | 72 | 12.1 | -/3.9 × 10–5 | (114) |
| 7-19 | IT-OH | –5.57 | –3.92 | 1.54 | PM6 | 1:1 | 0.92 | 17.4 | 70 | 11.2 | (123) | |
| 7-20 | IT-DOH | –5.58 | –3.93 | 1.53 | PM6 | 1:1 | 0.96 | 17.8 | 73 | 12.5 | (123) | |
| 7-21 | IO-4H | –5.61 | –3.65 | 1.88 | PBDB-T | 1:1.5 | 1.12 | 5.95 | 43 | 2.86 | -/6.5 × 10–7 | (125) |
| 7-22 | IO-4F | –5.65 | –3.83 | 1.85 | PBDB-T | 1:1.5 | 1.05 | 12.2 | 63 | 8.06 | -/4.5 × 10–7 | (125) |
| 7-23 | IO-4Cl | –5.72 | –3.83 | 1.80 | PM6 | 1:1.5 | 1.24 | 11.6 | 68 | 9.80 | (124) | |
| 7-24 | ITCPTC | –5.62 | –3.96 | 1.58 | PM6 | 1:1 | 0.97 | 17.1 | 74 | 12.3 | 6.5 × 10–4/3.6 × 10–4 | (127) |
| 7-25 | ITC-2Cl | –5.58 | –4.01 | 1.58 | PM6 | 1:1 | 0.91 | 20.1 | 74 | 13.6 | 7.8 × 10–4/4.2 × 10–4 | (128) |
| 7-26 | ITC-2Br2 | –5.59 | –4.02 | 1.59 | PM6 | 1:1 | 0.90 | 19.8 | 74 | 13.1 | 7.8 × 10–4/4.1 × 10–4 | (129) |
| 7-27 | MeIC | –5.57 | –3.92 | 1.58 | PM6 | 1:1 | 0.99 | 18.5 | 71 | 13.0 | 8.5 × 10–4/4.8 × 10–4 | (127) |
| 7-28 | ITCT-DM | –5.48 | –3.90 | 1.58 | PBDB-T | 1:1 | 0.90 | 17.4 | 65 | 10.6 | -/6.7 × 10–4 | (130) |
| 7-29 | IDTC | –5.47 | –3.76 | 1.68 | PBDB-T | 1:1 | 1.00 | 16.1 | 69 | 11.1 | -/2.5 × 10–4 | (131) |
| 7-30 | ITC-2Br | –5.73 | –3.93 | 1.73 | PM6 | 1:1 | 1.03 | 15.4 | 69 | 10.9 | 6.6 × 10–4/3.0 × 10–4 | (129) |
| 7-31 | ITC-2Br1 | –5.70 | –3.95 | 1.70 | PM6 | 1:1 | 1.01 | 16.6 | 71 | 11.9 | 6.9 × 10–4/3.4 × 10–4 | (129) |
| 7-32 | ITCCM-O | –5.67 | –3.26 | 2.00 | J52 | 1.34 | 9.2 | 44 | 5.50 | -/1.6 × 10–5 | (133) | |
| 7-33 | IDTTC | –5.40 | –3.72 | 1.58 | PBDB-T | 1:1 | 1.01 | 18.3 | 73 | 13.5 | -/5.4 × 10–4 | (131) |
| 7-34 | IDTTTC | –5.34 | –3.69 | 1.55 | PBDB-T | 1:1 | 1.03 | 14.5 | 43 | 6.46 | -/3.8 × 10–4 | (131) |
| 7-35 | ITBC | –5.49 | –3.90 | 1.59 | PBDB-T | 1:1.2 | 0.94 | 19.9 | 65 | 12.1 | -/5.8 × 10–5 | (134) |
| 7-36 | ITBC | –5.61 | –4.13 | 1.53 | FTAZ | 1:1 | 0.72 | 11.9 | 49 | 4.17 | (138) | |
| ITTBC | –5.61 | –4.13 | 1.53 | PM6 | 1:1 | 0.86 | 12.5 | 63 | 6.83 | -/6.7 × 10–5 | (139) | |
| 7-37 | IDTT-R | –5.32 | –3.49 | 1.84 | P3HT | 1:1 | 0.78 | 0.83 | 48 | 0.43 | (140) | |
| 7-38 | IDTT-T | –5.51 | –3.51 | PTB7-Th | 1:2 | 1.02 | 18.0 | 65 | 11.8 | -/4.0 × 10–3 | (107) | |
| IDTT-TBTA (b) | –5.51 | –3.72 | 1.89 | PTB7-Th | 1:1.5 | 1.00 | 13.7 | 64 | 8.77 | -/3.1 × 10–4 | (141) | |
| 7-39 | IDTT-TBTA (a) | –5.46 | –3.72 | 1.89 | PTB7-Th | 1:1.5 | 1.00 | 10.2 | 53 | 5.44 | -/4.4 × 10–5 | (141) |
| 7-40 | IDTT-TBTA (c) | –5.45 | –3.67 | 1.87 | PTB7-Th | 1:1.5 | 0.97 | 12.0 | 63 | 7.41 | -/2.2 × 10–4 | (141) |
| 7-41 | IDTT-TBTA (d) | –5.42 | –3.59 | 1.89 | PTB7-Th | 1:1.5 | 1.01 | 9.10 | 51 | 4.69 | (141) | |
| 7-42 | IDTT-TBTA (e) | –5.48 | –3.65 | 1.89 | PTB7-Th | 1:1.5 | 0.92 | 6.72 | 41 | 2.52 | (141) | |
| 7-43 | NIDT | –5.34 | –3.58 | 1.85 | PBDB-T | 1.2:1 | 1.14 | 13.3 | 66 | 10.0 | 4.7 × 10–5/2.1 × 10–5 | (142) |
| 7-44 | ITIC-Cl-δ-Th | –5.31 | –3.70 | PM6 | 1:1 | 0.89 | 17.3 | 73 | 11.1 | -/1.2 × 10–4 | (143) | |
| 7-45 | ITIC-Cl-γ-Th | –5.30 | –3.66 | PM6 | 1:1 | 0.91 | 18.3 | 73 | 12.3 | -/2.1 × 10–4 | (143) | |
| 7-46 | ITIC-2Cl-Th | –5.31 | –3.74 | PM6 | 1:1 | 0.86 | 18.6 | 72 | 11.5 | -/8.3 × 10–4 | (143) | |
| 7-47 | A2 | –5.37 | –3.67 | 1.61 | J71 | 1:1 | 0.98 | 11.6 | 40 | 4.52 | 9.9 × 10–5/5.7 × 10–5 | (144) |
Obtained from the oxidation/reduction potential of the cyclic voltammetry (CV) measurement if not otherwise stated.
Other method or method not defined.
Determined via the space-charge limited current (SCLC) technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
The simplest and most frequent modification of the end groups includes the addition of substituents, such as halogens or methyl groups, on the phenyl ring of the INCN unit. Upon the introduction of two (7-2, IT-2F)111 (one on each side: INCN-F) or four (7-4, IT-4F)111 (two on each side: INCN-2F) fluorines on the molecule, the band gap is narrowed compared to ITIC and the HOMO/LUMO energy levels are shifted downward.112,113 Gao et al. reported the introduction of three (7-3, IT-3F)111 fluorine atoms on INCN groups, which has the same effect on the energy levels as for 7-2 and 7-4. Regarding the photovoltaic parameters, the JSC, FF, and PCE are increased significantly, whereas the VOC is decreased, compared to ITIC in combination with PM6.108,111 In contrast, the asymmetric molecule 7-5 (a-IT-2F),114 where fluorine is only attached at one end group, shows a similar PCE, lower VOC and FF, as well as higher JSC.106,114 When one chlorine (INCN-Cl) or two chlorines (INCN-2Cl) are attached to INCN (7-6, ITIC-2Cl and 7-7, ITIC-4Cl, respectively),115 it has a similar effect on the photovoltaic parameters and the energy levels as the addition of fluorine.108,115 Lai et al. investigated an asymmetric acceptor unit substitution with fluorine and/or chlorine atoms. Upon the introduction of two fluorines on one and one chlorine on the other INCN (7-8, ITIC-γCl-2F),116 the energy levels stayed in the same range as ITIC and are therefore higher than those for the fluorinated compounds 7-2–7-4. Solar cells with PM6 as a donor polymer are reaching 12%.
When two chlorines are attached only on one side as in 7-10 (α-ITIC-2Cl),117 the HOMO level is upshifted compared to ITIC or compared to the fluorinated counterpart 7-5. With the polymer donor PM6, solar cells reach a higher efficiency (12%) than 7-5 with PBDB-T (10%).114,117 In 7-9 (ITIC-2Cl-β),117 one chlorine is attached on each side on a specific position on the phenyl ring. Comparing with molecule 7-6, where the chlorine atom can be present anywhere on the phenyl ring, the HOMO/LUMO energy levels lie higher. With PM6 as the polymer donor and the same solvent additive, 7-6 possesses a higher efficiency of 13.2%, due to a much higher FF of 75% than 7-9.115,117 Qu et al. showed that the photovoltaic parameters differ when bromine atoms are substituted at different positions on the phenyl ring. The group synthesized two compounds, 7-11 (ITIC-2Br-m) and 7-12 (ITIC-2Br-γ),118 where 7-12 has the bromine located on one position on the INCN phenyl ring and 7-11 consists of isomers, where the bromine atom can be located on different positions (INCN-Br). Comparing these materials to each other, 7-11 has a significantly lower PCE of 10.9% than 7-12 with 12.1%.118 The reason for the great difference in efficiency could be the higher absorption coefficient and consequently the higher EQE, which increased the JSC value. In addition, the charge dissociation and recombination for both molecules in blend film were measured, implying a better dissociation and less recombination for 7-12. When halogens are added to the INCN group, the absorption maximum in film as well as in solution is red-shifted. In the case of bromine substituents, the shift is notably increased in film but about the same in solution.
A methyl group on the INCN acceptor unit (7-13, IT-M)119 shifts the energy levels upward but gives a similar band gap as ITIC.119−121 Like in 7-13, methyl groups are added to INCN in 7-16 (IT-DM),112 with the difference that two groups instead of one group are substituted on each side. The additional methyl groups affect the LUMO energy level by shifting it upward, consequently extending the band gap. When the acceptor 7-13 is combined with the polymer PDTF-TZNT, the photovoltaic parameters are generally lower compared to ITIC, except for the FF, which reveals a value of 73%.1197-16 with the polymer J71 reveals a higher VOC and PCE but lower JSC and FF compared to 7-13.112 Yao et al. introduced a trifluoromethyl group on the end groups (7-14, IT-CF3)122 with the effect that the LUMO energy level is downshifted, leading to a smaller band gap. The addition of trifluoromethyl groups also leads to a higher red-shift than simple fluorine atoms; here observed in film and solution.112 The solar cell parameters, compared to the fluorinated compounds (7-2–7-4), showed a lower VOC, higher JSC, higher FF, and comparable PCE.111,1227-15 (ITEN)108 was designed and synthesized by Yu et al. through adding ethynyl groups to extend the π-conjugation of the acceptors. The molecule possesses similar energy levels and a similar band gap as ITIC, but combined with PM6, it shows a better performance in solar cells (PCE 10.9%).108
Hao et al. strived to combine the advantages of the addition of methyl groups and fluorine on the end group, resulting in a molecule with two methyl groups and two fluorines on different positions of the terminal groups (7-17, ITCF).112 An electron-withdrawing group, like fluorine, should narrow the optical band gap of the resulting acceptor, whereas the electron-donating group, like a methyl group, should heighten the LUMO energy level. Compared to ITIC, the HOMO/LUMO energy levels are downshifted equally and the photovoltaic parameters are enhanced compared to the ITIC/J71 blend.1127-17 achieved a similar PCE of 13.3% as 7-14, which lies between the efficiencies of 7-2 and 7-4 (see Table 1). The replacement of fluorine substituents in the asymmetric structure of 7-5 with electron-donating methoxy groups leads to the structure 7-18 (a-IT-2OM).114 In contrast to 7-5, 7-18 has higher energy levels and a higher band gap. Solar cells with PBDB-T as a donor gave an improved VOC of 0.93 V and FF of 72% but a lower JSC of 18.1 mA cm–2. However, this leads to PCE values of 12.1%, which are higher than those of devices with 7-5 as well as with ITIC (7-1) and the same donor.114
The NFAs 7-19 (IT-OH)123 and 7-20 (IT-DOH)123 contain hydroxy groups on one and two INCN units, respectively. Compared to ITIC, the HOMO/LUMO energy levels are not affected by the additional group. Moreover, the absorption spectra in solution show the same maxima, but in thin films, the maxima are red-shifted for 7-19 and 7-20. Photovoltaic parameters reveal the same (7-19, 10.4%) or higher (7-20, 11.0%) efficiency than ITIC when combined with the same donor PBDB-T. When the NFAs are blended with PM6, the efficiency increases for 7-19 to 11.2% and for 7-20 to 12.5%.106,123
The substitution of the dicyanomethylene group of the INCN end group with an oxo group leads to the structures 7-21–7-23.124,125 In addition, fluorine and chlorine were introduced on the phenyl ring. Comparing the molecules with their INCN counterparts, 7-21 (IO-4H)125 without halogenation shows a downshifted HOMO and upshifted LUMO level, widening the band gap to 1.88 eV, while the energy levels of both 7-22 (IO-4F)125 and 7-23 (IO-4Cl)124 are downshifted further due to their halogenation. Solar cells of PBDB-T/7-21 obtained a high VOC value of 1.12 V but low values of JSC and FF, resulting in an overall PCE of 2.86%, whereas the efficiency of PBDB-T/7-22 and PM6/7-23 was higher (8.06 and 9.80%, respectively). All three acceptors show higher VOC’s compared to their INCN-based counterparts combined with the polymer PBDB-T;124−126 however, 7-21 and 7-22 show very poor electron mobilities in the blend film, which may be the reason for the lower performance compared to their parent compounds.
An alternative strategy to modify the INCN acceptor group, is changing the aromatic ring system from benzene to thiophene, resulting in 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (CPTCN)-based end groups. Thereby, the sulfur can be oriented in different directions depending on the orientation of the thiophene ring. In the case of molecule 7-24 (ITCPTC)127 without any substituents on their acceptor units, the change in the structure leads to a similar optical band gap but lowered energy levels compared to ITIC (7-1). Solar cells with 7-24 lead to PCEs up to 12.3%; also, the JSC and FF are increased compared with ITIC-based devices.127 Based on the structure of 7-24, different substituents such as chlorine (7-25, ITC-2Cl),128 bromine (7-26, ITC-2Br2),129 or methyl (7-27, MeIC)127 were introduced on the thiophene ring. As expected, the attachment of halogens generally lowers the energy levels compared to 7-24, while the methyl group has the opposite effect. Blended with PM6, the PCE of both acceptors (7-25 and 7-26) exceeded 13% and showed a VOC slightly lower than 1 V.128,1297-27 was also combined with PM6 and gave a VOC approaching 1 V and a PCE of 13.0%.127 The attachment of two methyl groups on the acceptor unit leads to molecule 7-28 (ITCT-DM).130 This adjustment upshifts the HOMO energy level further but leads to a similar optical band gap as in 7-27. Combined with PBDB-T, 7-28 shows lower PV parameters than 7-27 with a PCE of 10.6%.127,130 Molecule 7-29 (IDTC)131 contains similar acceptor units as 7-24, with the difference in the position of the S in the thiophene ring. This change in the position affects the energy levels by shifting them upward and widening the optical band gap to 1.68 eV. PV parameters show, compared to 7-24 with the same polymer donor PBDB-T, a higher VOC and otherwise slightly lower values of JSC, FF, and PCE.131,132 Luo et al. investigated three isomeric structures containing a bromine substituent on the thiophene ring in the end group (7-26, 7-30, 7-31).129 If the end group is asymmetric regarding the sulfur in the thiophene ring, the LUMO levels are slightly higher and the HOMO levels slightly lower than in the symmetric molecule. Following this, the band gap is widened (i.e., 1.59 eV for 7-26 and 1.73 eV for 7-30, ITC-2Br). The VOC is reaching 1 V with lower JSC and FF; the PCE is also lower and settled between 10.9 and 11.9%. The better PCE is achieved with 7-31 (ITC-2Br1), where the bromine is on position 3 in the 5-membered ring.1297-27 with a methyl group on position 3 and sulfur on position 2 possesses a smaller band gap and better photovoltaic parameters than 7-32 (ITCCM-O)133 with methyl on position 2 and sulfur in position 3, with the exception of the VOC. The difference between these two molecules is not only the position of the sulfur and the methyl group but also that the malononitrile group was substituted with an oxygen atom. This leads to very poor photovoltaic parameters except for the VOC, which reached an outstanding 1.34 V.127,133 These results are consistent with 7-21–7-23, where the band gap is also enlarged and the solar cells achieved a VOC of over 1 V.
Deng et al. investigated the extension of the acceptor units in 7-29 by one (7-33, IDTTC)131 or two (7-34, IDTTTC)131 thiophene units. The extension heightens the HOMO/LUMO energy levels while reducing the optical band gap to 1.58 and 1.55 eV, respectively. However, the introduction of one more thiophene unit as in 7-33 leads to a higher JSC, FF, and PCE of 18.3 mA cm–2, 73%, and 13.5%, respectively, compared to 7-29, whereas two additional thiophenes (7-34) decrease the PCE to 6.46%. The reason for that lies in the coarser morphology of these donor/acceptor blend films.131 The substitution of the outer thiophene ring in 7-34 with a phenyl ring leads to structure 7-35 (ITBC)134 and affects the energy levels by lowering them again. Compared to 7-34, the PV parameters of 7-35 with PBDB-T are improved to a JSC of 19.9 mA cm–2, a FF of 65%, a PCE of 12.1%, and a decreased VOC of 0.94 V.131,134
In acceptor 7-36 (ITBC),138 the oxo group of INCN was replaced with a SO2 functionality. This downshifted the energy levels but maintained a similar band gap as ITIC. Combined with the polymer FTAZ, it leads to a quite low VOC, JSC, and FF with a PCE of 4.17%.138 In molecule 7-37 (IDTT-R),140 a 2-(1,1-dicyanomethylene)rhodanine (RCN) acceptor unit is introduced, which heightens the HOMO/LUMO energy levels compared to ITIC. Due to a blue-shift in the absorption of 7-37, the optical band gap is widened to 1.84 eV. In solar cell devices, the acceptor was combined with P3HT, which leads to poor PV parameters with the highest efficiency being 0.43%.
He et al. added a common acceptor group, a diethyl thiobarbituric acid (TBA), to the IDTT core unit. This has a pronounced influence on the LUMO energy level of 7-38 (IDTT-T),107 which is shifted upward. The PV parameters are improved compared to ITIC in combination with the same polymer PTB7-Th. 7-38 and ITIC show VOCs of 1.02 and 0.83 V, JSCs of 18.0 and 14.4 mA cm–2, similar FFs of 65 and 66%, as well as PCEs of 11.8 and 7.80%, respectively. 7-38 also has an outstanding electron mobility when blended with PTB7-Th.107 Xiao et al. designed similar molecules to 7-38, with differences in the N-annulated side chains of the TBA group (7-39–7-42). Besides the ethyl chains in 7-38, they introduced methyl (7-39, IDTT-TBA (a)),141 benzyl (7-40, IDTT-TBA (c)),141 octyl (7-41, IDTT-TBA (d)),141 and ethylhexyl (7-42, IDTT-TBA (e))141 chains. All five molecules (7-38–7-42) have comparable HOMO energy levels ranging from −5.42 to −5.51 eV and optical band gaps between 1.87 and 1.89 eV. Photovoltaic devices were built with PTB7-Th as a donor material which gave very different results on each acceptor. The acceptor with branched ethylhexyl chains (7-42) shows the lowest performance with an efficiency of 2.52%. While the PCE increased to 4.69% upon the introduction of octyl chains (7-41) and to 5.44% with methyl groups (7-39), a significant change in efficiency is observed when benzyl (7-40) and ethyl groups (7-38) are used, enhancing the PCE to 7.41 and 8.77%, respectively. The reason for these differences lies in the molecular packing in combination with the polymer donor and may be enhanced under the use of other donor materials.141
Naphthalene monoimide (NMI) was used as a terminal group in structure 7-43 (NIDT),142 which shifts the energy levels up, resulting in an optical band gap of 1.85 eV. Combined with PBDB-T, this molecule gives over 1 V in OSCs but a smaller JSC, FF, and PCE than ITIC with the same donor material.142 Lai et al. synthesized a series of asymmetric molecules (7-44–7-46) with an INCN on one side and a CPTCN end group on the other. They differ in the substitution of the number and position of chlorine atoms on their INCN unit. The molecules 7-44 (ITIC-Cl-δ-Th)143 and 7-45 (ITIC-Cl-γ-Th)143 possess one chlorine atom in the δ-position and the γ-position, respectively, whereas 7-46 (ITIC-2Cl-Th)143 contains two chlorines on the acceptor unit. These three compounds show similar HOMO energy levels, which are, compared to ITIC, heightened but differ in their LUMO levels being −3.70 eV for 7-44, −3.66 eV for 7-45, and −3.74 eV for 7-46. In solar cells blended with PM6 as a donor polymer, these molecules have a VOC approaching 0.90 V, a FF of 72–73%, and efficiencies over 11%. The highest PCE was reached by 7-44 with 12.3%, which shows that the position of the halogen on the end group is crucial for the device performance.143 The acceptor 7-47 (A2)144 is also asymmetric, having an INCN end group on one side and a fullerene (C60) on the other. The introduction of a fullerene raised the HOMO/LUMO levels while retaining a similar band gap. In contrast, the photovoltaic parameters are low except for the VOC and they show a moderate PCE of 4.52%.144
3.2. Impact of Side Chains
Side chain engineering is used to ensure the solubility of a compound, to improve molecular packing and the film morphology, and thus has an immense effect on the photovoltaic properties. The replacement of the p-hexylphenyl side chains in ITIC with alternatives leads to the structures 7-48–7-61 (see Figure 6 and Table 2). In general, the overall influence on the HOMO/LUMO energy levels is small and their values are similar or slightly downshifted compared to ITIC; consequently, also the optical band gap is in the same range.
Figure 6.
Structures of non-fullerene acceptors with the same donor backbone and accepting units and different side chains.
Table 2. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-48–7-61.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μec(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-48 | IDTTIC | –5.68 | –3.84 | 1.51 | PBDB-T | 1:1 | 0.92 | 17.3 | 70 | 11.2 | -/1.2 × 10–4 | (145) |
| 7-49 | C8-ITIC | –5.63 | –3.91 | 1.55 | PFBDB-T | 1:1.25 | 0.94 | 19.6 | 72 | 13.2 | -/6.9 × 10–4 | (146) |
| 7-50 | ITIC-Th | –5.66 | –3.93 | PTFB-O | 1:1.5 | 0.92 | 16.7 | 68 | 10.9 | -/4.5 × 10–4 | (147) | |
| ITIC-Th | –5.65 | –4.05b | 1.60 | P(Cl) | 1:1.25 | 0.90 | 18.6 | 68 | 11.4 | -/5.1 × 10–4 | (157) | |
| 7-51 | sp-mOEh-ITIC | –5.67 | –4.06b | 1.61 | PBDB-T | 1:1 | 0.87 | 12.1 | 61 | 6.44 | (148) | |
| 7-52 | POIT-IC | –5.57 | –3.92 | 1.58 | PM6 | 1:1 | 1.04 | 16.1 | 60 | 10.1 | 1.8 × 10–4/6.4 × 10–4 | (149) |
| 7-53 | ITIC-OE | –5.67 | –4.03 | 1.57 | PBDB-T | 1:1 | 0.85 | 14.8 | 67 | 8.50 | 4.8 × 10–4/1.2 × 10–5 | (106) |
| 7-54 | ITIC-OEG | –5.39 | –3.99 | 1.54 | PPDT2FBT | 1:1 | 0.90 | 3.56 | 49 | 1.58 | 1.9 × 10–5/- | (150) |
| 7-55 | m-ITIC | –5.68 | –3.95 | 1.59 | PBDS-T | 1:1.5 | 0.99 | 16.8 | 62 | 10.3 | (151) | |
| 7-56 | mO-ITIC | –5.50 | –3.74 | 1.63 | PTZ-DO | 1:1 | 0.92 | 15.2 | 66 | 9.28 | (152) | |
| m-ITIC-O-H | –5.25 | –3.65b | 1.60 | PBDB-T | 1:1 | 0.85 | 16.0 | 70 | 9.55 | -/9.4 × 10–6 | (153) | |
| 7-57 | m-ITIC-O-EH | –5.25 | –3.63b | 1.62 | PBDB-T | 1:1 | 0.88 | 15.9 | 68 | 9.77 | -/7.1 × 10–6 | (153) |
| 7-58 | FpO-ITIC | –5.61 | –3.72 | 1.64 | PTZ-DO | 1:1 | 0.88 | 12.6 | 61 | 6.69 | (152) | |
| 7-59 | oF-ITIC | –5.73 | –3.93 | 1.63 | PBTIBDTT | 1:1.2 | 0.94 | 13.5 | 71 | 9.01 | 3.6 × 10–4/3.8 × 10–4 | (154) |
| m-F-ITIC | –5.69 | –3.96 | 1.62 | PBDB-T | 1.3:1 | 0.88 | 15.8 | 64 | 8.90 | 9.4 × 10–5/3.7 × 10–5 | (155) | |
| 7-60 | mF-ITIC | –5.66 | –3.88 | 1.60 | PBTIBDTT | 1:1 | 0.96 | 14.8 | 67 | 9.55 | 3.0 × 10–4/3.0 × 10–4 | (154) |
| o-F-ITIC | –5.66 | –3.94 | 1.58 | PBDB-T | 1.3:1 | 0.92 | 18.1 | 67 | 11.1 | 4.8 × 10–4/4.1 × 10–4 | (155) | |
| 7-61 | ITC6-IC | –5.72 | –3.78 | 1.60 | PBDB-T | 1:1 | 0.94 | 16.2 | 71 | 10.9 | -/6.6 × 10–4 | (156) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Structures 7-48 (IDTTIC)145 and 7-49 (C8-ITIC)146 are the only acceptors with pure alkyl chains (C16 and C8, respectively) which reduce the band gaps significantly to 1.51 and 1.55 eV. The C16 chains give 7-48 a high crystallinity; however, in the blend film with PBDB-T, this crystallinity is largely suppressed. Compared to ITIC, solar cells based on 7-48 and PBDB-T show an improved PCE of 11.2% and enhanced VOC and JSC.145 When the alkyl chains only contain eight C atoms (7-49), the PCE of solar cells with PBDB-T is increased to 11.9%, and in comparison to 7-48, the VOC, JSC, and FF decreased slightly.145,146 Another prominent compound is ITIC-Th (7-50)147 in which the phenyl ring is substituted by a thiophene. Compared to ITIC, the energy levels are downshifted to −5.66 and −3.93 eV. Different polymers were combined with 7-50, showing that they greatly influence the crystallinity within the blend film. The molecular ordering is stronger in the PTFB-O/ITIC-Th blend film, which is also consistent with the PV parameters. PTFB-O/ITIC-Th blended active layers give a PCE over 10%, while all other tested polymers barely reached over 8%.147
Sung et al. added fluorene units with alkyl chains to the seven-ring backbone, giving 7-51 (sp-mOEh-ITIC)148 with two spirobifluorenes implemented in the acceptor. The solar cell parameters are better with chlorobenzene than with o-xylene; the highest PCE reached was 6.44%. GIWAXS measurements were done with the neat film of the acceptor, giving a wide halo pattern with a mixture of face-on and edge-on orientations.148
The introduction of an alkoxy side chain on the phenyl group (7-52, POIT-IC)149 leads to good PV parameters with a VOC over 1 V and a PCE above 10%. Upon measuring the crystallinity in pure films, the acceptor shows mixed face-on and edge-on orientations. Combined with PM6, the film favors the face-on orientation resulting from the arrangement of the polymer. PM6 also suppresses the crystallinity of 7-52 due to their good miscibility.149 Oligoethylene glycol side chains were introduced in acceptors 7-53 (ITIC-OE)106 and 7-54 (ITIC-OEG).150 They show a lower PCE than ITIC when blended with PBDB-T and PPDT2FBT, respectively. Thereby, 7-54 only reached PCEs of approx. 1.5%, which may be due to the usage of the polymer PPDT2FBT, as 7-53 led to an efficiency above 8%. Another reason could be the chain length of the oligoethylene glycol unit. The acceptors and polymers as well as ITIC itself were investigated with GIWAXS. ITIC, PPDT2FBT, and PBDB-T show strong lamellar stacking peaks in the in-plane and π–π stacking peaks in the out-of-plane direction. Both, 7-53 and 7-54 exhibit π–π stacking peaks in the out-of-plane direction, whereas the peak of 7-53 is much weaker compared to ITIC, which indicates less crystallinity. In contrast, 7-54 has a sharply resolved π–π stacking peak, suggesting a tighter stacking and the coherence length of 7-54 implying a promotion of the intermolecular packing. The same is present in the blend films. Acceptor 7-54 suffers from poor electron mobility in neat film, and 7-53 shows a low mobility in the blend film.106,150
Chen et al. investigated the influence of the hexyl chain position on the phenyl ring of the side chain (7-55, m-ITIC).151 They added the alkyl chain at the meta position which leads, combined with PBDS-T, to rather good photovoltaic properties in the same range as ITIC.151 Exchanging the alkyl side chain in the meta position with an alkoxy chain (7-56, mO-ITIC)164 leads to slightly blue-shifted absorption maxima and similar energy levels as ITIC. However, also the PV parameters are slightly lower. Compared to 7-52, where the alkoxy side chains are located in the para position, the energy levels are slightly upshifted and the band gap widened. The absorption maxima are also slightly blue-shifted. The PCE achieved for 7-56 is 9.3% and that for 7-52 is 10.1%; however, another conjugated polymer was used. Under the same conditions and the use of the same polymer, the acceptor 7-52 reached only 9.0% efficiency. Both acceptors were investigated using GIWAXS, giving an idea about the crystallinity, which is higher for 7-56 than for 7-52.149,152 Lee et al. investigated molecule 7-56 and reported another acceptor, where the hexyloxy chains are replaced with ethylhexyloxy chains (7-57, m-ITIC-O-EH). Blended with PBDB-T as a donor, solar cells show comparable parameters with 7-56 and 7-57. The higher PCE was achieved by 7-57 with 9.77% compared to 7-56 with 9.55%. However, the higher electron mobility in the blend film was reached by 7-56.153 Upon the introduction of a fluorine atom in the meta-position (7-58, FpO-ITIC),152 the device parameters decreased further, reaching only 6.69% efficiency. When comparing the acceptor to 7-52, the introduction of fluorine blue-shifts the absorption maxima of the NFA and downshifts the energy levels, as was expected for halogens.149,152 The neat film of 7-58 has a broad lamellar stacking peak indicating weak side chain packing; the absence of π–π stacking peaks in both directions suggests a weak crystallinity compared to ITIC, just like the para-substituted 7-52. Acceptor 7-56 shows a slightly higher crystallinity in pure film than 7-58.152 The additional fluorine in the side chain has no effect on the crystallinity of the NFA.
Finally, the influence of fluorine on the phenyl ring of the phenylhexyl side chain was investigated by introducing fluorine in either the meta (7-59, oF-ITIC/m-F-ITIC)154,155 or the ortho (7-60, mF-ITIC/o-F-ITIC)154,155 position to the backbone. Both molecules show similar energy levels and optical band gaps (1.58–1.63 eV); however, the efficiency of 7-60 is increased to 11.1% compared to 7-59 (PCE: 8.9%) when PBDB-T is used as a donor.155 Compared to ITIC, the absorption maxima of both acceptors are slightly blue-shifted, leading to a slightly higher band gap compared to ITIC, and the electron mobility in neat film is higher for ITIC than for 7-59 and 7-60.154,1557-61 (ITC6-IC)156 has a backbone and side chains similar to ITIC but additional hexyl chains on the outer thiophene rings of the donor unit. Compared to ITIC, the HOMO energy level is downshifted and the LUMO level and optical band gap are similar. Combined with PBDB-T, the solar cells show a VOC of 0.94 V, a JSC of 16.2 mA cm–2, a FF of 71%, and a PCE of 10.9%.156
3.3. Combined Effects in IDTT-Based Acceptors
The combination of modifying the side chain and variation of the acceptor end groups even further increases the structural diversity of IDTT-based acceptors, as shown in Figure 7. For example, the combination of the structure 7-13—ITIC methylated on the INCN-acceptor units—with variation in the end groups on the central core—hexyloxy groups in the para or meta position—leads to the structures 7-62 (POIT-M)120 and 7-63 (MOIT-M).120 As expected, their electrochemical and optical properties are comparable with 7-13. A comparison of solar cells using these three acceptors and PTZ1 as donor polymers leads to similar VOC values of approx. 0.97 V for all three acceptors, but the JSC as well as the FF increases from 14.2 mA cm–2 and 62% for 7-13 to 15.4 mA cm–2 and 65% for 7-62 and to 17.5 mA cm–2 and 69% for 7-63 (see also Table 3). Consequently, 7-63, with the alkoxy group in the meta position, leads to the highest PCE of 11.6% (compared to 9.10% for 7-13 and 9.70% for 7-62). GIWAXS data of the three acceptors indicate that 7-63 has a stronger intermolecular π–π stacking interaction and thus has a higher crystallinity and more ordered molecular orientation than 7-13 and 7-62. Moreover, the electron mobility of 7-63 is higher, which is also an indicator of why this acceptor works better than the other two.120 In very similar approaches, the influence of side chain variation on the IDTT core on the fluorinated ITIC analogues 7-2 and 7-4 was investigated. This leads to structures 7-64 (m-ITIC-2F)158 and 7-65 (m-ITIC-4F),158 where the hexyl side chains were placed in meta position on the phenyl ring. The HOMO and LUMO energy levels for 7-64 and 7-65 are −5.73 and −3.95 eV and −5.73 and −4.02 eV, respectively, which are lower than the values for structures 7-2 and 7-4. The fluorination leads to a reduction of the optical band gap to 1.56 and 1.53 eV for 7-64 and 7-65 compared to 1.59 eV for the unfluorinated 7-55. Organic solar cells with PTQ10 as a polymer donor exhibited efficiency values of 12.5%. 7-64 has a higher degree of self-organization and molecular packing than 7-65, which also agrees with the mobility data of the two materials.158
Figure 7.
Structures of non-fullerene acceptors with an IDTT core and different side chains and end groups.
Table 3. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-62–7-101.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μed(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-62 | POIT-M | –5.60 | –3.87 | 1.60 | PTZ1 | 1:1 | 0.97 | 15.4 | 65 | 9.70 | 6.2 × 10–4/3.7 × 10–4 | (120) |
| 7-63 | MOIT-M | –5.61 | –3.89 | 1.60 | PTZ1 | 1:1 | 0.96 | 17.5 | 69 | 11.6 | 8.3 × 10–4/6.0 × 10–4 | (120) |
| 7-64 | m-ITIC-2F | –5.73 | –3.95 | 1.56 | PTQ10 | 1:1 | 0.96 | 19.0 | 69 | 12.5 | 3.3 × 10–4/4.0 × 10–4 | (158) |
| 7-65 | m-ITIC-4F | –5.73 | –4.02 | 1.53 | PTQ10 | 1:1 | 0.90 | 19.8 | 70 | 12.5 | 2.9 × 10–4/5.1 × 10–4 | (158) |
| 7-66 | IT-4Cl-C8 | –5.94 | –4.10 | PM6 | 1:1 | 0.83 | 20.4 | 76 | 12.9 | (159) | ||
| 7-67 | IT-4Cl-C10 | –5.95 | –4.08 | PM6 | 1:1 | 0.87 | 20.3 | 77 | 13.5 | (159) | ||
| 7-68 | IDMIC-4F | –5.46 | –3.83 | PM6 | 1:1.2 | 0.89 | 16.6 | 61 | 9.40 | (160) | ||
| 7-69 | m-ITIC-OR-4Cl | –5.78 | –4.05 | 1.50 | PTQ10 | 1:1.5 | 0.87 | 21.8 | 67 | 12.7 | -/3.4 × 10–4 | (161) |
| 7-70 | POIT-IC2F | –5.60 | –3.98 | 1.55 | PM6 | 1:1 | 0.97 | 18.6 | 69 | 12.4 | 2.2 × 10–4/6.6 × 10–4 | (149) |
| 7-71 | POIT-IC4F | –5.65 | –4.07 | 1.49 | PM6 | 1:1 | 0.91 | 20.9 | 73 | 13.8 | 4.2 × 10–4/8.0 × 10–4 | (149) |
| 7-72 | IDTT-OB | –5.59 | –3.88 | 1.59 | PBDB-T | 1:1 | 0.91 | 16.4 | 74 | 11.2 | -/6.5 × 10–4 | (162) |
| 7-73 | ITPN | –5.69 | –3.91 | 1.60 | PM6 | 0.99 | 17.5 | 73 | 12.6 | 4.4 × 10–4/7.5 × 10–4 | (108) | |
| 7-74 | ITIC-Th1 | –5.70 | –4.12 | PBDB-T | 1:1 | 0.88 | 19.6 | 70 | 12.0 | -/9.8 × 10–3 | (135) | |
| 7-75 | ITThBC | –5.71 | –4.15 | 1.58 | PM6 | 1:1 | 0.87 | 13.4 | 65 | 7.59 | -/9.4 × 10–5 | (139) |
| 7-76 | ITzN-C9 | –5.62 | –3.78 | 1.65 | PM6 | 1:1 | 1.05 | 14.1 | 64 | 9.51 | 0.3 × 10–4/3.5 × 10–4 | (165) |
| 7-77 | ITzN-F4 | –6.00 | –4.20 | 1.58 | PM6 | 1:1 | 0.92 | 17.5 | 68 | 10.9 | -/3.5 × 10–4 | (166) |
| 7-78 | ITN-C9 | –5.78 | –3.92 | 1.54 | PM6 | 1:1 | 0.92 | 15.7 | 65 | 9.33 | 2.3 × 10–4/4.5 × 10–4 | (165) |
| 7-79 | ITN-F4 | –6.10 | –4.30 | 1.49 | PM6 | 1:0.8 | 0.82 | 19.6 | 67 | 10.7 | -/1.9 × 10–4 | (166) |
| 7-80 | IDTT-BH | –5.42 | –3.86 | 1.54 | J71 | 1:1.4 | 0.90 | 17.8 | 69 | 11.1 | -/1.6 × 10–4 | (164) |
| 7-81 | IDTT-OBH | –5.41 | –3.86 | 1.57 | PBDB-T | 1:0.8 | 0.87 | 17.5 | 72 | 10.9 | -/9.8 × 10–5 | (164) |
| 7-82 | C8-IT-4F | –5.57 | –4.04 | 1.47 | PM7 | 1:1 | 0.82 | 22.7 | 77 | 14.3 | 1.3 × 10–3/1.1 × 10–3 | (167) |
| 7-83 | MF2 | –5.61 | –3.93 | 1.49 | PM7 | 1:1 | 0.96 | 19.2 | 75 | 13.7 | 7.7 × 10–4/2.6 × 10–4 | (168) |
| 7-84 | C8IDTT-4Cl | –5.76 | –4.29 | 1.43 | PBDT-TPD | 1:1 | 0.67 | 19.1 | 59 | 7.55 | -/3.6 × 10–4 | (169) |
| 7-85 | IDTTA | –5.81c | –3.97 | 1.75 | PBDB-T | 1:1 | 0.98 | 15.8 | 69 | 10.8 | 1.8 × 10–4/1.0 × 10–4 | (170) |
| 7-86 | C6-IDTT-T | –5.71 | –3.78 | 1.82 | PTB7-Th | 1:1.1 | 1.05 | 14.4 | 56 | 8.51 | 1.3 × 10–4/8.0 × 10–5 | (171) |
| 7-87 | 2C6-IDTT-T | –5.74 | –3.72 | 1.84 | PTB7-Th | 1:1.1 | 1.07 | 13.3 | 53 | 7.52 | 7.3 × 10–5/4.2 × 10–5 | (171) |
| 7-88 | ITC6-2F | –5.73 | –3.81 | 1.58 | PBDB-T | 1:1 | 0.86 | 17.8 | 73 | 11.2 | -/1.6 × 10–5 | (156) |
| 7-89 | ITC6-4F | –5.74 | –3.84 | 1.54 | PBDB-T | 1:1 | 0.78 | 18.6 | 73 | 10.5 | -/2.7 × 10–4 | (156) |
| 7-90 | IM-4F | –5.69 | –4.19 | 1.46 | PM6 | 1:1 | 0.88 | 22.1 | 73 | 14.2 | -/5.2 × 10–4 | (172) |
| 7-91 | IOM-4F | –5.72 | –4.27 | 1.48 | PM6 | 1:1 | 0.86 | 21.7 | 72 | 13.4 | -/4.8 × 10–4 | (172) |
| 7-92 | sp-mOEh-ITIC-F | –5.77 | –4.23b | 1.54 | PBDB-T | 1:1 | 0.68 | 13.7 | 62 | 5.79 | (148) | |
| 7-93 | sp-mOEh-ITIC-Cl | –5.74 | –4.22b | 1.52 | PBDB-T | 1:1 | 0.69 | 14.2 | 59 | 5.78 | (148) | |
| 7-94 | sp-mOEh-ITIC-M | –5.69 | –4.07b | 1.62 | PBDB-T | 1:1 | 0.90 | 11.4 | 58 | 5.96 | (148) | |
| 7-95 | C8-ITCC | –5.45 | –3.85 | 1.66 | PM6 | 1:1 | 1.04 | 16.1 | 63 | 10.8 | -/2.7 × 10–4 | (173) |
| 7-96 | C8-ITCC-Cl | –5.50 | –3.93 | 1.58 | PM6 | 1:1 | 0.95 | 17.9 | 73 | 12.7 | -/6.7 × 10–4 | (173) |
| 7-97 | IDTT-C6-TIC | –5.55 | –3.99 | 1.60 | PBT1-C | 1:1.3 | 0.85 | 17.0 | 67 | 10.0 | 1.2 × 10–3/2.0 × 10–4 | (174) |
| 7-98 | IDTT-C8-TIC | –5.64 | –3.97 | 1.59 | PBT1-C | 1:1.3 | 0.88 | 20.3 | 75 | 13.7 | 2.2 × 10–4/1.2 × 10–4 | (174) |
| 7-99 | IDTT-C10-TIC | –5.71 | –3.91 | 1.61 | PBT1-C | 1:1.3 | 0.98 | 18.1 | 71 | 12.7 | 9.8 × 10–5/6.7 × 10–5 | (174) |
| 7-100 | m-ITTC | –5.62 | –3.89 | J71 | 1:1.2 | 0.87 | 19.3 | 74 | 12.4 | -/4.4 × 10–5 | (175) | |
| 7-101 | m-MeIC | –5.54 | –3.90 | 1.54 | J71 | 0.92 | 18.5 | 69 | 11.7 | -/2.5 × 10–4 | (176) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
HOMO obtained via photoelectron spectroscopy in air (PESA).
Determined via SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
7-66 (IT-4Cl-C8)159 and 7-67 (IT-4Cl-10)159 have chlorine atoms attached on their INCN units and octylphenyl and decylphenyl side chains, respectively. They differ only slightly in their energy levels and optical band gaps; compared to ITIC, the HOMO/LUMO energy levels are downshifted and the optical band gap is widened. In combination with PM6, solar cells with 7-66 and 7-67 show similar JSC and FF values, but PM6/7-66 gives a lower PCE with 12.9% than PM6/7-67 (13.5%).159 The NFA 7-68 (IDMIC-4F)160 has again fluorinated acceptor units, but its side chains are dimethylphenyl groups. The combination of the fluorines and the shorter side chains gave HOMO and LUMO energy levels of −5.46 and −3.83 eV. Blended with PM6, solar cells show a good performance with a PCE of 9.40%. Compared with 7-4, bearing longer side chains, in combination with PM6, the VOC is in the same range, but the PCE is exceeding 13%.111,160
Molecule 7-69 (m-ITIC-OR-4Cl)161 contains INCN-2Cl acceptor units and hexyloxyphenyl side chains, where the alkyloxy chain is present in the meta-position to the backbone. The HOMO/LUMO energy levels as well as the optical band gap are in the same range as for the chlorinated ITIC counterpart 7-7. Photovoltaic parameters of PM6/7-69 show a VOC of 0.83 V, a JSC of 21.6 mA cm–2, a FF of 70%, and a PCE of 12.9%, while the PM6/7-7 combination gives a lower VOC but a higher JSC, FF, and PCE.161 Replacing the p-hexyl chains in 7-2 and 7-4 with hexyloxy side chains leads to the structures 7-70 (POIT-IC2F) and 7-71 (POIT-IC4F).149 Compared to the unfluorinated compound 7-62, the energy levels are lowered, slightly more for 7-71 than 7-70, and the optical band gap is 1.49, 1.55, and 1.60 eV for 7-71, 7-70, and 7-62, respectively. Solar cells of the fluorinated acceptors with PM6 have high PCEs over 12%, with a maximum of 13.8%.149 Compound 7-72 (IDTT-OB)162 has INCN end groups with a methyl substituent. On each side of the backbone, two side chains are present, one being an octyl chain and the other being a hexylphenyl chain. The HOMO/LUMO energy levels and the optical band gap are similar to molecule 7-13, where similar side chains (hexylphenyl) are attached on each side. In combination with PBDB-T, an efficiency of 11.2% can be reached.119,162 In 7-73 (ITPN),108 the side chains were extended and branched to butyloctyl chains and the phenylene was changed to thiophenes. Compared to 7-72, the HOMO/LUMO energy levels are lowered; however, the optical band gap remains at about 1.60 eV. The PV parameters in combination with PM6 are enhanced (except the FF) to a VOC of 0.99 V and a PCE of 12.6%. The small molecule acceptor 7-74 (ITIC-Th1)135,163 has thiophene rings in the side chains instead of phenyl rings, and compared to 7-2, the energy levels are downshifted further. Solar cells based on 7-2 blended with PBDS-T reveal a VOC of 0.90 V, JSC of 18.6 mA cm–2, FF of 67%, and PCE of 11.2%. However, 7-74 reached 12.0% efficiency, a better FF of 70%, and JSC of 19.6 mA cm–2 but a slightly lower VOC of 0.88 V using the same donor. The non-fluorinated counterpart of 7-74, ITIC-Th (7-50), achieved 10.9% PCE.135 NFA 7-75 (ITThBC)139 has the same side chains as ITIC-Th (7-50) but contains a sulfone group instead of a carbonyl in the acceptor unit. In comparison with 7-50, 7-75 has downshifted energy levels and a similar optical band gap. Solar cell parameters with PM6 are also lower with the highest efficiency being 7.59% (P(Cl)/7-50 = 11.4%). This difference may lie in the selection of the polymer donor and the resulting low electron mobility of 9.4 × 10–5 cm2 V–1 s–1 of the PM6/7-75 blend film.139,157
The extension of the π-system on the acceptor units by using naphthyl instead of phenyl rings leads in combination with alternative side chains on the IDTT core to molecules 7-76–7-81.164−166 Compared to ITIC, it has the effect that the energy levels are slightly upshifted for 7-76 (ITzN-C9)165 and downshifted for 7-78 (ITN-C9).165 The fluorinated counterparts of 7-76 and 7-78, 7-77 (ITzN-F4),166 and 7-79 (ITN-F4),166 respectively, have much lower HOMO/LUMO energy levels (see Table 3). The [a]-annulated naphthalene derivative (7-76) has an optical band gap of 1.65 eV, while the [b]-annulated naphthalene derivative (7-78) has a band gap of 1.54 eV, and thereby, both are wider compared to 7-77 and 7-79. Solar cells of 7-76 and 7-78 with PM6 reveal a similar PCE of approx. 9.5%, with high VOC values of up to 1.05 V for 7-76. As expected, the lower band gap of 7-78 leads to lower VOC but higher JSC values with similar FFs in both cases (65%).165 Compared to that, the fluorine-containing molecules 7-77 and 7-79 combined with PM6 have similar to lower VOCs but reach PCEs of over 10.5%. The reason therefore may be the higher JSC and FF values, although the electron mobility in the blend film is lower.166 Structures 7-80 (IDTT-BH) and 7-81 (IDTT-OBH)164 differ from 7-78 only in the side chains on the central IDTT core, i.e., using a 2-butyloctyl-side chain in 7-80 and a 2-butyloctyloxy side chain in 7-81.164 As expected, the optical band gaps (1.54 and 1.57 eV) are very similar to the value of 7-78. By screening different donor polymers, solar cells based on 7-80 reached with J71 the best photovoltaic performance with PCEs up to 11.1%, whereas 7-81 worked best with PBDB-T as a donor, yielding a PCE of 10.9%.164,165
Compounds 7-82–7-84 have all octyl side chains and INCN acceptor units with different substitution on the phenyl ring. 7-82 (C8-IT-4F)167 is the fluorinated counterpart to 7-49; in comparison, the LUMO of 7-82 is downshifted, whereas the HOMO energy level is similar to the one of 7-49. Solar cells with PM7/7-82 revealed an efficiency of 14.3% with a high FF of 77% and high electron mobilities of 1.1 × 10–3 cm2 V–1 s–1 in the as-cast blend film. 7-83 (MF2)168 differs from 7-82 in the replacement of one fluorine by one methyl group on each acceptor unit. This leads to a heightened LUMO and an unchanged HOMO energy level with an optical band gap of 1.49 eV. Solar cells with PM7 lead to lower PCEs of 13.7%, due to lower FFs and electron mobilities.168 Replacing the fluorines in 7-82 with chlorines leads to molecule 7-84 (C8IDTT-4Cl).169 In comparison, the energy levels are further downshifted and 7-84 shows an optical band gap of 1.43 eV. Blended with PBDT-TPD, solar cells show an efficiency of 7.55%.169
The exchange of the acceptor unit in 7-49 and 7-82–7-84 from INCN to diethyl TBA leads to structure 7-85 (IDTTA).170 Compared to 7-49, 7-85 exhibits lower HOMO/LUMO energy levels and a wider optical band gap. Solar cell devices with the polymer PBDB-T gave a high VOC of 0.98 V and a PCE of 10.8%. 7-49/PBDB-T achieved a lower VOC of 0.86 V and a higher PCE of 11.9%, even though the electron mobility is higher for this blend film.146,170 The replacement of octyl with hexylphenyl side chains and the introduction of additional alkyl chains on one or two outer thiophene rings in the backbone leads to molecules 7-86 (C6-IDTT-T)171 and 7-87 (2C6-IDTT-T),171 respectively. These changes lower the energy levels compared to 7-85 and broaden the optical band gap further, thus leading to higher VOC values >1 V in solar cells with PTB7-Th. 7-86 with only one additional hexyl chain achieved a higher PCE of 8.51% compared to 7-87 (7.52%), which may be attributed to the higher electron mobility and thus faster charge transport.171 The same backbone with two additional hexyl side chains is present in structures 7-88 (ITC6-2F)156 and 7-89 (ITC6-4F).156 Like their parent compound, 7-61, the acceptor units are again INCN based with either one or two fluorines on each acceptor unit in 7-88 and 7-89, respectively. The fluorines shift the LUMO energy levels slightly downward; the optical band gap is narrowed with increasing fluorine content. Solar cells were built with PBDB-T, and the best PCE value (11.2%) of these three acceptors was achieved by 7-88.156
The structures 7-90 (IM-4F)172 and 7-91 (IOM-4F)172 have fluorines attached on their acceptor units and additional side chains, but unlike 7-88 and 7-89, these alkyl and alkyloxy units are located on the central phenyl ring of the backbone. Compared to 7-4, the additional methyl and methoxy groups lower the LUMO energy levels to −4.19 and −4.27 eV for 7-90 and 7-91, consequently narrowing the band gap to 1.46 and 1.48 eV, respectively. The photovoltaic parameters of PM6/7-90-based devices show a VOC of 0.88 V, a FF of 73%, and a high PCE of 14.2%, whereas PM6/7-91 reached a VOC of 0.86 V, a FF of 72%, and a PCE of 13.4%. The reason for the higher efficiency in 7-90 is attributed to the improved JSC of 22.1 mA cm–2 and the increased electron mobility.172
Sung et al. investigated the influence of substitution on the INCN acceptor units in spirobifluorene-containing IDTT-based acceptor structures (7-51, 7-92–7-94). Whereas the introduction of the methyl group does not significantly change the HOMO/LUMO energy levels and the optical band gap (1.62 eV for 7-94, sp-mOEh-ITIC-M,148 and 1.61 eV for 7-51), the halogenation shifted both to lower values (1.54 eV for the fluorinated structure 7-92, sp-mOEh-ITIC-F,148 and 1.52 eV for the chlorinated counterpart 7-93, sp-mOEh-ITIC-Cl).148 Overall, solar cells with these acceptors and PBDB-T as a donor lead to similar efficiencies with PCEs in the range of 5.80–6.40%.
Zhang et al. investigated IDTT-based acceptors with octyl side chains on the central core and CPTCN end groups (7-95, C8-ITCC) and compared them to 7-96 (C8-ITCC-Cl) with chlorinated CPTCN (CPTCN-Cl).173 In both cases, a mixture of structural isomers are obtained due to the different annulation of the thiophene ring. The chlorinated acceptor has lower energy levels and a smaller optical band gap than the non-halogenated counterpart. The photovoltaic parameters are also improved, giving a PCE of 12.7% and a slightly lower VOC of 0.95 V. In contrast, 7-95 reached a PCE of 10.8% and a VOC of 1.04 V.173 Similar structures with CPTCN acceptor units are designed by Ye et al., which differ in the length of their side chains on the central core being 6 (7-97, IDTT-C6-TIC),174 8 (7-98, IDTT-C8-TIC),174 or 10 (7-99, IDTT-C10-TIC)174 carbon atoms long. Upon the extension of the alkyl chain, the HOMO levels are lowered, whereas the LUMO levels are heightened. In solar cells with PBT1-C, 7-97 showed the lowest efficiency with 10.0%, followed by 7-99 with 12.7% and 7-98 with 13.7%. It shows that in this case the octyl-chain-containing structure yielded the best results, due to a more refined morphology in the blend film and therefore higher charge transport.174 Replacing the alkyl chain in 7-97–7-99 with hexylphenyl chains with the hexyl residue in the meta position leads to molecules 7-100 (m-ITTC)175 with HOMO/LUMO energy levels of −5.62 and −3.89 eV, respectively. The introduction of an additional methyl group on the CPTCN (CPTCN-Me) leads to 7-101 (m-MeIC)176 with downshifted HOMO/LUMO energies. Both acceptors were blended with J71 and implemented in solar cells, which yielded 12.4% for 7-100 and 11.7% for 7-101.175,176
The IDTT building block was used for the preparation of a variety of other NFAs, summarized in Figure 8 and Table 4. For example, IDTT was coupled with the electron-deficient benzothiadiazole (BT) π-bridge and N-alkylated RCN end groups with different alkyl chains leading to the NFAs 7-102–7-105 (ITBTR-C2–ITBTR-C8).177 All four acceptors have similar HOMO/LUMO energy levels of about −5.30 and −3.70 eV, respectively, which are upshifted in comparison to those of ITIC. The optical band gaps of all four NFAs show similar values around 1.52 eV, as the alkyl chains do not influence the electronic structure here. Solar cells with PBDB-T reveal similar values of all photovoltaic parameters. The PCEs varied slightly between the NFAs, giving 7.04, 7.43, 8.26, and 7.93% for 7-102, 7-103, 7-104, and 7-105, respectively. According to GIWAXS data of the neat acceptor films, 7-102 tends to crystallize more than the other three, whereas 7-104, with the highest efficiency, shows more ordered packing compared to 7-103 and 7-105.177
Figure 8.
Structures of non-fullerene acceptors with IDTT core units and π-spacer/dimeric compounds.
Table 4. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-102–7-113.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μec(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-102 | ITBTR-C2 | –5.30 | –3.70 | 1.52 | PBDB-T | 1:1 | 0.89 | 13.5 | 59 | 7.04 | -/8.6 × 10–6 | (177) |
| 7-103 | ITBTR-C4 | –5.30 | –3.71 | 1.52 | PBDB-T | 1:1 | 0.90 | 14.9 | 55 | 7.43 | -/2.4 × 10–5 | (177) |
| 7-104 | ITBTR-C6 | –5.30 | –3.71 | 1.52 | PBDB-T | 1:1 | 0.89 | 14.9 | 58 | 8.26 | -/6.6 × 10–5 | (177) |
| 7-105 | ITBTR-C8 | –5.28 | –3.70 | 1.49 | PBDB-T | 1:1 | 0.90 | 14.9 | 59 | 7.93 | -/3.4 × 10–5 | (177) |
| 7-106 | IDTTBM | –4.03b | PTB7-Th | 1:1 | 0.83 | 15.5 | 63 | 8.40 | (178) | |||
| 7-107 | IDTT-BO-MN | –5.98 | –3.81 | 2.17 | P3HT | 1:2 | 0.93 | 10.2 | 61 | 5.75 | -/9.1 × 10–4 | (179) |
| 7-108 | BTA13 | –5.34 | –3.62b | 1.72 | J52-F | 1:1 | 1.18 | 11.6 | 61 | 8.36 | -/6.2 × 10–5 | (180) |
| 7-109 | IDTT-CR | –5.25 | –3.38 | 1.74 | P3HT | 1:1 | 0.82 | 5.61 | 55 | 2.52 | (140) | |
| 7-110 | IDTT-CT | –5.28 | –3.65 | 1.63 | PTB7-Th | 1:1 | 0.93 | 12.6 | 51 | 5.89 | (140) | |
| 7-111 | ITOTIC-2F | –5.22 | –4.11 | 1.32 | PTB7-Th | 1:1.5 | 0.76 | 7.00 | 61 | 3.70 | -/5.1 × 10–6 | (181) |
| 7-112 | FDTBT-IDTT-FINCN | –5.46 | –3.92 | 1.51 | PBDB-T | 1:1 | 0.86 | 13.6 | 62 | 7.27 | -/3.9 × 10–5 | (182) |
| 7-113 | CNDTBT-IDTT-FINCN | –5.56 | –3.98 | 1.45 | PBDB-T | 1:1 | 0.82 | 17.1 | 65 | 9.13 | -/1.2 × 10–4 | (182) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
A similar design was used in NFA 7-106 (IDTTBM),178 by replacing the RCN groups with methylidene malonitrile groups. Replacing the BT group by a p-dimethoxyphenylene group leads to the structure 7-107 (IDTT-BO-MN).179 In contrast to ITIC, the HOMO energy level of 7-107 is downshifted, giving a larger band gap of 2.17 eV. Following this, the absorption of 7-107 is blue-shifted by about 200 nm in film and in solution. Solar cells with a combination of 7-106/PTB7-Th reveal 8.40% efficiency, whereas solar cells of 7-107/P3HT reach 5.75%, mainly due to the lower JSC caused by the limited absorption range of both the NFA 7-107 and P3HT.178,179 Exchanging the BT group in 7-102 with a N-octyl-benzotriazole unit leads to the NFA 7-108 (BTA13).180 This change slightly shifts the HOMO downward, whereas the LUMO energy level is shifted upward, thereby broadening the optical band gap to 1.72 eV. Blended with J52-F, it achieved a remarkable VOC value of 1.18 V. However, due to the limited JSC of 11.6 mA cm–2, an efficiency of only 8.36% was reached.1807-109 (IDTT-CR)140 comprises, like 7-102, RCN-based end groups but a different π-bridge (cyclohexene). The cyclohexene shifts the LUMO level upward, thus broadening the band gap compared to 7-102. However, in combination with P3HT, 7-109 shows a poor efficiency of 2.52% resulting from its low FF and JSC values.140 The replacement of the RCN-based end groups in 7-109 with a diethyl TBA group leads to structure 7-110 (IDTT-CT).140 The TBA unit lowers again the LUMO level and thereby narrows the optical band gap to 1.63 eV. Here, PTB7-Th was used as donor, giving a VOC of 0.93 V, a JSC of 12.6 mA cm–2, a FF of 51%, and a PCE of 5.89%.140
7-111 (ITOTIC-2F)181 features thiophenes with alkyloxy chains as a π-bridge and INCN-F end groups. Compared to 7-2, which has the same end-capped groups, the HOMO is upshifted and the LUMO level downshifted. In solar cells combined with PBDB-T, 7-111 gave a lower PCE of 3.70% with a VOC of 0.76 V, a JSC of 7.00 mA cm–2, and a FF of 61% (7-2 had a PCE of 9.3%). A more than 3 times higher efficiency (12.1% PCE) was reached with compound 5-149, which consists of a five-ring central core and the same π-bridge as 7-111. The poorer performance of the compound 7-111 despite its larger conjugation length was explained by an unfavorable active layer phase morphology.181
The structures 7-112 (FDTBT-IDTT-FINCN) and 7-113 (CNDTBT-IDTT-FINCN)182 are combining two IDTT units with INCN-2F end groups via a thieno-benzothiadiazole-thienyl bridge, thereby resulting in a more oligomer-like NFA species. The difference between these two acceptors are the substituents on the BT, which are two fluorine atoms in the molecule 7-112 and two nitrile groups in case of 7-113. The HOMO/LUMO energy levels of 7-112 and 7-113 are −5.46 and −3.92 eV as well as −5.56 and −3.98 eV, respectively. The optical band gaps are lower compared to ITIC with 1.51 eV for the fluorinated and 1.45 eV for the nitrile-containing counterpart. GIWAXS data showed that 7-113 has a reduced crystallinity in pristine film. The photovoltaic parameters are similar in terms of VOC and FF, but 7-113 has a better JSC of 17.1 mA cm–2 and PCE of 9.13%. On the contrary, 7-111 achieved 7.27% in photovoltaic devices.182
3.4. Impact of the Central Donor Unit
The central donor core is of utmost importance for the optical and electronical properties of the NFA. NFAs with INCN as acceptor subunits are the most common structures and thus are very suitable for the comparison of the influence of the central donor subunit (see Figure 9 and Table 5). By replacing the thienothiophene units with benzothiophene subunits in ITIC, the acceptor 7-114 (NIDBT)183 is obtained and exhibits a broad optical band gap of 1.84 eV due to the lower HOMO energy level of −5.87 eV compared to ITIC. Solar cells of 7-114 combined with PTB7-Th showed only moderate PCE values of max. 4.45%.183 The modification of ITIC by the introduction of selenophenothiophenes instead of the thienothiophene units leads to structure 7-115 (SeTIC).184 The selenium in the structure causes a slight reduction of the band gap compared to ITIC. Solar cells of 7-115 with PM6 as a donor gave similar JSC values than those with ITIC, but due to slightly lower VOC and significant lower FF values, the PCEs only achieved a maximum of 7.46%.184 In structure 7-116 (BDSeIC),185 the selenophene rings were swapped with the adjacent cyclopentadienyl rings bearing the hexylphenyl side chains. This leads to an even smaller band gap of 1.51 eV due to a slight downshift of the LUMO and an upshift of the HOMO energy level. Solar cells with PM6 gave similar results with maximum PCEs of about 7.10%.185 By replacing Se by S in this structure, acceptor 7-117 (BDT-IC)186,187 is obtained. NFA 7-117 is similar to ITIC but with exchanged thiophene and (dihexylphenyl)cyclopentadienyl units. The HOMO/LUMO energy levels as well as the optical band gap are comparable to ITIC. Organic solar cells were built with J71 as a polymer donor and revealed a VOC of 0.92 V, JSC of 17.3 mA cm–2, FF of 66%, and PCE of 10.5%. In contrast, solar cells of ITIC/J71 are only showing a PCE of 8.99%.186,188 The molecules 7-118–7-124 have the same conjugated donor core but have various side chains introduced at the central phenyl ring. Upon the introduction of hexyl side chains (7-118, CBT-IC),186 the LUMO level is downshifted, leading to a smaller band gap of 1.53 eV. However, hexylthio side chains (7-119, SBT-IC)186 lower the energy levels, and the band gap increased to 1.57 eV. GIWAXS data of the neat films indicate that the addition of hexyl or hexylthio side chains weakens the π–π stacking in the out-of-plane direction, which may lead to weaker crystallinity of the acceptor. Solar cells of 7-118 and 7-119 show PCEs up to 11.3% with similar VOC, JSC, and FF values as those of 7-117. The electron mobility is rather low for all three acceptors.1867-120 (BPIC)189 comprises hexylphenyl chains. Compared to the simple hexyl analogue 7-118, the LUMO level of 7-120 is heightened, but the HOMO energy level and the optical band gap are unchanged. Solar cells were built with PBDB-T and yielded efficiencies up to 10.7%, which is lower than for 7-118/J71 devices.186,1897-121 (BT-IC)190 features an ethylhexyloxy side chain (see Figure 9). The +M-effect heightens both energy levels, but especially the HOMO, leading to a reduced band gap of 1.43 eV. Blended with PBDB-T, the solar cells show PCE values like the solar cells based on 7-117.
Figure 9.
Structures of non-fullerene acceptors with the same end group and different donor units.
Table 5. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-114–7-144.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D/A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μeb(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-114 | NIDBT | –5.87 | –3.91 | 1.84 | PTB7-Th | 1:1 | 0.83 | 11.5 | 47 | 4.45 | 3.5 × 10–5/6.9 × 10–5 | (183) |
| 7-115 | SeTIC | –5.55 | –3.90 | 1.58 | PM6 | 1:1 | 0.95 | 15.5 | 51 | 7.46 | -/2.9 × 10–3 | (184) |
| 7-116 | BDSeIC | –5.53 | –3.92 | 1.51 | PM6 | 1:1 | 0.97 | 14.0 | 52 | 7.10 | 1.6 × 10–4/1.2 × 10–4 | (185) |
| 7-117 | BDT-IC | –5.51 | –3.90 | 1.61 | J71 | 0.92 | 17.3 | 66 | 10.5 | -/1.9 × 10–4 | (186) | |
| 7-118 | CBT-IC | –5.50 | –3.97 | 1.53 | J71 | 0.9:1 | 0.92 | 18.1 | 66 | 11.0 | -/8.6 × 10–5 | (186) |
| 7-119 | SBT-IC | –5.61 | –4.04 | 1.57 | J71 | 0.8:1.1 | 0.89 | 17.8 | 71 | 11.3 | -/1.1 × 10–4 | (186) |
| 7-120 | BPIC | –5.51 | –3.81 | 1.53 | PBDB-T | 1:1 | 0.88 | 17.9 | 68 | 10.7 | 6.3 × 10–4/3.8 × 10–4 | (189) |
| 7-121 | BT-IC | –5.39 | –3.87 | 1.43 | PBDB-T | 1:1 | 0.85 | 18.1 | 70 | 10.8 | (190) | |
| 7-122 | ITIC2 | –5.48 | –3.84 | 1.53 | PBDB-T-SF | 1:1 | 1.09 | 15.7 | 60 | 10.1 | 6.4 × 10–4/4.2 × 10–4 | (191) |
| 7-123 | ITIC-S | –5.50 | –3.86 | 1.55 | PBDB-T-SF | 1:1 | 1.06 | 16.4 | 67 | 11.6 | 7.0 × 10–4/4.4 × 10–4 | (191) |
| 7-124 | ITIC-SF | –5.57 | –3.92 | 1.58 | PBDB-T-SF | 1:1 | 1.04 | 16.8 | 69 | 12.1 | 5.3 × 10–4/4.0 × 10–4 | (192) |
| 7-125 | ArSiID | –5.37 | –3.86 | J51 | 1:1 | 0.84 | 16.2 | 60 | 8.30 | -/2.4 × 10–4 | (193) | |
| 7-126 | BDTIC | –5.30 | –3.85 | 1.38 | PBDB-T | 1:1 | 0.88 | 20.0 | 68 | 12.1 | -/2.3 × 10–4 | (194) |
| 7-127 | DTC(4R)-IC | –5.69 | –3.63 | J71 | 1:1.3 | 0.94 | 16.4 | 62 | 9.61 | (195) | ||
| 7-128 | DTC(4Ph)-IC | –5.75 | –3.67 | PBDB-T | 0.97 | 14.3 | 68 | 9.48 | (196) | |||
| 7-129 | DTCCIC | –5.72 | –3.58 | 1.62 | PFBDB-T | 1:2 | 1.01 | 12.6 | 48 | 6.20 | -/2.2 × 10–6 | (197) |
| 7-130 | HCN-C8 | –5.63 | –3.54 | 1.67 | J71 | 1:1 | 1.01 | 6.24 | 38 | 2.38 | -/9.4 × 10–7 | (198) |
| 7-131 | HCN-C16 | –5.63 | –3.57 | 1.66 | J71 | 1:1 | 1.03 | 12.3 | 44 | 5.51 | -/1.3 × 10–6 | (198) |
| 7-132 | DBTIC | –5.91 | –4.00 | 1.67 | PBDB-T | 1:1 | 1.00 | 15.5 | 62 | 9.66 | -/2.0 × 10–6 | (199) |
| 7-133 | F-H | –5.42c | –3.79 | 1.63 | PBDB-T | 0.94 | 15.0 | 67 | 9.59 | (200) | ||
| 7-134 | TPIC | –5.30 | –3.85 | 1.50 | PM7 | 1:1 | 1.00 | 18.8 | 71 | 13.3 | 6.7 × 10–4/3.7 × 10–4 | (201) |
| 7-135 | DTPPSe-IC | –5.38 | –3.84 | 1.46 | PBDB-T | 1:1.25 | 0.90 | 17.3 | 63 | 9.88 | -/4.5 × 10–4 | (202) |
| 7-136 | COj7IC | –5.66 | –3.92 | 1.60 | PTB7-Th | 1:1.4 | 0.82 | 13.1 | 59 | 6.36 | -/1.4 × 10–5 | (203) |
| 7-137 | Tr(Hex)6-3IC | –5.92 | –3.94 | 1.90 | PTB7-Th | 1:1 | 0.88 | 3.71 | 38 | 1.28 | -/5.4 × 10–6 | (204) |
| 7-138 | Tr(Dec)6-3IC | –5.88 | –3.90 | 1.92 | PTB7-Th | 1:1 | 0.74 | 2.55 | 28 | 0.55 | -/2.0 × 10–6 | (204) |
| 7-139 | TrBTIC | –5.56 | –3.62 | 1.80 | P3HT | 1:1.2 | 0.88 | 13.0 | 72 | 8.25 | -/3.3 × 10–4 | (205) |
| 7-140 | Y1 | –5.45 | –3.95 | 1.44 | PBDB-T | 1:1 | 0.87 | 22.4 | 69 | 13.4 | -/3.0 × 10–4 | (206) |
| 7-141 | Y16 | –5.25 | –3.66 | PBDB-T | 1:1 | 0.91 | 21.3 | 67 | 13.0 | -/5.0 × 10–4 | (207) | |
| 7-142 | Y9 | –5.59 | –3.78 | 1.36 | PBDB-T | 1:1 | 0.90 | 23.3 | 63 | 13.3 | 6.7 × 10–4/4.1 × 10–4 | (208) |
| 7-143 | Y5 | –5.55 | –3.87 | 1.38 | PBDB-T | 1:1.5 | 0.88 | 22.8 | 70 | 14.1 | 2.1 × 10–4/4.0 × 10–4 | (209) |
| 7-144 | BTP-IC | –5.58 | –3.90 | 1.42 | PM6 | 1:1.2 | 0.96 | 14.7 | 53 | 7.54 | 9.0 × 10–5/1.4 × 10–4 | (210) |
| BTP | –5.52 | –3.82 | 1.40 | PM6 | 1:1.2 | 0.96 | 16.3 | 57 | 8.85 | -/9.4 × 10–4 | (211) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Obtained via ultraviolet photoelectron spectroscopy (UPS).
7-122 (ITIC2)191 and 7-123 (ITIC-S)191 contain 2-ethylhexylthiophene and 2-ethylhexylthiothiophene side chains, respectively. Compared to 7-117, this does not have much impact on the band gap and energy levels. Regarding the PV parameters, the VOC is reaching over 1 V, and for 7-122, the FF and PCE are lower; for 7-123, they are higher than for 7-117. GIWAXS data indicate that the thioether bond reduces the crystallinity of the 7-123 neat film. The effect is also present in the blended films, but it is weakened.186,191 When a fluorine atom is added to the thiophene ring in the side chain of 7-123, it leads to structure 7-124 (ITIC-SF).192 The fluorine lowers the energy levels and widens the optical band gap to 1.58 eV. Solar cell devices with PBDB-T-SF as a donor reached a higher efficiency of 12.1% than 7-123 with a VOC of over 1 V.191,192
A variation of the core structure of 7-118–7-124 was introduced by Wang et al., who exchanged the cyclopentadiene rings with dioctylsiloles to give the structure 7-125 (ArSiID),193 shifting the HOMO/LUMO energy levels slightly to higher values. Solar cells using J51 as a donor polymer show a PCE of 8.30%, a VOC of 0.84 V, a JSC of 16.2 mA cm–2, and a FF of nearly 60%.193 Chen et al. exchanged the silole ring with 2-ethylhexyl pyrrole in the NFA 7-126 (BDTIC).194 This change upshifts the HOMO level, leading to a band gap of 1.38 eV. Combined with PBDB-T, solar cells with 7-126 reached PCE values up to 12.1%.194
Molecules 7-127–7-131 contain a pyrrole unit in the center of the donor unit but differ in the side chains. 7-127 (DTC(4R)-IC) comprise N-octylnonyl groups and octyl side chains,1957-128 (DTC(4Ph)-IC) N-octylnonyl and octylphenyl,1967-129 (DTCCIC) N-octyl and octyl,1977-130 (HCN-C8) N-octyl and hexylphenyl,198 and 7-131 (HCN-C16) N-hexadecyl and hexylphenyl groups.198 They have the most upshifted LUMO and downshifted HOMO energy levels compared to ITIC. Solar cells of 7-127 and 7-128 show PCEs of over 9% blended with J71 or PBDB-T, respectively,195,196 and with 7-129—bearing the shortest side chains—they show a VOC of 1.01 V but only an efficiency of 6.20%.197VOC values of over 1 V are also observed with 7-130/J71 and 7-131/J71 blends, but the PCE values decrease even further to 5.51 to 2.38%.198 Structure 7-132 (DBTIC)199 comprises a thiophene as a central unit. This leads to a further downshift of the HOMO level to −5.91 eV. Solar cells using PBDB-T as a donor polymer show comparable parameters to those of 7-128.196,199 Replacing this central thiophene unit in 7-132 with a cyclopentadienyl ring leads to structure 7-133 (F-H).200 The frontier orbital energy levels are upshifted, hereby the HOMO more than the LUMO, narrowing the band gap slightly. The PV devices of PBDB-T/7-133 gave a PCE of 9.59%.200
An asymmetric backbone can enhance the π–π stacking, thereby improving charge transport.212 The asymmetric 7-134 (TPIC)201 and 7-135 (DTPPSe-IC)202 feature the same backbone with the difference that 7-135 comprises a selenophene instead of one thiophene ring, leading to a slightly decreased optical band gap. Solar cells of 7-134/PM7 yield a good efficiency of 13.3% and a VOC of 1.00 V.201 Devices of 7-135/PBDB-T reached only PCE values up to 9.88%.202 Molecule 7-136 (COj7IC)203 contains carbon–oxygen bridges in the core unit. It possesses HOMO and LUMO energy levels of −5.66 and −3.92 eV and an optical band gap of 1.60 eV, which are all in the same range as ITIC. Solar cells were built with PTB7-Th as a donor and achieved a PCE of 6.36%.203
In a quite different seven-ring structure, truxene was used as the central core for the acceptors 7-137 (Tr(Hex)6-3IC),2047-138 (Tr(Dec)6-3IC),204 and 7-139 (TrBTIC)205 with the difference between 7-137 and 7-138 being only in the length of the side chains. 7-139 differs from the others regarding the side chains and π-bridge. In general, the energy levels of 7-137 and 7-138 are much lower than the ones of ITIC and the band gap is enlarged to values equal or over 1.9 eV. 7-139 shows a higher LUMO level than ITIC, thus enlarging the band gap to 1.80 eV. The solar cells of 7-137 and 7-138 were built with PTB7-Th as a donor material and the devices showed moderate PV parameters, having efficiencies of 1.28% for 7-137 and 0.55% for 7-138, respectively. Also, the electron mobilities of the active layer films are rather low. However, molecule 7-139 in combination with P3HT revealed a higher PV performance with a PCE of 8.25%.204,205
3.5. The Y Series and Related NFAs
A boost in efficiency was experienced by introducing the so-called Y-series by altering the classical A–D–A structural motif. By attaching electron-accepting units in the central core, the donor subunit is split in a DAD structure, resulting in an A–(DA′D)–A motif (see Figures 9–11 and Tables 5–7).213 This can be achieved by, e.g., introducing a benzotriazole group as the central core (7-140, Y1,2067-141, Y16,207 and 7-142, Y9,208 differing in the side chains on the central core) or a benzothiadiazole unit (7-143, Y5,209 and 7-144, BTP,211 differing also in the side chains on the central core) flanked by thienothienopyrrole units on both sides. Compared to ITIC, the energy values of the HOMO/LUMO are only slightly different (except 7-141); however, the optical band gap decreased for all four molecules, red-shifting the absorption by 40–90 nm in solution as well as in thin films (see also Table 5). Combined with PBDB-T, the solar cells of 7-140–7-143 achieved efficiencies over 13% (for 7-142, even more than 14% due to the increased JSC values above 22 mA cm–2), good FFs between 63 and 70%, and simultaneous high VOC values between 0.87 and 0.91 V.206−209 Only 7-144/PBDB-T gave, compared to the others, lower PCEs of 8.80%, even though the electron mobility in the blend is quite high with a value of 9.4 × 10–4 cm2 V–1 s–1.211 These last three structures have been shown to belong to the most efficient acceptor materials currently known and are in the center of today’s OPV research.214 Based on the above described A–(DA′D)–A motifs, this new class of materials can be divided into three main groups according to their central A′ unit, i.e., either (i) a benzothiadiazole unit, 7-145–7-205 and 7-222 (see Figures 10 and 11), (ii) a benzotriazole core, 7-207–7-214 and 7-221, or (iii) a quinoxaline unit (1,4 benzopyrazine) 7-215–7-220.
Figure 11.
Structures of non-fullerene acceptors with Y backbones containing Se atoms or benzotriazole core units.
Table 7. Optical, Electrical, and Photovoltaic Properties of the Non-Fullerene Acceptors 7-201–7-222.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μeb(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-201 | BPF-4F | –5.58 | –3.98 | 1.36 | SZ5 | 1:1.2 | 0.85 | 22.1 | 67 | 12.6 | -/3.2 × 10–4 | (251) |
| 7-202 | BPT-4F | –5.59 | –4.00 | 1.36 | SZ5 | 1:1.2 | 0.85 | 24.8 | 79 | 16.8 | -/6.4 × 10–4 | (251) |
| 7-203 | BPS-4F | –5.54 | –4.00 | 1.29 | SZ5 | 1:1.2 | 0.82 | 25.4 | 78 | 16.3 | -/4.8 × 10–4 | (251) |
| 7-204 | CH1007 | –5.59 | –3.97 | 1.30 | PM6 | 1:1.2 | 0.82 | 27.0 | 72 | 16.0 | 1.1 × 10–4/- | (252) |
| 7-205 | Y6-2Se | –5.58 | –3.84 | 1.34 | PM6 | 1:1.2 | 0.83 | 24.3 | 70 | 14.6 | -/3.3 × 10–4 | (253) |
| 7-206 | Y6Se | –5.70 | –4.15 | 1.32 | D18 | 1:1.6 | 0.84 | 28.0 | 75 | 17.7 | 2.7 × 10–4/- | (254) |
| 7-207 | Y3 | –5.56 | –4.03 | 1.32 | PM6 | 1:1.5 | 0.81 | 24.1 | 66 | 13.2 | -/7.5 × 10–5 | (255) |
| Y1-4F | –5.56 | –4.11 | 1.31 | PM6 | 1:1 | 0.83 | 25.2 | 69 | 14.8 | -/3.0 × 10–4 | (256) | |
| 7-208 | Y18 | –5.58 | –3.91 | 1.31 | PM6 | 1:1.5 | 0.84 | 25.7 | 77 | 16.5 | -/3.5 × 10–4 | (247) |
| 7-209 | Y18-DMO | –5.63 | –3.90 | 1.37 | PBDB-T | 1:1.5 | 0.81 | 24.0 | 73 | 14.2 | -/3.5 × 10–4 | (260) |
| 7-210 | Y11 | –5.61 | –3.92 | 1.32 | PM6 | 1:1.5 | 0.84 | 24.6 | 73 | 15.3 | -/6.2 × 10–4 | (255) |
| Y11 | –5.69 | –3.87 | 1.31 | PM6 | 0.83 | 26.7 | 74 | 16.5 | (259) | |||
| 7-211 | Y19 | –5.68 | –3.95 | 1.31 | PM6 | 1:1 | 0.84 | 22.4 | 68 | 12.8 | -/6.5 × 10–4 | (257) |
| 7-212 | Y15 | –5.56 | –3.93 | 1.30 | PM6 | 1:1.8 | 0.88 | 23.8 | 69 | 14.1 | -/5.2 × 10–4 | (258) |
| 7-213 | Y14 | –5.56 | –4.01 | 1.30 | PBDB-T | 0.80 | 26.2 | 72 | 14.9 | -/1.0 × 10–4 | (261) | |
| 7-214 | Y2 | –5.43 | –4.04 | 1.40 | PBDB-T | 1:1 | 0.82 | 23.6 | 69 | 13.4 | -/2.1 × 10–4 | (206) |
| 7-215 | QIP-4F | –5.75 | –3.86 | 1.54 | P2F-EHp | 1:1 | 0.94 | 18.3 | 71 | 12.1 | -/4.7 × 10–4 | (262) |
| 7-216 | QIP-4Cl | –5.77 | –3.89 | 1.48 | P2F-EHp | 1:1 | 0.94 | 19.6 | 72 | 13.3 | -/5.3 × 10–4 | (262) |
| 7-217 | AQx-2 | –5.62 | –3.88 | 1.35 | PM6 | 1:1.2 | 0.86 | 25.4 | 76 | 16.6 | -/2.9 × 10–4 | (263) |
| 7-218 | AQx-1 | –5.59 | –3.85 | 1.35 | PM6 | 1:1.2 | 0.89 | 22.2 | 67 | 13.3 | -/3.7 × 10–4 | (263) |
| 7-219 | TPQx-4F | –5.57 | –3.74 | 1.41 | PM6 | 1:1.2 | 0.94 | 15.0 | 54 | 7.72 | -/4.3 × 10–4 | (264) |
| 7-220 | TPQx-6F | –5.61 | –3.78 | 1.43 | PM6 | 1:1.2 | 0.92 | 21.8 | 71 | 14.3 | -/4.6 × 10–4 | (264) |
| 7-221 | Y18-ID | –5.43 | –3.75 | 1.28 | P | 1:1.5 | 0.84 | 24.5 | 74 | 15.3 | -/4.2 × 10–4 | (265) |
| 7-222 | BDTP-4F | –5.61 | –3.90 | 1.36 | PM6 | 1:1.2 | 0.90 | 22.5 | 76 | 15.2 | 7.2 × 10–4/4.1 × 10–4 | (266) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Figure 10.
Structures of non-fullerene acceptors with Y backbones containing benzothiadiazole units.
Table 6. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-145–7-200.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μed(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-145 | Y6 | –5.66 | –4.08 | 1.33 | PM6 | 1:1.2 | 0.82 | 25.2 | 76 | 15.7 | (247) | |
| BTP-4F | –5.65 | –4.02 | PM6 | 1:1.2 | 0.83 | 24.9 | 75 | 15.6 | -/7.3 × 10–5 | (225) | ||
| Y6 | –5.72 | –4.00 | PM6 | 1:1.2 | 0.85 | 25.5 | 75 | 16.2 | 5.7 × 10–4/3.4 × 10–4 | (222) | ||
| Y6 | –5.65 | –4.10 | D18 | 1:1.6 | 0.86 | 27.7 | 77 | 18.2 | -/1.4 × 10–4 | (1) | ||
| 7-146 | Y6-C2 | –5.65 | –4.09 | PM6-Ir1.5 | 1:1.2 | 0.84 | 26.1 | 78 | 17.1 | -/4.3 × 10–4 | (215) | |
| 7-147 | Y6-C3 | –5.68 | –4.07 | 1.41 | PM6 | 1:1.2 | 0.83 | 24.1 | 67 | 13.8 | 2.5 × 10–4/5.1 × 10–5 | (216) |
| 7-148 | BTP-4F-12 | –5.68 | –4.06 | PM6 | 1:1.2 | 0.86 | 25.3 | 76 | 16.4 | 7.4 × 10–4/-e | (217) | |
| 7-149 | Y6-nC8 | –5.71 | –4.08 | 1.41 | PM6 | 1:1.5 | 0.86 | 19.1 | 63 | 10.4 | 2.3 × 10–4/2.4 × 10–4 | (218) |
| 7-150 | C6 | –5.58 | –4.06 | 1.37 | PM6 | 1:1.2 | 0.84 | 23.8 | 73 | 14.5 | (219) | |
| 7-151 | Y6-phOC6 | –5.75 | –4.05 | 1.43 | PM6 | 1:1.5 | 0.84 | 21.3 | 62 | 11.1 | 7.1 × 10–4/4.7 × 10–5 | (218) |
| 7-152 | C4 | –5.60 | –4.05 | 1.39 | PM6 | 1:1.2 | 0.71 | 16.4 | 68 | 7.92 | (219) | |
| 7-153 | DTY6 | –5.67 | –4.04 | 1.40 | PM6 | 1:1.2 | 0.86 | 25.3 | 75 | 16.3 | 4.7 × 10–4/6.8 × 10–4 | (220) |
| 7-154 | BTPS-4F | –5.73 | –3.91 | 1.38 | PM6 | 1:1.2 | 0.82 | 24.8 | 76 | 16.2 | -/2.1 × 10–4 | (221) |
| 7-155 | BTP-PhC6 | –5.58 | –3.85 | 1.36 | PM6 | 1:1.2 | 0.87 | 25.0 | 77 | 16.7 | -/7.2 × 10–4 | (223) |
| 7-156 | BTP-C6Ph | –5.60 | –3.94 | 1.35 | PM6 | 1:1.2 | 0.84 | 24.3 | 76 | 15.5 | -/6.1 × 10–4 | (223) |
| 7-157 | N-C11 | PM6 | 1:1.2 | 0.85 | 21.5 | 71 | 12.9 | -/1.1 × 10–4 | (224) | |||
| 7-158 | BTP-4Cl | –5.68 | –4.12c | PM6 | 1:1 | 0.87 | 25.4 | 75 | 16.5 | -/1.6 × 10–4 | (225) | |
| 7-159 | N3-4Cl | –5.63 | –3.98 | 1.35 | PM6 | 1:1.2 | 0.85 | 25.9 | 75 | 16.5 | -/4.5 × 10–4 | (226) |
| 7-160 | BTP-4Cl-12 | –5.66 | –4.09 | 1.39 | PM6 | 1:1.2 | 0.86 | 25.6 | 78 | 17.0 | (227) | |
| BTP-BO-4Cl | PM6 | 1:1.2 | 0.85 | 26.1 | 78 | 17.3 | -/3.9 × 10–4 | (229) | ||||
| 7-161 | BTP-4Cl-16 | –5.68 | –4.09 | 1.40 | PM6 | 1:1.2 | 0.86 | 24.2 | 75 | 15.6 | (227) | |
| 7-162 | BTIC-C12-4Cl | –5.56 | –4.16 | 1.39 | PM6 | 0.84 | 19.8 | 68 | 11.4 | -/8.7 × 10–5 | (228) | |
| 7-163 | BTPS-4Cl | –5.65 | –3.93 | 1.36 | PM6 | 1:1.2 | 0.80 | 24.0 | 68 | 13.5 | -/1.9 × 10–4 | (221) |
| 7-164 | BT6IC-BO-4Cl | –5.54 | –4.05 | 1.35 | PM6 | 1:1.2 | 0.84 | 23.6 | 73 | 14.4 | -/4.8 × 10–5 | (230) |
| 7-165 | BT6IC-HD-4Cl | –5.57 | –4.10 | 1.35 | PM6 | 1:1.2 | 0.88 | 23.4 | 73 | 14.9 | -/6.4 × 10–5 | (230) |
| 7-166 | BT6IC-OD-4Cl | –5.55 | –4.11 | 1.35 | PM6 | 1:1.2 | 0.89 | 18.6 | 58 | 9.60 | -/5.6 × 10–6 | (230) |
| 7-167 | BTP-eC7 | –5.62 | –4.03 | 1.40 | PM6 | 1:1.2 | 0.84 | 24.1 | 74 | 14.9 | 1.3 × 10–4/- | (231) |
| 7-168 | BTP-eC9 | –5.64 | –4.05 | 1.40 | PM6 | 1:1.2 | 0.84 | 26.2 | 81 | 17.8 | 2.7 × 10–4/- | (231) |
| 7-169 | HD-4Cl | –5.68 | –4.09 | PM6 | 1:1.2 | 0.85 | 25.4 | 72 | 15.7 | 1.2 × 10–4/- | (232) | |
| 7-170 | HDO-4Cl | –5.70 | –3.91 | PM6 | 1:1.6 | 0.94 | 21.9 | 76 | 15.6 | 1.4 × 10–4/- | (232) | |
| 7-171 | BTIC-4Br | –5.57 | –4.11 | PM6 | 1:1.2 | 0.85 | 20.7 | 70 | 12.2 | 2.3 × 10–5/1.1 × 10–5 | (233) | |
| 7-172 | BTIC-BO-4Br | –5.53 | –4.09 | PM6 | 1:1.2 | 0.86 | 24.1 | 68 | 14.0 | 8.1 × 10–5/4.5 × 10–5 | (233) | |
| 7-173 | Y8 | –5.55 | –3.89 | 1.35 | PM6 | 1:1.2 | 0.88 | 22.2 | 73 | 14.3 | -/8.7 × 10–4 | (234) |
| 7-174 | BTIC-Cl-m | –5.42 | –3.91 | 1.34 | PM6 | 1:1.2 | 0.88 | 21.4 | 70 | 13.2 | -/3.8 × 10–4 | (235) |
| ZC | –5.69 | –4.12 | 1.38 | PM6 | 1:1 | 0.90 | 25.1 | 65 | 14.7 | -/1.5 × 10–4 | (248) | |
| 7-175 | BTIC-2Br-m | –5.56 | –4.07 | PM6 | 1:1.2 | 0.88 | 25.0 | 73 | 16.1 | 2.9 × 10–4/1.1 × 10–4 | (233) | |
| TPT10 | –5.52 | –3.99 | 1.36 | PTQ11 | 1:1.2 | 0.88 | 24.8 | 75 | 16.3 | (249) | ||
| 7-176 | BTIC-CF3-m | –5.45 | –3.97 | 1.31 | PM6 | 1:1.2 | 0.85 | 24.9 | 72 | 15.3 | -/6.6 × 10–5 | (235) |
| 7-177 | BTIC-CF3-γ | –5.45 | –3.96 | 1.30 | PM6 | 1:1.2 | 0.85 | 25.2 | 73 | 15.6 | -/4.5 × 10–4 | (235) |
| 7-178 | BTP-M | –5.48 | –3.81 | 1.42 | PM6 | 1:1.2 | 0.98 | 8.43 | 52 | 4.26 | -/5.2 × 10–4 | (236) |
| 7-179 | BTPIC-2Br-5 | –5.63 | –3.91 | 1.37 | PM6 | 1:1.2 | 0.90 | 22.9 | 68 | 14.0 | -/2.3 × 10–4 | (237) |
| 7-180 | BTPIC-2Br-6 | –5.60 | –3.93 | 1.34 | PM6 | 1:1.2 | 0.87 | 24.1 | 71 | 15.0 | -/2.7 × 10–4 | (237) |
| 7-181 | LY-Cl-1 | –5.66 | –3.82 | 1.39 | PM6 | 1:1.1 | 0.91 | 24.3 | 65 | 14.4 | 4.6 × 10–4/7.3 × 10–4 | (238) |
| 7-182 | LY-Cl-2 | –5.61 | –3.84 | 1.36 | PM6 | 1:1.1 | 0.88 | 24.2 | 71 | 15.2 | 7.1 × 10–4/1.0 × 10–3 | (238) |
| BTP-2Cl-δ | –5.57 | –3.85 | 1.36 | PM6 | 1:1.2 | 0.89 | 24.3 | 71 | 15.4 | -/5.8 × 10–4 | (211) | |
| 7-183 | N3-Cl-1 | –5.64 | –3.78 | 1.37 | PL1 | 1:1.5 | 0.88 | 23.9 | 71 | 15.1 | 5.8 × 10–4/6.2 × 10–4 | (238) |
| 7-184 | N3-Cl-2 | –5.58 | –3.80 | 1.33 | PL1 | 1:1.5 | 0.85 | 25.6 | 76 | 16.4 | 1.5 × 10–3/9.2 × 10–4 | (238) |
| 7-185 | SY2 | –5.67 | –3.99 | PM6 | 1:1.2 | 0.85 | 25.3 | 74 | 16.0 | 6.1 × 10–4/3.6 × 10–4 | (222) | |
| 7-186 | BTIC-γCl-2F | –5.39 | –3.87 | PM6 | 1:1.2 | 0.86 | 24.6 | 73 | 15.4 | -/1.8 × 10–4 | (116) | |
| 7-187 | BTIC-2Cl-γCF3 | –5.55 | –4.00 | 1.31 | PM6 | 1:1.2 | 0.84 | 25.1 | 77 | 16.3 | -/1.5 × 10–4 | (239) |
| 7-188 | SY1 | –5.68 | –3.95 | PM6 | 1:1.2 | 0.87 | 25.4 | 76 | 16.8 | 5.4 × 10–4/3.1 × 10–4 | (222) | |
| 7-189 | SY3 | –5.69 | –3.98 | PM6 | 1:1.2 | 0.86 | 25.5 | 74 | 16.2 | 6.5 × 10–4/3.6 × 10–4 | (222) | |
| 7-190 | BTP-S1 | –5.55 | –4.01 | 1.49 | PM6 | 1:1 | 0.93 | 22.4 | 73 | 15.2 | -/4.2 × 10–4 | (240) |
| 7-191 | BTP-S2 | –5.65 | –4.01 | 1.48 | PM6 | 1:1.2 | 0.95 | 24.1 | 72 | 16.4 | -/8.3 × 10–4 | (240) |
| 7-192 | BTP-ClBr | –5.79 | –4.00 | 1.38 | PM6 | 1:1.2 | 0.91 | 23.5 | 79 | 16.8 | 5.2 × 10–4/4.1 × 10–4 | (241) |
| 7-193 | BTP-ClBr1 | –5.79 | –4.03 | 1.33 | PM6 | 1:1.2 | 0.85 | 23.7 | 72 | 14.6 | 6.1 × 10–4/4.3 × 10–4 | (241) |
| 7-194 | BTP-ClBr2 | –5.80 | –4.04 | 1.33 | PM6 | 1:1.2 | 0.85 | 25.0 | 74 | 15.5 | 5.7 × 10–4/4.2 × 10–4 | (241) |
| 7-195 | ZY-4Cl | –5.64 | –3.67 | P3HT | 1:1 | 0.88 | 16.5 | 65 | 9.46 | -/3.6 × 10–5 | (242) | |
| 7-196 | BTP-IS | –5.65 | –4.02 | 1.38 | PM6 | 1:1.2 | 0.89 | 22.6 | 64 | 12.8 | 7.7 × 10–5/2.0 × 10–4 | (210) |
| 7-197 | Y10 | –5.56 | –3.76 | 1.35 | J11 | 1:1.2 | 0.89 | 21.2 | 72 | 13.5 | -/4.2 × 10–4 | (243) |
| Y6-T | –5.51 | –3.90 | 1.35 | PBDT-ST | 1:1.2 | 0.92 | 22.6 | 70 | 14.4 | -/3.8 × 10–4 | (250) | |
| 7-198 | BTCT-2Cl | –5.56 | –3.95 | 1.37 | PM6 | 1:1 | 0.88 | 24.4 | 70 | 15.1 | (244) | |
| 7-199 | BTP-2F-ThCl | –5.70 | –3.99 | 1.34 | PM6 | 1:1.2 | 0.87 | 25.4 | 77 | 17.1 | 5.6 × 10–4/3.3 × 10–4 | (245) |
| 7-200 | TPBT-RCN | –5.42 | –3.71b | 1.71 | P3HT | 1:1.5 | 0.81 | 10.3 | 61 | 5.11 | -/5.3 × 10–4 | (246) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Other method or method not defined.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Determined by other methods than SCLC.
The most common backbone type is based on BT as DA′D units, as already discussed with Y5 (7-143). This structural motif was systematically modified by introducing halogens on the accepting units, by changing the side chains, etc. Introducing INCN-2F units leads to the structure Y6 (7-145). As discussed above for ITIC derivatives, this modification leads to a lowering of the HOMO/LUMO energy levels and additionally to a slightly decreased optical band gap (1.33 eV) compared to the parent structure Y5 (1.38 eV) but also to the corresponding ITIC-4F (7-4). Solar cells based on a blend of 7-145/PM6 give high PCE values of 15.7% compared to 13.6% revealed by similar devices with 7-4/PM6. Although slightly lowering the VOC, the lower band gap allows efficient light harvesting above 900 nm and enables consequently high JSC values of 25 mA cm–2. Liu et al. investigated a new polymer donor, D18, and applied it in solar cells with Y6, leading to a PCE of 18.2%, which is the highest efficiency for single junction organic solar cells by now. This PCE is accompanied by a high JSC of 27.7 mA cm–2 and a FF of 77%. The VOC is limited by the narrow band gap and reached therefore not more than 0.86 V.1
The exchange of the ethylhexyl groups of the central N-alkyl side chains in 7-145 leads to the molecules 7-146–7-153. The substitution with ethylheptyl (7-146, Y6-C2),215 ethyloctyl (7-147, Y6-C3),216 or butyloctyl (7-148, BTP-4F-12)217 chains does not significantly affect the energy levels or the optical band gap but the crystallinity of the materials as well as their molecular packing order. 7-145, 7-146, and 7-148 show favorable face-on packing in neat film, whereas 7-147 has edge-on orientations, which is worse for vertical charge transport. 7-146 also shows a stronger π–π stacking and crystallization tendency. These results are consistent with the PV parameters; 7-146 gives the highest PCE of 16.3%, followed by 7-145 with 15.7% and 7-147 with 13.8%.216 NFA 7-146 was also tested using a modification of PM6 (addition of an iridium complex into the polymer backbone), leading to an enhanced efficiency of 17.1%. Also, the FF reached outstanding 78%, while the VOC is at 0.84 V.215 When the ethyloctyl side chain is changed to a butyloctyl, extending the alkyl chain length (7-148), the efficiency of the solar cells is also enhanced to 16.4%.217 The replacement of the butyloctyl chain to an unbranched octyl group leads to structure 7-149 (Y6-nC8).218 Again, the energy levels and the band gap are, compared to Y6, not affected by this change. However, the PV parameters of PM6/7-149-based solar cells are lower than those for 7-145–7-148 with a PCE of 10.4%. Upon the introduction of phenyl rings at the end of the alkyl side chains as in 7-150 (Y6-phC6)218 and 7-151 (Y6-phOC6),218 the energy levels and band gaps stay similar to 7-149. The PCEs of both acceptors with PM6 are enhanced to 11.8% for 7-150 and 11.1% for 7-151, whereas the additional oxygen in the 7-151 side chains lead to the lower efficiency, as well as lower JSC and FF. GIWAXS data of the neat acceptor films reveal a preferred “slant orientation” of 7-150, which is neither edge-on nor face-on. A similar characteristic, but not as distinctive, was observed in NFA 7-149.218 Han et al. investigated 7-150 (C6) and compared it with the counterpart 7-152 (C4)219 with shorter side chains on the nitrogen of the pyrrole units. As before, the energy levels and band gap are not affected, but the PV performance drops from 14.5% for 7-150 to 7.92% for 7-152 in PM6/NFA-based devices. Reasons therefore may lie in the lower absorption of 7-152 compared to 7-150, as well as in the more pronounced face-on orientation of the PM6/7-150 blend film which is beneficial for charge transport.219 In 7-153 (DTY6),220 2-decyltetradecyl is implemented as side chains on the pyrroles. The longer branched alkyl chains lead to a good device performance with PM6 reaching an efficiency of 16.3% when processed in chloroform. The same blend was processed in xylene too, giving a PCE as high as 16.1%. GIWAXS data also revealed a similar pattern of the chloroform and xylene processed solar cell devices with favored face-on orientation and alike line-cut profiles.220
In molecules 7-154–7-156, the alkyl chains on the pyrroles from Y6 remained while the outer alkyl chains were adjusted. Replacing the undecyl chains with undecylthio chains leads to structure 7-154 (BTPS-4F).221 The incorporation of sulfur upshifts the LUMO energy level, thus widening the band gap to 1.38 eV compared to Y6 with 1.33 eV. Solar cells with PM6 gave similar PCE values of 16.2% as Y6.221,222 Chai et al. used hexylphenyl (7-155, BTP-PhC6) or phenylhexyl (7-156, BTP-C6Ph) chains as outer side chains. Compared to Y6, the energy levels of both acceptors are heightened, and the optical band gap is slightly broadened. The PM6/7-155 blend film shows a higher electron mobility than the PM6/7-156 films. In addition, TEM images of PM6/7-155 show a smaller phase separation and clearer interpenetrating network nanostructures than PM6/7-156. These investigations as well as the GIWAXS pattern give rise to the higher PCE of solar cells with PM6/7-155 reaching values up to 16.7% compared to those of 7-156 with a PCE of 15.5%.223 Switching ethylhexyl and undecyl side chains in Y6 (7-145) leads to the structure 7-157 (N-C11)224 with an inverse substitution pattern. However, this affects the packing in a negative way, so that solar cells with PM6 showed decreased PCE values reaching 12.9%.217,224
Substitution of the fluorides in 7-145 (Y6) by chlorides leads to structure 7-158 (BTP-4Cl),225 and in analogy, variations on the N-alkyl side chains lead to molecules 7-159–7-162. Cui et al. investigated the difference between 7-158 and 7-145 in detail. The absorption spectra measured in neat film show a red-shift of 7-158 and a narrower absorption. In general, the chlorine attachment increases the PCE by about 1% under optimal conditions with enhanced VOC, JSC, and FF.225 Extending the alkyl chain like in 7-159 (N3-4Cl)226 does not significantly influence the PV parameters, resulting in identical PCE values of 16.5% for solar cells based on 7-158 or 7-159 and PM6.226
The structures 7-160 (BTP-4Cl-12),2277-161 (BTP-4Cl-16),227 and 7-162 (BTIC-C12-4Cl)228 have even more extended side chains, i.e., butyloctyl, hexyldecyl, and dodecyl chains, respectively. Solar cells of 7-160 showed increased PV parameters with outstanding efficiencies of 17.0–17.3%, whereas those with 7-161 and 7-162 have significantly lower PCEs of 15.6 and 11.4%, respectively (PM6 is used as a donor in all three cases).227,229 In the chlorinated NFAs 7-163–7-170, the outer alkyl chain was changed from undecyl to undecylthio or other alkyl substituents with different chain lengths. In 7-163 (BTPS-4Cl),221 undecylthio side chains were introduced, like in the fluorinated counterpart 7-154. As in 7-154, the LUMO energy level of 7-163 is upshifted compared to Y6, and thereby slightly broadening the band gap to 1.36 eV. Solar cells with PM6 exhibit lower PV parameters compared to 7-154, reaching a PCE of 13.5%. Mo et al. designed and synthesized the molecules 7-164–7-166, which contain hexyl chains on the outer thiophene ring and alkyl chains differing in their chain length on the nitrogen atom of the pyrrole units. Compared to Y6, the HOMO energy level of all three acceptors is heightened slightly and the optical band gap is similar at 1.35 eV for all three compounds. In solar cells with the polymer PM6, 7-164 (BT6IC-BO-4Cl)230 showed a PCE of 14.4%, 7-165 (BT6IC-HD-4Cl)230 reached 14.9%, and 7-166 (BT6IC-OD-4Cl)230 only 9.60%. One reason for the lower efficiency in PM6/7-166 cells is the by a factor of 10 lower electron mobility of the blend film, as well as the low JSC and FF values. Interestingly, 7-166 possesses the highest extinction coefficient followed by 7-164 and 7-165.230 Cui et al. used butyloctyl N-alkyl chains in 7-167 (BTP-eC7)231 and 7-168 (BTP-eC9)231 and changed the outer substituents from a heptyl to a nonyl chain, respectively. The energy levels of both acceptors are similar to Y6, but the PV parameters with the donor PM6 deviate from each other. 7-167 and 7-168 both show a VOC of 0.84 V and JSCs of over 24 mA cm–2. However, the FF of 7-168 based solar cells is with a value of 81% much higher, which leads to a PCE of 17.8% compared to 7-167 revealing a FF of 74% and a PCE of 14.9%. Molecules 7-169 (HD-4Cl)232 and 7-170 (HDO-4Cl)232 have a hexyldecyl chain on the nitrogen atom as well as decyl and decyloxy chains on the outer donor unit, respectively. 7-170 has a higher LUMO level than 7-169 due to the additional oxygen in the side chain. Blended with PM6, both show good performance with a PCE over 15.6%.232
NFAs 7-171 (BTIC-4Br)233 and 7-172 (BTIC-BO-4Br)233 are brominated structures, with either ethylhexyl or butyloctyl groups as N-alkyl side chains. Compared to their chlorinated and fluorinated counterpart, the HOMO energy level is slightly upshifted. Blended with PM6, 7-171 and 7-172 gave VOC’s of 0.85 and 0.86 V, JSC’s of 20.7 and 24.1 mA cm–2, FFs of 70 and 68%, and PCEs of 12.2 and 14.0%, respectively. Similarly, as found for the chlorinated and fluorinated counterparts, the acceptor with the butyloctyl chains gives a higher efficiency.233
The series 7-173–7-178 shows the difference of monosubstitution on the peripheral INCN units with either fluorine 7-173 (Y8),234 chlorine 7-174 (BTIC-Cl-m),235 bromine 7-175 (BTIC-2Br-m),233 trifluoromethyl, where 7-176 (BTIC-CF3-m)235 is an isomer mixture of γ- and δ-substituted acceptors and 7-177 (BTIC-CF3-γ)235 is the pure γ-isomer, and a methyl group 7-178 (BTP-M).236 The side chains of these NFAs are the same as in 7-143 (Y5) with undecyl and ethylhexyl chains. All acceptors exhibit downshifted LUMO levels in the order from −Me (−3.81 eV), −F (−3.89 eV), −Cl (−3.91 eV), −CF3 (−3.97 eV), and −Br (−3.97 eV) compared to 7-143 (−3.78 eV). The HOMO levels show similar values of around −5.45 eV for 7-174 and 7-176–7-178.234−236 For 7-173, conflicting values of −5.55 eV are reported,234 which are in the range as for 7-143 and 7-175. All six acceptors were blended with PM6 and investigated in PV devices, which are showing PCE values above 13% except for 7-178 (4.26%). Their VOC and FF values resemble each other (again except for 7-178), with VOC values between 0.85 and 0.88 V and FFs above 70%.235,236
7-173 shows a PCE of 14.3%, which is lower than the corresponding acceptor containing two fluorines on each side.234 The highest PCE of these five acceptors was reached by solar cells of the brominated 7-175. The obtained efficiency was 16.1% correlating with excellent JSC values of 25 mA cm–2, which is also enhanced compared to the other brominated acceptors 7-171 and 7-172.233 Molecules 7-176 and 7-177, having a trifluoromethyl group attached on the INCN phenyl ring, show the same high JSC values, but due to lower VOCs, the solar cells exhibit up to 15.6% efficiency.235
7-179–7-184 are a group of acceptors with one halogen on each end group but on different specific positions and with deviations in the N-alkyl chains. 7-179 (BTPIC-2Br-5)237 and 7-180 (BTPIC-2Br-6)237 share the same ethylhexyl chains on the nitrogen atom and have a bromine on each end group in position X and Y, respectively. They have similar optical band gaps and HOMO/LUMO energy levels, where the LUMO levels are upshifted compared to Y6. Solar cells were built with PM6; the devices yield similar VOCs, but PM6/7-179 has higher JSC, FF, and PCE values. 7-179 also shows a higher electron mobility in the blend film as well as weaker trap-assisted recombination in the devices.237 Molecules 7-181 (LY-Cl-1)238 and 7-182 (LY-Cl-2)238 are the counterparts of 7-179 and 7-180, respectively, with chlorine atoms attached on the end group. Compared to 7-179 and 7-180, the HOMO levels are slightly lowered and the LUMO levels heightened, which leads to higher optical band gaps of 1.39 and 1.36 eV for 7-181 and 7-182, respectively. Photovoltaic parameters with PM6 show again higher FF and PCE values for 7-182, as well as a higher electron mobility, while 7-181 reveals a slightly higher VOC and JSC. Li et al. also investigated the structures 7-183 (N3-Cl-1)238 and 7-184 (N3-Cl-2),238 which resemble 7-181 and 7-182 with longer N-alkyl chains. With ethylheptyl chains, the energy levels are slightly upshifted and the band gap narrowed. However, in solar cells with PL1, a similar trend is observed as for 7-181 and 7-182 with the difference that the PCEs are higher for both compounds.238
NFAs 7-185–7-188 have similar side chains but are asymmetric, concerning their acceptor units, as they have different atoms attached on each side. 7-185 (SY2)222 and 7-186 (BTIC-γCl-2F)116 share two fluorines on one end group and have two or one chlorine on the other (7-185 and 7-186), respectively. The results of 7-185 and 7-186 are difficult to compare, as completely different device structures were used for very similar molecules. However, in combination with PM6, 7-185 and 7-186 reached high FFs and PCEs of over 70% and over 15%, respectively.116,222 In 7-187 (BTIC-2Cl-γCF3),239 the fluorines from 7-186 were exchanged with chlorine atoms and the chlorine from 7-186 was replaced with a trifluoromethyl group. Photovoltaic cells based on PM6/7-187 achieved a good efficiency of 16.3% with a high FF of 77%.239 Liu et al. investigated three different molecules, 7-185, 7-188 (SY1),222 and 7-189 (SY3),222 where 7-188 and 7-189 contain one chlorine atom on one end group (on no specific position) and on the other side they have two fluorines or chlorines, respectively. Their HOMO/LUMO energy levels are in the same range, and the solar cells were built with PM6 as the donor. The photovoltaic parameters of all three combinations have VOCs over 0.85 V, JSCs over 25 mA cm–2, FFs over 74%, and PCEs over 16%. With 16.8%, 7-188 reached, compared to 7-185 and 7-189, the highest efficiency.222 Other asymmetric electron acceptors were introduced by Li et al. where four chlorines are attached on one INCN unit and two fluorines or chlorines on the other (7-190, BTP-S1,240 and 7-191, BTP-S2,240 respectively). In addition, the two molecules have different N-alkyl chain lengths. The HOMO energy level of 7-190 is heightened compared to 7-191, and the LUMO energy levels of both acceptors are higher than in Y6. This is also the reason for the higher VOCs in combination with PM6, which are between 0.93 and 0.95 V for 7-190- and 7-191-based solar cells. The devices show also similar FFs, but the JSC and PCE are 14.1 mA cm–2 and 16.4% for 7-191, which is elevated compared to 7-190.240
Luo et al. designed and synthesized three molecules 7-192–7-194 resembling 7-191 in their side chains and differing in the halogens attached on the INCN units. All three acceptors have one bromine and one chlorine atom on each end group on different positions on the phenyl ring. In 7-192 (BTP-ClBr),241 the bromine and chlorine atoms occupy positions next to each other, in 7-193 (BTP-ClBr1),241 they are randomly attached, whereas, in 7-194 (BTP-ClBr2),241 the halogens are attached in meta-substitution. The HOMO/LUMO energy levels are similar for all three acceptors, but the optical band gap has a higher value of 1.38 eV in 7-192 than for 7-193 and 7-194. The difference in the optical band gap indicates a stronger π–π stacking of 7-193 and 7-194, which is also consistent with the obtained GIWAXS data. Solar cells of PM6/7-192 reached a VOC of 0.91 V, an outstanding FF of 79%, and an efficiency of 16.8%. 7-193 and 7-194 have higher values for the JSC but lower ones for VOC, FF, and PCE.241
Replacing the INCN units with other electron-accepting groups leads to the structures 7-195–7-200, which all contain undecyl side chains and either ethylhexyl or butyloctyl N-alkyl chains. In acceptor 7-195 (ZY-4Cl),242 the malononitrile group in 7-160 was exchanged with a carbonyl unit, which heightens the LUMO level, whereas the HOMO energy level is similar in both 7-195 and 7-160. Due to the use of a different polymer donor (P3HT in 7-195 and PM6 in 7-160), the photovoltaic parameters are not easy to compare. However, P3HT/7-195-based devices possess a similar VOC as solar cells with PM6/7-160 absorber layers, but all other parameters are much lower with an efficiency of 9.46%. Exchanging the carbonyl unit at the INCN end groups of 7-144 with a sulfone group leads to 7-196 (BTP-IS).210 In comparison, the sulfone lowers both energy levels and gives a slightly lower band gap than 7-144. Combined with PM6, the 7-196-based device shows lower VOC but higher JSC, FF, and PCE values, enhancing the efficiency from 7.54 to 12.8%. A reason therefore can be the much higher electron mobility in the blend film of 7-196 compared to 7-144, as well as the more effective exciton dissociation and charge collection.210 The exchange of the benzene unit of the INCN group with a [c]-thiophene ring leads to the structures 7-197 (Y10)243 and 7-198 (BTCT-2Cl)244 with the latter having the same undecyl and ethylhexyl side chains as in 7-143 and 7-198 having additional chlorines on the acceptor units. The band gaps for 7-197 and 7-198 are 1.35–1.37 eV; their HOMO/LUMO energy levels are −5.56 eV/–3.76 eV and −5.56 eV/–3.95 eV, respectively. Solar cells were built with different polymers, namely, J11 and PM6, but in both cases, very similar VOC and FF values were reached; the PCE was higher for 7-198 (15.1% vs 13.5%) due to the higher JSC.243,244 The end groups in structure 7-199 (BTP-2F-ThCl)245 are a combination of the acceptor units of Y6 (7-145) and 7-198, having an INCN-2F unit on one side and a CPTCN-Cl end group on the other side. The energy levels as well as the optical band gap and absorption maxima of 7-199 lie between the values from 7-145 and 7-198. However, when it comes to the PV parameters in combination with PM6, 7-199 outperforms both 7-145 and 7-198 regarding the JSC, FF, and PCE, giving an efficiency of over 17% (7-145 with 16.4% and 7-198 with 14.5%). This study shows that asymmetric end group engineering may be an efficient way for enhancing the PCE.245 Yang et al. introduced RCN end groups on the Y backbone structure (7-200, TPBT-RCN),246 which heightens both energy levels compared to Y6 and thereby enlarges the band gap to 1.71 eV. In solar cells, 7-200 was blended with P3HT, which yielded 5.11% efficiency. Compared to the P3HT/Y6 blend, the electron mobility and PCE were enhanced.246
Chai et al. developed the molecules 7-201–7-203, which again have INCN-2F acceptor units but lack the outer alkyl chains usually present in the Y backbone structure (see Figure 11 and Table 7). Moreover, the thiophene rings of the base structure 7-202 (BPT-4F)251 were replaced with either furans (7-201, BPF-4F)251 or selenophenes (7-203, BPS-4F).251 All three molecules show similar LUMO energy levels, whereas the HOMO energy level of 7-203 is shifted upward, thus narrowing the optical band gap. The efficiency of 7-201/SZ5-based solar cells is lower (12.6%) than that for 7-202/SZ5- and 7-203/SZ5-based devices (16.8 and 16.3%, respectively). This is attributed to the lower electron mobility and worse blend film morphology, due to a less coplanar geometry of 7-201. Differences in 7-202 and 7-203 lie in lower EQE values for 7-203 due to inefficient charge transport compared to 7-202.251 Structure 7-204 (CH1007)252 is the selenium analogue of 7-148, with undecylselenophene outer rings and N-BO side chains. The HOMO/LUMO energy levels as well as the optical band gap are similar to those of 7-203. Solar cells based on 7-204 and PM6 exhibit a lower VOC and FF but a higher JSC (27.0 mA cm–2), yielding only a slightly lower overall PCE value than devices of PM6/7-148.252 Yu et al. introduced the selenium either by exchanging the inner thiophene of the Y-backbone with selenophene leading to structure 7-205 (Y6-2Se)253 or by changing the central benzothiadiazole to a benzoselenadiazole leading to structure 7-206 (Y6-Se).253 These particular molecules have, as the other selenium-containing compounds, slightly upshifted energy levels but similar optical band gaps as Y6. In combination with PM6, both reach a similar VOC, but 7-206 shows a higher JSC, FF, and PCE of 25.5 mA cm–2, 75, and 15.8%, respectively. Furthermore, the group of Zhang et al. combined 7-206 with D18 and achieved even higher PCEs up to 17.7%.254
The influence of halogenation as well as side chain variation was also investigated on benzotriazole-based acceptors of this type, leading to the structures 7-207–7-213. In all cases (except 7-209), an ethylhexyl side chain is attached on the central nitrogen atom. Their HOMO energy levels lie for all halogenated acceptors between −5.56 and −5.69 eV, and their LUMO energy levels between −3.87 and −4.11 eV. Compared to the parent non-halogenated structures 7-140 and 7-142, all of them exhibit low and similar band gaps of about 1.30 eV (except 7-209 with 1.37 eV). All acceptors, except 7-209 and 7-213, were blended with PM6 for the preparation of solar cells. 7-207 (Y3)255,256 derives from 7-140 with no alkyl chains on the outer thiophene rings but with INCN-2F units. Overall, the fluorination increased the PCE in this case from 13.4% for 7-140 to 14.8% for 7-207.206,256 The lower band gap leads to an increased photocurrent due to the higher absorption range but to lower VOC values.
Starting from the parental structure 7-142, the structures 7-210 (Y11),2557-211 (Y19),257 and 7-212 (Y15)258 contain INCN-2F or INCN-2Cl units and differ in their alkyl chain lengths. In contrast to 7-207, halogenation leads in both 7-210 and 7-212 to enhanced photovoltaic properties and PCE values of 16.5 and 14.1%, respectively.255,258,2597-211 with shorter alkyl chains yielded 12.8%. The chlorinated structures 7-211 and 7-212 reveal higher VOCs but lower JSCs and FFs. By exchanging the undecyl groups in 7-210 with shorter hexyl groups, the structure 7-208 (Y18)247 is obtained; however, the difference in the PV parameters of 7-208 and 7-210 is only marginal.2477-209 (Y18-DMO)260 differs from 7-208 by longer N-alkyl chains, which leads, in combination with PBDB-T, to overall lower PV parameters than PM6/7-209-based devices.2607-213 (Y14)261 is the monofluorinated counterpart of 7-210. Solar cells of this acceptor with PBDB-T showed a VOC of 0.80 V, a high JSC of 26.2 mA cm–2, a FF of 72%, and a PCE of 14.9%, which are, except for the JSC, all slightly lower compared to 7-210.261
Yuan et al. developed the molecule 7-214 (Y2),206 which resembled 7-197, with a benzotriazole instead of a BT unit and without the outer undecyl chains. Compared to 7-197, the HOMO/LUMO energy levels of 7-214 are further heightened and the optical band gap is widened to 1.40 eV. Solar cell devices were built with PBDB-T/7-214, which yielded an efficiency of 13.4%.206
Finally, as an alternative A′ unit in the (DA′D) core, quinoxaline (1,4-benzopyrazine) was introduced instead of benzotriazole or benzothiadiazole, leading to the structures 7-215–7-220. All six of them exhibit INCN-2X units and differ from each other in their backbone. 7-215 (QIP-4F)262 and 7-216 (QIP-4Cl)262 possess N-ethylhexyl quinoxaline-2,3-dicarboxylic acid amide as a central unit and either fluorines or chlorines on the INCN unit. This new backbone design leads to downshifted HOMO and upshifted LUMO energy levels compared to similar acceptors (7-145, 7-158, 7-210, 7-212), which also increases the optical band gap of the materials. In organic solar cells, the acceptors were blended with a new polymer called P2F-EHp. The devices give a high VOC of 0.94 V, a FF of about 70%, and PCEs of 12.1% for the fluorinated acceptor and 13.3% for the chlorinated acceptor.2627-217 (AQx-2),2637-218 (AQx-1),2637-219 (TPQx-4F),264 and 7-220 (TPQx-6F)264 have a quinoxaline unit in the center with or without two methyl groups on the pyrazine ring (7-217 and 7-218) or ethylhexylthienyl as well as fluorinated ethylhexylthienyl groups (7-219 and 7-220). These acceptors have about the same HOMO levels as the other Y backbone structures but have upshifted LUMO levels. The optical band gaps lie at about 1.40 eV for all four materials. The PV parameters of devices with PM6 as a donor deviate highly from each other, except for the VOC, which lies between 0.86 and 0.89 V for 7-217 and 7-218 and between 0.92 and 0.94 V for 7-219 and 7-220.263,264 Devices with 7-217 have the highest JSC, FF, and PCE and revealed 16.6% efficiency, while those with 7-218 only reached 13.3%. GIXD data revealed unbalanced crystallization properties of the donor/acceptor blend film with 7-218, which leads to reduced JSC and FF values. In addition, TEM images shows excessive aggregation of 7-218, which leads to worse exciton diffusion and charge transport compared to 7-217.263 Devices with 7-219 show a rather low efficiency of 7.72%, whereas the additional fluorines (in 7-220) enhance the PCE to 14.3%, due to more homogenous phase separation as well as higher exciton dissociation efficiency.264 Chen et al. developed a new DA′D backbone (7-221, Y18-ID)265 by exchanging the thienothiophene rings of 7-208 with indole groups. This leads to heightened energy levels and a smaller optical band gap of 1.28 eV. Solar cells with the polymer donor P reach a VOC of 0.84 V, a JSC of 24.5 mA cm–2, a FF of 74%, and a PCE of 15.3%, which is lower compared to PM6/7-208.265 Luo et al. introduced an asymmetric DA′D-backbone acceptor, 7-222 (BDTP-4F),266 with a BT core, INCN-2F acceptor units, and alkyl side chains of different length. Compared to Y6, the HOMO/LUMO energy levels of 7-222 are upshifted accompanied by a slightly broadened optical band gap. Solar cells with PM6 as a donor gave a high VOC of 0.90 V due to the broader band gap and a good PCE of 15.2%.
3.6. Other Seven-Ring Acceptors
As discussed in the section “Impact of the Central Donor Unit”, a promising group of acceptors are based on the benzodi(cyclopentadithiophene) structure as a donor unit (see above 7-117–7-124). Many structural variations of this donor motif have been presented in the last three years, as shown in Figure 12. Almost all benzodi(cyclopentadithiophene)-based NFAs depicted in Figure 12 comprise p-hexylphenyl side chains on the sp3 hybridized carbon on the cyclopentadiene ring. The structures differ in their substituents on the central phenyl ring and variations on the acceptor unit. The first nine acceptors (7-223–7-231) have INCN-F or methylated INCN (INCN-Me) end groups. Comparing structures 7-223 (NFBDT-F)267 and 7-224 (BDTIT-M),268 both exhibit similar HOMO/LUMO energy levels but the INCN-Me groups lead to a slightly larger band gap with a value of 1.55 eV in 7-224 compared to 1.50 eV in 7-223 (see also Table 8). Solar cells in combination with PBDB-T gave higher VOC (0.90 V) but lower JSC values (17.6 mA cm–2) for 7-224 compared to 7-223. With a slightly better FF, solar cells of 7-224 show better PCEs with a maximum of 11.3% (10.6% for 7-223). The introduction of 2-(2-ethylhexyl)thieno[3,2-b]thiophene side chains on the central benzene ring leads to the structure 7-225 (BTT-MIC)269 in the combination with INCN-F and to 7-226 (BTT-FIC)269 in combination with INCN-Me acceptor groups. The HOMO levels are downshifted for both structures and are exhibiting similar optical band gaps as 7-223 and 7-224. Solar cells based on both acceptors were built with PM6 showing high VOC values. Also, in this case, the methylated acceptor has a higher VOC of 1.03 V compared to 0.95 V for the fluorinated acceptor, but due to higher JSCs and higher FFs, the fluorinated acceptor 7-226 outperforms the methylated one, giving PCEs of 12.6% compared to 10.0% for 7-225.
Figure 12.
Structures of non-fullerene acceptors based on benzodi(cyclopentadithiophene) and related structures.
Table 8. Optical, Electrical, and Photovoltaic Properties of the Non-Fullerene Acceptors 7-223–7-270.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μec(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-223 | NFBDT-F | –5.43 | –3.88 | 1.50 | PBDB-T | 1:1 | 0.79 | 19.3 | 70 | 10.6 | -/1.3 × 10–4 | (267) |
| 7-224 | BDTIT-M | –5.40b | –3.85 | 1.55 | PBDB-T | 1:1 | 0.90 | 17.6 | 71 | 11.3 | 1.9 × 10–3/3.0 × 10–4 | (268) |
| 7-225 | BTT-MIC | –5.55 | –3.76 | 1.53 | PM6 | 1:1 | 1.03 | 15.9 | 62 | 10.0 | 1.6 × 10–3/5.2 × 10–4 | (269) |
| 7-226 | BTT-FIC | –5.64 | –3.87 | 1.47 | PM6 | 1:1 | 0.95 | 18.5 | 72 | 12.7 | 1.9 × 10–3/7.6 × 10–4 | (269) |
| 7-227 | BDTThIT-M | –5.35b | –3.82 | 1.53 | PBDB-T | 1:1 | 0.94 | 18.0 | 71 | 12.1 | 2.1 × 10–3/3.9 × 10–4 | (268) |
| 7-228 | NCBDT | –5.36 | –3.89 | 1.45 | PBDB-T | 1:0.8 | 0.84 | 20.3 | 71 | 12.1 | 2.0 × 10–4/1.6 × 10–4 | (270) |
| 7-229 | BT-SFIC | –5.46 | –4.04 | 1.44 | PTB7-Th | 1:2.5 | 0.77 | 18.5 | 67 | 9.52 | 3.5 × 10–4/3.1 × 10–4 | (271) |
| 7-230 | BTH-2F | –5.59 | –3.91 | 1.50 | PM6 | 1:1 | 0.92 | 19.5 | 63 | 11.3 | 7.1 × 10–4/5.6 × 10–4 | (272) |
| 7-231 | BTC-2F | –5.64 | –3.91 | 1.53 | PM6 | 1:1 | 0.92 | 20.3 | 69 | 12.9 | 8.6 × 10–4/6.5 × 10–4 | (272) |
| 7-232 | BDCPDT-FIC | –5.52 | –4.00 | 1.49 | PBDB-T | 1:1 | 0.70 | 19.1 | 61 | 8.12 | (273) | |
| 7-233 | BT-FIC | –5.48 | –4.10 | 1.39 | PTB7-Th | 1:1.5 | 0.73 | 21.3 | 65 | 10.1 | 3.8 × 10–4/2.0 × 10–4 | (271) |
| 7-234 | BP-4F | –5.63 | –3.90 | 1.49 | PM6 | 1:1 | 0.90 | 21.6 | 72 | 13.9 | 2.1 × 10–4/2.8 × 10–4 | (274) |
| 7-235 | SBT-FIC | –5.81 | –4.15 | PTB7-Th | 1:2 | 0.70 | 18.1 | 62 | 7.90 | (275) | ||
| 7-236 | HBDT-4Cl | –5.67 | –3.90 | 1.43 | PM6 | 1:1 | 0.90 | 17.8 | 65 | 10.4 | -/1.2 × 10–5 | (276) |
| 7-237 | FBDT-4Cl | –5.70 | –3.91 | 1.45 | PM6 | 1:1 | 0.89 | 19.8 | 70 | 12.4 | -/5.0 × 10–5 | (276) |
| 7-238 | ClBDT-4Cl | –5.72 | –3.92 | 1.44 | PM6 | 1:1 | 0.88 | 19.0 | 70 | 11.7 | -/4.0 × 10–5 | (276) |
| 7-239 | NCBDT-4Cl | –5.60 | –4.02 | 1.40 | PBDB-T-SF | 1:1 | 0.85 | 22.4 | 74 | 14.1 | -/1.9 × 10–4 | (277) |
| 7-240 | DPBDT-4Cl | –5.62 | –3.94 | 1.41 | PM6 | 1:1 | 0.90 | 19.2 | 66 | 11.4 | -/2.2 × 10–4 | (279) |
| BPIC-4Cl | –5.60 | –4.08 | 1.43 | PBDB-T | 1:1 | 0.74 | 17.1 | 63 | 8.01 | 5.7 × 10–4/3.4 × 10–4 | (189) | |
| 7-241 | NCBDT-4Cl | –5.60 | –4.02 | 1.40 | PM6 | 1:1 | 0.83 | 20.1 | 72 | 12.0 | -/2.0 × 10–4 | (278) |
| 7-242 | POBDT-4Cl | –5.58 | –3.97 | 1.39 | PM6 | 1:1 | 0.88 | 21.0 | 68 | 12.6 | -/3.6 × 10–4 | (279) |
| 7-243 | COBDT-4Cl | –5.57 | –3.98 | 1.39 | PM6 | 1:1 | 0.87 | 21.8 | 71 | 13.5 | -/3.7 × 10–4 | (279) |
| 7-244 | TOBDT | –5.56 | –3.97 | 1.41 | PM6 | 1:1 | 0.89 | 18.7 | 68 | 11.3 | -/3.1 × 10–4 | (280) |
| 7-245 | BPIC-2Cl | –5.57 | –3.90 | 1.47 | PBDB-T | 1:1 | 0.81 | 20.7 | 72 | 12.2 | 7.1 × 10–4/4.4 × 10–4 | (189) |
| 7-246 | BDTcIC-γCl | –5.29 | –4.01 | PM6 | 1:1.2 | 0.89 | 13.8 | 62 | 7.61 | -/4.2 × 10–5 | (281) | |
| 7-247 | BDTtIC-γCl | –5.21 | –3.89 | PM6 | 1:1.2 | 0.81 | 2.26 | 39 | 0.71 | -/4.6 × 10–6 | (281) | |
| 7-248 | ITIC5 | –5.48 | –3.95 | 1.53 | J71 | 1:1 | 0.90 | 18.5 | 76 | 12.5 | 1.2 × 10–3/8.3 × 10–4 | (188) |
| 7-249 | BTTIC-0M | –5.62 | –3.89 | 1.47 | PBDB-T | 1:1 | 0.86 | 19.0 | 73 | 11.9 | 6.7 × 10–4/3.7 × 10–4 | (282) |
| 7-250 | BDTSF-IC | –5.58 | –3.93 | 1.56 | PM6 | 1:1 | 0.90 | 20.4 | 72 | 13.1 | 1.5 × 10–4/6.7 × 10–4 | (283) |
| 7-251 | BTTIC-2M | –5.60 | –3.86 | 1.47 | PBDB-T | 1:1 | 0.90 | 19.4 | 75 | 13.2 | 8.2 × 10–4/4.6 × 10–4 | (282) |
| 7-252 | BTTIC-4M | –5.55 | –3.79 | 1.49 | PBDB-T | 1:1 | 0.97 | 15.7 | 64 | 9.60 | 9.4 × 10–4/4.0 × 10–4 | (282) |
| 7-253 | BTTIC-TT | –5.54 | –3.80 | 1.47 | PBDB-T | 1:1 | 0.92 | 19.6 | 74 | 13.4 | 9.3 × 10–4/5.1 × 10–4 | (284) |
| 7-254 | BTTIC-Ph | –5.48 | –3.78 | 1.45 | PBDB-T | 1:1 | 0.93 | 16.5 | 60 | 9.14 | 7.3 × 10–4/4.1 × 10–4 | (284) |
| 7-255 | BTOIC | –5.42 | –3.94 | 1.39 | PBDB-T | 1:1 | 0.86 | 18.6 | 68 | 11.0 | 9.4 × 10–4/4.1 × 10–4 | (285) |
| 7-256 | BDCPDT-TTC | –5.38 | –3.78 | 1.58 | PBDB-T | 1:1 | 0.94 | 17.7 | 62 | 10.3 | (273) | |
| 7-257 | BDCPDT-BC | –5.40 | –3.85 | 1.53 | PBDB-T | 1:1 | 0.92 | 18.6 | 63 | 10.8 | -/1.4 × 10–5 | (134) |
| 7-258 | BTOIPC | –5.47 | –3.79 | 1.45 | PBDB-T | 1:1 | 0.88 | 15.2 | 70 | 9.31 | -/5.2 × 10–4 | (190) |
| 7-259 | M2 | –5.60 | –3.96 | 1.39 | PM6 | 1:1 | 0.88 | 19.8 | 64 | 11.2 | -/1.3 × 10–4 | (286) |
| 7-260 | M36 | –5.62 | –3.95 | 1.39 | PM6 | 1:1 | 0.90 | 24.6 | 72 | 16.0 | -/5.8 × 10–4 | (286) |
| 7-261 | M38 | –5.65 | –3.93 | 1.47 | PM6 | 1:1 | 0.87 | 18.3 | 56 | 8.89 | -/5.7 × 10–5 | (286) |
| 7-262 | M4 | –5.61 | –3.97 | 1.38 | PM6 | 1:1 | 0.88 | 23.4 | 72 | 14.8 | -/4.2 × 10–4 | (287) |
| 7-263 | BDTN-BF | –5.58 | –3.90 | 1.41 | PM6 | 1:1 | 0.93 | 20.2 | 62 | 11.5 | 9.6 × 10–5/2.9 × 10–4 | (288) |
| 7-264 | BDTBO-4F | –5.52 | –3.90 | 1.43 | PM6 | 1:1 | 0.86 | 23.0 | 75 | 14.8 | -/5.4 × 10–4 | (289) |
| M34 | –5.60 | –3.91 | 1.39 | PM6 | 1:1 | 0.91 | 23.6 | 71 | 15.2 | -/2.7 × 10–4 | (290) | |
| 7-265 | BDTBO-4Cl | –5.54 | –3.94 | 1.39 | PM6 | 1:1 | 0.83 | 23.6 | 71 | 13.9 | -/3.8 × 10–4 | (289) |
| 7-266 | M8 | –5.49 | –3.91 | 1.28 | PM6 | 1:1 | 0.83 | 8.36 | 61 | 4.21 | -/6.3 × 10–5 | (290) |
| 7-267 | SNBDT1-F | –5.30 | –3.83 | 1.41 | PBDB-T | 1:1 | 0.86 | 21.0 | 70 | 12.7 | 4.0 × 10–5/3.6 × 10–5 | (291) |
| 7-268 | SNBDT2-F | –5.34 | –3.82 | 1.41 | PBDB-T | 1:1 | 0.88 | 13.9 | 54 | 6.57 | 4.4 × 10–5/3.0 × 10–5 | (291) |
| 7-269 | SNBDT3-F | –5.36 | –3.80 | 1.41 | PBDB-T | 1:1 | 0.90 | 12.5 | 52 | 5.87 | 8.5 × 10–5/1.7 × 10–5 | (291) |
| 7-270 | BDTN-Th | –5.50 | –3.85 | 1.41 | PM6 | 1:1 | 0.95 | 8.20 | 45 | 3.53 | 5.3 × 10–5/3.9 × 10–5 | (288) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
The introduction of ethylhexylthienyl substituents on the central benzene ring in the methylated structure 7-224 leads to NFA 7-227 (BDTThIT-M),268 with a slightly reduced band gap. Solar cells with PBDB-T gave enhanced PV performance mainly due to slightly better VOC values of 0.94 V for 7-227 instead of 0.90 V for 7-224 and better JSC values (18.0 vs 17.6 mA cm–2).268 Modification of the fluorinated acceptors (7-223, 7-226) by introduction of either octyl side chains or 2-ethylhexyloxy side chains on the central benzene ring leads to molecules 7-228 (NCBDT)270 and 7-229 (BT-SFIC),271 respectively, both having similar band gaps (1.45 and 1.44 eV). 7-228 reached quite promising PCEs of 12.1% in solar cells using PBDB-T as a donor polymer. Solar cells with acceptor 7-229 were prepared with PTB7-Th and led to lower PCEs of 9.52%. Ethylhexylthienyl and chlorinated ethylhexylthienyl groups on the central benzene ring result in the structures 7-230 (BTH-2F)272 and 7-231 (BTC-2F),272 respectively. Compared to 7-223 which has no additional side chains, both HOMO/LUMO energy levels are downshifted for 7-230 and 7-231, whereas the HOMO energy level of 7-231 is further lowered due to the chlorine atom on the side chains. Solar cells in combination with PM6 show that the additional halogens increase the efficiency by more than 1%, leading to a PCE of 12.9% for 7-231 (11.3% for 7-230).272
The molecule 7-232 (BDCPDT-FIC)273 contains INCN-2F units and is related to structure 7-223 (INCN-F groups). The additional fluorines lower both HOMO/LUMO energies. Solar cells with PBDB-T show a lower PCE of 8.12% for 7-232 than 7-223, due to lower VOC and FF values. However, different device architectures were used.2737-233 (BT-FIC)271 is the counterpart of 7-229 with INCN-2F units. Once more, the additional fluorine lowers the energy levels and band gap and the efficiency is improved to 10.1%.271
7-234 (BP-4F)274 and 7-235 (SBT-FIC)275 have ethylhexylphenyl and ethylhexylthio side chains, respectively. Compared to 7-232, 7-234 shows similar HOMO and slightly higher LUMO energy levels with an optical band gap of 1.49 eV, whereas 7-235 has both HOMO/LUMO levels downshifted. Solar cells based on PM6/7-234 yielded a high VOC of 0.90 V with a FF of 72% and a PCE of 13.9%. The combination of PTB7-Th/7-235 gave a rather low VOC of 0.70 V with an efficiency of 7.90%.274,275 Molecules 7-236–7-244 have INCN-2Cl acceptor units but different substituents on the central benzene ring of the donor unit. The NFAs 7-236–7-238 possess ethylhexylthienyl (7-236, HBDT-4Cl)276 or the fluorinated and chlorinated thienyl analogue 7-237 (FBDT-4Cl)276 and 7-238 (ClBDT-4Cl).276 All three molecules have a similar optical band gap of about 1.44 eV and similar HOMO/LUMO energies. Pristine films of the three acceptors were investigated using GIWAXS, showing that 7-236 has an amorphous nature and disordered packing. In the halogenated structures, the packing patterns are changed from amorphous to face-on orientation. Solar cells from blends with PM6 exhibit better PV performances for the halogenated and higher crystalline compounds 7-237 and 7-238 than the parent compound 7-236, reaching PCE values up to 12.4% (for 7-237).
7-239 (NCBDT-4Cl)277 contains octyl side chains on the central benzene ring, and solar cells with PBDB-T-SF as the donor polymer achieved a remarkable PCE of 14.1% exhibiting a VOC of 0.85 V.2777-241 (NCBDT-4Cl)278 resembles 7-239 but differs in the alkyl group in the side chains, being octylphenyl instead of hexylphenyl. The energy levels and optical band gaps of both acceptors are similar. Solar cells of PM6/7-241 give PCE values up to 12.0%. A study of Kan et al. compares the acceptors 7-240, 7-242, and 7-243 with different substituents on the central ring. Whereas 7-240 (DPBDT-4Cl)279 comprises two hexylphenyl side chains, 7-242 (POBDT-4Cl)279 and 7-243 (COBDT-4Cl)279 bear two different side chains, either a hexylphenyl and an octyloxy group in 7-242 or an octyl and octyloxy group in 7-243. The introduction of the alkyloxy groups together with the more flexible side chains improved the molecular packing, the light absorption, as well as the electron mobility. Consequently, solar cells with PM6 show higher JSC and FF values and thus higher PCEs for the octyloxy derivatives. Following, 7-243 possesses a PCE of 13.5%, which is the highest among these three acceptors.2797-244 (TOBDT)280 contains one octyloxy and one ethylhexylthienyl side chain on the middle benzene ring, which has no impact on the energy levels or the optical band gap compared to the acceptors 7-242 and 7-243. With PM6 as the polymer donor, a similar VOC was reached as that for 7-242 and 7-243 accompanied by a lower JSC, similar FF, and also a lower PCE of 11.3%.280 Yan et al. compared the hexylphenyl-substituted compounds 7-245 (BPIC-2Cl)189 and 7-240 (BPIC-4Cl),189 having either INCN-Cl or INCN-2Cl units with their parent compound 7-122 with INCN. The frontier orbital energies are lowered and the band gap narrowed with increasing chlorine substitution. The NFA 7-245 shows the highest electron mobilities and in combination with PBDB-T also the highest performance in solar cells (PCE: 12.2%) for these three acceptors, followed by 7-122 (10.7%) and 7-240 (8.01%).189
Lai et al. investigated different orientations of the thiophene rings in the backbone with the same end groups and side chains, leading to structures 7-246 (BDTcIC-γCl)281 and 7-247 (BDTtIC-γCl).281 Here, 7-247 has heightened energy levels compared to 7-246 and they show a quite different absorption behavior in solution as well as in thin films. For 7-247, a broader absorption but also a lower extinction coefficient is obtained. In solar cells with PM6, 7-246 shows a 10 times higher efficiency of 7.61% compared to 7-247, revealing a PCE of 0.71%, which is accompanied by low EQE values of PM6/7-247. Even though the thiophene orientation has little impact on the film morphology determined by AFM and TEM images, GIWAXS measurements indicate a more regular and ordered packing for 7-246 than 7-247, which is also accompanied by significantly higher electron mobilities of PM6/7-246 films.281
The NFAs 7-248–7-255 bear CPTCN end groups and show additional variations of the substituents on the central benzene ring of the donor unit. The parent compound 7-248 (ITIC5)188 without substituents reveals HOMO and LUMO energy levels of −5.48 and −3.95 eV, respectively, with an optical band gap of 1.53 eV. Solar cells with J71 as a donor gave a VOC of 0.90 eV, a FF of 70%, and an efficiency of 11.0%.188 Gao et al. investigated the influence of additional methyl substituents in the CPTCN unit in benzodi(cyclopentadithiophene) cores with ethylhexylthienyl substituents on the central ring by comparing the parent structure with CPTCN (7-249, BTTIC-0M),282 CPTCN-Me (7-251, BTTIC-2M),282 and CPTCN-2Me analogues (7-252, BTTIC-4M).282 The addition of methyl groups increases the energy levels and optical band gaps. Solar cells of these compounds with PBDB-T yield efficiencies of 11.9% (7-249), 13.2% (7-251), and 9.6% (7-252), respectively. GIWAXS data measured from the neat acceptor films show that all three molecules have a preferred face-on orientation, but 7-251 has a higher coherence length and π–π stacking, which is beneficial for charge transport and may explain why the efficiency is higher.282 Zhang et al. also investigated 7-249 and compared it with a similar structure 7-250 (BDTSF-IC),283 which has additional fluorines and sulfur atoms in the central side chains. Compared to 7-249, 7-250 comprises a larger band gap and PM6/7-250-based solar cells yield PCE values up to 13.1%.283 Structures 7-253 (BTTIC-TT),2847-254 (BTTIC-Ph),284 and 7-255 (BTOIC)285 are similar to 7-251 (CPTCN-Me) but bear ethylhexylthienothienyl, hexylphenyl, and ethylhexyloxy side chains, respectively, on the central ring. Whereas 7-253 and 7-254 exhibit similar optical band gaps as 7-251 with values of approx. 1.47 eV, the alkoxy chains in 7-255 narrow the band gap to 1.39 eV. All acceptors were implemented in solar cells with PBDB-T. Compared to 7-251, solar cells with 7-253 and 7-254 exhibit slightly higher VOC values, but while solar cells of 7-253 even reach slightly higher PCEs of 13.4%, those of 7-254 show only a maximum PCE of 9.14% due to lower JSC and FF values. The devices with 7-255 showed expectedly lower VOC values but still reached a PCE of 11.0%.
The extension of the aromatic ring system of the acceptor end groups either with a thiophene or phenyl ring leads to structures 7-256 (BDCPDT-TTC)273 and 7-257 (BDCPDT-BC),134 both without side chains on the central benzene ring. 7-256 has a higher lying LUMO as well as a broader band gap, whereas the HOMO energy levels are similar for 7-256 and 7-257. Besides, PV parameters (with PBDB-T) are similar with efficiencies of 10.3 and 10.8% for 7-256 and 7-257, respectively.134,273 Finally, the NFA 7-258 (BTOIPC)190 has ethylhexyloxy groups on the central group but replaces the INCN groups with ((2,3-dicyano-9H-indeno[1,2-b]pyrazine-9-ylidene)methyl) acceptor units. Compared to 7-255, the optical band gap is increased to 1.45 eV, whereas solar cells with the same donor polymer show lower PCEs of 9.31%.190
The NFAs 7-259–7-270 comprise two N-alkyl pyrroles instead of the cyclopentadiene rings in the central donor unit and an alkyloxy-substituted central benzene ring. They differ in their peripheral accepting units (INCN-based and CPTCN-based accepting units) as well as in their N-alkyl chains. Within this group of NFAs, some structures have shown promising high efficiencies in solar cells with PCEs up to 16%.
Ma et al. introduced molecules 7-259–7-261, which have INCN-2F groups but differ in the alkyl groups on the central benzene and the pyrrole unit, i.e., ethylhexyloxy and ethylhexyl chains (7-259, M2),286 butyloctyloxy and butyloctyl chains (7-260, M36),286 and decyltetradecyloxy and decyltetradecyl chains (7-261, M38).286 They all have similar HOMO/LUMO energy levels; the highest optical band gap is observed for 7-261 with 1.47 eV due to a blue-shifted absorption. The PV parameters of solar cells using the acceptors 7-259–7-261 blended with PM6 deviate highly from each other. The reason lies in the molecular packing of the donor/acceptor blend films. GIWAXS data revealed the smallest π–π stacking distance for 7-260, resulting in the highest electron mobility and remarkable efficiencies up to 16.0%. In comparison, 7-259 and 7-261 yielded 11.2 and 8.89%, respectively.286 Molecule 7-262 (M4) comprises mixed side chains, N-ethylhexyl groups (as in 7-259), and butyloctyloxy substituents on the central benzene ring (as in 7-260).287 This does not affect the HOMO/LUMO energy levels, nor the optical band gap. In combination with PM6, solar cells yielded high PCE values of 14.8%.287 NFA 7-263 (BDTN-BF)288 has butyloctyl chains on the nitrogen atoms and octyloxy chains on the central benzene ring. Solar cells of 7-263 in blend with PM6 reach efficiencies up to 11.5% with VOCs up to 0.93 V.288 By swapping the butyloctyl and ethylhexyl chains in 7-262, structure 7-264 (BDTBO-4F)289 is obtained; a further change to the INCN-2Cl groups leads to 7-265 (BDTBO-4Cl).289 Both acceptors share similar energy levels and optical band gaps, whereas the PV parameters (except the JSC) are higher for solar cells with 7-264, achieving an efficiency of 14.8% compared to the PCE of 7-265-based solar cells of 13.9%, due to better charge transport properties of PM6/7-264 blends.289 Ma et al. investigated structure 7-264 and compared it with the analogue 7-266 (M8),290 in which the inner thiophene rings are exchanged with furans. The oxygen leads to a heightened HOMO energy level, thus reducing the band gap to 1.28 eV. Solar cells of 7-264/PM6 showed much better parameters with a high PCE of 15.2% compared to 4.21% for 7-266.290 Zeng et al. prepared three acceptors with central ethylhexyloxy substituents and INCN-F groups, differing in their N-alkyl chains, i.e., ethylhexyl 7-267 (SNBDT1-F),291 ethylheptyl 7-268 (SNBDT2-F),291 and ethyloctyl 7-269 (SNBDT3-F).291 All three acceptors have the same optical band gap and similar HOMO/LUMO energy levels. With increasing side chain length, the PV efficiencies of solar cells with PBDB-T are decreasing from 12.7% (7-267) to 6.57% (7-268) and then further to 5.87% (7-269). The reason for the low PCEs lies in insufficient exciton dissociation of 7-268 and 7-269 due to edge-on orientation in blend films with the polymer PBDB-T.291
7-270 (BDTN-Th)288 resembles 7-263 with the difference of having CPTCN end groups instead of INCN-2F units. They are similar in their absorption behavior and both were evaluated in solar cells with PM6, which show a VOC of over 0.90 V in both cases; however, 7-270 only reached PCE values of 3.53% (compared to 11.5% for 7-263) This may be caused by the higher electron mobilities and more balanced electron/hole mobility values of 7-263.288
Other strategies of designing NFAs are the use of asymmetric donor units and/or the insertion of heteroatoms such as Se or Si in the core unit (7-271–7-303), as shown in Figure 13. Characteristic properties of these NFAs are summarized in Table 9. The asymmetric molecule 7-271 (T7Me)292 contains a backbone consisting of five thiophenes (T) and two cyclopentadienes (Cp) in the following order: T-T-Cp-T-T-Cp-T with CPTCN-Me acceptor units. It has a small optical band gap of 1.36 eV, and the HOMO and LUMO energy levels are −5.45 and −3.93 eV, respectively. Solar cells with PM6 as a donor revealed a PCE of 8.96%.
Figure 13.
Structures of non-fullerene acceptors with asymmetric backbones and/or additional heteroatoms implemented in the backbone.
Table 9. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-271–7-303.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μeb(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-271 | T7Me | –5.45 | –3.93 | 1.36 | PM6 | 1:1 | 0.89 | 17.1 | 59 | 8.96 | -/8.4 × 10–5 | (292) |
| 7-272 | MeIC1 | –5.59 | –3.89 | 1.54 | PBDB-T | 1:1 | 0.93 | 18.3 | 74 | 12.6 | 2.4 × 10–3/5.1 × 10–4 | (212) |
| 7-273 | TPTTT-T | –5.72 | –3.73 | 1.81 | PBT1-C | 1:1 | 1.11 | 11.1 | 61 | 7.42 | 2.6 × 10–4/2.2 × 10–4 | (293) |
| 7-274 | a-IT | –5.65 | –3.99 | 1.54 | PM6 | 1:1 | 0.91 | 16.6 | 76 | 11.5 | 4.3 × 10–4/- | (294) |
| 7-275 | TPIC-2Cl | –5.36 | –3.92 | 1.45 | PM7 | 1:1 | 0.94 | 21.4 | 72 | 14.5 | 7.5 × 10–4/4.5 × 10–4 | (201) |
| 7-276 | N7IT | –5.47 | –3.93 | 1.42 | PM6 | 1:1 | 0.93 | 21.0 | 71 | 13.8 | 7.2 × 10–4/7.0 × 10–4 | (294) |
| 7-277 | IPT2F-Th | –5.58 | –4.03 | 1.47 | PBDB-T | 1:1 | 0.86 | 20.4 | 71 | 12.5 | -/5.1 × 10–4 | (295) |
| 7-278 | IPT2F-Ph | –5.57 | –4.00 | 1.46 | PBDB-T | 1:0.8 | 0.86 | 21.2 | 72 | 13.1 | -/5.7 × 10–4 | (295) |
| 7-279 | IPT2F-TT | –5.60 | –4.00 | 1.45 | PBDB-T | 1:1 | 0.84 | 22.2 | 75 | 14.0 | -/6.1 × 10–4 | (295) |
| 7-280 | IDTP-4F | –5.54 | –3.98 | PM7 | 1:1 | 0.90 | 22.5 | 75 | 15.2 | 7.3 × 10–4/4.1 × 10–4 | (296) | |
| 7-281 | IPT-4F | –5.57 | –4.08 | 1.42 | PM6 | 1:1 | 0.91 | 22.1 | 74 | 15.0 | 5.8 × 10–4/1.3 × 10–4 | (297) |
| IPT-4F | –5.56 | –4.05 | 1.41 | PM6 | 1:1 | 0.88 | 22.2 | 75 | 14.6 | (298) | ||
| 7-282 | IPTBO-4F | –5.57 | –4.07 | 1.41 | PM6 | 1:1 | 0.92 | 22.1 | 73 | 14.7 | 3.7 × 10–4/1.0 × 10–4 | (297) |
| 7-283 | IPT-4Cl | –5.58 | –4.11 | 1.39 | PM6 | 1:1 | 0.88 | 23.2 | 70 | 14.4 | 7.0 × 10–4/1.1 × 10–4 | (297) |
| 7-284 | IPTBO-4Cl | –5.64 | –4.08 | 1.39 | PM6 | 1:1 | 0.89 | 23.2 | 73 | 15.0 | 5.2 × 10–4/1.2 × 10–4 | (297) |
| 7-285 | TPIC-4Cl | –5.35 | –3.97 | 1.40 | PM7 | 1:1 | 0.88 | 23.0 | 76 | 15.3 | 8.8 × 10–4/5.1 × 10–4 | (201) |
| 7-286 | IN-4F | –5.59 | –3.94 | 1.45 | PM6 | 1:1 | 0.92 | 19.5 | 70 | 12.5 | (298) | |
| 7-287 | INO-4F | –5.64 | –3.93 | 1.46 | PM6 | 1:1 | 0.93 | 20.5 | 72 | 13.7 | (298) | |
| 7-288 | IPCl-4F | –5.60 | –4.08 | 1.39 | PM6 | 1:1 | 0.83 | 21.2 | 61 | 10.8 | (298) | |
| 7-289 | DTPPSe-2F | –5.52 | –4.05 | 1.40 | PBDB-T | 1.25:1 | 0.84 | 22.2 | 74 | 13.8 | -/5.1 × 10–4 | (202) |
| 7-290 | DTPPSe-4F | –5.53 | –4.10 | 1.39 | PBDB-T | 1.25:1 | 0.78 | 21.2 | 73 | 12.0 | -/5.0 × 10–4 | (202) |
| 7-291 | TPTTT-2F | –5.69 | –4.01 | 1.56 | PBT1-C | 1:1 | 0.92 | 17.6 | 75 | 12.0 | 4.8 × 10–4/2.2 × 10–4 | (299) |
| 7-292 | SePTTT-2F | –5.66 | –3.97 | 1.50 | PBT1-C | 1:1.1 | 0.90 | 18.0 | 76 | 12.2 | 1.2 × 10–3/4.4 × 10–4 | (300) |
| 7-293 | SRID-4F | –5.52 | –3.90 | 1.44 | PBDB-T | 1:1 | 0.85 | 20.2 | 75 | 13.1 | -/2.2 × 10–4 | (126) |
| 7-294 | TRID-4F | –5.52 | –3.90 | 1.48 | PBDB-T | 1:1.5 | 0.89 | 18.5 | 75 | 12.3 | -/2.7 × 10–4 | (126) |
| 7-295 | TSeIC-4Cl | –5.75 | –3.99 | 1.44 | PM6 | 1:1 | 0.74 | 20.9 | 72 | 11.1 | 7.4 × 10–4/3.8 × 10–4 | (301) |
| 7-296 | TSeIC-4Br | –5.69 | –3.97 | 1.43 | PM6 | 1:1 | 0.77 | 21.3 | 72 | 11.9 | 7.9 × 10–4/4.2 × 10–4 | (301) |
| 7-297 | SeTIC4Cl | –5.65 | –4.08 | 1.44 | PM6 | 1:1 | 0.78 | 22.9 | 75 | 13.3 | -/3.0 × 10–3 | (184) |
| 7-298 | TSeTIC | –5.65 | –3.91 | 1.53 | PM6 | 1:1 | 0.93 | 19.4 | 76 | 13.7 | 4.1 × 10–4/3.1 × 10–4 | (302) |
| 7-299 | BDSeIC2Br | –5.63 | –3.99 | 1.41 | PM6 | 1:1 | 0.89 | 20.3 | 69 | 12.5 | 4.5 × 10–4/3.8 × 10–4 | (185) |
| 7-300 | BDSeIC4Br | –5.65 | –4.02 | 1.39 | PM6 | 1:1 | 0.85 | 16.4 | 69 | 9.60 | 4.2 × 10–4/2.2 × 10–4 | (185) |
| 7-301 | ArSiID-F | –5.42 | –3.90 | PBDB-T | 1:1 | 0.84 | 16.8 | 65 | 9.40 | -/3.1 × 10–4 | (193) | |
| 7-302 | ArSiID-Cl | –5.45 | –4.01 | PBDB-T | 1:1 | 0.79 | 16.4 | 60 | 7.90 | -/1.1 × 10–4 | (193) | |
| 7-303 | NFDTSB | –5.56 | –3.85 | 1.40 | PTB7-Th | 1:1 | 0.75 | 20.1 | 63 | 9.60 | -/9.3 × 10–5 | (303) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
NFAs 7-272–7-274 have an asymmetric backbone with six 5-membered rings—4 T and 2 Cp—and one 6-membered ring—a benzene (B)—in the order T-Cp-B-Cp-T-T-T and different acceptor end groups. The molecule 7-272 (MeIC1)212 shares the same end groups with 7-271; 7-273 (TPTTT-T)293 uses diethyl TBA as an acceptor unit, and 7-274 (a-IT)294 possesses INCN-Cl groups. 7-272 and 7-274 have the same optical band gap of 1.54 eV, whereas the TBA in 7-273 increases the band gap to 1.81 eV. The three acceptors were blended with different polymer donors in solar cells, which makes them difficult to compare. However, PBT1-C/7-273 achieved the highest VOC of these three combinations with 1.11 V but also the lowest PCE with 7.42%. PBDB-T/7-275 and PM6/7-274 reached VOCs of over 0.90 V with efficiencies of 12.6 and 11.5%, respectively.212,293,294
NFAs 7-275–7-288 have similar backbones as 7-272–7-274, but an N-alkylated pyrrole (P) substitutes a thiophene in the central donor unit, leading to the following motif: T-Cp-B-Cp-T-P-T. They are different in their accepting units, the N-alkyl groups, as well as the substituents on an outer thiophene (7-286–7-288). 7-275 (TPIC-2Cl)201 and 7-276 (N7IT)294 share INCN-Cl groups with 7-274, but the pyrrole ring leads to higher HOMO and lower LUMO energy levels, which reduces the band gaps of 7-275 and 7-276. Solar cells of PM7/7-275 and PM6/7-276 give VOCs of over 0.90 V, JSCs of over 21 mA cm–2, FFs of over 70%, and efficiencies up to 14.5 and 13.8%, respectively.201,294 Cao et al. synthesized a series of acceptors with the same backbone, INCN-F acceptor units, and different substituents on the nitrogen atom (7-277–7-279).295 The side chain has nearly no impact on the energy levels and the optical band gap of the acceptors. Solar cells were built in combination with PBDB-T, and the PCE increases from 7-277 (IPT2F-Th, 12.5%)295 over 7-278 (IPT2F-Ph, 13.1%)295 to 7-279 (IPT2F-TT, 14.0%)295 simultaneously with increasing FF and electron mobility values.2957-280–7-282 have INCN-2F units and differ in their N-alkyl chains. 7-280 (IDTP-4F)296 has HOMO/LUMO energy levels of −5.54 eV/–3.98 eV, respectively. Solar cells with PM7 yielded high efficiencies up to 15.2%.296 The two NFAs, 7-281 (IPT-4F)297 and 7-282 (IPTBO-4F),297 have similar HOMO/LUMO energy levels of −5.57 eV/–4.08 eV and −5.57 eV/–4.07 eV and optical band gaps of 1.42 and 1.41 eV, respectively. Acceptor 7-281 contains an octyl side chain and 7-282 a butyloctyl group. Solar cells with PM6 revealed similar VOC and JSC and efficiencies of 14.7 and 15.0% for 7-282 and 7-281, respectively. Devices of the analogous chlorinated NFAs, 7-283 (IPT-4Cl)297 and 7-284 (IPTBO-4Cl),297 have slightly lower band gaps of 1.39 eV. Solar cells of both acceptors, 7-283 and 7-284, with PM6 gave PCEs up to 15.0% (7-284) and 14.4% (7-283).297 The molecule 7-285 (TPIC-4Cl)201 resembles 7-275, with the difference of INCN-2Cl instead of INCN-Cl units. The monochlorinated 7-275 exhibits a higher optical band gap (1.45 eV) than 7-285 (1.40 eV) and the other dichlorinated compounds 7-283 and 7-284. Solar cells with 7-275 and 7-285 and PM7 achieved remarkable efficiencies of 14.5 and 15.3%, respectively. As expected from the band gap, the solar cells based on 7-285 show a lower VOC but higher JSC of 23.0 mA cm–2 and a better FF. The results align with GIWAXS data, as the higher crystallinity of the blend film PM7/7-285 indicates a better charge transport and thus better performance in OSCs and higher electron mobility.201 Zhang et al. studied the difference in the optical and electrochemical behavior when substituents are introduced on an outer thiophene ring of 7-281. This leads to molecules 7-286–7-288, which contain an additional ethylhexyl group (7-286, IN-4F),298 ethylhexyloxy group (7-287, INO-4F),298 or chlorine atom (7-288, IPCl-4F).298 The alkyl and alkoxy chains are shifting the HOMO down and the LUMO level upward, thus widening the band gap, while the chlorine atom lowers both energy levels, giving a slightly narrower band gap than 7-281. Photovoltaic devices of 7-286–7-288 combined with PM6 lead to lower JSC, FF, and PCE values than 7-281 but enhanced VOCs. Here, the highest efficiency was obtained with PM6/7-287 (13.7%) followed by PM6/7-286 (12.5%) and PM6/7-288 (10.8%).298
In the structures 7-289 (DTPPSe-2F)202 and 7-290 (DTPPSe-4F),202 one thiophene is replaced with a selenophene (T(Se)) ring, leading to the structure T(Se)-Cp-B-Cp-T-P-T. 7-290—comprising INCN-2F units—is thereby the Se analogue to 7-281, while 7-289 contains INCN-F units. This does not largely affect the positions of the energy levels, but the optical band gaps are slightly lower. The PCEs of PBDB-T/7-289- and PBDB-T/7-290-based solar cells are 13.8 and 12.0%, respectively; however, the PV parameters are difficult to compare to devices based on PM6/7-281 due to the different donors.202 The thiophene-based asymmetric structure 7-291 (TPTTT-2F)299 (T-Cp-B-Cp-T-T-T) can be compared with the selenium counterpart 7-292 (SePTTT-2F)300 (T(Se)-Cp-B-Cp-T-T-T). The selenium compound 7-292 provides slightly higher energy levels and a lower band gap (1.50 eV instead of 1.56 eV for 7-291). However, the PV parameters of solar cells with these molecules and PBT1-C as a donor are very similar. Both exhibit a PCE over 12%, a high VOC over 0.90 V, and FFs over 74%.299,300
Lin et al. compared the impact of the selenium position using structure 7-293 (SRID-4F)126 with T(Se)-T-Cp-B-Cp-T-T(Se) and structure 7-294 (TRID-4F)126 with T-T(Se)-Cp-B-Cp-T(Se)-T, both with INCN-2F acceptor units. The position of the selenophene does not affect the energy levels and only slightly the band gap in the case of 7-293 and 7-294. Combined with PBDB-T, the solar cells showed an efficiency of 13.1 and 12.3% for 7-293 and 7-294, respectively. These results are consistent with GIWAXS data, where a closer π–π stacking was observed for 7-293, leading to a better charge transport behavior.126 Molecules 7-295 (TSeIC-4Cl)301 and 7-296 (TSeIC-4Br)301 have a similar backbone to 7-294 but have INCN-2Cl and dibrominated INCN (INCN-2Br) acceptor units, respectively. Both the chlorinated and brominated species show lower HOMO/LUMO energy levels than the fluorinated counterpart, leading to smaller optical band gaps. The characteristic solar cell parameters (except for the JSCs) of blends with PM6 are lower for 7-295 and 7-296 than for 7-294; however, still PCEs of over 11% could be reached.301
The NFA 7-297 (SeTIC4Cl)184 is the INCN-Cl counterpart to 7-294. The chlorine atoms cause a further downshift of the energy levels and a slightly decreased optical band gap. Devices with PM6 as a donor give a high PCE of 13.3%, due to a high FF and a JSC of 22.9 mA cm–2.184 Molecule 7-298 (TSeTIC)302 comprises the same backbone as 7-293, but CPTCN end groups were used. Comparing them, 7-298 has a downshifted HOMO level and therefore a larger band gap than 7-293. Solar cells were assembled with PM6 as a donor and give a PCE of 13.7%.3027-299 (BDSeIC2Br)185 and 7-300 (BDSeIC4Br)185 feature the same backbone structure with two selenophene rings in the donor unit with a T-Cp-T(Se)-B-T(Se)-Cp-T core but either with INCN-Br (7-299) or INCN-2Br (7-300) acceptor units. The energy levels and the optical band gap resemble each other with a slightly lower band gap of 1.39 eV for the dibrominated molecule. However, the PV parameters of solar cells with PM6 as a donor show that the monobrominated derivative 7-299 yields higher PCE values (12.5%) than 7-300 (9.60%). Besides an expected higher VOC, also, higher JSCs and FFs are found. This is also reflected in the enhanced electron mobility in blend film and the more balanced electron/hole mobility.
Silicon can be used to replace carbon in conjugated structures. In 7-301–7-303, two silole (Si) rings substituted with two octyl chains were introduced into the donor unit, leading to the structure T-Si-T-B-T-Si-T. The central benzyl unit contains additional alkyl side chains. Wang et al. studied the difference of INCN-2F (7-301, ArSiID-F)193 and INCN-2Cl acceptor units (7-302, ArSiID-Cl)193 in this structure with hexyl chains on the central benzene ring. Both structures possess similar HOMO levels, but the LUMO is upshifted for the fluorinated compound 7-301. Solar cells with PBDB-T are showing maximum PCEs of 9.40% in the case of 7-301 and 7.90% for 7-302, caused especially by lower VOC and FF values.193 The exchange of the octyl chain on the central benzene ring in 7-301 with ethylhexyloxy side chains leads to structure 7-303 (NFDTSB).303 In comparison to 7-301, the HOMO energy level is downshifted and the LUMO upshifted, giving an optical band gap of 1.40 eV. Solar cells with PTB7-Th/7-303 gave a PCE of 9.6%.193,303
Figure 14 shows various other NFA motifs of the last years and Table 10 the corresponding solar cell data. 7-304–7-306 exhibit a “kinked” donor unit based on a central anthracene unit with two [b]-annulated cyclopenta[b]thiophene units on each side which lead to a double-kink structure in the arrangement of the seven rings. Acceptor 7-304 (ANT-4F)304 contains INCN-2F groups and hexylphenyl side chains, whereas 7-305 (AT-4Cl)305 has INCN-2Cl groups and (2-ethylhexyl)oxyphenyl side chains. 7-304 shows a broader band gap (1.68 eV) and lower LUMO, but their HOMO energies are similar. Solar cells with PM6 give PCEs above 13% with both compounds. Exchanging the dichlorobenzene rings in the INCN units of 7-305 with naphthalene results in structure 7-306 (AT-NC),305 which shows higher HOMO/LUMO energy levels as well as a slightly wider band gap. Solar cells (with PBDB-T) revealed, however, a lower performance. More kinked structures are represented by the structural isomers 7-307 (DTA-IC-S) and 7-308 (DTA-IC-M), with hexylphenyl side chains and INCN end groups.3067-307 has a large band gap of 1.67 eV accompanied by a much lower HOMO and higher LUMO energy level. Solar cells with the PBDB-T yielded a PCE of 4.20% (7-308) and 6.09% (7-307).306 The pyrane-bridged acceptor 7-309 (COi7DFIC)307 resembles in its double-kink structure the NFA 7-304, having INCN-2F units too. 7-309 shows HOMO/LUMO energy levels of −5.78 eV/–4.04 eV, respectively, and an optical band gap of 1.55 eV. Solar cells were built with PTB7-Th, giving moderate results with an efficiency of 8.70%.307
Figure 14.
Structures of acceptors with anthracene, pyran-bridged, fluorene, carbazole, dicyclopentathiophene, and dibenzothiophene-based donor units; structures with two seven-ring core units and truxene cores.
Table 10. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 7-304–7-335.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μed(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7-304 | ANT-4F | –5.69 | –4.01b | 1.68 | PM6 | 0.93 | 19.0 | 74 | 13.1 | -/1.4 × 10–4 | (304) | |
| 7-305 | AT-4Cl | –5.71 | –3.89 | 1.60 | PM6 | 1:1 | 0.90 | 19.5 | 76 | 13.3 | -/1.4 × 10–4 | (305) |
| 7-306 | AT-NC | –5.44 | –3.80 | 1.62 | PBDB-T | 1:0.8 | 0.92 | 17.1 | 69 | 10.9 | -/6.5 × 10–5 | (305) |
| 7-307 | DTA-IC-S | –5.73 | –3.83 | 1.67 | PBDB-T | 1:1.5 | 0.94 | 12.3 | 53 | 6.09 | 3.9 × 10–6/3.0 × 10–5 | (306) |
| 7-308 | DTA-IC-M | –5.53 | –4.05 | 1.35 | PBDB-T | 1:1 | 0.69 | 13.0 | 47 | 4.20 | 1.1 × 10–6/9.4 × 10–5 | (306) |
| 7-309 | COi7DFIC | –5.78c | –4.04c | 1.55 | PTB7-Th | 1:1.5 | 0.67 | 18.4 | 70 | 8.70 | (307) | |
| 7-310 | FCO-2F | –5.37 | –3.78 | PM6 | 1:1 | 0.88 | 20.9 | 72 | 13.4 | -/1.4 × 10–4 | (308) | |
| 7-311 | FO-2F | –5.72 | –3.94 | 1.47 | PM6 | 1:1 | 0.88 | 22.3 | 77 | 15.1 | -/2.4 × 10–4 | (309) |
| 7-312 | DTFBR | –5.54 | –3.68 | 1.74 | P3HT | 1:1 | 0.71 | 8.15 | 62 | 3.68 | 2.2 × 10–4/- | (310) |
| 7-313 | F-F | –5.45 | –3.77 | 1.59 | PBDB-T | 1:1.2 | 0.88 | 17.4 | 71 | 10.9 | -/1.0 × 10–4 | (200) |
| 7-314 | F-Cl | –5.46 | –3.75 | 1.58 | PBDB-T | 1:1 | 0.87 | 17.6 | 75 | 11.5 | -/1.2 × 10–4 | (200) |
| 7-315 | F-Br | –5.47 | –3.78 | 1.56 | PBDB-T | 1:1 | 0.87 | 18.2 | 76 | 12.1 | -/1.5 × 10–4 | (200) |
| 7-316 | F-N1 | –5.62 | –4.16 | 1.49 | PM6 | 1:1 | 0.74 | 20.1 | 72 | 10.7 | -/1.2 × 10–4 | (311) |
| 7-317 | F-N2 | –5.61 | –4.07 | 1.53 | PM6 | 1:1 | 0.94 | 18.3 | 69 | 11.9 | -/1.4 × 10–4 | (311) |
| 7-318 | F-2F | –5.78 | –3.89 | 1.60 | PM6 | 1:1 | 0.94 | 18.5 | 74 | 12.9 | -/1.5 × 10–4 | (309) |
| 7-319 | FXIC-1 | –5.78 | –3.96 | 1.67 | PTB7-Th | 1:1.5 | 0.79 | 13.4 | 67 | 7.13 | 5.5 × 10–4/3.5 × 10–4 | (312) |
| 7-320 | C8-DTC | –5.60 | –3.87 | PM6 | 1:1.2 | 0.95 | 16.9 | 75 | 12.1 | (313) | ||
| 7-321 | DTC(4R)-4FIC | –5.79 | –3.99 | J71 | 1:1.3 | 0.82 | 18.9 | 70 | 10.9 | (195) | ||
| 7-322 | DTC(4Ph)-4FIC | –5.80 | –3.94 | PM6 | 1:1 | 0.95 | 18.3 | 76 | 13.2 | (196) | ||
| 7-323 | DTCC-BC | –5.46 | –3.81 | 1.67 | PBDB-T | 1:1.5 | 0.98 | 17.2 | 64 | 10.7 | -/1.4 × 10–5 | (134) |
| 7-324 | DTCCIC-4F | –5.84 | –3.62 | 1.53 | PFBDB-T | 1:1.5 | 0.85 | 22.1 | 67 | 12.6 | -/4.3 × 10–6 | (197) |
| 7-325 | DTCC-4Cl | –5.71 | –4.16 | 1.50 | T1 | 1:1.4 | 0.94 | 20.0 | 76 | 14.4 | -/1.0 × 10–5 | (314) |
| 7-326 | DTSiC-4Cl | –5.73 | –4.15 | 1.55 | T1 | 1:1.4 | 1.00 | 19.6 | 74 | 14.5 | -/1.4 × 10–5 | (314) |
| 7-327 | FTBT | –5.67 | –3.95c | 1.59 | PM6 | 1:1 | 0.96 | 7.55 | 48 | 3.47 | 3.8 × 10–5/3.3 × 10–5 | (315) |
| 7-328 | DBTIC-2F | –5.94 | –4.08 | 1.62 | PBDB-T | 1:1 | 0.80 | 15.2 | 56 | 6.81 | -/1.3 × 10–6 | (199) |
| 7-329 | DBTTC | –5.92 | –4.03 | 1.64 | PBDB-T | 1:1 | 0.97 | 17.3 | 67 | 11.3 | -/3.6 × 10–5 | (199) |
| 7-330 | FXIC-2 | –5.79 | –4.00 | 1.67 | PTB7-Th | 1:1.5 | 0.72 | 10.7 | 68 | 5.22 | 1.8 × 10–4/4.0 × 10–5 | (312) |
| 7-331 | FXIC-3 | –5.79 | –3.98 | 1.68 | PTB7-Th | 1:1.5 | 0.76 | 12.0 | 67 | 6.12 | 1.8 × 10–4/1.2 × 10–4 | (312) |
| 7-332 | 99CZ-8F | –5.79 | –3.88 | 1.60 | PM6 | 1:1 | 0.94 | 11.2 | 63 | 6.60 | (316) | |
| 7-333 | meta-TrBRCN | –5.95 | –3.72 | 2.10 | PTB7-Th | 0.8:1 | 0.94 | 16.8 | 65 | 10.2 | -/2.0 × 10–4 | (317) |
| 7-334 | para-TrBRCN | –5.98 | –3.71 | 2.19 | PTB7-Th | 0.8:1 | 0.95 | 13.8 | 64 | 8.29 | -/1.2 × 10–4 | (317) |
| 7-335 | Tr(Hex)6-3BR | –5.79 | –3.74 | 2.15 | PTB7-Th | 1:1 | 1.02 | 5.92 | 33 | 2.10 | -/1.5 × 10–5 | (204) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Other method or method not defined.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
The donor unit of structures 7-310–7-319 comprises a central fluorene and two thiophene units linked either by a pyrane ring (7-310, FCO-2F,308 and 7-311, FO-2F309) or a cyclopentadiene unit (7-312–7-319). Compared to the above-described anthracene-based NFA 7-304, having the same INCN-2F units, 7-310 has much higher HOMO/LUMO energy levels, whereas 7-311 has slightly lower HOMO and higher LUMO levels, accompanied by a narrower optical band gap. Solar cells with PM6/7-310 absorber layers reached similar PCEs (13.4%) as PM6/7-304-based devices.308 PM6/7-311 blends reveal a similar VOC as PM6/7-310 but higher JSC and FF values with an enhanced efficiency of 15.1%.3097-312 comprises a BT π-spacer and RCN end groups. The material has a large optical band gap of 1.74 eV, and devices with P3HT as a polymer donor give an efficiency of 3.68%.310 In 7-313–7-315, the central donor unit is directly linked to a INCN-X unit. Wang et al. investigated the fluorinated (7-313, F-F),200 chlorinated (7-314, F-Cl),200 and brominated (7-315, F-Br)200 species, leading to the result that 7-313 possesses the lowest energy levels of these three compounds, whereas 7-314 and 7-315 have similar HOMO/LUMO energy levels. The optical band gap is alike for all of them. Interestingly, solar cells in combination with PBDB-T exhibited enhanced JSC, FF, PCE, and electron mobility values with increasing halogen atom size, giving the highest efficiency of 12.1% for 7-315.200 Wang et al. introduced molecules with a similar backbone as 7-313–7-315 but nitro-substituted end groups, where NO2 is situated either anywhere on the INCN phenyl ring (7-316, F-N1)311 or on a specific position (7-317, F-N2).311 The nitro substitution does not largely affect the energy levels and optical band gaps when compared to acceptors 7-313–7-315. Solar cells were built with PM6, and the VOCs of the 7-316- and 7-317-based cells deviate by 0.20 V, due to the higher energy loss of 0.85 eV for 7-316 compared to 0.70 eV for 7-317. The devices based on 7-316 yielded a lower VOC but higher JSC and FF, leading to an efficiency of 10.7%. The blend with the higher VOC of 0.94 V (PM6/7-317) reached an overall efficiency of 11.9%. Differences are also observed in the molecular packing of the two acceptors and acceptor/donor blends, indicating a more ordered packing for 7-317 neat and blend films.311 Molecule 7-318 (F-2F)309 resembles 7-317 in its backbone, but it contains INCN-2F units. 7-318 and the pyran-bridged molecule 7-311 have the same side chains and end groups, and in comparison, 7-318 has a higher LUMO and lower HOMO energy level, which widens the band gap to 1.60 eV. A great deviation is also visible in the PV parameters, as 7-318 gave a much higher VOC of 0.94 V, while the other parameters are lower. The highest efficiency reached with 7-318 was 12.9% (15.1% for 7-311). Absorption spectra show a red-shift of 7-311 and a broader EQE range.309
NFA 7-319 (FXIC-1)312 possesses a similar structure as 7-318 with the only difference of having other side chains. The LUMO energy level of 7-319 is lower than that for 7-318; thus, the optical band gap is widened. However, PV parameters with PTB7-Th are much lower for 7-319, giving an efficiency of 7.13%.312
The NFAs 7-320–7-325 have a central carbazole moiety in the donor unit with cyclopentathiophenes fused on both sides, and they differ in their end groups and side chains on the donor unit. 7-320 (C8-DTC)313 comprises octyl side chains on the cyclopentadienyl rings, 1-octylnonyl chains on the carbazole, and INCN-F units. 7-321 (DTC(4R)-4FIC)195 has the same side chains but INCN-2F end groups. 7-322 (DTC(4Ph)-4FIC)196 bears the same N-alkyl chain as 7-321 but octylphenyl side chains on the cyclopentadiene units. 7-320 and 7-322 were blended with PM6 and 7-321 with J71 for the solar cell preparation and are thus difficult to compare. The best photovoltaic performance is revealed by 7-322, showing a VOC of 0.95 V, a FF of 75%, a JSC of 18.3 mA cm–2, and a PCE of 13.2%. Also, the other molecules show their potential as NFAs in solar cells with PCEs of 12.1% (7-320) and 10.9% (7-321).195,196,313 Structure 7-323 (DTCC-BC)134 shares the backbone and side chains with 7-320 and 7-321 but has extended aromatic acceptor units (cf. also 7-35 and 7-257), leading to heightened HOMO/LUMO energy levels compared to 7-320 and 7-321. Solar cells yielded lower PCEs of 10.7%, but different donors were used; however, compared to the NFA with unsubstituted INCNs (7-128), the PV parameters are improved using for both the same donor (PBDB-T) (9.25% for 7-128).134 NFAs 7-324–7-326 are still carbazole-based but with an octyl chain on the nitrogen atom. 7-324 (DTCCIC-4F)197 comprises INCN-2F, whereas 7-325 (DTCC-4Cl)314 and 7-326 (DTSiC-4Cl)314 bear INCN-2Cl units. In 7-326, the cyclopentadiene rings are replaced with siloles. Solar cells of 7-324 with PFBDB-T yielded 12.6% PCE, which is much higher than the efficiency of the INCN-containing counterpart 7-129 (6.20%).197 The PV parameters of solar cells with 7-325 and 7-326 (with T1 as a donor) are further improved, leading to efficiencies of over 14%.314 The energy levels of these two acceptors are quite similar, but the band gap of 7-326 is slightly widened. 7-327 (FTBT)315 has the same aromatic rings but the benzyl and cyclopentadienyl units have exchanged positions, leading to the donor motif T-B-Cp-T-Cp-B-T with three 5-membered rings in the middle, shifting the side chains on the cyclopentadiene units closer together. Solar cells with PM6 give a VOC of 0.96 V, but all other parameters are significantly decreased, achieving PCEs of only 3.47%.315 The backbones of 7-328 (DBTIC-2F)199 and 7-329 (DBTTC)199 resemble the backbones of 7-320–7-325, but replacing the carbazole by a dibenzo[b,d]thiophene unit leads to the donor unit motif T-Cp-B-T-B-Cp-T. This widens the optical band gap to 1.6 eV. Blended with PBDB-T, solar cells of 7-328 and 7-329 achieved PCEs of 6.81 and 11.3%, respectively.199
Figure 14 also shows some acceptors with two seven-ring donor units (7-330–7-332) or a truxene donor unit (7-333–7-335). Molecules 7-330 (FXIC-2)312 and 7-331 (FXIC-3)312 are based on the backbone of 7-318 with the difference that two of these units are bound together via the central Cp-ring either through a double bond or by sharing one carbon atom (spiro-linkage). Between these three molecules, the variation of the energy levels and optical band gaps is negligible (Table 10). Solar cells were built with PTB7-Th as the donor and yielded quite low efficiencies, being the highest for 7-319 (7.06%), followed by 7-331 (6.12%) and 7-330 (5.22%).312 Jiang et al. introduced molecule 7-332 (99CZ-8F).316 It is similar to 7-330 and 7-331, but has central carbazole units, which are linked by a N–N single bond. It gave a slightly lower LUMO energy level, leading to a narrowed band gap. Solar cells with PM6 reached slightly higher PCE values than 7-330 and 7-332, achieving the highest efficiency with 6.60% with a much higher VOC of 0.94 V.316
Truxene-core-based acceptors 7-333–7-335 contain the same BT π-bridge and RCN end groups. 7-333 (meta-TrBRCN)317 and 7-334 (para-TrBRCN)317 are structural isomers with butyl side chains on the cyclopentadienyl units of the truxene. 7-335 (Tr(Hex)6-3BR)204 differs from the two others by the side chains having hexyl chains instead. All three acceptors are wide band gap materials (over 2.00 eV). All four molecules were tested in solar cells with the low band gap donor polymer PTB7-Th. 7-333 and 7-334 gave good results, achieving VOCs of 0.94 and 0.95 V and efficiencies of 10.2 and 8.29%, respectively. In contrast, 7-335 showed a VOC over 1 V but reached only a PCE of 2.10%.204,317
4. Five Fused Aromatic Ring Systems
Among the non-fullerene acceptors containing five aromatic rings in their central core, indacenodithiophene (IDT)-based structures have become the most investigated. Its rigid, coplanar, and electron-rich fused-ring structure impedes rotational freedom leading to a lower reorganization energy.318 A large variety of different chemical structures are available through the modification of the backbone, the side chains, and/or the end groups.
The side groups of the IDT mainly influence the morphological properties of the molecule. However, by changing the hybridization of the sp3 carbon atom or by replacing it with a heteroatom, also the (opto)electrochemical properties can be tuned. Though, variations of the electron-withdrawing end groups show a larger impact on those properties.37 The latest examples from research dealing with these aforementioned modifications are summarized in the first part of this section. This is followed by a larger set of compounds based on the IDT core but containing also a π-spacer unit. By elongation of the π-conjugation, such building blocks influence molecular geometry and therefore film morphology and have a significant effect on optical as well as electrochemical properties. Finally, acceptors based on other five-ring systems are summarized in the end of this section.
4.1. IDT-Based A–D–A Systems
IDIC (5-1) is structurally similar to ITIC (vide supra) and was designed and synthesized by Lin et al. in 2016 via a one-step Knoevenagel condensation of C6IDT-CHO with INCN.37 It comprises an IDT core decorated with hexyl side chains and INCN end groups (Figure 15).
Figure 15.
Structures of non-fullerene acceptors with IDT core units. Upper left, variations of side chains and INCN-based end groups; upper right, dimeric IDT-based structures with different linkers; lower left, IDT-based core units with different heteroatoms and/or side chain and end group variations; lower right, IDT core units with varying end groups.
The molecule has a strong absorption in the range from 500 to 800 nm, which is an important feature of all IDC-based compounds. Solar cells with PTBT-T1 obtain a VOC of 0.89 V, a JSC of 15.1 mA cm–2, a FF of 65%, and a PCE of 8.19%.37 Since then, IDIC has been tested in blends with various donors, to name a few—PTZP (PCE: 11.8%),319 FTAZ (PCE: 12.5%),320 PM6,321,322 PBDB-T,322 and PTQ10.323−325 These devices typically possess good VOC and FF; however, JSC values rarely reach 20 mA cm–2, since the optical band gap of IDIC is relatively high (1.6 eV). Interestingly, also bilayer-junction devices based on PTQ10/IDIC323 and PM6/IDIC326 blends reach efficiencies which are comparable to those of the BHJ-based devices with the same donors.
4.1.1. Side Chain and End Group Modifications on IDIC-Based NFAs
The role of the side chains in fused five-ring systems is very similar to what has been described in seven-ring systems; however, due to the central core being smaller, effects of the side chains are larger. Small elongation of the side chains on the IDT central core (5-2, IDIC-C8, R = octyl)327 has a minimal influence on the solar cell performance, while much longer side chains (5-3, IDIC16, R = hexadecyl)328 have a negative impact, particularly on the fill factor (cf. solar cell parameters with PM6, Table 11). para-Hexylphenyl groups (5-4, ITIC-2T/IDT-IC) are also often used as side chains on the IDT core.329,334 Due to their larger steric hindrance, IDT becomes less crystalline than its hexyl-substituted analogue but delivers a lower PCE (6.14%) than 5-1 in solar cells with PBDB-T.322 A lower FF is the main reason for the reduced PCE, while a higher optical band gap leads to a reduced current. Structure 5-5 (IDIC-C4Ph)322 comprises phenylbutyl side groups, where the butyl group acts as a spacer between the IDT backbone and the bulky phenyl ring. It combines the good properties of hexyl (as in structure 5-1) and hexylphenyl (as in 5-4) side chains and delivers similar or better PCEs with the same polymer donors (PM6, PBDB-T). Analysis of molecular stacking in the films of the three acceptors (5-1, 5-4, and 5-5) confirms that the acceptor with hexyl side chains (5-1) exhibits a face-on stacking orientation; the same is found in compound 5-5 with phenylbutyl side groups present albeit with a weakened π–π stacking/crystallization behavior. Compound 5-4 comprising hexylphenyl side groups, on the other hand, demonstrates the weakest crystallization behavior and has an edge-on orientation.322
Table 11. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 5-1–5-74.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μee(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5-1 | IDIC | –5.70 | –3.92 | 1.63 | PM6 | 1:1 | 0.95 | 18.2 | 70 | 12.0 | -/1.9 × 10–4 | (322) |
| IDIC | –5.74d | –3.90d | PTQ10 | 1:1 | 0.96 | 18.6 | 74 | 13.2 | -/7.8 × 10–4 | (324) | ||
| 5-2 | IDIC-C8 | –5.63 | –3.86 | PM6 | 0.97 | 14.1 | 75 | 10.2 | -/8.8 × 10–4 | (327) | ||
| 5-3 | IDIC16 | –5.82 | –3.87 | 1.64 | PM6 | 1:1 | 0.99 | 11.8 | 51 | 5.96 | 1.4 × 10–4/5.0 × 10–5 | (328) |
| 5-4 | ITIC-2T | –5.61 | –3.93b | 1.68 | PBDTsTh-BDD | 1:1 | 0.98 | 12.3 | 56 | 6.7 | (329) | |
| 5-5 | IDIC-C4Ph | –5.70 | –3.93 | 1.62 | PM6 | 1:1 | 0.94 | 19.1 | 78 | 14.0 | -/4.5 × 10–4 | (322) |
| 5-6 | IDT-PhIC | –5.67 | –3.89 | 1.68 | PBDB-T | 1:1 | 0.93 | 9.99 | 55 | 5.11 | -/1.8 × 10–5 | (330) |
| 5-7 | m-IDTV-PhIC | –5.63 | –3.78 | 1.59 | PBDB-T | 1:1 | 0.99 | 11.6 | 51 | 5.85 | -/2.0 × 10–5 | (330) |
| 5-8 | IDTV-PhIC | –5.52 | –3.84 | 1.55 | PBDB-T | 1:1 | 0.98 | 4.93 | 37 | 1.77 | -/4.3 × 10–6 | (330) |
| 5-9 | IDTV-ThIC | –5.43 | –3.72 | 1.52 | PBDB-T | 1:1 | 0.91 | 2.06 | 33 | 0.62 | -/4.8 × 10–6 | (330) |
| 5-10 | IDT-C6 | –5.70 | –3.98 | 1.65 | PM6 | 1:1 | 0.96 | 17.5 | 74 | 12.5 | 3.9 × 10–4/- | (331) |
| 5-11 | IDIC8-M | –5.51 | –3.72 | 1.66 | DRCN5T | 1:1 | 1.00 | 10.4 | 61 | 6.31 | -/1.1 × 10–4 | (332) |
| 5-12 | IDT-PhC6 | –5.69 | –3.94 | 1.74 | PM6 | 1:1 | 1.00 | 13.0 | 43 | 5.6 | 1.4 × 10–5/- | (331) |
| 5-13 | IDICO1 | –5.69 | –3.81 | 1.65 | PBDB-T | 1:1 | 0.92 | 15.1 | 68 | 9.40 | 4.1 × 10–5/4.5 × 10–5 | (333) |
| 5-14 | IDICO2 | –5.74 | –3.86 | 1.60 | PBDB-T | 1:1 | 0.92 | 14.8 | 69 | 9.35 | 4.8 × 10–5/2.4 × 10–5 | (333) |
| 5-15 | IDT-HN | –5.92 | –3.86 | 1.68 | PBDB-T | 1.5:1 | 0.93 | 14.4 | 76 | 10.2 | 2.8 × 10–4/4.1 × 10–4 | (334) |
| 5-16 | TPT-IN | –5.80 | –3.97 | 1.58 | PBT1-C | 1:1 | 0.88 | 13.9 | 73 | 8.91 | 4.7 × 10–4/4.0 × 10–4 | (335) |
| 5-17 | IDIC-2F | –5.72 | –3.99 | 1.60 | PTQ10 | 1:1 | 0.90 | 19.0 | 76 | 13.0 | 1.7 × 10–3/9.0 × 10–4 | (336) |
| 5-18 | IDIC-2F | –5.75 | –3.94 | 1.59 | BDTF-CA | 1:0.6 | 0.94 | 16.7 | 58 | 9.11 | -/1.6 × 10–3 | (337) |
| 5-19 | TPT-2F | –5.84 | –4.06 | 1.64 | PBT1-C | 1:1 | 0.87 | 13.9 | 69 | 8.33 | 4.2 × 10–4/7.1 × 10–5 | (299) |
| 5-20 | MF1 | –5.73 | –3.89 | 1.54 | PM7 | 1:1 | 0.94 | 16.8 | 78 | 12.4 | 1.3 × 10–3/5.2 × 10–4 | (168) |
| 5-21 | IDIC-4F | –5.78 | –4.00 | 1.60 | PTQ10 | 1:1 | 0.81 | 18.6 | 74 | 11.1 | 1.1 × 10–3/6.3 × 10–4 | (336) |
| 5-22 | IDIC-4F | ZR2-C3 | 1:0.6 | 0.78 | 19.0 | 70 | 10.4 | -/6.4 × 10–4 | (338) | |||
| 5-23 | ID4F | –5.83 | –3.86 | 1.64 | PM6 | 1:1.2 | 0.84 | 13.4 | 61 | 6.88 | -/3.4 × 10–5 | (339) |
| 5-24 | IDIC-4F | –5.79 | –4.11 | 1.63 | PM6 | 1:1 | 0.84 | 15.2 | 72 | 9.26 | -/4.3 × 10–5 | (340) |
| 5-25 | C4Ph-IDT-DFIC | –5.76 | –4.08 | 1.54 | PM6 | 1:1 | 0.80 | 17.8 | 74 | 10.5 | -/7.2 × 10–5 | (340) |
| 5-26 | BThIND-Cl | –5.64 | –3.96 | 1.49 | PM6 | 1:1.5 | 0.96 | 18.1 | 71 | 12.4 | -/1.2 × 10–5 | (341) |
| 5-27 | ID-4Cl | –5.81 | –4.01 | 1.51 | PM6 | 1:1 | 0.77 | 17.9 | 74 | 10.3 | -/4.3 × 10–5 | (342) |
| ID-4Cl | –5.81 | –4.01 | 1.51 | PTQ10 | 1:1.5 | 0.85 | 17.3 | 66 | 9.5 | -/4.7 × 10–5 | (161) | |
| 5-28 | IDIC-4Cl | –5.88 | –3.98 | 1.59 | BSC1 | 1:0.7 | 0.86 | 21.5 | 70 | 13.0 | -/7.7 × 10–5 | (343) |
| 5-29 | IDIC-4Cl | –5.78 | –4.02 | 1.58 | PM7 | 1:1 | 0.83 | 16.2 | 69 | 9.24 | -/2.7 × 10–4 | (344) |
| 5-30 | IDT2-DFIC | –5.39 | –3.97 | 1.42 | PBDB-T | 1:1.2 | 0.91 | 16.1 | 69 | 10.1 | -/4.9 × 10–4 | (345) |
| 5-31 | C6-IDT2-DFIC | –5.31 | –4.00 | 1.44 | PBDB-T | 1:1 | 0.81 | 22.6 | 68 | 12.3 | -/6.6 × 10–4 | (346) |
| 5-32 | DIDIC | –5.44 | –3.91 | 1.40 | FTAZ | 1:1.5 | 0.82 | 17.5 | 65 | 9.3 | 2.2 × 10–3/1.4 × 10–4 | (347) |
| 5-33 | TIDIC | –5.59 | –3.88 | 1.55 | FTAZ | 1:1.5 | 0.88 | 20.2 | 74 | 13.1 | 4.9 × 10–3/1.1 × 10–3 | (347) |
| 5-34 | DFB-dIDT | –5.53 | –3.87 | 1.64 | PBDB-T | 1:1 | 0.86 | 15.6 | 50 | 6.71 | -/1.1 × 10–5 | (348) |
| 5-35 | BT-dIDT | –5.34 | –3.87 | 1.54 | PBDB-T | 1:1 | 0.87 | 18.6 | 65 | 10.5 | -/1.2 × 10–4 | (348) |
| 5-36 | BTIDIC | –5.48 | –3.97 | 1.47 | J71 | 1:1 | 0.86 | 19.8 | 67 | 11.5 | 5.5 × 10–4/4.4 × 10–4 | (349) |
| 5-37 | BID-4F | –5.69 | –3.73 | 1.53 | PM6 | 1:1.2 | 0.92 | 17.8 | 74 | 12.3 | -/7.0 × 10–4 | (350) |
| 5-38 | BT2FIDT-4Cl | –5.70 | –3.88 | 1.56 | PM7 | 1:1 | 0.97 | 18.1 | 72 | 12.5 | -/1.2 × 10–4 | (351) |
| 5-39 | BO2FIDT-4Cl | –5.73 | –3.89 | 1.59 | PM7 | 1:1 | 0.96 | 16.1 | 61 | 10.4 | -/3.6 × 10–5 | (351) |
| 5-40 | TTIDIC | –5.43 | –3.92 | 1.46 | J71 | 1:1 | 0.91 | 10.7 | 67 | 6.54 | 3.7 × 10–4/2.2 × 10–4 | (349) |
| 5-41 | CNDTBT-C8IDT-INCN | –5.54 | –3.82 | 1.48 | PBDB-T | 1:1 | 0.86 | 19.4 | 67 | 11.2 | -/9.3 × 10–5 | (352) |
| 5-42 | CNDTBT-C8IDT-FINCN | –5.58 | –3.92 | 1.40 | PBDB-T | 1:1 | 0.79 | 22.1 | 70 | 12.3 | -/4.1 × 10–4 | (352) |
| 5-43 | BDT(IDT-IC)2 | –5.53 | –3.96 | 1.59 | PBDB-T | 1:1 | 0.89 | 11.0 | 51 | 4.98 | 5.5 × 10–5/4.8 × 10–6 | (353) |
| 5-44 | BDT(IDT-IC-2F)2 | –5.51 | –3.97 | 1.54 | PBDB-T | 1:1.5 | 0.83 | 13.6 | 55 | 6.21 | 2.2 × 10–4/1.1 × 10–5 | (353) |
| 5-45 | GDIC-C8 | –5.67 | –3.86 | PM6 | 1.02 | 11.4 | 58 | 6.76 | -/5.3 × 10–4 | (327) | ||
| 5-46 | SiIDT-IC | –5.47b | –3.78 | 1.69 | PBDB-T | 1:1 | 0.92 | 13.5 | 66 | 8.16 | -/1.0 × 10–4 | (354) |
| 5-47 | IDF-IC | –5.75 | –4.13 | 1.62 | PM6 | 1:1 | 0.91 | 14.6 | 59 | 7.80 | (355) | |
| 5-48 | IDF-4F | –5.83 | –4.27 | 1.56 | PM6 | 1:1 | 0.74 | 17.5 | 61 | 7.81 | (355) | |
| 5-49 | ID-MeIC | –5.68 | –3.58 | 1.51 | PBDB-T | 1:0.8 | 0.90 | 14.1 | 50 | 6.46 | 1.0 × 10–6/3.4 × 10–6 | (356) |
| 5-50 | MO-IDIC-Cl-1 | –5.80 | –3.91 | 1.55 | PTQ10 | 1:1.1 | 0.88 | 17.7 | 69 | 10.8 | 5.1 × 10–4/4.0 × 10–4 | (238) |
| 5-51 | MO-IDIC-Cl-2 | –5.79 | –3.92 | 1.54 | PTQ10 | 1:1.1 | 0.88 | 19.2 | 74 | 12.5 | 7.3 × 10–4/7.3 × 10–4 | (238) |
| 5-52 | MO-IDIC-2F | –5.80 | –3.93 | 1.55 | PM6 | 1:1 | 0.84 | 18.9 | 77 | 12.2 | 8.9 × 10–4/6.2 × 10–4 | (357) |
| 5-53 | HO-IDIC-2F | –5.81 | –3.91 | 1.55 | PM6 | 1:1 | 0.86 | 19.1 | 76. | 12.5 | 1.1 × 10–3/7.2 × 10–4 | (357) |
| HO-IDIC-2F | –5.81 | –3.91 | PTQ10 | 1:1 | 0.92 | 19.0 | 71 | 12.4 | -/1.3 × 10–3 | (358) | ||
| 5-54 | DO-IDIC-2F | –5.79 | –3.88 | 1.54 | PM6 | 1:1 | 0.86 | 19.6 | 77 | 13.0 | 1.1 × 10–3/7.3 × 10–4 | (357) |
| 5-55 | T-Se | –5.61 | –3.83 | 1.62 | PM6 | 1:1 | 0.93 | 12.8 | 62 | 7.44 | -/3.8 × 10–5 | (359) |
| 5-56 | SePT-IN | –5.77 | –4.00 | 1.54 | PBT1-C | 1:1 | 0.85 | 16.4 | 73 | 10.2 | 7.1 × 10–4/6.4 × 10–4 | (335) |
| 5-57 | T-Se-4F | –5.62 | –3.93 | 1.58 | PM6 | 1:1 | 0.78 | 18.1 | 67 | 9.41 | -/4.1 × 10–4 | (359) |
| 5-58 | T-Se-Th | –5.61 | –3.88 | 1.60 | PM6 | 1:1 | 0.91 | 16.9 | 67 | 10.3 | -/5.1 × 10–4 | (359) |
| 5-59 | A1 | –5.61 | –3.55 | 1.95 | P3HT | 1:0.8 | 0.93 | 8.69 | 66 | 5.39 | -/5.8 × 10–5 | (360) |
| 5-60 | IDTA | –5.88c | –3.90 | 1.90 | PBDB-T | 1:1 | 0.99 | 12.4 | 60 | 7.40 | 7.6 × 10–5/4.3 × 10–5 | (170) |
| 5-61 | IDT-TBA | –5.91c | –4.00 | 1.91 | PBDB-T | 1:1 | 1.00 | 11.2 | 57 | 6.70 | -/5 × 10–5 | (361) |
| 5-62 | IDT-R | –5.44 | –3.53 | 1.96 | P3HT | 1:1 | 0.73 | 3.73 | 51 | 1.57 | (140) | |
| 5-63 | IDT-CT | –5.39 | –3.67 | 1.63 | PTB7-Th | 1:1.4 | 0.93 | 11.2 | 42 | 4.62 | (140) | |
| 5-64 | IDT-CR | –5.34 | –3.44 | 1.78 | P3HT | 1:1 | 0.66 | 2.62 | 49 | 0.99 | (140) | |
| 5-65 | TIM-IDT | –5.36 | –4.01 | 1.43 | PTB7-Th | 1:1 | 0.59 | 6.04 | 48 | 1.96 | -/3.4 × 10–7 | (362) |
| 5-66 | IDNO-IDT | –5.45 | –3.65 | 1.95 | PBDB-T | 1:1 | 1.03 | 2.65 | 35 | 1.10 | -/2.4 × 10–7 | (363) |
| 5-67 | IM1-IDT | –5.46 | –4.02 | 1.48 | PBDB-T | 1:1.5 | 0.55 | 5.07 | 47 | 1.62 | -/8.4 × 10–7 | (363) |
| 5-68 | IM-IDT | –5.44 | –4.00 | 1.48 | PTB7-Th | 1:1 | 0.58 | 4.96 | 49 | 1.60 | -/1.9 × 10–7 | (362) |
| 5-69 | ITAN-IDT | –5.42 | –3.69 | 1.68 | PBDB-T | 1:1.5 | 1.18 | 3.71 | 39 | 2.00 | -/1.0 × 10–6 | (363) |
| 5-70 | IBCT | –5.50 | –3.70 | 1.65 | L1 | 1:1.2 | 1.02 | 15.1 | 74 | 11.3 | -/2.5 × 10–4 | (364) |
| 5-71 | IDTCN | –5.91 | –3.94 | 1.67 | PBT1-EH | 1:1 | 0.93 | 13.1 | 70 | 8.69 | -/2.3 × 10–3 | (365) |
| 5-72 | IDTPC | –5.84 | –3.98 | 1.52 | PTQ10 | 1:1 | 0.93 | 17.5 | 75 | 12.2 | 8.0 × 10–4/3.7 × 10–4 | (366) |
| 5-73 | IDTPC-Me | –5.62 | –3.92 | 1.55 | PTQ10 | 1:1 | 0.95 | 13.8 | 70 | 9.2 | 6.5 × 10–4/3.1 × 10–4 | (367) |
| 5-74 | IDTPC-DMe | –5.58 | –3.85 | 1.57 | PTQ10 | 1:1 | 1.02 | 13.5 | 68 | 9.3 | 5.4 × 10–4/2.3 × 10–4 | (367) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
HOMO obtained via PESA.
Other method or method not defined.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Introducing a hexyloxyphenyl side chain (5-6, IDT-PhIC)330 has practically no influence on the optical properties. This is not very surprising, as the side chains are attached to a sp3 hybridized carbon in the IDT backbone and, thus, they are not a part of π-conjugation. Solar cells with PBDB-T show a lower performance than those with 5-4 (cf. Table 11). In contrast to 5-6, the optical properties significantly change in structures 5-7–5-9, where one diphenylmethylene or dithienylmethylene group is attached to the IDT core instead of two alkyl or aryl side groups, thereby extending the conjugation, while decreasing the optical band gap. This effect scales with the electron-donating ability of the methylene moieties (5-7 < 5-8 < 5-9), leading to optical band gaps from 1.59 eV, over 1.55 eV to 1.52 eV. Solar cells of the meta-alkoxyl isomer (5-7, m-IDTV-PhIC) with PBDB-T exhibit a similar performance to those of 5-6 (with non-conjugated hexyloxyphenyl groups), while the para-alkoxyl isomer 5-8 (IDTV-PhIC) and the thiophene derivative 5-9 (IDTV-ThIC) show a drop of PCE.330
The influence of substituents on the INCN end group follows the same general characteristics as described in the previous section (Seven Fused Aromatic Ring Systems), e.g., for ITIC-based NFAs. Consequently, the introduction of methyl groups as in compounds 5-10 (IDT-C6), 5-11 (IDIC8-M), and 5-12 (IDT-PhC6) only marginally influences the optical or electrochemical properties, leading to slightly higher optical band gaps (compared to 5-1, 5-2, and 5-4, respectively).331,332 Solar cells of 5-10 and PM6 show the highest performance reaching PCE values of up to 12.5% compared to 5.6% for solar cells with 5-12, which was explained by a better film packing of 5-10 comprising hexyl side chains on the IDT core. A blend of 5-10 with PM6 had a favorable face-on orientation, while 5-12 with hexylphenyl side chains in a blend with PM6 had an amorphous morphology.331
Compounds 5-13 (IDICO1) and 5-14 (IDICO2) having octyl chains at different positions on the INCN unit reveal slightly raised LUMO levels compared to the unsubstituted 5-1. Solar cells of both acceptors with PBDB-T lead to similar PCEs values of approx. 9.4% by keeping the good electron mobility and FF, but increasing the JSC, compared to 5-1 (PCE 6.96%). The increased current density was attributed to a better light absorption of the octyl derivatives in film as well as improved intermixing with the donor polymer.333 Structure 5-15 (IDT-HN), which contains a cyclic alkyl substituent, demonstrated very similar improvements in the same parameters when compared to the unsubstituted compound 5-4, with nearly 70% increase in PCE (10.2% vs 6.11%) as 5-15 has an improved π–π stacking and a face-on orientation in film.334 The naphthalene-based analogue 5-16 (TPT-IN) has a packaging behavior very similar to that of 5-15 but smaller electrochemical and optical band gaps due to the extended aromatic system. Solar cells with PBT1-C reached a PCE of 8.91%.335 Halogenation of the INCN groups (compounds 5-17–5-29) has similar effects as described in the seven-ring section and leads to lower HOMO and LUMO energies and lower optical band gaps (in film). Here, fluorination leads only to a slight reduction in the band gap, whereas chlorination leads to molecules with optical band gaps of approx. 1.5 eV. As a result, solar cells with the halogenated derivative and the same donor material have usually decreased VOC values but an improved JSC. The electron mobilities in blends are similar or improved. However, there is up to now no clear trend on the final PCE values, probably also due to a lack of comparative device studies. For example, solar cells with the non-halogenated derivative 5-1 show a better performance with PCE values of 13.2%324 using PTQ10 as a donor polymer, compared to similar values of 13.0% for devices based on 5-17 (IDIC-2F) bearing INCN-F units.336 However, both are significantly better than those of the acceptor 5-21 (IDIC-4F, 11.1%) with INCN-2F end groups.336 Solar cell data for the chlorinated structures 5-26 (BThIND-Cl, INCN-Cl units)341 and 5-27 (ID-4Cl, INCN-2Cl units)342 with another donor, PM6, also show better PCE values of 12.4% for devices with the dichlorinated NFA 5-26. However, in the case of the octyl derivatives (5-2,3275-18, IDIC-2F,332,3375-22, IDIC-4F,337,338 and 5-28, IDIC-4Cl343), the best solar cell performances were obtained for solar cells with compound 5-28 bearing INCN-2Cl units (PCE values of 13% with BSC1 as a donor), whereas, for the hexylphenyl series, the best solar cells were reported for the combination of compound 5-24 (IDIC-4F) with PM6 (9.26%),340 which is very similar to devices based on 5-29 (IDIC-4CL9) and PM7.344 Regarding the phase separation and molecular arrangement, as expected, a face-on orientation is observed for the compounds which have alkyl side chains on the IDT core. Interestingly, for the tetrachloro-substituted compound with hexyl side chains (5-27), an edge-on orientation was observed in pristine film,161 while the octyl-substituted compound 5-28 had a face-on orientation.343,368−3705-24, a tetrafluoro-substituted compound with hexylphenyl side chains, showed no preferred orientation in film.371,372 A comparison of 5-19 (TPT-2F), which contains INCN-F units and hexylphenyl side chains on the IDT core, with its ring extended analogues TPTT-2F (fused additional thienothiophene, see 6-16) and TPTTT-2F (fused additional cyclopentadithiophene, see 7-291) demonstrates how small changes in conjugation length can already be enough to improve the π–π stacking. In the case of a six- and seven-ring central core (compounds 6-16 and 7-291, respectively), a dominant face-on orientation in film is observed, which is not the case for compound 5-19 with the five-ring central core.299 Since the size and shape of domains formed by the active layer components in the BHJ are mutually affected, changes in the acceptor structure can also influence the behavior of the polymer donor. This is nicely illustrated in a study, where the authors prepared blends of PTQ10 with four different NFAs bearing the same end group, 5-27 (ID-4Cl), 7-7, 7-69, and 7-158. The donor in each of the blends had rod-shaped domains but with different radius. When 5-27 was used as an acceptor, the radii of PTQ10 domains were the largest and the smallest in the case when 7-158 (Y7) is used. Consequently, PTQ10/5-27 had the smallest interfacial area. Thus, despite the high crystallinity of the PTQ10/5-27 blend, it had the most unbalanced hole and electron mobility, lowest JSC, FF, and PCE.161 This study underlines the importance of obtaining further understanding on how the blend morphology is influenced by relatively small changes in the NFA structure in order to design more efficient acceptors. To that end, the IDT central core is a suitable platform. Since it consists of only five fused rings, it permits the investigation of, for example, various end groups, without imposing too dominant effects itself. IDIC and its halogenated derivatives are often used for the development of new donor polymers157,373 or small molecule donors.370,374−377
4.1.2. Dimeric IDT Acceptors
The linking of two IDT units either directly or via a conjugated spacer (L) leads to the A–D–(L)–D–A motif. The structure 5-30 (IDT2-DFIC) has an extended absorption spectrum and a reduced optical band gap compared to the monomeric analogue 5-24. Furthermore, solar cells based on PBDB-T/5-30 blends reached a nearly doubled PCE of 10.1% and a higher VOC, JSC, and FF.345 Structure 5-31 with hexyl chains on the IDT core exhibits an improved crystallinity of the acceptor, thus reaching higher electron mobility and JSC values. This results in an improved PCE (12.3%) in solar cells using PBDB-T as a donor.346 Incorporation of a double bond (5-32, DIDIC) between the two IDT units has a minimal effect on the HOMO and LUMO levels and consequently also to the band gap. Insertion of a triple bond (5-33, TIDIC) lowers the HOMO level and thus increases the band gap. Solar cells of 5-33 and FTAZ as the donor reach PCE values up to 13.1%.347
If electron-withdrawing (A) or -donating (D′) units are incorporated, formal A′–D–A–D–A′- and A–D–D′–D–A-type structures are generated. The addition of electron-withdrawing difluorobenzene leads to structure 5-34 (DFB-dIDT) with a higher optical band gap (1.64 eV) than 5-33 (1.55 eV), whereas the BT-linked NFA 5-35 (BT-dIDT) exhibits a similar value (1.54 eV). Solar cells with blends of these compounds with a PBDB-T donor have nearly identical VOCs (consistent with the equal LUMO energies), but all other solar cell parameters are better for the benzothiadiazole linked compound 5-35 (PCE 10.5%). The better performance of this compound was assigned to its higher planarity.348 Exchanging the hexylphenyl groups on the IDT cores with hexyl side chains gives structure 5-36 (BTIDIC) whose solar cells with the donor J71 give a higher PCE of 11.5%.349 If the benzothiazole linker is additionally fluorinated as in compound 5-37 (BID-4F), solar cells with even higher PCE values up to 12.3% were achieved (blend with PM6).350 Replacing the fluorine atoms in the INCN group with chlorines leads to structure 5-38 (BT2FIDT-4Cl), and solar cells of this compound with PM7 as a donor give similar PCE values (12.5%).351 The substitution of benzothiadiazole with benzoxadiazole gives structure 5-39 (BO2FIDT-4Cl). Solar cells with the donor PM7 gave lower PCE values (10.4%) than the solar cells based on 5-38. However, this decrease of the PCE cannot be assigned only to the weaker electron-withdrawing strength of benzoxadiazole, as also morphological changes are observed: the benzothiadiazole-linked 5-38 has a face-on orientation (pristine and in blend with PM7), while the pristine film of benzoxadiazole-based 5-39 does not have a clear orientation.351 Kim et al. introduced the two NFAs 5-41 (CNDTBT-C8IDT-INCN) and 5-42 (CNDTBT-C8IDT-FINCN) bearing thiophene-flanked dicyanobenzothiazole linkers and either INCN or INCN-2F accepting units. The compounds have relatively low optical band gaps, 1.48 eV for 5-41 and 1.40 eV for 5-42. Solar cells with PBDB-T gave PCE values up to 11.2 and 12.2%, respectively.352 If electron-donating linkers are used, only moderate PCE values are reached. For example, 6.54% with the thienothiophene linked 5-40 (TTIDIC; A-D-D′-D-A-type structure)349 and 4.98 and 6.21% with benzodithiophene-based compounds 5-43 (BDT(IDT-IC)2) and 5-44 (BDT(IDT-IC-2F)2), respectively.353
Summarizing these dimerized structures, solar cells with the A′–D–A–D–A′ type of compounds have shown better efficiencies than those with the A–D–D′–D–A type. Fluorinated end groups also have better efficiencies than their non-halogenated counterparts. On average (disregarding the used linker, end groups, or the donor polymer), compounds with alkyl side groups on the IDT core have higher PCE values than those which bear hexylphenyl side groups on the IDT core.
4.1.3. Modification of the IDT Core Unit
Modification of the IDT core can be easily achieved by heterosubstitution or by additional side chains on the central benzene ring. First, replacing the sp3 hybridized carbon in the IDT backbone by germanium leads to structure 5-45 (GDIC-C8). The Ge–C bond is longer (1.97 Å) than its C–C analogue (1.53 Å), which makes the molecule more crystalline. This, in return, disrupts the crystallization of PM6 (PCE 6.76%).327 In contrast to 5-45, the silicon analogue 5-46 (SiIDT-IC) in thin film has a favorable face-on orientation. Also, the solar cell efficiency in a blend with PBDB-T is improved to 8.16%.354
The substitution of sulfur with oxygen (i.e., replacing thiophene with furan) in the structures 5-4 and 5-24 leads to the NFAs 5-47 (IDF-IC) and 5-48 (IDF-4F), with similar HOMO levels but significant lower LUMO energies consequently leading to smaller optical band gaps. Thus, solar cells with PM6 have lower VOCs and overall only moderate performances.355
Modification of the short axis of the IDT core by introduction of methyl groups on the central phenyl ring (5-49, ID-MeIC) results in a reduction of the optical band gap by 160 meV (in contrast to 5-4), suggesting significant changes in the molecular packing. In films, 5-49 has a dominant face-on orientation, while 5-4 does not show a favorable orientation (vide supra). Solar cells with PBDB-T revealed better PCE values compared to the unsubstituted compound (6.46% vs 4.94% for 5-4).356 The introduction of alkoxy groups on the central benzene in combination with halogenated INCN acceptor groups leads to compounds 5-50–5-54 with optical band gaps of about 1.55 eV. Optical and electrochemical properties are influenced minimally if chlorine-substituted (5-50, MO-IDIC-Cl-1, and 5-51, MO-IDIC-Cl-2)238 or fluorine-substituted (5-52, MO-IDIC-2F)357 INCN end groups are used. Also, the length of the alkyl chain of the alkoxy group has no influence (5-53, HO-IDIC-2F, and 5-54, DO-IDIC-2F).357 All compounds (5-50–5-54) have a face-on orientation in film, and solar cells with these compounds deliver similarly good efficiencies between 10.8 and 13%.
Asymmetrical NFAs are obtained if only one sulfur in the IDT unit is replaced with selenium: 5-55 (T-Se),3595-56 (SePT-IN),3355-57 (T-Se-4F),3595-58 (T-Se-Th), all with different acceptor end groups.359 Their energy levels are slightly raised if compared to their sulfur-containing analogues (5-4, 5-16, 5-24, and 5-71, respectively), but the optical band gap is smaller. Solar cells of these compounds with various donor polymers have lower VOCs but higher JSCs; thus, similar or higher PCEs are reached (values between 7.44 and 10.3%).335 In film, the selenium-containing compound 5-56 has a face-on orientation, just as its sulfur analogue, yet the π–π stacking appears stronger in the Se-containing compound.335
4.1.4. Other Acceptor End Groups
End-capping of the IDT core with 3-ethylrhodanine (5-59, A1) yields uplifted energy levels and a higher optical band gap compared to 5-2 with the classical INCN unit (1.95 eV vs 1.64 eV). Solar cells with P3HT reached PCE values of 5.39%.360 A similar large optical band gap (1.90 eV) is also obtained if a TBA derived end group is used as in 5-60 (IDTA), which reached a PCE of 7.1% with PBDB-T. The same end groups when combined with an IDTT central core (7-85) led to a PCE of 10.8%.170 The NFA 5-61 (IDT-TBA/TPT-T),293,361 in which the octyl side chains are replaced with hexylphenyl side chains, shows similar photovoltaic performance in devices with the same donor (PCE: 6.70%). Again, the NFA with the IDTT central core (7-86) reached higher PCE values (7.5%).361 Compounds 5-63 (IDT-CT) and 5-64 (IDT-CR) containing a cyclohexene linker were designed in order to increase the (photo)chemical stability of the exocyclic double bond between the IDT core and the end group.140 An elongation of the π-system leads to lower optical band gaps than in 5-61 and 5-62 (IDT-R): 1.63 eV for 5-63 and 1.78 eV for 5-64 compared to 1.91 eV for 5-61 and 1.96 eV for 5-62. Albeit the PCEs of solar cells of 5-63 and 5-64 were not high, these compounds exhibited improved stability to chemical degradation and photo-degradation. Furthermore, it is worth noting that their analogues with the IDTT central core (7-109 and 7-110) reached better PCE values.140 In these above-mentioned examples (5-60 to 5-64), the PCE is boosted upon the replacement of the IDT central core with the slightly larger IDTT core, which illustrates the advantages of an increased conjugation length. Acceptors 5-65–5-69 containing the (thio)isatylidene-based end groups have the peculiarity that they are attached to the IDT core by a C–C single bond instead of a vinylene unit. The compounds 5-65–5-69 have similar, relatively high lying HOMO energy levels, and the variations of the LUMO energies correspond to the electron-withdrawing strength of the acceptor unit (5-66, IDNO-IDT has the highest LUMO).362,363 All (thio)isatine-based acceptors suffer from low electron mobilities; thus, solar cells with them yield low PCEs below 2%.363 Structure 5-70 (IBCT) contains thiophene-based indandione analogue end groups. Combined with the donor L1, good PCEs up to 11.3% are reached.364 Compounds 5-71–5-74 comprise CPTCN end groups with slightly weaker electron-withdrawing strength compared to INCN. Compound 5-71 (IDTCN) in solar cells with PBDB-T exhibits PCE values of 6.40%,378 and with other donors even higher values (PTQ10, 7.4%; PBT1-EH, 8.69%).365,366 Similar to its INCN-based analogue 5-4, 5-71 does not show a preferential orientation in film.365,366,378 Compound 5-72 (IDTPC) on the other hand has a clear face-on orientation in film, due to a replacement of the bulky hexylphenyl side chains with hexyl groups.366 Solar cells with PTQ10/5-72 yielded higher PCE values (12.2%).366,367 A methyl substitution on the end group’s thiophene elevates the LUMO energy; thus, a higher VOC can be achieved in solar cells with compounds 5-73 (IDTPC-Me) and 5-74 (IDTPC-DMe). At the same time, the lower electron mobility and slightly higher charge carrier recombination lead to a lower PCE. Interestingly, the solubility of 5-74 is only half as good as that of 5-72 (<35 vs >65 mg mL–1, respectively) due to an increased crystallinity upon the introduction of the methyl substituents.367
4.2. π-Spacers
The discussion so far clearly revealed how modifications of the IDT backbone, side chains, or end groups influence the acceptor properties and photovoltaic performance. Also, an addition of a conjugated π-spacer was already described for the dimeric compounds (5-32–5-44) linked either via an electron-donating or electron-accepting π-linker.
Thus, in the following, we subdivided the π-spacers into two groups: electron-deficient (benzothiadiazoles, benzotriazoles, quinoxalines, and their derivates) and electron-rich (various thiophene derived compounds), as shown in Figure 16. Of course, this classification is not always unambiguous, especially in cases where in one π-spacer mixed electron-rich and electron-poor heteroatoms are used. Nevertheless, proceeding with a classification into electron-poor and electron-rich spacers gives two types of structural motifs: A′–A–D–A–A′ and A–D′–D–D′–A, respectively. Acceptors belonging in the first class have higher LUMO levels, which yields solar cells with high VOCs. Meanwhile, the increased electron-rich nature of the acceptors in the second class contributes to very low optical band gaps and thus large JSCs. π-Spacers can also be used as non-covalent conformational locks, in “like–acceptor–like–donor” strategy (incorporating similar building blocks in acceptor molecules as those used in donors) or for the preparation of asymmetric acceptors.
Figure 16.
Structures of non-fullerene acceptors based on IDT core units and its derivatives (indicated by symbols next to the compound number) with different π-spacers and end groups.
4.2.1. Electron-Deficient π-Spacers (Benzothiadiazoles, Benzotriazole, Quinoxaline)
A considerable number of acceptors (5-75–5-92) comprises IDT-based cores with a BT π-spacer and different accepting end groups (see Figure 16). Compounds 5-75–5-78 are among the few examples where the end groups are not based on heterocycles. With the simple nitrile end groups in 5-76 (IDT-BC) and 5-75 (IDT-C8-BC), 6.3–7.3% PCE values were achieved (blends with PBDB-T, see also Table 12).379 Malononitrile derived end groups like in 5-77 (O-IDTBCN) gave a nearly doubled JSC in solar cells (blend with PTB7-Th), leading to high PCE values of 11.1%.380 Solar cells of 5-78 (IDT-CA), which contains the less electron-withdrawing cyanoacetate end groups, give PCEs of 4.19% (blend with P3HT).381 The change to heterocyclic 3-ethylrhodanine as the end group leads to the NFA O-IDTBR (5-79), which exhibits higher PCE values in solar cells with P3HT (up to 7.10%).404,405 Solar cells of 5-79 with PTB7-Th reach PCEs between 8.8406 and 9.9%,380 while those with the donor 2TRA surpass 10%.407 Nevertheless, the highest PCE (10.4%) was achieved using PffBT2T-TT as a donor polymer, despite a small LUMO–LUMO offset.382 Further variations of this acceptor were prepared with different side chains: ethylhexyl (5-80, EH-IDTBR),383,408,409 hexylphenyl (5-81, IDT-BT-R),38,384 octyloxyphenyl (5-82, 1-IDTBTRh),385 and 2,7-bis(octyloxy)spirofluorene groups (5-83, DTFBT-1).386 However, none of the solar cells using these NFAs reached PCEs above 10%. Interestingly, if the thiophenes of the central core are swapped upside-down, to point in the same direction as the sp3 hybridized carbon (5-84, a-IDTBTRh, in Figure 16, see the upper right side for the structure of the central core), the efficiencies of the solar cells are reduced by half (2.53% vs 5.38% for 5-82 in blend with P3HT).385 Another pair of isomers are the compounds 5-85 (cis-IDT-BT-R) and 5-86 (trans-IDT-BT-R), both containing mixed side chains (hexylphenyl and methyl groups, located cis or trans to each other) on the sp3 hybridized carbon. The trans isomer (5-86) has a higher PCE (9.43% compared to 8.02%) in solar cells with PTB7-Th.387 NFA 5-87 (IDT-4CN) with a RCN end group does not exceed 3.5% efficiency in solar cells with P3HT,381,410 but devices with PBDB-T reached values over 8%.388 The RCN end group has a positive impact on the PCE (4.4%, blend with J71) if compounds 5-88 (DTFBT-2) and 5-83 are compared (the latter contains a rhodanine end group without cyano modification, PCE 3.35%).386 Introducing electron-donating substituents on the rhodanine units as in structures 5-89 (IDT-2) and 5-90 (IDT-3) causes an increase of the optical band gap and an upshift of the HOMO/LUMO energies. Solar cells with P3HT give neglible PCE values.389 Fluorination of the BT spacer (5-91, H-FFBR, and 5-92, O-FFBR) lowers the energy levels compared to 5-79. Solar cells with PTB7-Th have similar PCE values (up to 9.4% in both cases).390 The Zhou research group has developed new acceptors based on a benzotriazole spacer (BTA series). Benzotriazole is less electron-withdrawing than the BT unit and if two molecules with the same residual structure are compared; e.g., the BTA-based NFA 5-95 (BTA3) has higher frontier orbital energies and larger band gaps than the analogous BT-based 5-87. Similar permutations in the BTA-based acceptors are undertaken for BT-based compounds. Compound 5-94 (BTA2) with the less electron-withdrawing oxygenated rhodanine end groups has higher LUMO levels (thus, a higher band gap of 2.0 eV) than its sulfur analogue 5-93 (BTA1; 1.87 eV) or the RCN derivative 5-95 (1.76 eV), showing the influence of different end groups.391 Solar cells with all of these molecules with J61 reached very high VOCs ≥ 1.15 V and in the case of 5-95 PCEs up to 8.25%.391 Solar cells with the chlorinated donor J52-Cl reached a remarkable VOC of 1.24 V together with a PCE of 10.5%.386 Fluorination of BTA (5-96, F-BTA3) minimally lowers the HOMO/LUMO energies; thus, the VOC for solar cells based on P2F-EHp is reduced by 40 mV, but the JSC and PCE are nearly doubled (8.38%) compared to the non-fluorinated 5-95 (PCE 4.62%).393 Replacing the ethyl substituent on the end group’s nitrogen of 5-95 with phenyl (5-97, BTA4) or benzyl (5-98, BTA5) has differing outcomes. The phenyl substituent (5-97) elevates the HOMO/LUMO energies and the optical band gap; thus, solar cells with J52-F as a donor have a higher VOC than those of 5-95 but lower JSC, FF, and PCE values (5-97 gives 8.38%, while 5-95 yields 9.04%). On the other hand, the additional methylene group between the nitrogen and the phenyl ring in 5-98 results in lower HOMO/LUMO energies and a lower optical band gap. Thus, solar cells with J52-F have a lower VOC, but all other parameters are improved, giving a higher PCE (11.3%). Compared to 5-95, 5-98 has an enhanced face-on orientation in film with J52-F, while 5-97 has an amorphous stacking morphology.394 Replacing rhodanine-based end groups with a 4-cyano-styryl (5-99, BTA701) or cyanated furane-like end groups (5-100, BTA703) has a negative impact on the PCE values (blends with J71 reaching 0.07% for 5-99 and 1.21% for 5-100).395 Replacing the hexylphenyl side chains in 5-95 with hexyloxyphenyl side chains on the IDT core, such as in 5-101 (BTA43), has a positive impact on the PCE values in solar cells with P3HT (6.56% vs 5.64% for 5-95).396
Table 12. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 5-75–5-115.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μee(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5-75 | IDT-C8-BC | –5.39 | –3.50 | 1.83 | PBDB-T | 1:1.5 | 1.05 | 10.3 | 68 | 7.3 | -/3.0 × 10–4 | (379) |
| 5-76 | IDT-BC | –5.50 | –3.57 | 1.92 | PBDB-T | 1:1.5 | 1.08 | 10.5 | 56 | 6.3 | -/1.4 × 10–4 | (379) |
| 5-77 | O-IDTBCN | –5.70c | –4.18 | 1.52 | PTB7-Th | 1:2.5 | 0.72 | 19.8 | 73 | 11.1 | 2 × 10–4/2.2 × 10–4 | (380) |
| 5-78 | IDT-CA | –5.59 | –3.64 | 1.66 | P3HT | 1:0.8 | 0.84 | 7.77 | 64 | 4.19 | (381) | |
| 5-79 | O-IDTBR | –5.51 | –3.55 | 1.63 | PffBT2T-TT | 1:1.5 | 1.08 | 14.3 | 67 | 10.4 | (382) | |
| 5-80 | EH-IDTBR | PffBT4T-2OD | 1:1.5 | 1.04 | 14.5 | 63 | 9.5 | -/4.4 × 10–5 | (383) | |||
| 5-81 | IDT-BT-R | –5.30d | –3.59d | PTB7-Th | 1:1.5 | 1.05 | 14.6 | 60 | 9.39 | -/1.0 × 10–4 | (384) | |
| 5-82 | l-IDTBTRh | –5.42 | –3.68 | 1.65 | P3HT | 1:0.8 | 0.86 | 8.81 | 71 | 5.38 | -/1.2 × 10–5 | (385) |
| 5-83 | DTFBT-1 | –5.62 | –3.82 | J71 | 1:1 | 1.13 | 6.32 | 47 | 3.35 | -/1.3 × 10–4 | (386) | |
| 5-84 | a-IDTBTRh | –5.63 | –3.59 | 1.89 | P3HT | 0.8:1 | 0.92 | 5.00 | 55 | 2.53 | -/8.8 × 10–6 | (385) |
| 5-85 | cis-IDT-BT-R | –5.44 | –3.62 | 1.64 | PTB7-Th | 1:1.5 | 1.08 | 13.8 | 58 | 8.02 | -/1.0 × 10–6 | (387) |
| 5-86 | trans-IDT-BT-R | –5.43 | –3.61 | 1.63 | PTB7-Th | 1:1.3 | 1.05 | 15.1 | 60 | 9.43 | -/2.2 × 10–6 | (387) |
| 5-87 | IDT-4CN | –5.45 | –3.95 | PBDB-T | 1:1.2 | 0.91 | 14.7 | 61 | 8.13 | -/1.9 × 10–4 | (388) | |
| 5-88 | DTFBT-2 | –5.76 | –3.93 | J71 | 1:1 | 0.94 | 8.35 | 56 | 4.40 | -/3.1 × 10–4 | (386) | |
| 5-89 | IDT-2 | –5.34 | –3.49 | 1.65 | P3HT | 1:1 | 0.55 | 0.04 | 22 | 0.01 | -/5 × 10–9 | (389) |
| 5-90 | IDT-3 | –5.28 | –3.28 | 1.77 | P3HT | 1:1 | 0.68 | 0.29 | 25 | 0.05 | -/6 × 10–8 | (389) |
| 5-91 | H-FFBR | –5.54 | –3.93b | 1.61 | PTB7-Th | 1:1.5 | 0.94 | 16.9 | 57 | 9.1 | -/3.4 × 10–5 | (390) |
| 5-92 | O-FFBR | –5.59 | –3.95b | 1.64 | PTB7-Th | 1:1.5 | 0.94 | 17.0 | 59 | 9.4 | -/5.7 × 10–5 | (390) |
| 5-93 | BTA1 | –5.46 | –3.59 | 1.87 | J61 | 1:1 | 1.24 | 5.21 | 47 | 3.02 | -/5.4 × 10–5 | (391) |
| 5-94 | BTA2 | –5.43 | –3.43 | 2.00 | J61 | 1:1 | 1.29 | 0.84 | 24 | 0.26 | -/2.4 × 10–5 | (391) |
| 5-95 | BTA3 | –5.49 | –3.73 | 1.82 | J52-Cl | 1:1 | 1.24 | 13.2 | 67 | 10.5 | -/2.4 × 10–4 | (392) |
| 5-96 | F-BTA3 | –5.59 | –3.82b | 1.77 | P2F-EHp | 1:1 | 1.25 | 11.3 | 59 | 8.38 | -/1.5 × 10–5 | (393) |
| 5-97 | BTA4 | –5.50 | –3.65 | 1.79 | J52-F | 1:1 | 1.21 | 8.39 | 55 | 5.61 | -/1.3 × 10–5 | (394) |
| 5-98 | BTA5 | –5.55 | –3.71 | 1.76 | J52-F | 1:1 | 1.17 | 13.8 | 70 | 11.3 | -/3.4 × 10–5 | (394) |
| 5-99 | BTA701 | –5.27 | –2.83 | 2.10 | J71 | 1:1 | 1.32 | 0.21 | 25 | 0.07 | -/2.5 × 10–8 | (395) |
| 5-100 | BTA703 | –5.50 | –3.90 | 1.50 | J71 | 1:0.5 | 0.85 | 3.56 | 40 | 1.21 | -/1.1 × 10–7 | (395) |
| 5-101 | BTA43 | –5.44 | –3.47 | 1.78 | P3HT | 1:0.5 | 0.89 | 10.8 | 68 | 6.56 | -/3.2 × 10–6 | (396) |
| 5-102 | BTA100 | –5.32 | –3.23 | 2.05 | P3HT | 1:1 | 1.34 | 1.65 | 47 | 1.04 | -/6.9 × 10–9 | (397) |
| 5-103 | BTA101 | –5.41 | –3.55 | 1.88 | P3HT | 1:1 | 1.19 | 5.33 | 56 | 3.55 | -/1.4 × 10–7 | (397) |
| 5-104 | BTA103 | –5.37 | –3.64 | 1.77 | P3HT | 1:2 | 0.94 | 8.56 | 66 | 5.31 | -/3.5 × 10–5 | (397) |
| 5-105 | A1 | –5.66 | –3.70 | 1.40 | PTB7-Th | 1.5:1 | 0.89 | 12.5 | 52 | 5.79 | 3.0 × 10–4/6.3 × 10–5 | (398) |
| 5-106 | A2 | –5.70 | –3.78 | 1.36 | PTB7-Th | 1:2 | 0.70 | 20.8 | 63 | 9.07 | 7.5 × 10–4/2.3 × 10–4 | (398) |
| 5-107 | JC1 | –5.51 | –3.95 | 1.30 | P3HT | 1:0.6 | 0.48 | 10.5 | 56 | 2.80 | 5.0 × 10–4/1.5 × 10–4 | (399) |
| 5-108 | JC2 | –5.48 | –3.73 | 1.48 | P3HT | 1:0.8 | 0.71 | 14.0 | 63 | 6.24 | 4.1 × 10–4/4.5 × 10–5 | (399) |
| 5-109 | A3 | –5.60 | –3.71 | 1.59 | PTB7-Th | 1:1.5 | 0.96 | 17.3 | 66 | 11.0 | 6.1 × 10–4/2.0 × 10–4 | (400) |
| 5-110 | Qx1 | –5.42 | –3.60 | 1.74 | P3HT | 1:0.6 | 1.00 | 6.02 | 67 | 4.03 | -/7.5 × 10–7 | (401) |
| 5-111 | Qx1b | –5.35 | –3.66 | 1.68 | P3HT | 0.8:1 | 0.95 | 7.34 | 69 | 4.81 | -/1.1 × 10–5 | (401) |
| 5-112 | Qx3 | –5.38 | –3.56 | 1.64 | P3HT | 1:0.5 | 0.89 | 5.57 | 68 | 3.37 | -/4.2 × 10–6 | (402) |
| 5-113 | Qx3b | –5.27 | –3.63 | 1.59 | P3HT | 1:0.8 | 0.75 | 12.9 | 66 | 6.37 | -/2.0 × 10–5 | (402) |
| 5-114 | Qx3c | –5.30 | –3.59 | 1.60 | P3HT | 1:1 | 0.75 | 0.14 | 30 | 0.03 | -/1.3 × 10–4 | (402) |
| 5-115 | IDT-FBTR | –5.47 | –3.65 | 1.67 | PTB7-Th | 1:1.5 | 1.02 | 15.2 | 58 | 9.14 | -/1.5 × 10–4 | (403) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
HOMO obtained via PESA.
Other method or method not defined.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Three NFAs with dimethoxy-substituted BTAs and different end groups were prepared: 5-102 (BTA100) with 2,4-thiazolidinedione, 5-103 (BTA101) with rhodanine, and 5-104 (BTA103) with RCN.397 In this order, the optical band gaps were reduced to 2.05, 1.88, and 1.77 eV. Solar cells with compound 5-102/P3HT gave a remarkable VOC of 1.34 V with a PCE around 1%, but 5-104 had the highest PCE values up to 5.31% but a lower VOC of 0.94 V.397
The introduction of thienobenzothiadiazole and thienobenzotriazole spacer units (spacer units 6 and 7, NFAs 5-105–5-109) contributes to an extended conjugation length and strengthened intramolecular charge transfer due to an increased quinoid character. Benzothiadiazole derived molecules 5-105 (A1),3985-106 (A2),398 and 5-107 (JC1)399 have lower band gaps (1.40, 1.36, and 1.30 eV, respectively) than the benzotriazole-based 5-108 (JC2)399 and 5-109 (A3)400 with 1.48 and 1.59 eV due to the stronger electron-withdrawing nature of benzothiazole. Solar cells with PTB7-Th resulted in PCE values of 11.0% using 5-109 due to a VOC of 0.96 V, a FF of 66%, and a good JSC of 17.3 mA cm–2 compared to those with 5-106 with a higher JSC of 20.8 mA cm–2 but an overall lower PCE of 9.07%.398,400 Further representatives of π-spacers encompass the quinoxaline (π-spacer 9) and 2,3-diphenylquinoxaline (π-spacer 8) moiety. A number of acceptors were designed through the combination of different side chains (hexylphenyl or octyl) and rhodanine-based end groups (5-110–5-114). OCSs from blends with the P3HT give PCEs from 0.03 to 6.37% and VOCs in the range 0.75–1.00 V.401,402 Compound 5-115 (IDT-FBTR) has two spacers, fluoro-benzodithiophene and thiophene, leading to an A–D′–A′–D–A′–D′–A structure.403 This motif yields a PCE of 9.14% (blended with PTB7-Th).403
4.2.2. Electron-Rich π-Spacers
The thiazole spacer unit in molecules 5-116 (H-IDTzR, hexyl side chains)411 and 5-117 (P-IDTzR, hexylphenyl side chains) acts as a non-covalent conformational lock, thus improving the planarity (due to the interactions between the S atom of the central core and the N atom of the spacer).411 The 5-116/P3HT blend reaches PCE values of 3.53%, while 5-117 gave a slightly larger PCE of 5.01% (see Table 13). Replacement of the ethylrhodanine end group in 5-117 with INCN yields the structure 5-118 (DC-IDT2Tz) which reaches a PCE of 5.81% (blend with PTB7-Th).412
Table 13. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 5-116–5-204.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μee(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5-116 | H-IDTzR | –5.31b | –3.44 | 1.87 | P3HT | 1:1 | 1.04 | 6.67 | 53 | 3.53 | -/1.8 × 10–6 | (411) |
| 5-117 | P-IDTzR | –5.31b | –3.42 | 1.89 | P3HT | 1:1 | 1.02 | 9.00 | 55 | 5.01 | -/7.1 × 10–6 | (411) |
| 5-118 | DC-IDT2Tz | –5.63 | –4.03b | 1.60 | PTB7-Th | 1:1.5 | 0.82 | 10.8 | 66 | 5.81 | -/5.0 × 10–7 | (412) |
| 5-119 | T-TPT-T-2F | –5.50 | –3.99 | 1.51 | PBT1-C | 1:1 | 0.93 | 17.6 | 67 | 10.7 | 7.7 × 10–4/1.2 × 10–4 | (413) |
| 5-120 | IDTT2F | –5.57 | –4.03 | 1.46 | PBDB-T | 1:1 | 0.81 | 18.5 | 59 | 8.85 | -/3.1 × 10–6 | (414) |
| 5-121 | IDT-T | –5.57 | –4.01 | 1.56 | PBDB-T | 1:1 | 0.95 | 11.2 | 60 | 6.36 | -/8.2 × 10–6 | (415) |
| 5-122 | 3 | –5.47 | –3.67 | 1.69 | J61 | 1:1 | 0.92 | 8.09 | 43 | 3.17 | -/7.5 × 10–4 | (416) |
| 5-123 | 4 | –5.47 | –3.72 | 1.59 | J61 | 1:1 | 0.89 | 9.27 | 50 | 4.06 | -/6.4 × 10–4 | (416) |
| 5-124 | IDTOT2F | –5.54 | –3.94 | 1.44 | PBDB-T | 1:1 | 0.85 | 20.9 | 72 | 12.8 | -/4.0 × 10–5 | (414) |
| 5-125 | IDTO-T-4F | –5.49 | –3.88 | 1.45 | PBDB-T | 1:1.2 | 0.86 | 20.1 | 73 | 12.6 | -/6.4 × 10–5 | (417) |
| 5-126 | IDTO-Se-4F | –5.48 | –3.90 | 1.40 | PBDB-T | 1:1.1 | 0.83 | 18.6 | 69 | 10.7 | -/4.5 × 10–5 | (417) |
| 5-127 | IDT2Se | –5.41 | –3.87 | 1.45 | PBDB-T | 1:1 | 0.89 | 17.5 | 61 | 9.36 | -/4.8 × 10–5 | (418) |
| 5-128 | IDT2Se-4F | –5.51 | –4.00 | 1.39 | PBDB-T | 1:1 | 0.79 | 21.5 | 66 | 11.2 | -/1.6 × 10–4 | (418) |
| 5-129 | IDT2SeC2C4 | –5.30 | –3.91 | 1.44 | PBDB-T | 1:1 | 0.88 | 17.2 | 59 | 8.92 | 1.3 × 10–4/6.5 × 10–5 | (419) |
| 5-130 | IDT2SeC2C4-2F | –5.37 | –3.99 | 1.40 | PBDB-T | 1:1 | 0.81 | 19.2 | 66 | 10.2 | 1.8 × 10–4/1.5 × 10–4 | (419) |
| 5-131 | IDT2SeC2C4-4F | –5.40 | –4.08 | 1.30 | PBDB-T | 1:1 | 0.77 | 22.0 | 62 | 10.6 | 2.1 × 10–4/1.7 × 10–4 | (419) |
| 5-132 | IFIC | –5.11 | –3.53 | 1.58 | HFQx-T | 1:1 | 1.02 | 11.2 | 55 | 6.28 | -/9.9 × 10–5 | (420) |
| 5-133 | IFIC-F | –5.43 | –3.86 | 1.57 | HFQx-T | 1:1 | 0.95 | 12.0 | 51 | 5.87 | -/1.1 × 10–4 | (420) |
| 5-134 | ITMIC | –5.51b | –4.08 | 1.47 | PM6 | 1:1 | 0.88 | 12.6 | 54 | 5.95 | -/1.5 × 10–4 | (421) |
| 5-135 | IEIC | –5.41c | –3.84c | 1.57 | PBDB-T | 1:1 | 1.02 | 15.1 | 48 | 7.30 | -/1.1 × 10–4 | (422) |
| 5-136 | IE-4F | –5.51c | –4.07c | 1.44 | PBDB-T | 1:1 | 0.87 | 21.4 | 58 | 10.8 | -/3.0 × 10–4 | (422) |
| 5-137 | IE-4Cl | –5.54c | –4.11c | 1.43 | PBDB-T | 1:1 | 0.86 | 21.5 | 60 | 11.1 | -/3.7 × 10–4 | (422) |
| 5-138 | A1 | –5.15 | –3.73 | 1.55 | J71 | 1:1 | 0.97 | 5.71 | 30 | 1.63 | 3.5 × 10–5/1.2 × 10–5 | (144) |
| 5-139 | IDT-TN | –5.42 | –4.03 | 1.43 | PBDB-T | 1:1 | 0.97 | 13.2 | 45 | 5.89 | 1.2 × 10–6/6.8 × 10–5 | (423) |
| 5-140 | ERCN | –5.50 | –3.59 | 1.82 | P3HT | 1:1.3 | 0.90 | 5.87 | 50 | 2.64 | -/2.7 × 10–5 | (424) |
| 5-141 | IDTP-P-C | –5.37 | –3.72 | 1.42 | PTB7-Th | 1:1 | 0.79 | 18.8 | 55 | 8.21 | 7.2 × 10–4/8.6 × 10–4 | (425) |
| 5-142 | IDTP-O-C | –5.43 | –3.75 | 1.44 | PTB7-Th | 1:1 | 0.77 | 18.1 | 62 | 8.61 | 1.4 × 10–3/9.5 × 10–4 | (425) |
| 5-143 | IDTCN-C | –5.59 | –3.92 | 1.48 | PBDB-T | 1:1 | 0.84 | 20.3 | 70 | 11.9 | -/9.5 × 10–5 | (426) |
| 5-144 | p-IO1 | –5.46 | –4.13 | 1.34 | PTB7-Th | 1:1.5 | 0.78 | 22.3 | 62 | 10.8 | (427) | |
| 5-145 | o-IO1 | –5.44 | –4.15 | 1.28 | PTB7-Th | 1:1.5 | 0.74 | 26.3 | 67 | 13.1 | (427) | |
| 5-146 | o-IO2 | –5.41 | –4.21 | 1.20 | PTB7-Th | 1:1.5 | 0.68 | 21.8 | 63 | 9.3 | (427) | |
| 5-147 | p-IO2 | –5.44 | –4.19 | 1.24 | PTB7-Th | 1:1.5 | 0.70 | 23.0 | 67 | 10.8 | (427) | |
| IEICO-4F | –5.43d | –4.03d | Si25-H2 | 1:1.5 | 0.70 | 26.9 | 70 | 13.2 | 1.2 × 10–4/9.2 × 10–4 | (428) | ||
| 5-148 | IEICO-4Cl | –5.56 | –4.23 | D18 | 1:1.6 | 0.80 | 3.45 | 49 | 1.42 | (136) | ||
| 5-149 | IOTIC-2F | –5.34 | –4.06 | 1.31 | PTB7-Th | 1:1.5 | 0.82 | 21.9 | 65 | 12.1 | -/2.1 × 10–5 | (181) |
| 5-150 | IEICO | –5.32d | –3.95d | PTB7-Th | 1:1.5 | 0.90 | 12.5 | 59 | 6.7 | -/3.1 × 10–4 | (429) | |
| 5-151 | ORCN | –5.37 | –3.56 | 1.64 | P3HT | 1:1.3 | 0.87 | 11.5 | 62 | 6.40 | -/1.8 × 10–4 | (424) |
| 5-152 | IDTP-P-O | –5.21 | –3.75 | 1.28 | PTB7-Th | 1:1 | 0.73 | 19.1 | 59 | 8.17 | 3.7 × 10–3/2.6 × 10–3 | (425) |
| 5-153 | IDTP-O-O | –5.27 | –3.76 | 1.30 | PTB7-Th | 1:1 | 0.73 | 19.0 | 60 | 8.40 | 6.5 × 10–3/2.9 × 10–3 | (425) |
| 5-154 | IDTCN-O | –5.54 | –3.80 | 1.53 | PBDB-T | 1:1 | 0.91 | 20.0 | 73 | 13.3 | -/7.6 × 10–5 | (426) |
| 5-155 | IDTCN-S | –5.57 | –3.90 | 1.48 | PBDB-T | 1:1 | 0.85 | 19.0 | 66 | 10.6 | -/4.5 × 10–5 | (426) |
| 5-156 | ACS8 | –5.54 | –4.05 | 1.30 | PTB7-Th | 1:2 | 0.75 | 25.3 | 69 | 13.2 | 2.7 × 10–4/2.0 × 10–4 | (430) |
| 5-157 | A134 | –5.54 | –4.05 | PTB7-Th | 1:2 | 0.75 | 16.7 | 61 | 7.6 | (275) | ||
| 5-158 | i-IEICO | –5.31 | –3.68 | 1.60 | J52 | 1:1 | 0.96 | 18.8 | 58 | 10.5 | -/1.1 × 10–4 | (431) |
| 5-159 | i-IEICO-2F | –5.29b | –3.73 | 1.58 | J52 | 1:1 | 0.91 | 20.9 | 68 | 12.9 | -/2.0 × 10–4 | (432) |
| 5-160 | i-IEICO-F3 | –5.34b | –3.74 | 1.59 | J52 | 1:1 | 0.90 | 16.2 | 53 | 7.65 | -/2.8 × 10–5 | (432) |
| 5-161 | i-IEICO-4F | 1.56 | J52 | 1:1 | 0.85 | 22.9 | 68 | 13.2 | -/3.8 × 10–4 | (433) | ||
| 5-162 | i-cc23 | –5.34 | –3.64 | 1.69 | PBDB-T | 1:1 | 1.10 | 11.2 | 59 | 7.34 | -/1.8 × 10–5 | (434) |
| 5-163 | i-cc34 | –5.39 | –3.75 | 1.57 | PBDB-T | 1:1 | 0.96 | 15.7 | 63 | 9.51 | -/1.2 × 10–4 | (434) |
| 5-164 | i-mO-4F | –5.50 | –3.81 | 1.55 | PBDB-T | 1:1 | 0.92 | 21.6 | 71 | 14.0 | -/2.2 × 10–4 | (435) |
| 5-165 | i-mO-4Cl | –5.55 | –3.83 | 1.53 | PBDB-T | 1:1 | 0.87 | 15.1 | 56 | 7.41 | -/5.1 × 10–5 | (435) |
| 5-166 | IDT-OT | –5.11 | –3.46 | PBDB-T | 1:1.5 | 0.93 | 8.27 | 43 | 3.32 | -/1.1 × 10–4 | (436) | |
| 5-167 | IDTS-4F | –5.54 | –3.93 | 1.43 | PM6 | 1:1 | 0.89 | 21.0 | 70 | 12.9 | -/3.1 × 10–4 | (437) |
| 5-168 | IDT2ST-4F | –5.36 | –3.71 | 1.43 | PBDB-T | 1:1 | 0.85 | 19.4 | 69 | 11.4 | -/3.3 × 10–5 | (438) |
| 5-169 | ITOIC | –5.48 | –3.75 | 1.55 | PBDB-T | 1:1 | 1.02 | 15.7 | 55 | 8.87 | 2.5 × 10–5/2.4 × 10–4 | (439) |
| 5-170 | ITOIC-F | –5.52 | –3.82 | 1.50 | PBDB-T | 1:1 | 0.95 | 18.6 | 61 | 10.7 | 6.5 × 10–5/4.9 × 10–4 | (439) |
| 5-171 | ITOIC-2F | –5.57 | –3.87 | 1.45 | PBDB-T | 1:1.5 | 0.90 | 21.0 | 65 | 12.2 | 1.3 × 10–4/6.0 × 10–4 | (439) |
| 5-172 | IDT-EDOT | –5.43 | –3.80 | 1.41 | PBDB-T | 1:1.2 | 0.86 | 21.3 | 62 | 11.3 | -/6.6 × 10–5 | (440) |
| 5-173 | IDT-PDOT | –5.39 | –3.77 | 1.43 | PBDB-T | 1:0.5 | 0.85 | 5.26 | 49 | 2.18 | -/3.0 × 10–6 | (440) |
| 5-174 | IDT-PDOT-C6 | –5.59c | –4.17 | 1.42 | PBDB-T | 1:1 | 0.91 | 19.5 | 62 | 11.1 | -/3.1 × 10–4 | (441) |
| 5-175 | ITCIC | –5.59b | –4.12 | 1.43 | PM6 | 1:1 | 0.86 | 20.5 | 64 | 11.3 | -/4.3 × 10–4 | (421) |
| 5-176 | IDT-3MT | –5.68 | –4.16 | 1.52 | PBDB-T | 1:1 | 0.95 | 14.4 | 61 | 8.40 | -/1.0 × 10–5 | (415) |
| 5-177 | IDT-TiFIC | –5.57 | –4.05 | 1.41 | PBDB-T | 1:1 | 0.86 | 17.0 | 65 | 9.46 | -/2.1 × 10–4 | (442) |
| 5-178 | IDT-ToFIC | –5.55 | –3.86 | 1.50 | PBDB-T | 1:1 | 0.88 | 17.8 | 71 | 11.1 | -/3.9 × 10–4 | (442) |
| 5-179 | ITVT | –5.47 | –3.59 | 1.48 | PBZ | 1:2 | 0.96 | 14.2 | 43 | 5.84 | 1.9 × 10–4/1.4 × 10–4 | (443) |
| 5-180 | A401 | –5.43 | –3.53 | PBDB-T | 1:1 | 0.93 | 13.0 | 62 | 7.54 | -/3.1 × 10–6 | (444) | |
| 5-181 | IDTO-TT-4F | –5.39 | –3.89 | 1.38 | PBDB-T | 1:1.1 | 0.86 | 17.2 | 69 | 10.2 | -/2.2 × 10–5 | (417) |
| 5-182 | IDTC-4Cl | –5.50 | –3.79 | 1.35 | PBDB-T | 1:1 | 0.82 | 19.2 | 60 | 9.50 | 2.8 × 10–4/2.5 × 10–4 | (445) |
| 5-183 | 4TIC | –5.36 | –4.11 | 1.26 | PTB7-Th | 1:1.3 | 0.70 | 14.6 | 49 | 5.26 | -/4.4 × 10–4 | (446) |
| 5-184 | 4T4F | –5.38 | –4.13 | 1.22 | PTB7-Th | 1:1.3 | 0.60 | 17.3 | 60 | 6.58 | -/4.8 × 10–4 | (446) |
| 5-185 | 6TIC | –5.31 | –3.96 | 1.30 | PTB7-Th | 1:1.3 | 0.74 | 19.2 | 54 | 8.13 | -/9.8 × 10–4 | (446) |
| 5-186 | 6T4F | –5.45 | –4.00 | 1.23 | PTB7-Th | 1:1.3 | 0.60 | 24.9 | 69 | 10.7 | -/1.3 × 10–3 | (446) |
| 5-187 | ATT-5 | –5.41 | –3.60 | 1.50 | PBDB-T | 1:1 | 0.93 | 18.9 | 71 | 12.4 | 2.1 × 10–4/2.5 × 10–4 | (447) |
| 5-188 | IFIC-i-2F | –5.42 | –3.91 | 1.34 | PTB7-Th | 1:1.8 | 0.72 | 21.0 | 65 | 9.82 | -/2.3 × 10–4 | (448) |
| 5-189 | IFIC-i-6F | –5.31 | –4.00 | 1.27 | PTB7-Th | 1:1.8 | 0.61 | 22.0 | 70 | 9.43 | -/7.9 × 10–4 | (448) |
| 5-190 | IFIC-i-4F | –5.34 | –3.96 | 1.30 | PTB7-Th | 1:1.8 | 0.65 | 24.9 | 67 | 10.9 | -/5.1 × 10–4 | (448) |
| 5-191 | IFIC-o-4F | –5.36 | –4.01 | 1.27 | PTB7-Th | 1:1.8 | 0.61 | 18.6 | 62 | 7.01 | -/3.4 × 10–5 | (448) |
| 5-192 | ATT-1 | –5.51 | –3.68 | 1.54 | PBDB-T | 1:1 | 0.92 | 16.4 | 60 | 9.00 | 8.6 × 10–5/0.1 × 10–4 | (447) |
| 5-193 | ATT-4 | –5.41 | –3.62 | 1.49 | PBDB-T | 1:1 | 0.93 | 17.3 | 70 | 11.2 | 3.7 × 10–4/1.0 × 10–4 | (447) |
| 5-194 | ATT-6 | –5.48 | –3.66 | 1.53 | PBDB-T | 1:1 | 0.96 | 14.7 | 59 | 8.39 | -/2.4 × 10–5 | (449) |
| 5-195 | ATT-7 | –5.45 | –3.61 | 1.54 | PBDB-T | 1:1 | 0.97 | 16.0 | 67 | 10.3 | -/5.4 × 10–5 | (449) |
| 5-196 | ATT-8 | –5.46 | –3.71 | 1.55 | PBDB-T | 1:1 | 0.80 | 13.9 | 65 | 7.31 | -/1.9 × 10–5 | (450) |
| 5-197 | TPD3 | –5.65 | –3.79 | 1.71 | J52-Cl | 1:1.5 | 0.96 | 12.7 | 62 | 7.58 | -/1.1 × 10–6 | (451) |
| 5-198 | TPD8 | –5.60 | –4.10b | 1.46 | PTQ10 | 1:1.5 | 0.91 | 18.2 | 63 | 10.4 | -/1.3 × 10–4 | (452) |
| 5-199 | IDBTC | –5.25 | –3.54 | 1.57 | P3HT | 0.73 | 7.7 | 45 | 2.5 | (453) | ||
| 5-200 | IDBTCF | –5.27 | –3.55 | 1.56 | P3HT | 0.75 | 8.93 | 48 | 3.22 | (453) | ||
| 5-201 | IDTBF | –5.64 | –3.80 | 1.58 | PM6 | 1:1 | 0.94 | 17.0 | 65 | 10.4 | -/2.5 × 10–4 | (454) |
| 5-202 | 2FIFIC | –5.45 | –3.97 | 1.38 | PM6 | 1:1.4 | 0.80 | 22.4 | 62 | 11.3 | -/8.0 × 10–4 | (455) |
| 5-203 | ICIF2F | –5.45 | –4.02 | 1.36 | PM6 | 1:1.4 | 0.79 | 19.3 | 61 | 9.60 | -/5.7 × 10–4 | (455) |
| 5-204 | IDST-4F | –5.59 | –4.01 | 1.41 | PM6 | 1:1.25 | 0.82 | 24.9 | 70 | 14.3 | -/4.3 × 10–4 | (456) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Obtained via ultraviolet photoelectron spectroscopy (UPS).
Method not defined.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Li et al. investigated IDTT analogues by replacing the terminal condensed thiophene units of the electron-donating IDTT core in structure 7-2 with thiophene as a π-bridging unit.413 In the case of one thiophene unit being replaced, an asymmetric six-ring system is created (see the section “Six Fused Aromatic Ring Systems”, 6-26), while removal of the thiophene units from both sides yields the structure 5-119 (T-TPT-T-2F).413 The absorption spectrum of 5-119 is more red-shifted when compared to the parent seven-ring structure 7-2. The LUMO level is influenced minimally, while the HOMO has a higher energy than in the fully fused molecule. Solar cells built from 5-119 with donor PBT1-C reached similar VOC values than those with the six-ring analogue 6-26 or the seven-ring structure 7-2. The overall photovoltaic performance was similar to the devices based on 7-2 with PCE values of 10.7% vs 10.5%, but both were outperformed by those of the asymmetric 6-26 with the highest JSC and FF values leading to PCEs of 12.7%. Solar cells of 5-120, which has INCN-2F end groups, reached PCE values of 8.85%,414 while solar cells of 5-121 (IDT-T), with INCN groups, had a PCE of 6.36% (both in blend with PBDB-T).415 Li et al. developed NFAs with new electron-withdrawing groups based on difluoroboron(III)β-diketonate.416 Energy levels, particularly the LUMOs of compounds 5-122 (3, with a para-CF3-phenyl group)416 and 5-123 (4, with a para-CN-phenyl group)416 are upshifted in comparison to compounds with INCN-based end groups (5-119–5-121); thus, also the optical band gaps are larger (1.69 eV/1.59 eV). Solar cells with J61 as a donor had moderate PCEs of 3.17% (5-122) and 4.06% (5-123).416 Modification of the IDT central core by the introduction of an additional alkoxy substituent on the terminal thiophene rings leads to 5-124 (IDTOT2F)414 and 5-125 (IDTO-T-4F),417 which differ from each other by the length of the alkyl chain (hexyl, octyl). Both compounds have similar optical band gaps, also compared to the unsubstituted 5-120. Compound 5-124 has a better solubility than 5-120, and it is more crystalline (face-on orientation, just as 5-125). Solar cells of both compounds with PBDB-T gave increased VOC, JSC, as well as FF values compared to 5-120 and PCEs above 12.5%.414,417 Replacing the thiophene π-spacer in 5-125 with selenophene leads to structure 5-126 (IDTO-Se-4F)417 with a slightly decreased band gap of 1.40 eV. However, also the PCE values for solar cells with PBDB-T are lower (10.7%).417 Moreover, selenophene was introduced in several other IDT-based NFAs. Selenium increases the energy levels but reduces the optical band gap due to Se having a stabilizing influence on the LUMO. This can be seen by comparing 5-127 (IDT2Se; with an INCN end group and a band gap of 1.45 eV)418 and 5-128 (IDT2Se-4F; with INCN-2F, 1.39 eV)418 to their sulfur analogues 5-121 (INCN, 1.56 eV) and 5-120 (INCN-2F, 1.46 eV), respectively. Despite an increased LUMO energy, solar cells with the same donor polymer (PBDB-T) have a smaller VOC than their sulfur analogues. At the same time, an enhanced JSC and FF and a higher electron mobility enable higher efficiencies. Solar cells with 5-127 reached efficiencies of 9.36%, and devices based on the difluorinated analogue 5-128 revealed PCE values up to 11.2%.418 Similar observations are made for compounds in which the hexylphenyl side chains of the IDT core unit are replaced with 2-ethylhexyl groups, as shown in the compounds 5-129 (IDT2SeC2C4; INCN end group),4195-130 (IDT2SeC2C4-2F; INCN-F),419 and 5-131 (IDT2SeC2C4-4F; INCN-2F).419 The optical band gaps are further reduced to 1.30 eV for 5-131, and solar cells with PBDB-T as a donor gave similar PCE values as those of the hexylphenyl derivatives. Due to the different side chains on the central core, the furan-bridged molecules 5-132 (IFIC)420 and 5-133 (IFIC-F)420 cannot be directly compared to the thiophene- and selenophene-containing acceptors. Solar cells of these compounds with the donor HFQx-T, show high VOCs of approx. 1 V; however, due to the moderate JSCs and FFs, the PCEs are 6.28% for 5-132 and 5.87% for 5-133.420 Adding a methyl group on the thiophene spacer, as realized in structure 5-134 (ITMIC),421 significantly improves the electron mobility in comparison to the compound without the methyl group (5-120), but the optical properties remain the same. In a blend with PM6, 5-134 reaches a PCE of 5.95%.421 Hong et al. introduced a series of NFAs with 2-ethylhexylthiophene as a π-spacer and INCN end groups (5-135, IEIC),422 INCN-2F (5-136, IE-4F),422 and INCN-2Cl (5-137, IE-4Cl)422 with decreasing optical band gaps (1.57, 1.44, and 1.43 eV). The alkyl substituent on the thiophene spacer has a positive effect on the performance in solar cells. Despite the relatively similar optical properties, the solar cells based on the 3-(2-ethylhexyl) derivatives 5-135 and 5-136 (polymer donor: PBDB-T) deliver higher PCE values than the thiophene derivatives 5-121 and 5-120, 7.30% vs 6.36% for 5-135 and 5-121 (INCN groups) and 10.8% vs 8.85% for 5-136 and 5-120 (INCN-2F groups) due to the much higher electron mobility values.415,422 Solar cells with the chlorinated compound 5-137 reached the highest PCE (11.1%, blend with PBDB-T) due to even better electron mobility values.422 NFA 5-139 (IDT-TN)423 with extended INCN groups yields a lower electron mobility and weaker solar cell performance in blend with PBDB-T (PCE 5.89%).423 In structure 5-140 (ERCN),424 RCN end groups are used; in blend with P3HT, the material is yielding a moderate performance (2.64%).424 Also, the asymmetric IDT with an ethylhexylthiophene linker, one fullerene, and one INCN end group (5-138, A1) yields low PCE values (1.63% in a blend with J71).144 A further altnernative π-spacer is octylthiophene, present in the molecules 5-141 (IDTP-P-C)425 and 5-142 (IDTP-O-C).425 The combination with INCN-2F end groups leads to structure 5-141, with very similar properties as the analogue compounds with thiophene (5-120), methylthiophene (5-134), and 2-ethylhexylthiophene (5-136). Solar cells with PTB7-Th yield a PCE of 8.21%.425 Replacing one hexylphenyl side chain of the IDT core with hexyloxy groups leads to the modified structure 5-142 (for the central core modification, see the upper right corner in Figure 16) which has a slightly better PCE (8.61%).425 Structure 5-143 (IDTCN-C)426 has a (4-hexyl)-thiophene spacer and INCN-2F end groups. Compared to the (3-octyl)-thiophene derivative (5-141), the optical band gap is slightly larger, but the electron mobility is 1 order of magnitude lower. Nevertheless, solar cells with PBDB-T reached a relatively good PCE of almost 12% with a high JSC (20.3 mA cm–2) and FF (70%).426 Asymmetric compounds were prepared by using two different π-spacers simultaneously, 2-ethylhexylthiophene and 2-ethylhexyloxythiophene with INCN-2F end groups and either hexylphenyl (5-144, p-IO1) or octyl (5-145, o-IO1) side chains on the IDT core.427 Both compounds have similar HOMO and LUMO energy levels; however, since the octyl side chains allow closer molecular packing, compound 5-145 has a lower optical band gap (1.28 eV vs 1.34 eV). As a result, solar cells of 5-145 with PTB7-Th as a donor have a better JSC, FF, and PCE (13.1%) than those of 5-144 (PCE 10.8%). Interestingly, the symmetric molecules with two 3-(2-ethylhexyloxy)-thiophene spacers 5-146 (o-IO2; octyl chains on the IDT core) and 5-147 (p-IO2; with hexylphenyl groups on IDT) both exhibit even lower optical band gaps with values of 1.20 eV for 5-146 and 1.24 eV for 5-147.427 Compound 5-146, due to its symmetric nature, has a stronger crystallization tendency than the asymmetric 5-145, which leads to a higher degree of phase separation in the absorber layer with PTB7-Th as a donor, causing a negative influence on exciton dissociation, thus lowering the PCE (9.3%). The hexylphenyl derivative (5-147, IEICO-4F) shows an improved phase separation; i.e., the domain size is reduced compared to 5-146. As a result, solar cells with PTB7-Th yield an improved performance with PCE values up to 10.8% by the same authors427 and up to 12.6% by Corzo et al.457 NFA 5-147 has a low optical band gap (1.24 eV) and a comparably good electron mobility (10–4 cm2 V–1 s–1), ensuring a good JSC (typically over 20 mA cm–2) and FF (60–70% range). The highest PCE values of 13.2% were obtained in devices with Si25-H2 as donor.428 The INCN-2Cl analogue 5-148 (IEICO-4Cl) reached only a mediocre performance of 1.35% in solar cells with D18.136 Using the INCN-F group instead leads to 5-149 (IOTIC-2F) with an increased optical band gap due to the higher lying LUMO energy level. Thus, solar cells with PTB7-Th achieve higher VOC values compared to the before discussed acceptors and PCE values went up to 12.1%.181 The non-halogenated INCN compound 5-150 (IEICO) has a larger optical band gap, as expected. Solar cells with various donor polymers gave only moderate PCE values, e.g., with J52 a value of 5.13%431 and with PTB7-Th values between 6.0 and 6.7%,429,458 while 5-151 (ORCN) with RCN-based end groups reached a similar PCE with P3HT (6.40%).424 A further, structurally similar, π-spacer is octyloxythiophene (spacer unit 20), which is used in compounds 5-152 (IDTP-P-O) and 5-153 (IDTP-O-O). Both compounds in blend with PTB7-Th have lower efficiencies (>8.4%) than those of 5-147 (ethylhexyloxythiophene spacer derivatives), with the same donor (PCE above 10%).425 Compound 5-154 has a 4-(hexyloxy)-thiophene spacer (spacer unit 21 in Figure 16); thus, the alkoxy group is on the far side of the IDT unit in contrast to the octyloxy group in 5-152 or the 2-ethylhexyloxy unit in 5-147. Compared to the latter two, 5-154 (IDTCN-O) has a much larger optical band gap with a value of 1.53 eV. As expected, solar cells of 5-154 with PBDB-T exhibit a higher VOC of 0.91 V and a PCE of 13.3%.426 Replacing oxygen with sulfur, i.e., using (hexylthio)thiophene, the structure 5-155 (IDTCN-S) with a slightly lower band gap (1.48 eV) is obtained. Solar cells with PBDB-T show a lower performance (PCE of 10.6%) than those of the hexyloxythiophene derivative, probably due to the lower electron mobility.426 Another example of a sulfur-containing π-spacer is the 3-(2-ethylhexylthio)-thiophene spacer unit 23 (see Figure 16) which is the sulphur analogue to 3-(2-ethylhexyloxy)-thiophene (spacer unit 19). The NFA structure 5-156 (ACS8) is the sulfur analogue to 5-147, comprising INCN-2F groups, and has a slightly higher band gap of 1.30 eV. Solar cells with PTB7-Th are showing an increased JSC without reducing other solar cell parameters (compared to 5-147), and thus delivers a better PCE of 13.2%.430 The PCE values drop to 7.6% in solar cells of PTB7-Th/NFA 5-157 (A134), in which the hexylphenyl side chains are replaced with hexyl groups, mainly due to a much lower JSC.275 Structures 5-158 (i-IEICO),4315-159 (i-IEICO-2F),432 and 5-161 (i-IEICO-4F)433 use the 2-(2-ethylhexyloxythiophene) linker with INCN, INCN-F, and INCN-2F, but these acceptor end groups are attached at the 4-position of the thiophene ring and thus these compounds are regioisomers to compounds 5-150, 5-149, and 5-147, respectively. All of the 4-isomers have larger band gaps and thus larger VOC values than the 5-isomers in solar cells. In combination with J52, 5-158 reached a VOC of 0.96 V and a PCE of 10.5%, which is twice as high as the efficiencies of devices with the regioisomer 5-150.431 Solar cells of 5-159 (INCN-F groups) showed increased PCE values from 11.3459 to 12.9%,432 and further, those of 5-161 with the INCN-2F groups get even higher up to 13.8%, also exceeding the performance achieved with the isomer 5-147.433 Electrochemical and optical properties of 5-160 (i-IEICO-F3),432 in which the fluorine atom is located ortho to the carbonyl group of the INCN moiety, are nearly identical to its regioisomer 5-159. However, solar cells with the donor J52 have a lower performance (PCE of 7.65%), which might be due to the 1 order of magnitude reduced electron mobility.432 Structures 5-162 (i-cc23) and 5-163 (i-cc34) comprise the thiophene modifications (end group r and s in Figure 16) of the INCN end groups. Solar cells of the 3,4-fused thiophene end group (i-cc34) outperformed its 2,3-fused isomer i-cc23 (PCE 9.51% vs 7.34%, blends with PBDB-T).434 Compounds 5-164 (i-mO-4F) have meta-hexyloxyphenyl side chains on the IDT core and INCN-2F end groups and, thus, can be seen as modifications of structures 5-161 with para-hexylphenyl side chains on the core. Both acceptors have nearly identical optical band gaps, and also, the solar cell parameters are very similar (albeit with different donors). Blends of 5-164 with PBDB-T show a high JSC (21.6 mA cm–2) and an excellent FF (71%) and consequently reached a PCE of 14.0%.435 In acceptor 5-165 (i-mO-4Cl), the INCN-2F end group is replaced with INCN-2Cl units, which has a negative influence on solar cells with the same donor (PCE: 7.41%).435 Acceptor 5-166 (IDT-OT) contains 3,4-dimethoxythiophene spacer units, but its blends with PBDB-T did not reach high efficiencies (3.32%).436 Better results are obtained with 5-167 (IDTS-4F) with the same π-spacer but fluorinated end groups and oxygen replaced with sulfur in the central core side chains, i.e., para-hexylthiophenyl groups. Solar cells using a blend with the donor polymers PM6 or PM7 give efficiencies over 12%.4375-168 (IDT2ST-4F) contains a 3,4-di(octylthio)thiophene π-spacer (spacer unit 26) and INCN-2F groups.438 Oxygen analogous compounds having a 3,4-di(hexyloxy)thiophene π-spacer (spacer unit 27) are represented by the structures 5-169 (ITOIC) with INCN, 5-170 (ITOIC-F) with INCN-F, and 5-171 (ITOIC-2F) with INCN-2F groups.439 As expected, fluorination leads to lower frontier orbital energies, especially on the LUMO and thus to lower band gaps decreasing with the fluorine content from 1.55 eV for 5-169 to 1.50 eV for 5-170 and 1.45 eV for 5-171. Consequently, solar cells of these compounds with PBDB-T show an expected decrease in VOC values in this order but an increase in JSC and FF, thus making the devices with the INCN-2F groups 5-171 the most efficient ones, reaching PCEs of 12.2%.439 The sulfur analogue 5-168 has an even lower band gap of 1.40 eV, and thus, solar cells with PBDB-T gave even lower VOC values, but due to a FF of 69% and a JSC of 19.4 mA cm–2, they reached almost the same PCE values (11.4%).438 It is interesting to compare 5-171 with compounds 5-172 (IDT-EDOT), 5-173 (IDT-PDOT),440 and 5-174 (IDT-PDOT-C6)441 all having 3,4-alkoxy substituents on the thiophene unit. However, the latter three in different cyclic forms, i.e., 5-172, contain the classical ethylenedioxythiophene (EDOT) as a spacer, 5-173 propylenedioxythiophene, and 5-174 additional exocyclic hexyl side chains on the propylenedioxy bridge. The optical band gaps of all of these compounds are nearly identical (between 1.41 and 1.45 eV), and in solar cells, also the VOC values (with PBDB-T donor) do not differ significantly (0.88 ± 0.03 V). Whereas also the other solar cell parameters of 5-172 and 5-174 are very similar to the overall PCE values of 11.3 and 11.1%, respectively, the PCE of devices with 5-173 is significantly smaller (2.18%). The 3,4-propylenedioxythiophene derived molecule (5-173) has a lower molar absorption coefficient in solution than its 3,4-ethylenedioxythiophene-based counterpart 5-172; also, the π–π stacking of 5-172 was determined to be stronger. This might be the reason for the improved electron mobility, JSC, and FF.440 At the same time, adding two hexyl chains as in 5-174 again increases the electron mobility (this compound has a face-on orientation in film).441 The π-spacers used in acceptors 5-175 (ITCIC)421 and 5-176 (IDT-3MT)415 differ from other thiophene-based spacers reviewed so far as being the only ones with an electron-withdrawing substituent. 5-175 has a 2-chlorothiophene linker, which can form non-covalent interactions with sulfur atoms of the IDT core. This interaction improves the planarity of the molecule, which has a positive impact on the electron mobility. In comparison to the methyl-substituted analogue 5-134, which has nearly identical optoelectronic parameters, solar cells of 5-175/PM6 have an improved JSC, FF, and PCE (11.3% vs 5.95% for 5-134).4215-176 has an ester group in the 3-position of the thiophene linker, and when compared to 5-121 (with the unsubstituted thiophene linker IDT-T), the former yields a higher PCE (8.40% vs 6.36%) in blend with PBDB-T.415 Ming et al. attached (5-hexylthiophen-2-yl) groups to the thiophene π-bridge, once in the 3-position, i.e., pointing toward the IDT core (“inner position”, 5-177, IDT-TiFIC), and once in the 4-position, pointing toward the end group (“outer position”, 5-178, IDT-ToFIC). The outer isomer (5-178) is slightly better with a PCE of 11.1% (blend with PBDB-T).442 Linking the two thiophene molecules with a double bond as in 5-179 (ITVT) leads to a lower PCE of 5.84% (PBZ as donor).443 Also, the acceptor 5-180 (A401) with four INCN accepting units attached to a terthiophene branching unit (see spacer unit 36 in Figure 16) did not lead to a high PCE in solar cells with PBDB-T (7.54%).444 Compound 5-181 (IDTO-TT-4F) comprises thienothiophene spacer units. This modification leads to a slightly reduced PCE (10.2%, in blend with PBDB-T) if compared to a simple thiophene linked molecule (5-125, 10.7%).417 The cyclopentadithiophene π-bridge present in acceptor 5-182 (IDTC-4Cl) with INCN-2Cl groups shows relatively good absorption properties expanding until 900 nm, and thus, solar cells with PBDB-T have relatively high JSC values of 19.2 mA cm–2. However, the VOC of 0.82 V and FF of 60% do not allow a PCE over 9.5%.445 Alternative π-spacer units are thieno[3,4-b]thiophene bridges (spacer units 39–44) and have been introduced in the NFA structures 5-183–5-196. NFAs 5-183 (4TIC) and 5-184 (4T4F) combine an IDT core with central octyl chains with INCN and INCN-2F units, respectively, bridged with 2-ethylhexyloxycarbonyl-substituted thienothiophene spacers. Both compounds absorb into the near infrared regions with absorption maxima in thin films at 881 and 920 nm, respectively. Similar values are observed also for the thienothiophene π-spacer isomer (spacer unit 40) containing 5-185 (6TIC) and 5-186 (6T4F). Solar cells of the INCN-based compounds 5-185 and 5-183 with PBDB-T have, as expected, higher VOC but lower JSC values than the difluorinated compounds. Devices with 5-185 and 5-186 (comprising the spacer unit 40) exhibit higher JSC values, with 5-186 outperforming 5-185. The excellent electron mobility in combination with the red-shifted absorption maximum gives a high JSC of 24.9 mA cm–2 and a FF of 69%, resulting in a PCE of 10.7%.446 In addition to the change from octyl to hexyl groups on the IDT core, the INCN group has been replaced with RCN acceptor units in compound 5-187 (ATT-5).447 These changes lead to a blue-shift in the absorption maximum. Solar cells with PBDB-T reached a high VOC above 0.9 V and a good JSC and FF, leading to an overall PCE up to 12.4%.447 Compounds 5-188–5-191 have an additional fluorine on the π-spacer and combine hexylphenyl-IDT cores with INCN (5-188, IFIC-i-2F), INCN-2F (5-189, IFIC-i-6F), as well as INCN-F (5-190, IFIC-i-4F). Additionally, 5-191 (IFIC-o-4F) is the regioisomer of 5-190 by changing the thienothiophene orientation as discussed before. These compounds have higher optical band gaps in thin films than their non-halogenated analogous thienothiophenes; however, this effect may partly arise from the bulkier hexylphenyl side chains on the IDT core hindering closer packing. Solar cells with PTB7-Th show very similar efficiency values: 9.82% for 5-188, 9.43% for 5-189, 10.9% for 5-190, and 7.01% for 5-191.448 By linking RCN acceptor units with IDT cores with different side chains via an octyloxycarbonyl-substituted thienothiophene spacer (43), NFA structures 5-192 (ATT-1)447 with para-hexylphenyl side chains, 5-193 (ATT-4)447 with hexyl side chains, 5-194 (ATT-6)449 with meta-hexylphenyl groups, and 5-195 (ATT-7)449 with meta-hexyloxyphenyl groups have been introduced. The optical band gaps are within values between 1.49 and 1.54 eV, resulting in high VOCs around 0.95 V of solar cells with PBDB-T. Replacing p-hexylphenyl side chains on the IDT core (5-192) with its meta isomer (5-194) reduced the PCE from 9.00 to 8.39%, while the corresponding m-hexyloxyphenyl side chains (5-195) improved the PCE to 10.3%.449 However, the derivative with hexyl side chains 5-193 leads to the highest PCE of 11.2%.447 These results again emphasize the impact of the side chain modification on the film morphology and thus on the solar cell performance. Acceptor 5-196 contains a trifluoromethyl substitution on the thieno[3,4-b]thiophene bridge (spacer unit 44) and has very similar optical properties to the aforementioned ones; however, solar cells with PBDB-T show only moderate efficiencies of 7.3% mainly due to a lower VOC.450
The imide functionality in 5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (π-spacer 45) makes this spacer unit more electron-withdrawing. In structures 5-197 (TPD3)451 and 5-198 (TPD8),452 this π-spacer is used to combine hexylphenyl-IDT with either RCN451 or INCN452 end groups, respectively. Compound 5-198 in blend with PTQ10 reached good PCE values of 10.4%,452 while 5-197 in blends with J52-Cl gave smaller PCE values (7.58%).451 A thiophene–benzothiadiazole–thiophene bridge (spacer unit 46) and the fluorinated derivative (spacer unit 47) were incorporated between the IDT core and the RCN end group to give the structures 5-199 (IDBTC) and 5-200 (IDBTCF). However, these combinations of D–A–D building blocks in the π-bridging unit partly counteract with each other; thus, solar cells with P3HT reach moderate PCE values up to 2.5 and 3.22%, respectively.453
Asymmetric acceptors 5-201–5-204 were prepared by using only one π-spacer combined with an acceptor unit (spacer units 48-51) and using an INCN end group without a spacer on the other side. Structure 5-201 (IDTBF) was prepared by end-capping the IDT core with a INCN-2F group on one side and a BT π-spacer with a rhodanine end group (spacer unit 48) on the other side.454 This molecule can be roughly compared to O-IDTBR (5-79), the symmetric acceptor with two BT π-spacers and rhodanine end groups. The maximum PCE obtained with PM6 is 10.4%, and it is in the same performance range as the solar cells of 5-79 with different donors (see discussion above). Acceptors 5-202 (2FIFIC) and 5-203 (ICIF2F) differ in their end groups; in one case, fluorinated INCN is attached directly to the IDT core and a non-modified INCN group is attached to the 2-ethylhexyloxycarbonyl modified fluorothienothiophene spacer (5-202), while, in the other case, the end groups are exchanged (5-203). Solar cells of 5-202 with PM6 have higher PCE values than those with 5-203 (11.0% vs 9.42%).455
However, the best results are achieved by the molecule 5-204 (IDST-4F), which contains a 3-(2-ethylhexyl)thiothiophene π-spacer between one of the INCN-2F end groups and the IDT core. Solar cells of 5-204 with PM6 reach JSCs and FFs of almost 25 mA cm–2 and 70%, leading to PCE values of 14.3%.456
4.3. Miscellaneous Rings
In the last three years, also multiple other fused-ring cores have been prepared and tested in solar cells (Figure 17, Table 14). Some are based on well-known structural motifs, while others are completely new. Many of these pioneering structures still are lacking in efficiency; however, such initial material design is essential for the development of OPVs and might bring further breakthroughs.
Figure 17.
Structures of non-fullerene acceptors with miscellaneous five-ring core units and varying side chains, end groups, and π-spacers.
Table 14. Optical, Electrical, and Photovoltaic Properties of Non-Fullerene Acceptors 5-205–5-249.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μed(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5-205 | CC5 | –5.75 | –4.09 | 1.37 | PM6 | 1:0.7 | 0.65 | 18.0 | 60 | 6.91 | -/5.2 × 10–5 | (460) |
| 5-206 | CDTTIC | –5.62 | –3.98 | 1.44 | PFBDB-T | 1:1 | 0.78 | 25.8 | 57 | 11.5 | 4.2 × 10–3/3.5 × 10–4 | (461) |
| 5-207 | DTP-C17-4F | –5.59 | –4.16 | 1.38 | PM6 | 1:1 | 0.69 | 21.2 | 61 | 8.94 | -/3.5 × 10–3 | (462) |
| 5-208 | DTC-PDI | –5.39 | –3.88 | 1.69 | PTB7-Th | 1:1.25 | 0.90 | 7.38 | 39 | 2.63 | -/2.2 × 10–5 | (463) |
| 5-209 | DCB-4F | –5.50 | –3.86 | 1.55 | PM6 | 1:1 | 1.00 | 16.4 | 58 | 9.56 | -/1.3 × 10–4 | (464) |
| 5-210 | BTPT-4F | –5.73 | –4.00 | 1.45 | P2F-EHp | 1:1.2 | 0.78 | 3.20 | 44 | 1.09 | (213) | |
| 5-211 | H1 | –5.39 | –3.90 | 1.22 | PBDB-T | 1:1.2 | 0.78 | 16.8 | 53 | 6.98 | 1.1 × 10–4/1.0 × 10–4 | (465) |
| 5-212 | H2 | –5.38 | –3.90 | 1.22 | PBDB-T | 1:1.2 | 0.78 | 24.4 | 69 | 13.2 | 2.9 × 10–3/3.5 × 10–4 | (465) |
| 5-213 | H3 | –5.40 | –3.92 | 1.22 | PBDB-T | 1:1.2 | 0.76 | 25.8 | 70 | 13.8 | 3.3 × 10–3/4.2 × 10–4 | (465) |
| 5-214 | DCNQA-BT-T1 | –5.36 | –3.69 | 1.81 | P3HT | 1:1 | 0.51 | 3.75 | 38 | 0.79 | (466) | |
| 5-215 | DCNQA-BT-T2 | –5.31 | –3.70 | 1.80 | P3HT | 1:1 | 0.51 | 1.60 | 28 | 0.31 | (466) | |
| 5-216 | DCNQA-BT-T3 | –5.16 | –3.65 | 1.79 | P3HT | 1:1 | 0.50 | 1.21 | 33 | 0.28 | (466) | |
| 5-217 | 1-Tol | –6.01 | –3.71 | 2.38 | PTB7-Th | 1:1 | 0.96 | 1.4 | 31 | 0.4 | (467) | |
| 5-218 | 1-Far | –6.24 | –3.88 | 2.27 | PTB7-Th | 1:1 | 0.87 | 5.4 | 40 | 1.9 | (467) | |
| 5-219 | 2-Tol | –5.13 | 1.64 | PTB7-Th | 1:1 | 0.89 | 0.8 | 27 | 0.2 | (467) | ||
| 5-220 | 2-Far | –5.18 | –3.66 | 1.48 | PTB7-Th | 1:1 | 0.79 | 1.2 | 31 | 0.3 | (467) | |
| 5-221 | IPY-T-IC | –5.69 | –3.50 | 1.90 | PTB7-Th | 1:2 | 0.83 | 16.5 | 56 | 7.68 | -/1.9 × 10–4 | (468) |
| 5-222 | IPY-T-ICCl | –5.72 | –3.56 | 1.75 | PTB7-Th | 1:1.5 | 0.69 | 14.1 | 64 | 6.20 | -/1.1 × 10–4 | (468) |
| 5-223 | IPY-T-ICF | –5.71 | –3.53 | 1.82 | PTB7-Th | 1:1.5 | 0.71 | 13.5 | 60 | 5.81 | -/1.1 × 10–4 | (468) |
| 5-224 | ICz-Rd2 | –5.59 | –3.88b | 1.71 | P | 1:1 | 1.04 | 14.0 | 54 | 7.88 | 1.2 × 10–4/- | (469) |
| 5-225 | ICz-RdCN2 | –5.58 | –3.98b | 1.61 | P | 1:1 | 1.01 | 15.6 | 62 | 9.76 | 2.9 × 10–4/- | (469) |
| 5-226 | DICTFDT | –5.56 | –3.89b | 1.67 | PTB7-Th | 1:1.5 | 0.89 | 12.5 | 46 | 5.12 | -/2.6 × 10–5 | (470) |
| 5-227 | TFDTBR | –5.46 | –3.77b | 1.69 | PTB7-Th | 2:3 | 0.92 | 12.3 | 35 | 3.95 | -/3.5 × 10–6 | (470) |
| 5-228 | IDIDT-C8 | –5.50 | –3.86 | 1.64 | PBDB-T | 1:1.5 | 0.97 | 15.8 | 66 | 10.1 | -/2.4 × 10–5 | (471) |
| 5-229 | ICBF | –5.64 | –3.80 | 1.80 | PBDB-T | 1:1 | 0.99 | 10.3 | 60 | 6.07 | (472) | |
| 5-230 | FICBF | –5.66 | –3.88 | 1.72 | PBDB-T | 1:1 | 0.87 | 12.7 | 67 | 7.41 | (472) | |
| 5-231 | ICBF-O | –5.70 | –3.75 | 1.90 | PBDB-T | 1:1 | 1.06 | 6.82 | 43 | 3.11 | (472) | |
| 5-232 | FICBF-O | –5.72 | –3.85 | 1.81 | PBDB-T | 1:1 | 0.96 | 10.6 | 55 | 5.54 | (472) | |
| 5-233 | IF-TN | –5.80 | –3.93 | 1.81 | PBDB-T | 1:1 | 1.01 | 7.02 | 41 | 3.03 | 4.1 × 10–7/1.0 × 10–5 | (423) |
| 5-234 | i-IF-4F | –5.55 | –3.71 | 1.79 | PTB7-Th | 1:1.5 | 0.87 | 12.5 | 60 | 6.47 | -/1.3 × 10–5 | (473) |
| 5-235 | PTIC | –5.67 | –3.85 | 1.55 | PBDB-T | 1:1 | 0.84 | 14.2 | 64 | 7.66 | -/4.9 × 10–5 | (474) |
| 5-236 | COi5DFIC | –5.96c | –4.29c | 1.56 | PTB7-Th | 1:1.5 | 0.59 | 14.4 | 53 | 4.5 | (307) | |
| 5-237 | PTTIC | –5.48 | –4.04 | 1.53 | PBDB-T | 1:1 | 0.93 | 14.3 | 55 | 7.35 | (475) | |
| 5-238 | Ph-DTDPi-OT | –5.55 | –3.90 | 1.51 | PBDB-T | 1:1 | 0.90 | 18.7 | 68 | 11.4 | -/8.8 × 10–5 | (476) |
| 5-239 | PTBT-R | –5.59 | –3.70 | 1.64 | PBDB-T | 1:1 | 0.97 | 10.4 | 50 | 5.06 | (475) | |
| 5-240 | i-PTIC | –5.41b | –3.86 | 1.55 | PBDB-T | 1:1 | 0.83 | 11.0 | 65 | 5.80 | (477) | |
| 5-241 | i-PTIC-F | –5.66b | –4.18 | 1.48 | PBDB-T | 1:1 | 0.63 | 14.6 | 62 | 5.68 | (477) | |
| 5-242 | Ph-DTDPo-TE | –5.63 | –3.96 | 1.43 | PBDB-T | 1:1 | 0.82 | 20.8 | 71 | 12.2 | -/1.0 × 10–4 | (478) |
| 5-243 | Ph-DTDPo-OT | –5.50 | –3.87 | 1.46 | PBDB-T | 1:1 | 0.87 | 14.8 | 59 | 7.60 | -/1.9 × 10–5 | (476) |
| 5-244 | CO5DFIC-OT | –5.51 | –3.95 | 1.28 | PTB7-Th | 1:1.5 | 0.71 | 17.6 | 61 | 7.66 | -/8.6 × 10–6 | (479) |
| 5-245 | CO5DFIC-ST | –5.55 | –3.92 | 1.34 | PTB7-Th | 1:2.5 | 0.74 | 20.7 | 64 | 9.73 | -/2.0 × 10–5 | (479) |
| 5-246 | Ph-DTDPo-OTE | –5.52 | –3.88 | 1.46 | PBDB-T | 1:1 | 0.88 | 18.3 | 69 | 11.0 | -/4.1 × 10–5 | (478) |
| 5-247 | BTA53 | –5.48 | –3.58 | 1.68 | P3HT | 1:0.8 | 0.88 | 11.6 | 62 | 6.31 | -/1.8 × 10–6 | (396) |
| 5-248 | SiOTC | –5.60 | –3.81 | 1.61 | PM6 | 1:1.2 | 0.81 | 5.06 | 48 | 1.96 | -/5.3 × 10–6 | (480) |
| 5-249 | SiOTIC | –5.57 | –3.84 | 1.55 | PM6 | 1:1.2 | 0.92 | 14.5 | 75 | 10.0 | -/1.3 × 10–4 | (480) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Other method or method not defined.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Structure 5-205 (CC5)460 combines a di(hexylphenyl)-cyclopentadithienothiophene (CDTT) core with two INCN-2Cl units and exhibits a smaller optical band gap of 1.37 eV and lower lying HOMO and LUMO levels than the structurally similar 5-206 (CDTTIC),461 combining a dioctyl-CDTT core with an INCN end group exhibiting an optical band gap of 1.44 eV. The as-cast device built up from a blend of PFBDB-T/5-206 delivered a very good PCE of 11.5% and a JSC of 25.8 mA cm–2.461 The fusion of pyrrole and thienothiophenes leads to pentacyclic S,N-heteroacene compounds, for example, to structure 5-207 (DTP-C17-4F) with an optical band gap of 1.38 eV. A high JSC of 21.2 mA cm–2 was obtained for solar cells with PM6 as a donor (PCE 8.94%).462
Structure 5-208 (DTC-PDI) consists of an electron-rich pentacyclic di(benzothiophene)carbazole core and perylene diimides as end groups, but blends with PTB7-Th lacked in JSC and FF, yielding only a PCE of 2.60%.463 NFA 5-209 (DCB-4F) also comprises a carbazole as the central unit but flanked by two benzenes on the second and fourth rings of this central core. INCN-2F end groups are attached via a hexylthiophene π-spacer. The acceptor has an optical band gap of 1.55 eV, and its solar cells with PM6 reach a high VOC (1.00 V) and an overall PCE of 9.56%.464 Moreover, several acceptors contain a fused (D–A′–D) core, which is formed via fusion of an electron-deficient BT to two thienopyrrole units. Consequently, a prolonged conjugation length combined with a good delocalization of π-electrons is achieved. The combination of this core with INCN-2F end groups leads to structure 5-210 (BTPT-4F) having low lying energy levels and an optical band gap of 1.45 eV.213 Solar cells with the donor P2F-EHp yielded only a PCE of 1.09%, which is a hard contrast to the seven-ring analogue 7-145 (BTPTT-4F/Y6) which achieved a PCE of 16.0%. The partly edge-on orientation of 5-210 in the active layer being unfavorable for charge transfer could be the reason for the limited performance of 5-210 in solar cells. The incorporation of an additional alkylthiophene π-spacer between the dithienopyrrolobenzothiadiazole core and the INCN-2F end groups leads to the structures 5-211 (H1), 5-212 (H2), and 5-213 (H3) with narrow band gaps of 1.22 eV.465 The three structures differ in their alkyl substitution on the thiophene linker; i.e., 5-211 has a decyl group in the 3-position (pointing toward the core), whereas 5-212 and 5-213 have 2-ethylhexyl and decyl chains in the 4-position (pointing toward INCN). All three compounds have nearly identical electrochemical and optical properties, and blends with PBDB-T have good electron mobilities (5-213 > 5-212 > 5-211). As expected, solar cells of these compounds with PBDB-T have nearly the same VOC; however, the JSC values are different following the trend of the mobility data. 5-211 has the smallest JSC with 16.8 mA cm–2, 5-212 reached 22.4 mA cm–2, while 5-213-based devices revealed even 25.8 mA cm–2. Consequently, solar cells with the latter yield the highest PCE value (13.8%). Also, in this case, 5-211 had an edge-on orientation in neat film, 5-212 a mixed edge-on and face-on orientation, and the best performing NFA, 5-213, a face-on orientation.465 Quinacridone-based acceptors 5-214–5-216 (DCNQA-BT-Tx) are interesting due to their reversed structure; e.g., the electron-rich thiophenes are used as end groups. However, solar cells with P3HT as a donor gave only PCE values below 1% in all cases.466
Morgan et al. introduced boron-containing NFAs by the reaction of different 2,5-diphenylpyrazine derivatives with BCl3 and subsequent substitution of the chlorides with further aryl groups (structures 5-217–5-220).467 The best solar cells were assembled from 5-218 which, blended with PTB7-Th, gave a VOC of 0.87 V and a JSC of 5.4 mA cm–2, reaching a PCE of 1.9%; the solar cells of the other three compounds reached PCEs <1%.467 Acceptors 5-221–5-223 contain a 6,12-dihydrodiindeno[1,2-b:1′,2′-e]pyrazine as a core linked via a thiophene π-spacer to INCN-based end groups, i.e., INCN (5-221, IPY-T-IC), INCN-2Cl (5-222, IPY-T-ICCl), and INCN-2F (5-223, IPY-T-ICF).468 Structure 5-221 has the highest LUMO, and thus, also its solar cells with PTB7-Th as a donor yielded a VOC (0.83 V) higher than the solar cells of its halogenated analogues. Combined with a JSC of 16.5 mA cm–2, this results in the highest PCE values obtained in this series (7.68%).468 Suman et al. prepared acceptor structures by combining a 5,11-dihydroindolo[3,2-b]carbazole core with a BT spacer and the rhodanine (5-224, ICz-Rd2) or the RCN (5-225, ICz-RdCN2) end group. The latter had a better performance in solar cells, with polymer P giving a PCE of 9.76%.469 Fluorene fused with thiophene was introduced as an alternative electron-donating core in combination with a hexylthiophene π-spacer and INCN termini (5-226, DICTFDT) and with an additional benzothiadiazole π-spacer combined with a rhodanine end group (5-227, TFDTBR). The former yielded a better PCE (5.12%, in PTB7-Th).470 Structure 5-228 comprises a thiophene-based core, linked to INCN end groups by thiophene π-bridges. Rather untypical is the cis arrangement of the alkyl side chains of the central core. Solar cells prepared with PBDB-T revealed a PCE of 10.1%.471 Structures 5-229–5-234 contain a 6,12-dihydroindeno[1,2-b]fluorene core linked via different thiophene spacer units and INCN-based end groups. The lack of heteroatoms in the central core makes these compounds slightly less electron-rich; thus, the optical band gaps are rather high (between 1.72 and 1.90 eV). Structure 5-229 (ICBF) comprises the unmodified INCN and a simple thiophene linker. Solar cells in combination with PBDB-T achieved PCE values up to 6.07%, which were improved to 7.41% when INCN-2F end groups were used (5-230, FICBF). The introduction of an octyl substituent on the thiophene spacer leads to the derivatives 5-231 (ICBF-O) and 5-232 (FICBF-O). The planarity of the molecules is slightly reduced, whereby the optical band gaps increase and both compounds show lower performances in solar cells with the same donor.472 The efficiency is not improved with the compound 5-233, which has a 2-ethylhexyl chain on the linker and extended INCN end groups (PCEs 3.03% with PBDB-T).423 The electron-donating strength of the thiophene spacer was increased by alkoxy substitution as in compound 5-234 (i-IF-4F). Solar cells with PTB7-Th yielded a PCE of 6.47%.473
A further class of NFAs is based on 4,10-dihydrothieno[2′,3′:4,5]pyrano[2,3-g]thieno[3,2-c]chromene as a central core. Here, a thiophene–benzene–thiophene structure is additionally linked by (oxymethylene) bridges. The combination of this central core with different end groups gives the structures 5-235 (PTIC) with INCN474 and 5-236 (COi5DFIC) with INCN-2F307 accepting groups. Solar cells with 5-235 in blends with PBDB-T had a higher VOC (0.94 V, consequence of higher LUMO energy), improved FF, and a better overall PCE of 7.66%474 compared to 5-236-based devices. When compared to its higher analogue 7-309,3075-236 has lowered energy levels, leading also to a lower VOC (0.59 V) and PCE (4.5%) in solar cells with the polymer donor PTB7-Th.307 A variation of this acceptor motif is represented in structure 5-237 (PTTIC) in which the core and the INCN group are linked via a thiophene linker, leading to a slightly lower optical band gap of 1.53 eV. Upshifted energy levels lead to a higher VOC (0.93 V) and a PCE of 7.35% in solar cells with PBDB-T as a donor.475 The combination with a hexyloxythiophene π-spacer and INCN-2F groups leads to structure 5-238 (Ph-DTDPi-OT) with an even lower optical band gap of 1.51 eV. As expected, solar cells with PBDB-T showed a slightly lower VOC, but due to much higher values for the JSC (18.7 mA cm–2) and FF (68%), the devices reached a promising PCE of 11.4%.476 The combination of this core with rhodanine end groups linked by a BT bridge gives molecule 5-239 (PTBT-R), with the highest optical band gap in this series. Thus, solar cells with PBDB-T have a good VOC of 0.97 V, but due to reduced JSC, the PCE is smaller (5.06%).475 Changing the oxygen and carbon positions in the central core leads to the isomeric 5,11-dihydrothieno[2′,3′:5,6]pyrano[3,4-g]thieno[3,2-c]isochromene donor unit, used for building the acceptors 5-240–5-247. The acceptor molecule 5-240 (i-PTIC) is the isomeric compound to 5-235, but in solar cells, 5-240 has a lower PCE (5.80% vs 7.66%), which is only slightly improved if INCN-2F end groups are used (compound 5-241, PCE 5.86% with PBDB-T).477 Blends of 5-241 with PTB7-Th yield a similar PCE (5.58%).479 The introduction of a thiophene-based π-spacer between the core and INCN-2F groups has a positive influence on the photovoltaic performance. Structure 5-242 with ethylhexylthiophene spacers has a reduced optical band gap of 1.43 eV, and solar cells with PBDB-T reached the best PCE of this compound class with a value of 12.2%.478 The change to ethylhexyloxythiophene, i.e., the introduction of an additional oxygen, leads to the acceptor structure 5-246 (Ph-DTDPo-OTE) with a slightly larger band gap of 1.46 eV, thus giving slightly higher VOC values in solar cells with the same donor. However, due to a lower JSC, the PCE is reduced to 11.0%.478 The hexyloxythiophene derivative 5-243 (Ph-DTDPo-OT) and the octyloxythiophene derivative 5-244 (CO5DFIC-OT) showed a lower performance in solar cells, reaching PCEs of 7.6% in both cases (5-243 with PBDB-T,476,4785-244 with PTB7-Th479). Exchanging oxygen in compound 5-244 with sulfur using the 3-(octylthio)-thiophene spacer gives 5-245 (CO5DFIC-ST), which showed a good performance in solar cells with PTB7-Th of almost 10%.479 Using BTA as a π-spacer and RCN end groups yields NFA 5-247 (BTA53). Due to the electron-withdrawing character of BTA, the HOMO and LUMO energies are shifted upward, and the optical band gap is increased to 1.68 eV (in comparison to the thiophene-spacer-containing compounds). Solar cells with P3HT achieved a PCE of 6.31%.396 In compounds 5-248 (SiOTC) and 5-249 (SiOTIC), the sp3-hybridized carbons of the central core are replaced by Si(EH)2 groups. The inner isomer 5-248 has a slightly higher band gap than the outer isomer 5-249 (1.61 eV vs 1.55 eV), and the HOMO and LUMO energy levels are also quite similar. Nevertheless, solar cells of 5-249 with PM6 show a much better performance with a PCE reaching 10.0%, while those of 5-248 are below 2%. The better performance of 5-249 was explained by the much higher electron mobility and improved blend morphology, which had a positive impact on JSC and FF.480
5. Six Fused Aromatic Ring Systems
The hexacyclic fused-ring electron acceptors reported since 2018 can be assigned to four main groups based on the structural features they are containing. The characteristic structural unit of the first group is an angular shaped dithienonaphthalene (DTN) core. A typical representative of this group is 6-1 (DTNIF), exhibiting an optical band gap of 1.63 eV and reaching PCEs of 8.73% with PBDB-T (Figure 18, Table 15).481 The incorporation of a thiophene spacer into the backbone of this molecule results in 6-2 (DTNSF) and leads to a reduced optical band gap (1.47 eV). However, overall, this modification resulted in a reduced PCE (7.15%) compared to 6-1 due to a significantly decreased FF.481
Figure 18.
Structures of non-fullerene acceptors with six fused rings.
Table 15. Optical, Electrical, and Photovoltaic Properties of the Non-Fullerene Acceptors Containing a Six-Fused-Ring Structural Feature.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μed(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6-1 | DTNIF | –5.82 | –3.92 | 1.63 | PBDB-T | 1:1 | 0.90 | 13.3 | 73 | 8.73 | -/1.8 × 10–5 | (481) |
| 6-2 | DTNSF | –5.52 | –4.00 | 1.47 | PBDB-T | 1:1 | 0.92 | 14.5 | 55 | 7.15 | -/6.7 × 10–6 | (481) |
| 6-3 | O-NTIC | –5.51 | –3.67 | 1.70 | PBDB-T | 1:0.8 | 0.98 | 13.3 | 70 | 9.1 | -/8.3 × 10–5 | (482) |
| 6-4 | NT-4F | –5.88 | –3.86 | 1.68 | PM6 | 1:1.2 | 0.96 | 13.9 | 71 | 9.46 | -/3.3 × 10–4 | (278) |
| 6-5 | NT-4Cl | –5.89 | –3.92 | 1.64 | PM6 | 1:1.2 | 0.93 | 16.6 | 74 | 11.4 | -/7.8 × 10–4 | (278) |
| 6-6 | O-NTNC | –5.54 | –3.78 | 1.64 | PBDB-T | 1:0.8 | 0.94 | 16.0 | 73 | 11.0 | -/1.7 × 10–4 | (482) |
| 6-7 | TITI-4F | –5.81c | –4.25c | 1.56 | PM6 | 1:1 | 0.91 | 16.0 | 75 | 10.9 | -/3.5 × 10–4 | (483) |
| 6-8 | T6Me | –5.50 | –3.94 | 1.38 | PM6 | 1:1 | 0.87 | 21.3 | 65 | 12.1 | -/6.4 × 10–4 | (292) |
| 6-9 | 4TTIC | –5.46 | –3.73 | 1.46 | PBDB-ST | 1:1 | 0.93 | 18.6 | 67 | 11.5 | -/3.5 × 10–4 | (484) |
| 6-10 | 4TTIC-Cl | –5.50 | –3.79 | 1.42 | PBDB-ST | 1:1 | 0.88 | 20.2 | 74 | 13.1 | -/4.2 × 10–4 | (484) |
| 6-11 | 4TBA | –5.51 | –3.70b | 1.66 | PBDT-2TC | 1:1 | 0.96 | 15.4 | 71 | 10.5 | -/3.9 × 10–5 | (485) |
| 6-12 | STBA | –5.45 | –3.60b | 1.59 | (485) | |||||||
| 6-13 | F6IC | –5.68 | –4.04 | 1.36 | PTB7-Th | 1:1.2 | 0.61 | 18.2 | 63 | 7.00 | 9.1 × 10–4/9.1 × 10–5 | (486) |
| 6-14 | 4TO-T-4F | –5.30 | –3.85 | 1.30 | PTB7-Th | 1:1.5 | 0.75 | 20.4 | 58 | 8.87 | -/4.0 × 10–5 | (487) |
| 6-15 | 4TO-Se-4F | –5.29 | –3.85 | 1.27 | PTB7-Th | 1:1.5 | 0.70 | 19.1 | 55 | 7.40 | -/2.1 × 10–5 | (487) |
| 6-16 | TPTT-2F | –5.75 | –4.04 | 1.58 | PBT1-C | 1:1 | 0.88 | 15.8 | 73 | 10.2 | 4.5 × 10–4/1.6 × 10–4 | (299) |
| 6-17 | A201 | –5.69 | –3.93 | 1.64 | J71 | 1:1 | 0.88 | 13.2 | 67 | 9.36 | -/5.2 × 10–4 | (488) |
| ITIC-3T | –5.54 | –3.93b | 1.61 | PBDTBDD-Ph | 1:1 | 0.92 | 17.5 | 68 | 11.0 | -/1.6 × 10–4 | (329) | |
| 6-18 | IDT6CN-M | –5.62 | –3.90 | 1.63 | PBDB-T | 1:1 | 0.92 | 16.0 | 76 | 11.2 | 9.0 × 10–4/5.9 × 10–4 | (489) |
| 6-19 | T-TT-4F | –5.44 | –3.51 | 1.58 | PM6 | 1:1 | 0.86 | 18.5 | 66 | 10.5 | -/4.3 × 10–4 | (490) |
| 6-20 | T-TT-4Cl | –5.48 | –3.59 | 1.53 | PM6 | 1:1 | 0.81 | 19.0 | 66 | 10.2 | -/3.7 × 10–4 | (490) |
| 6-21 | IDT6CN | –5.68 | –3.97 | 1.63 | PBDB-T | 1:1 | 0.83 | 15.1 | 74 | 9.27 | 7.7 × 10–4/6.3 × 10–4 | (378) |
| 6-22 | IDT6CN-Th | –5.71 | –4.01 | 1.61 | PBDB-T | 1:1 | 0.81 | 16.8 | 77 | 10.4 | 9.0 × 10–4/7.1 × 10–4 | (378) |
| 6-23 | IDT6CN-TM | –5.70 | –3.96 | 1.60 | PM6 | 1:1 | 0.95 | 17.4 | 75 | 12.4 | 8.3 × 10–4/- | (491) |
| 6-24 | IDT6CN-4F | –5.78 | –4.12 | 1.58 | PM6 | 1:1 | 0.86 | 18.3 | 69 | 10.9 | 7.6 × 10–4/- | (491) |
| 6-25 | SePTT-2F | –5.71 | –4.00 | 1.50 | PBT1-C | 1:0.9 | 0.83 | 17.5 | 75 | 10.9 | 5.1 × 10–4/2.5 × 10–4 | (300) |
| 6-26 | TTPT-T-2F | –5.60 | –4.00 | 1.54 | PBT1-C | 1:1.5 | 0.92 | 18.5 | 75 | 12.7 | 6.6 × 10–4/3.6 × 10–4 | (413) |
| 6-27 | TTPT-T-4F | –5.63 | –4.08 | 1.45 | L2 | 1:1.5 | 0.86 | 22.2 | 74 | 14.0 | 3.6 × 10–4/3.4 × 10–4 | (492) |
| 6-28 | Y21 | –5.65 | –3.90 | 1.35 | PM6 | 1:1 | 0.83 | 24.9 | 74 | 15.4 | -/1.3 × 10–3 | (493) |
| 6-29 | Y22 | –5.69 | –3.94 | 1.36 | PM6 | 1:1.2 | 0.85 | 24.4 | 74 | 15.4 | -/1.0 × 10–3 | (494) |
| 6-30 | BP-4F | –5.68 | –3.88 | PM7 | 1:1.2 | 0.88 | 21.7 | 76 | 14.6 | -/3.3 × 10–5 | (495) | |
| 6-31 | TB-4Cl | –5.7 | –4.1 | PM6 | 0.85 | 23.0 | 75 | 14.7 | -/5.8 × 10–4 | (496) | ||
| 6-32 | CC10 | –5.72 | –4.07 | 1.38 | PM6 | 1:1.2 | 0.77 | 22.7 | 67 | 11.8 | -/1.0 × 10–4 | (460) |
| 6-33 | SN61C | –5.38 | –3.93 | 1.39 | PBDB-T | 1:1 | 0.88 | 16.5 | 66 | 9.6 | -/3.0 × 10–4 | (497) |
| 6-34 | SN61C-4F | –5.52 | –4.11 | 1.32 | PBDB-T | 1:1 | 0.78 | 23.2 | 73 | 13.2 | -/5.0 × 10–4 | (497) |
| 6-35 | PTTtID-Cl | –5.53 | –4.08 | PTB7-Th | 1:1.6 | 0.71 | 19.9 | 60 | 8.5 | (498) | ||
| 6-36 | SN6-2Br | –5.39 | –3.86 | 1.32 | PBDB-T | 0.73 | 19.4 | 71 | 10.0 | -/4.9 × 10–4 | (499) | |
| 6-37 | NFDTS | –5.36 | –3.92 | 1.41 | PTB7-Th | 1:1.2 | 0.67 | 18.8 | 49 | 6.15 | -/3.9 × 10–6 | (303) |
| 6-38 | Si4TIC-F | –5.58 | –4.21 | 1.37 | PBTIBDTT | 1:1.5 | 0.78 | 18.8 | 69 | 10.2 | 2.3 × 10–4/1.3 × 10–5 | (500) |
| 6-39 | P6IC | –5.43 | –3.94 | 1.30 | PTB7-Th | 1:1 | 0.69 | 25.0 | 70 | 12.2 | 8.8 × 10–4/1.9 × 10–4 | (501) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Obtained via UPS.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Feng et al. used non-fluorinated INCN units and solubilizing para-alkoxy-phenyl side chains leading to structure 6-3 (O-NTIC),482 which has a slightly increased band gap compared to the phenylhexyl-substituted analogue IHIC1.502 By combining this donor unit with INCN-2F (6-4, NT-4F) and INCN-2Cl (6-5, NT-4Cl) end groups, NFAs with lower optical band gaps and downshifted energy levels are obtained. With PM6/6-4-based solar cells, PCEs of 9.46% are reported; PM6/6-5 absorber layers lead to PCEs of 11.4%.278 Through extending the end groups of 6-3 from INCN to the benzannulated derivative 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile, a more ordered packing could be achieved in 6-6 (O-NTNC), which resulted in a significantly red-shifted absorption and increased electron mobility. Furthermore, an enhancement of the PCE of the respective solar cells (blend with PBDB-T) from 9.1 to 11.0% was obtained. Exchanging the bridging atom in the donor unit of 6-2 from carbon to nitrogen leads to 6-7 (TITI-4F). This acceptor, in combination with PM6, reached PCEs up to 10.9%.483
A further structural modification similar to the IDT unit was implemented by exchanging the central benzene ring with a thienothiophene moiety to give the donor unit T-Cp-T-T-Cp-T.503 Using CPTCN-Me end groups gives 6-8 (T6Me).292 The extension of the backbone conjugation compared to IDT-based NFAs such as, e.g., 5-73 causes, as expected, also a red-shift of the absorption with an optical band gap of 1.38 eV and an absorption maximum in thin film of 820 nm. Based on their energy levels, 6-8 and PM6 are well suited to be combined in the absorber layer and solar cells based on this material combination revealed high PCEs up to 12.1%. Chlorination of the CPTCN end group (6-9, 4TTIC, and 6-10, 4TTIC-Cl) could increase the solar cell efficiency from 11.5% (PBDB-ST/6-9) to 13.1% (PBDB-ST/6-10).484 The NFAs 6-11 (4TBA) and 6-12 (STBA) contain TBA derived end groups, and in 6-12, the outer thiophene units are replaced by selenophenes. These compounds reveal an optical band gap of 1.66 and 1.59 eV, respectively. Combined with the polymer PBDT-2TC, 6-11 shows a VOC of 0.96 V and a PCE of 10.5%.4856-13 (F6IC) consists of the T-Cp-T-T-Cp-T core unit with INCN-2F end groups and reveals an optical band gap of 1.36 eV, very similar to 6-8. For PTB7-Th/6-13-based solar cells, PCEs up to 7.00% are reported.486,504 Thiophene or selenophene π-spacers combined with additional alkoxy side chains on the peripheral thiophene rings of the central core yield very narrow band gap materials 6-14 (4TO-T-4F, 1.30 eV) and 6-15 (4TO-Se-4F, 1.27 eV).487 Solar cells prepared in combination with PTB7-Th had good JSC values, but PCEs did not exceed 8.87% (for compound 6-14).487
The second group of recently investigated six-fused-ring NFAs (6-16–6-24) contains the asymmetric moiety thieno[1,2-b]indaceno[5,6-b′]thienothiophene (TITT) as a donor unit (T-Cp-B-Cp-T-T). TITT in turn is obtained from a fusion of the IDT unit with an additional thiophene moiety. Asymmetric structures have the advantage of bearing higher dipole moments, which facilitates (i) intramolecular charge transfer in the A–D–A structure leading to a higher charge carrier mobility as well as (ii) the self-assembly of the molecules involving enhanced lamellar packing and π–π stacking.488,489,505−507 Moreover, a broader absorption range and an increased LUMO energy level are expected.299 A series of such molecules with different INCN acceptor units (INCN-F, 6-16 (TPTT-2F);299 INCN, 6-17 (A201488/ITIC-3T);329 monomethylated INCN, 6-18 (IDT6CN-M);378,489 INCN-2F, 6-19 (T-TT-4F);490,508 INCN-2Cl, 6-20 (T-TT-4Cl)490) are reported. Using different donor polymers, solar cells yielded relatively similar PCEs around 9–11%. These structures are also closely related to the seven-ring ITIC with the only difference that one of the two thienothiophene groups in the fused ITIC backbone is replaced by a thiophene unit, making the acceptor asymmetric and shorter. The shorter structure is discussed to possess advantages in forming beneficial morphologies in combination with certain weakly aggregated conjugated polymers, which could also be proved by an increased PCE.329 As a further variation of this structure, the terminal INCN was substituted by a CPTCN unit, resulting in 6-21 (IDT6CN), and an additional substitution of the hexylphenyl residue with thienyl side chains (5-hexylthiophen-2-yl) can facilitate the π–π stacking of the side chains in 6-22 (IDT6CN-Th).378,491 Using PBDB-T as a donor polymer, solar cells of 6-21 and 6-22 exhibit lower VOC values and thus lower PCEs (9.27 and 10.4%) than those of 6-18 (11.2%) with the same donor polymer. The NFA 6-23 (IDT6CN-TM)491 bears additional methyl groups at the termini and was compared to 6-24 (IDT6CN-4F) with INCN-2F end groups. A comparison of solar cells based on 6-23 and 6-24 with PM6 reveals a change in PCE from 12.4 to 10.9%.491 This enhanced performance of 6-23 is ascribed to the higher lying LUMO level, the higher dipole moment, and the slightly higher electron mobility compared to 6-24, which is beneficial for the VOC and the FF of the solar cells (cf. Table 15).
Replacing the singular thiophene unit in the donor core of 6-16 with a selenophene unit leads to 6-25 (SePTT-2F).300 Compared to the sulfur analogue 6-16,299 the optical band gap is reduced from 1.58 to 1.50 eV. In combination with the donor polymer PBT1-C, the PCE is improved from 10.2 to 10.9%. 6-26 (TTPT-T-2F) is a further modification of 6-16 by introducing a thiophene as the π-spacer.413 Solar cells of 6-26 with the donor polymer PBT1-C showed a balanced charge mobility and higher PCEs of 12.7%413 compared to those of 6-16 with the same donor (cf. Table 15). With 6-27 (TTPT-T-4F), the analogue of 6-26 with INCN-2F end groups, PCEs of 14.0% are obtained in combination with polymer L2.492
The NFAs 6-28–6-31 are modifications of the Y-series NFAs with one fused thiophene less on one end of the core, leading to an asymmetric molecular geometry. 6-28 (Y21) and 6-29 (Y22) contain a BTA unit instead of the commonly used BT group. Compared to 6-28, 6-29 bears only one additional hexyl side chain hardly affecting the optical and photovoltaic properties. Both NFAs have a narrow band gap of ∼1.35 eV and exhibit a PCE of 15.4% (polymer PM6).493,4946-30 (BP-4F) and 6-31 (TB-4Cl) contain INCN-2F and INCN-2Cl end groups, respectively, and can be additionally distinguished by their different alkyl side chains. In solar cells, PCEs of 14.6% (PM7/6-30) and 14.7% (PM6/6-31) are obtained.495,496
The asymmetric NFA 6-32 (CC10)460 comprises a donor core in a T-T-Cp-T-B-T motif. Compared to the symmetric five-ring structure T-T-Cp-T-T (5-205), this asymmetric acceptor leads to better π–π stacking, enhanced electron mobility and morphology, and also less trap assisted recombination. Overall, this is reflected in a simultaneous increase of VOC, JSC, and FF, resulting in a significant enhancement of the photovoltaic performance in solar cells with PM6 from 6.91% (PM6/5-205) to 11.8% (PM6:6-32).
The donor units of structures 6-33–6-36 replace the two cyclopentadiene units with N-alkylated pyrroles, leading to the symmetric motif T-P-T-T-P-T.497 The aromatic pyrrole ring can thereby enhance the conjugation and has a higher electron-donating character, leading to increased energy levels. The combination with INCN and INCN-2F acceptor units leads to the structures 6-33 (SN61C) and 6-34 (SN61C-4F), both introduced by Huang et al.,497 and with INCN-2Cl to 6-35 (PTTtID-Cl), published by Wang et al.4986-33 and 6-34 were evaluated in solar cells with the donor PBDB-T. The lower band gap significantly increased the JSC values of the devices based on difluorinated 6-34. Together with a higher fill factor, PCE values reach remarkable 13.2% compared to 9.6% for those of 6-33. In 6-35, the fluorinated INCN are replaced by INCN-2Cl units, giving a compound with rather similar HOMO and LUMO levels (see Table 15). However, the solar cell performance cannot be fully compared, as 6-35 was tested (i) in combination with PTB7-Th, whereby a PCE of 8.5% was obtained, and (ii) in a ternary solar cell architecture in combination with the donor PM7 and IT-4F (7-2) as a second acceptor, resulting in a PCE of 12.0%. 6-36 (SN6-2Br) contains brominated INCN end groups and reveals the same optical band gap (1.32 eV) as the fluorinated analogue (6-34); the performance in solar cells is, however, lower. For PBDB-T/6-36-based solar cells, PCEs of 10.0% are obtained.499
The replacement of the pyrrole rings with silole rings leads to the bisdithienosilole-based NFAs 6-37 (NFDTS) and 6-38 (Si4TIC-F). 6-37 in solar cells with PTB7-Th gave a photovoltaic performance of 6.15%,303 while the devices prepared from 6-38 with the conjugated polymer PBTIBDTT reveal a PCE of 10.2%.500 Lu et al. compared NFA 6-13, consisting of the donor unit T-Cp-T-T-Cp-T and INCN-2F units, with structure 6-39 (P6IC) where the two outermost thiophenes of the donor unit are replaced by N-alkylated pyrroles.501 Solar cells of the pyrrole compounds in combination with PTB7-Th reach PCE values up to 12.2% compared to only moderate 7.00% for 6-13. This was mainly ascribed to a narrower band gap, an enhanced crystallinity and electron mobility, as well as a slightly upshifted LUMO energy level.
6. Eight Fused Aromatic Ring Systems
A common design for the class of octocyclic acceptors is based on annulating three thieno[2,3-b]thiophene units via cyclopentadiene rings to the motif T-T-Cp-T-T-Cp-T-T and different acceptor end groups (see Figure 19, structures 8-1–8-6). Chen et al. investigated the influence of chlorination using INCN, INCN-Cl, and INCN-2Cl acceptor units, which led to NFAs 8-1 (IXIC), 8-2 (IXIC-2Cl), and 8-3 (IXIC-4Cl), respectively.509
Figure 19.
Structures of non-fullerene acceptors with eight fused rings.
The chlorination of the end group causes a decrease of the band gap from 1.35 eV (8-1) to 1.30 eV (8-2) to 1.25 eV (8-3) mainly by lowering the LUMO energies (Table 16). Consequently, the VOCs of solar cells with PBDB-T also decreased in this order from 0.82 to 0.69 V. Moreover, the presence of chlorine increases the crystallinity as well as the π–π stacking, which facilitates charge carrier mobility and leads to higher FFs in the solar cells. While the PCEs of the PBDB-T/8-1 as well as the PBDB-T/8-3-based solar cells were around 11.2%, the highest efficiencies of 12.2% in this series were obtained with the PBDB-T/8-2-based devices due to the best compromise of the VOC and JSC in this absorber material combination.509
Table 16. Optical, Electrical, and Photovoltaic Properties of the Non-Fullerene Acceptors Containing an Eight-Fused-Ring Structural Feature.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μee(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 8-1 | IXIC | –5.13 | –3.78b | 1.35 | PBDB-T | 1:1 | 0.82 | 20.9 | 65 | 11.3 | 9.3 × 10–4/4.1 × 10–4 | (509) |
| 8-2 | IXIC-2Cl | –5.20 | –3.90b | 1.30 | PBDB-T | 1:1 | 0.73 | 23.6 | 71 | 12.2 | 1.0 × 10–3/5.2 × 10–4 | (509) |
| 8-3 | IXIC-4Cl | –5.20 | –3.95b | 1.25 | PBDB-T | 1:1 | 0.69 | 22.9 | 71 | 11.2 | 1.1 × 10–3/4.9 × 10–4 | (509) |
| 8-4 | FOIC | –5.36 | –3.92 | 1.32 | PTB7-Th | 1:1.5 | 0.74 | 24.0 | 67 | 12.0 | 1.2 × 10–3/5.1 × 10–4 | (512) |
| 8-5 | FOIC1 | –5.39 | –3.99 | PTB7-Th | 1:1 | 0.69 | 23.4 | 69 | 11.0 | 1.8 × 10–3/3.9 × 10–4 | (513) | |
| 8-6 | F8IC | –5.43 | –4.00 | 1.27 | PTB7-Th | 1:1.7 | 0.64 | 25.1 | 68 | 10.9 | 1.5 × 10–3/1.6 × 10–4 | (504) |
| 8-7 | 3TT-FIC | –5.42 | –4.17 | 1.25 | PTB7-Th | 1:1.2 | 0.66 | 25.9 | 71 | 12.2 | -/1.7 × 10–4 | (514) |
| 8-8 | 3TT-CIC | –5.24 | –3.95 | 1.23 | PTB7-Th | 1:1.5 | 0.65 | 26.7 | 69 | 12.0 | -/1.2 × 10–4 | (515) |
| 8-9 | 3TT-OCIC | –5.22 | –3.91 | 1.29 | PTB7-Th | 1:1 | 0.68 | 26.5 | 69 | 12.4 | -/1.4 × 10–4 | (515) |
| 8-10 | 6TBA | –5.41 | –3.66 | 1.52 | PBDT-2TC | 1:1 | 1.0 | 13.9 | 63 | 8.76 | (485) | |
| 8-11 | T8Me | –5.39 | –3.89 | 1.35 | PM6 | 1:1 | 0.90 | 10.5 | 64 | 6.09 | -/2.5 × 10–4 | (292) |
| 8-12 | F8IC1 | –5.43 | –3.99 | 1.32 | PTB7-Th | 1:1.5 | 0.68 | 22.3 | 70 | 10.7 | 1.1 × 10-3 / 3.9 × 10-4 | (486) |
| 8-13 | DBTIC | –5.45 | –3.73 | 1.71 | J52 | 0.94 | 13.8 | 67 | 8.64 | -/9.3 × 10–5 | (516) | |
| 8-14 | Z1-aa | –5.67 | –3.84 | 1.68 | PM6 | 1:1 | 0.98 | 11.7 | 40 | 4.56 | -/5.5 × 10–6 | (517) |
| 8-15 | Z1-ab | –5.69 | –3.87 | 1.68 | PM6 | 1:1 | 1.00 | 14.8 | 61 | 9.60 | -/1.9 × 10–5 | (517) |
| 8-16 | Z1-bb | –5.70 | –3.92 | 1.61 | PM6 | 1:1 | 0.98 | 18.5 | 70 | 12.7 | -/3.0 × 10–4 | (517) |
| 8-17 | FTTBT | –5.57 | –4.02 | 1.54 | PM6 | 1:1 | 0.93 | 16.0 | 65 | 9.79 | -/1.1 × 10–4 | (315) |
| 8-18 | TTPTTT-IC | –5.64 | –3.87 | 1.60 | PBT1-C | 1:1.2 | 1.00 | 12.5 | 64 | 7.91 | 2.8 × 10–4/1.7 × 10–4 | (518) |
| 8-19 | TTPTTT-2F | –5.67 | –4.04 | 1.54 | PBT1-C | 1:1.2 | 0.92 | 16.8 | 75 | 11.5 | 5.0 × 10–4/2.6 × 10–4 | (518) |
| 8-20 | TTPTTT-4F | –5.69 | –4.12 | 1.52 | PBT1-C | 1:1.2 | 0.86 | 19.4 | 72 | 12.1 | 6.5 × 10–4/3.2 × 10–4 | (518) |
| 8-21 | AOIC | –5.50 | –3.93 | 1.39 | PTB7-Th | 1:1.25 | 0.74 | 24.5 | 75 | 13.7 | 2.0 × 10–3/2.3 × 10–3 | (371) |
| 8-22 | IDTTP-4F | –5.53 | –3.97 | PM7 | 1:1 | 0.91 | 21.2 | 72 | 13.8 | 6.2 × 10–4/3.5 × 10–4 | (296) | |
| 8-23 | N8IT | –5.41 | –3.90 | 1.42 | PM6 | 1:1 | 0.94 | 18.5 | 68 | 11.9 | 5.9 × 10–4/3.8 × 10–4 | (294) |
| 8-24 | BTDTP-4F | –5.56 | –3.93 | 1.30 | PM6 | 1:1.2 | 0.87 | 21.3 | 71 | 13.1 | 7.9 × 10–4/4.2 × 10–4 | (266) |
| 8-25 | a-BTTIC | –5.45 | –3.83 | 1.43 | PBDB-T | 1:1 | 0.90 | 20.3 | 74 | 13.6 | 1.1 × 10–3/5.5 × 10–4 | (519) |
| 8-26 | NTO-4F | –5.61 | –3.88 | 1.55 | PM6 | 1.25:1 | 0.99 | 19.1 | 61 | 11.5 | -/4.0 × 10–5 | (520) |
| 8-27 | NC-FIC | –5.43 | –3.94 | 1.51 | PBDB-T | 1:1 | 0.88 | 15.2 | 56 | 7.52 | -/7.0 × 10–7 | (521) |
| 8-28 | IOIC2 | –5.70c | –4.16c | 1.54 | PM6 | 1:1 | 0.97 | 16.3 | 66 | 10.5 | 1.0 × 10–3/0.5 × 10–4 | (522) |
| 8-29 | IOIC3 | –5.64c | –4.19c | 1.45 | PM6 | 1:1 | 0.92 | 20.0 | 70 | 12.8 | 1.5 × 10–3/0.6 × 10–4 | (522) |
| 8-30 | IOIC4 | –5.70c | –4.17c | 1.53 | PM6 | 1:1.2 | 0.96 | 17.3 | 67 | 11.1 | 1.1 × 10–3/0.4 × 10–4 | (522) |
| 8-31 | IOIC5 | –5.65c | –4.19c | 1.46 | PM6 | 1:1.2 | 0.92 | 20.9 | 72 | 13.8 | 1.5 × 10–3/1.4 × 10–4 | (522) |
| 8-32 | NOIC | –5.76 | –4.03 | 1.55 | PM6 | 1:1 | 0.89 | 18.1 | 71 | 11.4 | 6.2 × 10–4/1.1 × 10–4 | (523) |
| 8-33 | NOIC1 | –5.41 | –4.02 | 1.38 | PM6 | 1:1 | 0.86 | 21.9 | 66 | 12.5 | 7.1 × 10–4/2.4 × 10–4 | (523) |
| 8-34 | NOIC2 | –5.64 | –3.99 | 1.49 | PM6 | 1:1 | 0.93 | 20.6 | 74 | 14.1 | 9.0 × 10–4/2.5 × 10–4 | (523) |
| 8-35 | NOIC3 | –5.83 | –3.95 | 1.62 | PM6 | 1:1 | 0.93 | 12.9 | 60 | 7.15 | 6.6 × 10–5/3.4 × 10–5 | (523) |
| 8-36 | NOIC4 | –5.64 | –3.90 | 1.55 | PM6 | 1:1 | 0.94 | 16.8 | 64 | 10.1 | 1.2 × 10–4/8.6 × 10–5 | (523) |
| 8-37 | ZITI | –5.59 | –3.74 | 1.53 | J71 | 1:1 | 0.93 | 20.4 | 69 | 13.2 | -/1.1 × 10–4 | (524) |
| 8-38 | ZITI-3F | –5.64 | –3.76 | 1.50 | J71 | 1:1 | 0.90 | 20.7 | 72 | 13.2 | -/1.6 × 10–4 | (525) |
| 8-39 | ZITI-4F | –5.66 | –3.81 | 1.47 | J71 | 1:1 | 0.85 | 21.3 | 73 | 13.2 | -/1.9 × 10–4 | (525) |
| 8-40 | ZITI-N-CH3 | –5.53 | –3.82 | 1.41 | PffBT4T-2OD | 1:1 | 0.82 | 19.0 | 56 | 8.78 | -/2.1 × 10–4 | (526) |
| 8-41 | ZITI-N-C8H17 | –5.53 | –3.85 | 1.40 | PffBT4T-2OD | 1:1 | 0.80 | 21.0 | 72 | 12.1 | -/2.5 × 10–4 | (526) |
| 8-42 | ZITI-N-EH | –5.52 | –3.86 | 1.40 | PffBT4T-2OD | 1:1 | 0.81 | 22.1 | 73 | 13.1 | -/2.7 × 10–4 | (526) |
| 8-43 | COi8DFIC | –5.50d | –3.88 | PTB7-Th | 1:1 | 0.70 | 27.4 | 66 | 12.9 | -/5.5 × 10–5 | (527) | |
| 8-44 | CRIC | –5.44 | –3.82 | 1.53 | J52-2F | 1:1 | 0.94 | 13.4 | 49 | 6.12 | -/7.8 × 10–5 | (528) |
| 8-45 | CRIC-4F | –5.67 | –3.90 | 1.60 | J52-2F | 1:1.5 | 0.89 | 13.1 | 48 | 5.61 | -/1.9 × 10–5 | (528) |
| 8-46 | Py-1 | –5.79 | –3.76 | 1.95 | PTB7-Th | 1:1 | 0.74 | 13.0 | 60 | 5.75 | -/1.1 × 10–4 | (529) |
| 8-47 | Py-2 | –5.83 | –3.78 | 1.97 | PTB7-Th | 1:1 | 0.75 | 9.67 | 47 | 3.47 | -/1.9 × 10–5 | (529) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
HOMO/LUMO energy levels obtained from the LUMO/HOMO levels determined by CV and the optical band gap.
Obtained via UPS.
HOMO obtained via PESA.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Nian et al. obtained a PCE of 13.5% with solar cells based on the small molecular donor ZnP-TSEH and 8-1.510 The combination of 8-3 with PM7, which has a lower HOMO level compared to PBDB-T, led to an increase of the VOC from 0.69 V up to 0.80 V, which is also reflected in a higher PCE of 12.0%.511
The NFAs 8-4 (FOIC) and 8-6 (F8IC) contain INCN-F and INCN-2F end groups, respectively, and similar to the chlorinated compounds (8-2, 8-3), the LUMO energy levels are lowered and the optical band gaps are decreasing compared to 8-1.504,512 This is reflected in a lower VOC; however, a higher JSC is obtained in PTB7-Th/8-6, compared to PTB7-Th/8-4-based solar cells. The introduction of fluorine atoms into the phenyl moiety of the side chains of 8-4 leads to 8-5 (FOIC1). While in a bulk heterojunction absorber layer in combination with PTB7-Th as a donor the PCE is slightly decreased compared to the 8-4-based solar cells (11.0%), efficiencies of 12.0% were obtained by a sequentially deposited heterojunction.513 Changing the linear hexyl chains in 8-6 to branched ethylhexyl side chains results in 8-7 (3TT-FIC) and leaves the optical and photovoltaic properties almost unchanged; only the FF is increased, leading to a higher PCE of 12.2%.514
Structural variations of compound 8-3 bearing INCN-2Cl end groups were introduced by Gao et al.515 by replacing hexyl side chains with ethylhexyl chains, leading to structure 8-8 (3TT-CIC), or keeping the hexyl chains and introducing additional octyl side chains at the terminal thiophene units of the donor core, resulting in 8-9 (3TT-OCIC). As expected, structure 8-8 has a very similar optical band gap as 8-3, whereas structure 8-9 leads to a higher band gap, which entailed a 30 mV increase in VOC to 0.68 V compared to 8-8 in solar cells with PTB7-Th, resulting in a slight increase in PCE from 12.0 to 12.4%. Moreover, this terthieno[3,2-b]thiophene core structure was combined with a TBA end group, resulting in 8-10 (6TBA) as well as the CPTCN-Me terminal group leading to structure 8-11 (T8Me). In solar cells, 8-10 revealed a PCE of 8.76% in combination with PBDT-2TC.485 The NFA 8-11 was, however, tested in combination with PM6, which resulted in a lower PCE of 6.09% compared to similar hexacyclic (6-8) and heptacyclic structures (7-271); see Table 16. Although the absorption range is slightly broader, solar cells of 8-11 reached significantly lower JSC values due to the unfavorable energy level alignment of the donor and the acceptor material in the absorber layer.292
Changing the position of the cyclopentadienyl rings, resulting in a T-Cp-T-T-T-T-Cp-T motif, leads to the NFA 8-12 (F8IC1).486 Compared to 8-6, bearing the same side chains and end groups, no significant changes in the optoelectronic properties as well as the photovoltaic performance are revealed and PCEs of 10.7% are obtained in solar cells.
A further group of NFAs contains a central benzo[b]benzo[4,5]thieno[2,3-d]thiophene (BTTB) structural motif with annulated cyclopentathiophene units on each side (T-Cp-B-T-T-B-Cp-T) in combination with INCN units and is represented by the compounds 8-13–8-16. This core unit is covalently rigidified and has a large π-conjugation, which typically causes high lying energy levels. The electron-donating ability of the donor core is comparatively low, which reduces intramolecular charge transfer, resulting in a larger optical band gap with significantly higher values between 1.61 and 1.71 eV than the NFAs 8-1–8-12, discussed before with values around 1.3 eV. Solar cells of 8-13 (DBTIC, INCN end groups) with the donor J52 reached a maximum PCE of 8.64%.516 Using INCN-2F, the influence of different structural isomers was investigated on the structures 8-14–8-16, with the same sequentially fused-ring cores but with different annulation of the terminal cyclopentathiophenes to the BTTB, as shown in Figure 19.5178-14 (Z1-aa) has [2,3:b] and [6,7:b] annulation and thus a linear structure, 8-15 (Z1-ab) comprises a [1,2:b] and [6,7:b] annulation leading to a kink, whereas 8-16 (Z1-bb) has a double-kink due to a [1,2:b] and [5,6:b] annulation. Solar cells based on 8-16 and PM6 as a donor reveal clearly the highest photovoltaic performance (12.7%, see also Table 16) compared to 9.60 and 4.56% for solar cells with 8-15 and 8-14, respectively. This originates from several facts: First, 8-16 has a reduced optical band gap (1.61 eV) compared to 8-14 and 8-15 (both 1.68 eV), which leads to a broader absorption range in combination with PM6. Second, 8-16 possesses a better phase morphology, more ordered π–π stacking, and significantly higher electron mobilities in the blend with PM6 compared to the other isomers, leading to a more balanced electron and hole mobility.517
Wang et al. introduced the centrosymmetric acceptor 8-17 (FTTBT) with a T-B-Cp-T-T-Cp-B-T motif, which is very similar to the heptacyclic structure 7-327.315 Thereby, the central thiophene unit in 7-327 was replaced by a thienothiophene unit. Solar cells with a PM6/8-17 blend led to a significant increase of the PCE (9.79%) compared to devices using 7-327 in the same solar cell setup, which revealed a PCE of only 3.47%. This performance increase is based on a smaller optical band gap originating from an extended π-conjugation and a phase morphology more beneficial for charge transport.315
Li et al. investigated asymmetric NFAs combining a dithienothiophene unit and a thienothiophene via a s-indacene unit leading to the octocyclic motif T-T-T-Cp-B-Cp-T-T.518 They focused on the influence of the fluorination degree of the INCN end groups comparing INCN (8-18, TTPTTT-IC), INCN-F (8-19, TTPTTT-2F), and INCN-2F (8-20, TTPTTT-4F) acceptor units. Fluorination caused a red-shift of the absorption along with a downshift of the HOMO/LUMO energies. Moreover, the fluorination led to strong intra- and intermolecular interactions and consequently also to a higher electron mobility. Solar cells of these acceptors with PTB1-C show—despite a decrease in VOC—an overall increase of the PCE with increasing fluorinination degree caused by higher JSC and FF. While solar cells with the non-fluorinated analogue showed a PCE of 7.91%, the efficiency was increased to 11.5% for 8-19 and to 12.1% for the 8-20-based solar cells.518 The asymmetric compound 8-21 (AOIC) is based on the five-fused-ring compound 5-24 having an extension of the fused-ring system on one side of the molecule.371 This change in the chemical structure results in upshifted HOMO/LUMO levels, a reduced band gap, and, in particular, a higher electron mobility (2.0 × 10–3 cm2 V–1 s–1) compared to 5-24, which is reflected in a significantly increased photovoltaic performance (PTB7-Th/8-21, 13.7%; PTB7-Th/5-24, 5.61%).
In the NFA 8-22 (IDTTP-4F), one thiophene in the central unit is substituted by an N-alkylated pyrrole leading to a T-T-Cp-B-Cp-T-P-T structure.2968-22 has a C-shape and is related to 7-280, which reveals an S-shape confirmation. Solar cells using PM7 as a donor revealed higher PCEs with the S-shape (15.2%) compared to 13.8% obtained with 8-22. This difference is due to the more pronounced aggregation of the C-shaped 8-22 in contrast to the S-shaped 7-280.2968-23 (N8IT) bears INCN-Cl end groups and an n-octyl alkyl chain on the pyrrole moiety, which leads to slightly elevated HOMO and LUMO levels compared to 8-22 and an optical band gap of 1.42 eV; PM6/8-23-based solar cells reveal PCEs of 11.9%.294 The importance of the optimization and tuning of the molecular packing and the phase morphology is also thoroughly elaborated by Luo et al. based on the compounds 8-24 (BTDTP-4F) and 7-222 as well as 8-22 and 7-280.266 While in the latter mentioned compounds, the S-shape led to the higher PCE, in the case of 8-24 and 7-222, the NFA with the C-shape (7-222) revealed the higher photovoltaic performance compared to 8-24 outlining an S-shape. This is due to the fact that, in this case, the C-shape led to the more favorable phase morphology and more pronounced face-on orientation, which were identified to be the main reasons for the difference in the photovoltaic performance.266
A good photovoltaic performance was obtained in solar cells using 8-25 (a-BTTIC) combined with PBDB-T, yielding a PCE of 13.6%.519 This asymmetric compound is based on the symmetric structure 7-251 to which one thiophene ring is added, shows a T-Cp-T-B-T-Cp-T-T motif, and bears CPTCN-Me acceptor units. Compared to 7-251,282 the band gap is reduced and the LUMO level is elevated to −3.83 eV. This leads to simultaneously increased JSC and VOC values, and a lower energy loss was observed for PBDB-T/8-25-based devices.
The molecule 8-26 (NTO-4F) is based on a naphthodithiophene core bearing alkoxy side chains.520 In this structure, the oxygen in the side chain and the sulfur of the thiophene unit can form intramolecular non-covalent S–O interactions and can thereby increase the size of the conjugated system. 8-26 reveals an optical band gap of 1.55 eV and an elevated LUMO level (−3.88 eV) which leads to a high VOC (0.99 V) in solar cells with PM6 as a donor. These solar cells exhibit a PCE of 11.5% and good thermal stability.520 The NFA 8-27 bears linear dodecyl side chains instead of the ethylhexyloxy side chains of 8-26, and for PBDB-T/8-27 devices, PCEs of 7.52% are reported.521 Zhu et al. investigated the influence of the alkoxylation position on this core unit (8-28–8-31).522 Alkoxylation on the naphthodithiophene core leads to a reduced optical band gap, upshifted HOMO and downshifted LUMO levels, and slightly increased electron mobility, while the alkoxylation of the Cp unit does not have significant effects. Compared to 8-28 (IOIC2, no alkoxylation), hexyloxy chains on the core (8-29, IOIC3) lead to an increase of the PCE from 10.5 to 12.8%, while the alkoxylation of the Cp unit (8-30, IOC4) only leads to an increase to 11.1%. The highest photovoltaic performance (13.8%) was obtained with the NFA 8-31 (IOIC5, alkoxylation at both sites) in combination with PM6.522 Using FTAZ as a conjugated polymer, 8-28 reveals PCEs up to 12.3%530 and PTB7-Th/8-29-based solar cells show PCEs of 13.1%.531
Li et al. investigated the influence of the methoxylation of the naphthalene unit in NFAs with the structural motif T-T-Cp-B-B-Cp-T-T combined with INCN-2F end groups (8-32–8-36). Different methoxylation positions strongly influence the material properties; i.e., methoxy substitution at the core leads to red-shifted absorption, higher crystallinity, and charge carrier mobility, while carbon–oxygen bridges (present in 8-35 and 8-36) were found to be not beneficial regarding the material properties as well as the solar cell performance. Consequently, PM6/8-35 solar cells reveal a PCE of 7.15%, the lowest value within this series, and PCEs of 14.1% are obtained with PM6/8-34-based devices.523
The acceptors 8-37–8-39 contain a fused di(thienocyclopenta)indenoindene (ZIT) core (with the T-Cp-B-Cp-Cp-B-Cp-T motif) and two INCN-F (8-37, ZITI), one INCN-F, and one INCN-2F on each side (8-38, ZITI-3F) and two INCN-2F units (8-39, ZITI-4F) as termini.524,525 These materials reveal an optical band gap of ∼1.5 eV, whereby it is slightly lowered by the fluorination. Zhang et al. synthesized 8-38 and 8-39 in a one-pot synthesis, which yielded a molar ratio of 1:1.525 After separation of these compounds and preparation of solar cells in combination with the donor J71, very similar PCEs of 13.15 and 13.18% were obtained, which is also similar to the PCE of J71/8-37-based solar cells.524 When the mixture of 8-38 and 8-39 (ZITI-m), as yielded in the synthesis, is directly used for the device preparation, the obtained ternary solar cells revealed an improved PCE of 13.7% which was attributed to synergy effects in the ternary blend, e.g., a broader absorption range and a more balanced charge carrier transport.525 In the compounds 8-40–8-42, the outer Cp units in the ZIT core are replaced by pyrrole units bearing different alkyl chains (8-40, ZITI-N-CH3, methyl; 8-41, ZITI-N-C8, n-octyl; 8-42, ZITI-N-EH, ethylhexyl). While the different alkyl chains only hardly affect the positions of the energy levels and the optical properties, the solar cell efficiency based on these NFAs and PffBT4T-2OD increases remarkably from 8.78% (8-40) to 12.1% (8-41) and 13.1% (8-42), which is due to the fact that, in the PffBT4T-2OD/8-42-based absorber layers, the smallest domain sizes and the highest crystallinity are found within this series.5268-43 (COi8DFIC) contains two carbon–oxygen bridges in the fused-ring core and INCN-2F end groups and shows high crystallinity in the absorber layers. In combination with PTB7-Th, PCEs up to 12.9% are obtained.527 This acceptor was also used compared to its smaller sized central core analogues (five to seven fused rings). It could be seen that the PCE was increasing together with the conjugation length. Obviously, changes in the conjugation length have an impact on the optical band gap and energy levels of these acceptors; however, also significant changes in phase morphology are observed.307
Furthermore, fused octocyclic acceptors (8-44, CRIC, and 8-45, CRIC-4F) based on a chrysene core are reported by Zhao et al. and reveal optical band gaps of 1.53 and 1.60 eV, respectively.528 Solar cells based on J52-2F/8-44 showed a PCE of 6.12%, and an efficiency of 5.61% was obtained in the same device setup with the fluorinated analogue 8-45. The higher photovoltaic performance of the J52-2F/8-44 blend is ascribed to a higher LUMO energy level (−3.82 eV compared to −3.90 eV) and a more beneficial crystallinity and blend morphology.528
The molecules 8-46 (Py-1) and 8-47 (Py-2) bear a dithienocyclopentapyrene core and INCN-2F acceptor groups, whereby 8-46 is the centrosymmetric and 8-47 the axisymmetric isomer.529 While the optical band gap (8-46, 1.95 eV; 8-47, 1.97 eV) and the energy levels show no significant difference, the centrosymmetric isomer shows a significantly higher electron mobility in the blend with PTB7-Th, which also results in an increased PCE of 5.75% (compared to 3.47% for a PTB7-Th/8-47-based solar cell).529
7. Nine Fused Aromatic Ring Systems
One type of nonacyclic NFAs is based on combining two cyclopentathienothiophene units via annulation with a central fluorene or carbazole unit (9-1–9-11)—with a T-T-Cp-B-Cp (or Py)-B-T-T motif (see also Figure 20 and Table 17). 9-1 (BTTFIC) contains a bis(thieno[3,2-b]thieno)cyclopentafluorene donor core and INCN units as end groups and has an optical band gap of 1.58 eV and HOMO and LUMO energy levels of −5.56 and −3.95 eV.532 The carbazole-based analogue 9-5 (CZTT-IC) reveals rather similar optical properties, energy levels, and photovoltaic performance.533 In combination with PBDB-T, PCEs of 9.00% were obtained for the 9-1-based photovoltaic devices, while, for solar cells with PBDB-T/9-5, 9.87% are reached. Also, the compounds 9-3 (FTTCN) and 9-4 (FTTCN-M) with thiophene-based end groups have nearly identical optical and electrochemical properties, but the efficiencies of solar cells from blends with PBDB-T reach PCEs above 10%.534 Further improvement can be achieved if INCN-2F end groups are used. To that end, compound 9-2 (4TFIC-4F, BTTFIC4F-Ar) reaches a PCE above 11%, but its carbazole-based analogue 9-7 (4TCIC-4F) even 13.0%.535,536 Albeit, here a different donor polymer (PM7) was used than for compound 9-1. Fluorination of the end group results in a lower band gap and downshifted energy levels, as can be seen if acceptor 9-6 (CZTT-4F) is compared to its non-fluorinated analogue 9-5. Thus, solar cells with PBDB-T and 9-6 show lower VOC but higher JSC values, leading to a very similar overall PCE of 9.8–9.9%. However, a lower miscibility of the blend with PM6 is observed for 9-6, which leads to a beneficial polymer/NFA morphology in the bulk heterojunction film and a higher domain purity. Overall, this results in a PCE of 12.1% of PM6/9-6-based solar cells.533 Variations of the alkyl substituent on the carbazole nitrogen have a marginal influence on the optical and electrochemical properties of the molecule. This can be seen when 9-7 (4TC-4F-C8C8) is compared to 9-8 (4TC-4F-C6C8) and 9-9 (4TC-4F-C16). A linear alkyl chain (9-9) allows for the highest crystallinity, while the α- and β-branched chains have moderate and low crystallinities, respectively (9-7 and 9-8). This has a large influence on the PV performance in blends with the donor PBTIBDTT. The best PCE is achieved with α-branched 9-7 (12.1%), the lowest with the β-branched 9-8 (2.38%).537
Figure 20.
Structures of non-fullerene acceptors with nine fused rings.
Table 17. Optical, Electrical, and Photovoltaic Properties of the Non-Fullerene Acceptors Containing a Nine-Fused-Ring Structural Feature.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μec(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 9-1 | BTTFIC | –5.56 | –3.95 | 1.58 | PBDB-T | 1.5:1 | 0.94 | 17.7 | 54 | 9.00 | -/1.4 × 10–4 | (532) |
| 9-2 | 4TFIC-4F | –5.66 | –3.98 | 1.55 | PM7 | 1:1 | 0.93 | 16.6 | 73 | 11.2 | 1.7 × 10–4/1.9 × 10–4 | (535) |
| BTTFIC4F-Ar | –5.64 | –4.05 | 1.54 | PM7 | 1.5:1 | 0.95 | 19.5 | 64 | 11.8 | -/1.2 × 10–4 | (536) | |
| 9-3 | FTTCN | –5.54 | –3.95 | 1.56 | PBDB-T | 1:1 | 0.90 | 15.9 | 74 | 10.6 | -/6.6 × 10–4 | (534) |
| 9-4 | FTTCN-M | –5.53 | –3.91 | 1.57 | PBDB-T | 1:1 | 0.93 | 15.2 | 71 | 10.1 | -/5.5 × 10–4 | (534) |
| 9-5 | CZTT-IC | –5.54 | –3.97 | 1.56 | PBDB-T | 1:1 | 0.97 | 17.3 | 59 | 9.87 | -/8.6 × 10–4 | (533) |
| 9-6 | CZTT-4F | –5.61 | –4.11 | 1.48 | PM6 | 1:1.25 | 0.94 | 19.7 | 65 | 12.1 | -/1.9 × 10–3 | (533) |
| 9-7 | 4TCIC-4F | –5.53 | –3.95 | 1.51 | PM7 | 1:1 | 0.94 | 19.0 | 73 | 13.0 | 2.9 × 10–4/2.4 × 10–4 | (535) |
| 4TC-4F-C8C8 | –5.60 | –3.96 | 1.51 | PBTIBDTT | 1:1.5 | 0.90 | 18.6 | 72 | 12.1 | 2.3 × 10–4/2.0 × 10–4 | (537) | |
| 9-8 | 4TC-4F-C6C8 | –5.59 | –4.00 | 1.49 | PBTIBDTT | 1:1 | 0.84 | 6.75 | 42 | 2.38 | 7.2 × 10–5/1.5 × 10–6 | (537) |
| 9-9 | 4TC-4F-C16 | –5.53 | –3.96 | 1.48 | PBTIBDTT | 1:1.25 | 0.91 | 16.1 | 65 | 9.53 | 2.9 × 10–4/7.1 × 10–5 | (537) |
| 9-10 | DTTC-4F | –5.69 | –3.91 | 1.55 | PM6 | 1:1.1 | 0.95 | 21.7 | 68 | 13.9 | -/3.0 × 10–4 | (538) |
| 9-11 | DTTC-4Cl | –5.72 | –4.03 | 1.47 | PM6 | 1:1 | 0.92 | 22.6 | 74 | 15.4 | -/7.9 × 10–4 | (538) |
| 9-12 | INPIC | –5.36 | –3.82 | 1.46 | PBDB-T | 1:1 | 0.96 | 8.55 | 53 | 4.31 | 2.0 × 10–4/9.6 × 10–5 | (539) |
| 9-13 | INPIC-4F | –5.42 | –3.94 | 1.39 | PBDB-T | 1:1 | 0.85 | 21.6 | 72 | 13.1 | 1.3 × 10–3/5.0 × 10–4 | (539) |
| 9-14 | IPIC | –5.47 | –3.82 | 1.44 | PBDB-T | 1:1 | 0.95 | 7.16 | 59 | 3.98 | -/2.5 × 10–7 | (540) |
| 9-15 | IPIC-4F | –5.54 | –3.94 | 1.38 | PBDB-T | 1:1 | 0.84 | 19.8 | 67 | 11.1 | -/5.6 × 10–5 | (540) |
| 9-16 | IPIC-4Cl | –5.51 | –3.95 | 1.32 | PBDB-T | 1:1 | 0.81 | 22.2 | 74 | 13.4 | -/1.0 × 10–4 | (540) |
| 9-17 | 2F-C5 | –5.36 | –3.95 | 1.49 | PBDB-T | 1:1 | 0.77 | 19.6 | 67 | 10.2 | 2.4 × 10–4/1.3 × 10–4 | (541) |
| 9-18 | 2F-C6 | –5.36 | –3.97 | 1.48 | PBDB-T | 1:1 | 0.79 | 21.0 | 68 | 11.3 | 5.8 × 10–4/2.2 × 10–4 | (541) |
| 9-19 | 2F-C8 | –5.38 | –3.98 | 1.49 | PBDB-T | 1:1 | 0.80 | 21.3 | 72 | 12.3 | 7.9 × 10–4/2.6 × 10–4 | (541) |
| 9-20 | 2F-C10 | –5.37 | –3.97 | 1.49 | PBDB-T | 1:1 | 0.80 | 19.9 | 65 | 10.4 | 1.3 × 10–3/8.9 × 10–5 | (541) |
| 9-21 | INIC | –5.47 | –3.89 | 1.57 | PBD-SF | 1:1.4 | 1.05 | 9.89 | 49 | 5.1 | 9.0 × 10–5/2.1 × 10–5 | (542) |
| 9-22 | FINIC | –5.56 | –4.05 | 1.51 | PBD-SF | 1:1.4 | 0.87 | 22.0 | 73 | 14.0 | 8.3 × 10–4/4.5 × 10–4 | (542) |
| 9-23 | BCPT-4F | –5.36 | –3.91 | 1.32 | PBDB-T | 1:1 | 0.78 | 23.0 | 70 | 12.4 | -/2.9 × 10–4 | (543) |
| 9-24 | CPDT-4Cl | –5.32 | –3.78 | 1.35 | PTB7-Th | 1:1.5 | 0.74 | 23.2 | 69 | 11.9 | 3.6 × 10–4/3.1 × 10–4 | (544) |
| 9-25 | NNFA[0,6] | –5.27 | –3.86 | 1.38 | FTAZ | 1:1.4 | 0.84 | 19.8 | 57 | 9.52 | -/3.9 × 10–5 | (545) |
| 9-26 | NNFA[6,3] | –5.21 | –3.83 | 1.37 | FTAZ | 1:1.4 | 0.87 | 14.7 | 59 | 7.56 | -/7.2 × 10–6 | (545) |
| 9-27 | NNFA[6,6] | –5.23 | –3.84 | 1.37 | FTAZ | 1:1.4 | 0.87 | 19.9 | 61 | 10.6 | -/4.9 × 10–5 | (545) |
| 9-28 | NNFA[12,3] | –5.23 | –3.84 | 1.37 | FTAZ | 1:1.4 | 0.86 | 19.3 | 65 | 10.8 | -/5.7 × 10–5 | (545) |
| 9-29 | NNFA[12,6] | –5.21 | –3.83 | 1.37 | FTAZ | 1:1.4 | 0.86 | 18.9 | 59 | 9.54 | -/2.8 × 10–5 | (545) |
| 9-30 | TTC0-4F | –5.65b | –4.27b | 1.38 | PM6 | 1:1 | 0.81 | 19.5 | 67 | 10.6 | -/2.8 × 10–4 | (546) |
| 9-31 | TTC2-4F | –5.60b | –4.20b | 1.40 | PM6 | 1:1 | 0.84 | 20.3 | 67 | 11.4 | (546) | |
| 9-32 | TTC4-4F | –5.60b | –4.20b | 1.40 | PM6 | 1:1 | 0.88 | 21.0 | 68 | 12.7 | -/3.5 × 10–4 | (546) |
| 9-33 | TTC6-4F | –5.60b | –4.20b | 1.40 | PM6 | 1:1 | 0.89 | 21.7 | 69 | 13.2 | (546) | |
| 9-34 | TTC8-4F | –5.60b | –4.19b | 1.41 | PM6 | 1:1 | 0.90 | 22.4 | 69 | 14.0 | -/3.7 × 10–4 | (546) |
| 9-35 | TTC10-4F | –5.60b | –4.19b | 1.41 | PM6 | 1:1 | 0.90 | 22.6 | 65 | 13.2 | (546) | |
| 9-36 | TTC12-4F | –5.60b | –4.19b | 1.41 | PM6 | 1:1 | 0.91 | 22.3 | 62 | 12.5 | -/3.7 × 10–4 | (546) |
| 9-37 | TTC8-O1-4F | –5.42 | –3.86 | 1.31 | PBDB-T | 1:1 | 0.82 | 23.0 | 70 | 13.2 | -/2.5 × 10–4 | (547) |
| 9-38 | IN-4F | –5.56 | –3.99 | 1.38 | PM6 | 1:1 | 0.87 | 21.8 | 69 | 13.0 | 0.7 × 10–5/6.8 × 10–4 | (548) |
| 9-39 | ISI-4F | –5.65 | –4.01 | 1.43 | PM6 | 1:1 | 0.88 | 22.8 | 62 | 12.5 | -/4.3 × 10–4 | (549) |
| 9-40 | ISI-4Cl | –5.64 | –4.05 | 1.42 | PM6 | 1:1 | 0.87 | 20.5 | 68 | 12.1 | -/3.2 × 10–4 | (549) |
| 9-41 | FNIC2 | –5.56 | –4.00 | 1.38 | PTB7-Th | 1:1 | 0.74 | 23.9 | 73 | 13.0 | 1.7 × 10–3/1.4 × 10–3 | (550) |
| 9-42 | FNIC1 | –5.61 | –3.92 | 1.48 | PTB7-Th | 1:1 | 0.77 | 20.0 | 66 | 10.3 | 1.2 × 10–3/6.0 × 10–4 | (550) |
| 9-43 | TfIF-4F | –5.67 | –3.98 | 1.61 | PM7 | 1:1 | 0.97 | 18.5 | 78 | 14.0 | 3.7 × 10–4/3.5 × 10–4 | (551) |
| 9-44 | TfIF-4Cl | –5.71 | –4.09 | 1.57 | PM7 | 1:1 | 0.97 | 18.5 | 76 | 13.7 | 6.0 × 10–4/4.6 × 10–4 | (551) |
| 9-45 | IPY-T-IC | –5.64 | –3.48 | 1.71 | PTB7-Th | 1:1.5 | 0.91 | 13.1 | 52 | 6.19 | -/4.5 × 10–5 | (552) |
| 9-46 | IPY-T-ICF | –5.67 | –3.51 | 1.67 | PTB7-Th | 1:1.5 | 0.76 | 15.5 | 59 | 7.00 | -/9.7 × 10–5 | (552) |
| 9-47 | ITOT-4Cl | –5.49 | –3.90 | 1.28 | PBDB-T | 1:1 | 0.78 | 22.9 | 71 | 12.5 | -/1.8 × 10–4 | (553) |
| 9-48 | IDTODT-1 | –5.35 | –3.90 | 1.32 | PBDB-T | 1:1 | 0.80 | 14.5 | 59 | 6.87 | -/1.3 × 10–5 | (554) |
| 9-49 | IDTODT-2 | –5.38 | –3.95 | 1.34 | PBDB-T | 1:1 | 0.77 | 15.3 | 59 | 6.99 | -/6.1 × 10–5 | (554) |
| 9-50 | IDTODT-3 | –5.39 | –3.96 | 1.34 | PBDB-T | 1:1 | 0.79 | 17.3 | 61 | 8.34 | -/5.2 × 10–5 | (554) |
| 9-51 | DTPR-4Cl | –5.35 | –3.85 | 1.30 | PTB7-Th | 1:1.5 | 0.68 | 25.0 | 62 | 10.5 | 2.1 × 10–4/1.4 × 10–4 | (544) |
| 9-52 | X94FIC | –5.58 | –4.17 | 1.25 | PBDB-T | 1:1 | 0.73 | 14.7 | 66 | 7.08 | -/6.1 × 10–4 | (555) |
| 9-53 | X9IC | –5.53 | –4.06 | 1.29 | PBDB-T | 1:1 | 0.86 | 11.4 | 64 | 6.29 | -/6.2 × 10–4 | (555) |
| 9-54 | X9Rd | –5.51 | –3.80 | 1.45 | PBDB-T | 1:1 | 1.08 | 6.70 | 34 | 2.48 | -/6.6 × 10–4 | (555) |
| 9-55 | X9T4FIC | –5.51 | –4.10 | 1.29 | PBDB-T | 1:1 | 0.85 | 7.01 | 54 | 3.22 | -/5.2 × 10–4 | (555) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
Obtained via UPS.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Compound 9-10 (DTTC-4F) is very similar to 9-7, with the only difference being the use of octylphenyl instead of hexylphenyl side chains on the central core.538 Solar cells with PM6 gave PCE values of almost 14%. 9-10 was compared to the analogous compound 9-11 (DTTC-4Cl) containing an INCN-2Cl end group. The chlorinated 9-11 reveals a smaller optical band gap as well as slightly lower energy levels, stronger π–π interaction, and a significantly increased PCE of 15.4% for PM6/9-11-based solar cells (cf. Table 17).538
The NFAs 9-12 (INPIC) and 9-13 (INPIC-4F) are based on a donor core with two dithieno[3,2-b:2′,3′-d]pyrrole units annulated via a S-indacene unit (structural motif: T-P-T-Cp-B-Cp-T-P-T) and INCN as well as INCN-2F end groups, respectively.539 As expected, the fluorinated analogue has a narrower band gap of 1.39 eV, downshifted energy levels (−3.94 eV compared to −3.82 eV), a higher crystallinity, as well as a higher electron mobility. While solar cells based on PBDB-T/9-12 led to a moderate PCE of 4.31%, PBDB-T/9-13-based devices showed a PCE of 13.1%.539,556 Replacing the octyl chains on the pyrrole nitrogens in compounds 9-12 and 9-13 by 2-butyloctyl chains (compounds 9-14, IPIC, and 9-15, IPIC-4F) results in significantly lower electron mobilities (for blends with PBDB-T). Consequently, the solar cells yield lower PCEs. Nevertheless, replacing fluorine by chlorine in the end group as in compound 9-16 (IPIC-4Cl) restores the good electron mobility, and a remarkable PCE of 13.4% is achieved.540
Using dithienothiophene units instead of dithienopyrroles leads to a T-T-T-Cp-B-Cp-T-T-T motif.101 In a study by Zhao et al., the influence of increasing alkyl chain lengths was systematically investigated for pentyl (9-17, 2F-C5), hexyl (9-18, 2F-C6), octyl (9-19, 2F-C8), and decyl side chains (9-20, 2F-C10), as shown in Figure 20.541 Longer side chains increased the miscibility of the acceptor with the used polymer PBDB-T, which led to smaller domain sizes and lower domain purity. On the one hand, smaller domain sizes are beneficial for efficient exciton dissociation, but on the other hand, a too low domain purity leads to increased bimolecular recombination. Based on that, the highest PCE (12.3%) was obtained for the PBDB-T/9-19-based absorber layer with octyl side chains, which showed the best compromise of domain size and domain purity. Acceptor 9-21 (INIC) is structurally very similar to 9-18 but comprises unsubstituted INCN termini, leading to a higher optical band gap and a lower electron mobility. Therefore, solar cells with the donor PBD-SF yield a moderate PCE of 5.1%. Solar cells with the same donor and the difluorinated acceptor 9-22 (FINIC) exhibit a significantly increased PCE (14.0%). In comparison to devices of the structurally similar 9-18 with the PBDB-T donor, the largest improvement has been reached in VOC despite 9-22 having a lower LUMO level than 9-18.542
Exchanging the pyrrole ring in 9-13 with a cyclopentadiene ring leads to the conjugated structure of 9-23 (BCPT-4F).5439-23 showed a low band gap of 1.32 eV, and solar cells based on this acceptor and PBDB-T gave PCE values of 12.4%. Structurally similar 9-24 (CPDT-4Cl) has upshifted energy levels and an increased optical band gap (1.35 eV). Solar cells with PTB7-Th yield an overall similar performance (PCE: 11.9%).544
The influence of alkyl chain lengths was investigated in NFAs with a benzodithiophene-based central core, flanked by two cyclopentathienothiophene units (T-T-Cp-T-B-T-Cp-T-T) and INCN-F acceptor units (9-25–9-29).545 Alkyl chains were introduced on the central core (in-plane) and on the phenyl side chains (out-of-plane). All molecules had very similar optical and electrochemical properties, but the solubility was varying between 23 and 226 mg mL–1. Solar cells with the FTAZ donor achieved the best PCE values (10.8%) using 9-28 (NNFA[12,3]), which bears dodecyl chains in-plane and propyl side chains out-of-plane. In another study, various alkyl chains were introduced on the peripheral thiophene unit, keeping all other groups the same: octyl (central benzene) and hexylphenyl (on the Cp-rings) side chains and INCN-2F groups.546 Alkyl chains, regardless of their length (ethyl to dodecyl, compounds 9-31–9-36, TTCn-4F), slightly lower the energy levels and increase the optical band gap compared to the unsubstituted 9-30, TTC0-4F (1.40 eV vs 1.38 eV). Nevertheless, solar cells of alkyl derivatives (blends with PM6) show an improved electron mobility and high PCEs (11–14%).546 Replacing the octyl chains on the core with octyloxy groups gives compound 9-37 (TTC8-O1-4F),547 which shows a narrower optical band gap of 1.31 eV and a similar electron mobility of 2.5 × 10–4 cm2 V–1 s–1. In the acceptors 9-38 (IN-4F) and 9-39 (ISI-4F), a silyl-based side chain is introduced via thiophene (9-38) or triple bond (9-39) spacers. Compared to the octyl-substituted compound 9-30, the LUMO energies are higher. Thus, devices with PM6 have increased VOC values and due to a higher electron mobility also slightly higher JSC values, leading to overall improved PCEs of 13.0% (9-38)548 and 12.5% (9-39). Replacing fluorine atoms in the end group by chlorine (9-40, ISI-4Cl) does not improve the PCE (12.1%).549 Wang et al. studied the NFA 9-41 (FNIC2), bearing a hexylthiophene substituent at the central benzene unit and its isomer 9-42 (FNIC1) with the structure T-Cp-T-T-B-T-T-Cp-T.5509-41 exhibits a lower band gap of 1.38 eV and a higher electron mobility (1.7 × 10–3 cm2 V–1 s–1) compared to 9-42 (1.48 eV, 1.2 × 10–3 cm2 V–1 s–1). While consequently the VOC of PTB7-Th/9-41-based solar cells is 30 mV lower, the JSC is increased from 20.0 to 23.9 mA cm–2. This, together with a higher FF, leads to an increase in the PCE from 10.3% (PTB7-Th/9-42) to 13.0% (PTB7-Th/9-41).
A further nonacyclic structure is based on indenofluorene flanked by cyclopenta[b]thiophene units (T-Cp-B-Cp-B-Cp-B-Cp-T) at both sides and either INCN-2F or INCN-2Cl acceptor groups, as realized in 9-43 (TfIF-4F) and 9-44 (TfIF-4Cl).551 The chlorinated NFA shows a lower band gap, deeper energy levels, and better molecular packing. However, the photovoltaic properties of PM7/9-43- and PM7/9-44-based devices were rather similar, yielding PCEs of 14.0 and 13.7%, respectively. The solar cells based on the chlorinated analogue revealed a low layer thickness dependence of the PCE due to the higher electron mobility.551 Exchanging the central benzene ring with a pyrazine yields 9-45 (IPY-T-IC) and its end group fluorinated analogue 9-46 (IPY-T-ICF). NFA 9-46 has a significantly higher LUMO, and due to unchanged HOMO energy, a higher optical band gap is observed compared to 9-43. Solar cells of PTB7-Th/9-46 blends yielded PCEs of 7.00%.552
The dithienopyrane moieties containing acceptor 9-47 (ITOT-4Cl) have a low optical band gap (1.28 eV) and a good electron mobility in blend with PBDB-T. Due to its low LUMO energy, solar cells are not reaching high VOC values (0.78 V), but the low band gap and good electron mobility contribute to a high JSC (22.9 mA cm–2) and thus decent PCEs (12.5%).553 Acceptor 9-48 (IDTODT-1) also is comprised of a dithienopyrane moiety, but the oxygen atoms are located in the outer direction, i.e., closer to the periphery of the central core. Since INCN-F groups are used instead of INCN-2F, one cannot draw a conclusion about how the position of the oxygen atom influences the molecular properties. Nevertheless, compound 9-48 has the same LUMO energy as 9-47, but the optical band gap is higher (by 40 meV) and the electron mobility is one order of magnitude lower. Thus, solar cells with PBDB-T have much lower JSCs (14.5 mA cm–2), smaller FFs (59% vs 71%), and therefore reduced PCEs (6.87%). The morphology of the blend can be improved by side chain modifications; thus, both 9-49 (IDTODT-2) and 9-50 (DITODT-3) have an improved electron mobility and a higher PCE (6.99 and 8.34%, respectively).554 The acceptor 9-51 (DTPR-4Cl) has INCN-2Cl end groups and in comparison to 9-47 different side chains, leading to a slightly higher LUMO and a larger optical band gap. Good electron mobility ensures high JSC values for its devices with PTB7-Th (25.0 mA cm–2), but a low VOC (0.68 V) and FF (62%) limits the PCE (10.5%).544
Similar to the Y-series, the A–(DA′D)–A approach was introduced in nonacyclic NFAs, as realized in structures 9-52–9-55 by the insertion of additional benzene rings replacing the thienothiophenes with benzodithiophene units.555 These structures comprise different acceptor termini, 9-52 (X94FIC) INCN-2F (analogue to Y6, 7-145), 9-53 (X9IC) INCN (analogue to Y5, 7-142), and 9-54 (X9Rd) an RCN end group (analogue to TPBT-RCN, 7-200). 9-55 (X9T4FIC) bears additional thiophene π-spacers between the core unit and the INCN-2F termini. Within this series, 9-52 showed the highest performance in solar cells (PCE: 7.08%) in combination with PBDB-T. While 9-52, 9-53, and 9-55 revealed rather similar optical band gap values between 1.25 and 1.30 eV, 9-54 possesses an optical band gap of 1.45 eV due to a significantly upshifted LUMO level (−3.80 eV).555
8. Aromatic Ring Systems with 10–13 Fused Rings
A majority of the reported NFAs containing fused decacyclic aromatic systems as a core unit are based on two fused IDT units (see Figure 21). In 10-1 (p-IDTIDT-IC) and 10-2 (R10-Cl), the double IDT unit contains para-hexylphenyl side chains and INCN (10-1) or INCN-2Cl end groups (10-2).557,558 The INCN-2Cl units lead to a reduced optical band gap (1.43 eV compared to 1.53 eV for 10-1, see also Table 18). In combination with PTB7-Th, solar cells of 10-1 led to a PCE of 6.48%, while PBDB-T/10-2-based devices reached PCE values of 10.7%.557,558
Figure 21.
Structures of non-fullerene acceptors with 10–13 fused rings.
Table 18. Optical, Electrical, and Photovoltaic Properties of the Non-Fullerene Acceptors Containing 10- and 12-Fused-Ring Structural Features.
| NFA | original name | HOMOa (eV) | LUMOa (eV) | Egopt (eV) | donor | D:A ratio | VOC (V) | JSC(mA cm–2) | FF (%) | PCE (%) | μeb(cm2 V–1 s–1) | ref. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 10-1 | p-IDTIDT-IC | –5.42 | –3.82 | 1.53 | PTB7-Th | 1:1.5 | 0.94 | 14.5 | 48 | 6.48 | 5.3 × 10–6/- | (557) |
| 10-2 | R10-Cl | –5.35 | –3.91 | 1.43 | PBDB-T | 0.85 | 18.9 | 67 | 10.7 | -/2.3 × 10–5 | (558) | |
| 10-3 | m-IDTIDT-FIC | –5.47 | –3.94 | 1.51 | J71 | 1:1.5 | 0.92 | 18.0 | 68 | 11.3 | 2.8 × 10–5/1.9 × 10–4 | (557) |
| 10-4 | FBTIC | –5.70 | –3.85 | 1.70 | PM6 | 1:1 | 0.95 | 14.1 | 75 | 10.1 | 1.7 × 10–3/1.2 × 10–3 | (559) |
| 10-5 | B3T-TT-6F | –5.42 | –4.04 | 1.59 | PBDB-T | 1:1.5 | 0.82 | 18.3 | 66 | 9.94 | -/1.1 × 10–4 | (560) |
| 10-6 | FPIC | –5.75 | –3.97 | 1.63 | PTB7-Th | 1:2 | 0.76 | 15.3 | 73 | 8.45 | 1.7 × 10–3/4.7 × 10–4 | (561) |
| 10-7 | F10IC2 | –5.53 | –3.97 | 1.35 | PTB7-Th | 1:1 | 0.77 | 23.8 | 69 | 12.5 | -/1.2 × 10–4 | (562) |
| 10-8 | F10IC1 | –5.35 | –3.96 | 1.29 | PTB7-Th | 1:1.2 | 0.72 | 23.4 | 72 | 12.3 | -/6.1 × 10–4 | (486) |
| 11-1 | FUIC | –5.31 | –4.06 | 1.22 | PTB7-Th | 1:1.5 | 0.69 | 22.9 | 71 | 11.2 | 6.8 × 10–4/3.0 × 10–4 | (563) |
| 11-2 | i-FUIC | –5.31 | –3.99 | 1.28 | PTB7-Th | 1:1.5 | 0.78 | 20.9 | 63 | 10.3 | 3.0 × 10–4/1.4 × 10–4 | (563) |
| 11-3 | IUIC | –5.45 | –3.87 | 1.41 | PTB7-Th | 1:1.5 | 0.80 | 21.7 | 65 | 11.2 | 1.1 × 10–3/5.5 × 10–4 | (564) |
| 11-4 | IUIC2 | –5.32 | –3.86 | 1.25 | PTB7-Th | 1:1.5 | 0.76 | 11.0 | 53 | 4.48 | 3.9 × 10–4/1.4 × 10–5 | (371) |
| 12-1 | LC81 | –5.49 | –3.97 | 1.45 | PBT1-C | 1:1.1 | 0.88 | 19.8 | 73 | 12.7 | 7.0 × 10–4/5.5 × 10–4 | (565) |
| 12-2 | R12-4Cl | –5.26 | –3.95 | 1.35 | PBDB-T | 0.75 | 18.5 | 67 | 9.3 | -/2.1 × 10–5 | (558) | |
| 13-1 | BTTCTT-ICF | –5.64 | –4.08 | 1.54 | PM6 | 1:1.5 | 0.91 | 16.8 | 68 | 10.4 | -/1.6 × 10–5 | (566) |
| B3T-BT-6F | –5.62 | –4.03 | 1.69 | PBDB-T | 1:1.2 | 0.81 | 16.5 | 63 | 8.40 | -/1.2 × 10–4 | (560) |
Obtained from the oxidation/reduction potential of the CV measurement if not otherwise stated.
Determined via the SCLC technique from the neat acceptor/donor:acceptor blend films if not otherwise stated.
Compared to that, 10-3 (m-IDTIDT-FIC) bears meta-hexylphenyl side chains and INCN-2F termini.557 The difference in the position of the alkylphenyl side chain has a significant effect on the molecular packing and thus on the electron mobility. The meta-alkylphenyl side chains in 10-3 lead to a smaller π–π stacking distance, an increased crystalline coherence length, and a higher mobility. This is also reflected in the PCE of the respective solar cells, which is increased from 6.48 to 8.27% due to a significantly enhanced FF when 10-3 is used instead of 10-1 in combination with PTB7-Th. By replacing PTB7-Th with J71, the PCE of solar cells using 10-3 could be further improved to 11.3%.557
The star shaped acceptors 10-4 (FBTIC) in solar cells with PM6 reach a good VOC (0.95 V) and fill factor (75%). However, this blend has a narrow spectral response (<750 nm) which limits the JSC value (14.1 mA cm–2). Nevertheless, a PCE of 10.1% is reached.559 The spectral response is improved if a thiophene linker is incorporated between the central core and the end groups as in acceptor 10-5 (B3T-TT-6F). However, devices of 10-5 with PBDB-T have a lower electron mobility and thus lower JSC values (18.3 mA cm–2), resulting in lower PCEs (9.94%).560 A similar problem can be observed for the acceptor 10-6 (FPIC), which reaches 8.45% PCE when blended with PTB7-Th. The solar cells have reasonable VOC and FF values (0.76 V, 73%), and thus, the PCE seems mainly limited by the lack of the blend’s absorption above 800 nm.561 Compound 10-7 (F10IC2) is structurally similar, but the central core is made electron rich by replacing two of the benzene rings with thiophene and adding alkyloxy substituents to the other two benzene rings. This lowers the HOMO energy, resulting in a narrower band gap (1.35 eV). Since the LUMO energies of both acceptors are the same, also blends with PTB7-Th have the same VOC. The slightly lower electron mobility and FF (69%) are compensated by the large JSC (23.8 mA cm–2), yielding a good PCE of 12.5%.562
Acceptor 10-8 (F10IC1) combines a bis(thienothiophene) unit with two cyclopentathienothiophenes (T-T-Cp-T-T-T-T-Cp-T-T motif) and INCN-2F end groups and yields the smallest optical band gap for the fused 10-ring systems covered here. Solar cells with PTB7-Th reach good JSC (23.4 mA cm–2) and PCE values (12.3%). Its analogues with eight-fused-ring (8-12) and six-fused-ring (6-13) central cores reach lower efficiencies (10.7 and 7.00%, respectively).486 Incorporation of one more cyclopentadiene moiety in the centers gives the 11-ring system 11-1 (FUIC).563 Despite the very low optical band gap (1.22 eV), devices with PTB7-Th do not reveal an improved JSC (22.9 mA cm–2) and yield PCE values up to 11.2%. A reason for this could be the lower electron mobility. Linking one of the end groups to the β-carbon of the central core’s last thiophene gives the asymmetric 11-2 (i-FUIC). This acceptor has a higher LUMO level; thus, solar cells with PTB7-Th reach higher VOC values than for 11-1 (0.78 V vs 0.69 V), but the PCEs were not improved (10.3%).563
11-3 (IUIC) derives from 7-4 (ITIC-4F) and contains an extended conjugated core with the motif T-Cp-T-T-Cp-B-Cp-T-T-Cp-T. This leads to a smaller band gap, a higher charge carrier mobility, and an improved photovoltaic performance of 11.2% using PTB7-Th as a polymer.564 However, 11-4 (IUIC2), which is an extended version of 5-24 (IDIC-4F) with the structure T-T-Cp-T-Cp-B-Cp-T-Cp-T-T, led to neither increased charge carrier mobility nor improved PCEs, whereas the unsymmetrically extended compound 8-21 showed better mobility and a significantly higher PCE of 13.7%.371
The reported 12-fused-ring systems reveal a similarly replicated IDT structure as the aforementioned 10-fused-ring systems with additional peripheral thiophenes on both sides.565 Acceptor 12-1 (LC81) has INCN-2F end groups,565 whereas NFA 12-2 (R12-4Cl) comprises INCN-2Cl units.558 The chlorinated analogue 12-2 has a narrower optical band gap of 1.35 eV, while the LUMO energy levels of both acceptors are similar. Solar cells with 12-1 and PBT1-C reach efficiencies up to 12.7%, whereas those of 12-2 in combination with PBDB-T give 9.3%. The 10-fused-ring analogue 10-2 with the donor PBDB-T reached a PCE of 10.7%, serving as an example where an acceptor with a shorter conjugation length performs better than its higher analogue.558
The star shaped acceptor 13-1 (BTTCTT-ICF/B3T-BT-6F) is based on a truxene core and consists of 13 fused rings. The truxene core is not a strong electron donor; thus, despite the large π-system, this molecule has a rather high optical band gap. Blends with PBDB-T yield 8.40% PCE. This result is slightly outperformed by 10-5 (PCE 9.94%). Both molecules have the same number of conjugated rings, but the terminal thiophene unit is not fused to the central core, which is beneficial to the phase morphology.560 At the same time, blends of 13-1 with PM6 reach a higher PCE (10.4%) with the main improvement being found in the higher VOC (0.91 V vs 0.81 V for the blend with PBDB-T).566
9. Conclusion and Outlook
Summarizing the large number of fused-ring molecules reported in the last three years enables the possibility of making more general conclusions about non-fullerene acceptor design. Some of the design strategies are universal to all of the acceptors discussed in this Review. For example, the length and sterical bulk of the side chains allows tuning of the material processability and blend morphology. Here, linear alkyl chains are to be favored for an increased crystallinity, while bulky (such as phenylhexyl) or branched units (ethylhexyl) are preferable for a decreased crystallinity. Longer alkyl chains often facilitate the processability. However, when choosing the side chains, also the properties of the desired donor material need to be taken into account, i.e., to have appropriate miscibility and phase separation of both materials in the absorber layer blend.
The data covered in this Review reveal the trend that NFAs with larger fused-ring systems (≥6 rings), as can be seen in Figure 22A, show a higher fraction of PCE values above 10% compared to solar cells based on five-fused-ring NFAs, but they also reveal that PCEs above 15% are mainly reached with seven-fused-ring systems, in particular with NFAs related to the Y-series (highlighted in magenta), whereas only two compounds containing a six-fused-ring core and one with a nine-fused-ring core show PCEs above 15%. The photovoltaic performance of all NFAs belonging to the groups of 5-, 8-, and 10+-fused-ring systems stays below this threshold.
Figure 22.
(A) PCEs obtained with the NFAs covered in this Review grouped by the number of fused rings in their donor core (entries of the Y-series are marked in magenta), (B) PCEs obtained with NFAs bearing different commonly used end groups, and (C) conjugated polymers applied in non-fullerene OSCs with PCEs exceeding 11%. (D) ΔHOMO versus ΔLUMO levels of donor and acceptor; the data of representative donor:acceptor combinations from each core size revealing PCEs above 11% are shown. (E) Eloss versus ΔLUMO levels and (F) Eloss versus ΔHOMO levels of donor and acceptor. The respective PCE ranges of the solar cells are indicated by the color scale.
This is an interesting fact, as generally an increased conjugation length of the fused-ring core leads to narrower band gaps, upshifted HOMO and LUMO levels, as well as increased charge carrier mobility due to enhanced molecular packing, which is beneficial for the performance in solar cells. However, the length of the fused-ring core also significantly influences the solubility of the compounds and the phase morphology of the donor and the acceptor. Therefore, an extension of the conjugation length does not automatically lead to increased PCE values. In this regard, some studies reveal that it is more difficult to obtain sufficient solubility or ideal phase morphology with the NFAs containing larger conjugated fused-ring cores.371,372
Moreover, often the preparation of these larger conjugated fused-ring systems is synthetically more demanding compared to smaller ones and the synthesis routes of many five- and seven-fused-ring acceptors are well established, with many building blocks being commercially available. As, in addition, the majority of the reported seven-ring systems combined with a suitable donor polymer can reach PCEs above 10% (65% of the seven-ring compounds in this Review) and the current record efficiencies are obtained with NFAs of this core size, it comes as no surprise that in the last years seven-fused-ring systems have been studied and applied in solar cells very frequently.
The herein discussed acceptor systems usually comprise an A–D–A-type structure. By analyzing the solar cell data of devices with PCE values of more than 11%, there is a rather large variety of different electron-donating cores with the A–DA′D–A structure of the Y-series as the most promising motifs. However, also other seven-ring-based cores, in particular, asymmetric structures with the motif T-Cp-B-Cp-T-P-T, as well as lower- and higher-numbered fused-ring molecules gave attractive values, whereby IDT-based compounds with thiophene π-spacers, asymmetric derivatives of Y-series structures, or some carbon–oxygen-bridged NFAs can be highlighted in this context.
A further important aspect to note is that the high photovoltaic performance of certain NFAs often cannot only be ascribed to their well suited optical and electronic properties, but also photophysical and structural peculiarities play crucial roles. Regarding Y6, one of the currently most efficient NFAs, major reasons for the low energy losses in solar cells comprising the Y6 acceptor have been found to be the conformational rigidity and uniformity. The alkyl chains at the outer thiophene units hinder the rotation of the end groups, leading to lower charge carrier trapping into electronic intra-bandgap tail states.567 Moreover, exciton delocalization, charge transfer, and transport in Y6 are facilitated due to a particular crystal packing of the Y6 molecules.568,569 This example clearly shows the intricate correlations of device physics with the molecular design as well as the crystalline structures of the NFAs and the importance of understanding these correlations for the further development of this field.
Furthermore, a large amount of research in this field in the recent years has been devoted to modifications of the end groups and a huge variety of different structures have been reported. In almost all highly efficient solar cells, which reveal PCE values above 11% (see Figure 22B), the used acceptors comprise INCN-based accepting groups, especially halogenated INCN units. Alternatively, acceptors with the thiophene analogue, the CPTCN motif, are much less investigated but also give promising solar cell efficiencies up to 15%. There are only three other electron-accepting units (a simple malonitrile-based end group, as well as RCN and TBA derivatives, summarized as “others” in Figure 22B) in this category, which reached PCEs up to 13.5% in solar cells.
Although not being the focus of this Review, the donor materials, in most cases a conjugated polymer, are of the same importance and a large number of materials have been introduced during the last years. By analyzing the solar cell data discussed in this Review, it is obvious that most of the solar cell optimization was carried out using only a few polymers. Figure 22C shows all donor materials used in solar cells with PCEs >11%. Within this category, PBDB-T-type polymers (such as PBDB-T, PM6, and PM7) are the most often used donor materials. Out of about 350 reported non-fullerene acceptors leading to solar cell efficiencies above 11%, approximately 280 contained a PBDB-T-type polymer in the absorber layer. Moreover, several of these solar cells showed PCEs above 17%. A higher PCE (18.2%) could only be otained with the recently designed donor material D18. Alternatively, polymers of the J-series and PTB7-Th have been used reaching PCEs up to 14%. Furthermore, it should be mentioned that PTQ10 is a very promising donor, especially due to its simple structure (synthetic considerations) and solar cells have revealed already very good performance.
As discussed above, it is generally accepted that both the LUMO and HOMO energies of the acceptor should be bigger than those of the donor to facilitate exciton separation. Generally, a value of 0.3 eV was postulated and this empirical driving energy has been regularly questioned for non-fullerene solar cells.570 Thus, we have extracted the LUMO–LUMO and HOMO–HOMO offsets of various NFA-conjugated polymer combinations from each of the fused-ring core sizes discussed in this Review (PCE values above 10%, representative examples from each chapter, the HOMO and LUMO values of both the donor and the acceptor have been taken from the respective articles) and have compared these data in regard to the power conversion efficiency and the energy loss (band gap of the solar cell - VOC). Figure 22D correlates the donor/acceptor HOMO and LUMO offsets of these highly efficient solar cells. The entries are grouped and color-coded regarding their efficiency. In general, high energy level offsets lead to lower energy losses within the charge separation process; however, a high offset in the HOMO values deteriorates the maximum possible VOC. Thus, by looking at the data, the differences in the HOMO offsets are generally small and exhibit values below 0.25 eV for the most efficient solar cells (PCEs above 15%). There are even data showing a negligible offset; however, as there are certain uncertainties in the determination of the exact orbital energies, this has to still be verified by further investigations. In contrast, the LUMO offset values vary in a larger range from 0.15 up to 1.4 eV. This supports the picture that the HOMO offset values are more crucial to reach higher PCE values due to the correlation with the VOC. Parts E and F of Figure 22 contain the energy loss data (optical band gap minus VOC) plotted against HOMO and LUMO orbital offsets, respectively. The approach of using the optical band gap of the component in the absorber layer with the lowest band gap for the calculation was chosen as an approximation for using the inflection point of the onset of the EQE spectrum.571 As expected, the best solar cells have a low energy loss (<0.6 eV). The distribution of orbital offsets is much smaller for the HOMO, up to 0.5 eV, with the best solar cells having values below 0.3 eV. Regarding the LUMO, the orbital offset distribution is much broader (up to 1.5 eV) and highly efficient cells can be found over the entire range.
Finally, we analyzed the dependence of the characteristic solar cell parameters on the band gap of the absorber layer. Parts A–C of Figure 23 show the JSC, VOC, and PCE plotted against the optical band gap of the longest wavelength absorbing component of the devices. Moreover, the respective data points are color coded to additionally show the PCE (Figure 23A,B) and the FF (Figure 23C) of the respective solar cell. It can be seen from Figure 23A that only absorber layer blends with band gaps below 1.7 eV yield solar cells with efficiencies above 12% and particularly high JSCs (>25 mA cm–2) have been obtained for active layers made of acceptors with band gaps around 1.45 eV. The maximal open circuit voltage the solar cell can reach is reduced (Figure 23B) when lower band gap absorbers are used. Therefore, highly efficient solar cells with VOCs above 1 V are hardly obtained. As is shown in Figure 23D, all solar cells with PCEs above 15% exhibit very similar photovoltages ranging between 0.80 and 0.90 V.
Figure 23.
(A) JSC, (B) VOC, and (C) PCE of the solar cells covered in this Review plotted vs the optical band gap of the component with the lower band gap in the absorber layer. While in parts A and B the data points are color coded by the PCE of the solar cells, in part C, they are color coded according to the respective FF value. The line represents the theoretical Shockley–Queisser limit. (D) VOC plotted against the PCE with the JSC value in the color code information.
In the graphs in Figure 23A and B, also the maximum theoretical JSC and VOC values according to the Shockley–Queisser (SQ) limit are included to indicate the distance of the already obtained values in organic solar cells compared to the maximum achievable ones.3,572,573 From both graphs, it becomes clear that the currently most efficient solar cells have band gaps between 1.30 and 1.40 eV. In both values, VOC and JSC, OPV devices are reaching up to 80% of the theoretical limit. These differences to the SQ limits in VOC and JSC are currently larger than in other PV technologies such as GaAs and Si but also perovskite solar cells.3 This might be caused by the intrinsic donor–acceptor heterojunction of OPVs with expected higher VOC and recombination losses compared to other technologies. The FF values of the best OPV devices are also reaching approx. 80% of the theoretical limit, and thus, overall, the best efficiencies of organic solar cells are currently slightly above 50% of the SQ limit (see Figure 23C).
Despite the significant progress in the last decade, organic photovoltaics have yet to reach and to exceed the milestone efficiency of 20%. An even more detailed understanding of the materials, processes, and mechanisms involved in non-fullerene organic solar cells will be very beneficial to guide material design toward this ambitious target. Besides, the power conversion efficiency, aspects such as device stability, and the scalability of materials synthesis and coating processes become increasingly important in view of the applicability of OPV and its commercialization. In particular, some issues have to be addressed in the acceptor molecule design and synthesis, such as the research toward simple, large scale, and high yield synthesis methods for the most efficient non-fullerene acceptor molecules. Regarding the stability of NFAs, the majority of compounds reported in this Review have a good thermal stability: however, this is not always associated with good (photo)chemical stability. Thus, improving the latter is another crucial hurdle, which has to be overcome in NFA design. Finally, a facile and reproducible processability and also the use of green solvents in the coating processes are crucial aspects to be taken into account in future research on non-fullerene acceptors.
Acknowledgments
Financial support by the Austrian Climate and Energy Fund within the program Energy Emission Austria (Project: ALTAFOS, FFG No. 865 072) and the Austrian Science Fund (FWF) via Meitner-Programm grant M 2738 is gratefully acknowledged.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
The authors declare no competing financial interest.
References
- Liu Q.; Jiang Y.; Jin K.; Qin J.; Xu J.; Li W.; Xiong J.; Liu J.; Xiao Z.; Sun K.; Yang S.; Zhang X.; Ding L. 18% Efficiency organic solar cells. Sci. Bull. 2020, 65, 272–275. 10.1016/j.scib.2020.01.001. [DOI] [PubMed] [Google Scholar]
- Green M.; Dunlop E.; Hohl-Ebinger J.; Yoshita M.; Kopidakis N.; Hao X. Solar cell efficiency tables (version 57). Prog. Photovoltaics 2021, 29, 3–15. 10.1002/pip.3371. [DOI] [Google Scholar]
- Almora O.; Baran D.; Bazan G. C.; Berger C.; Cabrera C. I.; Catchpole K. R.; Erten-Ela S.; Guo F.; Hauch J.; Ho-Baillie A. W. Y.; Jacobsson T. J.; Janssen R. A. J.; Kirchartz T.; Kopidakis N.; Li Y.; Loi M. A.; Lunt R. R.; Mathew X.; McGehee M. D.; Min J.; Mitzi D. B.; Nazeeruddin M. K.; Nelson J.; Nogueira A. F.; Paetzold U. W.; Park N.-G.; Rand B. P.; Rau U.; Snaith H. J.; Unger E.; Vaillant-Roca L.; Yip H.-L.; Brabec C. J. Device Performance of Emerging Photovoltaic Materials (Version 1). Adv. Energy Mater. 2021, 11, 2002774. 10.1002/aenm.202002774. [DOI] [Google Scholar]
- Colsmann A.; Röhm H.; Sprau C. Shining Light on Organic Solar Cells. Sol. RRL 2020, 4, 2000015. 10.1002/solr.202000015. [DOI] [Google Scholar]
- Qin J.; Lan L.; Chen S.; Huang F.; Shi H.; Chen W.; Xia H.; Sun K.; Yang C. Recent Progress in Flexible and Stretchable Organic Solar Cells. Adv. Funct. Mater. 2020, 30, 2002529. 10.1002/adfm.202002529. [DOI] [Google Scholar]
- Fukuda K.; Yu K.; Someya T. The Future of Flexible Organic Solar Cells. Adv. Energy Mater. 2020, 10, 2000765. 10.1002/aenm.202000765. [DOI] [Google Scholar]
- Sampaio P. G. V.; González M. O. A.; Oliveira Ferreira P.; Cunha Jácome Vidal P.; Pereira J. P. P.; Ferreira H. R.; Oprime P. C. Overview of printing and coating techniques in the production of organic photovoltaic cells. Int. J. Energy Res. 2020, 44, 9912–9931. 10.1002/er.5664. [DOI] [Google Scholar]
- Park S.; Kim T.; Yoon S.; Koh C. W.; Woo H. Y.; Son H. J. Progress in Materials, Solution Processes, and Long-Term Stability for Large-Area Organic Photovoltaics. Adv. Mater. 2020, 32, 2002217. 10.1002/adma.202002217. [DOI] [PubMed] [Google Scholar]
- Wang G.; Adil M. A.; Zhang J.; Wei Z. Large-Area Organic Solar Cells: Material Requirements, Modular Designs, and Printing Methods. Adv. Mater. 2019, 31, 1805089. 10.1002/adma.201805089. [DOI] [PubMed] [Google Scholar]
- Gusain A.; Thankappan A.; Thomas S. Roll-to-roll printing of polymer and perovskite solar cells: compatible materials and processes. J. Mater. Sci. 2020, 55, 13490–13542. 10.1007/s10853-020-04883-1. [DOI] [Google Scholar]
- Brus V. V.; Lee J.; Luginbuhl B. R.; Ko S.-J.; Bazan G. C.; Nguyen T.-Q. Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance. Adv. Mater. 2019, 31, 1900904. 10.1002/adma.201900904. [DOI] [PubMed] [Google Scholar]
- Hu Z.; Wang J.; Ma X.; Gao J.; Xu C.; Yang K.; Wang Z.; Zhang J.; Zhang F. A critical review on semitransparent organic solar cells. Nano Energy 2020, 78, 105376. 10.1016/j.nanoen.2020.105376. [DOI] [Google Scholar]
- Dai S.; Zhan X. Nonfullerene Acceptors for Semitransparent Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1800002. 10.1002/aenm.201800002. [DOI] [Google Scholar]
- La Notte L.; Giordano L.; Calabrò E.; Bedini R.; Colla G.; Puglisi G.; Reale A. Hybrid and organic photovoltaics for greenhouse applications. Appl. Energy 2020, 278, 115582. 10.1016/j.apenergy.2020.115582. [DOI] [Google Scholar]
- Mainville M.; Leclerc M. Recent Progress on Indoor Organic Photovoltaics: From Molecular Design to Production Scale. ACS Energy Lett. 2020, 5, 1186–1197. 10.1021/acsenergylett.0c00177. [DOI] [Google Scholar]
- Cui Y.; Hong L.; Hou J. Organic Photovoltaic Cells for Indoor Applications: Opportunities and Challenges. ACS Appl. Mater. Interfaces 2020, 12, 38815–38828. 10.1021/acsami.0c10444. [DOI] [PubMed] [Google Scholar]
- Hashemi S. A.; Ramakrishna S.; Aberle A. G. Recent progress in flexible–wearable solar cells for self-powered electronic devices. Energy Environ. Sci. 2020, 13, 685–743. 10.1039/C9EE03046H. [DOI] [Google Scholar]
- Yu K.; Rich S.; Lee S.; Fukuda K.; Yokota T.; Someya T. Organic Photovoltaics: Toward Self-Powered Wearable Electronics. Proc. IEEE 2019, 107, 2137–2154. 10.1109/JPROC.2019.2929797. [DOI] [Google Scholar]
- Subramaniam S. V.; Kutsarov D.; Sauermann T.; Meier S. B. Commercialization of Organic Photovoltaics and Its Product Integration: A Perspective Focusing on Durability. Energy Technol. 2020, 8, 2000234. 10.1002/ente.202000234. [DOI] [Google Scholar]
- Vijayan R.; Azeez A.; Narayan K. S. Toward reliable high performing organic solar cells: Molecules, processing, and monitoring. APL Mater. 2020, 8, 040908. 10.1063/5.0002979. [DOI] [Google Scholar]
- Speller E. M.; Clarke A. J.; Luke J.; Lee H. K. H.; Durrant J. R.; Li N.; Wang T.; Wong H. C.; Kim J.-S.; Tsoi W. C.; Li Z. From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells. J. Mater. Chem. A 2019, 7, 23361–23377. 10.1039/C9TA05235F. [DOI] [Google Scholar]
- Duan L.; Uddin A. Progress in Stability of Organic Solar Cells. Adv. Sci. 2020, 7, 1903259. 10.1002/advs.201903259. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Wang Y.; Lee J.; Hou X.; Labanti C.; Yan J.; Mazzolini E.; Parhar A.; Nelson J.; Kim J.-S.; Li Z. Recent Progress and Challenges toward Highly Stable Nonfullerene Acceptor-Based Organic Solar Cells. Adv. Energy Mater. 2021, 11, 2003002. 10.1002/aenm.202003002. [DOI] [Google Scholar]
- Angmo D.; Gevorgyan S. A.; Larsen-Olsen T. T.; Søndergaard R. R.; Hösel M.; Jørgensen M.; Gupta R.; Kulkarni G. U.; Krebs F. C. Scalability and stability of very thin, roll-to-roll processed, large area, indium-tin-oxide free polymer solar cell modules. Org. Electron. 2013, 14, 984–994. 10.1016/j.orgel.2012.12.033. [DOI] [Google Scholar]
- Landerer D.; Bahro D.; Röhm H.; Koppitz M.; Mertens A.; Manger F.; Denk F.; Heidinger M.; Windmann T.; Colsmann A. Solar Glasses: A Case Study on Semitransparent Organic Solar Cells for Self-Powered, Smart, Wearable Devices. Energy Technol. 2017, 5, 1936–1945. 10.1002/ente.201700226. [DOI] [Google Scholar]
- Tang C. W. Two-layer organic photovoltaic cell. Appl. Phys. Lett. 1986, 48, 183–185. 10.1063/1.96937. [DOI] [Google Scholar]
- Sariciftci N. S.; Smilowitz L.; Heeger A. J.; Wudl F. Photoinduced electron transfer from a conducting polymer to buckminsterfullerene. Science 1992, 258, 1474–1476. 10.1126/science.258.5087.1474. [DOI] [PubMed] [Google Scholar]
- Collavini S.; Delgado J. L. Fullerenes: the stars of photovoltaics. Sustain. Energy Fuels 2018, 2, 2480–2493. 10.1039/C8SE00254A. [DOI] [Google Scholar]
- Gaspar H.; Figueira F.; Pereira L.; Mendes A.; Viana J. C.; Bernardo G. Recent Developments in the Optimization of the Bulk Heterojunction Morphology of Polymer: Fullerene Solar Cells. Materials 2018, 11, 2560. 10.3390/ma11122560. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Deng D.; Zhang Y.; Zhang J.; Wang Z.; Zhu L.; Fang J.; Xia B.; Wang Z.; Lu K.; Ma W.; Wei Z. Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells. Nat. Commun. 2016, 7, 13740. 10.1038/ncomms13740. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Li W.; Cai J.; Cai F.; Yan Y.; Yi H.; Gurney R. S.; Liu D.; Iraqi A.; Wang T. Achieving over 11% power conversion efficiency in PffBT4T-2OD-based ternary polymer solar cells with enhanced open-circuit-voltage and suppressed charge recombination. Nano Energy 2018, 44, 155–163. 10.1016/j.nanoen.2017.12.005. [DOI] [Google Scholar]
- Zhao J.; Li Y.; Yang G.; Jiang K.; Lin H.; Ade H.; Ma W.; Yan H. Efficient organic solar cells processed from hydrocarbon solvents. Nat. Energy 2016, 1, 15027. 10.1038/nenergy.2015.27. [DOI] [Google Scholar]
- Armin A.; Li W.; Sandberg O. J.; Xiao Z.; Ding L.; Nelson J.; Neher D.; Vandewal K.; Shoaee S.; Wang T.; Ade H.; Heumüller T.; Brabec C.; Meredith P. A History and Perspective of Non-Fullerene Electron Acceptors for Organic Solar Cells. Adv. Energy Mater. 2021, 11, 2003570. 10.1002/aenm.202003570. [DOI] [Google Scholar]
- Lin Y.; Wang J.; Zhang Z.-G.; Bai H.; Li Y.; Zhu D.; Zhan X. An electron acceptor challenging fullerenes for efficient polymer solar cells. Adv. Mater. 2015, 27, 1170–1174. 10.1002/adma.201404317. [DOI] [PubMed] [Google Scholar]
- Holliday S.; Ashraf R. S.; Wadsworth A.; Baran D.; Yousaf S. A.; Nielsen C. B.; Tan C.-H.; Dimitrov S. D.; Shang Z.; Gasparini N.; Alamoudi M.; Laquai F.; Brabec C. J.; Salleo A.; Durrant J. R.; McCulloch I. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor. Nat. Commun. 2016, 7, 11585. 10.1038/ncomms11585. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Nielsen C. B.; Holliday S.; Chen H.-Y.; Cryer S. J.; McCulloch I. Non-fullerene electron acceptors for use in organic solar cells. Acc. Chem. Res. 2015, 48, 2803–2812. 10.1021/acs.accounts.5b00199. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lin Y.; He Q.; Zhao F.; Huo L.; Mai J.; Lu X.; Su C.-J.; Li T.; Wang J.; Zhu J.; Sun Y.; Wang C.; Zhan X. A Facile Planar Fused-Ring Electron Acceptor for As-Cast Polymer Solar Cells with 8.71% Efficiency. J. Am. Chem. Soc. 2016, 138, 2973–2976. 10.1021/jacs.6b00853. [DOI] [PubMed] [Google Scholar]
- Wu Y.; Bai H.; Wang Z.; Cheng P.; Zhu S.; Wang Y.; Ma W.; Zhan X. A planar electron acceptor for efficient polymer solar cells. Energy Environ. Sci. 2015, 8, 3215–3221. 10.1039/C5EE02477C. [DOI] [Google Scholar]
- Lin Y.; Zhang Z.-G.; Bai H.; Wang J.; Yao Y.; Li Y.; Zhu D.; Zhan X. High-performance fullerene-free polymer solar cells with 6.31% efficiency. Energy Environ. Sci. 2015, 8, 610–616. 10.1039/C4EE03424D. [DOI] [Google Scholar]
- Zhang H.; Li Y.; Zhang X.; Zhang Y.; Zhou H. Role of interface properties in organic solar cells: from substrate engineering to bulk-heterojunction interfacial morphology. Mater. Chem. Front. 2020, 4, 2863–2880. 10.1039/D0QM00398K. [DOI] [Google Scholar]
- Fahlman M.; Fabiano S.; Gueskine V.; Simon D.; Berggren M.; Crispin X. Interfaces in organic electronics. Nat. Rev. Mater. 2019, 4, 627–650. 10.1038/s41578-019-0127-y. [DOI] [Google Scholar]
- Li P.; Lu Z.-H. Interface Engineering in Organic Electronics: Energy-Level Alignment and Charge Transport. Small Sci. 2021, 1, 2000015. 10.1002/smsc.202000015. [DOI] [Google Scholar]
- Sorrentino R.; Kozma E.; Luzzati S.; Po R. Interlayers for non-fullerene based polymer solar cells: distinctive features and challenges. Energy Environ. Sci. 2021, 14, 180–223. 10.1039/D0EE02503H. [DOI] [Google Scholar]
- Zhao F.; Zhang H.; Zhang R.; Yuan J.; He D.; Zou Y.; Gao F. Emerging Approaches in Enhancing the Efficiency and Stability in Non-Fullerene Organic Solar Cells. Adv. Energy Mater. 2020, 10, 2002746. 10.1002/aenm.202002746. [DOI] [Google Scholar]
- Brus V. V.; Lee J.; Luginbuhl B. R.; Ko S.-J.; Bazan G. C.; Nguyen T.-Q. Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance. Adv. Mater. 2019, 31, 1900904. 10.1002/adma.201900904. [DOI] [PubMed] [Google Scholar]
- Sajjad M. T.; Zhang Y.; Geraghty P. B.; Mitchell V. D.; Ruseckas A.; Blaszczyk O.; Jones D. J.; Samuel I. D. W. Tailoring exciton diffusion and domain size in photovoltaic small molecules by annealing. J. Mater. Chem. C 2019, 7, 7922–7928. 10.1039/C9TC00951E. [DOI] [Google Scholar]
- Gurney R. S.; Lidzey D. G.; Wang T. A review of non-fullerene polymer solar cells: from device physics to morphology control. Rep. Prog. Phys. 2019, 82, 036601. 10.1088/1361-6633/ab0530. [DOI] [PubMed] [Google Scholar]
- Zhu L.; Zhang M.; Zhong W.; Leng S.; Zhou G.; Zou Y.; Su X.; Ding H.; Gu P.; Liu F.; Zhang Y. Progress and prospects of the morphology of non-fullerene acceptor based high-efficiency organic solar cells. Energy Environ. Sci. 2021, 14, 4341. 10.1039/D1EE01220G. [DOI] [Google Scholar]
- Zhang J.; Tan H. S.; Guo X.; Facchetti A.; Yan H. Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors. Nat. Energy 2018, 3, 720–731. 10.1038/s41560-018-0181-5. [DOI] [Google Scholar]
- Firdaus Y.; Le Corre V. M.; Karuthedath S.; Liu W.; Markina A.; Huang W.; Chattopadhyay S.; Nahid M. M.; Nugraha M. I.; Lin Y.; Seitkhan A.; Basu A.; Zhang W.; McCulloch I.; Ade H.; Labram J.; Laquai F.; Andrienko D.; Koster L. J. A.; Anthopoulos T. D. Long-range exciton diffusion in molecular non-fullerene acceptors. Nat. Commun. 2020, 11, 5220. 10.1038/s41467-020-19029-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Haneef H. F.; Zeidell A. M.; Jurchescu O. D. Charge carrier traps in organic semiconductors: a review on the underlying physics and impact on electronic devices. J. Mater. Chem. C 2020, 8, 759–787. 10.1039/C9TC05695E. [DOI] [Google Scholar]
- Mikhnenko O. V.; Blom P. W. M.; Nguyen T.-Q. Exciton diffusion in organic semiconductors. Energy Environ. Sci. 2015, 8, 1867–1888. 10.1039/C5EE00925A. [DOI] [Google Scholar]
- Xie Y.; Wu H. Balancing charge generation and voltage loss toward efficient nonfullerene organic solar cells. Mater. Today Adv. 2020, 5, 100048. 10.1016/j.mtadv.2019.100048. [DOI] [Google Scholar]
- Xu Y.; Yao H.; Ma L.; Wang J.; Hou J. Efficient charge generation at low energy losses in organic solar cells: a key issues review. Rep. Prog. Phys. 2020, 83, 082601. 10.1088/1361-6633/ab90cf. [DOI] [PubMed] [Google Scholar]
- Sajjad M. T.; Ruseckas A.; Samuel I. D. Enhancing Exciton Diffusion Length Provides New Opportunities for Organic Photovoltaics. Matter 2020, 3, 341–354. 10.1016/j.matt.2020.06.028. [DOI] [Google Scholar]
- Zhao F.; Wang C.; Zhan X. Morphology Control in Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1703147. 10.1002/aenm.201703147. [DOI] [Google Scholar]
- Wang Q.; Qin Y.; Li M.; Ye L.; Geng Y. Molecular Engineering and Morphology Control of Polythiophene:Nonfullerene Acceptor Blends for High-Performance Solar Cells. Adv. Energy Mater. 2020, 10, 2002572. 10.1002/aenm.202002572. [DOI] [Google Scholar]
- Roncali J.; Grosu I. The Dawn of Single Material Organic Solar Cells. Adv. Sci. 2019, 6, 1801026. 10.1002/advs.201801026. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Wu Y.; Guo J.; Wang W.; Chen Z.; Chen Z.; Sun R.; Wu Q.; Wang T.; Hao X.; Zhu H.; Min J. A conjugated donor-acceptor block copolymer enables over 11% efficiency for single-component polymer solar cells. Joule 2021, 5, 1800. 10.1016/j.joule.2021.05.002. [DOI] [Google Scholar]
- Jiang X.; Yang J.; Karuthedath S.; Li J.; Lai W.; Li C.; Xiao C.; Ye L.; Ma Z.; Tang Z.; Laquai F.; Li W. Miscibility-Controlled Phase Separation in Double-Cable Conjugated Polymers for Single-Component Organic Solar Cells with Efficiencies over 8. Angew. Chem., Int. Ed. 2020, 59, 21683–21692. 10.1002/anie.202009272. [DOI] [PubMed] [Google Scholar]
- Bi P.; Hao X. Versatile Ternary Approach for Novel Organic Solar Cells: A Review. Sol. RRL 2019, 3, 1800263. 10.1002/solr.201800263. [DOI] [Google Scholar]
- Xu X.; Li Y.; Peng Q. Recent advances in morphology optimizations towards highly efficient ternary organic solar cells. Nano Select 2020, 1, 30–58. 10.1002/nano.202000012. [DOI] [Google Scholar]
- Gasparini N.; Paleti S. H. K.; Bertrandie J.; Cai G.; Zhang G.; Wadsworth A.; Lu X.; Yip H.-L.; McCulloch I.; Baran D. Exploiting Ternary Blends for Improved Photostability in High-Efficiency Organic Solar Cells. ACS Energy Lett. 2020, 5, 1371–1379. 10.1021/acsenergylett.0c00604. [DOI] [Google Scholar]
- Cheng P.; Wang J.; Zhan X.; Yang Y. Constructing High-Performance Organic Photovoltaics via Emerging Non-Fullerene Acceptors and Tandem-Junction Structure. Adv. Energy Mater. 2020, 10, 2000746. 10.1002/aenm.202000746. [DOI] [Google Scholar]
- Zhang K.; Ying L.; Yip H.-L.; Huang F.; Cao Y. Toward Efficient Tandem Organic Solar Cells: From Materials to Device Engineering. ACS Appl. Mater. Interfaces 2020, 12, 39937–39947. 10.1021/acsami.0c09909. [DOI] [PubMed] [Google Scholar]
- Xu X.; Li Y.; Peng Q. Recent Advances Toward Highly Efficient Tandem Organic Solar Cells. Small Struct. 2020, 1, 2000016. 10.1002/sstr.202000016. [DOI] [Google Scholar]
- Di Carlo Rasi D.; Janssen R. A. J. Advances in Solution-Processed Multijunction Organic Solar Cells. Adv. Mater. 2019, 31, 1806499. 10.1002/adma.201806499. [DOI] [PubMed] [Google Scholar]
- Kini G. P.; Jeon S. J.; Moon D. K. Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review. Adv. Mater. 2020, 32, 1906175. 10.1002/adma.201906175. [DOI] [PubMed] [Google Scholar]
- Fan Q.; Méndez-Romero U. A.; Guo X.; Wang E.; Zhang M.; Li Y. Fluorinated Photovoltaic Materials for High-Performance Organic Solar Cells. Chem. - Asian J. 2019, 14, 3085–3095. 10.1002/asia.201900795. [DOI] [PubMed] [Google Scholar]
- Ragni R.; Punzi A.; Babudri F.; Farinola G. M. Organic and Organometallic Fluorinated Materials for Electronics and Optoelectronics: A Survey on Recent Research. Eur. J. Org. Chem. 2018, 2018, 3500–3519. 10.1002/ejoc.201800657. [DOI] [Google Scholar]
- Chen J.; Yang K.; Zhou X.; Guo X. Ladder-Type Heteroarene-Based Organic Semiconductors. Chem. - Asian J. 2018, 13, 2587–2600. 10.1002/asia.201800860. [DOI] [PubMed] [Google Scholar]
- Zhao C.; Guo Y.; Zhang Y.; Yan N.; You S.; Li W. Diketopyrrolopyrrole-based conjugated materials for non-fullerene organic solar cells. J. Mater. Chem. A 2019, 7, 10174–10199. 10.1039/C9TA01976F. [DOI] [Google Scholar]
- Cui Y.; Zhu P.; Liao X.; Chen Y. Recent advances of computational chemistry in organic solar cell research. J. Mater. Chem. C 2020, 8, 15920–15939. 10.1039/D0TC03709E. [DOI] [Google Scholar]
- Levine A. M.; Biswas S.; Braunschweig A. B. Photoactive organic material discovery with combinatorial supramolecular assembly. Nanoscale Adv. 2019, 1, 3858–3869. 10.1039/C9NA00476A. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Duan L.; Elumalai N. K.; Zhang Y.; Uddin A. Progress in non-fullerene acceptor based organic solar cells. Sol. Energy Mater. Sol. Cells 2019, 193, 22–65. 10.1016/j.solmat.2018.12.033. [DOI] [Google Scholar]
- Dey S. Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells. Small 2019, 15, 1900134. 10.1002/smll.201900134. [DOI] [PubMed] [Google Scholar]
- Wang J.; Zhan X. Fused-Ring Electron Acceptors for Photovoltaics and Beyond. Acc. Chem. Res. 2021, 54, 132–143. 10.1021/acs.accounts.0c00575. [DOI] [PubMed] [Google Scholar]
- Fujimoto K.; Takahashi M.; Izawa S.; Hiramoto M. Development of Perylene-Based Non-Fullerene Acceptors through Bay-Functionalization Strategy. Materials 2020, 13, 2148. 10.3390/ma13092148. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Zink-Lorre N.; Font-Sanchis E.; Sastre-Santos Á.; Fernández-Lázaro F. Perylenediimides as more than just non-fullerene acceptors: versatile components in organic, hybrid and perovskite solar cells. Chem. Commun. 2020, 56, 3824–3838. 10.1039/D0CC00337A. [DOI] [PubMed] [Google Scholar]
- Wang J.; Zhan X. Rylene Diimide Electron Acceptors for Organic Solar Cells. Trends Chem. 2019, 1, 869–881. 10.1016/j.trechm.2019.05.002. [DOI] [Google Scholar]
- Macedo A. G.; Christopholi L. P.; Gavim A. E. X.; Deus J. F. de; Teridi M. A. M.; Yusoff A. R. b. M.; da Silva W. J. Perylene derivatives for solar cells and energy harvesting: a review of materials, challenges and advances. J. Mater. Sci.: Mater. Electron. 2019, 30, 15803–15824. 10.1007/s10854-019-02019-z. [DOI] [Google Scholar]
- Li S.; Li C.-Z.; Shi M.; Chen H. New Phase for Organic Solar Cell Research: Emergence of Y-Series Electron Acceptors and Their Perspectives. ACS Energy Lett. 2020, 5, 1554–1567. 10.1021/acsenergylett.0c00537. [DOI] [Google Scholar]
- Yuan J.; Zhang H.; Zhang R.; Wang Y.; Hou J.; Leclerc M.; Zhan X.; Huang F.; Gao F.; Zou Y.; Li Y. Reducing Voltage Losses in the A-DA′D-A Acceptor-Based Organic Solar Cells. Chem. 2020, 6, 2147–2161. 10.1016/j.chempr.2020.08.003. [DOI] [Google Scholar]
- Cheng P.; Yang Y. Narrowing the Band Gap: The Key to High-Performance Organic Photovoltaics. Acc. Chem. Res. 2020, 53, 1218–1228. 10.1021/acs.accounts.0c00157. [DOI] [PubMed] [Google Scholar]
- Forti G.; Nitti A.; Osw P.; Bianchi G.; Po R.; Pasini D. Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells. Int. J. Mol. Sci. 2020, 21, 8085. 10.3390/ijms21218085. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Zhao C.; Wang J.; Jiao J.; Huang L.; Tang J. Recent advances of polymer acceptors for high-performance organic solar cells. J. Mater. Chem. C 2020, 8, 28–43. 10.1039/C9TC05567C. [DOI] [Google Scholar]
- Yin H.; Yan C.; Hu H.; Ho J. K. W.; Zhan X.; Li G.; So S. K. Recent progress of all-polymer solar cells – From chemical structure and device physics to photovoltaic performance. Mater. Sci. Eng., R 2020, 140, 100542. 10.1016/j.mser.2019.100542. [DOI] [Google Scholar]
- Wan X.; Li C.; Zhang M.; Chen Y. Acceptor-donor-acceptor type molecules for high performance organic photovoltaics - chemistry and mechanism. Chem. Soc. Rev. 2020, 49, 2828–2842. 10.1039/D0CS00084A. [DOI] [PubMed] [Google Scholar]
- Liu Z.; Zhang X.; Li P.; Gao X. Recent development of efficient A-D-A type fused-ring electron acceptors for organic solar. Sol. Energy 2018, 174, 171–188. 10.1016/j.solener.2018.09.008. [DOI] [Google Scholar]
- Cui C. Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells. Front. Chem. 2018, 6, 404. 10.3389/fchem.2018.00404. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Wang H.; Cao J.; Yu J.; Zhang Z.; Geng R.; Yang L.; Tang W. Molecular engineering of central fused-ring cores of non-fullerene acceptors for high-efficiency organic solar cells. J. Mater. Chem. A 2019, 7, 4313–4333. 10.1039/C8TA12465E. [DOI] [Google Scholar]
- Luo Z.; Liu T.; Yan H.; Zou Y.; Yang C. Isomerization Strategy of Nonfullerene Small-Molecule Acceptors for Organic Solar Cells. Adv. Funct. Mater. 2020, 30, 2004477. 10.1002/adfm.202004477. [DOI] [Google Scholar]
- Pan Y.-Q.; Sun G.-Y. Star-Shaped Non-Fullerene Small Acceptors for Organic Solar Cells. ChemSusChem 2019, 12, 4570–4600. 10.1002/cssc.201901013. [DOI] [PubMed] [Google Scholar]
- Li C.; Fu H.; Xia T.; Sun Y. Asymmetric Nonfullerene Small Molecule Acceptors for Organic Solar Cells. Adv. Energy Mater. 2019, 9, 1900999. 10.1002/aenm.201900999. [DOI] [Google Scholar]
- Lin Y.; Zhan X. Designing Efficient Non-Fullerene Acceptors by Tailoring Extended Fused-Rings with Electron-Deficient Groups. Adv. Energy Mater. 2015, 5, 1501063. 10.1002/aenm.201501063. [DOI] [Google Scholar]
- Wadsworth A.; Moser M.; Marks A.; Little M. S.; Gasparini N.; Brabec C. J.; Baran D.; McCulloch I. Critical review of the molecular design progress in non-fullerene electron acceptors towards commercially viable organic solar cells. Chem. Soc. Rev. 2019, 48, 1596–1625. 10.1039/C7CS00892A. [DOI] [PubMed] [Google Scholar]
- Lin Y.; Zhao F.; He Q.; Huo L.; Wu Y.; Parker T. C.; Ma W.; Sun Y.; Wang C.; Zhu D.; Heeger A. J.; Marder S. R.; Zhan X. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138, 4955–4961. 10.1021/jacs.6b02004. [DOI] [PubMed] [Google Scholar]
- Wei Q.; Yuan J.; Yi Y.; Zhang C.; Zou Y. Y6 and its derivatives: molecular design and physical mechanism. Natl. Sci. Rev. 2021, 8, nwab121. 10.1093/nsr/nwab121. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Nie Q.; Tang A.; Guo Q.; Zhou E. Benzothiadiazole-based non-fullerene acceptors. Nano Energy 2021, 87, 106174. 10.1016/j.nanoen.2021.106174. [DOI] [Google Scholar]
- Yue Q.; Liu W.; Zhu X. n-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications.. J. Am. Chem. Soc. 2020, 142, 11613–11628. 10.1021/jacs.0c04084. [DOI] [PubMed] [Google Scholar]
- Dai S.; Zhao F.; Zhang Q.; Lau T.-K.; Li T.; Liu K.; Ling Q.; Wang C.; Lu X.; You W.; Zhan X. Fused Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 1336–1343. 10.1021/jacs.6b12755. [DOI] [PubMed] [Google Scholar]
- Aldrich T. J.; Matta M.; Zhu W.; Swick S. M.; Stern C. L.; Schatz G. C.; Facchetti A.; Melkonyan F. S.; Marks T. J. Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response. J. Am. Chem. Soc. 2019, 141, 3274–3287. 10.1021/jacs.8b13653. [DOI] [PubMed] [Google Scholar]
- Cui C.; Li Y. High-performance conjugated polymer donor materials for polymer solar cells with narrow-bandgap nonfullerene acceptors. Energy Environ. Sci. 2019, 12, 3225–3246. 10.1039/C9EE02531F. [DOI] [Google Scholar]
- Guo X.; Facchetti A. The journey of conducting polymers from discovery to application. Nat. Mater. 2020, 19, 922–928. 10.1038/s41563-020-0778-5. [DOI] [PubMed] [Google Scholar]
- Sun L.; Xu X.; Song S.; Zhang Y.; Miao C.; Liu X.; Xing G.; Zhang S. Medium-Bandgap Conjugated Polymer Donors for Organic Photovoltaics. Macromol. Rapid Commun. 2019, 40, 1900074. 10.1002/marc.201900074. [DOI] [PubMed] [Google Scholar]
- Liu X.; Xie B.; Duan C.; Wang Z.; Fan B.; Zhang K.; Lin B.; Colberts F. J. M.; Ma W.; Janssen R. A. J.; Huang F.; Cao Y. A high dielectric constant non-fullerene acceptor for efficient bulk-heterojunction organic solar cells. J. Mater. Chem. A 2018, 6, 395–403. 10.1039/C7TA10136H. [DOI] [Google Scholar]
- He B.; Yang B.; Kolaczkowski M. A.; Anderson C. A.; Klivansky L. M.; Chen T. L.; Brady M. A.; Liu Y. Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics. ACS Energy Lett. 2018, 3, 1028–1035. 10.1021/acsenergylett.8b00366. [DOI] [Google Scholar]
- Yu R.; Yao H.; Hong L.; Xu Y.; Gao B.; Zhu J.; Zu Y.; Hou J. Enhancing the Photovoltaic Performance of Nonfullerene Acceptors via Conjugated Rotatable End Groups. Adv. Energy Mater. 2018, 8, 1802131. 10.1002/aenm.201802131. [DOI] [Google Scholar]
- Bin H.; Gao L.; Zhang Z.-G.; Yang Y.; Zhang Y.; Zhang C.; Chen S.; Xue L.; Yang C.; Xiao M.; Li Y. 11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor. Nat. Commun. 2016, 7, 13651. 10.1038/ncomms13651. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Li X.; Liang Z.; Wang H.; Qiao S.; Liu Z.; Jiang H.; Chen W.; Yang R. Fluorinated D1 (0.5) −A–D2 (0.5) −A model terpolymer: ultrafast charge separation kinetics and electron transfer at the fluorinated D/A interface for power conversion. J. Mater. Chem. A 2020, 8, 1360–1367. 10.1039/C9TA10610C. [DOI] [Google Scholar]
- Gao B.; Yao H.; Hou J.; Yu R.; Hong L.; Xu Y.; Hou J. Multi-component non-fullerene acceptors with tunable bandgap structures for efficient organic solar cells. J. Mater. Chem. A 2018, 6, 23644–23649. 10.1039/C8TA09830A. [DOI] [Google Scholar]
- Hao M.; Liu T.; Xiao Y.; Ma L.-K.; Zhang G.; Zhong C.; Chen Z.; Luo Z.; Lu X.; Yan H.; Wang L.; Yang C. Achieving Balanced Charge Transport and Favorable Blend Morphology in Non-Fullerene Solar Cells via Acceptor End Group Modification. Chem. Mater. 2019, 31, 1752–1760. 10.1021/acs.chemmater.8b05327. [DOI] [Google Scholar]
- Yao H.; Qian D.; Zhang H.; Qin Y.; Xu B.; Cui Y.; Yu R.; Gao F.; Hou J. Critical Role of Molecular Electrostatic Potential on Charge Generation in Organic Solar Cells. Chin. J. Chem. 2018, 36, 491–494. 10.1002/cjoc.201800015. [DOI] [Google Scholar]
- Li M.; Zhou Y.; Zhang J.; Song J.; Bo Z. Tuning the dipole moments of nonfullerene acceptors with an asymmetric terminal strategy for highly efficient organic solar cells. J. Mater. Chem. A 2019, 7, 8889–8896. 10.1039/C8TA12530A. [DOI] [Google Scholar]
- Zhang H.; Yao H.; Hou J.; Zhu J.; Zhang J.; Li W.; Yu R.; Gao B.; Zhang S.; Hou J. Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene Small-Molecule Acceptors. Adv. Mater. 2018, 30, 1800613. 10.1002/adma.201800613. [DOI] [PubMed] [Google Scholar]
- Lai H.; Chen H.; Zhu Y.; Chen L.; Huang H.-H.; He F. An asymmetrical A–DAD–A-type acceptor simultaneously enhances voltage and current for efficient organic solar cells. J. Mater. Chem. A 2020, 8, 9670–9676. 10.1039/D0TA03128C. [DOI] [Google Scholar]
- Lai H.; Chen H.; Zhou J.; Qu J.; Wang M.; Xie W.; Xie Z.; He F. 3D Interpenetrating Network for High-Performance Nonfullerene Acceptors via Asymmetric Chlorine Substitution. J. Phys. Chem. Lett. 2019, 10, 4737–4743. 10.1021/acs.jpclett.9b01931. [DOI] [PubMed] [Google Scholar]
- Qu J.; Li D.; Wang H.; Zhou J.; Zheng N.; Lai H.; Liu T.; Xie Z.; He F. Bromination of the Small-Molecule Acceptor with Fixed Position for High-Performance Solar Cells. Chem. Mater. 2019, 31, 8044–8051. 10.1021/acs.chemmater.9b02501. [DOI] [Google Scholar]
- Tang D.; Wan J.; Xu X.; Lee Y. W.; Woo H. Y.; Feng K.; Peng Q. Naphthobistriazole-based wide bandgap donor polymers for efficient non-fullerene organic solar cells: Significant fine-tuning absorption and energy level by backbone fluorination. Nano Energy 2018, 53, 258–269. 10.1016/j.nanoen.2018.08.059. [DOI] [Google Scholar]
- Su W.; Fan Q.; Guo X.; Chen J.; Wang Y.; Wang X.; Dai P.; Ye C.; Bao X.; Ma W.; Zhang M.; Li Y. Significant enhancement of the photovoltaic performance of organic small molecule acceptors via side-chain engineering. J. Mater. Chem. A 2018, 6, 7988–7996. 10.1039/C8TA01509K. [DOI] [Google Scholar]
- Doumon N. Y.; Houard F. V.; Dong J.; Yao H.; Portale G.; Hou J.; Koster L. J. A. Energy level modulation of ITIC derivatives: Effects on the photodegradation of conventional and inverted organic solar cells. Org. Electron. 2019, 69, 255–262. 10.1016/j.orgel.2019.03.037. [DOI] [Google Scholar]
- Yao C.; Zhao J.; Zhu Y.; Liu B.; Yan C.; Perepichka D. F.; Meng H. Trifluoromethyl Group-Modified Non-Fullerene Acceptor toward Improved Power Conversion Efficiency over 13% in Polymer Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 11543–11550. 10.1021/acsami.9b20544. [DOI] [PubMed] [Google Scholar]
- Liu X.; Wang X.; Xiao Y.; Yang Q.; Guo X.; Li C. H-Bonds-Assisted Molecular Order Manipulation of Nonfullerene Acceptors for Efficient Nonannealed Organic Solar Cells. Adv. Energy Mater. 2020, 10, 1903650. 10.1002/aenm.201903650. [DOI] [Google Scholar]
- Cui Y.; Wang Y.; Bergqvist J.; Yao H.; Xu Y.; Gao B.; Yang C.; Zhang S.; Inganäs O.; Gao F.; Hou J. Wide-gap non-fullerene acceptor enabling high-performance organic photovoltaic cells for indoor applications. Nat. Energy 2019, 4, 768–775. 10.1038/s41560-019-0448-5. [DOI] [Google Scholar]
- Xian K.; Cui Y.; Xu Y.; Zhang T.; Hong L.; Yao H.; An C.; Hou J. Efficient Exciton Dissociation Enabled by the End Group Modification in Non-Fullerene Acceptors. J. Phys. Chem. C 2020, 124, 7691–7698. 10.1021/acs.jpcc.0c01003. [DOI] [Google Scholar]
- Lin F.; Zuo L.; Gao K.; Zhang M.; Jo S. B.; Liu F.; Jen A. K.-Y. Regio-Specific Selenium Substitution in Non-Fullerene Acceptors for Efficient Organic Solar Cells. Chem. Mater. 2019, 31, 6770–6778. 10.1021/acs.chemmater.9b01242. [DOI] [Google Scholar]
- Liu T.; Luo Z.; Fan Q.; Zhang G.; Zhang L.; Gao W.; Guo X.; Ma W.; Zhang M.; Yang C.; Li Y.; Yan H. Use of two structurally similar small molecular acceptors enabling ternary organic solar cells with high efficiencies and fill factors. Energy Environ. Sci. 2018, 11, 3275–3282. 10.1039/C8EE01700J. [DOI] [Google Scholar]
- Luo Z.; Liu T.; Wang Y.; Zhang G.; Sun R.; Chen Z.; Zhong C.; Wu J.; Chen Y.; Zhang M.; Zou Y.; Ma W.; Yan H.; Min J.; Li Y.; Yang C. Reduced Energy Loss Enabled by a Chlorinated Thiophene-Fused Ending-Group Small Molecular Acceptor for Efficient Nonfullerene Organic Solar Cells with 13.6% Efficiency. Adv. Energy Mater. 2019, 9, 1900041. 10.1002/aenm.201900041. [DOI] [Google Scholar]
- Luo Z.; Liu T.; Xiao Y.; Yang T.; Chen Z.; Zhang G.; Zhong C.; Ma R.; Chen Y.; Zou Y.; Lu X.; Yan H.; Yang C. Significantly improving the performance of polymer solar cells by the isomeric ending-group based small molecular acceptors: Insight into the isomerization. Nano Energy 2019, 66, 104146. 10.1016/j.nanoen.2019.104146. [DOI] [Google Scholar]
- Yan D.; Liu W.; Yao J.; Zhan C. Fused-Ring Nonfullerene Acceptor Forming Interpenetrating J -Architecture for Fullerene-Free Polymer Solar Cells. Adv. Energy Mater. 2018, 8, 1800204. 10.1002/aenm.201800204. [DOI] [Google Scholar]
- Deng M.; Xu X.; Lee Y. W.; Ericsson L. K. E.; Moons E.; Woo H. Y.; Li Y.; Yu L.; Peng Q. Fine regulation of crystallisation tendency to optimize the BHJ nanostructure and performance of polymer solar cells. Nanoscale 2020, 12, 12928–12941. 10.1039/D0NR00698J. [DOI] [PubMed] [Google Scholar]
- Liu W.; Li W.; Yao J.; Zhan C. Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor. Chin. Chem. Lett. 2018, 29, 381–384. 10.1016/j.cclet.2017.11.018. [DOI] [Google Scholar]
- Gao B.; Yao H.; Hong L.; Hou J. Efficient Organic Solar Cells with a High Open-Circuit Voltage of 1.34 V. Chin. J. Chem. 2019, 37, 1153–1157. 10.1002/cjoc.201900269. [DOI] [Google Scholar]
- Chang S.-L.; Hung K.-E.; Cao F.-Y.; Huang K.-H.; Hsu C.-S.; Liao C.-Y.; Lee C.-H.; Cheng Y.-J. Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving Improved VOC and JSC of Nonfullerene Organic Solar Cells via End Group Manipulation. ACS Appl. Mater. Interfaces 2019, 11, 33179–33187. 10.1021/acsami.9b08462. [DOI] [PubMed] [Google Scholar]
- Huang B.; Hu L.; Chen L.; Chen S.; Hu M.; Zhou Y.; Zhang Y.; Yang C.; Chen Y. Morphological optimization by rational matching of the donor and acceptor boosts the efficiency of alkylsilyl fused ring-based polymer solar cells. J. Mater. Chem. A 2019, 7, 4847–4854. 10.1039/C9TA00114J. [DOI] [Google Scholar]
- Wang Z.; Peng Z.; Xiao Z.; Seyitliyev D.; Gundogdu K.; Ding L.; Ade H. Thermodynamic Properties and Molecular Packing Explain Performance and Processing Procedures of Three D18:NFA Organic Solar Cells. Adv. Mater. 2020, 32, 2005386. 10.1002/adma.202005386. [DOI] [PubMed] [Google Scholar]
- Gao Y.; Shen Z.; Tan F.; Yue G.; Liu R.; Wang Z.; Qu S.; Wang Z.; Zhang W. Novel benzo[1,2-b:4,5-b′]difuran-based copolymer enables efficient polymer solar cells with small energy loss and high VOC. Nano Energy 2020, 76, 104964. 10.1016/j.nanoen.2020.104964. [DOI] [Google Scholar]
- Cao H.; Bauer N.; Pang C.; Rech J.; You W.; Rupar P. A. End-cap Group Engineering of a Small Molecule Non-Fullerene Acceptor: The Influence of Benzothiophene Dioxide. ACS Appl. Energy Mater. 2018, 1, 7146–7152. 10.1021/acsaem.8b01576. [DOI] [Google Scholar]
- Deng M.; Xu X.; Yu L.; Li R.; Peng Q. Fused ring non-fullerene acceptors with benzothiophene dioxide end groups and their side chain effect investigations. Dyes Pigm. 2020, 180, 108452. 10.1016/j.dyepig.2020.108452. [DOI] [Google Scholar]
- Liu H.; Wang W.; Zhou Y.; Li Z. A ring-locking strategy to enhance the chemical and photochemical stability of A–D–A-type non-fullerene acceptors. J. Mater. Chem. A 2021, 9, 1080–1088. 10.1039/D0TA09924D. [DOI] [Google Scholar]
- Xiao L.; Kolaczkowski M. A.; Min Y.; Liu Y. Substitution Effect on Thiobarbituric Acid End Groups for High Open-Circuit Voltage Non-Fullerene Organic Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 41852–41860. 10.1021/acsami.0c11828. [DOI] [PubMed] [Google Scholar]
- Kolhe N. B.; West S. M.; Tran D. K.; Ding X.; Kuzuhara D.; Yoshimoto N.; Koganezawa T.; Jenekhe S. A. Designing High Performance Nonfullerene Electron Acceptors with Rylene Imides for Efficient Organic Photovoltaics. Chem. Mater. 2020, 32, 195–204. 10.1021/acs.chemmater.9b03329. [DOI] [Google Scholar]
- Lai H.; Chen H.; Shen Y.; Wang M.; Chao P.; Xie W.; Qu J.; Yang B.; He F. Using Chlorine Atoms to Fine-Tune the Intermolecular Packing and Energy Levels of Efficient Nonfullerene Acceptors. ACS Appl. Energy Mater. 2019, 2, 7663–7669. 10.1021/acsaem.9b01667. [DOI] [Google Scholar]
- Zhao Y.; Luo Z.; Li G.; Luo J.; Zhang Z.-G.; Li Y.; Yang C. De novo design of small molecule acceptors via fullerene/non-fullerene hybrids for polymer solar cells. Chem. Commun. 2018, 54, 9801–9804. 10.1039/C8CC04845B. [DOI] [PubMed] [Google Scholar]
- Song X.; Gasparini N.; Nahid M. M.; Chen H.; Macphee S. M.; Zhang W.; Norman V.; Zhu C.; Bryant D.; Ade H.; McCulloch I.; Baran D. A Highly Crystalline Fused-Ring n-Type Small Molecule for Non-Fullerene Acceptor Based Organic Solar Cells and Field-Effect Transistors. Adv. Funct. Mater. 2018, 28, 1802895. 10.1002/adfm.201802895. [DOI] [Google Scholar]
- Fei Z.; Eisner F. D.; Jiao X.; Azzouzi M.; Röhr J. A.; Han Y.; Shahid M.; Chesman A. S. R.; Easton C. D.; McNeill C. R.; Anthopoulos T. D.; Nelson J.; Heeney M. An Alkylated Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptor with High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies Greater than 13% with Low Voltage Losses. Adv. Mater. 2018, 30, 1705209. 10.1002/adma.201705209. [DOI] [PubMed] [Google Scholar]
- Hu H.; Jiang K.; Chow P. C. Y.; Ye L.; Zhang G.; Li Z.; Carpenter J. H.; Ade H.; Yan H. Influence of Donor Polymer on the Molecular Ordering of Small Molecular Acceptors in Nonfullerene Polymer Solar Cells. Adv. Energy Mater. 2018, 8, 1701674. 10.1002/aenm.201701674. [DOI] [Google Scholar]
- Sung M. j.; Park B.; Choi J. Y.; Kim J.; Sun C.; Kang H.; Kwon S.; Jang S.-Y.; Kim Y.-H.; Lee K.; Kwon S.-K. Spirobifluorene-based non-fullerene acceptors for the environmentally benign process. Dyes Pigm. 2020, 180, 108369. 10.1016/j.dyepig.2020.108369. [DOI] [Google Scholar]
- Fan Q.; Su W.; Zhang M.; Wu J.; Jiang Y.; Guo X.; Liu F.; Russell T. P.; Zhang M.; Li Y. Synergistic Effects of Side-Chain Engineering and Fluorination on Small Molecule Acceptors to Simultaneously Broaden Spectral Response and Minimize Voltage Loss for 13.8% Efficiency Organic Solar Cells. Sol. RRL 2019, 3, 1900169. 10.1002/solr.201900169. [DOI] [Google Scholar]
- Jang B.; Lee C.; Lee Y. W.; Kim D.; Uddin M. A.; Kim F. S.; Kim B. J.; Woo H. Y. A High Dielectric N-Type Small Molecular Acceptor Containing Oligoethyleneglycol Side-Chains for Organic Solar Cells. Chin. J. Chem. 2018, 36, 199–205. 10.1002/cjoc.201700629. [DOI] [Google Scholar]
- Chen S.; Lee S. M.; Xu J.; Lee J.; Lee K. C.; Hou T.; Yang Y.; Jeong M.; Lee B.; Cho Y.; Jung S.; Oh J.; Zhang Z.-G.; Zhang C.; Xiao M.; Li Y.; Yang C. Ultrafast Channel II process induced by a 3-D texture with enhanced acceptor order ranges for high-performance non-fullerene polymer solar cells. Energy Environ. Sci. 2018, 11, 2569–2580. 10.1039/C8EE01546E. [DOI] [Google Scholar]
- Zhao B.; Wang W.; Xin J.; Wu H.; Liu H.; Guo Z.; Cong Z.; Ma W.; Gao C. Absorptive Behaviors and Photovoltaic Performance Enhancements of Alkoxy-Phenyl Modified Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptors. ACS Sustainable Chem. Eng. 2018, 6, 2177–2187. 10.1021/acssuschemeng.7b03606. [DOI] [Google Scholar]
- Lee S.; Park K. H.; Lee J.-H.; Back H.; Sung M. j.; Lee J.; Kim J.; Kim H.; Kim Y.-H.; Kwon S.-K.; Lee K. Achieving Thickness-Insensitive Morphology of the Photoactive Layer for Printable Organic Photovoltaic Cells via Side Chain Engineering in Nonfullerene Acceptors. Adv. Energy Mater. 2019, 9, 1900044. 10.1002/aenm.201900044. [DOI] [Google Scholar]
- Xin Y.; Zeng G.; OuYang J.; Zhao X.; Yang X. Enhancing thermal stability of nonfullerene organic solar cells via fluoro-side-chain engineering. J. Mater. Chem. C 2019, 7, 9513–9522. 10.1039/C9TC02700A. [DOI] [Google Scholar]
- Lee J.; Go E. M.; Dharmapurikar S.; Xu J.; Lee S. M.; Jeong M.; Lee K. C.; Oh J.; Cho Y.; Zhang C.; Xiao M.; Kwak S. K.; Yang C. Insights into constitutional isomeric effects on donor–acceptor intermolecular arrangements in non-fullerene organic solar cells. J. Mater. Chem. A 2019, 7, 18468–18479. 10.1039/C9TA07133D. [DOI] [Google Scholar]
- Zhang C.; Ming S.; Wu H.; Wang X.; Huang H.; Xue W.; Xu X.; Tang Z.; Ma W.; Bo Z. High-efficiency ternary nonfullerene organic solar cells with record long-term thermal stability. J. Mater. Chem. A 2020, 8, 22907–22917. 10.1039/D0TA07887E. [DOI] [Google Scholar]
- Jeon S. J.; Han Y. W.; Kim Y. H.; Moon D. K. Case Study on the Correlation between Crystal Packing and Miscibility of Chlorinated Thiophene–Based Donor Polymers for Nonfullerene Organic Solar Cells with Long Shelf Life. Sol. RRL 2020, 4, 2000074. 10.1002/solr.202000074. [DOI] [Google Scholar]
- Li X.; Yao J.; Angunawela I.; Sun C.; Xue L.; Liebman-Pelaez A.; Zhu C.; Yang C.; Zhang Z.-G.; Ade H.; Li Y. Improvement of Photovoltaic Performance of Polymer Solar Cells by Rational Molecular Optimization of Organic Molecule Acceptors. Adv. Energy Mater. 2018, 8, 1800815. 10.1002/aenm.201800815. [DOI] [Google Scholar]
- Liu X.; Zheng Z.; Xu Y.; Wang J.; Wang Y.; Zhang S.; Hou J. Quantifying Voc loss induced by alkyl pendants of acceptors in organic solar cells. J. Mater. Chem. C 2020, 8, 12568–12577. 10.1039/D0TC01941K. [DOI] [Google Scholar]
- Li D.; Chen X.; Cai J.; Li W.; Chen M.; Mao Y.; Du B.; Smith J. A.; Kilbride R. C.; O’Kane M. E.; Zhang X.; Zhuang Y.; Wang P.; Wang H.; Liu D.; Jones R. A. L.; Lidzey D. G.; Wang T. Non-fullerene acceptor fibrils enable efficient ternary organic solar cells with 16.6% efficiency. Sci. China: Chem. 2020, 63, 1461–1468. 10.1007/s11426-019-9681-8. [DOI] [Google Scholar]
- Wang H.-C.; Chen C.-H.; Li R.-H.; Lin Y.-C.; Tsao C.-S.; Chang B.; Tan S.; Yang Y.; Wei K.-H. Engineering the Core Units of Small-Molecule Acceptors to Enhance the Performance of Organic Photovoltaics. Sol. RRL 2020, 4, 2000253. 10.1002/solr.202000253. [DOI] [Google Scholar]
- Feng S.; Zhang C.; Bi Z.; Liu Y.; Jiang P.; Ming S.; Xu X.; Ma W.; Bo Z. Controlling Molecular Packing and Orientation via Constructing a Ladder-Type Electron Acceptor with Asymmetric Substituents for Thick-Film Nonfullerene Solar Cells. ACS Appl. Mater. Interfaces 2019, 11, 3098–3106. 10.1021/acsami.8b19596. [DOI] [PubMed] [Google Scholar]
- Li Z.; Dai S.; Xin J.; Zhang L.; Wu Y.; Rech J.; Zhao F.; Li T.; Liu K.; Liu Q.; Ma W.; You W.; Wang C.; Zhan X. Enhancing the performance of the electron acceptor ITIC-Th via tailoring its end groups. Mater. Chem. Front. 2018, 2, 537–543. 10.1039/C7QM00547D. [DOI] [Google Scholar]
- Chang M.; Wang Y.; Yi Y.-Q.-Q.; Ke X.; Wan X.; Li C.; Chen Y. Fine-tuning the side-chains of non-fullerene small molecule acceptors to match with appropriate polymer donors. J. Mater. Chem. A 2018, 6, 8586–8594. 10.1039/C8TA00764K. [DOI] [Google Scholar]
- Swick S. M.; Zhu W.; Matta M.; Aldrich T. J.; Harbuzaru A.; Lopez Navarrete J. T.; Ponce Ortiz R.; Kohlstedt K. L.; Schatz G. C.; Facchetti A.; Melkonyan F. S.; Marks T. J. Closely packed, low reorganization energy π-extended postfullerene acceptors for efficient polymer solar cells. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, E8341–E8348. 10.1073/pnas.1807535115. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Swick S. M.; Alzola J. M.; Sangwan V. K.; Amsterdam S. H.; Zhu W.; Jones L. O.; Powers-Riggs N.; Facchetti A.; Kohlstedt K. L.; Schatz G. C.; Hersam M. C.; Wasielewski M. R.; Marks T. J. Fluorinating π-Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells. Adv. Energy Mater. 2020, 10, 2000635. 10.1002/aenm.202000635. [DOI] [Google Scholar]
- Fang J.; Liu Q.; Zhang J.; Ye L.; Wu J.; Wei Z.; Guo X.; Zhang M.; Li Y. Efficient As-Cast Polymer Solar Cells with High and Stabilized Fill Factor. Sol. RRL 2020, 4, 2000275. 10.1002/solr.202000275. [DOI] [Google Scholar]
- Gao W.; An Q.; Hao M.; Sun R.; Yuan J.; Zhang F.; Ma W.; Min J.; Yang C. Thick-Film Organic Solar Cells Achieving over 11% Efficiency and Nearly 70% Fill Factor at Thickness over 400 nm. Adv. Funct. Mater. 2020, 30, 1908336. 10.1002/adfm.201908336. [DOI] [Google Scholar]
- Xie L.; Yang C.; Zhou R.; Wang Z.; Zhang J.; Lu K.; Wei Z. Ternary Organic Solar Cells Based on Two Non-fullerene Acceptors with Complimentary Absorption and Balanced Crystallinity. Chin. J. Chem. 2020, 38, 935–940. 10.1002/cjoc.201900554. [DOI] [Google Scholar]
- Firdaus Y.; He Q.; Lin Y.; Nugroho F. A. A.; Le Corre V. M.; Yengel E.; Balawi A. H.; Seitkhan A.; Laquai F.; Langhammer C.; Liu F.; Heeney M.; Anthopoulos T. D. Novel wide-bandgap non-fullerene acceptors for efficient tandem organic solar cells. J. Mater. Chem. A 2020, 8, 1164–1175. 10.1039/C9TA11752K. [DOI] [Google Scholar]
- Xia T.; Li C.; Ryu H. S.; Sun X.; Woo H. Y.; Sun Y. Asymmetrically Alkyl-Substituted Wide-Bandgap Nonfullerene Acceptor for Organic Solar Cells. Sol. RRL 2020, 4, 2000061. 10.1002/solr.202000061. [DOI] [Google Scholar]
- Zhang Z.; Wang H.; Yu J.; Sun R.; Xu J.; Yang L.; Geng R.; Cao J.; Du F.; Min J.; Liu F.; Tang W. Modification on the Indacenodithieno[3,2-b]thiophene Core to Achieve Higher Current and Reduced Energy Loss for Nonfullerene Solar Cells. Chem. Mater. 2020, 32, 1297–1307. 10.1021/acs.chemmater.9b04911. [DOI] [Google Scholar]
- Zhang J.; Li Y.; Hu H.; Zhang G.; Ade H.; Yan H. Chlorinated Thiophene End Groups for Highly Crystalline Alkylated Non-Fullerene Acceptors toward Efficient Organic Solar Cells. Chem. Mater. 2019, 31, 6672–6676. 10.1021/acs.chemmater.9b00980. [DOI] [Google Scholar]
- Ye L.; Weng K.; Xu J.; Du X.; Chandrabose S.; Chen K.; Zhou J.; Han G.; Tan S.; Xie Z.; Yi Y.; Li N.; Liu F.; Hodgkiss J. M.; Brabec C. J.; Sun Y. Unraveling the influence of non-fullerene acceptor molecular packing on photovoltaic performance of organic solar cells. Nat. Commun. 2020, 11, 6005. 10.1038/s41467-020-19853-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Li S.; Ma Q.; Chen S.; Meng L.; Zhang J.; Zhang Z.; Yang C.; Li Y. A low boiling-point and low-cost fluorinated additive improves the efficiency and stability of organic solar cells. J. Mater. Chem. C 2020, 8, 15296–15302. 10.1039/D0TC03217D. [DOI] [Google Scholar]
- Luo Z.; Li G.; Gao W.; Wu K.; Zhang Z.-G.; Qiu B.; Bin H.; Xue L.; Liu F.; Li Y.; Yang C. A universal nonfullerene electron acceptor matching with different band-gap polymer donors for high-performance polymer solar cells. J. Mater. Chem. A 2018, 6, 6874–6881. 10.1039/C7TA11339K. [DOI] [Google Scholar]
- Qu J.; Mu Z.; Lai H.; Chen H.; Liu T.; Zhang S.; Chen W.; He F. Alkyl Chain End Group Engineering of Small Molecule Acceptors for Non-Fullerene Organic Solar Cells. ACS Appl. Energy Mater. 2018, 1, 4724–4730. 10.1021/acsaem.8b00851. [DOI] [Google Scholar]
- Alamoudi M. A.; Khan J. I.; Firdaus Y.; Wang K.; Andrienko D.; Beaujuge P. M.; Laquai F. Impact of Nonfullerene Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells. ACS Energy Lett. 2018, 3, 802–811. 10.1021/acsenergylett.8b00045. [DOI] [Google Scholar]
- Irfan A.; Mahmood A.; Al-Sehemi A. G.; Ahmad F. Experimental and theoretical study of planar small molecule acceptor for organic solar cells. J. Mol. Struct. 2019, 1196, 169–175. 10.1016/j.molstruc.2019.06.035. [DOI] [Google Scholar]
- Tang A.; Xiao B.; Chen F.; Zhang J.; Wei Z.; Zhou E. The Introduction of Fluorine and Sulfur Atoms into Benzotriazole-Based p-Type Polymers to Match with a Benzotriazole-Containing n-Type Small Molecule: “The Same-Acceptor-Strategy” to Realize High Open-Circuit Voltage. Adv. Energy Mater. 2018, 8, 1801582. 10.1002/aenm.201801582. [DOI] [Google Scholar]
- Lee J.; Ko S.-J.; Seifrid M.; Lee H.; McDowell C.; Luginbuhl B. R.; Karki A.; Cho K.; Nguyen T.-Q.; Bazan G. C. Design of Nonfullerene Acceptors with Near-Infrared Light Absorption Capabilities. Adv. Energy Mater. 2018, 8, 1801209. 10.1002/aenm.201801209. [DOI] [Google Scholar]
- Kim H. S.; Song C. E.; Ha J.-W.; Lee S.; Rasool S.; Lee H. K.; Shin W. S.; Hwang D.-H. Synthesis of ITIC Derivatives with Extended π-Conjugation as Non-Fullerene Acceptors for Organic Solar Cells. ACS Appl. Mater. Interfaces 2019, 11, 47121–47130. 10.1021/acsami.9b15247. [DOI] [PubMed] [Google Scholar]
- Wang P.; Fan H.; Zhang C.; Zhu X. A large-bandgap small-molecule electron acceptor utilizing a new indacenodibenzothiophene core for organic solar cells. Mater. Chem. Front. 2018, 2, 136–142. 10.1039/C7QM00403F. [DOI] [Google Scholar]
- Wang J.-L.; Liu K.-K.; Hong L.; Ge G.-Y.; Zhang C.; Hou J. Selenopheno[3,2-b]thiophene-Based Narrow-Bandgap Nonfullerene Acceptor Enabling 13.3% Efficiency for Organic Solar Cells with Thickness-Insensitive Feature. ACS Energy Lett. 2018, 3, 2967–2976. 10.1021/acsenergylett.8b01808. [DOI] [Google Scholar]
- Wan S.-S.; Chang C.; Wang J.-L.; Yuan G.-Z.; Wu Q.; Zhang M.; Li Y. Effects of the Number of Bromine Substitution on Photovoltaic Efficiency and Energy Loss of Benzo[1,2-b:4,5-b′]diselenophene-based Narrow-Bandgap Multibrominated Nonfullerene Acceptors. Sol. RRL 2018, 1800250. 10.1002/solr.201800250. [DOI] [Google Scholar]
- Lin J.-D.; Zhong L.; Wu F.-P.; Li Y.; Yuan Y.; Bin H.; Zhang Z.; Liu F.; Fan J.; Zhang Z.-G.; Liao L.-S.; Jiang Z.-Q.; Li Y. Short-axis substitution approach on ladder-type benzodithiophene-based electron acceptor toward highly efficient organic solar cells. Sci. China: Chem. 2018, 61, 1405–1412. 10.1007/s11426-018-9275-6. [DOI] [Google Scholar]
- Li Y.; Zhong L.; Lin J.-D.; Wu F.-P.; Bin H.-J.; Zhang Z.; Xu L.; Jiang Z.-Q.; Zhang Z.-G.; Liu F.; Russell T. P.; Li Y.; Liao L.-S.; Forrest S. R. Isomeric Effects of Solution Processed Ladder-Type Non-Fullerene Electron Acceptors. Sol. RRL 2017, 1, 1700107. 10.1002/solr.201700107. [DOI] [Google Scholar]
- Yan C.; Wang W.; Lau T.-K.; Li K.; Wang J.; Liu K.; Lu X.; Zhan X. Enhancing the performance of non-fullerene organic solar cells via end group engineering of fused-ring electron acceptors. J. Mater. Chem. A 2018, 6, 16638–16644. 10.1039/C8TA05800H. [DOI] [Google Scholar]
- Yan C.; Yang T.; Gao W.; Xiao Y.; Li Y.; Lu X.; Yang C.; Li G. Chlorination Strategy-Induced Abnormal Nanomorphology Tuning in High-Efficiency Organic Solar Cells: A Study of Phenyl-Substituted Benzodithiophene-Based Nonfullerene Acceptors. Sol. RRL 2019, 3, 1900262. 10.1002/solr.201900262. [DOI] [Google Scholar]
- Gao W.; Zhang M.; Chen Z.; Liu X.; Zheng K.; Zhong C.; Zhang F.; Yang C. A novel 9H-indeno[1,2-b]pyrazine-2,3-dicarbonitrile end group for an efficient non-fullerene small molecule acceptor. J. Mater. Chem. C 2019, 7, 10111–10118. 10.1039/C9TC01214A. [DOI] [Google Scholar]
- Zhang Y.; Wang Y.; Yang T.; Liu T.; Xiao Y.; Lu X.; Yan H.; Yuan Z.; Chen Y.; Li Y. Thioether Bond Modification Enables Boosted Photovoltaic Performance of Nonfullerene Polymer Solar Cells. ACS Appl. Mater. Interfaces 2019, 11, 32218–32224. 10.1021/acsami.9b11700. [DOI] [PubMed] [Google Scholar]
- Zhang Y.; Shi L.; Yang T.; Liu T.; Xiao Y.; Lu X.; Yan H.; Yuan Z.; Chen Y.; Li Y. Introducing an identical benzodithiophene donor unit for polymer donors and small-molecule acceptors to unveil the relationship between the molecular structure and photovoltaic performance of non-fullerene organic solar cells. J. Mater. Chem. A 2019, 7, 26351–26357. 10.1039/C9TA09241B. [DOI] [Google Scholar]
- Wang C.-K.; Jiang B.-H.; Su Y.-W.; Jeng R.-J.; Wang Y.-J.; Chen C.-P.; Wong K.-T. Si-Bridged Ladder-Type Small-Molecule Acceptors for High-Performance Organic Photovoltaics. ACS Appl. Mater. Interfaces 2019, 11, 1125–1134. 10.1021/acsami.8b17753. [DOI] [PubMed] [Google Scholar]
- Chen Z.; Ma S.-S.; Zhang K.; Hu Z.-C.; Yin Q.-W.; Huang F.; Cao Y. A Near-infrared Non-fullerene Acceptor with Thienopyrrole-expanded Benzo[1,2-b:4,5-b′]dithiophene Core for Polymer Solar Cells. Chin. J. Polym. Sci. 2021, 39, 35–42. 10.1007/s10118-020-2440-8. [DOI] [Google Scholar]
- Chen T.-W.; Hsiao Y.-T.; Lin Y.-W.; Chang C.-C.; Chuang W.-T.; Li Y.; Hsu C.-S. Fluorinated heptacyclic carbazole-based ladder-type acceptors with aliphatic side chains for efficient fullerene-free organic solar cells. Mater. Chem. Front. 2019, 3, 829–835. 10.1039/C9QM00005D. [DOI] [Google Scholar]
- Chen T.-W.; Chang C.-C.; Hsiao Y.-T.; Chan C.; Hong L.; Zhong L.; Chuang W.-T.; Hou J.; Li Y.; Hsu C.-S. Single-Junction Organic Solar Cell Containing a Fluorinated Heptacyclic Carbazole-Based Ladder-Type Acceptor Affords over 13% Efficiency with Solution-Processed Cross-Linkable Fullerene as an Interfacial Layer. ACS Appl. Mater. Interfaces 2019, 11, 31069–31077. 10.1021/acsami.9b09044. [DOI] [PubMed] [Google Scholar]
- He Q.; Shahid M.; Jiao X.; Gann E.; Eisner F. D.; Wu T.; Fei Z.; Anthopoulos T. D.; McNeill C. R.; Heeney M. Crucial Role of Fluorine in Fully Alkylated Ladder-Type Carbazole-Based Nonfullerene Organic Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 9555–9562. 10.1021/acsami.0c00981. [DOI] [PubMed] [Google Scholar]
- Tu Q.; Ma Y.; Zhou X.; Ma W.; Zheng Q. Enhancing the Photovoltaic Performance of Ladder-Type Dithienocyclopentacarbazole-Based Nonfullerene Acceptors through Fluorination and Side-Chain Engineering. Chem. Mater. 2019, 31, 5953–5963. 10.1021/acs.chemmater.9b02355. [DOI] [Google Scholar]
- Tang C.; Cai D.; Zheng Q. Dithienocyclopentadibenzothiophene: a C2v -symmetric core for nonfullerene acceptors with tunable bandgaps. J. Mater. Chem. A 2019, 7, 9609–9617. 10.1039/C9TA02544H. [DOI] [Google Scholar]
- Wang Y.; Zhang Y.; Qiu N.; Feng H.; Gao H.; Kan B.; Ma Y.; Li C.; Wan X.; Chen Y. A Halogenation Strategy for over 12% Efficiency Nonfullerene Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1702870. 10.1002/aenm.201702870. [DOI] [Google Scholar]
- Li G.; Li D.; Ma R.; Liu T.; Luo Z.; Cui G.; Tong L.; Zhang M.; Wang Z.; Liu F.; Xu L.; Yan H.; Tang B. Efficient modulation of end groups for the asymmetric small molecule acceptors enabling organic solar cells with over 15% efficiency. J. Mater. Chem. A 2020, 8, 5927–5935. 10.1039/D0TA01032D. [DOI] [Google Scholar]
- Cao J.; Qu S.; Yu J.; Zhang Z.; Geng R.; Yang L.; Wang H.; Du F.; Tang W. 13.76% efficiency nonfullerene solar cells enabled by selenophene integrated dithieno[3,2-b:2′,3′-d]pyrrole asymmetric acceptors. Mater. Chem. Front. 2020, 4, 924–932. 10.1039/C9QM00775J. [DOI] [Google Scholar]
- Jin K.; Deng C.; Zhang L.; Li D.; Li T.; Wang F.; Yuan Y.; Xiao Z.; Ding L. A heptacyclic carbon–oxygen-bridged ladder-type building block for A–D–A acceptors. Mater. Chem. Front. 2018, 2, 1716–1719. 10.1039/C8QM00285A. [DOI] [Google Scholar]
- Lin K.; Xie B.; Wang Z.; Xie R.; Huang Y.; Duan C.; Huang F.; Cao Y. Star-shaped electron acceptors containing a truxene core for non-fullerene solar cells. Org. Electron. 2018, 52, 42–50. 10.1016/j.orgel.2017.10.009. [DOI] [Google Scholar]
- Xu X.; Zhang G.; Yu L.; Li R.; Peng Q. P3HT-Based Polymer Solar Cells with 8.25% Efficiency Enabled by a Matched Molecular Acceptor and Smart Green-Solvent Processing Technology. Adv. Mater. 2019, 31, 1906045. 10.1002/adma.201906045. [DOI] [PubMed] [Google Scholar]
- Yuan J.; Huang T.; Cheng P.; Zou Y.; Zhang H.; Yang J. L.; Chang S.-Y.; Zhang Z.; Huang W.; Wang R.; Meng D.; Gao F.; Yang Y. Enabling low voltage losses and high photocurrent in fullerene-free organic photovoltaics. Nat. Commun. 2019, 10, 570. 10.1038/s41467-019-08386-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Ma X.; Luo M.; Gao W.; Yuan J.; An Q.; Zhang M.; Hu Z.; Gao J.; Wang J.; Zou Y.; Yang C.; Zhang F. Achieving 14.11% efficiency of ternary polymer solar cells by simultaneously optimizing photon harvesting and exciton distribution. J. Mater. Chem. A 2019, 7, 7843–7851. 10.1039/C9TA01497G. [DOI] [Google Scholar]
- Luo M.; Zhou L.; Yuan J.; Zhu C.; Cai F.; Hai J.; Zou Y. A new non-fullerene acceptor based on the heptacyclic benzotriazole unit for efficient organic solar cells. J. Energy Chem. 2020, 42, 169–173. 10.1016/j.jechem.2019.07.002. [DOI] [Google Scholar]
- Yuan J.; Zhang Y.; Zhou L.; Zhang C.; Lau T.-K.; Zhang G.; Lu X.; Yip H.-L.; So S. K.; Beaupré S.; Mainville M.; Johnson P. A.; Leclerc M.; Chen H.; Peng H.; Li Y.; Zou Y. Fused Benzothiadiazole: A Building Block for n-Type Organic Acceptor to Achieve High-Performance Organic Solar Cells. Adv. Mater. 2019, 31, 1807577. 10.1002/adma.201807577. [DOI] [PubMed] [Google Scholar]
- Tao L.; Liu X.; Deng C.; Zhang W.; Song W. Highly Efficient Nonfullerene Acceptor with Sulfonyl-Based Ending Groups. ACS Appl. Mater. Interfaces 2020, 12, 49659–49665. 10.1021/acsami.0c14774. [DOI] [PubMed] [Google Scholar]
- Li Y.; Meng H.; Huang J.; Zhan C. Structural Cutting of Non-fullerene Acceptors by Chlorination: Effects of Substituent Number on Device Performance. ACS Appl. Mater. Interfaces 2020, 12, 50541–50549. 10.1021/acsami.0c16389. [DOI] [PubMed] [Google Scholar]
- Gao W.; An Q.; Zhong C.; Luo Z.; Ming R.; Zhang M.; Zou Y.; Liu F.; Zhang F.; Yang C. Designing an asymmetrical isomer to promote the LUMO energy level and molecular packing of a non-fullerene acceptor for polymer solar cells with 12.6% efficiency. Chem. Sci. 2018, 9, 8142–8149. 10.1039/C8SC02018C. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Fan B.; Zhang D.; Li M.; Zhong W.; Zeng Z.; Ying L.; Huang F.; Cao Y. Achieving over 16% efficiency for single-junction organic solar cells. Sci. China: Chem. 2019, 62, 746–752. 10.1007/s11426-019-9457-5. [DOI] [Google Scholar]
- Wei Q.; Liu W.; Leclerc M.; Yuan J.; Chen H.; Zou Y. A-DA′D-A non-fullerene acceptors for high-performance organic solar cells. Sci. China: Chem. 2020, 63, 1352–1366. 10.1007/s11426-020-9799-4. [DOI] [Google Scholar]
- Sun R.; Wang T.; Luo Z.; Hu Z.; Huang F.; Yang C.; Min J. Achieving Eco-Compatible Organic Solar Cells with Efficiency >16.5% Based on an Iridium Complex-Incorporated Polymer Donor. Sol. RRL 2020, 4, 2000156. 10.1002/solr.202000156. [DOI] [Google Scholar]
- Luo Z.; Sun R.; Zhong C.; Liu T.; Zhang G.; Zou Y.; Jiao X.; Min J.; Yang C. Altering alkyl-chains branching positions for boosting the performance of small-molecule acceptors for highly efficient nonfullerene organic solar cells. Sci. China: Chem. 2020, 63, 361–369. 10.1007/s11426-019-9670-2. [DOI] [Google Scholar]
- Hong L.; Yao H.; Wu Z.; Cui Y.; Zhang T.; Xu Y.; Yu R.; Liao Q.; Gao B.; Xian K.; Woo H. Y.; Ge Z.; Hou J. Eco-Compatible Solvent-Processed Organic Photovoltaic Cells with Over 16% Efficiency. Adv. Mater. 2019, 31, 1903441. 10.1002/adma.201903441. [DOI] [PubMed] [Google Scholar]
- Yi L.; Dai S.; Sun R.; Wang W.; Wu Y.; Jiao X.; Zhang C.; Min J. Alkyl chain engineering of non-fullerene small molecule acceptors for solution-processable organic solar cells. Org. Electron. 2020, 87, 105963. 10.1016/j.orgel.2020.105963. [DOI] [Google Scholar]
- Han Y.; Song W.; Zhang J.; Xie L.; Xiao J.; Li Y.; cao L.; Song S.; Zhou E.; Ge Z. Annealing-free efficient organic solar cells via an alkylbenzene side-chain strategy of small-molecule electron acceptors. J. Mater. Chem. A 2020, 8, 22155–22162. 10.1039/D0TA05787H. [DOI] [Google Scholar]
- Dong S.; Jia T.; Zhang K.; Jing J.; Huang F. Single-Component Non-halogen Solvent-Processed High-Performance Organic Solar Cell Module with Efficiency over 14%. Joule 2020, 4, 2004–2016. 10.1016/j.joule.2020.07.028. [DOI] [Google Scholar]
- Cheung A. M. H.; Yu H.; Luo S.; Wang Z.; Qi Z.; Zhou W.; Arunagiri L.; Chang Y.; Yao H.; Ade H.; Yan H. Incorporation of alkylthio side chains on benzothiadiazole-based non-fullerene acceptors enables high-performance organic solar cells with over 16% efficiency. J. Mater. Chem. A 2020, 8, 23239–23247. 10.1039/D0TA08830G. [DOI] [Google Scholar]
- Liu T.; Zhang Y.; Shao Y.; Ma R.; Luo Z.; Xiao Y.; Yang T.; Lu X.; Yuan Z.; Yan H.; Chen Y.; Li Y. Asymmetric Acceptors with Fluorine and Chlorine Substitution for Organic Solar Cells toward 16.83% Efficiency. Adv. Funct. Mater. 2020, 30, 2000456. 10.1002/adfm.202000456. [DOI] [Google Scholar]
- Chai G.; Chang Y.; Peng Z.; Jia Y.; Zou X.; Yu D.; Yu H.; Chen Y.; Chow P. C.; Wong K. S.; Zhang J.; Ade H.; Yang L.; Zhan C. Enhanced hindrance from phenyl outer side chains on nonfullerene acceptor enables unprecedented simultaneous enhancement in organic solar cell performances with 16.7% efficiency. Nano Energy 2020, 76, 105087. 10.1016/j.nanoen.2020.105087. [DOI] [Google Scholar]
- Jiang K.; Wei Q.; Lai J. Y. L.; Peng Z.; Kim H. K.; Yuan J.; Ye L.; Ade H.; Zou Y.; Yan H. Alkyl Chain Tuning of Small Molecule Acceptors for Efficient Organic Solar Cells. Joule 2019, 3, 3020–3033. 10.1016/j.joule.2019.09.010. [DOI] [Google Scholar]
- Cui Y.; Yao H.; Zhang J.; Zhang T.; Wang Y.; Hong L.; Xian K.; Xu B.; Zhang S.; Peng J.; Wei Z.; Gao F.; Hou J. Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages. Nat. Commun. 2019, 10, 2515. 10.1038/s41467-019-10351-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Yu H.; Ma R.; Xiao Y.; Zhang J.; Liu T.; Luo Z.; Chen Y.; Bai F.; Lu X.; Yan H.; Lin H. Improved organic solar cell efficiency based on the regulation of an alkyl chain on chlorinated non-fullerene acceptors. Mater. Chem. Front. 2020, 4, 2428–2434. 10.1039/D0QM00151A. [DOI] [Google Scholar]
- Cui Y.; Yao H.; Hong L.; Zhang T.; Tang Y.; Lin B.; Xian K.; Gao B.; An C.; Bi P.; Ma W.; Hou J. Organic photovoltaic cell with 17% efficiency and superior processability. Natl. Sci. Rev. 2020, 7, 1239–1246. 10.1093/nsr/nwz200. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Mo D.; Chen H.; Zhou J.; Tang N.; Han L.; Zhu Y.; Chao P.; Lai H.; Xie Z.; He F. Alkyl chain engineering of chlorinated acceptors for elevated solar conversion. J. Mater. Chem. A 2020, 8, 8903–8912. 10.1039/C9TA12558B. [DOI] [Google Scholar]
- Xu X.; Yu L.; Yan H.; Li R.; Peng Q. Highly efficient non-fullerene organic solar cells enabled by a delayed processing method using a non-halogenated solvent. Energy Environ. Sci. 2020, 13, 4381–4388. 10.1039/D0EE02034F. [DOI] [Google Scholar]
- Mo D.; Chen H.; Zhu Y.; Huang H.-H.; Chao P.; He F. Synergistic Effect of Alkyl Chain and Chlorination Engineering on High-Performance Nonfullerene Acceptors. ACS Appl. Mater. Interfaces 2020, 12, 28329–28336. 10.1021/acsami.0c07856. [DOI] [PubMed] [Google Scholar]
- Cui Y.; Yao H.; Zhang J.; Xian K.; Zhang T.; Hong L.; Wang Y.; Xu Y.; Ma K.; An C.; He C.; Wei Z.; Gao F.; Hou J. Single-Junction Organic Photovoltaic Cells with Approaching 18% Efficiency. Adv. Mater. 2020, 32, 1908205. 10.1002/adma.201908205. [DOI] [PubMed] [Google Scholar]
- Xu Y.; Yao H.; Ma L.; Wu Z.; Cui Y.; Hong L.; Zu Y.; Wang J.; Woo H. Y.; Hou J. Organic photovoltaic cells with high efficiencies for both indoor and outdoor applications. Mater. Chem. Front. 2021, 5, 893–900. 10.1039/D0QM00633E. [DOI] [Google Scholar]
- Wang H.; Liu T.; Zhou J.; Mo D.; Han L.; Lai H.; Chen H.; Zheng N.; Zhu Y.; Xie Z.; He F. Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors. Adv. Sci. 2020, 7, 1903784. 10.1002/advs.201903784. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Li J.; Zhang Y.; Yuan J.; Zhu C.; Peng H.; Zou Y. Fine-tuning of non-fullerene acceptor gives over 14% efficiency for organic solar cells. Dyes Pigm. 2020, 181, 108559. 10.1016/j.dyepig.2020.108559. [DOI] [Google Scholar]
- Lai H.; Zhao Q.; Chen Z.; Chen H.; Chao P.; Zhu Y.; Lang Y.; Zhen N.; Mo D.; Zhang Y.; He F. Trifluoromethylation Enables a 3D Interpenetrated Low-Band-Gap Acceptor for Efficient Organic Solar Cells. Joule 2020, 4, 688–700. 10.1016/j.joule.2020.02.004. [DOI] [Google Scholar]
- Zhan L.; Li S.; Lau T.-K.; Cui Y.; Lu X.; Shi M.; Li C.-Z.; Li H.; Hou J.; Chen H. Over 17% efficiency ternary organic solar cells enabled by two non-fullerene acceptors working in an alloy-like model. Energy Environ. Sci. 2020, 13, 635–645. 10.1039/C9EE03710A. [DOI] [Google Scholar]
- Jia J.; Jing J.; Jia T.; Zhang K.; Zhang J.; Zhang J.; Huang F.; Yang C. The regioisomeric bromination effects of fused-ring electron acceptors: modulation of the optoelectronic property and miscibility endowing the polymer solar cells with 15% efficiency. J. Mater. Chem. A 2020, 8, 25101–25108. 10.1039/D0TA09707A. [DOI] [Google Scholar]
- Li X.; Angunawela I.; Chang Y.; Zhou J.; Huang H.; Zhong L.; Liebman-Pelaez A.; Zhu C.; Meng L.; Xie Z.; Ade H.; Yan H.; Li Y. Effect of the chlorine substitution position of the end-group on intermolecular interactions and photovoltaic performance of small molecule acceptors. Energy Environ. Sci. 2020, 13, 5028–5038. 10.1039/D0EE02251A. [DOI] [Google Scholar]
- Chen H.; Lai H.; Chen Z.; Zhu Y.; Wang H.; Han L.; Zhang Y.; He F. 17.1%-Efficient Eco-Compatible Organic Solar Cells from a Dissymmetric 3D Network Acceptor. Angew. Chem., Int. Ed. 2021, 60, 3238–3246. 10.1002/anie.202013053. [DOI] [PubMed] [Google Scholar]
- Li S.; Zhan L.; Jin Y.; Zhou G.; Lau T.-K.; Qin R.; Shi M.; Li C.-Z.; Zhu H.; Lu X.; Zhang F.; Chen H. Asymmetric Electron Acceptors for High-Efficiency and Low-Energy-Loss Organic Photovoltaics. Adv. Mater. 2020, 32, 2001160. 10.1002/adma.202001160. [DOI] [PubMed] [Google Scholar]
- Luo Z.; Ma R.; Chen Z.; Xiao Y.; Zhang G.; Liu T.; Sun R.; Zhan Q.; Zou Y.; Zhong C.; Chen Y.; Sun H.; Chai G.; Chen K.; Guo X.; Min J.; Lu X.; Yang C.; Yan H. Altering the Positions of Chlorine and Bromine Substitution on the End Group Enables High-Performance Acceptor and Efficient Organic Solar Cells. Adv. Energy Mater. 2020, 10, 2002649. 10.1002/aenm.202002649. [DOI] [Google Scholar]
- Yang C.; Zhang S.; Ren J.; Gao M.; Bi P.; Ye L.; Hou J. Molecular design of a non-fullerene acceptor enables a P3HT-based organic solar cell with 9.46% efficiency. Energy Environ. Sci. 2020, 13, 2864–2869. 10.1039/D0EE01763A. [DOI] [Google Scholar]
- Zhang Y.; Cai F.; Yuan J.; Wei Q.; Zhou L.; Qiu B.; Hu Y.; Li Y.; Peng H.; Zou Y. A new non-fullerene acceptor based on the combination of a heptacyclic benzothiadiazole unit and a thiophene-fused end group achieving over 13% efficiency. Phys. Chem. Chem. Phys. 2019, 21, 26557–26563. 10.1039/C9CP05015A. [DOI] [PubMed] [Google Scholar]
- Li X.; Pan M.-A.; Lau T.-K.; Liu W.; Li K.; Yao N.; Shen F.; Huo S.; Zhang F.; Wu Y.; Li X.; Lu X.; Yan H.; Zhan C. Roles of Acceptor Guests in Tuning the Organic Solar Cell Property Based on an Efficient Binary Material System with a Nearly Zero Hole-Transfer Driving Force. Chem. Mater. 2020, 32, 5182–5191. 10.1021/acs.chemmater.0c01245. [DOI] [Google Scholar]
- Luo Z.; Ma R.; Liu T.; Yu J.; Xiao Y.; Sun R.; Xie G.; Yuan J.; Chen Y.; Chen K.; Chai G.; Sun H.; Min J.; Zhang J.; Zou Y.; Yang C.; Lu X.; Gao F.; Yan H. Fine-Tuning Energy Levels via Asymmetric End Groups Enables Polymer Solar Cells with Efficiencies over 17%. Joule 2020, 4, 1236–1247. 10.1016/j.joule.2020.03.023. [DOI] [Google Scholar]
- Yang J.; Geng Y.; Li J.; Zhao B.; Guo Q.; Zhou E. A-DA′D-A-Type Non-fullerene Acceptors Containing a Fused Heptacyclic Ring for Poly(3-hexylthiophene)-Based Polymer Solar Cells. J. Phys. Chem. C 2020, 124, 24616–24623. 10.1021/acs.jpcc.0c07162. [DOI] [Google Scholar]
- Zhu C.; Yuan J.; Cai F.; Meng L.; Zhang H.; Chen H.; Li J.; Qiu B.; Peng H.; Chen S.; Hu Y.; Yang C.; Gao F.; Zou Y.; Li Y. Tuning the electron-deficient core of a non-fullerene acceptor to achieve over 17% efficiency in a single-junction organic solar cell. Energy Environ. Sci. 2020, 13, 2459–2466. 10.1039/D0EE00862A. [DOI] [Google Scholar]
- Zhang M.; Zeng M.; Ye L.; Tan S.; Zhao B.; Ryu H. S.; Woo H. Y.; Sun Y. Effects of monohalogenated terminal units of non-fullerene acceptors on molecular aggregation and photovoltaic performance. Sol. Energy 2020, 208, 866–872. 10.1016/j.solener.2020.07.100. [DOI] [Google Scholar]
- Sun C.; Qin S.; Wang R.; Chen S.; Pan F.; Qiu B.; Shang Z.; Meng L.; Zhang C.; Xiao M.; Yang C.; Li Y. High Efficiency Polymer Solar Cells with Efficient Hole Transfer at Zero Highest Occupied Molecular Orbital Offset between Methylated Polymer Donor and Brominated Acceptor. J. Am. Chem. Soc. 2020, 142, 1465–1474. 10.1021/jacs.9b09939. [DOI] [PubMed] [Google Scholar]
- Xu X.; Feng K.; Lee Y. W.; Woo H. Y.; Zhang G.; Peng Q. Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss. Adv. Funct. Mater. 2020, 30, 1907570. 10.1002/adfm.201907570. [DOI] [Google Scholar]
- Chai G.; Zhang J.; Pan M.; Wang Z.; Yu J.; Liang J.; Yu H.; Chen Y.; Shang A.; Liu X.; Bai F.; Ma R.; Chang Y.; Luo S.; Zeng A.; Zhou H.; Chen K.; Gao F.; Ade H.; Yan H. Deciphering the Role of Chalcogen-Containing Heterocycles in Nonfullerene Acceptors for Organic Solar Cells. ACS Energy Lett. 2020, 5, 3415–3425. 10.1021/acsenergylett.0c01688. [DOI] [Google Scholar]
- Lin F.; Jiang K.; Kaminsky W.; Zhu Z.; Jen A. K.-Y. A Non-fullerene Acceptor with Enhanced Intermolecular π-Core Interaction for High-Performance Organic Solar Cells. J. Am. Chem. Soc. 2020, 142, 15246–15251. 10.1021/jacs.0c07083. [DOI] [PubMed] [Google Scholar]
- Yu H.; Qi Z.; Zhang J.; Wang Z.; Sun R.; Chang Y.; Sun H.; Zhou W.; Min J.; Ade H.; Yan H. Tailoring non-fullerene acceptors using selenium-incorporated heterocycles for organic solar cells with over 16% efficiency. J. Mater. Chem. A 2020, 8, 23756–23765. 10.1039/D0TA06658C. [DOI] [Google Scholar]
- Zhang Z.; Li Y.; Cai G.; Zhang Y.; Lu X.; Lin Y. Selenium Heterocyclic Electron Acceptor with Small Urbach Energy for As-Cast High-Performance Organic Solar Cells. J. Am. Chem. Soc. 2020, 142, 18741–18745. 10.1021/jacs.0c08557. [DOI] [PubMed] [Google Scholar]
- Yuan J.; Zhang C.; Chen H.; Zhu C.; Cheung S. H.; Qiu B.; Cai F.; Wei Q.; Liu W.; Yin H.; Zhang R.; Zhang J.; Liu Y.; Zhang H.; Liu W.; Peng H.; Yang J.; Meng L.; Gao F.; So S.; Li Y.; Zou Y. Understanding energetic disorder in electron-deficient-core-based non-fullerene solar cells. Sci. China: Chem. 2020, 63, 1159–1168. 10.1007/s11426-020-9747-9. [DOI] [Google Scholar]
- Wang R.; Yuan J.; Han G.; Huang T.; Huang W.; Xue J.; Wang H.-C.; Zhang C.; Zhu C.; Cheng P.; Meng D.; Yi Y.; Wei K.-H.; Zou Y.; Yang Y. Rational Tuning of Molecular Interaction and Energy Level Alignment Enables High-Performance Organic Photovoltaics. Adv. Mater. 2019, 31, 1904215. 10.1002/adma.201904215. [DOI] [PubMed] [Google Scholar]
- Cao R.; Chen Y.; Cai F.; Chen H.; Liu W.; Guan H.; Wei Q.; Li J.; Chang Q.; Li Z.; Zou Y. A new chlorinated non-fullerene acceptor based organic photovoltaic cells over 12% efficiency. J. Cent. South Univ. 2020, 27, 3581–3593. 10.1007/s11771-020-4501-0. [DOI] [Google Scholar]
- Luo M.; Zhu C.; Yuan J.; Zhou L.; Keshtov M. L.; Godovsky D. Y.; Zou Y. A chlorinated non-fullerene acceptor for efficient polymer solar cells. Chin. Chem. Lett. 2019, 30, 2343–2346. 10.1016/j.cclet.2019.07.023. [DOI] [Google Scholar]
- Liu S.; Yuan J.; Deng W.; Luo M.; Xie Y.; Liang Q.; Zou Y.; He Z.; Wu H.; Cao Y. High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder. Nat. Photonics 2020, 14, 300–305. 10.1038/s41566-019-0573-5. [DOI] [Google Scholar]
- Chen Y.; Koukaras E. N.; Zou Y.; Dahiya H.; Liu Y.; Liu H.; Sharma G. D. A–DA′D–A Nonfullerene Acceptor Obtained by Fine-Tuning Side Chains on Pyrroles Enables PBDB-T-Based Organic Solar Cells with over 14% Efficiency. ACS Appl. Energy Mater. 2020, 3, 11981–11991. 10.1021/acsaem.0c02165. [DOI] [Google Scholar]
- Luo M.; Zhao C.; Yuan J.; Hai J.; Cai F.; Hu Y.; Peng H.; Bai Y.; Tan Z.; Zou Y. Semitransparent solar cells with over 12% efficiency based on a new low bandgap fluorinated small molecule acceptor. Mater. Chem. Front. 2019, 3, 2483–2490. 10.1039/C9QM00499H. [DOI] [Google Scholar]
- Zhu C.; An K.; Zhong W.; Li Z.; Qian Y.; Su X.; Ying L. Design and synthesis of non-fullerene acceptors based on a quinoxalineimide moiety as the central building block for organic solar cells. Chem. Commun. 2020, 56, 4700–4703. 10.1039/D0CC00896F. [DOI] [PubMed] [Google Scholar]
- Zhou Z.; Liu W.; Zhou G.; Zhang M.; Qian D.; Zhang J.; Chen S.; Xu S.; Yang C.; Gao F.; Zhu H.; Liu F.; Zhu X. Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation. Adv. Mater. 2020, 32, 1906324. 10.1002/adma.201906324. [DOI] [PubMed] [Google Scholar]
- Zhao Y.; Chen H.; Zhu C.; Yuan J.; Li Y.; Hai J.; Hu Y.; Jiang L.; Chen G.; Zou Y. Fine-tuning the energy levels and morphology via fluorination and thermal annealing enable high efficiency non-fullerene organic solar cells. Mater. Chem. Front. 2020, 4, 3310–3318. 10.1039/D0QM00034E. [DOI] [Google Scholar]
- Chen Y.; Cao R.; Liu H.; Keshtov M. L.; Koukaras E. N.; Dahiya H.; Zou Y.; Sharma G. D. Indole-based A–DA′D–A type acceptor-based organic solar cells achieve efficiency over 15 % with low energy loss. Sustain. Energy Fuels 2020, 4, 6203–6211. 10.1039/D0SE01343A. [DOI] [Google Scholar]
- Luo Z.; Ma R.; Xiao Y.; Liu T.; Sun H.; Su M.; Guo Q.; Li G.; Gao W.; Chen Y.; Zou Y.; Guo X.; Zhang M.; Lu X.; Yan H.; Yang C. Conformation-Tuning Effect of Asymmetric Small Molecule Acceptors on Molecular Packing, Interaction, and Photovoltaic Performance. Small 2020, 16, 2001942. 10.1002/smll.202001942. [DOI] [PubMed] [Google Scholar]
- Ke X.; Kan B.; Wan X.; Wang Y.; Zhang Y.; Li C.; Chen Y. Substituents on the end group subtle tuning the energy levels and absorptions of small-molecule nonfullerene acceptors. Dyes Pigm. 2018, 155, 241–248. 10.1016/j.dyepig.2018.03.061. [DOI] [Google Scholar]
- An Q.; Gao W.; Zhang F.; Wang J.; Zhang M.; Wu K.; Ma X.; Hu Z.; Jiao C.; Yang C. Energy level modulation of non-fullerene acceptors enables efficient organic solar cells with small energy loss. J. Mater. Chem. A 2018, 6, 2468–2475. 10.1039/C7TA10763C. [DOI] [Google Scholar]
- Jia Y.; Zhang Y.; Jiang W.; Su B.; Liu C.; Zhu E.; Che G. A fused-ring non-fullerene acceptor based on a benzo[1,2-b:4,5-b′]dithiophene central core with a thieno[3,2-b]thiophene side-chain for highly efficient organic solar cells. J. Mater. Chem. A 2019, 7, 10905–10911. 10.1039/C9TA01888C. [DOI] [Google Scholar]
- Kan B.; Zhang J.; Liu F.; Wan X.; Li C.; Ke X.; Wang Y.; Feng H.; Zhang Y.; Long G.; Friend R. H.; Bakulin A. A.; Chen Y. Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells. Adv. Mater. 2018, 30, 1704904. 10.1002/adma.201704904. [DOI] [PubMed] [Google Scholar]
- Li Y.; Su W.; Wu F.; Xu Z.; Wang Y.-K.; Zhang M.; Jiang Z.-Q.; Liao L.-S. Near-infrared non-fullerene acceptors based on dithienyl[1,2-b:4,5-b′]benzodithiophene core for high performance PTB7-Th-based polymer solar cells. Org. Electron. 2019, 65, 63–69. 10.1016/j.orgel.2018.10.042. [DOI] [Google Scholar]
- Li G.; Wu J.; Fang J.; Guo X.; Zhu L.; Liu F.; Zhang M.; Li Y. A Non-Fullerene Acceptor with Chlorinated Thienyl Conjugated Side Chains for High-Performance Polymer Solar Cells via Toluene Processing. Chin. J. Chem. 2020, 38, 697–702. 10.1002/cjoc.201900495. [DOI] [Google Scholar]
- Chang S.-L.; Cao F.-Y.; Huang W.-C.; Huang P.-K.; Huang K.-H.; Hsu C.-S.; Cheng Y.-J. New Thieno[3,2-b]thiophene-Based Acceptor: Tuning Acceptor Strength of Ladder-Type N-Type Materials to Simultaneously Achieve Enhanced VOC and JSC of Nonfullerene Solar Cells. ACS Energy Lett. 2018, 3, 1722–1729. 10.1021/acsenergylett.8b00563. [DOI] [Google Scholar]
- Guo Q.; Zhu X.; Dong X.; Zhu Q.; Fang J.; Guo X.; Ma W.; Zhang M.; Li Y. A non-fullerene acceptor based on alkylphenyl substituted benzodithiophene for high efficiency polymer solar cells with a small voltage loss and excellent stability. J. Mater. Chem. A 2019, 7, 24366–24373. 10.1039/C9TA08636F. [DOI] [Google Scholar]
- Li Y.; Guo X.; Peng Z.; Qu B.; Yan H.; Ade H.; Zhang M.; Forrest S. R. Color-neutral, semitransparent organic photovoltaics for power window applications. Proc. Natl. Acad. Sci. U. S. A. 2020, 117, 21147–21154. 10.1073/pnas.2007799117. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Yuan G.-Z.; Fan H.; Wan S.-S.; Jiang Z.; Liu Y.-Q.; Liu K.-K.; Bai H.-R.; Zhu X.; Wang J.-L. A two-dimensional halogenated thiophene side-chain strategy for balancing VOC and JSC and improving efficiency of non-fullerene small molecule acceptor-based organic solar cells. J. Mater. Chem. A 2019, 7, 20274–20284. 10.1039/C9TA06311K. [DOI] [Google Scholar]
- Kan B.; Feng H.; Yao H.; Chang M.; Wan X.; Li C.; Hou J.; Chen Y. A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss. Sci. China: Chem. 2018, 61, 1307–1313. 10.1007/s11426-018-9334-9. [DOI] [Google Scholar]
- Yan J.; Yi Y.-Q.-Q.; Zhang J.; Feng H.; Ma Y.; Wan X.; Li C.; Wei Z.; Chen Y. A privileged ternary blend enabling non-fullerene organic photovoltaics with over 14% efficiency. J. Mater. Chem. C 2020, 8, 15135–15141. 10.1039/D0TC02778B. [DOI] [Google Scholar]
- Kan B.; Chen X.; Gao K.; Zhang M.; Lin F.; Peng X.; Liu F.; Jen A. K.-Y. Asymmetrical side-chain engineering of small-molecule acceptors enable high-performance nonfullerene organic solar cells. Nano Energy 2020, 67, 104209. 10.1016/j.nanoen.2019.104209. [DOI] [Google Scholar]
- Chen X.; Kan B.; Kan Y.; Zhang M.; Jo S. B.; Gao K.; Lin F.; Liu F.; Peng X.; Cao Y.; Jen A. K.-Y. As-Cast Ternary Organic Solar Cells Based on an Asymmetric Side-Chains Featured Acceptor with Reduced Voltage Loss and 14.0% Efficiency. Adv. Funct. Mater. 2020, 30, 1909535. 10.1002/adfm.201909535. [DOI] [Google Scholar]
- Lai H.; Guo M.; Zhu Y.; Chen L.; Tan P.; Yang C.; He F. The cis - and trans -orientation of benzo[1,2-b:4,5-b′]dithiophene-based isomers in organic solar cells. Mater. Chem. Front. 2021, 5, 1486–1494. 10.1039/D0QM00778A. [DOI] [Google Scholar]
- Gao W.; Liu T.; Luo Z.; Zhang L.; Ming R.; Zhong C.; Ma W.; Yan H.; Yang C. Regulating exciton bonding energy and bulk heterojunction morphology in organic solar cells via methyl-functionalized non-fullerene acceptors. J. Mater. Chem. A 2019, 7, 6809–6817. 10.1039/C9TA00597H. [DOI] [Google Scholar]
- Zhang Y.; Cho Y.; Zhong L.; Wang Y.; Huang B.; Yuan Z.; Yang C.; Chen Y.; Li Y. Two-Dimension Conjugated Acceptors Based on Benzodi(cyclopentadithiophene) Core with Thiophene-Fused Ending Group for Efficient Polymer Solar Cells. Sol. RRL 2020, 4, 2000071. 10.1002/solr.202000071. [DOI] [Google Scholar]
- Liu T.; Gao W.; Wang Y.; Yang T.; Ma R.; Zhang G.; Zhong C.; Ma W.; Yan H.; Yang C. Unconjugated Side-Chain Engineering Enables Small Molecular Acceptors for Highly Efficient Non-Fullerene Organic Solar Cells: Insights into the Fine-Tuning of Acceptor Properties and Micromorphology. Adv. Funct. Mater. 2019, 29, 1902155. 10.1002/adfm.201902155. [DOI] [Google Scholar]
- Gao W.; Liu T.; Ming R.; Luo Z.; Wu K.; Zhang L.; Xin J.; Xie D.; Zhang G.; Ma W.; Yan H.; Yang C. Near-Infrared Small Molecule Acceptor Enabled High-Performance Nonfullerene Polymer Solar Cells with Over 13% Efficiency. Adv. Funct. Mater. 2018, 28, 1803128. 10.1002/adfm.201803128. [DOI] [Google Scholar]
- Ma Y.; Cai D.; Wan S.; Yin P.; Wang P.; Lin W.; Zheng Q. Control over π-π stacking of heteroheptacene-based nonfullerene acceptors for 16% efficiency polymer solar cells. Natl. Sci. Rev. 2020, 7, 1886–1895. 10.1093/nsr/nwaa189. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Cai D.; Zhang J.; Wang J.-Y.; Ma Y.; Wan S.; Wang P.; Wei Z.; Zheng Q. Enhancing the photovoltaic performance of heteroheptacene-based nonfullerene acceptors through the synergistic effect of side-chain engineering and fluorination. J. Mater. Chem. A 2020, 8, 24543–24552. 10.1039/D0TA09351C. [DOI] [Google Scholar]
- Li G.; Xu C.; Luo Z.; Ning W.; Liu X.; Gong S.; Zou Y.; Zhang F.; Yang C. Novel Nitrogen-Containing Heterocyclic Non-Fullerene Acceptors for Organic PhotovoltaicCells: Different End-Capping Groups Leading to a Big Difference of Power Conversion Efficiencies. ACS Appl. Mater. Interfaces 2020, 12, 13068–13076. 10.1021/acsami.9b22093. [DOI] [PubMed] [Google Scholar]
- Ma S.; Wu S.; Zhang J.; Song Y.; Tang H.; Zhang K.; Huang F.; Cao Y. Heptacyclic S,N-Heteroacene-Based Near-Infrared Nonfullerene Acceptor Enables High-Performance Organic Solar Cells with Small Highest Occupied Molecular Orbital Offsets. ACS Appl. Mater. Interfaces 2020, 12, 51776–51784. 10.1021/acsami.0c19033. [DOI] [PubMed] [Google Scholar]
- Ma Y.; Cai D.; Wan S.; Wang P.; Wang J.; Zheng Q. Ladder-Type Heteroheptacenes with Different Heterocycles for Nonfullerene Acceptors. Angew. Chem., Int. Ed. 2020, 59, 21627–21633. 10.1002/anie.202007907. [DOI] [PubMed] [Google Scholar]
- Zeng G.; Xin Y.; Zhang B.; OuYang J.; Zhao X.; Yang X. Significantly Increasing the Power Conversion Efficiency by Controlling the Orientation of Nonfullerene Small Molecular Acceptors via Side Chain Engineering. Sol. RRL 2020, 4, 2000234. 10.1002/solr.202000234. [DOI] [Google Scholar]
- Ming R.; Wang J.; Gao W.; Zhang M.; Gao J.; Ning W.; Luo Z.; Liu X.; Zhong C.; Zhang F.; Yang C. Fused-Ring Core Engineering for Small Molecule Acceptors Enable High-Performance Nonfullerene Polymer Solar Cells. Small Methods 2019, 3, 1900280. 10.1002/smtd.201900280. [DOI] [Google Scholar]
- Xia T.; Li C.; Ryu H. S.; Sun X.; Woo H. Y.; Sun Y. Effect of Extended π-Conjugation of Central Cores on Photovoltaic Properties of Asymmetric Wide-Bandgap Nonfullerene Acceptors. Org. Mater. 2020, 02, 173–181. 10.1055/s-0040-1709999. [DOI] [Google Scholar]
- Gao W.; Liu T.; Sun R.; Zhang G.; Xiao Y.; Ma R.; Zhong C.; Lu X.; Min J.; Yan H.; Yang C. Dithieno[3,2-b:2′,3′]-dpyrrol-Fused Asymmetrical Electron Acceptors: A Study into the Effects of Nitrogen-Functionalization on Reducing Nonradiative Recombination Loss and Dipole Moment on Morphology. Adv. Sci. 2020, 7, 1902657. 10.1002/advs.201902657. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Cao J.; Wang H.; Qu S.; Yu J.; Yang L.; Zhang Z.; Du F.; Tang W. 2D Side-Chain Engineered Asymmetric Acceptors Enabling Over 14% Efficiency and 75% Fill Factor Stable Organic Solar Cells. Adv. Funct. Mater. 2020, 30, 2006141. 10.1002/adfm.202006141. [DOI] [Google Scholar]
- Guo Q.; Ma R.; Hu J.; Wang Z.; Sun H.; Dong X.; Luo Z.; Liu T.; Guo X.; Guo X.; Yan H.; Liu F.; Zhang M. Over 15% Efficiency Polymer Solar Cells Enabled by Conformation Tuning of Newly Designed Asymmetric Small-Molecule Acceptors. Adv. Funct. Mater. 2020, 30, 2000383. 10.1002/adfm.202000383. [DOI] [Google Scholar]
- Yang L.; Hu Z.; Zhang Z.; Cao J.; Wang H.; Yu J.; Zhang F.; Tang W. Molecular engineering of acceptors to control aggregation for optimized nonfullerene solar cells. J. Mater. Chem. A 2020, 8, 5458–5466. 10.1039/D0TA00651C. [DOI] [Google Scholar]
- Zhang Z.; Yang L.; Hu Z.; Yu J.; Liu X.; Wang H.; Cao J.; Zhang F.; Tang W. Charge density modulation on asymmetric fused-ring acceptors for high-efficiency photovoltaic solar cells. Mater. Chem. Front. 2020, 4, 1747–1755. 10.1039/D0QM00123F. [DOI] [Google Scholar]
- Song J.; Li C.; Ye L.; Koh C.; Cai Y.; Wei D.; Woo H. Y.; Sun Y. Extension of indacenodithiophene backbone conjugation enables efficient asymmetric A–D–A type non-fullerene acceptors. J. Mater. Chem. A 2018, 6, 18847–18852. 10.1039/C8TA07334A. [DOI] [Google Scholar]
- Li C.; Xia T.; Song J.; Fu H.; Ryu H. S.; Weng K.; Ye L.; Woo H. Y.; Sun Y. Asymmetric selenophene-based non-fullerene acceptors for high-performance organic solar cells. J. Mater. Chem. A 2019, 7, 1435–1441. 10.1039/C8TA11197A. [DOI] [Google Scholar]
- Zhang C.; Liu T.; Wang J.-L.; Liu K.-K.; Yang C.; Zhang H.-J.; Ma R.; Yan H. Tetrabromination versus Tetrachlorination: A Molecular Terminal Engineering of Nonfluorinated Acceptors to Control Aggregation for Highly Efficient Polymer Solar Cells with Increased VOC and Higher JSC Simultaneously. Sol. RRL 2020, 4, 2000212. 10.1002/solr.202000212. [DOI] [Google Scholar]
- Liu K.-K.; Xu X.; Wang J.-L.; Zhang C.; Ge G.-Y.; Zhuang F.-D.; Zhang H.-J.; Yang C.; Peng Q.; Pei J. Achieving high-performance non-halogenated nonfullerene acceptor-based organic solar cells with 13.7% efficiency via a synergistic strategy of an indacenodithieno[3,2-b]selenophene core unit and non-halogenated thiophene-based terminal group. J. Mater. Chem. A 2019, 7, 24389–24399. 10.1039/C9TA08328F. [DOI] [Google Scholar]
- Zhang Z.; Zhu X. Dithienosilole-based non-fullerene acceptors for efficient organic photovoltaics. J. Mater. Chem. A 2018, 6, 4266–4270. 10.1039/C7TA10957A. [DOI] [Google Scholar]
- Yao C.; Liu B.; Zhu Y.; Hong L.; Miao J.; Hou J.; He F.; Meng H. Highly fluorescent anthracene derivative as a non-fullerene acceptor in OSCs with small non-radiative energy loss of 0.22 eV and high PCEs of over 13%. J. Mater. Chem. A 2019, 7, 10212–10216. 10.1039/C9TA02332A. [DOI] [Google Scholar]
- Feng H.; Yi Y.-Q.-Q.; Ke X.; Yan J.; Zhang Y.; Wan X.; Li C.; Zheng N.; Xie Z.; Chen Y. New Anthracene-Fused Nonfullerene Acceptors for High-Efficiency Organic Solar Cells: Energy Level Modulations Enabling Match of Donor and Acceptor. Adv. Energy Mater. 2019, 9, 1803541. 10.1002/aenm.201803541. [DOI] [Google Scholar]
- Luo J.; Wang Y.; Liu B.; Wu Z.; Zhang Y.; Tang Y.; Chen P.; Liao Q.; Woo H. Y.; Guo X. Isomerization enabling near-infrared electron acceptors. RSC Adv. 2019, 9, 37287–37291. 10.1039/C9RA07911D. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Li W.; Xiao Z.; Cai J.; Smith J. A.; Spooner E. L.; Kilbride R. C.; Game O. S.; Meng X.; Li D.; Zhang H.; Chen M.; Gurney R. S.; Liu D.; Jones R. A.; Lidzey D. G.; Ding L.; Wang T. Correlating the electron-donating core structure with morphology and performance of carbon oxygen-bridged ladder-type non-fullerene acceptor based organic solar cells. Nano Energy 2019, 61, 318–326. 10.1016/j.nanoen.2019.04.053. [DOI] [Google Scholar]
- Ke X.; Meng L.; Wan X.; Sun Y.; Guo Z.; Wu S.; Zhang H.; Li C.; Chen Y. An oxygen heterocycle-fused fluorene based non-fullerene acceptor for high efficiency organic solar cells. Mater. Chem. Front. 2020, 4, 3594–3601. 10.1039/D0QM00287A. [DOI] [Google Scholar]
- Ke X.; Meng L.; Wan X.; Li M.; Sun Y.; Guo Z.; Wu S.; Zhang H.; Li C.; Chen Y. The rational and effective design of nonfullerene acceptors guided by a semi-empirical model for an organic solar cell with an efficiency over 15%. J. Mater. Chem. A 2020, 8, 9726–9732. 10.1039/D0TA03087B. [DOI] [Google Scholar]
- Song J.; Xue X.; Fan B.; Huo L.; Sun Y. A novel bifunctional A–D–A type small molecule for efficient organic solar cells. Mater. Chem. Front. 2018, 2, 1626–1630. 10.1039/C8QM00223A. [DOI] [Google Scholar]
- Wang J.; Ding Y.; Li C.; Zheng N.; Xie Z.; Ma Z.; Lu Y.; Wan X.; Chen Y. Effect of Nitro-Substituted Ending Groups on the Photovoltaic Properties of Nonfullerene Acceptors. ACS Appl. Mater. Interfaces 2020, 12, 41861–41868. 10.1021/acsami.0c11698. [DOI] [PubMed] [Google Scholar]
- Li T.; Yang L.; Wu Y.; Wang J.; Jia B.; Hu Q.; Russell T. P.; Zhan X. Comparison of Fused-Ring Electron Acceptors with One- and Multidimensional Conformations. ACS Appl. Mater. Interfaces 2020, 12, 23976–23983. 10.1021/acsami.0c04674. [DOI] [PubMed] [Google Scholar]
- Ma Q.; Jia Z.; Meng L.; Zhang J.; Zhang H.; Huang W.; Yuan J.; Gao F.; Wan Y.; Zhang Z.; Li Y. Promoting charge separation resulting in ternary organic solar cells efficiency over 17.5%. Nano Energy 2020, 78, 105272. 10.1016/j.nanoen.2020.105272. [DOI] [Google Scholar]
- Chen T.-W.; Yu J.-Y.; Lin Y.-W.; Peng S.-H.; Wu S.-H.; Su Y.-J.; Karapala V. K.; Hong L.; Yao H.; Hou J.; Hsu C.-S. Chlorinated Carbon-Bridged and Silicon-Bridged Carbazole-Based Nonfullerene Acceptors Manifest Synergistic Enhancement in Ternary Organic Solar Cell with Efficiency over 15%. Sol. RRL 2020, 4, 2000357. 10.1002/solr.202000357. [DOI] [Google Scholar]
- Wang P.; Jiao X.; Xu S.; Wu H.; McNeill C. R.; Fan H.; Zhu X. Boosted photovoltaic performance of indenothiophene-based molecular acceptor via fusing a thiophene. J. Mater. Chem. C 2020, 8, 630–636. 10.1039/C9TC03407B. [DOI] [Google Scholar]
- Jiang C.; Huang X.; Sun B.; Li Y.; Gao M.; Ye L.; Ade H.; Forrest S. R.; Fan J. A 3D nonfullerene electron acceptor with a 9,9′-bicarbazole backbone for high-efficiency organic solar cells. Org. Electron. 2020, 84, 105784. 10.1016/j.orgel.2020.105784. [DOI] [Google Scholar]
- Wu W.; Zhang G.; Xu X.; Wang S.; Li Y.; Peng Q. Wide Bandgap Molecular Acceptors with a Truxene Core for Efficient Nonfullerene Polymer Solar Cells: Linkage Position on Molecular Configuration and Photovoltaic Properties. Adv. Funct. Mater. 2018, 28, 1707493. 10.1002/adfm.201707493. [DOI] [Google Scholar]
- Li Y.; Gu M.; Pan Z.; Zhang B.; Yang X.; Gu J.; Chen Y. Indacenodithiophene: a promising building block for high performance polymer solar cells. J. Mater. Chem. A 2017, 5, 10798–10814. 10.1039/C7TA02562A. [DOI] [Google Scholar]
- Guo X.; Li W.; Guo H.; Guo B.; Wu J.; Ma W.; Zhang M.; Wong W.-Y. A wide-bandgap polymer based on the alkylphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene unit with high power conversion efficiency of over 11%. J. Mater. Chem. A 2018, 6, 16529–16536. 10.1039/C8TA05868G. [DOI] [Google Scholar]
- Lin Y.; Zhao F.; Prasad S. K. K.; Chen J.-D.; Cai W.; Zhang Q.; Chen K.; Wu Y.; Ma W.; Gao F.; Tang J.-X.; Wang C.; You W.; Hodgkiss J. M.; Zhan X. Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with 12% Efficiency. Adv. Mater. 2018, 30, 1706363. 10.1002/adma.201706363. [DOI] [PubMed] [Google Scholar]
- Fan Q.; Wang Y.; Zhang M.; Wu B.; Guo X.; Jiang Y.; Li W.; Guo B.; Ye C.; Su W.; Fang J.; Ou X.; Liu F.; Wei Z.; Sum T. C.; Russell T. P.; Li Y. High-Performance As-Cast Nonfullerene Polymer Solar Cells with Thicker Active Layer and Large Area Exceeding 11% Power Conversion Efficiency. Adv. Mater. 2018, 30, 1704546. 10.1002/adma.201704546. [DOI] [PubMed] [Google Scholar]
- Li Y.; Zheng N.; Yu L.; Wen S.; Gao C.; Sun M.; Yang R. A Simple Phenyl Group Introduced at the Tail of Alkyl Side Chains of Small Molecular Acceptors: New Strategy to Balance the Crystallinity of Acceptors and Miscibility of Bulk Heterojunction Enabling Highly Efficient Organic Solar Cells. Adv. Mater. 2019, 31, 1807832. 10.1002/adma.201807832. [DOI] [PubMed] [Google Scholar]
- Sun R.; Guo J.; Sun C.; Wang T.; Luo Z.; Zhang Z.; Jiao X.; Tang W.; Yang C.; Li Y.; Min J. A universal layer-by-layer solution-processing approach for efficient non-fullerene organic solar cells. Energy Environ. Sci. 2019, 12, 384–395. 10.1039/C8EE02560F. [DOI] [Google Scholar]
- Wu Y.; Zheng Y.; Yang H.; Sun C.; Dong Y.; Cui C.; Yan H.; Li Y. Rationally pairing photoactive materials for high-performance polymer solar cells with efficiency of 16.53%. Sci. China: Chem. 2020, 63, 265–271. 10.1007/s11426-019-9599-1. [DOI] [Google Scholar]
- Sun C.; Pan F.; Bin H.; Zhang J.; Xue L.; Qiu B.; Wei Z.; Zhang Z.-G.; Li Y. A low cost and high performance polymer donor material for polymer solar cells. Nat. Commun. 2018, 9, 743. 10.1038/s41467-018-03207-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lee T. H.; Park S. Y.; Du X.; Park S.; Zhang K.; Li N.; Cho S.; Brabec C. J.; Kim J. Y. Effects on Photovoltaic Characteristics by Organic Bilayer- and Bulk-Heterojunctions: Energy Losses, Carrier Recombination and Generation. ACS Appl. Mater. Interfaces 2020, 12, 55945–55953. 10.1021/acsami.0c16854. [DOI] [PubMed] [Google Scholar]
- Zhang Y.; Liu Z.; Shan T.; Wang Y.; Zhu L.; Li T.; Liu F.; Zhong H. Tuning the molecular geometry and packing mode of non-fullerene acceptors by altering the bridge atoms towards efficient organic solar cells. Mater. Chem. Front. 2020, 4, 2462–2471. 10.1039/D0QM00277A. [DOI] [Google Scholar]
- Fan Q.; Su W.; Chen S.; Liu T.; Zhuang W.; Ma R.; Wen X.; Yin Z.; Luo Z.; Guo X.; Hou L.; Moth-Poulsen K.; Li Y.; Zhang Z.; Yang C.; Yu D.; Yan H.; Zhang M.; Wang E. A Non-Conjugated Polymer Acceptor for Efficient and Thermally Stable All-Polymer Solar Cells. Angew. Chem., Int. Ed. 2020, 59, 19835–19840. 10.1002/anie.202005662. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Zhu Q.; Liu D.; Lu Z.; Gu C.; Zhang K.; Bao X.; Li Q.; Yang R. Length evolution of fused-ring electron acceptors toward optimal blend morphology in polymer solar cells incorporating asymmetric benzodithiophene-based donors. J. Mater. Chem. A 2019, 7, 4823–4828. 10.1039/C8TA11363G. [DOI] [Google Scholar]
- Liang J.; Yin P.; Zheng T.; Wang G.; Zeng X.; Cui C.; Shen P. Conjugated side-chain optimization of indacenodithiophene-based nonfullerene acceptors for efficient polymer solar cells. J. Mater. Chem. C 2019, 7, 10028–10038. 10.1039/C9TC02237F. [DOI] [Google Scholar]
- Gao B.; Yao H.; Jang B.; Zhu J.; Yu R.; Cui Y.; Wang F.; Hou J.; Woo H. Y.; Hou J. The crucial role of intermolecular π–π interactions in A–D–A-type electron acceptors and their effective modulation. J. Mater. Chem. A 2018, 6, 2664–2670. 10.1039/C7TA10461H. [DOI] [Google Scholar]
- Wang Y.; Chang M.; Kan B.; Wan X.; Li C.; Chen Y. All-Small-Molecule Organic Solar Cells Based on Pentathiophene Donor and Alkylated Indacenodithiophene-Based Acceptors with Efficiency over 8%. ACS Appl. Energy Mater. 2018, 1, 2150–2156. 10.1021/acsaem.8b00205. [DOI] [Google Scholar]
- Ryu H. S.; Lee H. G.; Shin S.-C.; Park J.; Kim S. H.; Kim E. J.; Shin T. J.; Shim J. W.; Kim B. J.; Woo H. Y. Terminal alkyl substitution in an A–D–A-type nonfullerene acceptor: simultaneous improvements in the open-circuit voltage and short-circuit current for efficient indoor power generation. J. Mater. Chem. A 2020, 8, 23894–23905. 10.1039/D0TA07684H. [DOI] [Google Scholar]
- Li R.; Liu G.; Xie R.; Wang Z.; Yang X.; An K.; Zhong W.; Jiang X.-F.; Ying L.; Huang F.; Cao Y. Introducing cyclic alkyl chains into small-molecule acceptors for efficient polymer solar cells. J. Mater. Chem. C 2018, 6, 7046–7053. 10.1039/C8TC01780H. [DOI] [Google Scholar]
- Li C.; Song J.; Cai Y.; Han G.; Zheng W.; Yi Y.; Ryu H. S.; Woo H. Y.; Sun Y. Heteroatom substitution-induced asymmetric A–D–A type non-fullerene acceptor for efficient organic solar cells. J. Energy Chem. 2020, 40, 144–150. 10.1016/j.jechem.2019.03.009. [DOI] [Google Scholar]
- Pan F.; Li X.; Bai S.; Liu T.; Wei X.; Li Y.; Chen S.; Yang C.; Chen X.; Lv M.; Li Y. High electron mobility fluorinated indacenodithiophene small molecule acceptors for organic solar cells. Chin. Chem. Lett. 2021, 32, 1257–1262. 10.1016/j.cclet.2020.08.051. [DOI] [Google Scholar]
- Wu Q.; Deng D.; Zhang J.; Zou W.; Yang Y.; Wang Z.; Li H.; Zhou R.; Lu K.; Wei Z. Fluorination-substitution effect on all-small-molecule organic solar cells. Sci. China: Chem. 2019, 62, 837–844. 10.1007/s11426-018-9437-7. [DOI] [Google Scholar]
- Zhou R.; Jiang Z.; Shi Y.; Wu Q.; Yang C.; Zhang J.; Lu K.; Wei Z. Moving Alkyl-Chain Branching Point Induced a Hierarchical Morphology for Efficient All-Small-Molecule Organic Solar Cells. Adv. Funct. Mater. 2020, 30, 2005426. 10.1002/adfm.202005426. [DOI] [Google Scholar]
- Yu Z.-P.; Liu Z.-X.; Chen F.-X.; Qin R.; Lau T.-K.; Yin J.-L.; Kong X.; Lu X.; Shi M.; Li C.-Z.; Chen H. Simple non-fused electron acceptors for efficient and stable organic solar cells. Nat. Commun. 2019, 10, 2152. 10.1038/s41467-019-10098-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Tan H.; Yu X.; Zhu Z.; Yu J.; Zhu W. Highly efficient polymer solar cells with improved molecular stacking and appropriate active layer morphology by side chain engineering of small molecular acceptors. Synth. Met. 2021, 271, 116625. 10.1016/j.synthmet.2020.116625. [DOI] [Google Scholar]
- Keshtov M. L.; Kuklin S. A.; Dou C.; Koukaras E. N.; Singhal R.; Malhotra P.; Sharma G. D. Enhancement of photovoltaic efficiency through fine adjustment of indacene-based non-fullerene acceptor by minimal chlorination for polymer solar cells. Nano Select 2020, 1, 320–333. 10.1002/nano.202000027. [DOI] [Google Scholar]
- Li X.; Meng H.; Shen F.; Su D.; Huo S.; Shan J.; Huang J.; Zhan C. Semitransparent fullerene-free polymer solar cell with 44% AVT and 7% efficiency based on a new chlorinated small molecule acceptor. Dyes Pigm. 2019, 166, 196–202. 10.1016/j.dyepig.2019.03.024. [DOI] [Google Scholar]
- Zhang Z.; Wu Q.; Deng D.; Wu S.; Sun R.; Min J.; Zhang J.; Wei Z. The post-treatment effects on open circuit voltages and device performances in a high efficiency all-small-molecule organic solar cell. J. Mater. Chem. C 2020, 8, 15385–15392. 10.1039/D0TC03043K. [DOI] [Google Scholar]
- Qu J.; Chen H.; Zhou J.; Lai H.; Liu T.; Chao P.; Li D.; Xie Z.; He F.; Ma Y. Chlorine Atom-Induced Molecular Interlocked Network in a Non-Fullerene Acceptor. ACS Appl. Mater. Interfaces 2018, 10, 39992–40000. 10.1021/acsami.8b15923. [DOI] [PubMed] [Google Scholar]
- Tan H.; Zheng X.; Zhu J.; Yu J.; Zhu W. An A–D–D–A-type non-fullerene small-molecule acceptor with strong near-infrared absorption for high performance polymer solar cells. J. Mater. Chem. C 2019, 7, 13301–13306. 10.1039/C9TC04898G. [DOI] [Google Scholar]
- Tan H.; Zheng X.; Zhu J.; Yu J.; Zhu W. Side-chain engineering of A-D-D-A-type small-molecule electron acceptors for high-performance polymer solar cells with JSC exceeding 22 mA cm–2. Dyes Pigm. 2020, 181, 108581. 10.1016/j.dyepig.2020.108581. [DOI] [Google Scholar]
- Cai G.; Xiao Y.; Li M.; Rech J. J.; Wang J.; Liu K.; Lu X.; Tang Z.; Lian J.; Zeng P.; Wang Y.; You W.; Zhan X. Effects of linking units on fused-ring electron acceptor dimers. J. Mater. Chem. A 2020, 8, 13735–13741. 10.1039/D0TA03058A. [DOI] [Google Scholar]
- Zhang M.; Zeng M.; Chen H.; Li L.; Zhao B.; Tan S. A2-D-A1-D-A2-type small molecule acceptors incorporated with electron-deficient core for non-fullerene organic solar cells. Sol. Energy 2020, 197, 511–518. 10.1016/j.solener.2020.01.032. [DOI] [Google Scholar]
- Cai G.; Li Y.; Wang J.; Zhang Y.; Lu X.; Lian J.; Zeng P.; Wang Y.; Zhan X. Effects of π-Bridge on Fused-Ring Electron Acceptor Dimers. ACS Appl. Polym. Mater. 2021, 3, 23–29. 10.1021/acsapm.0c00748. [DOI] [Google Scholar]
- Chang Q.; Chen H.; Yuan J.; Hu Y.; Hai J.; Liu W.; Cai F.; Hong J.; Xiao X.; Zou Y. Diflurobenzothiadiazole core-based noncovalently fused small molecule acceptor exhibiting over 12% efficiency and high fill factor. J. Energy Chem. 2020, 51, 7–13. 10.1016/j.jechem.2020.03.036. [DOI] [Google Scholar]
- Zhang C.; Song X.; Liu K.-K.; Zhang M.; Qu J.; Yang C.; Yuan G.-Z.; Mahmood A.; Liu F.; He F.; Baran D.; Wang J.-L. Electron-Deficient and Quinoid Central Unit Engineering for Unfused Ring-Based A1-D-A2-D-A1-Type Acceptor Enables High Performance Nonfullerene Polymer Solar Cells with High Voc and PCE Simultaneously. Small 2020, 16, 1907681. 10.1002/smll.201907681. [DOI] [PubMed] [Google Scholar]
- Kim H. S.; Rasool S.; Shin W. S.; Song C. E.; Hwang D.-H. Alkylated Indacenodithiophene-Based Non-fullerene Acceptors with Extended π-Conjugation for High-Performance Large-Area Organic Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 50638–50647. 10.1021/acsami.0c13277. [DOI] [PubMed] [Google Scholar]
- Zhu J.; Zheng X.; Tan H.; Tan H.; Yang J.; Yu J.; Zhu W. Small molecule acceptors with indacenodithiophene–benzodithiophene–indacenodithiophene as donating cores for solution-processed non-fullerene solar cells. Chem. Phys. Lett. 2019, 726, 7–12. 10.1016/j.cplett.2019.04.024. [DOI] [Google Scholar]
- Nian Y.; Wang Z.; Jiang H.; Feng S.; Li S.; Zhang L.; Cao Y.; Chen J. Silaindacenodithiophene-Based Fused-Ring Non-Fullerene Electron Acceptor for Efficient Polymer Solar Cells. Chin. J. Chem. 2018, 36, 495–501. 10.1002/cjoc.201700809. [DOI] [Google Scholar]
- He E.; Wang X.; Lu Y.; Yin Y.; Guo F.; Gao S.; He Z.; Zhao L.; Cao Y.; Zhang Y. Indacenodifuran-Based Non-Fullerene Electron Acceptors for Efficient Polymer Solar Cells. ACS Appl. Energy Mater. 2020, 3, 6133–6138. 10.1021/acsaem.0c01247. [DOI] [Google Scholar]
- Li Y.; Wang M.; Wu F.; Gao X.; Huettner S.; Tao Y.; Jiang Z.-Q. Short-Axis Methyl Substitution Approach on Indacenodithiophene: A New Multi-Fused Ladder-Type Arene for Organic Solar Cells. Front. Chem. 2019, 7, 372. 10.3389/fchem.2019.00372. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Li X.; Huang H.; Angunawela I.; Zhou J.; Du J.; Liebman-Pelaez A.; Zhu C.; Zhang Z.; Meng L.; Xie Z.; Ade H.; Li Y. Effects of Short-Axis Alkoxy Substituents on Molecular Self-Assembly and Photovoltaic Performance of Indacenodithiophene-Based Acceptors. Adv. Funct. Mater. 2020, 30, 1906855. 10.1002/adfm.201906855. [DOI] [Google Scholar]
- Huang H.; Li X.; Sun C.; Angunawela I.; Qiu B.; Du J.; Qin S.; Meng L.; Zhang Z.; Ade H.; Li Y. Green solvent-processed organic solar cells based on a low cost polymer donor and a small molecule acceptor. J. Mater. Chem. C 2020, 8, 7718–7724. 10.1039/D0TC01313G. [DOI] [Google Scholar]
- Liu S.; Zhao B.; Cong Z.; Wang W.; Cheng Q.; Liu J.; Wu H.; Gao C. Performance of asymmetric non-fullerene acceptors containing the 4,4,9,9-tetramethyl-4,9-dihydroselenopheno[2′,3′:5,6]-s-indaceno[1,2-b]thiophene core. Dyes Pigm. 2021, 186, 108988. 10.1016/j.dyepig.2020.108988. [DOI] [Google Scholar]
- Andersen T. R.; Weyhe A. T.; Tao Q.; Zhao F.; Qin R.; Zhang S.; Chen H.; Yu D. Novel cost-effective acceptor:P3HT based organic solar cells exhibiting the highest ever reported industrial readiness factor. Mater. Adv. 2020, 1, 658–665. 10.1039/D0MA00133C. [DOI] [Google Scholar]
- Gong Y.; Kan Z.; Xu W.; Wang Y.; AlShammari S. H.; Laquai F.; Lai W.-Y.; Huang W. Wide-Bandgap Small Molecular Acceptors Based on a Weak Electron-Withdrawing Moiety for Efficient Polymer Solar Cells. Sol. RRL 2018, 2, 1800120. 10.1002/solr.201800120. [DOI] [Google Scholar]
- Zhao D.; Hu J.; Liu Z.; Xiao B.; Wang X.; Zhou E.; Zhang Q. Isatylidene malononitrile derived acceptors for fullerene free organic solar cells. Dyes Pigm. 2018, 151, 102–109. 10.1016/j.dyepig.2017.12.054. [DOI] [Google Scholar]
- Zhao D.; Hu J.; Cao K.; Xiao B.; Zhou E.; Zhang Q. Isatin-derived non-fullerene acceptors towards high open circuit voltage solar cells. Dyes Pigm. 2019, 162, 898–904. 10.1016/j.dyepig.2018.11.027. [DOI] [Google Scholar]
- Liu Q.; Jin K.; Li W.; Xiao Z.; Cheng M.; Yuan Y.; Shi S.; Jin Z.; Hao F.; Yang S.; Ding L. An efficient medium-bandgap nonfullerene acceptor for organic solar cells. J. Mater. Chem. A 2020, 8, 8857–8861. 10.1039/D0TA02427A. [DOI] [Google Scholar]
- Xie D.; Liu T.; Lee T. H.; Gao W.; Zhong C.; Huo L.; Luo Z.; Wu K.; Xiong W.; Kim J. Y.; Choi H.; Sun Y.; Yang C. A new small molecule acceptor based on indaceno[2,1-b:6,5-b′]dithiophene and thiophene-fused ending group for fullerene-free organic solar cells. Dyes Pigm. 2018, 148, 263–269. 10.1016/j.dyepig.2017.09.009. [DOI] [Google Scholar]
- Luo Z.; Sun C.; Chen S.; Zhang Z.-G.; Wu K.; Qiu B.; Yang C.; Li Y.; Yang C. Side-Chain Impact on Molecular Orientation of Organic Semiconductor Acceptors: High Performance Nonfullerene Polymer Solar Cells with Thick Active Layer over 400 nm. Adv. Energy Mater. 2018, 8, 1800856. 10.1002/aenm.201800856. [DOI] [Google Scholar]
- Li G.; Luo Z.; Sun C.; Wu K.; Gong S.; Qiu B.; Li Y.; Yang C. Systematic investigation of methyl substitution effect on physicochemical properties and photovoltaic performance in nonfullerene small-molecule electron acceptors. Dyes Pigm. 2019, 164, 126–132. 10.1016/j.dyepig.2019.01.018. [DOI] [Google Scholar]
- Zhou R.; Jiang Z.; Yang C.; Yu J.; Feng J.; Adil M. A.; Deng D.; Zou W.; Zhang J.; Lu K.; Ma W.; Gao F.; Wei Z. All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies. Nat. Commun. 2019, 10, 5393. 10.1038/s41467-019-13292-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Huang Z.; Zhou R.; Lv M.; Zhang H.; Yang C.; Shi Y.; Tang Y.; Zhang J.; Lu K.; Wei Z. Constructing high efficiency non-fullerene all-small-molecule ternary organic solar cells by employing structurally similar acceptors. Mater. Chem. Front. 2021, 5, 1405–1409. 10.1039/D0QM00814A. [DOI] [Google Scholar]
- Wu Q.; Deng D.; Zhou R.; Zhang J.; Zou W.; Liu L.; Wu S.; Lu K.; Wei Z. Modulation of Donor Alkyl Terminal Chains with the Shifting Branching Point Leads to the Optimized Morphology and Efficient All-Small-Molecule Organic Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 25100–25107. 10.1021/acsami.0c03977. [DOI] [PubMed] [Google Scholar]
- Jia B.; Wang J.; Wu Y.; Zhang M.; Jiang Y.; Tang Z.; Russell T. P.; Zhan X. Enhancing the Performance of a Fused-Ring Electron Acceptor by Unidirectional Extension. J. Am. Chem. Soc. 2019, 141, 19023–19031. 10.1021/jacs.9b08988. [DOI] [PubMed] [Google Scholar]
- Dai S.; Xiao Y.; Xue P.; James Rech J.; Liu K.; Li Z.; Lu X.; You W.; Zhan X. Effect of Core Size on Performance of Fused-Ring Electron Acceptors. Chem. Mater. 2018, 30, 5390–5396. 10.1021/acs.chemmater.8b02222. [DOI] [Google Scholar]
- Zhang Y.; Zhou P.; Wang J.; Zhan X.; Chen X. Designing a thiophene-fused quinoxaline unit to build D–A copolymers for non-fullerene organic solar cells. Dyes Pigm. 2020, 174, 108022. 10.1016/j.dyepig.2019.108022. [DOI] [Google Scholar]
- Tang H.; Yan C.; Karuthedath S.; Yin H.; Gao Y.; Gao J.; Zhang L.; Huang J.; So S. K.; Kan Z.; Laquai F.; Li G.; Lu S. Deciphering the Role of Fluorination: Morphological Manipulation Prompts Charge Separation and Reduces Carrier Recombination in All-Small-Molecule Photovoltaics. Sol. RRL 2020, 4, 1900528. 10.1002/solr.201900528. [DOI] [Google Scholar]
- Angunawela I.; Ye L.; Bin H.; Zhang Z.-G.; Gadisa A.; Li Y.; Ade H. Multi-length scale morphology of nonfullerene all-small molecule blends and its relation to device function in organic solar cells. Mater. Chem. Front. 2019, 3, 137–144. 10.1039/C8QM00503F. [DOI] [Google Scholar]
- Xu T.; Chang Y.; Yan C.; Yang Q.; Kan Z.; Singh R.; Kumar M.; Li G.; Lu S.; Duan T. Fluorinated oligothiophene donors for high-performance nonfullerene small-molecule organic solar cells. Sustain. Energy Fuels 2020, 4, 2680–2685. 10.1039/D0SE00335B. [DOI] [Google Scholar]
- Pradhan R.; Dahiya H.; Bag B. P.; Keshtov M. L.; Singhal R.; Sharma G. D.; Mishra A. Energy-level modulation of coumarin-based molecular donors for efficient all small molecule fullerene-free organic solar cells. J. Mater. Chem. A 2021, 9, 1563–1573. 10.1039/D0TA10334A. [DOI] [Google Scholar]
- Gao W.; Zhang M.; Liu T.; Ming R.; An Q.; Wu K.; Xie D.; Luo Z.; Zhong C.; Liu F.; Zhang F.; Yan H.; Yang C. Asymmetrical Ladder-Type Donor-Induced Polar Small Molecule Acceptor to Promote Fill Factors Approaching 77% for High-Performance Nonfullerene Polymer Solar Cells. Adv. Mater. 2018, 30, 1800052. 10.1002/adma.201800052. [DOI] [PubMed] [Google Scholar]
- Zhang J.; Lv J.; Dong X.; Xu T.; Dai X.; Duan T.; Kan Z.; Liu P.; Lu S. Cyano-functionalized small-molecule acceptors for high-efficiency wide-bandgap organic solar cells. J. Mater. Chem. C 2020, 8, 9195–9200. 10.1039/D0TC02061C. [DOI] [Google Scholar]
- Wadsworth A.; Bristow H.; Hamid Z.; Babics M.; Gasparini N.; Boyle C. W.; Zhang W.; Dong Y.; Thorley K. J.; Neophytou M.; Ashraf R. S.; Durrant J. R.; Baran D.; McCulloch I. End Group Tuning in Acceptor–Donor–Acceptor Nonfullerene Small Molecules for High Fill Factor Organic Solar Cells. Adv. Funct. Mater. 2019, 29, 1808429. 10.1002/adfm.201808429. [DOI] [Google Scholar]
- Liu R.; Du Z.; Wen S.; Wu Y.; Zhu D.; Yang R. Energy levels modulation of small molecule acceptors for polymer solar cells. Synth. Met. 2018, 235, 131–135. 10.1016/j.synthmet.2017.12.005. [DOI] [Google Scholar]
- Chen S.; Wang Y.; Zhang L.; Zhao J.; Chen Y.; Zhu D.; Yao H.; Zhang G.; Ma W.; Friend R. H.; Chow P. C. Y.; Gao F.; Yan H. Efficient Nonfullerene Organic Solar Cells with Small Driving Forces for Both Hole and Electron Transfer. Adv. Mater. 2018, 30, 1804215. 10.1002/adma.201804215. [DOI] [PubMed] [Google Scholar]
- Zhang G.; Xia R.; Chen Z.; Xiao J.; Zhao X.; Liu S.; Yip H.-L.; Cao Y. Overcoming Space-Charge Effect for Efficient Thick-Film Non-Fullerene Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1801609. 10.1002/aenm.201801609. [DOI] [Google Scholar]
- Xiao J.; Chen Z.; Zhang G.; Li Q.-Y.; Yin Q.; Jiang X.-F.; Huang F.; Xu Y.-X.; Yip H.-L.; Cao Y. Efficient device engineering for inverted non-fullerene organic solar cells with low energy loss. J. Mater. Chem. C 2018, 6, 4457–4463. 10.1039/C8TC00705E. [DOI] [Google Scholar]
- Huang H.; Xiao B.; Huang C.; Zhang J.; Liu S.; Fu N.; Zhao B.; Qin T.; Zhou E.; Huang W. Enhanced open circuit voltage of small molecule acceptors containing angular-shaped indacenodithiophene units for P3HT-based organic solar cells. J. Mater. Chem. C 2018, 6, 12347–12354. 10.1039/C8TC04608E. [DOI] [Google Scholar]
- Xie L.; Xiao J.; Wu L.; Zhang W.; Ge Z.; Tan S. Synthesis and photovoltaic properties of small molecule electron acceptors with twin spiro-type core structure. Dyes Pigm. 2019, 168, 197–204. 10.1016/j.dyepig.2019.04.064. [DOI] [Google Scholar]
- Cao J.; Shan T.; Wang J.-K.; Xu Y.-X.; Ren X.; Zhong H. Stereoisomerism of ladder-type acceptor molecules and its effect on photovoltaic properties. Dyes Pigm. 2019, 165, 354–360. 10.1016/j.dyepig.2019.02.046. [DOI] [Google Scholar]
- Yang L.; Gu W.; Yang Y.; Hong L.; Zhang X.; Xiao Y.; Wu X.; Peng A.; Huang H. A Highly Planar Nonfullerene Acceptor with Multiple Noncovalent Conformational Locks for Efficient Organic Solar Cells. Small Methods 2018, 2, 1700330. 10.1002/smtd.201700330. [DOI] [Google Scholar]
- Che Y.; Zhang Y.; Yang Y.; Liu C.-H.; Izquierdo R.; Xiao S. S.; Perepichka D. F. Understanding the Photovoltaic Behavior of A-D-A Molecular Semiconductors through a Permutation of End Groups. J. Org. Chem. 2020, 85, 52–61. 10.1021/acs.joc.9b01654. [DOI] [PubMed] [Google Scholar]
- Chen M.; Nian Y.; Hu Z.; Zhang L. Alkyl side chain engineering for difluorinated benzothiadiazole flanked non-fullerene acceptors toward efficient polymer solar cells. J. Mater. Sci.: Mater. Electron. 2021, 32, 219–231. 10.1007/s10854-020-04757-x. [DOI] [Google Scholar]
- Tang A.; Xiao B.; Wang Y.; Gao F.; Tajima K.; Bin H.; Zhang Z.-G.; Li Y.; Wei Z.; Zhou E. Simultaneously Achieved High Open-Circuit Voltage and Efficient Charge Generation by Fine-Tuning Charge-Transfer Driving Force in Nonfullerene Polymer Solar Cells. Adv. Funct. Mater. 2018, 28, 1704507. 10.1002/adfm.201704507. [DOI] [Google Scholar]
- Tang A.; Song W.; Xiao B.; Guo J.; Min J.; Ge Z.; Zhang J.; Wei Z.; Zhou E. Benzotriazole-Based Acceptor and Donors, Coupled with Chlorination, Achieve a High VOC of 1.24 V and an Efficiency of 10.5% in Fullerene-Free Organic Solar Cells. Chem. Mater. 2019, 31, 3941–3947. 10.1021/acs.chemmater.8b05316. [DOI] [Google Scholar]
- Nie Q.; Tang A.; Cong P.; Chen L.; Zhang Q.; Ji H.; Li G.; Guo Q.; Zhou E. Wide Band Gap Photovoltaic Polymer Based on Pyrrolo[3,4-f]benzotriazole-5,7-dione (TzBI) with Ultrahigh VOC Beyond 1.25 V. J. Phys. Chem. C 2020, 124, 19492–19498. 10.1021/acs.jpcc.0c05914. [DOI] [Google Scholar]
- Wang X.; Tang A.; Yang J.; Du M.; Li J.; Li G.; Guo Q.; Zhou E. Tuning the intermolecular interaction of A2-A1-D-A1-A2 type non-fullerene acceptors by substituent engineering for organic solar cells with ultrahigh VOC of ∼1.2 V. Sci. China: Chem. 2020, 63, 1666–1674. 10.1007/s11426-020-9840-x. [DOI] [Google Scholar]
- Xiao B.; Geng Y.; Tang A.; Wang X.; Chen Y.; Zeng Q.; Zhou E. Controlling the Cyano-Containing A2 Segments in A2-A1-D-A1-A2 Type Non-Fullerene Acceptors to Combine with a Benzotriazole-Based p-Type Polymer: “Same-Acceptor-Strategy” for High VOC Organic Solar Cells. Sol. RRL 2019, 3, 1800332. 10.1002/solr.201800332. [DOI] [Google Scholar]
- Xiao B.; Du M.; Wang X.; Xiao Z.; Li G.; Tang A.; Ding L.; Geng Y.; Sun X.; Zhou E. Effects of Oxygen Atoms Introduced at Different Positions of Non-Fullerene Acceptors in the Performance of Organic Solar Cells with Poly(3-hexylthiophene). ACS Appl. Mater. Interfaces 2020, 12, 1094–1102. 10.1021/acsami.9b16662. [DOI] [PubMed] [Google Scholar]
- Zhang Q.; Xiao B.; Du M.; Li G.; Tang A.; Zhou E. A2–A1–D–A1–A2 type non-fullerene acceptors based on methoxy substituted benzotriazole with three different end-capped groups for P3HT-based organic solar cells. J. Mater. Chem. C 2018, 6, 10902–10909. 10.1039/C8TC03963A. [DOI] [Google Scholar]
- Xu H.; Yang Y.; Zhong C.; Zhan X.; Chen X. Narrow bandgap non-fullerene acceptor based on a thiophene-fused benzothiadiazole unit with a high short-circuit current density of over 20 mA cm–2. J. Mater. Chem. A 2018, 6, 6393–6401. 10.1039/C8TA00704G. [DOI] [Google Scholar]
- Wang J.; Li T.; Wang X.; Xiao Y.; Zhong C.; Wang J.; Liu K.; Lu X.; Zhan X.; Chen X. Enhancing the JSC of P3HT-Based OSCs via a Thiophene-Fused Aromatic Heterocycle as a ″π-Bridge″ for A-π-D-π-A-Type Acceptors. ACS Appl. Mater. Interfaces 2019, 11, 26005–26016. 10.1021/acsami.9b08007. [DOI] [PubMed] [Google Scholar]
- Wang J.; Xu H.; Xiao Y.; Li T.; Wang J.; Liu K.; Lu X.; Zhan X.; Chen X. A thiophene-fused benzotriazole unit as a “π-bridge” in A-π-D-π-A type acceptor to achieve more balanced JSC and VOC for OSCs. Org. Electron. 2020, 82, 105705. 10.1016/j.orgel.2020.105705. [DOI] [Google Scholar]
- Xiao B.; Tang A.; Yang J.; Mahmood A.; Sun X.; Zhou E. Quinoxaline-Containing Nonfullerene Small-Molecule Acceptors with a Linear A2-A1-D-A1-A2 Skeleton for Poly(3-hexylthiophene)-Based Organic Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 10254–10261. 10.1021/acsami.8b00216. [DOI] [PubMed] [Google Scholar]
- Xiao B.; Zhang Q.; Li G.; Du M.; Geng Y.; Sun X.; Tang A.; Liu Y.; Guo Q.; Zhou E. Side chain engineering of quinoxaline-based small molecular nonfullerene acceptors for high-performance poly(3-hexylthiophene)-based organic solar cells. Sci. China: Chem. 2020, 63, 254–264. 10.1007/s11426-019-9618-7. [DOI] [Google Scholar]
- Yang Y.; Wang J.; Xu H.; Zhan X.; Chen X. Nonfullerene Acceptor with ″Donor-Acceptor Combined π-Bridge″ for Organic Photovoltaics with Large Open-Circuit Voltage. ACS Appl. Mater. Interfaces 2018, 10, 18984–18992. 10.1021/acsami.8b04541. [DOI] [PubMed] [Google Scholar]
- Luo P.; An K.; Ying L.; Li G.; Zhu C.; Fan B.; Huang F.; Cao Y. Design and synthesis of an amino-functionalized non-fullerene acceptor as a cathode interfacial layer for polymer solar cells. J. Mater. Chem. C 2020, 8, 5273–5279. 10.1039/D0TC00435A. [DOI] [Google Scholar]
- An K.; Zhong W.; Ying L. Enhanced performance of P3HT-based non-fullerene polymer solar cells by optimizing film morphology using non-halogenated solvent. Org. Electron. 2020, 82, 105701. 10.1016/j.orgel.2020.105701. [DOI] [Google Scholar]
- Yi X.; Ho C. H. Y.; Gautam B.; Lei L.; Chowdhury A. H.; Bahrami B.; Qiao Q.; So F. Effects of polymer crystallinity on non-fullerene acceptor based organic solar cell photostability. J. Mater. Chem. C 2020, 8, 16092–16099. 10.1039/D0TC03969A. [DOI] [Google Scholar]
- Chen Z.; Hu Z.; Liang Y.; Zhou C.; Xiao J.; Zhang G.; Huang F. Highly efficient, green-solvent processable, and stable non-fullerene polymer solar cells enabled by a random polymer donor. Org. Electron. 2020, 85, 105874. 10.1016/j.orgel.2020.105874. [DOI] [Google Scholar]
- Lv J.; Feng Y.; Fu J.; Gao J.; Singh R.; Kumar M.; Kim M.; Tang H.; Lu S.; Zhang W.; McCulloch I.; Li J.; Kan Z. Energetic Disorder and Activation Energy in Efficient Ternary Organic Solar Cells with Nonfullerene Acceptor Eh-IDTBR as the Third Component. Sol. RRL 2020, 4, 1900403. 10.1002/solr.201900403. [DOI] [Google Scholar]
- Jia X.; Liu G.; Chen S.; Li Z.; Wang Z.; Yin Q.; Yip H.-L.; Yang C.; Duan C.; Huang F.; Cao Y. Backbone Fluorination of Polythiophenes Improves Device Performance of Non-Fullerene Polymer Solar Cells. ACS Appl. Energy Mater. 2019, 2, 7572–7583. 10.1021/acsaem.9b01532. [DOI] [Google Scholar]
- Xiao B.; Tang A.; Zhang Q.; Li G.; Wang X.; Zhou E. A2-A1-D-A1-A2 Type Non-Fullerene Acceptors with 2-(1,1-Dicyanomethylene)rhodanine as the Terminal Groups for Poly(3-hexylthiophene)-Based Organic Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 34427–34434. 10.1021/acsami.8b10312. [DOI] [PubMed] [Google Scholar]
- Ye P.; Chen Y.; Wu J.; Wu X.; Xu Y.; Li Z.; Hong S.; Sun M.; Peng A.; Huang H. Combination of noncovalent conformational locks and side chain engineering to tune the crystallinity of nonfullerene acceptors for high-performance P3HT based organic solar cells. Mater. Chem. Front. 2019, 3, 64–69. 10.1039/C8QM00461G. [DOI] [Google Scholar]
- Mahmood A.; Hu J.; Tang A.; Chen F.; Wang X.; Zhou E. A novel thiazole based acceptor for fullerene-free organic solar cells. Dyes Pigm. 2018, 149, 470–474. 10.1016/j.dyepig.2017.10.037. [DOI] [Google Scholar]
- Li X.; Li C.; Ye L.; Weng K.; Fu H.; Ryu H. S.; Wei D.; Sun X.; Woo H. Y.; Sun Y. Asymmetric A–D−π–A-type nonfullerene small molecule acceptors for efficient organic solar cells. J. Mater. Chem. A 2019, 7, 19348–19354. 10.1039/C9TA06476A. [DOI] [Google Scholar]
- Liu Y.; Li M.; Zhou X.; Jia Q.-Q.; Feng S.; Jiang P.; Xu X.; Ma W.; Li H.-B.; Bo Z. Nonfullerene Acceptors with Enhanced Solubility and Ordered Packing for High-Efficiency Polymer Solar Cells. ACS Energy Lett. 2018, 3, 1832–1839. 10.1021/acsenergylett.8b00928. [DOI] [Google Scholar]
- Park S. H.; Park G. E.; Choi S.; Kim Y. U.; Park S. Y.; Park C. G.; Cho M. J.; Choi D. H. Effect of a methyl thiophene-3-carboxylate bridge in an indacenodithiophene-based acceptor–donor–acceptor-type molecule on the performance of non-fullerene polymer solar cells. J. Mater. Chem. C 2018, 6, 7549–7556. 10.1039/C8TC01508B. [DOI] [Google Scholar]
- Li P.; Liang Q.; Hong E. Y.-H.; Chan C.-Y.; Cheng Y.-H.; Leung M.-Y.; Chan M.-Y.; Low K.-H.; Wu H.; Yam V. W.-W. Boron(iii ) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices. Chem. Sci. 2020, 11, 11601–11612. 10.1039/D0SC04047A. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Liu D.; Kan B.; Ke X.; Zheng N.; Xie Z.; Lu Di; Liu Y. Extended Conjugation Length of Nonfullerene Acceptors with Improved Planarity via Noncovalent Interactions for High-Performance Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1801618. 10.1002/aenm.201801618. [DOI] [Google Scholar]
- Liang Z.; Li M.; Zhang X.; Wang Q.; Jiang Y.; Tian H.; Geng Y. Near-infrared absorbing non-fullerene acceptors with selenophene as π bridges for efficient organic solar cells. J. Mater. Chem. A 2018, 6, 8059–8067. 10.1039/C8TA00783G. [DOI] [Google Scholar]
- Zhang X.; Wang Q.; Liang Z.; Li M.; Geng Y. Low-bandgap non-fullerene acceptors based on selenophene π spacer and alkylated indaceno[1,2-b:5,6-b′]dithiophene for organic solar cells. Org. Electron. 2019, 69, 200–207. 10.1016/j.orgel.2019.03.038. [DOI] [Google Scholar]
- Li J.; Feng L.; Cai F.; Zhang Z.; Wei Q.; Peng H.; Zou Y. Synthesize non-fullerene acceptors of five fused-ring by modifying side chain. Sol. Energy 2019, 191, 566–573. 10.1016/j.solener.2019.09.013. [DOI] [Google Scholar]
- Jiang S.; Qin L.; Chen H.; Wu X.; Li Y.; Lv L.; Chen J.; Tao Y.; Zhang S.; Lu X.; Shi Q.; Huang H. S···Cl intramolecular interaction: An efficient strategy to improve power conversion efficiency of organic solar cells. Dyes Pigm. 2020, 179, 108416. 10.1016/j.dyepig.2020.108416. [DOI] [Google Scholar]
- Hong L.; Yao H.; Yu R.; Xu Y.; Gao B.; Ge Z.; Hou J. Investigating the Trade-Off between Device Performance and Energy Loss in Nonfullerene Organic Solar Cells. ACS Appl. Mater. Interfaces 2019, 11, 29124–29131. 10.1021/acsami.9b10243. [DOI] [PubMed] [Google Scholar]
- Li R.; Liu G.; Fan B.; Du X.; Tang X.; Li N.; Ying L.; Brabec C. J.; Huang F.; Cao Y. Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells. Org. Electron. 2018, 59, 366–373. 10.1016/j.orgel.2018.05.050. [DOI] [Google Scholar]
- Wu J.; Xu Y.; Yang Z.; Chen Y.; Sui X.; Yang L.; Ye P.; Zhu T.; Wu X.; Liu X.; Cao H.; Peng A.; Huang H. Simultaneous Enhancement of Three Parameters of P3HT-Based Organic Solar Cells with One Oxygen Atom. Adv. Energy Mater. 2019, 9, 1803012. 10.1002/aenm.201803012. [DOI] [Google Scholar]
- Pang Z.; Zhang W.; Wu J.; Luo Y.; Liu J.; Zhao S.; Xu Z.; Lu Z.; Huang Y. Insight into the effects of alkoxy side chain position in nonfullerene electron acceptors on the morphological stability of organic solar cells. Dyes Pigm. 2020, 181, 108562. 10.1016/j.dyepig.2020.108562. [DOI] [Google Scholar]
- Liu Y.; Li M.; Yang J.; Xue W.; Feng S.; Song J.; Tang Z.; Ma W.; Bo Z. High-Efficiency As-Cast Organic Solar Cells Based on Acceptors with Steric Hindrance Induced Planar Terminal Group. Adv. Energy Mater. 2019, 9, 1901280. 10.1002/aenm.201901280. [DOI] [Google Scholar]
- Lee J.; Song S.; Huang J.; Du Z.; Lee H.; Zhu Z.; Ko S.-J.; Nguyen T.-Q.; Kim J. Y.; Cho K.; Bazan G. C. Bandgap Tailored Nonfullerene Acceptors for Low-Energy-Loss Near-Infrared Organic Photovoltaics. ACS Mater. Lett. 2020, 2, 395–402. 10.1021/acsmaterialslett.9b00512. [DOI] [Google Scholar]
- Wang Z.; Jiang H.; Liu X.; Liang J.; Zhang L.; Qing L.; Wang Q.; Zhang W.; Cao Y.; Chen J. Significantly enhanced electron transport of a nonfullerene acceptor in a blend film with a high hole mobility polymer of high molecular weight: thick-film nonfullerene polymer solar cells showing a high fill factor. J. Mater. Chem. A 2020, 8, 7765–7774. 10.1039/D0TA01340D. [DOI] [Google Scholar]
- Chen M.; Liu D.; Li W.; Gurney R. S.; Li D.; Cai J.; Spooner E. L. K.; Kilbride R. C.; McGettrick J. D.; Watson T. M.; Li Z.; Jones R. A. L.; Lidzey D. G.; Wang T. Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells. ACS Appl. Mater. Interfaces 2019, 11, 26194–26203. 10.1021/acsami.9b07317. [DOI] [PubMed] [Google Scholar]
- Chen J.; Li G.; Zhu Q.; Guo X.; Fan Q.; Ma W.; Zhang M. Highly efficient near-infrared and semitransparent polymer solar cells based on an ultra-narrow bandgap nonfullerene acceptor. J. Mater. Chem. A 2019, 7, 3745–3751. 10.1039/C8TA11484F. [DOI] [Google Scholar]
- Cong Z.; Zhao B.; Chen Z.; Wang W.; Wu H.; Liu J.; Wang J.; Wang L.; Ma W.; Gao C. Efficient Polymer Solar Cells Having High Open-Circuit Voltage and Low Energy Loss Enabled by a Main-Chain Twisted Small Molecular Acceptor. ACS Appl. Mater. Interfaces 2019, 11, 16795–16803. 10.1021/acsami.9b03499. [DOI] [PubMed] [Google Scholar]
- Wu H.; Zhao B.; Zhao H.; Wang L.; Wang W.; Cong Z.; Liu J.; Ma W.; Gao C. Effects of Monofluorinated Positions at the End-Capping Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 789–797. 10.1021/acsami.9b18301. [DOI] [PubMed] [Google Scholar]
- Wang W.; Zhao B.; Cong Z.; Xie Y.; Wu H.; Liang Q.; Liu S.; Liu F.; Gao C.; Wu H.; Cao Y. Nonfullerene Polymer Solar Cells Based on a Main-Chain Twisted Low-Bandgap Acceptor with Power Conversion Efficiency of 13.2%. ACS Energy Lett. 2018, 3, 1499–1507. 10.1021/acsenergylett.8b00627. [DOI] [Google Scholar]
- Wu H.; Bian Q.; Zhao B.; Zhao H.; Wang L.; Wang W.; Cong Z.; Liu J.; Ma W.; Gao C. Effects of the Isomerized Thiophene-Fused Ending Groups on the Performances of Twisted Non-Fullerene Acceptor-Based Polymer Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 23904–23913. 10.1021/acsami.0c03842. [DOI] [PubMed] [Google Scholar]
- Zhao B.; Wang W.; Xie Y.; Zhao H.; Wang L.; Liu S.; Xin J.; Cong Z.; Wu H.; Liang Q.; Ma W.; Gao C.; Wu H.; Cao Y. Efficient polymer solar cells enabled by alkoxy-phenyl side-chain-modified main-chain-twisted small molecular acceptors. J. Mater. Chem. A 2020, 8, 22335–22345. 10.1039/D0TA07960J. [DOI] [Google Scholar]
- Yin P.; Wang L.; Liang J.; Yu Y.; Chen L.; Weng C.; Cui C.; Shen P. A small-molecule/fullerene acceptor alloy: a powerful tool to enhance the device efficiency and thermal stability of ternary polymer solar cells. J. Mater. Chem. C 2020, 8, 11223–11238. 10.1039/D0TC00329H. [DOI] [Google Scholar]
- Zeng A.; Pan M.; Lin B.; Lau T.-K.; Qin M.; Li K.; Ma W.; Lu X.; Zhan C.; Yan H. A Nonfullerene Acceptor with Alkylthio- and Dimethoxy-Thiophene-Groups Yielding High-Performance Ternary Organic Solar Cells. Sol. RRL 2020, 4, 1900353. 10.1002/solr.201900353. [DOI] [Google Scholar]
- Wang Z.; Huang K.; Chen X.; Xu Y.; Xu Z.; He T.; Yin S.; Qiu H.; Li M.; Zhang Q. A high-performance non-fullerene electron acceptor with bisalkylthiothiophene π-bridges for organic photovoltaics. J. Mater. Chem. C 2019, 7, 14499–14503. 10.1039/C9TC05166J. [DOI] [Google Scholar]
- Liu Y.; Zhang C.; Hao D.; Zhang Z.; Wu L.; Li M.; Feng S.; Xu X.; Liu F.; Chen X.; Bo Z. Enhancing the Performance of Organic Solar Cells by Hierarchically Supramolecular Self-Assembly of Fused-Ring Electron Acceptors. Chem. Mater. 2018, 30, 4307–4312. 10.1021/acs.chemmater.8b01319. [DOI] [Google Scholar]
- Jiang P.; Ming S.; Jia Q.-Q.; Liu Y.; Lu H.; Li M.; Xu X.; Li H.-B.; Bo Z. The influence of the π-bridging unit of fused-ring acceptors on the performance of organic solar cells. J. Mater. Chem. A 2018, 6, 21335–21340. 10.1039/C8TA08410F. [DOI] [Google Scholar]
- Zhang C.; Jiang P.; Zhou X.; Feng S.; Bi Z.; Xu X.; Li C.; Tang Z.; Ma W.; Bo Z. Efficient Ternary Organic Solar Cells with a New Electron Acceptor Based on 3,4-(2,2-Dihexylpropylenedioxy)thiophene. ACS Appl. Mater. Interfaces 2020, 12, 40590–40598. 10.1021/acsami.0c11128. [DOI] [PubMed] [Google Scholar]
- Ming S.; Zhang C.; Jiang P.; Jiang Q.; Ma Z.; Song J.; Bo Z. Impact of the Bonding Sites at the Inner or Outer π-Bridged Positions for Non-Fullerene Acceptors. ACS Appl. Mater. Interfaces 2019, 11, 19444–19451. 10.1021/acsami.9b02964. [DOI] [PubMed] [Google Scholar]
- Li X.; Xu Z.; Guo X.; Fan Q.; Zhang M.; Li Y. Synthesis and photovoltaic properties of a small molecule acceptor with thienylenevinylene thiophene as π bridge. Dyes Pigm. 2019, 160, 227–233. 10.1016/j.dyepig.2018.08.009. [DOI] [Google Scholar]
- Mahmood A.; Yang J.; Hu J.; Wang X.; Tang A.; Geng Y.; Zeng Q.; Zhou E. Introducing Four 1,1-Dicyanomethylene-3-indanone End-Capped Groups as an Alternative Strategy for the Design of Small-Molecular Nonfullerene Acceptors. J. Phys. Chem. C 2018, 122, 29122–29128. 10.1021/acs.jpcc.8b09336. [DOI] [Google Scholar]
- Yi Y.-Q.-Q.; Feng H.; Ke X.; Yan J.; Chang M.; Wan X.; Li C.; Chen Y. A cyclopentadithiophene-bridged small molecule acceptor with near-infrared light absorption for efficient organic solar cells. J. Mater. Chem. C 2019, 7, 4013–4019. 10.1039/C9TC00162J. [DOI] [Google Scholar]
- Zhang Z.; Shan T.; Zhang Y.; Zhu L.; Kong L.; Liu F.; Zhong H. Isomerizing thieno[3,4-b]thiophene-based near-infrared non-fullerene acceptors towards efficient organic solar cells. J. Mater. Chem. C 2020, 8, 4357–4364. 10.1039/D0TC00269K. [DOI] [Google Scholar]
- Liu F.; Zhang J.; Wang Y.; Chen S.; Zhou Z.; Yang C.; Gao F.; Zhu X. Modulating Structure Ordering via Side-Chain Engineering of Thieno[3,4-b]thiophene-Based Electron Acceptors for Efficient Organic Solar Cells with Reduced Energy Losses. ACS Appl. Mater. Interfaces 2019, 11, 35193–35200. 10.1021/acsami.9b10641. [DOI] [PubMed] [Google Scholar]
- Chen F.-X.; Xu J.-Q.; Liu Z.-X.; Chen M.; Xia R.; Yang Y.; Lau T.-K.; Zhang Y.; Lu X.; Yip H.-L.; Jen A. K.-Y.; Chen H.; Li C.-Z. Near-Infrared Electron Acceptors with Fluorinated Regioisomeric Backbone for Highly Efficient Polymer Solar Cells. Adv. Mater. 2018, 30, 1803769. 10.1002/adma.201803769. [DOI] [PubMed] [Google Scholar]
- Zhang J.; Liu F.; Chen S.; Yang C.; Zhu X.; Zhu D. High-Performance Polymer Solar Cells Achieved by Introducing Side-Chain Heteroatom on Small-Molecule Electron Acceptor. Macromol. Rapid Commun. 2019, 40, 1800393. 10.1002/marc.201800393. [DOI] [PubMed] [Google Scholar]
- Wang P.; Fan H.; Zhu X. A 2-(trifluoromethyl)thieno[3,4-b]thiophene-based small-molecule electron acceptor for polymer solar cell application. Dyes Pigm. 2018, 155, 179–185. 10.1016/j.dyepig.2018.03.032. [DOI] [Google Scholar]
- Li J.; Li F.; Zhang B.; Zhou E. Synthesis of 1-Formyl-3-bromo-thieno[3,4-c]pyrrole-4,6-dione and the Application in A2–A1–D–A1–A2 Type Non-Fullerene Acceptor. J. Phys. Chem. C 2020, 124, 9795–9801. 10.1021/acs.jpcc.0c02111. [DOI] [Google Scholar]
- Li J.; Sun C.; Tang A.; Zhang B.; Guo Q.; Zhou E.; Li Y. Utilizing an electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) unit as a π-bridge to improve the photovoltaic performance of A−π–D−π–A type acceptors. J. Mater. Chem. C 2020, 8, 15981–15984. 10.1039/D0TC04601A. [DOI] [Google Scholar]
- He A.; Qin Y.; Zhou D.; Ni C. Effective design of novel low band gap acceptors for non-fullerene solar cells via modulating molecular planarity and F atom substitution. Mater. Lett. 2020, 258, 126785. 10.1016/j.matlet.2019.126785. [DOI] [Google Scholar]
- Duan T.; Hou L.; Fu J.; Kan Z.; Yang Q.; Chen Q.; Zhong C.; Xiao Z.; Yu D.; Lu S. An asymmetric end-capping strategy enables a new non-fullerene acceptor for organic solar cells with efficiency over 10. Chem. Commun. 2020, 56, 6531–6534. 10.1039/D0CC01739F. [DOI] [PubMed] [Google Scholar]
- Zhang Y.; Wang Y.; Xie Z.; Shan T.; Zhu L.; Liu F.; Zhong H. Preparation of non-fullerene acceptors with a multi-asymmetric configuration in a one-pot reaction for organic solar cells. J. Mater. Chem. C 2020, 8, 17229–17236. 10.1039/D0TC04749J. [DOI] [Google Scholar]
- Guo Q.; Lin J.; Liu H.; Dong X.; Guo X.; Ye L.; Ma Z.; Tang Z.; Ade H.; Zhang M.; Li Y. Asymmetrically noncovalently fused-ring acceptor for high-efficiency organic solar cells with reduced voltage loss and excellent thermal stability. Nano Energy 2020, 74, 104861. 10.1016/j.nanoen.2020.104861. [DOI] [Google Scholar]
- Corzo D.; Bihar E.; Alexandre E. B.; Rosas-Villalva D.; Baran D. Ink Engineering of Transport Layers for 9.5% Efficient All-Printed Semitransparent Nonfullerene Solar Cells. Adv. Funct. Mater. 2021, 31, 2005763. 10.1002/adfm.202005763. [DOI] [Google Scholar]
- Karki A.; Vollbrecht J.; Gillett A. J.; Selter P.; Lee J.; Peng Z.; Schopp N.; Dixon A. L.; Schrock M.; Nádaždy V.; Schauer F.; Ade H.; Chmelka B. F.; Bazan G. C.; Friend R. H.; Nguyen T.-Q. Unifying Charge Generation, Recombination, and Extraction in Low-Offset Non-Fullerene Acceptor Organic Solar Cells. Adv. Energy Mater. 2020, 10, 2001203. 10.1002/aenm.202001203. [DOI] [Google Scholar]
- Cho H. W.; An N. G.; Park S. Y.; Shin Y. S.; Lee W.; Kim J. Y.; Song S. Thermally Durable Nonfullerene Acceptor with Nonplanar Conjugated Backbone for High-Performance Organic Solar Cells. Adv. Energy Mater. 2020, 10, 1903585. 10.1002/aenm.201903585. [DOI] [Google Scholar]
- Jiao C.; Guo Z.; Sun B.; Yi Y.-Q.-Q.; Meng L.; Wan X.; Zhang M.; Zhang H.; Li C.; Chen Y. An acceptor–donor–acceptor type non-fullerene acceptor with an asymmetric backbone for high performance organic solar cells. J. Mater. Chem. C 2020, 8, 6293–6298. 10.1039/D0TC00981D. [DOI] [Google Scholar]
- He Q.; Shahid M.; Wu J.; Jiao X.; Eisner F. D.; Hodsden T.; Fei Z.; Anthopoulos T. D.; McNeill C. R.; Durrant J. R.; Heeney M. Fused Cyclopentadithienothiophene Acceptor Enables Ultrahigh Short-Circuit Current and High Efficiency >11% in As-Cast Organic Solar Cells. Adv. Funct. Mater. 2019, 29, 1904956. 10.1002/adfm.201904956. [DOI] [Google Scholar]
- Chen X.; Liu H.; Xia L.; Hayat T.; Alsaedi A.; Tan Z. A pentacyclic S,N-heteroacene based electron acceptor with strong near-infrared absorption for efficient organic solar cells. Chem. Commun. 2019, 55, 7057–7060. 10.1039/C9CC03536B. [DOI] [PubMed] [Google Scholar]
- Li Z.; Wang J.; Huang H.; Liu Y.; Yun Y.; Cheng Z.; Liu S.; Ding Z.; Zhao B.; Huang W. A fully fused non-fullerene acceptor containing angular-shaped S,N-heteroacene and perylene diimide for additive-free organic solar cells. New J. Chem. 2019, 43, 13775–13782. 10.1039/C9NJ02918D. [DOI] [Google Scholar]
- Du F.; Wang H.; Zhang Z.; Yang L.; Cao J.; Yu J.; Tang W. An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells. Mater. Horiz. 2021, 8, 1008–1016. 10.1039/D0MH01585G. [DOI] [PubMed] [Google Scholar]
- He C.; Li Y.; Liu Y.; Li Y.; Zhou G.; Li S.; Zhu H.; Lu X.; Zhang F.; Li C.-Z.; Chen H. Near infrared electron acceptors with a photoresponse beyond 1000 nm for highly efficient organic solar cells. J. Mater. Chem. A 2020, 8, 18154–18161. 10.1039/D0TA06907H. [DOI] [Google Scholar]
- Su J.; Wen X.; Chen W.; Miao Y.; Li F.; Wang Y. Benzothiadiazole-oligothiophene flanked dicyanomethylenated quinacridone for non-fullerene acceptors in polymer solar cells. New J. Chem. 2018, 42, 5005–5013. 10.1039/C8NJ00102B. [DOI] [Google Scholar]
- Morgan M. M.; Nazari M.; Pickl T.; Rautiainen J. M.; Tuononen H. M.; Piers W. E.; Welch G. C.; Gelfand B. S. Boron-nitrogen substituted dihydroindeno[1,2-b]fluorene derivatives as acceptors in organic solar cells. Chem. Commun. 2019, 55, 11095–11098. 10.1039/C9CC05103A. [DOI] [PubMed] [Google Scholar]
- Chen L.; Zeng M.; Weng C.; Tan S.; Shen P. Nonhalogenated-Solvent-Processed Efficient Polymer Solar Cells Enabled by Medium-Band-Gap A-π-D-π-A Small-Molecule Acceptors Based on a 6,12-Dihydro-diindolo1,2-b:10,20-epyrazine Unit. ACS Appl. Mater. Interfaces 2019, 11, 48134–48146. 10.1021/acsami.9b17185. [DOI] [PubMed] [Google Scholar]
- Suman S.; Siddiqui A.; Keshtov M. L.; Sharma G. D.; Singh S. P. New indolo carbazole-based non-fullerene n-type semiconductors for organic solar cell applications. J. Mater. Chem. C 2019, 7, 543–552. 10.1039/C8TC05318A. [DOI] [Google Scholar]
- Wu J.; Wang W.; Zhan C.; Xiao S. 5H-Fluoreno [3,2-b:6,7-b′] Dithiophene Based Non-fullerene Small Molecular Acceptors for Polymer Solar Cell Application. J. Wuhan Univ. Technol., Mater. Sci. Ed. 2019, 34, 1220–1227. 10.1007/s11595-019-2181-0. [DOI] [Google Scholar]
- Hou R.; Li M.; Feng S.; Liu Y.; Wu L.; Bi Z.; Xu X.; Ma W.; Bo Z. Fused pentacyclic electron acceptors with four cis -arranged alkyl side chains for efficient polymer solar cells. J. Mater. Chem. A 2018, 6, 3724–3729. 10.1039/C7TA10026D. [DOI] [Google Scholar]
- Lin Z.; Huang K.; Wang Z.; Chen X.; Sun J.; Xu Z.; He T.; Yin S.; Li M.; Zhang Q.; Qiu H. Alkyl side-chain and fluorination engineering in the indeno[1,2-b]fluorene-based small-molecule acceptors for efficient non-fullerene organic solar cells. Dyes Pigm. 2019, 160, 432–438. 10.1016/j.dyepig.2018.08.039. [DOI] [Google Scholar]
- Wang L.; Cheng Q.; Li P.; Wang W.; Liu J.; Zhao B.; Wu H.; Cong Z.; Gao C. A wide-bandgap nonplanar small molecule acceptor having indenofluorene core for non-fullerene polymer solar cells. Dyes Pigm. 2020, 180, 108409. 10.1016/j.dyepig.2020.108409. [DOI] [Google Scholar]
- Wen S.; Wu Y.; Wang Y.; Li Y.; Liu L.; Jiang H.; Liu Z.; Yang R. Pyran-Bridged Indacenodithiophene as a Building Block for Constructing Efficient A-D-A-Type Nonfullerene Acceptors for Polymer Solar Cells. ChemSusChem 2018, 11, 360–366. 10.1002/cssc.201701917. [DOI] [PubMed] [Google Scholar]
- Liu Z.; Zhang X.; Ren P.; Wu Y.; Zeng Di; Duan X.; Gao X.; Cai P. Pyran-bridged A-D-A type small molecular acceptors for organic solar cells. Sol. Energy 2019, 183, 463–468. 10.1016/j.solener.2019.03.046. [DOI] [Google Scholar]
- Li M.; Liu Y.; Zhou Y.; Yang L.; Shen S.; Song J.; Bo Z. Photovoltaic Performances of Fused Ring Acceptors with Isomerized Ladder-Type Dipyran Cores. ACS Appl. Mater. Interfaces 2020, 12, 4887–4894. 10.1021/acsami.9b19676. [DOI] [PubMed] [Google Scholar]
- Li J.; Wu Y.; Zhang X.; Zeng Di; Luo Y.; Duan X.; Gao X.; Cai P.; Zhang Q.; Zhang J.; Liu Z. A new fluorinated pyran-bridged A-D-A type small molecular acceptor for organic solar cells. Dyes Pigm. 2020, 175, 108165. 10.1016/j.dyepig.2019.108165. [DOI] [Google Scholar]
- Li M.; Zhou Y.; Yang L.; Shen S.; Liu Y.; Chen Y.; Song J.; Bo Z. Regulating molecular orientations of dipyran-based nonfullerene acceptors through side-chain engineering at the π-bridge. J. Mater. Chem. A 2020, 8, 22416–22422. 10.1039/D0TA06394K. [DOI] [Google Scholar]
- Zhang L.; Jin K.; Xiao Z.; Wang X.; Wang T.; Yi C.; Ding L. Alkoxythiophene and alkylthiothiophene π-bridges enhance the performance of A–D–A electron acceptors. Mater. Chem. Front. 2019, 3, 492–495. 10.1039/C8QM00647D. [DOI] [Google Scholar]
- Qin Y.; Chen H.; Yao J.; Zhou Y.; Cho Y.; Zhu Y.; Qiu B.; Ju C.-W.; Zhang Z.-G.; He F.; Yang C.; Li Y.; Zhao D. Silicon and oxygen synergistic effects for the discovery of new high-performance nonfullerene acceptors. Nat. Commun. 2020, 11, 5814. 10.1038/s41467-020-19605-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Zhang M.; Ma Y.; Zheng Q. Dithienonaphthalene-Based Non-fullerene Acceptors With Different Bandgaps for Organic Solar Cells. Front. Chem. 2018, 6, 427. 10.3389/fchem.2018.00427. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Feng H.; Yi Y.-Q.-Q.; Ke X.; Zhang Y.; Wan X.; Li C.; Chen Y. Synergistic Modifications of Side Chains and End Groups in Small Molecular Acceptors for High Efficient Non-Fullerene Organic Solar Cells. Sol. RRL 2018, 2, 1800053. 10.1002/solr.201800053. [DOI] [Google Scholar]
- Zhang X.; Ding Y.; Wan X.; Ke X.; Li C.; Chen Y. A novel acceptor with a N,N-dialkyl thieno[3′,2′:2,3]indolo[7,6-g]thieno[3,2-b]indole (TITI) core for organic solar cells with a high fill factor of 0.75. Chem. Commun. 2020, 56, 751–753. 10.1039/C9CC08098H. [DOI] [PubMed] [Google Scholar]
- Ni S.-S.; Xu X.; Wang J.-L.; Wan S.-S.; Liu K.-K.; Bai H.-R.; Yang C.; Lv G.; Peng Q. Regioisomer-Free Chlorinated Thiophene-Based Ending Group for Thieno[3,2-b]thiophene Central Unit-Based Acceptor Enabling Highly Efficient Nonfullerene Polymer Solar Cells with High Voc Simultaneously. Sol. RRL 2020, 4, 1900446. 10.1002/solr.201900446. [DOI] [Google Scholar]
- Liao X.; Cui Y.; Shi X.; Yao Z.; Zhao H.; An Y.; Zhu P.; Guo Y.; Fei X.; Zuo L.; Gao K.; Lin F.; Xie Q.; Chen L.; Ma W.; Chen Y.; Jen A. K.-Y. The role of dipole moment in two fused-ring electron acceptor and one polymer donor based ternary organic solar cells. Mater. Chem. Front. 2020, 4, 1507–1518. 10.1039/D0QM00016G. [DOI] [Google Scholar]
- Wang W.; Lu H.; Chen Z.; Jia B.; Li K.; Ma W.; Zhan X. High-performance NIR-sensitive fused tetrathienoacene electron acceptors. J. Mater. Chem. A 2020, 8, 3011–3017. 10.1039/C9TA13128K. [DOI] [Google Scholar]
- Liu D.; Wang T.; Ke X.; Zheng N.; Chang Z.; Xie Z.; Liu Y. Ultra-narrow bandgap non-fullerene acceptors for organic solar cells with low energy loss. Mater. Chem. Front. 2019, 3, 2157–2163. 10.1039/C9QM00505F. [DOI] [Google Scholar]
- Zhai W.; Tang A.; Xiao B.; Wang X.; Chen F.; Zhou E. A small molecular electron acceptor based on asymmetric hexacyclic core of thieno[1,2-b]indaceno[5,6-b′]thienothiophene for efficient fullerene-free polymer solar cells. Sci. Bull. 2018, 63, 845–852. 10.1016/j.scib.2018.05.025. [DOI] [PubMed] [Google Scholar]
- Gao W.; Liu T.; Zhong C.; Zhang G.; Zhang Y.; Ming R.; Zhang L.; Xin J.; Wu K.; Guo Y.; Ma W.; Yan H.; Liu Y.; Yang C. Asymmetrical Small Molecule Acceptor Enabling Nonfullerene Polymer Solar Cell with Fill Factor Approaching 79%. ACS Energy Lett. 2018, 3, 1760–1768. 10.1021/acsenergylett.8b00825. [DOI] [Google Scholar]
- Liu S.; Zhao B.; Cong Z.; Cheng Q.; Wang W.; Pan H.; Liu J.; Wu H.; Gao C. Influences of the terminal groups on the performances of asymmetric small molecule acceptors-based polymer solar cells. Dyes Pigm. 2020, 178, 108388. 10.1016/j.dyepig.2020.108388. [DOI] [Google Scholar]
- Gao W.; Wu F.; Liu T.; Zhang G.; Chen Z.; Zhong C.; Zhu L.; Liu F.; Yan H.; Yang C. Multifunctional asymmetrical molecules for high-performance perovskite and organic solar cells. J. Mater. Chem. A 2019, 7, 2412–2420. 10.1039/C8TA10975C. [DOI] [Google Scholar]
- Li X.; Weng K.; Ryu H. S.; Guo J.; Zhang X.; Xia T.; Fu H.; Wei D.; Min J.; Zhang Y.; Woo H. Y.; Sun Y. Non-Fullerene Organic Solar Cells Based on Benzo[1,2-b:4,5-b′]difuran-Conjugated Polymer with 14% Efficiency. Adv. Funct. Mater. 2020, 30, 1906809. 10.1002/adfm.201906809. [DOI] [Google Scholar]
- Cai F.; Zhu C.; Yuan J.; Li Z.; Meng L.; Liu W.; Peng H.; Jiang L.; Li Y.; Zou Y. Efficient organic solar cells based on a new ″Y-series″ non-fullerene acceptor with an asymmetric electron-deficient-core. Chem. Commun. 2020, 56, 4340–4343. 10.1039/C9CC10076H. [DOI] [PubMed] [Google Scholar]
- Cai F.; Peng H.; Chen H.; Yuan J.; Hai J.; Lau T.-K.; Wang J.; Hu Y.; Liu W.; Lu X.; Zou Y. An asymmetric small molecule acceptor for organic solar cells with a short circuit current density over 24 mA cm–2. J. Mater. Chem. A 2020, 8, 15984–15991. 10.1039/D0TA01636E. [DOI] [Google Scholar]
- Gao J.; Gao W.; Ma X.; Hu Z.; Xu C.; Wang X.; An Q.; Yang C.; Zhang X.; Zhang F. Over 14.5% efficiency and 71.6% fill factor of ternary organic solar cells with 300 nm thick active layers. Energy Environ. Sci. 2020, 13, 958–967. 10.1039/C9EE04020J. [DOI] [Google Scholar]
- Zhang J.; Han Y.; Zhang W.; Ge J.; Xie L.; Xia Z.; Song W.; Yang D.; Zhang X.; Ge Z. High-Efficiency Thermal-Annealing-Free Organic Solar Cells Based on an Asymmetric Acceptor with Improved Thermal and Air Stability. ACS Appl. Mater. Interfaces 2020, 12, 57271–57280. 10.1021/acsami.0c17423. [DOI] [PubMed] [Google Scholar]
- Huang C.; Liao X.; Gao K.; Zuo L.; Lin F.; Shi X.; Li C.-Z.; Liu H.; Li X.; Liu F.; Chen Y.; Chen H.; Jen A. K.-Y. Highly Efficient Organic Solar Cells Based on S,N-Heteroacene Non-Fullerene Acceptors. Chem. Mater. 2018, 30, 5429–5434. 10.1021/acs.chemmater.8b02276. [DOI] [Google Scholar]
- Wang C.-K.; Jiang B.-H.; Lu J.-H.; Cheng M.-T.; Jeng R.-J.; Lu Y.-W.; Chen C.-P.; Wong K.-T. A Near-Infrared Absorption Small Molecule Acceptor for High-Performance Semitransparent and Colorful Binary and Ternary Organic Photovoltaics. ChemSusChem 2020, 13, 903–913. 10.1002/cssc.201903087. [DOI] [PubMed] [Google Scholar]
- Yu Z.-P.; Li X.; He C.; Wang Di; Qin R.; Zhou G.; Liu Z.-X.; Andersen T. R.; Zhu H.; Chen H.; Li C.-Z. High-efficiency organic solar cells with low voltage-loss of 0.46 V. Chin. Chem. Lett. 2020, 31, 1991–1996. 10.1016/j.cclet.2019.12.003. [DOI] [Google Scholar]
- Zeng G.; OuYang J.; Xin Y.; Zhao X.; Yang X. A small-molecule acceptor incorporating a silicon bridging atom for efficient nonfullerene polymer solar cells. J. Mater. Chem. C 2018, 6, 13211–13217. 10.1039/C8TC04883E. [DOI] [Google Scholar]
- Lu B.; Chen Z.; Jia B.; Wang J.; Ma W.; Lian J.; Zeng P.; Qu J.; Zhan X. Enhancing Performance of Fused-Ring Electron Acceptor Using Pyrrole Instead of Thiophene. ACS Appl. Mater. Interfaces 2020, 12, 14029–14036. 10.1021/acsami.0c00733. [DOI] [PubMed] [Google Scholar]
- Zhu J.; Wu Y.; Rech J.; Wang J.; Liu K.; Li T.; Lin Y.; Ma W.; You W.; Zhan X. Enhancing the performance of a fused-ring electron acceptor via extending benzene to naphthalene. J. Mater. Chem. C 2018, 6, 66–71. 10.1039/C7TC04520D. [DOI] [Google Scholar]
- Wang W.; Yan C.; Lau T.-K.; Wang J.; Liu K.; Fan Y.; Lu X.; Zhan X. Fused Hexacyclic Nonfullerene Acceptor with Strong Near-Infrared Absorption for Semitransparent Organic Solar Cells with 9.77% Efficiency. Adv. Mater. 2017, 29, 1701308. 10.1002/adma.201701308. [DOI] [PubMed] [Google Scholar]
- Dai S.; Li T.; Wang W.; Xiao Y.; Lau T.-K.; Li Z.; Liu K.; Lu X.; Zhan X. Enhancing the Performance of Polymer Solar Cells via Core Engineering of NIR-Absorbing Electron Acceptors. Adv. Mater. 2018, 30, 1706571. 10.1002/adma.201706571. [DOI] [PubMed] [Google Scholar]
- Intemann J. J.; Yao K.; Ding F.; Xu Y.; Xin X.; Li X.; Jen A. K.-Y. Enhanced Performance of Organic Solar Cells with Increased End Group Dipole Moment in Indacenodithieno[3,2-b]thiophene-Based Molecules. Adv. Funct. Mater. 2015, 25, 4889–4897. 10.1002/adfm.201501600. [DOI] [Google Scholar]
- Walker B.; Tamayo A. B.; Dang X.-D.; Zalar P.; Seo J. H.; Garcia A.; Tantiwiwat M.; Nguyen T.-Q. Nanoscale Phase Separation and High Photovoltaic Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2009, 19, 3063–3069. 10.1002/adfm.200900832. [DOI] [Google Scholar]
- Yao H.; Cui Y.; Yu R.; Gao B.; Zhang H.; Hou J. Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra-Narrow Band Gap. Angew. Chem., Int. Ed. 2017, 56, 3045–3049. 10.1002/anie.201610944. [DOI] [PubMed] [Google Scholar]
- Zhu K.; Zhong Y.; Wang X.; Li F.; Yu L.; Chu L.; Sun M. Asymmetric ITIC acceptor for asymmetric benzodithiophene polymer solar cells. Dyes Pigm. 2020, 183, 108727. 10.1016/j.dyepig.2020.108727. [DOI] [Google Scholar]
- Chen Y.; Liu T.; Hu H.; Ma T.; Lai J. Y. L.; Zhang J.; Ade H.; Yan H. Modulation of End Groups for Low-Bandgap Nonfullerene Acceptors Enabling High-Performance Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1801203. 10.1002/aenm.201801203. [DOI] [Google Scholar]
- Nian L.; Kan Y.; Gao K.; Zhang M.; Li N.; Zhou G.; Jo S. B.; Shi X.; Lin F.; Rong Q.; Liu F.; Zhou G.; Jen A. K.-Y. Approaching 16% Efficiency in All-Small-Molecule Organic Solar Cells Based on Ternary Strategy with a Highly Crystalline Acceptor. Joule 2020, 4, 2223–2236. 10.1016/j.joule.2020.08.011. [DOI] [Google Scholar]
- Liu T.; Luo Z.; Chen Y.; Yang T.; Xiao Y.; Zhang G.; Ma R.; Lu X.; Zhan C.; Zhang M.; Yang C.; Li Y.; Yao J.; Yan H. A nonfullerene acceptor with a 1000 nm absorption edge enables ternary organic solar cells with improved optical and morphological properties and efficiencies over 15%. Energy Environ. Sci. 2019, 12, 2529–2536. 10.1039/C9EE01030K. [DOI] [Google Scholar]
- Li T.; Dai S.; Ke Z.; Yang L.; Wang J.; Yan C.; Ma W.; Zhan X. Fused Tris(thienothiophene)-Based Electron Acceptor with Strong Near-Infrared Absorption for High-Performance As-Cast Solar Cells. Adv. Mater. 2018, 30, 1705969. 10.1002/adma.201705969. [DOI] [PubMed] [Google Scholar]
- Xue P.; Dai S.; Lau T.-K.; Yu J.; Zhou J.; Xiao Y.; Meng K.; Xie Z.; Lu G.; Lu X.; Han R. P. S.; Zhan X. High-Performance Nonfullerene Organic Solar Cells with Unusual Inverted Structure. Sol. RRL 2020, 4, 2000115. 10.1002/solr.202000115. [DOI] [Google Scholar]
- Gao H.-H.; Sun Y.; Wan X.; Ke X.; Feng H.; Kan B.; Wang Y.; Zhang Y.; Li C.; Chen Y. A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary-Blend Organic Solar Cells with Efficiency over 13. Adv. Sci. 2018, 5, 1800307. 10.1002/advs.201800307. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Gao H.-H.; Sun Y.; Cai Y.; Wan X.; Meng L.; Ke X.; Li S.; Zhang Y.; Xia R.; Zheng N.; Xie Z.; Li C.; Zhang M.; Yip H.-L.; Cao Y.; Chen Y. Achieving Both Enhanced Voltage and Current through Fine-Tuning Molecular Backbone and Morphology Control in Organic Solar Cells. Adv. Energy Mater. 2019, 9, 1901024. 10.1002/aenm.201901024. [DOI] [Google Scholar]
- Wang Y.; Chang M.; Ke X.; Wan X.; Yang G. A new medium-bandgap fused-[1]benzothieno[3,2-b][1]benzo-thiophene (BTBT) nonfullerene acceptor for organic solar cells with high open-circuit voltage. Polymer 2019, 185, 121976. 10.1016/j.polymer.2019.121976. [DOI] [Google Scholar]
- Zhou Z.; Duan J.; Ye L.; Wang G.; Zhao B.; Tan S.; Shen P.; Ryu H. S.; Woo H. Y.; Sun Y. Simultaneously improving the photovoltaic parameters of organic solar cells via isomerization of benzo[b]benzo[4,5]thieno[2,3-d]thiophene-based octacyclic non-fullerene acceptors. J. Mater. Chem. A 2020, 8, 9684–9692. 10.1039/D0TA00451K. [DOI] [Google Scholar]
- Li C.; Song J.; Ye L.; Koh C.; Weng K.; Fu H.; Cai Y.; Xie Y.; Wei D.; Woo H. Y.; Sun Y. High-Performance Eight-Membered Indacenodithiophene-Based Asymmetric A-D-A Type Non-Fullerene Acceptors. Sol. RRL 2019, 3, 1800246. 10.1002/solr.201800246. [DOI] [Google Scholar]
- Gao W.; Liu T.; Li J.; Xiao Y.; Zhang G.; Chen Y.; Zhong C.; Lu X.; Yan H.; Yang C. Simultaneously increasing open-circuit voltage and short-circuit current to minimize the energy loss in organic solar cells via designing asymmetrical non-fullerene acceptor. J. Mater. Chem. A 2019, 7, 11053–11061. 10.1039/C9TA02283J. [DOI] [Google Scholar]
- Dong X.; Guo Q.; Liu Q.; Zhu L.; Guo X.; Liu F.; Zhang M. A naphthodithiophene-based nonfullerene acceptor for high-performance polymer solar cells with a small energy loss. J. Mater. Chem. C 2020, 8, 6513–6520. 10.1039/D0TC00270D. [DOI] [Google Scholar]
- Chang S.-L.; Cao F.-Y.; Huang K.-H.; Lee W.-L.; Lee M.-H.; Hsu C.-S.; Cheng Y.-J. 2-Dimensional cross-shaped tetrathienonaphthalene-based ladder-type acceptor for high-efficiency organic solar cells. J. Mater. Chem. A 2020, 8, 12141–12148. 10.1039/D0TA04240D. [DOI] [Google Scholar]
- Zhu J.; Xiao Y.; Zhang C.; Jia B.; Lu H.; Wang J.; Lu X.; Li Z.; Zhan X. Effects of alkoxylation position on fused-ring electron acceptors. J. Mater. Chem. C 2020, 8, 15128–15134. 10.1039/D0TC02158J. [DOI] [Google Scholar]
- Li T.; Wu Y.; Zhou J.; Li M.; Wu J.; Hu Q.; Jia B.; Pan X.; Zhang M.; Tang Z.; Xie Z.; Russell T. P.; Zhan X. Butterfly Effects Arising from Starting Materials in Fused-Ring Electron Acceptors. J. Am. Chem. Soc. 2020, 142, 20124–20133. 10.1021/jacs.0c09800. [DOI] [PubMed] [Google Scholar]
- Liu W.; Zhang J.; Zhou Z.; Zhang D.; Zhang Y.; Xu S.; Zhu X. Design of a New Fused-Ring Electron Acceptor with Excellent Compatibility to Wide-Bandgap Polymer Donors for High-Performance Organic Photovoltaics. Adv. Mater. 2018, 30, 1800403. 10.1002/adma.201800403. [DOI] [PubMed] [Google Scholar]
- Zhang J.; Liu W.; Chen S.; Xu S.; Yang C.; Zhu X. One-pot synthesis of electron-acceptor composite enables efficient fullerene-free ternary organic solar cells. J. Mater. Chem. A 2018, 6, 22519–22525. 10.1039/C8TA08961B. [DOI] [Google Scholar]
- Zhang J.; Liu W.; Zhang M.; Xu S.; Liu F.; Zhu X. PCE11-based polymer solar cells with high efficiency over 13% achieved by room-temperature processing. J. Mater. Chem. A 2020, 8, 8661–8668. 10.1039/D0TA02271C. [DOI] [Google Scholar]
- Zhang X.; Wang H.; Li D.; Chen M.; Mao Y.; Du B.; Zhuang Y.; Tan W.; Huang W.; Zhao Y.; Liu D.; Wang T. Modulation of J-Aggregation of Nonfullerene Acceptors toward Near-Infrared Absorption and Enhanced Efficiency. Macromolecules 2020, 53, 3747–3755. 10.1021/acs.macromol.0c00469. [DOI] [Google Scholar]
- Zhao J.; Yao C.; Zhu Y.; Cai J.; Ali M. U.; Miao J.; Meng H. Synthesis and characterization of new nonfullerene electron acceptors with a chrysene core. Dyes Pigm. 2020, 174, 108012. 10.1016/j.dyepig.2019.108012. [DOI] [Google Scholar]
- Wu J.; Chen Y.; Hu B.; Pang Z.; Lu Z.; Huang Y. Isomers of Dithienocyclopentapyrene-Based Non-Fullerene Electron Acceptors: Configuration Effect on Photoelectronic Properties. Chem. - Eur. J. 2019, 25, 6385–6391. 10.1002/chem.201900296. [DOI] [PubMed] [Google Scholar]
- Zhu J.; Ke Z.; Zhang Q.; Wang J.; Dai S.; Wu Y.; Xu Y.; Lin Y.; Ma W.; You W.; Zhan X. Naphthodithiophene-Based Nonfullerene Acceptor for High-Performance Organic Photovoltaics: Effect of Extended Conjugation. Adv. Mater. 2018, 30, 1704713. 10.1002/adma.201704713. [DOI] [PubMed] [Google Scholar]
- Zhu J.; Xiao Y.; Wang J.; Liu K.; Jiang H.; Lin Y.; Lu X.; Zhan X. Alkoxy-Induced Near-Infrared Sensitive Electron Acceptor for High-Performance Organic Solar Cells. Chem. Mater. 2018, 30, 4150–4156. 10.1021/acs.chemmater.8b01677. [DOI] [Google Scholar]
- Fan X.; Gao J.; Wang W.; Xiao S.; Zhan C.; Lu X.; Zhang Q. Ladder-Type Nonacyclic Arene Bis(thieno[3,2-b]thieno)cyclopentafluorene as a Promising Building Block for Non-Fullerene Acceptors. Chem. - Asian J. 2019, 14, 1814–1822. 10.1002/asia.201801669. [DOI] [PubMed] [Google Scholar]
- Wang H.; Zhang Z.; Yu J.; Liu X.; Qu S.; Guang S.; Tang W. Nonacyclic carbazole-based non-fullerene acceptors enable over 12% efficiency with enhanced stability for organic solar cells. J. Mater. Chem. A 2019, 7, 21903–21910. 10.1039/C9TA08573D. [DOI] [Google Scholar]
- Ming R.; Zhang M.; Gao W.; Ning W.; Luo Z.; Zhong C.; Zhang F.; Yang C. Fluorene-fused ladder-type non-fullerene small molecule acceptors for high-performance polymer solar cells. Mater. Chem. Front. 2019, 3, 709–715. 10.1039/C9QM00013E. [DOI] [Google Scholar]
- Ouyang J.; Zeng G.; Xin Y.; Zhao X.; Yang X. A Novel Carbazole-Based Nonfullerene Acceptor for High-Efficiency Polymer Solar Cells. Sol. RRL 2020, 4, 1900417. 10.1002/solr.201900417. [DOI] [Google Scholar]
- Wang W.; Li Y.; Zhan C.; Xiao S.; Tang C.; Li G.; Lu X.; Zhang Q. Bis(thieno[3,2-b]thieno)cyclopentafluorene-Based Acceptor with Efficient and Comparable Photovoltaic Performance under Various Processing Conditions. ACS Appl. Mater. Interfaces 2020, 12, 49876–49885. 10.1021/acsami.0c13109. [DOI] [PubMed] [Google Scholar]
- Ouyang J.; Zhang B.; Zeng G.; Zhang J.; Zhao X.; Yang X. Improving Efficiency of Organic Solar Cells by Restricting the Rotation of Side Chain on Small Molecule Acceptor. Sol. RRL 2020, 4, 2000359. 10.1002/solr.202000359. [DOI] [Google Scholar]
- Chen T.-W.; Peng K.-L.; Lin Y.-W.; Su Y.-J.; Ma K.-J.; Hong L.; Chang C.-C.; Hou J.; Hsu C.-S. A chlorinated nonacyclic carbazole-based acceptor affords over 15% efficiency in organic solar cells. J. Mater. Chem. A 2020, 8, 1131–1137. 10.1039/C9TA12605H. [DOI] [Google Scholar]
- Sun J.; Ma X.; Zhang Z.; Yu J.; Zhou J.; Yin X.; Yang L.; Geng R.; Zhu R.; Zhang F.; Tang W. Dithieno[3,2-b:2′,3′-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells. Adv. Mater. 2018, 30, 1707150. 10.1002/adma.201707150. [DOI] [PubMed] [Google Scholar]
- Geng R.; Song X.; Feng H.; Yu J.; Zhang M.; Gasparini N.; Zhang Z.; Liu F.; Baran D.; Tang W. Nonfullerene Acceptor for Organic Solar Cells with Chlorination on Dithieno[3,2-b:2′,3′-d]pyrrol Fused-Ring. ACS Energy Lett. 2019, 4, 763–770. 10.1021/acsenergylett.9b00147. [DOI] [Google Scholar]
- Zhao F.; He D.; Xin J.; Dai S.; Xue H.; Jiang L.; Wei Z.; Ma W.; Zhan X.; Li Y.; Wang C. Modulating morphology via side-chain engineering of fused ring electron acceptors for high performance organic solar cells. Sci. China: Chem. 2019, 62, 790–796. 10.1007/s11426-019-9453-7. [DOI] [Google Scholar]
- Dai S.; Zhou J.; Chandrabose S.; Shi Y.; Han G.; Chen K.; Xin J.; Liu K.; Chen Z.; Xie Z.; Ma W.; Yi Y.; Jiang L.; Hodgkiss J. M.; Zhan X. High-Performance Fluorinated Fused-Ring Electron Acceptor with 3D Stacking and Exciton/Charge Transport. Adv. Mater. 2020, 32, 2000645. 10.1002/adma.202000645. [DOI] [PubMed] [Google Scholar]
- Cai Y.; Meng L.; Gao H.; Guo Z.; Zheng N.; Xie Z.; Zhang H.; Li C.; Wan X.; Chen Y. Achieving organic solar cells with efficiency over 14% based on a non-fullerene acceptor incorporating a cyclopentathiophene unit fused backbone. J. Mater. Chem. A 2020, 8, 5194–5199. 10.1039/D0TA00677G. [DOI] [Google Scholar]
- Zhang J.; Li Y.; Peng Z.; Bai F.; Ma L.-K.; Ade H.; Li Z.; Yan H. Near-infrared electron acceptors with fused nonacyclic molecular backbones for nonfullerene organic solar cells. Mater. Chem. Front. 2020, 4, 1729–1738. 10.1039/C9QM00754G. [DOI] [Google Scholar]
- Liu Q.; Xiao Z.; Li T.; Yang S.; You W.; Wang M.; Ding L. Understanding the side-chain effects on A–D–A acceptors: in-plane and out-of-plane. Mater. Chem. Front. 2018, 2, 1563–1567. 10.1039/C8QM00238J. [DOI] [Google Scholar]
- Zhang X.; Ding Y.; Feng H.; Gao H.; Ke X.; Zhang H.; Li C.; Wan X.; Chen Y. Side chain engineering investigation of non-fullerene acceptors for photovoltaic device with efficiency over 15%. Sci. China: Chem. 2020, 63, 1799–1806. 10.1007/s11426-020-9820-2. [DOI] [Google Scholar]
- Ding Y.; Zhang X.; Feng H.; Ke X.; Meng L.; Sun Y.; Guo Z.; Cai Y.; Jiao C.; Wan X.; Li C.; Zheng N.; Xie Z.; Chen Y. Subtle Morphology Control with Binary Additives for High-Efficiency Non-Fullerene Acceptor Organic Solar Cells. ACS Appl. Mater. Interfaces 2020, 12, 27425–27432. 10.1021/acsami.0c05331. [DOI] [PubMed] [Google Scholar]
- Su D.; Pan M.-A.; Liu Z.; Lau T.-K.; Li X.; Shen F.; Huo S.; Lu X.; Xu A.; Yan H.; Zhan C. A Trialkylsilylthienyl Chain-Substituted Small-Molecule Acceptor with Higher LUMO Level and Reduced Band Gap for Over 16% Efficiency Fullerene-Free Ternary Solar Cells. Chem. Mater. 2019, 31, 8908–8917. 10.1021/acs.chemmater.9b02943. [DOI] [Google Scholar]
- Liu Z.; Liu W.; Su D.; Shen F.; Huo S.; Han M.; Xu A.; Zhan C. Synthesizing alkynyl-decorated 2D conjugated nonfullerene acceptors for efficient polymer solar cells. Mater. Chem. Front. 2021, 5, 1836–1846. 10.1039/D0QM00882F. [DOI] [Google Scholar]
- Wang J.; Zhang J.; Xiao Y.; Xiao T.; Zhu R.; Yan C.; Fu Y.; Lu G.; Lu X.; Marder S. R.; Zhan X. Effect of Isomerization on High-Performance Nonfullerene Electron Acceptors. J. Am. Chem. Soc. 2018, 140, 9140–9147. 10.1021/jacs.8b04027. [DOI] [PubMed] [Google Scholar]
- Liu G.; Jia T.; Zhang K.; Jia J.; Yin Q.; Zhong W.; Jia X.; Zheng N.; Ying L.; Huang F.; Cao Y. Chlorinated Fused Nonacyclic Non-Fullerene Acceptor Enables Efficient Large-Area Polymer Solar Cells with High Scalability. Chem. Mater. 2020, 32, 1022–1030. 10.1021/acs.chemmater.9b03694. [DOI] [Google Scholar]
- Chen L.; Zeng M.; Tang X.; Weng C.; Tan S.; Shen P. Development of A–DA′D–A Small-Molecular Acceptors Based on a 6,12-Dihydro-diindolo[1,2-b:10,20-e]pyrazine Unit for Efficient As-Cast Polymer Solar Cells. J. Phys. Chem. C 2020, 124, 21366–21377. 10.1021/acs.jpcc.0c07557. [DOI] [Google Scholar]
- Ke X.; Meng L.; Wan X.; Cai Y.; Gao H.-H.; Yi Y.-Q.-Q.; Guo Z.; Zhang H.; Li C.; Chen Y. A nonfullerene acceptor incorporating a dithienopyran fused backbone for organic solar cells with efficiency over 14%. Nano Energy 2020, 75, 104988. 10.1016/j.nanoen.2020.104988. [DOI] [Google Scholar]
- Xia C.; Wu H.; Yue Q.; Chen S.; Shui L.; Fan H.; Zhu X. Oxygen heterocycle-fused indacenodithiophenebithiophene enables an efficient non-fullerene molecular acceptor. J. Mater. Chem. C 2019, 7, 15344–15349. 10.1039/C9TC05041H. [DOI] [Google Scholar]
- Xiao J.; Yan T.; Lei T.; Li Y.; Han Y.; cao L.; Song W.; Tan S.; Ge Z. Organic solar cells based on non-fullerene acceptors of nine fused-ring by modifying end groups. Org. Electron. 2020, 81, 105662. 10.1016/j.orgel.2020.105662. [DOI] [Google Scholar]
- Li W.; Chen M.; Zhang Z.; Cai J.; Zhang H.; Gurney R. S.; Liu D.; Yu J.; Tang W.; Wang T. Retarding the Crystallization of a Nonfullerene Electron Acceptor for High-Performance Polymer Solar Cells. Adv. Funct. Mater. 2018, 1807662. 10.1002/adfm.201807662. [DOI] [Google Scholar]
- Wu F.; Zhong L.; Hu H.; Li Y.; Zhang Z.; Li Y.; Zhang Z.-G.; Ade H.; Jiang Z.-Q.; Liao L.-S. A decacyclic indacenodithiophene-based non-fullerene electron acceptor with meta -alkyl-phenyl substitutions for polymer solar cells. J. Mater. Chem. A 2019, 7, 4063–4071. 10.1039/C8TA11972D. [DOI] [Google Scholar]
- Qu J.; Zhao Q.; Zhou J.; Lai H.; Liu T.; Li D.; Chen W.; Xie Z.; He F. Multiple Fused Ring-Based Near-Infrared Nonfullerene Acceptors with an Interpenetrated Charge-Transfer Network. Chem. Mater. 2019, 31, 1664–1671. 10.1021/acs.chemmater.8b05047. [DOI] [Google Scholar]
- Cai G.; Wang W.; Zhou J.; Xiao Y.; Liu K.; Xie Z.; Lu X.; Lian J.; Zeng P.; Wang Y.; Zhan X. Comparison of Linear- and Star-Shaped Fused-Ring Electron Acceptors. ACS Mater. Lett. 2019, 1, 367–374. 10.1021/acsmaterialslett.9b00253. [DOI] [Google Scholar]
- Liu D.; Wang T.; Chang Z. C.; Zheng N.; Xie Z.; Liu Y. Fused or Unfused? Two-dimensional Non-fullerene Acceptors for Efficient Organic Solar Cells. J. Mater. Chem. A 2021, 9, 2319–2324. 10.1039/D0TA10901K. [DOI] [Google Scholar]
- Cai G.; Xue P.; Chen Z.; Li T.; Liu K.; Ma W.; Lian J.; Zeng P.; Wang Y.; Han R. P. S.; Zhan X. High-Performance Mid-Bandgap Fused-Pyrene Electron Acceptor. Chem. Mater. 2019, 31, 6484–6490. 10.1021/acs.chemmater.8b04668. [DOI] [Google Scholar]
- Han X.; Zhu J.; Xiao Y.; Jiang H.; Zhang Z.; Wang J.; Li Z.; Lin Y.; Lu X.; Zhan X. An Alkoxy-Solubilizing Decacyclic Electron Acceptor for Efficient Ecofriendly As-Cast Blade-Coated Organic Solar Cells. Sol. RRL 2020, 4, 2000108. 10.1002/solr.202000108. [DOI] [Google Scholar]
- Li T.; Yang L.; Xiao Y.; Liu K.; Wang J.; Lu X.; Zhan X. Facile synthesis of high-performance nonfullerene acceptor isomers via a one stone two birds strategy. J. Mater. Chem. A 2019, 7, 20667–20674. 10.1039/C9TA07917C. [DOI] [Google Scholar]
- Jia B.; Dai S.; Ke Z.; Yan C.; Ma W.; Zhan X. Breaking 10% Efficiency in Semitransparent Solar Cells with Fused-Undecacyclic Electron Acceptor. Chem. Mater. 2018, 30, 239–245. 10.1021/acs.chemmater.7b04251. [DOI] [Google Scholar]
- Xia T.; Li C.; Ryu H. S.; Guo J.; Min J.; Woo H. Y.; Sun Y. Efficient Fused-Ring Extension of A-D-A-Type Non-Fullerene Acceptors by a Symmetric Replicating Core Unit Strategy. Chem. - Eur. J. 2020, 26, 12411–12417. 10.1002/chem.202000889. [DOI] [PubMed] [Google Scholar]
- Wang W.; Wang T.; Wu X.; Tong H.; Wang L. An efficient star-shaped fused-ring electron acceptor with C3h -symmetric core via thieno[3,2-b]thiophene extending conjugation strategy. Mater. Chem. Front. 2020, 4, 3328–3337. 10.1039/D0QM00514B. [DOI] [Google Scholar]
- Wu J.; Lee J.; Chin Y.-C.; Yao H.; Cha H.; Luke J.; Hou J.; Kim J.-S.; Durrant J. R. Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells. Energy Environ. Sci. 2020, 13, 2422–2430. 10.1039/D0EE01338B. [DOI] [Google Scholar]
- Zhang G.; Chen X.-K.; Xiao J.; Chow P. C. Y.; Ren M.; Kupgan G.; Jiao X.; Chan C. C. S.; Du X.; Xia R.; Chen Z.; Yuan J.; Zhang Y.; Zhang S.; Liu Y.; Zou Y.; Yan H.; Wong K. S.; Coropceanu V.; Li N.; Brabec C. J.; Bredas J.-L.; Yip H.-L.; Cao Y. Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells. Nat. Commun. 2020, 11, 3943. 10.1038/s41467-020-17867-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Zhu L.; Zhang M.; Zhou G.; Hao T.; Xu J.; Wang J.; Qiu C.; Prine N.; Ali J.; Feng W.; Gu X.; Ma Z.; Tang Z.; Zhu H.; Ying L.; Zhang Y.; Liu F. Efficient Organic Solar Cell with 16.88% Efficiency Enabled by Refined Acceptor Crystallization and Morphology with Improved Charge Transfer and Transport Properties. Adv. Energy Mater. 2020, 10, 1904234. 10.1002/aenm.201904234. [DOI] [Google Scholar]
- Karuthedath S.; Gorenflot J.; Firdaus Y.; Chaturvedi N.; Castro C. S. P. de; Harrison G. T.; Khan J. I.; Markina A.; Balawi A. H.; Peña T. A. D.; Liu W.; Liang R.-Z.; Sharma A.; Paleti S. H. K.; Zhang W.; Lin Y.; Alarousu E.; Anjum D. H.; Beaujuge P. M.; Wolf S. de; McCulloch I.; Anthopoulos T. D.; Baran D.; Andrienko D.; Laquai F. Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nat. Mater. 2021, 20, 378–384. 10.1038/s41563-020-00835-x. [DOI] [PubMed] [Google Scholar]
- Wang Y.; Qian D.; Cui Y.; Zhang H.; Hou J.; Vandewal K.; Kirchartz T.; Gao F. Optical Gaps of Organic Solar Cells as a Reference for Comparing Voltage Losses. Adv. Energy Mater. 2018, 8, 1801352. 10.1002/aenm.201801352. [DOI] [Google Scholar]
- Rühle S. Tabulated values of the Shockley–Queisser limit for single junction solar cells. Sol. Energy 2016, 130, 139–147. 10.1016/j.solener.2016.02.015. [DOI] [Google Scholar]
- Shockley W.; Queisser H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510–519. 10.1063/1.1736034. [DOI] [Google Scholar]