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. 2020 Jul 15;5(29):18449–18457. doi: 10.1021/acsomega.0c02333

Differently Linked Perylene Bisimide Dimers with Various Twisting and Phase Structures for Nonfullerene All-Small-Molecule Organic Solar Cells

Rui Xin †, Cheng Zeng ‡, Dong Meng ‡, Zhongjie Ren §,*, Wei Jiang ‡,*, Zhaohui Wang ∥, Shouke Yan †,§,*
PMCID: PMC7391947  PMID: 32743222

Abstract

graphic file with name ao0c02333_0007.jpg

Nonfullerene all-small-molecule organic solar cells (NF all-SMSCs) are an important classification in the organic solar cell system. However, the application and research of NF all-SMSCs are limited due to the easy aggregation of small molecules to form large-phase domains. Perylene bisimides (PBIs) have been widely used as nonfullerene acceptors. Simply changing the link position of the PBI dimer can control the accumulation of molecules to regulate the size of the phase domain. Herein, the bay-linked, ortho-linked, and hydrazine-linked PBI dimers as nonfullerene acceptors, named as B-SdiPBI, O-SdiPBI, and H-SdiPBI, respectively, were chosen. The link position of the PBI dimer can lead to diverse molecular torsion and planarity, which affects the film-forming ability, phase separation, and thus optoelectronic properties. NF all-SMSCs based on B-SdiPBI, O-SdiPBI, and H-SdiPBI as nonfullerene acceptors and a small molecule DR3TBDTT as the donor achieve the best power conversion efficiencies of 1.93, 3.30, and 4.05%, respectively. The result is consistent with the sequence of inter-PBI twist and phase domain size of the corresponding blend films in the device. The DR3TBDTT:H-SdiPBI system has the best efficiency with the largest dihedral angle of H-SdiPBI (ψ = 90°) and an appropriate phase size (10–40 nm), whereas the smaller dihedral angle of O-SdiPBI (ψ = 86°) produces a larger phase size (20–50 nm) and the smallest dihedral angle of B-SdiPBI (ψ = 71°) gives the largest phase size (30–80 nm). This illustrates that the twist angle can effectively increase the phase separation between the acceptor and donor to obtain an effective phase size in this system. The work provides a guide for designing the acceptors and controlling phase domains of high-performance NF all-SMSCs.

1. Introduction

Solution-processed bulk-heterojunction organic solar cells (BHJ-OSCs) have been considered as a significant academic and prospective photovoltaic technology owing to its low-cost, large printable area, and mechanical flexibility.1−3 Until now, typical polymer/fullerene-based solar cells (PSCs) have significantly contributed to the development of BHJ-OSCs.4,5 Fullerene derivatives, [6,6]-phenyl-C61(C71)-butyric acid methyl ester (PC61BM and PC71BM), play a significant role in acceptor materials. Nonfullerene acceptors also have several advantages over fullerene acceptors, like the strong and high light-harvesting ability and the controlled energy levels, which can be easily tuned by chemical functionalization.6 Therefore, much more nonfullerene acceptors have been manufactured as a kind of good candidate in BHJ-OSCs. Until now, the power conversion efficiency (PCE) of polymer-based nonfullerene OSCs is more than 15%, even a higher efficiency for a single junction.7−10 Compared with the polymeric donors or acceptors, small molecular donors or acceptors possess some advantages, including well-defined molecular weight, convenient purification, and less batch-to-batch variation.11−13 Thus, recently, nonfullerene all-small-molecule organic solar cells (NF all-SMSCs, nonfullerene acceptors in combination with a small molecular donor) are considered significant. However, small molecular donors tend to form large aggregates that affect exciton separation, so there are only a few reports on NF all-SMSCs with PCEs more than 10%.14−17 In addition, the relationship between chemical structure, morphology, and electro-optical properties has not been fully addressed yet.

Perylene bisimide (PBI) derivatives as a type of significant nonfullerene acceptors have got excellent success in recent years, and their PCEs are even better than those of the typical fullerene-based OSCs.18−24 However, PBIs usually have a strong tendency to form exaggerated microscale aggregates, which hinders efficient charge separation and transport. To solve the problem, several methods have been used to adjust the intermolecular π–π stacking, including introducing substituents at the imide, bay, and headland positions of the PBI monomer25 and building PBI dimers26−32 and multimer PBI derivatives.23,33−35 Very recently, the main research focused on PBI-based nonfullerene acceptors used in PSCs, such as a PCE of above 9% has been achieved by triperylene hexaimide derivative (TPH-Se) acceptor-based PSCs33 and tetramer hexaimide derivative (FTTB-PDI4) acceptor-based PSCs achieves a PCE of above 10%.34 The introduction of intramolecular distortion is conducive to construct small domain sizes, and at the same time, intermolecular π–π stacking will also limit the electron transfer efficiency of the acceptor in the blend.34 For perylene-based PSCs, there are some literature works on the relationship between intramolecular twisting and photoelectric properties, such as the photoelectric efficiency of a series of PBI tetramers (TPC-PDI4, TPE-PDI4, TPPz-PDI4) increases with decreasing intermolecular distortion.35 However, small molecules are more likely to form a large domain size than polymers. For perylene-based all-SMSCs, an intermolecular twist could become the principal factor, and a more twisted structure may be more beneficial to the efficiency of small-molecule devices. However, PBI dimer-based OSCs with small molecular donors have rarely been reported. Recently, SdiPBI-S and SdiPBI-Se with the introduction of halogen bridges into the PBI core as a nonfullerene acceptor and small molecule DR3TBDTT as an electron donor achieve PCEs of 5.80 and 6.22%, respectively, which is one of the highest PCEs to date for PBI acceptor-based NF all-SMSCs.36 This result indicates that the twisted singly linked PBI dimers are the effective nonfullerene acceptors in NF all-SMSCs. As we know, the differently linked position of the PBI dimer could affect the energy levels, flexibility, molecular configuration, and thus photoelectrical properties. Bay-linked PBI derivatives (B-SdiPBI) are usually reported, and ortho-linked PBI derivatives (O-SdiPBI) are less investigated owing to the low reactivity toward electrophilic substitutions.29,37 Nevertheless, O-SdiPBI shows excellent photovoltaic performance due to better planarity and weakly coupled interactions than B-SdiPBI.29,37 In addition, hydrazine-linked PBI derivatives (H-SdiPBI) display high intensity and red-shifted absorption, where two PBI segments are perpendicularly connected with each other.27,31,32 Therefore, it is necessary to study B-SdiPBI, O-SdiPBI, and H-SdiPBI systematically to detect the effect of molecular geometry on the morphology and photovoltaic properties of their NF all-SMSCs.

In this study, three differently linked PBI dimers with various degrees of twisting and planarity, B-SdiPBI, O-SdiPBI, and H-SdiPBI, were chosen as electron acceptors. Also, DR3TBDTT with benzo[1,2-b:4,5-b′]dithiophene (BDT) blocks was used as the donor for NF all-SMSCs.38 Solvent vapor annealing (SVA) was also adopted to optimize the morphology of blended films in NF all-SMSCs.39−41 The effect of molecular aggregation and phase separation size, which influence hole/electron mobility and exciton separation/transport efficiency dramatically, on the photovoltaic properties of OSCs has been studied in detail. The molecular twisting of singly linked diPBI derivatives can reduce the formation of large aggregates, which improves exciton separation/transport efficiency and thus a high PCE when blended with DR3TBDTT donors to manufacture NF all-SMSCs.

2. Results and Discussion

2.1. Molecular Structures and Optoelectronic Properties

Molecular structures of B-SdiPBI, O-SdiPBI, and H-SdiPBI are illustrated in Figure 1. The molecular simulation was performed using the density functional theory (DFT) at the B3LYP/6-31G(d) level to determine the optimized geometric and energetic difference of B-SdiPBI, O-SdiPBI, and H-SdiPBI. As exhibited in Figure 1, the two PBI units of B-SdiPBI are twisted relative to one another (as quantified by ψ defined as the dihedral angle). The torsion angle (ψ) across the PBI–PBI bond of B-SdiPBI is 71° (Table 1), which is almost consistent with the previous calculations.29,37 Owing to the large steric repulsion, the dihedral angle are increased to 86 and 90° for O-SdiPBI and H-SdiPBI, respectively. In addition, the two naphthalene subunits of B-SdiPBI are somewhat twisted from the coplanar configuration, whereas O-SdiPBI and H-SdiPBI are virtually planar (as quantified by ϕ). So, the torsion angles between the two PBI subunits and the planar extent of a single PBI subunit for the three molecules are in the following order: H-SdiPBI > O-SdiPBI > B-SdiPBI, suggesting that the conformations of the singly linked diPBI derivatives are facilely tuned by linking at different sites.

Figure 1.

Figure 1

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(a) Chemical structures and molecular conformations of B-SdiPBI, O-SdiPBI, and H-SdiPBI in side views using DFT calculations at the B3LYP/6-31G(d) level. (b) Chemical structure of the donor DR3TBDTT. (c) UV–vis absorption spectra of B-SdiPBI, O-SdiPBI, and H-SdiPBI in chloroform solution. (d) Absorption spectra of the three SdiPBI-based acceptors and the DR3TBDTT donor.

Table 1. Geometric and Energetic Characteristics of B-SdiPBI, O-SdiPBI, and H-SdiPBI from DFT Calculations (B3LYP/6-31G(d)).

2.1.

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The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of the three PBI-based acceptors B-SdiPBI, O-SdiPBI, and H-SdiPBI are −3.84, −3.90, and −3.86 eV and −5.94, −6.02, and −5.90 eV, respectively, and the energy level diagrams of the three SdiPBI acceptors and the DR3TBDTT donor are shown in Figure 2a. Furthermore, as shown in Table 1, theoretical calculations for B-SdiPBI, O-SdiPBI, and H-SdiPBI were also performed by the density functional theory (DFT, B3LYP/6-31G(d)). The wavefunctions and energies for B-SdiPBI and O-SdiPBI are similar to those previously reported.29,37 The bonding and antibonding combinations of the symmetric SdiPBI can be estimated by LUMO and LUMO + 1, as shown in Table 1. Also, the LUMO/LUMO + 1 energetic separation of the three molecules increased in the following order: B-SdiPBI > O-SdiPBI > H-SdiPBI. These could be ascribed to the inter-PBI π-overlap, which is related to ψ. The large ψ will result in a small π-overlap. At the same time, HOMO and HOMO – 1 are much alike in energetic separation.

Figure 2.

Figure 2

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(a) Energy level diagrams of B-SdiPBI, O-SdiPBI, H-SdiPBI, and DR3TBDTT. (b) Schematic illustration of a traditional device architecture for NF all-SMSCs. (c) Current density–voltage (J–V) and (d) external quantum efficiency (EQE) curves of the NF all-SMSC devices based on B-SdiPBI, O-SdiPBI, and H-SdiPBI as the acceptors and DR3TBDTT as the donor.

The absorption spectra of three SdiPBI derivatives in solution present the obvious characteristic absorption of PBI, as shown in Figure 1c. Among them, the spectra of B-SdiPBI are significantly broader (400–600 nm) and have lower intensity (εmax = 0.6 × 105 M–1 cm–1) than those of O-SdiPBI and H-SdiPBI, which is partly attributed to the nonplanar configuration of PBI units in B-SdiPBI.29 However, the absorption spectra of O-SdiPBI is similar to those of H-SdiPBI, implying the similar stereostructures of PBI subunits in O-SdiPBI and H-SdiPBI with more planar configuration and less coupling than those in B-SdiPBI. This result is consistent with DFT calculations. However, the spectrum of H-SdiPBI shows a slight red shift and possesses a much higher absorption coefficient of 2.46 × 105 M–1 cm–1 than that of O-SdiPBI (εmax = 1.89 × 105 M–1 cm–1), suggesting that the decreased proportion of alkyl chains caused high absorption capability owing to the relatively greater oscillation strength.31 However, as shown in Figure 1d, all of the UV–vis absorption spectra of three neat SdiPBI films display a similar shape from 400 to 600 nm, which is complementary to the absorption spectrum of DR3TBDTT located at 400–750 nm. This would be beneficial to form a broadband light-harvesting active layer of OSCs.

2.2. NF All-SMSC Device Performance

NF all-SMSC devices were fabricated based on the normal device conformation of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/active layer/electron transport layer (ETL)/Al with DR3TBDTT as the donor and B-SdiPBI, O-SdiPBI, and H-SdiPBI as the acceptors. The device properties of NF all-SMSC devices with different manufacturing conditions are summarized in Table 2 and Figure 2. With the optimal processing by adjusting the weight ratio of the donor and acceptor (D/A) and SVA parameters, the best device parameters are summarized in Table 2 and the current density–voltage (J–V) characteristics and external quantum efficiency (EQE) spectra are shown in Figure 2d. As shown in Table S1, we have initially optimized the donor–acceptor weight ratio (D/A) in the Supporting Information. B-SdiPBI-, O-SdiPBI-, and H-SdiPBI-based NF all-SMSC devices with the best D/A exhibit the PCEs of 0.78, 1.86, and 2.30%, with VOC values of 0.93, 0.93, and 0.88 V; JSC values of 2.81, 5.87, and 5.82 mA cm–2; and FFs of 29.83, 34.16, and 44.65%, respectively. The higher performance of O-SdiPBI and H-SdiPBI is due to higher short-circuit current and fill factor, implying that the acceptor with a larger dihedral angle has higher miscibility with the small molecular donor and is more likely to form the appropriate phase domain size. Compared with the unprocessed device, the devices processed by SVA show much high PCEs of 1.93, 3.30, and 4.05%, with much high JSC values of 5.16, 9.03, and 8.75 mA cm–2 and high FFs of 42.71, 42.07, and 53.21%, respectively. This is because solvent annealing further increases the crystallinity of small molecular phase domains, thereby improving the electron mobility and imparting higher efficiency. The device performance could be useful for further understanding the relationship between the structure and property of these three acceptors.

Table 2. Photovoltaic Performances of NF All-SMSCs Based on DR3TBDTT:B-SdiPBI, DR3TBDTT:O-SdiPBI, and DR3TBDTT:H-SdiPBI Blend Films with and without SVA.

blends SVA VOC [V] JSC [mA cm–2] FF [%] PCEmax [%] PCEavg [%]c
DR3TBDTT:B-SdiPBI Na 0.93 2.81 29.83 0.78 0.70 ± 0.08
Yb 0.88 5.16 42.71 1.93 1.88 ± 0.06
DR3TBDTT:O-SdiPBI Na 0.93 5.87 34.16 1.86 1.74 ± 0.12
Yb 0.87 9.03 42.07 3.30 3.29 ± 0.03
DR3TBDTT:H-SdiPBI Na 0.88 5.82 44.65 2.30 2.26 ± 0.05
Yb 0.87 8.75 53.21 4.05 3.94 ± 0.11
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a

Without SVA.

b

With SVA by CHCl3 (Vsol. = 50 μL, time = 60 s).

c

The average values were obtained over six devices.

2.3. Charge Transport

To get deep insights into the physical mechanism of devices, we have used the space-charge-limited current (SCLC) model to evaluate charge transport properties of DR3TBDTT:B-SdiPBI, DR3TBDTT:O-SdiPBI, and DR3TBDTT:H-SdiPBI blend films before and after SVA (Figures S1 and S2 and Table 3). As shown in Table 3, all of the NF all-SMSC devices displayed better hole mobility than electron mobility, illustrating that electron mobility is the limiting factor for attaining high device performance, JSC, and FF. After the SVA process, both the hole and electron mobilities were obviously enhanced, which may explain the improved device performances by the SVA process. In addition, the hole and electron mobilities of three devices increased with the planarity of two PBI subunits, resulting in their much high JSC and PCE.

Table 3. SCLC Mobility Data for DR3TBDTT:B-SdiPBI, DR3TBDTT:O-SdiPBI, and DR3TBDTT:H-SdiPBI Blend Films.

  DR3TBDTT:B-SdiPBI (cm2 V–1 S–1)
 DR3TBDTT:O-SdiPBI (cm2 V–1 S–1)
 DR3TBDTT:H-SdiPBI (cm2 V–1 S–1)
μh μe μh μe μh μe
without SVA 4.32 × 10–5 8.33 × 10–6 6.81 × 10–5 8.54 × 10–6 1.31 × 10–4 1.52 × 10–5
SVA 9.61 × 10–5 1.31 × 10–5 1.52 × 10–4 1.37 × 10–5 2.54 × 10–4 5.59 × 10–5
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2.4. Film Morphology

To further investigate the influence of the distinguished structure on the photovoltaic performance, the film morphologies of three different acceptors with DR3TBDTT blend films were probed by atomic force microscopy (AFM), as shown in Figure 3. Without SVA, the as-cast blend films showed the smaller root-mean-square (RMS) roughnesses of 1.76 nm for B-SdiPBI, 1.31 nm for O-SdiPBI, and 1.21 nm for H-SdiPBI than those with SVA (2.00, 1.63, and 1.43 nm, respectively). The twisted structure enabled good miscibility with the donors, and thus, the blend films exhibited quite smooth surfaces and uniform morphologies with small RMS. Meanwhile, the obvious phase separation and appropriate aggregation domains would be obtained with SVA, indicating that the suitable hole–electron-transporting pathway will be formed.

Figure 3.

Figure 3

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AFM height images (a, b, e, f, i, j) and phase images (c, d, g, h, k, l) of DR3TBDTT:B-SdiPBI (a–d), DR3TBDTT:O-SdiPBI, (e–h) and DR3TBDTT:H-SdiPBI (i–l) blend films without (a, c, e, g, i, k) and with (b, d, f, h, j, l) SVA. The area is 2 × 2 μm2.

To understand the relationship between phase size and device efficiency, we further analyzed the AFM phase diagram and transmission electron microscopy (TEM) pictures (Figures 3 and S3). As shown in Figure 3, the phase size for the NF-SCMS films reduces in the following order: DR3TBDTT:B-SdiPBI (30–80 nm) > DR3TBDTT:O-SdiPBI (20–50 nm) > DR3TBDTT:H-SdiPBI (10–40 nm), which is consistent with the twisted extent of three acceptors, implying that the molecular torsion between the two PBI units can effectively increase the phase separation, thereby obtaining a suitable phase size for high device performance. For the three all-SMSC blend systems, solvent annealing will lead to an increase in the size of phase separation. As shown in Figure S3, TEM images show similar results to AFM phase images. DR3TBDTT:O-SdiPBI and DR3TBDTT:H-SdiPBI possess a more suitable phase size than DR3TBDTT:B-SdiPBI, which is beneficial to electron transport and thus to device efficiency. It is concluded that the different bonding positions change the torsion of PBI units so that different phase sizes are obtained after blending with the donors. Moreover, the appropriate phase size directly affects the charge transport and separation, thereby affecting the device efficiency.

2.5. Molecular Packing

The grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed to investigate the crystallinity and molecular packing of the films. As shown in Figures S4 and 4, the GIWAXS scattering profiles of the neat donor, acceptor, and the blend films with and without SVA can be further investigated by dividing into in-plane (IP) and out-of-plane (OOP). For the pure donor films, the π–π stacking (010) peak (qz = 1.707 Å–1, d = 3.68 Å) and the lamellar scattering (100) peak (qz = 0.301 Å–1, d = 20.87 Å) appear in the in-plane direction and the out-of-plane direction, respectively. The result indicates that the typical edge-on-oriented crystallites exist in DR3TBDTT films, which is not beneficial to the vertical charge transport in the OSC device. In addition, both B-SdiPBI and O-SdiPBI films showed the amorphous state whether with or without SVA. However, only the H-SdiPBI films presented the amorphous state before SVA, but the crystallinity was dramatically enhanced after SVA. With solvent vapor annealing, the lamellar scattering peak (100) at qz = 0.303 Å–1 for H-SdiPBI appeared in OOP. All of the three acceptors displayed no obvious π–π stacking peaks both in OOP and IP, which was attributed to the twisted structure destroying the easy-to-stack characteristics of PBI units.

Figure 4.

Figure 4

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2D GIWAXS patterns and scattering profiles of out-of-plane (OOP) and in-plane (IP) for DR3TBDTT:B-SdiPBI blend active layer (a, d, g), DR3TBDTT:O-SdiPBI blend active layer (b, e, h), and DR3TBDTT:H-SdiPBI blend active layer (c, f, i) without (a–c) and with (d–f) SVA treatment.

For the all-blend systems, the films with and without SVA all display the lamellar scattering peak (100) in OOP at about qz = 0.31 Å–1 (DR3TBDTT:B-SdiPBI, d = 20.26 Å), 0.30 Å–1 (DR3TBDTT:O-SdiPBI, d = 20.93 Å), and 0.30 Å–1 (DR3TBDTT:H-SdiPBI, d = 20.93 Å), demonstrating a major orientation of the a-axis parallel to the active layer. Indeed, the lamellar scattering peaks (100) of the three systems were obviously improved after annealing for 60 s, indicating that the SVA treatment can successfully improve the crystallinity and molecular ordering. In addition, the π–π stacking peaks cannot be discovered without SVA in both blend films. However, after annealing, the π–π stacking peaks appeared in IP for DR3TBDTT:B-SdiPBI blend films only, indicating the addition of the twisted PBI units destroyed the major edge-on crystallites in neat donor films and thus promoted the vertical charge transport. The crystal coherence length (CCL) can be used to characterize the crystallite size from the Scherrer equation42

2.5.

where L is the crystal coherence length (CCL), K is the Scherrer constant, λ is the incident wavelength, β is the full width at half-maximum of the diffraction peak in radians, and Q is the diffraction vector.

As shown in Table 4 and Figure 5, without SVA, the DR3TBDTT:B-SdiPBI blend film possessed a large CCL of 40 nm. As the link position changes and the degree of molecular twist increases, the crystallite sizes of DR3TBDTT:O-SdiPBI and DR3TBDTT:H-SdiPBI are reduced to 33 and 32 nm, respectively, which is well consistent with the results derived from AFM and TEM. In addition, after SVA, the crystallite sizes of all of the blend systems slightly increase, which is consistent with the AFM topographic phase images and TEM images as well. The improved crystallinity and molecular order of donors and acceptors lead to the increased device efficiency.

Table 4. GIWAXS Parameters of NF All-SMSCs Based on DR3TBDTT:B-SdiPBI, DR3TBDTT:O-SdiPBI, and DR3TBDTT:H-SdiPBI Blend Films with and without SVA.

blends SVA Q (Å–1)c d (Å)d CCL (nm)e
DR3TBDTT:B-SdiPBI Na 0.31 20.26 40
Yb 0.31 20.26 44
DR3TBDTT:O-SdiPBI Na 0.30 20.93 33
Yb 0.30 20.93 35
DR3TBDTT:H-SdiPBI Na 0.30 20.93 32
Yb 0.30 20.93 34
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a

Without SVA.

b

With SVA by CHCl3 (Vsol. = 50 μL, time = 60 s).

c

The diffraction vector in the (100) direction.

d

Spacing calculated by d = 2π/Q.

e

The crystal coherence length (CCL) assessed by the Scherrer Equation.

Figure 5.

Figure 5

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CCL of NF all-SMSCs based on DR3TBDTT:B-SdiPBI, DR3TBDTT:O-SdiPBI, and DR3TBDTT:H-SdiPBI blend films without and with SVA, where CCL was calculated using the Scherrer equation. The illustration above the column is the corresponding AFM phase image (0.5 × 0.5 μm2).

3. Conclusions

In this work, we selected three differently linked (bay-linked, ortho-linked, and hydrazine-linked) perylene bisimide dimers as acceptors and DR3TBDTT as a donor to prepare PBI-based NF all-SMSCs. The effects of PBI link positions on twisting, planarity, and device performance were explored to further understand the relationship between structure and performance in PBI-based NF all-SMSCs. Solvent annealing is used to improve device efficiency. The B-SdiPBI-, O-SdiPBI-, and H-SdiPBI-based devices achieve the best PCEs of 1.93, 3.30, and 4.05%, respectively. The extent of twisting of three acceptors affects the corresponding device efficiency and phase domain size. H-SdiPBI has the largest dihedral angle of 90° with the best efficiency and the appropriate phase size of 10–40 nm, O-SdiPBI has a smaller dihedral angle of 86° and a larger phase size of 20–50 nm, and B-SdiPBI has the smallest dihedral angle of about 71° and the largest phase size of 30–80 nm. This is because the distortion conformation of acceptors prevents the aggregation of small molecules and increases the phase separation of acceptors and the donor to obtain the small phase size in NF all-SMSCs. The work provides a guide for designing the acceptors and controlling phase domains of high-performance NF all-SMSCs.

4. Experimental Section

4.1. Fabrication of Organic Photovoltaic Devices

The device structure of the NF all-SMSCs manufactured in this experiment is ITO/PEDOT:PSS/donor:acceptor/ETL/Al.38 First, the glass substrate with ITO coating was washed twice with detergent, deionized water, acetone, and isopropanol twice for 20 min each and placed at 150 °C for drying. It was then treated with UV–ozone for 20 min. After that, PEDOT:PSS (Clevios P VP AI 4083, filtered at 0.45 μm) was spin-coated onto the clean surface at 3500 rpm for 30 s and then placed at 150 °C for 20 min. Afterward, the substrate was moved into a glovebox filled with a N2 atmosphere and spin-coated with different blend chloroform solutions. The active layer was spin-coated onto the surface at 3000 rpm for 60 s from a chloroform solution, and the thickness of the active layer is about 100 nm. The cathode modification layer was coated on the active layer by spinning at 3000 rpm for 30 s using a methanol solution of ETL (0.5 mg mL–1).38 Then, the electrode of Al was evaporated under high vacuum (<1.5 × 10–4 Pa) with a thickness of 100 nm. A Precision Source/Measure Unit (B2912A; Agilent Technologies) with an AAA grade solar simulator (XES-70S1, 7 × 7 cm2 beam size; SAN-EI Electric Co. Ltd.), coupled with AM 1.5G solar spectrum filters, was used as the light source, measured the J–V curve for the photovoltaic device. A standard single-crystal Si reference cell (SRC-1000-TC-QZ, 2 × 2 cm2; VLSI Standards Inc.) was used to calibrate photocurrent to be 100 mW cm–2, and a solar cell spectral response measurement system (QE-R3011; Enlitech Co. Ltd.) was used to measure the EQE curve.

The space-charge-limited current (SCLC) method was used to measure the charge transport. The hole and electron mobilities were determined with the device construction of ITO/PEDOT:PSS/donor:acceptor/Au and Al/donor:acceptor/Al, respectively, and calculated by J = 9ε0εrμV2/8L3 exp [0.891γ(V/L)0.5].

4.2. Materials and Measurements

B-SdiPBI, O-SdiPBI, and H-SdiPBI are synthesized using methods reported in the literature.29,31,37 DR3TBDTT, all chemicals, and solvents in this work were bought from commercial suppliers and used without extra purification except otherwise specified.

The UV–vis spectrum was measured with a Hitachi (model U-3010) UV–vis spectrophotometer. Atomic force microscopy (AFM) was performed using a Nanoscope III Multi-Mode atomic force microscope operating in the tapping mode. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 TEM operated at 200 kV. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were recorded on a Xenocs-SAXS/WAXS system using an X-ray wavelength of 1.5418 Å, and the samples were treated at a fixed angle of 0.2°.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 21274009, 51221002, and 51473009).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02333.

  • Device performance parameters for the solar cells with different D/A ratios, TEM images, 2D GIWAXS patterns, and charge transport (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c02333_si_001.pdf (1.3MB, pdf)

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