3‐Dicyanomethylene‐Substituted Thienothiophenes as the Terminal Units in Non‐Fullerene Electron Acceptor Molecules for Organic Photovoltaics

Abstract

A series of dicyanomethylene‐substituted thieno[3,2‐b]‐ and thieno[2,3‐b]‐thiophenes and their α‐halogenated derivatives (CNTTs) were developed as electron‐withdrawing terminal units of non‐fullerene acceptors (NFAs) for bulk heterojunction organic photovoltaics (OPVs). The series of CNTTs was combined with a [1,2,5]thiadiazolo[3,4‐e]thieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo[3,2‐g]thieno[2',3':4,5]thieno[3,2‐b]indole (BTP) unit, the electron‐donating core of Y6, a representative NFA, to afford a series of NFAs (BTP‐CNTTs). The BTP‐CNTTs, particularly the ones with α‐chlorinated or brominated CNTT units, exhibit longer absorption wavelengths than that of Y6 and have low‐lying LUMO energy levels comparable to that of Y6. Bulk heterojunction OPV devices based on the BTP‐CNTTs blended with a representative donor polymer, PBDB‐T, exhibited power conversion efficiencies of over 10% under AM1.5 irradiation. These results suggest that the CNTTs are good candidates as electron acceptor building units for NFAs or n‐type organic semiconductors.

3‐Dicyanomethylene thienothiophenes (CNTTs), a new series of electron acceptor units, were developed and incorporated into non‐fullerene acceptors (NFAs). The CNTT‐based NFAs exhibited favorable electronic structures and packing structures and thus enabled organic photovoltaic devices to achieve a power conversion efficiency of 10% when blended with PBDB‐T.

1. Introduction

Through intensive research efforts in the last three decades, organic photovoltaics (OPV) have achieved competitive performance with other photovoltaic technologies; the power conversion efficiencies (PCE) of single‐junction OPVs in the laboratory exceed 20% at the cell level.^{[ 1} , ² , ^{3 ]} The discovery and advancement of so‐called non‐fullerene acceptors (NFAs) have been the fuel for these recent advancements in OPVs.^{[ 4} , ^{5 ]} The molecular structures of NFAs that prevail most are the triad structures with the central electron‐rich π‐systems (often called donor, “D”) terminated with electron‐deficient units (acceptor, “A”) at both sides, which enables panchromatic photo‐absorption to reach the near‐infrared (NIR) region and adjustment of the energy levels of the frontier molecular orbitals. For these reasons, it is important to properly choose both the donor and acceptor units in the A–D–A triad structure. Particularly, selecting an acceptor unit often becomes crucial, as it can predominantly affect the LUMO energy levels (E _LUMOs) of the resulting NFA molecules, which contributes to the formation of effective electron transport paths through the intermolecular interaction between the terminal acceptor units.

Since the emergence of the NFAs, various acceptor moieties have been examined,^{[ 5 ]} and the most successful and widely used terminal acceptor unit is 3‐(dicyanomethylidene)indan‐1‐one (IC) and its derivatives (Figure 1a). Two electron‐withdrawing groups, dicyanomethylene and carbonyl groups, ensure the strong electron‐withdrawing nature of the IC units, which, in the A–D–A triad structure, effectively induces intramolecular charge transfer (ICT), which results in reduced bond length alternation and NIR photoabsorption. For example, the representative IC‐based NFAs, ITIC,^{[ 6 ]} and Y6, exhibit NIR absorption up to 750 and 900 nm, respectively. Furthermore, the electron‐withdrawing nature of IC moieties can be tuned by modifying them with halogen substituents, which affect the overall NFA properties and, thereby, the performance of OPV devices. To date, a great number of studies on NFAs having IC‐related acceptor units have been carried out, which provides guidelines for designing novel NFA molecules.^{[ 7 ]}

Figure 1.

Chemical structures of IC2F (a) and anti‐ (b) and syn‐ (c) CNTT terminal units and the corresponding NFAs with the BTP donor unit, and PBDB‐T (d), a representative donor polymer.

In our studies of a series of organic semiconducting molecules based on quinoidal thiophenes,^{[ 8 ]} we have demonstrated that dicyanomethylene‐ (or other related electron‐withdrawing groups) terminated molecules can act as superior building blocks for n‐type organic semiconductors.^{[ 9} , ^{10 ]} Such quinoidal oligothiophenes share a common feature: the resonance structures are well balanced between quinoidal and aromatic forms thanks to the moderately high aromatic stabilization of the thiophene ring. This enables the bistable nature of quinoidal thiophene‐based compounds with the quinoidal structure in the neutral state and with 6π‐aromatic structure in the radical anion (and dianion) states, resulting in stable redox‐active systems with low‐lying LUMO energy levels.^{[ 11} , ^{12 ]} Another merit of thiophene moieties in organic semiconductors is that they provide a chance to enhance the intermolecular orbital overlap through the sulfur atoms with large atomic radii.^{[ 13 ]} Furthermore, the thiophene α‐position is easily modified to extend the structural variety of compounds, including π‐extended structures.

Considering these features of electron‐deficient quinoidal thiophenes, we have searched for a new terminal acceptor units with a dicyanomethylene‐substituted thiophene substructure with the aid of theoretical calculations. Among the potential terminal acceptor units, we focused on 3‐dicyanomethylene‐substituted thieno[3,2‐b]‐ and thieno[2,3‐b]‐ thiophenes (anti‐ and syn‐CNTTs, Figure 1b,c) for the following reasons. First, they have a structural resemblance to the IC terminal units. The carbonyl group and fused benzene ring in the IC unit are substituted with the sulfur atom and a thiophene ring, respectively, keeping a similar molecular shape. Second, the fused thiophene ring holds a vacant α‐position, which is readily modified chemically. Recent reports on the thiophene‐fused IC‐related analogues have shown that the α‐positions of the fused thiophenes can be chlorinated or brominated to tune their electronic structures,^{[ 14} , ¹⁵ , ¹⁶ , ¹⁷ , ^{18 ]} some of which were further used as reactive sites for π‐extension to polymers.^{[ 19 ]} In fact, halogen substituents (Cl and Br groups) were conveniently introduced to tune the electron‐withdrawing nature of the CNTT terminal units (Figure 1b,c). Furthermore, the choice of the ring‐fusion manner, e.g., the direction and position, could be another way of tuning their electronic structures and thus is of great interest. In this work, we report the synthesis and characterization of isomeric pairs of the anti‐ and syn‐CNTT‐based NFAs with the [1,2,5]thiadiazolo[3,4‐e]thieno[2'',3'':4',5']thieno[2',3':4,5]pyrrolo[3,2‐g]thieno[2',3':4,5]thieno[3,2‐b]indole (BTP) unit as the central donor unit, and the device characteristics of bulk‐heterojunction OPV devices based on the NFAs, which were blended with a representative donor polymer, PBDB‐T (Figure 1d).

2. Results and Discussion

2.1. Synthesis of Terminal Units and NFAs

The synthesis of anti‐ and syn‐CNTTs and their α‐halogenated derivatives, CNTTCls and CNTTBrs, is shown in Scheme 1. For the synthesis of anti‐CNTTs, thieno[3,2‐b]thiophen‐3(2H)‐one (1)^{[ 20 ]} was the starting compound, which was easily converted into 3‐dicyanomethylidene‐2H‐thieno[3,2‐b]thiophene, anti‐CNTT, via the Knoevenagel condensation with malononitrile. The direct introduction of chlorine or bromine groups to 1 via the electrophilic aromatic halogenation failed, and thus, 1 was converted into the corresponding triisopropylsilyl‐ (TIPS) protected 3‐hydoroxythienothiophene (2), which was then lithiated at the 5‐position, reacted with hexachloroethane or 1,2‐tetrabromoethane to give the corresponding α‐chloro or α‐bromo thieno[3,2‐b]thiophens (3a and 3b), respectively. Then, the TIPS group was deprotected with tetrabutylammonium fluoride to give the α‐halogenated thieno[3,2‐b]thiophen‐3(2H)‐ones (4a and 4b). The Knoevenagel condensation with malononitrile gave the corresponding α‐halogenated 3‐dicyanomethylidene‐2H‐thieno[3,2‐b]thiophenes (anti‐CNTTCl and anti‐CNTTBr). On the other hand, unsubstituted thieno[2,3‐b]thiophen‐3(2H)‐one could not be prepared by a similar procedure to that for 1, probably because the vacant thiophene α‐position is more reactive than the β‐position in the Friedel–Crafts intramolecular electrophilic cyclization of the corresponding acid chloride. We thus first synthesized α‐halogenated thieno[2,3‐b]thiophen‐3(2H)‐one (5a and 5b), which were readily converted into α‐halogenated syn‐CNTTCl and syn‐CNTTBr in reasonable isolated yields. Unsubstituted syn‐CNTT was synthesized via the debromination of 5b by the action of zinc dust to prepare thieno[2,3‐b]thiophen‐3(2H)‐one (6), followed by the Knoevenagel condensation with malononitrile. The ring‐fusion manner and the position of the halogen substituent in anti‐ and syn‐CNTTs were unambiguously elucidated by single‐crystal X‐ray analysis, where the CNTTs have coplanar structures including both thienothiophene and dicyanomethylene moieties (Table S1, Figure S23).^{[ 20 ]}

Scheme 1.

Syntheses of anti‐ and syn‐CNTTs and their corresponding BTP‐based NFAs.

Interestingly, ¹H and ¹³C NMR spectroscopy revealed that all the CNTTs exhibited tautomerisation with the 3‐dicyanomethylthieno[3,2‐ or 2,3‐b]thiophene forms as the minor tautomers in solution (Scheme 1, Figures S1, S7, S8, S13, S14, and S16), which is unlike the IC unit. Density functional theory (DFT) calculations revealed that the differences in the total energies between the dicyanomethylidene (major) and dicyanomethyl (minor) forms of the CNTTs (22–26 kJ mol⁻¹) are much smaller than that of the IC2F (73 kJ mol⁻¹), which could be the major reason for the tautomerisation observable only for the CNTTs. The significantly different tautomeric energetics between the CNTT and IC2F can be explained by the difference in the aromaticity gain via tautomerisation from the major dicyanomethylidene forms to the minor dicyanomethyl forms. Namely, the number of aromatic rings found in the CNTTs increases from one for the dicyanomethylidene form to two for the dicyanomethyl form, which is not the case for IC2F. Although the difference between the total energy of the two tautomers of the CNTTs is still too large to show the observable population of the minor form based on the Boltzmann distribution at room temperature (300 K), the rotatable dicyanomethyl group in the minor form, in contrast to the rigid dicyanomethylene group in the major form, could entropically contribute to reducing the Gibbs free energy of the system in solution.

Although each CNTT consists of two tautomers, the deprotonation of these tautomers both afforded the same nucleophilic species, which reacted with the diformylated BTP core, similarly to the synthesis of Y6, to give the corresponding BTP‐CNTTs in moderate yields (Scheme 1).^{[ 21 ]} Note that we introduced 2‐butyloctyl groups instead of 2‐ethylhexyl groups in the BTP core since the solubility of BTP‐CNTTs with 2‐ethylhexyl groups was found to be poor. The chemical structures of BTP‐CNTTs were fully characterized by spectroscopic and combustion elemental analyses. Among BTP‐CNTTs, the crystal structures of BTP‐syn‐CNTTCl and BTP‐syn‐CNTTBr were elucidated by single‐crystal X‐ray analysis to provide structural evidence of the target compound, though the quality of structural analyses was not high owing to the poor crystal quality and heavy disorder at the alkyl parts in the molecule. It is interesting to note that the conformation of all the CNTT terminal units in the crystal structures was found to be s‐cis in terms of the single bond between the donor and acceptor units; namely, the terminal CNTT units and the sulfur atom of the outermost thiophene ring of the BTP core are on the same side (Figure 2). Dihedral‐angle‐dependent energy profiles calculated for the NFAs showed that in the BTP‐CNTTs, the s‐cis conformation is more stable than s‐trans conformation by 18 kJ mol⁻¹, which is much larger than the thermal energy (2.5 kJ mol⁻¹), thus indicating that the conformational disorder of the conjugated backbone in the thin‐film state could be minimal (Figure S25).

Figure 2.

Molecular structures with ORTEP representation in the single‐crystal structures of BTP‐syn‐CNTTCl (a) and BTP‐syn‐CNTTBr (b). The alkyl groups are omitted as their exact positions were not determined in the actual structural analyses. Thermal ellipsoids are shown at the 50% probability level.

2.2. Electronic Structures

The electrochemical behaviors of BTP‐CNTTs in solution were investigated by cyclic voltammetry to estimate the energy levels of their frontier molecular orbitals. All the BTP‐CNTTs showed quasi‐reversible oxidation and irreversible reduction current responses similar to that of Y6 (Figure S26). The HOMO and LUMO energy levels (E _HOMO and E _LUMO) were estimated from the oxidation and reduction onset potentials, respectively, and were summarized in Table 1, together with those of Y6 measured under identical conditions. The E _HOMOs and E _LUMOs of BTP‐anti‐CNTT (−5.22 and −3.79 eV) and BTP‐syn‐CNTT (−5.23 and −3.76 eV) are slightly higher than those of Y6 (−5.38 and −3.88 eV), which can be attributed to the substitution of the electron‐withdrawing carbonyl groups with the electron‐donating sulfur atoms. Particularly, the electronic perturbation by the substitution on the E _HOMO seems more significant than the E _LUMO, which can be rationalized by the large orbital coefficients from the sulfur atoms on the HOMOs but the small coefficient on the LUMOs (Figure 3). As a result, the HOMO–LUMO energy gaps of the BTP‐anti‐CNTT and BTP‐syn‐CNTT are smaller than that of Y6. The introduction of electron‐withdrawing chlorine or bromine atoms at the α‐position of the outer thiophene ring of the CNTT units effectively lowers the E _LUMOs by 0.04–0.07 eV from the unsubstituted BTP‐CNTTs to afford the E _LUMOs of −3.83 to −3.84 eV, which are comparable to that of Y6 (−3.88 eV). In contrast, the halogen substitution did not affect the E _HOMOs probably because the outer thiophene rings have only a marginal contribution to the HOMOs (Figure 3). The different effects of the α‐halogenation on the E _LUMOs and E _HOMOs can be rationalized by the different distribution of the LUMOs and HOMOs of the terminal groups. Specifically, the LUMOs of the BTP‐CNTTs extend to the outer fused thiophene rings of the CNTT units, and thus the chloro or bromo group effectively lowers the orbital energy levels via an inductive effect. In contrast, the HOMOs do not extend to the outer thiophene rings, and thus their energy levels remain largely unaffected.

Table 1.

Electrochemically estimated HOMO (E _H ^CV) and LUMO (E _L ^CV) energy levels and their energy gap (E _g ^CV) and maximum absorption wavelengths (λ _max ^sol) of BTP‐CNTTs in solution, maximum absorption wavelengths (λ _max ^film) and ionization potentials (IP) of thin films of BTP‐CNTTs, and quantum chemically calculated HOMO (E _H ^calc) and LUMO (E _L ^calc) energy levels and their energy gap (E _g ^calc) and absorption energy of the electronic transition from S₀ to S₁ state (λ _max ^calc) of the model compounds for BTP‐CNTTs by using DFT and TD‐DFT at the level of B3LYP/6–311G(d,p).

Compound	E H CV a) (eV)	E L CV a) (eV)	E g CV b) (eV)	λ max sol c) (nm)	λ max film d) (nm)	IP (eV)	E H calc (eV)	E L calc (eV)	E g calc (eV)	λ max calc (nm)
BTP‐anti‐CNTT	−5.22	−3.79	1.43	759	864	5.55	−5.55	−3.64	1.91	700
BTP‐anti‐CNTTCl	−5.23	−3.83	1.40	770	891	5.58	−5.65	−3.74	1.91	706
BTP‐anti‐CNTTBr	−5.23	−3.84	1.39	772	884	5.56	−5.63	−3.74	1.89	712
BTP‐syn‐CNTT	−5.23	−3.76	1.47	742	833	5.54	−5.59	−3.63	1.96	686
BTP‐syn‐CNTTCl	−5.23	−3.84	1.40	755	871	5.60	−5.69	−3.75	1.94	691
BTP‐syn‐CNTTBr	−5.24	−3.83	1.42	755	866	5.53	−5.67	−3.74	1.93	696
Y6	−5.36	−3.88	1.48	733	848	5.92	−5.86	−3.82	2.04	670
PBDB‐T	–	–	–	615	624	4.95	–	–	–	–

^a) E _HOMO and E _LUMO were estimated from the oxidation (E _ox ^onset) and reduction onset potentials (E _red ^onset) with the following equations: E _H ^CV = −4.8 −E _ox ^onset and E _L ^CV = −4.8 − E _red ^onset.

^b) E _g ^CV = E _L ^CV − E _H ^CV

^c) In chloroform solution.

^d) Spin‐coated from chloroform solutions and annealed at 160 °C.

Figure 3.

Kohn–Sham HOMO and LUMO around the terminal acceptor units of BTP‐anti‐CNTTs (a–c), BTP‐syn‐CNTTs (d–f), and Y6 (g), calculated at the level of B3LYP/6–311G(d,p).

Figure 4a,b shows the UV–vis–NIR absorption spectra of BTP‐CNTTs, Y6, and a representative donor polymer, PBDB‐T, in solution. BTP‐CNTTs in solution exhibited intense and sharp absorption bands around 700–800 nm, similar to that of Y6, and longer maximum absorption wavelengths than those of Y6, which are complementary to that of PBDB‐T around 600 nm. Among the BTP‐CNTTs, the BTP‐anti‐CNTTs exhibited slightly longer maximum absorption wavelengths than the corresponding BTP‐syn‐CNTTs by 15–17 nm. The ring‐fusion manner of the outer thiophene appeared to be responsible for this small but significant difference, where the linearly conjugated structures in anti‐CNTTs may result in better effective conjugation than syn‐CNTTs that have cross‐conjugated structures.^{[ 22 ]} Furthermore, α‐halogenated BTP‐CNTTs showed longer maximum absorption wavelengths than the parent unsubstituted BTP‐CNTTs, which are consistent with the trend observed in electrochemically estimated HOMO–LUMO energy gaps as well as the theoretical calculation results (Table 1).

Figure 4.

UV–vis–NIR absorption spectra of chloroform solution and thin films of BTP‐anti‐CNTTs (a and c) and BTP‐syn‐CNTTs (b and d), and Y6 and PBDB‐T.

In the annealed thin films, the main absorption bands of BTP‐CNTTs red‐shifted by approximately 100 nm from their solution state, similar to Y6 (Figure 4c,d). Thus, most of the BTP‐CNTTs, except for BTP‐syn‐CNTT, showed longer maximum absorption wavelengths than Y6 by 22–30 nm. Furthermore, the thin films are stable under AM 1.5 photoirradiation with an inert atmosphere, although they are not as stable as Y6 under the photoirradiation in air (Figure S27). These results indicate that the CNTTs are suitable choices as electron acceptor units for designing NFAs with NIR‐absorbing properties as well as low‐lying E _LUMOs.

2.3. Packing Structures of BTP‐syn‐CNTTCl and BTP‐syn‐CNTTBr

Figure 5 shows the packing structures of BTP‐syn‐CNTTCl and BTP‐syn‐CNTTBr in the single crystals grown by slow diffusion of methanol or n‐hexane into a chlorobenzene solution, respectively. Both crystal structures have the same space group of P2₁/c with very similar lattice parameters (Table S2) and molecular packing motifs consisting of three crystallographically independent molecules in the unit cell (Figure 5a,d). In the packing structures, the two arms of a U‐shaped conjugated backbone stack with those of two, three, or four different neighboring molecules to form mesh‐like 3D‐networking structures with channels occupied by disordered alkyl chains (Figure 5b,e), which are often observed for IC‐based high‐performance NFAs.^{[ 23 ]} It should be noted that in both crystal structures, the CNTT terminal units stack to form column structures with stacking distances around 3.4–3.5 Å, which should be favorable for efficient electron transport (Figure 5c,f). These structures indicate that the CNTT terminal units could be beneficial for enhancing intermolecular attractive interaction thanks to the fused thiophene ring containing a large sulfur atom, as reported for NFAs having thiophene‐fused terminal groups.^{[ 16 ]}

Figure 5.

Asymmetric units (a, d), packing structures viewed along the c‐axis (b, e), and stacking structures of terminal groups viewed along the b‐axis with stacking distances (c, f) in the single‐crystal structures of BTP‐syn‐CNTTCl and BTP‐syn‐CNTTBr, respectively. The alkyl groups are omitted as their exact positions were not decided in the actual structural analyses in all the figures. Red, green, and blue colors denote the three independent molecules in the asymmetric units (a, c, d, f). Terminal groups were highlighted in blue color in (b) and (e). Stacking terminal groups were represented in a space‐filling model (c, f).

Furthermore, differential scanning calorimetry revealed that the BTP‐CNTTs are thermally stable at least up to 250 °C, and that the chlorinated and brominated ones have comparably high melting and decomposition temperatures to those of Y6 likely due to the stronger dispersion interaction with the heavy atoms (Figure S29, Table S3).

2.4. Fabrication and Evaluation of OPV Devices

The electronic structures and crystal packing structures of BTP‐s suggest their potential as electron acceptors for bulk heterojunction OPV applications. We thus fabricated and evaluated OPV devices with PBDB‐T as the donor polymer, which has complementary absorption bands to those of the NFAs and sufficient offsets between the E _HOMOs of the donor and acceptors in the solid state, judged from the ionization potentials of the thin films (Table 1, Figure S30). The details of the device fabrication are described in the Supporting Information (Table S4). The J–V characteristics under the AM1.5 illumination and EQE spectra of the representative devices are shown in Figure 6, and the device metrics are summarized in Table 2.

Figure 6.

J–V characteristics (a and b) and EQE spectra (c and d) of the OPV devices based on BTP‐anti‐CNTTs and BTP‐syn‐CNTTs as well as Y6 blended with PBDB‐T.

Table 2.

OPV characteristics ^{a )} of the devices based on BTP‐CNTTs and Y6 with PBDB‐T.

NFA	J SC (mA cm−2)	V OC (V)	FF	PCE (%)
BTP‐anti‐CNTT	19.35 (0.29)	0.75 (0.01)	0.64 (0.01)	9.29 (0.12)
BTP‐anti‐CNTTCl	21.60 (0.21)	0.72 (0.01)	0.66 (0.01)	10.19 (0.06)
BTP‐anti‐CNTTBr	21.47 (0.17)	0.73 (0.01)	0.65 (0.01)	10.18 (0.11)
BTP‐syn‐CNTT	21.24 (0.18)	0.77 (0.01)	0.63 (0.01)	10.32 (0.14)
BTP‐syn‐CNTTCl	23.20 (0.29)	0.72 (0.01)	0.63 (0.01)	10.51 (0.19)
BTP‐syn‐CNTTBr	20.84 (0.27)	0.74 (0.01)	0.64 (0.01)	9.99 (0.15)
Y6	23.46 (0.37)	0.71 (0.01)	0.59 (0.01)	9.73 (0.21)

^a) Averaged over more than eight devices. Values in parentheses are standard deviations.

The OPV devices with the CNTT‐based NFAs showed J–V characteristics, and the integrated EQE spectra agreed well with the short‐circuit current density (J _SC), indicating that the evaluation of the OPV devices was appropriate. The optimal devices mostly showed J _SCs larger than 20 mA cm⁻², V _OC higher than 0.7 V, and decent FF higher than that of Y6‐based devices, and consequently, the power conversion efficiency (PCE) around 10%, which is comparable to PBDB‐T/Y6‐based OPVs evaluated in our laboratory.^{[ 24} , ^{25 ]} Particularly, the OPV devices with the unsubstituted NFAs exhibited higher V _OC than the ones with the halogenated NFAs, which is consistent with the trend of E _LUMOs of the NFAs. In contrast, the devices with the halogenated NFAs tended to exhibit higher J _SC than the devices with the unsubstituted NFAs. Overall, the devices based on BTP‐syn‐CNTTCl showed the best averaged PCE of 10.5%.

The EQE spectra seem to reflect the absorption characteristics of the NFAs. OPV devices based on unsubstituted BTP‐anti‐CNTT and BTP‐syn‐CNTT exhibited similar responses of up to 900 nm, while the ones with BTP‐CNTTCls and ‐CNTTBrs exhibited photocurrent generation of up to slightly longer wavelengths of 950 nm. Although in the wavelength range of up to 850 nm, the EQE values of the devices based on BTP‐CNTTs are lower than those of the Y6‐based devices, the extended EQE spectral windows of the chlorinated and brominated NFAs brought about a positive effect on the J _SC. Overall, the device based on BTP‐syn‐CNTT showed comparable J _SC to that of the Y6‐based device. These results suggest that the CNTTs well function as terminal acceptor units comparably to Y6, at least in combination with PBDB‐T as the donor polymer.

On the other hand, the blend of Y6 and PM6 has been one of the most widely used active layers for high‐performance OPVs with PCEs of >16%.^{[ 21} , ²⁶ , ^{27 ]} When the CNTT‐based acceptors were combined with PM6 with a relatively low‐lying E _HOMO of −5.5 eV, the OPV device showed rather poorer performance than that with PBDB‐T, although they showed higher V _OCs (Figure 31 and Table S5). This could originate in the lack of sufficient offset between the E _HOMOs of the donor polymer and the acceptors.

2.5. Thin‐Film Microstructures

To understand the OPV performance of the devices based on BTP‐CNTTs/PBDB‐T, the microstructures of the blend thin films were investigated by atomic force microscopy (Figures S32 and S33) and out‐of‐plane thin‐film X‐ray diffraction (Figure S34). The blend thin films based on the BTP‐CNTTs showed mostly similar surface morphology with well‐mixed small grains and root‐mean‐square surface roughness (3.5–4.3 nm), among which the thin films with the chlorinated and brominated BTP‐CNTT showed fine fibrous surface morphology (Figure S32c–f) similar to the one observed for the blend thin film of Y6/PBDB‐T (Figure S32g). In the out‐of‐plane XRD, the blend thin film with BTP‐syn‐CNTTCl showed significantly intense peaks (2θ = 25.4 °) due to the π‐stacking, which could be beneficial for electron transport in the vertical direction, although the space–charge‐limited current mobility of the thin film was not significantly higher than those of the other thin films (Tables S6 and S7). Thus, the direction of the fused thiophene rings and α‐halogenation of the CNTT terminal group affect the intermolecular interaction and packing structure, although the correlation between the morphology and OPV performance is not totally clear.

3. Conclusion

In summary, we have designed and synthesized a series of CNTT derivatives as building blocks for the terminal acceptor units of NFAs and investigated a series of BTP‐based NFAs. Although the CNTTs have a substructural motif in which the carbonyl group in the IC unit is replaced by an electron‐donating sulfur atom, the E _LUMOs of the BTP‐CNTTs are not significantly higher than that of Y6. Moreover, the α‐chlorination or bromination on the outer thiophene ring effectively lowered their E _LUMOs to values comparable to that of Y6. On the other hand, E _HOMOs of the BTP‐CNTTs are higher than that of Y6 due to the electron‐donating sulfur atom, resulting in smaller HOMO–LUMO energy gaps and, thus, longer maximum absorption wavelengths. Although the performance of the OPV devices with the BTP‐CNTTs are not comparable to the state‐of‐the‐art, preliminary evaluations of the OPV devices afforded PCEs higher than 10%, indicating their potential as building blocks for the terminal acceptor units in NFAs and n‐type organic semiconductors. Particularly, α‐brominated CNTT units can be promising platforms for further π‐extended oligomers and polymers thanks to the bromo group readily available for palladium‐catalyzed cross‐coupling reaction as well as the small steric bulk of the fused thiophene ring for high backbone coplanarity and thus highly effective π‐conjugation.

Supporting Information

The authors have cited additional references within the Supporting Information.^{[ 28} , ²⁹ , ^{30 ]}

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Shibahashi K., Nakamura M., Kawabata K., Takimiya K., Chemistry - An Asian Journal. 2025, e00524. 10.1002/asia.202500524

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.