Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES26 Mar 2022Enhancing the Intermolecular Interactions of Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors for Efficient Polymer Solar Cells by Incorporating Asymmetric Side Chains Qisheng Tu, Wenjun Zheng, Yunlong Ma, Ming Zhang, Zhijian Li, Dongdong Cai, Pan Yin, Jinyun Wang, Shan-Ci Chen, Feng Liu and Qingdong Zheng Qisheng Tu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Wenjun Zheng State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Yunlong Ma State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Ming Zhang Frontiers Science Center for Transformative, Molecules and In-Situ Center for Physical Science, and the Center of Hydrogen Science, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author , Zhijian Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Dongdong Cai State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Pan Yin State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Jinyun Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Shan-Ci Chen State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Feng Liu Frontiers Science Center for Transformative, Molecules and In-Situ Center for Physical Science, and the Center of Hydrogen Science, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author and Qingdong Zheng *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101524 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Asymmetric nonfullerene acceptors (NFAs) possess larger dipole moments and stronger intermolecular bonding energy than their symmetric counterparts thereby making them promising candidates for high-performance polymer solar cells (PSCs). Herein, we report two efficient acceptor–donor–acceptor (A–D–A) type NFAs (M14 and M18) with asymmetric side chains that show enhanced intermolecular interactions compared with their corresponding counterparts (M17 and M19) based on symmetric side chains. Furthermore, M14 and M18 exhibit elevated lowest unoccupied molecular orbitals and smaller π–π stacking distances in comparison with M17 and M19, respectively. In combination with the benchmark polymer donor of PM6, the PM6:M14 blend affords superior charge transport properties, and more importantly, an increased power conversion efficiency (PCE) of 15.49% in comparison with the M17-based counterpart (13.01% PCE). Similarly, the asymmetric M18-based blend also shows a higher PCE of 13.00% than the M19-based blend (11.55%). Through further interface engineering, the best-performing M14-based device delivers an enhanced PCE of 16.46%, which represents a record value among all asymmetric A–D–A type NFAs. Our results provide new insights into the design of asymmetric NFAs with enhanced intermolecular interactions for high-performance PSCs. Download figure Download PowerPoint Introduction With the salient features of low-cost, light-weight, semitransparency, and good flexibility, bulk heterojunction (BHJ) polymer solar cells (PSCs) have drawn increasing attention from numerous researchers around the world.1–5 The active layer of a PSC, which consists of a polymer donor material and a small-molecule acceptor material, plays a vital role in determining the power conversion efficiency (PCE) of a PSC.6 According to the molecular symmetry, the small-molecule acceptors can be divided into two categories: symmetric acceptors and asymmetric acceptors. For a long period of time, asymmetric fullerene derivatives, such as phenyl-C71-butyric acid methyl ester (PC71BM) and phenyl-C61-butyric acid methyl ester (PC61BM), have been the predominant acceptor materials for PSCs.5,7,8 However, these fullerene-based single-junction PSCs can barely achieve a PCE beyond 12% due to the intrinsic drawbacks of fullerene acceptors such as relatively fixed energy levels and bandgaps, weak absorption in the long wavelength region, and morphological instability.9–11 Therefore, nonfullerene acceptor (NFA) materials (asymmetric or symmetric) are emerging as promising candidates to replace traditional fullerene derivatives, thereby leading to the significantly boosted PCE from around 6% in 2015 to over 18% currently.12–20 Among all the NFAs with different molecular design motifs, acceptors with an acceptor–donor–acceptor (A–D–A) configuration have been the most widely adopted.21–25 For the NFA with an A–D–A configuration, its energy levels and bandgap could be easily tuned by selecting different donor and acceptor moieties.11 Furthermore, the strong intramolecular charge transfer (ICT) can occur between the electron-rich core (D) and electron-withdrawing ending groups (A), which would help to extend the absorption band of the A–D–A type NFA to the near-infrared region.11 Currently, tremendous efforts have been devoted to the development of symmetric A–D–A type NFAs.11,26–30 However, in comparison with symmetric A–D–A type NFAs, relatively less attention has been paid to asymmetric A–D–A type NFAs despite the fact that they might show larger dipole moments (δ) and stronger intermolecular interactions that are beneficial for more efficient charge transport and therefore enhancing the photovoltaic performance of the corresponding PSC. Generally, an asymmetric A–D–A type NFA can be developed by using an asymmetric electron-rich core, or using different end groups and side chains.31–38 For example, our group has previously utilized asymmetric indenothiophene as an electron-rich core to construct A–D–A type NFAs with a decent PCE of 8.00%.39 Recently, Sun and co-workers40 successfully designed and synthesized an asymmetric A–D–A type NFA (IDT-2F-Th) with two different ending groups of 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (TIC) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN2F). Benefiting from the two distinct ending groups, IDT-2F-Th exhibited a broader absorption band, reduced crystallinity, and more balanced charge mobilities in blend films in comparison with the symmetric acceptor based on TIC ending groups. Consequently, an increased PCE of 12% was achieved for the IDT-2F-Th-based device.40 Additionally, a new NFA (IDT-OB) with asymmetric side chains was developed by Bo and co-workers.41 They effectively tuned solubility, energy levels, bandgaps, and molecular packing properties of the NFA via asymmetric side chains. The IDT-OB-based PSC exhibited a higher PCE of 10.12% in comparison with the PSCs based on the two symmetric counterparts.41 Over the past few years, other important progress on asymmetric NFAs has been achieved, and the best PCE of PSCs based on asymmetric A–D–A type NFAs has gradually increased to more than 15% currently.42–44 However, the development of asymmetric NFAs still lags behind that of their symmetric counterparts. Recently, our group reported a series of symmetric A–D–A type NFAs with PCEs over 15% using benzo[1,2-b:4,5-b′]bis(4-H-dithieno[3,2-b:2′,3′-d]pyrrole) (BDTPT) as the electron-rich core and INCN2F as the electron-deficient ending groups.45–47 For example, with two pairs of symmetric bulky side chains, the NFA (M34 in Chart 1) afforded an excellent PCE of 15.24% with an open-circuit voltage (VOC) of 0.91 V, a short-circuit current density (JSC) of 23.63 mA cm−2, and a fill factor (FF) of 70.7%.45 It is expected that incorporating asymmetric side chains into the BDTPT core would further improve the photovoltaic performance of these ladder-type heteroheptacene-based NFAs. In addition, halogenation, especially fluorination, has been proven to be one of the most successful strategies to tune the absorption, energy level, crystallinity, and mobility of NFA.28,37 In this context, we report two asymmetric NFAs (M14 and M18 in Chart 1) that have one alkoxy side-chain and one alkylthio side-chain on the BDTPT core with different halogenated ending groups (fluorinated and chlorinated). For comparison purposes, we also report two symmetric NFAs (M17 and M19 in Chart 1) that have two identical alkylthio side chains on the BDTPT core. Compared with symmetric NFAs, their asymmetric counterparts may be endowed with increased solubility. More importantly, the additional dipole moments induced by the side-chain difference (alkoxy vs alkylthio side chains) may enhance the intermolecular interactions thereby improving charge transport of the resulting NFA. When blended with a wide bandgap polymer donor of PM6, the optimized PSC based on PM6:M14 delivered a higher PCE of 15.49% with a VOC of 0.906 V, a JSC of 24.08 mA cm−2, and a FF of 71.0% compared with the counterpart based on PM6:M17 which gave a PCE of 13.01% with a VOC of 0.872 V, a JSC of 21.50 mA cm−2, and a FF of 69.4%. Similar performance variation was also observed for M18- and M19-based devices (13.00% for M18 and 11.55% for M19). Furthermore, by optimizing on the electron transporting layer (ETL), the efficiency of the best-performing PM6:M14-based PSC was further improved to 16.46% with a VOC of 0.889 V, a JSC of 24.16 mA cm−2, and a FF of 76.6%. To the best of our knowledge, the 16.46% PCE is the highest among all A–D–A type asymmetric NFAs reported in the literature. Chart 1 | Chemical structures of M34, M14, M17, M18, and M19 (R1 = 2-ethylhexyl, R2 = 2-butyloctyl). Download figure Download PowerPoint Experimental Methods All chemicals are commercially available and were used without further treatment unless otherwise specifically stated. The synthesis and characterization of the NFAs, absorption spectra measurement, cyclic voltammetry (CV) measurement, photoluminescence (PL) quenching experiments, preparation and characterization of PSCs, space-charge-limited current (SCLC) mobility measurement, atomic force microscopy (AFM) measurement, grazing incident wide-angle X-ray scattering (GIWAXS) measurement, and X-ray diffraction measurement, are provided in detail in the Supporting Information. Computational Methods The density functional theory (DFT) calculations were carried out using the Gaussian 16 program package. The details are provided in the Supporting Information. Results and Discussion Synthesis and characterization The synthetic procedures for M14, M17, M18, and M19 are depicted in Scheme 1, and the details are provided in the Supporting Information. M14 was synthesized using 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione as the starting material. The monothionation of 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophen-4,8-dione with Lawesson’s reagent occurred rapidly to afford a yellow oligomer precursor (1a) in 80.2% yield. After the reduction and subsequent alkylation of compound 1a, the asymmetric compound 2a was obtained in 30.6% yield. Compound 2a was then reacted with Br2 to produce compound 3a in 95.7% yield. The Negishi coupling reaction of compound 3a with (3-bromothiophen-2-yl)zinc chloride afforded compound 4a in 82.6% yield. The Buchwald–Hartwig amination reaction of compound 4a with 2-butyloctan-1-amine using bis(dibenzylideneacetone)palladium(0) (Pd(dba)2) and 1,1′-ferrocenediyl-bis(diphenylphosphine) (dppf) as the catalyst system afforded compound 5a in 25.0% yield. In the presence of POCl3 and dimethylformamide (DMF), the dialdehyde intermediate 6a was obtained in 88.1% yield via the standard Vilsmeier–Haack reaction. Finally, the asymmetric NFA (M14) was synthesized by the Knoevenagel condensation between compound 6a and INCN2F in 71.0% yield. For the synthesis of M17, the key intermediate S1 was similarly prepared as described above for compound 6a using dialkylthiol-substituted benzo[1,2-b:4,5-b′]dithiophene as the starting material. Symmetric NFA (M17) was obtained in 73.0% yield via the condensation between compound S1 and INCN2F. The other two NFAs (M18 and M19) with chlorinated ending groups (INCN2Cl) were also prepared from the condensation reactions between INCN2Cl and 6a and S1 in 68.1% and 73.3% yields, respectively. The four NFAs were characterized by 1H NMR, elemental analysis, and high-resolution mass spectroscopy. All the NFAs are highly soluble in common organic solvents, such as dichloromethane, chloroform, and chlorobenzene at room temperature. For example, the solubilities of M14 and M17 in chloroform at room temperature were determined to be 53 and 44 mg mL−1, suggesting slightly improved solubility via the introduction of asymmetric side chains. Similar to the variation in fluorinated NFAs, M18 and M19 (chlorinated NFAs) have solubilities of 48 and 40 mg mL−1 in chloroform, respectively. This same phenomenon in solubility variation between symmetric and asymmetric molecules has also been reported by other research groups.41,48 Scheme 1 | Synthesis of M14, M17, M18, and M19: (i) Lawesson’s reagent, chlorobenzene, reflux; (ii) tetrahydrofuran (THF), NaOH, NaBH4, 2-ethylhexyl bromide, reflux; (iii) Br2, CHCl3, CH3COOH, 60 °C; (iv) (3-bromothiophen-2-yl)zinc chloride, Pd(dppf)Cl2, ether, reflux; (v) 2-butyloctan-1-amine, Pd(dba)2, dppf, sodium tert-butoxide, toluene, reflux; (vi) POCl3, DMF, 1,2-dichloroethane, 60 °C; (vii) pyridine, CHCl3, 50 °C. Download figure Download PowerPoint Optical and electrochemical properties The absorption spectra for M14, M17, M18, and M19 in dilute chloroform solution (5 × 10−6 M) and in thin film were measured to investigate the asymmetric effect of NFAs on their optical properties. As shown in Figure 1a, all four NFAs show strong absorption bands in the wavelength range from 600 to 800 nm, which are mainly induced by the strong ICT from the electron-rich core to the electron-withdrawing ending groups. M17 has an absorption peak (λmax) at 732 nm with a molar extinction coefficient (ε) of 2.23 × 105 M−1 cm−1. When one of the alkylthio side chains on the BDTPT core was replaced with an alkoxy side-chain, the asymmetric NFA (M14) exhibits a slightly red-shifted absorption peak (λmax) of 735 nm. Moreover, the ε value of M14 increases to 2.31 × 105 M−1 cm−1. With the chlorinated ending groups, M18 exhibits an increased ε of 2.52 × 105 M−1 cm−1 and a red-shifted λmax of 748 nm. Similar to variations in the fluorinated NFAs, M19 with symmetric side chains displays a slightly blue-shifted and weakened absorption with reduced λmax and ε values of 744 nm and 2.34 × 105 M−1 cm−1, respectively. In going from solution to thin film states, all absorption bands of the NFAs are bathochromically shifted by 49–66 nm, suggesting enhanced intermolecular interactions in films. As shown in Figure 1b, M14 and M17 films have absorption onsets of 856 and 849 nm, corresponding to the optical bandgaps (Eg) of 1.45 and 1.46 eV, respectively. The asymmetric NFA (M14) has a slightly lower bandgap than M17, which benefits enhanced light-harvesting of the device based on M14. The same phenomenon is observed for M18 and M19 (chlorinated counterparts) which have Egs of 1.40 and 1.41 eV, respectively. Furthermore, all four NFAs have complementary absorptions with the benchmark wide bandgap polymer donor of PM6 (Figure 1b) which are beneficial for enhanced light-harvesting of the resulting PSCs. Figure 1 | (a) Absorption spectra of M14, M17, M18, and M19 in dilute chloroform solutions; (b) absorption spectra of four NFAs and PM6 in pure film; (c) CV curves of four NFAs; (d) energy level diagram of four NFAs and PM6. Download figure Download PowerPoint The electrochemical properties of M14, M17, M18, and M19 were investigated by CV measurements using thin films deposited on a platinum plate electrode, and the related data are summarized in Table 1. As presented in Figure 1c, the onset oxidation/reduction potentials (φox/φred) deduced from the CV curves are 0.82/−0.85 and 0.86/−0.82 V for M14 and M17, respectively. Thus, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for M17 were determined to be −5.68 and −4.00 eV, respectively, according to the equation of EHOMO/LUMO = −(φox/red + 4.82) eV. However, for M14, the HOMO and LUMO energy levels were estimated to be −5.64 and −3.97 eV, respectively, which are both relatively higher-lying compared to those for M17. The elevated LUMO energy level of M14 favorably achieves a larger VOC in comparison with M17 when blended with a given polymer donor material. From the fluorinated end groups to the chlorinated end groups, both HOMO and LUMO energy levels for M18 and M19 are down-shifted to some extent, especially for the latter with symmetric alkylthio side chains on the BDTPT core. DFT calculations were employed to further confirm the effect of side-chain asymmetry on the energy levels of NFAs. To simplify the DFT calculations, all long alkyl side chains were replaced with shorter 2-methylpropyl groups. When one of the alkylthio groups is substituted by the alkoxy group ( Supporting Information Figure S1), the HOMO/LUMO energy levels are simultaneously up-shifted from −5.78/−3.72 eV (M17) to −5.76/−3.70 eV (M14). Similarly, the calculated HOMO/LUMO energy levels for the chlorinated NFAs are also elevated from −5.82/−3.78 eV (M19) to −5.79/−3.76 eV (M18). These calculated results agree with those obtained from the CV measurements. The energy diagram for the four NFAs, as well as the polymer donor PM6, is depicted in Figure 1d. When M14, M17, M18, and M19 are blended with PM6, the LUMO and HOMO energy offsets are all larger than 0.10 eV which are sufficient for efficient exciton dissociation.49,50 In addition, to gain more insights into the effect of molecular symmetry on the dipole moment, quantum chemistry calculations were also conducted for the four molecules (as shown in Supporting Information Figure S2, M34 was also calculated for the comparison purpose). With asymmetric side chains, M14 exhibits a larger dipole moment value (δ = 1.12 D) than that of M17 (δ = 0.40 D), which helps to enhance the intermolecular interactions of the corresponding NFA (asymmetric M14). Similarly, the δ value of M18 with asymmetric side chains is 1.16 D which is also larger than that for M19 with symmetric side chains (δ = 0.40 D). Furthermore, the dipole moment of M14 is also larger than that of M34 (δ = 0.32 D) with symmetric alkoxy side chains. Table 1 | Optical and Electrochemical Properties of M14, M17, M18, and M19 Acceptor ε (105 M−1 cm−1) λmaxsolution (nm) λmaxfilm (nm) Eg (eV)a HOMOb (eV) LUMOc (eV) M14 2.31 735 784 1.45 −5.64 −3.97 M17 2.23 732 781 1.46 −5.68 −4.00 M18 2.52 748 814 1.40 −5.69 −4.04 M19 2.34 744 807 1.41 −5.72 −4.06 aOptical bandgaps were estimated from the onsets of the absorption spectra in thin film. bEHOMO = −(φox + 4.82) eV. cELUMO = −(φred + 4.82) eV. PL quenching analysis PL quenching experiments were performed to probe the electron and hole transfer behaviors between the donor and the acceptor in blend films. The wavelengths of 750 and 580 nm were chosen to excite the NFAs, PM6, and corresponding blend films. The PL spectra of neat PM6, neat NFAs, and PM6:NFA blend films under the same excitation conditions are shown in Supporting Information Figure S3. When excited at 750 nm, all four NFAs show strong PL emissions in the wavelength range from 800 to 1000 nm. For the blend films (PM6:M14 and PM6:M17) under the same excitation wavelength of 750 nm, their PL emission spectra are significantly quenched by PM6, suggesting efficient hole transfer from both the NFAs to PM6. However, the PL quenching efficiency is slightly higher in the PM6:M14 blend (95.4%) in comparison with that in the PM6:M17 blend (93.8%). As shown in Supporting Information Figure S3b, PM6 exhibits a strong PL emission in the wavelength range from 600 to 850 nm when excited at 580 nm. However, for the blend films of PM6:M14 and PM6:M17 under the same excitation wavelength of 580 nm, the strong PL emission of PM6 is greatly quenched by the acceptors (M14 and M17), indicating efficient electron transfer from PM6 to both two NFAs. Nonetheless, the PL quenching efficiency is slightly higher in the PM6:M14 blend (98.6%) than that in the PM6:M17 blend (97.9%). As presented in Supporting Information Figure S3, both hole and electron transfer are relatively less efficient between the chlorinated NFAs (M18 and M19) and the donor polymer (PM6) when excited at 580 and 750 nm. However, the PM6:M18 blend films were more efficiently quenched (94.0% at 750 nm, 96.9% at 580 nm) in comparison with the M19-based counterparts (89.7% at 750 nm, 95.9% at 580 nm). These results suggest that the asymmetric NFAs (M14 and M18) have improved electron and hole transfer efficiencies in the corresponding blend films, in comparison with the corresponding symmetric NFAs (M17 and M19) by which enhanced photovoltaic performances are expected for the former two. Crystallinity and molecular orientation behaviors GIWAXS measurements were employed to investigate the microstructure and molecular packing behaviors of M14, M17, M18, and M19 pristine films. The two-dimensional (2D) GIWAXS images and corresponding one-dimensional (1D) line cuts are shown in Figure 2, and the related structural parameters are summarized in Supporting Information Table S1. As depicted in Figure 2, all four pure films exhibit strong π–π stacking peaks (010) along the out-of-plane (OOP) direction and strong lamellar stacking peaks (100) along the in-plane (IP) direction, indicating that M14, M17, M18, and M19 adopt a “face-on” orientation with respect to the substrate. Interestingly, the 2D GIWAXS results demonstrate that the asymmetric side chains on the BDTPT core could induce some changes in the crystallinity and molecular orientation behaviors of the corresponding films. As shown in Figures 2a–2e, the (100) peaks of M14, M17, M18, and M19 are located at about 0.322 Å−1, suggesting similar lamellar stacking distances (dls) in different pure films. However, the (010) diffraction peaks in OOP direction are found at 1.804, 1.751, 1.819, and 1.796 Å−1, with the corresponding π–π stacking distances (dπs) of 3.48, 3.59, 3.45, and 3.50 Å, for M14, M17, M18, and M19, respectively. The reduced dπs for the asymmetric M14 and M18 in comparison with the symmetric M17 and M19 implies that the asymmetric alkoxy and alkylthio side chains were able to strengthen the intermolecular interactions of the resulting acceptor molecules. Furthermore, the coherence lengths (CLs) of the (100) and (010) diffraction peaks for M14, M17, M18, and M19 were estimated to be 166.32, 152.83, 91.21, and 84.40 and 20.93, 19.49, 19.36, 18.90 Å, respectively. The asymmetric M14 and M18 show slightly larger CLs in both lamellar stacking and π–π stacking compared with their symmetric counterparts. In addition, the π–π stacking distance of M14 is also smaller than that of M34 (dπ = 3.51 Å) with symmetric alkoxy side chains.45 These results suggest that the asymmetric alkoxy and alkylthio side chains modulate the molecular crystallinity and also effectively reduce the intermolecular interaction distance of the resulting NFA, all of which are beneficial for the enhanced charge transport property of NFAs. Figure 2 | Molecular packing behaviors of neat films. (a–d) 2D GIWAXS images of (a) M14, (b) M17, (c) M18, and (d) M19. (e) Corresponding 1D line cuts in the OOP and IP directions. Download figure Download PowerPoint To further reveal the effect of asymmetric side chains on molecular packing behaviors, single-crystal structures of M14 and M17 were obtained by X-ray diffraction experiments. In this work, the diffraction quality single crystals (M14 and M17) were cultivated with a solvent diffusion method using a toluene/methanol system. The detailed single-crystal parameters are provided in Supporting Information Table S2. Although both M14 and M17 crystallize in monoclinic lattices, the asymmetric M14 shows a P21/n space group, which differs from M17 with a C2/c space group. The single-crystal structures of M14 and M17 from the top view (Figures 3a and 3b) reveal that both M14 and M17 have nearly planar conformations because of the noncovalent intramolecular S···O interactions.51 However, the absolute values of torsion angles for M14 were determined to be 4.54° and 1.10°, respectively, which are both less than the values of 6.33° and 1.52° for the symmetric M17 (Figures 3a and 3b). Crystallographic analysis (Figures 4a and 4c) shows that there are clearly π–π interactions (3.31 Å for M14, 3.41 Å for M17) between the “D” core of one nonfullerene molecule and the “A” end group of another nonfullerene molecule, which would cause the molecules to be arranged in “A-to-D” type J-aggregation (slip angles smaller than 40°) with large rotational angles.52,53 In contrast with traditional J-aggregation, the presence of many strong intermolecular interactions (four different F···S and two different O···H interactions) can embrittle the intramolecular vibrations thereby leading to dipole-rotated J-aggregation patterns (Figures 3c and 3d and Figures 4a and 4c). For M14 and M17, the corresponding rotational angles (θ) between the dimers relative to their long molecular axis are 134° and 132°, respectively (Figures 3c and 3d). The relatively smaller rotational angle for M17 can be partially attributed to the stronger intermolecular interactions with shorter F···S distances (type III, 2.93 and 3.07 Å for M17 vs 3.07 and 3.12 Å for M14). Interestingly, the slight changes in intermolecular interactions as well as the resulting aggregation states have a large influence on the patterns of the corresponding interpenetrating three-dimensional (3D) networks. As shown in Figures 4b and 4d and Supporting Information Figure S4, the 3D packing structures of M14 and M17 crystals are both constituted by repetitive elliptical frameworks with different length/width values of 14.88/7.74 Å and 20.10/10.67 Å, respectively. These highly ordered network packing structures are beneficial to efficient charge transport. Together with the smaller π–π stacking distance, the more compact framework for M14 facilitates its enhanced charge transport in comparison with M17. Figure 3 | (a) The single-crystal structure of M14, (b) the single-crystal structure of M17, (c) the molecular packing of M14 crystal, and (d) the molecular packing of M17 crystal. Download figure Download PowerPoint Photovoltaic performance Conventional PSCs with a device configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/PM6:acceptor/2,9-bis(3-(dimethylamino)propyl)anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetraone (PDIN)/Ag were fabricated to investigate the effect of molecular symmetry on the photovoltaic performance of NFAs. The wide bandgap polymer PM6 was used as an electron donor material because of its matched absorption and energy level