Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Jun 2021Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics Xiaozhang Zhu, Songjun Liu, Qihui Yue, Wuyue Liu, Shaoming Sun and Shengjie Xu Xiaozhang Zhu *Corresponding author: E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of the Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Songjun Liu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of the Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Qihui Yue Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of the Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Wuyue Liu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of the Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Shaoming Sun Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of the Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author and Shengjie Xu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100956 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail High-performance donor–acceptor electron acceptors containing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN)-type terminals are labile toward photooxidation and basic conditions, and new molecular designs toward electron acceptors that can achieve both high power conversion efficiencies and high stability are urgently needed. By replacing the central benzene ring in the classical ladder-type n-type semiconductor, 2,2′-(indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile, with the electron-rich 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene, we report herein the design of 2,2′-(7,7,15,15-tetrahexyl-7,15-dihydro-s-indaceno[1,2-b:5,6-b′]diindeno[1,2-d]thiophene-2,10(2H)-diylidene)dimalononitrile (ITYM), a new type of all-fused-ring electron acceptor (AFRA). A three-step reaction including a key Pd-catalyzed double C–H activation/intramolecular cyclization is established for the efficient synthesis of such type of electron acceptors. ITYM is confirmed by single-crystal X-ray analysis, which shows a planar nonacyclic structure with strong π–π stacking. Compared with the classical carbon-bridged INCN-type acceptors, ITYM exhibits extraordinary stability with very promising performance. The AFRA concept opens a new avenue toward high-efficiency and -stability organic photovoltaics (OPVs). Download figure Download PowerPoint Introduction Benefiting from the excellent tunability of the electronic structure and blend morphology for donor (D)–acceptor (A)-type electron acceptors,1–4 organic photovoltaics (OPVs) have achieved high power conversion efficiencies (PCEs) above 17% within a short period of time,5–19 indicating a bright future for commercial applications, provided that the three key issues, efficiency, cost, and stability, can be synergistically addressed.20–28 Currently, most high-performance D–A acceptors contain an electron-rich core and strong electron-deficient 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) terminals (Figure 1a),29 recognized as one of the main factors that limit the device lifetime because the exocyclic double bonds formed by the kinetically reversible Knoevenagel condensation reaction (KCR) are highly labile upon photooxidation,30–34 ZnO-catalyzed photodegradation,35,36 and base-induced37–39 decomposition (Figure 1a). To increase the device stability, stabilizers,30,40 doped or modified ZnO,41–45 or replacement of ZnO with SnO236,46–48 are applied to suppress photodegradation. When aqueous polyethylenimine ethoxylated (PEIE) is utilized to form the interfacial layer, the base-induced decomposition can be temporarily solved.41 While new molecular designs for highly stable electron acceptors are a more fundamental and effective approach, there are few options.31,49–51 It was reported that the incorporation of side alkyl chains to increase the steric hindrance of the exocyclic double bonds can improve the photostability.49 With a ring-locking strategy, Liu et al.50 reported a new electron acceptor, 3,9-bis(cyclohex-2-en-4-yl-1-ylidene))bis(1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (IDTT-CT), that shows considerably decreased decay under the ethanolamine (EA) treatment and light illumination. Zhang et al.51 reported unique electron acceptors [2,2′-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(5,6-difluoro-8H-indeno[2,1-b]thiophene-2-yl-8-ylidene))dimalononitrile (Q1-nF)] in which the electron-rich core and the electron-deficient terminals are formed by Stille coupling, and the robust carbon–carbon single bonds improved the photostability to a great extent. Despite the improved stability, the PCEs of IDTT-CT- and Q1-4F-based OPVs are only 6.13% and 7.93%, respectively, meaning new designs that can achieve both high PCEs and stability are urgently needed. Figure 1 | (a) Traditional design toward D–A-type fused-ring electron acceptors and the new design toward AFRAs proposed in this work. (b) The frontier molecular orbitals of IFDM and ITYM with the calculated energy levels and absorptions for comparison. Download figure Download PowerPoint N-type organic semiconductors featuring a rigid and planar ladder-type structure have been widely investigated for more than a decade.52–57 We envisioned that such n-type semiconductors should be very promising as nonfullerene acceptors in terms of their excellent solution processability and electron mobility with high air stability. However, before the exploration of their OPV application, the excessively deep lowest unoccupied molecular orbital (LUMO) energy level and very weak absorption that are sufficient for organic field-effect transistors but would be unbeneficial for achieving a high open-circuit voltage (Voc) and short-circuit current (Jsc) should be solved. By replacing the central benzene ring in the classical ladder-type n-type semiconductor, 2,2′-(indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile (IFDM)58 with the electron-rich 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT) (Figure 1b), we report herein the design and synthesis of a new type of electron acceptor, 2,2′-(7,7,15,15-tetrahexyl-7,15-dihydro-s-indaceno[1,2-b:5,6-b′]diindeno[1,2-d]thiophene-2,10(2H)-diylidene)dimalononitrile (ITYM). This new type of electron acceptor differs from the traditional INCN-type acceptors that possess flexible σ bonds, and shows an all-fused-ring molecular framework,59 resulting in a considerably reduced reorganization energy compared with IDIC,60 a classical small-molecule electron acceptor, 0.16 versus 0.20 eV based on density functional theory (DFT) calculations at the B3LYP/6-31G** level. Compared with IFDM, ITYM exhibits significantly elevated highest occupied molecular orbital (HOMO) (−5.77 vs −6.49 eV) and LUMO energy levels (−3.59 vs −3.95 eV) that are suitable for the OPV application. We established a convenient three-step reaction containing an efficient and green Pd-catalyzed double C–H activation/intramolecular cyclization61 for the synthesis of the ITYM acceptor with a consecutive nonacyclic structure, which is confirmed by the single-crystal X-ray analysis. Compared with the classical carbon-bridged INCN-type acceptors, IDIC, ITIC,29 and 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (IT-4F),62 ITYM exhibits excellent thermal, chemical, and photochemical stability because of its completely fused structure. By combining ITYM with a large-bandgap polymer donor, ITYM-based OPVs deliver a promising efficiency approaching 10% and, together with their high stability, confirming the effectiveness of our design. Results and Discussion As shown in Scheme 1, we established an efficient three-step reaction for the synthesis of ITYM including a Pd-catalyzed double C–H activation and intramolecular cyclization. (4,4,9,9-Tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis((2-bromophenyl)methanone) 2 is synthesized by the deprotonation of the commercially available IDT 1, followed by a nucleophilic reaction with 2-bromo-N,N-dimethylbenzamide 70%. 7,7,15,15-Tetrahexyl-7,15-dihydro-s-indaceno[1,2-b:5,6-b′]diindeno[1,2-d]thiophene-2,10(2H)-dione 3 is synthesized by the double Pd-catalyzed C–H activation/intramolecular cyclization in 67% yield as a red solid, followed by the clean KCR catalyzed by a Lewis acid, using titanium chloride yields ITYM in 89% yield as a dark solid. All new compounds have been unambiguously characterized using 1H NMR, 13C NMR, and high-resolution mass spectrometry. ITYM shows good solubility in various commonly used solvents such as chloroform (20 mg/mL), which facilitates device fabrication. This three-step synthesis is universal for the synthesis of all-fused-ring electron acceptors (AFRAs) with different electron-rich cores. Scheme 1 | Synthesis of AFRA ITYM. Reagents and conditions: (a) 1. n-BuLi, THF, –78 °C; 2. 2-bromo-N,N-dimethylbenzamide; (b) Pd(OAc)2, tricyclohexylphosphine tetrafluroborate, K2CO3, N,N-dimethylacetamide, reflux; (c) TiCl4, pyridine, chlorobenzene, r.t. Download figure Download PowerPoint We obtained high-quality single crystals that are suitable for X-ray analysis by the slow diffusion of n-hexane into ITYM solution in CH2Cl2 (CCDC: 2058454). As shown in Figures 2a and 2b, ITYM exhibits a nearly planar nonacyclic structure with the four n-hexyl groups perpendicularly protruding outside the molecular π plane. As indicated in Figure 2c, because of the large steric hindrance induced by the carbon bridges, ITYM shows one-dimensional π–π stacking between the electron-deficient terminals along the a-axis direction, leading to a short distance of 3.31 Å and facilitating electron transport. Figure 2 | The single-crystal structure of ITYM: (a) top view and (b) side view. (c) The crystal packing diagram with the π–π distance. Download figure Download PowerPoint The photophysical and electrochemical properties of ITYM were investigated using UV–vis–near infrared (NIR), fluorescence (FL), and cyclic voltammetry (CV) measurements, and the results are summarized in Figure 3. As shown in Figure 3a, ITYM in chloroform exhibits a structured optical absorption with a peak (λabsmax) at 668 nm [molar absorption coefficient (ε): 7.43 × 104 M−1 cm−1]. For comparison, 2,2′-(5,11-didodecylindeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile (IFDM-C12)52 exhibits a very weak absorption at 579 nm, and the main absorption is only 426 nm. According to the time-dependent DFT (TD-DFT) calculations (Figure 1b), the maximum absorption of ITYM is from the favorable HOMO → LUMO excitation [638.49 nm, oscillator strength (f): 0.8584], which, however, is quite weak for IDFM-C12 (657.36 nm, f: 0.002). The main absorption of IDFM-C12 can be assigned to the HOMO-1 → LUMO excitation (408.06 nm, f: 0.4501). ITYM absorption is further redshifted to a longer wavelength of 680 nm in the solid state, resulting in a medium-optical bandgap (Egopt) of 1.68 eV based on the absorption onset at 740 nm. ITYM in chloroform shows a FL property with a peak at 717 nm resulting in a small Stokes shift of 1023 cm−1 (49 nm), which is a typical characteristic for ladder-type molecules63,64 and consistent with its low reorganization energy. The FL quantum yields of ITYM in CHCl3 solution (3.18%) and thin film (4.26%) are determined to be 3.18% and 4.26% ( Supporting Information Figure S1) by the absolute method, respectively. The HOMO and LUMO energy levels, key parameters determining the exciton dissociation efficiency in OPV blends, are estimated to be −5.76 and −3.74 eV according to the CV curve, respectively (Figure 3b). For comparison, the HOMO and LUMO levels of IDFM-C12 are quite deep, −6.16 and −4.30 eV, respectively. Thus, the incorporation of IDT not only enhances light absorption from the improved excitation property but also forms a favorable energy alignment. ITYM yields complementary absorption with the large-bandgap polymer donor PM6 possessing a deep HOMO level and HOMO/LUMO offsets of 0.26/0.08 eV that are sufficient for achieving efficient exciton dissociation. Accordingly, as shown in Figure 3c, the FL of both PM6 and ITYM in as-cast blend film is almost totally quenched upon the 630-nm excitation of either D or A (Figure 3d), indicating efficient exciton dissociation, a typical Type II heterojunction for both hole and electron transfer, and the possibility for OPV applications. Figure 3 | (a) UV–vis absorption and FL spectra of ITYM in solution and thin-film states and the absorption spectrum of the PM6 thin film. (b) Cyclic voltammograms of ITYM and PM6 in the solid state. (c) Photoluminescence quenching spectrum for the PM6:ITYM blend (excited at 630 nm). Download figure Download PowerPoint We systematically investigated the thermal, chemical, and photochemical stability of ITYM together with the classical carbon-bridged D–A acceptors, IDIC, ITIC, and IT-4F as the control group for comparison (Figure 4). Despite its lowest molecular weight, ITYM shows the highest melting point among the four acceptors, 324 °C (ITYM) versus 282 °C (IDIC), 314 °C (ITIC), and 316 °C (IT-4F), which is related to its all fused molecular structure. According to the thermogravimetric analysis (TGA, Figure 4a), ITYM shows the highest decomposition temperature at 5% weight loss (5 wt %) of 353 versus 332 °C for IDIC, 346 °C for ITIC, and 336 °C for IT-4F. Carefully studying the TGA curves of ITYM and the control group, we found that all the acceptors show 5 wt % temperatures above the corresponding melting point, meaning that besides thermal decomposition, the initial weight loss might also be because of the evaporation of melted acceptors. Thus, we checked the quality of acceptors heated at the individual 5 wt % temperatures. We found that the control group completely decomposed into insoluble black solids, but no decomposition was observed for ITYM according to the 1H NMR analysis ( Supporting Information Figure S2), consistent with our assumption. Further heating analysis of ITYM at 370 °C and the transition temperature (400 °C) defined as the second slope change, we found ITYM partially decomposed at 370 °C and completely decomposed at 400 °C. According to the above analysis, we conclude that the real thermal stability for ITYM should be much higher than the control group. Since the decomposition temperature is quite similar for IDIC, ITIC, and IT-4F with quite different molecular weights, we suppose that the low thermal stability of the control group is because of the highly polarized single bonds that enable the strong push–pull effect, which does not exist in ITYM. By treating the diluted solution (10−5 M) of acceptors in a mixed tetrahedrofuran (THF)/H2O solvent (v/v = 96:4) with 100 equiv EA, we examined the chemical resistance of the acceptors against nucleophilic attack (Figure 4b and Supporting Information Figure S3). Because the exocyclic double bonds in INCN-type acceptors are highly labile under basic conditions, the maximum absorptions of IDIC, ITIC, and IT-4F immediately decayed by 36%, 66%, and 79%, respectively, and the decay ratios further increased to 66%, 79%, and 94% after 3 h. After 12 h, the decay ratios increased to 81%, 97%, and 99%, respectively. Notably, only a minor decay of <5% was observed for ITYM after 12 h, affirming its highest chemical stability. The stability of nonfullerene acceptors toward photooxidation is crucial for the realization of stable OPV devices. We examined the absorption decay of all four electron acceptors in diluted THF upon illumination with simulated air mass (AM) 1.5G solar light (Figure 4c and Supporting Information Figure S4). ITYM exhibits a higher 50% decay time (100 min) than the control group, 55 min for IDIC, <2 min for ITIC and IT-4F, suggesting that it has the best photochemical stability. According to the results in the control group, we found that the photostability is decreased with the elongated π-conjugation and the incorporation of fluorine atoms. Thus, ITYM shows excellent stability (Figure 4d) that is urgently needed, suggesting its great potential of AFRAs in OPV applications. Figure 4 | (a) TGA curves of ITYM and the control group. The time-dependent absorption decays of ITYM and the control group at the corresponding maximum absorptions upon the EA treatment (b) and AM 1.5G illumination (c). (d) Photos of the ITYM and the control group before and after the 12-h EA treatment and 1-h AM 1.5G illumination, respectively. Download figure Download PowerPoint To evaluate the photovoltaic performance of ITYM as an electron acceptor, conventional devices with an architecture of ITO/PEDOT:PSS/PM6:ITYM/PDINN/Ag were fabricated, where ITO and PEDOT:PSS refer to indium tin oxide and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), respectively, PDINN is a PDI derivative serving as a cathode interlayer, and PM6 is a well-known polymer electron donor. The effects of different D/A ratios, dosages of additives, posttreatments, and layer thicknesses on the photovoltaic performance were thoroughly investigated ( Supporting Information Tables S1–S7). Table 1 lists the detailed photovoltaic parameters of the as-cast device, the device with the various dosage of 1-chloronaphthalene (1-CN), and the optimized device, and the relevant current density–voltage (J–V) curves are shown in Figure 5a. The as-cast device exhibits a PCE of 6.47% with a Voc of 0.93 V, a Jsc of 12.40 mA cm−2, but an inferior fill factor (FF) of 56.46%. After the addition of 0.25% (v/v) 1-CN, with the obvious enhancements in Jsc and FF, the PCE is increased to over 8%, and under the synergistic effect of 1-CN and thermal annealing (TA), the final PCE of 9.51% is achieved with further improved Jsc and FF (14.65 mA cm−2 and 71.73%, respectively) with a slightly decreased Voc of 0.91 V. The increased performance is strongly related to changes of the blend morphology. From the external quantum efficiency (EQE) curves (Figure 5b), the integrated Jscs are in great agreement with the results in the J–V tests (12.22, 13.76, and 14.48 mA cm−2, within 2% mismatch). All the devices show a wide-range (300–750 nm) photoresponse, and under the effects of 1-CN and 1-CN + TA, the photoresponse is improved over the entire range, which also shows the apparent morphology improvement after these treatments. Table 1 | Photovoltaic Parameters of PM6:ITYM-Based Solar Cells under Illumination of AM 1.5G, 100 mW cm−2a Treatment Voc (V) Jsc (mA cm−2) FF (%) PCE (%) As-cast 0.93 (0.93 ± 0.01) 12.40 (12.30 ± 0.08) 56.46 (55.88 ± 0.36) 6.47 (6.38 ± 0.06) 1-CN 0.94 (0.94 ± 0.01) 13.92 (13.71 ± 0.13) 62.32 (62.19 ± 0.61) 8.14 (7.99 ± 0.13) 1-CN + TA 0.91 (0.91 ± 0.01) 14.65 (14.41 ± 0.18) 71.73 (71.60 ± 0.37) 9.51 (9.35 ± 0.09) aThe values in parentheses are averages from more than 10 devices. Figure 5 | (a) J–V and (b) EQE curves for PM6:ITYM-based photovoltaic devices under AM1.5G irradiation, and (c–e) morphology of the PM6:ITYM blend films by TEM. Download figure Download PowerPoint The exciton dissociation and charge extraction capabilities were also investigated with the relationship between the photocurrent density (Jph) on the effective voltage (Veff), where Jph represents the difference between the photocurrent density under illumination (Jlight) and dark conditions (Jdark) and Veff equals the difference between the voltage when Jph = 0 (V0) and the applied voltage (Vapp). As shown in Supporting Information Figure S5a, at a high Veff of 2 V, Jphs reached saturation (Jsats) in all devices, indicating that all carriers generated by light absorption are swept out. With the equation P(E,T) = Jph/Jsat, exciton dissociation and charge collection probabilities under different conditions can be calculated. Under the short-circuit condition, exciton dissociation and charge extraction are quite efficient, with P(E,T)s of over 90% (the highest was 97.4% in the optimal device). However, in actual applications, P(E,T)s under the maximum power output condition are more important. The values of the three devices exhibit a clear increasing trend (72.5%, 75.8%, and 84.8%, respectively), which is consistent with the gradually increased FF. We also analyzed the bimolecular recombination behavior under the short-circuit condition in these devices using the proportional relationship between Jsc and Pα, where α is the exponential factor. If all free charges are swept out without any bimolecular recombination, then α = 1. Supporting Information Figure S5b shows that the α values are calculated to 0.892, 0.945, and 0.962 in the as-cast device and devices with 1-CN and 1-CN + TA, respectively. The presented results suggest that the optimal device exhibits superior exciton dissociation probability and charge collection ability and suppresses bimolecular recombination most effectively, explaining the highest Jsc and FF among the three devices, which may be attributed to the improved morphology. We investigated the charge-transport property using the space-charge-limited current method. The hole- and electron-only devices have structures of ITO/PEDOT:PSS/PM6:ITYM/Au and ITO/ZnO/PM6:ITYM/PFN/Al, respectively ( Supporting Information Figure S6). In the as-cast blend film, the electron mobility (μe) is only one-tenth of the hole mobility (μh) (2.73 × 10−5 vs 2.72 × 10−4 cm2 V−1 s−1), meaning that the hole–electron transport is extremely unbalanced, which is also a possible reason for the low FF in the related device. Under the 1-CN effect, μe and μh both slightly increased to values of 7.55 × 10−5 and 3.16 × 10−4 cm2 V−1 s−1, respectively, while the cooperation of 1-CN and TA can also slightly increase the μh (4.44 × 10−4 cm2 V−1 s−1) but significantly increase the μe (1.27 × 10−4 cm2 V−1 s−1), which is almost an order of magnitude improvement compared with the initial situation, suggesting a more balanced charge transport, which will inhibit charge recombination and contribute to a higher FF. To understand better the reason behind the performance improvements, transmission electron microscopy (TEM), which is helpful to characterize the bulk morphology, and atomic force microscopy (AFM), which describes the surface morphology features, were used. From the TEM images (Figures 5c–5e), the as-cast blend film is homogeneous, and no obvious phase separation is observed, which may lead to severe recombination during the charge-transport process and thus the poor FF. Under the effect of 1-CN, bicontinuous interpenetrating networks are formed. Further TA leads to a more well-defined phase separation, which is conducive to charge generation and continuous charge transport, leading to the highest Jsc and FF in the related device. The phase separation change consistent with the TEM results can also be observed in the AFM images ( Supporting Information Figure S7). In addition, from the height images, the as-cast blend exhibited the most uniform surface with a root mean square (RMS) of 1.38 nm, while 1-CN-treated and 1-CN + TA-treated blend showed much higher RMSs of 8.51 nm and 6.29 nm, respectively. Conclusion We proposed an all-fused-ring strategy for the design of nonfullerene acceptors, based on which a new nonacyclic electron acceptor ITYM is developed by replacing the central benzene ring in the classical n-type semiconductor IFDM with the electron-rich IDT. ITYM is confirmed by the single-crystal X-ray analysis forming strong π–π stacking along the two ends of the molecule. Compared with the classical carbon-bridged nonfullerene acceptors, ITYM exhibits extraordinary thermal, chemical, and photochemical stability. The preliminary OPV evaluation has delivered a very promising performance with efficiencies approaching 10%, which we expect can be effectively promoted by the incorporation of other electron-rich cores to further elevate the HOMO level and decrease the optical bandgap. The following distinct advantages can be expected for such an all-fused-ring design: (1) feasible and efficient three-step synthesis from various electron-rich cores65; (2) low reorganization energy, favorable aggregation, and thus high electron mobility that can be expected from rigid and planar structures66,67; and (3) high thermal, chemical, and photochemical stability for achieving stable OPVs.63,64 Based on a judicious selection of electron-rich cores and electron-deficient terminals, we may establish a rich library of AFRAs, providing a new opportunity to achieve high-efficiency, low-cost, and high-stability OPVs that are suitable for commercial application. Supporting Information Supporting Information is available including the supplemental experimental procedures, Figures S1–S7, and Table S1–S7. Conflict of Interest The authors declare no competing interests. Funding Information The authors thank the National Key R&D Program of China (nos. 2019YFA0705900 and 2017YFA0204701) and the National Natural Science Foundation of China (nos. 21661132006 and 91833304) for their financial support.