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The intra and intermolecular interactions (J- and H-type aggregation) in the conjugated polymer films are found to readily facilitate the electron and hole transport, respectively. However, how those different aggregation types influence the photocurrent generation at the heterojunction is still mysterious, especially for the newly developed semicrystalline conjugated polymers. Here, the prototypical copolymer PM6 is used to tune the relative content of aggregation types with various halogen-free processing solvents. Various measurements reveal that the toluene-processed PM6 film exhibits the increased H-aggregates and crystallinity in the π-π stacking direction compared to its o-xylene- and trimethylbenzene (TMB)-processed counterparts. This is partly resulted from the weak steric effect and good solubility in the PM6 solution prepared with toluene, which strengthens the intermolecular interaction of adjacent polymer segments. After analyzing the photovoltaic properties of PM6/Y6 bilayer devices, the faster charge carrier transport, smaller charge recombination, lower energy losses, and interfacial energetic disorder can be observed in the toluene-processed device, leading to the synergistically improved short-circuit current density (J SC ) and open-circuit voltage (V OC ). These findings indicate the control of the molecular packing structure in terms of aggregation types is a powerful strategy to promote the photocurrent generation process at the conjugated polymer-based heterojunction., (© 2022 Wiley-VCH GmbH.)

Currently, most of the disclosed ternary strategies to improve photovoltaic performance of all‐polymer solar cells (all‐PSCs) commonly focus on the guest polymers having similar structures with the host polymer donors or acceptors. Herein, this work develops a distinctive ternary method that adding an amorphous B←N embedded polymer named BN‐Cl‐2fT to a crystallized host polymer blend of PM6 (a commercialized polymer donor) and PY‐TT (a copolymer of Y6 and thieno[3,2‐b]thiophene). Although the structures between BN‐Cl‐2fT and PM6 and PY‐TT are completely different, excellent miscibility is found between BN‐Cl‐2fT and both of the host PM6 and PY‐TT, which can be interpreted by the crowded phenyl groups anchoring along the backbone of BN‐Cl‐2fT, leading to weak self‐aggregation. Glazing incidence wide‐angle X‐ray diffraction (GIWAXS) measurements explicitly confirm the crystallization of PM6 and PY‐TT and amorphous feature of BN‐Cl‐2fT. Furthermore, adding 10 wt% BN‐Cl‐2fT to PM6:PY‐TT can significantly enhance the crystallization of the host polymers. Thus the ternary devices based on PM6:PY‐TT:BN‐Cl‐2fT afford promote short‐circuit current density (JSC, 23.29 vs. 21.80 mA cm−2), fill factor (FF, 62.4% vs. 60.0%), and power conversion efficiency (PCE, 13.70% vs. 12.23%) in contrast to these parameters of binary devices based on PM6:PY‐TT. This work provides a unique and enlightening avenue to design high performance all‐PSCs by adding amorphous B←N embedded polymers as guest component to enhance host‐crystallization. [ABSTRACT FROM AUTHOR]

Organic solar cells (OSCs) have achieved great progress, driven by the rapid development of wide bandgap electron donors and narrow bandgap non‐fullerene acceptors (NFAs). Among a large number of electron‐accepting (A) building blocks, thiazole (Tz) and its derived fused heterocycles have been widely used to construct photovoltaic materials, especially conjugated polymers. Benefiting from the electron deficiency, rigidity, high planarity, and enhanced intra/intermolecular interactions of Tz‐containing heterocycles, some related photovoltaic materials exhibit proper energy levels, optimized molecular aggregation, and active layer morphology, leading to excellent photovoltaic performance. This review focuses on the progress of Tz‐based photovoltaic materials in the field of OSCs. First, the Tz‐based donor and acceptor photovoltaic materials are reviewed. Then, the materials based on promising Tz‐containing heterocycles, mainly including thiazolo[5,4‐d]thiazole (TzTz), benzo[1,2‐d:4,5‐d']bis(thiazole) (BBTz), and benzo[d]thiazole (BTz) are summarized and discussed. In addition, the new emerging Tz‐fused structures and their application in OSCs are introduced. Finally, perspectives and outlooks for the further development of Tz‐containing heterocycle‐based photovoltaic materials are proposed. [ABSTRACT FROM AUTHOR]

In the past 20 years, organic solar cells (OSCs) have made great progress in pursuing high power-conversion efficiencies, reaching the application threshold. Instead, device stability is becoming particularly important toward commercialization. There are many factors influencing the stability of OSCs, such as light, heat, humidity, oxygen, as well as device structure. Active layer materials, as the most critical functional layer in the devices, are greatly affected by these factors in terms of both efficiency and stability. Herein, it is desirable and urgent to summarize methods for obtaining active layer materials with long-term stability, mainly focusing on the chemical structure and blending morphology. Meanwhile, the corresponding degraded mechanism of OSCs is concluded and analyzed. In this outlook, challenges for developing high-performance and stable OSCs are discussed., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nonfused ring acceptors (NFRAs) have attracted significant attention for nonfullerene organic solar cells (OSCs) owing to their chemical tunability and facile synthesis. In this study, a benzotriazole‐based NFRA with chlorinated end groups (Triazole‐4Cl) is developed to realize highly efficient and thermally stable NFRA‐based OSCs; an analogous NFRA with nonchlorinated end groups (Triazole‐H) is synthesized for comparison. Triazole‐4Cl film exhibits the high‐order packing structure and the near‐infrared absorption capability, which are advantageous in charge transport and light harvesting of the resulting OSCs. In particular, the strong crystalline behavior of Triazole‐4Cl results in enhanced self‐aggregation, leading to high charge carrier mobility. Owing to these properties, a PBDB‐T (polymer donor):Triazole‐4Cl OSC demonstrates a high short‐circuit current, fill factor, and power conversion efficiency (PCE = 10.46%), outperforming a PBDB‐T:Triazole‐H OSC (PCE = 7.65%). In addition, the thermal stability of a PBDB‐T:Triazole‐4Cl OSC at an elevated temperature of 120 °C exceeds that of a PBDB‐T:Triazole‐H OSC. This is mainly attributed to the significantly higher cold crystallization temperature of Triazole‐4Cl (205.9 °C). This work provides useful guidelines for the design of NFRAs to achieve efficient and thermally stable NFRA‐based OSCs. [ABSTRACT FROM AUTHOR]

Wide bandgap polymers with a donor–acceptor alternating structure play a key role in constructing high‐efficiency organic solar cells (OSCs). However, only a handful of high‐performance polymers are available owing to the limited choices of acceptor units. 5,6‐Difluorobenzo[c][1,2,5]oxadiazole (ffBX) is a promising acceptor unit with high ionization potential, and can afford high charge carrier mobility and strong aggregation for the resulting polymers. Historically, ffBX is successfully used in constructing high‐performance polymer donors for fullerene‐based OSCs. However, this unit is far less been explored in non‐fullerene OSCs. In this work, three ffBX‐based wide bandgap polymers (Oc00, Oc25, and Oc50) with varied solubilizing side chain content for application in non‐fullerene OSCs are reported. The polymers show matched energy levels and complementary optical absorption with the state‐of‐the‐art non‐fullerene acceptor Y6. Moreover, the polymer solubility, solid state packing, and bulk‐heterojunction morphology are finely tuned via side chain engineering. Encouragingly, a decent efficiency of 14.25% is realized by the polymer Oc25 when blended with Y6 due to the efficient charge transport and favorable active layer morphology. These results suggest the promising prospect of ffBX in constructing high‐performance polymer donors for non‐fullerene OSCs. [ABSTRACT FROM AUTHOR]

Molecular structural modifications are utilized to improve the short‐circuit current (JSC) of high‐voltage organic photovoltaics (OPVs). Herein, the classic non‐fullerene acceptor (NFA), BTA3, is chosen as a benchmark, with BTA3b containing the linear alkyl chains on the middle core and JC14 fusing thiophene on the benzotriazole (BTA) unit as a contrast. The photovoltaic devices based on J52‐F: BTA3b and J52‐F: JC14 achieve wider external quantum efficiency responses with band edges of 730 and 800 nm, respectively than that of the device based on J52‐F: BTA3 (715 nm). The corresponding JSC increases to 14.08 and 15.78 mA cm−2, respectively, compared to BTA3 (11.56 mA cm−2). The smaller Urbach energy and higher electroluminescence efficiency guarantee J52‐F: JC14 a decreased energy loss (0.528 eV) and a high open‐circuit voltage (VOC) of 1.07 V. Finally, J52‐F: JC14 combination achieves an increased power conversion efficiency (PCE) of 10.33% than that of J52‐F: BTA3b (PCE = 9.81%) and J52‐F: BTA3 (PCE = 9.04%). Overall, the research results indicate that subtle structure modification of NFAs, especially introducing fused rings, is a simple and effective strategy to extend the photoelectric response, boosting the JSC and ensuring a high VOC beyond 1.0 V. [ABSTRACT FROM AUTHOR]

Currently, rational design of polymer acceptors is desirable but there is still a challenge to develop high‐performance all‐polymer solar cells (all‐PSCs). In this work, brominated thienyl‐fused malononitrile‐based monomer is employed to copolymerize with indacenodithiophene (IDT) and benzodithiophene (BDT)‐based linking units to develop two polymerized small molecule acceptors (PSMAs) PIDT and PBDT, respectively, for all‐PSCs. The two PSMAs show similar absorption edges, while PBDT shows a slightly higher lowest unoccupied molecular orbital (LUMO) energy level than PIDT. Benefitted from the relatively high LUMO levels of the two polymer acceptors, notable open‐circuit voltage (Voc) values over 1.0 V are achieved when using them as acceptor to blend with PTQ10 as polymer donor. Particularly, the all‐PSC based on PTQ10:PIDT demonstrates a power conversion efficiency of 10.19%, with an outstanding Voc of 1.10 V benefitted from the higher LUMO energy level of PIDT acceptor. The results demonstrate a feasible strategy to design PSMAs by selecting appropriate linking units for increasing the Voc and improving the efficiency of all‐PSCs. [ABSTRACT FROM AUTHOR]

Singlet fission (SF) is a spin-allowed process in which a higher-energy singlet exciton is converted into two lower-energy triplet excitons via a triplet pair intermediate state. Implementing SF in photovoltaic devices holds the potential to exceed the Shockley-Queisser limit of conventional single-junction solar cells. Although great progress has been made in exploiting the underlying mechanism of SF over the past decades, the scope of materials capable of SF, particularly polymeric materials, remains poor. SF-capable polymer is one of the most potential candidates in the implementation of SF into devices due to their distinct superiorities in flexibility, solution processability and self-assembly behavior. Notably, recent advancements have demonstrated high-performance SF in isolated donor-acceptor (D-A) copolymer chains. This review provides an overview of recent progress in the development of SF-capable polymeric materials, with a significant focus on elucidating the mechanisms of SF in polymers and optimizing the design strategies for SF-capable polymers. Additionally, the paper discusses the challenges encountered in this field and presents future perspectives. It is expected that this comprehensive review will offer valuable insights into the design of novel SF-capable polymeric materials, further advancing the potential for SF implementation in photovoltaic devices., (© 2023 Wiley-VCH GmbH.)

Molecular ordering of conjugated polymers both in solution‐state aggregates and in solid‐state microstructures is a determining factor of the charge transport properties in optoelectronic devices. However, the effect of backbone conformation in conjugated polymers on assembly structures is still unclear. Herein, to understand such backbone conformation effect, three novel chlorinated benzodifurandionge‐based oligo(p‐phenylene vinylene) (BDOPV) polymers are systematically developed. These BDOPV‐based polymers exhibit significantly twisted backbone conformation (near 90° interunit torsion angle) between conjugated units, which can prevent polymer chains from forming ordered assembly structures by increasing conformational energy penalty in closely packed chains. A higher rotational barrier of the torsion angle would further prevent polymer chains from assembling, finally resulting in nonaggregated chains in solution and highly disordered solid‐state packing structures. This work will deepen the understanding of the relationship between polymer backbone conformation and assembly structures, contributing to the exploration of the structure–property relationship of polymers. [ABSTRACT FROM AUTHOR]

Design and development of wide bandgap (WBG) polymer donors with low‐lying highest occupied molecular orbitals (HOMOs) are increasingly gaining attention in non‐fullerene organic photovoltaics since such donors can synergistically enhance power conversion efficiency (PCE) by simultaneously minimizing photon energy loss (Eloss) and enhancing the spectral response. In this contribution, three new WBG polymer donors, P1, P2, and P3, are prepared by adding phenylene cores with a different number of fluorine (F) substituents (n = 0, 2, and 4, respectively) to dicarboxylate bithiophene‐based acceptor units. As predicted, fluorination effectively aides in the lowering of HOMO energy levels, tailoring of the coplanarity and molecular ordering in the polymers. Thus, fluorinated P2 and P3 polymers show higher coplanarity and more intense interchain aggregation than P1, leading to higher charge carrier mobilities and superior phase‐separated morphology in the optimized blend films with IT‐4F. As a result, both P2:IT‐4F and P3:IT‐4F realize the best PCEs of 6.89% and 7.03% (vs 0.16% for P1:IT‐4F) with lower Eloss values of 0.65 and 0.55 eV, respectively. These results signify the importance of using phenylene units with sequential fluorination in polymer backbone for modifying the optoelectronic properties and realizing low Eloss values by synergistically lowering the HOMO energy levels. [ABSTRACT FROM AUTHOR]

Two highly crystalline polymer donors (PBTz4T2C‐a, PBTz4T2C‐b) with isomers (4T2C‐a, 4T2C‐b) are synthesized and applied in polymer solar cells. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor IT2F. It is interesting that the photophysical properties, crystallinity, and active layer morphology characteristic can be significantly changed by just slightly regulating the substitution position of the carboxylate groups. A series of simulation calculations of the two isomers are conducted in the geometry and electronic properties to explore the difference induced by the position adjustment of carboxylate groups. The results decipher that 4T2C‐b moiety features much stronger intramolecular noncovalent S⋯O interactions compared to that of 4T2C‐a, implying a higher coplanarity and much stronger crystallinity, and leading to excessive phase separation in PBTz4T2C‐b:IT2F blend film. In contrast, PBTz4T2C‐a with 4T2C‐a moiety exhibits suitable crystallinity with a lower the highest occupied molecular orbital level, higher film absorption coefficient, and charge mobilities, resulting in a much higher power conversion efficiency of 11.02%. This research demonstrates that the molecular conformation is of great importance to be considered for developing high‐performance polymer donors. [ABSTRACT FROM AUTHOR]

Obtaining a large open‐circuit voltage (VOC) and high short‐circuit current density (JSC) simultaneously is important in improving power conversion efficiency (PCE) of organic photovoltaics. The ternary strategy with using a higher lowest unoccupied molecular orbital (LUMO) level nonfullerene acceptor (NFA) guest can achieve increased VOC, yet JSC is decreased or maintained, so it's still a challenge to offer increased VOC and JSC values concurrently via the newly presented VOC‐increased ternary strategy. To overcome this issue, a new narrow bandgap NFA TT‐S‐4F is reported by introducing 3,6‐dimethoxylthieno[3,2‐b]thiophene (TT) as π‐spacers to connect electron‐rich core with terminal groups, so as to upshift the LUMO level and extend π‐system. When adding 10% TT‐S‐4F into binary system based on PTB7‐Th:IEICO‐4F, the higher‐LUMO‐level of TT‐S‐4F, the increased charge mobilities, the reduced trap‐assisted combination loss, and a finer nanofiber structure and increased phase separation size are obtained, which simultaneously promotes JSC, VOC, and fill factor (FF), thus obtaining an optimal PCE (12.5% vs 11.5%). This work illustrates that an extending conjugated backbone with large π‐spacers and inclusion of alkylthiophenyl side‐chains is a concept to synthesize NFA guests for use on the VOC‐increased ternary strategy that enables to realize simultaneously increased JSC, VOC, and FF. [ABSTRACT FROM AUTHOR]

Two wide‐bandgap (WBG) conjugated polymers (PBPD‐p and PBPD‐m) based on phenyl‐substituted benzodithiophene (BDT) with the different substitution position of the alkyl side chain and benzodithiophene‐4,8‐dione (BDD) units are designed and synthesized to investigate the influence of alkyl substitution position on the photovoltaic performance of polymers in polymer solar cells (PSCs). The thermogravimetric analysis, absorption spectroscopy, molecular energy level, X‐ray diffraction, charge transport and photovoltaic performance of the polymers are systematically studied. Compared with PBPD‐p, PBPD‐m exhibits a slight blue‐shift but a deeper highest occupied molecular orbital (HOMO) energy level, a tighter alkyl chain packing and a higher hole mobility. The PBPD‐m‐based PSCs blended with acceptor IT‐4F shows a higher power conversion efficiency (PCE) of 11.95% with a high open‐circuit voltage (Voc) of 0.88 V, a short‐circuit current density (Jsc) of 19.76 mA cm−2 and a fill factor (FF) of 68.7% when compared with the PCE of 6.97% with a Voc of 0.81 V, a Jsc of 15.97 mA cm−2 and an FF of 53.9% for PBPD‐p. These results suggest that it is a feasible and effective strategy to optimize photovoltaic properties of WBG polymers by changing the substitution position of alkyl side chain in PSCs. [ABSTRACT FROM AUTHOR]

Molecular structural modifications are utilized to improve the short-circuit current (J SC ) of high-voltage organic photovoltaics (OPVs). Herein, the classic non-fullerene acceptor (NFA), BTA3, is chosen as a benchmark, with BTA3b containing the linear alkyl chains on the middle core and JC14 fusing thiophene on the benzotriazole (BTA) unit as a contrast. The photovoltaic devices based on J52-F: BTA3b and J52-F: JC14 achieve wider external quantum efficiency responses with band edges of 730 and 800 nm, respectively than that of the device based on J52-F: BTA3 (715 nm). The corresponding  J SC increases to 14.08 and 15.78 mA cm -2 , respectively, compared to BTA3 (11.56 mA cm -2 ). The smaller Urbach energy and higher electroluminescence efficiency guarantee J52-F: JC14 a decreased energy loss (0.528 eV) and a high open-circuit voltage (V OC ) of 1.07 V. Finally, J52-F: JC14 combination achieves an increased power conversion efficiency (PCE) of 10.33% than that of J52-F: BTA3b (PCE = 9.81%) and J52-F: BTA3 (PCE = 9.04%). Overall, the research results indicate that subtle structure modification of NFAs, especially introducing fused rings, is a simple and effective strategy to extend the photoelectric response, boosting the  J SC and ensuring a high V OC beyond 1.0 V., (© 2022 Wiley-VCH GmbH.)

Abstract: A series of quinoxaline‐based conjugated polymers, in which the electron‐donating benzodithiophene (BDT) unit is linked to the electron‐accepting 6,7‐difluorinated quinoxaline (DFQ) derivatives by a thiophene bridge, is synthesized. To investigate their effects on the intrinsic properties of polymers, strong electron‐withdrawing trifluoromethyl (CF3) groups were incorporated into the meta‐position of the phenyl ring at the 2,3‐positions of the DFQ unit of the reference polymer, labelled PEhB‐FQx, to yield the target polymer PEhB‐FQxCF3. In addition, the 2‐ethylhexyloxy substituents on the BDT donor in PEhB‐FQxCF3 are changed to the more planar 2‐ethylhexyl thiophene units to produce another target polymer PThB‐FQxCF3. Owing to the significant contributions of the CF3 moiety, PEhB‐FQxCF3 exhibits quite discernible optical and electrochemical properties along with significant enhancement in photovoltaic performances compared to the reference polymer PEhB‐FQx. Furthermore, the incorporation of the alkylthienyl side chains on the BDT moiety confers on the resultant PThB‐FQxCF3 to possess the maximum power conversion efficiency of 7.26% with an open circuit voltage of 0.88 V, short‐circuit current density of 12.20 mA cm−2, and fill factor of 67.80%. [ABSTRACT FROM AUTHOR]

Abstract: Three novel copolymers based on zigzag naphthodithiophene (zNDT) with different aromatic rings as π bridges and different core side substitutions are designed and synthesized (PzNDT‐T‐1,3‐bis(4‐(2‐ethylhexyl)‐thiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]‐dithiophene‐4,8‐dione (BDD), PzNDT‐TT‐BDD, and PzNDTP‐T‐BDD, respectively). The 2D conjugation structure and molecular planarity of the polymers can be effectively altered through the modification of conjugated side chains and π‐bridges. These alterations contribute to the variation in energy levels, light absorption capacity, and morphology compatibility of the polymers. When blended with the nonfullerene acceptor (2,2′‐[(4,4,9,9‐tetrahexyl‐4,9‐dihydro‐sindaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis[methylidyne(3‐oxo‐1H‐indene‐2,1(3H)‐diylidene)]]bis‐propanedinitrile) (IDIC), PzNDT‐T‐BDD exhibits the highest power conversion efficiency (PCE) of 9.72% among the three polymers. This result can be attributed to its superior crystallinity and more obvious face‐on orientation in blending film. PzNDT‐TT‐BDD and PzNDTP‐T‐BDD present PCE values of 8.20% and 4.62%, respectively. The alteration of polymer structure, particularly the modification of conjugated side chains and π‐bridges, is an effective strategy for designing NDT‐based polymers with high photovoltaic performance and potential applications in fullerene‐free solar cells. [ABSTRACT FROM AUTHOR]

Abstract: Naphthalene diimide (NDI) dimers, NDI–Ph–NDI with a phenyl linker and NDI–Xy–NDI with a xylene linker, are designed and synthesized. The influence of the xylene and phenyl linkers on optical properties, electrochemical properties, morphology, and device performance is systematically investigated. Non‐fullerene organic solar cells (OSCs) with NDI–Ph–NDI show poor device efficiency due to aggregation of polymer chains and/or NDI dimers caused by the highly planar structure of NDI–Ph–NDI. Although NDI–Xy–NDI is a non‐planar structure, uniform surface morphology and weak bimolecular recombination lead to high power conversion efficiencies of 3.11%, which is the highest value in non‐fullerene OSCs with NDI small molecules. [ABSTRACT FROM AUTHOR]

Abstract: Push–pull‐type conjugated polymers applied in organic electronics do not always contain a perfect alternation of donor and acceptor building blocks. Misscouplings can occur, which have a noticeable effect on the device performance. In this work, the influence of homocoupling on the optoelectronic properties and photovoltaic performance of PDTSQxff polymers is investigated, with a specific focus on the quinoxaline acceptor moieties. A homocoupled biquinoxaline segment is intentionally inserted in specific ratios during the polymerization. These homocoupled units cause a gradually blue‐shifted absorption, while the highest occupied molecular orbital energy levels decrease only significantly upon the presence of 75–100% of homocouplings. Density functional theory calculations show that the homocoupled acceptor unit generates a twist in the polymer backbone, which leads to a decreased conjugation length and a reduced aggregation tendency. The virtually defect‐free PDTSQxff affords a solar cell efficiency of 5.4%, which only decreases substantially upon incorporating a homocoupling degree over 50%. [ABSTRACT FROM AUTHOR]

Abstract: A nonconjugated, alcohol‐soluble zwitterionic polymer, poly(sulfobetaine methacrylate) (denoted by PSBMA), is employed as cathode interfacial layer (CIL) in polymer solar cells (PSCs) based on PTB7‐Th:PC71BM. Compared with the control device without CIL, PSCs with PSBMA CILs show significant enhancement on the resulting performance, and the highest power conversion efficiency (PCE) of 8.27% is achieved. Under parallel conditions, PSCs with PSBMA as CIL show comparable performance than those with widely used poly[(9,9‐bis(30‐(N,N‐dimethylamino)propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐ioctylfluorene)] as CIL. The polar groups of PSBMA not only provide a solvent orthogonal solubility in the process of preparation of the devices but also lead to interfacial dipole to the electrode, which promises a better energy level alignment. In addition, PSBMA‐based devices show better abilities of hole blocking. These results indicate that the zwitterionic polymer PSBMA should be a promising CIL in PSC‐based narrow‐bandgap polymers. [ABSTRACT FROM AUTHOR]

Abstract: The design and synthesis of three n‐type conjugated polymers based on a naphthalene diimide–thiophene skeleton are presented. The control polymer, PNDI‐2HD, has two identical 2‐hexyldecyl side chains, and the other polymers have different alkyl side chains; PNDI‐EHDT has a 2‐ethylhexyl and a 2‐decyltetradecyl side chain, and PNDI‐BOOD has a 2‐butyloctyl and a 2‐octyldodecyl side chain. These copolymers with different alkyl side chains exhibit higher melting and crystallization temperatures, and stronger aggregation in solution, than the control copolymer PNDI‐2HD that has the same side chain. Polymer solar cells based on the electron‐donating copolymer PTB7‐Th and these novel copolymers exhibit nearly the same open‐circuit voltage of 0.77 V. Devices based on the copolymer PNDI‐BOOD with different side chains have a power‐conversion efficiency of up to 6.89%, which is much higher than the 4.30% obtained with the symmetric PNDI‐2HD. This improvement can be attributed to the improved charge‐carrier mobility and the formation of favorable film morphology. These observations suggest that the molecular design strategy of incorporating different side chains can provide a new and promising approach to developing n‐type conjugated polymers. [ABSTRACT FROM AUTHOR]

Abstract: Polymer solar cells have received considerable attention due to the advantages of low material cost, tunable band gaps, ultralight weight, and high flexible properties, and they have been a promising organic photovoltaic technology for alternative non‐renewable fossil fuels for the past decade. Inspired by these merits, numerous state‐of‐the‐art organic photovoltaic materials have been constructed. Among them, indaceno‐based polymer materials have made an impact in obtaining an impressive power conversion efficiency of more than 11%, which shows the momentous potential of this class of materials for commercial applications. In this review, recent progress of indaceno‐based organic polymer solar cells are reviewed, and the structure–property device performance correlations of the reported materials are highlighted. Then, common regularities of these successful cases are collected, and encouraging viewpoints on the further development of more exciting indaceno‐based organic photovoltaic materials are provided. [ABSTRACT FROM AUTHOR]

Abstract: A novel n‐type polymer of PTDI‐T based on asymmetric rylene diimide and thiophene is designed and synthesized. The highest power conversion efficiency of 4.70% is achieved for PTB7‐Th:PTDI‐T‐based devices, which is obviously higher than those of the analogue polymers of PPDI‐2T and PDTCDI. When using PBDB‐T as a donor, an open‐circuit voltage (VOC) as high as 1.03 V is obtained. The results indicate asymmetric rylene diimide is a kind of promising building block to construct n‐type photovoltaic polymers. [ABSTRACT FROM AUTHOR]

Abstract: Benzo[c][1,2,5]oxadiazole (BO) moiety is a strong electron‐withdrawing unit compared to benzo[c][1,2,5]thiadiazole (BT). It is usually introduced as an acceptor to construct narrow band‐gap donor‐acceptor (D‐A) materials. Herein, the π‐extended conjugated moiety dithieno[3′,2′:3,4″;2,3″:5,6]benzo[1,2‐c][1,2,5]oxadiazole (BOT) was adopted as the acceptor moiety to design D‐A polymers. Considering the more extended π‐conjugated molecular system of BOT compared to the BO unit, a narrower optical band‐gap is expected for BOT‐based IDT polymer (PIDT‐BOT). Unexpectedly, the UV‐vis absorption spectra of PIDT‐BOT films display a great hypochromatic shift of about 60 nm compared to a BO‐based analog (PIDT‐BO). The optical band‐gaps of the materials are broadened from 1.63 eV (PIDT‐BO) to 2.00 eV (PIDT‐BOT) accordingly. Although the range of external quantum efficiency (EQE) of PIDT‐BOT‐based polymer solar cell (PSC) devices is not as wide as for PIDT‐BO‐based devices, the EQE response intensities of the PIDT‐BOT based device are evidently high. As a result, PSC devices based on PIDT‐BOT reveal the best power conversion efficiency at 6.08%. [ABSTRACT FROM AUTHOR]

Abstract: Main chain donor and side chain acceptor (D‐s‐A) copolymers are an important branch of the D–A copolymer family. However, the development of D‐s‐A copolymers significantly falls behind the alternative D–A copolymers, especially for organic solar cells, because a breakthrough in device performance is not yet obtained with a reported power conversion efficiency (PCE) of 2%–4%. Herein, a newly developed D‐s‐A copolymer PDRCNBDT, bearing 2‐(1, 1‐dicyanomethylene) rhodanine pendant group as the donor material, delivers a high PCE of 5.3% for nonfullerene solar cells. To the best of our knowledge, this is the best value reported for D‐s‐A copolymers to date. The improved PCE is observed to be associated with a very small energy loss (Eloss) of 0.57 eV, accompanied by a high open‐circuit voltage (Voc) of 1.015 eV. It is important to note that this efficient D‐s‐A copolymer is employed in organic solar cells (OSCs), free of additive and annealing treatments. [ABSTRACT FROM AUTHOR]

Abstract: Recently, research on nonfullerene acceptors in organic solar cells has gradually become a hot topic due to such superior characteristics of light absorption and energy‐level‐convenient manipulation, multiformity of the photoactive material structures, as well as the extensive area in production compared to the fullerene derivatives. However, the nonfullerene acceptors evolved slowly before 2012 and, as a matter of fact, the power conversion efficiency values could only bear 2.0%. Strikingly, nonfullerene acceptors have developed at a fast pace since 2013, with the best device performance of 13.1% now. In this review, recent research progress on nonfullerene acceptors, including small molecules and polymers, are sorted and summarized on the basis of the different characteristics. [ABSTRACT FROM AUTHOR]

Two donor-acceptor (D-A) type conjugated copolymers, P1 and P2, are designed and synthesized. A classical benzothiadiazole acceptor is used to replace a thiophene unit in the polymer chain of P1 to obtain P2 terpolymer. Compared with P1, P2 exhibits broader absorption spectra, higher absorption coefficient, deeper lowest unoccupied molecular orbital level, and a relatively lower band gap. As a result, the P2-based solar cell exhibits a high power conversion efficiency (PCE) of 6.60%, with a short-circuit current (Jsc) of 12.43 mA cm-2, and a fill factor (FF) of 73.1%, which are higher than those of the P1-based device with a PCE of 4.70%, aJsc of 9.43 mA cm-2, and an FF of 61.6%. [ABSTRACT FROM AUTHOR]

This communication reports the synthesis of two novel chalcogen-atom-annulated perylene diimide (PDI) trimers with twisted structures, TriPDI-S and TriPDI-Se, for efficient nonfullerene polymer solar cells. TriPDI-Se exhibits more compact molecular arrangement due to the stronger intermolecular interactions induced by the selenium atom. This selenium annulation endows TriPDI-Se with improved absorption and crystallinity in its blend film. The resulting devices exhibit enhanced Jsc of 17.15 mA cm−2 and fill factor (FF) of 66.8%, which are much higher than those of TriPDI-S devices ( Jsc = 16.71 mA cm−2; FF = 63.6%). Although TriPDI-Se exhibits lower-lying energy levels, TriPDI-Se devices still obtain a higher Voc of 0.77 V compared to TriPDI-S devices ( Voc = 0.74 V), which is mainly originated from the reduced recombination in the related devices. Finally, the power conversion efficiency is significantly elevated from 7.86% for TriPDI-S devices to 8.82% for TriPDI-Se devices. [ABSTRACT FROM AUTHOR]

The halogen-free solvent additive, 1,4-butanedithiol (BT) has been incorporated into PTB7-Th:PC71BM, leading to higher power conversion efficiency (PCE) value as well as substantially enhanced thermal stability, as compared with the traditional 1,8-diiodooctane (DIO) additive. More importantly, the improved thermal stability after processing with BT contributes to a higher glass transition temperature ( Tg) of PTB7-Th, as determined by dynamic mechanical analysis. After thermal annealing at 130 °C in nitrogen atmosphere for 30 min, the PCE of the specimen processed with BT reduces from 9.3% to 7.1%, approaching to 80% of its original value. In contrast, the PCE of specimens processed with DIO seriously depresses from 8.3% to 3.8%. These findings demonstrate that smart utilization of low-boiling-point solvent additive is an effective and practical strategy to overcome thermal instability of organic solar cells via enhancing the Tg of donor polymer. [ABSTRACT FROM AUTHOR]

Variations in the open-circuit voltage ( Voc) of ternary organic solar cells are systematically investigated. The initial study of these devices consists of two electron-donating oligomers, S2 (two units) and S7 (seven units), and the electron-accepting [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and reveals that the Voc is continuously tunable due to the changing energy of the charge transfer state ( Ect) of the active layers. Further investigation suggests that Voc is also continuously tunable upon change in Ect in a ternary blend system that consists of S2 and its corresponding polymer (P11):PC71BM. It is interesting to note that higher power conversion efficiencies can be obtained for both S2:S7:PC71BM and S2:P11:PC71BM ternary systems compared with their binary systems, which can be ascribed to an improved Voc due to the higher Ect and an improved fill factor due to the improved film morphology upon the incorporation of S2. These findings provide a new guideline for the future design of conjugated polymers for achieving higher performance of ternary organic solar cells. [ABSTRACT FROM AUTHOR]

Two novel copolymers based on benzothiadiazole (BT) or difluorobenzothiadizole (ffBT) with 2,2′-(perfluoro-1,4-phenylene)dithiophene (2TPF4), namely PBT-2TPF4 and PffBT-2TPF4, are synthesized for applications in polymer solar cells (PSCs). A noticeably high open-circuit voltage ( Voc) of 1.017 and 0.87 V are achieved for PffBT-2TPF4 and PBT-2TPF4-based devices, respectively. Although only a moderate efficiency (5.7%) of PBT-2TPF4-based devices is obtained, it is first demonstrated that 2TPF4 is a promising acceptor block for construction of the donor copolymers which possess high Voc, prominent crystallinity, and long-term stability, simultaneously. Besides, two thienyl flanking the tetrafluorophenylene can decrease torsion angle between conjugated units, resulting in a high coplanar structure of copolymers to enhance their charge carrier mobility. The findings may open a promising and practical way to accelerate the commercialization of PSCs by developing a series of new donor copolymers for efficient and long-term stable thickness bulk heterojunction PSCs. [ABSTRACT FROM AUTHOR]

Systematic optimization of the chemical structure of wide-bandgap (≈2.0 eV) 'donor-acceptor' copolymers consisting of indacenodithiophene or indacenodithieno[3,2- b]thiophene as the electron-rich unit and thieno[3,4- c]pyrrole-4,6-dione as the electron-deficient moiety in terms of alkyl side chain engineering and distance of the electron-rich and electron-deficient monomers within the repeat unit of the polymer chain results in high-performance electron donor materials for organic photovoltaics. Specifically, preliminary results demonstrate extremely high open circuit voltages ( Vocs) of ≈1.0 V, reasonable short circuit current density ( Jsc) of around 11 mA cm−2, and moderate fill factors resulting in efficiencies close to 6%. All the devices are fabricated in an inverted architecture with the photoactive layer processed by doctor blade equipment, showing the compatibility with roll-to-roll large-scale manufacturing processes. From the correlation of the chemical structure-optoelectronic properties-photovoltaic performance, a rational guide toward further optimization of the chemical structure in this family of copolymers, has been achieved. [ABSTRACT FROM AUTHOR]

A new broad bandgap and 2D-conjugated D-A copolymer, PBDTBTz-T, based on bithienyl-ben zodithiophene donor unit and bithiazole (BTz) acceptor unit, is designed and synthesized for the application as donor material in polymer solar cells (PSCs). The polymer possesses highly coplanar and crystalline structure with a higher hole mobility and lower HOMO energy level which is beneficial to achieve higher open circuit voltage (Voc) of the PSCs with the polymer as donor. The PSCs based on PBDTBTz-T:PC71BM blend film with a lower PC71BM content of 40% demonstrate a power conversion efficiency (PCE) of 6.09% with a relatively higher Voc of 0.92 V. These results indicate that the lower HOMO energy level of the BTz-based D-A copolymer is beneficial to a high Voc of the PSCs. The polymer, with highly coplanar and crystalline structure, can effectively reduce the content of fullerene acceptor in the active layer and can enhance the absorption and PCE of the PSCs. [ABSTRACT FROM AUTHOR]

The molecular weight of an electron donor-conjugated polymer is as essential as other well-known parameters in the chemical structure of the polymer, such as length and the nature of any side groups (alkyl chains) positioned on the polymeric backbone, as well as their placement, relative strength, the ratio of the donor and acceptor moieties in the backbone of donor-acceptor (D-A)-conjugated polymers, and the arrangement of their energy levels for organic photovoltaic performance. Finding the 'optimal' molecular weight for a specific conjugated polymer is an important aspect for the development of novel photovoltaic polymers. Therefore, it is evident that the chemistry of functional conjugated polymers faces major challenges and materials have to adopt a broad range of specifications in order to be established for high photovoltaic performance. In this review, the approaches followed for enhancing the molecular weight of electron-donor polymers are presented in detail, as well as how this influences the optoelectronic properties, charge transport properties, structural conformation, morphology, and the photovoltaic performance of the active layer. [ABSTRACT FROM AUTHOR]

Two hydrophilic conjugated polymers, PmP-NOH and PmP36F-NOH, with polar diethanol­amine on the side chains and main chain structures of poly( meta-phenylene) and poly( meta-phenylene- alt-3,6-fluorene), respectively, are successfully synthesized. The films of PmP-NOH and PmP36F-NOH show absorption edges at 340 and 343 nm, respectively. The calculated optical bandgaps of the two polymers are 3.65 and 3.62 eV, respectively, the largest ones so far reported for hydrophilic conjugated polymers. PmP-NOH and PmP36F-NOH also possess deep-lying highest occupied molecular orbital levels of −6.19 and −6.15 eV, respectively. Inserting PmP-NOH and PmP36F-NOH as a cathode interlayer in inverted polymer solar cells with a PTB7/PC71BM blend as the active layer, high power conversion efficiencies of 8.58% and 8.33%, respectively, are achieved, demonstrating that the two hydrophilic polymers are excellent interlayers for efficient inverted polymer solar cells. [ABSTRACT FROM AUTHOR]

Designing the bulk-heterojunction structure (binary, ternary and quaternary) is of great fundamental interest for relaxing the trade-off between open circuit voltage (V oc ) and short circuit current density (   J sc ). Herein, a new quaternary blended material system is reported with a nonfullerene and PC 71 BM as the third and fourth component of PBDB-T:ITCT, leading to simultaneously increased V oc and J sc and maintained fill factor (FF). The guest IT-T-IC has upshifted LUMO energy level helping to obtain a higher V oc . The further addition of 0.2 PC 71 BM as the fourth component yields an even higher V oc because the LUMO of PC 71 BM is higher than that of ITCT. Interestingly, the blend of PC 71 BM leads to the formation of an unprecedented neuron-like morphology, which acts as new centers not only performing light absorption and charge separation but also charge transport through their surrounding donor and acceptor fibers. The increased hole and electron mobilities and the reduced bimolecular loss results in an even larger J sc and FF. These results indicate that a combination using a structurally similar higher-LUMO-level non-fullerene acceptor and PCBM is a simple yet effective quaternary material approach to simultaneously increase V oc and J sc while maintaining FF, improving final device performance., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A wide‐bandgap conjugated polymer, PNQx‐2F2T, based on a ring‐fused unit of quinoxalino[6,5‐f  ]quinoxaline (NQx), is synthesized for use as electron donor in polymer solar cells (PSCs). The polymer shows intense light absorption in the range from 300 to 740 nm and favorable energy levels of frontier molecular orbitals. The polymer has afforded decent device performance when blended with either fullerene‐based acceptor [6,6]‐phenyl‐C71‐butylric acid methyl ester ([70]PCBM) or non‐fullerene acceptor 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone‐methyl))‐5,5,11,11‐tetrakis(4‐n‐hexylphenyl)‐dithieno[2,3‐d: 2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IT‐M). The highest PCEs of 7.9% and 7.5% have been achieved for [70]PCBM or IT‐M based PSCs, respectively. Moreover, the influence of molecular weight of PNQx‐2F2T on solar cell performance has been investigated. It is found that fullerene‐based devices prefer higher polymer molecular weight, while non‐fullerene devices are not susceptible to the molecular weight of PNQx‐2F2T. The device results are extensively explained by electrical and morphological characterizations. This work not only evidences the potential of NQx for constructing high‐performance photovoltaic polymers but also demonstrates a useful structure–performance relationship for efficiency enhancement of non‐fullerene PSCs via the development of new conjugated polymers. [ABSTRACT FROM AUTHOR]

A trade‐off between open‐circuit voltage (VOC) and high short‐circuit (JSC) becomes one of the most vital problems limiting further improvement in polymer solar cells' (PSCs) efficiency. In this work, two asymmetric polymer donors PBDT‐F‐2TC and PBDT‐SF‐2TC are designed and synthesized. When blended with a state‐of‐the‐art acceptor IT‐4F with low lowest‐unoccupied molecular orbital level, simultaneously high VOC (up to 0.94 V) and JSC (up to 20.73 mA cm−2) are obtained for both copolymers. Note that the VOC value of 0.94 V is the highest value of PSCs based on IT‐4F reported so far. The simultaneously improved VOC and JSC in resulting devices are discovered from the deep highest‐occupied molecular orbital levels (−5.5 to −5.7 eV) and the hyperchromic effect of the polymers, the small driving force, and the small energy loss during the charge transfer, due to the synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms. More importantly, thanks to the asymmetric 2TC, both PBDT‐F‐2TC‐ and PBDT‐SF‐2TC‐based PSCs can be successfully processed by non‐halogenated solvent 1,2,4‐trimethylbenzene (TMB) to yield device efficiencies of 10.29% and 10.39%, respectively, which are the maximum values for non‐fullerene PSCs fabricated using the eco‐friendly solvent TMB. [ABSTRACT FROM AUTHOR]

The 2D asymmetric benzodithiophene (BDT) unit is used as a donor unit to construct one new polymer PBDTBDD‐Th with benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) as acceptor building block. In comparison to the polymer PBDTsTh‐BDD with a side chain containing a sulfur atom, the devices based on PBDTBDD‐Th/ITIC show better performance due to the introduction of carbon atoms in the side chain, which could weaken the self‐aggregations of polymer chains. As a result, the devices based on PBDTBDD‐Th/ITIC blends yield power conversion efficiencies (PCEs) over 10%, much higher than those based on PBDTsTh‐BDD/ITIC blends (7.09%). The exciton dissociation probabilities (Pdiss) of a device based on PBDTBDD‐Th/ITIC blends is 95.3%, which suggests that the device achieves good exciton dissociation and charge transfer. In general, the polymer PBDTBDD‐Th shows capability to increase the PCEs of polymer solar cells (PSCs) with a non‐fullerene acceptor. Side chain engineering weakens the polymer donor aggregation for high‐performance non‐fullerene polymer solar cells (PSCs), yielding power conversion efficiencies (PCEs) of over 10%. [ABSTRACT FROM AUTHOR]