of these FREAs can be fine-tuned by optimizing the intramolecular charge transfer character while maintaining their key characteristics as efficient electron acceptor materials. [4-6] The FREAs with fine-tailored properties enable the state-of-the-art power conversion efficiencies (PCEs) surpassing 16% in OSCs. [7-12] Among various OSCs, all-polymer solar cells (all-PSCs) employing conjugated polymers as both the electron donors and acceptors, have recently attracted great attention because of some unique advantages including superior stability and mechanical robustness. [13-20] However, only a few polymer acceptors can yield PCEs over 8% till now (see Figure S1 and Table S1, Supporting Information) due to the scarcity of highly electron-deficient building blocks. For instance, classical naphthalene diimide and perylene diimidebased donor-acceptor (D-A)-type polymer acceptors (see Figure S2a, Supporting Information) suffer from poor absorption coefficient in the long-wavelength region and (or) the localized lowest unoccupied molecular orbital (LUMO), which limits the short-circuit currents density (J sc) and V oc , together with Narrow-bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow-bandgap polymer acceptor L14, featuring an acceptor-acceptor (A-A) type backbone, is synthesized by copolymerizing a dibrominated fused-ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A-A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low-lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open-circuit voltage (V oc), which is attributed to a small nonradiative recombination loss (E loss,nr) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V oc , an excellent efficiency of 14.3% is achieved, which is among the highest values for all-polymer solar cells (all-PSCs). The results demonstrate the superiority of narrow-bandgap A-A type polymers for improving all-PSC performance and pave a way toward developing high-performance polymer acceptors for all-PSCs.

the power conversion efficiencies (PCEs) of OSCs show rapid increase and the values have increased to over 18%. [16][17][18][19][20][21][22][23][24] However, owing to the brittle nature of small molecules, [25] the mechanical properties of polymer:NF-SMA blends are generally insufficient and can hardly meet the requirement of stretchable electronics. [26,27] The mechanical imperceptibility of OSCs requires low stiffness and high extensibility for wearable and portable applications. The human skin exhibits a ductility of about 30%, which is the benchmark for skin-wearable devices. [28] Studies have been carried out to determine the mechanical properties of nonfullerene OSCs [29][30][31] and drive the development of stretchable OSCs. [32][33][34][35] For instance, the fracture strain of the well-known PTB7-Th:(3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene) (ITIC) blend films decreased dramatically with the increase of ITIC, and the blend films became significantly stiffer as a result of increased elastic modulus. [26,36] Therefore, methods should be adopted to improve the stretchability and reduce the stiffness of OSCs based on polymer:NF-SMA blends.As the mechanical performance of polymer:NF-SMA blends are often poor, a representative high-efficiency Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of highefficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

This research underscores the importance of considering both miscibility and molecular ordering in the design of higher efficiency polythiophene (PT):nonfullerene solar cells. We find that ITIC-Th1 exhibits proper miscibility and relatively highly ordered molecular packing with PDCBT-Cl, affording the record performance of >12%. Conversely, due to the excessively high miscibility of PDCBT-Cl and Y6, the system exhibits abysmal photovoltaic efficiency of 0.5%. The results provide efficient guidelines for materials matching of higher efficiency PT:nonfullerene solar cells.

Understanding the key factors influencing the mechanical and electrical properties of semiconducting polymers is crucial to the development of stretchable electronics. In this work, a high-mobility diketopyrrolopyrrole-based conjugated polymer with varied number-average molecular weights (M n ) was used as the model system to explore the impact of molecular weight on the electrical and mechanical properties. Higher-M n films are more ductile and stretchable. Both hole mobilities of thin-film transistors and elastic modulus become maximum at a moderate M n of 88 kg/mol. It was found that film continuity, entanglements, and relative degree of crystallinity are critical factors for approaching the best device performance and highest elastic modulus. The transition molecular weight of a given polymer semiconductor is the key to achieving stretchable high-mobility transistors that are practically useful. This work would help to offer guidance to manipulate the mechanical and electrical properties for other polymer semiconductors.

Ternary solar cells comprising both fullerene and nonfullerene acceptors have shown a rapid increase in power conversion efficiency, which holds promise in commercial applications. Despite the rapid progress, there is still a lack of fundamental understanding of the relations between microstructure and (photovoltaic/mechanical) properties in these ternary blend systems. In this work, the dependence of molecular packing, phase separation, mechanical properties, and photovoltaic performance on acceptor composition of a recently certificated ternary system is thoroughly investigated by combined scattering and microscopy characterizations. It is demonstrated that incorporating a small amount (20% by weight) PC71BM to the PM6:N3 binary blend can afford the best device efficiency and the highest ductility simultaneously. This maximum performance is due to the optimized molecular order, orientational texture, and phase separation. Additionally, increasing the amount of PC71BM results in higher elastic modulus, as probed by two distinct methods. A more crucial observation is that the elastic modulus of ternary blends can be well captured by an extended Halpin–Tsai model. This finding is expected to enable the prediction of the elastic modulus of various kinds of ternary blends that are widely used in solar cells and other electronics.

Regulating molecular structure to optimize the active layer morphology is of considerable significance for improving the power conversion efficiencies (PCEs) in organic solar cells (OSCs). Herein, we demonstrated a simple ternary copolymerization approach to develop a terpolymer donor PM6‐Tz20 by incorporating the 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz, 20 mol%) unit into the backbone of PM6 (PM6‐Tz00). This method can effectively tailor the molecular orientation and aggregation of the polymer, and then optimize the active layer morphology and the corresponding physical processes of devices, ultimately boosting FF and then PCE. Hence, the PM6‐Tz20: Y6‐based OSCs achieved a PCE of up to 17.1% with a significantly enhanced FF of 0.77. Using Ag (220 nm) instead of Al (100 nm) as cathode, the champion PCE was further improved to 17.6%. This work provides a simple and effective molecular design strategy to optimize the active layer morphology of OSCs for improving photovoltaic performance.

The rapid development of low bandgap polymer acceptors has promoted the efficiency up to ≈17% for all-polymer solar cells (all-PSCs). Nevertheless, the polymeric blend film, core to the photoelectric conversion of all-PSCs, has not been thoroughly understood in terms of the influence and regulatory factors of mechanical properties, which hinders the advances in flexible and wearable applications. Herein, a range of characterization methods is combined to investigate the mechanical properties, miscibility, and film microstructure of the blends based on several representative polymer donors (PTzBI-Si, PTVT-T, PM6 and PTQ10) and a benchmark polymer acceptor N2200, and to further reveal the miscibility-property relationships of the miscibility property. The results stress that fracture behaviors and elastic moduli of these blends with varied compositions show different changing trends, which are affected by molecular interactions and aggregated structure of the blends. The elastic moduli of the four all-polymer blends can be nicely predicted by different models that are deduced from macromolecular mechanics. Most crucially, the correlations between elastic modulus, morphology, and miscibility of all-polymer blends are elucidated for the first time. The derived relationships is validated with another high-efficiency blend and will be the key to the successful fabrication of mechanically robust and stretchable all-PSCs with high efficiency.

Developing novel solid additives has been regarded as a promising strategy to achieve highly efficient organic solar cells with good stability and reproducibility. Herein, a small molecule, 2,2′-(perfluoro-1,4-phenylene)dithiophene (DTBF), designed with high volatility and a strong quadrupole moment, is applied as a solid additive to implement active layer morphology control in organic solar cells. Systematic theory simulations have revealed the charge distribution of DTBF and its analog and their non-covalent interaction with the active layer materials. Benefitting from the more vital charge-quadrupole interaction, the introduction, and volatilization of DTBF effectively induced more regular and condensed molecular packing in the active layer, leading to enhanced photoelectric properties. Thus, high efficiency of over 17% is obtained in the DTBF-processed devices, which is higher than that of the control devices. Further application of DTBF in different active layer systems contributed to a deeper comprehension of this type of additive. This study highlights a facile approach to optimizing the active layer morphology by finely manipulating the quadrupole moment of volatile solid additives.