Modulating morphology via side-chain engineering of fused ring electron acceptors for high performance organic solar cells
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In this work, four fused ring electron acceptors (FREAs), 2F-C5, 2F-C6, 2F-C8 and 2F-C10, are developed to investigate the effect of side-chain size on the molecular properties and photovoltaic performance of FREA systematically. The elongation of side-chains in the FREAs not only improves their solubility in the processing solvent, but also enhances their miscibility with the donor PBDB-T. It helps the FREA diffuse into the donor PBDB-T during film-formation, thus leading to the decrease in domain size and domain purity from PBDB-T:2F-C5 to PBDB-T:2F-C10 blend films in sequence. The smaller domain size affords more D/A interfaces to benefit exciton dissociation and inhibit monomolecular recombination. However, severe bimolecular recombination occurs when the domain purity decreases to a critical point. Due to the dual function of the increment of side-chain length, both short-circuit current density (JSC) and fill factor (FF) of devices exhibit an evolution of first increasing then decreasing from 2F-C5, 2F-C6, 2F-C8 to 2F-C10 based OSCs. The PBDB-T:2F-C8 based OSCs get a fine balance in morphology with moderate domain size as well as high domain purity simultaneously for the least charge carrier recombination, thus achieving the highest power conversion efficiency of 12.28% with the best JSC (21.27 mA cm−2) and FF (71.96%).
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This work was supported by the National Postdoctoral Program for Innovative Talents (BX201700253), the China Postdoctoral Science Foundation (2017M620068, 2018M630208), the National Natural Science Foundation of China (21673257, 21805288), and the Ministry of Science and Technology (2016YFA0200700).
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