D–A Copolymer Donor Based on Bithienyl Benzodithiophene D‑Unit and Monoalkoxy Bifluoroquinoxaline A‑Unit for High-Performance Polymer Solar Cells

Molecular frontier orbital energy level and aggregation behavior regulation of polymer donors are feasible ways to improve the photovoltaic performance of polymer solar cells (PSCs). Here, we design and synthesize a new D–A copolymer donor PBQ10 based on bithienyl benzodithiophene D-unit and monoalkoxy-substituted bifluoroquinoxaline A-unit, which shows an obviously downshifted highest occupied molecular orbital energy level in comparison with the control polymer PBQ7 with a dialkoxyphenyl substituent on the bifluoroquinoxaline A-unit. Moreover, PBQ10 exhibits more preferential face-on molecular orientation and tighter π–π stacking in the vertical direction of the substrate than PBQ7, which significantly improves the hole mobility of PBQ10 to 5.22 × 10^–4 cm² V^–1 s^–1 in comparison with that (1.71 × 10^–4 cm² V^–1 s^–1) of PBQ7. As a result, the PBQ10-based PSC with Y6 as the acceptor demonstrates an impressive power conversion efficiency (PCE) of 16.34% with simultaneously increased open-circuit voltage and fill factor, which is significantly increased compared with the PBQ7-based PSC with a PCE of 13.45% and is one of the highest PCEs in binary PSCs. The result suggests that rational side-chain optimization of the polymer donor is an efficient way to regulate the molecular energy level and self-assembly feature and thus to improve the PCE of PSCs.