Open-circuit voltage (VOC) is an important parameter in determining the performance of polymer solar cells (PSCs). Given the desire for superior VOC values in PSCs, we have designed and synthesized a series of ‘medium bandgap’ donor–acceptor (D–A) copolymers containing carbazole (Cz) and quinoxaline (Qx) (PCzDT-Qx, PCzDT-fQx, and PCzDT-ffQx). As a result of their deep-lying HOMO levels (−5.45 to −5.61 eV), high VOC values are achieved in PSCs with the resulting copolymers, despite the expense of short-circuit current density (JSC) and fill factor (FF) parameters. In this study, in addition to the best power-conversion efficiency (PCE) of up to 4.03% from PCzDT-fQx-based on PSCs, we have demonstrated a VOC value exceeding 1.0 V with PSCs of PCzDT-ffQx, which is among the highest VOC values achieved to date. Moreover, a comprehensive investigation on the mechanism of charge recombination and transport characteristics can determine a clear structure–property correlation in this class of molecules, which is helpful for designing better materials with maximum VOC without scarifying other key photovoltaic parameters.
