Two Isomeric Benzo[d]imidazole-Based Nonfullerene Acceptors for Organic Photovoltaics

In this study, we designed and synthesized two nonfullerene acceptors (NFAs) based on 1H- and 2H-benzo[d]imidazole (BIz) isomers, named 1-OBIA and 2-OBIA, for organic photovoltaics (OPVs). Whereas 1-OBIA possesses an electron-donating amine-type nitrogen atom and an electron-withdrawing imine-type nitrogen atom, 2-OBIA possesses two electron-withdrawing imine-type nitrogen atoms in the benzimidazole moiety. Owing to its more electron-rich central core, 1-OBIA showed higher HOMO and LUMO energy levels than 2-OBIA. Photovoltaic cells based on 2-OBIA, when paired with a benchmark polymer donor, PM6, provided a power conversion efficiency of 10.3%, which was much higher than that of cells based on 1-OBIA (4.0%). This can be explained by the more efficient hole transfer for the PM6:2-OBIA blend than that for the PM6:1-OBIA blend. Studies on the morphology, thin-film structure, and charge carrier transport properties further validated the high photovoltaic performance of PM6:2-OBIA cells. This study demonstrates the interesting properties and potential of BIz-based NFAs for OPVs.