Impact of z-bridge unit in fully nonfused ring electron acceptor on the photovoltaic performance of organic solar cells

Due to the advantages of simple synthesis and low cost, fully nonfused ring electron acceptors (FN-FREAs) have been developed in recent years. Almost all reported FN-FREAs use thiophene derivatives as z-bridging unit, while other z-bridging units have not yet been developed. Here, an FN-FREA M01 with 2,5-bis(alkyloxy)phenylene as the z-bridge unit is designed and synthesized to compare its photovoltaic property with an analogue M05 which uses 3,4-bis(alkyloxy)thiophene as the z-bridge unit. Although the only variation between M01 and M05 is the z-bridge unit, these two FN-FREAs demonstrate significantly different photovoltaic performances. Organic solar cells (OSCs) based on M01 exhibit a high power conversion efficiency of 13.70%, while M05 based one shows a much lower efficiency of 3.96% with a markedly reduced short-circuit current density. Fluorescence, Raman, electron paramagnetic resonance, and femtosecond time-resolved transient absorption spectroscopy prove that M05 tends to be oxidized to form radical cations which aggravates exciton quenching and charge carrier recombination in the active layer, while in M01 such radical cations are absent. All in all, we have demonstrated that the z-bridging unit of FN-FREAs is pivotal to the photovoltaic performance and provided a useful design strategy to low-cost, high-efficiency, and chemically stable acceptor materials.