Engineering ultrafast exciton dynamics to boost organic photovoltaic performance

State-of-the-art organic photovoltaic (OPV) devices are based on Y-type acceptors, with power conversion efficiencies now exceeding 20%. However, the basic structure-photophysics-performance relationship of these materials remains unclear, hindering rational material development and engineering. Here we investigate a broad range of Y-type acceptors using a combination of experimental and theoretical studies. We first show that a transient electroabsorption (TEA) signal is universal in neat Y-type acceptor films upon photoexcitation, which is caused by the formation of intermolecular charge-transfer (ICT) states in tightly packed molecular aggregates (i.e. ordered regions of the film). Tracking the TEA signal growth dynamics can monitor the migration of excitons from disordered to ordered regions in various Y-type acceptor films on the sub-picosecond timescale. Importantly, our results reveal that Y-type acceptors with moderately reduced intermolecular interaction strength can generally achieve faster exciton migration, better structural uniformity and higher device performance, thereby providing insights for future OPV material development and engineering.