Physical insights into non-fullerene organic photovoltaics

Boosted by the fast development of non-fullerene acceptors, organic photovoltaics (OPVs) have achieved breakthrough power conversion efficiencies — in excess of 20% and approaching those of state-of-the-art crystalline silicon photovoltaics. New physical properties, unusual phenomena and critical mechanisms have been uncovered in non-fullerene acceptors and related devices, all contributing to deliver advances in OPV technologies. In this Review, we summarize the photophysics and device physics of non-fullerene-acceptor-based OPVs, with emphasis on the comparison between fullerene and non-fullerene acceptors of the physical processes that affect device performance. We discuss the processes of exciton generation, diffusion, transport and separation and charge recombination in OPVs and present recent interpretations of the physics of non-fullerene-acceptor-based OPVs, looking at how driving energy affects exciton separation and how charge recombination affects voltage loss. Compiling these mechanisms — especially those that can overcome the intrinsic limitations imposed by the energy-gap law — we provide a strategy for minimizing voltage loss and discuss future research directions and challenges in the fundamentals and performance of OPVs, including new modes of operation for non-fullerene-acceptor-based OPVs.