Charge separation boosts exciton diffusion in fused ring electron acceptors

Abstract

Non-fullerene acceptors (NFAs) are highly promising materials for organic photovoltaics (OPVs). Exciton diffusion in NFAs is crucial to their photovoltaic performance, but is not yet well understood. Here we systematically examine exciton diffusion in a fused-ring electron acceptor (IDIC) based on a first-principles framework. We discover that low-energy excitons in disordered IDIC are charge-separated with electrons and holes residing on neighboring molecules, yielding long exciton lifetimes. With low energetic disorder, high exciton density of states (DOS) and long lifetimes, the disordered IDIC is predicted to exhibit large exciton diffusion lengths and high quantum efficiency. The temperature and energy dependences of exciton diffusion are explored and the manner in which various materials properties (exciton energy, DOS, energetic disorder, and phonon frequency) conspire to influence exciton diffusion is elucidated. Finally, we show that dilation could be an effective strategy to increase the exciton diffusion length in IDIC.