Steric Hindrance Induced Low Exciton Binding Energy Enables Low-Driving-Force Organic Solar Cells

Exciton binding energy ( E b ) has been regarded as a critical parameter in charge separation during photovoltaic conversion. Minimizing the E b of the photovoltaic materials can facilitate the exciton dissociation in low-driving force organic solar cells (OSCs) and thus improve the power conversion efficiency (PCE), nevertheless, diminishing the E b with deliberate design principles remains a significant challenge. Herein, bulky side chain as steric hindrance structure was inserted into Y-series acceptors to minimize the E b by modulating the intra- and inter-molecular interaction. Theoretical and experimental results indicate that steric hindrance-induced weaker intra-molecular interactions but stronger inter-molecular interaction can strengthen the molecular polarizability, increase the overlap of electronic orbitals between molecules and facilitate delocalized pathway charge transfer, thereby resulting in a low E b . The conspicuously reduced E b obtained in Y-ChC5 with pinpoint steric hindrance modulation can minimize the detrimental effects on exciton dissociation in low-driving-force OSCs, achieving a remarkable PCE of 19.1% with over 95% internal quantum efficiency. Our study provides a new molecular design rationale to reduce the exciton binding energy.