High-performance binary organic solar cells by simultaneously enhancing exciton diffusion and charge transport in small molecule acceptors

Efficient exciton diffusion and charge transport play a crucial role in boosting the photovoltaic performance of organic solar cells (OSCs). Herein, a feasible strategy is demonstrated to simultaneously enhance long-range exciton migration and charge transport for small molecule acceptors (SMAs) by chlorination of terminal groups, and a chlorinated SMA K3 is developed. Compared to fluorinated K1, K3 exhibits red-shifted and enhanced absorption with improved intramolecular charge transfer effects owing to the longer transition dipole moment, the prolonged exciton diffusion length because of the longer exciton lifetime and larger exciton diffusion coefficient, and stronger charge transport benefitting from more long-range ordered molecular pi-pi packing. By employing polymer PM6 as donor, the K3 based devices deliver more efficient charge generation, less charge recombination, longer nanosecond carrier lifetimes, and the higher and more balanced charge mobilities than K1 based devices. Eventually, the PM6:K3 based binary OSC demonstrates a superior efficiency of 19.27%. This work highlights the importance of chlorination in constructing efficient exciton diffusion and charge transport processes for OSCs and also provides insights into the molecular design of high-performance acceptor materials.