Modulating Exciton Delocalization in Organic Solar Cells Through Tailored Local Structures and Dipole Offset of Acceptor

The charge-transfer states are strongly associated with the charge recombination in organic solar cells (OSCs), leading to severe energy losses in relevant devices. Enhancing exciton delocalization could mitigate these losses by facilitating dissociation of excitons into free charge carriers. However, the correlation between exciton delocalization and single-molecule properties is currently unclear, and the influence of exciton delocalization on non-geminate recombination and associated triplet formation is rarely studied. Herein, we demonstrate that a larger offset between the ground- and excited-state dipole moments will facilitate the exciton delocalization of non-fullerene acceptors (NFAs). In addition, the hole transfer efficiency is not solely affected by the population of delocalized excitons. Among the three NFAs studied, only the perfluorinated one, FTh-4F, with moderate exciton delocalization, shows simultaneously suppressed triplet formation and prolonged carrier lifetime, resulting in an impressive power conversion efficiency (PCE) of 20.2% in the derived OSC device. We also highlight the feasibility of applying tailored halogenation on NFAs to modulate exciton delocalization for mitigating non-geminate recombination and triplet formation in OSCs.