Halogenation‐Regulated Electron‐Phonon Coupling Stabilizes Delocalized Excitation Toward High‐Performance Organic Solar Cells

Abstract

ABSTRACT Non‐radiative energy loss remains a critical limitation to further improving the efficiency of organic solar cells (OSCs), as charge generation is typically governed by charge‐transfer (CT) states that intrinsically suffer from non‐radiative recombination. Intra‐moiety excitation (i‐EX), a delocalized excitation capable of dissociating within acceptor domains, provides an alternative pathway that can reduce CT‐mediated loss. However, its molecular regulation and intrinsic photophysical characteristics remain unclear. In this work, the competition between CT‐ and i‐EX‐mediated charge generation pathways is systematically regulated by halogen substitution, where enhanced i‐EX contribution suppresses CT‐related spin‐triplet recombination and substantially reduces non‐radiative energy loss. Increasing halogen atomic numbers from fluorine to iodine promotes i‐EX formation while suppressing CT participation. Reduced CT involvement diminishes spin‐triplet recombination, leading to decreased non‐radiative energy loss. Meanwhile, heavier halogen substitution weakens electron–phonon coupling and stabilizes delocalized excitations against vibration‐induced dissipation, enabling efficient i‐EX‐mediated charge generation at room temperature. By balancing i‐EX generation and CT processes, the brominated acceptor achieves 20.3% power conversion efficiency in binary OSCs and over 21% in ternary OSCs. This work reveals a clear structure‐property relationship for stabilizing delocalized excitations and minimizing energy loss toward high‐efficiency OSCs.