Terminal Fluorination Regulates Molecular Assembly Kinetics toward Optimized Bulk-Heterojunction Morphology.

Understanding how molecular design governs assembly kinetics during film formation is essential for optimizing bulk-heterojunction morphology in organic solar cells (OSCs), yet the molecular mechanism by which terminal-group engineering regulates this process remains poorly understood. Herein, a series of non-fullerene acceptors (NFAs) with progressively reduced terminal fluorination were developed to systematically elucidate the role of terminal fluorination in molecular assembly and photovoltaic performance. Among them, the moderately fluorinated acceptor N3-3F delivered the highest power conversion efficiency (PCE) of 19.11%, achieving an optimal balance among open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF). By integrating in situ absorption spectroscopy with multiscale structural characterization and theoretical calculations, we demonstrate that moderate terminal defluorination regulates molecular assembly kinetics during film formation. The resulting optimized assembly process promotes ordered molecular packing and an optimized bulk-heterojunction morphology, thereby facilitating efficient charge generation and extraction while maintaining balanced charge transport. Theoretical calculations further reveal that the regulated assembly originates from an optimized intermolecular electrostatic environment established through moderate terminal fluorination. This work uncovers the molecular origin of assembly kinetics regulation by terminal fluorination, providing a fundamental insight into how intermolecular interactions govern morphology evolution in OSCs.