Fluorination‐Controlled Crystallization Kinetics in Donor Polymers for 20% Efficiency Organic Solar Cells

ABSTRACT Controlling the crystallization dynamics of polymer donors is essential for achieving high‐performance organic solar cells (OSCs). While polymers like PM6 and D18 possess excellent electronic properties, their inherently rapid aggregation during film formation often leads to insufficient nucleation sites and excessive microscale aggregation, which impede charge dissociation and lower device efficiency. To overcome this issue, we designed a series of polymer donors, PBT, PfBT, and PffBT, by systematically modulating the fluorination degree of the benzothiadiazole (BT) acceptor unit. Through combined theoretical and experimental investigations, we demonstrate that optimal fluorination, particularly in PfBT, fine‐tunes the average electrostatic potential and molecular polarizability, effectively regulating temperature‐dependent aggregation and prolonging nucleation behavior. This results in a higher density of nucleation sites, balanced crystallization, enhanced molecular packing, and improved nanoscale phase separation. These favorable morphological characteristics facilitate efficient exciton dissociation and suppress charge recombination. As a result, PfBT‐based ternary OSCs achieve a remarkable power conversion efficiency of 20.12%. This work not only establishes a clear structure–property relationship but also highlights rational molecular fluorination as an effective strategy for optimizing active layer morphology in high‐performance OSCs.