Understanding the Effect of Regioselective Fluorination in Tuning Miscibility and Crystallinity of Polymer Donors for Efficient Organic Solar Cells.

Fluorination is a widely adopted strategy for modifying polymer donors (PDs) in organic solar cells (OSCs). The incorporation of fluorine atoms is known to enhance crystallinity and facilitate charge transport of PDs, thereby boosting the short-circuit current density  of related OSCs. However, improperly executed fluorination can impair miscibility with acceptors, leading to excessive self-aggregation, unfavorable phase separation, serious charge reorganization, and increased energy loss. It is crucial to balance the crystallinity and miscibility of PDs for efficient OSCs. In this study, four PDs, SL1 to SL4, are designed with varying degrees of fluorination by precisely controlling the fluorination sites within D-A conjugate polymers. Notably, SL2, fluorinated on the A unit, exhibits optimal aggregation behavior and crystallinity while maintaining good miscibility with acceptors. Consequently, SL2 achieved reduced energy loss and delivered an impressive power conversion efficiency of 15.35% in binary devices. Furthermore, D18:Y6:SL2 based ternary device achieved a remarkable PCE of over 19%. This work offers new insights into the fluorination of PDs and a strategy for synergistic enhancement in crystallinity and miscibility for OSCs.