Mitigating Photo‐Aging of Donor Polymer via Crosslinking for Long‐Term Organic Solar Cell Performance

Abstract The benzodithiophene‐based donor polymer PM6 is widely used in organic solar cells (OSCs), but its practical application is hindered by poor photostability, mainly due to structural twisting and photo‐oxidation under prolonged light exposure. To address this limitation, a crosslinkable donor polymer, PM6‐Br is designed and synthesized to investigate the effect of crosslinking on photostability. Although PM6‐Br exhibits optical and photoelectrical properties comparable to those of PM6, its crosslinked form showed significantly improved resistance to photo‐oxidation and enhanced morphological stability under continuous irradiation. X‐ray photoelectron spectroscopy (XPS), resonance Raman spectroscopy and time‐dependent density functional theory (TD‐DFT) analyses reveal that PM6 undergoes substantial photo‐oxidation—specifically sulfur oxidation—resulting in energy level shifts, the formation of trap states, and molecular aggregation. Raman signals associated with the conjugated backbone decrease more rapidly in PM6 than in PM6‐Br, indicating more extensive structural degradation. The enhanced photostability of the crosslinked donor polymer led to improve device stability, with OSCs based on crosslinked PM6‐Br maintaining significantly better operational performance under continuous 1‐sun irradiation for 1000 h. These findings highlight crosslinking as a promising strategy to improve the photostability of OSCs, advancing their potential for commercial applications.