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.