Role of the donor on the light-induced degradation of Y6 non-fullerene acceptors in PM6:Y6 blend films

Abstract

Despite the rapid advancements in the performance of organic solar cells (OSCs), improving their operational lifetime remains a significant challenge. The photodegradation of donor polymer PM6, small molecule non-fullerene acceptor (NFA) Y6, and their blend was investigated under ambient conditions. To photodegrade the spin-coated thin films, samples were exposed to AM 1.5 illumination, as well as UV-filtered and long-wavelength-filtered light. The evolution of their properties with increasing exposure time up to 45 h was monitored using UV-vis absorption, Fourier transform infrared (FTIR), and photoemission spectroscopy. The results demonstrate that neat PM6 films exhibit faster absorbance loss than the neat Y6 films. This is accompanied by the formation of new carbonyl groups on PM6, while only minor indications of photooxidation were observed in degraded Y6 films. The valence band spectra of Y6 remain unchanged upon photodegradation. Interestingly, the photobleaching rate of Y6 in PM6:Y6 blend films was found to be higher than that of neat Y6 films. XPS spectra of C 1s and S 2p confirm photooxidation products formed in PM6 and PM6:Y6 films, evidenced by new oxidized carbonyl C 1s and oxidized sulfur S 2p peaks. Under AM 1.5 illumination, several photooxidation pathways can be active, involving the formation of both superoxide radicals and singlet oxygen species, and their subsequent oxidation reactions with conjugated molecules. Using filtered light conditions, these different degradation pathways could be separated. Upon exposure to long-wavelength-filtered light, which is predominantly absorbed by the Y6 acceptor, the generation of superoxide radicals is significantly suppressed, resulting in enhanced photostability of the blend compared to illumination with unfiltered light. The remaining photodegradation of the blend components under these illumination conditions can therefore be ascribed to energy transfer from the photosensitizing acceptor, feeding into the singlet oxygen formation. These insights could inspire the design of new donor and acceptor materials with improved photostability by tuning the positions of their singlet and triplet states to minimize the formation of oxygen-mediated reactive species.