The limited operational lifetime of organic solar cells remains an obstacle to their commercial development and is largely due to the poor intrinsic photostability of the conjugated molecules that constitute the photoactive layer. Here, we selected a series of state-of-the-art donor and acceptor materials including PBDB-T, Y5, PF5-Y5, and PYT to study their photostability under AM1.5 simulated sunlight in ambient conditions. Their properties are monitored over time, using various spectroscopy techniques, including UV-Vis absorption, Fourier-transform infrared (FTIR), and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). We found that the absorption spectra of Y5 and PYT films remain almost intact even after 30 hours of light exposure in air, while the PF5-Y5 and PBDB-T films undergo rapid photobleaching. The absorption losses observed in blend films of PBDB-T with Y5 and with PF5-Y5 can be understood as composed of contributions from the separate blend components that are similar to the absorption losses in neat films. The new peaks emerging in the FTIR spectra of PBDB-T, PF5-Y5, and their blend films witness the formation of new carbonyl groups, while these are absent in the spectra of the Y5 and PYT films. The XPS C 1s spectra of the PF5-Y5 and PBDB-T films confirm this carbonyl formation and the S 2p spectra reveal that sulphone groups are formed after 30 hours of exposure of these films. These results confirm that films of Y5 and the copolymer PYT are significantly more resistant to photooxidation, compared to the copolymer PF5-Y5. The comparison of these results suggests that the benzo[1,2-b:4,5-b ']dithiophene moiety with alkylated thiophenes as side chains (BDT-T) accelerates the photodegradation of PBDB-T and PF5-Y5. The replacement of the BDT-T unit by thiophene contributes to the enhanced stability of PYT, demonstrating that the nature of the co-monomer has a significant effect on the intrinsic photostability of Y5-based copolymers. These new insights are expected to stimulate the design of stable donors and acceptor polymers for the development of long-lived OPV devices. Absorption spectra show the photobleaching of acceptor copolymer PF5-Y5. The replacement of BDT-T by thiophene strongly improves the photostability.

Despite the rapid advancements in organic solar cells (OSCs) performance, improving their operational lifetime remains a significant challenge. The photodegradation of donor polymers PTQ10 and PM6, small molecule non-fullerene acceptor (NFA) Y6, and their blends were investigated under ambient atmospheric conditions. To degrade thin spin-coated films of these materials, the samples were exposed under ambient conditions to AM 1.5 illumination, as well as to UV- and long-wavelength filtered light. The photodegraded films were investigated using UV-vis absorption spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). For neat films, we found that the donor polymer PTQ10 and acceptor Y6 are relatively stable, compared to donor polymer PM6. However, the degradation rate of Y6 was found to accelerate in blend films of PM6:Y6 and PTQ10:Y6, as compared to neat Y6 film. In the case of AM 1.5 illumination, several pathways of photodegradation of the conjugated molecules can be active, involving both superoxide radicals and singlet oxygen species. We show that exposure under filtered light conditions provides a method to distinguish these different pathways. The enhanced photostability of the blends upon exposure to long-wavelength filtered light as compared to unfiltered light indicates that the electron transfer pathway from the donor is suppressed, blocking the formation of superoxide radicals, while the remaining photodegradation under these illumination conditions can 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.