Electrification of societies and the rapid development of artificial intelligence are predicted to significantly increase global energy demand in the coming years. Meeting this demand sustainably requires renewable energy sources. Organic solar cells (OSCs) have emerged as a promising candidate, achieving power conversion efficiencies exceeding 20% in laboratory settings through the development of non-fullerene acceptors (NFAs). However, limited long-term stability remains a major challenge for commercialization.

In this work, the photodegradation of the NFAs Y6 and IT4F (outdoor photovoltaics), GS-ISO and FTCC-Br (indoor photovoltaics), and the donor polymer PTQ10, was investigated in neat films, bulk heterojunction (BHJ) blends and solar cell devices under standardized AM1.5G LED illumination without UV in air. A combination of UV-Vis-NIR absorption spectroscopy, photoluminescence (PL), transient absorption spectroscopy (TAS) and device characterization was used to correlate material degradation with device performance.

The results showed wavelength dependent degradation for IT4F, GS-ISO and FTCC-Br, with higher energy photons causing more severe damage, while Y6 exhibited high photostability. Although the absorption of PTQ10 remained stable after light soaking, a reduced photoluminescence quantum yield (PLQY) and shortened lifetimes indicated increased non-radiative recombination. TAS revealed accelerated formation of bound polaron pairs in light soaked IT4F, GS-ISO and FTCC-Br, while no such features were observed for PTQ10. Despite efficient charge transfer in BHJ blends, the devices showed a dramatic loss of performance and external quantum efficiency after 12 hours of light soaking. This loss in performance could not be explained solely by the photodegradation of the active layer, and indicated additional problems within the device stack, including increased series resistance and reduced shunt resistance. Overall, the results demonstrated that a dominant degradation pathway is the formation of non-radiative recombination channels driven by photoinduced degradation of the NFAs.