Organic solar cells (OSC), a promising renewable energy technology, have gained significant attention due to their lightweight, flexibility, transparency, and low manufacturing cost. OSCs have recently achieved power conversion efficiencies exceeding 20% thanks to the development of novel high-performing electron acceptors and electron donors [1]. However, their operational lifetime remains limited. Improving device stability requires a fundamental understanding of the intrinsic photostability of active layer materials. Recent studies report that the electron acceptor PF5-Y5 films are more prone to photo-oxidation, whereas the PYIT films remained more stable under the same conditions. The rapid photodegradation of the PF5-Y5 is due to the presence of the benzo[1,2-b:4,5-b′]dithiophene (BDT-T) moiety, which appears to accelerate the process, though the mechanism is not yet fully understood [2].

In this work, we extend these investigations to gain detailed insight into the electronic structure of PF5-Y5 and PYIT and its evolution under photo-oxidation by combining photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS), and resonant PES (ResPES). While PES probes the occupied valence electronic states and NEXAFS maps the unoccupied states, ResPES inherently links the unoccupied states to the occupied states [3]. Hence, ResPES with its element-, site-, and orbital selectivity, is a powerful tool for identifying the degradation pathways and providing guidance for improving the operational lifetime of materials used in OSCs.

For PF5-Y5, PES reveals significant DoS losses in valence electronic states, and NEXAFS shows the losses of electronic transitions of the unoccupied states upon photo-oxidation, while PYIT exhibits no significant changes under the same conditions. ResPES further reveals the complete disappearance of resonant enhancement in PF5-Y5 upon photo-oxidation. Density functional theory (DFT) calculations establish that the first two resonances of NEXAFS spectra of both materials are associated with the Y5 end-groups, while the third resonance in PF5-Y5 is associated with N-containing moieties and the BDT unit. In PF5-Y5, the transition probability of the third resonance is already significantly affected after 10 hours of photo-oxidation, identifying the BDT unit and N-containing moieties as the primary sites of degradation. After 30 hours, this resonance has become severely affected, and the first resonance is also found to be affected, revealing the photo-oxidation in the stronger donating BDT group alters the electronic structure of the molecule as a whole and makes it more prone to degradation. Consistent with this, the DoS losses are observed in HOMO of PF5-Y5 upon photo-oxidation. As reported in the literature, the HOMO is predominantly associated with the BDT unit (80 %) with only 20 % contribution from the Y5 moiety, suggesting that photo-oxidation primarily affects the BDT unit [4]. These findings provide valuable insight into the degradation pathways and offer guidance for designing more durable OSC materials.