Stability of organic solar cells: photon energy and atmosphere

Abstract

Non-fullerene acceptors have enabled organic solar cells (OSCs) with power conversion efficiencies exceeding 20%, yet their operational stability remains a key challenge. This work investigates degradation mechanisms in OSCs under both ambient and inert conditions using the PM6:Y6 system. In air, molecular oxygen is identified as the dominant agent causing photo-oxidation of the PM6 donor, particularly at the BDT unit, while the Y6 acceptor is largely unaffected. The formation of charge-transfer states in the bulk heterojunction is sufficient to trigger this process, and interface engineering alone cannot suppress it. Under inert conditions, stability strongly depends on photon energy. Devices remain stable for thousands of hours under illumination with wavelengths above ~500nm, whereas higher-energy photons induce degradation. This degradation originates from photo-induced twisting of the donor backbone, leading to phase separation, excessive NFA aggregation, increased disorder, and enhanced charge recombination. Adjusting the donor–acceptor ratio mitigates charge-extraction losses and significantly improves operational stability.