The efficiency of perovskite solar cells has surpassed 26%, approaching the fundamental efficiency limits for single-junction cells. Tandem technology, combining two solar cells with different bandgaps, offers a solution to overcome detailed-balance limits by reducing thermalization loss. Although the best perovskite/silicon and all-perovskite tandem devices have exceeded 33% and 29% efficiencies respectively, the high energy consumption of silicon wafers leads to significant carbon emissions, and the self-doping oxidation effect (Sn²⁺ to Sn⁴⁺) in narrow-bandgap perovskites poses an instability issue. These challenges can be addressed by changing the narrow bandgap cells to non-fullerene organic photovoltaics (OPVs). In such structures, solution-processed OPVs are energy-friendly and do not have stability concerns related to self-doping oxidation. The recent emergence of near-infrared acceptor Y6 further positions perovskite/organic tandem solar cells as a promising technology.

In previous work, we integrated PM6:Y6:PC₆₁BM ternary OPVs into a perovskite/organic tandem devices, yielding a record efficiency of 24%. Notably, OPVs maintained approximately 95% original efficiency after 5000 hours of continuous operation under irradiation with low-energy photons (850 nm), but degraded rapidly when illuminated with a white light-emitting diode (LED), indicating that the visible spectral region could be responsible for device degradation^[1]. In this work, we utilize monochromatic LED sources covering ultraviolet, visible, and near-infrared spectral region to systematically explore the photostability of Y6 ternary OPVs. Our results reveal that under continuous operation in the maximum-power point under irradiation with low-energy photons (λ > 590 nm), the devices show long-term stability (>1000 hours), while high-energy photons (λ < 530 nm) infer degradation, with the device's T90/T80 lifetime strongly correlated with the photon energy. Additionally, the distinct degradation behaviors of single polymers or NFAs under monochromatic light exposure elucidate their respective contributions to degradation in bulk heterojunction devices.

Interestingly, ternary OPVs featuring other Y-family NFAs with various  energy gaps (Y18 (1.31 eV), CH1007 (1.30 eV), mBzS-4F (1.25 eV)) exhibit similar degradation behaviors under the same illumination conditions,  verifying that the photo-degradation behavior evidenced above is generally valid. The distinctive photostable property of OPVs under low-energy photons can eliminate concerns regarding the stability of the rear cell in perovskite-based tandem structures, because the front perovskite cells function as spectral low-pass filters. We integrates these OPVs as sub-cells in a tandem structure with a wide-bandgap perovskite cells, the corresponding photostability of perovskite/organic tandem devices strongly depends on that of the wide-bandgap perovskite sub-cells, highlighting the advantage of organic rear cells and directing community focus more towards the stability of wide-bandgap perovskite in the future.