Perovskite-based multi-junction solar cells allow to overcome efficiency limits of single-junction cells by reducing thermalization losses. Even though, impressive progress has been achieved by perovskite/silicon and all-perovskite tandem devices, significant challenges persist, including the substantial carbon emissions due to energy-intensive silicon wafer production or fundamental stability concerns due to the oxidation of Sn²⁺ to Sn⁴⁺ in narrow-gap perovskites. On the contrary, organic solar cells (OSCs) based on narrow-gap non-fullerene acceptors (NFA) present an attractive alternative as rear cells in perovskite-based tandem devices. In our previous work, the benchmark PM6:Y6:PC₆₁BM ternary OSCs maintained approximately 95% of its efficiency after 5000 hours of continuous operation under irradiation with low-energy photons (l = 850 nm), but some notable degradation was found when illuminated with a white light-emitting diode (LED), indicating that photons in the visible spectral region infer device degradation.^[1] In a perovskite-organic tandem solar cell, the wide bandgap perovskite sub-cell serves as a low-pass filter that protects the organic sub-cell against high-energy photons.^[2] While the photostability of the perovskite-organic tandem devices is still limited by the photostability of the wide gap perovskite, for the narrow-gap sub-cell, NFA based organic solar cells might be a better choice compared to narrow-gap Pb-Sn perovskite solar cells. 

In this work we generalize our study to include a wider range of Y-type acceptors (Y18 (E_g = 1.31 eV), CH1007 (E_g = 1.30 eV), mBzS-4F (E_g = 1.25 eV)), that we identified to show great promise in our perovskite/organic tandem solar cells. Most importantly, we could evidence that the remarkable photostability found for the Y6 NFA is generally valid for the entire Y-family. Using monochromatic light sources covering the ultraviolet, visible, and near-infrared spectral regions we are able to identify in detail the influence of photon energy on device stability. By varying the device architecture (e.g. hole extraction layer either MoO3 or PEDOT-F) we are able to identify the impact of the photoactive organic absorber in the degradation. Under continuous operation in the maximum-power point under irradiation with low-energy photons (λ > 590 nm), the devices show excellent long-term stability (well above 1000 hours), while higher-energy photons (λ < 530 nm) infer increasingly severe degradation. Combining this wavelength-selective degradation studies with in-situ photoluminescence, Raman spectroscopy, photoluminescence quantum yield investigations, transient absorption, and suns-Voc device analytics, we systematically investigate the degradation pathway.