Perovskite–organic tandem solar cells have emerged as one of the most promising thin-film photovoltaic technologies. Although their efficiencies have only recently begun to rival those of all-perovskite tandems, their greatest advantage may lie in their potential for superior operational stability. In particular, all-perovskite tandem solar cells remain subject to the fundamental stability limitations associated with tin-containing narrow-bandgap perovskite absorbers. For organic solar cells based on PM6 and non-fullerene acceptors (NFAs), we demonstrated early on their potential for excellent photostability under the spectrally filtered illumination conditions encountered in perovskite-organic tandem devices.^1,2 Building on our observations, we unravelled the mechanisms governing the wavelength-dependent stability of PM6 organic solar cells. Upon illumination with photons below a critical wavelength of approximately 500 nm, we observe twisting of the PM6 backbone, which initiates the aggregation of Y6 and is accompanied by an increase in its π–π stacking distance. These structural changes ultimately compromise the extraction and transport of photogenerated charge carriers. The pronounced spectral dependence of this degradation pathway eventually explains why PM6:NFAs cells can reach operational lifetimes of several thousand hours under one-sun illumination while exhibiting virtually no performance degradation under tandem-relevant illumination conditions. Beyond photostability, resilience against other technologically relevant operating conditions, e.g. elevated temperatures and exposure to ambient air, is essential. This is especially important for thin-film perovskite tandems as the narrow-bandgap subcell typically terminates the device stack and is therefore particularly exposed to the environment. Under these conditions, PM6 again emerges as the critical weak link.³ Its benzo[1,2-b:4,5-b′]dithiophene (BDT) units undergo photo-oxidation in the presence of oxygen. Moisture exposure, by contrast, induces reversible twisting of the PM6 backbone, which proves considerably less detrimental to device performance. Interestingly, the oxygen-induced photo-oxidation is largely independent of the photon energy. We pinpoint the underlying mechanism to most likely involve reactive triplet oxygen species, generated through excitation from the charge-transfer state. Consequently, unlike the wavelength-dependent degradation pathway observed under inert conditions, photo-oxidation in ambient air is highly detrimental under both single-junction and tandem operating conditions and therefore requires effective mitigation. To address this challenge, we developed PM6 solar cells incorporating an atomic-layer-deposited nanolaminate barrier.⁴ The fabrication of this barrier, as well as operation under damp-heat conditions, requires the device to withstand elevated temperatures. We identify the limited thermal resilience of the conventional device architecture below 120 °C as originating primarily from the thermal instability of the bathocuproine electron extraction interlayer. By replacing this layer with Al:ZnO nanoparticles as the electron-extraction layer, we achieve the thermal robustness required for subsequent nanolaminate deposition, which ultimately enables, the first damp-heat-stable NFA-based organic solar cell - without the need for additional encapsulation. We therefore conclude that an PM6:NFA based solar cell does carry immense stability promise under multiple harsh stressors, especially in a case where it is combined as a narrow-gap subcell in a tandem with a wide-gap device that acts as a spectral filter for the incident light.