Moisture‐Induced Non‐Equilibrium Phase Segregation in Triple Cation Mixed Halide Perovskite Monitored by In Situ Characterization Techniques and Solid‐State NMR

Environmental stability is a major bottleneck of perovskite solar cells. Only a handful of studies are investigating the effect of moisture on the structural degradation of the absorber. They mostly rely on ex situ experiments and on completely degraded samples, which restrict the assessment on initial and final stage. By combining in situ X‐ray diffraction under controlled 85% relative humidity, and live observations of the water‐induced degradation using liquid‐cell transmission electron microscopy, we reveal two competitive degradation paths leading on one hand to the decomposition of state‐of‐the‐art mixed cation/anion (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(Br_0.17I_0.83)₃ (CsMAFA) into PbI₂ through a dissolution/recrystallization mechanism and, on the other hand, to a non‐equilibrium phase segregation leading to CsPb₂Br₅ and a Cesium‐poor/iodide‐rich Cs_0.05‐x(MA_0.17FA_0.83)_0.95Pb(Br_0.17−2yI_0.83+2y)₃ perovskite. This degradation mechanism is corroborated at atomic‐scale resolution through solid‐state ¹H and ¹³³Cs NMR analysis. Exposure to moisture leads to a film containing important heterogeneities in terms of morphology, photoluminescence intensities, and lifetimes. Our results provide new insights and consensus that complex perovskite compositions, though very performant as champion devices, are comparatively metastable, a trait that limits the chances to achieve long‐term stability.