As a promising solar energy harvesting technology, solution-processed metal halide perovskites (MHPs) are of great current interest in developing low-cost and efficient photovoltaic cells. Despite their excellent optoelectronic properties and the nascent advancements in compositional tailoring and interfacial engineering to develop high-performance MHPs, issues associated with the long-term environmental stability of these materials are yet to be addressed. Here we examine the moisture-induced cascade degradation reactions over a year for methylammonium l e a d i o d i d e ( M A P b I 3 ) -a n d f o r m a m i d i n i u m -r i c h [Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(Br 0.17 I 0.83 ) 3 ] formulations at 40 and 85% relative humidity (RH) in the air. The transformative reactions at 85% RH lead to chemical degradation process in both MA-rich and FA-rich perovskites, yielding to the different organic and inorganic byproducts within a few hours, but the exposure to 40% RH retains the longevity of these materials up to several months. The defect passivation by the tetrapropylammonium cation (TPA + ) imparts enhanced stability of MAPbI 3 particles, irrespective of the exposure conditions to water vapor. By resolving thin-film morphology at sub-nanometer to nanometer resolution using solid-state (ss)NMR spectroscopy and X-ray diffraction techniques, kinetics of degradation reactions and structural insights into the inorganic/organic interfaces and degradation products are obtained and compared. Our findings provide mechanistic details into the cascade degradation reactions in pristine and defect-passivated MHPs, enabling guidance for novel passivating and interfacial engineering strategies to further improve the robustness of the MHPs with respect to environmental stressors.

Hybrid halide perovskite has established its credibility as high performance thin film photovoltaic technology. Perovskite based on formamidinium cation is at the core composition to top performances and stability. Herein, a depth study based on temperature‐controlled in situ X‐ray diffraction focusing on the photo‐active formamidinium lead iodide (α‐FAPbI3) is reported. In particular, the thermal stability of the latter and the degradation pathways under different experimental conditions are clarified. Based on this in situ technique, the lattice thermal expansion coefficient is reported that provides relevant information on possible mechanical stress created upon temperature cycling or damp heat test. The results support that α‐FAPbI3 degradation is substantially accelerated when temperature is combined to illumination and when it is interfaced with the extraction layers. In addition, by contrast to in darkness for which α‐FAPbI3 degrades directly into PbI2, the existence of a temperature gap under illumination involving an intermediate step with a non‐crystalline phase resulting from the perovskite degradation and contributing to the formation of PbI2 by‐product is revealed.