Deciphering the degradation mechanism of the lead-free all inorganic perovskite Cs₂SnI₆

Organic-inorganic perovskite materials are revolutionizing photovoltaics with high power conversion efficiencies, but experience significant environmental degradation and instability. In this work, the phase stability and decomposition mechanisms of lead-free all inorganic Cs₂SnI₆ perovskite upon water and moisture exposure were systematically investigated via in situ synchrotron X-ray diffraction, environmental SEM, and micro-Raman spectroscopy. A critical relative humidity (80%) is identified below which Cs₂SnI₆ perovskite is stable without decomposition. Under higher humidity or aqueous environment, Cs₂SnI₆ perovskite decomposes into SnI₄ and CsI through etch pits formation and stepwave propagation, leading to rapid crystal dissolution. A partial reversibility of the Cs₂SnI₆ perovskite upon dissolution and re-precipitation with subsequent dehydration was identified, suggesting a self-healing capability of Cs₂SnI₆ and thus enhanced air stability. Mechanistic understanding of the Cs₂SnI₆ degradation behavior can be a vital step towards developing new perovskites with enhanced environmental stability and materials performance.