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.