Abstract
Solar cells composed of methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that (i) hydrated crystal phases are formed when MAPI is exposed to water vapor at room temperature and (ii) these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH3NH3PbI3·H2O followed by (CH3NH3)4PbI6·2H2O (upon long exposure times) was observed using time-resolved XRD and ellipsometry of thin films prepared using “solvent engineering”, single crystals, and state-of-the-art solar cells. In contrast to water vapor, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI2. MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH3NH3PbI3·H2O formed on single crystals were determined, with a bandgap at 3.1 eV. Using the single-crystal optical constants and thin-film ellipsometry measurements, the time-dependent changes to MAPI films exposed to moisture were m...
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