Comparing the Effect of Mesoporous and Planar Metal Oxides on the Stability of Methylammonium Lead Iodide Thin Films

The family of lead halide perovskites has revolutionized the field of solution-processed photovoltaics; however, the parent member of this family, CH₃­NH₃­PbI₃, is known to be unstable in the presence of water vapor. It degrades through a series of discrete hydrate phases (CH₃­NH₃­PbI₃·​H₂O and (CH₃­NH₃)₄­PbI₆·​2H₂O), eventually forming PbI₂ after long exposures. While this decomposition process has limited the commercial utility of perovskite-based devices, recent studies have suggested that the electron-transport layer can have a pronounced effect on device longevity. In this work, we combine <i>in situ</i> absorbance spectroscopy and <i>in situ</i> grazing incidence wide-angle X-ray scattering measurements to quantitatively investigate how the choice of metal oxide support (planar TiO₂, mesoporous TiO₂, or mesoporous Al₂O₃) affects the stability of the perovskite film in the presence of moisture. We demonstrate that the monohydrate phase forms rapidly when methyl­ammonium lead iodide is deposited on a compact TiO₂ layer, but that the rate of perovskite decomposition is much slower when mesoporous supports are used. Furthermore, mesoporous Al₂O₃ layers act as better protective barriers than mesoporous TiO₂, suggesting a route toward robust perovskite solar cells with better resistance to humidity and longer device lifetimes