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 center dot H2O and (CH3NH3)(4)PbI6 center dot 2H(2)O), eventually forming PbI, 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 in situ absorbance spectroscopy and in situ 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.