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, CH 3 NH 3 PbI 3, is known to be unstable in the presence of water vapor. It degrades through a series of discrete hydrate phases (CH 3 NH 3 PbI 3 ·H 2 O and (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O), eventually forming PbI 2 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 TiO 2, mesoporous TiO 2, or mesoporous Al 2 O 3 ) 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 TiO 2 layer, but that the rate of perovskite decomposition is much slower when mesoporous supports are used. Furthermore, mesoporous Al 2 O 3 layers act as better protective barriers than mesoporous TiO 2, suggesting a route toward robust perovskite solar cells with better resistance to humidity and longer device lifetimes.