Shedding Light on the Moisture Stability of 3D/2D Hybrid Perovskite Heterojunction Thin Films

To increase the moisture stability of hybrid perovskite photovoltaics, a combination of three-dimensional (3D) and a thin layer of two-dimensional (2D) perovskite incorporating long-chained organic cations is often employed as photoabsorber. However, the detailed interaction between water and 3D/2D perovskite heterojunctions has not been elucidated yet. Using in situ neutron and X-ray scattering techniques, we reveal surprisingly strong water uptake into 3D/2D perovskite films despite the presence of hydrophobic bulky cations. Our results show rapid disproportionation of the initial 2D phase (number of layers, m = 5) in methylammonium lead iodide (MAPI)/2D films into lower m phases under humidity. Nevertheless, the 2D perovskite inhibits the irreversible PbI2 formation, which suggests that the suppression of land MA(+) ion migration and consequently of MAI escape is related to the improved moisture stability of MAPI/2D perovskite films. In comparison, quadruple-cation perovskites including Rb+ exhibit poor stability toward phase segregation upon exposure to moisture regardless of the 2D perovskite layer.