Open-Air Stability of Methylammonium Lead Iodide Perovskite Films Capped with 2D Ruddlesden–Popper Phase Butylammonium Methylammonium Lead Iodide and the Role of Interlayer Diffusion

The use of low-dimensional perovskites to stabilize perovskite films and interlayers in photovoltaic devices (PVs) has become a common strategy. Here, we present a detailed examination of open-air stability of methylammonium lead iodide (MAPbI3) films capped with two-dimensional (2D) butylammonium methylammonium lead iodide (BA2MAn-1PbnI3n+1). MAPbI3 decomposes to PbI2 within days at ambient lab conditions (20%–75% RH and 19–24 °C), even when capped with BA2MAn-1PbnI3n+1. Films of 2D mixed n BA2MAn-1PbnI3n+1 and BA2PbI4 also readily degraded, even though bulk crystals of these materials were found to be very stable. X-ray diffraction and UV–vis absorbance spectra indicate that the BA+ cation interlayers readily diffuse in the films, explaining why BA2MAn-1PbnI3n+1-capped MAPbI3 and 2D mixed n BA2MAn-1PbnI3n+1 films lack open-air stability.