The presence of CH₃NH₂ neutral species in organometal halide perovskite films

We report the presence of CH₃NH₂ neutral species not only on the surface but also at grain boundaries in the interior of thin polycrystalline films of methylammonium lead iodide perovskite CH₃NH₃PbI₃ (thickness ∼ 50 nm) that were prepared using a standard solution method. Different chemical states for C K-edge were observed at the surfaces and in the interiors of perovskite films. Salient features of σ∗(CH₃-NH₃⁺: methylammonium cation) and σ∗(CH₃-NH₂: methylamine neutral species) were observed at 290.3 and 292.8 eV in both partial (surface-sensitive) and total (bulk) electron yield modes by near-edge x-ray absorption fine structure measurements. Consistently, two chemical states originated from CH₃NH₃⁺ and CH₃NH₂ in C 1s and N 1s core-level spectra were observed using high-resolution x-ray photoelectron spectroscopy. Using density functional theory calculations, we show that CH₃NH₂ cannot reside stably in the MAPbI₃ perovskite crystal structure. Therefore, we propose that these CH₃NH₂ neutral species are mainly located on the surface or at grain boundaries in the interior of CH₃NH₃PbI₃ films.