Degradation by Exposure of Coevaporated CH₃NH₃PbI₃ Thin Films

Degradation of coevaporated CH₃NH₃PbI₃ thin films were investigated with X-ray photoelectron spectroscopy and X-ray diffraction as the films were subjected to exposure of oxygen, low pressure atmospheric air, atmospheric air, or H₂O. The coevaporated thin films have consistent stoichiometry and crystallinity suitable for detailed surface analysis. The results indicate that CH₃NH₃PbI₃ is not sensitive to oxygen. Even after 10¹³ Langmuir (L, one L equals 10^–6 Torr s) oxygen exposure, no O atoms could be found on the surface. The film is not sensitive to dry air as well. A reaction threshold of about 2 × 10¹⁰ L is found for H₂O exposure, below which no CH₃NH₃PbI₃ degradation takes place, and the H₂O acts as an n-dopant. Above the threshold, the film begins to decompose, and the amount of N and I decrease quickly, leaving the surface with PbI₂, hydrocarbon complex, and O contamination.