Electronic structures and chemical states of methylammonium lead triiodide thin films and the impact of annealing and moisture exposure

Methylammonium lead triiodide (CH₃NH₃PbI₃) is the fundamental material used in perovskite solar cells, and its electronic properties have, therefore, attracted a great deal of attention as a potential key to highly efficient solar cell performance. However, the deterioration of perovskite solar cells when exposed to high temperature and humidity remains a serious obstacle to the material's use, and the clarification of the degradation mechanisms has been keenly anticipated. In this study, the valence electronic structures and depth-dependence of the chemical states of CH₃NH₃PbI₃ thin films are investigated using ultraviolet photoelectron spectroscopy and excitation energy dependent X-ray photoelectron spectroscopy. Additionally, the effects of high temperature and a moisture rich atmosphere on the CH₃NH₃PbI₃ thin films are examined. It is confirmed that the high temperature and moist atmosphere facilitate the oxidation of CH₃NH₃PbI₃, whereas the Pb:I stoichiometry of the CH₃NH₃PbI₃ thin films is found to be preserved at its original ratio (1:3) after thermal annealing and exposure to a moist atmosphere.