The stability of CH3NH3PbI3 was investigated by observing the degradation in a coevaporated film irradiated by a blue laser in ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were employed to investigate the effects of irradiation on the surface. The core levels of CH3NH3PbI3 were observed to shift toward a higher binding energy (BE) during the irradiation, suggesting that the surface became more n-type. A new metallic Pb component in the XPS spectrum appeared after 120 min of irradiation, indicating that the film had started to decompose. The decomposition saturated after about 480 min of irradiation when the ratio of metallic Pb to total Pb was about 33%. Furthermore, the film was no longer continuous after irradiation, as the elements gold and oxygen from the substrate were detected by XPS. SEM images also show a roughened surface after irradiation. The results strongly indicate that CH3NH3PbI3 is sensitive to the laser irradiation and that the light induced decomposition is a self-limiting process.

Degradation of coevaporated CH 3 NH 3 PbI 3 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 H 2 O. The coevaporated thin films have consistent stoichiometry and crystallinity suitable for detailed surface analysis. The results indicate that CH 3 NH 3 PbI 3 is not sensitive to oxygen. Even after 10 13 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 × 10 10 L is found for H 2 O exposure, below which no CH 3 NH 3 PbI 3 degradation takes place, and the H 2 O 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 PbI 2 , hydrocarbon complex, and O contamination.

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