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

Light-Induced Degradation of CH₃NH₃PbI₃ Hybrid Perovskite Thin Film

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

Air-Induced High-Quality CH₃NH₃PbI₃ Thin Film for Efficient Planar Heterojunction Perovskite Solar Cells

Efficient planar heterojunction perovskite solar cells (PHJ–PSCs) with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al were fabricated using air-induced high-quality CH₃NH₃PbI₃ perovskite thin films, in which the air-inducing process was controlled with a humidity of ∼40%. The air exposure of CH₃NH₃PbI₃ thin films could dramatically improve their properties with large grains and smooth surface, as well as uniform morphology, resulting in an impressive enhancement in carrier lifetime. The ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy results proved that the CH₃NH₃PbI₃ film was <i>n</i>-doped by the absorption of H₂O on the surface but was very stable without obvious degradation for 10 days’ exposure in air. The power conversion efficiency (PCE) of PHJ–PSCs with an air exposure process showed a significant increase up to 16.21% as compared to reference PHJ–PSCs with a PCE of 12.02%. The research work demonstrated that an air-exposure process with a suitable humidity could produce high-quality perovskite thin film for efficient PHJ–PSCs, which may pave a boulevard for fabricating high-efficiency PHJ–PSCs in atmospheric environment