Degradation Mechanism of Halide Perovskite Crystals under Concurrent Light and Humidity Exposure

For the large-scale application of perovskite solar cells, it is crucial to improve their durability. However, this improvement is limited because their degradation mechanism under concurrent conditions, such as exposure to light radiation and humidity, has not yet been clarified. Therefore, it is essential to understand this complicated degradation process, which is accelerated by a combination of multiple factors. In this study, we observed degradation process in CH3NH3PbI3 crystals by using the in situ 2D wide-angle X-ray scattering technique by simultaneously exposing the perovskite crystals to light and humidity, and revealed the degradation process based on crystallography. We found that NH4PbI3·2H2O was formed during the degradation process under these conditions. The results indicated that CH3NH3I and NH4PbI3·2H2O suppressed the degradation of the perovskite crystals to PbI2. Thus, we revealed one of the complicated degradation processes that occur upon simultaneous light and humidity exposure. The results of this study suggest that the protection of perovskite crystals from water molecules is crucial for improving the photodurability of perovskite solar cells.