Probing the origins of photodegradation in organic–inorganic metal halide perovskites with time-resolved mass spectrometry

Operational and long-term stability of perovskite solar cells are critical for their commercialization on a large scale. To mitigate stability issues, a fundamental understanding of the physicochemical processes associated with the degradation of perovskite materials is needed. Here, we perform time resolved mass spectrometry of the gas species evolved during the photoinduced degradation of organic–inorganic lead trihalide perovskites made with commonly used monovalent cations, including methylammonium (MA), formamidinium (FA), and Cs. Our results indicate that the hot-carrier-induced deprotonation of MA+ cations is the fundamental origin of the photodegradation, which inevitably leads to the release of volatile species such as ammonia (NH3), aminocarbyne fragments (CNH2), hydrogen (H2), and iodine/hydrogen iodide (I/HI) from methylammonium lead iodide (MAPbI3) at different rates under simulated one sun solar illumination. Photodegradation processes can be mitigated by applying ultra-violet (UV) filters with suitable cutoff wavelengths to the light source. Additionally, we demonstrate that the incorporation of FA reduces the release of organic species but does not prevent the formation of I/HI. However, the addition of Cs effectively suppresses the release of all volatile gases. The best photostability is obtained with the FA/Cs mixed perovskites, showing that the complete removal of MA from mixed-cation perovskites is preferred for more photostable perovskites.