Towards reliable oxidation state analysis of tin halide perovskites with XPS

Tin is the most promising replacement for lead in hybrid halide perovskite solar absorbers. A disadvantage of tin perovskites is ease of oxidation of Sn(II) hampering optoelectronic performance and long-term stability. Quantification of the extent of Sn oxidation in hybrid halide perovskites is therefore an important indicator of material stability but remains a significant challenge. X-ray photoemission spectroscopy (XPS) is commonly used to measure Sn chemical environments; we show here that conventional approaches to fitting the Sn 3d spectra from perovskite surfaces can easily lead to erroneous conclusions about the tin oxidation state. We consider several approaches to developing a robust fitting model for Sn 3d spectra. Furthermore, we identify that tin halide perovskite surfaces can be unstable under XPS measurement conditions, and the chemical state of tin will change significantly over typical analysis times. We use correlation analysis to validate our fitting model and identify phases present.