Best Practice of Identifying Chemical Constituents and Evolution of Sn-Containing Perovskites by Photoelectron Spectroscopy.

Tin-containing halide perovskites are promising for tandem solar cells but face stability issues due to tin oxidation, even under low-oxygen conditions. A comprehensive understanding of the tin chemical states at the surface and within the bulk after oxidation is essential for developing strategies to mitigate tin oxidation. In this study, we investigate the nature and evolution of oxidation products near the surface of tin-containing perovskites using X-ray photoelectron spectroscopy (XPS) and gas-cluster ion-beam (GCIB) sputter profiling. We demonstrate that peaks previously attributed to Sn2+ and Sn4+ are now reassigned to Sn2+(-I) and Snx+(-O), respectively, with a thin Sn oxide layer formed on the perovskite surface. Under inert conditions, Snx+ and O species do not penetrate the bulk but significantly alter the Sn/Pb and I/(Sn+Pb) ratios. Furthermore, Sn diffusion from the bulk to the surface occurs alongside the A-site cation (N species and Cs+) and iodine depletion, even without external stimuli. These findings provide critical insights into the complex interplay between tin's oxidation states and the stability of tin-based perovskite solar cells.