Fig. 2

XPS characterization. a Ge 3d XPS spectra, at different incidence angles, from CsSn_0.5Ge_0.5I₃ perovskite thin film that has been exposed to air. b Corresponding plot of the fraction of Ge(II) vs. the incidence angle. c, d XPS maps of Ge 3d (33 eV) and O 1s (532 eV), respectively, from the same area of the thin film