This section provides supplementary figures and notes supporting the main text of the article. The figures cover various aspects of perovskite films, including XRD patterns, 3D depth profiles for oxygen, aging solar cells under oxygen and light, absorbance profiles of different crystallite sizes, oxygen diffusion, salt coating impact, absorbance profiles of different salt treatments, photoluminescence, stability enhancement, morphology, normalized PCE degradation, J-V curves, and photovoltaic parameters. The notes include calculations for the oxygen diffusion coefficient and the formation energy of superoxide insertion and reduction in MAPI films.This section provides supplementary figures and notes supporting the main text of the article. The figures cover various aspects of perovskite films, including XRD patterns, 3D depth profiles for oxygen, aging solar cells under oxygen and light, absorbance profiles of different crystallite sizes, oxygen diffusion, salt coating impact, absorbance profiles of different salt treatments, photoluminescence, stability enhancement, morphology, normalized PCE degradation, J-V curves, and photovoltaic parameters. The notes include calculations for the oxygen diffusion coefficient and the formation energy of superoxide insertion and reduction in MAPI films.