| Summary: | Measurements of XPS survey, core levels (N 1s, O 1s, Pb 4f, I 3d), and valence band (VB) spectra of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (MAPbI<sub>3</sub>) hybrid perovskite prepared on different substrates (glass, indium tin oxide (ITO), and TiO<sub>2</sub>) aged under different light-soaking conditions at room temperature are presented. The results reveal that the photochemical stability of MAPbI<sub>3</sub> depends on the type of substrate and gradually decreases when glass is replaced by ITO and TiO<sub>2</sub>. Also, the degradation upon exposure to visible light is accompanied by the formation of MAI, PbI<sub>2</sub>, and Pb<sup>0</sup> products as shown by XPS core levels spectra. According to XPS O 1s and VB spectra measurements, this degradation process is superimposed on the partial oxidation of lead atoms in ITO/MAPbI<sub>3</sub> and TiO<sub>2</sub>/MAPbI<sub>3</sub>, for which Pb–O bonds are formed due to the diffusion of the oxygen ions from the substrates. This unexpected interaction leads to additional photochemical degradation.
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