| Summary: | In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI<sub>3</sub>, MA<sub>0.7</sub>FA<sub>0.3</sub>PbI<sub>3</sub>, and (FAPbI<sub>3</sub>)<sub>0.95</sub>(MAPbBr<sub>3</sub>)<sub>0.05</sub> perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI<sub>3</sub> and MA<sub>0.7</sub>FA<sub>0.3</sub>PbI<sub>3</sub> films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI<sub>3</sub>)<sub>0.95</sub>(MAPbBr<sub>3</sub>)<sub>0.05</sub> films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.
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