Optical Scattering Evolution during Ambient Aging of Cs/FA Alloyed Perovskite Thin Films

This work investigates the optical stability of formamidinium-cesium lead halide perovskite thin films deposited on fluorine-doped tin oxide substrates and aged under ambient conditions for 21 days. The optical response was analyzed through specular and diffuse transmittance and reflectance measurements, collected with light incident from both sides of the heterostructure. Specular transmittance exhibits nonmonotonic variations with an initial increase followed by a gradual decrease over time, while diffuse transmittance increases systematically across the full spectral range, indicating the progressive formation of scattering centers. Total reflectance decreases monotonically with aging, revealing that degradation is primarily governed by absorption-related optical losses. Despite these changes, the absorption edge remains stable, and the optical bandgap and Urbach tail show no significant variation. Direction-dependent measurements demonstrate that the fluorine-doped tin oxide substrate is the dominant source of initial scattering whereas the perovskite layer initially reduces optical contrast and later introduces disorder as degradation progresses. Haze values remain nearly constant over time, indicating that changes in scattering efficiency are moderate compared to absorption losses. These results demonstrate that integrating sphere-based optical spectroscopy provides a nondestructive and effective framework for monitoring early stage degradation in perovskite thin films.