In Situ Monitoring of Thermal Degradation of CH3NH3PbI3 Films by Spectroscopic Ellipsometry

During the last few years, the organic–inorganic hybrid methylammonium lead halide perovskite, CH3NH3PbX3 (MAPbX3, X = I–, Cl–, Br–), has received great interest in the field of photovoltaics. Relevant researches develop rapidly due to the excellent properties of MAPbI3, such as high charge mobility, suitable band gap, and long carrier diffusion length. However, the instability of MAPbI3 has been a key issue hindering practical application. Here, we present in situ spectroscopic ellipsometry measurement to monitor the thermal degradation process of MAPbI3. The dynamic evolution of dielectric functions of the as-prepared MAPbI3 films through degradation is extracted by an effective medium approximation model fitting the ellipsometry spectrum. The content proportion of MAPbI3 and PbI2 is also obtained from the modeling results. The thickness of the film decreases in two steps corresponding to the degradation and melting–recrystallization process. The current work shows that ellipsometry provides a potentially fast and non-damage tool to monitor the degradation process and to determine the degree of degradation for MAPbI3 perovskite films. Our work exhibits the first in situ monitoring of the optical properties through the degradation process of MAPbI3 films, which can be consulted for further testing of the degree of degradation for MAPbI3 solar cells and improving the stability of MAPbI3.