Unveiling the Chemistry: A Dive into Wet Chemical Perovskite Thin Film Creation Revealed by Spectral in-situ Reflectance

In this paper, we demonstrate how in-situ reflectance data can be applied to monitor compositional changes and formation sequences during perovskite formation processes. Here, the time-dependent evolution of the refractive index $n$ and the extinction coefficient $k$ are used as representatives for the chemical changes occurring during the process. Furthermore, direct evidence for the impact of an anti-solvent-drip during a 3-cation perovskite deposition process is given by means of optical in-situ reflectance data. It is also shown that generally both, the liquid and the solid components of the thin film contribute to the reflectance signal and their contribution and thickness changes during the process time are analyzed. Finally, we identify fluctuations in the reflectance data, caused by inevitable discrepancies in manual dispensing of the anti-solvent solution within different experimental runs. In summary, it is demonstrated that optical in-situ metrology is able to (i) yield a complete fingerprint of wet-chemical perovskite deposition processes, (ii) provide quantitative information about the cumulative liquid and solid optical thin film thickness and (iii) assess further quantities like solution composition and thin film roughness.