Revealing Intermediate PbI₂-DMF Solvated Crystallites via UV-VIS In Situ Spectroscopy on Perovskite Precursor Solution

We investigated the precursor solution for the fabrication of lead iodide perovskite thin films via UV-VIS absorption spectroscopy during the spin coating process (in situ). The optical absorption spectra of the precursor solution at concentrations suitable for thin film fabrication are difficult to measure and evaluate. However, we use thin film techniques to monitor the changes in absorption spectra of the precursor solution at concentrations relevant for device fabrication. The iodoplumbate complexes that are present in the precursor solution absorb light in the UV range, providing insights into the precursor chemistry. Since the iodoplumbate complexes are very sensitive to their environment and can alter their coordination considerably when changing parameters like the solvent or concentration, we conducted an optical in situ study during the critical phase before the nucleation. We observe the emergence of PbI₂-DMF solvated (PDS) crystallites during spin coating. These PDS crystallites mainly form from PbI₃^- complexes and act as a predecessor for subsequent crystalline perovskite phases. The amount of PDS phase is closely connected to the concentration of the precursor solution film, which we estimate using white light interference.