Perovskite Crystallization Dynamics during Spin-Casting: An In Situ Wide-Angle X-ray Scattering Study

[Image: see text] In situ wide-angle X-ray scattering (WAXS) has been measured during the spin coating process used to make the precursor films required for the formation of thin films of perovskite. A customized hollow axis spin coater was developed to permit the scattered X-rays to be collected in transmission geometry during the deposition process. Spin coating is the technique most commonly used in laboratories to make thin perovskite films. The dynamics of spin-casting MAPbI(3–x)Cl(x) and FAPbI(3–x)Cl(x) films have been investigated and compared to investigate the differences between the dynamics of MAPbI(3–x)Cl(x) and FAPbI(3–x)Cl(x) film formation. In particular, we focus on the crystallization dynamics of the precursor film formation. When casting MAPbI(3–x)Cl(x), we observed relatively fast 1D crystallization of the intermediate product MA(2)PbI(3)Cl. There was an absence of the desired perovskite phase formed directly; it only appeared after an annealing step that converted the MA(2)PbI(3)Cl to MAPbI(3). In contrast, slower crystallization via a 3D precursor was observed for FAPbI(3–x)Cl(x) film formation compared to MAPbI(3–x)Cl(x). Another important finding was that some FAPbI(3–x)Cl(x) perovskite was generated directly during spin-casting before annealing. These findings indicate that there are significant differences between the crystallization pathways for these two perovskite materials. These are likely to explain the differences in the lifetimes of the resulting perovskite solar cell devices produced using FA and MA cations.