Unraveling the crystallization process in mix halide wide bandgap perovskite by in-situ dynamic optical probing

Controlling the crystallization to achieve a high-quality homogeneous film of mixed halide wide bandgap (>1.7 eV) perovskites is vital for the development of tandem and semitransparent solar cells. Since nucleation from the precursor ions is sensitive and typically completed in a few seconds, monitoring the crystallization during the perovskite film formation is still challenging. Here, we demonstrate an in-situ dynamic optical-probing technique which can monitor the crystallization without affecting the process by laser excitation. It is found that the generally observed low film quality is due to the inefficient nucleation of MAPbI2Br during the spin-coating process. Once the film is transferred to the annealing process, we find that there is a fast ion-exchange of halide ions within the first few second, and large amount of PbI2 is formed simultaneously which strongly deteriorate the film homogeneity. We find that an additional preheating process at a moderate temperature is crucial to promote the nucleation of perovskite and retard the forming of PbI2. Such process can significantly increase the number of MAPbI2Br nuclei, and therefore facilitates the formation of uniformly composited perovskite film. Eventually, only a small amount of PbI2 is observed around the grain boundary which in turn passivates the perovskite. This presentation would provide insight into identifying the limiting factors which could affect the crystallization of the organo-metal halide perovskites films during the fabrication processes by in-situ optical techniques.