On the Ion Coordination and Crystallization of Metal Halide Perovskites by In Situ Dynamic Optical Probing

Controlling the crystallization to achieve high-quality homogeneous perovskite film is the key strategy in developing perovskite electronic devices. Here, an in situ dynamic optical probing technique is demonstrated that can monitor the fast crystallization of perovskites and effectively minimize the influence of laser excitation during the measurement. This study finds that the typical static probing technique would damage and induce phase segregation in the perovskite films during the excitation. These issues can be effectively resolved with the dynamic probing approach. It also found that the crystallization between MAPbI3 and MAPbI2Br is strikingly different. In particular, MAPbI2Br suffers from inefficient nucleation during the spin-coating that strongly affects the uniform crystal growth in the annealing process. The commonly used pre-heating process is found at a lower temperature not only can further promote the nucleation but also to complete the crystallization of MAPbI2Br. The role of further annealing at a higher temperature is to facilitate ion-dissociation on the crystal surface to form a passivation layer to stabilize the MAPbI2Br lattices. The device performance is strongly correlated with the film formation mechanism derived from the in situ results. This work demonstrates that the in situ technique can provide deep insight into the crystallization mechanism, and help to understand the growth mechanism of perovskites with different compositions and dimensionalities. A non-destructive in situ dynamic photoluminescence probing technique is developed to investigate the fast ion-coordination, nucleation, and crystallization processes during the organo-metal halide perovskite film formation. The technique is able to uncover the limiting factors in achieving high-quality perovskite films, providing guidance to develop the optimal processing protocols.image