In Situ Monitoring of Nanocrystal Formation and Ion Migration in Lead Halide Perovskite Metal–Organic Framework Composites

Abstract Combining all‐inorganic perovskites (CsPbX₃) with metal–organic frameworks (MOFs) offers great promise for efficient optoelectronic applications, but a lack of understanding of the microscopic mechanisms of host‐guest interactions that lead to stability limits their further advancement. Herein, a series of CsPbX₃‐MOF composites is studied based on twinned Pb‐X‐based MOFs (X = Cl, Br) to explore the effect of the local ionic environment on their crystallization kinetics and the local photoluminescence (PL) properties by in situ fluorescence microscopy. Real‐time monitoring of the crystal growth process and PL characterization after complete crystallization shows a large compositional heterogeneity in the embedded mixed halogen perovskites. This heterogeneity is a result of the local ionic environment of the individual nanocrystals (NCs). In addition, the compositional heterogeneity is a dynamic phenomenon, manifested as spectral fluctuations of individual NCs on the time scales of tens of seconds under excitation. Compositional changes correlated over distances of micrometers point to collective ionic diffusion present in the system. This use of the in situ PL spectroscopy enables unique insights into the fundamental photophysical properties of perovskite‐MOF composites and will provide essential feedback for their further development and potential applications such as light‐emitting diodes and ion sensors.