Interface-Specific In Situ Photoluminescence Resolves Depth-Dependent Crystallization in Solution-Processed Metal Halide Perovskites

Abstract Crystallization in solution-processed metal halide perovskites proceeds through an interplay among multiple precursor-derived species, including solvent molecules, ionic monomers, complexes, intermediate phases, and perovskite nuclei. Their abundance, composition, size, and stability govern crystal growth, defect formation, and film morphology. However, the spatially heterogeneous evolution of these species and their role in governing perovskite film quality remain insufficiently understood. Here, we investigate vertical crystallization heterogeneity in two prototypical methylammonium- and formamidinium-based perovskites using an in situ photoluminescence (PL) platform that selectively tracks crystallization at the film top surface and buried interface. We find that nucleation predominantly initiates earlier at the buried interface, whereas nuclei near the top surface exhibit faster transport and coarsening dynamics during crystal growth. In particular, we show that anti-solvent dripping can effectively trigger nucleation throughout the precursor film. Buried-interface nucleation is suppressed during high-quality film fabrication, suggesting that stabilizing buried-interface species is essential to minimize competing secondary nucleation and ensure unidirectional crystal growth. More broadly, this work demonstrates the capability of interface-specific in situ PL to resolve vertical crystallization heterogeneity, providing mechanistic insight and practical guidance for achieving high-quality perovskite thin films.