Epitaxial growth of FAPbI3 thin films on MAPbBr3 single crystals via vapor phase deposition with suppressed halide ion migration

Achieving epitaxial heterostructures of metal halide perovskites (MHPs) remains a significant challenge due to the high mobility of halide ions and the dissolution of underlayer perovskites in solution processes. Here, we demonstrate the vapor-phase epitaxial growth of FAPbI₃ thin films on MAPbBr₃ single-crystal substrates by co-evaporation of FAI and PbI₂. The use of different A-site cations (FA⁺ for the thin film and MA⁺ for the substrate) effectively suppressed interfacial halide ion migration, enabling the formation of a stable iodine-rich perovskite layer. Structural analyses including X-ray diffraction, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry confirm the epitaxial nature and compositional stability of the films. Reciprocal space mapping reveals a relaxed epitaxial relationship with nanometer-scale grain domains. Importantly, the heterostructure maintained its crystallinity over 70 days without the formation of δ-phase FAPbI₃, highlighting enhanced long-term stability. These findings offer insights into controlling interdiffusion in MHP epitaxy and open pathways for developing next-generation perovskite-based heterostructures.