High-entropy perovskite halides: design principles, properties, and emerging applications.

High-entropy perovskite halides are emerging as a compelling extension of compositional engineering for overcoming the persistent instability and performance limitations of conventional metal perovskite halides. By incorporating multiple principal components at sufficiently high concentrations, configurational entropy can act as a thermodynamic stabilizing factor enabling access to compositionally complex phases with modified structural, ionic, and optoelectronic properties. This review examines the design principles, synthesis strategies, and structure-property relationships that define this rapidly developing class of materials. We discuss how entropy-driven compositional complexity influences phase stability, lattice distortion, defect chemistry, ion migration, and band-edge electronic structure, with particular attention to the distinct roles of A-site and B-site disorder. We further summarize current synthetic routes, as well as emerging computational frameworks for screening and guiding the design of high-entropy compositions. Recent advances in light-emitting devices, photovoltaics, photodetection, imaging, sensing and electrochemical applications are then highlighted to illustrate how entropy engineering can improve both functionality and durability. Finally, we identify key unresolved challenges in compositional control, structural characterization, and device integration, and outline future directions for the rational development of high-entropy perovskite halides as robust optoelectronic materials.