Mixed-valence halide perovskites

Mixed-valence compounds—which feature an element in at least two different oxidation states—can display emergent optical and transport phenomena stemming from electron transfer between the different valences (intervalence charge-transfer; IVCT). As halide perovskites show promise as active materials in numerous optoelectronic devices, it is an opportune moment to incorporate and study the effects of mixed-valence in this versatile materials family, to access tunable electronic structures ranging from insulators to semiconductors to metals. Herein, we introduce the basic concepts of mixed-valence in molecules and discuss how these concepts may be extended to mixed-valence in extended solids. We then review the few studies of mixed valence in 3D and 2D halide perovskites and halide perovskites with mixed-valence impurities, ranging from studies in the early 1900s to the present day. Through judicious choice of metal ion , its coordinating ligands and their geometry, and overall structural dimensionality, chemists can exert powerful synthetic control over electronic delocalization in mixed-valence perovskites, and we hope to see this intriguing materials class expand to encompass new compositions . • Ligand field theory provides a powerful platform for understanding bandgap transitions. • The basic concepts of mixed-valence in molecules are readily extended to solids. • Intervalence charge transfer (IVCT) can tune solids from insulators to semiconductors to metals. • Through judicious choice of metal, ligand, and dimensionality, chemists can control IVCT. • We expect that many more mixed-valence halide perovskites can be synthesized.