Understanding Complex Layered Halide Perovskites: Double Perovskites and Mosaic Perovskites

Layered halide perovskites offer a flexible platform for tuning optoelectronic properties through changes in composition. One of the most intriguing aspects of 2D halide double perovskites is the change in bandgap symmetry with dimension, observed for certain compositions. For example, the indirect gap of Cs₂AgBiX₆ converts to a direct gap at the <i>n</i> = 1 (monolayer) 2D perovskite, whereas the direct gap of Cs₂AgTlX₆ transitions to an indirect gap at the <i>n</i> = 1 limit. I will present a molecular orbital analysis that explains this transition and allows us to predict which double perovskite compositions will feature such a change in band dispersion pattern upon dimensional reduction. I will also present our recent studies on further increasing the accessible electronic structures of 2D perovskites by incorporating three stoichiometric octahedral metals. We refer to this family as “mosaic perovskites” due to the numerous local packing arrangements of the three different metal-halide octahedra that nevertheless maintain a fixed metal ratio. Notably, mosaic perovskites allow for Cu(I/II) mixed valence and sets the stage for many more mixed-valence metal combinations to be incorporated into halide perovskites.