Fine Structure\nof Excitons in Vacancy-Ordered Halide\nDouble Perovskites

Abstract

Vacancy-ordered halide double perovskites (VODPs) have\nbeen widely\nexplored throughout the past few years as promising lead-free alternatives\nfor optoelectronic applications. Yet, the atomic-scale mechanisms\nthat underlie their optical properties remain elusive. In this work,\na thorough investigation of the excitonic properties of key members\nwithin the VODP family is presented. We employ state-of-the-art ab\ninitio calculations and unveil critical details regarding the role\nof electron–hole interactions in the electronic and optical\nproperties of VODPs. The materials family is sampled by picking prototypes\nbased on the electronic configuration of the tetravalent metal at\nthe center of the octahedron. Hence, groups with a valence comprised\nof s, p, and d closed shells are represented by the known materials\nCs₂SnX₆, Cs₂TeX₆, and\nCs₂ZrX₆ (with X = Br, I), respectively. The\nelectronic structure is investigated within the G₀W₀ many-body Green’s function method, while the Bethe–Salpeter\nequation is solved to account for electron–hole interactions\nthat play a crucial role in the optical properties of the family.\nA detailed symmetry analysis unravels the fine structure of excitons\nfor all compounds. The exciton binding energy, excitonic wavefunctions,\nand the dark–bright splitting are also reported for each material.\nIt is shown that these quantities can be tuned over a wide range,\nfrom Wannier- to Frenkel-type excitons, through for example substitutional\nengineering. In particular, Te-based materials, which share the electronic\nvalency of corner-sharing Pb halide perovskites, are predicted to\nhave exciton binding energies of above 1 eV and a dark–bright\nsplitting of the excitons reaching over 100 meV. Our findings provide\na fundamental understanding of the optical properties of the entire\nfamily of VODP materials and highlight how these are not, in fact,\nsuitable Pb-free alternatives to traditional halide perovskites.