Fine Structure of Excitons in Vacancy Ordered Double Perovskites

A₂BX₆ vacancy ordered double perovskites (VODP) are air-stable Pb-free materials that have been proposed as alternatives to halide perovskites. Yet, to-date VODP are far away from the high performance of their Pb-based counterparts. In this work, we analyse the role of electron-hole (e-h) interactions in VODP by employing state-of-art G₀W₀ method and Bethe-Salpeter equation (BSE) calculations. We unveil key details of their electronic structure and show the strong impact of e-h interactions on their optical properties. The family is sampled by picking prototypes based on the valency of the tetravalent metal that sits at the B-site of the A₂BX₆ lattice. The structural properties are investigated, and we show that the size of the vacancy depends solely on the size of the halogen. We report exciton binding energies ranging from 190 to 1240 meV and analyse the exciton fine structure for each material. We perform a symmetry analysis of the band structures and exciton wavefunctions, on which a direct link between these and the metal site species is established. Finally, we identify the most and least promising candidates to act as photo-active materials for selective charge transport layers in light-emitting and solar-cell applications. Our findings provide fundamental understanding of the optical properties of the entire VODP family of materials and highlight how these are not suitable Pb-free alternatives to traditional halide perovskites.