Structural distortions control scaling of exciton binding energies in two-dimensional Ag/Bi double perovskites

Three-dimensional metal halide double perovskites such as Cs2AgBiBr6 exhibit pronounced excitonic effects which have been linked to their anisotropic electronic structure leading to chemical localization effects. Previous experimental and computational studies reported unexpected trends for the optical band gap and exciton binding energies of the quasi two-dimensional derivatives of this material, with organic molecules separating inorganic perovskite layers. Contrary to expectations based on dielectric and quantum confinement, it was found that the exciton binding energy is not monotonously increasing as the thickness of the inorganic layer tends to the monolayer limit. We use ab initio Green's functions-based many-body perturbation theory in the GW and Bethe-Salpeter Equation approximation to elucidate the origin of these observations. We systematically compare the electronic and excitonic properties of experimental and a series of model structures built to isolate the effects of octahedral distortions, layer separation and layer stacking patterns. Our calculations unambiguously identify structural distortions in the experimental systems as the origin of the previously reported trends. Furthermore, by changing layer separation and layer stacking, we suggest possible pathways for tuning the optoelectronic properties of such materials by using organic spacers of different thicknesses in their synthesis.