Strong out-of-plane excitons in 2D hybrid halide double perovskites

y means of an ab initio investigation, exploiting many-body perturbation theory approaches on top of ground-state density functional theory calculations, we disclose the impact of the electronic correlation on the band structure and optical properties of a class of recently synthesized Pb-free 2D-layered halide perovskites, namely, (BA)4AgBiBr8 and (BA)2CsAgBiBr7. We observe strong out-of-plane excitonic dipoles characterizing the optical spectra, a result that suggests the potential applicability of these layered hybrid materials in photonics and plasmonics, where the perpendicular field direction plays a relevant role.