Exciton ground state fine structure and excited states landscape in layered halide perovskites from combined BSE simulations and symmetry analysis

Layered halide perovskites are solution-processed natural heterostructures where quantum and dielectric confinement down to the nanoscale strongly influence the optical properties, leading to stabilization of bound excitons. Detailed understanding of the exciton properties is crucial to boost the exploitation of these materials in energy conversion and light emission applications, with on-going debate related to the energy order of the four components of the most stable exciton. To provide theoretical feedback and solve among contrasting literature reports, this work performs ab initio solution of the Bethe-Salpeter equation (BSE) for symmetrized reference Cs2PbX4 (X = I and Br) models, with detailed interpretation of the spectroscopic observables based on group-theory analysis. Simulations predict the following E-dark Ein-plane Eout-of-plane fine-structure assignment, consistent with recent magneto-absorption experiments and obtain similar increase in dark/bright splitting when going from lead-iodide to a lead-bromide composition as found experimentally. The authors further suggest that polar distortions may lead to stabilization of the in-plane component and end-up in a bright lowest exciton component, discuss exciton landscape over a broad energy range and clarify the exciton spin-character, when large spin-orbit coupling is in play, to rationalize the potential of halide perovskites as triplet sensitizers in combination with organic dyes.