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

TL;DR

This study uses advanced ab-initio simulations and symmetry analysis to clarify the exciton fine structure and excited states in layered halide perovskites, aiding their application in energy and light emission technologies.

Contribution

It provides a detailed theoretical analysis of exciton energy order and spin-character in layered halide perovskites, resolving existing debates and highlighting the role of symmetry and polar distortions.

Findings

Confirmed the energy order of exciton fine-structure components

Suggested polar distortions stabilize the in-plane exciton component

Clarified exciton spin-character in the presence of strong spin-orbit coupling

Abstract

Layered halide perovskites are solution-processed natural heterostructures where quantum and dielectric confinement effects down to the nanoscale strongly influence the optical properties, leading to stabilization of bound excitons. Achieving a detailed understanding of the exciton properties is crucial to boost the exploitation of these materials in energy conversion and light emission applications, with current 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, we perform here ab-initio solution of the Bethe Salpeter Equation (BSE), with detailed interpretation of the spectroscopic observables based on symmetry-analysis. We confirm the Edark < Ein plane < Eout-of-plane fine-structure assignment, as from recent magneto-absorption experiments. We further suggest that polar distortions may lead to stabilization of the in-plane component and ultimately end-up in a bright lowest exciton component. Also, we discuss the 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. In addition to contributing to the current understanding of the exciton properties of layered halide perovskites, the work further evidence the in-depth knowledge gained by combining advanced ab-initio simulations and group theory.