Size-Dependent Lattice Symmetry Breaking Determines the Exciton Fine Structure of Perovskite Nanocrystals

The order of bright and dark excitonic states in lead-halideperovskitenanocrystals is debated. It has been proposed that the Rashba effect,driven by lattice-induced symmetry breaking, causes a bright excitonicground state. Direct measurements of excitonic spectra, however, showthe signatures of a dark ground state, bringing the role of the Rashbaeffect into question. We use an atomistic theory to model the excitonfine structure of perovskite nanocrystals, accounting for realisticlattice distortions. We calculate optical gaps and excitonic featuresthat compare favorably with experimental works. The exciton fine structuresplittings show a nonmonotonic size dependence due to a structuraltransition between cubic and orthorhombic phases. Additionally, theexcitonic ground state is found to be dark with spin triplet character,exhibiting a small Rashba coupling. We additionally explore the effectsof nanocrystal shape on the fine structure, clarifying observationson polydisperse nanocrystals.