Exchange-driven excitonic circular dichroism in two-dimensional hybrid perovskite

The introduction of chiral cations into two-dimensional hybrid perovskites opens a new pathway for designing chiroptical devices. However, the exact mechanism of chirality transfer from the chiral cations to the inorganic sublattice is still unclear, since it involves a complex interplay of structural distortion, electronic excitation, and dynamical many-body effects that are challenging to capture in theory. Here, we address the role of many-body excitonic interactions in the circular dichroism (CD) of (S-NEA)₂PbBr₄, where NEA stands for 1-(1-naphthyl)ethylammonium. The CD of this material exhibits a pronounced Cotton effect at the bandedge. We perform first-principles calculations based on the GW plus Bethe-Salpeter equation (GW-BSE) framework to calculate the exciton states, absorption spectrum, and CD of (S-NEA)₂PbBr₄. Our calculations reveal that the observed Cotton effect arises from a pair of bright exciton states localized on the inorganic sublattice whose energies are split by a quantum mechanical electron-hole exchange interaction between states on degenerate Rashba-Dresselhaus split bands. Our first-principles calculations agree well with experimental results obtained from bulk (S-NEA)₂PbBr₄ single crystals and provide insight into the requisite conditions for the emergence of exchange-driven Cotton effect in hybrid and organic crystals.