Understanding Exciton Effects in Optical Absorption and Circular Dichroism of Chiral 2D Hybrid Perovskites

In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling control of light, charge, and electron spin. Understanding the microscopic mechanism of chirality transfer from the chiral organic spacers to the inorganic sublattice require predictive first principles theories that can correlate complex experimental signatures with underlying physical phenomena. Here, we extend the first principle GW plus Bethe Salpeter equations (GW-BSE) approach to the study of the chiroptical properties of hybrid perovskites including excitons effects. In such calculation, the construction of the dielectric matrix is the principal bottleneck. We develop a fully ab initio approximation for the dielectric matrix, known as IPSA-2C, in which we separate the polarizability of the organic/inorganic layers into minimal building blocks, thus circumventing the undesirable power-law scaling. The IPSA-2C method reproduces the quasi-particle band structures and absorption spectra for a series of Ruddlesden–Popper perovskites to high accuracy, by including critical nonlocal effects neglected in simpler models, and sheds light on the complicated interplay of screening between the organic and inorganic sublattices. Then, by implementing calculations of the circular dichroism (CD) in the exciton basis, we study the CD of several layered perovskites. We reveal that a specific chiral perovskite, (S-NEA)₂PbBr₄ , has a large CD arising from an exchange-driven energy splitting of two nearly degenerate exciton states. We also present a numerical study of differences between the formulation of the circular dichroism (CD) in the length gauge and the velocity gauge.