Giant excitonic effects in bulk vacancy-ordered double perovskites

TL;DR

This study reveals exceptionally strong excitonic effects with giant binding energies in bulk vacancy-ordered double perovskites, challenging existing understanding of electron-hole interactions in semiconductors.

Contribution

First-principles GW and Bethe-Salpeter calculations uncover giant exciton binding energies in bulk perovskites, highlighting their Frenkel-like excitons and the roles of dielectric screening and spin-orbit coupling.

Findings

Giant exciton binding energies (~1 eV) in bulk perovskites.

Excitons are highly localized and resemble Frenkel excitons.

Spin-orbit coupling significantly affects exciton energies.

Abstract

Using first-principles GW plus Bethe-Salpeter equation calculations, we identify anomalously strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X = I, Br). Giant exciton binding energies about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of electron-hole (e-h) interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical 2D semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energy agrees well with the experimental optical band gap. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly bonded [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in red-shifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the strong exciton binding energies.