Direct, indirect, and self-trapped excitons in Cs$_2$AgBiBr$_6$

TL;DR

This study uses advanced computational methods to analyze exciton states in Cs$_2$AgBiBr$_6$, revealing the importance of self-trapped excitons and correcting previous band gap estimates, with implications for optoelectronic applications.

Contribution

It demonstrates the effectiveness of TD-DFT with hybrid functionals in accurately predicting excitonic properties and the absorption spectrum of Cs$_2$AgBiBr$_6$, providing new insights into its excited states.

Findings

Accurate prediction of absorption spectrum using TD-DFT with hybrid functionals.

Identification of self-trapped excitons as the source of photoluminescence signals.

Revealing a complex landscape of competing excitonic states in the material.

Abstract

Cs$_2$AgBiBr$_6$ exhibits promising photovoltaic and light-emitting properties, making it a candidate for next-generation solar cells and LED technologies. Additionally, it serves as a model system within the family of halide double perovskites, offering insights into the broader class of materials. Here, we study various possible excited states of this material to understand its absorption and emission properties. We use Time-Dependent Density Functional Theory (TD-DFT) coupled with non-empirical hybrid functionals, specifically PBE0($\alpha$) and dielectric-dependent hybrids (DDH) to explore direct, indirect, and self-trapped excitons in this material. Based on comparison with experiment, we show that these methods can give excellent prediction of the absorption spectrum and that the fundamental band gap has been underestimated in previous computational studies. We connect the experimental photoluminescence signals at 1.9-2.0 eV to the emission from self-trapped excitons and electron polarons. Finally, we reveal a complex landscape with energetically competing direct, indirect, and self-trapped excitons in the material.