# Optoelectronic Properties and Defect Physics of Lead-free Photovoltaic Absorbers Cs$_2$Au$^{I}$Au$^{III}$X$_6$ (X=I, Br)

## TL;DR

This study explores lead-free gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X=I, Br, Cl) as promising, stable, and non-toxic photovoltaic materials with high efficiency potential, while analyzing defect physics that could impact performance.

## Contribution

It introduces a new class of lead-free perovskites with favorable optoelectronic properties and provides insights into defect physics affecting their photovoltaic performance.

## Key findings

- High optical absorption near the band gap.
- Potential deep-level defects that can trap carriers.
- Higher simulated efficiency than MAPbI$_3$.

## Abstract

Stability and toxicity issues with the hybrid lead iodide perovskite MAPbI$_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X = I, Br, Cl). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very high short-circuit current density and hence higher simulated efficiency compared with MAPbI$_3$. However, careful investigation of defect physics reveals the possibility of deep-level defects (such as V$_X$ , V$_{Cs}$, X$_{Au}$, X$_{Cs}$, Au$_i$, and Au$_X$ , X = I, Br), depending on the growth conditions. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help in taking necessary precautions in synthesizing these compounds in a controlled chemical environment, which should minimize performance limiting defects and pave the way for future studies on this class of materials.