  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.
