LAUSR

Stability and toxicity issues of hybrid lead iodide perovskite MAPbI$_3$ necessitate potential alternatives. Here we shed new light on promising photovoltaic properties of Gold mixed valence halide perovskites, satisfying fundamental requirements i.e. non-toxicity, stability, band gap in visible range, low excitonic binding energy etc.. Our study shows, favourable electronic structure resulting in high optical transition strength, thus sharp rise in absorption spectra near band gap yielding very high short circuit current density and hence gives higher simulated efficiency than MAPbI$_3$, for Cs$_2$Au$_2$I$_6$ and Cs$_2$Au$_2$Br$_6$. Careful investigation of defect physics reveals possibility of deep level halide vacancies in Cs$_2$Au$_2$I$_6$, but fairly less for Cs$_2$Au$_2$Br$_6$. Keeping Iodine moderate to rich and moderate cation in the growth environment for Cs$_2$Au$_2$I$_6$, while keeping anion moderate to poor and cation moderate to rich for Cs$_2$Au$_2$Br$_6$, may help achieve best possible efficiencies from both. We propose Cs$_2$Au$_2$X$_6$; (X=I, Br) to be the next generation potential solar absorber, with proper precautions taken during synthesis.