Theoretical study of lead-free mixed-valence mixed-halide Au-based perovskites for optoelectronics

Abstract

Inorganic mixed-valence Au-based perovskites have received considerable attention due to their nontoxic elements, good stability and superior properties. Hence it is of significant interest to explore more novel Au-based perovskite materials for solar cell applications. This study presents a detailed theoretical exploration on the structural features, thermodynamic and mechanical stability, optoelectronic characteristics, and photovoltaic performance of Cs2AuIAuIIICl6-xBrx, Cs2AuIAuIIICl6-xIx, and Cs2AuIAuIIIBr6-xIx (x = 2, 4) in comparison with that of Cs2AuIAuIIIX6 (X = Cl, Br, I). Through the comprehensive evaluation of thermodynamic and mechanical stability, Cs2AuIAuIIIX2Y4 (X2Y4Cl2Br4, Cl2I4, and Br2I4) is identified to be stable. The impact of anion substitution on the electronic characteristics of these mixed-halide Au-based perovskites is thoroughly disclosed. It is observed from band structure calculations that Cs2AuIAuIIIX4Y2 (X4Y2Cl4Br2, Cl4I2, and Br4I2) exhibits a lower direct-gap within 0.7–1.0 eV, while Cs2AuIAuIIIX2Y4 (X2Y4Cl2Br4, Cl2I4, and Br2I4) possesses an indirect-gap with ∼1.1 eV. The nonlinear band gap variation is disclosed and reasonably explained. Furthermore, Cs2AuIAuIIIX2Y4 displays strong dipole-allowed visible-light absorption. In addition, the simulated efficiency of Cs2AuIAuIIIX2Y4 is within ∼28–30 %, which is comparable to those of GaAs and CH3NH3PbI3. This theoretical exploration is helpful for developing more novel inorganic Au-based perovskites with desirable photovoltaic performance.