Phase diagrams and stability of lead-free halide double perovskites Cs2BB′X6: B = Sb and Bi, B′ = Cu, Ag, and Au, and X = Cl, Br, and I

Lead-free pnictogen/noble metal halide double perovskites Cs 2 BiAgCl 6 , Cs 2 BiAgBr 6 , and Cs 2 SbAgCl 6 are some of the most promising environmentally friendly alternatives to lead-halide perovskites. However, due to their relatively large band gaps (1.9-2.2 eV), they are not yet competitive candidates for use in photovoltaic devices. In this work, we perform a systematic study of the thermodynamic stability of the entire family of Cs 2 BB′X 6 compounds (B = Bi and Sb, B′ = Cu, Ag, and Au, and X = Cl, Br, and I), and we explore the possibility of chemical mixing as a route to stabilize pnictogen/noble metal halide perovskites with low band gaps. Our calculations indicate that Cs 2 BiAg 1-x Cu x Cl 6 mixes should be amenable to synthesis and could reduce the band gap down to 1.6-1.9 eV