A DFT study of the stability and optoelectronic properties of all-inorganic lead-free halide perovskites

In recent years, all-inorganic lead-free CsGe0.5Sn0.5I3 has been proposed as a promising candidate for perovskite solar cells. In the present study, the stability and optoelectronic properties of the CsGeX3 and Cs2GeX6 (X = Cl, Br, I) compounds and mixed CsGe1-xSnxI3 perovskites have been explored using first-principles calculations. The results indicate that the CsGeX3 compounds and Cs2GeCl6 are stable at room temperature, while Cs2GeBr6 and Cs2GeI6 are unstable. A phase transition is revealed for the CsGe1-xSnxI3 systems when the ratio of Sn doping is over 0.53. In addition, both CsGe2/3Sn1/3I3 and CsGe0.25Sn0.75I3 show the potential tendency for phase segregation. CsGe2/3Sn1/3I3 and CsGe0.25Sn0.75I3 have suitable band gaps and exhibit strong absorption coefficients, which are the promising candidates for solar cells. Our work demonstrates the favorable optoelectronic properties of novel lead-free mixed Ge–Sn perovskites for single-junction solar cells.