First-principles investigation of stable lead-free halide perovskite materials CsSnCl _x Br _y I_{3- x- y}for solar cell applications

Perovskite solar cells based on hybrid organic-inorganic lead halide materials have attracted immense interest in recent years due to their enhanced power conversion efficiency. However, the toxic lead element and unstable property of the material limit their applications. With first-principles calculations based on density functional theory, we studied a series of ten lead-free perovskite materials made of cesium, tin, and halogen elements, chlorine (Cl), bromine (Br), and iodine (I). We found that the relative concentrations of the halogen atoms determine the crystal structures and the relative stability of the halide perovskites. Chlorine tends to increase the structural stability, while iodine plays the role of reducing the band gaps of the mixed halide perovskites. Considering the stability and the requirement of suitable band gaps, we identify that, among the ten lead-free halide perovskites, CsSnCl₂I, CsSnBr₂I, CsSnClBrI, CsSnClI₂, CsSnBrI₂, and CsSnI₃are the appropriate choices for solar cell applications.