Effect of the decrease of Pb concentration on the properties of pentarnary mixed-halide perovskites CsPb_8-xSn_xIBr₂ and CsPb_8-xSn_xI₂Br (1≤x≤7) for solar-cell applications: A DFT study

In order to investigate the possibility of producing novel perovskite materials for fabricating solar cells with low toxicity, the influence of Sn substitution for Pb on the structural, electronic, and optical properties of the pentarnary compounds CsPb_8-xSn_xIBr₂ and CsPb_8-xSn_xI₂Br (for 1 ≤ x ≤ 7) was investigated by density functional theory (DFT) calculations using the GGA-PBE approximation. The predicted structure of these new pentarnary materials is cubic. Their optical absorption coefficient is predicted to be in the range of 10⁵ cm⁻¹, which is a good match with that of the Sn-free CsPbIBr₂ and CsPbI₂Br compounds. The increase in Pb concentration causes an increase in the bandgap as well as in the optical absorption coefficient, which reveal the key role of Pb in perovskite solar cells. The compounds with the highest Pb concentration, CsPb₇SnIBr₂ and CsPb₇SnI₂Br, exhibited a promising bandgap of 1.31 and 1.29 eV, respectively, which is close to the Shockley-Queisser bandgap limit that is the best for solar-cell applications. The above properties, in conjunction with their relatively easy synthesis process, qualify these new predicted materials for being included in the family of the mixed-halide perovskites.