Reducing lead toxicity in the methylammonium lead halide MAPbI3 : Why Sn substitution should be preferred to Pb vacancy for optimum solar cell efficiency

Methylammonium lead halide (MAPbI₃) perovskite has emerged as one of the frontier optoelectronic semiconductors. To avoid lead toxicity, the role of Sn substitution and Pb vacancy (Pb-) are addressed in regulating stability and solar cell efficiency of MAPb_1-X-YSnXYI₃ perovskite using hybrid density functional theory (DFT). The role of spin-orbit coupling (SOC) and the electron's self-interaction error are examined carefully. We find to reduce the Pb content from pristine MAPbI₃, Sn substitution has a more favorable thermodynamic stability than creating Pb-. Moreover, on substituting Sn, due to strong s-p and p-p couplings, the lower parts of the conduction band gets shifted downwards, which results in the reduction of the band gap (direct). This further helps us to get a high optical absorption coefficient (redshifted) and maximum solar cell efficiency in MAPb_1-XSnXI₃ for $0