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

Abstract

Methylammonium lead halide $({\mathrm{MAPbI}}_{3})$ perovskite has emerged as one of the frontier optoelectronic semiconductors. To avoid lead toxicity, the role of Sn substitution and Pb vacancy (Pb-$\ensuremath{\boxtimes})$ are addressed in regulating stability and solar cell efficiency of ${\mathrm{MAPb}}_{1\ensuremath{-}X\ensuremath{-}Y}{\mathrm{Sn}}_{X}{\ensuremath{\boxtimes}}_{Y}{\mathrm{I}}_{3}$ 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 ${\mathrm{MAPbI}}_{3}$, Sn substitution has a more favorable thermodynamic stability than creating Pb-$\ensuremath{\boxtimes}$. Moreover, on substituting Sn, due to strong $s\text{\ensuremath{-}}p$ and $p\text{\ensuremath{-}}p$ 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 ${\mathrm{MAPb}}_{1\ensuremath{-}X}{\mathrm{Sn}}_{X}{\mathrm{I}}_{3}$ for $0<X\ensuremath{\le}0.5$.