Tuning the optoelectronic properties of MASnI3 perovskite via Br and Cl doping and Br/Cl co-doping: a first principles DFT study

Abstract

Lead-free tin-based halide perovskites are promising light-absorbing materials for photovoltaic and optoelectronic applications, but their electronic structures and optical responses require further optimization. Using first-principles density functional theory (DFT) calculations performed with the CASTEP code, employing the GGA-PBE exchange-correlation functional, on-the-fly generated ultrasoft pseudopotentials, a 550 eV plane-wave cutoff, and a 2 × 4 × 2 Gamma-centered Monkhorst-Pack k-point mesh, this work systematically investigates the effect of Br and Cl substitution on the electronic and optical properties of CH₃NH₃SnI₃ (MASnI₃). Pristine, Br-doped, Cl-doped, and Br/Cl co-doped MASnI₃ are found to exhibit direct band gaps of 1.055, 0.675, 0.714, and 0.722 eV, respectively. The total and partial densities of states show that substitution of I by Br and Cl redistributes the electronic states and modifies the orbital contributions near the band edges. Br and Cl incorporation also shifts the principal optical response toward longer wavelengths, reduces the ultraviolet response, and enhances absorption in the visible region, with consistent changes observed in the dielectric function, complex refractive index, optical conductivity, and energy-loss function. These results indicate that Br and Cl substitution is an effective strategy for tuning the optoelectronic properties of MASnI₃, supporting its further evaluation as a lead-free absorber material for photovoltaic and photodetector applications.