Novel Lead-free double perovskite Cs2LiFeBr6 as photovoltaics: a DFT study of structural, mechanical, spin-resolved electronic and optical properties

Lead-free halide double perovskites are grabbing attention as these compounds are environmentally friendly, stable and prominent for their high-performances in photovoltaics. Here we report a comprehensive first-principles study of the double perovskite Cs2LiFeBr6 using density functional theory with an on-site Hubbard correction. Cs2LiFeBr6 is geometrically relaxed and crystallizes in the cubic elpasolite structure (Fm-3m, No. 225). Thermodynamic stability is confirmed formation energy calculations. The dynamic stability is calculated by phonon calculation. The elastic constants indicate ductile mechanical behavior. The band gap is 0.444 eV; inclusion of the Hubbard correction yields a fully spin-polarized semiconducting ground state with an indirect band gap of 0.927 eV. Carrier effective masses are asymmetric. The static dielectric constant is 5.06, and the absorption coefficient exceeds 104 cm−1 from 1.17 eV, indicating Cs2LiFeBr6 is effective in visible-range solar energy harvesting. Although the band gap is indirect, the lowest direct allowed transition lies only 0.029 eV above it, so the spectroscopic limited maximum efficiency reaches 24.9% for a 1 µm film and maximum of 26.18% for a 5 µm film. This study unveils the structural stability, mechanical properties, chemical stability, and potential of solar cell applications of the structure.