Electronic and optical properties of lead‐free double perovskites A₂BCl₆ (A = Rb, Cs; B = Si, Ge, Sn) for solar cell applications: A systematic computational study

Abstract In recent years, lead‐free double perovskite materials have attracted much attention due to their probable applications in photovoltaic and optoelectronic devices. In this work, the electronic and optical properties of lead‐free double perovskites A 2 BCl 6 (A = Rb, Cs; B = Si, Ge, and Sn) are studied using density functional theory (DFT) methodology. The result shows that the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO‐LUMO) energy gaps of these compounds vary between 0.524 and 0.919 eV, which agrees with the previously reported data. HOMO‐LUMO gap for Rb 2 SiCl 6 is observed as 0.919 eV, which falls in the optimal energy gap range, that is, 0.9 to 1.6 eV for double perovskite material. Conceptual DFT‐based descriptors—molecular hardness, softness, electronegativity, electrophilicity index, and dipole moment of these compounds—are studied. The tolerance factor of A 2 BCl 6 is observed in the range of 1.00 to 1.26. Rb 2 SnCl 6 is almost a perfect fit with a value of 1.00. Cs 2 SiCl 6 shows the maximum value of the refractive index and dielectric constant. Optical electronegativity is found between 0.178 and 0.246 eV. The suitable band gap and high value of the refractive index and dielectric constant make double perovskites A 2 BCl 6 effective for solar cells and optoelectronic devices.