Structural, electronic and optical properties of lead free Rb based triiodide for photovoltaic application: an ab initio study

Abstract

The instability of organic–inorganic hybrid halide perovskites due to light, heat, and moisture restricts them for practical use despite having most suitable photovoltaic properties and higher power conversion efficiency. Several methods such as surface engineering, carbon electrode utilization, and optimization of the components are applied to increase the stability; still, it is far from the practical implementation. Moreover, the toxicity of Pb in most of the efficient hybrid halide perovskites is another major issue. It motivates us to search for a stable and Pb free perovskite solar cell. Hence, a systematic investigation within density functional theory has been made on the structural, electronic, and optical properties of RbMI3 compounds (where M = Ge and Sn). The structural properties such as lattice parameters, formation energy are calculated. The calculated negative formation energy confirms the chemical stability for both compounds. The electronic properties like partial density of states, band structures with Perdew–Burke–Ernzerhof, Tran–Blaha modified Becke–Johnson (TB-mBJ) and HSE06 are discussed. Band gaps are calculated for RbGeI3 (2.645 eV) and RbSnI3 (2.544 eV) with TB-mBJ potential, which is proved to estimate the band gap values accurately for inorganic solids. SOC influences the conduction band minimum without any changes in the valence band maximum and thus reduces band gaps to 2.021 for RbGeI3 and 1.865 eV for RbSnI3. The optical properties like the real and imaginary part of the dielectric constants, absorption coefficients, refractive indices, and reflectivities have also been discussed. Further, transport properties like effective masses, binding energy of excitons, and spectroscopic limited maximum efficiency (SLME) are calculated for both RbMI3 compounds. The excitons for both structures are found to be Frenkel type. SLME for RbGeI3 and RbSnI3 having the thickness of 0.5 μm at a temperature of 300 K are found as 16.5% and 18%, respectively. Finally, the possibility of RbGeI3 and RbSnI3 in the solar cell configuration with TiO2 as the hole transporting material have been explored.