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

Abstract

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 Rb M I 3 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 RbGeI 3 (2.645 eV) and RbSnI 3 (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 RbGeI 3 and 1.865 eV for RbSnI 3 . 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 Rb M I 3 compounds. The excitons for both structures are found to be Frenkel type. SLME for RbGeI 3 and RbSnI 3 having the thickness of 0.5 μ m at a temperature of 300 K are found as 16.5% and 18%, respectively. Finally, the possibility of RbGeI 3 and RbSnI 3 in the solar cell configuration with TiO 2 as the hole transporting material have been explored.

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