First-Principles Study on Structural, Electronic, and Optical Properties of Inorganic Ge-Based Halide Perovskites

Using density functional theory calculations, we explore the structural, electronic, and optical properties of the inorganic Ge-based halide perovskites AGeX₃ (A = Cs, Rb; X = I, Br, Cl) that can possibly be used as light absorbers. We calculate the lattice parameters of the rhombohedral unit cell with an R3 m space group, frequency-dependent dielectric constants, photoabsorption coefficients, effective masses of charge carriers, exciton binding energies, and electronic band structures by use of PBEsol and HSE06 functionals with and without SOC effect. We also predict the absolute electronic energy levels with respect to the external vacuum level by using the (001) surfaces with AX and GeX₂ terminations, demonstrating their strong dependence on the surface terminations. The calculated results are found to be in reasonable agreement with the available experimental data for the cases of CsGeX₃, while for the cases of RbGeX₃ they are predicted for the first time in this work. We reveal that replacement of Cs with Rb can offer reasonable flexibility in optoelectronic properties matching for solar cell design and optimization, while X anion exchange gives rise to large changes.