Crystal structure and optoelectronic properties of Rb-based metal halide perovskites RbSiI₃ and RbGeI₃: GGA–PBE study

In this paper, the crystal structure, optical and electronic properties of two related rubidium iodide halide perovskites RbSiI₃ and RbGeI₃ are investigated and discussed thoroughly. The calculations of these properties are performed using the generalized gradient approximation under Perdew–Burke–Ernzerhof functional (GGA–PBE). Also, the structural optimizations and accurate optoelectronic properties have been achieved by exploiting full-potential linearized augmented plane wave method (FP-LAPW). Analysis of optimization results exposed that the volume per unit cell and lattice parameter (a₀ = 5.8348 Å (RbSiI₃)) and (a₀ = 5.9631 Å (RbGeI₃)) are closely in agreement with the previous results. In addition, the calculated values of tolerance factor (T_F ≈ 1.0) satisfy the creation criterion for perovskites, and the negative and small values of formation energy (ΔF_E) confirm the chemical stability of studied compounds. The results of density of states and band structures reveal that RbSiI₃ and RbGeI₃ are nonmagnetic semiconductors having a proper direct energy gap (E_g) of 0.465 and 0.953 eV, respectively, along the M–M symmetry directions in the first Brillouin zone. The 2-D distributions of charge density confirm that the chemical bonding of Rb–I and Si/Ge–I bonds obey the covalent and ionic nature. Moreover, we have calculated and discussed the optoelectronic properties, real ε₁(ω) and imaginary ε₂(ω) functions, optical absorption α(ω), reflectivity R(ω) and refractivity n(ω). The results obtained in this study like structural stability, suitable E_g and highly accurate optical absorption α(ω) of visible light waves, indicate the possible exploitation of semiconductors RbSiI₃ and RbGeI₃ and make them candidate materials for optoelectronics, such as photovoltaic solar cells, photosensors, photodetectors and photodiodes devices.