Halide-Induced Band Gap Modulation in RbPbCl3-XBrX Perovskites: Insights from a First-Principles Study

Abstract

First-principles calculations based on density functional theory (DFT) as implemented in Quantum ESPRESSO were used to systematically investigate the structural and electronic properties of inorganic halide perovskites RbPbCl3-xBrx (x = 0.00, 0.25, 1.50, 2.75, and 3.00). The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was employed to describe exchange correlation effects. The optimized lattice parameters reveal that bromine incorporation significantly influences the crystal structure, leading to variations in lattice constants, bond lengths, and unit cell volume due to the difference in atomic radii between Cl and Br atoms. The calculated bulk modulus indicates a reduction in mechanical stiffness with increasing bromine concentration, suggesting enhanced structural flexibility. Electronic band structure analysis shows that all compounds have a direct band gap at the Γ-point. Notably, the band gap values obtained were 2.22 eV, 1.87 eV, 1.62 eV, 1.32 eV, and 1.21 eV for RbPbCl3-xBrx (x = 0.00, 0.25, 1.50, 2.75, and 3.00), respectively. The decrease in the band gap becomes prominent as the Br doping concentration increases. This indicates effective band gap tuning via halide substitution. The density of states (DOS) and partial density of states (PDOS) analysis reveal that the valence band is predominantly contributed to by halide p-orbitals, while the conduction band is mainly composed of Pb p-states with minor contributions from Rb states. These findings demonstrate that bromine doping effectively modulates the electronic properties of RbPbCl3, making RbPbCl3-xBrx a promising candidate for optoelectronic and photovoltaic applications.