Indirect to direct band gap transition of ABI3 (A = Rb, Cs; B = Ca, Sr) perovskites under hydrostatic pressure for photovoltaic and optoelectronic applications: A DFT study

The objective of this study is to enhance the utilization of ABI₃ (A = Rb, Cs; B = Ca, Sr) metal-halide perovskites in multiple forms of technological applications by investigating their structural, electronic, optical, and mechanical characteristics using the ab initio method. At ambient pressure, the geometry-optimized lattice constants for RbCaI₃, RbSrI₃, CsCaI₃, and CsSrI₃ are 6.19 Å, 6.45 Å, 6.22 Å, and 6.47 Å, respectively, consistent with prior research. Additionally, a significant decrease in lattice parameters and bond length is observed, which leads to increased levels of atomic interactions. Pressure narrows the band gap from the ultraviolet-to-visible region. This phenomenon promotes the transfer of electrons from the valence-to-conduction band, hence improving the performance of optoelectronic devices. Furthermore, a shift from an indirect to a direct band gap is induced by the application of pressure, which improves the material's suitability for photovoltaic applications. The covalent and ionic nature of Ca/Sr-I and Rb/Cs-I under pressure and without pressure conditions was determined using the charge density maps. Enhanced pressure significantly boosts optical absorption and conductivity in the visible spectrum, implying substantial potential for improving the efficacy of perovskite solar cells and photonic devices. Additionally, applied pressure significantly influences the mechanical properties of the entitled perovskites, leading to higher ductility and anisotropy. As a whole, this study offers insights into the scientific applications of non-toxic perovskites in different pressure environments.