The Effect of Phase Changes on Optoelectronic Properties of Lead-Free CsSnI₃ Perovskites

First-principles calculations were carried out within the framework of density functional theory to investigate the influence of phase changes on the electronic and optical properties of CsSnI₃. The lattice parameter and band gap of four different phases, i.e., α-, β-, γ-, and δ-phases, of CsSnI₃ are estimated by employing different exchange–correlation functionals in order to explore their ability to reproduce geometric and electronic structures adequately. Comparison of the calculated total energies shows the non-perovskite orthorhombic (δ-phase) modification of CsSnI₃ is the most stable, followed by the orthorhombic (γ-CsSnI₃) perovskite phase. Thermal stability calculations in the form of temperature dependence of entropy as well as the absence of imaginary frequencies in the phonon dispersion diagrams also confirmed the dynamical stability of the δ-CsSnI₃. The influence of the structural phase changes on the band gap and Fermi level shifts of CsSnI₃ were assessed. Contribution of the electronic states on the formation of the valence and conduction band of four phases of CsSnI₃ were determined, which were calculated using various exchange–correlation functionals, including the high-precision hybrid functional HSE06, and compared with available experimental ones. The calculated energy band distribution diagrams showed that all three perovskite modifications of CsSnI₃ have direct transitions, while δ-CsSnI₃ has an indirect transition. It was found that during the transition from δ- to α-phase, the Fermi level descends to the low energy region (towards the valence band), and the band gap decreases from 2.99 eV to 1.33 eV. During the transition from α- to β-phase, the band gap width again decreases to 1.23 eV and the Fermi level mixes by 1.65 eV towards the conduction band (CB). On the contrary, the band gap increases from β- to γ-phase and the Fermi level shifts by 0.41 eV towards the conduction band. The values of the complex dielectric constant and the refractive index of four phases of CsSnI₃ were also calculated.