New Insights Into the Structural, Electronic, Optical and Phonon Properties of Cubic and Orthorhombic Cspbi3 Perovskite Through First-principles Method for Photovoltaic Applications

Abstract

This study has been done using the first-principles technique based on the plane wave self-consistent field method as implemented in the quantum espresso computational code. The GGA has been used as the exchange-correlation potential while the exchange-correlation functionals used in this work are those based on Perdew Burke Ernzerhof (GGA-PBE), and Perdew Burke Ernzerhof for solids (GGAPBEsol). The other exchange-correlation functionals used were norm-conserving (NC) scalar relativistic type used in calculating the structural, elastic, electronic, phonon-induced thermal stability, and anisotropic optical properties of cubic (α-CsPbI3) and orthorhombic (δ-CsPbI3), with consideration that incident radiation is oriented in the x, y and z directions. For the orthorhombic CsPbI3, the GGAPBE and GGA-PBEsol with PAW pseudopotentials were also used to calculate the electronic properties. The results showed the following direct band gaps for the orthorhombic structure: 2.4579 eV, 2.321346 eV and 2.39372 eV for GGA-PBE with PAW, GGA-PBE and GGA-PBEsol with NC pseudopotentials, respectively. On the other hand, for cubic structure, the direct band gaps calculated were 1.195219 eV and 1.169895 eV for GGA-PBE and GGA-PBEsol, respectively, with NC pseudopotentials. For the cubic structure, the states that dominated the conduction band are Cs 3(s), I 2(s), I 3(p), and Pb 5(p) in the case of GGA-PBE with NC pseudopotentials, and Cs 3(s), Pb 3(p), I 1(s) and I 2(p) for GGA-PBEsol with NC pseudopotential. For the orthorhombic structure, the conduction band was dominated by Cs 3(d) and Pb 2(p) for GGA-PBE with NC pseudopotentials. Pb 3(p) and I 2(p) dominated the conduction band in the case of PBEsol. The bulk modulus values ranged from 1317 - 1498 kbar for the orthorhombic structure and 159 kbar for the cubic structure. The calculated equilibrium volume and lattice constants for cubic and orthorhombic CsPbI3 agree with the experimental results. Both orthorhombic and cubic structures have been found to have optimal absorptivity within the visible radiation range extending to the edge of the UV radiation, whence the refractive index peaks. The calculated elastic properties depict the cubic structure as flexible. The imaginary phonon frequencies at low temperatures reveal slight structural instability for cubic (α- CsPbI3).