First principle study of X2GaAgCl6 (X = Cs, Rb) double perovskites: structural, mechanical, vibrational, electronic, optical, SLME, thermoelectric, and thermodynamic properties for solar cell applications

Abstract

The structural, mechanical, vibrational, electronic, optical, SLME, thermoelectric, and thermodynamic properties of X₂GaAgCl₆ (X = Cs, Rb), a double perovskite material, were computed by employing Density Functional Theory (DFT). CASTEP and Quantum ESPRESSO were used to perform first-principles calculations. X₂GaAgCl₆ possesses a cubic structure with the space-group symmetry Fm-3 m. The lattice parameters of Cs₂GaAgCl₆ and Rb₂GaAgCl₆ were optimized using the energy-volume curves, resulting in values of 7.357 Å and 7.365 Å, respectively. The population analysis confirmed the charge transfer among transition metals and halogen atoms. The stability of crystal X₂GaAgCl₆ (X = Cs, Rb) is effectively demonstrated by analyzing phonon dispersion curves with no negative frequencies. The band structure calculations indicated the semiconducting nature of compounds with energy gaps of 0.96 eV and 0.88 eV for Cs₂GaAgCl₆ and Rb₂GaAgCl₆, respectively. The optical characteristics results confirm that the examined materials are suitable for devices working, primarily in the electromagnetic spectrum's visible region. SLME results showed that Cs₂GaAgCl₆ has 30% and Rb₂GaAgCl₆ has 27% efficiency, respectively, suggesting their use in photovoltaics. The thermoelectric properties of X₂GaAgCl₆ (X = Cs, Rb) were calculated by using the BoltzTraP code in the temperature range of 300 to 800 K. The quasi-harmonic Debye model was applied to calculate the thermodynamic characteristics.

Keywords: DFT; Direct band gap; Double perovskite halides; SLME; Thermodynamics.