| Summary: | CsGeI<sub>3</sub> has been widely studied as an important photoelectric material. Based on the density functional theory (DFT), we use first-principles to study the photoelectric properties of CsGeI<sub>3</sub> by applying successive hydrostatic pressure. It has been found that CsGeI<sub>3</sub> has an optimal optical band gap value of 1.37 eV when the applied pressure is −0.5 GPa, so this paper focuses on the comparative study of the photoelectric properties when the pressure is −0.5 GPa and 0 GPa. The results showed that CsGeI<sub>3</sub> has a higher dielectric value, conductivity, and absorption coefficient and blue shift in absorption spectrum when the pressure is −0.5 GPa. By calculating and comparing the effective masses of electrons and holes and the exciton binding energy, it was found that their values are relatively small, which indicates that CsGeI<sub>3</sub> is an efficient light absorbing material. CsGeI<sub>3</sub> was found to be stable under both pressure conditions through multiple calculations of the Born Huang stability criterion, tolerance factor T, and phonon spectrum with or without virtual frequency. We also calculated the elastic modulus of both pressure conditions and found that they are both soft, ductile, and anisotropic. Finally, the thermal properties of CsGeI<sub>3</sub> under two kinds of pressure were studied. It was found that the Debye temperature and heat capacity of CsGeI<sub>3</sub> increased with the increase of thermodynamic temperature, and the Debye temperature increased rapidly after pressure, while the heat capacity slowly increased and finally stabilized. Through the calculation of enthalpy, entropy, and Gibbs free energy of CsGeI<sub>3</sub>, it was found that the Gibbs free energy decreases faster with the increase of temperature without applied pressure, which indicates that CsGeI<sub>3</sub> has a higher stability without pressure. Through the comparative analysis of the photoelectric properties of CsGeI<sub>3</sub> under pressure, it was found that CsGeI<sub>3</sub> after applied pressure is a good photoelectric material and suitable for perovskite solar cells (PSCs) material.
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