First-Principles Study on Structural, Elastic, Mechanical, Electronic, Optical, and Thermoelastic Properties of Ge-Based Doped Halide Perovskites CsGeCl3-xBrx: Emerging Semiconductor Materials for Solar Cell and Photovoltaic Applications

In this paper, we present first-principles DFT calculations of the structural, elastic, mechanical, thermoelastic, optical, and electronic properties of mixed halide perovskites CsGeCl_3-xBr_x (x = 0, 1, 2, 3). CsGeCl_3-xBr_x crystallizes in a cubic (Pm-3m) structure at (x = 0, 3) and in a tetragonal (P4/mmm) when (x = 1, 2). CsGeCl_3-xBr_x are mechanically stable with intrinsic ductility and a Debye temperature \({\theta }_{D}\) above 97.4 ± 300 K. Using DFT with GGA-PBE and TB-mBJ functionals, we predict that semiconductor compounds CsGeCl_3-xBr_x are stable structures with direct band gap \({E}_{g}\), suitable for solar cells, photovoltaics, and related optoelectronic applications. The direct band gaps of CsGeCl_3-xBr_x are \({E}_{g}\) = 1.105–1.431 eV (PBE) and \({E}_{g}\) = 1.260–1.762 eV (mBJ). Also, the optical properties study reveals that the original peaks of CsGeCl_3-xBr_x materials lie in the visible light spectrum, confirming their candidate as a good absorber for solar cells. The results of this study confirm that through band gap tuning, we can obtain higher optical absorption ranges and greater efficiency for halide perovskite-based optoelectronics.