A density-functional-theory-based study of the lead-free perovskite materials CsGeX3 and CsGeX2X′ (X, X′ = Cl, Br, I) for photovoltaic applications

In this report, the perovskite materials CsGeX3 and CsGeX2X' (X and X'=Cl, Br, I) are studied using density functional theory (DFT) and a time-dependent (TD)-DFT approach. The structural, optoelectronic, and thermal characteristics of these materials are analysed using B3LYP/LANL2DZ and CAM-B3LYP/LANL2DZ functionals. The lattice constants and volume get intensified from CsGeCl3 to CsGeBr3 to CsGeI3. In the mixed halides CsGeX2X', the lattice constants and volume also follow a similar trend. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, computed from CAM-B3LYP, is higher compared to the B3LYP. For CsGeX3, HOMO-LUMO gaps computed from the functionals B3LYP/LanL2DZ and Cam-B3LYP/LanL2DZ are in the range of 1.13-2.12 eV and 1.62-3.03 eV, respectively, and for mixed halides, CuGeX2X', they vary from 1.66 eV to 2.60 eV and 1.90 eV to 2.75 eV, respectively. For mixed halide perovskites, the maximum HOMO-LUMO gap is found for CsGeBr2Cl. The HOMO-LUMO gaps of these perovskite materials obtained from the functional Cam-B3LYP/LanL2DZ are in line with the previously stated data and in the range needed for optoelectronic and photovoltaic devices. Quantum chemical descriptors and conceptual density-functional-based parameters are computed. The optical electronegativity values of CsGeX3 and CsGeX2X' are found to be directly proportional to the HOMO-LUMO gaps of these materials. The absorption spectra of mixed halides obtained from B3LYP/LanL2DZ are high compared to CAM-B3LYP/LanL2DZ. The computed data reveal a systematic reduction in thermal energy, Gibbs energy, and Zero-Point Vibrational Energy (ZPVE) as a consequence of substituting X-site atoms from Cl to Br to I.