A Density Functional Theory based Study of Lead-free Perovskite Materials CsGeX3 and CsGeX2X’ (X, X' = Cl, Br, I) for Photovoltaics Applications

Abstract In this report, perovskite materials CsGeX3 and CsGeX2X’ (X and X’ = Cl, Br, I) are examined by using Density Functional Theory (DFT) and Time-dependent (TD)- DFT approach. These materials' structural, optoelectronic, and thermal characteristics are analyzed using B3LYP/ LanL2DZ and Cam-B3LYP/LanL2DZ. Lattice constant and volume intensifies from CsGeCl3 to CsGeBr3 to CsGeI3. In the case of mixed halide CsGeX2X’, lattice constant and volume also follow a similar trend. The HOMO-LUMO gap computed from Cam-B3LYP is higher as compared to the B3LYP. In the case of CsGeX3, HOMO-LUMO gap computed from functional B3LYP/ LanL2DZ and Cam-B3LYP/LanL2DZ is in the range of 1.13 eV to 2.12 eV and 1.62 eV to 3.03 eV respectively, and for mixed halide CsGeX2X’, it varies from 1.66 eV to 2.60 eV and 1.90 eV to 2.75 eV correspondingly. For mixed halide perovskite, the maximum HOMO-LUMO gap is found for CsGeBr2Cl. The HOMO-LUMO gaps of these perovskite materials obtained from functional Cam-B3LYP/LanL2DZ are aligned with the previously stated data and the range needed for optoelectronic and photovoltaic devices. Quantum chemical descriptors and conceptual density functional-based parameters are computed. The optical electronegativity of CsGeX3 and CsGeX2X’ is directly proportional to the HOMO-LUMO gap of the materials. The absorption spectra of mixed halides obtained from B3LYP/LanL2DZ are high compared to Cam-B3LYP/LanL2DZ. The computed data reveals a systematic reduction in thermal energy, Gibbs energy, and ZVPE as a consequence of substituting X-site atoms from Cl to Br to I.