A DFT study of electronic, optical and thermoelectric properties of Ge-halide perovskites CsGeX3 (X=F, Cl and Br)

In this paper, the structural, electronic, optical and thermoelectric properties of the inorganic Ge-based halide perovskites CsGeX3 (X = F, Cl; Br) have been investigated using density functional theory (DFT) and semi-classical Boltzmann transport theory. Electronic calculations show that the CsGeF3, CsGeCl3 and CsGeBr3 perovskites exhibit a direct bandgap of 2.10 eV, 1.25 eV, and 0.79 eV, respectively. Optical properties were calculated and discussed, including real and imaginary parts of dielectric functions, reflectivity, absorption coefficient, and refractive index, have been calculated and discussed. The calculation of the electrical conductivity (σ/τ), the electronic thermal conductivity (ke/ σ), the Seebeck coefficients (S) and the electronic figure of merit (ZTe) of CsGeF3, CsGeCl3 and CsGeBr3 systems show interesting results. The highest Seebeck coefficient of 2422 μv/k accompanied by the highest electronic figure of merit (0.99), was found for CsGeBr3 system at low temperatures, which indicates its high potential for thermoelectric applications.