Thermoelectric behaviour of CuFeS₂ from DFT simulations

We will present a density functional theory (DFT) study of the electronic structure and thermoelectric behaviour of CuFeS₂. Using Hubbard U corrections and Grimme’s dispersion corrections, we find values of bandgaps and cell parameters in good agreement with experiment. The electronic structure suggests significant covalent character of the Cu-S bonding and a more ionic Fe-S bonding in the lattice. We calculate electronic transport coefficients based on the Boltzmann’s transport equation with the relaxation time approximation. A model for energy- and temperature-dependent carrier relaxation time, based on the electronic density of states (DOS), gives a significant improvement over the constant relaxation time approximation (CRTA), giving a better fit to the electrical conductivities and a better estimate of the Seebeck coefficients. Lattice thermal conductivities (κ_L) calculated from DFT also compare well with the experimental values. We predict that nanostructuring would be an effective way to reduce κ_L in this material, although the effect is much less pronounced at the high temperatures of interest for applications, than at room temperature. We show that a figure of merit (zT) as high as 0.44 can be reached for the bulk sample at 673 K at a carrier concentration of 5×10²⁰ cm^-3. With the reduction of the particle size, a significant enhancement of the ZT values is predicted (up to zT ≈ 1.5 at 673 K and optimal carrier concentration).