Thermoelectric properties of high power factor sulfide NiSbS and Co substitution system Ni1−xCoxSbS

This paper describes experimental and theoretical investigations of electronic and thermoelectric (TE) properties of high power factor sulfide Ni1−xCoxSbS (x = 0, 0.10, 0.20, and 0.40). For NiSbS, even in the metallic behavior, the power factor PF of NiSbS is 1.9 mW·K−2·m−1 at 300 K, which exceeds the high performance TE sulfide materials as tetrahedrites or colusites. For the Ni1−xCoxSbS system, the residual electrical resistivity ρresidual increases. However, the thermopower S decreases in comparison with NiSbS. For density functional theory (DFT) calculations, the chemical potential μ for NiSbS is located at the edge of the pseudo-gap in the electronic density of states (DOS). Electronic structure μ is located at the peak of PF, as understood by the large transmission R(E) and ∂R(E)/∂E at Fermi energy EF for NiSbS. For Co-substitution, μ shifts to the valley of PF from the peak, indicating the importance of electron filling control for TE properties.