Large thermoelectric power factor in TiS2 crystal with nearly stoichiometric composition

A TiS₂ crystal with a layered structure was found to have a large thermoelectric power factor. The in-plane power factor S²/ρ at 300 K is 37.1 μW/K²cm with resistivity (ρ) of 1.7 mΩ cm and thermopower (S) of -251 μV/K, and this value is comparable to that of the best thermoelectric material, Bi₂Te₃ alloy. The electrical resistivity shows both metallic and highly anisotropic behaviors, suggesting that the electronic structure of this TiS₂ crystal has a quasi-two-dimensional nature. The large thermoelectric response can be ascribed to the large density of states just above the Fermi energy and inter-valley scattering. In spite of the large power factor, the figure of merit ZT of TiS₂ is 0.16 at 300 K, because of relatively large thermal conductivity, 68 mW/K cm. However, most of this value comes from reducible lattice contribution. Thus, ZT can be improved by reducing lattice thermal conductivity, e.g., by introducing a rattling unit into the inter-layer sites.