Improvement of thermoelectric properties and their correlations with electron effective mass in Cu 1.98 S x Se 1-x .

Sulphur doping effects on the crystal structures, thermoelectric properties, density-of-states, and effective mass in Cu1.98 Sx Se1-x were studied based on the electrical and thermal transport property measurements, and first-principles calculations. The X-ray diffraction patterns and Rietveld refinements indicate that room temperature Cu1.98 Sx Se1-x (x = 0, 0.02, 0.08, 0.16) and Cu1.98 Sx Se1-x (x = 0.8, 0.9, 1.0) have the same crystal structure as monoclinic-Cu2 Se and orthorhombic-Cu2 S, respectively. Sulphur doping can greatly enhance zT values when x is in the range of 0.8≤ × ≤1.0. Furthermore, all doped samples show stable thermoelectric compatibility factors over a broad temperature range from 700 to 1000 K, which could greatly benefit their practical applications. First-principles calculations indicate that both the electron density-of-sates and the effective mass for all the compounds exhibit non-monotonic sulphur doping dependence. It is concluded that the overall thermoelectric performance of the Cu1.98 Sx Se1-x system is mainly correlated with the electron effective mass and the density-of-states.