The Effects of Te^(2−) and I^− Substitutions on the Electronic Structures, Thermoelectric Performance, and Hardness in Melt-Quenched Highly Dense Cu_(2-x)Se

A systematic study has been carried out on the electronic band structure and density of states, crystal structures, thermoelectric properties, and hardness of the Cu_(2-x)Se system with and without Te^(2−) or I^− substitutions for Se^(2−). Density functional theory calculations indicate that stoichiometric Cu_2Se is a zero-gap material, and copper-deficient Cu_(1.875)Se is a p-type conductor. Te^(2–) substitution increases the total density of states at the Fermi level, whereas, the I^− substitution leads to the reduction of the total and partial density of states for both Se and Cu. Highly dense undoped, Te-doped, and I-doped Cu_(2-x)Se bulks have been fabricated by a melt-quenching method which only takes a few minutes. Rietveld refinements of the X-ray diffraction patterns reveal that the unit cells are expanded after doping. All the fabricated bulks are p-type conductors in accordance with band structure calculations, and they all have figure of merit, zT, values over or close to 1.0 at T = 973 K, except for the Cu_(2-x)Te_(0.16)Se_(0.84). Furthermore, the hardness is distinctly improved by the doping approach, with a maximum value of ca. 0.66 GPa for the Cu_(2-x)Te_(0.16)Se_(0.84), which is higher than those of polycrystalline Bi_2Te_3 and PbTe bulks.