Effects of Ag doping and thermal annealing temperature on thermoelectric transport properties of Bi2(Se,Te)3 compounds are investigated. On the basis of the comprehensive analysis of carrier concentration, Hall mobility, and lattice parameter, we identified two Ag-related interstitial (Agi) and substitutional (AgBi) defects that modulate in different ways the thermoelectric properties of Ag-doped Bi2(Se,Te)3 compounds. When Ag content is less than 0.5 wt %, Agi plays an important role in stabilizing crystal structure and suppressing the formation of donor-like Te vacancy (VTe) defects, leading to the decrease in carrier concentration with increasing Ag content. For the heavily doped Bi2(Se,Te)3 compounds (>0.5 wt % Ag), the increasing concentration of AgBi is held responsible for the increase of electron concentration because formation of AgBi defects is accompanied by annihilation of hole carriers. The analysis of Seebeck coefficients and temperature-dependent electrical properties suggests that electrons in Ag-doped Bi2(Se,Te)3 compounds are subject to a mixed mode of impurity scattering and lattice scattering. A 10% enhancement of thermoelectric figure-of-merit at room temperature was achieved for 1 wt % Ag-doped Bi2(Se,Te)3 as compared to pristine Bi2(Se,Te)3.

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Thermoelectric properties of Bi-Sb-Te and Bi-Se-Te compounds with graded Ag doping profiles are reported. A junction structure with graded doping is formed in the Bi-Te based compounds through thermally driven Ag diffusion, which has demonstrated a greatly enhanced Seebeck coefficient when a thermal gradient is applied in the same direction of carrier concentration gradient. A mechanism based on the spatial variation of bandgap narrowing induced by heavy-doping effect is proposed to explain the anomalous thermoelectric property of Bi-Te based compounds with graded doping profiles.

The transverse thermoelectric effect is generally found in a material system with anisotropic electrical/thermal properties. Herein, we reported a simple way of forming thermoelectric anisotropy in a single piece of Bi-Sb-Te compound by partial doping of Ag elements. A transverse Seebeck effect is experimentally observed on an asymmetrically doped Bi-Sb-Te pellet with an effective Seebeck coefficient of 216 ± 4 μV/K. A two-dimensional distribution of electrical potential and temperatures in the pellet is modeled numerically. The size dependence of thermoelectric power and electrical resistance for asymmetrically doped Bi-Sb-Te compounds is also investigated.