Probing the thermoelectric transport properties of n-type Bi₂Te₃ close to the limit of constitutional undercooling

Bulk n-type Bi₂Te₃ single crystals with optimized chemical composition were successfully prepared by a high temperature-gradient directional solidification method. We investigate the influence of alloy microstructure, chemical composition, and growth orientation on the thermoelectric transport properties. The results show that the composition of single-crystal Bi₂Te₃ alloy, along the c axis direction, could be slightly tuned by changing the growth rate of the crystal. At a rate of 18 mm/h, the formed Bi₂Te₃ crystal exhibits good thermoelectric properties. At 300 K, a maximum Seebeck coefficient of -245 μV/K and an electrical conductivity of 5.6 × 10⁴ S/m are acquired. The optimal power factor is obtained as 3.3 × 10^-3 W/K²m, with a figure of merit of 0.74. It can be attributed to the increased tellurium allocation in the Bi₂Te₃ alloys, as verified well by the density functional theory calculations.