Synergistic effect of Zn doping on thermoelectric properties to realize a high figure-of-merit and conversion efficiency in Bi2−xZnxTe3 based thermoelectric generators

Abstract

In state-of-the-art bismuth telluride based thermoelectric devices, the n-type composition shows a lower figure-of-merit (zT) than the p-type counterpart due to intrinsic electrical anisotropy and bipolar conduction at elevated temperatures. Herein, we demonstrate a significant improvement in the zT values of n-type Bi_2−xZn_xTe₃ (x = 0–0.3) materials through aliovalent Zn doping at the Bi-site in the rhombohedral Bi₂Te₃ structure. A simultaneous enhancement in the electrical conductivity (σ) and Seebeck coefficient (α) with Zn doping is observed, which is attributed to the increased carrier concentration (n) and effective mass (m*), respectively. Concomitantly, a drastic enhancement in the power factor (α²σ) from 2.4 mW m⁻¹ K⁻² (x = 0) to 3.1 mW m⁻¹ K⁻² (x = 0.2) at 430 K is achieved. In addition, the incorporation of Zn shows a significant reduction in the lattice and bipolar contributions to the total thermal conductivity (κ), which drastically enhances the zT value to ∼1.2 (x = 0.2) at 450 K. The achieved zT value is ∼70% higher than that of the undoped Bi₂Te₃ sample. A unicouple module fabricated using n-type Bi_1.8Zn_0.2Te₃ in combination with the compatible high zT p-type Bi_0.5Sb_1.495Cu_0.005Te₃ material shows a record-high conversion efficiency (η) of ∼8.1% at a temperature gradient (ΔT) of 224 K. The achieved η is amongst the best-reported values till date in Bi₂Te₃ lab-scale thermoelectric power generators.