Electrical, Thermal, and Thermoelectric Transport Properties of Co-Doped n-type Cu_0.008Bi₂Te_2.6Se_0.4 Polycrystalline Alloys

Bi₂Te₃-based alloys have been extensively studied as thermoelectric materials near room temperature. In this study, the electrical, thermal, and thermoelectric transport properties of a series of Co-doped n-type Cu_0.008Bi₂Te_2.6Se_0.4 polycrystalline alloys (Cu_0.008Bi_2−xCo_xTe_2.6Se_0.4, x = 0, 0.03, 0.06, 0.09 and 0.12) are investigated. The electrical conductivity of the Cu_0.008Bi_1.97Co_0.03Te_2.6Se_0.4 (x = 0.03) sample was significantly enhanced, by 34%, to 1199 S/cm compared to 793 S/cm of the pristine Cu_0.008Bi₂Te_2.6Se_0.4 (x = 0) sample at 300 K, and gradually decreased to 906 S/cm for x = 0.12 upon further doping. Power factors of the Co-doped samples decreased compared to the 3.26 mW/mK² of the pristine Cu_0.008Bi₂Te_2.6Se_0.4 sample at 300 K. Meanwhile, the power factor of the Cu_0.008Bi_1.97Co_0.03Te_2.6Se_0.4 (x = 0.03) sample became higher at 520 K. The lattice thermal conductivities of the Co-doped samples decreased due to additional point defect phonon scattering by the Co dopant. Consequently, the zT for the Cu_0.008Bi_1.97Co_0.03Te_2.6Se_0.4 alloy at 520 K was 0.83, which is approximately 15% larger than that of pristine Cu_0.008Bi₂Te_2.6Se_0.4, while the zT of the Cu doped samples at 300 K was smaller than that of the pristine Cu_0.008Bi₂Te_2.6Se_0.4 sample. Electrical transport properties of the Co-doped Cu_0.008Bi_2−xCo_xTe_2.6Se_0.4 samples were analyzed by experimental phenomenological parameters, including the density-of-state, effective mass, weighted mobility, and quality factor.