Studies on phase formation behavior and thermoelectric transport properties of Cu-doped Bi₂Te₃–Bi₂S₃ system

Herein, we investigated the influence of sulfur (S) addition on the phase formation behavior and thermoelectric transport properties of n-type Cu_0.008Bi₂Te₃. A complete single phase of rhombohedral Bi₂Te₃ was formed in Cu_0.008Bi₂Te_2.8S_0.2, whereas a mixture of Bi₂Te₃ and Bi₂Te₂S₁ phases was generated at higher S contents. We elucidated the complex effects of S substitution at Te-site on transport properties of Cu_0.008Bi₂Te₃; (1) increase in density-of-states effective mass, (2) decrease in weighted mobility ratio, (3) reduction in lattice thermal conductivity, and (4) reduction in bipolar thermal conductivity. Despite the slight deterioration in electronic transport properties, a maximum thermoelectric figure of merit (zT) of 0.82 was achieved at 400 K in Cu_0.008Bi₂Te_2.8S_0.2 due to the synergetic effect of intensified point defect phonon scattering in a wide temperature range and suppressed bipolar thermal conduction at elevated temperatures.