Concentration-dependent excess Cu doping behavior and influence on thermoelectric properties in Bi₂Te₃

Excess Cu has been reported as an effective way to enhance the thermoelectric performance of n-type Bi₂Te₃-based alloys as well as to secure the reproducibility of their electronic properties. However, the effect of Cu doping into Bi₂Te₃ lattice is also known to be complex since Cu can occupy either interlayer or cation/anion sites, depending on conditions. Herein, Cu doping behavior in a binary Bi₂Te₃ prepared by a conventional melt-solidification process was demonstrated, and corresponding changes in electronic and thermal transport properties were investigated. We found that the mechanism behind electronic transport properties improvements was different depending on Cu doping behavior: (a) power factor enhancement (especially at elevated temperatures) at low Cu concentrations (x ≤ 0.008 in Cu_xBi₂Te₃) is mainly due to interlayer intercalation of Cu, which optimizes electron concentration and increases the effective mass and (b) power factor enhancement at higher Cu concentrations (0.012 ≤ x ≤ 0.02 in Cu_xBi₂Te₃) is due to the substituted Cu at Bi-site, which increases the weighted mobility ratio, as well as intercalated Cu. Enhanced room temperature zT ~ 0.68 and average zT ~ 0.53 were obtained in Cu_0.02Bi₂Te₃ due to synergetic effect of intensified point defect (intercalated and substituted Cu) phonon scattering.