Influence of Cu Doping on Bipolar Conduction Suppression for p-type Bi0.5Sb1.5Te3 and Bi0.4Sb1.6Te3 Alloys

In this study, we report how Cu doping can modify the thermoelectric performance of p-type Bi0.5Sb1.5Te3 and Bi0.4Sb1.6Te3 thermoelectric alloys, including their electronic and thermal transport properties. For electronic transport, the power factors of both Bi0.5Sb1.5Te3 and Bi0.4Sb1.6Te3 compositions were increased by Cu doping. The origins of the enhanced power factors were examined using a single parabolic band model, by estimating the changes in deformation potential, effective mass, nondegenerate mobility and weighted mobility in both valence and conduction bands. The weighted mobility of the valence band was increased by Cu doping and increased Sb ratio, while the weighted mobility of the conduction band decreased, suggesting bipolar conduction was greatly reduced. For thermal transport, Cu0.0075Bi0.4Sb1.6Te3 and Bi0.4Sb1.6Te3 had a lower lattice thermal conductivity than Cu0.0075Bi0.5Sb1.5Te3 and Bi0.5Sb1.5Te3, respectively, due to an increase in Umklapp scattering. In addition, Cu doping suppressed bipolar thermal conductivity at high temperatures, by increasing hole concentration. It was also confirmed that Cu-doped samples had a lower lattice thermal conductivity than undoped samples due to additional point defect scattering. As a result, the thermoelectric figure of merit (zT) was greatly enhanced by 0.0075 mol of Cu doping, from 0.80 to 1.11 in Bi0.5Sb1.5Te3, while the zT is increased from 1.0 to 1.05 for Bi0.4Sb1.6Te3.