Characterization of Bipolar Transport in Hf(Te_1−xSe_x)₂ Thermoelectric Alloys

Control of bipolar conduction is essential to improve the high-temperature thermoelectric performance of materials for power generation applications. Recently, Hf(Te_1−xSe_x)₂ alloys have gained much attention due to their potential use in thermoelectric power generation. Increasing the Se alloying content significantly increases the band gap while decreasing its carrier concentration. These two factors affect bipolar conduction substantially. In addition, the weighted mobility ratio is estimated from the experimental electronic transport properties of Hf(Te_1−xSe_x)₂ alloys (x = 0.0, 0.025, 0.25, 0.5, 1.0) by using the Two-Band model. From the bipolar thermal conductivity also calculated using the Two-Band model, we find that it peaks near x = 0.5. The initial bipolar conductivity increase of x < 0.5 is mostly due to the decrease in the weighted mobility ratio and carrier concentration with increasing x. For x > 0.5, the drop in the bipolar conductivity can be understood with significant band gap enlargement.