Investigation on p-type conversion of n-type Cu_0.008Bi₂Te_2.4Se_0.6 thermoelectric alloys by slight Pb doping

Preventing the mechanical failure caused by differences in the thermomechanical properties of n- and p-type materials during thermal cycle is crucial for the successful utilization of thermoelectric devices. Therefore, realizing p–n conversion in a similar composition by doping would provide maximum mechanical stability and further understanding of the p–n conversion mechanism. In this study, the p-type conversion of n-type Cu_0.008Bi₂Te_2.6Se_0.4 alloys by Pb doping was investigated by synthesizing a series of Cu_0.008Bi_2-xPb_xTe_2.6Se_0.4 (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) compositions. It was found that the small amount of Pb dopants substituting the Bi sites provided a number of holes very effectively by generating multiple holes per dopant. Consequently, the electron concentration decreased as x increased to 0.01, and holes became the majority carriers at 300 K, switching to the p-type, beyond x = 0.01. When converted to p-type, a p-type power factor as high as 1.5 mW/mK² was achieved for x = 0.02 at 300 K, which was approximately 58% of the n-type power factor of the pristine sample. The maximum p-type zT of 0.55 was achieved for x = 0.02 (Cu_0.008Bi_1.98Pb_0.02Te_2.6Se_0.4), which was approximately 60% of the n-type zT of 0.91 for the pristine sample. A relatively comparable performance of p-type zT with similar compositions was observed in n-type Cu_0.008Bi₂Te_2.4Se_0.6 owing to the generation of holes through slight Pb doping.