n-type thermoelectric performance in Co-doped Bi2Te3 polycrystalline alloys

Bi2Te3-based alloys are studied as most suitable thermoelectric alloys at room temperature. However, the lower zT values of Se-doped n-type Bi2Te3-xSex than those of p-type BixSb2-xTe3 still hinder provision of high device performance. In this study, in effort to search for n-type Bi2Te3-based alloys other than Se-doped Bi2Te3, electrical, thermal, and thermoelectric transport properties of Bi2-xCoxTe3 (x = 0, 0.03, 0.06, 0.09, and 0.12) alloys were examined systematically. By Co doping, power factor of Bi2Te3 alloys was increased by 65% to 2.89 mW/mK2 at 300 K. The enhanced electric transport properties were analyzed by experimental phenomenological parameters deduced from two-band model. It was also found that Co doping reduced bipolar and lattice thermal conductivities. Consequently, zT at 300 K was improved to 0.48 for Bi1.91Co0.09Te3 (x = 0.09) by 37% compared to that of 0.35 for Bi2Te3. The maximum zT of 0.62 was obtained for Bi1.91Co0.09Te3 at 400 K. The two-band model predicts that the zT of Co-doped Bi2Te3 can be further improved by optimizing carrier concentration.