Enhanced Thermoelectric Properties in Highly Co-Doped Bi₂Se₃by Density-of-State Effective Mass Increase

Bi2Se3 alloys are promising thermoelectric materials for use at near room temperature. In this study, we investigated the thermoelectric transport properties of Co-doped Bi2Se3, Bi2-xCoxSe3 (x=0, 0.03, 0.06, 0.09, and 0.12) polycrystalline alloys. The electrical conductivity of Bi2Se3 increased with Co doping as the electron concentration increased. The power factor increased by 83% (1.17 mW/mK2) with the Co doping compared with 0.64 mW/mK2 for pristine Bi2Se3 at room temperature. The density-of-states effective mass significantly increased to 0.30 m0 (free electron mass) by high Co doping of x =0.12 compared to 0.14 m0 for pristine Bi2Se3. The total thermal conductivity of the Co-doped samples increased owing to the increase in electronic thermal conductivity, despite the systematic decrease in the lattice thermal conductivity caused by Co doping due to additional phonon scattering. Nevertheless, an enhancement in zT was observed for all the Co-doped samples. The maximum zT value of 0.39 in highly Co-doped Bi1.88Co0.12Se3 is observed at 520K, which is 18% higher than that of pristine Bi2Se3. The zT value of the Co-doped sample could be further increased with the decrease in carrier concentration, which could be attained with the decrease in total thermal conductivity owing to the decreased electronic thermal conductivity while maintaining a high power factor originating from the increased effective mass.