Enhanced Thermoelectric Properties in Highly Co‐Doped Bi2Se3 by Density‐of‐State Effective Mass Increase

Bi 2 Se 3 alloys are promising thermoelectric materials for use at near room temperature. In this study, we investigated the thermoelectric transport properties of Co‐doped Bi 2 Se 3 , Bi 2−x Co x Se 3 ( x = 0, 0.03, 0.06, 0.09, and 0.12) polycrystalline alloys. The electrical conductivity of Bi 2 Se 3 increased with Co doping as the electron concentration increased. The power factor increased by 83% (1.17 mW/mK 2 ) with the Co doping compared with 0.64 mW/mK 2 for pristine Bi 2 Se 3 at room temperature. The density‐of‐states effective mass significantly increased to 0.30 m 0 (free electron mass) by high Co doping of x =0.12 compared to 0.14 m 0 for pristine Bi 2 Se 3 . 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 z T was observed for all the Co‐doped samples. The maximum z T value of 0.39 in highly Co‐doped Bi 1.88 Co 0.12 Se 3 is observed at 520K, which is 18% higher than that of pristine Bi 2 Se 3 . The z T 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.