Multiple Roles of Unconventional Heteroatom Dopants in Chalcogenide Thermoelectrics: The Influence of Nb on Transport...

Improvements in the thermoelectric performance of n-type Bi₂Te₃ materials to more closely match their p-type counterparts are critical to promote the continued development of bismuth telluride thermoelectric devices. Here the unconventional heteroatom dopant, niobium, has been employed as a donor in Bi₂Te₃. Nb substitutes for Bi in the rhombohedral Bi₂Te₃ structure and exhibits multiple roles in its modulation of electrical transport and defect-induced phonon scattering. The carrier concentration is significantly increased as electrons are afforded by aliovalent doping and formation of vacancies on the Te sites. In addition, incorporation of Nb in the pseudoternary Bi_2-NbTe_3-δ system increases the effective mass, m*, which is consistent with cases of "conventional" elemental doping in Bi₂Te₃. Lastly, inclusion of Nb induces both point and extended defects (tellurium vacancies and dislocations, respectively), enhancing phonon scattering and reducing the thermal conductivity. As a result, an optimum zT of 0.94 was achieved in n-type Bi_0.92Nb_0.08Te₃ at 505 K, which is dramatically higher than an equivalent undoped Bi₂Te₃ sample. This study suggests not only that is Nb an exciting and novel electron dopant for the Bi₂Te₃ system but also that unconventional dopants might be utilized with similar effects in other chalcogenide thermoelectrics.