Electronic structure and volume effect on thermoelectric transport inp-type Bi and Sb tellurides

Thermoelectric transport properties (Seebeck coefficient, S, and electrical conductivity, σ) of p-type Bi and Sb tellurides are investigated using a first-principles all-electron density-functional approach. We demonstrate that the carrier concentration, band gap, and lattice constants have an important influence on the temperature behavior of S and that the volume expansion by 5.5% in Sb₂Te₃ results in an increase in S by 33 μV/K at 300 K. We argue that in addition to the electronic structure characteristics, the volume also affects the value of S and hence should be considered as an origin of the experimental observations that S can be enhanced by doping Sb₂Te₃ with Bi (which has a larger ionic size) in Sb sites or by the deposition of thick Bi₂Te₃ layers alternating with thinner Sb₂Te₃ layers in a superlattice, Bi₂Te₃/Sb₂Te₃. We show that the optimal carrier concentration for the best power factor of Bi₂Te₃ and Sb₂Te₃ is approximately 10¹⁹ cm^-3.