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

Thermoelectric transport properties (Seebeck coefficient, S, and electrical conductivity, sigma) 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 Sb(2)Te(3) results in an increase in S by 33 mu V/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 Sb(2)Te(3) with Bi (which has a larger ionic size) in Sb sites or by the deposition of thick Bi(2)Te(3) layers alternating with thinner Sb(2)Te(3) layers in a superlattice, Bi(2)Te(3)/Sb(2)Te(3). We show that the optimal carrier concentration for the best power factor of Bi(2)Te(3) and Sb(2)Te(3) is approximately 10(19) cm(-3).