Co-doping for significantly improved thermoelectric figure of merit in p-type Bi1-2xMgxPbxCuSeO oxyselenides

The microstructure, oxidation state and thermoelectric transport properties of Mg and Pb co-doped p-type Bi1-2,MgxPbxCuSeO oxyselenides have been investigated. Pure single BiCuSeO phase are obtained for all the samples without preferential growth in the crystallites. X-ray photoelectron spectroscopy (XPS) results indicate that both Mg and Pb dopants are in the 2+ oxidation state whereas other than the expected Bi3+, higher oxidation -state Bi ions also exist in both the pristine and doped BiCuSeO compounds. The Mg and Pb co-doping yields a remarkable enhancement in the electrical conductivity coupled with a moderate Seebeck coefficient. A pronounced increment in power factor is achieved from 2.54 mu W cm(-1) K-2 for the pristine BiCuSeO to 11.1 mu W cm(-1) K-2 for Bi0.88MK0.06Ph0.06CuSeO at 750 K. In addition to increasing the power factor, Mg and Pb co-doping reduces the lattice thermal conductivity owing to the dopant-induced point defect scattering of phonon. The combination of optimized power factor and intrinsically low lattice thermal conductivity results in a high ZT of 1.19 at 750 K for Bi0.88Mg0.06Pb0.06CuSeO, which is about 3.1 times as large as that for the pristine BiCuSeO.