Improved electrical conductivity and thermoelectric performance of ZnO by doping with NaCl and CdO

Abstract

ZnO is a promising thermoelectric (TE) material for high-temperature applications; however, its TE performance is limited by strong coupling between the electrical and thermal transport properties. The Schottky barrier at the grain boundaries, via Zn vacancies (an electron killer) and interstitial oxygen, forms the acceptor defects that restrict carrier mobility within ZnO. Increasing the carrier concentration by increasing the solubility of a trivalent/divalent/monovalent dopant in ZnO is also important. In this study, NaCl- and CdO- are co-doped with Al into ZnO samples synthesized by spark plasma sintering, and establish a strong correlation with TE properties. NaCl's alloy with ZnAlO is doped simultaneously at cationic and anionic sites, while the embedded CdO contributes to doping and surface modification. The methodology produced an ultrahigh σ ≥ 3600 S/cm at 300 K with significantly lower lattice thermal conductivity ~3.1 W/mK at high temperature. The structural, electrical, and thermal properties are analyzed to elucidate CdO's influence, and a correlation between the power factor and the energy barrier height discussed. This yields the best power factor of ~10.9 µW/cm K2 at 1173 K in the ZnAlCdO matrix, resulting because of carrier energy filtering effect, which is 1.5 times above the reported data.