Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn1−xTaxO by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa2O6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT~0.07 at 1175 K observed for Zn0.998Ta0.002O is mainly provided by high Seebeck coefficient (−464 μV/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice.

This research was funded by FCT individual grant SFRH/BPD/124238/2016; project CICECO-Aveiro Institute of Materials (ref. UID/CTM/50011/2019), Project of Bilateral Cooperation between FCT and DAAD (Germany) and the project POCI-01-0145-FEDER-031875, financed by COMPETE 2020 Program and National Funds through the FCT/MEC and when applicable cofinanced by FEDER under the PT2020 Partnership Agreement. TEM work was financed by H2020-MSCA-RISE-2016 SPICOLOST (Grant No. 734187) and has been conducted at Advanced Microscopy Laboratory INA- Universidad de Zaragoza. David M. Tobaldi is grateful to the Portuguese National Funds (OE), through FCT and IP, within the scope of the framework contract provided for by numbers 4, 5 and 6 of Article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19.