Unusual role of ZnO nanoparticles in enhancing the thermoelectric performance of GeTe

ZnO nanoparticles were incorporated into the GeTe samples. Unexpectedly, the ZnO nanoparticles reacted with the GeTe phase to produce ZnTe and Ge impurities. Ge precipitates promoted self compensation in GeTe, which in turn reduced the Ge vacancies and decreased the carrier concentration. The resulting increase in the Seebeck coefficient led to a higher power factor, despite an increase in the electrical resistivity. Due to the optimized power factor and a lower carrier thermal conductivity, the average zT increased from 0.64 for the pure GeTe to 0.79 for GeTe + 0.5 wt.% ZnO. Additionally, irreversibility of the reaction between GeTe and ZnO during the high-temperature thermoelectric measurements emphasizes importance of the proper sequence for the property measurements; the Hall data should be recorded once the thermopower and electrical conductivity were measured to high temperatures.