High thermoelectric performance in copper telluride

Recently, Cu2-delta S and Cu2-delta Se were reported to have an ultralow thermal conductivity and high thermoelectric figure of merit zT. Thus, as a member of the copper chalcogenide group, Cu2-delta Te is expected to possess superior zTs because Te is less ionic and heavy. However, the zT value is low in the Cu2Te sintered using spark plasma sintering, which is typically used to fabricate high-density bulk samples. In addition, the extra sintering processes may change the samples' compositions as well as their physical properties, especially for Cu2Te, which has many stable and meta-stable phases as well as weaker ionic bonding between Cu and Te as compared with Cu2S and Cu2Se. In this study, high-density Cu2Te samples were obtained using direct annealing without a sintering process. In the absence of sintering processes, the samples' compositions could be well controlled, leading to substantially reduced carrier concentrations that are close to the optimal value. The electrical transports were optimized, and the thermal conductivity was considerably reduced. The zT values were significantly improved-to 1.1 at 1000 K-which is nearly 100% improvement. Furthermore, this method saves substantial time and cost during the sample's growth. The study demonstrates that Cu2-delta X (X= S, Se and Te) is the only existing system to show high zTs in the series of compounds composed of three sequential primary group elements.