Realizing high figure-of-merit in Cu2Te using a combination of doping, hierarchical structure, and simple processing

Abstract

The Cu2Te chalcogenide alloy is doped with 2 at. % Ni to increase the charge carrier concentration and then is further doped with 3 at. % Se to reduce the thermal conductivity. The alloys processing is kept simple–vacuum arc melting only to make a dense alloy for characterization. This also results in retaining the as-solidified highly layered structure. The alloys are found to have two polymorphic forms: hexagonal and orthorhombic at room temperature with a superstructure. The fractured surface shows clearly the layered structure with ∼300 nm thick platelet like features stacked together to form large defect free grains. The electrical conductivity increases to ∼7 × 103 S cm−1 due to Ni-doping compared to ∼5 × 103 S cm−1 for the undoped alloy at room temperature. This however decreases to ∼2.5 × 103 S cm−1 due to double doping, i.e., Ni and Se. In both cases, the alloys exhibit a weak metallic behavior with the conductivity decreasing with increasing temperature. The Seebeck coefficient however increases with temperature and with double doping resulting in the highest Seebeck coefficient, which increases from 40 μVK−1 to 110 μVK−1 when the temperature varies from 300 K to 1000 K. The hole carrier concentration in the two alloys, Ni-doped and double doped, is found to be nearly identical, 7 × 1020 cm−3 and 8.52 × 1020 cm−3, respectively, while the mobility of carriers decreased by 5 times from 283 cm2 V−1 s−1 to 52 cm2 V−1 s−1 due to double doping. These factors together with multiple scale phonon scattering resulted in the double doped alloy having the lowest thermal conductivity in the range of 1–2 Wm−1 K−1 in the complete temperature range. The thermal conductivity reduction due to the layered structure and alloy scattering results in increasing the figure of merit zT steeply to 0.65 at 950 K which at 1100 K can reach 1.0.