Cu vacancy engineering of cage-compound BaCu2Se2: Realization of temperature-dependent hole concentration for high average thermoelectric figure-of-merit

Abstract

Vacancy engineering offers an alternative route to aliovalent and interstitial doping for optimization of the carrier concentration in thermoelectric materials. For the wide-bandgap semiconductor BaCu2Se2, the Cu vacancy is dynamically stable; this feature can be rationally manipulated to maximize the thermoelectric figure of merit zT. In this work, we show that at room temperature, Cu-deficient BaCu2−xSe2 samples exhibit increased hole effective mass and mobility, attributed to the energy band modulation, which are favorable for improved electrical transport properties. More importantly, the defect energy level resulting from the Cu vacancies continually contributes holes at high temperature, thereby allowing the hole concentration to approach an optimal concentration. This effect leads to an increase of the power factors over a wide temperature range. The artificial reduction of the Cu content in BaCu2Se2 results in the strengthened point-defect scattering, suppressing the lattice thermal conductivity. This strategy allows simultaneous optimization of the electrical and thermal transport properties, with a thermoelectric figure of merit zT = 1.08 achieved for BaCu1.94Se2 at 823 K, which is 38% higher than that of stoichiometric BaCu2Se2. Within the measured temperature range, the average zT value for BaCu1.94Se2 is 0.494, which is 52.9% higher than that of BaCu2Se2.