Ultralow Lattice Thermal Conductivity and Significantly Enhanced Near-Room-Temperature Thermoelectric Figure of Merit in α-Cu₂Se through Suppressed Cu Vacancy Formation by Overstoichiometric Cu Addition

Finding alternatives for Bi2Te3, the only thermoelectric material for near-room-temperature (RT) applications, is of great importance in thermoelectrics. Here, we report a very promising near-RT thermoelectric figure of merit (ZTmax = 0.9 at 390 K, ZTave = 0.68 between RT and 390 K) for Cu-excess α-Cu2+xSe, comprising low-cost, abundant, and nontoxic elements. Although α-Cu2+xSe has a propensity to form a large number of Cu vacancies to stabilize its structure by diminishing Cu–Cu interactions, excess Cu leads to a decrease in hole concentration by suppressing the formation of Cu vacancies, resulting in a power factor optimization in Cu-excess compounds. These effects of Cu addition were also elucidated by density functional theory calculations and the Boltzmann transport equation. Furthermore, we directly measured the Lorentz number (2.12 × 10–8 V2 K–2 at RT) of α-Cu2Se for the first time and determined the origin of its very low lattice thermal conductivity (0.27 W m–1 K–1 at RT). On the basis of phonon calculations, it is suggested that this ultralow lattice thermal conductivity is associated with the structural instability of α-Cu2Se, as evidenced by the existence of negative phonon frequency in its phonon dispersion. On the basis of our findings, we propose a new way to control the thermoelectric transport properties of α-Cu2+xSe through overstoichiometric Cu addition, and we also suggest that Cu-excess α-Cu2Se is a very promising thermoelectric material to replace Bi2Te3 for near-RT applications.