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 (ZT_max = 0.9 at 390 K, ZT_ave = 0.68 between RT and 390 K) for Cu-excess α-Cu_2+xSe, comprising low-cost, abundant, and nontoxic elements. Although α-Cu_2+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-Cu_2+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.