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

Abstract

Finding alternatives for Bi 2 Te 3 , 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+x Se, comprising low-cost, abundant, and nontoxic elements. Although α-Cu 2+x Se 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 Cuexcess 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 V 2 K −2 at RT) of α-Cu 2 Se 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-Cu 2 Se, 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+x Se through overstoichiometric Cu addition, and we also suggest that Cu-excess α-Cu 2 Se is a very promising thermoelectric material to replace Bi 2 Te 3 for near-RT applications.

Promising and Eco‐Friendly Cu2X‐Based Thermoelectric Materials: Progress and Applications

“…Figure a shows the calculated n H ‐dependent zT at 300 K, plotted with the related experimental results of the pristine Cu 2 Se, Cu 2‐ y Se 1− x Br x and those with overstoichiometric Cu 2+ x Se . As can be seen, n H,opt of Cu 2 Se at 300 K is estimated as ≈1 × 10 20 cm −3 , where a maximum zT (zT max ) of ≈0.55 can be achievable through effective optimization of n H .…”

“…Similar effect has also been found in the Cl‐doped Cu 2 Se, in which Cl substitutes Se site . On the contrary, introducing additional Cu or Na can reduce n H by reducing VECs. Small amount of Bi doping on Cu site can reduce n H .…”

“…In the case of Cu 2 X‐based thermoelectric materials, n H can be effectively tuned through VECs engineering and bond engineering as well as introducing additional secondary phases (possibly functioning as either modulation doping or energy filtering effect).…”

“…As can be seen, n H,opt of Cu 2 Se at 300 K is estimated as ≈1 × 10 20 cm −3 , where a maximum zT (zT max ) of ≈0.55 can be achievable through effective optimization of n H . Comparing with the pristine Cu 2 Se, both Br‐doping and introducing overstoichiometric Cu have effectively reduced n H and approached n H,opt . The peak zT value of ≈0.53 was achieved for Cu 2.075 Se at 300 K, close to zT max …”

Enhancement of the performance of the Cu2Se band filters via Yb nanosandwiching

“…The enhancement in the structural parameters upon nanosandwiching of Yb‐ slab could be assigned to the chemical dynamics that are associated with the metal‐ semiconductor interfacing. Cu 2 Se is known to exhibit Cu vacancy . It is reported that α‐Cu 2 + x Se has a tendency to form a large number of Cu vacancies to stabilize its structure by detraction of Cu − Cu interactions.…”