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.

The electronic and thermoelectric properties of Bi2Te3, PbBi2Te4 and PbBi4Te7 were examined on the basis of density functional theory (DFT) calculations and thermoelectric transport property measurements. The layered phase PbBi4Te7 is composed of the slabs forming the layered phases Bi2Te3 and PbBi2Te4. The electronic structure of PbBi4Te7 around the valence band maximum and conduction band minimum exhibits those of Bi2Te3 and PbBi2Te4. The band gap of PbBi4Te7 lies in between those of Bi2Te3 and PbBi2Te4, and the density of states of PbBi4Te7 is well approximated by the sum of those of Bi2Te3 and PbBi2Te4. In terms of the carrier concentration, the carrier mobility, the carrier lifetime, the electrical conductivity normalized to the carrier lifetime, and the effective mass, the layered phases Bi2Te3, PbBi4Te7 and PbBi2Te4 form a group of thermoelectrics, which have the structures composed of several different slabs and whose thermoelectric properties are approximated by the average of those of the constituent slabs. We propose to use the term “LEGO thermoelectrics” to describe such a family of thermoelectric materials that operate in a desired temperature range and possess predictable thermoelectric properties.

No abstract