High thermoelectric performance in Cu-doped Bi2Te3 with carrier type transition

For decades, the Bismuth-telluride (Bi2Te3) has been intensively studied as the thermoelectric (TE) cooler. Still, new ideas and emerging results are put forward for raising the conversion efficiency. Herein, we revisit the Cu-doped Bi2Te3, and report the high zT values nearing the room temperature, for both the p-type (Cu2Te)(0.01)(Bi2Te3)(0.99)/Cu0.01Bi1.99Te3 (zT similar to 1.2 at 300 K) as well as the n-type (Cu2Te)(0.09)(Bi2Te3)(0.91) (zT similar to 1.09 at 363 K), respectively. Given that the phase boundary mapping is essential for the optimization of high-efficiency TE materials, the isothermal section of ternary Bi-Cu-Te at 523 K is constructed, by collecting the phase equilibria information of various thermally-equilibrated alloys; it further guides the alloying directions for (Cu2Te)(x) (Bi2Te3)(1-x) and CuyBi2-yTe3, respectively. Small modulation in the stoichiometry leads to the carrier type transition. As a consequence, the lamellae composed of Bi2Te3 and Cu7Te5 precipitate along the grain boundary of the n-type (Cu2Te)(0.09)(Bi2Te3)(0.91), resulting in the reduced kappa, due the stronger interfacial phonon scattering, and the higher PF, owing to the higher amounts of Te vacancy (V-Te). On the contrary, the promising p-type (Cu2Te)(0.01)(Bi2Te3)(0.99)/Cu0.01Bi1.99Te3 features the single-phase Bi2Te3, whereas the soluble Cu introduces extra holes and might therefore promote the p-type conduction. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.