Thermoelectric power factor of Bi-Sb-Te and Bi-Te-Se alloys and doping strategy: First-principles study

Abstract

The Bi2Te3-related binary compounds (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and ternary alloys [(Bi1-xSbx)2Te3 (BST, 0 ≤ x ≤ 1) and Bi2(Te1-ySey)3 (BTSe, 0 ≤ y ≤ 1)] are known as the high performance room temperature thermoelectric materials. Here, for the first time, we systematically study the thermoelectric transport properties of BST and BTSe alloys by calculating their thermoelectric power factor (PF) as a function of alloy composition ratio x and y, carrier concentration n, and the absolute temperature T. The band valley degeneracy and the band gap are critical to determine the thermoelectric transport properties of ternaries. We find that PFs of p-type BST are comparable to those of binaries, while those of n-type BTSe are not, due to the band structure similarity. And the p-type BST performances are superior to the n-type BTSe due to the longer carrier relaxation time, transport anisotropy, and the band valley degeneracy. We also find that the optimal carrier concentrations which maximize the PFs (nopt) depend on the ternary composition and the transport direction. The bipolar effect is found to be less significant for n-type BTSe due to the large band gap and the large nopt. For p-type poly-crystalline BST, the nopt is between 3 and 4 × 1019 cm−3 and it is achievable by Sb alloying and controlling the concentration of intrinsic defect. However, for n-type polycrystalline BTSe, nopt is ranging between 6 × 1019 cm−3 and 1 × 1020 cm−3 and thereby we need additional extrinsic dopant beyond Se alloying. The defect formation energy calculations reveal that Cl, Br, and I impurities are potential candidates for n-type carrier sources without forming any compensating defect, while F as well as Au is the compensating defect.