Bi₂Te₃-based alloys are the main materials for the construction of low- and medium-temperature thermoelectric modules. In this work, the microstructure and thermoelectric properties of Cl-doped Bi₂Te_3-xSe_x alloys were systematically investigated considering the high anisotropy inherent in these materials. The prepared samples have a highly oriented microstructure morphology, which results in very different thermal transport properties in two pressing directions. To accurately separate the lattice, electronic, and bipolar components of the thermal conductivity over the entire temperature range, we employed a two-band Kane model to the Cl-doped Bi₂Te_3-xSe_x alloys. It was established that Cl atoms act as electron donors, which tune the carrier concentration and effectively suppress the minority carrier transport in Bi₂Te_3-xSe_x alloys. The estimated value of the lattice thermal conductivity was found to be as low as 0.15 Wm^-1 K^-1 for Bi₂Te_3-x-ySe_xCl_y with x = 0.6 and y = 0.015 at 673 K in parallel to the pressing direction, which is among the lowest values reported for crystalline materials. The large reduction of the lattice thermal conductivity in both pressing directions for the investigated Bi₂Te_3-xSe_x alloys is connected with the different polarities of the Bi-(Te/Se)1 and Bi-(Te/Se)2 bonds, while the lone-pair (Te/Se) interactions are mainly responsible for the extremely low lattice thermal conductivity in the parallel direction. As a result of the enhanced power factor, suppressed bipolar conduction, and ultralow lattice thermal conductivity, a maximum ZT of 1.0 at 473 K has been received in the Bi₂Te_2.385Se_0.6Cl_0.015 sample.

Lead telluride is one of the most attractive state-of-the-art thermoelectric (TE) materials. Therefore, any improvement of its average thermoelectric figure of merit (ZT)_av over a broad temperature range is a great challenge. Here, we show that advanced electronic structure engineering using the In quasi-local level with simultaneously optimized E_F leads to a significant enhancement of energy conversion. Electronic structure calculations confirmed the formation of a half-occupied indium quasi-local level in PbTe, which, together with I-implemented electrons, provides the optimal chemical potential in terms of energy conversion, near the conduction band edge over the entire temperature range. As a result, an extremely high average thermoelectric figure of merit (ZT)_av of up to ∼1.05 in the temperature range of 298 K to 773 K was achieved. Thermoelectric efficiency η was enhanced up to 14.2% for an n-type PbTe-based leg, which is one of the record-high values for a single-phase TE material. The performed theoretical analyses and experimental data prove that the large (ZT)_av found in our system comes from the electronic structure and chemical potential engineering, offering a novel route for improving the efficiency of thermoelectric devices.

High-efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z = S²σ/κ, where S is the Seebeck coefficient, and σ and κ are the electrical conductivity and thermal conductivity, respectively, over a wide temperature range. Lead telluride alloys were some of the first materials investigated and commercialized for generators; however, their full potential for thermoelectrics has only recently been revealed to be greater than commonly believed. The maximal value of Z, as a function of electron density, is attained only for a specific location of the Fermi level E_F relative to the conduction band edge E_C. A systematic study of structural, microstructural, and thermoelectric properties of bulk PbTe doped with indium is presented. Samples were prepared by the pulsed electric current sintering technique. The high dimensionless figure of merit ZT ≈ 0.8 over 200–500 °C temperature range for PbTe doped with 0.05–0.1 at. % of In was obtained. Moreover, ZT is practically the same for Pb_0.9995In_0.0005Te and Pb_0.99In_0.01Te compounds at high temperature. Therefore, indium dopant in PbTe stabilizes the optimal location of the Fermi level. The effect of the negative process of indium diffusion into the matrix during the long service time of the TE generator could be avoided by doping heavily with indium the hot side of n-type functionally graded PbTe:In leg.

The lattice thermal conductivity of Pb _1−x Sn _x Te solid solutions was determined directly from the lattice dynamics data obtained by XAFS spectroscopy.