For several decades, thermoelectric advancements have largely relied on the reduction of lattice thermal conductivity (k(L)). According to the Boltzmann transport theory of phonons, k(L) mainly depends on the specific heat, the velocity, and the scattering of phonons. Intensifying the scattering rate of phonons is the focus for reducing the lattice thermal conductivity. Effective scattering sources include 0D point defects, 1D dislocations, and 2D interfaces, each of which has a particular range of frequencies where phonon scattering is most effective. Because acoustic phonons are generally the main contributors to k(L) due to their much higher velocities compared to optical phonons, many low-k(L) thermoelectrics rely on crystal structure complexity leading to a small fraction of acoustic phonons and/or weak chemical bonds enabling an overall low phonon propagation velocity. While these thermal strategies are successful for advancing thermoelectrics, the principles used can be integrated with approaches such as band engineering to improve the electronic properties, which can promote this energy technology from niche applications into the mainstream.

Band convergence has been proven as an effective approach for enhancing thermoelectric performance, particularly in p-type IV VI semiconductors, where the superior electronic performance originates from the contributions of both L and Sigma band valleys when they converge to have a small energy offset. When alloying with cubic IV VI semiconductors, CdTe has been found as an effective agent for achieving such a band convergence. This work focuses on the effect of CdTe-alloying on the thermoelectric transport prciperties of GeTe, where the carrier concentration can be tuned in a broad range through Bi-doping on Ge site. It is found that CdTe-alloying indeed helps to converge the valence bands of GeTe in both low-T rhombohedral and high-T cubic phases for an increase in Seebeck coefficient with a decrease in mobility. In addition, the strong phonon scattering due to the existence of high-concentration Cd/Ge and Bi/Ge substitutions leads the lattice thermal conductivity to be reduced to as low as 0.6 W/(m-K). These lead to an effectively increased average thermoelectric figure of merit (ZT(ave) similar to 1.2) at 300-800 K, which is higher than that of many IV VI materials with CdTe-alloying or alternatively with MnTe-, MgTe-, SrTe-, EuTe-, or YbTe-alloying for a similar band convergence effect.

Semiconducting manganese ditelluride (MnTe2) crystalizes in a high symmetry cubic structure with sufficient band gap and consists of nontoxic elements only, therefore is focused on in this work for its potential thermoelectric applications. This material intrinsically comes with a very low hole concentration of 10(19) cm(-3), which can be successfully increased to 4 x 10(20) cm(-3) through Ag-doping at Mn site. Such a broad carrier concentration enables an effective optimization on thermoelectric power factor, and the doping process effectively reduces the lattice thermal conductivity down to similar to 0.5 W/m-K due to the phonons scattered by additional point defects. As a result, a peak zT of similar to 0.7 is obtained in p-type conduction. Moreover, the SPB model with acoustic scattering estimates the electronic properties well, which also enables insight into the underlying physical parameters related to the thermoelectric performance. Importantly, band structure calculation suggests a potentially higher thermoelectric performance for n-type conduction due to both higher band degeneracy and lower band effective mass. This work reveals MnTe2 is a novel promising thermoelectric material. (C) 2018 The Chinese Ceramic Society. Production and hosting by Elsevier B.V.

Over the past couple of decades, thermoelectric Mg3Sb2 and its derivatives have attracted increasing attention for thermoelectric applications. This is enabled by the richness in composition for manipulating both electronic and thermal properties and by the intrinsic low lattice thermal conductivity. With existing efforts on these materials, the thermoelectric figure of merit has been significantly improved to compete with conventional thermoelectrics, while many of these materials keep the compositions cheap and less-toxic elements only. Here, not only the control of defects, band structure, electronic transport properties, and lattice thermal conductivity for these materials, but also the proven strategies on transport property manipulation are summarized. These strategies are well demonstrated for advancing thermoelectric Mg3Sb2 and its derivatives, and the principles used are believed to be equally applicable for many other thermoelectric materials. In addition, perspectives for possible further advancements in this class of thermoelectric materials are shown.

SnSe has attracted increasing attention as a promising thermoelectric material. In this work, a horizontal vapor transfer method was developed to synthesize high-quality, fully dense, and stoichiometric SnSe single crystals, which enables an evaluation of the transport properties inherent to SnSe along the bc-plane. The electronic transport properties can be well-understood by a single parabolic band model with acoustic phonon scattering, enabling insights into the fundamental material parameters determining the electronic properties. The lattice thermal conductivity (kappa(1)) decreases from 2.0 W m(-1) K-1 at 300 K to 0.55 W m(-1) K-1 at 773 K. It is revealed that an increase in hole concentration, an involvement of low-lying bands for transport, and a further reduction in kappa(1) would all enable p-type SnSe to be a promising eco-friendly thermoelectric material. This work not only provides a fundamental understanding of the charge transport but also guides the further improvement of thermoelectric SnSe.

As a typical class of Zintl thermoelectrics, AB(2)C(2) (A =3D Eu, Yb, Ba, Ca, Mg; B =3D Zn, Cd, Mg, and C =3D Sb, Bi) compounds have shown a superior thermoelectric performance, largely stemming from the existence of multiple transporting bands in both conduction types. Being similar to many III-V and elemental semiconductors, the transport of holes in AB(2)C(2) Zintls usually involves multiple valence bands with extrema at the Brillouin zone center Gamma. However, these valence bands, originating from different orbitals, are unnecessarily aligned in energy due to the crystal field splitting. Formation of solid solutions between constituent compounds having opposite arrangements in energy of band orbitals is believed to be particularly helpful for thermoelectric enhancements, because orbital alignment increases band degeneracy while alloy defects scatter phonons. These effects are simultaneously realized in this work, where the p orbitals of anions in YbCd2-xZnxSb2 alloys are well-aligned for maximizing the electronic performance, and meanwhile high-concentration Cd/Zn substitutions are introduced for minimizing the lattice thermal conductivity. As a result, a significantly enhanced thermoelectric figure of merit, zT similar to 1.3, is achieved, being a record among AB(2)C(2) Zintls in p-type. This work demonstrates not only YbCd2-xZnxSb2 alloys as efficient thermoelectrics but also orbital alignment as an effective strategy for advancing thermoelectrics.

Tin sulfide (SnS), a low-cost compound from the IV-VI semiconductors, has attracted particular attention due to its great potential for large-scale thermoelectric applications. However, pristine SnS shows a low carrier concentration, which leads to a low thermoelectric performance. In this work, sodium is utilized to substitute Sn to increase the hole concentration and consequently improve the thermoelectric power factor. The resultant Hall carrier concentration up to similar to 10(19) cm(-3) is the highest concentration reported so far for this compound. This further leads to the highest thermoelectric figure of merit, zT of 0.65, reported so far in polycrystalline SnS. The temperature-deperident Hall mobility shows a transition of carrier-scattering source firom a grain boundary potential below 400 K to acoustic phonons at higher temperatures. The electronic transport properties can be well understood by a single parabolic band (SPB) model, enabling a quantitative guidance for maximizing the thermoelectric power factor. Using the experimental lattice thermal conductivity, a maximal zT of 0.8 at 850 K is expected when the carrier concentration is further increased to similar to 1 X 10(20) cm(-3) , according to the SPB model. This work not only demonstrates SnS as a promising low-cost thermoelectric material but also details the material parameters that fundamentally determine the thermoelectric properties.

The doping effects of the boron group (Al, Ga, In and Tl) and the nitrogen group elements (As and Sb) on BiCuSeO are studied combining a band unfolding technique and density functional theory. Substitutional site preferences of these dopants are predicted based on the formation energy. It is found that significant resonant states near the conduction band minimum and valence band maximum of BiCuSeO are induced by In and Tl, respectively, providing a guideline for the enhancement of thermoelectric efficiency through band engineering. Effective band structures of the doped systems have been obtained for direct examination of the resonant states. Arsenic tends to substitute either Bi or Se atoms, while the electronic structure strongly depends on the substitutional sites. The decomposed density of states and charge densities are also calculated to unveil the origins of the resonant effects.

It is known that phonon scattering by point defects is effective for reducing the lattice thermal conductivity due to the mass and strain fluctuations between the host and guest atoms. Therefore a high concentration of defects having big mass and strain fluctuations is desired. Based on this strategy, this work focuses on the effect of Ag/Cu substitution on reducing the lattice thermal conductivity in CuGaTe2. It is seen that the lattice thermal conductivity can be significantly reduced by a factor of 4 when >30% Cu is substituted by isovalent Ag, which further leads to a great enhancement in the thermoelectric figure of merit, zT in the entire temperature range. The peak zT of similar to 1.0 at 750 K is obtained in the samples with an optimal carrier concentration, which is one of the highest reported so far for this material in a single phase at the same temperature. This work demonstrates CuGaTe2 as a promising thermoelectric material and the point defect scattering as an effective strategy for enhancing its zT.

PbTe has been leading the advancements in the field of thermoelectricity due to its capability for demonstrating and integrating various new concepts. However, the toxicity of Pb is always a concern for terrestrial applications, which inspired great advancement to be achieved very recently in its alternative analogue SnTe. Challenges making p-type SnTe as thermoelectrically efficient as PbTe rely on a reduction of its carrier concentration, valence band offset, and lattice thermal conductivity. Utilization of newly developed concepts including both band and defect engineering amazingly increases the thermoelectric figure of merit, zT, from 0.4 up to 1.6 while remaining a nontoxic composition. The corresponding conceptual route diagram is surveyed, and future considerations on composition, crystal structure, and microstructure for further advancements are discussed in this Perspective. Concepts discussed here not only have promoted SnTe as a highly efficient environment-friendly thermoelectric material but also guided advancements in many other thermoelectrics.

While numerous improvements have been achieved in thermoelectric materials by reducing the lattice thermal conductivity (kappa(L)), electronic approaches for enhancement can be as effective, or even more. A key challenge is decoupling Seebeck coefficient (S) from electrical conductivity (s). The first order approximation - a single parabolic band assumption with acoustic scattering - leads the thermoelectric power factor (S-2 sigma) to be maximized at a constant reduced Fermi level (eta similar to 0.67) and therefore at a given S of similar to 167 mu V/K. This simplifies the challenge of maximization of s at a constant., leading to a large number of degenerate transport channels (band degeneracy, N-v) and a fast transportation of charges (carrier mobility, mu). In this paper, existing efforts on this issue are summarized and future prospectives are given.

Zintl compounds are usually rich in composition and thus enable a large degree of manipulation in both electronic and phononic properties for potential thermoelectric applications. This is typified by the AB(2)C(2) compounds (A =3D Eu, Yb, Ba, Ca; B =3D Zn, Cd, Mg and C =3D As, Sb, Bi) that have attracted extensive attention. Among this class of compounds, a few existing works indicate that the high thermoelectric performance in p-type EuZn2Sb2 relies on its relatively high mobility and the reported figures of merit are scattered. This has motivated this study to focus on the thermoelectric transport properties of p-EuZn2 xAgxSb2 (Ag-doped) in a broad carrier concentration range (3.5-14 x 10(19) cm(-3)), which reveals a single parabolic band (SPB) behavior in this material and enables insights into the fundamental material parameters determining the thermoelectric performance. This finding has guided a further enhancement to be achieved by a reduction in the lattice thermal conductivity, which is realized by a strong phonon scattering through the Ca/Eu isovalent substitutional defects. The achieved peak figure of merit, zT, was as high as unity, demonstrating EuZn2Sb2 as a promising thermoelectric material.