Siqi Lin

Tellurium as a high-performance elemental thermoelectric

High-efficiency thermoelectric materials require a high conductivity. It is known that a large number of degenerate band valleys offers many conducting channels for improving the conductivity without detrimental effects on the other properties explicitly, and therefore, increases thermoelectric performance. In addition to the strategy of converging different bands, many semiconductors provide an inherent band nestification, equally enabling a large number of effective band valley degeneracy. Here we show as an example that a simple elemental semiconductor, tellurium, exhibits a high thermoelectric figure of merit of unity, not only demonstrating the concept but also filling up the high performance gap from 300 to 700 K for elemental thermoelectrics. The concept used here should be applicable in general for thermoelectrics with similar band features.

Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics

To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb1−xSb2x/3Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm−1 K−1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.

Promoting SnTe as an Eco‐Friendly Solution for p‐PbTe Thermoelectric via Band Convergence and Interstitial Defects

Compared to commercially available p-type PbTe thermoelectrics, SnTe has a much bigger band offset between its two valence bands and a much higher lattice thermal conductivity, both of which limit its peak thermoelectric figure of merit, zT of only 0.4. Converging its valence bands or introducing resonant states is found to enhance the electronic properties, while nanostructuring or more recently introducing interstitial defects is found to reduce the lattice thermal conductivity. Even with an integration of some of the strategies above, existing efforts do not enable a peak zT exceeding 1.4 and usually involve Cd or Hg. In this work, a combination of band convergence and interstitial defects, each of which enables a ≈150% increase in the peak zT, successfully accumulates the zT enhancements to be ≈300% (zT up to 1.6) without involving any toxic elements. This opens new possibilities for further improvements and promotes SnTe as an environment-friendly solution for conventional p-PbTe thermoelectrics.

Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6

Conventional strategies for advancing thermoelectrics by minimizing the lattice thermal conductivity focus on phonon scattering for a short mean free path. Here, a design of slow phonon propagation as an effective approach for high‐performance thermoelectrics is shown. Taking Ag8SnSe6 as an example, which shows one of the lowest sound velocities among known thermoelectric semiconductors, the lattice thermal conductivity is found to be as low as 0.2 W m−1 K−1 in the entire temperature range. As a result, a peak thermoelectric figure of merit zT > 1.2 and an average zT as high as ≈0.8 are achieved in Nb‐doped materials, without relying on a high thermoelectric power factor. This work demonstrates not only a guiding principle of low sound velocity for minimal lattice thermal conductivity and therefore high zT, but also argyrodite compounds as promising thermoelectric materials with weak chemical bonds and heavy constituent elements.

Vacancy scattering for enhancing the thermoelectric performance of CuGaTe2 solid solutions

Enhancing the thermoelectric performance through an effective phonon scattering by point defects has long been proven to be successful in many materials. This type of phonon scattering relies on the mass and strain fluctuations between the host and guest atoms, both of which can be maximized when the dominant point defects are vacancies. Besides the intrinsic vacancies in some compounds by nature, the formation of solid solutions with a solvent having a smaller cation-to-anion ratio as compared to the matrix is expected to create vacancies because the crystal structure needs to be stabilized in that of the matrix compound. In this work, In2Te3 and Ga2Te3i, compounds with a smaller cation-to-anion ratio as compared to the CuGaTe2 matrix, are chosen as molecular solvents to form solid solutions. The resulting high concentration vacancies on the cation sites, which can act as the phonon scattering centers, significantly reduce the lattice thermal conductivity and therefore enhance the thermoelectric performance by up to ∼75% in the entire temperature range. This work demonstrates a useful strategy for enhancing thermoelectric performance by vacancy creation in solid solutions.

Single parabolic band behavior of thermoelectric p-type CuGaTe2

The existence of a noticeable discrepancy in thermoelectric properties reported in the literature motivates the current work on the transport properties of CuGaTe2. Taking Zn- and Mn-doping at the Ga site as an example, the hole concentration can be effectively tuned within 1018–1020 cm−3 that enables a reliable assessment of the transport properties. It is evident that both temperature and carrier concentration dependent transport properties follow well within the framework of a single parabolic band approximation with a dominant carrier scattering by acoustic phonons. This work helps distinguish the effects that contribute to the high thermoelectric figure of merit zT in CuGaTe2. The modeling further suggests that this compound can show a thermoelectric figure of merit of unity or higher, when further strategies are taken for reducing the lattice thermal conductivity and engineering the band structure.

Substitutional defects enhancing thermoelectric CuGaTe2

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 ∼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.

Sb induces both doping and precipitation for improving the thermoelectric performance of elemental Te

As one of the focus areas, phonon scattering by boundary interfaces enables an effective reduction in the lattice thermal conductivity for improving thermoelectrics. Introduction of boundary interfaces is expected through precipitation by low-temperature processing of over-saturated solids or melts. In this study, a single step of both doping and precipitation is achieved in elemental tellurium with a few percent of Sb, leading to a high thermoelectric figure of merit, zT, of 0.9. This is quite comparable with the As-doped Te reported previously, but without involving any toxic elements. Evolutionarily, Sb-substitution of Te within 0.5% sufficiently increases the hole concentration leading to an optimized thermoelectric power factor, while higher concentration of Sb introduces Sb2Te3 precipitates, enabling an effective phonon scattering for a reduced lattice thermal conductivity by ∼25%. This study further demonstrates tellurium as a promising elemental thermoelectric material from 300 to 700 K.

Thermoelectric Properties of SnS with Na-Doping

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 ∼1019 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-dependent Hall mobility shows a transition of carrier-scattering source from 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 ∼1 × 1020 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.

Performance optimization and single parabolic band behavior of thermoelectric MnTe

Semiconducting MnTe has long been considered a potential thermoelectric material for p-type conduction. However, its low carrier concentration limits its peak thermoelectric performance, zT, which is only ∼0.6. In this study, Na and Ag are found to enable a significant increase in hole concentration from ∼1019 cm−3 in pristine MnTe to ∼1021 cm−3 in the doped samples, leading to an effective enhancement in power factor in the entire temperature range investigated. Moreover, doping simultaneously introduces additional phonon scattering by point defects and secondary phases, leading to a reduction in lattice thermal conductivity, as low as ∼0.6 W m−1 K−1, approaching the amorphous limit. The synergic effects of carrier concentration optimization and lattice thermal conductivity reduction realize a peak zT as high as 1.0, which is one of the highest reported thus far for this thermoelectric material. The broad carrier concentration achieved in this study enables a reasonable assessment of the electronic transport properties of MnTe, which reveals effective single parabolic band behavior with dominant carrier scattering by acoustic phonons. It is further expected, according to the model, that a peak zT up to 1.1 could be achieved once the lattice thermal conductivity is further reduced. Moreover, the valence band structure strongly suggests the probability of a well-improved zT through a further band engineering approach due to the existence of low-lying bands with large valley degeneracies. This study not only demonstrates that MnTe is a promising thermoelectric material, but also provides guidance for its further optimization.