Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

Thermoelectrics have long been recognized as a potentially transformative energy conversion technology due to their ability to convert heat directly into electricity. Despite this potential, thermoelectric devices are not in common use because of their low efficiency, and today they are only used in niche markets where reliability and simplicity are more important than performance. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties, making it plausible that thermoelectrics could start being used in new settings in the near future. Of the various types of nanostructured materials, bulk nanostructured materials have shown the most promise for commercial use because, unlike many other nanostructured materials, they can be fabricated in large quantities and in a form that is compatible with existing thermoelectric device configurations. The first generation of these materials is currently being developed for commercialization, but creating the second generation will require a fundamental understanding of carrier transport in these complex materials which is presently lacking. In this review we introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions. Finally, we discuss several research directions which could lead to the next generation of bulk nanostructured materials.

Bulk nanostructured thermoelectric materials: current research and future prospects

Distortions of the electronic density of states (DOS) are a potent mechanism to increase the thermopower of thermoelectric semiconductors, thereby increasing their power factor. We review band-structure engineering approaches that have been used to achieve this, resonant impurity levels, dilute Kondo effects, and hybridization effects in strongly correlated electron systems. These can increase the thermoelectric power of metals and semiconductors through two mechanisms: (1) the added density of states increases the thermopower in a nearly temperature-independent way; (2) resonant scattering results in a strong electron energy filtering effect that increases the thermopower at cryogenic temperatures where the electron–phonon interactions are weaker. Electronic structure calculation results for Tl:PbTe and Ti:PbTe are contrasted and identify the origin of the thermopower enhancement in Tl:PbTe. This leads to a discussion of the conditions for DOS distortions to produce thermopower enhancements and illustrates the existence of an optimal degree of delocalization of the impurity states. The experimentally observed resonant levels in several III–V, II–VI, IV–VI and V2-VI3 compound semiconductor systems are reviewed.

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Resonant levels in bulk thermoelectric semiconductors

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

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Nanostructured thermoelectric materials: current research and future challenge

Thermoelectric clathrates hold significant promise for high temperature applications with zT values exceeding 1.3. The inorganic clathrates have been shown to be both chemically and thermally stable at high temperatures, and high performance can be obtained from both single crystals and processed powders. The clathrates also show excellent compatibility factors in segmented module applications. For a materials chemist it is furthermore of great importance that the clathrates exhibit a very rich chemistry with the ability for substitution of many different elements. This allows delicate tuning of both the crystal structure as well as the physical properties. With all these assets, it is not surprising that clathrates have been intensely investigated in the thermoelectric community during the past decade. The present perspective provides a review of the many studies concerned with the synthesis, crystal structure and thermoelectric properties of clathrates with emphasis on the type I clathrate.

Thermoelectric clathrates of type I

For more than a decade strongly correlated semiconductors and Kondo insulators have been considered as potential thermoelectric materials Such materials have large d- or f-character of the electronic band structure close to the Fermi level that theoretically leads to Seebeck coefficients (S) with large magnitudes In this work it is shown for the first time that the strongly correlated semiconductor FeSb2 exhibits a colossal Seebeck coefficient of ~−45000 μVK−1 at 10 K The thermoelectric power factor PF=S2·ρ−1, where ρ is the electrical resistivity, reaches a record high value of ~2300 μWK−2 cm−1 at 12 K and is 65 times larger than that of the state-of-the-art Bi2Te3-based thermoelectric materials However, due to a large lattice thermal conductivity the dimensionless thermoelectric figure of merit is only 0005 at 12 K Nonetheless, the potential of FeSb2 as a future solid-state thermoelectric cooling device at cryogenic temperatures is underlined

https://iopscience.iop.org/article/10.1209/0295-5075/80/17008/pdf

Colossal Seebeck coefficient in strongly correlated semiconductor FeSb2

The extraordinary thermoelectric properties of lead chalcogenides have attracted huge interest in part due to their unexpected low thermal conductivity. Here, it is shown that anharmonicity and large cation disorder are present in both PbTe and PbS, based on elaborate charge density visualization using synchrotron powder X-ray diffraction (SPXRD) data analyzed with the maximum entropy method (MEM). In both systems, the cation disorder increases with increasing temperature, whereas the Te/S anions appear to be centered on the expected lattice positions. Even at the lowest temperatures of 105 K, the lead ion is on average displaced by ≈0.2 A from the rock-salt lattice position, creating a strong phonon scattering mechanism. These findings provide a clue to understanding the excellent thermoelectric performance of crystals with atomic disorder. The SPXRD–MEM approach can be applied in general opening up for widespread characterization of subtle structural features in crystals with unusual properties.

Direct Evidence of Cation Disorder in Thermoelectric Lead Chalcogenides PbTe and PbS

Thermopower, resistivity, and heat capacity have been measured on eight Sn-substituted ${\mathrm{FeSb}}_{2\ensuremath{-}x}{\mathrm{Sn}}_{x}$ samples with $x=0--0.15$. A giant peak in the low temperature thermopower of ${\mathrm{FeSb}}_{2}$ is observed and is similar to what is found in the Kondo insulator $\mathrm{FeSi}$. The ab initio calculated band structure of ${\mathrm{FeSb}}_{2}$ shows striking similarities to that of $\mathrm{FeSi}$. For the samples with Sn substitution the data indicate that as the Sn content increases ${\mathrm{FeSb}}_{2\ensuremath{-}x}{\mathrm{Sn}}_{x}$ evolves from a Kondo insulator into a heavy fermion metal similar to what is observed for ${\mathrm{FeSi}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}$.

Experimental and theoretical investigations of strongly correlated FeSb 2 − x Sn x

FeSb2 was recently identified as a narrow-gap semiconductor with indications of strong electron–electron correlations. In this manuscript, we report on systematic thermoelectric investigation of a number of FeSb2 single crystals with varying carrier concentrations, together with two isoelectronically substituted FeSb2−xAsx samples (x = 0.01 and 0.03) and two reference compounds FeAs2 and RuSb2. Typical behaviour associated with narrow bands and narrow gaps is only confirmed for the FeSb2 and the FeSb2−xAsx samples. The maximum absolute thermopower of FeSb2 spans from 10 to 45 mV/K at around 10 K, greatly exceeding that of both FeAs2 and RuSb2. The relation between the carrier concentration and the maximum thermopower value is in approximate agreement with theoretical predictions of the electron-diffusion contribution which, however, requires an enhancement factor larger than 30. The isoelectronic substitution leads to a reduction of the thermal conductivity, but the charge-carrier mobility is also largely reduced due to doping-induced crystallographic defects or impurities. In combination with the high charge-carrier mobility and the enhanced thermoelectricity, FeSb2 represents a promising candidate for thermoelectric cooling applications at cryogenic temperatures.

Simon Johnsen

Papers

Thermoelectric clathrates of type I

Colossal Seebeck coefficient in strongly correlated semiconductor FeSb2

Direct Evidence of Cation Disorder in Thermoelectric Lead Chalcogenides PbTe and PbS

Experimental and theoretical investigations of strongly correlated FeSb 2 − x Sn x

Narrow band gap and enhanced thermoelectricity in FeSb2.