The escalating demand for alternative clean energy sources requires the development of new and effective materials for energy recovery, conversion, storage, and transfer. A survey conducted by the Lawrence Livermore National Lab shows that 59% of all energy generated in 2014 in the US was lost to the environment in the form of heat. While the exergetic value of much of this heat is low, there are nevertheless opportunities for waste heat recovery. Thermoelectric (TE) materials convert heat into electrical energy and vice versa and, as such, are promising materials for waste heat reduction or recovery. Further advances in thermoelectric materials could enable stand-alone solid state heat engines. Today, applications of TE materials range from portable refrigerating bags and outdoor cell phone chargers to modern units designed for power generation in space utilizing heat from nuclear sources to power solar system exploration missions.

Such thermoelectric generators depend upon the Seebeck effect, in which a temperature gradient applied across p-type and n-type materials generates an electrical voltage as a result of the diffusive flow of charge carriers. A similar phenomenon is found in thermoelectric coolers, in which external voltage bias drives charge carriers and induces a temperature gradient. The dimensionless figure-of-merit, ZT, describing the efficiency of TE devices, is expressed as ZT = (S²·T·σ)/κ, where S is the Seebeck coefficient, T is the absolute temperature, σ is the electrical conductivity, and κ is the thermal conductivity. Thus, a TE material should be a good electrical conductor (high σ) and induce high voltage in response to a temperature gradient (high S) but should be a poor heat conductor to maintain the applied temperature gradient (low κ). An optimization of TE properties is compromised in most materials because S, σ, and κ are strongly coupled.

In this virtual issue (http://pubs.acs.org/page/vi/thermoelectric-materials.html) we selected 25 recent papers (Table 1) representing the current trends and approaches toward improving the efficiency of thermoelectric materials. Computing the temperature dependent heat and charge transport properties is not a trivial task and requires state-of-the-art methods to be applied that are still quite computationally expensive. Despite of these limitations, theory provides useful guidelines in both the optimization of known thermoelectric materials and the identification of potential thermoelectrics among known compounds for which TE properties are not yet characterized.^1,2

Doping, defects, and electronic structure control is a major theme in the recent literature. For example, deleterious defects are found in Cu_2–ySe_1–xBr_x that compensate the dopants and lead to Fermi level pinning.³ On the other hand, modifications to SrTiO₃ and BaGa₂Sb₂ through defect chemistry engineering yield performance enhancements.^4–6 Together, these efforts highlight the lack of intuition we have for defects in materials and the need for defect design strategies in thermoelectric materials. This trend continues as codopants and alloys are explored. For example, codoping in the tetrahedrite Cu₁₂Sb₄S₁₃ allows for simultaneous band structure engineering and reduced lattice thermal conductivity.⁷ Similar phenomena are found in several papers on controlling the electronic structure of SnTe with Ca, Mg, and In alloying.^8–10

Doping can be also performed on the nano- and macroscale, which results in the formation of thermoelectric nanocomposites that incorporate nanoparticle precipitates within a bulk matrix. In such materials heat-carrying phonons are scattered on the interfaces, which may be “transparent” for charge carriers. Effective examples of applying this approach to classical TE materials are PbTe- and AgSnBiTe-based heterogeneous nanocomposites.^8,11–13 Nanostructrual discontinuities can be also introduced on a significantly smaller scale, as shown in the example of natural and engineered nanolattices formed in layered chalcogenides.^14,15

Nanostructuring materials has led to significant increases in ZT, but revolutions in thermoelectric performance will require new bulk materials. Thus, novel bulk materials are in high demand in the field of TE. In the search for new thermoelectric materials, we see both traditional, experimentally led searches, e.g., MgAgSb,¹⁶ and high throughput, computationally led efforts.² Analysis of the large computational data set generated in the second paper² identifies chemical trends related to cation valence that facilitate good thermoelectric performance.

The “phonon glass–electron crystal” (PGEC) approach suggests to look for new bulk thermoelectrics among narrow band semiconductors having cage-like structures with inclusion atoms or molecules trapped inside the cages. The rattling of guest atoms will provide effective scattering of heat carrying phonons, thus decreasing the lattice thermal conductivity while charge carrier transport will occur through the covalent framework. Thus, the thermal and electric transport properties of PGEC phases may to some extent be optimized independently. This concept is actively explored nowadays with respect to cage-like compounds with cationic guests.^17–21 Beyond rattling, anisotropic bonding is explored in CdSb as a strategy to enhance phonon–phonon scattering through anharmonic bonding.²² Another, more traditional approach, to achieve low lattice thermal conductivity is to increase structural complexity for compounds containing heavy elements. Those compounds often can tolerate a significant degree of doping or incorporation of interstitial atoms which allows for optimization of the charge transport properties.^23–25 Local disorder plays an important role in the reduction of lattice thermal conductivities of those compounds.

Our intention with this issue is to represent the diversity and complexity of the thermoelectric field. As evidenced by the affiliations of our authors the thermoelectrics research lies at the interface of solid state chemistry, condensed matter physics, materials science, and engineering. Collaboration between theoretical and experimental groups within each of those disciplines and between the disciplines is the key to success in thermoelectric research.