What Are Thermoelectric Materials?

Thermoelectric materials are semiconducting or semimetallic solids that exhibit a strong thermoelectric response, meaning they can generate a substantial electrical voltage when subjected to a temperature gradient, or produce a significant temperature difference when driven by an electric current. The selection and optimization of these materials is the primary factor governing the efficiency of thermoelectric devices, which have applications in solid-state cooling, waste heat recovery, and autonomous power generation. Unlike photovoltaic or electrochemical energy conversion, thermoelectric conversion occurs in a single solid-state structure with no interfaces between distinct phases, giving thermoelectric materials a uniquely direct role in device performance.

The performance of a thermoelectric material is characterized by the dimensionless figure of merit ZT, defined as the square of the Seebeck coefficient (thermopower) multiplied by the electrical conductivity, divided by the thermal conductivity, all evaluated at absolute temperature T. Because the Seebeck coefficient, electrical conductivity, and thermal conductivity are physically coupled through carrier concentration, achieving a high ZT requires careful manipulation of the electronic band structure and phonon scattering mechanisms. For most of the twentieth century, the practical ceiling for ZT hovered near 1 in the best known materials; recent work with nanostructuring and band engineering has pushed values substantially higher.

Figure of Merit and Material Properties

The three properties entering ZT respond differently to changes in carrier concentration. The Seebeck coefficient decreases as carrier concentration increases, while electrical conductivity increases. Thermal conductivity has two components: the electronic contribution, which tracks with electrical conductivity through the Wiedemann-Franz law, and the lattice (phonon) contribution, which can in principle be reduced independently through structural disorder, grain boundaries, or nanostructuring. Achieving high ZT therefore requires finding the carrier concentration that maximizes the power factor (Seebeck coefficient squared times electrical conductivity) while simultaneously suppressing lattice thermal conductivity as much as possible. Research reviewed in PMC on strategies for improving thermoelectric figure of merit describes band convergence, resonant levels, and phonon engineering as the principal levers available to materials scientists.

Chalcogenide-Based Materials

The most commercially successful thermoelectric materials are chalcogenides: compounds containing sulfur, selenium, or tellurium. Bismuth telluride (Bi2Te3) and its alloys with antimony telluride (Sb2Te3) for p-type legs and bismuth selenide (Bi2Se3) for n-type legs dominate near-room-temperature applications, achieving ZT values near 1 for bulk polycrystalline material. Studies reviewed in PMC on bismuth telluride thermoelectric properties and applications document how anisotropy in the crystal structure affects both electrical and thermal transport. Lead telluride (PbTe) and its alloys operate efficiently in the 300 to 600 degree Celsius range and are the materials of choice for mid-temperature power generation. Tin selenide (SnSe) single crystals achieved a ZT of 2.6 along the crystallographic b-axis at 923 K, reported in a 2014 Nature paper, representing one of the highest values measured in a bulk thermoelectric material.

Emerging Material Systems

Research beyond conventional chalcogenides has identified several classes of materials with the potential to surpass ZT values of 2 in practically relevant temperature ranges. Skutterudites, cage-structured compounds with the formula MX3 where M is cobalt or iron and X is a pnictogen, can be filled with heavy atoms that rattle inside the cage and scatter phonons without disrupting electron transport. Half-Heusler alloys offer high power factors and mechanical robustness at elevated temperatures. Organic thermoelectrics based on conducting polymers such as poly(3,4-ethylenedioxythiophene) have attracted attention for flexible and wearable applications. The Frontiers in Chemistry review of thermoelectric figure of merit strategies covers nanocomposite approaches and multiscale phonon engineering as paths to materials with ZT above 2 at device-relevant temperatures.

Applications

Thermoelectric materials have applications in a wide range of disciplines, including:

• Solid-state refrigeration modules for photonic detectors and laboratory instruments
• Radioisotope thermoelectric generators for deep-space spacecraft
• Automotive exhaust heat recovery for auxiliary electrical power
• Wearable sensors and health monitors powered by body heat gradients
• Industrial waste heat recovery in high-temperature process industries