Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/AdvMater_19_1043_2007.pdf

New Directions for Low-Dimensional Thermoelectric Materials**

The efficiency of thermoelectric energy converters is limited by the material thermoelectric figure of merit (zT). The recent advances in zT based on nanostructures limiting the phonon heat conduction is nearing a fundamental limit: The thermal conductivity cannot be reduced below the amorphous limit. We explored enhancing the Seebeck coefficient through a distortion of the electronic density of states and report a successful implementation through the use of the thallium impurity levels in lead telluride (PbTe). Such band structure engineering results in a doubling of zT in p-type PbTe to above 1.5 at 773 kelvin. Use of this new physical principle in conjunction with nanostructuring to lower the thermal conductivity could further enhance zT and enable more widespread use of thermoelectric systems.

http://science.sciencemag.org/content/sci/321/5888/554.full.pdf

Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States

Thermoelectric generators, which directly convert heat into electricity, have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. Although these devices can be very reliable and compact, the thermoelectric materials themselves are relatively inefficient: to facilitate widespread application, it will be desirable to identify or develop materials that have an intensive thermoelectric materials figure of merit, zT, above 1.5 (ref. 1). Many different concepts have been used in the search for new materials with high thermoelectric efficiency, such as the use of nanostructuring to reduce phonon thermal conductivity, which has led to the investigation of a variety of complex material systems. In this vein, it is well known, that a high valley degeneracy (typically ≤6 for known thermoelectrics) in the electronic bands is conducive to high zT, and this in turn has stimulated attempts to engineer such degeneracy by adopting low-dimensional nanostructures. Here we demonstrate that it is possible to direct the convergence of many valleys in a bulk material by tuning the doping and composition. By this route, we achieve a convergence of at least 12 valleys in doped PbTe_(1) − _(x)Se_(x) alloys, leading to an extraordinary zT value of 1.8 at about 850 kelvin. Band engineering to converge the valence (or conduction) bands to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials, because it simultaneously leads to a high Seebeck coefficient and high electrical conductivity.

Convergence of electronic bands for high performance bulk thermoelectrics

Lead chalcogenides have long been used for space-based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl-type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.

Band Engineering of Thermoelectric Materials

Despite the widespread use of solution-processable hybrid organic-inorganic perovskites in photovoltaic and light-emitting applications, determination of their intrinsic charge transport parameters has been elusive due to the variability of film preparation and history-dependent device performance. Here we show that screening effects associated to ionic transport can be effectively eliminated by lowering the operating temperature of methylammonium lead iodide perovskite (CH3NH3PbI3) field-effect transistors. Field-effect carrier mobility is found to increase by almost two orders of magnitude below 200 K, consistent with phonon scattering-limited transport. Under balanced ambipolar carrier injection, gate-dependent electroluminescence is also observed from the transistor channel, with spectra revealing the tetragonal to orthorhombic phase transition. This demonstration of CH3NH3PbI3 light-emitting field-effect transistors provides intrinsic transport parameters to guide materials and solar cell optimization, and will drive the development of new electro-optic device concepts, such as gated light-emitting diodes and lasers operating at room temperature.

/pdf/lead-iodide-perovskite-light-emitting-field-effect-4wswvsib8i.pdf

Lead iodide perovskite light-emitting field-effect transistor.

Scattering of Current Carriers and Transport Phenomena in Lead Chalcogenides

Scattering of Current Carriers and Transport Phenomena in Lead Chalcogenides II. Experiment

Scattering of current carriers and transport phenomena in lead chalkogenides (II)

The first investigations of the thermoelectric properties of lead chalcogenides were carried out by Ioffe, Maslakovets et al. [207, 488–490]. The outcome of these investigations was the use of lead sulfide in the first thermoelectric generators and of lead telluride in various cooling units [490, 491]. Later it was found that alloys based on antimony and bismuth tellurides were more suitable materials for cooling purposes. Lead chalcogenides, particularly lead telluride and its alloys, were found to be the most efficient materials for thermoelectric generators working between room temperature and 600–650°C.

Applications of Lead Chalcogenides

In the presence of a temperature gradient VT, the distribution of carrier velocities varies across a semiconductor. Consequently, even in the absence of an electric field, a macroscopic carrier flux is produced because the temperature gradient acts as an effective temperature field, which has some of the effects of the electric field. If the circuit is open and there is no current, an electric field is produced at each point in the sample and this field is found from the condition that the thermal current must be balanced by the ohmic current. The temperature gradient thus generates an electric field which gives rise to a thermo-emf or the Seebeck effect. Carriers with higher energies move from the hotter to the colder parts of the sample, and those with lower energies move in the opposite direction so that the total current is equal to zero.

B. A. Efimova

Papers

Scattering of Current Carriers and Transport Phenomena in Lead Chalcogenides

Scattering of Current Carriers and Transport Phenomena in Lead Chalcogenides II. Experiment

Scattering of current carriers and transport phenomena in lead chalkogenides (II)

Applications of Lead Chalcogenides

Thermoelectric and Thermal Properties