Perspectives on thermoelectrics: from fundamentals to device applications

This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.MIT Energy InitiativeSolid-State Solar-Thermal Energy Conversion Center (funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577)United States. Dept. of Energy (DOE Grant No. DE-FG02-08ER46516)Robert Bosch Gmb

High Thermoelectric Performance and Defect Energetics of Multipocketed Full Heusler Compounds

We report a first-principles density-functional study of electron-phonon interactions in and thermoelectric transport properties of the full Heusler compounds Sr2BiAu and Sr2SbAu. Our results show that ultrahigh intrinsic bulk thermoelectric performance across a wide range of temperatures is physically possible and point to the presence of multiply degenerate and highly dispersive carrier pockets as the key factor for achieving this. Sr2BiAu, which features ten energy-aligned low-effective-mass pockets (six along Γ-X and four at L), is predicted to deliver n-type zT=0.4-4.9 at T=100-700 K. Comparison with the previously investigated compound Ba2BiAu shows that the additional L pockets in Sr2BiAu significantly increase its low-temperature power factor to a maximum value of 12 mW m-1 K-2 near T=300 K. However, at high temperatures the power factor of Sr2BiAu drops below that of Ba2BiAu because the L states are heavier and subject to strong scattering by phonon deformation, as opposed to the lighter Γ-X states, which are limited by polar-optical scattering. Sr2SbAu is predicted to deliver a lower n-type zT=3.4 at T=750 K due to appreciable misalignment between the L and Γ-X carrier pockets, generally heavier scattering, and a slightly higher lattice thermal conductivity. Soft acoustic modes, which are responsible for the low lattice thermal conductivity, also increase the vibrational entropy and high-temperature stability of these Heusler compounds, suggesting that their experimental synthesis may be feasible. The dominant intrinsic defects are found to be Au vacancies, which drive the Fermi level towards the conduction band and work in favor of n-doping

Electronic transport simulations of thermoelectric nanostructures

Thermoelectric materials have the unusual but highly desirable property of converting between heat and electricity. While their poor efficiencies have so far limited them to niche applications such as space exploration, they have great potential for waste heat recovery. Recent years have seen the demonstration of large performance improvements through the use of nanostructured materials. These materials have shown efficiency gains predominantly through dramatic decreases in the thermal conductivity, however it is desirable to achieve these thermal conductivity reductions without also impacting on the material’s electronic properties. For this, a high level of understanding of the electronic transport through such structures is needed. This thesis uses a variety of simulation methods—ranging from the classical to the quantum mechanical and including a Monte Carlo simulator constructed as part of the work—to explore the electronic transport through nanostructured systems. We identify key optimization guidelines to maintain and even enhance the electronic transport in the presence of nanostructures. Most significantly we outline a new concept we term “clean filtering” which shows the potential to provide substantial increases in thermoelectric performance

Effectiveness of nanoinclusions for reducing bipolar effects in thermoelectric materials

Bipolar carrier transport is often a limiting factor in the thermoelectric efficiency of narrow bandgap materials (such as Bi2Te3 and PbTe) at high temperatures due to the introduction of an additional term to the thermal conductivity and a reduction in the Seebeck coefficient. In this work, we present a theoretical investigation into the ability of nanoinclusions to reduce the detrimental effect of bipolar transport. Using the quantum mechanical non-equilibrium Green’s function (NEGF) transport formalism, we simulate electronic transport through two-dimensional systems containing densely packed nanoinclusions, separated by distances similar to the electron mean-free-path. Specifically, considering an n-type material, where the bipolar effect comes from the valence band, we insert nanoinclusions that impose potential barriers only for the minority holes. We then extract the material’s electrical conductivity, Seebeck coefficient, and electronic thermal conductivity including its bipolar contribution. We show that nanoinclusions can indeed have some success in reducing the minority carrier transport and the bipolar effect on both the electronic thermal conductivity and the Seebeck coefficient. The benefits from reducing the bipolar conductivity are larger the more conductive the minority band is to begin with (larger hole mean-free-path in particular), as expected. Interestingly, however, the benefits on the Seebeck coefficient and the power factor are even more pronounced not only when the minority mean-free-path is large, but when it is larger compared to the majority conduction band mean-free-path. Finally, we extract an overall estimate for the benefits that nanoinclusions can have on the ZT figure of merit

Multi-terminal far-from-equilibrium thermoelectric nano-devices in the Kondo regime

The quest for good thermoelectric materials and/or high-efficiency thermoelectric devices is of primary importance from theoretical and practical points of view. Low-dimensional structures with quantum dots or molecules are promising candidates to achieve the goal. Interactions between electrons, far-from-equilibrium conditions and strongly non-linear transport are important factors affecting the usefulness of the devices. This paper analyses the thermoelectric power of a two-terminal quantum dot under large thermal $\Delta T$ and voltage $V$ biases as well as the performance of the three-terminal system as a heat engine. To properly characterise the non-linear effects under these conditions, two different Seebeck coefficients are introduced, generalizing the linear response expression. The direct calculations of thermally induced electric and heat currents show, in agreement with recent work, that the efficiency of the thermoelectric heat engine as measured by the delivered power is maximal far from equilibrium. Moreover, the strong Coulomb interactions between electrons on the quantum dot are found to diminish the efficiency at maximum power and the maximal value of the delivered power, both in the Kondo regime and outside of it.Comment: 29 pages, 8 figure

Energy Transport and Conversion in Semiconductor Nanocrystal Solids

Solids constructed with single and multicomponent nanocrystal represent an exciting new form of condensed matter, as they can potentially capture not only the quantum features of the individual building blocks but also novel collective properties that arise from coupling of nanocrystal components. In this thesis, measurement and interpretation of temperature-dependent thermopower in semiconductor nanocrystal solids are used to elucidate the Fermi energy level and the density of state distribution. The physical understating of temperature dependence of thermopower is, in turn, utilized to develop a powerful tool with which to monitor doping in PbTe nanocrystal solids with different concentrations of Ag2Te nanocrystal dopants. Combining the temperature-dependent thermopower and electrical conductivity measurements provides a unique electronic spectroscopy tool with which to reveal the carrier distribution and dynamics in semiconductor nanocrystal solids