High Thermoelectric Power Factor Research Articles

Ultra-High Thermoelectric Power Factors in Narrow Gap Materials with Asymmetric Bands

We theoretically unveil the unconventional possibility to achieve extremely high thermoelectric power factors in lightly doped narrow gap semiconductors with asymmetric conduction/valence bands operated in the bipolar transport regime. Specifically, using Boltzmann transport simulations, we show that narrow bandgap materials, rather than suffering from performance degradation due to bipolar conduction, if they possess highly asymmetric conduction and valence bands in terms of either effective masses, density of states, or phonon scattering rates, then they can deliver very high power factors. We show that this is reached because, under these conditions, electronic transport becomes phonon scattering limited, rather than ionized impurity scattering limited, which allows large conductivities. We explain why this effect has not been observed so far in the known narrow-gap semiconductors, interpret some recent related experimental findings, and propose a few examples from the half-Heusler materials family for which this effect can be observed and power factors even up to 50 mW/m$K^2$ can be reached.

Effective dopants for CuI single nanocrystals as a promising room temperature thermoelectric material

Room temperature (RT) thermoelectric (TE) materials are of great importance for harvesting energy from solar heat or other hot environments. The nontoxic, earth-abundant copper iodide (CuI) has a high Seebeck coefficient at RT; however, its electrical conductivity needs to be further enhanced for better TE performance. In this work, pure and doped CuI single nanocrystals were synthesized by the precipitation method in ethanol medium. The formed CuI nanoparticles (NPs) have high crystallinity in the gamma phase. Among several metallic dopants, Tb and Fe were found to be effective for enhancing the TE performance of CuI NPs. The Tb-doped sample was found to have the highest TE power factor. The concentration of this dopant was optimized for maximum TE performance. The TE power factor and Figure of merit values at RT were enhanced from approximately 30 μW/m K2 and 0.01 for the pure CuI NPs to approximately 210 μW/m K2 and 0.08 for the 0.05 mol% Tb-doped sample, respectively. This enhancement is correlated with the incorporation of this dopant in mixed chemical states. The pure and Tb-doped CuI NPs were characterized by several techniques, including XDR, XPS, TEM, SEM, and UV–visible spectroscopy. The Tb-doped sample might be suitable for RT TE devices and generators.

High Thermoelectric Power Factor of Poly(3-hexylthiophene) through In-Plane Alignment and Doping with a Molybdenum Dithiolene Complex.

We report a record thermoelectric power factor of up to 160 μW m–1 K–2 for the conjugated polymer poly(3-hexylthiophene) (P3HT). This result is achieved through the combination of high-temperature rubbing of thin films together with the use of a large molybdenum dithiolene p-dopant with a high electron affinity. Comparison of the UV–vis–NIR spectra of the chemically doped samples to electrochemically oxidized material reveals an oxidation level of 10%, i.e., one polaron for every 10 repeat units. The high power factor arises due to an increase in the charge-carrier mobility and hence electrical conductivity along the rubbing direction. We conclude that P3HT, with its facile synthesis and outstanding processability, should not be ruled out as a potential thermoelectric material.

Flexible Thermoelectric Double‐Layer Inorganic/Organic Composites Synthesized by Additive Manufacturing

AbstractThis study shows an approach to combine a high electrical conductivity of one composite layer with a high Seebeck coefficient of another composite layer in a double‐layer composite, resulting in high thermoelectric power factor. Flexible double‐layer‐composites, made from Bi2Te3‐based‐alloy/polylactic acid (BTBA/PLA) composites and Ag/PLA composites, are synthesized by solution additive manufacturing. With the increase in Ag volume‐ratio from 26.3% to 41.7% in Ag/PLA layers, the conductivity of the double‐layer composites increases from 12 S cm−1 to 1170 S cm−1, while the Seebeck coefficient remains ≈80 μV K−1 at 300 K. With further increase in volume ratio of Ag until 45.6% in Ag/PLA composite layer, the electrical conductivity of the double‐layer composites increases to 1710 S cm−1, however, with a slight decrease of the Seebeck coefficient to 64 μV K−1. The electrical conductivity and Seebeck coefficient vary only to a limited extent with the temperature. The high Seebeck coefficient is due to scattering of low energy charge carriers across compositionally graded interfaces. A power factor of 875 μW m−1 K−2 is achieved at 360 K for 41.7 vol.% Ag in the Ag/PLA layers. Solution additive manufacturing can directly print this double‐layer composite into intricate geometries, making this process is promising for large‐scale fabrication of thermoelectric composites.

Chemical Bonding in Colossal Thermopower FeSb2.

FeSb2 exhibits a colossal Seebeck coefficient ( ) and a record-breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb2 are based on either ligand-field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb2 . Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X-ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single-electron Sb-Sb bond implied by the covalent model, and the large formal charge and expected d-orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences.

Effects of Octahedral Tilting on Band Structure and Thermoelectric Power Factor of Titanate Perovskites: A First-Principles Study on SrTiO3

Doped SrTiO3 and other perovskite structured titanates are attracting interest as n-type thermoelectric materials due to their relatively high thermoelectric power factor, low toxicity, and modest cost. Taking SrTiO3 as an example, the effects of octahedral tilting on the electronic band structure and thermoelectric power factor of titanate perovskites have been studied from first-principles calculations. By utilizing Glazer’s notation, six representative tilt systems, including three out-of-phase (a0a0c–, a0b–b–, and a–a–a–) and three in-phase tilt systems (a0a0c+, a0b+b+, and a+a+a+), were investigated. It is found that out-of-phase tilting improves the optimum power factor as compared to the cubic aristotype, while in-phase tilting marginally lowers the optimum power factor. The largest increase in power factor (∼100%) is obtained in the one-tilt system a0a0c– at a tilt angle of 15°, which can be achieved with an energy cost of only 44 kJ mol–1 per formula unit. These findings agree with the experimental evidence that increased power factors are found in a0a0c– and a–a–a– tilt systems of titanate perovskites. The predicted increase of Seebeck coefficient as a function of tilt angle in the a–a–a– tilt system of SrTiO3 is also consistent with the experimental increase of Seebeck coefficient in a–a–a– titanates of La0.55K0.45TiO3 and La0.5Na0.5Ti0.9Nb0.1O3. Our simulations provide valuable insights into tuning the thermoelectric power factor of titanate perovskites by controlling octahedral tilting.

High Thermoelectric Power Factor Realization in Si-Rich SiGe/Si Superlattices by Super-Controlled Interfaces

A Si-based superlattice is one of the promising thermoelectric films for realizing a stand-alone single-chip power supply. Unlike a p-type superlattice (SL) achieving a higher power factor due to strain-induced high hole mobility, in the n-type SL, the strain can degrade the power factor due to lifting conduction band degeneracy. Here, we propose epitaxial Si-rich SiGe/Si SLs with ultrathin Ge segregation interface layers. The ultrathin interface layers are designed to be sufficiently strained, not to give strainto the above Si layers. Therein, a drastic thermal conductivity reduction occurs by larger phonon scattering at the interfaces with the large atomic size difference between Si layers and Ge segregation layers, while unstrained Si layers preserve a high conduction band degeneracy leading to a high Seebeck coefficient. As a result, the n-type Si0.7Ge0.3/Si SL with controlled interfaces achieves a higher power factor of ∼25 μW cm-1 K-2 in the in-plane direction at room temperature, which is superior to ever reported SiGe-based films: SiGe-based SLs and SiGe films. The Si0.7Ge0.3/Si SL with controlled interfaces also exhibits a low thermal conductivity of ∼2.5 W m-1 K-1 in the cross-plane direction, which is ∼5 times lower than the reported value in a conventional Si0.7Ge0.3/Si SL. These results demonstrate that strain and atomic differences controlled by ultrathin layers can bring a breakthrough for realizing high-performance light-element-based thermoelectric films.

High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy

Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelectric applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial single-crystalline metal/semiconductor superlattices for thermionic energy conversion. Solid-solution alloys of ScN with traditional III-nitrides such as AlxSc1−xN have demonstrated piezoelectric and ferroelectric properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and physical and chemical vapor depositions for thermoelectric applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concentrations, and achieve high-mobility in epitaxial films. Here, we report the high-mobility and high-thermoelectric power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted molecular beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Electrical measurements demonstrated a high room-temperature mobility of 127 cm2/V s and temperature-dependent mobility in the temperature range of 50–400 K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coefficients (−175 μV/K at 950 K) and, along with a moderately high electrical conductivity, a large thermoelectric power factor (2.3 × 10−3 W/m-K2 at 500 K) was achieved, which makes ScN a promising candidate for thermoelectric applications. The thermal conductivity of the films, however, was found to be a bit large, which resulted in a maximum figure-of-merit of 0.17 at 500 K.

Enargite Cu3PS4: A Cu–S‐Based Thermoelectric Material with a Wurtzite‐Derivative Structure

AbstractCompound semiconductors derived from ZnS (zincblende and wurtzite) with tetrahedral framework structures have functions for various applications. Examples of such materials include Cu–S‐based materials with zincblende‐derivative structures, which have attracted attention as thermoelectric (TE) materials over the past decade. This study illuminates superior TE performance in polycrystalline samples of enargite Cu3P1−xGexS4 with a wurtzite‐derivative structure. The substitution of Ge for P dopes holes into the top of the valence band composed of Cu‐3d and S‐3p, whereby its multiband characteristic leads to a high TE power factor. Furthermore, a reduction in the grain size to 50–300 nm can effectively decrease phonon mean free paths, leading to low thermal conductivity. These features result in a dimensionless TE figure of merit ZT of 0.5 at 673 K for the x = 0.2 sample. Environmentally benign and low‐cost characteristics of the constituent elements of Cu3PS4, as well as its high‐performance thermoelectricity, make it a promising candidate for large‐scale TE applications. Furthermore, this finding extends the development field of Cu–S‐based TE materials to those with wurtzite‐derivative structures.

High Thermoelectric Power Factor of Si–Mg2Si Nanocomposite Ribbons Synthesized by Melt Spinning

Si has been considered as a promising thermoelectric material because of its many advantages, such as its nontoxic characteristics and abundance. In this study, the concept of modulation doping is ...

Thermoelectric polymer films with a significantly high Seebeck coefficient and thermoelectric power factor obtained through surface energy filtering

Coating with Rhodamine 101 can significantly enhance the Seebeck coefficient of PEDOT:PSS, and surface energy filtering is proposed to be the reason for this effect.

Theoretical search for high-performance thermoelectric donor–acceptor copolymers: the role of super-exchange couplings

First-principles calculations of a series of representing D–A copolymers demonstrated the strong Super-Exchange couplings induce not only small hole effective mass but also weak electron-phonon couplings, and eventually high thermoelectric power factor.

Electrochemical doping engineering tuning of the thermoelectric performance of a π-conjugated free-standing poly(thiophene-furan) thin-film

Promising free-standing poly(thiophene-furan) gains a high thermoelectric power factor by facile electrochemical doping engineering.

Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors

Ca3Co4O9 ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca3Co4O9 ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)2 as a template and CaCO3 as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m−1 K−2.

Efficient Debundling of Few-Walled Carbon Nanotubes by Wrapping with Donor-Acceptor Polymers for Improving Thermoelectric Properties.

Organic thermoelectric (TE) materials have great potential as sustainable energy sources for powering flexible and wearable electronic devices via harvesting of human body heat. Recent advances in soluble conjugated polymer/carbon nanotube (CNT) composites have facilitated achievement of high TE power factors. However, the effects of conjugated polymers on the debundling and electrical percolation of CNTs and on the TE properties of their composites are not yet fully understood. Herein, we introduce a novel type of polymer/CNT composite composed of a donor-acceptor (D-A)-type polymer and few-walled CNTs (FWCNTs). Three kinds of D-A polymers are employed to disperse FWCNTs, and the photophysical, morphological, and TE properties of the resulting polymer/FWCNT composites are compared with those of composites composed of FWCNTs dispersed with conventional donor-only poly(3-hexylthiophene). The results reveal that the strong intermolecular interaction forces and high backbone planarity of the D-A polymers facilitate effective debundling of FWCNTs, which results in much smaller bundle sizes. Consequently, the D-A polymer/FWCNT composite films show superior electrical percolation and TE performances with improved power factors of up to 459 μW/mK2. Finally, we demonstrate the feasibility of the D-A polymer/FWCNT composites for use in the fabrication of a flexible TE generator, which shows a maximum power output of 210 nW at a temperature gradient of 20 K.

Nanostructured potential well/barrier engineering for realizing unprecedentedly large thermoelectric power factors

This work describes, through the semi-classical Boltzmann transport theory and simulation, a novel nanostructured material design that can lead to unprecedentedly high thermoelectric power factors, with improvements of more than an order of magnitude compared to optimal bulk material power factors. The design is based on a specific grain/grain-boundary (potential well/barrier) engineering such that: i) carrier energy filtering is achieved using potential barriers, combined with ii) higher than usual doping operating conditions such that high carrier velocities and mean-free-paths are utilized, iii) minimal carrier energy relaxation is achieved after passing over the barriers to propagate the high Seebeck coefficient of the barriers into the potential wells, and, importantly, iv) an intermediate dopant-free (depleted) region is formed. Thus, the design consists of a ‘three-region geometry’, in which the high doping resides in the center/core of the potential well, with a dopant-depleted region separating the doped region from the potential barriers. It is shown that the filtering barriers are optimal when they mitigate the reduction in conductivity they introduce, and this can be done primarily when they are ‘clean’ from dopants during the process of filtering. The potential wells, on the other hand, are optimal when they mitigate the reduced Seebeck coefficient they introduce by: i) not allowing carrier energy relaxation, and ii) mitigating the reduction in mobility that the high concentration of dopant impurities causes. It is shown that dopant segregation, with ‘clean’ dopant-depletion regions around the potential barriers, serves this key purpose of improved mobility toward the phonon-limited mobility levels in the wells. Using quantum transport simulations based on the non-equilibrium Green's function method as well as semi-classical Monte Carlo simulations, we also verify the important ingredients and validate this ‘clean-filtering’ design.

Anisotropic thermal conductivity in 2D tellurium

Two-dimensional tellurium (2D-Te) has been recently synthesized and shown potential in electronics, optoelectronics, and thermoelectric applications, with the merits of high mobility, environmental stability, high thermoelectric power-factor, and simplicity of mass production. These 2D-Te films have unique atomic structures: the Te atoms form trigonal helical chains and are then stacked into hexagonal lattice by van der Waals force, which brings up distinctive transport behaviors. Here we report anisotropic thermal conductivity of suspended 2D-Te films measured by micro-Raman thermometry and the time-domain thermal reflectance (TDTR) method. The in-plane along-chain and cross-chain thermal conductivities are found to be around 2.5 and 1.7 W m−1 K−1, respectively, for thicker films (>100 nm), and reduced to 1.6 and 0.64 W m−1 K−1 for the thinner films (<20 nm). The measured anisotropy is >1.3 for all the films studied. The cross-plane (also across-chain) thermal conductivity is found to be around 0.8 to 1.2 W m−1 K−1 for thicker films, slightly lower than that along the in-plane across-chain direction due to the stronger suppression by the thin film boundary. Theoretical modeling reveals that the anisotropy mainly originates from anisotropic phonon dispersion. The long mean-free-path phonons in Te are also shown to be strongly suppressed by boundary scattering. The large reduction of anisotropic thermal conductivity from the bulk makes it the best single-element thermoelectric material and enables potential thermoelectric generation or cooling devices at room temperature. Our results also provide critical information for thermal management of 2D-Te electronic devices.

Magnon drag effect in Fe-Co alloys

We report here a systematic study of the magnon-drag induced thermoelectric properties of Fe-rich, Fe-Co body-centered-cubic alloys. The sign of the low temperature behavior of thermopower is explained well by the hydrodynamic theory for magnon-drag, informed by density functional theory calculations of the ground state of Fe-Co alloys. The high-temperature thermopower of some of the alloys, and indeed that of elemental iron, changes the sign, as previously observed. We propose a mechanism to elucidate this hitherto unexplained observation. Further, the power factor of Fe72Co28 peaks around 35 μV/cm K2 at 500 K, comparable to the standard thermoelectric material Bi2Te3. Because of their high thermoelectric power factor, Fe-Co alloys are potential candidate thermoelectric metals for active cooling of electronic devices.

Origin of the magnetic ground state developed upon Si, Ge, and Sb-substitution at Sn site in Fe2TiSn

A nonmagnetic full Heusler alloy, Fe2TiSn, was substituted with Si, Ge, and Sb at the Sn site with a primary aim of investigating the physical properties of these compositions that are much anticipated to result in a high thermoelectric power factor and generate a half-metallic ground state. The X-ray diffraction (XRD) study carried out using the synchrotron source confirms the solubility limit of substituent atoms to be a mere 5% for Si and Ge and 60% for Sb that results in a pure L21 Heusler phase. Rietveld refinement of the XRD profiles yields the lattice parameters. Despite the fact that chemical substitution should lead to atomic disorder and an increase in the scattering of charge carriers, resistivity measurements show a significant decrease in the magnitude with as little as 5% substitution of Sn by Si or Ge. The weak localization present in Fe2TiSn seems to drastically reduce in the substituted compositions, and the long range magnetic order seems to affect the transport of the charge carriers. The origin of magnetic interactions is investigated using extended X-ray absorption fine structure spectroscopy. The changes in the local crystal structure in terms of the Fe–Fe bond distance are determined. The inference drawn from the experimental study is further complemented with exchange coupling parameter, Jij, and orbital projected density of states calculations, thus presenting an overall understanding of the origin of magnetic interactions in these technologically relevant compositions.

Highly Passivated n‐Type Colloidal Quantum Dots for Solution‐Processed Thermoelectric Generators with Large Output Voltage

AbstractColloidal quantum dots (CQDs) are attractive materials for thermoelectric applications due to their simple and low‐cost processing; advantageously, they also offer low thermal conductivity and high Seebeck coefficient. To date, the majority of CQD thermoelectric films reported upon have been p‐type, while only a few reports are available on n‐type films. High‐performing n‐ and p‐type films are essential for thermoelectric generators (TEGs) with large output voltage and power. Here, high‐thermoelectric‐performance n‐type CQD films are reported and showcased in high‐performance all‐CQD TEGs. By engineering the electronic coupling in the films, a thorough removal of insulating ligands is achieved and this is combined with excellent surface trap passivation. This enables a high thermoelectric power factor of 24 µW m−1 K−2, superior to previously reported n‐type lead chalcogenide CQD films operating near room temperature (&lt;1 µW m−1 K−2). As a result, an all‐CQD film TEG with a large output voltage of 0.25 V and a power density of 0.63 W m−2 at ∆T = 50 K is demonstrated, which represents an over fourfold enhancement to previously reported p‐type only CQD TEGs.