Large Thermoelectric Power Factor Research Articles

Large thermoelectric power factor at low temperatures in one-dimensional telluride Ta4SiTe4

We report the discovery of a very large thermoelectric power over –400 μV K−1 in the whisker crystals of a one-dimensional telluride Ta4SiTe4, while maintaining a low electrical resistivity of ρ = 2 mΩ cm, yielding a very large power factor of P = 80 μW cm−1 K−2 at an optimum temperature of 130 K. This temperature is widely controlled from the cryogenic temperature of 50 K to room temperature by chemical doping, resulting in the largest P of 170 μW cm−1 K−2 at 220–280 K. These P values far exceed those of the Bi2Te3-Sb2Te3 alloys at around room temperature, offering an avenue for realizing the practical-level thermoelectric cooling at low temperatures. The coexistence of a one-dimensional electronic structure and a very small band gap appearing in the vicinity of the Dirac semimetals probably causes the very large power factors in Ta4SiTe4, indicating that the “one-dimensional Dirac semimetal” is a promising way to find high-performance thermoelectric materials for the low temperature applications.

High thermoelectricpower factor in graphene/hBN devices

Fast and controllable cooling at nanoscales requires a combination of highly efficient passive cooling and active cooling. Although passive cooling in graphene-based devices is quite effective due to graphene's extraordinary heat conduction, active cooling has not been considered feasible due to graphene's low thermoelectric power factor. Here, we show that the thermoelectric performance of graphene can be significantly improved by using hexagonal boron nitride (hBN) substrates instead of SiO2 We find the room temperature efficiency of active cooling in the device, as gauged by the power factor times temperature, reaches values as high as 10.35 W⋅m-1⋅K-1, corresponding to more than doubling the highest reported room temperature bulk power factors, 5 W⋅m-1⋅K-1, in YbAl3, and quadrupling the best 2D power factor, 2.5 W⋅m-1⋅K-1, in MoS2 We further show that the Seebeck coefficient provides a direct measure of substrate-induced random potential fluctuations and that their significant reduction for hBN substrates enables fast gate-controlled switching of the Seebeck coefficient polarity for applications in integrated active cooling devices.

A Monte Carlo Study on the Effect of Energy Barriers on the Thermoelectric Properties of Si

Abstract Energy filtering by energy barriers has been proposed to interpret observations on large thermoelectric power factor (TPF) enhancement in highly doped nanocrystalline Si (nc-Si). Previous Boltzmann transport equation (BTE) modeling indicated that high TPFs could be explained as the result of the presence of energy barriers at the grain boundaries, the high Fermi energy due to the high doping level, and the formation of a low thermal conductivity second phase. To test the assumptions of the BTE modeling and provide more realistic simulations, we have performed Monte Carlo (MC) simulations on the transport properties of composite nc-Si structures. Here, we report on (i) the effect of an energy barrier, and (ii) the effect of multiple barriers on the conductivity and the Seebeck coefficient. In short structures, a TPF enhancement was found and it has been attributed to energy filtering by the energy barrier. The MC indicated that the TE performance can be improved by multiple barriers in close separation. It has been shown that TPF enhancement is possible even when the condition for thermal conductivity non-uniformity across the composite structure is not-fulfilled.

High Power Factor and Enhanced Thermoelectric Performance of SnTe-AgInTe2: Synergistic Effect of Resonance Level and Valence Band Convergence.

Understanding the basis of electronic transport and developing ideas to improve thermoelectric power factor are essential for production of efficient thermoelectric materials. Here, we report a significantly large thermoelectric power factor of ∼31.4 μW/cm·K2 at 856 K in Ag and In co-doped SnTe (i.e., SnAgxInxTe1+2x). This is the highest power factor so far reported for SnTe-based material, which arises from the synergistic effects of Ag and In on the electronic structure and the improved electrical transport properties of SnTe. In and Ag play different but complementary roles in modifying the valence band structure of SnTe. In-doping introduces resonance levels inside the valence bands, leading to a significant improvement in the Seebeck coefficient at room temperature. On the other hand, Ag-doping reduces the energy separation between light- and heavy-hole valence bands by widening the principal band gap, which also results in an improved Seebeck coefficient. Additionally, Ag-doping in SnTe enhances the p-type carrier mobility. Co-doping of In and Ag in SnTe yields synergistically enhanced Seebeck coefficient and power factor over a broad temperature range because of the synergy of the introduction of resonance states and convergence of valence bands, which have been confirmed by first-principles density functional theory-based electronic structure calculations. As a consequence, we have achieved an improved thermoelectric figure of merit, zT ≈ 1, in SnAg0.025In0.025Te1.05 at 856 K.

(Invited) Thermoelectric Properties of Semiconducting Single-Walled Carbon Nanotube Networks

Nanostructured organic semiconductors (OSCs), including single-walled carbon nanotubes (SWCNTs), offer a number of intriguing technological characteristics for thermoelectric applications, such as earth-abundant raw materials, low-cost deposition, and flexible form factors. We will initially present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a wide bandgap semiconducting polymer matrix, followed by more recent work aimed at understanding the role that the semiconducting polymer plays in the observed transport properties. Rational choice of the starting SWCNT material and the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. Consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs, we observe a distinct dependence of the thermopower and thermoelectric power factor on the bandgap (or diameter) of the carbon nanotubes. We have measured large thermopower values (as high as ~2,500 µV/K for s-SWCNT networks with very low electrical conductivity) and impressive thermoelectric power factor as large as ~350 µW/m·K2. By varying the carrier density injected into the s-SWCNT networks by a stable charge-transfer dopant, we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. For carbon nanotubes prepared by high-pressure carbon monoxide (HiPCO) conversion, as we tune the carrier density, we are able to maintain a thermopower above 100 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 40,000 S/m, resulting in a thermoelectric power factor of >300 µW/m·K2. These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors. By modifying an approach that allows us to strip the dispersing polymer from the s-SWCNTs we were able to demonstrate that the polymer appears to play no role in modifying the barriers to electrical transport present at tube-tube junctions, and simply controls the extent of nanotube bundling and thereby the surface area available to the charge-transfer dopant. This study also indicates that densification of the s-SWCNT network results in a two-fold enhancement of the thermoelectric power factor, suggesting that careful control of the amount and nature of the matrix material is required for high-performance s-SWCNT thermoelectric materials. Finally, we present a data from sensitive transport measurement technique, based on a microfabricated silicon nitride thermal isolation platform, to probe transport in the s-SWCNT networks, showing that the thermoelectric figure of merit (zT) is positively correlated with the measurement temperature, increasing by a factor of ~2.5 from 300 K to 350 K. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by controlling the composition of the s-SWCNT network and tuning the carrier density (i.e., Fermi energy), and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability.

On the basis of an excellent transistor material, [1]benzothieno[3,2-b][1]benzothiophene (BTBT), a series of highly conductive organic metals with the composition of (BTBT)2XF6 (X = P, As, Sb, and Ta) are prepared and the structural and physical properties are investigated. The room-temperature conductivity amounts to 4100 S cm(-1) in the AsF6 salt, corresponding to the drift mobility of 16 cm(2) V(-1) s(-1). Owing to the high conductivity, this salt shows a thermoelectric power factor of 55-88 μW K(-2) m(-1), which is a large value when this compound is regarded as an organic thermoelectric material. The thermoelectric power and the reflectance spectrum indicate a large bandwidth of 1.4 eV. These salts exhibit an abrupt resistivity jump under 200 K, which turns to an insulating state below 60 K. The paramagnetic spin susceptibility, and the Raman and the IR spectra suggest 4kF charge-density waves as an origin of the low-temperature insulating state.

Large thermoelectric power factors and impact of texturing on the thermal conductivity in polycrystalline SnSe

Large thermoelectric power factors and low thermal conductivities linked to changes in texturing have been observed in consolidated polycrystalline SnSe ingots.

The Fragility of Thermoelectric Power Factor in Cross-Plane Superlattices in the Presence of Nonidealities: A Quantum Transport Simulation Approach

Energy filtering has been put forth as a promising method for achieving large thermoelectric power factors in thermoelectric materials through Seebeck coefficient improvement. Materials with embedded potential barriers, such as cross-plane superlattices, provide energy filtering, in addition to low thermal conductivity, and could potentially achieve high figure of merit. Although there exist many theoretical works demonstrating Seebeck coefficient and power factor gains in idealized structures, experimental support has been scant. In most cases, the electrical conductivity is drastically reduced due to the presence of barriers. In this work, using quantum-mechanical simulations based on the nonequilibrium Green’s function method, we show that, although power factor improvements can theoretically be observed in optimized superlattices (as pointed out in previous studies), different types of deviations from the ideal potential profiles of the barriers degrade the performance, some nonidealities being so significant as to negate all power factor gains. Specifically, the effect of tunneling due to thin barriers could be especially detrimental to the Seebeck coefficient and power factor. Our results could partially explain why significant power factor improvements in superlattices and other energy-filtering nanostructures mainly fail to be realized, despite theoretical predictions.

Calculated transport properties of CdO: Thermal conductivity and thermoelectric power factor

We present first-principles calculations of the thermal and electronic transport properties of the oxide semiconductor CdO. In particular, we find from theory that the accepted thermal conductivity $\ensuremath{\kappa}$ value of $0.7\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ is approximately one order of magnitude too small; our calculations of $\ensuremath{\kappa}$ of CdO are in good agreement with recent measurements. We also find that alloying of MgO with CdO is an effective means to reduce the lattice contribution to $\ensuremath{\kappa}$, despite MgO having a much larger thermal conductivity. We further consider the electronic structure of CdO in relation to thermoelectric performance, finding that large thermoelectric power factors may occur if the material can be heavily doped $p$ type. This work develops insight into the nature of thermal and electronic transport in an important oxide semiconductor.

Thermoelectricity Generation and Electron–Magnon Scattering in a Natural Chalcopyrite Mineral from a Deep‐Sea Hydrothermal Vent

AbstractCurrent high‐performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+xFe1−xS2, obtained from a deep‐sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron–magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earth’s crust.

Thermoelectricity Generation and Electron-Magnon Scattering in a Natural Chalcopyrite Mineral from a Deep-Sea Hydrothermal Vent.

Current high-performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+x Fe1-x S2 , obtained from a deep-sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron-magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earth's crust.

Synthesis, structure, and properties of Ca3Co3.85M0.15O9 + δ (M = Ti–Zn, Mo, W, Pb, Bi) layered thermoelectrics

We have prepared Ca3Co3.85M0.15O9 + δ (M = Ti–Zn, Mo, W, Pb, Bi) solid solutions, investigated their crystal structure and microstructure, assessed their thermal stability in air, and measured their thermal expansion, electrical conductivity, and thermoelectric power in air at temperatures from 300 to 1100 K. The results demonstrate that the Ca3Co3.85M0.15O9 + δ cobaltites are p-type semiconductors and that their unitcell parameters decrease with an increase in the number of d electrons in the 3d transition metal ion and with an increase in the average oxidation state of the cobalt. Their thermoelectric power increases with increasing temperature, reaching the highest value in the Ca3Co3.85Pb0.15O9 + δ solid solution: 380 μV/K at a temperature of 1100 K. The Ca3Co3.85Bi0.15O9 + δ solid solution has the largest thermoelectric power factor among the materials studied, 206 μW/(m K2) at a temperature of 1100 K, which is twice the power factor of the unsubstituted calcium cobaltite Ca3Co4O9 + δ.

Low-dimensional transport and large thermoelectric power factors in bulk semiconductors by band engineering of highly directional electronic states.

Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures and the introduction of resonant states were suggested as possible solutions to this paradox, but with limited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. It exploits the highly directional character of some orbitals to engineer the band structure and produce a type of low-dimensional transport similar to that targeted in nanostructures, while retaining isotropic properties. Using first-principle calculations, the theoretical concept is demonstrated in Fe2YZ Heusler compounds, yielding power factors 4 to 5 times larger than in classical thermoelectrics at room temperature. Our findings are totally generic and rationalize the search of alternative compounds with similar behavior. Beyond thermoelectricity, these might be relevant also in the context of electronic, superconducting, or photovoltaic applications.

PANI/graphene nanocomposite films with high thermoelectric properties by enhanced molecular ordering

A combination of in situ polymerization and a solution process was adopted to prepare PANI/graphene nanocomposites with a large thermoelectric power factor.

Antisite-disorder, magnetic and thermoelectric properties of Mo-rich Sr2Fe1-yMo1+yO6 (0 ≤y≤ 0.2) double perovskites.

Structure analysis using X-ray and neutron powder diffraction and elemental mapping has been used to demonstrate that nominal A-site deficient Sr(2-x)FeMoO(6-δ) (0 ≤x≤ 0.5) compositions form as Mo-rich Sr(2)Fe(1-y)Mo(1+y)O(6) (0 ≤y≤ 0.2) perovskites at high temperatures and under reducing atmospheres. These materials show a gradual transition from the Fe and Mo rock salt ordered double perovskite structure to a B-site disordered arrangement. Analysis of the fractions of B-O-B' linkages revealed a gradual increase in the number of Mo-O-Mo linkages at the expense of the ferrimagnetic (FIM) Fe-O-Mo linkages that dominate the y = 0 material. All samples contain about 10-15% antiferromagnetic (AF) Fe-O-Fe linkages, independent of the degree of B-site ordering. The magnetic susceptibility of the y = 0.2 sample is characteristic of a small domain ferrimagnet (T(c)∼ 250 K), while room temperature neutron powder diffraction demonstrated the presence of G-type AF ordering linked to the Fe-O-Fe linkages (m(Fe) = 1.25(7)μ(B)). The high temperature thermoelectric properties are characteristic of a metal with a linear temperature dependence of the Seebeck coefficient, S (for all y) and electrical resistivity ρ (y≥ 0.1). The largest thermoelectric power factor S(2)/ρ = 0.12 mW m(-1) K(-1) is observed for Sr(2)FeMoO(6) at 1000 K.

Type VIII Si based clathrates: prospects for a giant thermoelectric power factor.

Although clathrate materials are known for their small thermal conductivity, they have not shown a large thermoelectric power factor so far. We present the band structures of type VIII Si, Ge, and Sn clathrates as well as the alkali and alkaline-earth intercalated type VIII Si clathrates. Our calculations revealed that this group of materials has potentially large power factors due to the existence of a large number of carrier pockets near their band edges. In particular, we calculated the charge carrier transport properties of Si46-VIII both for n-type and p-type materials. The exceptionally high multi-valley band structure of Si46-VIII near the Fermi energy due to the high crystallographic symmetry resulted in a giant power factor in this material. It was shown that the intercalation of Si46-VIII with alkali and alkaline-earth guest atoms shifts the Fermi energy close to the conduction band edge and, except for Be8Si46 and Mg8Si46, they weakly influence the band structure of Si46. Among these clathrate systems, Ca8Si46, Sr8Si46, and Ba8Si46 showed negative formation energy, which should facilitate their synthesis. Our results imply that the intercalation affects the conduction band of Si46-VIII more than its valence band. Also, interestingly, the type VIII clathrates of Si46 and its derivatives (except Be8Si46 and Mg8Si46), Sn46, and Ge46 all have 26 carrier pockets near their valence band edge. Among the different derivatives of Si46-VIII, Rb8Si46 and Ba8Si46 have the highest number of electron pockets near their band edges. The thermoelectric power factor was predicted using a multiband Boltzmann transport equation linked with parameters extracted from density functional calculations. It was shown that both the increment of charge mobility and the existence of multiple band extrema contribute to the enhancement of the thermoelectric power factor considerably. Such a large power factor along with their inherently low thermal conductivity can make this group of clathrates promising thermoelectric materials.

Prediction of giant thermoelectric power factor in type-VIII clathrate Si46.

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

Large theoretical thermoelectric power factor of suspended single-layer MoS2

We have calculated the semi-classical thermoelectric power factor of suspended single-layer (SL)- MoS2 utilizing electron relaxation times derived from ab initio calculations. Measurements of the thermoelectric power factor of SL-MoS2 on substrates reveal poor power factors. In contrast, we find the thermoelectric power factor of suspended SL-MoS2 to peak at ∼2.8 × 104 μW/m K2 at 300 K, at an electron concentration of 1012 cm−2. This figure is higher than that in bulk Bi2Te3, for example. Given its relatively high thermal conductivity, suspended SL-MoS2 may hold promise for in-plane thin-film Peltier coolers, provided reasonable mobilities can be realized.

Local Structure and Energetics of Pr- and La-Doped SrTiO3 Grain Boundaries and the Influence on Core–Shell Structure Formation

Using atomistic simulation techniques, we investigate the structural and energetic effects of Pr- and La-doping on the grain boundaries of SrTiO3. Recent experimental studies have shown that Pr-doping of SrTiO3 produces a large thermoelectric power factor and that this is a result of Pr-rich grain boundaries formed in a core–shell structure. This behavior was not observed for La-doping and dopant-rich grain boundaries were not formed. In this work, for both the infinitely dilute limit and solid solutions with dopant concentrations of up to ∼15 mol %, Pr-doping at the grain boundaries is found to be far more favorable than doping in the bulk. La-doping at the grain boundaries is also found to be favored over bulk doping, however, to a far lesser extent than that calculated for Pr. The binding energy between Pr ions and charge compensating Sr vacancies is also stronger than for La ion defect clusters. By analyzing the local structures around these defect clusters, we provide possible explanations as to why there is a significant energetic benefit for grain-boundary doping.

Gated Si nanowires for large thermoelectric power factors

We investigate the effect of electrostatic gating on the thermoelectric power factor of p-type Si nanowires (NWs) of up to 20 nm in diameter in the [100], [110], and [111] crystallographic transport orientations. We use atomistic tight-binding simulations for the calculation of the NW electronic structure, coupled to linearized Boltzmann transport equation for the calculation of the thermoelectric coefficients. We show that gated NW structures can provide ∼5× larger thermoelectric power factor compared to doped channels, attributed to their high hole phonon-limited mobility, as well as gating induced bandstructure modifications which further improve mobility. Despite the fact that gating shifts the charge carriers near the NW surface, surface roughness scattering is not strong enough to degrade the transport properties of the accumulated hole layer. The highest power factor is achieved for the [111] NW, followed by the [110], and finally by the [100] NW. As the NW diameter increases, the advantage of the gated channel is reduced. We show, however, that even at 20 nm diameters (the largest ones that we were able to simulate), a ∼3× higher power factor for gated channels is observed. Our simulations suggest that the advantage of gating could still be present in NWs with diameters of up to ∼40 nm.