Showing papers on "Seebeck coefficient published in 2015"

Characterization of Lorenz number with Seebeck coefficient measurement

Abstract: In analyzing zT improvements due to lattice thermal conductivity (κ_L ) reduction, electrical conductivity (σ) and total thermal conductivity (κ_(Total)) are often used to estimate the electronic component of the thermal conductivity (κ_E) and in turn κ_L from κ_L = ∼ κ_(Total) − LσT. The Wiedemann-Franz law, κ_E = LσT, where L is Lorenz number, is widely used to estimate κ_E from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10^(−8) WΩK^(−2) (degenerate limit). However, significant deviations from the degenerate limit (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10^(−8) WΩK^(−2) for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: L=1.5+exp[−_(|S|)_(116)] (where L is in 10^(−8) WΩK^(−2) and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si_(0.8) Ge _(0.2) where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity.

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

Abstract: We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically struc...

Mg Alloying in SnTe Facilitates Valence Band Convergence and Optimizes Thermoelectric Properties

Abstract: SnTe, a lead-free rock-salt analogue of PbTe, having valence band structure similar to PbTe, recently has attracted attention for thermoelectric heat to electricity generation. However, pristine SnTe is a poor thermoelectric material because of very high hole concentration resulting from intrinsic Sn vacancies, which give rise to low Seebeck coefficient and high electrical thermal conductivity. In this report, we show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient. Mg (2–10 mol %) alloying in SnTe modulates its electronic band structure by increasing the band gap of SnTe and results in decrease in the energy separation between its light and heavy hole valence bands. Thus, solid solution alloying with Mg enhances the contribution of the heavy hole valence band, leading to significant improvement in the Seebeck coefficient in Mg alloyed SnTe, which in turn results in re...

Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe

Abstract: We demonstrate a high solubility limit of >9 mol% for MnTe alloying in SnTe. The electrical conductivity of SnTe decreases gradually while the Seebeck coefficient increases remarkably with increasing MnTe content, leading to enhanced power factors. The room-temperature Seebeck coefficients of Mn-doped SnTe are significantly higher than those predicted by theoretical Pisarenko plots for pure SnTe, indicating a modified band structure. The high-temperature Hall data of Sn1–xMnxTe show strong temperature dependence, suggestive of a two-valence-band conduction behavior. Moreover, the peak temperature of the Hall plot of Sn1–xMnxTe shifts toward lower temperature as MnTe content is increased, which is clear evidence of decreased energy separation (band convergence) between the two valence bands. The first-principles electronic structure calculations based on density functional theory also support this point. The higher doping fraction (>9%) of Mn in comparison with ∼3% for Cd and Hg in SnTe gives rise to a muc...

Extraordinary role of Hg in enhancing the thermoelectric performance of p-type SnTe

Abstract: We report several synergistic effects in Hg alloying of SnTe to enhance the power factor and overall figure of merit ZT. Hg alloying decreases the energy separation between the two valence bands, leading to pronounced band convergence that improves the Seebeck coefficient. Hg alloying of SnTe also significantly enlarges the band gap thereby effectively suppressing the bipolar diffusion. Collectively, this results in high ZT of ∼1.35 at 910 K for 2% Bi-doped SnTe with 3%HgTe. The solubility limit of Hg in SnTe is less than 3 mol%, and above this level we observe HgTe precipitates in the SnTe matrix, typically trapped at grain boundary triple junctions. The strong point defect scattering of phonons caused by Hg alloying coupled with mesoscale scattering via grain boundaries contributes to a great reduction of lattice thermal conductivity. The multiple synergistic roles that Hg plays in regulating the electron and phonon transport in SnTe provide important new insights into continued optimization of SnTe-based and related materials.

Impact of the Doping Method on Conductivity and Thermopower in Semiconducting Polythiophenes

Abstract: The development of organic semiconductors for use in thermoelectrics requires the optimization of both their thermopower and electrical conductivity. Here two fundamentally different doping mechanisms are used to investigate the thermoelectric properties of known high hole mobility polymers: poly 3-hexylthiophene (P3HT), poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14), and poly(2,5-bis(thiphen-2-yl)-(3,7-diheptadecantyltetrathienoacene)) (P2TDC17-FT4). The small molecule tetrafluorotetracyanoquinodimethane (F4TCNQ) is known to effectively dope these polymers, and the thermoelectric properties are studied as a function of the ratio of dopant to polymer repeat unit. Higher electrical conductivity and values of the thermoelectric power factor are achieved by doping with vapor-deposited fluoroalkyl trichlorosilanes. The combination of these data reveals a striking relationship between thermopower and conductivity in thiophene-based polymers over a large range of electrical conductivity that is independent of the means of electrical doping. This relationship is not predicted by commonly used transport models for semiconducting polymers and is demonstrated to hold for other semiconducting polymers as well.

Phosphorene nanoribbon as a promising candidate for thermoelectric applications.

Abstract: In this work, the electronic properties of phosphorene nanoribbons with different width and edge configurations are studied by using density functional theory It is found that the armchair phosphorene nanoribbons are semiconducting while the zigzag nanoribbons are metallic The band gaps of armchair nanoribbons decrease monotonically with increasing ribbon width By passivating the edge phosphorus atoms with hydrogen, the zigzag series also become semiconducting, while the armchair series exhibit a larger band gap than their pristine counterpart The electronic transport properties of these phosphorene nanoribbons are then investigated using Boltzmann theory and relaxation time approximation We find that all the semiconducting nanoribbons exhibit very large values of Seebeck coefficient and can be further enhanced by hydrogen passivation at the edge Taking pristine armchair nanoribbons and hydrogen-passivated zigzag naoribbons with width N = 7, 8, 9 as examples, we calculate the lattice thermal conductivity with the help of phonon Boltzmann transport equation and evaluate the width-dependent thermoelectric performance Due to significantly enhanced Seebeck coefficient and decreased thermal conductivity, we find that at least one type of phosphorene nanoribbons can be optimized to exhibit very high figure of merit (ZT values) at room temperature, which suggests their appealing thermoelectric applications

Studies on Thermoelectric Properties of n‐type Polycrystalline SnSe1‐xSx by Iodine Doping

Abstract: Iodine-doped n-type SnSe polycrystalline by melting and hot pressing is prepared. The prepared material is anisotropic with a peak ZT of ≈0.8 at about 773 K measured along the hot pressing direction. This is the first report on thermoelectric properties of n-type Sn chalcogenide alloys. With increasing content of iodine, the carrier concentration changed from 2.3 × 1017 cm−3 (p-type) to 5.0 × 1015 cm−3 (n-type) then to 2.0 × 1017 cm−3 (n-type). The decent ZT is mainly attributed to the intrinsically low thermal conductivity due to the high anharmonicity of the chemical bonds like those in p-type SnSe. By alloying with 10 at% SnS, even lower thermal conductivity and an enhanced Seebeck coefficient were achieved, leading to an increased ZT of ≈1.0 at about 773 K measured also along the hot pressing direction.

High Thermoelectric Performance and Enhanced Mechanical Stability of p-type Ge1–xSbxTe

Abstract: High thermoelectric figure of merit, zT, of ∼1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge1–xSbxTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300–773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge0.9Sb0.1Te sample shows mechanical stability (Vickers microhardness) of ∼206 Hv, which is significantly higher compared to other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and TAGS.

Measuring thermoelectric transport properties of materials

Abstract: In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, zT. Advice for improving the data quality in Seebeck coefficient, electrical resistivity, and thermal conductivity (from flash diffusivity and heat capacity) measurements are given together with methods for identifying possible erroneous data. Measurement of the Hall coefficient and calculation of the charge carrier concentration and mobility is also included due to its importance for understanding materials. It is not intended to be a complete record or comparison of all the different techniques employed in thermoelectrics. Rather, by providing an overview of common techniques and their inherent difficulties it is an aid to new researchers or students in the field. The focus is mainly on high temperature measurements but low temperature techniques are also briefly discussed.

Synergistically optimized electrical and thermal transport properties of SnTe via alloying high-solubility MnTe

Abstract: Lead chalcogenides are the most efficient thermoelectric materials. In comparison, SnTe, a lead-free analogue of PbTe, exhibits inferior thermoelectric performance due to low Seebeck coefficient and high thermal conductivity. In this report, we show that we can synergistically optimize the electrical and thermal transport properties of SnTe via alloying Mn. We report that the introduction of Mn (0–50%) induces multiple effects on the band structure and microstructure of SnTe: for the former, it can tune the Fermi level and promote the convergence of the two valence bands, concurrently enhancing the Seebeck coefficient; for the latter, it can profoundly modify the microstructure into an all-scale hierarchical architecture (including nanoscale precipitates/MnTe laminates, stacking faults, layered structure, atomic-scale point defects, etc.) to scatter phonons with a broad range of mean free paths, strongly reducing the lattice thermal conductivity. Meanwhile, most significantly, the Mn alloying enlarges the energy gap of the conduction band (C band) and the light valence band (L band), thereby suppressing the bipolar thermal conductivity by increasing the band gap. The integration of these effects yields a high ZT of 1.3 at 900 K for 17% Mn alloyed SnTe.

Completely Organic Multilayer Thin Film with Thermoelectric Power Factor Rivaling Inorganic Tellurides

Abstract: Composed exclusively of organic components, polyaniline (PANi), graphene, and double-walled nanotubes (DWNTs) are alternately deposited from aqueous solutions using a layer-by-layer assembly. The 40 quadlayer thin film (470 nm thick) exhibits electrical conductivity of 1.08 × 10(5) S m(-1) and a Seebeck coefficient of 130 μV K(-1) , producing a thermoelectric power factor of 1825 μW m(-1) K(-2) .

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

Abstract: Polyaniline (PANI)/graphene (GP) thermoelectric (TE) composite films were prepared by a combination of in situ polymerization and a solution process. It was found that there existed a large number of graphene–polyaniline nano-interfaces in the composite films with graphene nanoplates aligned in the PANI matrix in the direction parallel to the substrate. SEM, TEM, Raman, XPS and UV-Vis analyses indicated that polyaniline coated on the surface of graphene by the strong π–π conjugation interactions during in situ polymerization, and then the PANI molecular chains were expanded by the chemical interactions between polyaniline and solution. Both the in situ polymerization process and solution process contributed to the uniform dispersion of graphene in the PANI matrix, which not only increased the number of graphene–polyaniline nano-interfaces in the composite, but also strengthened the π–π conjugation interactions between graphene and polyaniline, resulting in more ordered regions forming in the composite films. Consequently, the Seebeck coefficient of the composite films was remarkably improved and higher than the values calculated based on the series-connected two-component mixture model. The optimal electrical conductivity and Seebeck coefficient of the composite with 48 wt% graphene reached 814 S cm−1 and 26 μV K−1, respectively, resulting in a maximum power factor of 55 μW m−1 K−2, which is the highest value among the reported polymer/graphene composite TE materials.

High-efficient thermoelectric materials: The case of orthorhombic IV-VI compounds

Abstract: Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS, and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity, and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Importantly, GeS, GeSe, and SnS exhibit comparative thermoelectric performance compared to SnSe. Especially, the Seebeck coefficients of GeS, GeSe, and SnS are even larger than that of SnSe under the studied carrier concentration and temperature region. We also use the Cahill's model to estimate the lattice thermal conductivities at the room temperature. The large Seebeck coefficients, high power factors, and low thermal conductivities make these four orthorhombic IV-VI compounds promising candidates for high-efficient thermoelectric materials.

Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants

Abstract: For inorganic thermoelectric materials, Seebeck coefficient and electrical conductivity are interdependent, and hence optimization of thermoelectric performance is challenging. In this work we show that thermoelectric performance of PEDOT:PSS can be enhanced by greatly improving its electrical conductivity in contrast to inorganic thermoelectric materials. Free-standing flexible and smooth PEDOT:PSS bulky papers were prepared using vacuum-assisted filtration. The electrical conductivity was enhanced to 640, 800, 1300, and 1900 S cm(-1) by treating PEDOT:PSS with ethylene glycol, polyethylene glycol, methanol, and formic acid, respectively. The Seebeck coefficient did not show significant variation with the tremendous conductivity enhancement being 21.4 and 20.6 μV K(-1) for ethylene glycol- and formic acid-treated papers, respectively. This is because secondary dopants, which increase electrical conductivity, do not change oxidation level of PEDOT. A maximum power factor of 80.6 μW m(-1) K(-2) was shown for formic acid-treated samples, while it was only 29.3 μW m(-1) K(-2) for ethylene glycol treatment. Coupled with intrinsically low thermal conductivity of PEDOT:PSS, ZT ≈ 0.32 was measured at room temperature using Harman method. We investigated the reasons behind the greatly enhanced thermoelectric performance.

Significant electronic thermal transport in the conducting polymer poly(3,4-ethylenedioxythiophene).

Abstract: Suspended microdevices are employed to measure the in-plane electrical conductivity, thermal conductivity, and Seebeck coefficient of suspended poly(3,4-ethylenedioxythiophene) (PEDOT) thin films. The measured thermal conductivity is higher than previously reported for PEDOT and generally increases with the electrical conductivity. The increase exceeds that predicted by the Wiedemann-Franz law for metals and can be explained by significant electronic thermal transport in PEDOT.

Band gap estimation from temperature dependent Seebeck measurement - Deviations from the 2e|S|maxTmax relation

Abstract: In characterizing thermoelectric materials, electrical and thermal transport measurements are often used to estimate electronic band structure properties such as the effective mass and band gap. The Goldsmid-Sharp band gap, Eg = 2e|S|_(max)T_(max), is a tool widely employed to estimate the band gap from temperature dependent Seebeck coefficient measurements. However, significant deviations of more than a factor of two are now known to occur. We find that this is when either the majority-to-minority weighted mobility ratio (A) becomes very different from 1.0 or as the band gap (Eg) becomes significantly smaller than 10 kBT. For narrow gaps (Eg ≲ 6 kBT), the Maxwell-Boltzmann statistics applied by Goldsmid-Sharp break down and Fermi-Dirac statistics are required. We generate a chart that can be used to quickly estimate the expected correction to the Goldsmid-Sharp band gap depending on A and S_(max); however, additional errors can occur for S < 150 μV/K due to degenerate behavior.

Large Seebeck effect by charge-mobility engineering

Abstract: The Seebeck effect describes the generation of an electric potential in a conducting solid exposed to a temperature gradient In most cases, it is dominated by an energy-dependent electronic density of states at the Fermi level, in line with the prevalent efforts towards superior thermoelectrics through the engineering of electronic structure Here we demonstrate an alternative source for the Seebeck effect based on charge-carrier relaxation: a charge mobility that changes rapidly with temperature can result in a sizeable addition to the Seebeck coefficient This new Seebeck source is demonstrated explicitly for Ni-doped CoSb3, where a marked mobility change occurs due to the crossover between two different charge-relaxation regimes Our findings unveil the origin of pronounced features in the Seebeck coefficient of many other elusive materials characterized by a significant mobility mismatch When utilized appropriately, this effect can also provide a novel route to the design of improved thermoelectric materials

Thermoelectric effects in graphene nanostructures.

Abstract: The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.

Ionic Seebeck Effect in Conducting Polymers

Abstract: Conducting polymers display an ionic thermoelectric effect in addition to the known electronic thermoelectric effect. Their Seebeck coefficient is as large as ≈200 μV K−1. This finding discloses a ...

Increasing the thermoelectric figure of merit of tetrahedrites by Co-doping with nickel and zinc

Abstract: We report a more than 30% increase in the thermoelectric figure of merit of Ni-doped tetrahedrite (Cu12Sb4S13) by the addition of Zn. We show that this enhancement is due to the combination of two effects: (1) a tuning of the Fermi energy to optimize the Seebeck coefficient and thermoelectric power factor and (2) a reduction in thermal conductivity due to decreases in both the electronic and lattice thermal conductivities. Unlike tetrahedrites doped solely with Zn, which can become electrically insulating at high Zn concentrations, Ni doped samples remain conducting due to the existence of an additional valence band from spin–split Ni states. By adding Zn to these Ni-doped materials, electrons can fill the holes in the valence band, allowing for tuning of the thermoelectric properties. Tetrahedrites optimally doped with both Ni and Zn have thermal conductivity approaching theoretical minimum values, which helps boost the thermoelectric figure of merit zT in these compounds above unity.

Dual Vacancies: An Effective Strategy Realizing Synergistic Optimization of Thermoelectric Property in BiCuSeO

Abstract: Vacancy is a very important class of phonon scattering center to reduce thermal conductivity for the development of high efficient thermoelectric materials. However, conventional monovacancy may also act as an electron or hole acceptor, thereby modifying the electrical transport properties and even worsening the thermoelectric performance. This issue urges us to create new types of vacancies that scatter phonons effectively while not deteriorating the electrical transport. Herein, taking BiCuSeO as an example, we first reported the successful synergistic optimization of electrical and thermal parameters through Bi/Cu dual vacancies. As expected, as compared to its pristine and monovacancy samples, these dual vacancies further increase the phonon scattering, which results in an ultra low thermal conductivity of 0.37 W m(-1) K(-1) at 750 K. Most importantly, the clear-cut evidence in positron annihilation unambiguously confirms the interlayer charge transfer between these Bi/Cu dual vacancies, which results in the significant increase of electrical conductivity with relatively high Seebeck coefficient. As a result, BiCuSeO with Bi/Cu dual vacancies shows a high ZT value of 0.84 at 750 K, which is superior to that of its native sample and monovacancies-dominant counterparts. These findings undoubtedly elucidate a new strategy and direction for rational design of high performance thermoelectric materials.

Reduced graphene oxide-polyaniline composites—synthesis, characterization and optimization for thermoelectric applications

Abstract: Reduced graphene oxide (rGO) can improve the thermoelectric properties of polyaniline (PANI) by varying its concentration in composites of rGO nanosheets and PANI. The figure of merit (ZT) of rGO–PANI composites is increased with an increasing percentage of rGO (up to 50%), which is 7.5 times higher as compared to pure PANI. High resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) analyses show a uniform growth of PANI over the surface of rGO as a template, leading to a more ordered structure with high crystallinity during polymerization. Compared to pure PANI, both the electrical conductivity and thermoelectric power of the rGO–PANI composite is higher due to the increased carrier mobility as confirmed by a Hall effect measurement. Fourier transform infrared spectroscopy (FTIR), ultra-violet visible range spectroscopy (UV-Vis) and Raman spectroscopy analyses reveal that strong π–π interactions assisted the uniform distribution of PANI on the rGO nanosheets. Other strong interactions include electrostatic forces and hydrogen bonding between rGO and PANI, which provide a route for constructing highly ordered chain structures with improved thermoelectric performance of PANI. There is no significant change in the thermal conductivity of the rGO–PANI composite as compared to pure PANI, which improves the thermoelectric performance of composite.

Unravelling Doping Effects on PEDOT at the Molecular Level: From Geometry to Thermoelectric Transport Properties.

Abstract: Tuning carrier concentration via chemical doping is the most successful strategy to optimize the thermoelectric figure of merit. Nevertheless, how the dopants affect charge transport is not completely understood. Here we unravel the doping effects by explicitly including the scattering of charge carriers with dopants on thermoelectric properties of poly(3,4-ethylenedioxythiophene), PEDOT, which is a p-type thermoelectric material with the highest figure of merit reported. We corroborate that the PEDOT exhibits a distinct transition from the aromatic to quinoid-like structure of backbone, and a semiconductor-to-metal transition with an increase in the level of doping. We identify a close-to-unity charge transfer from PEDOT to the dopant, and find that the ionized impurity scattering dominates over the acoustic phonon scattering in the doped PEDOT. By incorporating both scattering mechanisms, the doped PEDOT exhibits mobility, Seebeck coefficient and power factors in very good agreement with the experimental data, and the lightly doped PEDOT exhibits thermoelectric properties superior to the heavily doped one. We reveal that the thermoelectric transport is highly anisotropic in ordered crystals, and suggest to utilize large power factors in the direction of polymer backbone and low lattice thermal conductivity in the stacking and lamellar directions, which is viable in chain-oriented amorphous nanofibers.

Anisotropic giant magnetoresistance in NbSb2.

Abstract: The magnetic field response of the transport properties of novel materials and then the large magnetoresistance effects are of broad importance in both science and application. We report large transverse magnetoreistance (the magnetoresistant ratio ~ 1.3 × 105% in 2 K and 9 T field, and 4.3 × 106% in 0.4 K and 32 T field, without saturation) and field-induced metal-semiconductor-like transition, in NbSb2 single crystal. Magnetoresistance is significantly suppressed but the metal-semiconductor-like transition persists when the current is along the ac-plane. The sign reversal of the Hall resistivity and Seebeck coefficient in the field, plus the electronic structure reveal the coexistence of a small number of holes with very high mobility and a large number of electrons with low mobility. The large MR is attributed to the change of the Fermi surface induced by the magnetic field which is related to the Dirac-like point, in addition to orbital MR expected for high mobility metals.

Tuning of the Thermoelectric Figure of Merit of CH3NH3MI3 (M=Pb,Sn) Photovoltaic Perovskites

Abstract: The hybrid halide perovskites, the very performant compounds in photovoltaic applications, possess large Seebeck coefficient and low thermal conductivity, making them potentially interesting high figure of merit (ZT) materials. For this purpose, one needs to tune the electrical conductivity of these semiconductors to higher values. We have studied the CH3NH3MI3 (M═Pb,Sn) samples in pristine form showing very low ZT values for both materials; however, photoinduced doping (in M═Pb) and chemical doping (in M═Sn) indicate that, by further doping optimization, ZT can be enhanced toward unity and reach the performance level of the presently most efficient thermoelectric materials.

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

Abstract: 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.

Development of n-type cobaltocene-encapsulated carbon nanotubes with remarkable thermoelectric property.

Abstract: Direct conversion from heat to electricity is one of the important technologies for a sustainable society since large quantities of energy are wasted as heat. We report the development of a single-walled carbon nanotube (SWNT)-based high conversion efficiency, air-stable and flexible thermoelectric material. We prepared cobaltocene-encapsulated SWNTs (denoted CoCp2@SWNTs) and revealed that the material showed a negative-type (n-type) semiconducting behaviour (Seebeck coefficient: −41.8 μV K−1 at 320 K). The CoCp2@SWNT film was found to show a high electrical conductivity (43,200 S m−1 at 320 K) and large power factor (75.4 μW m−1 K−2) and the performance was remarkably stable under atmospheric conditions over a wide range of temperatures. The thermoelectric figure of merit (ZT) value of the CoCp2@SWNT film (0.157 at 320 K) was highest among the reported n-type organic thermoelectric materials due to the large power factor and low thermal conductivity (0.15 W m−1 K−1). These characteristics of the n-type CoCp2@SWNTs allowed us to fabricate a p-n type thermoelectric device by combination with an empty SWNT-based p-type film. The fabricated device exhibited a highly efficient power generation close to the calculated values even without any air-protective coating due to the high stability of the SWNT-based materials under atmospheric conditions.