Showing papers on "Thermoelectric effect published in 2017"
TL;DR: In this paper, a review of thermoelectric generators is presented, as well as the future applications which are currently being studied in research laboratories or in industry and the main purpose of this paper is to clearly demonstrate that, almost anywhere in industry or in domestic uses, it is worth checking whether a TEG can be added whenever heat is moving from a hot source to a cold source.
854 citations
TL;DR: It is shown that together these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.
Abstract: The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials. Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal conductivity of the lattice through the design of either interface structures at nanometre or mesoscopic length scales or multiscale hierarchical architectures. A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelectric performance. Here we show that by embedding nanoparticles of a soft magnetic material in a thermoelectric matrix we achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles-in particular, their superparamagnetic behaviour (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)-lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. We show that together these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.
432 citations
TL;DR: This work reports a low-cost n-type material, Te-doped Mg3Sb1.5Bi0.5, that exhibits a very high figure of merit zT ranging from 0.56 to 1.65 at 300−725 K and shows that the high thermoelectric performance originates from the significantly enhanced power factor because of the multi-valley band behaviour dominated by a unique near-edge conduction band with a sixfold valley degeneracy
Abstract: Widespread application of thermoelectric devices for waste heat recovery requires low-cost high-performance materials. The currently available n-type thermoelectric materials are limited either by their low efficiencies or by being based on expensive, scarce or toxic elements. Here we report a low-cost n-type material, Te-doped Mg3Sb1.5Bi0.5, that exhibits a very high figure of merit zT ranging from 0.56 to 1.65 at 300−725 K. Using combined theoretical prediction and experimental validation, we show that the high thermoelectric performance originates from the significantly enhanced power factor because of the multi-valley band behaviour dominated by a unique near-edge conduction band with a sixfold valley degeneracy. This makes Te-doped Mg3Sb1.5Bi0.5 a promising candidate for the low- and intermediate-temperature thermoelectric applications. Zintl-phase thermoelectrics are predominantly p-type. Here, Zhanget al. use tellurium to n-dope Mg3Sb1.5Bi0.5and obtain thermoelectric figures of merit up to 1.6 at 700 K. Calculations show that these performances result from a conduction band with sixfold valley degeneracy.
406 citations
TL;DR: This work demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL, but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.
Abstract: Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97 Eu0.03 Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm-2 ) are particularly effective at reducing κL . When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.
394 citations
TL;DR: The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.
Abstract: To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb1-xSb2x/3Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm-1 K-1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.
355 citations
TL;DR: A combination of band convergence and interstitial defects, each of which enables a ≈150% increase in the peak zT, successfully accumulates the zT enhancements to be ≈300% (zT up to 1.6) without involving any toxic elements.
Abstract: Compared to commercially available p-type PbTe thermoelectrics, SnTe has a much bigger band offset between its two valence bands and a much higher lattice thermal conductivity, both of which limit its peak thermoelectric figure of merit, zT of only 0.4. Converging its valence bands or introducing resonant states is found to enhance the electronic properties, while nanostructuring or more recently introducing interstitial defects is found to reduce the lattice thermal conductivity. Even with an integration of some of the strategies above, existing efforts do not enable a peak zT exceeding 1.4 and usually involve Cd or Hg. In this work, a combination of band convergence and interstitial defects, each of which enables a ≈150% increase in the peak zT, successfully accumulates the zT enhancements to be ≈300% (zT up to 1.6) without involving any toxic elements. This opens new possibilities for further improvements and promotes SnTe as an environment-friendly solution for conventional p-PbTe thermoelectrics.
329 citations
TL;DR: In this article, a novel and facile method is reported to significantly enhance the thermoelectric (TE) property of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through sequential post-treatments with common acids and bases.
Abstract: Thermoelectric (TE) materials are important for the sustainable development because they enable the direct harvesting of low-quality heat into electricity. Among them, conducting polymers have attracted great attention arising from their advantages, such as flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. In this work, a novel and facile method is reported to significantly enhance the TE property of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through sequential post-treatments with common acids and bases. Compared with the as-prepared PEDOT:PSS, both the Seebeck coefficients and electrical conductivities can be remarkably enhanced after the treatments. The oxidation level, which significantly impacts the TE property of the PEDOT:PSS films, can also be well tuned by controlling the experimental conditions during the base treatment. The optimal PEDOT:PSS films can have a Seebeck coefficient of 39.2 µV K−1 and a conductivity of 2170 S cm−1 at room temperature, and the corresponding power factor is 334 µW (m−1 K−2). The enhancement in the TE properties is attributed to the synergetic effect of high charge mobility by the acid treatment and the optimal oxidation level tuned by the base treatment.
295 citations
TL;DR: In this article, the authors demonstrate that by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials, it is possible to noticeably improve the Hall mobility, from ∼19 to ∼77 cm2 V−1 s−1, and hence substantially increase the power factor by a factor of 3.
Abstract: A high thermoelectric power factor not only enables a potentially high figure of merit ZT but also leads to a large output power density, and hence it is pivotal to find an effective route to improve the power factor. Previous reports on the manipulation of carrier scattering mechanisms (e.g. ionization scattering) were mainly focused on enhancing the Seebeck coefficient. In contrast, here we demonstrate that by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials, it is possible to noticeably improve the Hall mobility, from ∼19 to ∼77 cm2 V−1 s−1, and hence substantially increase the power factor by a factor of 3, from ∼5 to ∼15 μW cm−1 K−2. The enhancement in mobility is mainly due to the reason that ionization scattering has been converted into mixed scattering between ionization and acoustic phonon scattering, which less effectively scatters the carriers. The strategy of tuning the carrier scattering mechanism to improve the mobility should be widely applicable to various material systems for achieving better thermoelectric performance.
291 citations
TL;DR: In this article, the authors highlight the key strategies to enhance the thermoelectric performance of tin telluride (SnTe) materials through band engineering, carrier concentration optimization, synergistic engineering, and structure design.
Abstract: As a key type of emerging thermoelectric material, tin telluride (SnTe) has received extensive attention because of its low toxicity and eco-friendly nature. The recent trend shows that band engineering and nanostructuring can enhance thermoelectric performance of SnTe as intermediate temperature (400-800 K) thermoelectrics, which provides an alternative for toxic PbTe with the same operational temperature. This review highlights the key strategies to enhance the thermoelectric performance of SnTe materials through band engineering, carrier concentration optimization, synergistic engineering, and structure design. A fundamental analysis elucidates the underpinnings for the property improvement. This comprehensive review will boost the relevant research with a view to work on further performance enhancement of SnTe materials.
283 citations
TL;DR: In this paper, the authors defined the figure of merit for a thermoelectric generator using the maximum efficiency of the generator using a simple algorithm that can be performed on a spreadsheet calculator.
Abstract: While the thermoelectric materials figure of merit is a well defined metric to evaluate thermoelectric materials, it can be a poor metric for maximum thermoelectric device efficiency because of the temperature dependence of the Seebeck coefficient S, the electrical resistivity ρ, and the thermal conductivity κ where T is the absolute temperature Historically the field has used a thermoelectric device figure of merit ZT to characterize a device operating between a hot side temperature Th and cold side temperature Tc While there are many approximate methods to calculate ZT from temperature dependent materials properties, an exact method is given here that uses a simple algorithm that can be performed on a spreadsheet calculator The figure of merit is defined for a thermoelectric generator using the maximum efficiency of the thermoelectric device η calculated from the exact method
275 citations
TL;DR: In this paper, the in situ growth of metal Cu2Se on the surface of CNTs was realized and a series of hybrid materials were fabricated using the special interaction between metal Cu and multi-walled carbon nanotubes.
Abstract: Here, by utilizing the special interaction between metal Cu and multi-walled carbon nanotubes (CNTs), we have successfully realized the in situ growth of Cu2Se on the surface of CNTs and then fabricated a series of Cu2Se/CNT hybrid materials. Due to the high degree of homogeneously dispersed molecular CNTs inside the Cu2Se matrix, a record-high thermoelectric figure of merit zT of 2.4 at 1000 K has been achieved.
TL;DR: Inspired by the findings, the compact-configuration flexible TE modules are designed and successfully fabricate, which own great advantages compared with the conventional π-type configuration modules and well integrate the superior thermoelectric properties of p-type and n-type carbon nanotube films resulting in a markedly high performance.
Abstract: It is a great challenge to substantially improve the practical performance of flexible thermoelectric modules due to the absence of air-stable n-type thermoelectric materials with high-power factor. Here an excellent flexible n-type thermoelectric film is developed, which can be conveniently and rapidly prepared based on the as-grown carbon nanotube continuous networks with high conductivity. The optimum n-type film exhibits ultrahigh power factor of ∼1,500 μW m−1 K−2 and outstanding stability in air without encapsulation. Inspired by the findings, we design and successfully fabricate the compact-configuration flexible TE modules, which own great advantages compared with the conventional π-type configuration modules and well integrate the superior thermoelectric properties of p-type and n-type carbon nanotube films resulting in a markedly high performance. Moreover, the research results are highly scalable and also open opportunities for the large-scale production of flexible thermoelectric modules. Thermoelectric modules can generate electricity directly from heat and have applications to waste heat-energy conversion. Here Zhouet al. have fabricated a thermoelectric module based on an air-stable n-type single-walled carbon nanotube sheet which can reach a high power factor of 1500 μWm−1K−2.
TL;DR: In this paper, the 2D layered CaAl 2 Si 2 -type Zintl phases are discussed in more detail, from fundamental crystal structure and electronic band structure to the approaches that have been successfully used to enhance the thermoelectric performance.
TL;DR: Important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics are introduced.
Abstract: The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with F n TCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.
TL;DR: The superior room-temperature thermoelectric performance of p-type transparent copper iodide (CuI) thin films is presented and opens a path for multifunctional technologies combing transparent electronics, flexible electronics and thermoeLECTricity.
Abstract: Thermoelectric devices that are flexible and optically transparent hold unique promise for future electronics. However, development of invisible thermoelectric elements is hindered by the lack of p-type transparent thermoelectric materials. Here we present the superior room-temperature thermoelectric performance of p-type transparent copper iodide (CuI) thin films. Large Seebeck coefficients and power factors of the obtained CuI thin films are analysed based on a single-band model. The low-thermal conductivity of the CuI films is attributed to a combined effect of the heavy element iodine and strong phonon scattering. Accordingly, we achieve a large thermoelectric figure of merit of ZT=0.21 at 300 K for the CuI films, which is three orders of magnitude higher compared with state-of-the-art p-type transparent materials. A transparent and flexible CuI-based thermoelectric element is demonstrated. Our findings open a path for multifunctional technologies combing transparent electronics, flexible electronics and thermoelectricity. Flexible thermoelectric devices with high optical transparency may enable new applications; however, the lack of a p-type counterpart has hitherto hindered its development. Yanget al., report a transparent p-type thermoelectric based on polycrystalline copper iodide thin film with record performance.
TL;DR: In this paper, the authors show that reduced graphene oxide (rGO) increases the grain boundary thermal resistivity by a factor of 3 to 5 compared to grain boundaries without graphene.
Abstract: Skutterudite materials are widely considered for thermoelectric waste heat recovery. While the skutterudite structure effectively scatters the high frequency phonons, grain-boundary engineering is needed to further reduce the thermal conductivity beyond simply decreasing grain size. Here, we show that reduced graphene oxide (rGO) increases the grain boundary thermal resistivity by a factor of 3 to 5 compared to grain boundaries without graphene. Wrapping even micron sized grains with graphene leads to such a significant reduction in the thermal conductivity that a high thermoelectric figure of merit zT = 1.5 was realized in n-type YbyCo4Sb12, while a zT of 1.06 was achieved in p-type CeyFe3CoSb12. A 16 leg thermoelectric module was made by using n- and p-type skutterudite–graphene nanocomposites that exhibited conversion efficiency 24% higher than a module made without graphene. Engineering grain boundary complexions with 2-D materials introduces a new strategy for advanced thermoelectric materials.
TL;DR: An entirely new approach is presented and the results suggest that this novel approach can finally produce flexible TEGs that have the potential to challenge the rigid T EGs and provide a pathway for the realization of self-powered wearable electronics.
TL;DR: In this paper, three compositions of Mo-based MXenes (Mo2CTx, Mo2TiC2Tx, and Mo2 TiC3Tx) have been synthesized and processed into free-standing binder-free papers by vacuum-assisted filtration.
Abstract: MXenes are an interesting class of 2D materials consisting of transition metal carbides and nitrides, which are currently a subject of extensive studies. Although there have been theoretical calculations estimating the thermoelectric properties of MXenes, no experimental measurements have been reported so far. In this report, three compositions of Mo-based MXenes (Mo2CTx, Mo2TiC2Tx, and Mo2Ti2C3Tx) have been synthesized and processed into free-standing binder-free papers by vacuum-assisted filtration, and their electrical and thermoelectric properties are measured. Upon heating to 800 K, these MXene papers exhibit high conductivity and n-type Seebeck coefficient. The thermoelectric power reaches 3.09 × 10–4 W m–1 K–2 at 803 K for the Mo2TiC2Tx MXene. While the thermoelectric properties of MXenes do not reach that of the best materials, they exceed their parent ternary and quaternary layered carbides. Mo2TiC2Tx shows the highest electrical conductivity in combination with the largest Seebeck coefficient of...
TL;DR: The thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory showed that the thermoeLECTric performance of monolayer of these compounds is improved in comparison compared to their bulk phases.
Abstract: We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures, Seebeck coefficients, electrical conductivities, lattice thermal conductivities, and figures of merit of these two-dimensional materials, which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT = 2.63, 2.46), SnS (ZT = 1.75, 1.88), GeSe (ZT = 1.99, 1.73), and GeS (ZT = 1.85, 1.29) at 700 K along armchair and zigzag directions, respectively. Phonon dispersion calculations confirm the dynamical stability of these compounds. The calculated lattice thermal conductivities are low while the electrical conductivities and Seebeck coefficients are high. Thus, the properties of the monolayers show high potential toward thermoelectric applications.
TL;DR: In this article, the authors demonstrate that by manipulating the carrier scattering mechanism in n-type Mg3Sb2-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved.
Abstract: Achieving higher carrier mobility plays a pivotal role for obtaining potentially high thermoelectric performance. In principle, the carrier mobility is governed by the band structure as well as by the carrier scattering mechanism. Here, we demonstrate that by manipulating the carrier scattering mechanism in n-type Mg3Sb2-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved. In this work, Fe, Co, Hf, and Ta are doped on the Mg site of Mg3.2Sb1.5Bi0.49Te0.01, where the ionized impurity scattering crosses over to mixed ionized impurity and acoustic phonon scattering. A significant improvement in Hall mobility from ∼16 to ∼81 cm2⋅V-1⋅s-1 is obtained, thus leading to a notably enhanced power factor of ∼13 μW⋅cm-1⋅K-2 from ∼5 μW⋅cm-1⋅K-2 A simultaneous reduction in thermal conductivity is also achieved. Collectively, a figure of merit (ZT) of ∼1.7 is obtained at 773 K in Mg3.1Co0.1Sb1.5Bi0.49Te0.01 The concept of manipulating the carrier scattering mechanism to improve the mobility should also be applicable to other material systems.
TL;DR: The discovery of ultralow thermal conductivity in single-crystalline, all-inorganic halide perovskite nanowires, which is comparable to their amorphous limit value is reported, which holds promise for diverse applications such as phononic and thermoelectric devices.
Abstract: Controlling the flow of thermal energy is crucial to numerous applications ranging from microelectronic devices to energy storage and energy conversion devices. Here, we report ultralow lattice thermal conductivities of solution-synthesized, single-crystalline all-inorganic halide perovskite nanowires composed of CsPbI3 (0.45 ± 0.05 W·m-1·K-1), CsPbBr3 (0.42 ± 0.04 W·m-1·K-1), and CsSnI3 (0.38 ± 0.04 W·m-1·K-1). We attribute this ultralow thermal conductivity to the cluster rattling mechanism, wherein strong optical-acoustic phonon scatterings are driven by a mixture of 0D/1D/2D collective motions. Remarkably, CsSnI3 possesses a rare combination of ultralow thermal conductivity, high electrical conductivity (282 S·cm-1), and high hole mobility (394 cm2·V-1·s-1). The unique thermal transport properties in all-inorganic halide perovskites hold promise for diverse applications such as phononic and thermoelectric devices. Furthermore, the insights obtained from this work suggest an opportunity to discover low thermal conductivity materials among unexplored inorganic crystals beyond caged and layered structures.
TL;DR: The phenomenological transport mechanism in these materials indicates that thermal conductivities are reduced in 2D materials with intrinsically short mean free paths, and possible research directions to enhance the thermoelectric performance of bulk materials with 2D structures are briefly considered.
Abstract: Given that more than two thirds of all energy is lost, mostly as waste heat, in utilization processes worldwide, thermoelectric materials, which can directly convert waste heat to electricity, provide an alternative option for optimizing energy utilization processes. After the prediction that superlattices may show high thermoelectric performance, various methods based on quantum effects and superlattice theory have been adopted to analyze bulk materials, leading to the rapid development of thermoelectric materials. Bulk materials with two-dimensional (2D) structures show outstanding properties, and their high performance originates from both their low thermal conductivity and high Seebeck coefficient due to their strong anisotropic features. Here, the advantages of superlattices for enhancing the thermoelectric performance, the transport mechanism in bulk materials with 2D structures, and optimization methods are discussed. The phenomenological transport mechanism in these materials indicates that thermal conductivities are reduced in 2D materials with intrinsically short mean free paths. Recent progress in the transport mechanisms of Bi2 Te3 -, SnSe-, and BiCuSeO-based systems is summarized. Finally, possible research directions to enhance the thermoelectric performance of bulk materials with 2D structures are briefly considered.
TL;DR: In this paper, the authors studied the thermoelectric transport properties inherent to p-type germanium tellurides (GeTe), a member of the group IV monotellurides that is relatively less studied.
Abstract: PbTe and SnTe in their p-type forms have long been considered high-performance thermoelectrics, and both of them largely rely on two valence bands (the first band at L point and the second one along the Σ line) participating in the transport properties. This work focuses on the thermoelectric transport properties inherent to p-type GeTe, a member of the group IV monotellurides that is relatively less studied. Approximately 50 GeTe samples have been synthesized with different carrier concentrations spanning from 1 to 20 × 1020 cm−3, enabling an insightful understanding of the electronic transport and a full carrier concentration optimization for the thermoelectric performance. When all of these three monotellurides (PbTe, SnTe and GeTe) are fully optimized in their p-type forms, GeTe shows the highest thermoelectric figure of merit (zT up to 1.8). This is due to its superior electronic performance, originating from the highly degenerated Σ band at the band edge in the low-temperature rhombohedral phase and the smallest effective masses for both the L and Σ bands in the high-temperature cubic phase. The high thermoelectric performance of GeTe that is induced by its unique electronic structure not only provides a reference substance for understanding existing research on GeTe but also opens new possibilities for the further improvement of the thermoelectric performance of this material. A team has uncovered the electronic reasons why germanium tellurides (GeTe) are superior energy harvesters to their periodic counterparts. Efforts to improve GeTe thermoelectric efficiency focus mostly on reducing heat conductivity inside its phase-changing crystal lattice. Yanzhong Pei from Tongji University in Shanghai, China, and colleagues now show how semiconductor band tuning may also improve this compound. They synthesized over 50 samples of GeTe with a broad range of carrier concentrations. The researchers then compared transport measurements and theoretical band calculations with those for the more common lead and tin telluride thermoelectrics. The GeTe samples with the best conversion efficiency had charge carriers with lower effective masses than usual and access to numerous energy bands — a combination that enables more heat to be converted to electricity compared to lead telluride or tin telluride. This work focuses on the thermoelectric properties inherent to p-type GeTe by tuning the carrier concentration. Compared with PbTe and SnTe, GeTe shows the highest Seebeck coefficient (a) and power factor (b) in the important carrier concentration range of 1–3 × 1020 cm−3. Provided all are fully optimized in carrier concentrations, GeTe shows the highest thermoelectric figure of merit, zT at all temperatures (c). This originates from its highest degeneracy of the first valence band (along Σ line) in the low temperature phase, and the smallest effective masses for both L and Σ bands in the high temperature phase (d).
TL;DR: In this article, the thermoelectric transport properties inherent to p-type GeTe-based alloys have been investigated, and it was shown that further alloying Te with Se with an optimal carrier concentration enables a reduction on the lattice thermal conductivity by ∼40% and eventually leads to a further enhancement on zT (up to 2.0) by ∼20%.
Abstract: GeTe-based alloys have been intensively considered as p-type thermoelectrics for about 50 years, yet existing literature barely discussed the thermoelectric properties of pristine GeTe at high temperatures (300–800 K). This work first backs to a fundamental understanding on the thermoelectric transport properties inherent to p-type GeTe, based on more than 50 samples synthesized with expected carrier concentrations ranging from 1 × 1020 to 3 × 1021 cm–3. A thermoelectric figure of merit zT as high as ∼1.7 is found inherent to this compound when it is optimally doped with a Hall carrier concentration of 2.2 ± 10% × 1020 cm–3, offering a reference substance to expose the origins for the high zT in historical GeTe-based alloys. Guided by the above knowledge, further alloying Te with Se in samples with an optimal carrier concentration enables a reduction on the lattice thermal conductivity by ∼40% and eventually leads to a further enhancement on zT (up to 2.0) by ∼20%. This work demonstrates not only GeTe as ...
TL;DR: This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing Cu2Te inclusions.
Abstract: High thermoelectric performance of n-type PbTe is urgently needed to match its p-type counterpart. Here, we show a peak ZT ∼ 1.5 at 723 K and a record high average ZT > 1.0 at 300–873 K realized in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing Cu2Te inclusions. Cu performs several outstanding roles: Cu atoms fill the Pb vacancies and improve carrier mobility, contributing to an unexpectedly high power factor of ∼37 μW cm–1 K–2 at 423 K; Cu atoms filling Pb vacancies and Cu interstitials both induce local disorder and, together with nano- and microscale Cu-rich precipitates and their related strain fields, lead to a very low lattice thermal conductivity of ∼0.38 Wm–1 K–1 in PbTe-5.5%Cu2Te, approaching the theoretical minimum value of ∼0.36 Wm–1 K–1. This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties.
TL;DR: This study successfully develops a strategy of using entropy as the global gene-like performance indicator that shows how multicomponent thermoelectric materials with high entropy can be designed via a high-throughput screening method.
Abstract: High-throughput explorations of novel thermoelectric materials based on the Materials Genome Initiative paradigm only focus on digging into the structure-property space using nonglobal indicators to design materials with tunable electrical and thermal transport properties. As the genomic units, following the biogene tradition, such indicators include localized crystal structural blocks in real space or band degeneracy at certain points in reciprocal space. However, this nonglobal approach does not consider how real materials differentiate from others. Here, this study successfully develops a strategy of using entropy as the global gene-like performance indicator that shows how multicomponent thermoelectric materials with high entropy can be designed via a high-throughput screening method. Optimizing entropy works as an effective guide to greatly improve the thermoelectric performance through either a significantly depressed lattice thermal conductivity down to its theoretical minimum value and/or via enhancing the crystal structure symmetry to yield large Seebeck coefficients. The entropy engineering using multicomponent crystal structures or other possible techniques provides a new avenue for an improvement of the thermoelectric performance beyond the current methods and approaches.
TL;DR: Results demonstrate that the modulation of the conjugated backbone represents a powerful strategy for tuning the electronic structure and mobility of organic semiconductors toward a maximum thermoelectric performance.
Abstract: Conjugated backbones play a fundamental role in determining the electronic properties of organic semiconductors. On the basis of two solution-processable dihydropyrrolo[3,4-c]pyrrole-1,4-diylidenebis(thieno[3,2-b]thiophene) derivatives with aromatic and quinoid structures, we have carried out a systematic study of the relationship between the conjugated-backbone structure and the thermoelectric properties. In particular, a combination of UV–vis–NIR spectra, photoemission spectroscopy, and doping optimization are utilized to probe the interplay between energy levels, chemical doping, and thermoelectric performance. We found that a moderate change in the conjugated backbone leads to varied doping mechanisms and contributes to dramatic changes in the thermoelectric performance. Notably, the chemically doped A-DCV-DPPTT, a small molecule with aromatic structure, exhibits an electrical conductivity of 5.3 S cm–1 and a high power factor (PF373 K) up to 236 μW m–1 K–2, which is 50 times higher than that of Q-DCM...
TL;DR: In this paper, a review of the recent advances in organic polymer thermoelectric composites is presented, where their preparation strategies have been discussed and an outlook on future investigations is provided.
Abstract: Thermoelectric materials can realize the direct energy conversion between heat and electricity, having diverse applications in energy harvesting (especially for waste heat and low-grade heat) and local cooling and sensing. In recent years, organic polymer thermoelectric composites have received extensive attention and have experienced a rapid development because of their low densities, low thermal conductivities, high flexibilities, and the synergistic combination of the advantages of both constituents. This review covers the recent advances in organic polymer thermoelectric composites. Herein, their preparation strategies have been discussed. In addition, the non-conducting polymer-based composites, ternary composites, and the devices have also been discussed. Finally, an outlook on future investigations is provided.
TL;DR: In this article, the 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to their large effective masses and valley degeneracies, but also due to the unique density of states (DOS) of confined electrons and holes.
Abstract: The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces toward renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric power factor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to a high power factor due to the unique density of states (DOS) of confined electrons and holes. The 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects but especially due to their large effective masses and valley degeneracies. Here, we report a power factor of $\mathrm{Mo}{\mathrm{S}}_{2}$ as large as $8.5\phantom{\rule{0.28em}{0ex}}\mathrm{mW}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ at room temperature, which is among the highest measured in traditional, gapped thermoelectric materials. To obtain these high power factors, we perform thermoelectric measurements on few-layer $\mathrm{Mo}{\mathrm{S}}_{2}$ in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, resulting in a large Seebeck coefficient. The demonstrated high, electronically modulated power factor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.
01 Sep 2017
TL;DR: It is demonstrated that by manipulating the carrier scattering mechanism in n-type Mg3Sb2-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved.
Abstract: Significance Higher carrier mobility can contribute to a larger power factor, so it is important to identify effective means for achieving higher carrier mobility. Since carrier mobility is governed by the band structure and the carrier scattering mechanism, its possible enhancement could be obtained by manipulating either or both of these. Here, we report a substantial enhancement in carrier mobility by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials. The ionized impurity scattering in these materials has been shifted into mixed scattering by acoustic phonons and ionized impurities. Our results clearly demonstrate that the strategy of tuning the carrier scattering mechanism is quite effective for improving the mobility and should also be applicable to other material systems. Achieving higher carrier mobility plays a pivotal role for obtaining potentially high thermoelectric performance. In principle, the carrier mobility is governed by the band structure as well as by the carrier scattering mechanism. Here, we demonstrate that by manipulating the carrier scattering mechanism in n-type Mg3Sb2-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved. In this work, Fe, Co, Hf, and Ta are doped on the Mg site of Mg3.2Sb1.5Bi0.49Te0.01, where the ionized impurity scattering crosses over to mixed ionized impurity and acoustic phonon scattering. A significant improvement in Hall mobility from ∼16 to ∼81 cm2⋅V−1⋅s−1 is obtained, thus leading to a notably enhanced power factor of ∼13 μW⋅cm−1⋅K−2 from ∼5 μW⋅cm−1⋅K−2. A simultaneous reduction in thermal conductivity is also achieved. Collectively, a figure of merit (ZT) of ∼1.7 is obtained at 773 K in Mg3.1Co0.1Sb1.5Bi0.49Te0.01. The concept of manipulating the carrier scattering mechanism to improve the mobility should also be applicable to other material systems.