Conversion of waste heat to voltage has the potential to significantly reduce the carbon footprint of a number of critical energy sectors, such as the transportation and electricity-generation sectors, and manufacturing processes. Thermal energy is also an abundant low-flux source that can be harnessed to power portable/wearable electronic devices and critical components in remote off-grid locations. As such, a number of different inorganic and organic materials are being explored for their potential in thermoelectric-energy-harvesting devices. Carbon-based thermoelectric materials are particularly attractive due to their use of nontoxic, abundant source-materials, their amenability to high-throughput solution-phase fabrication routes, and the high specific energy (i.e., W g-1 ) enabled by their low mass. Single-walled carbon nanotubes (SWCNTs) represent a unique 1D carbon allotrope with structural, electrical, and thermal properties that enable efficient thermoelectric-energy conversion. Here, the progress made toward understanding the fundamental thermoelectric properties of SWCNTs, nanotube-based composites, and thermoelectric devices prepared from these materials is reviewed in detail. This progress illuminates the tremendous potential that carbon-nanotube-based materials and composites have for producing high-performance next-generation devices for thermoelectric-energy harvesting.

Carbon-Nanotube-Based Thermoelectric Materials and Devices.

Polymers are usually considered as thermal insulators, and their applications are limited by their low thermal conductivity. However, recent studies have shown that certain polymers have surprisingly high thermal conductivity, some of which are comparable to that in poor metals or even silicon. Here, the experimental achievements and theoretical progress of thermal transport in polymers and their nanocomposites are outlined. The open questions and challenges of existing theories are discussed. Special attention is given to the mechanism of thermal transport, the enhancement of thermal conductivity in polymer nanocomposites/fibers, and their potential application as thermal interface materials.

Thermal Conductivity of Polymers and Their Nanocomposites.

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations.

https://www.tandfonline.com/doi/pdf/10.1080/14686996.2018.1530938

Thermoelectric materials and applications for energy harvesting power generation

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.

/pdf/morphology-controls-the-thermoelectric-power-factor-of-a-1fw026wz6m.pdf

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

Conducting polymers are potential candidates for thermoelectric (TE) applications owing to their low thermal conductivity, non-toxicity and low cost. However, the coil conformation and random aggregation of polymer chains usually degrade electrical transport properties, thus deteriorating TE performance. In this work, we fabricated poly(3-hexylthiophene) (P3HT) films with highly oriented morphology using 1,3,5-trichlorobenzene (TCB), an organic small-molecule, as a template for polymer epitaxy under a temperature gradient crystallization process. The resulting P3HT molecules, which were confirmed to be highly anisotropic by a combination of scanning electron microscopy, atomic force microscopy, polarizing microscope, polarized Raman spectroscopy, and two-dimensional-grazing incidence X-ray diffraction (GIXRD) analysis, not only markedly reduced the conjugated defects along the polymer backbone, but also effectively increased the degree of electron delocalization. These combined phenomena produced an efficient, 1D path for carrier movement and therefore resulted in enhanced carrier mobility in the TCB-treated P3HT films. The maximum values of the electrical conductivity and Seebeck coefficient were 320 S cm−1 and 269 μV K−1, respectively. Consequently, the maximum TE power factor and ZT value at 365 K reached 62.4 μW mK−2 and 0.1, respectively, in the parallel direction of the TCB-treated P3HT film. To the best of our knowledge, these are the highest values reported for pure P3HT TE materials. The method of using organic small-molecule epitaxy to generate highly anisotropic polymer films is expected to be valid for many conducting polymers. A process for stretching twisted polymer chains into ordered fibers can benefit production of low-cost materials that harvest heat losses. Most thermoelectric energy converters are made from inorganic semiconductors. Conductive polymers, however, are an attractive alternative because of their simple manufacturing requirements and intrinsic thermal insulating properties. Lidong Chen from the Shanghai Institute of Ceramics and co-workers have developed a way to reduce the defects that often plague plastic thermoelectrics with a dissolvable organic template. The team co-crystallized the conductive polymer poly(3-hexylthiophene) with small, trichlorobenzene-based molecules that solidify into needle-like structures under a temperature gradient. The aromatic stacking forces within the trichlorobenzene crystals lock the polymer chains into elongated positions that persist after template removal. Experiments on the resulting poly(3-hexylthiophene) filaments revealed near-record thermoelectric conversion efficiencies for this material. Herein, we fabricated pure poly(3-hexylthiophene) (P3HT) film with highly oriented morphology using a small-molecule 1,3,5-trichlorobenzene (TCB) as the template for polymer epitaxy with a temperature gradient crystallization process. The strong π–π conjugated interactions as well as the very close matching between the repeat distance of the thiophene units in P3HT and the repeat distance of TCB molecules have successfully induced the epitaxy process, allowing the P3HT polymer chains to lock onto the lattice of TCB, forming highly ordered P3HT chains in the direction parallel to the fiber axis. As a result, the electrical conductivity in the direction parallel to the fiber axis was significantly improved. The maximum thermoelectric power factor and ZT value at 365 K reached 62.4 μW mK-2 and 0.1 in the parallel direction of the TCB-treated P3HT film.

/pdf/highly-anisotropic-p3ht-films-with-enhanced-thermoelectric-2tlnomljgo.pdf

Highly anisotropic P3HT films with enhanced thermoelectric performance via organic small molecule epitaxy

Tip-bias-induced domain evolution in PMN–PT transparent ceramics via piezoresponse force microscopy

The nonlinear response of a material to an external stimulus is vital in fundamental science and technical applications. The power-law current-voltage relationship of a varistor is one such example. An excellent example of such behavior is the power-law current-voltage relationship exhibited by Bi2O3-doped ZnO varistor ceramics, which are the cornerstone of commercial varistor materials for overvoltage protection. Here, we report on a sustainable, ZnO-based varistor ceramic, without the volatile Bi2O3, that is based on Cr2O3 as the varistor former and oxides of Ca, Co, and Sb as the performance enhancers. The material has an ultrahigh α of up to 219, a low IL of less than 0.2 μA/cm2, and a high Eb of up to 925 V/mm, making it superior to state-of-the-art varistor ceramics. The results provide insights into the design of materials with specific characteristics by tailoring states at the grain boundaries. The discovery of this ZnO-Cr2O3-type varistor ceramic represents a major breakthrough in the field of varistors for overvoltage protection and could drastically affect the world market for overvoltage protection.

Novel Ultrahigh-Performance ZnO-Based Varistor Ceramics.

In this paper we develop scanning thermoelectric microscopy (STeM) on the basis of commercial atomic force microscope. The nanoscale thermoelectric behaviors of (Bi,Sb) 2 Te 3 (BST) thin films were studied. 3 ω -technique was used for thermal conductivity imaging and quantitative thermal characterization. By acquiring the unique Seebeck information from 2 ω frequency component, nanoscale thermoelectric images were firstly obtained, exhibiting remarkably inhomogeneous distribution of local Seebeck coefficient in the thin films. Positive thermoelectric response is revealed by the modulation of temperature difference between thermal tip and sample, corresponding to p-type conduction within BST sample.

Scanning thermoelectric microscopy of local thermoelectric behaviors in (Bi,Sb)2Te3 films

In the past decade, the development of high-performance p-type flexible organic–inorganic thermoelectric composites based on nanocarbons (e.g., carbon nanotubes and graphene) has achieved unprecedented success, but progress in the n-type counterpart lags far behind because carbon nanotubes and graphene usually demonstrate p-type behavior. In this work, beyond carbon nanotubes and graphene, we conduct a proof-of-principle study using semiconducting graphdiyne (GDY) to fabricate high-performance n-type organic–inorganic flexible thermoelectric composites. Based on the chemical interactions between GDY and the quasi-one-dimensional semiconductor Ta4SiTe4, we successfully fabricate n-type Ta4SiTe4/polyvinylidene fluoride (PVDF)/GDY composite films. GDY is homogeneously distributed inside the composites, acting as bridges among the Ta4SiTe4 whiskers to significantly improve the electrical conductivity. Combining the well-maintained large Seebeck coefficient and low thermal conductivity, the Ta4SiTe4/PVDF/GDY composite films demonstrate a maximum ZT value of 0.2 at 300 K, among the highest reported in organic–inorganic flexible thermoelectric composites. The protype thermoelectric module that consists of n-type Ta4SiTe4/PVDF/GDY legs and p-type PEDOT:PSS/CNT legs shows the highest normalized maximum power density among the reported organic material-based flexible thermoelectric modules including both n- and p-type legs.

Kunqi Xu

Papers

Highly anisotropic P3HT films with enhanced thermoelectric performance via organic small molecule epitaxy

Tip-bias-induced domain evolution in PMN–PT transparent ceramics via piezoresponse force microscopy

Novel Ultrahigh-Performance ZnO-Based Varistor Ceramics.

Scanning thermoelectric microscopy of local thermoelectric behaviors in (Bi,Sb)2Te3 films

High-performance n-type Ta4SiTe4/polyvinylidene fluoride (PVDF)/graphdiyne organic–inorganic flexible thermoelectric composites