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of thermoelectric generators. They can be used in thermoelectric cooling systems in terms of the Peltier effect as well.
The conventional thermoelectric materials are inorganic semiconductors or semimetals such as Bi2Te3 and its derivatives or analogues. These materials can exhibit high Seebeck coefficient and high electrical conductivity, but they also have a problem of high thermal conductivity. Hence, great effort has been made on lowering the thermal conductivity of inorganic thermoelectric materials besides on the improvement in the Seebeck coefficient and/or electrical conductivity. Consider for practical application, inorganic thermoelectric materials are usually brittle, and thus they are not suitable for flexible thermoelectric systems.
Polymers emerge as the next‐generation thermoelectric materials mainly due to their high mechanical flexibility. Flexible thermoelectric materials can enable the realization of flexible or even portable/wearable thermoelectric generators or coolers. They are significant because of the ubiquitous heat on earth and the convenient heat collection related to the mechanical flexibility. For example, a wearable thermoelectric generator can harvest heat from human body and provide electricity to other wearable or implanted devices that are used for communication or healthcare. Polymers have much lower thermal conductivity than their inorganic counterparts, but their Seebeck coefficient is remarkably inferior to the latter. Therefore, the research focus on developing high‐performance thermoelectric polymers is very different from developing high‐performance inorganic thermoelectric materials.
Thermoelectric polymers are intrinsically conducting polymers in the doped state. They have conjugated backbone, and their charge carriers are produced through the chemical oxidation or reduction. Although intrinsically conducting polymers have been studied for more than 40 years since the discovery of conducting polyacetylene in 1970s, their thermoelectric properties only recently gained considerable attention. Hence, some researchers in the area of organic/polymer electronics may not be very familiar with the knowledge of thermoelectrics. The first chapter of this book introduces the fundamental knowledge of thermoelectric materials, thermoelectric effects, thermoelectric generators, and Peltier coolers. Some fundament knowledge of thermoelectric polymers is notably different from that of inorganic thermoelectric materials. Chapter 2 provides an overall review on the thermoelectric polymers. The thermoelectric properties of polymers depend not only on their chemical structure but also on their doping level and morphology. The most popular thermoelectric polymers are the poly(3,4‐ethylenedioxythiophene) (PEDOT) family, and they are reviewed in Chapter 3. Although intrinsically conducting polymers can be prepared through chemical synthesis, electrochemical polymerization is also popular for the preparation of intrinsically conducting polymers. The Chapter 4 reviews the thermoelectric polymers by electrochemical synthesis. Apart from electronic conductors including semiconductors, semimetals and intrinsically conducting polymers, ionic conductors recently attracted great attention owing to their high thermovoltage that is usually higher than that of electronic conductors by two to three orders by magnitude. In addition, they can affect the thermoelectric properties of polymers as well. This is covered in Chapter 5. Chapter 6 provides a comprehensive review on the thermoelectric polymer composites. The nanofillers in the composites can greatly improve the thermoelectric properties of polymers. The last chapter introduces the thermoelectric properties of low‐dimensional thermoelectric materials including zero‐dimensional nanoparticles, one‐dimensional nanowire or nanotubes, and two‐dimensional materials. These low‐dimensional materials can exhibit interesting thermoelectric materials, and they are often used as the nanofillers in thermoelectric polymer composites for practical application.
I am grateful to all the authors for their contributions to this book particularly in this special time of the COVID‐19 pandemic. I also would like to thank my wife and my two sons for their support.
Jianyong Ouyang
1 Fundamental Knowledge on Thermoelectric Materials
Jianyong Ouyang and Hanlin Cheng
Department of Materials Science and Engineering, National University of Singapore, Singapore, Singapore
1.1 Properties of Thermoelectric Materials
Thermoelectric materials are electronic materials that have charge carrier distributions sensitive to temperature and produce a voltage at the presence of a temperature gradient. In terms of the electrical conductivity, electronic materials can be classified into conductors, semiconductors, and insulators. Conductors like metals usually have a conductivity of >102 S cm−1, while insulators like plastics usually have a conductivity of below 10−7 S cm−1. The conductivity of semiconductors is between those of the conductors and insulators. The conductivity of a semiconductor can be dramatically increased by doping. The different electrical conductivities of these materials arise from their different electronic band structures. As shown in Figure 1.1, conductors have a partially filled band, and the maximum electron energy at the absolute 0 K is the Fermi energy (E F). There is a bandgap between the conduction band and valence band for semiconductors. For a semiconductor, the valence band is fully occupied while the conduction band is completely empty at 0 K in dark. The Fermi level is between the top of the valence band and the bottom of the conduction band for an intrinsic or lightly doped semiconductor. Thermal excitation or light exposure can induce electron transition from the valence band to the conduction band. This leads to increase in the electrical conductivity. The bandgap of insulators is much larger than semiconductors. Insulators always have very low conductivity because temperature or light cannot usually excite the electrons from the valence band to the conduction band.
Figure 1.1 Band structures of (a) conductors, (b) semiconductors, and (c) insulators.
In terms of the energy band structure, the conventional conductors are metals or heavily doped semiconductors that can have the Fermi level higher than the minimum of the conduction band or lower than the maximum of the valence band. Some charge‐transfer organic salts, intrinsically conducting polymers, multi‐walled carbon nanotubes (MWCNTs), single‐walled carbon nanotubes (SWCNTs) with certain chirality, graphene, and MXenes can have partially filled band structure, and they are thus conductors. The conventional semiconductors include Si, Ge, and compound semiconductors. Conjugated organic compounds and conjugated polymers in neutral state are considered as organic semiconductors. Single‐walled carbon nanotubes can be semiconductive or metallic depending on the chirality.
Apart from the electrical conductivity, the Seebeck coefficient and thermal conductivity are important parameters for thermoelectric materials. The thermoelectric performance of a material is usually characterized by the dimensionless figure of merit (ZT),
where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. S2 σ is called the power factor. To have a high ZT value, the thermoelectric material should have a high power factor and a low thermal conductivity. Because thermoelectric polymers or organic molecules usually have very low thermal conductivity, the power factor is the important parameter for their thermoelectric performance.