Recent Development of Thermoelectric ... - Wiley Online Library

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Jan 22, 2018 - Hongyan Yao, Zeng Fan, Hanlin Cheng, Xin Guan, Chen Wang, Kuan Sun,* ... Dr. H. Yao, Dr. Z. Fan, Dr. H. Cheng, X. Guan, Prof. J. Ouyang.
REVIEW Thermoelectric Materials

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Recent Development of Thermoelectric Polymers and Composites Hongyan Yao, Zeng Fan, Hanlin Cheng, Xin Guan, Chen Wang, Kuan Sun,* and Jianyong Ouyang* is called power factor (PF). TE materials can be classified into p-type and n-type materials in terms of the charge carriers. Holes and electrons are the major charge carriers for the former and the latter, respectively. Efficient inorganic TE materials are semiconductors or semimetals. They usually have high Seebeck coefficient and moderate electrical conductivity. Although many organic conductors and semiconductors also exhibit thermoelectric behavior, in early days these materials have no practical significance due to their low ZT values.[5,6] However, great progress was made on organic thermoelectric materials recently. High thermoelectric properties close to inorganic TE materials were observed on some intrinsically conductive polymers. Polymers are promising to be the next generation TE materials arising from their unique advantages including low cost, high mechanical flexibility, light weight, low or no toxicity, and intrinsically low thermal conductivity.[5,7,8] The ZT value depends on the PF value and the thermal conductivity. The thermal conductivity of polymers is usually less than 1 W m−1 K−1 and difficult to further decrease. In contrast, the PF value of the conductive polymers can be significantly enhanced by several orders of magnitude. Therefore the PF value is usually used to evaluate the TE properties of polymers. The PF depends on both the Seebeck coefficient and the electrical conductivity. The electrical conductivity of a material is generally expressed by the equation, σ = nqμ, where q is the elementary charge, n is the charge carrier concentration, and μ is the charge carrier mobility. As PF is proportional to the electrical conductivity, almost all the TE polymers are conductive polymers. The charge carrier concentration of conductive polymers is affected by the doping level, and the charge carrier mobility is related to the chemical structure and morphology.[6] S represents the entropy of a carrier with unit charge, which is governed by the following equation in a simplified system of charge carriers without strong interactions, qS = kBln[(1 − ρ)/ρ], where ρ is the charge density.[5] Thus the Seebeck coefficient and electrical conductivity are interdependent. Generally, lowering the charge carrier concentration can increase the Seebeck coefficient while decrease the electrical conductivity. Hence, there is an optimal doping level for high PF. Apart from approaches to improve the TE properties of neat polymers, another popular approach is to form composites of

Thermoelectric materials can be used as the active materials in thermoelectric generators and as Peltier coolers for direct energy conversion between heat and electricity. Apart from inorganic thermoelectric materials, thermoelectric polymers have been receiving great attention due to their unique advantages including low cost, high mechanical flexibility, light weight, low or no toxicity, and intrinsically low thermal conductivity. The power factor of thermoelectric polymers has been continuously rising, and the highest ZT value is more than 0.25 at room temperature. The power factor can be further improved by forming composites with nanomaterials. This article provides a review of recent developments on thermoelectric polymers and polymer composites. It focuses on the relationship between thermoelectric properties and the materials structure, including chemical structure, microstructure, dopants, and doping levels. Their thermoelectric properties can be further improved to be comparable to inorganic counterparts in the near future.

1. Introduction Thermoelectric (TE) materials can exhibit electrical voltage when there is a temperature gradient at two ends. They can be used as the active materials of TE generators and Peltier coolers that can directly convert energy between heat and electricity. Because there is no mechanically moving parts in these TE devices, they are quiet and compact. The energy conversion efficiency of a TE generator or Peltier cooler depends on the figure of merit (ZT) of the TE materials, ZT = S2 σT/κ, where S is Seebeck coefficient or thermopower, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature.[1–4] S2σ Dr. H. Yao, Dr. Z. Fan, Dr. H. Cheng, X. Guan, Prof. J. Ouyang Department of Materials Science and Engineering National University of Singapore Singapore 117574, Singapore E-mail: [email protected] C. Wang, Prof. K. Sun Key Laboratory of Low-Grade Energy Utilization Technologies and Systems Ministry of Education School of Power Engineering Chongqing University Chongqing 400044, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/marc.201700727.

DOI: 10.1002/marc.201700727

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polymers with nanomaterials. The nanofillers can increase the electrical conductivity and/or the Seebeck coefficient of the polymer matrix, but they usually do not increase the thermal conductivity remarkably when their loading is not too high. Therefore, TE polymer composites can have high mechanical flexibility and low thermal conductivity similar to the thermoelectric polymers. Great progress has been made on p-type and n-type TE polymers as well as polymer composites (Figure 1). This article provides a review on the recent development of TE polymers and composites. We believe that the TE properties of polymers can be comparable to their inorganic counterparts in near future. This will enable the development of flexible and portable TE systems.

Kuan Sun is an assistant professor at the School of Power Engineering, Chongqing University, China. He received his B.Appl.Sci. (Hon.) and Ph.D. degree from the National University of Singapore (NUS), and postdoc trainings at the University of Melbourne and NUS. He was also a visiting scholar at Karlsruhe Institute of Technology and the Max Planck Institute for Polymer Research. His research focuses on printable solar cells, transparent electrodes, thin-film thermoelectric materials, as well as novel functional materials and devices. Jianyong Ouyang received his Ph.D., master’s, and bachelor’s degrees from the Institute for Molecular Science in Japan, the Institute of Chemistry of the Chinese Academy of Science, and Tsinghua University in Beijing, respectively. He worked as an assistant professor at the Japanese Advanced Institute of Science and Technology and as a postdoctoral researcher at the University of California, Los Angeles (UCLA), before joining the National University of Singapore as an assistant professor in 2006. He was promoted to associate professor in 2012. His research interests include flexible electronics and energy materials and devices.

2. p-Type TE Polymers Intrinsically conductive polymers usually have conjugated backbone in either oxidized or reduced state. The charge carriers are holes when the polymers are in the oxidized state. The representative p-type TE polymers include poly(3,4-ethylenedioxythiophene) (PEDOT), polythiophene derivatives, polyacetylene (PA), polyaniline (PANi), polypyrrole (PPy), polycarbazole, and polyphenylenevinylene (PPV) derivatives.

2.1. PEDOT:PSS The PEDOT family is the most popular TE polymers because of their high TE properties. In particular, PEDOT:poly(styrenesulfonate) (PEDOT:PSS, chemical structure shown in Figure 2a) has received the greatest attention because it can be processed by solution processing techniques when in its doped state. The TE properties of some typical PEDOT:PSS are listed in Table 1. PEDOT:PSS can be dispersed in water, and its aqueous solutions are commercially available. But as-prepared PEDOT:PSS film from its aqueous solution has low TE properties with

Macromol. Rapid Commun. 2018, 39, 1700727

the electrical conductivity