Conjugated polymers

Typical conjugated polymers used as thermoelectric materials include poly(3,4-ethylenedioxythiophene) (PEDOT), polyanilines (PANIs), polythiophenes, polyacetylenes, polypyrrole, and polycarbazole, which have been intensively studied and reviewed.^{Reference Bubnova and Crispin1–Reference Culebras, Gómez and Cantarero5} Among these, p-type PEDOT:PSS (polystyrene sulfonate) and PEDOT-Tos (Tosylate) exhibit encouraging thermoelectric performance through careful chemical control of the oxidation level where the optimal power factor (PF) can reach 460 and 450 μW/mK²,^{Reference Kim, Shao, Zhang and Pipe6,Reference Bubnova, Khan, Wang, Braun, Evans, Fabretto, Hojati-Talemi, Dagnelund, Arlin, Geerts, Desbief, Breiby, Andreasen, Lazzaroni, Chen, Zozoulenko, Fahlman, Murphy, Berggren and Crispin7} and the values are among the highest PF values that the organic-based thermoelectric materials can achieve. However, air-stable n-type organic conductors are rare as most of them have relatively small electron affinity and are likely to react with water and oxygen.^{Reference Anthopoulos, Anyfantis, Papavassiliou and de Leeuw8–Reference Nicolai, Kuik, Wetzelaer, De Boer, Campbell, Risko, Brédas and Blom10} Recently, it was found that electrochemical polymerization can be used to synthesize high-performance n-type conjugated polymers by controlling the oxidation (and thus the carrier concentration) at the anode. For instance, Sun et al. fabricated an n-type poly(Ni-ethylenetetrathiolate), which exhibited a large PF of 453 μW/mK² at 300 K.^{Reference Sun, Qiu, Tang, Geng, Wang, Zhang, Huang, Xu, Yue and Guan11}

Hybrid thermoelectric composites

In addition to optimizing the oxidation level, conductive polymers can be blended with other components to improve the overall thermoelectric performance, including with highly conductive materials such as CNTs and graphene as well as inorganic materials with large Seebeck coefficients. These hybrid composites can be fabricated by either solution processing or in situ polymerization.^{Reference Kim, Kim, Choi, Grunlan and Yu12–Reference Ju and Kim16} The typical synthesis methods are shown in Figure 1. Thermoelectric performance can be tailored by tuning the morphology and distribution of the polymer in the composites. For instance, PANI can grow along the wall of a CNT to form a cable-like structure by either solution processing or in situ polymerization, which results in stronger π–π interaction between PANI and CNT, leading to enhanced carrier mobility that improves both the electrical conductivity and Seebeck coefficient.^{Reference Yao, Chen, Zhang, Liufu and Chen17–Reference Wang, Yao, Chang and Chen20} In some cases, segregated CNT bundles can be coated with polymer particles, such as PEDOT:PSS or poly(3-hexylthiophene) (P3HT) by solution processing, which helps bridge neighboring nanotubes and allows charge carriers to transfer at the junctions.^{Reference Kim, Kim, Choi, Grunlan and Yu12,Reference Yu, Choi, Yin and Grunlan21–Reference Bounioux, Díaz-Chao, Campoy-Quiles, Martín-González, Goni, Yerushalmi-Rozen and Müller23} Distinct from CNT-based composites, conducting polymers such as PANI and PEDOT can be intercalated into stacked graphene or simply grow on the surface of graphene nanosheets during in situ polymerization, which will promote intramolecular conjugation and facilitate the hopping transport of charge carriers at the interface.^{Reference Xu, Chen and Qiu14,Reference Wang, Yao, Bi, Huang, Wang and Chen15}

Figure 1.

Schematics of various synthesis methods for organic-based thermoelectric composites. (a) (Left to right) The random coil-like polyaniline (PANI) molecules become extended chains when treated with m-cresol solvent. After subsequent treatment with a solvent containing dispersed single-walled carbon nanotubes (CNTs), a highly ordered cable-like PANI/CNT composite forms due to synergetic effects and intramolecular conjugation;^{Reference Yao, Wang, Wang and Chen13} (b) (Left to right) EDOT monomers are dispersed in a reduced graphene oxide (rGO) aqueous solution to form a pie-like PEDOT/rGO composite during in situ polymerization;^{Reference Xu, Chen and Qiu14} (c) (Left to right) Li-intercalated SnSe powder was exfoliated into nanosheets in water, and then mixed with aqueous PEDOT:PSS. The obtained SnSe/PEDOT:PSS composite was deposited onto a glass substrate by drop casting.¹⁶ Note: PEDOT^, poly(3,4-ethylenedioxythiophene); PSS, poly(styrene sulfonate); SWNT, single-walled nanotube; EDOT, 3,4-ethylenedioxythiophene.

For inorganic–organic hybrid composites, the size, morphology, and distribution of inorganic nanomaterials in the polymer matrix as well as the adhesion at the interfaces are crucial for improving the thermoelectric performance. In addition, special attention has been paid to inorganic nanoparticles, which can be easily oxidized during synthesis. To minimize such oxidation, processing methods such as ball milling should be conducted in an inert atmosphere. Alternatively, an acid treatment has proven to be an effective and low-cost method to reduce the oxidized surface of the alloy-based nanoparticles.^{Reference Zhang, Sun, Katz, Fang and Opila24}

Figure 2 summarizes the thermoelectric properties of organic-based hybrid composites reported in the literature, including polymer/CNTs,^{Reference Kim, Kim, Choi, Grunlan and Yu12,Reference Yao, Wang, Wang and Chen13,Reference Yao, Chen, Zhang, Liufu and Chen17–Reference Bounioux, Díaz-Chao, Campoy-Quiles, Martín-González, Goni, Yerushalmi-Rozen and Müller23,Reference Yu, Kim, Kim and Grunlan25–Reference Wang, Yi, Pu and Yu31} polymer/graphene,^{Reference Xu, Chen and Qiu14,Reference Wang, Yao, Bi, Huang, Wang and Chen15,Reference Kim, Hwang and Woo32–Reference Yoo, Kim, Lee, Cho, Choi, Kim and Kim44} and polymer/inorganic composites.^{Reference Ju and Kim16,Reference Zhang, Sun, Katz, Fang and Opila24,Reference Song, Liu, Zhu, Kong, Lu, Xu, Wang and Zhao45–Reference Zhou, Dun, Wang, Wang, Shi, Carroll, Liu and Qiao60} In addition, the thermoelectric PFs of some inorganic–organic superlattice materials^{Reference Wan, Gu, Dang, Itoh, Wang, Sasaki, Kondo, Koga, Yabuki, Snyder, Yang and Koumoto61–Reference Wan, Tian, Azizi, Huang, Wei, Sasai, Wasusate, Ishida and Koumoto63} have been included in Figure 2 for comparison, and this will be discussed hereafter in detail. Compared to the CNT or graphene composites (circles), the polymer/inorganic composites (triangles), particularly PEDOT:PSS/Te (nanowires or nanorods) composites, possess a relatively large S (>160 μV/K).^{Reference See, Feser, Chen, Majumdar, Urban and Segalman50–Reference Yee, Coates, Majumdar, Urban and Segalman52} However, their σ needs to be significantly enhanced to achieve PF > 100 μW/mK². Among the polymer–inorganic composites (triangles), the highest PFs were achieved for p-type PEDOT:PSS/SnSe composite (380 μW/mK²),^{Reference Ju and Kim16} and n-type PVDF [poly(vinylidene fluoride)]/Cu_0.1Bi_1.9Se₃ composite (103 μW/mK²).^{Reference Dun, Hewitt, Huang, Xu, Zhou, Huang, Cui, Zhou, Jiang and Carroll59} On the other hand, PF of polymer/graphene composites (squares) is generally below 50 μW/mK², due to the low S (<50 μV/K in most cases). For the CNT-based composites, particularly the PEDOT/CNTs composites, the high σ outweighs the drawback of the low S. Among them, the maximum PF reaches 500 μW/mK^{Reference Chen, Zhao and Liang2} by p-type PEDOT:PSS mixed with 40 wt% double-wall CNTs.^{Reference Moriarty, Briggs, Stevens, Yu and Grunlan22}

Figure 2.

Thermoelectric properties of a wide range of organic-based composites compiled from data reported in the literature. The plot includes polymer/CNT (carbon nanotube) composites (circle symbols),^{Reference Kim, Kim, Choi, Grunlan and Yu12,Reference Yao, Wang, Wang and Chen13,Reference Yao, Chen, Zhang, Liufu and Chen17–Reference Bounioux, Díaz-Chao, Campoy-Quiles, Martín-González, Goni, Yerushalmi-Rozen and Müller23,Reference Yu, Kim, Kim and Grunlan25–Reference Wang, Yi, Pu and Yu31} polymer/graphene composites (square symbols),^{Reference Xu, Chen and Qiu14,Reference Wang, Yao, Bi, Huang, Wang and Chen15,Reference Kim, Hwang and Woo32–Reference Yoo, Kim, Lee, Cho, Choi, Kim and Kim44} and polymer/inorganic composites (triangle symbols).^{Reference Ju and Kim16,Reference Zhang, Sun, Katz, Fang and Opila24,Reference Song, Liu, Zhu, Kong, Lu, Xu, Wang and Zhao45–Reference Zhou, Dun, Wang, Wang, Shi, Carroll, Liu and Qiao60} Among these, composites containing poly(3,4-ethylenedioxythiophene) (PEDOT) are shown with hollow symbols, and other polymer materials, including polyaniline, polypyrrole, PVDF, and P3HT, are shown with solid symbols. Power factor (PF) values of organic–inorganic superlattices (A–C)^{Reference Wan, Gu, Dang, Itoh, Wang, Sasaki, Kondo, Koga, Yabuki, Snyder, Yang and Koumoto61–Reference Wan, Tian, Azizi, Huang, Wei, Sasai, Wasusate, Ishida and Koumoto63} are also presented (solid diamond) for comparison. The plot shows that the polymer/inorganic composites possess relatively large S as compared to polymer blended with CNTs or graphene. However, their σ (electrical conductivities) need to be significantly enhanced to achieve a large PF. Note: HA, hexylammonium; DMSO, dimethylsulfoxide; NMF, n-methylformamide; TBA, tetrabutyl ammonium; P3HT, poly(3-hexylthiophene); PVDF, poly(vinylidene fluoride); A, electrochemically intercalated TiS₂[(HA)(H₂O)(DMSO)] superlattice crystal; B, optimized TiS₂[(HA)(TBA)] superlattice crystal through heat treatment; C, solution-processed TiS₂[(HA)(NMF)] superlattice film.

It should be noted that the vast majority of composites presented in Figure 2 are p-type materials, and the development of air-stable n-type organic materials/composites is highly desired to make efficient p–n coupled thermoelectric devices. In addition to incorporating n-type nanomaterials into polymers, single-wall CNTs treated with alkali salts or hydroxides (such as NaCl, NaOH, and KOH) and crown ether could be negatively charged and exhibited excellent stability in air up to 150°C, with the optimized PF reaching ∼230 μW/mK² at 310 K.^{Reference Nonoguchi, Nakano, Murayama, Hagino, Hama, Miyazaki, Matsubara, Nakamura and Kawai64}

Flexible thermoelectric module from Fujifilm Corporation

The flexibility of organic materials and advanced processing techniques offer great opportunities to fabricate novel thermoelectric modules that are beyond traditional designs. Fujifilm has developed various organic-based thermoelectric modules using printing techniques since 2010. Recently, a high-quality p-type CNT composite paste was successfully produced using a dispersion method. Parameters such as viscosity, viscoelasticity, and permeability of the paste were carefully adjusted for stencil printing. Based on this, a flexible single-leg type thermoelectric module (10 cm × 10 cm) featuring an in-plane configuration was fabricated (Figure 3).^{Reference Hayashi, Sugiura, Nomura and Aoai65}

Figure 3.

The configuration and power output of a carbon nanotube (CNT)-based thermoelectric (TE) module fabricated by Fujifilm Corporation.^{Reference Hayashi, Sugiura, Nomura and Aoai65} (a) Photograph of a CNT-based TE pattern fabricated by stencil printing, and (b) a schematic illustration of a single leg in the pattern. (c) The generated power output at different airflow rates. The inset shows a photograph of the TE module consisting of p-type CNT-based composite material. The TE module can achieve a maximum power output of 120 μW when the hot side is 120°C and the flow rate of the cool air is 3.9 m/s.^{Reference Hayashi, Sugiura, Nomura and Aoai65}

To evaluate the power generation of the device, the thermoelectric module was mounted onto an aluminum fin with flowing air for cooling, and heat was extracted from a hot pipe with a diameter of 80 mm. The power output was measured at different airflow rates in the range of 0–3.9 m/s (Figure 3). The results demonstrate that the module can generate a power output of 10 μW by natural air cooling when the hot side temperature is 120°C, and the value can remarkably increase to 50 and 120 μW when the airflow rate was 1.2 and 3.9 m/s, respectively.^{Reference Hayashi, Sugiura, Nomura and Aoai65}