Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Review

Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration Hong Wang1,* and Choongho Yu2

Recent rapid development of inorganic thermoelectric (TE) materials has aroused enthusiasm for exploring low-cost, flexible, lightweight, and non-toxic organic TE materials. Great progress has been achieved in developing organic materials with high TE performance (figure of merit, ZT) over the past decade. However, it is still extremely challenging to obtain organic materials with high TE performance and a ZT over 0.5 because of the strong interrelationship between the three TE parameters: electrical conductivity, Seebeck coefficient, and thermal conductivity. In this review, we discuss current trends in developing strategies to decouple the electrical conductivity, Seebeck coefficient, and thermal conductivity, which to the best of our knowledge have not been discussed in previously published reviews. Methods such as solvent treatment, electrochemical doping, and nanostructure formation are analyzed. In addition, incorrect thermal conductivity values for highly electrically conducting organic materials are still frequently reported, even in papers published in high-impact journals. A description of this puzzling phenomenon is provided in this review. Finally, a discussion of the advantages of state-of-the-art fabrication techniques of organic TE modules is presented, which highlights the unique advantages of organic TE materials in supporting wearable/portable devices. Introduction Thermoelectric (TE) materials have attracted much attention after being overlooked for almost a century. The first work on TE materials is the report of the Seebeck effect by Thomas Johann Seebeck in 1822.1 Later, the Peltier effect and Thomson effect were reported by Jean Charles Peltier in 18342 and Lord Kelvin in 1854.3 However, the development of high-TE-performance materials has been slow in the following century and beyond.4 The performances of TE materials seldom reached ZT = 0.1, mainly due to the adversely correlated TE parameters. In the 2000s, this research field began to flourish with the utilization of nano-techniques to decrease the thermal conductivity.4,5 Numerous TE materials with a high figure of merit (ZT) of >1 have been reported in the past decade.6–9 Nevertheless, many high-performance TE materials work at high temperatures and are composed of inorganic materials that are typically expensive, brittle, and heavy and contain toxic elements such as Pb, Bi, and Te. Now is the time to develop low-cost, flexible, lightweight, and non-toxic TE materials for applications at low temperature (300 K < T < 600 K). Organic TE materials have recently been of great interest to researchers because of their natural advantages, such as low material cost, mechanical flexibility, low toxicity, and light weight, in contrast to traditional inorganic TE materials. Figure 1 schematically illustrates a flexible TE device, which is ideal for wearable/portable

Context & Scale Green technology has attracted much attention in recent years due to the fear of exhaustion of traditional fossil sources and the rising awareness of environmental protection. Thermoelectric (TE) technology is an auxiliary energy technique that can directly convert waste heat to electricity through TE generators. This green energy-saving technology is considered to be a promising way to relieve the pressure of energy and environment. TE generators have significant advantages of durability and simplicity over conventional power generators because of their unique solidstate structures. The global market growth of TE generators is rapid and will reach over $950 million by 2024 (Zervos). Specifically, organic TE generators offer additional benefits of light weight, flexibility, and low-cost production. The performance of TE generators depends on the performance of TE materials, which is determined by the Seebeck coefficient, electrical conductivity, and thermal conductivity. A good TE material requires a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, these three parameters are strongly

Joule 3, 1–28, January 16, 2019 ª 2018 Elsevier Inc.

1

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

correlated with each other, which inhibits the improvement of the performance of TE materials. In this review, recent advances in materials preparation, performance optimization, and device integration are highlighted, and an outlook on the future development in organic thermoelectrics is provided.

Figure 1. Illustration of an Organic TE Device and Its Potential Applications (A) Schematic illustration of a flexible TE device with pairs of p- and n-type legs. The device may be made of wearable functional fabric, which can be used for converting waste heat from the human body into electricity. High-energy carriers move from the hot face to the cold face, generating electricity. (B) Illustration of the potential applications of organic TEGs to support wearable devices such as watches, health monitors, and toys.

devices such as health-monitoring sensors. Despite the benefits of organic TE materials, their low TE performances have hindered the commercialization of organic thermoelectric generators (TEGs). It is very challenging to enhance the performance of organic TE materials because organic material systems are complex and their performances are affected by various factors.10,11 Fundamental studies into organic TE materials are also lacking, and there is no theoretical model that can guide us in the development of high-TE-performance organic materials. TE performance is typically indicated by the TE figure of merit, ZT = sS2T/k, where s, S, T, and k are the electrical conductivity, Seebeck coefficient (or thermopower), absolute temperature, and thermal conductivity, respectively. An ideal TE material is expected to have a high Seebeck coefficient to improve energy conversion,12–14 a high electrical conductivity to reduce joule heating,15,16 and a low thermal conductivity to maintain a temperature gradient.17–19 However, the conditions for obtaining ideal s, S, and k values conflict.4 For example, increasing s is usually accompanied by a reduction of S and an increase of k, imposing restrictions on improving the ZT. In the 1990s, the ZT for most organic TE materials lay in the range of 10
3–10
6.20,21 Notable progress has been made in the past decade in developing high-TE-performance organic materials. Insightful work by Yu et al.22,23 on decoupling the electrical conductivity and Seebeck coefficient opened up the organic TE field in 2008 and 2010. A large enhancement in the TE power factor was also reported by Bubnova et al. in 201124 by carefully tuning the carrier concentration. These results have aroused the enthusiasm of researchers in related fields. Various promising organic TE materials with high power factors from 70 mW/m-K2 to 2,710 mW/m-K2 have been reported recently (Table 1). A record-high ZT of 0.5 was reported by Wang et al, in 2015, which is now the highest ZT among organic TE materials.19,28 Although the current highest ZT of organic TE materials is not comparable with that of their inorganic counterparts (ZT  2.7),6 the advantages in material cost and scalability make organic materials competitive in various applications.

2

Joule 3, 1–28, January 16, 2019

1Frontier

Institute of Science and Technology & State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710054, China

2Department

of Mechanical Engineering, Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2018.10.012

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Table 1. Selected High-TE-Performance Organic Materials Fabricated in the Past Decade, Including Polymer-Only Samples, Composites, and Organometallic Polymers Sample

Electrical Conductivity (S/cm)

Seebeck Coefficient (mV/K)

Thermal Conductivitya (W/m-K)

Power Factor (mW/m-K2)

Reference

PEDOT:PSS

6.20 3 102

33.4 G 2.2

0.42 G 0.07

469

Kim et al.25

PEDOT:Tos

0.8

200

0.33

324

Bubnova et al.24

PEDOT:Tos

9.23

117

–

1,270

Park et al.26

PANI/SWCNT

6.8 3 102

51

0.43

176

Yao et al.27

PEDOT/CNT

8


1,140

0.6

1,050

Wang et al.28

PEI/SWCNT

4.68 3 103


69

–

1,500

Zhou et al.29

PEDOT:PSS/Te

1.93 3 10

163

0.22–0.30

70.9

See et al.30

Poly[Kx(Ni-ett)] 400 K

5.7 3 101


152

0.22

132

Sun et al.31

PEDOT:PSS-PVAc/CNT composite

1.35 3 10

28

0.2–0.4

160

Yu et al.32

PANI/graphene-PEDOT:PSS/ PANI/DWNT

1.93 3 103

120

–

2,710

Cho et al.33

PANI/graphene/PANI/DWNT

10.80

130

–

1,825

Cho et al.34

1

3

PEDOT, poly(3,4-ethylenedioxythiophene); PSS, poly(styrenesulfonate); Tos, tosylate; PANI, polyaniline; PEI, poly(ethyleneimine); Poly[Kx(Ni-ett), ett, 1,1,2,2ethenetetrathiolate; PVAc, poly(vinyl acetate); SWCNT, single-walled carbon nanotube; CNT, carbon nanotube; DWNT, double-walled carbon nanotube. a Thermal conductivity data are controversial.

The promising widespread applications have led to surging interest in organic TE materials, and some reviews on organic TE materials have been published.21,35–41 For example, Bubnova, Katz and colleagues provided a good introduction to the solid-state physics of conjugated polymers.35,38 Kroon et al. discussed the most studied building blocks of organic TE materials.39 Zhang et al. summarized the TE properties of the most popular polymers and small molecules with charts of the TE parameters.37 Toshima introduced recent progress to improve the TE performance over the past two decades in the field of organic and hybrid TE materials.41 Here, we discuss current trends in developing strategies to decouple the electrical conductivity, Seebeck coefficient, and thermal conductivity to improve the TE performance of organic materials. The strong correlation between these parameters restricts the development of high-performance organic TE materials. Various approaches that have been proposed and employed to decouple the parameters to improve the performance of organic TE materials are included. For example, a modified density of states due to quantum confinement can increase S without reducing s.42 Phonon scattering can help reduce k without a significant reduction in s.32 Low-dimensional nanostructures give additional control over the three parameters.30,43 We aim to discuss the most promising ways to improve the performance of organic TE materials rather than reiterating all the details in the previous reports. Meanwhile, debates on the thermal conductivity of high-performance organic TE materials are analyzed to give readers a full description of the TE properties of organic materials. This review paper is organized as follows. The first part describes the three most important TE parameters. In the second part, we describe strategies used to decouple the three parameters for polymer-only materials and polymer composites, and include current state-of-the-art sample preparation and optimization methods to maximize the power factor and minimize the thermal conductivity for improving the ZT. The third part is related to applications, including the key parameters and recent developments in TEG fabrication. Finally, a future outlook on organic TE materials and devices is given.

Joule 3, 1–28, January 16, 2019

3

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Key Parameters for Organic TE Materials Thermal Conductivity of Organic Materials Thermal conductivity (k) is one of the three key parameters in TE materials. In general, the thermal conductivity can be described by Equation 1: k = ke + kL ;

(Equation 1)

where ke and kL are the electronic thermal conductivity and the lattice thermal conductivity, respectively. The electronic thermal conductivity is proportional to the electrical conductivity of a material according to the Wiedemann-Franz law (WFL)44,45: ke = LTs;

(Equation 2)
8

where L is the Lorenz number (typically, L = 2.44 3 10 WU/K ). Equation 2 is suitable for many organic materials. Liu et al. reported that WFL is valid for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the Lorenz number is consistent with the conventional Sommerfeld value of the Lorenz number.26 Salamon et al. found that the WFL is valid in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) with a similar Lorenz number.46 However, some researchers suggested that the Lorenz number may not be the same as the conventional Sommerfeld value because of the dissimilar electrical transport as a result of polarons.47 The Lorenz number may deviate from the Sommerfeld value, and could be dependent on the structure and doping level of the material.47–49 2

The lattice thermal conductivity of a material is due to energy transport by phonons and is often estimated by Equation 3: 1 kL = cvl; 3

(Equation 3)

where c, l, and v are the heat capacity, the phonon mean free path, and the sound velocity, respectively. For PEDOT:PSS, c and l have been reported to be 1.8 3 106 J/K-m3 and 0.1 nm, respectively.50 The sound velocity of organic materials is often in the range of 103–104 m/s. For instance, the sound velocities of PEDOT:PSS and poly(3-hexylthiophene) (P3HT) are 1.58 3 103 and 2.87 3 103 m/s, respectively.51,52 The lattice thermal conductivities of PEDOT:PSS and P3HT are estimated to be 0.3 W/m-K and 0.5 W/m-K. These values lie in the range of reported thermal conductivities of various polymers (0.1–0.7 W/m-K).53,54 Typically, organic materials have a low thermal conductivity in the range of 0.1–0.7 W/m-K53,54 because their electrical conductivities are often lower than 1 S/cm. The low electrical conductivity leads to orders-of-magnitude lower electronic thermal conductivity compared with the lattice thermal conductivity of organic materials. Therefore, the lattice thermal conductivity often equals the total thermal conductivity for organic materials. Caution should be exercised when determining the thermal conductivity of highly conducting polymers because the electronic thermal conductivity is likely to be comparable with the lattice thermal conductivity. Liu et al. measured the in-plane thermal conductivity of highly conducting PEDOT, the electrical conductivities of which are 900 S/cm and 820 S/cm,48 and found total thermal conductivities of 1.1 W/m-K and 0.84 W/m-K. The authors suggested that the electronic thermal conductivity according to the WFL is 0.6–0.7 W/m-k, which is larger than the typical lattice thermal conductivity for PEDOT (0.3 W/m-K). Wei et al. reported that PEDOT with an electrical conductivity of 810 S/cm can reach a thermal conductivity of 0.94 W/m-K.55 Shi et al. reported that the measured total thermal conductivity is 1.5 W/m-K for PEDOT at 500 S/cm.49 The electronic thermal

4

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

conductivity could be higher than 1 W/m-K. High thermal conductivities of >2 W/m-K have also been reported for heavily doped organic materials.40,49 Therefore, the thermal conductivity of organic materials could be high, particularly when the electrical conductivity is high. The electrical and thermal conductivities are often anisotropic, especially in organic thin films. To evaluate the TE performance of such materials, the electrical and thermal conductivities should be measured along the same direction. In particular, the electrical conductivity is typically measured along the in-plane direction for thin films, and the corresponding thermal conductivity is measured along the out-of-plane direction because of measurement difficulties. When the electronic thermal conductivity is estimated by WFL, it is necessary to use the electrical conductivity measured along the same direction. Seebeck Coefficient and Electrical Conductivity The Seebeck coefficient and electrical conductivity are typically interrelated as a function of carrier concentration. One common strategy to optimize the TE performance of a material is to find the maximum power factor (S2s) by varying the carrier concentration (n). The interrelationship between S and n of highly conducting semiconductors can be described using a model that is typically used to explain electron transport in metals56: S

8p2 kB2 
p 2=3 mT ; 3n 3eh2

(Equation 4)

where kB is the Boltzmann constant, e is the electron charge, h is the Planck constant, and m* is the effective mass of the carrier. This model is generally used to estimate the change in S as a function of n. A low carrier concentration leads to a large Seebeck coefficient but results in a low electrical conductivity because the electrical conductivity is proportional to the carrier concentration, as shown in Equation 5: s = nem;

(Equation 5)

where m is the carrier mobility. Obtaining a high power factor normally suffers from the tradeoff relationship between S and s. A compromise between the Seebeck coefficient and electrical conductivity can result in a maximal power factor, which typically occurs at a carrier concentration between 1019 and 1021 per cm3 for inorganic TE materials.57 The preferred carrier concentration for organic materials has not yet been firmly established. The optimum carrier concentration could be different from that of inorganic materials since the conduction mechanisms of organic TE materials are different from those of traditional inorganic TE materials.47 Conduction in organic materials, especially in conducting polymers, often arises from polaron hopping caused by overlap of the electron wave functions on adjacent sites. The strong electron-phonon interaction may cause lattice distortion around the electron that moves along the chain but is also trapped by the polarized field. This type of electrical transport leads to a low carrier mobility (typically less than 10
4–101 cm2/V-s) compared with inorganic materials (in the range of 1–103 cm2/V-s),58 subsequently resulting in a relatively large carrier concentration at the maximal power factor for organic materials. To optimize the carrier concentration for high-performance organic TE materials, it is essential to understand the electrical transport mechanism in organic materials, especially the electrical transport mechanism in conjugated polymers. For example, doping in organic materials is not the same as doping in inorganic semiconductors. In general, electrically resistive neutral or undoped conjugated polymers are doped by oxidation to form p-type semiconductors since reduced polymers (n-type) are

Joule 3, 1–28, January 16, 2019

5

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

normally unstable.59 Electrochemical oxidation and chemical oxidation are the most popular ways to dope conjugated polymers, in which counter ions are introduced into the polymer chains to make them charge neutral, resulting in solitons, polarons, and bipolarons. Among them, solitons are rare and are only found in trans-polyacetylene. Polarons and bipolarons are more common in conducting polymers, which can be identified by electron paramagnetic resonance (EPR).60 Taking polythiophene as an example, a polaron is formed when an electron is removed from the valence band, antibonding character appears in some p bonds and a distortion is induced locally in the conjugation backbone. Unlike inorganic semiconductors, doping in conjugated polymers may create new polaron or bipolaron bands due to the generation of polarons and/or bipolarons. Therefore, the band gap may decrease as the doping level is increased. At high doping levels, conducting polymers can have a vanishingly small gap, exhibiting metallic-like electrical properties at room temperature.61 The Seebeck coefficient is intimately related to the electronic structure. For example, in Mott’s formula, the Seebeck coefficient is proportional to the derivative of density of states at the Fermi level energy EF, assuming that the diffusion coefficient is constant with energy. At high doping levels, the wave function of the polarons/bipolarons localized on the same chain can overlap, creating one-dimensional ‘‘intra-chain’’ bands.62 The intrachain bands do not typically extend through all three dimensions of the entire material because organic materials are often amorphous with disordered structures and the inter-chain electronic coupling is weak.63 Therefore, polaronic/bipolaronic bands are spatially localized with a spread in energy distribution. The Fermi level lies in a smooth region of the density of states and among localized states between the valence band and the polaron/bipolaron band for a disordered polaronic/bipolaronic polymer solid. The slope of lnN(E) at EF is small, leading to a small Seebeck coefficient for amorphous polaronic/bipolaronic systems. A tradeoff between the Seebeck coefficient and electrical conductivity is typically required to obtain the maximum power factor for amorphous polymers when optimizing the doping level. More details about polarons can be found in the literature.35 Another way to improve the power factor is to increase the carrier mobility of materials. According to Equations 4 and 5, carrier mobility has less effect on the Seebeck coefficient. Therefore, increasing the carrier mobility to raise the electrical conductivity may not noticeably effect the Seebeck coefficient. Organic materials with high carrier mobility are promising for developing high-performance TE materials. The design of novel organic materials with intrinsically high carrier mobility and the preparation of highly oriented structures to increase the carrier mobility of organic materials have recently become attractive research topics.20 Fundamental and Theoretical Studies on Improving the TE Performance In recent years, fundamental studies have been performed to provide research directions on how to maximize ZT, particularly for traditional inorganic materials. For organic materials, however, it is still an open question since the transport properties can be dramatically changed by a number of parameters. In terms of general TE properties, Snyder et al. gave their perspectives about the relationships among a variety of conflicting parameters in TE materials57 and thought nanostructured materials to be promising. The work of Mahan et al. provides a clue about the best electronic structure for TE materials, while taking the ZT value as a function of the transport distribution.64 The calculation results indicate that the best TE material should have a narrow distribution of energy carriers and high carrier mobility in the direction of the applied electric field.

6

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Progress has been made on establishing fundamental models to predict TE properties to guide researchers in the development of high-performance organic materials. One recent interest is one-dimensional nanocrystals. For example, Wang et al. reported the TE properties of pentacene and rubrene crystals. The ZT of these crystals was calculated by coupling the Boltzmann transport theory with the first-principles method, and decent values in the range of 0.8–1.1 were estimated.65 Casian et al. estimated that quasi-one-dimensional organic crystals might have a ZT of approximately 20 using the Boltzmann transport equations.66 Yang et al. used the Kubo formula with the Holstein model to theoretically predict the ZT of quasi-one-dimensional self-assembled molecular nanowires and found that the ZT might be greater than 15 at room temperature.47 However, the experimental ZT value is much lower than the theoretical ZT values. The perfect packing structure has not yet been realized with current self-assembly techniques due to the variation in the molecular weight of polymers and the complexity of intermolecular forces between polymer chains. Polymer-Only and Small-Molecule-Only TE Materials Development of Polymer-Only TE Materials The performance of polymer-only TE materials is mainly determined by the power factor (sS2) since their thermal conductivities are typically low. Therefore, efforts have been made to improve the power factor of polymers. In contrast, inorganic TE materials can experience major improvement by controlling thermal conductivity.67,68 The thermal conductivity of polymers is typically approximately one order of magnitude lower than those of traditional inorganic TE materials.69–73 Tuning the Carrier Concentration via Doping Polymers and Improving Mobility. Numerous polymers have been tested, such as polyacetylene,53,74–76 polypyrrole,77–80 polyaniline (PANI),81–84 poly(3,4-ethlenedioxythiophene) (PEDOT),85–88 and poly(3-hexylthiophene) (P3HT).89,90 The chemical structures of conducting polymers used for thermoelectrics are shown in Figure 2. Doping is an effective approach to improve the power factor by raising the electrical conductivity of polymers and is one of the most widely used strategies at an early stage to improve the TE performance of polymer-based materials. For pristine conducting polymers, the electrical conductivity is dominated by phonon-assisted hopping through polymer chains, and the carrier concentration is often too low to make current flow.91 Their electrical conductivities are often in the range of 10
3–10
8 S/cm,92,93 resulting in low power factors. Although pristine polymers may possess high Seebeck coefficients in the range of 102–103 mV/K,90,94,95 the power factors are often less than 1 mW/m-K2. Doping provides extra charge carriers for intra- or inter-chain charge transport,96 which can dramatically raise the electrical conductivity to 104 S/cm, subsequently improving the power factor. For example, pristine polyacetylene has a low electrical conductivity of 0.0015 S/cm and a high Seebeck coefficient of 1,077 mV/K, leading to a power factor of 0.17 mW/m-K2.74 After iodine doping, the electrical conductivity reaches 1 3 104 S/cm with a decreased Seebeck coefficient of 20 mV/K. The power factor increases to 400 mW/m-K2, which is more than 2,300 times larger than that of the pristine sample.74 However, polyacetylene is typically not stable in air, which inhibits its practical applications. In general, doped polymers possess higher power factors than pristine polymers.74,77,81,84,90 The maximum power factor is usually realized in the highly doped region because the electrical conductivity of polymers can be easily enhanced by several orders of magnitude, while the variation in the Seebeck coefficient is relatively small.60,97,98

Joule 3, 1–28, January 16, 2019

7

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 2. Candidates of Organic TE Mateirals Chemical structures of several popular conducting polymers in the organic TE field, including fully hydrocarbon-based polymers and polymers with heteroatoms such as nitrogen and sulfur.

Therefore, many polymers show the maximum power factors81,90 when heavily doped. Pristine polymers often have high Seebeck coefficients, but the values drop rapidly outside of narrow doping ranges. For example, pristine polyacetylene has a Seebeck coefficient of 1,000 mV/K, which drops to <50 mV/K when the dopant concentration is over 0.1% for trans-[CH(AsF5)y]x.60 Similar changes have also been observed in trans-[CH(I3)y]x60,97 and (C4H2SIy)x.98 Pristine polymers are typically semiconductors with relatively large energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), leading to high Seebeck coefficients. After doping, the semiconductors become metallic because the extra carriers destroyed the bound states.60 Energy-gap changes between semiconductors and metals can be observed in the EPR spectra.60 In the metallic state, the Seebeck coefficient becomes relatively insensitive to carrier concentration. Therefore, the maximum power factor often appears in the highly doped region. However, the maximum power factor does not always appear at the highest electrical conductivity.24,26 For example, PEDOT:Tos was treated with tetrakis (dimethylamino)ethylene (TDAE).24 The highest electrical conductivity of PEDOT:Tos obtained in the paper is 300 S/cm, while the maximum power factor of 324 mW/m-K2 (Figure 3) was obtained at an electrical conductivity of 60 S/cm. The power factor of PEDOT:Tos was optimized by tuning the oxidation level (doping level) of PEDOT. TDAE acts as both a post-treatment solvent to improve the carrier mobility and a reduction reagent to adjust the oxidation level. Kim et al. reported an electrochemical method to control the carrier concentration of PEDOT.26 A maximum power factor of up to 1,270 mW/m-K2 was obtained when the electrical conductivity was 1,000 S/cm, which is only half of the highest electrical conductivity of PEDOT (shown in Figure 4A). A poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer (PEPG) was used as a

8

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 3. Tuning the Oxidation Level of PEDOT:Tos (A) Illustration of the structure of poly(3,4-ethylenedioxythiophene) doped with tosylate. The carrier concentration is further optimized with tetrakis(dimethylamino)ethylene (TDAE) vapor. (B) Power factor as a function of the oxidation level. 24 Seebeck coefficient (filled triangles), electrical conductivity (open triangles), and corresponding power factors (red squares). The maximum power factor of 324 mW/m-K2 appears at the oxidation level of 22%. Adapted with permission from Bubnova et al. 24 Copyright 2011, Springer Nature.

post-treatment solvent to improve the carrier mobility by controlling the morphology of PEDOT and reducing film defects.99 The ZT value of the material is unknown because the thermal conductivity was not measured, but the thermal conductivity is likely to be >1 W/m-K due to the relatively high electronic thermal conductivity according to the WFL. Solvent treatments have been employed to optimize the power factor of polymers. In fact, this is an effective and widely used method for improving the electrical conductivity of polymers. For example, dimethyl sulfoxide (DMSO), diethylene glycol (DEG), N,N-dimethylformamide (DMF), and formic acid can dramatically increase the electrical conductivity of PEDOT:PSS from 0.2 S/cm to 890–2,100 S/cm.100–106 The significant improvement in electrical conductivity can be attributed to several reasons, such as the improvement in the carrier mobility of the polymers upon changing the polymer backbone from a coil conformation to a linear or expandedcoil conformation,105,107–109 the reduction of defects in the polymer films,99 or the removal of insulating counter-ion molecules.106 In addition to the solvent treatment method, a stretch treatment method has been reported as an effective way to improve the TE performance of polymers by increasing the carrier mobility of organic materials and hybrids. Toshima et al. reported that 2,5-dialkoxy-substituted phenylenevinylene (P(ROPV-co-PV)) and its derivatives could reach a high ZT of up to 0.1 after being stretched.110 The high ZT was attributed to the improvement in the carrier mobility, which increased the electrical conductivity without considerably affecting the Seebeck coefficient. Tuning the Molecular Structure of Polymers. In addition to optimizing the carrier concentration and increasing the carrier mobility to improve the TE performance of polymers, the structure-property relationship in organic TE materials for molecular structures such as backbones, side substitutes, and counter ions111,112 have also been studied. Polymer backbones such as thiophene, thieno[3,2-b]thiophene (TT), and 3,4-ethylenedithiathiophene (EDTT) have been studied. The power factor is typically less than 1 mW/m-K2.113–115 Le´vesque et al. reported high TE performances for poly(2,7carbazole) (PC), polyindolocarbazole (PIC), and a series of their derivatives

Joule 3, 1–28, January 16, 2019

9

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 4. TE Properties of PEDOT Prepared from the Mixture of Pyridine and PEPG with Controlled Oxidation Level by Electrochemical Process (A) The electrical conductivity (black squares) and Seebeck coefficient (blue squares) of PEDOT prepared from the mixture of pyridine and PEPG (PP-PEDOT) at different applied potentials. The electrical conductivity increases up to 2,120 S/cm at 1.1 V and then decreases sharply at applied potentials above 1.1 V due to damage from the high potential. (B) The corresponding power factor of PP-PEDOT versus the applied potential. The power factor shows a maximum value at 0.1 V. (C) Photographic images of the PP-PEDOT self-electrode after EC reduction at different potentials (indicated).26 The color gradually changes from light blue to dark blue. Adapted with permission from Park et al. 26 Copyright 2013, The Royal Society of Chemistry.

(Figure 5).111 PC and PIC (shown in Figure 5, top) had electrical conductivities of less than 1 S/cm, leading to a small power factor of 0.1 mW/m-K2. After further modifying the carbazole backbones with benzothiadiazole and bithiophene units, poly [N-90 -heptadecanyl-2,7-carbazole-alt-5,50 -(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (PCDTBT) showed an electrical conductivity of 160 S/cm, leading to a power factor of 19 mW/m-K2.116,117 In addition to traditional conducting polymers, organometallic polymers have also been studied. In 2012, Sun et al. reported better performing materials, named poly(M-ett), shown in Figure 6.31 Among them, poly[Kx(Ni-ett)]s (ett = 1,1,2,2-ethenetetrathiolate), exhibited a power factor of 135 mW/m-K2. A higher performance was obtained by the same group in 2016 when the organometallic polymers were prepared as thin films. The reported ZT reaches 0.32 at 400 K for the poly(Ni-ett) complex shown in Figure 7.118 Side substituents and counter ions have also been found to greatly affect the TE properties of polymers. Shinohara et al. investigated the effect of the length of side substituents of polymers with thiophene backbones.119 Polymers such as polythiophene, poly(3-hexylthiophene), poly(3-octylthiophene), and poly(3-dodecylthiophene) were synthesized, and the power factor was found to decrease with an increase in the length of the side substituents.120 Mai et al. compared the TE properties of a series of conjugated polyelectrolytes (CPEs) with different counter ions

10

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 5. Chemical Structure of Poly(2,7-Carbazole) and Polyindolocarbazole Derivatives The poly(2,7-carbazole) (PC) derivatives show a high electrical conductivity up to 160 S/cm and a relatively high Seebeck coefficient up to 34 mV/K, leading to a maximum power factor of 19 mW/m-K 2 . Adapted from Le´ vesque et al. 111 and Aı¨ ch et al. 116

and different lengths of alkyl chains (Table 2).120 Polymers with smaller counter ions and shorter side chains had a higher power factor. Smaller counter ions and shorter side substituents exhibit higher doping ability and form more ordered structures in polymer films, leading to higher electrical conductivity.120 Great progress has been made in exploring the relationship between polymer structures and TE properties. However, we still lack principles to guide us in the design of suitable organic molecules with high TE performance. Development of TE Materials Based on Small Organic Molecules Small organic molecules are also good candidates for developing organic TE materials. The TE properties of many crystals formed by small molecules have been studied in the past decades. Pentacene crystals showed a Seebeck coefficient of 265 mV/K.121 (BEDTTTF)Cu2Br4122 and TTF-Ni(mnt)2 (mnt = maleonitriledithiolato)123 showed high Seebeck coefficients of
850 mV/K and
655 mV/K, respectively. (Tetrathiotetracene)-triiodide (TTT2-I3) can have a high electrical conductivity over 1,000 S/cm.124 Despite the improvement in the Seebeck coefficient and electrical conductivity, the power factor of small molecules is typically less than 100 mW/m-K2. For example, tetrathiafulvalene:tetracyanoquinodimethane (TTF:TCNQ) has an electrical conductivity of 500 S/cm and a negative Seebeck coefficient of
28 mV/K, resulting in a power factor of 39 mW/m-K2.125 Only a few small molecules showed high power factors.126 Nevertheless, small molecules are still very attractive because some can behave as stable n-type materials in air, while most polymers are p-type materials. In addition, molecular orientations can be readily controlled,127 which can help clarify the relationship between the arrangement of organic molecules and the TE properties. A few groups have compared the TE properties of amorphous and crystalline materials made of small molecules. Generally, amorphous materials have poorer TE performance than crystals due to their low electrical conductivities.128

Joule 3, 1–28, January 16, 2019

11

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 6. Illustration of the Synthesis Process of Poly(M-ett) Chemical structure of poly(M-ett), where M could be copper or nickel and A could be sodium, potassium, tetrabutylammonium, etc. 31 These organometallic polymers show good TE performance; in particular, poly[K x (Ni-ett)]s films reveal n-type TE performance with a high ZT of 0.32 at 400 K. Adapted with permission from Sun et al. 31 Copyright 2016, John Wiley and Sons.

Single-molecule junctions have been built with conjugated organic molecules to experimentally and theoretically study the TE transport mechanism in organic materials.129–135 Yee et al. reported the first negative Seebeck coefficient for single-molecule junctions, with a value of up to
33 mV/K (Figure 8A).131 TE measurements of fullerene molecules trapped between a series of metallic electrodes, such as Pt, Au, and Ag, were performed. Widawsky et al. reported the TE measurements of amine-terminated molecules and pyridine-terminated molecules.129 Notably, a positive Seebeck coefficient was observed for amine-Au junctions, and a negative Seebeck coefficient was observed for pyridine-Au junctions. Chang et al. reported the Seebeck coefficients for a series of newly synthesized thiophene derivatives with different lengths of alkyl chains and numbers of thiophene rings (Figure 8B).134 A higher Seebeck coefficient and conductance were obtained when the HOMO energy was closer to the Fermi level (EF) of Au. The Seebeck coefficient decreased with longer alkyl chains and increased with more thiophene rings. Thus, the TE performance was improved when the distribution of carriers was narrow and the carrier mobility in the direction of the applied electric field was high.64 Interestingly, Widawsky et. al. found that the Seebeck coefficients of pyridine-Au linked junctions are negative and of amine-Au linked junctions are positive,129 because pyridine-Au junctions are LUMO-conducting junctions while amine-Au junctions are HOMO-conducting junctions according to theoretical calculations. In summary, great progress has been made in exploring high-TE-performance polymers and small molecules. Various approaches have been studied to maximize the TE performance, such as optimization of the carrier concentrations, raising the carrier mobility, preparing highly oriented molecular structures, and synthesizing highly conductive organic molecules. Currently, a popular strategy is to pursue high electrical conductivity first and then adjust the Seebeck coefficient and thermal conductivity. Based on the knowledge gained about electrical conductivities obtained from organic electronics, several different methods have been employed to tune the band gap of organic materials to increase the electrical conductivity, such as the donoracceptor approach, the addition of electron-withdrawing units, the use of fused heterocycles to increase backbone planarity and extend delocalization, and twodimensional conjugation.136 While pursuing high TE performance, the cost, processability, and stability of materials should also be taken into account to accelerate the practical application of organic thermoelectrics. Polymer Nanocomposites Polymer nanocomposites can possess polymer characteristics such as low thermal conductivity, solution-based processability, and mechanical flexibility. Meanwhile,

12

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 7. Electrical Properties of Poly(Ni-ett) Films (A and B) Electrical conductivity s (A) and Seebeck coefficient S (B) versus temperature.118 The electrical conductivity and the Seebeck coefficient increase almost linearly over the measured temperature range. Adapted with permission from Sun et al.118 Copyright 2016, John Wiley and Sons.

nanofillers can control carrier transport to provide an alternative way of optimizing the tradeoff between the electrical and thermal properties. The electrical conductivity of polymers is typically increased by increasing the carrier concentration, but this method suppresses the Seebeck coefficient. Introducing nanofillers with high carrier mobility into a polymer matrix could improve the electrical conductivity of polymers without considerably affecting the Seebeck coefficient, subsequently leading to a high power factor.42,137 It is possible to maintain the low thermal conductivity in polymer composites because heat transport is typically limited by the polymers. Therefore, the composite approach could be a very promising strategy to achieve high TE performance in organic materials. Nanocomposites with Carbon Nanotubes and Polymers Carbon nanotubes (CNTs) are one of the most widely studied nanofillers due to their excellent charge transport properties.138,139 Decoupling of Electrical Conductivity and Thermal Conductivity. The addition of a small amount of CNTs to polymers can induce a large enhancement in the electrical conductivity while maintaining polymer-like low thermal conductivity. Yu et al. reported the TE performance of a series of segregated-network CNT-polymer nanocomposites with different CNT concentrations (Figure 9).22 CNT fillers were introduced into a non-conducting poly(vinyl acetate) (PVAc) emulsion matrix. The electrical conductivity was dramatically enhanced with negligible changes in the low thermal conductivity of the polymer matrix. At a CNT concentration of 20%, the polymer nanocomposites exhibited an electrical conductivity of 48 S/cm with a low thermal conductivity of 0.34 W/m-K.22 The electrical conductivity of the nanocomposites is lower than that of the pure CNT since the filler used here is non-conducting PVAc. The electrical conductivity can be explained with a power law as a function of the conductive filler fraction140: s = s0 jVCNT
V  j ; p

(Equation 7)

where s0 is a proportionality constant related to the intrinsic conductivity of CNT, VCNT is the volume fraction of CNT, and V* is the critical volume fraction of CNT associated with the percolation threshold. When p = 3.37 and s0 = 1.09 3 106 S/m, the experimental results fits well with the theoretical calculation as shown in Figure 9B. The obtained coefficient of determination is R2 = 0.9994. Despite the large increase in electrical conductivity, S does not change much with increasing CNT concentration which is in the range of 40–50 mV/K. This value is close to the S of a metallic

Joule 3, 1–28, January 16, 2019

13

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Table 2. Summary of TE Properties of CPEs with Different Counter Ions and Lengths of Alkyl Chains

s (S/cm)

S (mV/K)

sS2 (mW/m-K2)

k (W/m-K)

0.16 G 0.005

165 G 12

0.44

0.26

CPE-K

0.024 G 0.001

230 G 10

0.13

0.23

CPE-C4-TBA

–

CPE-Na

CPE-C3-Na CPE-C3-K

–

–

0.22

0.22 G 0.02

195 G 5

0.84

–

0.048 G 0.004

200 G 18

0.19

0.27

CPEs with smaller counter ions and shorter side chains exhibit better TE performance. Adapted with permission from Mai et al.120 Copyright 2014, American Chemical Society.

CNT141 because S does not depend on filler dimensions and is strongly affected by more electrically conductive paths. Further improvement in the electrical conductivity was reported by Yu’s group when replacing the electrically insulating PVAc matrix with conducting PEDOT:PSS.23,32 The electrical conductivity was increased from 48 S/cm for the PVAc-CNT nanocomposite to over 1 3 103 S/cm. The highest electrical conductivity of the nanocomposite is comparable with that of the optimized CNT-only samples reported in previous work. For example, the electrical conductivity of the films with similar CNTs was measured to be 1 3 102 to 1 3 103 S/cm. The large variation in electrical conductivity is due to the nanotube dispersion with different solvents and surfactant, which affects the energy barrier and the number of tube-tube junctions for charge transport.142 The high electrical conductivity of the nanocomposite is attributed to the fact that the spaces existing in the CNT network have been filled by the conducting polymer. The Seebeck coefficient of the PEDOT:PSS-CNT nanocomposite was 30 mV/K. The maximum power factor was 160 mW/m-K2 at a CNT concentration of 60 wt%,32 leading to a great enhancement over that of previously reported polymer composites.143,144 The proposed charge transport mechanism, as shown in Figure 10A, is that the PEDOT:PSS particles decorated on the surface of the CNTs are electrically conducting and deterred heat transfer by the dissimilar bonding and vibrational spectra between the CNTs and PEDOT:PSS.32 Therefore, the thermal conductivity remained comparable with that of polymers. The decoupled electrical and thermal transport properties significantly increased the power factor.32 Decoupling of Electrical Conductivity and Seebeck Coefficient. An inversely proportional relationship between the electrical conductivity and Seebeck coefficient is one of the major hurdles in improving the TE performance. There have been efforts to decouple the two parameters. Wang et al. reported the simultaneous improvement of the electrical conductivity and the Seebeck coefficient in polyaniline (PANI)-CNT nanocomposites.42 By adding CNTs into PANI, the electrical conductivity was increased to 610 S/cm, and the Seebeck coefficient was increased to 61 mV/K (Figure 11A). The maximum power factor was measured to be 220 mW/m-K2 (Figure 11B). The illustration of the composite is shown in Figure 11C. The simultaneous increase in the electrical conductivity and Seebeck

14

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 8. Testing the TE Properties of Single Molecules (A) Schematic of the experimental setup for measuring the conductance and Seebeck coefficient with a modified scanning tunneling microscopy (STM) break junction. 131 (B) Chemical structures of organic small molecules used in single-metal junctions. 129,132,134 Adapted with permission from Yee et al. 131 Copyright 2011, American Chemical Society.

coefficient was further studied with Hall measurements to obtain the carrier mobility and concentration. With the addition of CNTs, the carrier mobility of PANI increased from 0.15 to 7.3 cm2/V-s (50-fold improvement), while the carrier concentration decreased from 2.1 3 1021 to 5.6 3 1020 cm
3 (4-fold reduction) (Figure 11D).42 The improvement in the carrier mobility could be attributed to the band alignment, which attracts hole carriers to the CNTs, the mobility of which is much higher than that of PANI. The Fermi levels of the CNT-only and PANI-only samples were found to be 5.01 and 4.81 eV, respectively. After CNTs were added to PANI, band bending in the HOMO and LUMO of PANI is thought to occur, equilibrating the Fermi levels. In this band alignment, the hole carriers in PANI are likely to be attracted to the Fermi level of the CNTs. The large increase in mobility increased the electrical conductivity, and the reduction of the carrier concentration increased the Seebeck coefficient, raising the power factor to 220 mW/m-K2, which is more than two orders of magnitude higher than those of the PANI-only samples and the previously reported PANI composites. The optimized power factor of the PANI-CNT composite is higher than that of the CNT-only film. For example, the PANI-single/double-wall CNTs (S/DWCNTs) can reach a power factor of 34 mW/m-K2, which is higher than that of

Joule 3, 1–28, January 16, 2019

15

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 9. TE Properties of PVAc-CNT Nanocomposites (A) Schematic of CNTs suspended in an aqueous emulsion. (B) Electrical conductivity and Seebeck coefficient of polymer composites with different CNT concentrations at room temperature. The electrical conductivity increases with increasing CNT concentration, while the Seebeck coefficient does not change much with increasing CNT concentration. (C) Thermal conductivities of CNT-polymer composites at room temperature as the CNT concentrations is varied. 22 Adapted with permission from Yu et al.22 Copyright 2008, American Chemical Society.

the S/DWCNT-only film of 10 mW/m-K2.42 Similar decoupling behavior between the electrical conductivity and the Seebeck coefficient has been observed in other polymer-CNT composites.42,145,146 High-Performance TE Composites by Decoupling the TE Parameters. An approach for balancing the three TE parameters was recently reported by Wang et al.28 A record ZT of 0.5 at room temperature was reported by tuning the carrier concentration and introducing non-percolated CNT networks to decouple the electrical conductivity and thermal conductivity in PEDOT-CNT nanocomposites. The nonpercolated structure was identified by atomic force microscopy (AFM) and tunneling AFM (TUNA) images.28 A large power factor of 1,050 mW/m-K2 (Figure 12B) was obtained by introducing CNTs, the carrier mobility of which is very high and can raise the electrical conductivity of the composite. The high power factor was further investigated by Hall measurements to obtain the carrier mobility and concentration. Similar to previous work,42 the electrical conductivity was enhanced over that of the polymer-only samples because the addition of CNTs increased the mobility to 1 cm2/V-s after an 80-s CNT spray, and the mobility was further increased to 2.7 cm2/V-s after TDAE treatment. The mobility enhancement is probably due to the conformational change of PEDOT chains. Similar results have been observed for organic solvents such as ethylene glycol,107 dimethyl sulfoxide,23 and methanol.147 Meanwhile, TDAE treatment decreased the carrier concentration to 3.5 3 1019 cm
3, resulting in an increase in the Seebeck coefficient owing to the inversely

16

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 10. Electrical Properties of PEDOT:PSS-CNT Nanocomposites (A) Schematic illustration of the junctions of CNTs coated by PEDOT:PSS particles. The presence of the junction is thought to deter heat transport rather than create favorable pathways for electrons. (B) Electrical conductivity and Seebeck coefficient of the composites at different CNT concentrations. The inset shows the thermoelectric power factor. 32 A maximum power factor of 100 mW/m-K 2 is observed at a CNT concentration of 60 wt%. Adapted with permission from Yu et al. 32 Copyright 2011, American Chemical Society.

proportional relationship, S  1/n. The square term in the power factor (S2s) boosted the power factor to 1,050 mW/m-K2. The thermal conductivity of the composite was measured to be 0.67 W/m-K, which is due to the thermally resistive CNT junctions intervened by PEDOT via in situ polymerization. The measurements for all three properties were performed in the in-plane direction. A large ZT of 0.5 was obtained at room temperature, which is considered the highest ZT value among organic TE materials.148 A similar approach was employed for composites containing CNT-PANI-CNT junctions.137 The power factor of nanocomposites made of a PANI emeraldine base and CNTs was increased 6-fold after HCl doping because the electrical conductivity increased and the Seebeck coefficient remained high. PANI at the CNT junctions had a high Seebeck coefficient because it was difficult to be doped due to the strong p-p interactions between the CNTs and PANI. Increasing the carrier concentration by doping increased the electrical conductivity and increased the power factor effectively.137 This approach could be applied to other polymer-nanofiller systems. Comparing the CNT-PANI-CNT composites with the above PEDOT-CNT composites, we can see that the non-percolated structure should be the key for high-performance TE materials. Nanocomposites with Inorganic Materials and Polymers Inorganic materials with high TE performances could be candidates as fillers for synthesizing polymer-inorganic nanocomposites. Various inorganic nanostructures have been introduced into polymer matrices to improve the TE performance of composites. The obtained composites are expected to be mechanically flexible to make them more practical.149 Moreover, nanostructures could improve the TE properties due to the quantum confinement effect and the change in the density of states.4 Polymer-Nanoparticle Composites. Inorganic TE materials such as Bi, Te, and Bi2Te3 nanostructures have been investigated as fillers.30,67,150–152 PEDOT-BiTe3 nanocomposites showed a power factor of 131 mW/m-K2 at a polymer concentration of 10 vol%.149 An estimated ZT of 0.08 was obtained with an assumption of 0.558 W/m-K according to the WFL. Small polymer fractions were necessary to

Joule 3, 1–28, January 16, 2019

17

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 11. Characterization of PANI-CNT Composites (A and B) Electrical conductivity and Seebeck coefficient (A) and power factor (B) of a PANI-CNT nanocomposite with various amounts of double-walled CNTs. The electrical conductivity and Seebeck coefficient increase simultaneously as the CNT concentration is increased. (C) Schematic illustration of the structure of the PANI-CNT nanocomposite. (D) Carrier mobility and carrier concentration of the composite at different CNT concentrations. 42 The Hall measurements indicate that the large increase in mobility increases the electrical conductivity despite the reduced carrier concentration, which enlarges the Seebeck coefficient. Adapted with permission from Wang et al. 42 Copyright 2015, American Chemical Society.

obtain better TE properties, and at least 20 vol% PEDOT was required to realize mechanical flexibility. Polymer-Nanowire/Nanorod Composites. Polymer-nanowire/nanorod-based composites have advantageous mechanical flexibility due to their high aspect ratios compared with those of polymer-nanoparticle composites.43 Segalman et al. reported that PEODT-BiTe3 nanorod composites showed a high ZT of 0.1 at room temperature. The high ZT value is due to the low thermal conductivity of 0.2 W/m-K for the composite, which was thought to be caused by phonon scattering by the nanowires/nanorods.43,152–154 The relationship between the length of the nanostructures and TE performance was further investigated,43,154 but it remains unclear because an increase in the nanowire length is always accompanied by an increase in the diameter. The diameter of the nanostructures may considerably alter the TE properties, so further investigation is needed to understand the size-property relationship of the composites (Figure 13). The organic-inorganic interface was utilized to selectively filter transport carriers to improve the TE performance of the composites.155,156 The TE properties of P3HT-Bi2Te3 composites152 reported by Lin et al. showed a higher Seebeck coefficient at a higher electrical conductivity and a lower Seebeck coefficient at a lower electrical conductivity. When P3HT is heavily doped (s > 2 S/cm), an interfacial energy barrier of 0.1 eV forms at the polymer-inorganic interface, which is thought to scatter low-energy carriers, leading to a high Seebeck coefficient. When P3HT is lightly doped (s < 2 S/cm), the polymerinorganic interface blocks charge carrier transport.152

18

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 12. Characterization of the Electrical Properties before (Hollow Symbols) and after (Filled Symbols) TDAE Treatment (A and B) Electrical conductivity and Seebeck coefficient (A) and power factor (B) as a function of CNT concentration for concentrations of 4.5%, 6.1%, 7.9%, 10.7%, and 15.8% (corresponding to 20, 40, 60, 80, 100 s CNT spray, respectively). 28 A maximum power factor of 1,050 mW/m-K 2 is observed with a CNT spray time of 80 s. Adapted with permission from Wang et al.28 Copyright 2015, John Wiley and Sons.

Extreme caution should be exercised when preparing polymer-inorganic nanocomposites because the nanostructured inorganic semiconductors are very easy to oxidize due to their high surface-to-volume ratio.149,157 Post-treatment methods could be utilized to improve the TE performance. Zhang et al. reported that HCl solution could remove the oxidation layer,56 resulting in a 5-fold enhancement in the power factor of PEDOT-Bi2Te3 nanocomposites. The enhancement is due to the increase in electrical conductivity after removal of the oxidation layer.56 Nanocomposites with Graphene/Graphene Oxide and Polymers Graphene and graphene oxide (GO) are also attractive fillers because of their high carrier mobility. Zhao et al. reported TE properties of 7.5 S/cm and 28 mV/K for PANI/GO nanocomposites.158 The carrier mobility was dramatically increased from 0.6 to 2.25 cm2/V-s at a GO concentration of 10 wt%. The improvement in the carrier mobility is due to the intrinsically high mobility of GO and the strong molecular interactions between GO and PANI. A better TE performance was obtained with graphene instead of GO due to the higher electrical conductivity of graphene compared with that of GO. The electrical conductivity can be increased to 58.89 S/cm without significantly suppressing the Seebeck coefficient.159 Graphene is a good candidate to be introduced into a polymer matrix due to high carrier mobility. The charge transport along the in-plane direction is excellent, and the charge transfer between layers is limited. Fabrication of Organic Thermoelectric Generators TEGs are composed of simple two-leg structures and are advantageous for fabricating low-cost and disposable energy-conversion devices. In particular, organic TE devices are ideal for this type of application. Key Parameters of Thermoelectric Generators The performance of a TEG can be determined by the energy-conversion efficiency (h)160: pffiffiffiffiffiffiffiffiffiffiffiffiffi 1 + ZT
1 Thot
Tcold pffiffiffiffiffiffiffiffiffiffiffiffiffi T ; h= (Equation 8) Thot 1 + ZT + Tcold hot

where Thot and Tcold represent the absolute temperature of the hot side and the cold side, respectively. The efficiency as a function of temperature is shown in Figure 14. Note that the above equation is for an ideal TEG. In reality, the efficiency is affected

Joule 3, 1–28, January 16, 2019

19

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 13. Electrical Properties of PEDOT-BiTe3 Nanorod Composites (A) AFM images of nanowire films. a 1 : L = 595 G 81 nm; a 2 : L = 867 G 101 nm; a 3 : L = 970 G 134 nm. (B) Relationship between the nanowire length and diameter and the thermoelectric properties. 154 AFM images of several nanowires were used to determine the Gaussian distribution in length and diameter, and the standard deviation in length is represented in the horizontal error bars. Adapted with permission from Yee et al. 154 Copyright 2013, The Royal Society of Chemistry.

by various factors, including joule heating, heat loss, the electrical and thermal contact resistances, and so forth.161,162 LeBlanc et al. reported that the efficiency of actual systems could be 30%–60% lower than the calculated maximum efficiency for water heater, automotive exhaust, and industrial furnace applications.163 Therefore, the calculated h is often considered to be the maximum efficiency (hmax) of a TEG.164 The power output is another important parameter that has been used as one of the primary factors for evaluating TEGs since the power generated by a TEG is valuable to the end user.164,165 The maximum power output of a TEG (Pmax) can be obtained when the resistance of an external load (RL) is close to the internal resistance of the device (RTEG), RL z RTEG. Then, the output voltage (VL) is half of the open-circuit voltage (VOC), and the maximum power output can be written as35 Pmax =

2 VL2 VOC S 2 ðThot
Tcold Þ2 = = : RL 4RL 4RL

(Equation 9)

In general, the thermal power input at the hot side is divided into three parts: the heat conducted through the device, the energy consumed by joule heating due to the internal resistance, and the TE power output. The power output increases as the internal resistance is reduced (e.g., shorter TE legs) because the joule heating is reduced. On the other hand, longer legs are advantageous in improving efficiency. Therefore, TEG modules are typically designed to operate under optimal conditions with a tradeoff between these parameters. The power density and efficiency are two important parameters commonly considered when evaluating the performance of a TEG. However, working environment, space, and mass should also be considered for practical applications because these often limit the ability to integrate TEGs into existing systems and architectures.164,166 Organic TEG Fabrication Techniques Printing Techniques. Printing is one of the main advantages of organic TEGs over inorganic TEGs since solution processes can be used for polymers at room temperature. Several printing techniques have been investigated in recent years.

20

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 14. TE Devices Characterization (A) Illustration of a thermoelectric module with p-type and n-type legs. (B) The conversion efficiency of the module as a function of temperature. 35 T is the temperature of the hot side, while the cold side is maintained at 293 K. 1, 2, 3, and 4 are the numbers of TE p-n modules used for fabricating TE devices. Adapted with permission from Bubnova and Crispin. 35 Copyright 2012, The Royal Society of Chemistry.

Screen-Printing Technique. Wusten et al. reported the performance of TEGs composed of polyvinylchloride (PVC)/graphite as a p-type TE material and PVC with TTF-TCNQ as an n-type TE material.167 A TE voltage of 120 mV/K was reported, but the combination of the insulating polymer resulted in a high internal resistance (R = 380 KU for a 6 3 6 cm2 area). Wei et al. screen-printed PEDOT:PSS (p-type) and silver paste (n-type) on paper.168 The power output of the device can reach up to 50 mW. The device showed sufficient power to illuminate light-emitting diodes (LEDs) for 100 hr without any encapsulation at a temperature gradient of 100 K. The screen-printing technique is simple and easy to use, which is promising for fabricating organic TE modules.169 Ink-Jet Printing Technique. Ink-jet printing was used by Crispin et al. to fabricate TEG modules from PEDOT:Tos and TTF-TCNQ.24 The p-type ink is an EDOT monomer with a polymerization catalyst, and the n-type ink is TTF-TCNQ blended with PVC in toluene (Figure 15A). A 54-leg TEG was created with an area of 25 mm 3 25 mm and a leg length of 30 mm. The maximum power output was 0.128 mW at a temperature gradient of 10 K. The power density of the designed TEG was approximately 0.27 mW/cm2 (Figure 15B). Roll-to-Roll Printing Technique. Søndergaard et al. demonstrated the scalability of organic TE materials with a roll-to-roll (R2R) printing technique (Figure 16).170 PEDOT:PSS was used as the p-type TE material, and silver was used as the n-type material. The TEG was printed on a 60-mm polyethylene terephthalate substrate and was composed of 18,000 serially connected silver/PEDOT:PSS junctions, as shown in Figure 16. Although the power output of this device was estimated to be low, this work presented the potential for the fast processing of organic TE devices at low cost. Pellets and Vacuum Filtration or Spraying. Making organic TEG pellets is an alternative way to fabricate TE devices, since organic TE materials are not always printable. For example, some materials have very low solubility in common solvents. This method was used in the earlier stage of TE research,123 and the TE performance of pellets is typically lower than that of films because of the low electrical conductivity due to their poor crystallinity. The pellet approach is effective for organometallic polymers. Impressive results were reported by Zhu et al.31 TEG pellets of poly(metal 1,1,2,2-ethenetetrathiolate)s with a high

Joule 3, 1–28, January 16, 2019

21

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 15. Power Output of Organic TEG Modules from PEDOT:Tos and TTF-TCNQ (A) Illustration of the fabrication process of an all-organic TEG through photolithography patterning and ink-jet printing. Thermogenerator fabrication process steps: cavities are formed above the bottom Au electrodes using the photoresist SU8; the cavities are filled with a solution of EDOT and TTF-TCNQ to form the p-legs and n-legs, respectively; the TEG is completed by depositing the top Au contacts. (B) Maximum power output per area of the TEG with packing densities of 0.47 (solid line) and 0.94 (dashed line). 24 The inset figure represents a thermocouple formed by one PEDOT-Tos and one TTF-TCNQ leg. Adapted with permission from Bubnova et al. 24 Copyright 2011, Springer Nature.

power density have been fabricated.31 The electrical conductivity of the pellets can reach 44 S/cm with a Seebeck coefficient of
122 mV/K. With 35 p-n junctions, the TEG provided a TE voltage of 0.26 V at a temperature gradient of 80 K. The power density of the TEG reaches 2.8 mW/cm2 with a temperature gradient of 30 K. They claimed that the high performance of the TEG could be attributed to the high ZT of 0.25. For composite materials, vacuum filtration has also been used to fabricate thin films. For example, using different types of polymers, both p- and n-type polymer composites have been made, and alternating p/n modules were fabricated to generate power from low-grade heat and operate sensors.171,172 Conclusion and Perspective Significant progress has been made in this field, with various novel ideas for improving the TE performance of organic materials in recent years. For example, the electrochemical method was used to precisely control the oxidation level of conducting polymer thin films, leading to a large power factor of 1,270 mW/m-K2.26 The reducing agent TDAE was used to control the carrier concentration of PEDOT with short-chain counter ions, resulting in a high power factor of 324 mW/m-K2.24 Combining the decoupling method and the carrier concentration strategy in PEDOT-CNT composites realized a record ZT value for organic TE materials of 0.5.28 The TE performances of organic materials are comparable with those of many inorganic TE materials at room temperature. However, further improvement is still necessary to reach the commercial goal of ZT R 2 for organic materials. Current trends to improve the TE performance of organic materials have focused on the decoupling of the three parameters s, S, and k. One notable strategy is improving s without considerably affecting S by solvent treatment. The properties can also be decoupled by improving the crystallinity of polymers.101,102,104,105 High-crystallinity materials, such as nanocrystals, often have high charge mobility, resulting in an enhancement of s without a reduction of S.88 As the thermal conductivity decreases, PEDOT nanowires are predicted to have a ZT as high as 15.2.47 However, it is very challenging to obtain ideally packed molecules. There have

22

Joule 3, 1–28, January 16, 2019

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Figure 16. Film-Based TEG Employing Only One Type of Thermoelectric Material Processed by R2R Printing As illustrated above, the prints are aligned in a manner in which each junction is serially connected to the two adjacent ones, creating an infinite connection along the web. Adapted with permission from Søndergaard et al.170 Copyright 2013, John Wiley and Sons.

been systematic studies on the influence of the structure of conjugated polymer backbones, length or conformation of the side chains, polarity, molecular weight of the polymers, etc., on the TE properties. An alternative way to improve the TE performance of organic materials is to decouple s and k by forming nanocomposites. Highly conducting nanofillers could improve s by raising the carrier mobility, while a low k can be obtained due to the intrinsically low k of organic materials. The introduced nanofillers may scatter phonons, leading to a low k of the nanocomposites. High-TE-performance nanocomposites made of PEDOT and CNT with a ZT of 0.5 at room temperature were reported by Wang et al.28 Non-percolated nanofiller networks were used in the sample preparation. Notably, non-percolated networks are essential, otherwise the TE properties may follow those of the nanofillers rather than having synergistic effects from the matrix and fillers.28,173 This method could be extended to other nanofillers and polymers. Core-shell structures, whose shells are polymers and cores are nanofillers, could be a promising strategy to pursue high-TE-performance organic materials.30,154 In summary, organic TE materials are very promising in heat-to-electricity conversion near room temperature for low-cost and disposable applications. Attempts to fabricate organic TEG demonstrated that it is feasible to power small devices such as sensors, LEDs, batteries, and radiofrequency identifier tags.171,174 It is also possible to fabricate large-scale TE materials using R2R processes. Continuous efforts in this area could bring this green sustainable energy technology closer to commercial applications.

ACKNOWLEDGMENTS H.W. acknowledges financial support from the National Natural Science Foundation of China (grant number 51876151) and the start-up funding from Xi’an Jiaotong University, China (grant number PY3A010, QY1J003). C.Y. acknowledges financial support from the Creative Materials Discovery Program through the National

Joule 3, 1–28, January 16, 2019

23

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

Research Foundation of Korea funded by the Ministry of Science and ICT (2018M3D1A1057844).

REFERENCES 1. Seebeck, T.J. (1822). Ueber den Magnetismus der galvanischen Kette (Deutsche Akademie der Wissenschaften zu Berlin). 2. Peltier, J.C. (1834). Nouvelles experiences sur la caloricite des courants electriques. Ann. Chim. Phys. 56, 371–386. 3. Thomson, W. (1857). 4. On a mechanical theory of thermo-electric currents. Proc. R. Soc. Edinburgh 3, 91–98. 4. Dresselhaus, M.S., Chen, G., Tang, M.Y., Yang, R.G., Lee, H., Wang, D.Z., Ren, Z.F., Fleurial, J.P., and Gogna, P. (2007). New directions for low-dimensional thermoelectric materials. Adv. Mater. 19, 1043–1053. 5. Chen, G., Dresselhaus, M.S., Dresselhaus, G., Fleurial, J.P., and Caillat, T. (2003). Recent developments in thermoelectric materials. Int. Mater. Rev. 48, 45–66. 6. Zhao, L.-D., Lo, S.-H., Zhang, Y., Sun, H., Tan, G., Uher, C., Wolverton, C., Dravid, V.P., and Kanatzidis, M.G. (2014). Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 508, 373–377. 7. Biswas, K., He, J., Blum, I.D., Wu, C.-I., Hogan, T.P., Seidman, D.N., Dravid, V.P., and Kanatzidis, M.G. (2012). High-performance bulk thermoelectrics with all-scale hierarchical architectures. Nature 489, 414–418. 8. Tan, G., Shi, F., Hao, S., Zhao, L.-D., Chi, H., Zhang, X., Uher, C., Wolverton, C., Dravid, V.P., and Kanatzidis, M.G. (2016). Nonequilibrium processing leads to record high thermoelectric figure of merit in PbTe-SrTe. Nat. Commun. 7, 12167. 9. Venkatasubramanian, R., Siivola, E., Colpitts, T., and O’Quinn, B. (2001). Thin-film thermoelectric devices with high roomtemperature figures of merit. Nature 413, 597–602. 10. Heremans, J.P., Thrush, C.M., and Morelli, D.T. (2001). Geometrical magnetothermopower in semiconductors. Phys. Rev. Lett. 86, 2098–2101. 11. Lampinen, M.J. (1991). Thermodynamic analysis of thermoelectric generator. J. Appl. Phys. 69, 4318–4323. 12. Heremans, J.P., Jovovic, V., Toberer, E.S., Saramat, A., Kurosaki, K., Charoenphakdee, A., Yamanaka, S., and Snyder, G.J. (2008). Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 321, 554–577. 13. Ohta, H., Kim, S., Mune, Y., Mizoguchi, T., Nomura, K., Ohta, S., Nomura, T., Nakanishi, Y., Ikuhara, Y., Hirano, M., et al. (2007). Giant thermoelectric Seebeck coefficient of a twodimensional electron gas in SrTiO3. Nat. Mater. 6, 129–134. 14. Reddy, P., Jang, S.-Y., Segalman, R.A., and Majumdar, A. (2007). Thermoelectricity in molecular junctions. Science 315, 1568–1571.

24

Joule 3, 1–28, January 16, 2019

15. Coleman, J.N., Lotya, M., O’Neill, A., Bergin, S.D., King, P.J., Khan, U., Young, K., Gaucher, A., De, S., Smith, R.J., et al. (2011). Twodimensional nanosheets produced by liquid exfoliation of layered materials. Science 331, 568–571. 16. Shi, L., Li, D., Yu, C., Jang, W., Kim, D., Yao, Z., Kim, P., and Majumdar, A. (2003). Measuring thermal and thermoelectric properties of one-dimensional nanostructures using a microfabricated device. J. Heat Transfer 125, 881–888. 17. Vashaee, D., and Shakouri, A. (2004). Improved thermoelectric power factor in metal-based superlattices. Phys. Rev. Lett. 92, 106103. 18. Boukai, A.I., Bunimovich, Y., Tahir-Kheli, J., Yu, J.-K., Goddard, W.A., III, and Heath, J.R. (2008). Silicon nanowires as efficient thermoelectric materials. Nature 451, 168–171. 19. Tang, J., Wang, H.-T., Lee, D.H., Fardy, M., Huo, Z., Russell, T.P., and Yang, P. (2010). Holey silicon as an efficient thermoelectric material. Nano Lett. 10, 4279–4283. 20. Zhang, Q., Sun, Y., Xu, W., and Zhu, D. (2014). What to expect from conducting polymers on the playground of thermoelectricity: lessons learned from four high-mobility polymeric semiconductors. Macromolecules 47, 609–615. 21. Chen, Y., Zhao, Y., and Liang, Z. (2015). Solution processed organic thermoelectrics: towards flexible thermoelectric modules. Energy Environ. Sci. 8, 401–422. 22. Yu, C., Kim, Y.S., Kim, D., and Grunlan, J.C. (2008). Thermoelectric behavior of segregated-network polymer nanocomposites. Nano Lett. 8, 4428–4432. 23. Kim, D., Kim, Y., Choi, K., Grunlan, J.C., and Yu, C. (2010). Improved thermoelectric behavior of nanotube-filled polymer composites with poly(3,4ethylenedioxythiophene) poly(styrenesulfonate). ACS Nano 4, 513–523. 24. Bubnova, O., Khan, Z.U., Malti, A., Braun, S., Fahlman, M., Berggren, M., and Crispin, X. (2011). Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene). Nat. Mater. 10, 429–433. 25. Kim, G.H., Shao, L., Zhang, K., and Pipe, K.P. (2013). Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. Nat. Mater. 12, 719–723. 26. Park, T., Park, C., Kim, B., Shin, H., and Kim, E. (2013). Flexible PEDOT electrodes with large thermoelectric power factors to generate electricity by the touch of fingertips. Energy Environ. Sci. 6, 788–792. 27. Yao, Q., Wang, Q., Wang, L., and Chen, L. (2014). Abnormally enhanced thermoelectric transport properties of SWNT/PANI hybrid

films by the strengthened PANI molecular ordering. Energy Environ. Sci. 7, 3801–3807. 28. Wang, H., Hsu, J.-H., Yi, S.-I., Kim, S.L., Choi, K., Yang, G., and Yu, C. (2015). Thermally driven large N-type voltage responses from hybrids of carbon nanotubes and poly(3,4ethylenedioxythiophene) with Tetrakis(dimethylamino)ethylene. Adv. Mater. 27, 6855–6861. 29. Zhou, W., Fan, Q., Zhang, Q., Cai, L., Li, K., Gu, X., Yang, F., Zhang, N., Wang, Y., Liu, H., et al. (2017). High-performance and compactdesigned flexible thermoelectric modules enabled by a reticulate carbon nanotube architecture. Nat. Commun. 8, 14886. 30. See, K.C., Feser, J.P., Chen, C.E., Majumdar, A., Urban, J.J., and Segalman, R.A. (2010). Water-processable polymer-nanocrystal hybrids for thermoelectrics. Nano Lett. 10, 4664–4667. 31. Sun, Y., Sheng, P., Di, C., Jiao, F., Xu, W., Qiu, D., and Zhu, D. (2012). Organic Thermoelectric materials and devices based on p- and n-type poly(metal 1,1,2,2ethenetetrathiolate)s. Adv. Mater. 24, 932–937. 32. Yu, C., Choi, K., Yin, L., and Grunlan, J.C. (2011). Light-weight flexible carbon nanotube based organic composites with large thermoelectric power factors. ACS Nano 5, 7885–7892. 33. Cho, C., Wallace, K.L., Tzeng, P., Hsu, J.-H., Yu, C., and Grunlan, J.C. (2016). Outstanding low temperature thermoelectric power factor from completely organic thin films enabled by multidimensional conjugated nanomaterials. Adv. Energy Mater. 6, 1502168. 34. Cho, C., Stevens, B., Hsu, J.-H., Bureau, R., Hagen, D.A., Regev, O., Yu, C., and Grunlan, J.C. (2015). Completely organic multilayer thin film with thermoelectric power factor rivaling inorganic tellurides. Adv. Mater. 27, 2996– 3001. 35. Bubnova, O., and Crispin, X. (2012). Towards polymer-based organic thermoelectric generators. Energy Environ. Sci. 5, 9345–9362. 36. He, M., Qiu, F., and Lin, Z. (2013). Towards high-performance polymer-based thermoelectric materials. Energy Environ. Sci. 6, 1352–1361. 37. Zhang, Q., Sun, Y., Xu, W., and Zhu, D. (2014). Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently. Adv. Mater. 26, 6829–6851. 38. Poehler, T.O., and Katz, H.E. (2012). Prospects for polymer-based thermoelectrics: state of the art and theoretical analysis. Energy Environ. Sci. 5, 8110–8115. 39. Kroon, R., Mengistie, D.A., Kiefer, D., Hynynen, J., Ryan, J.D., Yu, L., and Muller, C. (2016). Thermoelectric plastics: from design to synthesis, processing and

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

structure-property relationships. Chem. Soc. Rev. 45, 6147–6164. 40. Russ, B., Glaudell, A., Urban, J.J., Chabinyc, M.L., and Segalman, R.A. (2016). Organic thermoelectric materials for energy harvesting and temperature control. Nat. Rev. Mater. 1, 16050. 41. Toshima, N. (2017). Recent progress of organic and hybrid thermoelectric materials. Synth. Met. 225, 3–21. 42. Wang, H., Yi, S.-I., Pu, X., and Yu, C. (2015). Simultaneously improving electrical conductivity and thermopower of polyaniline composites by utilizing carbon nanotubes as high mobility conduits. ACS Appl. Mater. Interfaces 7, 9589–9597. 43. Coates, N.E., Yee, S.K., McCulloch, B., See, K.C., Majumdar, A., Segalman, R.A., and Urban, J.J. (2013). Effect of interfacial properties on polymer-nanocrystal thermoelectric transport. Adv. Mater. 25, 1629–1633. 44. Franz, R., and Wiedemann, G. (1853). Ueber die Wa¨rme-Leitungsfa¨higkeit der Metalle. Ann. Phys. 15, 497–531. 45. Kittel, C. (2004). Introduction to Solid State Physics (Wiley). 46. Salamon, M.B., Bray, J.W., DePasquali, G., Craven, R.A., Stucky, G., and Schultz, A. (1975). Thermal conductivity of tetrathiafulvalenetetracyanoquinodimethane (TTF-TCNQ) near the metal-insulator transition. Phys. Rev. B 11, 619–622. 47. Wang, Y., Zhou, J., and Yang, R. (2011). Thermoelectric properties of molecular nanowires. J. Phys. Chem. C 115, 24418– 24428. 48. Liu, J., Wang, X., Li, D., Coates, N.E., Segalman, R.A., and Cahill, D.G. (2015). Thermal conductivity and elastic constants of PEDOT: PSS with high electrical conductivity. Macromolecules 48, 585–591. 49. Weathers, A., Khan, Z.U., Brooke, R., Evans, D., Pettes, M.T., Andreasen, J.W., Crispin, A., and Shi, L. (2015). Significant electronic thermal transport in the conducting polymer poly(3,4-ethylenedioxythiophene). Adv. Mater. 27, 2101–2106. 50. Pietralla, M. (1996). High thermal conductivity of polymers: possibility or dream? J. Comput. Aided Mol. Des. 3, 273–280. 51. Semaltianos, N.G., Koidis, C., Pitsalidis, C., Karagiannidis, P., Logothetidis, S., Perrie, W., Liu, D., Edwardson, S.P., Fearon, E., Potter, R.J., et al. (2011). Picosecond laser patterning of PEDOT: PSS thin films. Synth. Met. 161, 431–439. 52. Cagnolati, R., Fioretto, D., Ruggeri, G., and Socino, G. (1993). Elastic characterization of poly(3-n.decylpyrrole) thin films by Brillouin light scattering. Synth. Met. 60, 255–258. 53. Moses, D., and Denenstein, A. (1984). Experimental determination of the thermal conductivity of a conducting polymer: pure and heavily doped polyacetylene. Phys. Rev. B 30, 2090–2097.

54. Jin, J., Manoharan, M.P., Wang, Q., and Haque, M.A. (2009). In-plane thermal conductivity of nanoscale polyaniline thin films. Appl. Phys. Lett. 95, 033113.

69. Han, Z., and Fina, A. (2011). Thermal conductivity of carbon nanotubes and their polymer nanocomposites: a review. Prog. Polym. Sci. 36, 914–944.

55. Wei, Q., Uehara, C., Mukaida, M., Kirihara, K., and Ishida, T. (2016). Measurement of in-plane thermal conductivity in polymer films. AIP Adv. 6, 045315.

70. Alaghemandi, M., Gharib-Zahedi, M.R., Spohr, E., and Bo¨hm, M.C. (2012). Thermal conductivity of polyamide-6,6 in the vicinity of charged and uncharged graphene layers: a molecular dynamics analysis. J. Phys. Chem. C 116, 14115–14122.

56. Cutler, M., Leavy, J.F., and Fitzpatrick, R.L. (1964). Electronic transport in semimetallic cerium sulfide. Phys. Rev. 133, A1143–A1152. 57. Snyder, G.J., and Toberer, E.S. (2008). Complex thermoelectric materials. Nat. Mater. 7, 105–114. 58. Coropceanu, V., Cornil, J., da Silva Filho, D.A., Olivier, Y., Silbey, R., and Bre´das, J.-L. (2007). Charge transport in organic semiconductors. Chem. Rev. 107, 926–952. 59. Yamamoto, T., Maruyama, T., Zhou, Z.-H., Ito, T., Fukuda, T., Yoneda, Y., Begum, F., Ikeda, T., and Sasaki, S. (1994). p-conjugated poly(pyridine-2,5-diyl), poly(2,2’-bipyridine5,5’-diyl), and their alkyl derivatives. Preparation, linear structure, function as a ligand to form their transition metal complexes, catalytic reactions, n-type electrically conducting properties, optical properties, and alignment on substrates. J. Am. Chem. Soc. 116, 4832–4845.

71. Kamseu, E., Nait-Ali, B., Bignozzi, M.C., Leonelli, C., Rossignol, S., and Smith, D.S. (2012). Bulk composition and microstructure dependence of effective thermal conductivity of porous inorganic polymer cements. J. Eur. Ceram. Soc. 32, 1593–1603. 72. Huang, X., Iizuka, T., Jiang, P., Ohki, Y., and Tanaka, T. (2012). Role of interface on the thermal conductivity of highly filled dielectric epoxy/AlN composites. J. Phys. Chem. C 116, 13629–13629. 73. Yan, H., Sada, N., and Toshima, N. (2002). Thermal transporting properties of electrically conductive polyaniline films as organic thermoelectric materials. J. Therm. Anal. Calorim. 69, 881–887. 74. Park, Y.W. (1991). Structure and morphology: relation to thermopower properties of conductive polymers. Synth. Met. 45, 173–182.

60. Chien, J.C.W., Warakomski, J.M., Karasz, F.E., Chia, W.L., and Lillya, C.P. (1983). Homogeneous doping and semiconductorto-‘‘metal’’ transition in polyacetylene. Phys. Rev. B 28, 6937–6952.

75. Park, Y.W., Yoon, C.O., Lee, C.H., Shirakawa, H., Suezaki, Y., and Akagi, K. (1989). Conductivity and thermoelectric power of the newly processed polyacetylene. Synth. Met. 28, D27–D34.

61. Lee, K., Cho, S., Park, S.H., Heeger, A.J., Lee, C.-W., and Lee, S.-H. (2006). Metallic transport in polyaniline. Nature 441, 65–68.

76. Zuzok, R., Kaiser, A.B., Pukacki, W., and Roth, S. (1991). Thermoelectric power and conductivity of iodine-doped ‘‘new’’ polyacetylene. J. Chem. Phys. 95, 1270–1275.

62. Stafstro¨m, S., and Bre´das, J.L. (1988). Evolution of the electronic structure of polyacetylene and polythiophene as a function of doping level and lattice conformation. Phys. Rev. B 38, 4180–4191. 63. Prigodin, V.N., and Efetov, K.B. (1993). Localization transition in a random network of metallic wires: a model for highly conducting polymers. Phys. Rev. Lett. 70, 2932–2935. 64. Mahan, G.D., and Sofo, J.O. (1996). The best thermoelectric. Proc. Natl. Acad. Sci. U S A 93, 7436–7439. 65. Wang, D., Tang, L., Long, M., and Shuai, Z. (2009). First-principles investigation of organic semiconductors for thermoelectric applications. J. Chem. Phys. 131, 224704. 66. Casian, A. (2010). Violation of the Wiedemann-Franz law in quasi-onedimensional organic crystals. Phys. Rev. B 81, 155415. 67. Scheele, M., Oeschler, N., Meier, K., Kornowski, A., Klinke, C., and Weller, H. (2009). Synthesis and thermoelectric characterization of Bi2Te3 nanoparticles. Adv. Funct. Mater. 19, 3476–3483. 68. Li, J.-F., Liu, W.-S., Zhao, L.-D., and Zhou, M. (2010). High-performance nanostructured thermoelectric materials. NPG Asia Mater. 2, 152–158.

77. Maddison, D.S., Unsworth, J., and Roberts, R.B. (1988). Electrical conductivity and thermoelectric power of polypyrrole with different doping levels. Synth. Met. 26, 99–108. 78. Kemp, N.T., Kaiser, A.B., Liu, C.J., Chapman, B., Mercier, O., Carr, A.M., Trodahl, H.J., Buckley, R.G., Partridge, A.C., Lee, J.Y., et al. (1999). Thermoelectric power and conductivity of different types of polypyrrole. J. Polym. Sci. B Polym. Phys. 37, 953–960. 79. Gangopadhyay, R., De, A., and Das, S. (2000). Transport properties of polypyrrole-ferric oxide conducting nanocomposites. J. Appl. Phys. 87, 2363–2371. 80. Maddison, D.S., Roberts, R.B., and Unsworth, J. (1989). Thermoelectric power of polypyrrole. Synth. Met. 33, 281–287. 81. Wang, H., Yin, L., Pu, X., and Yu, C. (2013). Facile charge carrier adjustment for improving thermopower of doped polyaniline. Polymer 54, 1136–1140. 82. Holland, E.R., Pomfret, S.J., Adams, P.N., Abell, L., and Monkman, A.P. (1997). Doping dependent transport properties of polyaniline-CSA films. Synth. Met. 84, 777–778. 83. Yan, H., Ohta, T., and Toshima, N. (2001). Stretched polyaniline films doped by

Joule 3, 1–28, January 16, 2019

25

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

(G)-10-camphorsulfonic acid: anisotropy and improvement of thermoelectric properties. Macromol. Mater. Eng. 286, 139–142. 84. Anno, H., Hokazono, M., Akagi, F., Hojo, M., and Toshima, N. (2013). Thermoelectric properties of polyaniline films with different doping concentrations of (G)-10camphorsulfonic acid. J. Electron. Mater. 42, 1346–1351. 85. Groenendaal, L., Jonas, F., Freitag, D., Pielartzik, H., and Reynolds, J.R. (2000). Poly(3,4-ethylenedioxythiophene) and its derivatives: past, present, and future. Adv. Mater. 12, 481–494. 86. Chang, K.-C., Jeng, M.-S., Yang, C.-C., Chou, Y.-W., Wu, S.-K., Thomas, M.A., and Peng, Y.-C. (2009). The thermoelectric performance of poly(3,4-ethylenedioxythiophene)/poly(4styrenesulfonate) thin films. J. Electron. Mater. 38, 1182–1188. 87. Liu, C., Lu, B., Yan, J., Xu, J., Yue, R., Zhu, Z., Zhou, S., Hu, X., Zhang, Z., and Chen, P. (2010). Highly conducting free-standing poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate) films with improved thermoelectric performances. Synth. Met. 160, 2481–2485. 88. Taggart, D.K., Yang, Y., Kung, S.-C., McIntire, T.M., and Penner, R.M. (2011). Enhanced thermoelectric metrics in ultra-long electrodeposited PEDOT nanowires. Nano Lett. 11, 125–131. 89. McCullough, R.D., and Lowe, R.D. (1992). Enhanced electrical conductivity in regioselectively synthesized poly(3alkylthiophenes). J. Chem. Soc. Chem. Commun. 70, 70–72. 90. Xuan, Y., Liu, X., Desbief, S., Lecle`re, P., Fahlman, M., Lazzaroni, R., Berggren, M., Cornil, J., Emin, D., and Crispin, X. (2010). Thermoelectric properties of conducting polymers: the case of poly(3-hexylthiophene). Phys. Rev. B 82, 115454. 91. Reedijk, J.A., Martens, H.C.F., Brom, H.B., and Michels, M.A.J. (1999). Dopant-induced crossover from 1D to 3D charge transport in conjugated polymers. Phys. Rev. Lett. 83, 3904–3907. 92. Hiroshige, Y., Ookawa, M., and Toshima, N. (2006). High thermoelectric performance of poly(2,5-dimethoxyphenylenevinylene) and its derivatives. Synth. Met. 156, 1341–1347. 93. Shakouri, A., and Suquan, L. (1999). Thermoelectric power factor for electrically conductive polymers. Proceedings of the Eighteenth International Conference on Thermoelectrics 402. 94. Kaiser, A.B. (2001). Electronic transport properties of conducting polymers and carbon nanotubes. Rep. Prog. Phys. 64, 1. 95. Du, Y., Shen, S.Z., Cai, K., and Casey, P.S. (2012). Research progress on polymerinorganic thermoelectric nanocomposite materials. Prog. Polym. Sci. 37, 820–841. 96. Bredas, J.L., and Street, G.B. (1985). Polarons, bipolarons, and solitons in conducting polymers. Acc. Chem. Res. 18, 309–315.

26

Joule 3, 1–28, January 16, 2019

97. Park, Y.W., Denenstein, A., Chiang, C.K., Heeger, A.J., and MacDiarmid, A.G. (1979). Semiconductor-metal transition in doped (CH)x: thermoelectric power. Solid State Commun. 29, 747–751. 98. Hayashi, S., Kaneto, K., Yoshino, K., Matsushima, R., and Matsuyama, T. (1986). Electrical conductivity and ESR studies in iodine-doped polythiophene from semiconductor to metallic regime. J. Phys. Soc. Jpn. 55, 1971–1980. 99. Evans, D., Fabretto, M., Mueller, M., Zuber, K., Short, R., and Murphy, P. (2012). Structuredirected growth of high conductivity PEDOT from liquid-like oxidant layers during vacuum vapor phase polymerization. J. Mater. Chem. 22, 14889–14895. 100. Yu, Z., Xia, Y., Du, D., and Ouyang, J. (2016). PEDOT: PSS films with metallic conductivity through a treatment with common organic solutions of organic salts and their application as a transparent electrode of polymer solar cells. ACS Appl. Mater. Interfaces 8, 11629– 11638. 101. Jiang, F.-X., Xu, J.-K., Lu, B.-Y., Xie, Y., Huang, R.-J., and Li, L.-F. (2008). Thermoelectric performance of poly(3,4ethylenedioxythiophene): poly(styrenesulfonate). Chin. Phys. Lett. 25, 2202. 102. Jimison, L.H., Hama, A., Strakosas, X., Armel, V., Khodagholy, D., Ismailova, E., Malliaras, G.G., Winther-Jensen, B., and Owens, R.M. (2012). PEDOT: TOS with PEG: a biofunctional surface with improved electronic characteristics. J. Mater. Chem. 22, 19498– 19505. 103. Crispin, X., Jakobsson, F.L.E., Crispin, A., Grim, P.C.M., Andersson, P., Volodin, A., van Haesendonck, C., Van der Auweraer, M., Salaneck, W.R., and Berggren, M. (2006). The origin of the high conductivity of poly(3,4ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT-PSS) plastic electrodes. Chem. Mater. 18, 4354–4360. 104. Cao, Y., Qiu, J., and Smith, P. (1995). Effect of solvents and co-solvents on the processibility of polyaniline: I. solubility and conductivity studies. Synth. Met. 69, 187–190. 105. Wei, Q., Mukaida, M., Naitoh, Y., and Ishida, T. (2013). Morphological change and mobility enhancement in PEDOT: PSS by adding co-solvents. Adv. Mater. 25, 2831–2836. 106. Hsu, J.-H., Choi, W., Yang, G., and Yu, C. (2017). Origin of unusual thermoelectric transport behaviors in carbon nanotube filled polymer composites after solvent/acid treatments. Org. Electron. 45, 182–189. 107. Ouyang, J., Xu, Q., Chu, C.-W., Yang, Y., Li, G., and Shinar, J. (2004). On the mechanism of conductivity enhancement in poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) film through solvent treatment. Polymer 45, 8443–8450. 108. Lin, Y.-J., Lee, J.-Y., and Chen, S.-M. (2016). Changing electrical properties of PEDOT: PSS by incorporating with dimethyl sulfoxide. Chem. Phys. Lett. 664, 213–218. 109. Ichikawa, S., and Toshima, N. (2015). Improvement of thermoelectric properties of

composite films of PEDOT-PSS with xylitol by means of stretching and solvent treatment. Polym. J. 47, 522–526. 110. Hiroshige, Y., Ookawa, M., and Toshima, N. (2007). Thermoelectric figure-of-merit of iodine-doped copolymer of phenylenevinylene with dialkoxyphenylenevinylene. Synth. Met. 157, 467–474. 111. Le´vesque, I., Bertrand, P.-O., Blouin, N., Leclerc, M., Zecchin, S., Zotti, G., Ratcliffe, C.I., Klug, D.D., Gao, X., Gao, F., and Tse, J.S. (2007). Synthesis and thermoelectric properties of polycarbazole, polyindolocarbazole, and polydiindolocarbazole derivatives. Chem. Mater. 19, 2128–2138. 112. Zhang, Q., Sun, Y., Jiao, F., Zhang, J., Xu, W., and Zhu, D. (2012). Effects of structural order in the pristine state on the thermoelectric power-factor of doped PBTTT films. Synth. Met. 162, 788–793. 113. Yue, R., Lu, B., Xu, J., Chen, S., and Liu, C. (2011). Electrochemistry, morphology, thermoelectric and thermal degradation behaviors of free-standing copolymer films made from 1,12-bis(carbazolyl)dodecane and 3,4-ethylenedioxythiophene. Polym. J. 43, 531–539. 114. Yue, R., Chen, S., Lu, B., Liu, C., and Xu, J. (2011). Facile electrosynthesis and thermoelectric performance of electroactive free-standing polythieno[3,2-b]thiophene films. J. Solid State Electrochem. 15, 539–548. 115. Chen, S., Lu, B., Duan, X., and Xu, J. (2012). Systematic study on chemical oxidative and solid-state polymerization of poly(3,4ethylenedithiathiophene). J. Polym. Sci. A Polym. Chem. 50, 1967–1978. 116. Aı¨ch, R.B., Blouin, N., Bouchard, A., and Leclerc, M. (2009). Electrical and thermoelectric properties of poly(2,7carbazole) derivatives. Chem. Mater. 21, 751–757. 117. Wakim, S., Aı¨ch, B.R., Tao, Y., and Leclerc, M. (2008). Charge transport, photovoltaic, and thermoelectric properties of poly(2,7carbazole) and poly(indolo[3,2-b]carbazole) derivatives. Polym. Rev. 48, 432–462. 118. Sun, Y., Qiu, L., Tang, L., Geng, H., Wang, H., Zhang, F., Huang, D., Xu, W., Yue, P., Guan, Y.-S., et al. (2016). Flexible n-type highperformance thermoelectric thin films of poly(nickel-ethylenetetrathiolate) prepared by an electrochemical method. Adv. Mater. 28, 3351–3358. 119. Kentaro, H., Akito, M., Hachiro, N., Hidetoshi, O., and Yosikazu, S. (2009). Evaluation of thermoelectric properties of polythiophene films synthesized by electrolytic polymerization. Jpn. J. Appl. Phys. 48, 071501. 120. Mai, C.-K., Schlitz, R.A., Su, G.M., Spitzer, D., Wang, X., Fronk, S.L., Cahill, D.G., Chabinyc, M.L., and Bazan, G.C. (2014). Side-chain effects on the conductivity, morphology, and thermoelectric properties of self-doped narrow-band-gap conjugated polyelectrolytes. J. Am. Chem. Soc. 136, 13478–13481.

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

121. von Mu¨hlenen, A., Errien, N., Schaer, M., Bussac, M.-N., and Zuppiroli, L. (2007). Thermopower measurements on pentacene transistors. Phys. Rev. B 75, 115338. 122. Ito, H., Yokochi, Y., Tanaka, H., Kuroda, S., Kanehama, R., Umemiya, M., Miyasaka, H., Sugiura, K.I., and Yamashita, M. (2005). Electrical conduction in divalent BEDT-TTF salts. Synth. Met. 153, 441–444. 123. Miles, M.G., and Wilson, J.D. (1975). Semiconducting metal-organic salts. Inorg. Chem. 14, 2357–2360. 124. Isett, L.C. (1978). Magnetic susceptibility, electrical resistivity, and thermoelectric power measurements of bis (tetrathiotetracene)triiodide. Phys. Rev. B 18, 439–447. 125. Chaikin, P.M., Greene, R.L., Etemad, S., and Engler, E. (1976). Thermopower of an isostructural series of organic conductors. Phys. Rev. B 13, 1627–1632. 126. Huewe, F., Steeger, A., Kostova, K., Burroughs, L., Bauer, I., Strohriegl, P., Dimitrov, V., Woodward, S., and Pflaum, J. (2017). Low-cost and sustainable organic thermoelectrics based on low-dimensional molecular metals. Adv. Mater. 29, 1605682. 127. Sato, R., Kiyota, Y., Kadoya, T., Kawamoto, T., and Mori, T. (2016). Thermoelectric power of oriented thin-film organic conductors. RSC Adv. 6, 41040–41044. 128. Itahara, H., Maesato, M., Asahi, R., Yamochi, H., and Saito, G. (2009). Thermoelectric properties of organic charge-transfer compounds. J. Electron. Mater. 38, 1171–1175. 129. Widawsky, J.R., Darancet, P., Neaton, J.B., and Venkataraman, L. (2012). Simultaneous determination of conductance and thermopower of single molecule junctions. Nano Lett. 12, 354–358. 130. Wang, R.-Q., Sheng, L., Shen, R., Wang, B., and Xing, D.Y. (2010). Thermoelectric effect in single-molecule-magnet junctions. Phys. Rev. Lett. 105, 057202. 131. Yee, S.K., Malen, J.A., Majumdar, A., and Segalman, R.A. (2011). Thermoelectricity in fullerene-metal heterojunctions. Nano Lett. 11, 4089–4094. 132. Baheti, K., Malen, J.A., Doak, P., Reddy, P., Jang, S.-Y., Tilley, T.D., Majumdar, A., and Segalman, R.A. (2008). Probing the chemistry of molecular heterojunctions using thermoelectricity. Nano Lett. 8, 715. 133. Aradhya, S.V., and Venkataraman, L. (2013). Single-molecule junctions beyond electronic transport. Nat. Nanotechnol. 8, 399–410. 134. Chang, W.B., Mai, C.-K., Kotiuga, M., Neaton, J.B., Bazan, G.C., and Segalman, R.A. (2014). Controlling the thermoelectric properties of thiophene-derived single-molecule junctions. Chem. Mater. 26, 7229.

polymers for polymeric photovoltaics. Chem. Soc. Rev. 45, 4825–4846. 137. Wang, H., Yi, S.-I., and Yu, C. (2016). Engineering electrical transport at the interface of conjugated carbon structures to improve thermoelectric properties of their composites. Polymer 97, 487–495. 138. Spitalsky, Z., Tasis, D., Papagelis, K., and Galiotis, C. (2010). Carbon nanotube-polymer composites: chemistry, processing, mechanical and electrical properties. Prog. Polym. Sci. 35, 357–401. 139. Du, F., Scogna, R.C., Zhou, W., Brand, S., Fischer, J.E., and Winey, K.I. (2004). Nanotube networks in polymer nanocomposites: rheology and electrical conductivity. Macromolecules 37, 9048–9055. 140. Kirkpatrick, S. (1973). Percolation and conduction. Rev. Mod. Phys. 45, 574–588. 141. Hone, J., Ellwood, I., Muno, M., Mizel, A., Cohen, M.L., Zettl, A., Rinzler, A.G., and Smalley, R.E. (1998). Thermoelectric power of single-walled carbon nanotubes. Phys. Rev. Lett. 80, 1042–1045. 142. Ryu, Y., Yin, L., and Yu, C. (2012). Dramatic electrical conductivity improvement of carbon nanotube networks by simultaneous de-bundling and hole-doping with chlorosulfonic acid. J. Mater. Chem. 22, 6959– 6964. 143. Marconnet, A.M., Yamamoto, N., Panzer, M.A., Wardle, B.L., and Goodson, K.E. (2011). Thermal conduction in aligned carbon nanotube-polymer nanocomposites with high packing density. ACS Nano 5, 4818–4825. 144. Yang, L., Liu, F., Xia, H., Qian, X., Shen, K., and Zhang, J. (2011). Improving the electrical conductivity of a carbon nanotube/ polypropylene composite by vibration during injection-moulding. Carbon 49, 3274–3283. 145. Yao, Q., Chen, L., Zhang, W., Liufu, S., and Chen, X. (2010). Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites. ACS Nano 4, 2445–2451. 146. Meng, C., Liu, C., and Fan, S. (2010). A promising approach to enhanced thermoelectric properties using carbon nanotube networks. Adv. Mater. 22, 535–539. 147. Alemu, D., Wei, H.-Y., Ho, K.-C., and Chu, C.-W. (2012). Highly conductive PEDOT: PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells. Energy Environ. Sci. 5, 9662–9671. 148. Song, H., and Cai, K. (2017). Preparation and properties of PEDOT: PSS/Te nanorod composite films for flexible thermoelectric power generator. Energy 125, 519–525.

135. Bergfield, J.P., and Stafford, C.A. (2009). Thermoelectric signatures of coherent transport in single-molecule heterojunctions. Nano Lett. 9, 3072–3076.

149. Zhang, B., Sun, J., Katz, H.E., Fang, F., and Opila, R.L. (2010). Promising thermoelectric properties of commercial PEDOT: PSS materials and their Bi2Te3 powder composites. ACS Appl. Mater. Interfaces 2, 3170–3178.

136. Liu, C., Wang, K., Gong, X., and Heeger, A.J. (2016). Low bandgap semiconducting

150. Zhang, Z., Sun, X., Dresselhaus, M.S., Ying, J.Y., and Heremans, J. (2000). Electronic

transport properties of single-crystal bismuth nanowire arrays. Phys. Rev. B 61, 4850–4861. 151. Yu, H., Gibbons, P.C., and Buhro, W.E. (2004). Bismuth, tellurium, and bismuth telluride nanowires. J. Mater. Chem. 14, 595–602. 152. He, M., Ge, J., Lin, Z., Feng, X., Wang, X., Lu, H., Yang, Y., and Qiu, F. (2012). Thermopower enhancement in conducting polymer nanocomposites via carrier energy scattering at the organic-inorganic semiconductor interface. Energy Environ. Sci. 5, 8351–8358. 153. Zebarjadi, M., Esfarjani, K., Dresselhaus, M.S., Ren, Z.F., and Chen, G. (2012). Perspectives on thermoelectrics: from fundamentals to device applications. Energy Environ. Sci. 5, 5147–5162. 154. Yee, S.K., Coates, N.E., Majumdar, A., Urban, J.J., and Segalman, R.A. (2013). Thermoelectric power factor optimization in PEDOT: PSS tellurium nanowire hybrid composites. Phys. Chem. Chem. Phys. 15, 4024–4032. 155. Soni, A., Yanyuan, Z., Ligen, Y., Aik, M.K.K., Dresselhaus, M.S., and Xiong, Q. (2012). Enhanced thermoelectric properties of solution grown Bi2Te3-xSex nanoplatelet composites. Nano Lett. 12, 1203–1209. 156. Kajikawa, Y. (2013). Effects of potential barrier height and its fluctuations at grain boundaries on thermoelectric properties of polycrystalline semiconductors. J. Appl. Phys. 114, 053707. 157. Poudel, B., Hao, Q., Ma, Y., Lan, Y., Minnich, A., Yu, B., Yan, X., Wang, D., Muto, A., Vashaee, D., et al. (2008). High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys. Science 320, 634–638. 158. Zhao, Y., Tang, G.-S., Yu, Z.-Z., and Qi, J.-S. (2012). The effect of graphite oxide on the thermoelectric properties of polyaniline. Carbon 50, 3064–3073. 159. Du, Y., Shen, S.Z., Yang, W., Donelson, R., Cai, K., and Casey, P.S. (2012). Simultaneous increase in conductivity and Seebeck coefficient in a polyaniline/graphene nanosheets thermoelectric nanocomposite. Synth. Met. 161, 2688–2692. 160. Heikes, R.R., and Ure, R.W. (1961). Thermoelectricity: Science and Engineering (Interscience Publishers). 161. Yee, S.K., LeBlanc, S., Goodson, K.E., and Dames, C. (2013). $ per W metrics for thermoelectric power generation: beyond ZT. Energy Environ. Sci. 6, 2561–2571. 162. LeBlanc, S., Yee, S.K., Scullin, M.L., Dames, C., and Goodson, K.E. (2014). Material and manufacturing cost considerations for thermoelectrics. Renew. Sustain. Energy Rev. 32, 313–327. 163. LeBlanc, S.A. (2012). Electrothermal Properties of Nanowire Materials for Energy Conversion Systems Electronic Resource (Stanford University). 164. LeBlanc, S. (2014). Thermoelectric generators: linking material properties and systems

Joule 3, 1–28, January 16, 2019

27

Please cite this article in press as: Wang and Yu, Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration, Joule (2018), https://doi.org/10.1016/j.joule.2018.10.012

engineering for waste heat recovery applications. Sustain. Mater. Technol. 1–2, 26. 165. Park, S.H., Jo, S., Kwon, B., Kim, F., Ban, H.W., Lee, J.E., Gu, D.H., Lee, S.H., Hwang, Y., Kim, J.-S., et al. (2016). High-performance shapeengineerable thermoelectric painting. Nat. Commun. 7, 13403. 166. Rowe, D.M., Smith, J., Thomas, G., and Min, G. (2011). Weight penalty incurred in thermoelectric recovery of automobile exhaust heat. J. Electron. Mater. 40, 784. 167. Jens, W., and Karin, P.-K. (2008). Organic thermogenerators for energy autarkic systems on flexible substrates. J. Phys. D Appl. Phys. 41, 135113.

28

Joule 3, 1–28, January 16, 2019

168. Wei, Q., Mukaida, M., Kirihara, K., Naitoh, Y., and Ishida, T. (2014). Polymer thermoelectric modules screen-printed on paper. RSC Adv. 4, 28802–28806. 169. Wei, Q., Mukaida, M., Kirihara, K., Naitoh, Y., and Ishida, T. (2015). Recent progress on PEDOT-based thermoelectric materials. Materials 8, 732. 170. Søndergaard, R.R., Ho¨sel, M., Espinosa, N., Jørgensen, M., and Krebs, F.C. (2013). Practical evaluation of organic polymer thermoelectrics by large-area R2R processing on flexible substrates. Energy Sci. Eng. 1, 81–88. 171. Kim, S.L., Choi, K., Tazebay, A., and Yu, C. (2014). Flexible power fabrics made of carbon

nanotubes for harvesting thermoelectricity. ACS Nano 8, 2377–2386. 172. Yu, C., Murali, A., Choi, K., and Ryu, Y. (2012). Air-stable fabric thermoelectric modules made of N- and P-type carbon nanotubes. Energy Environ. Sci. 5, 9481–9486. 173. Yoon, C.O., Reghu, M., Moses, D., Heeger, A.J., and Cao, Y. (1994). Electrical transport in conductive blends of polyaniline in poly(methyl methacrylate). Synth. Met. 63, 47–52. 174. Wang, C., Niu, Y., Jiang, J., Chen, Y., Tian, H., Zhang, R., Zhou, T., Xia, J., Pan, Y., and Wang, S. (2018). Hybrid thermoelectric battery electrode FeS2 study. Nano Energy 45, 432–438.