Organic thermoelectric materials: emerging green energy materials converting heat to electricity directly and efficiently.

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Organic Thermoelectric Materials: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently Qian Zhang, Yimeng Sun, Wei Xu,* and Daoben Zhu* interest from both scientific and industrial communities. The thermodynamic limit of the energyconversion efficiency (η) of a TEG device is defined by reversible Carnot cycles. The energy-conversion efficiency, namely the output electrical energy over the input thermal energy, is a function of the materials’ figure-of-merit (ZT), average working temperature of the device, and temperature difference between the hot (TH) and cold (TC) ends.[2] Although η can be improved via thermal engineering[2e] (e.g., increase TH and TH- TC), it is ZT that fundamentally predominates.[2a-d] The figure-of-merit of thermoelectric materials is ZT = S2σT/κ, where S (V K−1) is Seebeck coefficient or thermopower, σ (S m−1) is electrical conductivity, κ (W m−1 K−1) is thermal conductivity and T is absolute temperature. When variation in thermal conductivity is secondary to modulation in electrical conductivity and Seebeck coefficient, thermoelectric property of a material can be weighted with a parameter called power factor (PF) as PF = S2σ in Wm−1 K−2. Furthermore, it should be noted that electrical conductivity, Seebeck coefficient and thermal conductivity are not independent, namely it is hardly possible to optimize one parameter without affecting the other parameters.[2a-d,3] Basically, electrical conductivity is the product of carrier charge q, carrier concentration n (cm−3) and carrier mobility µ (cm2 V−1 s−1) as σ = nqµ. Since charge carriers also conduct heat as they move, thermal conduction is expected to increase with electrical conduction of a given material. Actually according to Wiedemann-Franz relation, the ratio between its contribution to thermal conduction and electrical conduction of a charge carrier follows the equation σT/κ = (3/π)2 (q/kB)2, where kB is Boltzmann constant. As to the Seebeck coefficient, it is a fundamental electronic transport property of materials; specifically it measures the entropy of a carrier with unit charge. Considering a simple system of charge carriers without strong interactions, the entropy of a carrier is statistically determined to be qS = kBln[(1–ρ)/ρ], where ρ is the charge density for each state. In the framework of energyband theory and Boltzmann distribution, the above equation is transformed into S = (kB/q)[(E–Ef)/kBT], where Ef is Fermi energy level and E is the energy level occupied by the carrier. Obviously, Seebeck coefficients largely decrease with increase in carrier concentrations. Experimentally, a Seebeck voltage (ΔV) can be measured between two ends of the sample along with the corresponding

The abundance of solar thermal energy and the widespread demands for waste heat recovery make thermoelectric generators (TEGs) very attractive in harvesting low-cost energy resources. Meanwhile, thermoelectric refrigeration is promising for local cooling and niche applications. In this context there is currently a growing interest in developing organic thermoelectric materials which are flexible, cost-effective, eco-friendly and potentially energy-efficient. In particular, the past several years have witnessed remarkable progress in organic thermoelectric materials and devices. In this review, thermoelectric properties of conducting polymers and small molecules are summarized, with recent progresses in materials, measurements and devices highlighted. Prospects and suggestions for future research efforts are also presented. The organic thermoelectric materials are emerging candidates for green energy conversion.

1. Introduction Energy is indispensible for industrial production and daily life. Currently the majority sources of energy are produced from fossil fuels. These fuels are combusted in heat engines to generate mechanical energy, which is then transformed through electrical generators into electricity for utilization and storage. In contrast, there is a neat heat engine called thermoelectric generator (TEG) which is able to convert temperature differences directly into electrical voltages via the Seebeck effect.[1] Upon supply of electrical energy, TEG devices can also work as heat pumps driven by Peltier effect, which is roughly an inverse process of the Seebeck effect. According to its working principles, a TEG device neither requires moving parts nor consumes liquid or gas media, implying favorable qualities, such as high reliability and eco-friendliness. Moreover, TEG devices can employ low-quality heat, which is otherwise dissipated. Therefore, the research of thermoelectricity has attracted great

Q. Zhang, Dr. Y. M. Sun, Prof. W. Xu, Prof. D.B. Zhu Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]; [email protected] Q. Zhang University of Chinese Academy of Sciences Beijing 100049, P. R. China

DOI: 10.1002/adma.201305371

Adv. Mater. 2014, DOI: 10.1002/adma.201305371

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Qian Zhang graduated with a B.S.E. in materials science and engineering from Shandong University in 2009. She is currently a Ph.D. candidate in chemistry under the supervision of Professor Daoben Zhu. Figure 1. (a) A typical set-up for measurement of Seebeck coefficients (S = ΔV/ΔT). (b) Schematic illustration of a thermoelectric element composed of a pair of p- and n- type legs. The diffusion of carriers (the inside narrow lines) under temperature gradient leads to the formation of current (the outside wide lines) in the circuit and drives the load.

Yimeng Sun is a research assistant in the Organic Solids Laboratory. He received his B.S. degree in chemistry from University of Science and Technology of China in 2006 and Ph.D. in physical chemistry from Institute of Chemistry, CAS in 2012, under the supervision of Prof. Daoben Zhu. Currently his research focuses on organic thermoelectric materials and devices.

temperature difference (ΔT) after the establishment of a thermal gradient over the sample, as illustrated in Figure 1a. Thereby the Seebeck coefficient of the sample can be obtained via S = ΔV/ΔT. Typically a TEG element consists of a pair of legs connected electrically in series and thermally in parallel. As shown in Figure 1b, the two legs are respectively composed of p- and n- type thermoelectric materials, which should meet the following criteria: a. the thermal conductivity is as low as possible to maintain larger temperature difference, b. a higher Seebeck coefficient is preferred to achieve higher Seebeck voltages and c. a higher electrical conductivity is required for larger shortcircuit current. Generally, insulators and pristine semiconductors are too resistive (criterion c); on the other hand, ordinary metals are inappropriate because their Seebeck coefficients are small (ca. ±10 µV K−1) while thermal conductivities are high (criterions a and b). As a consequence, the most investigated candidates so far are doped semiconductors and semimetals. Taking inorganic thermoelectric materials as an example. In the mid of last century, an extensive screening of materials led to the identification of several alloys and intermetallics based on elements like Bi, Te, Sb, Pb, etc., and ZT of unity at low temperatures (300–500 K) was achieved.[4] For these materials the carrier concentration is tuned to be within the range from 1019 cm−3 to 1020 cm−3. As to thermal conductivity, both electrical carriers and phonons (quantinization of lattice vibration) are heat carriers. In inorganic materials, the electronic contribution to thermal conduction generally follows the Wiedemann-Franz relation, and the phonon part is determined by composing elements, crystal structures, various microstructure boundaries, ect..[2d,4] Actually, suppressing thermal conductivity without sacrificing electrical conductivity has been one of the main driving-forces of enhancement in ZT values of inorganic thermoelectric materials. For example, bulk nanocomposites have been studied extensively since the 1990s, which are designed to restrict thermal conduction with boundaries between nanocrystallines remained after hot press.[2c,d,4] Nowadays, ZT values above unity are well established in bulk nanocomposites, while the commercialization and wide application of TEG

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Wei Xu is a Professor in the Organic Solids Laboratory in ICCAS. He received his Ph.D. degree from Wuhan University in 1997. After working with Prof. Daoben Zhu as a Postdoc for two years, he joined Prof. Zhu’s group in 1999. His research interests include design, synthesis and transport properties of organic semiconductors. Daoben Zhu is a Professor and Director of the Organic Solids Laboratory in the Institute of Chemistry, CAS. He finished his graduate courses at the East China University of Science and Technology in 1968. He was selected as an Academician of the CAS in 1997. His research interests include molecular materials and devices.

devices are still anticipated mainly because of limitations in the materials.[2c,4] Firstly, the composing elements such as Bi, Te, Sb and Pb are toxic and not abundant. Second, laboratory





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processing of inorganic materials, including melt-spinning, ball milling, hot pressing, etc., is costly in terms of energy and instruments, and hence requires a long payback time. Lastly, most inorganic materials are too heavy and brittle to be of use in everyday life. Given the aforementioned obstacles met by current inorganic thermoelectric materials, organic candidates are attracting more and more attention. After all, organic materials are potentially abundant, light-weight, flexible, solution-processable and low-cost. Moreover, the marvelous development of organic electronics has provided hundreds of organic semiconductors with the highest field-effect mobilities approaching that of polycrystalline silicon.[5] In particular, there are two features of organic solids that justify serious consideration and devoted endeavor to exploit organic thermoelectric materials. Firstly, thermal conductivity of organic materials is typically below 1 W m−1K−1, approaching the lower limit of thermal conductivity of inorganic thermoelectric materials. Second, electronic structures of organic (semi)conductors are fairly tunable through molecular chemistry and doping treatments.[6] Therefore, organic thermoelectric materials hold much promise in applications at low temperatures. Despite all the above advantages, further development in organic thermoelectric materials is impeded by bottleneck problems including low electrical carrier mobilities and mediocre Seebeck coefficients. In fact, research into organic thermoelectric materials is a topic where new insights accumulated over decades, in that the thermoelectric measurement of organic materials dates back to the early days of organic semiconductors[7] and conducting polymers[8] in the 1970s. Back then Seebeck coefficients (thermopower) of a wide range of organic electronic materials were characterized, including organic photo-conducing dyes,[9] organic charge transfer salts,[10] derivatives of phthalocyanines,[11] conducting polymers,[12] etc., under various conditions of measurements, doping treatments and film processing methods. Nevertheless, thermoelectric measurements performed at that time were intended for study of electronic structures rather than the search for potential organic thermoelectric materials. Similarly, in the 1990s Seebeck coefficients were characterized to learn about doping processes of films of small molecular semiconductors prepared by thermal deposition, where doping was performed to improve device performance of organic light-emitting diodes (OLEDs) or organic photovoltaic cells (OPVs).[13] Later on, the measurement of Seebeck coefficients was combined with field-effect transistor (FET) architectures to resolve electrical transport mechanisms in crystalline organic semiconductors.[14] Therefore, there are both practical and fundamental interests in the research of organic thermoelectric materials. Since the dawn of the 21st century there have been ongoing efforts to employ organic materials especially conductive polymers as thermoelectric materials.[15] Meanwhile, small conjugated molecules like pentacene[16] and fullerene (C60)[17] have been doped and used as p- and n- type TEG legs respectively. After incubation over a decade, the highest ZT values achieved to date are respectively 0.42[18] and 0.20[19] for p- and n- type organic thermoelectric materials. Moreover, TEG devices fabricated by printing have been demonstrated using organic thermoelectric materials.[20] On the molecular level, thermoelectric

measurement of a series of benzene and fullerene derivatives in molecular junctions has been experimentally realized through scanning probe microscopy (SPM).[21] Aside from heterostructuring on the molecular level, hybrid materials that incorporate organic materials with inorganic thermoelectric materials have been developed, showing better thermoelectric performance than either component.[22] Most of these hybrids are fabricated using conducting polymers as binders or linkers for inorganic or carbon-based matrixes. Hybridization is important not only because it provides a pathway to address processing issues of inorganic materials and to reduce thermal conductivities; more importantly, it suggests a possibility to utilize advantages of both components of a hybrid through careful design of the organic/inorganic interfaces.[22] In fact, an analogy can be drawn between hybrid materials and molecular heterojunctions. And research of the latter model has suggested significant influence of energy level alignment between molecular orbitals and electrode Fermi levels on the thermoelectric performance of the junction.[21] There have been several good reviews published recently with emphasis on hybrid thermoelectric materials.[23] In this review, we focus on thermoelectric properties of organic materials, including that of conductive polymers (Section 2), conducting charge transfer complexes and semiconductors based on small molecules (Section 3). Common methods of the measurement of Seebeck coefficient, electrical conductivity and especially thermal conductivity are discussed over representative examples of organic materials especially conjugated polymers (Section 4). Moreover, all-organic thermoelectric devices that have been demonstrated so far are summarized (section 5), with devices in film architectures highlighted. Finally, approaches for the development of each type of organic thermoelectric materials are proposed (Section 2 and 3), and a future outlook of this field is briefly discussed (Section 6).

2. Organic Thermoelectric Materials Based on Conductive Polymers The majority of organic thermoelectric materials reported to date are based on conductive polymers, including conjugated polymers and certain coordination polymers, where roughly three types of research objects can be identified. The first type is highly conducting polymers that have been well studied previously for other applications. In this case, thermoelectric properties of the polymers are strongly affected by the synthesis and/or processing conditions. This efficient approach is the most adopted, and yet the monomer species available are relatively limited. The second type of objects is selected from the tens of solution-processable polymeric semiconductors developed in the past two decades for organic electronics, whose thermoelectric properties are optimized through doping, blending, and so on. The third approach is to tailor new molecular structures through derivation and novel design, in search of potential building blocks appropriate for thermoelectric purposes. This is an elegant demonstration of chemical versatility of organic electronic materials although it is largely trial and error at the moment. Progress in each of the three research objects offers a unique perspective, shedding light on the



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2.1.1. Materials

Figure 2. Chemical structures of some benchmark conjugated polymers.

ultimate goal of clarifying the structure-property relationships of polymers for high-performance thermoelectric materials.

2.1. Conjugated Polymers: Materials and Optimization Strategies Polymers, widely known as plastics like nylon and resin, have been commonplaces in our everyday lives. Conjugated polymers differ from these traditional plastics by the nature of chemical bonds in the backbones, namely in addition to covalent σ-bonds the conjugated repeat units are connected by non-localized π-bonds. Consequently conjugated polymers are pristinely semiconductive rather than being insulative and can become conductive and even metallic after appropriate doping treatments. Since the discovery of metal-like behaviors in highly doped polyacetylene (PA) films at the end of 1970s, three generations of conjugated polymers have been developed.[6,24] For further information readers are referenced to a recent review presented by Crispin et al. who gave a very good introduction to most of the fundamental aspects of conjugated polymers.[25] Herein, we briefly introduce conjugated polymers, highlighting chemical and physical features that are potentially correlated with thermoelectric performance. Thereafter research progresses are summarized in an outline classified into three strategies for optimization of thermoelectric properties. Finally suggestions on future research efforts are tentatively proposed.

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Conjugated Polymers: Chemical structures of some benchmark conjugated polymers ever employed in thermoelectrics are sketched in Figure 2. Generally conjugated polymers have marvelous doping reversibility and wide doping ranges; they can display superior mechanical flexibility and decent charge conductivity along the polymer backbone. On the other hand, polymeric materials are often inhomogeneous, because amorphous and crystalline regions are usually coexistent, the stacking of polymer chains is hard to control, and distribution in molecular weight is broad. To overcome these shortcomings, various synthesis schemes are developed and good solubility in common solvents is highly desired. Historically, the first generation of conjugated polymers is neither soluble nor fusible because of very strong intermolecular interactions in the absence of side substitutes. Typical examples include PA, polypyroles (PPy), polythiophenes (PTh), polyanilines (PANi) and poly(3,4-ethylenedioxythiophene) (PEDOT), as shown in Figure 2. These polymers can be prepared by chemical or electrochemical polymerization with or without catalyses. The as-prepared polymers are usually highly doped, incorporating counterions to balance charges on the backbones. Moreover, polymerization conditions can be tuned to obtain various microscopic and macroscopic morphologies. Thereafter, great efforts are devoted to addressing the solubility issue. One scheme is to adopt counterions that can facilitate emulsion polymerizations. Typical examples are PEDOT: PSS and PANi: CSA, where PSS− and CSA− are respectively poly(styrenesulfonate) anion and (±)10-camphorsulfonic acid anion. Nowadays, PEDOT: PSS of various qualities is commercially available in the form of dispersion in water. The second solution for solublization is to polymerize monomers with longer side substitutes, leading to the second generation of conjugated polymers as represented by poly[2methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] MEHPPV and poly(3-hexylthiophene-2,5-diyl) P3HT shown in Figure 2. Owing to improved solubility, the purification, processing and characterizations of these polymers are facilitated. Besides, better solubility makes it easier to synthesize polymers with higher regioregularity and higher molecular weights. In particular, it is now possible to include repeat units with more complex structures into the backbones, giving rise to the third generation of conjugated polymers. As a result, the electronic structures of conjugated polymers are greatly enriched. Moreover, the electronic structures can be influenced by side substitutes because of their subtle effects on polymer conformation and stacking manner in solids, bulk density of conjugated units, chemical/thermal stability and so on.





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Finally, it should be noted that the as-prepared polymers of the second and third generations are neutral and soluble, and can be doped chemically or electrochemically with insertion of counterions and lost of solubility. As such, two basic mechanisms of (electro)chemical doping are recognized for conjugated polymers. One is based on redox reactions where electrons are transferred between polymer backbones and dopant molecules (in chemical doping) or electrodes (in electrochemical doping). The other, often referred to as protonic acid doping, is a base-acid reaction where proton ions are added to the polymer backbones without accompanied variation in electron numbers. Electrical Conduction: Electrical carriers in conjugated polymers are acknowledged as polarons and bipolarons, which are approximately the summation of electrons (holes) with local distortion in molecular structures. Polarons and bipolarons can be detected with electron spin resonance (ESR) techniques, ultraviolet-visible and near infrared (UV-vis NIR) spectroscopy, and Raman spectroscopy. Transport mechanisms of (bi)polarons in conducting polymers, e.g. localized hopping or delocalized diffusion, are dependent on crystallinity and morphologies of materials, doping levels, conformation and stacking manners of molecules, ect.. In this respect, the values of carrier mobility can be used as an indicator of the extent of delocalization of (bi)polarons. Given that electronic structures in the reciprocal space are determined by the structures of molecules and materials in the real space, both electrical conductivities and Seebeck coefficients are related with carrier mobility. From this point of view, not only are carrier concentrations and chemical structures of monomers and counterions responsible for thermoelectric properties, but how the molecules are arranged in materials is also critical. Thermal Conduction: Similar to the case in inorganic materials, heat carriers in conductive polymers include phonons and charge carriers. However, the Wiedemann-Franz law, which has been widely acknowledged in most inorganic metals and semiconductors, is often invalid in conjugated polymers because of their stronger charge-lattice coupling. Furthermore, the electronic contribution to thermal conduction is marginally small compared with the phonon part, given the low electrical conductivities of most polymers. Therefore thermal conduction in conjugated polymers is generally recognized to be dominated by phonons. For example, only minor variation in thermal conductivity values was observed in PA, PANi, and PPy samples, from less than 0.1 W m−1K−1 to below 1.0 W m−1K−1, when electrical conductivity values were tuned over three orders of magnitudes.[26] Although these results are desirable for organic thermoelectricity, thermal conduction property of conductive polymers remains to be studied extensively. To this end, it seems that thermal conductivity of polymers is susceptible to synthesis conditions and plausibly to molecular structures but there are no definite conclusions for the lack of systematic investigations.[26] Besides, recent research suggests remarkable impact of crystallinity, alignment of polymer chains, interfaces or boundaries on thermal conductivity of polymer films or fibers.[27] Given the huge impact of thermal conductivity on overall ZT value, thermal conduction property of crystalline conductive polymers deserves further investigations.

2.1.2. Optimization Strategies for Thermoelectric Polymers No.1: Carrier Concentration As mentioned in Introduction, it has been well-established in inorganic thermoelectric materials that the best performance is found in highly doped semiconductors, typically with carrier concentration ranging from 1019 to 1020 cm−3. In contrast, the situation in conjugated polymers is still perplexing. On the one hand, tuning of carrier concentration has been proved to be powerful in optimizing thermoelectric properties of the polymers. Note that instead of specified values of carrier concentration, it is doping levels that are mostly cited in the description of conjugated polymers because accurate measurement of carrier concentration in polymers is nontrivial. Experimentally more than one definitions can be found for doping levels, but the basic idea is to identify the number of counterions per repeat unit of the polymer. Since it is formally proposed and demonstrated by Crispin et al. in the year 2011 through reducing chemically polymerized PEDOT: Tos (Tos = tosylate anion) with tetrakis(dimethylamino)ethylene (TDAE) vapor[20] (Figure 3), intentional tuning of carrier concentration or doping level has become a routine step to optimize thermoelectric property of polymers. So far most of the best results are obtained through tuning doping levels, i.e. step-wisely increasing or decreasing the apparent ratio of monomer/counterion, as summarized in Table 1. In the case of PEDOT, various reducing methods have been reported, such as electrochemical reducing under constant potentials,[29,30] charge-annialation reducing with gating in electrochemical transistors,[33] solvent-induced removing of counterions[18] and exposure to active agents like TDAE[20] and ammonium formate.[42] In comparison, step-wise oxidation is more appropriate for polymers with deeper HOMO levels like P3HT whose oxidized state is less stable than the neutral state.[35,43] Along with thermoelectric measurement, characterizations of UV-vis NIR spectroscopy and X-ray photoelectronic spectroscopy (XPS) are often performed to infer doping levels and carrier nature (polaron and/or bipolaron). Several representative examples of controlling doping levels with methods aforementioned are depicted in Figure 3–7, illustrating how thermoelectric property evolves with oxidation or reduction and how the optical absorption and/or XPS spectra change correspondingly. On the other hand, mechanisms of the optimization in thermoelectric properties by tuning carrier concentration are not clarified. This judgement is corroborated by three common observations in published results, which are also evident in the examples shown in Figure 3–7. Firstly, the optimal doping levels vary with polymers, counterions and processing methods. For example, as shown in Figure 3,4 and 6 the optimized doping levels are around 22%, 14.5%, 20–30% and 30% respectively in PEDOT: Tos, PEDOT: PSS, P3HT: PF6 and P3HT: TFSI (TFSI− = triflimide anion, inset in Figure 6f).[20,33,35,43] Second, the evolution of optical absorption spectra as doping proceeds is indicative of coexsistence of polarons, bipolarons and neutral units. As shown in Figure 3–6, the ratios between either two of the three components vary with doping levels. It seems that the PF peak values are achieved around the critical point where polaron absorption is just beyond its maximum. The third observation is that electrical conductivity of a sample does



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Figure 3. Optimization of thermoelectric property of PEDOT: Tos by TDAE vapor treatment. Variation in doping levels is verified by XPS (a) and UV-VIS spectroscopy (b), and the resultant thermoelectric property is maximized at doping level around 22% (c). Adapted with permission.[20] Copyright 2011, Nature Publishing Group.

not necessarily decrease with reduction in doping levels. Actually as shown in Figure 5 and 7 both electrical conductivity and Seebeck coefficient increased with reduction in doping levels from the highest value. The summarized three observations imply that thermoelectric properties of conjugated polymers are significantly affected by the nature of charge carriers, with carrier concentration being one of all carrier properties. Therefore it is important to identify other charge carrier natures of key importance to thermoelectric performance of conjugated polymers. To this end, three possible approaches can be generalized as follows: i. direct characterizations of carrier concentrations are preferred to indirect methods like XPS or optical absorption, ii. the ratios between either two of (bi)polarons and neutral segments should be controlled to clarify their roles on thermoelectric performance, and iii. effects of doping mechanisms (redox doping and protonic acid doping) on thermoelectric properties are subtly important. However, it should be noted that the second point is very complicated. Although it has been mentioned previously by Crispin et al.,[25] the related research is barely reported. Nevertheless, experimental results corresponding to the first and third points are available and are reviewed in the following. As characterizations of carrier concentration or doping level are concerned, the most adopted method is XPS where the peak area (or height) of distinguishing elements from the polymer and counterion are compared quantitatively. However, XPS is highly surface-sensitive, and there are often massive

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ineffective counterions in polymers as well as large amounts of deep-trapped carriers which barely contribute to electrical transport. As shown in the case of PEDOT: PSS (Figure 7), the concentration of free carriers extracted from the measured Seebeck coefficients via numerical simulation is 0.13N0 while the counterion/monomer ratio (S168/S164) determined by XPS is 0.96, where N0 is the total density of states of neutral polymer.[18] Another regular but less common method is to derive injected charges from electrical signals in electrochemical or transistor configurations (Figure 4d). The subtle effect of doping mechanisms on thermoelectric properties is less noticed. As aforementioned, redox reaction and proton addition are two basic reaction mechanisms for chemical doping of conjugated polymers. The former is predominant in polythiophenes like PEDOT and P3HT while the latter is primarily observed in polyanilines. Previously Park et al. measured the Seebeck coefficients of PANi pellets as they systematically varied doping levels, temperatures, doping methods (hydrochloride acid HCl and Iodine vapor), and substitutes including Cl, OCH3, CH3.[44] A transition from positive to negative in the sign of Seebeck coefficient at high doping levels was observed in HCl-doped PANi but not in Iodine-doped samples. Similar transition in the sign of thermopower at high doping levels was observed in PEDOT: PSS which was used as the active layer in an electrochemical transistor with PSS: H as the dielectric layer (Figure 4).[33] To conclude, tuning carrier concentration is an important and effective method to modulate thermoelectric property of conjugated polymers and insights into the working mechanisms require further exploration.




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Table 1. Summary of thermoelectric results of polymers with PF above 1 µW m−1K−2 around 300 K. Polymers with PF above 100 µW m−1K−2 around 300 K

Part A −1

σ (S cm )

S (µV K )

PF (µW m−1K−2)

κ (W m−1K−1)a)

Treatmentb)

11110

28.4

900

0.69 (steady-state)

As-synthesized

PA: FeCl4

7530

15.3

180

/

As-synthesized

PEDOT: Tos[20]

238

40

38

/

As-synthesized (36%)

80

200

324

0.33 (0.37)

TDAE-reduced (22%)

PEDOT: Tos[29]

1354

79.7

861

/

As-synthesized (24%)

923

117

1270

/

Electrochemically Reduced

639

27.3

47.6

0.23 (0.52)

Spun-casted, EG

620

33.4

69.2

0.24 (0.42)

Spun-casted, DMSO

830

60

300

0.23 (0.37)

EG treated

957

70

469

0.22 (0.31)

DMSO treated

PA: Ix[26a,28] [28]

PEDOT: PSS[18] (Clevios PH1000)

PEDOS-C6: ClO4[30]

180

30

16

/

As-synthesized

335

103

354.7

/

Electrochemically Reduced

Polymers with PF ranging from 10 to 100 µW m−1K−2 around 300 K

Part B

S (µV K−1)

PF (µW m−1K−2)

κ (W m−1K−1)

Treatment

2.9

40.8

0.48

0.66 (laser-flash)

As-synthesized

349.2

47.3

78.1

σ (S EtOPV-co-PV I2

−1

vapor[31]

[32]

cm−1)

Stretching ratio 3.1

930.4

18

30.1

/

DMSO post-treatment

720

23

38.5

/

(EMIM)BF4-DMSO

PEDOT: PSS (GmbH)[33]

224

9

2.3

/

Spun-casted

23

101

23.5

/

PSSH-gated (14.5%)

PEDOT: PSS (PH1000)c)[34]

559

23.3

30.3

0.20 (laser-flash)

622

34.5

37.3

90

50

22.5

PEDOT: PSS (PH1000)

P3HT: TFSI[35]

Batch No. 1 Batch No. 2

/

As-synthesized

Polymers with PF ranging from 1.0 to 10 µW m−1K−2 around 300 K

Part C

σ (S cm−1)

S (µV K−1)

PF (µW m−1K−2)

κ (W m−1K−1)

Treatment

[36]

570

13.5

10.4

0.34 (TDTR)

5 vol.% DMSO and 10 vol.% isopropanol

PEDOT: PSS (PH500)[37]

175

23

8.5

/

Mixed with (bmim)BF4

123

28.5

9.9

/

Mixed with (bmim)Br

PEDOT: ClO4[38]

16.8

74

9.2

/

Nano-sized channel

PTh: ClO4[39]

201

23

10.3

/

Electrochemical Polymerization

PANi: CSA[40]

260

14

5

/

Drawing ratio 78%

PPy: Tos[41]

170

11

2

0.20 (laser-flash)

Additional oxidation

PEDOT: PSS (PH750)

a)Cross-plane thermal conductivity values with in-plane conductivity included in brackets. The cross-plane thermal conductivities are obtained with laser-flash method, time domain thermal reflectance (TDTR) method or differential three-omega (3w) method as detailed in the maintext. In-plane thermal conductivities are obtained with steadystate method or 3ω method; b)Doping levels experimentally determined are enclosed in brackets; c)Measured at 120 °C.

2.1.3. Optimization Strategies for Thermoelectric Polymers No.2: Microscopic Morphologies Microscopic morphologies as characterized with atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), etc., are usually closely correlated with such features as bulk mobility, density and crystallinity of materials, conformation and ordering of polymer chains and microstructure


heterogeneity. Therefore microscopic morphologies are expected to remarkably impact thermoelectric property of polymeric materials. Chemically, polymerization methods that improve material crystallinity and carrier mobility often change material morphologies simultaneously. For example, ZT values of β-Naphthalene sulfonic acid (β-NSA) doped polyaniline with and without nanotube structures were respectively 4.86 × 10−5 µW m−1K−2 and 4.58 × 10−7 µW m−1K−2.[45]



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Figure 4. Optimization of the thermoelectric property of PEDOT: PSS (a) with electrochemical transistor architectures (b). The UV-VIS absorption spectra (c) and doping levels (d) are both varied with gating voltages. (e) (f) The polymer thermoelectric property is optimized at doping level of 14.5%. Adapted with permission.[33] Copyright 2012, American Chemical Society.

Figure 5. Thermoelectric property of electrochemically polymerized PEDOS-C6. When doping level is tuned electrochemically, both UV-VIS absorption (a), (b) and thermoelectric property (c), (d) are modified. Morphologies of CVP (e) and CPP (f) films after dedoping at −0.1 V are characterized with SEM (left) and AFM (right). The scale bar in SEM images is 1µm and the scanning scale of AFM is 3µm. Adapted with permission.[30]

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REVIEW Figure 6. P3HT films step-wisely doped with NOPF6 (a-c) and Fe(TFSI)3 (d-f). (a) Scheme of carriers generated, (b) UV-VIS absorption spectra, and (c) variation in thermoelectric property. Adapted with permission.[43] Copyright 2010, American Physical Society. Evolution in (d) XPS, (e) XRD results with increased doping time and (f) temperature-dependence of thermoelectric property. Adapted with permission.[35] Copyright 2012, The Royal Society of Chemistry.

Similarly, Taggart et al. reported that PF values of PEDOT electrochemically deposited in nano-sized channels were on average three times that of PEDOT deposited on unpatterned substrates, owing to improved crystallinity and carrier mobility.[38] In another case, the Kim group compared PEDOT: Tos films synthesized with and without triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-85 blockpoly(ethylene glycol) (PEPG) in the precursor solution and found improved crystallinity (TEM) and higher density (13% increase) in the former.[29] As a result the PF value was improved from 861 µW m−1K−2 to 1,270 µW m−1K−2. More cases demonstrating the necessity to control polymer morphologies are reported in electrochemical polymerization. For instance, Hiraishi et al.[39] and Yan et al.[41] prepared films of PTh and PPy respectively through electrochemical polymerization. By varying current density (or oxidation potential), temperature, concentration of monomer and electrolyte, they found the best thermoelectric property in films with smoother and denser morphologies. Recently, Kim et al. prepared poly(3,4-ethylenedioxyselenophene) PEDOS-C6 by electrochemical polymerization.[30] They compared thermoelectric property of films synthesized with cyclic-voltammetric polymerization (CVP) and constant potential polymerization (CPP), showing that films obtained under constant potential were smoother and denser (Figure 5e, f). After electrochemical reducing, the PF was enhanced from the as-deposited 16.2 µW m−1K−2 to 354.7 µW m−1K−2 (Figure 5c, d).


The impact of morphology is also evident in the most studied PEDOT: PSS, where secondary doping treatment (addition of polar solvents, acids or salts) is widely adopted to increase the electrical conductivity from 0.1 − 1 S cm−1 to over 1,000 S cm−1. Such remarkable increase in electrical conductivity is primarily ascribed to improvement in mobility as a result of more extended conformation of polymer chains, thinning of insulating PSS shells, improved orientation and coherence of conductive PEDOT grains, etc.[46] Often the solvent of choice is dimethyl sulfoxide (DMSO) or ethylene glycol (EG) which leads to more significant fibril morphologies as revealed by AFM and TEM characterizations.[46a,47] In contrast, Seebeck coefficients are less sensitive to secondary doping treatment than electrical conductivity. Since the year 2002,[48] there have been about ten papers published (see a recent review[49]) suggesting that the effect of solvent treatment be increasing electrical conductivity without remarkable effects on Seebeck coefficients (specifically around 10 – 20 µV K−1), where solvents including water, EG, DMSO, dimethylformamide (DMF) and N-Methyl-2-pyrrolidone (NMP) have been examined.[49,50] Thereafter, ionic liquids are found to increase Seebeck coefficients to be around 30 µV K−1, accompanied with obvious modifications in film morphologies, optical absorption spectra and electrical conductivities.[32] Very recently for the first time, Pipe et al.[18] proved that Seebeck coefficient of spun-cast PEDOT: PSS film can be increased from 20 µV K−1 to 60–70 µV K−1 by immersing the film in EG



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Figure 7. Optimization of thermoelectric property of PEDOT: PSS by solvent-induced dedoping treatment. By immersing the polymer film in EG or DMSO bath, the film thickness is decreased (a) owing to selective removal of PSS as demonstrated by XPS (b). The values of ZT (c), Seebeck coefficient (d) and electrical conductivity (e) are increased while thermal conductivity is decreased (f) with dedoping. Simulation of variation in S2σ (normalized, the inset scale bar) as the dopant volume ratio (versus total host volume, rχ) and the carrier concentration (normalized, n/N0) are changed, as shown in the two-dimensional plot (g) and three-dimensional plot (h). Adapted with permission.[18] Copyright 2013, Nature Publishing Group.

or DMSO bath for a certain time in inert atmosphere. As shown in Figure 7, owing to the selective removal of counterions PSS− under such treatment, the film thickness was reduced by half, the doping level was decreased (according to XPS), and the thermal conductivity was also suppressed (Table 1). Consequently a maximum ZT value of 0.42 was obtained in DMSOtreated PEDOT: PSS for which the authors proposed an optimization scheme based on minimization of the dopant volume. Nevertheless, it should be noted that commercial sources are not consistent in all the aforementioned reports, and hence disparities in molecular weights, fed ratios of 3,4-ethylenedioxythiophene (EDOT) versus PSS and PH values are expected.[51] Overall, materials morphology is a manifestation of molecular organization in solids, offering another path to optimize thermoelectric property of conjugated polymers. More importantly, morphology in real space is closely related with electronic states in energy space. In this respect, characterizations related to crystallinity, density and mobility should be performed along with chemical synthesis and thermoelectric measurement. Furthermore, morphologies may have additional impact on thermal conductivities.

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2.1.4. Optimization Strategies for Thermoelectric Polymers No.3: Other Methods Explored The strategies aforementioned are effective in most cases. Whenever a polymer is synthesized, carrier concentration and morphology of the material can be tuned carefully to optimize thermoelectric performance. There are some other research results, though yet to achieve high thermoelectric result so far, revealing information valuable for future optimization of thermoelectric properties of polymers. Enhancement of Seebeck Coefficients: As mentioned in Introduction, Seebeck coefficient would be decreased when electrical conductivity is increased as carrier concentration or doping level is increased. However, several research results have demonstrated that it is possible to enhance Seebeck coefficient and electrical conductivity simultaneously, by means of modulating transport pathways of charge carriers. An early example is reported in P3HT which is blended with poly(3-hexylthiothiophene) (P3HTT). Compared to P3HT, a sulfur atom is inserted between the hexane side chain and the backbone in P3HTT, resulting in an upward shift of HOMO





REVIEW

level by 0.15 eV.[52] The solution of P3HT - P3HTT blend is doped by gradual addition of 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4TCNQ) solution and is thereafter casted into films. In the blend of P3HT - 20 wt.% P3HTT, electrical conductivity and Seebeck coefficient were increased simultaneously with addition of F4TCNQ when the dopant ratio is below 1.0 wt.%. It is suggested by the authors that Fermi level is pinned by P3HTT while the major contribution to electrical conductivity is from carriers in P3HT given its higher weight ratio and carrier mobility. A second method is to utilize vertical architectures where a thin polymer film is sandwiched between electrode layers, with temperature gradient applied vertically over the top and bottom electrodes.[53] This device architecture was designed to suppress thermal conductivity while keeping electrical conductivity intact so as to improve Seebeck coefficient.[53a] Specifically, thermal conduction is prohibited because of acoustic mismatch between polymers and inorganic electrodes. Meanwhile, Ohm contact is established to insure electrical conduction. Seebeck coefficient is improved because the disparity in carrier entropy at given temperature difference is enlarged between carriers at the hot end and carriers at the cold, in the presence of chargephonon interactions which is expected to occur at the polymerelectrode interface.[53] Furthermore, it is shown that film thickness, working temperature, and electrode materials at the hot end should be considered together to maximize the benefits from the interface. So far this sandwich device architecture has been demonstrated in films of Ppy[53a] and PEDOT: PSS[53b] with thickness around 200 nm. Recently, this multilayer structure is applied to MEH-PPV, showing that light illumination can simultaneously increase Seebeck coefficient and electrical conductivity in the ITO/MEHPPV/Au device.[53c] Orientation of Polymer Chains: The shortcoming shared by the above two methods is undesirable electrical conductivity (below 10−4 S cm−1), which is primarily caused by low carrier concentrations of chosen materials. So it remains whether these methods would be effective at higher density of charge carriers. A third method achieving high thermoelectric performance is to utilize anisotropy of polymers, e.g., to induce orientation of polymer chains through mechanical stretching. Toshima et al.[31,54] synthesized a series of copolymers of phenylenevinylene and dialkoxyphenylenevinylene ROPV-co-PV (RO = MeO, EtO, BuO). After systematic optimization in the ratio of the two monomers (ca. 30 mol.% of dialkoxy-substituted units) and in the length of side-chain substitutes, the PF of polymer EtOPV-co-PV were optimized to 78.1 µW m−1K−2 with elongation ratio of 3.1. Anisotropy in conductive polymers is an interesting attribute for thermoelectricity. Effects of Chemical Structures: Last but not least, reports related to the relationships between chemical structures and thermoelectric properties of the materials are summarized. In the year 2005, Gao et al.[55] proposed carbazole unit as a promising building-block for high Seebeck coefficients. Because according to their band structure calculation, oxidation was expected to firstly occur on nitrogen atom where electron cloud was more localized. Thereafter, poly(2,7-carbazole), polyindolocarbazole and a series of their derivatives were designed, synthesized and doped.[56] Regretfully, electrical conductivities of these soluble polymers were below 1.0 S cm−1 except for

poly(2,7-carbazole-alt-bithiophene)s. By introducing a benzothiadiazole unit between the carbazole and bithiophene units, the PF value was optimized to be 19 µW m−1K−2.[57] Similarly, Xu et al. synthesized a series of polymers with monomers composed of units including carbazole, thiophene, thieno[3,2-b]thiophene (TT), EDOT, and 3,4-ethylenedithiathiophene (EDTT) by electrochemical polymerization or solid-state polymerization.[58] The best performance around 1.0 µW m−1K−2 was obtained in poly(thieno[3,2-b]thiophene).[58b] In addition to molecular structures of the backbones, side substitutes and counterion species are also reported to affect thermoelectric properties significantly. Shinohara et al. compared thermoelectric property of electrochemically polymerized polythiophene, poly(3-hexylthiophene), poly(3-octylthiophene) and poly(3-dodecylthiophene), and found that PF values decreased with increasing length of side chains.[39,59] Zhu et al. found that PF values of P3HT: TFSI and PBTTT: TFSI (TFSI− = triflimide anion, inset in Figure 6f) were roughly ten times that of P3HT: PF6 and PBTTT: PF6 respectively (Figure 6).[35,43,60] Meanwhile, stability of the polymers against moisture and heat was obviously improved. Besides, Park prepared heavily-doped PA samples with iodine complexes, FeCl4−, ZrCl4-, MoCl5-, NbCl5- as counterions and found different temperature dependence of Seebeck coefficients assumingly caused by different scattering effects from counterions.[28] In summary, much more extensive research is required to generalize the effect of chemical structures of polymers (both backbones and side substitutes) and counterions on the thermoelectric properties. At the moment, it seems that once decent carrier mobility is insured by crafting polymer structures for appropriate molecular stacking and materials microstructures, polymer backbones with relatively localized distribution of electron cloud are preferred for higher Seebeck coefficients.[57,60b]

2.2. Thermoelectric Generators Based on Coordination Polymers Coordination polymers (CPs) are polymers constructed from metal ions and ligands, with metal ions acting as connectors and ligands as linkers. CPs can also be regarded as polymers whose repeat units are coordination complexes. Since the 1960s, CPs have been widely studied in chirality, luminescence, non-linear optics, electrical conductivity, magnetism, catalysis, molecular storage, etc.[61] However, the research on thermoelectric and related properties is very limited, much less than that on traditional conjugated polymers.[62] In 1979, D’Sa et al. prepared a series of CPs based on 4,4′-dihydroxy-3,3′-diacetyl biphenyl bis-thiosemicarbazone (L-1) and 4,4′-dihydroxy-3,3′-dipropionyl biphenyl bis-thiosemicarbazone (L-2),[63] as shown in Figure 8a. The electrical conductivity of Cu(L-1) was around 10−5 S cm−1. Its Seebeck coefficient was positive with values around 1.1 – 1.2 mV K−1 in the temperature range of 310 - 450 K, indicating that Cu(L-1) was a p-type semiconductor. Then Patel[64] found that the Seebeck coefficient of the polymer barely changed in the given temperature range, implying a hopping mechanism for charge carriers in the material. A second type of CPs of interest is based on bisphenolic ligands, and the general formula are given in Figure 8b. Electrical conductivities of these polymers were rather



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Figure 8. Chemical structures of (a) Cu(L-1), Cu(L-2), M = Cu(II), Co(II), Ni(II), R = CH3, C2H5; (b) bisphenolic CPs; (c) PSF-Cs; (d) poly[Ax(M-ett)].

low (

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