Thermoelectric (TE) systems are very effective in harvesting electricity from waste heat or heat sources with low-temperature gradients relative to environmental temperature.1, 2 These low-temperature gradients are often present in the environment (e.g., geothermal and solar energy) or generated from various power generating or consuming systems. TE materials turn heat into electricity without any pollution and, therefore, could be a green option for everything from power generation to microprocessor cooling. TE devices can be operated in a cooling mode, and their simple leg-type structures, without moving parts, provide enormous advantages over conventional turbines, engines, and compressors, particularly for the applications that require robustness and silence. Thus, larger cooling systems using vapor compression cycles could be built to replace the classic units. This would reduce the use of toxic chlorofluorocarbons and their substitutes; it would also reduce the weight and expense of coolers and save energy. Although current devices have a low-conversion efficiency of ∼10%,3 they are strongly advantageous when compared with conventional energy technologies. Several classes of compounds have been investigated for TE applications. A few examples are skutterudites,4 half-Heusler alloys,5 clathrates,6 and pentatellurides.7 Moreover, bismuth telluride has already been commercially available as one of the raw materials for a Peltier cooler. Bismuth telluride alloys have shown to be the best TE materials as far as their efficiency is concerned.8 However, the use of these materials gets restricted due to the fact that they are toxic and also Te, being one of the rarest elements on earth, makes the production of the alloys an uneconomical process.9
The fundamental problem in creating efficient TE materials is that they must be very good at conducting electricity but not heat. That way, one end of the apparatus can get hot while the other remains cold, instead of the material quickly equalizing the temperature. In most materials, electrical and thermal conductivity go hand in hand. Hence, the main focus of the research on TE materials is to improve electrical conductivity without an increase in the thermal conductivity. The efficiency of a TE material is related to the so-called dimensionless TE figure-of-merit (ZT) defined by the equation ZT = (S2σ) T/κ where S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ is the thermal conductivity. A good TE material has a high ZT value at the operating temperature. Materials with ZT > 1 are expected to be competitive against other methods of refrigeration and electric power generation.10 However, in the absence of precise data on the thermal conductivity, the performance is assessed with the TE power factor (PF).10, 11 It is given by the following equation:
This factor is generally used when the materials have similar thermal conductivities.
Conjugated Polymers for TE Applications
Inorganic conductors and semiconductors are no doubt efficient TE materials, but they are associated with issues like high cost of production, scarcity of materials, and toxicity. Because of these problems associated with inorganic conductors and semiconductors, organic electronic materials have come to the fore front. Conducting polymers have attracted considerable attention as important polymer materials since the initial discovery of doped polyacetylene in the late 1970s.12 In general, conjugated polymers are semiconductors that provide the electronic conductivity after doping with suitable dopants (see Scheme 1). They have been extensively studied for their applications as light-emitting diodes,13, 14 transistors,15, 16 sensors,17 and photovoltaic cells.18–20 They are advantageous over the inorganic materials in the fact that they are much easier to process. Through simple modifications of their molecular structure, it is also possible to tune their physical and chemical proprieties in a fairly large range, providing great material flexibility to meet the requirements of the targeted applications. Moreover, carbon is present in abundance in nature; hence, the synthesis of organic electronic materials is more economical. As polymers are characterized by poor thermal conductivity (ca. 0.2 W m−1 K−1), they prove to be ideal for TE applications.21, 22 A number of such conjugated semiconducting polymers, such as polyacetylene,23, 24 polypyrroles,25, 26 polyanilines,26–28 polythiophenes (PT),29, 30 poly(2,7-carbazole)s,11, 31 and so on, have been studied for their TE applications.
Doping of Polymers
Conducting polymers possess several attractive features for use as TE elements. They are lightweight, flexible, and cheap. However, they are associated with a major disadvantage of poor efficiency, that is, a relatively low TE ZT. The improvement of the electrical conductivity of these polymers can be achieved by doping the polymer with a sufficient quantity of a suitable doping agent. The doping may be p-type if conduction is through holes or n-type if the conduction is through electrons. For n-type conduction, the value of TE power or Seebeck coefficient ‘S’ is negative, and for p-type conduction, it is positive. The polymers studied for TE applications are usually doped electrochemically or chemically. Previously, the main focus for improved TE performance was on improving the electrical conduction with high level of doping. However, it is found that the Seebeck coefficient decreases when doping increases. This is because as the Fermi energy is pushed inside the conduction band due to increase number of charge carriers, the number of electronic states above and below the Fermi energy becomes more equal. This reduces considerably the transport energy of the charge carriers (the Seebeck coefficient).32 Therefore, it is required to balance these two parameters with a sufficient level of doping that improves the electrical conductivity without compromising the Seebeck coefficient.
There are a variety of p-dopants or oxidizing agents that have been used to dope the polymers for improved electrical conductivity: iodine,30 Fe(III)chloride,11, 30, 31 camphor sulfonic acid (CSA),26 methane sulfonic acid,26 arsenic pentachloride, hexafluorophosphate,25 and so forth. The doping procedures for polymers differ from conventional ion implantation, which is used for three-dimensional semiconductors. The doping process is carried out electrochemically or chemically by exposing the films to vapors or solutions of the dopants. Some doped conjugated polymers have shown very high electrical conductivity.
TE Properties of Various Conducting Polymers
Polyacetylene doped with iodine vapor has shown tremendous improvement in electrical conductivity reaching a value up to 10,000 S cm−1.23 Temperature-dependent electrical conductivity and TE power were measured for the polyacetylene film doped with transition metal halides.33 These results showed a maximum conductivity, δmax = 30,000 S cm−1 at T = 220 K in the FeCl3-doped stretch-oriented polyacetylene. Among all reported conjugated polymers, polyacetylene exhibits a TE PF up to 2 × 10−3 W m−1 K−2 when doped with iodine34 and 8.3 × 10−5 W m−1 K−2 when iron trichloride was used as the doping agent.35 However, polyacetylene is insoluble and unstable in air.
Research was then devoted to more stable aromatic polymers. Among these stable conjugated polymers, polyaniline doped with CSA shows a very good electrical conductivity of 300 S cm−1.36 Hydrochloric acid-doped polyaniline (PANI) were prepared by chemical oxidative polymerization. The maximum ZT can reach 2.67 × 10−4 at 423 K when HCl-doping concentration is 1.0 M.37 Sun et al.38 synthesized naphthalene sulfonic acid (NSA)-doped polyaniline nanotubes (PANI-NT). A sample without specific nanostructure was prepared as a reference. The Seebeck coefficient and the electrical and thermal conductivities of both samples were studied. For a PANI-NT prepared with an aniline/NSA ratio of 4:1, the Seebeck coefficient had a value of 212.4 μV K−1 at 300 K, which was seven times higher than that of the reference sample. Meanwhile, electrical conductivity almost doubled from 4.5 × 10−3 to 7.7 × 10−3 S cm−1, whereas the thermal conductivity reduced from 0.29 to 0.21 W m−1 K−1. Finally, TE performance was evaluated by calculating the TE PF and ZT, and there was an increase by two orders of magnitude for the nanotube-like PANI. Tubular nanostructure was proved to be effective for enhancing the TE performance. This idea might be applicable to other organic TE materials as well.
The electrical conductivity and absolute TE power of polypyrrole were measured between ∼4 and 350 K. Normally doped polypyrrole films had a conductivity of 26 S cm−1, whereas the lightly doped films had a conductivity of 8 S cm−1. The Seebeck coefficient reaches about 5 μV K−1 at 200 K.39 The Seebeck coefficient and conductivity were measured as a function of temperature for a number of polypyrrole samples, including soluble polypyrrole films chemically synthesized and wrinkled films synthesized using indium–tin-oxide electrodes. Other investigated samples included high-conductivity polypyrrole films synthesized at different temperatures and current densities, films grown on nonconducting substrates, and polypyrrole gas sensors. The maximum conductivity of 350 S cm−1 was obtained for polypyrrole doped with PF6.25
Mateeva et al.26 did a comparative study of air-stable polyaniline and polypyrrole at different doping levels. They found by a standard model that conduction by charge carriers of both signs may occur in these doped polymers, which thus leads to reduced TE efficiency. They found presence and near parity of the “ambipolar” conductivities in these polymers. This does not, however, occur in polyacetylene. Thus, polyacetylene, possibly because of conduction with carriers of one sign only, may be a better polymer for TE applications.
TE performances of freestanding and high tensile strength PT and poly(3-methylthiophene) (PMeT) nanofilms electrosynthesized from boron trifluoride diethyl etherate were investigated. They display decent electric conductivity (47 and 73 S cm−1), high Seebeck coefficient (130 and 76 μV K−1), and low thermal conductivity (0.17 and 0.15 W m−1 K−1) at room temperature. Their figure of merit, ZT, is good and reaches 3.0 × 10−2 at 250 K.40 These freestanding PT and PMeT exhibit much better TE performances than those obtained with pressed pellets due to the good arrangement of the polymer chains and preferably oriented structure in the films. Xuan et al.41 measured the electrical conductivity and Seebeck coefficient of a series of heavily doped regioregular poly(3-hexylthiophene) (P3HT) films between 220 and 370 K. The polymer films were doped by immersing them in a freshly prepared 0.001 M NOPF6/acetonitrile solution for a prescribed time interval. The electrical conductivity and Seebeck coefficient of the conducting polymer, P3HT, are very sensitive to the doping produced through its reaction with the oxidant NOPF6. This reaction yields positive charge carriers and PF counterions. The counterions produce a disordered environment within which the p-type electronic carriers can move. Increasingly, heavy doping progressively mitigates the localizing effects of the counterions and decreases the disorder. The electrical conductivity then rises sharply while the Seebeck coefficient falls. The importance of the disorder in conducting polymer explains both the decreasing activation energy of the conductivity as well as the weak temperature dependence of the Seebeck coefficient at high doping level. As a result, the TE PF reaches a broad maximum at between 20 and 30% doping.
Recently, the research group of Katz proposed novel polymer blends of poly(alkylthiophene) in which ground-state hole carriers, created by doping a minor additive component, are mainly at an orbital energy set below the hole energy of the major component of the blend.42 Transport occurs through the major component, which leads to a regime in which hole conductivity and Seebeck coefficient increased together. Although the absolute conductivity and the ZT value of this composite are not particularly high, the work still holds importance by demonstrating a route for designing TE materials in which increases in Seebeck coefficient and conductivity do not cancel each other.
The TE performance of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) has been explored by many researchers. The effects of the dielectric solvent, dimethyl sulfoxide (DMSO), and of the ratio of PEDOT to PSS in the polymeric PEDOT/PSS thin films were studied.43 The change of the PF was mainly dominated by the electrical conductivity, because the Seebeck coefficient remained small and unchanged within a wide range of DMSO and PSS concentrations. The main reason for the poor PF was the low Seebeck coefficient in the PEDOT:PSS system. A figure of merit, ZT, of 9.2 × 10−3 was obtained at room temperature on the addition of 5 vol % DMSO to the PEDOT:PSS formulation. In another study, the TE performance of thin films of PEDOT:PSS was investigated in which the high-boiling solvent DMSO was added to enhance the electrical conductivity.44 By changing the content of DMSO, the electrical conductivity was increased by a factor of two without changing the Seebeck coefficient or the thermal conductivity.
As shown above, polyacetylene shows a good electrical conductivity and PF but its poor stability is a major problem. Other conjugated polymers show better stability but their lower Seebeck coefficient and/or conductivity limit their TE applications. Therefore, the development of new stable polymers in the doped state with a high Seebeck coefficient and electrical conductivity is still needed. In the next section, we will discuss the possibility of using some poly(2,7-carbazole), poly(indolo[3,2-b]carbazole), and poly(diindolocarbazole) derivatives as TE materials.
Poly(2,7-carbazole), Poly(indolo[3,2-b]carbazole), and Poly(diindolocarbazole) Derivatives
After the discovery of conducting polymers in 1977, many carbazole-based conjugated polymers have been reported. In most cases, the carbazole unit is linked at the 3,6-positions when introduced in the main chain and at the 9-position when used as a pendant group, which is mainly related to the easy substitution of the carbazole unit at these positions.45, 46 In 2001, Leclerc et al.46, 47 reported the first synthesis of conjugated polymers based on 2,7-carbazole units. The flexible synthetic route of the 2,7-carbazole moiety allows to easily modify the nature of the reactive groups at the 2- and 7-positions providing ample opportunities to alternate the carbazole unit with other aryl groups to modulate the physical and chemical properties of the resulting copolymers.
Electronic band structure calculations performed on poly(2,7-carbazole) derivatives have revealed good potential for these materials in thermoelectrical applications. Moreover, it was recently found for poly(2,7-carbazole) derivatives that the nitrogen atom is oxidized before the backbone, and the charge is then localized.45–49 This characteristic is very favorable to produce large Seebeck coefficients. However, it could have a negative effect on the polymer conductivity due to charge-carrier pinning.
To overcome this problem, poly(2,7-carbazolylenevinylene) derivatives were investigated. The vinylene unit could increase the conductivity of the polymers, without lowering their Seebeck coefficients. To increase the solubility of N-substituted poly(2,7-carbazolylenevinylene)s, alkyl substituents were added at the 3,6-positions of the carbazole unit (Scheme 2). This polymer 1 is soluble in chloroform or tetrahydrofuran and forms flexible freestanding films when cast from a chloroform solution. The polymer shows very good thermal stability.49
The best compromise obtained between the electrical conductivity and the Seebeck coefficient led to a maximum TE PF of 7.5 × 10−8 W m−1 K−2 for polymer 1.49 The doped polymer is neither very stable in air nor in dry nitrogen atmosphere. The vinylene unit may be responsible for lowered stability of the doped film.
A promising approach to increase the electric conductivity and stability of 2,7-carbazole-based polymers was to replace the vinylene unit by electron-donating conjugated units such as thiophene or bis(3,4-ethylenedioxythiophene), as shown with polymers 2–6 (Scheme 3). The (indolo[3,2-b]carbazole)-based copolymers 7–9 and polydiindolocarbazoles 10 and 11 have relatively high electrical conductivity. Moreover, as the first oxidation occurs on the nitrogen atom, the charge will be more localized for the para-coupled copolymers 7, 8, and 10, which should have a higher Seebeck coefficient than their corresponding isomers (copolymers 9 and 11).50, 51 To obtain higher electrical conductivities, different poly(2,7-carbazole) and poly(indolo[3,2-b]carbazole) copolymers bearing N-benzoyl side chains were prepared.52
This study shows that the introduction of conjugated moieties such as bithiophene units as comonomer facilitates the doping process and improve the structure organization of the polymers in the films.53 The Seebeck coefficients of all poly(2,7-carbazole) derivatives and para-coupled poly(indolo [3,2-b]carbazole)s studied here were in the range of 53 and 71 μV K−1. Therefore, their PFs are mainly controlled by their respective conductivities.
The performance was improved by further modifying the structure of the poly(2,7-carbazole). Poly(2,7-carbazole-alt-bithiophene)s 12 exhibit better performances probably due to their longer π-conjugation and planar backbone allowing better interchain interactions (Scheme 4). It is also shown that the nature of the side chain on the nitrogen is important not only for increasing the solubility of the polymers but also for improving their molecular organization in thin films. Keeping these requirements in mind, three high molecular weight poly(2,7-carbazole) derivatives were synthesized.20, 54 The polymer containing structuring alkyl chain (A) was noted as 12(A). To further improve the structural organization in polymer 12, we introduced a second structuring benzene unit (B) between the carbazole and bi-thiophene units of our copolymers and noted it as 12(B). The polymer 12 having benzothiadiazole unit(C) was termed as 12(C). The resulting polymers show good thermal stability (Td), high glass transition temperature (Tg) values, and excellent solubility in chloroform, chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene.11
The X-ray data confirmed that well-structured films were obtained with this new family of polymers.55–59 TE materials must combine both high electrical conductivities and high Seebeck coefficients. Unfortunately, as mentioned above, the Seebeck coefficient usually decreases when the electrical conductivity is very high. The maximum as well as the optimized TE data for 12(A), 12(B), and 12(C) are given in Table 1.
The evaluation of their TE properties in doped films revealed high electrical conductivity (up to 500 S cm−1) and a relatively high Seebeck coefficient (ca. 70 μV K−1). The best compromise between these two TE parameters led to a maximum value of 19 μW m−1 K−2 as the PF. Good air stability was also observed with these TE polymers.11
Polymer–CNT Composites as TE Materials
Since their discovery, carbon nanotubes (CNTs) are being studied extensively in all fields of applied and basic research.60 Among all nanostructures that are available, CNTs are unique materials. They have remarkable electronic, physical, and mechanical properties. There are two types of CNTs; the one which have single graphene sheet wrapped as a cylinder are called the single-walled nanotubes (SWNTs). The other type is the multi-walled nanotubes (MWNTs), which are an array of concentric cylinders. CNTs are the stiffest materials known to date and exhibit novel electronic properties. The extraordinary electronic, mechanical, and adsorption properties of CNTs have suggested many possible applications.61, 62 They also represent a valuable starting point for preparing new nanocomposites. SWNT are one-dimensional nanowires that can be metallic or semiconducting. They have much better structure and low density, and this characteristic helps in formation of better composites, because the dispersion of CNTs in conjugated polymers is good as the conjugated polymer matrix also has delocalized bonds.62 The SWNT readily accept electrons, which can then be transported under nearly ideal conditions along the main axis.63 These days, much attention has been paid to the use of CNTs in conjugated polymer composites to harness their exceptional intrinsic properties.64, 65 Conjugated polymers with the incorporation of CNTs show great potential for electronic device applications, such as organic field emitting displays,66 photovoltaic cells,67, 68 and gas-sensing devices.69
As discussed in the previous sections, their relatively low electrical conductivity has excluded conjugated polymers as feasible candidates for TE applications. To solve this problem, many attempts have been made to prepare polymer–CNT composites. Composites of polyamide-6 and CNTs have been prepared on a corotating twin screw extruder. The electrical conductivity of these composites was analyzed and compared with carbon black-filled polyamide-6. It was found that the CNT-filled polyamide-6 showed an onset of the electrical conductivity at low filler loadings (4–6 wt %). In agreement with rheological measurements, this onset in the conductivity was attributed to a percolation of nanotubes. The mechanical properties of these composites were investigated and compared with carbon black-filled polyamide-6. It was shown that the nanotubes cause a significant increase in stiffness, but on the other hand, this material shows a brittle behavior indicated by the low elongation at break in the tensile test.65
There are different approaches by which these composites could be prepared. Poly(vinyl acetate)–CNT composites have been prepared by a segregated network approach. These materials are light weight and the electrical conductivity is greatly enhanced after the incorporation of CNT in the interstitial spaces of the aqueous polymer emulsion.64 With a CNT concentration of 20 wt %, these composites exhibit an electrical conductivity of 4800 S m−1, thermal conductivity of 0.34 W m−1 K−1, and a TE ZT greater than 0.006 at room temperature.70
The electrical properties of single-walled carbon nanotubes (SWNTs) embedded in a poly-3-octylthiophene matrix were also investigated as a function of SWNT concentration. It was found that doping of a conjugated polymer with single-walled CNTs to form a composite increases the conductivity by six orders of magnitude. This was mainly due to the introduction of conducting paths to the polymer.64 The TE properties of CNT-filled polymer composites was enhanced by modifying junctions between CNTs using PEDOT:PSS, yielding high electrical conductivities (up to 40,000 S m−1) without significantly altering Seebeck coefficient. Carrier transport at the junction was found to be strongly dependent on the type and concentration of stabilizers. The crucial role of stabilizers was revealed by characterizing transport characteristics of composites synthesized by electrically conducting PEDOT:PSS and insulating gum Arabic (GA) with 1:1–1:4 weight ratios of CNT to stabilizers. The highest TE ZT in this study was estimated to be ∼0.02 at room temperature, which was at least one order of magnitude higher than most polymers and higher than that of bulk Si.71
The electrical conductivity, thermoelectrical, and optical properties of the polyaniline containing boron/double-walled carbon nanotubes (DWNTs) composites were investigated. The electrical conductivities of the composites prepared with 1, 5, and 8% CNT concentrations at 300 K were found to be 5.3 × 10−6, 2.7 × 10−4, and 1.1 × 10−3 S cm−1, respectively. The thermoelectrical results indicate that all the samples exhibit n-type electrical conductivity. The obtained results suggest that the electrical conductivity of PANI-B polymer is improved by DWNT doping.72
In parallel, we have decided to investigate composites based on CNTs and polymer 12(C). The conducting polymer that we have chosen is 12(C), because it has given a high electrical conductivity and a good Seebeck coefficient on doping with FeCl3. The new composite of this polymer with CNTs has an enhanced conductivity and a better TE performance than that of the polymer 12(C) alone. In this work, the composites containing well-dispersed CNTs in different weight percentage have been prepared in freestanding film form to investigate their TE properties.
The CNTs were purified and functionalized using the hydrothermal method.73 The TE measurements of the composites were made after a p-doping with different concentrations of FeCl3 (0.01 to 0.1 M) at various intervals of time. Table 2 shows the best values of TE parameters for different percentages of CNTs in the nanocomposites at various concentrations of FeCl3 at different intervals of time.73 It is seen that 12(C)/CNT 10 wt % composite gives the highest PF at 26 μW m−1 K−2. The electrical conductivity of this composite is also very high at 440 S cm−1.
From Table 2, it is also seen that the conductivity increases when the wt % of CNTs is increased from 5 to 10% in the polymer matrix. This indicates that for weight fractions of CNT below 10%, the nanotubes are almost isolated with the electrical conductivity governed mainly by the electrical characteristics of the polymer. By increasing the weight fraction of CNTs, the average distance between the nanotube bundles becomes sufficiently small to favor electron tunneling in the polymer through the formation of physical contacts between the nanotube bundles at high concentrations.64 However, one can see that as the concentration of CNTs is increased to 20%, the electrical conductivity decreases. On the basis of scanning electron microscope images, this result could be explained by a nonhomogeneous distribution of the CNTs at higher concentrations.
Organic–Inorganic Composite Materials for Future TE Applications
As a new strategy, recent studies have been devoted toward development of composite materials consisting of both organic and inorganic entities. The composites formed by two molecular networks have gained much attention due to the fact that such materials possess unusual physical properties of their own or a combination of both species. Composites of organic–inorganic polymer have gained importance as TE materials, because they can be tuned for achieving high electrical conductivity, which is characteristic of inorganic materials and a poor thermal conductivity that is an inherent property of organic polymers.
For instance, the research group of Katz74 has developed a novel material comprising Bi2Te3 and PEDOT:PSS products. Newly commercialized PEDOT: PSS products CLEVIOS PH1000 and FE-T show unexpectedly higher Seebeck coefficients leading to promising TE PFs ∼47 and 30 μW m−1 K−2, respectively. By incorporating both n- and p-type Bi2Te3 ball-milled powders into the PEDOT:PSS products, PF enhancements for both p and n polymer composite materials were achieved.
In another study, the synthesis and TE characterization of composite nanocrystals composed of a tellurium core functionalized with the conducting polymer PEDOT:PSS have been reported.75 The electrical conductivity of solution-processed nanocrystal films outperformed both PEDOT:PSS and unfunctionalized Te nanorods while retaining a polymeric thermal conductivity, resulting in an impressive room temperature ZT close to 0.1.