Thermoelectricity, by converting heat energy directly into useable electricity, offers a promising technology to convert heat from solar energy and to recover waste heat from industrial sectors and automobile exhausts. In recent years, most of the efforts have been done on improving the thermoelectric efficiency using different approaches, that is, nanostructuring, doping, molecular rattling, and nanocomposite formation. The applications of thermoelectric polymers at low temperatures, especially conducting polymers, have shown various advantages such as easy and low cost of fabrication, light weight, and flexibility. In this review, we will focus on exploring new types of polymers and the effects of different structures, concentrations, and molecular weight on thermoelectric properties. Various strategies to improve the performance of thermoelectric materials will be discussed. In addition, a discussion on the fabrication of thermoelectric devices, especially suited to polymers, will also be given. Finally, we provide the challenge and the future of thermoelectric polymers, especially thermoelectric hybrid model.
Global energy uncertainty and the limited resources coupled with increased energy demands provide the impetus for improving the efficiency of energy conversion technologies [
The performance of the thermoelectric material is evaluated by the dimensionless figure of merit
Early thermoelectrical devices developed in the early 1960s earned some popularity given the solid state nature of the devices, that is, no moving parts compared to generators and motors. These devices were mainly based on Bi2Te3 [
Numerous research works have been carried out to address these issues by replacing the metal- and alloy-based TE materials with organic and polymer materials [
In this paper, a review on the use of polymers in thermoelectric materials and devices will be given. The effect of different polymer structures, molecular concentration, and weight on thermoelectric properties will also be highlighted. Next, useful fabrication methods for solution-process-based fabrication, such as spin coating, inkjet printing, and electrospinning, shall be provided.
Polymers as TE materials have attracted a lot of attention recently due to its easy fabrication processes and low material cost [
Different types of polymers have been used in thermoelectric devices, such as polyaniline (PANI) [
Thermoelectric property of various polymers.
Polymer | Conductivity |
Seebeck coeffient S |
Thermal conductivity |
---|---|---|---|
Polyacetylene [ |
~1.53 × 10−3–2.85 × 104 | ~−0.5–1077 | — |
poly( |
10−5 | 7 | 7.2 × 10−11 |
Polyaniline [ |
7000 | 7 | 5.1 × 10−2 |
poly(2,7-carbazolenevinylne) [ |
5 × 10−3 | 230 | 8.0 × 10−5 |
poly(2,5-dimethoxyphenylenevinylene) [ |
46.3 | 39.1 | — |
Various polymer structures of (a) polyaniline, (b) poly(2,7-carbazolenevinylene) [
Hiroshige et al. [
Conductivity dependence of monomer concentration [
It is found that the molecular weights of the polymers have a substantial effect on the electron mobility and consequently affect the electrical conductivity. Kline et al. [
Field-effect mobility versus the number average molecular weight [
As a semiconducting polymers, the electrical conductivity increases with increasing temperature [
Temperature dependence of electrical conductivities for polythieno[3,2-b]thiophene (PTT), poly(1,12-bis(carbazolyl)dodecane) (P2Cz-D), and copolymers synthesized with monomer feed ratios of TT/2Cz-
Thermoelectric properties of conductive polymers tend to change with humidity. When humidity is high, the polymer forms carriers from the chemical dopants to result in high electrical conductivity. However, this also causes a corresponding decrease in the Seebeck coefficient. Thus, the improvement of the overall
Thermoelectric properties of poly(3-hexylthiophene) P3HT-triflimide anion (TFSI) samples were filled and open symbols are samples that were kept inside and outside of desiccator, respectively [
Alignment of the polymer chains is also known to increase the electrical conductivity of conductive polymers and thus increasing the
Thermoelectric power factor and
The electric conductivity can be improved by doping the polymers with a sufficient amount of suitable dopants. However, an increase in dopants leads to the decrease in the Seebeck coefficient. As the number of charge carriers increases, the Fermi energy is forced deep inside the conduction band [
Seebeck coefficient and resistivity as a function of the doping level [
In 2011, Bubnova et al. [
CNTs are known to have a stable one-dimensional nanostructure and excellent electrical and mechanical properties. When it is incorporated into a polymer matrix, the thermoelectric properties of the materials are affected. Kim et al. [
Effect of CNT doping on the electrical conductivity and Seebeck coefficient [
Unlike CNTs, the addition of inorganic materials such as Te nanorods [
CNT is hydrophobic and tends to entangle in water which hinders complete dispersion and/or exfoliation in water [
According to Moriarty et al. [
(a) and (b) show schematic diagram of carbon nanotubes dispersed in two different stabilizing agents, and (c) and (d) show the formation of network after water is dried out [
Thermal conductivity of sodium deoxycholate (DOC) and meso-tetra(4-carboxyphenyl)porphine (TCPP) stabilized systems containing different percentages of carbon nanotubes [
The use of nanofibers in thermoelectric devices can be potentially useful in achievement of a higher thermoelectric
There is an extensive body of theoretical work concerned with efforts to improve
The use of nanostructures also allows selective blocking of phonons whilst allowing transportation of charge carriers. This allows decoupling of thermal and electrical conductivity, which usually increase in tandem in normal conducting materials. This is attributed to the fact that if one increases the concentration of charge carriers in a thermoelectric medium, then this presents a higher possibility of collision of charge carriers with the crystal lattice, and hence thermal conductivity correspondingly increases. On the other hand, if the effective wavelengths of phonon and electrons are recognized and their effects separated, then the effects of thermal and electrical conductivity may be decoupled. The motion of phonons that carries most of the heat has mean free paths of hundreds of nanometers, whereas electrons have 10 nm or less. Hence, it is feasible to confine the movement of phonons without obstructing the electron transportation [
In the previous section, it is anticipated that conductive polymers can be potential as thermoelectric materials, given their low thermal conductivity and improved electrical transport through doping with different nanoparticles [
The electrospinning technique is a simple and elegant method to produce nanofibers. In 1934, Formhals patented a process to produce polymer filaments using electrostatic force. Later on, the process evolved and was named as electrospinning [
A basic electrospinning setup [
In the electrospinning process a charged liquid polymer solution is introduced into an electric field. A high voltage (HV) direct current (DC) power supply is used to generate the potential differences in the range between 10 and 30 KV. A needle attached to a syringe is used to dispense the liquid polymer solution at a desired voltage between 10 and 30 kV. After that it is deposited on a collector which is grounded. The cathode of the HV power supply is attached to a wire and inserted into the syringe containing the polymer solution and the anode is attached to the ground. A rotating drum, usually wrapped with aluminum foil can be used as a collector. The tip to collector distance is maintained between the ranges of 10–30 cm. The inner diameter of a needle can be between 0.5–1.5 mm. The ejected polymer solution forms a continuous nanofiber when the high voltage overcomes the surface tension. Once the ejection starts, at the tip of the needle, the pendant droplet of the polymer solution forms a conical shape, typically referred to as Taylor cone. Whilst the fluid is charged, the surface charge and the surface tension operate in opposite relation. Therefore, the fluid changes shape and the formed structure is known as the Taylor cone [
Formation of the Taylor cone [
The key parameters which affect the formation of nanofibers are (1) solution parameters such as viscosity, conductivity, surface tension, and vapor pressure; (2) process parameters such as shape of collector, needle diameter, solution flow rate, tip to collector distance, and applied voltage; (3) ambient parameters such as solution temperature, humidity, and air velocity in the electrospinning chamber. By varying these parameters the thickness and smoothness of the fibers can be controlled [
SEM micrographs of a polypyrrole electrospun nanofibers, formed from aqueous solutions of 1.5 wt% poly(ethylene oxide) as carrier, with (a) and without (b) 0.5 wt% Triton X-100 surfactant. The polypyrrole content of the nanofibers is 71.5 wt% [
Recently the inkjet printing approach has been adopted for fabrication of electronics devices, where the colored ink cartridge is replaced with a functional electronic ink, for example, semiconducting ink and conductive ink. Today, inkjet printing is used in electronic industries and research for fabrication of printed circuit boards (PCB), light-emitting diodes (LED), thin film transistors (TFT), solar cells, thermoelectric devices, and many others [
With that being said, inkjet printing is an attractive method to be used in the fabrication of the whole or part of the structure of thermoelectric devices. Traditionally, thermoelectric devices structure is produced by conventional printing such as lithography and roll-to-roll printing [
Step by step process for fabrication of device using inkjet printing [
The field of printable electronics is becoming more significant. This technology requires the active electronic materials, such as metals, organometallics, nanoparticles, and biopolymers in solution form, and thus soluble organic or polymeric materials are attractive candidates for this processing method. The inkjet printing method is also tuned for room temperature processing, and only a small amount (in the region of picoliters for a cartridge) is required to produce a device, thus resulting in substantial cost reductions [
Printing quality is strongly influenced by the ink properties. Printing resolution depends on the ink viscosity and surface tension [
Stroboscopic images of droplets produced by inkjet printing [
Inkjet printing inks including sol-gel, conducting polymers, ceramics, metals, nanoparticles, and biopolymers ink have been used widely for various inkjet-printed devices [
Examples of conducting polymers commonly used in inkjet printing are polyaniline (PANI) and PEDOT : PSS [
Comparison of (a) PEDOT and (b) F8 ink droplets [
Printing quality is also controlled by substrates surfaces and ink interaction [
As is the case with polymeric thermoelectric materials in general, the inkjet printing process is best suited for low temperature thermoelectric materials, for applications in ambient temperature such as hybrid solar cells, and body heat electricity generation. Even higher resolutions may be attained using electrohydrodynamic jet printing, which allows for high resolution, precision, and speed printing [
Thus, given the attractiveness of the inkjet printing method, that is, solution processability, low amounts of ink, direct patterning of device structure onto substrate, the possibility of flexible substrates, and on-demand fabrication, there is much potential for fabrication of thermoelectric devices using inkjet printing.
In 1890, Mond, Langer, and Quincke developed the Chemical Vapor Deposition (CVD) technique for large scale applications as a carbonyl process to refine nickel. Numerous earliest applications were implicated in refining or purification of metals and a limited number of nonmetals by carbonyl or halide processes. In recent times, CVD, research, and development efforts have been more concentrated towards the thin-film deposition. It is actually widely used in materials-processing technology. Mostly, it is involved in solid thin-film coatings to surfaces. However, this technique is recurrently used in producing carbon nanotubes (CNTs) high-purity bulk materials and powders, as well as fabricating composite materials via infiltration techniques. So far, the majority of the elements in the periodic table have been deposited by CVD techniques, with some being in the form of the pure elements, but mostly in combinations to form compounds [
In CVD process, precursor gases are brought into a reaction chamber in an activated (light, plasma, and heat) environment and directed towards a heated substrate. Thus, a controlled chemical reaction is induced. The chemical reactions result in the deposition of a solid thin film material onto the substrate surface. It is a very useful processing method for the deposition of polycrystalline, amorphous, and single-crystalline thin films and coatings for a wide range of applications [
Schematic diagram of a simple CVD setup [
A high-temperature tube furnace with a quartz tube can be used to fabricate nanofibers at temperatures ranging from 300°C to 1200°C. The reaction duration can be varied in the range of 15 min to 8 h according to the desired length of nanowires and thickness of the thin films. The synthesis can be regulated through several parameters such as hydrocarbon’s concentration, catalyst, temperature, pressure, gas-flow rate, deposition time, and reactor’s geometry. Recently, CVD has attracted a lot of attention in producing CNTs through thermal CVD or catalytic CVD (to distinguish it from many other kinds of CVD used for various purposes) [
High magnification SEM pictures of vertically aligned CNTs on samples ((a)–(d)) indicating the different degrees of tube alignment [
Electrochemical deposition or in short electrodeposition has been used in producing thin films and has been intensively used for the last 35 years [
This technique is used as an electrochemical liquid phase thin film preparation method. Usually, the design of an electrochemical cell depends on the specific needs of the experiment. In general, 25–50 mL cell can be used for the laboratory experiments for the sake of handiness. In this process, the reactions are either reduction or oxidation, completed by using an external current source. To carry out the deposition process, an electrochemical cell consists of a reaction vessel and two or three electrodes. In the three-electrode cell, a reference electrode is used to control or measure the potential of the working electrode. However, the current passes between the working electrode and a separate auxiliary or counter electrode. Depositions are controlled by regulating either current or potential. All the compartments of the cell can be separated by using glass frit to reduce the interference of electrochemical reactions. This setup is used when the cell resistivity is relatively higher. The working electrodes work as cathode. Gold, platinum, carbons, mercury, and some semiconductors can be used as working electrodes. Most commonly, reference electrode is “Saturated Calomel Electrode (SCE)” or the Ag/AgCl electrode. Platinum wires or mesh can be used as counter electrodes or anodes. The reactions can occur in room temperature [
Recently, thermoelectric material composed of conductive polymer polyaniline (PANI) and Bi2Te3 nanocomposites was prepared using a simultaneous electrochemical reaction and deposition method. The three-electrode system was used in the fabrication process (Figure
The schematic representation of electrochemical deposition system for PANI/Bi2Te3 [
SEM images of (a) pure polyaniline, (b) PANI/Bi2Te3, and (c) PANI/Bi2Te3 [
The developments of polymer thermoelectric materials have seen dramatic increase over the last half decade. Despite that, there is reasonable development to be carried out before real commercialization of polymer-based thermoelectric devices can be realized. Low
There have been a few drawbacks in polymers to be potential TE materials thus far. Lower Seebeck coefficients and lower electrical conductivity are yet to address to employ the polymer TE material as efficient TE device. Low Seebeck coefficient is improved by the introduction of nanostructures. In addition, the effect of humidity on electrical conductivity is also a constraint to increase the
For the past few decades, research on dye-sensitized solar cells (DSSCs) has attracted the attention of many academics due to the simple fabrication process, low cost, and potentially high efficiency of converting sunlight energy to electric energy [
There have been many efforts have been applied to TE materials to harvest electricity from solar energy [
Schematic structure of TE generator-DSSC hybrid system [
Wang et al. [
The developments of polymer thermoelectric materials have seen dramatic increase over the last half decade. The applications of thermoelectric polymers at low temperatures, especially conductive polymers, have shown various advantages compared to inorganic materials such as easy and low cost of fabrication, light weight, flexibility, and low thermal conductivity. However, thermoelectric polymers have shown some drawbacks such as low electrical conductivity and Seebeck coefficient. Nanostructuring, polymer composites, nanotubes, and addition of semiconducting stabilizers have been shown as important approaches to improve the performance of thermoelectric polymers. By taking advantage of different fabrication techniques, the morphology and structure of the thermoelectric materials and devices can be fine-tuned to suit the intended application. Not only that, but also the control of the morphology will allow for an improvement in the performance of the thermoelectric materials. The use of thermoelectricity with on dye-sensitized solar cells and utilizing the waste heat produced by on dye-sensitized solar cells will be a promising technology to generate electricity from solar energy and low temperature heat sources.
The authors would like to thank the University of Malaya (UM.C/1/625/HIR/MOHE/ENG/29) and Malaysia Toray Science Foundation (55-02-03-1061) for supporting this work.