Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity.[1] Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive polymers are generally not plastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. They can offer high electrical conductivity but do not show mechanical properties as other commercially used polymers do. The electrical properties can be fine-tuned using the methods of organic synthesis [2] and by advanced dispersion techniques.[3]
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The linear-backbone "polymer blacks" (polyacetylene, polypyrrole, and polyaniline) and their copolymers are the main class of conductive polymers. Historically, these are known as melanins. Poly(p-phenylene vinylene) (PPV) and its soluble derivatives have emerged as the prototypical electroluminescent semiconducting polymers. Today, poly(3-alkylthiophenes) are the archetypical materials for solar cells and transistors.[2]
The following table presents some organic conductive polymers according to their composition. The well-studied classes are written in bold and the less well studied ones are in italic.
| The main chain contains | Heteroatoms present | ||
|---|---|---|---|
| No heteroatom | Nitrogen-containing | Sulfur-containing | |
| Aromatic cycles |
|
The N is in the aromatic cycle:
The N is outside the aromatic cycle:
|
The S is in the aromatic cycle:
The S is outside the aromatic cycle: |
| Double bonds |
|
||
| Aromatic cycles and double bonds | |||
There are many methods for the synthesis of conductive polymers. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. Such reactions entail dehydrogenation:
The low solubility of most polymers presents challenges. Some researchers have addressed this through the formation of nanostructures and surfactant-stabilized conducting polymer dispersions in water. These include polyaniline nanofibers and PEDOT:PSS. These materials have lower molecular weights than that of some materials previously explored in the literature. However, in some cases, the molecular weight need not be high to achieve the desired properties.
The conductivity of such polymers is the result of several processes. E.g., in traditional polymers such as polyethylenes, the valence electrons are bound in sp3 hybridized covalent bonds. Such "sigma-bonding electrons" have low mobility and do not contribute to the electrical conductivity of the material. However, in conjugated materials, the situation is completely different. Conducting polymers have backbones of contiguous sp2 hybridized carbon centers. One valence electron on each center resides in a pz orbital, which is orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have high mobility when the material is "doped" by oxidation, which removes some of these delocalized electrons. Thus, the conjugated p-orbitals form a one-dimensional electronic band, and the electrons within this band become mobile when it is partially emptied. The band structures of conductive polymers can easily be calculated with a tight binding model. In principle, these same materials can be doped by reduction, which adds electrons to an otherwise unfilled band. In practice, most organic conductors are doped oxidatively to give p-type materials. The redox doping of organic conductors is analogous to the doping of silicon semiconductors, whereby a small fraction silicon atoms are replaced by electron-rich (e.g., phosphorus) or electron-poor (e.g. boron) atoms to create n-type and p-type semiconductors, respectively.
Although typically "doping" conductive polymers involves oxidizing or reducing the material, conductive organic polymers associated with a protic solvent may also be "self-doped."
The most notable difference between conductive polymers and inorganic semiconductors is the electron mobility, which until very recently was dramatically lower in conductive polymers than their inorganic counterparts. This difference is diminishing with the invention of new polymers and the development of new processing techniques. Low charge carrier mobility is related to structural disorder. In fact, as with inorganic amorphous semiconductors, conduction in such relatively disordered materials is mostly a function of "mobility gaps"[4] with phonon-assisted hopping, polaron-assisted tunneling, etc., between localized states. Recently, it has been reported that Quantum Decoherence on localized electron states might be the fundamental mechanism behind electron transport in conductive polymers.[5]
The conjugated polymers in their undoped, pristine state are semiconductors or insulators. As such, the energy gap can be > 2 eV, which is too great for thermally activated conduction. Therefore, undoped conjugated polymers, such as polythiophenes, polyacetylenes only have a low electrical conductivity of around 10−10 to 10−8 S/cm. Even at a very low level of doping (< 1%), electrical conductivity increases several orders of magnitude up to values of around 0.1 S/cm. Subsequent doping of the conducting polymers will result in a saturation of the conductivity at values around 0.1–10 kS/cm for different polymers. Highest values reported up to now are for the conductivity of stretch oriented polyacetylene with confirmed values of about 80 kS/cm.[6][7][8][9][10][11] Although the pi-electrons in polyactetylene are delocalized along the chain, pristine polyacetylene is not a metal. Polyacetylene has alternating single and double bonds which have lengths of 1.44 and 1.36 Å, respectively.[12] Upon doping, the bond alteration is diminished in conductivity increases. Non-doping increases in conductivity can also be accomplished in a field effect transistor (organic FET or OFET) and by irradiation. Some materials also exhibit negative differential resistance and voltage-controlled "switching" analogous to that seen in inorganic amorphous semiconductors.
Despite intensive research, the relationship between morphology, chain structure and conductivity is still poorly understood.[13] Generally, it is assumed that conductivity should be higher for the higher degree of crystallinity and better alignment of the chains, however this could not be confirmed for PEDOT and polyaniline, which are largely amorphous.
Conductive polymers enjoy few large-scale applications due to their poor processability. They have been known to have promise in antistatic materials[2] and they have been incorporated into commercial displays and batteries, but there have had limitations due to the manufacturing costs, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly melt process. Literature suggests they are also promising in organic solar cells, printing electronic circuits, organic light-emitting diodes, actuators, electrochromism, supercapacitors, biosensors, flexible transparent displays, electromagnetic shielding and possibly replacement for the popular transparent conductor indium tin oxide.[14] Conducting polymers are rapidly gaining attraction in new applications with increasingly processable materials with better electrical and physical properties and lower costs. The new nanostructured forms of conducting polymers particularly, provide fresh air to this field with their higher surface area and better dispersability.
With the availability of stable and reproducible dispersions, PEDOT and polyaniline have gained some large scale applications. While PEDOT (poly(3,4-ethylenedioxythiophene)) is mainly used in antistatic applications and as a transparent conductive layer in form of PEDOT:PSS dispersions (PSS=polystyrene sulfonic acid), polyaniline is widely used for printed circuit board manufacturing – in the final finish, for protecting copper from corrosion and preventing its solderability.[3]
Electroluminescence is light emission stimulated by electrical current. In organic compounds, electroluminescence has been known since the early 1950s, when Bernanose and coworkers first produced electroluminescence in crystalline thin films of acridine orange and quinacrine. In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doping. In some cases, similar light emission is observed when a voltage is applied to a thin layer of a conductive organic polymer film. While electroluminescence was originally mostly of academic interest, the increased conductivity of modern conductive polymers means enough power can be put through the device at low voltages to generate practical amounts of light. This property has led to the development of flat panel displays using organic LEDs, solar panels, and optical amplifiers.
Since most conductive polymers require oxidative doping, the properties of the resulting state are crucial. Such materials are salt-like (polymer salt), which diminishes their solubility in organic solvents and water and hence their processability. Furthermore, the charged organic backbone is often unstable towards atmospheric moisture. Compared to metals, organic conductors can be expensive requiring multi-step synthesis. The poor processability for many polymers requires the introduction of solubilizing or substituents, which can further complicate the synthesis.
Experimental and theoretical thermodynamical evidence suggests that conductive polymers may even be completely and principally insoluble so that they can only be processed by dispersion.[15]
Most recent emphasis is on organic light emitting diodes and organic polymer solar cells.[16] The Organic Electronics Association is an international platform to promote applications of organic semiconductors.[17] Conductive polymer products with embedded and improved electromagnetic interference (EMI) and electrostatic discharge (ESD) protection have led to both prototypes and products. For example, Polymer Electronics Research Center at University of Auckland is developing a range of novel DNA sensor technologies based on conducting polymers, photoluminescent polymers and inorganic nanocrystals (quantum dots) for simple, rapid and sensitive gene detection. Typical conductive polymers must be "doped" to produce high conductivity. To date, there remains to be discovered an organic polymer that is intrinsically electrically conducting.[18]
There are multiple reviews of the history of the field.[1][20] The first report on polyaniline goes back to the discovery of aniline. In the mid-19th century, Letheby reported the electrochemical and chemical oxidation products of aniline in acidic media, noting that reduced form was colourless but the oxidized forms were deep blue. In the early 20th century, German chemists named several compounds "aniline black" and "pyrrole black" and used them industrially. Classically, such polymer "blacks", their parent compound polyacetylene, and their co-polymers were called "Melanins".[21]
The first highly-conductive organic compounds were the charge transfer complexes. In the 1950s, researchers reported that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens.[2] In 1954, researchers at Bell Labs and elsewhere reported organic charge transfer complexes with resistivities as low as 8 ohms-cm.[22][23] In the early 1970s, salts of tetrathiafulvalene were shown to exhibit almost metallic conductivity, while superconductivity was demonstrated in 1980. Broad research on charge transfer salts continues today. While these compounds were technically not polymers, this indicated that organic compounds can carry current. While organic conductors were previously intermittently discussed, the field was particularly energized by the prediction of superconductivity[24] following the discovery of BCS theory.
In 1963 Australians Bolto, DE Weiss, and coworkers reported iodine-doped oxidized polypyrrole blacks with resistivities as low as 1 ohm·cm.[25][26][27] This Australian group eventually claimed to reach resistivities as low as 0.03 ohm·cm with other conductive organic polymers. This resistivity is roughly equivalent to present-day efforts. The 1964 monograph Organic Semiconductors[28] cites multiple reports of similar high-conductivity oxidized polyacetylenes. With the notable exception of Charge transfer complexes (some of which are even superconductors), organic molecules were previously considered insulators or at best weakly conducting semiconductors. Subsequently, DeSurville and coworkers reported high conductivity in a polyaniline.[29] Likewise, in 1980, Diaz and Logan reported films of polyaniline that can serve as electrodes.[30]
Similarly, much early work on the physics and chemistry of conductive polymers was done under the melanin rubrick. This was because of the medical relevance of this material. For example, in the 1960s Blois et al. showed semiconduction in melanins, as well as further defining their physical structures and properties[31] Nicolaus et al. further defined the conductive polymer structures.[32] Classically, all polyacetylenes, polypyrroles and polyanilines are melanins, "The most simple melanin can be considered the acetylene-black from which it is possible to derive all the others.. Substitution does not qualitatively influence the physical properties like conductivity, colour, EPR, which remain unaltered."[33]
However, while mostly operating in the quantum realm of less than 100 nanometers, "molecular" electronic processes can collectively manifest on a macro scale. Examples include quantum tunneling, negative resistance, phonon-assisted hopping, polarons, and the like. Thus, macro-scale active organic electronic devices were described decades before molecular-scale ones. E.g., in 1974, John McGinness and his coworkers described the putative "first experimental demonstration of an operating molecular electronic device".[20] This was an "active" organic-polymer electronic device, a voltage-controlled bistable switch.[34] As its active element, this device used DOPA-melanin, a well-characterized self-doping copolymer of polyaniline, polypyrrole, and polyacetylene. The "ON" state of this device exhibited almost metallic conductivity, and exhibited low conductivity with switching, with as much as five orders of magnitude shifts in current. Their material also exhibited classic negative differential resistance.
In 1977, Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa reported similar high conductivity in oxidized iodine-doped polyacetylene. This research earned them the 2000 Nobel prize in Chemistry "For the discovery and development of conductive polymers."[21] The Nobel citation made no reference to Weiss et al.'s similar earlier work (see Nobel Prize controversies). Because of the numerous earlier reports of similar compounds, reviewers question the Nobel citation's discovery assignment. Thus, Inzelt notes that,[1] while the Nobelists deserve credit for publicising and popularizing the field, conductive polymers were " ..produced, studied and even applied " [35] well before their work.