Conductive polymer

Conductive polymers are organic polymers that conduct electricity.^[1] Such compounds may be true metallic conductors or semiconductors. It is generally accepted that metals conduct electricity well and that organic compounds are insulating, but this class of materials combines the properties of both. The biggest advantage of conductive polymers is their processibility. Conductive polymers are also plastics (which are organic polymers) and therefore can combine the mechanical properties (flexibility, toughness, malleability, elasticity, etc.) of plastics with high electrical conductivities. Their properties can be fine-tuned using the exquisite methods of organic synthesis.^[2]

Correlation of chemical structure and electrical conductivity

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. The situation is completely different in conjugated materials. Conducting polymers have backbones of contiguous sp² hybridized carbon centers. One valence electron on each center resides in a p_z 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 p-orbitals form a band, and the electrons within this band become mobile when it is partially emptied. 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" the 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 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"^[3] with phonon-assisted hopping, polaron-assisted tunneling, etc., between localized states.

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 of 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.^{[4][5][6][7][8][9]} 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.^[10] 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.

Classes of materials

Structures of various conductive organic polymers. Polyphenylenevinylene, polyacetylene, polythiophene (X = S) and polypyrrole (X = NH), polyaniline (X = N, NH) and polyphenylene sulfide (X = S).

Well-studied classes of organic conductive polymers include poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, polyanilines, polythiophenes, poly(p-phenylene sulfide), and poly(para-phenylene vinylene)s (PPV). 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. Other less well studied conductive polymers include polyindole, polypyrene, polycarbazole, polyazulene, polyazepine, poly(fluorene)s, and polynaphthalene.^[2]

Synthesis of conductive polymers

Many methods for the synthesis of conductive polymers have been developed. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. Such reactions entail dehydrogenation:

n H-[X]-H → H-[X]_n-H + 2(n-1) H⁺ + 2(n-1) e^-

One challenge is usually the low solubility of the polymer. However, in some cases, the molecular weight need not be high to achieve the desired properties.

Properties and applications

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're also promising in organic solar cells, organic light-emitting diodes, actuators, electrochromism, supercapacitors, biosensors, flexible transparent displays, electromagnetic shielding and possibly replacement for the popular transparent conductor indium tin oxide.^[11] Conducting polymers are rapidly gaining traction 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.

Electroluminescence

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 OLEDs, solar panels, and optical amplifiers.

Barriers to applications

Since most conductive polymers require oxidative doping, the properties of the resulting state are crucial. Such materials are often salt-like, which diminishes their solubility in organic solvents and hence their processibility. 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 processibility for many polymers requires the introduction of solubilizing substituents, which can further complicate the synthesis.

History and trends

voltage-controlled switch, an organic polymer electronic device from 1974. Now in the Smithsonian Chip collection.

The history of the field has been recounted from several perspectives.^[12][13] The first report on polyaniline goes back to the discovery of aniline. In the mid 1800s, 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 1900s, 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".

In the 1950s, it was discovered that polycyclic aromatic compounds formed semi-conducting charge-transfer complex salts with halogens.^[2] This finding indicated that organic compounds could carry current. Organic conductors had been intermittently discussed and the area was energized by the prediction of superconductivity, following the discovery of BCS theory.^[14]

Beginning in 1963, Bolto and co-workers reported conductivity in iodine-doped polypyrroles.^[15][16][17]. This Australian group eventually claimed to reach resistivities as low as 0.03 ohm·cm with other conductive polymers. This resistivity is roughly equivalent to present-day efforts.

For these early discoveries, the polymerization mechanisms were poorly understood. Likewise, primarily because they preceded Neville Mott's work on conduction in disordered materials, conductivity mechanisms were not well modeled. Subsequently, DeSurville and coworkers reported high conductivity in a polyaniline.^[18] In 1980, Diaz and Logan reported films of polyaniline that could serve as electrodes.^[19]

Similarly, because of its medical relevance, much early work on the physics and chemistry of conductive polymers was done under the melanin rubrick. E.g., in the 1960s Blois et al. showed semiconduction in melanins, as well as further defining their physical structures and properties^[20] Nicolaus et al. further defined structures.^[21] Classically, all polyacetylenes, polypyrroles and polyanilines are Melanins. e.g., R. Nicolaus: "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." from The Nature of Animal Blacks ("acetylene-black" = polyacetylene).

In 1974, McGinness and coworkers described^[22] an "active" organic-polymer electronic device, a voltage-controlled bistable switch. This device used DOPA-melanin, a well-characterized self-doping copolymer of polyaniline, polypyrrole, and polyacetylene. The "ON" state of this device 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 and subsequent studies advanced knowledge of the structures and conduction mechanisms in conductive polymers. This research earned them the 2000 Nobel prize in Chemistry "For the discovery and development of conductive polymers."^[23]

Trends

Most recent emphasis is on organic light emitting diodes and organic polymer solar cell.^{[citation needed]} The Organic Electronics Association is an international platform to promote applications of organic semiconductors.^[24] Conductive polymer products with embedded and improved electromagnetic interference (EMI) and electrostatic discharge (ESD) protection has led to prototypes and products. 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. To date, there remains to be discovered an organic polymer that is intrinsically electrically conducting.^[25]

References