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Conductive polymer

Semiconducting and metallic “organic” polymers are based on sp2 hybridized linear carbon chains. The carbon atom has 6 electrons outside the nucleus, of which 4 are valence electrons; i.e. four (the 2s and the 2p electrons) take part in chemical bonds. In free space, or where the potential is spherically symmetric, the 1s and 2s orbitals of the carbon atom are filled, and the 2p orbitals are occupied with 2 electrons. In order to lower the total energy and form spatially-directed chemical bonds, in molecules and crystals, carbon can form two primary hybrid structures; tetrahedrally directed covalent bonds (sp3 hybridized)as in diamond and saturated polymers, and hexagonally directed covalent bonds (sp2 hybridized) as in graphite and conjugated polymers.

Traditional polymers such as polyethylenes are electrical insulators. Since all of the valence electrons are bound in sp3 hybridized covalent bonds, there are no mobile electrons to participate in electronic transport. Conjugated, conducting polymers are formed from sp2 hybridized carbons. Polyacetylene is the simplest conjugated polymer (CH)x. The three in-plane sigma-orbitals of the sp2 hybridized carbon create the “backbone”; two of them bonded to the neighboring carbons and the third sigma-orbital bonded to a hydrogen atom. The fourth electron resides in the pz orbital and, because of its orthogonality to the plane defined by the other three sigma-bonds, it is in first approximation independent of them. This one-electron picture of the pz electron being decoupled from the backone sigma-orbitals gives these polymers special electronic properties.

Although the pi-electrons in polyacteylene are delocalized along the chain, pristine polyacetylene is not a metal. The polymerization of polyacetylene from the monomer acetylene yields a dimerized (bond alternating) structure. The resulting polymer is insoluble and intractable. Consequently, the molecular weight cannot be directly determined. Thus, because the Staudinger index, N, is unknown, polyacetylene is typically designated as (CH)x. The molecular structure of “real” polyacetylene has alternating single and double bonds which are, respectively, longer and shorter than the equilibrium value of the bond length in uniform (CH)x. In this structure, the pi-electrons on neighboring carbon atoms form a weak pi-pi bond resulting in the bond alternating structure (short bond length associated with and indicative of the pi-bond). The bond alternation has been determined from analysis of X-ray diffraction data and from analysis of nuclear magnetic resonance data; the shorter bond length is 1.35 Å and the longer bond length is 1.45 Å. Such a bond alternating structure doubles the unit cell thereby opening a gap in the electronic structure. As a result, because of the bond alternating structure, polyacetylene is a semiconductor in its ground state. Semiconducting polymers can be doped to sufficiently high carrier densities that metallic polymers are achieved. The metallic state of doped conjugated polymers (conducting polymers) is stabilized by interchain interactions sufficiently strong that the systems are anisotropic three-dimensional metals.

When charge carriers (from the addition or removal of electrons) are introduced into the conduction or valence bands (see below) the electrical conductivity increases dramatically. 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, though recent advancements in molecular self-assembly are closing that gap. This low charge carrier mobility is related to amorphous and disordered nature of the solid state nanostructure in the conducting polymers. In fact, as with inorganic amorphous semiconductors, conduction in such relatively disordered materials is mostly a function of "mobility gaps" with phonon-assisted hopping, polaron-assisted tunnelling, etc. between localized states.

Typically "doping" the conductive polymers involves actually oxidizing/reducing of the compound. Conductive organic polymers associated with a protic solvent may also be "self-doped". Melanin is the classic example of both types of doping, being both an oxidized polyacetylene and likewise commonly being hydrated.

The conjugated polymers in their undoped, pristine state are semiconductors/insulators. As such the energy gap is around 2 eV and higher is too big for a considerable excitation of the charge carriers thermally. Therefore, the undoped conjugated polymer such as polythiophene, polyacetylene etc has only a conductivity of around 10^-10 to 10^-8 S/cm . Upon doping the conjugated polymers there is a rapid increase of electrical conductivity of several orders of magnitude up to values of around 10^-1 S/cm even at a very low level of doping such as < 1 %. Subsequent doping of the conducting polymers will result in a saturation of the conductivity at values around 100-10000 S/cm for different polymers. Highest values reported up to now are for the conductivity of stretch oriented polyacetylene with confirmed values of around 80.000 S/cm.

Poly(phenylene vinylene), PPV, is an alternating copolymer of the repeat units of polyacteylene and poly(paraphenylene). PPV and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Today, poly(3-alkylthiophenes) are the archetypical materials for solar cells and transistors.

History

See An Overview of the First Half-Century of Molecular Electronics by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003)

The first report on polyaniline goes back to mid 1800, when a medical doctor Letheby first reported the electrochemical and chemical oxidation products of aniline in acidic media such as human stomach. Early 1900 the German chemistry named the several compounds like "aniline black" of "pyrrolle black" and used them in industrial scale. The electrical properties of such powders as well as the relation of their pi-conjugation with the semiconducting and conducting properties were not investigated and scientifically unknown. In 1963, Australians DE Weiss and coworkers reported high conductivity in oxidized iodine-doped polypyrrole, a polyacetylene derivative. They achieved the quite low resistivity of 1.0 ohm-cm. In a series of detailed papers, they also described the effects of doping with iodine on conductivity, the conductivity type (n or p), and electron spin resonance studies on polypyrrole. The same authors noted an Australia patent application (5246/61, June 5, 1961) for conducting polypyrrole. In 1965 , the Australian group reached resistances as low as .03 ohm/cm with other conductive polymers. This is roughly equivalent to present-day efforts. This extensive work was "lost" until recently. E.g., Diaz et al. are often wrongly credited with discovering conductive polypyrrole in 1979. In 1974, as a "proof of concept" for their version of the now-accepted model of conduction in such materials, John McGinness and his coworkers built and reported a voltage-controlled organic-polymer switch. This device used melanin-- here, a self-doped mixed copolymer of oxidized polyacetylene, polypyrrole and polyaniline. It is now in the Smithsonian's collection of early electronic devices. In the "ON" state, this material has almost metallic conductivity. As Hush notes, this device also exhibited negative differential resistance, now a well-characterized hallmark of electronically-active organic materials. Though in a major journal and (e.g.) the subject of a contemporary news article in the journal Nature, this work was also "lost" until similar devices emerged decades later.

In 1977 Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa reported metallic conductivity in iodine doped polyacetylene similar to that reported a decade earlier by Weiss and coworkers for iodine-doped polypyrolle, which they did not cite. The time finally being ripe. this was followed by extensive research and development in the semiconducting and conducting properties of a large family of conjugated, sp2 hybridized polymers. This large international effort resulted in development of organic, polymeric light emitting diodes, solar cells, transistors.

This work of Heeger, MacDiarmid and Shirakawa eventually resulted in the award of the 2000 Nobel prize in Chemistry. According to the citation, this was "For the discovery and development of conductive polymers" .

Chemistry

Common classes of organic conductive polymers include poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(aniline)s, poly(fluorene)s, poly(3-alkylthiophene)s, polytetrathiafulvalenes, polynaphthalenes, poly(p-phenylene sulfide), and poly(para-phenylene vinylene)s. Classically, these linear backbone polymers are known as polyacetylene, polyaniline, etc. "blacks" or "melanins". The melanin pigment in animals is generally a mixed copolymer of polyacetylene, polypyrrole, and polyaniline. Some fungal melanins are pure polyacetylene.

Doping

In silicon semiconductors, a few of the 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. In contrast, there are two primary methods of doping a conductive polymer, both through an oxidation-reduction (redox) process. The first method, chemical doping, involves exposing a polymer, such as melanin (typically a thin film), to an oxidant (typically iodine or bromine) or reductant (far less common, but typically involves alkali metals). The second is electrochemical doping in which a polymer-coated, working electrode is suspended in an electrolyte solution in which the polymer is insoluble along with separate counter and reference electrodes. An electric potential difference is created between the electrodes which causes a charge (and the appropriate counter ion from the electrolyte) to enter the polymer in the form of electron addition (n doping) or removal (p doping). Polymers may also be self-doped, e.g., when associated with a protonic solvent such as water or an alcohol.

The reason n doping is so much less common is that Earth's atmosphere is oxygen-rich, which creates an oxidizing environment. An electron-rich n doped polymer will react immediately with elemental oxygen to de-dope (re-oxidize to the neutral state) the polymer. Thus, chemical n doping has to be done in an environment of inert gas (e.g., argon). Electrochemical n doping is far less common in research, because it is much more difficult to exclude oxygen from a solvent in a sealed flask; therefore, although very useful, there are likely to be no commercialized n doped conductive polymers.

Electroluminescence

Electroluminescence and photoconductivity in organic compounds has been known since the early 1950s. However, the very poor conductivity of such materials limited current flow and thus light output. In contrast, the increased current flow through conductive polymers and improvements in their efficiency has led to the rapid development of practical polymer-based light emitting devices (OLEDs) and organic photovoltaic devices and polymer solar cell.

Properties

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 the high electrical conductivities of a doped conjugated polymer.

Physics

In addition to "switching", an increase in conductivity can also be accomplished in a field effect transistor (organic FET or OFET), or by irradiation (originally-demonstrated in the 1960s ). Strong coupling can also occur between electrons and phonons (mechanical oscillations such as heat vibrations, particles of sound) since both are constrained to travel along the polymer backbone.

Applications of conducting polymers

Electroluminescence (light emission) in organic compounds 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. 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 has led to the development of flat panel displays using OLEDs, solar panels and optical amplifiers. Conducting polymers are also successfully used in polymer solar cells.

Biological applications

Conductive polymers such as DOPA melanin are present in most mammalian tissues where electrical conduction or transduction from light or sound are necessary, including the skin, eye, inner ear, and Midbrain. Melanin's electronic conductivity and allied phenomena such as strong electron-phonon coupling seems to be the underlying mechanism for absorption of light, and electron-phonon interactions are exploited in hearing . See the main article: Melanin.

Current Research

There are a great number of research projects and research groups at the academic and industrial laboratories worldwide. Most focus is given to organic light emitting diodes and organic polymer solar cell. Organic electronic associationis an international platform to promote applications of organic semiconductors. The European Union is currently funding a pan-European project into the development of conductive Polymers. The Polycondproject involves a consortium of trade associations and SME's from across Europe and is due for completion in January 2009. The main aim for this project is to develop conductive polymer products that have embedded and improved Electromagnetic Interference (EMI) and Electrostatic Discharge (ESD) protection. Research is now at an advanced stage and prototypes of products have been produced and are now being taken through a rigorous testing process to assess the Polymers performance and characterize it. PERC(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.