Conductive polymers
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A conductive polymer is an organic polymer semiconductor, or an organic semiconductor. Roughly, there are two classes-- the Charge transfer complexes and the conductive polyacetylenes. The latter include polyacetylene itself as well as polypyrrole, polyaniline, and their derivatives.
Most commercially produced organic polymers are electrical insulators. Conductive organic polymers often have have extended delocalized bonds (often composed of aromatic units). At least locally, these create a band structure similar to silicon, but with localized states. 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. Technically almost all known conductive polymers are semiconductors due to the band structure and low electronic mobility. However, so-called zero band gap conductive polymers may behave like metals. 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 self assembly are closing that gap.
Delocalization can be accomplished by forming a conjugated backbone of continuous overlapping orbitals. For example, alternating single and double carbon-carbon bonds can form a continuous path of overlapping p orbitals. In polyacetylene, but not in most other conductive polymers, this creates degeneracy in the frontier molecular orbitals (the highest occupied and lowest unoccupied orbitals named HOMO and LUMO respectively). This leads to the filled (electron containing) and unfilled bands (valence and conduction bands respectively) resulting in a semiconductor.
However, conductive polymers generally exhibit very low conductivities. 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 and not band gaps as in crystalline semiconductors.
In more ordered materials, it is not until an electron is removed from the valence band (p-doping) or added to the conduction band (n-doping, which is far less common) does a conducting polymer become highly conductive. Doping (p or n) generates charge carriers which move in an electric field. Positive charges (holes) and negative charges (electrons) move to opposite electrodes. This movement of charge is what is actually responsible for electrical conductivity in crystaline matterials.
In contrast, typically "doping" in the polyacetylene-derived conductive polymers involves actually oxidizing 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.
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Early 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).
In 1963, Weiss et al reported [1] [2] high conductivity in oxidized iodine-doped polypyrrole, a polyacetylene derivative. They achieved the quite low conductivity of 1S/cm. They also described the effects of doping with iodine on conductivity, the conductivity type (n or p), and electron spin resonance studies on polypyrrole. This work was too early to be properly appreciated. Thus, it likely did not have an audience wide enough to reach critical mass and keep interest in the topic. The same authors noted an Australia patent application (5246/61, June 5, 1961) for conducting polypyrrole. (Interestingly, Diaz et al.[3]) are often credited with discovering conductive polypyrrole in 1979).
In an "active" solid-state device, a current or voltage controls current flow. In 1974, as a "proof of concept" for the classic model of conduction in such materials, John McGinness and his coworkers built and reported [4] 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 exhibits 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[5] in the journal Nature, this work was also "lost" until similar devices emerged decades later.
The 2000 Chemistry Nobel prize was awarded to Shirakawa et al for the later [6] discovery and characterization of high conductivity in oxidized, iodine-"doped" polyacetylene. There is no evidence the Nobel committee knew of Weiss et al's similar demonstration of high conductivity in iodine-"doped" polypyrrole or of the earlier construction of an actual organic-polymer electronic device. See Nobel Prize controversies.
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-hexylthiophene), polynaphthalenes, poly(p-phenylene sulfide), and poly(para-phenylene vinylene)s. Classically, these compounds 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).
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.
Electroluminesence
Electroluminescence and photoconductivity in organic compounds has been known since the early 1950's. However, the very poor conductivity of such materials limited current flow and thus light output. In contrast, the increased currrent flow through conductive polymers has led to the rapid development of practical polymer-based light emitting devices (OLEDs) and organic photovoltaic devices.
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
This increase in conductivity can also be accomplished in a field effect transistor (organic FET or OFET), or by irradiation. 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
Electroluminescence (light emission) in organic compounds has been known since the early 1950's, but practical application was limited by their low conductivity. 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 more 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.
Conductive polymers such as melanin are present in most mammal tissues where electrical conduction or transduction from light or sound are necessary, including the skin, eye, inner ear, and brain. 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 [7]. See the main article: Melanin.
See also
- Organic semiconductor
- "An Overview of the First Half-Century of Molecular Electronics" by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003).
- Australian Journal of Chemistry, 1963 16: 1056-89
