Conducting polymer

The majority of the Polymère S organics produced are excellent electrical insulators. The polymeric drivers , almost always organic, have delocalized bonds (often in an aromatic group) which form a structure similar to that of the Silicium. When one applies a tension between the two bands, electric conductivity increases: it is a Transistor. Almost all the known conducting polymers are Semi-conducteur S thanks to their structure in bands, whereas the polymers with step zero behave like metals, in drivers. The principal difference between conducting polymers and the inorganic semiconductors is the mobility of the electrons, until recently quite lower within conducting polymers - a ditch than science does not cease reducing. In addition to their interests in fundamental chemistry, this research gave rise to many recent applications, like the electroluminescent diodes, of many video screens, new markings of products in the supermarkets, the treatments of photographic films, etc

The conducting plastics have a great future in information technologies.

Discovered

In the years 1970, three American scientists showed that by doping a film of Polyacétylène (in fact, by oxidizing it with iodine vapor), its electric conductivity increases a billion times, then becoming comparable with that of metals as copper and the money. The optical properties of material are also modified, since this one emits light.

The Nobel Prize of chemistry was decreed into 2000 with the American Alan J. Heeger, Alan G. MacDiarmid and with the Japanese Hideki Shirakawa for their discovery and their study of conducting polymers.

Chemistry

The principal organic conducting polymer classes are the polycetylenes, the polypyrroles, the polythiophenes, the polyanilines and the polyvinyls of paraphenylene (PVPP).

Doping

In the semiconductors containing silicon, some silicon atoms are replaced by ions in excess (for example the Phosphore) or at fault (for example the Bore) of electrons, which gets the type NR to them or P. This technique is different from doping. The polymers can be doped by the addition of a chemical reagent which oxidizes (or reduced) the system, which makes forward the electrons of the valence band to the band of conduction, making the system more conducting.

There exist two principal methods of doping of conducting polymers, both being based on an oxydoreduction.

Chemical doping

The first method, called chemical doping, exposes polymer, for example a film of Mélanine, with an oxidant (of iodine or bromine) or with a reducer (rarer, the use of alkaline metals implies).

Electrochemical doping

The second method, called electrochemical doping, uses an electrode covered with a polymer and bathing in an electrolytic solution in which the polymer is insoluble. One applies a tension between the electrodes which causes a movement of the ions of the solution and electrons which are fixed then on treated polymer, or escape from it. One thus obtains an excess (doping NR) or a defect (doping P) of electrons on the level of the band of conduction of polymer.

This method is one of most effective, but especially that which profits from more research: the doping NR, which cannot occur in the presence of oxygen, is easier to carry out: one can make the vacuum in the containers with suitable means.

Weakness of doping NR

The doping NR which consists in obtaining an excess of electrons is much less current than doping P; indeed, the terrestrial atmosphere is rich in Oxygène and is presented in the form of an oxidizing environment. A polymer doped NR reacts then with oxygen in air and loses the electrons in excess, becoming again neutral. Thus, doping NR implies that the polymer is maintained in an inert gas (generally the Argon).

This is why one finds today no polymer conducting of type NR in the trade, their lifespan being too short for an unspecified use.

Conjugation

The conjugation (or hybridization) of a conducting polymer supports the Fluorescence, which allows the system development transmitting Lumière (LEDs or LED S, OLED S) and of the organic photovoltaic systems.

One thus saw being born from the screens and the extremely fine sensors (less than one centimetre) and very flexible devices, whose record of size is held by the company Samsung with a diagonal of 105 cm in May 2005.

Properties

The main advantage of polymers is their facility of production. The conducting polymers are simple plastics, and thus combine flexibility, resistance, the elasticity of the plastics to conductivities of a metal or a doped hybrid polymer.

Physics

This increase in conductivity is characteristic of a transistif system and can be simulated by a field-effect transistor (FET). These polymers are thus organic FET or OFET.

Applications

In certain cases, one can emit light by applying a tension to a fine layer of a conducting polymer. This discovery allowed the settling of extra-flat screens, such as the screens using of OLED S, the solar panels or the amplifying optics.

Surprisingly, one finds certain conducting polymers within the body of some Mammifère S, where they allow the transduction light or sound in electrical signal, for example in the skin, the eyes, the ear or the brain. Their conductivity seems to allow the absorption of the light by the Peau. The Mélanine, which belongs to the polycetylenes, has such properties and remains currently one of the most promising molecules in this field.

Flexibility, resistance, the elasticity and the facility of production of conducting polymers did of it one of the fields of research privileged for the Nanotechnologie. Following the example current processors, one hopes to be able to use these polymers to create circuits with the molecular scales.

See too

Polymeric
Conductivity
Semiconductor
Nanotechnology

External bonds

Organicsemiconductors.com
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