Conductive polymers
From Free net encyclopedia
Conductive polymers are organic polymer semiconductors, or organic semiconductors.
For a history of the field, see: "An Overview of the First Half-Century of Molecular Electronics" by Noel S. Hush, Ann. N.Y. Acad. Sci. 1006: 1–20 (2003).
Most commercially produced organic polymers are electrical insulators. Conductive polymers, which are almost always organic, have extended delocalized bonds (often comprised of aromatic units) that creates 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, 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, which leaves a continuous path of overlapping p orbitals. In polyacetylene, but not in most other conductive polymers, this continuous string of orbitals creates degeneracy in the frontier molecular orbitals (the highest occupied and unoccupied orbitals named HOMO and LUMO respectively) which leads to the filled (electron containing) and unfilled bands (valence and conduction bands respectively) that define a semiconductor.
However, conductive polymers generally exhibit very low conductivities. In fact, conduction in such relatively disordered matererials 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.
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.
Following up an earlier paper defining conduction mechanisms in such polymers, McGinness, Corry, and Proctor reported a high conductivity state in a polyacetylene (melanin) and the first organic electronic device. This was a voltage-controlled switch (Science, vol 183, 853-855 (1974)). Their original "gadget" is now in the Smithsonian's collection of early electronic devices. This device also exiibted negative differential resistance, now a well-characterized hallmark of such materials. Though in a major journal, this work was "lost" and went generally-uncited. The chemistry Nobel prize in 2000 was awarded to others for the later discovery and charaterization of passive high conductivity in other polyacetylenes.
Contents |
Chemistry
Common classes of organic conductive polymers include poly(acetylene)s, poly(pyrrole)s, poly(thiophene)s, poly(aniline)s, poly(fluorene)s, 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.
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 more common in research, because it is easier to exclude oxygen from a solvent in a sealed flask; however, there are likely no commercialized n doped conductive polymers.
Conjugation
The extended conjugation of a conductive polymer tends to give rise to fluorescence which has led to the rapid development of polymer-based light emitting devices (OLEDs) and organic photovoltaic devices.
Properties
The biggest advantage of conductive polymers is processibility. Conductive polymers are also plastics (which are organic polymers) and therefore can combine the mechanical properties (flexibility, toughness, 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
In some cases, light emission is observed when a voltage is applied to a thin layer of a conductive organic polymer film. It has led to the development of flat panel displays using OLEDs, solar panels and optical amplifiers.
Conductive polymers 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. Its electronic conductivity seems to be the underlying mechanism for absorption of light, and electron-phonon interactions are exploited in hearing [1]. See the main article: Melanin.