Definition of Electrical Conductivity
Electrical conductivity or specific conductance is a measure of a material's ability to
conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current. The conductivity σ is defined as the ratio of the current density J to the electric field strength E
J = σ E
Conductivity is the reciprocal (inverse) of electrical resistivity, ρ, and has the SI units of siemens per metre (S·m-1):
σ = 1/ρ
Electrical conductivity is commonly represented by the Greek letter σ, but κ or γ are also occasionally used.
An EC meter is normally used to measure conductivity in a solution.
The current is due to the motion of the conduction electrons under the influence of the field. because these particles are charged, their motion leads to an electrical current ; the motion of neutral particles does not lead to an electrical current.
It is the conduction electrons which are responsible for the current because the ions are attached to and vibrate about the lattice sites.
Classification of materials by conductivity
A conductor such as a metal has high conductivity and a low resistivity.
An insulator like glass or a vacuum has low conductivity.
The conductivity of a semiconductor is generally intermediate, but varies widely under different conditions, such as exposure of the material to electric fields or specific frequencies of light, and, most important, with temperature and composition of the semiconductor material.
The degree of doping in solid state semiconductors makes a large difference in conductivity. More doping leads to higher conductivity. The conductivity of a solution of water is highly dependent on its concentration of dissolved salts and sometimes other chemical species which tend to ionize in the solution. Electrical conductivity of water samples is used as an indicator of how salt-free, ion-free, or impurity-free the sample is; the purer the water, the lower the conductivity (the higher the resistivity).
Conjugated Conducting Polymer
Traditional polymers(plastic) such as polyethylenes. PVC (polyvinylchloride),ABS 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.
but conjugated, conducting polymers are formed from sp2 hybridized carbons. Polyacetylene is the simplest conjugated polymer (CH)x. In polyacetylene, 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 backbone sigma-orbitals gives these polymers special electronic properties.
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 polypyrrole. Unfortunately, they did not cite the earlier papers. There followed 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 and so forth.
Though their priority is questioned, Heeger, MacDiarmid and Shirakawa eventually received the 2000 Nobel prize in Chemistry "For the discovery and development of conductive polymers"
Families of conducting polymer
Conducting polymers exhibit the behaviour of metals or semi-conductors.
Their backbones have a delocalised bond structure which can be augmented through “doping” with an appropriate agent (e.g. iodine), in a manner analogous to silicon. This not only controls conductivity, but can affect optical and physical properties in response to electrical stimulation (e.g. expansion/contraction, light emission, ingress or egress of molecular species). Such changes are the key features behind their potential application.
Most conducting polymers belong to one of five families: polyacetylene, polyaniline, polypyrrole, polythiophene, and polyphenylvinylenes.
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.
Electrical Conductivity of Conducting Polymers
When charge carriers (from the addition or removal of electrons) are introduced into the conduction or valence bands the electrical conductivity increases dramatically.
Typically "doping" the conductive polymers involves actually oxidizing/reducing of the compound.
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.
The conjugated polymers in their undoped, pristine state are semiconductors/ insulators. As such the energy gap is around 2 eV and higher and is too big for a considerable excitation of the charge carriers thermally. I will explain this bandgap of polymer in next final term paper.
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 80000 S/cm.
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-type doping is so much less common is that Earth's atmosphere is oxygen-rich, which creates an oxidizing environment. An electron-rich n-type polymer will react immediately with elemental oxygen to de-dope (re-oxidize to the neutral state) the polymer. Thus, chemical n-type doping has to be done in an environment of inert gas (e.g., argon). Electrochemical n-type 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-type conductive polymers
Application of Conducting Polymer
There are many application areas of conductive polymer. Currently Most focus is given to organic light emitting diodes(OLED) and organic polymer solar cell.
1. Nobel Chemistry 2000. http://nobelprize.org/nobel_prizes/chemistry/laureates/2000/index.html
2. Wikipedia Conductive Polymer.
3. Dr.Sotzing Class Materials. UCONN IMS Polymer Program.
4. Electrochimica Acta, Volume 50, Issues 7-8, 15 February 2005, Pages 1739-1745
Electrochemistry of Electrocactive Materials
" Application Potential of conducting polymers "
5. IBM Journal of Research and Development Volume 45, Number 1, 2001, Organic electronics " Conducting Polymers in Microelectronics "