Most organic polymers are insulators by nature. However, a few intrinsically conducting polymers (ICPs) exist that have alternating single and double bonds along the polymer backbone (conjugated bonds) or that are composed of aromatic rings such as phenylene, naphthalene, anthracene, pyrrole, and thiophene which are connected to one another through carbon-carbon single bonds.
The first polymer with significant conductivity synthesized was polyacetylene (polyethyne). Its electrical conductivity was discovered by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid who received the Nobel Prize in Chemistry in 2000 for this discovery. They synthesized this polymer for the first time in the year 1974 when they prepared polyacetylene as a silvery film from acetylene, using a Ziegler-Natta catalyst. Despite its metallic appearance, the first attempt did not yield a very conductive polymer. However, three years later, they discovered that oxidation with halogen vapor produces a much more conductive polyacetylene film.1 Its conductivity was significantly higher than any other previously known conductive polymer. This discovery started the development of many other conductive organic polymers.
The conductivity of non-doped, conjugated polymers such as polyacetylene is due to the existence of a conducting band similar to a metal. In a conjugated polymer three of the four valence electrons from strong σ bonds through sp2 hybridization where elctrons are strongly localized. The remaining unpaired electron of each carbon atom remains in a pz orbital. It overlaps with a neighboring pz orbital to form a π bond. The π electrons of these conjugated pz orbitals overlap to form an extended pz orbital system through which electrons can move freely (delocalization of π electrons). However, non-doped polymers have a rather low conductivity. Only when an electron is removed from the valence band by oxidation (p-doping) or is added to the conducting band by reduction (n-doping) does the polymer become highly conductive. The four main methods of doping are
Redox p-doping: Some of the π-bonds are oxidized by treating the polymer with an oxidizing agent such as iodine, chlorine, arsenic pentafluoride etc.
Redox n-doping2: Some of the π-bonds are reduced by treating the polymer with a reducing agents such as lithium, and sodium naphthaline.
Electrochemical p- and n-doping: Doping is achieved by cathodic reduction (p) or by anodic oxidation (n)
Photo-Induced Doping: The polymer is exposed to high energy radiation that allows electrons to jump to the conducting band. In this case, the positive and negative charges are localized over a few bonds.
Doping increases the conductivity by many orders of magnitude. Values as high as 102 - 104 S/m have been reported. Another method to increase conductivity is mechanical alignment of the polymer chains. In the case of polyacetylene, conductivities as high as 105 S/m have been found which is still several magnitudes lower than the conductivity of silver and copper (108 S/m) but more than sufficient for electronic applications such as polymer-based transistors, light-emitting diodes and lasers.
The table below lists typical conductivities of some common conjugated polymers and their repeat units. The actual conductivity not only depends on the structure and morphology of the polymer but also on the type of dopant and its concentration.
|Compound||Repeating Unit||Conductivity (S cm-1)|
|trans-Polyacetylene||103 - 105|
|Polypyrrole||102 - 7.5 · 103|
|Poly(p-phenylene)||102 - 103|
|Polyaniline||2 · 102|
|Poly(p-phenylene vinylene)||2 · 104|
1H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang, and A.J. Heeger, J Chem. Soc. Chem. Comm., 579 (1977)
2n-doping is less common because the double bonds get readily oxidized when exposed to air and other oxidizing atmoshperes, i.e. radical anions are less stable than radical cations.