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1.2 Conducting Polymers

Although electrical conduction is generally associated with metals, another unique set of conductors is conducting polymers.  In metals, there are electrons that are not localized, so they can freely move throughout the conductor.  Insulators and semiconductors have their electrons localized and have an energy barrier to overcome before the free carriers are delocalized to any great extent.  Illustration 1.5 shows the simplified band diagrams for metals, semiconductors, and insulators.  The fermi level is defined as the point in which there is a 50% probability of finding an electron at that energy.  As can be seen in Illustration 1.5(b) and (c), the amount of energy required for an electron to enter the conduction band of an insulator is greater than that required for a semiconductor.  Metals, as seen in Illustration 1.5(a), enter the conduction band freely since there is no band gap to overcome.  Doping of semiconductors is another way to increase electrical conduction through the creation of free carriers.  Likewise, semiconductors like silicon are often doped with P or Al  to increase the number of extra electrons or create a deficiency of electrons (or holes).  Since doping is more or less uniformly distributed throughout the material, the extra electrons in P doped silicon are able to move.  This is considered n-type doping.  In the case of Al doped silicon, holes are created that can move.  This is considered p-type doping.  Simple band diagrams for these systems are shown in Illustration 1.6. 

On a molecular level, the formation of bands arises from the combination of a large number molecular orbitals.  One molecular analogy to the solid-state concept of bands would be that the energy required to promote an electron across the HOMO-LUMO (highest occupied molecular orbital - lowest unoccupied molecular orbital) gap is analogous to the energy required to promote an electron from the valance band into the conduction band. 

One limitation with the doping of traditional semiconductors like Si and GaAs is that the process is not easily reversed.  Conducting polymers like polypyrrole and polythiophene also possess semiconductor-like band structures and likewise can be doped into a conductive state.  The advantage in these systems is the fact that they can be doped and undoped in a reversible fashion.  This is accomplished by the introduction of species like BF₄^- into the polymer matrix upon oxidation of the polymer.  The process leads to a delocalization of the electrons along the polymer backbone.  This procedure can then be reversed by reduction of the polymer and removal of the doping species from the porous polymer matrix.  This unique ability to reversibly alter the conducting properties makes it an excellent choice to be interfaced with high-temperature superconductors.  

The discovery¹¹ of the metallic properties of the conducting polymer, (SN)_x, began a new era of research in the field of conducting polymers.¹²  This material was later found to be a superconductor at low temperatures.¹³  In 1977, it was first reported¹⁴ that the conjugated polymer, polyacetylene, could have its electrical conductivity dramatically altered upon exposure to halogen species.  A wide array of conjugated polymer systems have since been synthesized.  One of the most notable is polypyrrole, which was discovered by Diaz and coworkers¹⁵ in 1979.  The incorporation of conducting polymers into hybrid superconductor systems may provide pathways to produce novel composite materials as well as unique electrical phenomena.

Illustration 1.5:     Band diagrams for a) metal, b) semiconductor, and c) insulator.

Illustration 1.6: Band diagrams for a) p-type semiconductor and b) n-type semiconductor.