Conjugated Polymers: Electronic Conductors

The most important aspect of conjugated polymers from an electrochemical perspective is their ability to act as electronic conductors. Not surprisingly $\pi$-electron polymers have been the focus of extensive research [16], ranging from applications of ``conventional'' polymers (e.g., polythiophene, polyaniline, polypyrrole) in charge storage devices such as batteries and supercapacitors, to new polymers with specialized conductivity properties such as low bandgap and intrinsically conducting polymers. Indeed, many successful commercial applications of these polymers have been available for more than fifteen years, including electrolytic capacitors, ``coin'' batteries, magnetic storage media, electrostatic loudspeakers, and anti-static bags. It has been estimated [6] that the annual global sales of conducting polymers in the year 2000 will surpass one billion US dollars. Clearly these materials have considerable commercial potential both from the continued development of well established technologies and from the generation of new concepts such as those to be presented in this thesis.

The genesis of the field can be traced back to the mid 1970s when the first polymer capable of conducting electricity -- polyacetylene -- was reportedly prepared by accident by Shirakawa [17]. The subsequent discovery by Heeger and MacDiarmid [18] that the polymer would undergo an increase in conductivity of 12 orders of magnitude by oxidative doping quickly reverberated around the polymer and electrochemistry communities, and an intensive search for other conducting polymers soon followed. The target was (and continues to be) a material which could combine the processibility, environmental stability, and weight advantages of a fully organic polymer with the useful electrical properties of a metal.

The essential structural characteristic of all conjugated polymers is their quasi-infinite $\pi$ system extending over a large number of recurring monomer units. This feature results in materials with directional conductivity, strongest along the axis of the chain [19]. The simplest possible form is of course the archetype polyacetylene (CH)_x shown in Figure 1.2. While polyacetylene itself is too unstable to be of any practical value, its structure constitutes the core of all conjugated polymers. Owing to its structural and electronic simplicity, polyacetylene is well suited to ab initio and semi-empirical calculations and has therefore played a critical role in the elucidation of the theoretical aspects of conducting polymers.

Figure 1.2: Conjugated polymer structure: (a) trans- and (b) cis-polyacetylene, and (c) polythiophene

a	b	c

Electronically conducting polymers are extensively conjugated molecules, and it is believed that they possess a spatially delocalized band-like electronic structure [7,20]. These bands stem from the splitting of interacting molecular orbitals of the constituent monomer units in a manner reminiscent of the band structure of solid-state semiconductors (Figure 1.3).

Figure 1.3: Band structure in an electronically conducting polymer

It is generally agreed [16,21] that the mechanism of conductivity in these polymers is based on the motion of charged defects within the conjugated framework. The charge carriers, either positive p-type or negative n-type, are the products of oxidizing or reducing the polymer respectively. The following overview describes these processes in the context of p-type carriers although the concepts are equally applicable to n-type carriers.

Figure 1.4: Positively charged defects on poly(p-phenylene). A: polaron B: bipolaron

Oxidation of the polymer initially generates a radical cation with both spin and charge. Borrowing from solid state physics terminology, this species is referred to as a polaron and comprises both the hole site and the structural distortion which accompanies it. This condition is depicted in Figure 1.4A. The cation and radical form a bound species, since any increase in the distance between them would necessitate the creation of additional higher energy quinoid units. Theoretical treatments [22,23] have demonstrated that two nearby polarons combine to form the lower energy bipolaron shown in Figure 1.4B. One bipolaron is more stable than two polarons despite the coulombic repulsion of the two ions. Since the defect is simply a boundary between two moieties of equal energy -- the infinite conjugation chain on either side -- it can migrate in either direction without affecting the energy of the backbone, provided that there is no significant energy barrier to the process. It is this charge carrier mobility that leads to the high conductivity of these polymers.

The conductivity $\sigma$ of a conducting polymer is related to the number of charge carriers n and their mobility $\mu$:

$$\sigma \propto \mu$$ (1.1)

Because the band gap of conjugated polymers is usually fairly large, n is very small under ambient conditions. Consequently, conjugated polymers are insulators in their neutral state and no intrinsically conducting organic polymer is known at this time. A polymer can be made conductive by oxidation (p-doping) and/or, less frequently, reduction (n-doping) of the polymer either by chemical or electrochemical means, generating the mobile charge carriers described earlier. The cyclic voltammetry of electronically conducting polymers is characterized by broad non-Nernstian waves. A typical example is shown in Figure 1.5 for an N-substituted pyrrole based conducting polymer [24].

Figure 1.5: Cyclic voltammogram of a substituted polypyrrole [24]. Reprinted with permission from J. Chem. Soc. Faraday Trans., 1990, 86, 3631. Copyright 1990 The Royal Society of Chemistry