Doped phases and their structural evolution

All known conducting polymers are semiconductors in their pristine unperturbed state. Photoexcitation, charge injection and/or doping are required to create the local electronic excitations necessary for charge transport. Of these three mechanisms only the process of doping, or more precisely intercalation, yields a permanent transition to the conductive state. Doping can be accomplished through chemical, electrochemical, and vapor methods and, unlike the relatively light doping concentrations typical of conventional semiconductor compounds, the dopant levels in conducting polymer hosts may approach one dopant ion per monomer unit. This process involves essentially unit transfer of charge between the polymer host backbone and the guest dopant specie. In order to guaranty overall charge neutrality there must be interdiffusion of the guest specie into the host matrix. Thus there can be massive local structural reorganizations within the polymer matrix in order to accommodate the uptake of dopant.

The peculiar anisotropy of the polymer host, covalent bonding along the backbone with weaker interchain interactions in the two orthogonal directions, in combination with the local structural ordering introduces considerable complexity into the doping induced structural evolution. Even before the availability of detailed scattering data, indirect measurements found strong evidence for the existence of multiple guest-host structural phases exhibiting periodic structures both in the directions parallel and perpendicular to the polymer chain axis[60]. The earliest scattering studies showed the step-wise existence of high-symmetry structures in close analogy to the well-known stage-n, n=1,2,3..., transformations of quasi-two-dimensional layered materials[61,62,63]. In conducting polymers the actual response can be significantly more complex. For layered materials intercalation by a guest specie requires only dilation of the host matrix to enable the formation of intercalant galleries. On the other hand, as seen in Fig. 8, intercalation of conducting polymers hosts potentially involves both rotational and translation motions of the individual constituents and in the host lattice itself.

 

Figure 8: Schematic 2D model depicting the three most common structural responses to conducting polymer intercalation. (a) Transformation to a three fold channel structure; (b) transformation to a four fold channel structure; c) transformation to a layer compound.

The final quoted compositions are also highly variable. In general the relative guest ion concentrations are given in terms of their mole weight with respect to a monomer unit basis. PA has a relatively short skeletal c-axis repeat (2.45Å) and so the nominal mole weights are considerably less than those of, for instance, PPV (with a 6.6Å c-axis repeat).