Doping level, polymer

Polyacetylene in the doped state is sensitive to air and moisture. Other polymers (e.g., those of pyrrole, thiophene, and benzene) are stable in air and/or toward humidity in their doped and undoped states. Generally, when stored in the doped state, the polymers lose doping level by mechanisms not fully understood in most cases the loss is reversible. 

Instabilities of doped conductive polymers are largely an environmental problem Some polymers when p-doped to the limit have oxidation potentials high enough to attack the most inert solvents. Most will attack water even at much lower doping levels. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

An important point concerning the spectra in Figure 3.71 is that at intermediate doping levels three principal absorption bands can be seen, at c. l.OeV, 2.7eV and 3.6eV, that are not simply the superposition of the as-grown and neutral polymer absorptions. The authors interpreted this observation in terms of the homogeneous doping of the film throughout its bulk, not just the oxidation of the surface layer or the layer next to the electrode. 

The ratio of the integrated currents for the first reduction wave of V2+ and the oxidation wave of the polythiophene from 0.4 V to 1.0 V vs. Ag+/Ag is about 4. This value means that upon oxidation of poly(I) one electron is withdrawn from four repeat units in the backbone of the polymer upon scanning to +1.0 V vs. Ag+/Ag. At this potential, the polythiophene achieves its maximum conductivity (vide infra). The level of oxidation to achieve maximum conductivity is consistent with the result reported by Gamier and co-workers (31-33) that the doping level of oxidized polythiophene is about 25%, but the Garnier work did not establish that the 25% doping level corresponds to maximum conductivity. Scheme III illustrates the electrochemical processes of poly(I) showing reversible oxidation of the polythiophene backbone and reversible reduction of the pendant V2+ centers. 

Electrochemical oxidation of X produces a polymer film with polythiophene as the backbone and viologen centers as pendant redox groups. The electrochemical properties of the polymer are the combination of polythiophene and viologen. Using viologen subunits as the internal standard (one per repeat unit of the polymer), the "doping level" of the oxidized polythiophene backbone at its maximum conductivity can be measured and is about 25%. The charge transport via the pendant V2+/+ of poly(l) has been studied by... 

In line with the terminology adopted, X is commonly called the dopant counter anion and y, which represents the ratio between dopant ion and polymer repeating unit, is commonly called the doping level. 

The bipolarons are energetically described as spinless bipolaron levels (scheme (9.30a)) which are empty and which, at high doping levels, may overlap with the formation of bipolaronic bands (9.30b). Finally, for polymers with band gap, values smaller than that of polypyrrole - such as polythiophene - the bipolaronic bands may also overlap with the valence and conduction bands, thus approaching the metallic regime. 

The occurrence of bipolaronic states in the polymer chains promotes optical absorption prior to the n-n gap transitions. In fact, referring to the example (9.30) of the band structure of doped heterocyclic polymers, transitions may occur from the valence band to the bipolaronic levels. These intergap transitions are revealed by changes in the optical absorptions, as shown by Fig. 9.8 which illustrates the typical case of the spectral evolution of polydithienothiophene upon electrochemical doping (Danieli et al., 1985). 

The charging process implies the oxidation of the cathode polymer with the concurrent insertion of the C104 anions from the electrolyte and the deposition of lithium at the anode. In the discharging process the electroactive cathode material releases the anion and the lithium ions are stripped from the metal anode to restore the initial electrolyte concentration. Therefore, the electrochemical process involves the participation of the electrolyte salt to an extent which is defined by the doping level y. 

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