Transport Properties and Doping

The diffusion of dopants into, and out of, conducting polymers is important for possible applications in batteries, and as conductors or semiconductors. For conductors or semiconductors, the chief requirements are that the material can be doped in a reasonable time, but that it will then not lose dopant over periods of years. This is particularly important in determining junction stability in devices. In the case of batteries, on the other hand, rapid and reversible uptake and loss of dopants is needed, since the diffusion rate controls charging and discharging rates. In addition, the accessibility of the structure to oxygen, and other degradants, will be a factor in the stability of the polymer. 

In many cases the important property will actually be the permeability, which is the product of the diffusion coefficient and the solubility of the dopant in the polymer. The solubility is determined by the degree of interaction between the diffusant and the polymer. The picture is further complicated, since reactions may take place so that several different species are diffusing. The reaction of a gaseous dopant with a conducting polymer is a complicated diffusion and reaction process. We must consider the solubility and diffusion of the molecular gas, the charge-transfer reaction to dope the polymer, the diffusion of the resulting ions in the doped (intercalated) structure and any reaction between the dopant ions and the polymer which may lead to covalent bonding. 

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