Charge transfer reactions, doped conductive

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. 

For heavily doped materials, either notp type, the surface is degenerated and the material behaves like a metal electrode, meaning that the charge transfer reaction in the Helmholtz double layer is the rate-determining step. This is supported by the lack of an impedance loop associated with the space charge for the heavily doped materials. Also, for heavily doped n-Si large current in the dark is due to electron injection, which is not characterized by a slope of 60 mV/decade. For p-Si, electron injection into the conduction band may also occur during the anodic dissolution. 

When an organic semiconductor is weakly n- or p-doped, thus when dopants lead to the charge-transfer reactions mentioned above, it becomes an Ohmic conductor. A measurement of the specific conductivity a can then in optimal cases permit the concentration of the dopants to be computed, if the mobility /x is known. 

Self-doping has also been studied with poly(3-alkylsulfonate-thiophenes) [7-9]. The self-doped polythiophenes have a SO3H group at the end of the alkyl side chain. However, in order to obtain good conductivities, the charge transfer reaction has to be driven electrochemically to remove the cation (H+ or Na+) of the sulfonic acid out of the polymer. Otherwise this kind of self-doped polythiophenes have a very low conductivity in the dried state due to the protonation of the thiophene rings [75]. 

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