In-situ conductivity and UV-VIS absorption monitoring of iodine doping-dedoping processes in poly(3-hexylthiophene) (P3HT)

Poly(3-hexylthiophene) (P3HT) belongs to the family of conducting polymers whose conductivity can be controlled via doping. This paper is intended to study iodine doping-dedoping processes in P3HT films via in-situ measurements of the conductivity and light absorption. The film was spin-coated on ITO (indium tin oxide) patterned in the form of four-probe configuration. The sample was doped by iodine via exposure to iodine vapours in chamber and then de-doped by spontaneous release of the dopant under ambient conditions. The doping process was found to be several orders of magnitude faster than dedoping, which is attributed to the different mechanisms of diffusion. The theory of diffusion was applied to approximate the conductivity relaxation as measured experimentally. The best-fit parameters for the approximation were used to determine the dopant diffusivity D and beta parameter of the semi-empirical relation between conductivity and dopant concentration (sigma proportional to N-D(beta)). Their values were found to be (3.9+/-0.9) 10(-13) cm(2)/s and 4.28+/-0.04 respectively. The latter agrees with the value know from the literature. The change of the polaron band in the absorption spectrum correlated with the conductivity changes in a complex manner and cannot be used as a measure of the concentration of free carriers contributing into the conductivity.