Semiconducting properties of polypyrrole films in aqueous solution

The influence of the nature of the anions on the conductivity of polypyrrol e films in aqueous solution was investigated by photocurrent spectroscopy c ombined with electrochemical impedance spectroscopy in dependence on the po tential. As demonstrated, the conductivity of polypyrrole films at negative potentials can vary from a semiconducting to an ionic conducting state, de pending on the size of the charge-compensating counter-anion incorporated d uring the electropolymerization. The reduced polypyrrole shows semiconducti ng properties when small anions are inserted, releasing the polymer matrix during the reduction process. The polymer can than be considered as a two-l ayer system, consisting of a semiconducting layer and a porous layer. Measu rements at different thickness of polypyrrole films have shown that the pos ition of the semiconducting layer is in the electrode/polymer interface. Th e ohmic resistance of the semiconducting layer is in the range 1-5 k Omega, the capacitance approaches a value of 100-500 nF and the flatband potentia l is -0.62 V-SCE. If large anions are incorporated, cation insertion takes place during reduction. the electrolyte content in the polymer then is rela tively high and the polymer's behaviour is similar to that of an ionic cond uctor. The results are presented and discussed together with the example of methylsulfonate as a relatively small anion and polystyrenesulfonate as a large anion.