Faradaic impedance behaviour of oxidized and reduced poly (2,5-di-(-2-thienyl)-thiophene) films

Poly(2,5-di-(-2-thienyl)-thiophene) films with thickness less than 1.32 mu m were electrogenerated on Pt from 5 mM and 20 mM monomer concentrations in 0.1 M LiClO4 + acetonitrile at constant current. Cyclic voltammograms recorded between 0 and 1.10 V vs. SSCE in the electrolyte showed a redox couple associated to the reduced and oxidized states. The polymer retains its reduced form until 0.6 V, its oxidized form from 0.8 V and a mixture of both states in the potential region 0.6 to 0.8 V. Faradaic impedance measurements for both states were performed at a constant potential between 0.3 and 1.0 V after reducing the electrogenerated polymer at 0 V. A progressive decrease in impedance was always found with increasing the film thickness and more positive potential. An equivalent circuit involving two complex elements is proposed to simulate adequately the Bode plots, Nyquist diagrams and complex plane admittance diagrams obtained. The film resistance is very high for all polymers, suggesting that the mass transport of ClO4-, ions through the polymer is the rate-determining of the oxidation-doping process. The film capacitance is low for the reduced form and gradually increases when the polymer is oxidized. Deposits obtained from a 20 mM monomer solution have higher charge-storage capacity than those generated from a 5 mM one, particularly for the oxidized polymer where large amounts of ClO4- counterions are accumulated. (C) 1997 Elsevier Science S.A.