ELECTROCHEMICALLY DEPOSITED POLYTHIOPHENE .1. OHMIC DROP COMPENSATION AND THE POLYTHIOPHENE PARADOX

High-quality polythiophene cannot be directly electrodeposited on many metallic substrates such as titanium or even gold, because at the required high potential the oxidation of those substrates is preferred to polymerization. We found that a thin polypyrrole coating ensures the deposition of polythiophene on these substrates. Due to the low conductivity of 1-100 mM thiophene + 0.1 M tetrabutylammonium tetrafluoroborate in acetonitrile or 1:1 acetonitrile:water (about 95 Ω cm in acetonitrile as determined in this work), a significant ohmic (IR) voltage drop occurs between the working and reference electrode at any reasonable distance during deposition. To maintain a constant and known voltage drop across the solution/polymer interface, IR-drop control must be applied. By using dynamic IR compensation with current interrupts and 100% error compensation, the quality of the resulting film is better controlled. At these deposition conditions, the so-called "polythiophene paradox" has been proven to be the result of the degradation of polythiophene which starts solely at the solution/film interface and reaches the bulk of the film only at a further stage. This overoxidation reaction is competing with polymerization and oxidation of the monomer. In the presence of the thiophene monomer these latter "useful" reactions prevail while in the absence of thiophene the degradation takes place. The cell geometry, solution composition, and deposition potential have been optimized. © 1990 American Chemical Society.