Growth of the conducting polymer polythiophene on semiconducting substrates and its application to the fabrication of energy conversion devices

Derivatives of polythiophene (PT) are organic polymers that can be electrochemically grown over metallic or semiconducting substrates. Their conductivity can be easily tuned over more than ten orders of magnitude, by chemical or electrochemical treatments. Furthermore, they are environmentally stable in both their oxidized (conducting) and neutral (semiconducting) states. The electrodeposition of PT on an n-type semiconducting substrate like GaAs or CdS was carried out in a classical three-electrode electrochemical setup. Illumination of the semiconductor electrode was necessary in order to get the anodic current required for the electropolymerization. As the polymer grows at a relatively high potential (greater than ca. 1 V/sce), photocorrosion of the semiconductor inevitably occurs even in an organic solvent like acetonitrile. The competition between corrosion and polymerization prevents the formation of the polymer. One way to overcome this problem is the pre-deposition of a catalytic layer in order to enhance the polymerization process. This was done for instance on n-GaAs by electroreduction of hexachloroplatinate in an aqueous solution at a potential of -0.6 V/sce. Very low amounts of platinum (less than a monolayer) were sufficient to allow the subsequent formation of very adhesive and homogeneous PT coverages