Photoelectrochemical studies of vanadyl phthalocyanine (VOPc) thin film electrodes

Photoactive vanadyl phthalocyanine (VOPc) thin films have been prepared by vacuum deposition on gold metallized plastic optically transparent electrode (MPOTE) substrates. The morphology of the film as determined by scanning electron microscopy (SEM) revealed the existence of two types of crystallites at the surface. Photoelectrochemical studies of these thin films were undertaken using different redox couples in aqueous solution. For redox couples with standard electrochemical potentials negative of +0.33 V vs. Ag/AgCl, positive photopotentials were observed (i.e., the films behave as photocathodes). Those redox couples with E°'s situated positive of +0.33 V exhibited reversible electrochemical behavior independent of illumination at the VOPc electrode. These results suggest a clear p-type semiconducting nature for the VOPc system. Mott-Schottky capacitance data revealed an acceptor density of 6×10¹⁸ cm^-3 which is speculated to be attributed to the inclusion of dioxygen within the VOPc lattice as supported by X-ray photoelectron spectroscopy (XPS). A power conversion efficiency of 0.1% was observed for the photoassisted evolution of hydrogen which did not require the addition of micro-islands of platinum to the VOPc surface.