Chlorogallium phthalocyanine thin-film photoelectrochemistry. Effect of surface coverage and physical distribution of microcrystallites on the photoelectrochemical response

Chlorogallium phthalocyanine (GaPc-Cl) thin films have been prepared by vacuum deposition on gold-metallized plastic substrates. Depending on deposition rate, two types of films resulted: type A GaPc-Cl films, consisting of small crystallites, 100-300 nm in diameter, which completely covered the substrate; and type B films, consisting of larger crystallites, 500-700 nm, isolated on the exposed Au substrate. Photoelectrochemical studies of the redox reactions of hydroquinone and ferrocyanide solutions showed that the type A films behave as photoconductors with a film thickness too small to support an appreciable photopotential, while the type B films can be made to exhibit a photopotential of -220 mV vs. a Pt cathode in the hydroquinone solutions. The difference in behavior of these two chemically equivalent photoelectrodes was attributable to differences in the thickness of the films in relation to the thickness of the space charge regions formed in the film. However, the exposed gold substrate on the type B films was found to diminish photopotentials by acting as a back-reaction site for the solution redox reaction. Photopotentials could be increased on the type B films by electrochemically polymerizing phenol to form an insulating layer over the gold substrate. Photoaction spectra identified holes as the majority charge carriers with an estimated mean free path of 150 nm.