Effect of adsorbed gases on the dark and photo-conductivity of FeII phthalocyanine crystals

Abstract

We report the effect of evacuation (10^–6 Torr) and baking at 200 °C (10^–8 Torr) and of O₂, H₂, CO and NO on the dark and photo-conductivity of Fe^II phthalocyanine (FePc) crystals grown by entrainer sublimation. The unusual non-ohmicity of FePc in air and the increase in dark conductivity on evacuation and baking are attributed to oxidation of surface molecules to give a Fe^III derivative. Correlation of the conductivity spectrum with the optical absorption is made between 6000 and 12000 cm^–1. The effects of O₂ and H₂ are explained in terms of the alteration of the charge-carrier recombination rate by donor–acceptor interactions between the gas and the solid. In other cases (air on baked FePc and NO on FePc) reactions between the gas and the solid complicate the observations. CO has opposite effects on the dark and photo-currents, while NO appears to react with the crystal surface. The kinetics of CO and NO adsorption measured by changes in the conductivity follows the Elovich equation. The reaction of O₂ with baked FePc follows a bimolecular rate law. It is suggested that two neighbouring FePc molecules participate in the rate-determining oxidation step.