PHTHALOCYANINE THIN-FILMS AS SEMICONDUCTOR ELECTRODES

Metal-free, zinc, and nickel phthalocyanines (H2Pc, ZnPc, and NiPc) have been studied electrochemically. The electrode assemblies involved a glass substrate overlaid by a gold contact layer, which in turn was covered by the phthalocyanine at nominal thicknesses of 40-3000 Å. Interfacial capacitances were determined as functions of potential and frequency. Plots of capacitance vs. potential showed sharp rises for potentials more positive than 0.3 V vs. SCE and (for H2Pc and ZnPc) peaks at more negative potentials. The capacities were linear with the inverse square root of frequency in the negative potential regime. Cyclic voltammetric studies of six nonlabile complexes of Fe(II) were carried out, and chronocoulometry was used to measure their standard heterogeneous rate constants. The complexes with very negative standard potentials could be oxidized irreversibly on H2Pc but could not be reduced again. Systems with more positive standard potentials were reversibly oxidized and reduced. The irreversible systems could be reduced on H2Pc at potentials more negative than 0.35 V vs. SCE when the electrode was irradiated by light at 6328Å. The observations have been rationalized by considering the phthalocyanines as relatively well behaved p-type semiconductor electrodes. Band edges and interfacial states have been mapped and the nature of interfacial charge transfer has been discussed in terms of Gerischer's model modified by inclusion of surface states in the gap region. © 1979, American Chemical Society. All rights reserved.