Photoelectrochemical effects in the electrolyte-pigment-metal system. II. Metal-free phthalocyanine film action spectra of short-circuit photocurrents with increase of the film thickness.

The electrolyte-pigment-metal system can be described as analogous to a photosensitive junction region. When the thickness of the pigment film is increased, the action spectra of the maximum short-circuit photo-current under continuous illumination differ from the absorption spectra both in direct illumination (pigment-electrolyte) and in back illumination (metal-pigment). One is led to believe that there exist two photoactive regions in the system for the production of the short-circuit photocurrent; these two active regions are associated respectively with each interface. When the metallic semitransparent electrode is made of aluminum the two interfaces have opposite sign contributions to the photocurrent; this allows the determination of conditions in which one can observe specifically the contribution of the pigment-electrolyte interface, that is the interactions between excited pigment molecules and the redox system in the electrolyte.