Photovoltaic effects of metalfree and zinc phthalocyanines. II. Properties of illuminated thin-film cells

Abstract

Rather strong rectification and photovoltaic effects are observed for sandwich cells in which a thin film (∼3000 Å) of metalfree or zinc phthalocyanine is interposed between an ohmic contact (Au) and a blocking contact (Al or In). The action spectra of the short‐circuit photocurrent suggest that only light absorbed near the blocking contact is effective in producing carriers for the external circuit. Cells with indium contacts show performance that is quantitatively consistent with the presence of a Schottky junction at that contact. The quantum efficiency for light incident on the ZnPc phase of an (In/ZnPc/Au) cell can be as high as 14% at 6328 Å. For light actually absorbed in the barrier region, the yield approaches unity. The efficiencies of cells with aluminum contacts are 2–3 orders smaller, apparently because an insulating layer (∼65 Å) of Al₂O₃ occupies the Al/ZnPc junction. This interfacial insulating film also affects the relation between the short‐circuit photocurrent and the light intensity. In cells with In contacts, there is a linear, or nearly linear, current response; but for devices with Al contacts, the photocurrent rises as the cube root of the intensity. The latter dependence can be analyzed in terms of coupled diffusion and second‐order decay of light‐generated carriers produced near the blocking contact.