Evidence for charge trapping at the gold/chlorogallium phthalocyanine interface using photocurrent spectroscopy with one or two illumination sources

Photocurrent vs. wavelength spectra were reported for thin films of chlorogallium phthalocyanine (GaPc-Cl) on optically transparent gold substrates in contact with aqueous hydroquinone solutions. Charge carriers were produced which depending upon the absorptivity of the film and direction of illumination, were localized near either the Au interface or the solution interface. Under positive bias (hole harvesting at the solution interface), with illumination of the Au/Pc interface first, photons near the absorbance maximum in the visible range (600-750 nm) produced charge which was trapped at the Au/Pc interface. This trapping arose because of the formation of a potential well brought about by a mismatch in the Fermi levels of bare gold and the Pc film. The Fermi level of the Pc layer before contact with either phase is ca. 0.5 eV above he valence band edges allowing the formation of space charge layers at both interfaces. The potential well at the Au/Pc interface extends ca 250 nm into the 1-μm-thickness films for bias potentials 200-300-mV positive of the flat-band potential (e.g the equilibrium potential for H₂O/BQ couple). Carriers generated by the addition of a second CW light source attenuated the effects of the potential well at the Au/Pc interface and thus enhanced the photocurrent yields from the primary, modulated light source.