Electrochemical and Photoelectrochemical Processes on Thin Films of Perylenetetracarboxylic Dianhydride

Thin films of perylene derivative perylenetetracarboxylic dianhydride(PTCDA) have been examined as electrodes and photoelectrodes on both metal and metal oxide substrates. These thin films gave only anodic photocurrents(reverse bias condition) in contact with various aqueous electrolytes and facile dark currents negative of ca. −0.1 V vs. Ag/AgCl(forward bias condition). The origin of this photoeffect appears to be due to differential rates of charge injection at the PTCDA/solution interface and/or an asymmetric distribution of traps at that same interface. Electron microscopy of the PTCDA films indicated that they were deposited as elongated crystallites, with spaces between individual crystallites that strongly affected their dark and photoelectrochemical behavior, especially on Au substrates. Electrochemical polymerization with α-naphthol was carried out to passivate sites that were electrochemically active in the dark, a treatment that greatly enhanced the overall photoelectrochemical activity of the PTCDA thin films. Cathodic polarization of the PTCDA thin films caused surface bound electrochemically active redox couples to be produced at the film’s surface, and this polarization step further enhanced the photoelectrochemical activity. These experiments suggest that the PTCDA photoelectrochemical activity is strongly dependent upon near-surface composition and morphology and that, because of the highly porous nature of these thin films, the photoactive interfaces extend throughout most of the bulk of the PTCDA film.