Characterization of molecular semiconductor and multilayer molecular organic photoconductor interfaces by photoelectrochemistry and surface analytical techniques.

Abstract

Organic semiconductor thin films are of interest to us for a variety of molecular electronic applications, including solar cells, chemical sensors, and nonlinear optical devices. We have been seeking unusual new ways of controlling the composition and long-range molecular structure of these materials through the use of vacuum deposition techniques which mimic, in some ways, those used for epitaxial layer growth in inorganic materials. Thin films of perylene tetracarboxylic dianhydride (PTCDA) have been examined as electrodes and photoelectrodes on both metal and metal oxide substrates. In contrast to most previous studies of phthalocyanine thin films, these materials behaved in such a way as to suggest n-type character, i.e. dark electron transfer reactions were facile in negative potential regions with solution redox couples, and little dark electrochemistry could be observed in regions of positive potentials. It is likely that junction formation occurs only as a result of illumination, with different rates of interfacial hole and electron injection and transport, at the PTCDA/electrolyte interface. Electron microscopy of the PTCDA films indicated that they were deposited as elongated crystallites, with relatively large spaces between individual crystallites, which strongly affected their dark and photoelectrochemical behavior, especially on Au substrates. Electrochemical polymerization of α-napthol was carried out to passivate sites that were electrochemically active in the dark, a treatment which greatly enhanced the overall electrochemical activity of these PTCDA thin films. A variety of p-n heterojunction-like structures, created from thin film molecular materials (vanadyl phthalocyanine (VOPc) and perylene tetra-carboxylic dianhydride (PTCDA)), have been nondestructively explored by photoelectrochemical techniques and UHV surface analytical techniques. Vacuum deposited bilayers and multilayers of these thin films behave like "p-n" diodes over a narrow potential window. The open circuit photopotential is determined by the junction potential formed at the Pc/PTCDA interface. It was found that the transient photocurrent (using a modulated light source) in multilayer VOPc/PTCDA assemblies was directly related to the number of interfaces present, consistent with the idea that exciton dissociation is localized primarily to such an interface, and is the photocurrent limiting process.