Spectroscopic Srudies of Model Organic Photovoltaic and Organic Light Emitting Diode Organic-Organic' and Metal-Organic Heterojunctions

Abstract

The purpose of this Dissertation was to present fundamental approaches to expand the general knowledge of the chemistry that occurs at both the organic-organic' (O-O') and the metal-organic (M-O) interfaces in organic optoelectronic devices. In order to simplify the interactions in the initial studies presented herein, simple model molecules that represent the larger, highly conjugated molecules used in device construction were considered.UPS, reductive-desorption electrochemistry, and Raman surface spectroscopy were used to determine monolayer characteristics of thiophenol and pentafluorothiophenol on Ag. Proposed interfacial orientations and molecular spacing of the TP and F5TP were proposed. Benzene and hexafluorobenzene (F6-benzene) were then condensed and forcibly dewet onto the monolayers in an effort to understand the solid-liquid interfacial interactions. Benzene films on alkanethiol (UDT) and perfluorinated thiophenol (F5TP) were prone to rupturing, and spectroscopically appeared to be liquid-like in character, while molecular spacing of TP and adsorbed benzene on unmodified Ag template ordered benzene films. Polycrystalline films of F6-benzene forms at the interfaces of TP and unmodified substrates. F6-benzene induces a reorientation of F5TP molecules, but is subsequently unable to induce long range order. F6-benzene on UDT appears liquid-like. These studies show that fixed molecules can stimulate order or disorder at a molecular heterojunction, which may have profound effects in device efficiency.In an effort to begin to understand the complicated reaction chemistry that occurs at the organic-metal interfaces in optoelectronic devices, thin benzene films were reacted with typical device cathode metals, Ag, Mg, Al, and Ca, and studied using Raman vibrational spectroscopy. Ag and Mg form metal clusters and some adduct formation. Al undergoes an insertion reaction, forming a substituted benzene ring. Ca reacts with benzene to form a phenyl radical, which then decomposes the film into regions of ordered graphitic carbon. The results of these studies are attributed to atomic properties of the metal atoms.