Interfaces in organic electronic devices: Surface characterization and modification and their effect on microstructure in molecular assemblies

Abstract

This dissertation has focused on (i) the characterization and optimization of the near-surface region of indium-tin oxide (ITO) thin films, and (ii) the characterization of the microstructure and electrical properties of thin films of several new self-organizing liquid crystalline phthalocyanines (Pcs). Commercial ITO surfaces were explored through a combination of high resolution X-ray photoelectron spectroscopy and electrochemical techniques. It was determined that sputter-deposited ITO films undergo hydrolysis immediately upon exposure to atmosphere, creating InOOH and In(OH)₃ species, which appear to inhibit charge transfer reactions. The surface coverage of these InOOH and In(OH)₃-like species can be controlled by various solution and vacuum pretreatments, including etching with EDTA solutions, and RF-plasmas. Characterization of new discotic mesophase Pc materials has focused on modifications of the original Pc in this series, CuPc(OCH₂CH₂OBz)₈, including a polymerizable version, CuPc(OCH₂CH₂OCH₂CH=CH-Ph)₈, and the sulfur analogs of these molecules, CuPc(SCH₂CH₂OBz)₈ and CuPc(SCH₂CH₂OCH₂CH=CH-Ph)₈. The self-organizing properties of these new Pcs are altered by the changes in side chain composition, but still show the same "column-forming" tendencies as the parent Pc, with long range order. The polymerizable Pc materials can be photolithographically patterned with features as small as 2 microns. Electrical anisotropies in these films were measured with a conductive tip AFM and with OFETs, and anisotropies in current (j(∥)/j(⊥)) were ca. 10 on the micron scale, and up to 1000 on the submicron scale. OFET measurements showed low hole mobilities, which are attributed to poor contact between the Pc column and the Au electrodes. Chemical modification of these electrodes shows that considerable improvements in OFET performance result from this modification strategy. Understanding and controlling the microscopic structure of these Pc films is important for optimizing their electrical properties. A considerable effort was focused on developing a quantitative protocol to combine transmission and reflectance vibrational spectroscopic data to determine the three Euler angles that determine the orientation of these Pcs in an LB-deposited film on a planar substrate. Changes in orientation upon annealing and polymerization were observed, but in general these molecules display tilt angles away from the surface normal of <20° and twists about the surface normal of ca. 25°.