Molecular Structural and Electrical Characterization of Rodlike Aggregates of Discotic Phthalocyanines

Abstract

This dissertation focuses on structural and electrical characterization of self-organizing discotic molecular materials, specifically alkoxy and thioether side chain modified copper phthalocyanines, both in bulk and at organic/dielectric and organic /metal interfaces. A great deal of effort has been focused on understanding the self-organizing nature in these materials since molecular ordering is believed to control the intrinsic physical as well as electrical properties of these molecular aggregates. It was determined that side chains in these Pcs have a significant impact on the general ordering in these materials: alkoxy side chain modification favors a columnar hexagonal phase with a cofacial intracolumn alignment; thioether side chain modification, however, favors a tilted intracolumn alignment and much rigid columnar packing, driven by sulfur-sulfur interactions among adjacent molecular disks. Incorporation of styrene functionality in the side chain has been shown to enable photopolymerization. An optimal hexagonal columnar packing has been proved to be stabilized via photolysis at the mesophase. It is critical to explore the molecular ordering as well as the charge transport characteristics at interfaces since the organic/dielectric interface controls the charge accumulation in organic field-effect transistors (OFETs) and the metal/organic interface determines the charge injection in devices such as organic photovoltaic cells (OPVs). Two analytical tools have been developed in this dissertation work that successfully address these interfacial issues from a molecular level. 1) Probing interfacial structures at the organic/dielectric interface with X-ray reflectometry (XRR). Surface chemistry has shown a drastic impact on the ordering of the initially deposited materials. Surface engineering strategies, i.e. chemical modification, have been shown to significantly improve the coherence of molecular assemblies thereby optimizing charge transport properties of these molecular materials in an OFET platform. 2) Exploring charge injection and transport characteristics at molecular junctions with conductive-probe AFM (C-AFM). Charge injection processes at the metal/organic molecular junction have shown a strong dependence on the microstructure of these molecular materials. Thermionic emission and field emission were shown to be competing processes at these junctions. One dimensional charge transport is realized only with the appropriate molecular ordering in these discotic materials at metal/organic junctions. 3) Exploring structural and electrical properties of ITO with C-AFM. The ITO surfaces have shown both structural and electrical heterogeneity at the nanometer scale. A tunneling model has been proposed and the presence of thin insulating layers was believed to be the cause of electrically inactive regions of ITO. Aggressive chemical etching protocols have been developed and shown to improve the percentage of surface electrically active area, thereby, enhancing the electrode performance.