| In the past ten years, much progress has been made in producing organic semiconductors with properties that rival amorphous silicon. Organic semiconductors are sought for low-cost or large-area electronic devices such as organic field-effect transistors (OFETs), light-emitting diodes (OLEDs), photovoltaic cells, sensors, and radio frequency identification (RFID) tags. Low temperature processing options and the ductility of organic materials also allow flexible plastics or fabrics to be used as substrates. However, the majority of the newly developed materials are insoluble requiring the use of thermal evaporation to obtain thin films. In order for these organic semiconductors to compete as 'low cost' alternatives to silicon, manufacturers must be able to use large-area, continuous, reel-to-reel methods for production, which would likely involve the solution-based deposition methods such as spin coating, stamping, or printing. Therefore, our work has focused on methods for solubilizing and controlling the self-assembly of conjugated oligomers in order to maximize the semiconducting properties from solution-based processes.; Chapter 1 gives a general overview of charge transfer in organic semiconductors, and describes how devices are fabricated and tested. An extensive review of the literature on solution processed p- and n-type oligomers is also given.; Chapter 2 and 3 discuss our initial attempts to produce soluble oligothiophenes capable of self-assembly. Chapter 2 focuses on beta-functionalized heptathiophene oligomers, while Chapter 3 deals with (x-(x'-substituted oligothiophene derivatives.; A synthetic method for incorporating thermally removable solubilizing groups into organic semiconductors is outlined in Chapter 4, and demonstrated using a sexithiophene oligomer. The synthesis, chemical characterization, as well as an in-depth characterization of solution-processed films of this oligomer using AFM and NEXAFS spectroscopy is given. This theme is carried over to Chapter 5, where a series of oligomers functionalized with these solubilizing groups were prepared in order to deduce various structure-property relationships.; In Chapter 6, we report the first OFET devices fabricated from self-assembled monolayers of end-functionalized thiophene oligomers. Oligomers containing organosilane and thiol binding groups were prepared and characterized. Monolayer films were made using solution or Lang muir-Blodgett deposition techniques, and characterized by AFM, ellipsometry and NEXAFS spectroscopy. The performance and stability of these monolayers were investigated in bottom contact field-effect transistors.; Finally, Chapter 7 discusses the synthetic strategy we have developed to access both regiorandom and regioregular polythiophenes containing electron-withdrawing carboxylate substituents. (Abstract shortened by UMI.)... |
