Structure and Properties of Defects in Organic Molecular Semiconductors

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TECHNICAL SUMMARY:There is intense continuing interest in the use of polymer and organic molecular materials for the construction of a variety of optoelectronically-active devices. Despite the importance of these materials, the relationship between the morphology of the organic molecular film and its macroscopic properties remains incompletely understood. The nature of the structural defects in organic molecular crystals and their influence on macroscopic properties still needs to be determined in detail. Because of the extended molecular connectivity of these solids, the energetics, structure, and mobility of the grain boundary (2-D), dislocation (1-D), and vacancy (0-D) defects can be quite different than those seen in inorganic crystals. We have been focusing on fundamental studies of specific defect structures and properties in organic molecular and polymer materials that are optoelectronically active. Most recently, we have investigated the processing and structure of TIPS-pentacene, a soluble variant that packs with the acene rings face-to-face. We have developed Low Dose High Resolution Electron Microscopy (HREM) imaging techniques for the direct observation of defect microstructures in crystalline and liquid crystalline polymers and organic molecular films. We now propose additional studies on the structure and properties of pentacene variants with functionalities designed to better control their solid-state packing and macroscopic properties. We will concentrate our efforts on a series of pentacene derivatives designed to enhance solubility in organic solvents, stabilize the formation of liquid crystalline mesophases, and promote face-to-face packing between the acene rings in the solid-state. The structural characterization of these new materials will take advantage of a variety of instrumentation available at the University of Michigan including Optical Microscopy, Wide and Small Angle X-ray Scattering, Scanning Electron Microscopy, Scanned Probe Microscopy, and Transmission Electron Microscopy. The intellectual merit of this proposal is its focus on the characteristic morphologies and defect structures of a new series of highly crystalline, functionalized pentacene derivatives of interest for organic electronic devices. We will determine relationships between the microstructure and the macroscopic properties of these new materials. We expect that our results will continue to be useful for optimizing processing conditions of existing materials and for motivating future molecular designs. NON-TECHNICAL SUMMARY: There is considerable interest in the development of organic molecular and polymer materials for "plastic electronic" applications such as radio-frequency identification tags, flexible displays, and biosensors. This project will investigate the detailed microstructure of these materials using advanced microscopic techniques available at the University of Michigan. The information obtained will make it possible to design new materials with enhanced performance. The project will involve full-time support for a graduate student in Materials Science and Engineering, and will also provide research experience for undergraduates through the University Research Opportunity Program (UROP), the Marian Sarah Parker program for female engineering students, the Minority Engineering Program Office (MEPO), the NASA/Sharp summer program for high school students, and an outreach summer research program with Greenhills High School in Ann Arbor.