Relating Structure and Electrostatic Potentials in Organic Semiconductor Thin Films

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TECHNICAL SUMMARY: In this renewal proposal to the Solid State and Materials Chemistry Program, in the NSF Division of Materials Research, the Principal Investigators (PIs) will continue their experimental efforts to uncover fundamental microstructure-property relationships in organic semiconductors. In particular, the goal is to probe the connection between film structure and surface electrostatic potential. Surface potentials impact charge transport at interfaces and thus are directly relevant to the performance of organic electronic devices. For example, in a thin film transistor (TFT) the surface potential at the organic semiconductor/insulator interface determines the charge concentration, and the gradient in the surface potential determines the direction of carrier flow. Surface potentials reflect many factors associated with surfaces (or interfaces) including crystal structures, electronic energy levels, defects, dipoles, fixed charges, contaminants, and illumination conditions. The PIs will use high resolution Electric Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KFM) to measure and map surface potentials in organic semiconductor films, and to correlate surface potential domains and gradients with structural features. The work is designed to address a number of questions including: How do epitaxial growth modes and grain morphologies in organic semiconductor films impact surface potentials? Where do trapped charges reside in organic semiconductors and can the trapping zones be correlated with surface potential peaks or valleys? A principal outcome of this work will be significantly improved understanding of electrostatic complexity in organic semiconductor films and interfaces and correlation of this complexity with microstructure. Films and interfaces that are central to the operation of OTFTs and organic solar cells will be investigated, so that the relevance of the results will be immediately apparent to the organic electronics research community. NON-TECHNICAL SUMMARY: Organic semiconductors are an important class of thin film electronic materials that have many attractive properties including efficient luminescence, liquid phase processability, and compatibility with plastic substrates; these advantages are driving new applications ranging from flexible, rugged e-readers and smart cards to low cost solar cells. Organic light emitting diodes (OLEDs), in particular, have attained performance suitable for display technologies and are undergoing commercialization, and there are also exciting prospects for organic semiconductors in biosensors and printed electronics. Importantly, the development of a mature organic semiconductor technology hinges on thorough understanding of structure-processing-performance relationships. The overarching goal of this proposal is to advance the materials science of organic semiconductors by uncovering fundamental microstructure-property correlations in model organic semiconductor systems. The work will be carried out by two University of Minnesota faculty researchers in collaboration with PhD students. Thus, a principal broader impact will be graduate level training of students in materials science and engineering. In addition, the PIs will provide summer research experiences for one Minneapolis area high school student and one minority undergraduate each summer over the course of the award. The minority undergraduate will be selected from a pool of science and engineering sophomores and juniors at the University of Texas Pan American (UTPA), a largely Hispanic serving institution. For student selection, the PIs will be assisted by a UTPA faculty member who has performed summer research previously at Minnesota. The goal will be to excite these young potential scientists about the opportunities in materials research, while providing them real hands-on technical training. The PIs will also continue to provide demonstrations of scanning probe techniques to K-12 students, as they have done under their previous award.