2014 COPE Fellowship Award Winners
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Thomas Bougher, a graduate student in Mechanical Engineering presents "Thermal Transport in Chain-oriented Amorphous Polymers".Caroline Grand, a graduate student in Chemistry, presents "Tuning Solubility of Isoindigo Polymers to Control Thin Film Morphology in Organic Photovoltaics". In this contribution we discuss the hypothesis that the thin-film morphology in the active layer of organic photovoltaics (OPVs) is dictated by the state of the polymer in the coating solution, and that tuning aggregation of the active material in solution via its structure, or via processing additives, leads to a predictable morphology in thin films. We have synthesized and characterized the optoelectronic properties of a family of poly(terthiophene-alt-isoindigo) P(T3-iI) with varying side chains. Through this structural modification we are able to control polymer solubility, and intermolecular interactions among polymer chains leading to increased mobilities in OFETs and increased short-circuit currents in blends with fullerene derivatives for OPVs. Alternatively, processing additives such as 1,8-diiodooctane are shown to lead to drastic changes in the morphology and OPV device parameters. Characterization of polymer aggregation in solution is compared to the thin-film morphology observed by AFM with smaller domain sizes achieved with a wide range of additive structures, which leads to 6.5% power conversion efficiency in OPV devices using P(T3-iI):PC71BM blends.Nabil Kleinhez, a graduate student in Chemical and Biomolecular Engineering, presents "Employing Liquid Crystal Phases for Enhanced Semiconducting Polymer Morphologies for Organic Solar Cell and Transistor Applications". Polymer-based electronic devices including organic field-effect transistors (OFET) and organic photovoltaics (OPV) are attractive due to their potentially low cost, light-weight, large area and flexible nature. In order to improve device performance, processes must be developed to enhance the degree of intermolecular ordering between polymer chains in order to facilitate hopping of charges through the material. We have studied poly(3-hexylthiophene) (P3HT) as a model π-conjugated semiconducting polymer and have investigated the emergence of a lyotropic liquid crystalline phase that could serve as a means of intermolecular ordering in solution and as a potential precursor to well-ordered thin films of P3HT for device applications. As P3HT dissolved in trichlorobenzene ages, the polymers self-assemble into nano-rods which are subsequently aligned when filled into capillary tubes. Using polarized optical microscopy, we have observed long range order as evidenced by near monodomain-like orientation behavior and other birefringent textures indicating the presence of a liquid crystalline phase under certain solution conditions. Raman spectroscopy performed on capillaries of these solutions allows quantification of the degree of ordering in the aligned P3HT, and we have observed an orientational order parameter that increases with solution aging time. UV-Vis spectroscopy, dynamic light scattering and cryogenic transmission electron microscopy were employed to characterize the formation of P3HT nanostructures that give rise to ordering in the fluid state which can be transferred to the solid state for device applications.Keith Knauer, a graduate student in Electrical and Computer Engineering, presents "The Operational Lifetime of Inverted Top-Emitting Organic Light-Emitting Diodes". Recent research results focused on inverted top-emitting organic light-emitting diodes (OLEDs) of both the single-unit and stacked varieties are presented. An OLED is an electroluminescent device consisting of organic semiconductor material placed between two electrodes, an anode and a cathode. When a voltage is applied across the device, current is injected and light is emitted. OLEDs are of growing interest in the application areas of full-color high definition displays and solid-state white lighting. Inverted top-emitting OLEDs have many advantages over conventional bottom-emitting OLEDs. Inverted OLEDs are more convenient to integrate with the n-type driving electronics used in active-matrix displays and top-emitting OLEDs can be fabricated on opaque substrates and can lead to a maximized pixel aperature ratio when fabricated on the driving electronics in active-matrix displays. Furthermore, these inverted top-emitting OLEDs can be stacked vertically leading to devices with enhanced operational lifetime, higher efficiency, and simpler color-mixing schemes than single-unit OLEDs.