Browsing by Author "Davis, Ruth M."

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    Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion
    Drees, M.; Premaratne, K.; Graupner, W.; Heflin, James R.; Davis, Ruth M.; Marciu, D.; Miller, M. (AIP Publishing, 2002-12-01)
    Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C-60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface. (C) 2002 American Institute of Physics.
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    Discrete Event Simulation with Pascal
    Davis, Ruth M. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1986-06-01)
    Pascal_SIM is a collection of Pascal constants, types, variables, functions and procedures for developing event, activity, three-phase of process oriented discrete-event simulation models. Facilities are provided for queue processing, time advance and event list maintenance, control of entities and resources, random number generation and streams, sampling from parametric and empirical distributions, statistics collection, and visual displays.
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    Thickness dependence, in situ measurements, and morphology of thermally controlled interdiffusion in polymer-C-60 photovoltaic devices
    Drees, M.; Davis, Ruth M.; Heflin, James R. (American Physical Society, 2004-04-28)
    A series of detailed studies is presented in which heat-induced interdiffusion is used to create a gradient bulk-heterojunction of 2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) copolymer and C-60. Starting from a bilayer of spin-cast MEH-PPV and sublimed C-60, films are heated in the vicinity of the glass transition temperature of the polymer to induce an interdiffusion of polymer and fullerene. Variation of the polymer layer thickness shows that the photocurrents increase with decreasing layer thickness within the examined thickness regime as transport of the separated charges out of the film is improved. The interdiffusion was observed in situ by monitoring the photocurrents during the heating process and exhibited a rapid rise during the first five minutes. Cross-sectional transmission electron microscopy studies show that C-60 forms clusters of up to 30 nm in diameter in the polymer bulk of the interdiffused devices. This clustering of the fullerene molecules puts a significant constraint on the interdiffusion process that can be alleviated by use of donor-acceptor combinations with better miscibility.