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Enhanced Exciton Diffusion in Semiconducting Carbon Nanotube Films Michael S. Arnold, University of Wisconsin-Madison, DMR 0905861 .

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  1. Enhanced Exciton Diffusion in Semiconducting Carbon Nanotube FilmsMichael S. Arnold, University of Wisconsin-Madison, DMR 0905861 Understanding how excitons – bound, photogenerated charges – migrate through films of semiconducting carbon nanotubes is a critical aspect of implementing these appealing materials as next-generation photoabsorbers in photovoltaic solar cells and photodetectors. We have studied the relationship between carbon nanotube coupling and exciton diffusion length by varying the coverage of a solubilizing polymer that wraps nanotubes and thereby tuning inter-nanotube interactions. By reducing the polymer to nanotube ratio from 3:1 to 1:1 as shown below, we can increase inter-tube coupling and exciton diffusion by a factor of three. This increase results in greatly improved photocurrent responsivity (external quantum efficiency, EQE) in thin film photovoltaic and photodetector devices, and has enabled the manufacture of proof-of-principal photovoltaic devices with 7% monochromatic power conversion efficiency at 1.05 μm. Increased efficiency and diffusivity 3:1 hP,mono = 7% 1:1 Coupling between carbon nanotubes is controlled by the amount of solubilizing polymer in cast films (SEM image, center). Varying coupling, in turn, affects the distance excitons travel, measured roughly as the peak in thickness dependence of external quantum efficiency (EQE, electron/hole pairs out per photon in, center). Monochromatic power conversion efficiencies have been subsequently improved to >7%, (Right).

  2. Sorting Carbon Nanotubes According to Electronic Type from Aqueous Surfactant SolutionsMichael S. Arnold, University of Wisconsin-Madison, DMR 0905861 Hosting students for summer research internships provides an excellent opportunity to teach research skills and course-related material in a collaborative, tactile environment. These experiences also develop the technical communication and teaching skills of graduate researchers. This summer, high-school student Paul Dieterle worked with graduate students to optimize a scalable method for separating carbon nanotubes according to electronic type via column chromatography. In addition to wet chemistry techniques, sedimentation differential centrifugation, and carbon nanotubephotophysics, Paul was exposed to optoelectronic measurement techniques, electronic device physics and became acquainted with the vast literature on carbon nanotubes. High-school senior Paul Dieterle measures spectrally resolved optical absorption of carbon nanotube solutions as part of his summer research internship.

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