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19 May 2010 Island size effects in organic optoelectronic devices
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For diode device structures, the vertical transport of charge carriers across the device is expected to dominate its' performance. In planar organic molecules, where the π-stacking direction is normal to the main body of the molecule, charge transport is highly anisotropic and related to the orientation of the molecules relative to the substrate. On ITO, we have confirmed with AFM and X-ray scattering (reflectivity and grazing incidence X-ray diffraction ) that PTCDI-C8, an electron accepting photoactive molecule, forms large crystalline domains consisting of tilted, up-right standing molecules. Similarly to that observed on SiO2 substrates, the domain size can be tuned by adjusting the substrate temperature during growth. In such a configuration, the charge transport is dominated by carrier movement parallel to the substrate, along the π-stacking direction, with vertical transport limited by hopping between layers. We observed that in diodes using such films, the developed charge density increased with the lateral island size. In the vertical direction, charge transport is best described by a thermally activated hopping mechanism. This type of behaviour can have significant implications for nanostructured bulk heterojunctions of such films, when combined with electron donors such as pentacene or diindenoperylene, to be used in small molecule solar cells.
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Minh Nguyen, Ayse Z. Turak, Felix Maye, Jonathan Heidkamp, Jörg Wrachtrup, and Helmut Dosch "Island size effects in organic optoelectronic devices", Proc. SPIE 7722, Organic Photonics IV, 77221O (19 May 2010);

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