The potential of organic semiconductors as active materials in electronic and optoelectronic devices has attracted basic scientific as well as technological interest. They often consist of a multiplicity of organic layers, thus making the understanding of the organic interface and organic-on-organic growth a scientifically and technologically important issue. On the one hand details of the interfaces are important for charge injection, while the films morphology and crystallinity together with the molecular/crystallite orientation determine charge transport and light emission/absorption characteristics. To optimise device performance these key factors need to be controlled for the specific device as OLEDs, OFETs and photovoltaic cells have different requirements.
Here we report the growth of epitaxially ordered heterostructures of sexithiophene (6T), sexiphenyl (6P) and pentacene (5A). We have produced crystalline films, where the molecules are either exclusively parallel or near perpendicular to the inorganic supports. These crystalline films are then used as substrates for the 6P, 6T, 5A heterostructures. The geometric structure and morphology of these heterostructures was obtained from near edge x-ray absorption fine structure spectroscopy, synchrotron radiation x-ray diffraction and atomic force microscopy studies, together with the electronic band alignment at the organic interface, as measured with photoemission. We find that the orientation of the molecules in the organic substrate film determines not only the orientation in the heterolayer, but also leads to a second film which is crystalline with a well defined epitaxial relationship. This demonstrates that highly oriented crystalline molecular films provide excellent, stable templates for organic heteroepitaxial growth. However, our results clearly show that the epitaxial growth of organics on organics cannot be simply understood in terms of lattice match, but appears to be mainly driven by the alignment of the overlayer molecules with respect to the substrate molecules [1].
Supported by the Austrian Science Foundation (FWF).
[1] G. Koller, S. Berkebile, J.R. Krenn, F.P. Netzer, M. Oehzelt, T. Haber, R. Resel, M.G. Ramsey, Nano Lett. 6, 1207 (2006).
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