Aggregate States and Energetic Disorder in Highly Ordered Nanostructures of para-Sexiphenyl Grown by Hot Wall Epitaxy

Authors


  • We thank O. Zrzavecka, D. Lysacek, and L. Valek for helping with the growth experiments. Research was supported by the Austrian Foundation for Advancement of Scientific Research (projects P-15155, P-15627, P-15629, and P-15630-N08). Part of this work was performed within the Christian Doppler Society's dedicated laboratory on Plastic Solar Cells funded by the Austrian Ministry of Economic Affairs and Konarka Austria GmbH. The support by NATO grant PST.CLG 978952 is also gratefully acknowledged. The work was also supported by the National Academy of Science of Ukraine under the program on basic research of structure and material properties on microscopic level. The work was also supported by the National Academy of Science of Ukraine under the research program “Nanostructured systems, nanomaterials, and nanotechnologies”.

Abstract

We report on photoluminescence (PL) and thermally stimulated luminescence (TSL) in highly ordered nanostructures of para-sexiphenyl (PSP) grown by hot wall epitaxy (HWE). A low-energy broad band is observed in the PL spectra that can be attributed to the emission from molecular aggregates. While the intrinsic exciton emission in steady-state PL dominates at low temperatures, the emission from aggregates increases with elevating temperature and its magnitude depends sensitively on film preparation conditions. Time-resolved PL measurements showed that the aggregate emission decays with a life-time of ≈ 4 ns, which is approximately an order of magnitude larger than the lifetime of singlet excitons. TSL data suggests the presence of an energetically disordered distribution of localized states for charge carriers in PSP films, which results from an intrinsic disorder in this material. A low-temperature TSL peak with the maximum at around 30 K evidences for a weak energy disorder in PSP films, and has been interpreted in terms of a hopping model of TSL in disordered organic materials.

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