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HL: Fachverband Halbleiterphysik

HL 36: Poster III

HL 36.7: Poster

Mittwoch, 27. Februar 2008, 16:30–19:00, Poster D

Transport Gap and exciton binding energy determination in organic semiconductors — •Stefan Krause¹, Benedetta Casu², Achim Schöll¹, Friedrich Reinert¹, and Eberhard Umbach¹ — ¹University of Würzburg, Experimental Physics II, Am Hubland, 97074 Würzburg, Germany — ²Inst. f. Physik. u. Theor. Chemie, Auf der Morgenstelle 8 , 72076 Tübingen , Germany

The transport gap of an organic semiconductor is defined as the energy difference between the HOMO and LUMO levels in the presence of a hole or electron, respectively, after relaxation has occurred. Its knowledge is mandatory for the optimisation of electronic devices based on these materials. UV photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) are routinely applied to measure these molecular levels. However, the precise determination of the transport gap on the basis of the respective data is not an easy task. It involves fundamental questions about the properties of organic molecules and their condensates, about their reaction on the experimental probe, and on the evaluation of the spectroscopic data. In particular electronic relaxation processes, which occur on the time scale of the photo excitation, have to be considered adequately. We determined the transport gap for the organic semiconductors PTCDA, Alq3, DIP, CuPc, and PBI-H4. After careful data analysis and comparison to the respective values for the optical gap we obtain values for the exciton binding energies between 0.1 - 0.5 eV. This is considerably smaller than commonly believed and indicates a significant delocalisation of the excitonic charge over various molecular units.