Berlin 2005 – scientific programme

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O: Oberflächenphysik

O 53: Organische Dünnschichten V

O 53.5: Talk

Wednesday, March 9, 2005, 11:45–12:00, TU EB420

A new approach to the determination of the transport gap in organic semiconductors using a combination of IPES and UPS — •Stefan Krause, Benedetta Casu, and Eberhard Umbach — Experimentelle Physik II, Universität Würzburg, Am Hubland, 97074 Würzburg

Organic semiconductors are in general believed to be strongly correlated systems. The value for the transport gap is somewhere between the measured value of optical absorption and the one, which one gets from the energetic distance of the HOMO in UPS and the LUMO in IPES. If one uses the peak positions of the HOMO and LUMO the gap is about 1 eV larger than the one of optical absorption. This is then explained with the strong correlation and polarization [1].

We compared the measurements of organic semiconductors with those of inorganic to find a hint on which side of these two boundaries one has to search. Surprisingly the data for the inorganic semiconductors showed the same trend and the UPS-IPES gap was much larger than the optical gap. This can no longer be explained with correlation because these materials are solids with strong covalent bonds and small (Mott-Wannier-) exciton binding energies. However, the distance of the peak onsets perfectly fits the optical data.

[1] E.V. Tsiper et al. Chem. Phys. Lett. 360 (2002), 47-52