Dresden 2006 – scientific programme

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O: Oberflächenphysik

O 25: Organic films II

O 25.8: Talk

Tuesday, March 28, 2006, 17:30–17:45, PHY C213

Determination of the transport gap by UPS and IPES — •Stefan Krause, Detlef Eich, Benedetta Casu, and Eberhard Umbach — Experimentelle Physik II, Universität Würzburg

The transport gap in semiconductors is defined as the minimum energy needed to produce a separated electron and hole. In a conventional semiconductor such as Si or GaAs this gap can be determined by means of optical absorption taking into account that the optical gap is only slightly smaller than the transport gap because of the small exciton binding energy (few meV). Organic materials are in general believed to have very weak intermolecular interaction leading to a localized charge and hence to a large exciton energy. Thus transport and optical gap may differ by several 100 meV. Then, other methods like UPS and IPES are needed to measure the position of the highest occupied and lowest unoccupied molecular orbital directly. We did such measurements for the organic molecules PTCDA, Alq3 and Cu-Pc and compared the results to those obtained from GaAs, Si, HgTe, and CdTe. In both groups of materials, the peak-to-peak distances of the leading peaks in UPS and IPES are much larger (1-3 eV) than the optical gap, i.e. also much larger than the known or expected exciton energies. The agreement is much better if one uses the peak onsets in all cases. However, then the exciton energies in organic materials are surprisingly small. The origin of the failure of the peak-to-peak approach and the implications of the findings will be discussed.