Regensburg 2004 – scientific programme

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SYOH: Organic and Hybrid Systems for Future Electronics

SYOH 2: Spectroscopy

SYOH 2.1: Talk

Thursday, March 11, 2004, 14:45–15:00, H37

Relation between optical transition energies and the transport gap in α-PTCDA — •Reinhard Scholz and Thorsten U Kampen — Institut für Physik, Technische Universität Chemnitz

Recently, the slowest photoluminescence (PL) channels in α-PTCDA were assigned to pairs of oppositely charged molecules [1]. Based on detailed model calculations of molecular dimers with time-dependent density functional theory, we can determine the Stokes shift arising from the internal deformation of the ionized molecules, resulting in an improved estimate for the energy of the charge transfer states in the periodic crystal. These findings are compared with the results of high-resolution photoemission spectroscopy and electrical characterization techniques applied to PTCDA films grown on n-doped GaAs(100) substrates. Depending on the substrate treatment, the ionization potential of the GaAs surfaces varies by more than 1 eV, resulting in a sign change of the interface dipole within this interval. From a linear fit of the interface dipole vs. the electron affinity of the substrate, an electron affinity of 4.12 ± 0.10 eV can be deduced for the organic layer. This procedure allows for an estimate of the energy offset between the LUMO of PTCDA and the conduction band minimum of the substrate, and the resulting HOMO-LUMO gap of 2.44 - 2.55 eV is in agreement with the lower limit estimated from the electrical measurements. The energetic difference between the observed PL energy and the transport gap can be related to the energy required for the separation of the two opposite charges to infinite distance.

[1] A. Yu. Kobitski, R. Scholz, H. P. Wagner, and D. R. T. Zahn, Phys. Rev. B 68, 155201 (2003).