Hamburg 2001 – scientific programme

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HL: Halbleiterphysik

HL 6: Optische Eigenschaften

HL 6.7: Talk

Monday, March 26, 2001, 12:00–12:15, S9/10

Experimental and theoretical analysis of the Stokes shift between optical absorption and photoluminescence spectra of crystalline PTCDA — •A. Yu. Kobitski, R. Scholz, H. P. Wagner, and D. R. T. Zahn — Institut für Physik, TU Chemnitz, 09107 Chemnitz, Germany

By means of optical absorption studies, PL, and time-resolved PL, we investigate the increase of the Stokes shift between optical absorption and photoluminescence (PL) in crystalline 3,4,9,10,-perylene tetracarboxylic dianhydride (PTCDA) compared to the dissolved monomer.

This increased Stokes shift can be analysed further with calculations including the new degrees of freedom in the crystal: Transfer of excitons between equivalent molecules in different unit cells, and the interaction among the two molecules in the crystal basis. The most critical input in any of these models is the interaction between consecutive molecules in the quasi-1D stacks because the intermolecular distance is very small.

The optical properties of a single molecule can be calculated with good accuracy using time-dependent density functional theory (TD-DFT) on the B3LYP level. In order to analyse the possible optical transitions in PTCDA molecular crystals, these calculations are extended towards molecular complexes consisting of 2 (dimer) and 3 (trimer) molecules in a stack arrangement. Our calculations are performed for stacks based on different molecular geometries: Neutral ground state, relaxed excited state, anion, and cation. The possiblity to assign the observed PL spectra to relaxed excited monomers or to recombination between molecules in anionic and cationic geometries are discussed in some detail.