Excitons in organic molecular crystals

A photoluminescence (PL) study has been performed from 10 to 330 K on a 3,4:9, 10-perylenetetracarboxylic dianhydride (PTCDA) and aluminum tris (8-hydroxyquinoline) (Alq3) thin layers, grown on Si substrates by thermal evaporation. The observed anisotropy of the excitation energies, the spectral shape and the dispersion indicates the presence of two types of excitons near the excitation onset. These are associated with mixed intramolecular Frenkel and intermolecular charge-transfer excitons. In particular, the excitons exhibit a negative dispersion along the molecular stacks which has important consequences for their recombination.