Exciton emission in PTCDA thin films under uniaxial pressure

We study the strain dependent photoluminescence (PL) of a 90-nm-thick polycrystalline 3,4,9,10-perylene tetracarboxylic dianhydride film on Si(001) between 20 and 300 K. Uniaxial pressure up to similar to 5 kbar is applied along the molecular stacking direction using a specially designed pressure cell. With increasing pressure, we find a quenching of the integrated PL intensity, which is mainly attributed to the creation of defects. At low temperature, the charge transfer exciton emission (CT2) gains intensity relative to the Frenkel exciton emission. Furthermore, the CT2 transition reveals a shift to lower energies by up to 24 meV. At room temperature, the PL is dominated by the excimer transition, which also shows a redshift of similar to 24 meV at the highest uniaxial pressure. The relative increase of the CT2 transition at low temperature and the redshift of the emission bands are attributed to an increased exciton trapping probability and an enhanced binding energy with reduced distance between stacked molecules. This explanation is supported by a comparison with total energy calculations for pairs of adjacent molecules. Moreover, the calculated pressure-induced energy shifts of the CT2 and excimer transitions are in good agreement with the experimentally observed values.