Spectroscopic properties of a prototypic organic semiconductor: The case of PTCDA

摘要

In the present work, we have investigated the optical properties of PTCDA, both for dissolved monomers and crystalline films. Our calculations of the vibrational modes of a single molecule and the deformation in the relaxed excited state show good agreement with the observed absorption of dissolved molecules and resonant Raman spectra of thin films. The subsequent development of microscopic approaches for the transfer of Frenkel excitons between different molecules has allowed for a quantitative modelling of the observed linear optical properties of poly-crystalline PTCDA films. Furthermore, the corresponding k-space dispersion of the excitonic transitions, together with a detailed analysis of low-frequency internal vibrations and external phonon modes as observed in resonant Raman spectra, gives a hint for the origin of the very high Stokes shift observed between absorption and low-temperature PL spectra. A contribution to the PL spectra at lower energy can be interpreted on the basis of exciton dissociation and subsequent recombination between positively and negatively charged molecules. The required charge separation seems to involve a small barrier, so that the recombination between molecules in opposite charge states becomes dominant at higher temperatures. The application of the model of Frenkel exciton transfer to heterostructures with only a few monolayers revealed the existence of surface states, probably depending on the type of the surrounding material.