Excitons in organic molecular crystals

In this paper, we present a study of the nature of the lowest lying electronic states in crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) layers grown by thermal evaporation from effusion cells on p-type (0 0 1) Si substrates. Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy were employed to investigate the excitons in the temperature range from 10 to 330 K. In all the investigated samples the thermal quenching of PL has been observed. It was found that the PL thermal quenching is mainly caused by the strong exciton-phonon coupling which is typical for organic molecular crystals such as PTCDA. The strong temperature dependence of the photoluminescence spectra can be attributed to the reduced exciton-phonon interactions with decreasing temperature.