Photoluminescence study of the exciton dynamics at PTCDA/noble-metal interfaces

Charge and energy transport across organic/metal interfaces play a decisive role for the functionality of organic semiconductor devices. As well-defined model systems for such heterointerfaces, we consider thin films of the organic semiconductor perylene-tetracarboxylicacid-dianhydride (PTCDA) deposited on the single-crystalline (111)-surfaces of the noble metals silver and gold. By means of time-resolved photoluminescence we investigate the exciton dynamics of these systems in the energy and time domain. Systematic variation of the PTCDA film thickness enables us to follow the exciton relaxation rates as a function of the molecule--metal separation from several nanometers down to a few angstroms. Spatially localized excitations, such as excimers, are found to relax by nonradiative energy transfer to the metal. In contrast, the relaxation of charge-transfer (CT) excitons can be explained by exciton diffusion and subsequent annihilation at the organic/metal interface. Both mechanisms are found to be much more efficient on Ag(111) than on Au(111). For excimers, the faster relaxation on the silver substrate presumably involves the excitation of intraband transitions inside the metal. The higher relaxation rate of CT excitons is explained in terms of enhanced charge transfer across the PTCDA/Ag(111) interface, which is mediated by the electronic interface state inherent to this organic/metal interface.