The unoccupied electronic structure of stacked layers of copper(II)phthalocyanine (CuPc) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on Ag(1 1 1) has been investigated by means of two-photon photoemission (2PPE). We find a rich electronic structure comprising at least five unoccupied electronic states which we identify based on their energetic position and their dispersion in momentum space. More specifically, we observe the first and the second image-potential states of the modified Ag(1 1 1) surface, as well as the metal-organic interface state (IS) inherent to the PTCDA/Ag(1 1 1) interface. Moreover, two additional molecular features are observed for the CuPc/PTCDA/Ag(1 1 1) system which we attribute to an unoccupied molecular orbital (LUMO + 2) of CuPc. The 2PPE intensity of the IS exhibits a pronounced dependence on the pump photon energy, which closely follows the optical absorption of the outer molecular layer. This strongly points to charge transfer from the optically excited molecules to the interface state.
The theoretical modelling of metal-organic interfaces represents a formidable challenge, especially in consideration of the delicate balance of various interaction mechanisms and the large size of involved molecular species. In the present study, the energies of interface states, which are known to display a high sensitivity to the adsorption geometry and electronic structure of the deposited molecular species, have been used to test the suitability and reliability of current theoretical approaches. Two well-ordered overlayer structures (relaxed and compressed monolayer) of NTCDA on Ag(111) have been investigated using two-photon-photoemission to derive precise interface state energies for these closely related systems. The experimental values are reproduced by our DFT calculations using different treatments of dispersion interactions (optB88, PBE-D3) and basis set approaches (localized numerical atomic orbitals, plane waves) with remarkable accuracy. Our results underline the trustworthiness, and some of the limitations, of current DFT based methods regarding the description of geometric and electronic properties of metal-organic interfaces.
The functionality of organic electronic devices is governed
by
the dynamics of charge carriers and excited states in organic semiconductors.
In particular, the relaxation of excitons and the transfer of charge
carriers at metal electrodes crucially determine the performance of
organic optoelectronic devices. In a combined experimental study we
apply time-resolved photoluminescence and two-photon photoemission
to reveal the ultrafast exciton dynamics and charge transfer at prototype
organic/metal contacts comprising thin molecular films on single-crystalline
noble-metal surfaces. On the basis of experiments with systematically
varied film thicknesses, we relate the strong quenching of Frenkel
excitons and charge-transfer excitons to the wave function overlap
with the metal, indicating charge transfer as the dominant relaxation
pathway. Moreover, the presence of an electronic interface state is
found to facilitate the transfer of excited carriers across the organic/metal
interface.
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