This work reports on combining current-voltage characteristics, electroluminescence (EL) measurements, and modeling to identify the selectivity of the electrodes in bulk-heterojunction organic solar cells. Devices with the same photoactive layer but different contact materials are compared and the impact of surface recombination at the contacts on their performance is determined. The open-circuit voltage, V OC , depends strongly on the selectivity of the electrodes and it is observed that the EL signal of cells with lower V OC is dramatically reduced. This is ascribed to an enhanced rate of surface recombination, which is a non-radiative recombination pathway and does therefore not contribute to the EL yield. In addition, these cells have a lower current in forward direction despite the fact that the surface recombination occurs in addition to the recombination in the bulk. A theoretical model was set up and in the corresponding numerical simulations all three fi ndings (lower V OC , strongly reduced EL signal and lower forward current) could be clearly reproduced by varying just one single parameter which determines the selectivity of the electrode.

Mixed films of perfluorinated pentacene and diindenoperylene are studied for their structural and optical properties on SiO 2 and glass substrates. Grazing incidence Xray diffraction shows that the compounds mix on the molecular level. Two structures are identified. Their out-ofplane lattice spacing is determined by X-ray reflectivity, and their growth morphology is characterized using atomic force microscopy. It is found that one structure consists of molecules in a standing-up and the other in a lying-down orientation. The uniaxial dielectric function of mixed films is determined with spectroscopic ellipsometry, and the in-plane optical extinction coefficient is measured by real-time differential reflectance spectroscopy during growth. The question of intermolecular coupling between perfluorinated pentacene and diindenoperylene is discussed.

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