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.

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Niobium-containing diamond-like carbon films (a-C:H:Nb) were deposited in a hybrid plasma-enhanced chemical vapor deposition (PECVD) and direct-current magnetron sputtering (DCMS) process, using a niobium target in an argon/acetylene atmosphere on industrial polyimide foil, spin-coated polyimide foil, and alumina substrates. While the coating on the industrial foil tends to crack, the thin films on the spin-coated polyimide and the ceramic substrates showed good adhesion, as well as a smooth and dense topography. Investigations of the electrical properties revealed their suitability as biocompatible sensor materials in strain gauges, with a gauge factor of up to 3.2 and a temperature coefficient of the electrical resistance (TCR) of −200 ppm/K.