Full colour electroluminescence using dye-dispersed polymer blends

By blending suitable dyes into poly(N-vinylcarbazole) (PVK) we have fabricated devices which emit light in the whole visible spectrum. Their current-voltage (I–V) characteristics can be described by space-charge limited currents (SCLC) with effective trapping of the charge carriers by the dye molecules, while the light intensity shows a Fowler-Nordheim-like behaviour as a function of the mean electric field. We were able to model the electroluminescence characteristics by assuming tunnelling of the minority charge carriers through a triangular barrier. The obtained barrier heights showed a strong dependence on the dye molecules, suggesting that the injection of minority charge carriers takes place directly into the LUMO levels of the chromophors. The recombination along with the space charge leads to a steeper increase of the current and a Fowler-Nordheim-like I–V characteristic in the double injection regime though the majority carrier current is not injection limited.