Polymer light-emitting diodes with an emitting layer based on a nano-confined semiconducting polymer blend

Blending a visible light-emitting organic semiconductor with an insulator alleviates the trap-limited nature of the electron current. Organic light emitting diodes (OLEDs) comprising such a blend as emissive layer exhibit a two-fold increase in luminous efficiency with only 10% semiconductor. Due to this low content of semiconductor, polymer-LEDs are more attractive than small molecule-based devices. However, polymers impose the difficulty of an inherently low miscibility. In a plain blend macro-phase separation can be avoided if the molecular weight is kept low, which, in case of the semiconductor, is a disadvantage as it suppresses charge carrier mobility. An alternative strategy is to impose a nano-confinement. We prepare aqueous nanodispersions of red (PPV) and blue (polyfluorene) emitting polymers, blended with polystyrene as insulator. We seem to fully suppress macro-phase separation in both cases. For the latter, the combination of nano-confinement and blending influences the phase morphology of the semiconductor in an unprecedented way. Fabricating OLEDs with an emitting layer consisting of nanoparticles poses a considerable challenge due to high operational current densities. We now succeed in fabricating such devices in a reproducible way at very decent efficiencies.