Blue luminescence induced by confinement in self-assembled films

The fabrication and characterization by means of photoluminescence (PL), UV-vis absorption, electro-luminescence (EL) and X-ray reflectivity of multilayer heterostructures consisting of alternate layers of conjugated and non-conjugated polymers have been studied. The heterostructures are prepared by the layer-by-layer self-assembly technique, using two types of polyelectrolytes. The first are precursors of conjugated polymers such as poly(phenylenevinylene) (PPV) and other poly(arylenevinylene) polymers, and the second are non-conjugated polymers such as poly(styrene-4-sulfonate) (SPS), polyacrylic acid (PAA) and poly(allylamine hydrochloride) (PAH). The heterostructures consist of a repeated sequence of bilayers (layer pair) or multilayers, where the conjugated polymer is formed by heat treatment under vacuum. The thickness of each bilayer or multilayer was controlled by changing the non-conjugated polymer layer. Most importantly, we have found that the PL and EL spectral emissions can be ‘tuned’ by a proper ‘design’ of the heterostructure. Particularly, heterostructures in which the bilayer thickness is rather small and the electroluminescent layers are practically in contact show a blue shift upon decreasing the thickness of the assembly for ultrathin assemblies. In contrast, for assemblies where the electroluminescent layers are well separated by one or several non-conjugated layers (polyelectrolyte spacers), the emission is in the blue and independent of the assembly thickness (number of bilayers). We interpret the results as being due to confinement effects. Using this assembly technique, we were able to fabricate light emitting diodes (LEDs) which emit in the blue region.