The Application of Poly(Phenylene) Type Polymers and Oligomers in Electroluminescent Color Displays

Due to their high photoluminescence efficiency (>30%), high environmental stability and the good charge transport properties the derivatives of poly(paraphenylene) (PPP), as the laddertype PPP (LPPP) and the oligomer hexaphenyl, are very suitable materials to realise efficient, stable, large area blue organic light-emitting diodes (OLEDs). The emission of blue OLEDs can be efficiently converted into all other emission colors either by an external color conversion technique (ECCT) or an internal color conversion technique (ICCT) and hence are very interesting for a number of display applications Firstly, we demonstrate the realisation of efficient red-green-blue (RGB) emission colors (representing the RGB-pixels in a multicolor display) by an external CCT. In this case the blue EL device is covered with highly fluorescenct dye/matrix layers, which are excited by the blue emission and emit photoluminescence light in a lower energetic range. Secondly, a new method for producing efficient white light-emitting polymer diodes (which are interesting for e.g. backlight sources in liquid crystal displays) based on a blend of two polymers is presented: a blue light-emitting m-LPPP and a red-orange emitter poly(perylene-co-diethynylbenzene) (PPDB). The red-orange emission is created within the EL device (ICCT) by an excitation energy transfer from m-LPPP into the energetically lower lying states of PPDB. This internal excitation energy transfer is very efficient, so that only a concentration of 0.05 weight % PPDB in the polymer blend is required in order to obtain white light emission.