Organic light-emitting diodes using spherical aggregates

We have succeeded in fabricating submicrometer-sized organic light emitting diodes (OLED) with bright emission using a polymer blend. The cell structure was composed of ITO (indium-tin oxide)/hole transport layer (polymer blend layer)/emitter layer/MgAg. We used poly (methyl methacrylate) (PMMA) as an inert host material and N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine-polycarbonate (PC-TPD) as a guest material. The polymer blend layers showed various morphology depending on the concentration of PC-TPD. With 2wt% of PC-TPD, the PC-TPD formed submicrometer-sized spherical domains, actually prolate spheroids, in the PMMA. host layer. The domain diameters were around 200 nm according to AFM and SEM observations. The OLED including such a blend structure showed aggregation of bright submicrometer-sized electroluminescence spots. The number of working diodes exceeded four million per square centimeter.