Abstract
Light outcoupling for top-emitting organic light-emitting devices is improved by using a semitransparent multilayer cathode structure of Al(2.5nm)∕Ag(0.5nm)∕Al(1nm)∕Ag(0.5nm)∕Al(0.5nm)∕Ag(11nm). In addition, an excellent transparency of 70% is achieved with a 78-nm-thick tris-(8-hydroxyquinoline) aluminum (Alq3) film as a top-capping layer, which is overlaid onto the cathode. The electroluminescence intensity with this Alq3-covered cathode is increased by a factor of 1.9∼3.1 (applied voltage from 7 to 21V) compared with the conventional cathode Al(4nm)∕Ag(12nm). This enhancement can be attributed to a gradual change of index refraction and extinction coefficient in the multilayer cathode (actually, not multilayer Al∕Ag, but an Al:Ag alloylike configuration) and to the reduction of top Ag reflection by overlaying an Alq3-capping layer onto the cathode. Besides, photoluminescence of the top-capping Alq3 layer is helpful to improve device luminance.
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