Improved light outcoupling for phosphorescent top-emitting organic light-emitting devices

Light outcoupling for top-emitting organic light-emitting devices is improved by using a semitransparent multilayer cathode structure of Al(2.5 nm)/Ag(0.5 nm)/Al(1 nm)/Ag(0.5 nm)/Al(0.5 nm)/Ag(11 nm). In addition, an excellent transparency of 70% is achieved with a 78-nm-thick tris-(8-hydroxyquinoline) aluminum (Alq(3)) film as a top-capping layer, which is overlaid onto the cathode. The electroluminescence intensity with this Alq(3)-covered cathode is increased by a factor of 1.9 similar to 3.1 (applied voltage from 7 to 21 V) compared with the conventional cathode Al(4 nm)/Ag(12 nm). 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 Alq(3)-capping layer onto the cathode. Besides, photoluminescence of the top-capping Alq(3) layer is helpful to improve device luminance. (c) 2006 American Institute of Physics.