Influence of charge carrier injection on the device performance of blue organic light-emitting diodes

Abstract

Multilayer organic light-emitting diodes with a well-defined emission region are ideal systems to investigate the underlying mechanisms of device operation. In this paper we give a brief overview of our studies on the injection and recombination processes in blue organic light-emitting diodes. Our diodes consist of copper phthalocyanine as the hole-injection/buffer layer, the aromatic diamine N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine as the hole-transport layer, a distyrylarylene derivative as the emitting layer, tris(8-hydroxy-quinoline)aluminum as the electron-injection/transport layer sandwiched between indium tin oxide (ITO) as the transparent hole injector, and low work-function metals as the electron-injecting electrode. To reveal the limiting mechanisms in device operation we investigated the influence of different anode (ITO, optimized ITO) and cathode materials (Mg:Ag, Ca, LiF/Mg:Ag) as well as different layer thicknesses on the device characteristics. These investigations revealed that the device characteristics in the low and medium voltage range are determined by the injection properties of the electrodes, whereas at higher voltages (i.e. at higher current densities) a transition from an electrode-limited injection to a bulk-limited conduction process is detectable.