Loss process in efficiency of blue organic electroluminescent devices

We report EL characteristics of blue organic electroluminescent (EL) devices with a hole transport layer (HTL) as an emitter i.e. ITO/NPD/BCP/Alq(3)/Cs(0.5 nm)/Al. Here, ITO, NPD, BCP, and Alq(3) are abbreviations for indium-tin-oxide, 4,4'-bis[N-(I-naphthyl)-N-phenyl-amino]-biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, and tris(8-hydroxyquinoline) aluminum, respectively. Three different chemically modified ITO electrodes were used. As a hole-injecting buffer layer, copper phthalocyanine (CuPc) or 4,4',4"-tris-(3-metbylphenylphenylamino)triphenylamine (MTDATA) was used in this study. We found that the luminance increased less than linearly with an increase in current for all EL devices studied even when hole and electron injection were enhanced. The luminance loss in the device is attributed to quenching singlet excited states ((NPD)-N-1*) by large excess radical cations (NPD+) accumulated in the emission zone due to large overlap between a fluorescence spectrum of (NPD)-N-1* and an absorption spectrum of NPD+. In addition, (NPD)-N-1* can be also quenched by radical anions (NPD-) when excess electrons are injected.