Emission shift by recombination effect in a three-layered oeld

Organic electroluminescent devices (OELDs) with the structure of indium-tin -oxide (ITO)/N,N-diphenyl-N . N-(3-methylphenyl)1,1-biphenyl-4,4-diamine (T PD)/2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD)/tris(8-q uinolinolato)aluminum (Alq(3))/Al:Li were fabricated and we investigated th e effects of applied voltage, thickness of organic layer, and ITO anode to probe the emission mechanism. Electroluminescence (EL) spectra and Commissi on Internationale de l'Eclairage (CIE) coordinates of these devices were me asured to observe emitting characteristics in various voltages. The emissio n color gradually changes from yellowish-green to greenish-blue due to the shift of recombination region of holes and electrons as driving voltage is increased in the same structure. The thickness of each organic layer and ho le injection capabilities of the anode are also among major factors to expa nd the recombination region in the device. Especially, the total amount of holes provided by the ITO anode affects the probability of recombination an d can shift the CIE coordinates. It is possible in an OELD for holes and el ectrons to recombine in the emitting layer (PBD) as well as there are to be contributions to the emission from the electron transport and hole transpo rt layers (HTLs). (C) 2000 Published by Elsevier Science S.A. All rights re served.