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 (TPD)/2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD)/tris(8-quinolinolato)aluminum (Alq3)/Al:Li were fabricated and we investigated the effects of applied voltage, thickness of organic layer, and ITO anode to probe the emission mechanism. Electroluminescence (EL) spectra and Commission Internationale de l'Eclairage (CIE) coordinates of these devices were measured to observe emitting characteristics in various voltages. The emission 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 hole injection capabilities of the anode are also among major factors to expand the recombination region in the device. Especially, the total amount of holes provided by the ITO anode affects the probability of recombination and can shift the CIE coordinates. It is possible in an OELD for holes and electrons to recombine in the emitting layer (PBD) as well as there are to be contributions to the emission from the electron transport and hole transport layers (HTLs).