Energy transfer vs. carrier trapping: emission mechanism in dye-doped organic light emitting diodes

A dye-doped double layer device with a structure of ITO:NPB:TPBI:dopant:TPBI:Mg:Ag, where the NPB and TPBI are 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl and 2,2',2-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole], respectively, and the dopant includes various blue, green and red fluorescent dyes, was constructed. The photoluminescence as well as electroluminescence behaviors were examined for each device. At a low dopant concentration (2-3%), the dopant emission, which was attributable to carrier trapping at the dopant sites, was observed mainly for the green and red dyes but not for the blue dyes used. However, by inserting an intermediate layer of 4,4'-dicarbazolyl-1,1'-biphenyl (CBP) between the NPB and TPBI layers, dopant emission from all dyes can be observed. Energy transfer between TPBI and the dopant is suggested to be the major channel of dopant excitation. An analysis of HOMO/LUMO levels of various layers was presented to rationalize the results. (C) 2002 Elsevier Science B.V. All rights reserved.