Electroluminescence (EL) of bilayer organic light-emitting diodes based on N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) and 2-(4'-biphenyl)-5-(4”-tert-butylphenyl)-1,3,4-oxadiazole (PBD) were reported. The EL spectra of bilayer device ITO/TPD/PBD/Al consist of monomolecular emission from TPD, exciplex emission and charge carriers cross recombination at the TPD/PBD interface. By varying the thickness of each organic layer while keeping the thickness of the whole device constant, three kinds of bilayer devices were fabricated and their EL and photoluminescence spectra were compared with each other, and our experimental data show that charge tunneling and cross recombination coexist at the TPD/PBD interface, and these two processes compete with each other under high electric fields. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200773.20.-r Electron states at surfaces and interfaces, 73.40.-c Electronic transport in interface structures, 74.78.Fk Multilayers, superlattices, heterostructures, 77.84.Jd Polymers; organic compounds,
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