Organic light emitting diodes with nanostructured ultrathin layers at the interface between electron- and hole-transport layers

Organic light emitting diodes (OLEDs) with nanostructured ultrathin layers inserted at the interface between electron- and hole-transport layers were investigated. The fundamental structure of the OLEDs fabricated by a vacuum evaporation method was indium-tin-oxide (ITO) anode/copper phthalocyanine (CuPc)/N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD)/8-hydroxyquinoline aluminum (Alq(3))/LiF/Al cathode. Fullerene (C-60) and rhodamine B (RhB) molecules were used as the nanosutructured ultrathin layers inserted at the interface between the Alq(3) and TPD layers. The electroluminescent (EL) properties have been measured for the OLEDs with C-60 and RhB ultrathin layers and the dependences on the thickness and the position of the inserted layers were examined. For the OLEDs with the C-60 ultrathin layer, the improvements of the drive voltage and EL efficiency were observed. The OLED with the inserted C-60 ultrathin film of a monolayer thickness showed the highest efficiency, which was twice as large as that without C-60 layer. On the contrary, the improvements were not observed for the OLEDs with the RhB ultrathin layer. (c) 2004 Elsevier B.V. All rights reserved.