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 (Alq3)/LiF/Al cathode. Fullerene (C60) and rhodamine B (RhB) molecules were used as the nanosutructured ultrathin layers inserted at the interface between the Alq3 and TPD layers. The electroluminescent (EL) properties have been measured for the OLEDs with C60 and RhB ultrathin layers and the dependences on the thickness and the position of the inserted layers were examined. For the OLEDs with the C60 ultrathin layer, the improvements of the drive voltage and EL efficiency were observed. The OLED with the inserted C60 ultrathin film of a monolayer thickness showed the highest efficiency, which was twice as large as that without C60 layer. On the contrary, the improvements were not observed for the OLEDs with the RhB ultrathin layer.