This paper reports on the separately-doped structures of organic light-emitting diodes (OLED) in which red dye 4-(dicyanomethylene)-2-methyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJT) was doped into the host emitting layer of tris(8-hydroxyquinoline)aluminum (Alq3) by means of ultrathin separate doping (like quantum wells). The parameters in the ultrathin layer, including its width, position, number and spacing between separately doped layers, were changed to study their effects on electroluminescence (EL) characteristics. Thin doped layers increase the luminance efficiency. When the ultrathin separately doped layers were in the vicinity of the NPB/Alq3 interface, where N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) is used as a hole transport layer, luminance efficiency also increased. The highest luminance efficiency reaching 4 cd/A was achieved using a double ultrathin separately doped structure with optimum doping thickness and 2 nm of space between the two layers. The study also found that double ultrathin separately doped structures achieved better EL intensity than single or triple ultrathin separately doped structures.

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