Enhancement of red organic light-emitting diodes via cascade energy transfer

We have demonstrated that efficient red electroluminescence is obtained via cascade energy transfer from Alq to fluorescent dye Coumarin(C545) and then from C545 to 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). The cell structure was indium tin oxide (ITO)/ N,N′-bis-(1-naphenyl)-N,N′-biphenyl-1,1′-bipheny1-4-4′-diamine (NPB)/ tris (8-hydroxyquinoline) aluminum (Alq): C545: DCJTB/Alq/LiF/Al. An additional dopant, C545, was used to assist the energy transfer from Alq to the red dopant. Compared with the devices where the emitting layer is only composed of Alq and DCJTB, the emission efficiency and color purity were improved. We attribute these improvements to the assistant dopant C545 which leads to the more efficient energy transfer from Alq to DCJTB. The co-doping system is a promising method for red organic light-emitting diodes.