Organic light-emitting diodes using cascade energy transfer process

In order to increase the energy transfer from host to guest in doping organic light-emitting diodes (OLEDs), bis(2-biphenyl-4'-yl-8-quinolinolato) zinc Zn[2-(p-biPh)-8-Q-O]₂ was synthesized and used to assist the energy transfer from N,N'-bis-(3-naphthyl)-N,N'-biphenyl-(1,1'-biphenyl)-4,4'- diamine (NPB) to red fluorescent dye 4-(dicyanomethylene) -2-t-butyl-6 (1,1,7,7-tetramethyljulolidyl-9-enyl) -4H-pyran (DCJTB). OLED with the structure of ITO/NPB/NPB:DCJTB/Zn[2-(p-biPh)-8-Q-O]₂/BCP/Al were fabricated. Without the Zn[2-(p-biPh)-8-Q-O]₂ layer, incomplete energy transfer from NPB to DCJTB led to both emission from host and guest. After Zn[2-(p-biPh)-8-Q-O]₂ was added in to the device, only red emission of DCJTB material was detected in electroluminescence spectra. The efficient emission of DCJTB is achieved via energy transfer with a cascade process from NPB to Zn[2-(p-biPh)-8-Q-O]₂ and then to DCJTB, which results in a complete energy transfer from host to guest. In particular, as the concentration of DCJTB is only 0.5%, red-light-emitting OLED is successfully fabricated using Forster transfer theory at twice. Traditionally, the fabrication of OLED using the assistance material needs thermal evaporation with three sources. However the red OLED only needs two sources, which is very useful in application in the future. In addition, by varying the distance between Zn[2-(p-biPh)-8-Q-O] ₂ and the doping system, the efficiency of energy transfer is changed. When the distance is below 10 nm, Zn[2-(p-biPh)-8-Q-O]₂ has an influence on the energy transfer of the doping system. And the longer the distance is, the lower the efficiency is.