White Organic Electroluminescence Devices

Recently,white organic electroluminescence devices(WOLED) have attracted most attention for their wide applications.In this paper we not only fabricated the multi-layer WOLEDs based on small molecule materials and single-layer WOLEDs based on fluorescence-doped polymers,but also studied their performances and luminescence mechanism.For the multi-layer devices,we chose DCJTB,Alq3 and NPB as red,green and blue fluorescence materials,respectively,and fabricated two groups of devices with structures of ITO/NPB(30 nm)/Alq₃(X nm)/BCP(5 nm)/Alq₃:DCJTB(2 wt%)(30 nm)/Alq₃(50 nm)/Al(X=5,10,20) and ITO/NPB(30 nm)/BCP(5 nm)/Alq₃(X nm)/Alq₃:DCJTB(2 wt%)(30 nm)/Alq₃(50 nm)/Al(X=10,20,30).It was found that the thickness of BCP hole blocking layer,the location and thickness of Alq₃ layer and the energy transfer between different materials played very important roles in the electroluminescence spectra of the devices.The device ITO/NPB(30 nm)/Alq₃(X nm)/BCP(5 nm)/Alq₃:DCJTB(2 wt%)(30 nm)/Alq₃(50 nm)/Al could not emit blue light while the device ITO/NPB(30 nm)/BCP(6 nm)/Alq₃(30 nm)/Alq₃:DCJTB(2 wt%)(30 nm)/Alq₃(30nm)/Al could generate a pure white light with the CIE chromaticity coordinates of(0.33,0.33) at the driving voltage of 25 V.The spectrum of this device covered the entire visible region.Its brightness and efficiency reached 939 cd/m² and 0.36cd/A,respectively.Moreover,The CIE coordinates of the white emission were insensitive when the driving voltage increased from 16 to 28 V.We also fabricated single-layer WOLEDs for simplifying device structure.The results showed that the devices ITO/PVK:TAPC(100:X in wt.)(X=1,2,3,4,10)(100 nm)/Al could only emit yellow light peaking at 580 nm.The yellow light is attributed to directly capturing electron and hole pairs of the TTA subunits in TAPC.To obtain white light,perylene was added to the devices.The energy transfer between PVK and perylene is almost completely.When the mass ratio of PVK:TAPC:perylene is 100:9:1 in the complex,white light with CIE coordinates of(0.33,0.34) and maxima bightness of 3727 cd/A was obtained from the device ITO/PVK:TAPC:perylene(100:9:1 in wt.)(100 nm)/Alq₃(5 nm)/Al.Besides,we studied the stability of the CIE coordinates by changing electron transporting materials.The results indicated the cause for instability of the CIE coordinates is the number of electrons captured by TTA in TAPC since the electron transporting materials have different HUMO levels.The more electrons transporting to TTA,the more instable of the device spectra.