White color emitting organic electroluminescence (EL) device is a convenient means to realize full-color display because the three necessary primary colors could be obtained by color filters from white back light. In this paper a new three-layer device is presented. We used N, N′-bis-(lnaphhyl)-N , N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB) as the hole transport layer, Alq as the electron transport layer, CuPc as the buffer layer, and TPBi as the blocking layer inserted between NPB and Alq.The 5,6,11,12-tetraphenyltetracene (Rubrene) as dopant was doped into the blocking layer.\;It was thought initially that if energy transfer existed between the host and dopant in photoluminescence (PL) of NPB doped with Rubrene, white emission might be obtained in a device such as ITO/CuPc/NPB:Rubrene/TPBi/Alq/Mg:Ag. Thus the PL of NPB:Rubrene system excited by NPB absorption band with different dopant concentrations were investigated in advance. There are two bands in the PL spectra, the blue one is emitted by NPB and the yellow one is by Rubrene. The ratio of these two bands can be adjusted by changing the Rubrene concentration. This shows that energy transfer does occur from NPB to Rubrene. Time-resolved spectra of this system, which will be given in another paper, confirm further that the energy transfer exists and is quite effective. White emission can be produced by adjusting the concentration of Rubrene in PL. However, devices of structure ITO/CuPc/NPB:Rubrene/TPBi/Alq/Mg:Ag were made according to these results with TPBi as the blocking layer, white emission can not be obtained by changing the Rubrene concentration.\;For another device of structure ITO/CuPc/NPB/TPBi:Rubrene/Alq/Mg : Ag, with a blocking layer TPBi doped by Rubrene concentration 1.5% inserted between NPB and Alq, white emission can be obtained. The highest luminance and maximum efficiency are 8635cd/m 2 and 1.39lm/W respectively , CIE coordinate x =0.31, y =0.32. The color of the emitted light can still be optimized by adjusting the relative th
In order to use organic light emitting devices (OLEDs) in display application , it is very important to obtain red emitting light. There are two methods for obtaining red emitting light: doping high fluorescent dyes in host or using metal complexes. Phosphorescent dyes has been used efficiently recently. In this letter, we demonstrate red organic light emitting devices (OLED) with the electroluminescent layers consisting of aluminum tris(8 hydroxyquinoline) (Alq 3) doped with the dye DCM and DCJTB, which the emission color depends on the concentration of DCM and DCJTB. The typical cell structure is as follows: [ITO/ hole transport layer (60nm, TPD) /emitting layer(60nm, Alq 3 + red dopant) /LiF(0.5～2nm) /Al(150nm)]. For DCM doped devices, the maximum luminance of 148000cd/m 2 (chromaticity coordinates: x =0.51, y =0.47) and 5730cd/m 2 (chromaticity coordinates: x =0.58, y =0.42) are measured for DCM concentration of 0.2% and 2% in Alq 3, respectively; and for DCJTB doped devices, 17400 cd/m 2 (chromaticity coordinates : x =0.51, y =0.46) and 3846cd/m 2 (chromaticity coordinates: x =0. 63, y =0. 37) are obtained for DCJTB concentration of 0. 2 % and 2% in Alq 3, respectively.
The operating mechanisms of the OLEDs involve injection of electrons and holes from the electrodes, and electron hole recombination which emits the light. To balance the numbers of electrons and holes injected from electrode and obtain high emission efficiency, several hole injecting buffer layers such as CuPc, Carbon, Al 2O 3 and SiO 2 have been used to improve the efficiency and lifetime of the OLEDs. In this study, highly efficienct and bright organic electroluminescent devices were developed using lithium fluoride (LiF) film as hole and electron injecting layers. Typical OLEDs have the structure of ITO glass/LiF/NPB(70nm)/Alq(70nm)/LiF(0.5nm)/Al(200nm). The device with a 2.0nm LiF hole injecting layer showed the luminance of 1210cd/m 2 at 20mA/cm 2 which corresponds to an efficiency of 6.0cd/A. In contrast, the device without LiF hole injecting layer exhibited 617cd/m 2 at the same current density which showed an efficiency of 3.2cd/A. These results suggest that the LiF hole injecting layer with a proper thickness can enhance the efficiency of the OLEDs due to blocking the injection of holes.
