Study On Improving The Efficiency And Color Purity Of White Light Emitting Devices

Organic light-emitting diodes (OLEDs) have been attracting more and more attention as an new flat panel display technology for the merits of light weight, thin thickness, low cost, broad visual angle, fast response speed, active emitting, low energy consume, high brightness and efficiency, broad operating temperature, more choice of materials, availability for full color display and flexible display, etc. The research works in this field gained rapid development especially after 1987 when C.W.Tang for the first time reported the high brightness OLED at low operating voltage. In recent ten years or so, OLED has become a project on the cutting-edge of scientific research that relates to many intercrossed branches of science and advanced technology. Owing to the research on new materials, optimization of device structure and improvement of fabrication processes, OLEDs have developed rapidly.Compared with homochromous devices, the structure of white OLEDs (WOLEDs) are complex due to the fact that the spectra of WOLEDs should fully span the entire visible spectrum. WOLEDs can be used in not only white displays but also full color displays combined with color filters, backlights for liquid crystal displays and even illumination light sources. In this thesis, we introduce two kinds of WOLEDs with different materials and structures, and study the characteristics of these devices and the principles for adjusting the color purity of white emission.WOLEDs employing phosphorescent materials are most effective because phosphorescent materials can harvest both singlet and triplet excitons which leads to the potential for achieving 100% internal emission efficiency. But the instability and the low efficiency of the blue phosphorescent dyes, as well as their demand for wide band-gap host materials, will hamper their application in field of display or lighting. So the combined use of blue fluorescent and other color phosphorescent dyes may solve these problems and obtain efficient and stable WOLEDs. Based on above principle, we demonstrate an efficient white organic light-emitting device (WOLED) which combines phosphorescent and fluorescent materials. The WOLED has a structure of ITO /m-MTDATA (15 nm)/NPB(25 nm)/DPVBi(X nm) /3wt% Ir(piq)2(acac): CBP (Y nm)/BPhen(3 nm)/3wt%Ir(ppy)3: CBP (10nm)/ Bphen (30nm)/LiF (0.8nm)/ Al. The triplets have longer diffusion lengths than singlets because of their longer lifetimes than singlets, so the triplets generated in green phosphor doped layer will easily diffuse out of the green emitting region instead of being captured by Ir(ppy)3, which leads to the quenched green emission and the unbalanced output color. In order to confine the diffusion of charge and triplets and obtain balanced color, a thin 4,7-diphenyl-1,10-phenanthroline (Bphen) layer is inserted between green emission layer and red emission layer. By adjusting the thickness of blue fluorescent layer and red phosphorescent layer, we obtain efficient white device with good color stability. When the thickness of blue emitting layer and red one are both 13nm, efficient white light emission is obtained. The device has good color stability over a wide range of luminance: when the luminance of WOLEDs changes from3.90cd/m2 to 20850 cd/m2, the CIE changes from (0.34,0.38) to (0.30,0.35), which is very close to Equi-energy white point. Futhermore, the device has a maximum current efficiency of 8.31cd/A.Since the current efficiency and luminance can scale linearly with the number of emitting units, stacked OLEDs consisting of vertically stacked multiple emitting units in a device in series via charge generating layer (CGL) attract particular interest in recent years. Stacked white organic light-emitting devices (WOLEDs) comprising of blue fluorescent and orange phosphorescent emissive units employing Mg: tri(8-hydroxyquinoline) aluminum(Alq3)/MoO3 as the charge generation layer (CGL) are demonstrated. The structure of the stacked WOLEDs is ITO/ m-MTDATA (30 nm)/NPB(30 nm)/DPVBi(30 nm)/ Bphen (20 nm)/Alq3 (10 nm)/Alq3: 20wt% Mg (X nm)/MoO3 (Y nm)/ m-MTDATA(30 nm)/NPB(30 nm)/CBP: 8% (F-BT)2Ir(acac)(30 nm)/Bphen (20nm)/Alq3 (20 nm)/LiF (0.8 nm)/Al. Y=3 nm, X=5,10,30,40 nm for device A,B,C,D; X=30 nm, Y=2,5,15,30 nm for device E, F, G, H. The maximum current efficiencies of the device C and H are 38.53cd/Aå'Œ23.66cd/A, respectively. Our devices can obtain high efficiency at high luminance. For example, the current efficiencies at 1000 cd/m2 are 38.15cd/A and 23.66cd/A for device C and H, respectively. Moreover, both the devices have excellent color stability over a wide range of luminance. The good stability of the stacked WOLEDs is attributed to the avoidance of the movement of excitons recombination zones because the blue excitons recombination zone and the orange one are separated by the CGL.