Study On White Organic Light-Emitting Devices Based On A New Iridium Complex

Organic light emitting devices (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.White organic light emitting devices (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. The research works in this field gained rapid development especially after 1994 when J. Kido reported the WOLEDs. Among these works, WOLEDs employing phosphorescent materials are most effective because both singlet and triplet excited states participate in phosphorescent materials, and the internal efficiency of WOLEDs can, in principle, be increased to nearly quadruple that of fluorescent one. At present, heavy-metal complexes, such as iridium complexes and platinum complexes, have been widely used as phosphorescent materials in WOLEDs. The highly efficient phosphorescence emission of heavy-metal complexes is attributed to strong spin-orbit coupling that enhances intersystem crossing and mixes the singlet and triplet states. In this thesis, we demonstrate two types of WOLEDs based on a new Iridium complex, and study the characteristics of them.We demonstrate an efficient white organic light-emitting device based on the new Iridium complex, (F-BT)2Ir(acac). The emissive layers of the WOLED are all phosphor-doped, bis[(4,6-difluorophenyl)-pyridinato-N,C2)](picolinato) Ir(III) (FIrpic), bis[1-(phenyl)isoquinoline] iridium (III) acetylanetonate [Ir(piq)2(acac)] andbis(2-(2-fluorphenyl)-1,3-benzothiozolato-N,C2′)iridium(acetylacetonate) [(F-BT)2Ir(acac)] are doped into a common host 4,4'_N,N'_dicarbazole-biphenyl (CBP) as blue, red and orange emission, respectively. In this device, white light can be obtained just by adjusting the thickness of emitting layers and the concentration of dopant. However, in the film of CBP doped with FIrpic, triplet energy of (2.56±0.1) eV for CBP is lower than that of (2.62±0.1) eV for FIrpic, suggesting the triplet excitons are primarily on the host. The triplets have longer diffusion lengths than singlets because of their longer lifetime than singlets, so the triplets generated in blue phosphor doped layer will easily diffuse out of the blue emitting region instead of being captured by blue dopant, which lead to the quenched blue emission and the unbalanced output color. In order to confine the diffusion of charge and triplets, and obtain a balanced color, a thin 4, 7-diphenyl-1, 10-phenanthroline (Bphen) layer is inserted between blue emission layer and red emission layer.By adjusting layer thickness and phosphor dopants concentration, we obtain efficient white light emission. The WOLED has a structure of ITO/m-MTDATA (30 nm)/NPB (20 nm)/6 wt% FIrpic:CBP (15 nm)/Bphen (2 nm)/5 wt%Ir(prq)2(acac):CBP (4 nm)/8 wt% (F-BT)2Ir(acac):CBP (10 nm)/Bphen (30 nm)/LiF (0.8 nm)/Al. The WOLED has a maximum current efficiency of 17.6 cd/A at 7 V and a maximum luminance of 39050 cd/m2 at 14 V. Power efficiency is 10.2 lm/W at 100 cd/m2 and 7.78 lm/W at 1000 cd/m2. There is almost no difference in electroluminescent spectra when luminance changes from 630 cd/m2 to 4200 cd/m2, corresponding to Commission Internationale de l'Eclairage coordinates from (0.39, 0.42) to (0.39, 0.43). It indicates that the introducing of a thin Bphen balances the distribution of charge and excitons in emission regions. According to the good color stability and EL performance, our device can be used as illumination source.Although the WOLEDs with multiple all-phosphor doped emission layers can easily generate efficient white light, the instability and the inefficiency of the blue phosphorescent dyes, as well as their demand for wide band-gap host materials, will set barriers to improve the stability of WOLEDs. In order to solve this problem, we demonstrate white organic light-emitting devices based on an Iridium complex in combination with blue fluorescent dye. In this device, the layer of (F-BT)2Ir(acac) doped into CBP is used as an orange phosphorescent emission layer, DPVBi is used as an blue fluorescent emission layer, an undoped CBP is inserted between blue fluorescent layer and orange phosphorescent emission layer, and the color of the light emission can be turned by adjusting the thickness of the undoped CBP layer. Furthermore, the maximum current efficiency changes from 5.9 cd/A to 12.4 cd/A when the thickness changes from 0 nm to 8 nm. The device with 5 nm undoped CBP layer produces fairly pure white light with Commission Internationale de l'Eclairage (CIE) coordinates (0.33, 0.34) at 1000 cd/m2 and the maximum current efficiency is 10.4 cd/A obtained at 7 V. Furthermore, the device has good color stability from 6 V to 14 V.