Studies On Non-doped-type White Organic Light Emitting Devices

Organic light-emitting diodes (OLEDs) have been attracting more and more attention as an novel flat panel display technology for the merits of light weight, low cost, broad visual angle, fast response speed, active emitting, high brightness, high efficiency, availability for full color 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. The design of EL structure for used in OLEDs is critical to device's performance. Great strides have been made towards the development and improvement of small molecular OLEDs for display applications. Intense research in both academia and industry over the last 4–5 years has yielded OLEDs with remarkable full color, device efficiencies and operational stability.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 1984 when J.Kido reported the WOLEDs. A variety of methods have been proposed to achieve the WOLEDs. In this dissertation, we have made some meaningful works: achievement of the efficiency, chromaticitycoordinates (CIE) and color rendering index (CRI) improvement by non-doped method. In this thesis, we introduce some different structure WOLEDs with different materials, and study the characteristics of these devices. Tsuji et al introduces a 10 ? DCM1 thin film to the WOLEDs and fabricated the non-doped WOLED in 2003. However, the maximum luminance of the devices can only reach 1000 cd/m2, and they also did not report the efficiency of the devices. We studied the effect of the DCM1 thickness on the luminance and efficiency of the devices whose structure is ITO/NPB/DCM1/Alq3/LiF/Al, and we think that the low-efficiency and low-luminance of the WOLEDs fabricated by Tsuji et al is attributed to so-called concentration quenching. Thus, we fabricated the WOLEDs with an ultrathin DCM2 layer .The structure of the device is ITO/m-MTDATA / NPB/DPVBi/ DCM2/ BPhen/Alq3/LiF/Al. The EL spectrum of the device can be adjusted by changing the thickness of DCM2. As a result, we obtained the WOLEDs whose maximum efficiency and luminance are 4.80lm/W at 5 V and 11497 cd/m2 at 15 V, respectively. And the chromaticity coordinates, varying from (0.36, 0.31) to (0.28, 0.27) with increasing forward bias from 5V to 15V, are well within the white region. The characteristic of the device is to use the DCM2 ultrathin layer in the device, which lowers the effect of the concentration quench. And as a result, the efficiency of the device is improved. In order to further improve the luminance, efficiency, colour rendering index (CRI) and chromaticity coordinates of the non-doped-type WOLEDs, firstly, we introduce the high-efficiency fluorescent dye QAD and DCJTB with ultrathin layers to the non-doped WOLEDs. Thus, a non-doped-type pure white organic light-emitting device with high colour rendering index has been reported. The structure of the device is ITO / m-MTDATA/ NPB/ DPVBi/ DCJTB/ Alq3/ QAD/ Alq3 / BPhen/Alq3/LiF/Al. Through the optimization the thickness of QAD and DCJTB, pure white emission with CIE coordinatesof (0.33, 0.34) at 7 V was obtained, and the colour temperature and colour rendering index were 6712 K and 91 at 8V, respectively. The CIE coordinates of the device change from (0.37, 0.36) at 4 V to (0.326, 0.33) at 15 V that are well in the white region. Its maximum luminance was 27360 cd/ m2 at 15 V and maximum power efficiency was 3.16 lm /W at 7 V. Moreover, the current efficiency is largely insensitive to the current density. In order to improve the efficiency of the devices, non-doped-type white organic electroluminescent devices based on DCM2 and C545T ultrathin layer have been fabricated. When the thickness of ultrathin C545T layer is 0.05 nm, the device has a maximum efficiency of 8.06 cd/ A at 6 V and a maximum power efficiency of 4.79 lm/W at 5V, its maximum luminance was 24610 cd/m2 at 15 V, respectively. White emission with CIE coordinates of (0.40, 0.38) at 5V and (0.32, 0.31) at 14 V was obtained, respectively. Moreover, the colour rendering index (CRI) was higher than 84 in all devices. The performance of this WOLED is superior in the non-doped type WOLEDs.