Studies On Novel Structure And High Performance Of White Organic Light Emitting Diveces

Organic light-emitting diodes (OLEDs) have been attracting more and more attention as an important flat panel display technology for the merits of light weight, low cost, broad visual angle, fast response speed, active emitting, high brightness, high efficiency, capability for rich color emitting, etc. In addition, White OLEDs is also regarded as a potential candidate in solid-lighting field. 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. In this thesis, we will report novel structures in whtie,blue and red colors organic EL research, and a technique for the OLED display whtie balance.Recently, White Organic light emitting devices (WOLEDs) with high brightness properties is expected, not only for a display device, but also for the back light of a liquid crystal display and source of solid-lighting. The WOLEDs show excellent properties of low driving voltage, high efficiency, etc. A varity of methods have been proposed to achieve the WOLEDs. In this thesis, we report several kinds of WOLEDs. Firstly, we have fabaricated a voltage independently single-emissive-layer WOLED based on an incomplete energy transfer, in which emission layer composes of a layer of blue-emitting material ADN, uniformly doped with orange emitting rubrene. The thickness of the emitting layer and rubrene concentration in the ADN layer are varied to obtain the desired emission color. The structure of the WOLED is ITO/m-MTDATA/NPB/ADN:rubrene/Alq3/LiF/Al. The device's highest luminance is 20050cd/m2 at 19V and maximum efficiency is 6.4cd/A at 6V. Commission International De L'Eclairage (CIE) coordinates range from (0.32, 0.34) to (0.31, 0.34) when the current density changes from 20mA/cm2 to 500mA/cm2.Thanks to the incomplete energy transfer between Alq3 and DCM2, we have fabricated WOLED used DPVBi as emitting layer (EML) for blue light and DCM2 doped Alq3 as EML for red and green light, the device structure is ITO/m-MTDATA/NPB/DPVBi/Alq3:DCM2/Alq3/LiF/Al. Because blue, green and red lights all participated in white light emission, the WOLED has high color rendering index (CRI) of 91. To improve the devices'performance such as luminance, efficiency et al, QAD was introduced into the DCM2 doped layer for green light emitting. Device structure is ITO/m-MTDATA /NPB/ DPVBi/Alq3:QAD:DCM2/Alq3/LiF/Al. When doping concentration of QAD and DCM2 are both 0.5 wt%, device has highest luminance of 25350cd/m2, highest luminous efficiency of 7.9cd/A and CRI of 85. Device's CIE coordinates at 9V is (0.3338,0.3360).In order to realize solid-state lighting, high efficient phosphorescent materials should be adopt to fabricate WOLEDs. We use new phosphorescent material (F-BT)2Ir(acac) as yellow dopant, Ir(piq)2acac as red dopant, Ir(ppy)3 as green dopant and fluorescent material NPB as blue emitting layer to fabricate high efficiency, high CRI WOLED. The device structure is ITO/m-MTDATA/NPB/CBP:Ir(ppy)3/CBP:(F-Bt)2Ir(acac):Ir(piq)2acac/Bphen/Alq3/LiF. Based on this device we introduce 4nm CBP layer between the two phosphorescent EML and then exchange these two EMLs'locations, finally WOLED which has highest luminance of 50030cd/m2(17V), highest efficiencies of 21.6cd/A(at 7V, 1862 cd/m2) 10.3lm/W(at 6V, 790cd/m2) and CRI of 85. In addtion, its CIE coordinates only slightly changes from (0.3867, 0.4075) to (0.3638, 0.4070) when the drive voltage shifts from 7V(1862cd/m2) to 13V(39270cd/m2). Our methods of adjusting EML structures are original and lead to satisfactory effect.Highly efficient phosphorescent OLEDs based on three Re-ligand complexes have been fabricated(deviceI based on Re-Bphen, deviceII based on Re-Dppp, deviceIII based on Re-TPIP), the devices had highest efficiencies of 13.8cd/A,8.69lm/W(deviceI), 14.2cd/A,8.92lm/W(deviceII) and 17.6 cd/A,9.2lm/W(deviceIII). Such high efficiencies achieved from OLEDs based on Re-ligand have not been reported yet. In addition to characterized the devices'performances, we discussed the emission mechnism based on works of S.R.Forrest and R.H.Friend and believe that trapping contribute mostly to these relatively much higher efficiencies compared with Forster and Dexter energy transfers.