高效顶发射有机发光器件-电子工程指导essay Highly Efficient Top-Emitting Organic Light-Emitting DevicesShih-Feng Hsu*, Shiao-Wen Hwang** and Chin H. Chen***Department of Applied Chemistry** Display Institute, MIRC, National Chiao Tung University, Hsinchu, TaiwanShi-Hao Lee, Chung-Chun LeeOLED Technology Div., AU Optronics Corporation, Hsinchu, TaiwanAbstract我们已经开发出高效的红，绿和蓝（RGB）顶发射有机发光器件（TOLEDs），使用Ag作为阳极和阴极。We have developed highly efficient red, green and blue (RGB)top-emitting organic light-emitting devices (TOLEDs) by using Agas anode and cathode. We optimized microcavity effect in thedevices by tuning suitable optical length for RGB emissions. http://www.ukassignment.org/dxessay/Avery saturated RGB color with CIE coordinates of (0.646, 0.353),(0.227, 0.721) and (0.135, 0.056) for RGB respectively weredemonstrated and shown a color gamut of NTSC 102%. We alsointroduced a new hole-injection layer to get better carrier balancein the TOLEDs with which one of the best efficiencies for red wasachieved at 37.5 cd/A .1. IntroductionOrganic light-emitting devices (OLEDs)  have been wellrecognized in recent years as one of the best flat panel displaytechnologies that are capable of meeting the most stringent demandof future display applications. 为了实现全彩显示技术的全部潜力，顶部发光OLED结构（TOLED）加上一个LTPS-TFT有源矩阵（AM）背板似乎是最匹配的。To realize the full potential of thisdisplay technology, full color top-emitting OLED structure(TOLED) coupled with a LTPS-TFT active matrix (AM) backplaneappears to be the most attractive match. This is because TOLED canprovide not only higher aperture ratio (AR) than the usual bottomemitting one, but also higher display image quality that oftennecessitates a more complicated drive circuit in AMOLEDs. Inaddition, it is also well established that pixels with high AR in thepanel invariably lead to prolonged operational stability owing toless current density needed to drive each pixel in order to achieve adesired level of display luminescence.TOLEDs also have the advantage of improving color saturation andluminance efficiency because of strong microcavity effect producedbetween the two electrodes. A device with NTSC color gamut over100% was often observed in TOLEDs. Furthermore, theenhancement of device efficiency in the normal direction reducespower consumption of the display effectively. As a result, highlyefficient RGB TOLEDs are one of the most promising technologiesto meet the requirements of personal mobile display applications.2. Optical simulationThe principal of microcavity is realized that the spontaneousemission resonates in a cavity composed of the total reflectivemirror and semitransparent thin film, only certain wavelength isallowed cavity modes, and emits light in a given direction.Intensity enhancement and spectra narrowing are the mostcommon phenomena caused by microcavity effect. Taking theadvantage of microcavity effect, high efficiency devices withsaturated color can be achieved easily.Outcoupling light int. @ specific wavelengthFIG. 1. Calculated luminance intensity of RGB microcavitydevices as a function of NPB hole-transport-layer thickness.Figure 1 shows calculated luminance intensity of RGBmicrocavity devices as a function of NPB hole-transport-layerthickness. As higher carrier mobility of HTL than that of ETL,tuning NPB thickness in a reasonable range won’t sacrifice devicevoltage. The modeled structure is glass/Ag(100nm)/NPB(vary)/EML(30nm)/Alq(30nm)/Ag(20nm). The curves show theluminance predicted by theoretical model for blue (460 nm), green(530 nm),and red (640 nm) emitting layers.Thin hole-injecting orelectron-injecting layers were neglected which are irrelevant froman optical point of view. The simulated results show the RGBdevices with intense intensity when NPB thickness are 85, 55 and45 nm, respectively. It is noted that intensity of the red device isstronger than those of green and blue one. This implicates thatlight of shorter wavelength tends to be trapped in the device.3. High efficiency TOLED devicesThe device structures of three top-emitting devices were designedand fabricated as shown in Figure 2. In device A of theconventional top-emitting device, 200-nm-thick Ag as reflectiveanode coated with a polymerized fluorocarbon film (CFx) as holeinjectionlayer was used and Ca/Ag was used as semi-transparentcathode [2-3]. Compared with device A, Ca was replaced by an ndopedETL in device B. Finally, in device C, two new holeinjectionmaterials, HIM1 and HIM2 were used in place of CFx.Optical length of all devices were optimized for RGB with properthickness of NPB.3.1 Ca/Ag system设备A是TOLEDs第一款器件结构DCJTB，C545T和DSA-PH的RGB发光掺杂剂层。Device A is our first-type device structure for TOLEDs in whichDCJTB, C545T and DSA-Ph are the dopants for RGB emissivelayers. The RGB devices were optimized with fixed EML andETL and different thickness of NPB according to simulationresults. RGB devices show the best efficiency as NPB thicknessare 60, 50 and 10 nm, respectively. DCJTB doped in co-host consisting of Alq and rubrene, a luminance yield of 17.4 cd/Awith CIEx,y coordinates of (0.67, 0.33) has been achieved. Greenand blue devices with luminance yields of 26.7 and 3.7 cd/A andthe corresponding CIEx,y of (0.22, 0.72) and (0.12, 0.18) weredemonstrated, respectively. High color gamut of 92% was alsoobserved.3.2 n-doing ETL/Ag systemHowever, in mass production consideration, evaporation ofcalcium metal is not compatible with existing manufacturingprocess. An n-doped ETL combined with metal cathode wereemployed instead in the second type TOLEDs, as device B inwhich we replaced Ca/Ag to Cs-doped ETL/Ag cathode. Aphosphorescent red dopant, ER33, a deep blue dopant, EB512 anda deep blue host EB46 were introduced in the TOLEDs. Thedetailed EL performances of top and bottom-emitting RGBdevices are compared in Table 2 and CIE colors and EL spectra ofRGB TOLEDs are plotted in Figure 3 and Figure 4. Highlysaturated color of CIEx,y (0.646, 0.353), (0.227, 0.721) and (0.135,0.056) for RGB, respectively were demonstrated and shown aNTSC color gamut of 102%. High efficiencies of the green andred TOLEDs of 26.2 and 31.5 cd/A which are more than 2 timeshigher than those of the bottom-emitting devices were achieved.Blue devices with CIEx,y color of (0.132, 0.139) and (0.135,0.056) reaching luminance yields of 3.8 cd/A and 1.5 cd/A,respectively were also achieved. Although the blue devices showrelatively lower luminance efficiency than that of the bottomemittingdevice, stronger radiance of much deeper blue deviceswith CIEy exceeding NTSC blue color was demonstrated for thefirst time.Table 2. Comparison of EL performance between bottom andtop emitting devices measured at 20 mA/cm2.FIG. 4. Saturated CIE color of RGB devices (Device B).Table 3 shows detailed EL performance of RGB devices usingdevice structures B. It is also particularly noted that optimalthickness of NPB for RGB devices are 60, 40 and 160 nm,respectively. Introducing n-doping ETL system does changeposition of emission dipole of the device. That is the reason whythere is a slight different between device A and B. A thick NPBthickness of 160 nm was the second mode of optimal opticallength for blue emission.3.3 New HIMs system与设备B相反，我们更换CFX，并推出了两款新的空穴注入材料，HIM-1和HIM-2，大力发展第三类TOLEDs。Contrary to the device B, we replaced CFx and introduced twonew hole-injection materials, HIM-1 and HIM-2, to develop thethird type TOLEDs whose structure is shown as device C. Wefind these new HIMs not only improve hole-injection from metalanode, Ag, but also provide good hole and electron balances in theTOLEDs. Efficiency of RGB device were shown in Table 4. Thesame RGB dopants were used as in device B. We demonstrate thatwe can achieve similar chromaticity (NTSC 105%) but betterluminance efficiency than that of device B. In particular, theefficiency of the red device using HIM-1 and HIM-2 reached 37.5cd/A and 39.3 cd/A, respectively. We believe that these redTOLEDs’ luminance efficiencies with more than 3 timesenhancement were among the best ever reported in the literatures.Not only efficiency and color saturation are improved , but alsodriving voltage. Table 5 shows driving voltage for the greendevice at a luminance of 1000 nits with three differentHIMs.Using HIM-1and HIM-2 as hole injection layer, asignificantly improvement of driving voltage around 1.5-2 V wasachieved.Viewing angle preporty of top-emitting devices were alwayshighly corcerned for a full color display. Compared with device Band C in table 3 and 5, there is also a large improvement from10% to 30%. This can be rationalized that our new HIMs withdifferent optical properties from organic layer results in betterlight coupling.#p#分页标题#e#Table 4. EL performance of RGB using new HIMs measuredat 20 mA/cm2.FIG. 5. EL spectra of green device (device C-HIM2) underviewing angle of 0o, 30o and 60o off the surface normal.Although light-outcoupling of 30% at viewing angle of 60 degreeis still not satisfied, color change is encouraged. A CIE color shiftof 0.05 is the maximum tolerance for human eyes. All devicesshow a good CIE color shift smaller than 0.05. Red and Bluedevices are even better. A CIE shift of 0.02 for red and 0.03 forblue were achieved. Figure 5 shows the EL spectra of greendevice under different viewing angle. This is the evidence showsthere is no significant color change under different viewingangles. This also implicates that there is no viewing angleproblem in top-emitting devices. As RGB TOLEDs have veryhigh contrast and maintain good colors, images still can bedisplayed clearly.结论 4. ConclusionsRGB TOLEDs比底部发光器件提供更大的AR，更长的寿命，更高的效率，更低的功耗和更好的色彩饱和度。RGB TOLEDs provides larger AR, longer lifetime, higherefficiency, lower power consumption and better color saturationthan bottom-emitting devices. In this paper, we demonstrate ahighly efficient RGB TOLED structure that is expected to becompatible with the existing manufacturing process and expeditethe commercialization of personal mobile full color AMOLEDs.5. AcknowledgementsThis work was supported by the JD research grant ofIndustry/Academia Cooperation Project provided by AUOptronics Corporation. The generous supply of OLED materialsprovided by e-Ray Optoelectronics Technology Co., Ltd. isgratefully acknowledged.6. References C. W. Tang and S. A. VanSlyke, “Organicelectroluminescent diodes” Appl. Phys. Lett. 51, 913(1987). S. F. Hsu, S. W. Hwang and C. H. Chen, “Highly efficienttop-emitting white organic electroluminescent devices” SID05 Digest, 32 (2005). S. F. Hsu , C. C. Lee, A. T. Hu, S. W. Hwang, H. H. Chenand C. H. Chen, “Color-tunable top-emitting organic lightemittingdevices by microcavity effect” Thin Solid Films,478/1-2, 271 (2005). T. H.Liu, C. Y. Iou, C. H. Chen, “Doped red organicelectroluminescent devices based on a cohost emittersystem” Appl. Phys. Lett. 83, 5241 (2003).