Highly Efficient Top-Emitting Organic Light-Emitting Devices
Shih-Feng Hsu*, Shiao-Wen Hwang** and Chin H. Chen**
*Department of Applied Chemistry
** Display Institute, MIRC, National Chiao Tung University, Hsinchu, Taiwan
Shi-Hao Lee, Chung-Chun Lee
OLED Technology Div., AU Optronics Corporation, Hsinchu, Taiwan
Abstract
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 Ag as anode and cathode. We optimized microcavity effect in the devices by tuning suitable optical length for RGB emissions. http://www.ukAUS.org/dxessay/ A very saturated RGB color with CIE coordinates of (0.646, 0.353),(0.227, 0.721) and (0.135, 0.056) for RGB respectively were demonstrated and shown a color gamut of NTSC 102%. We also introduced a new hole-injection layer to get better carrier balance in the TOLEDs with which one of the best efficiencies for red was achieved at 37.5 cd/A .
1. Introduction
Organic light-emitting devices (OLEDs) [1] have been well recognized in recent years as one of the best flat panel display technologies that are capable of meeting the most stringent demand of 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 this display technology, full color top-emitting OLED structure (TOLED) coupled with a LTPS-TFT active matrix (AM) backplane appears to be the most attractive match. This is because TOLED can provide not only higher aperture ratio (AR) than the usual bottom emitting one, but also higher display image quality that often necessitates a more complicated drive circuit in AMOLEDs. In addition, it is also well established that pixels with high AR in the panel invariably lead to prolonged operational stability owing to less current density needed to drive each pixel in order to achieve a desired level of display luminescence.
TOLEDs also have the advantage of improving color saturation and luminance efficiency because of strong microcavity effect produced between the two electrodes. A device with NTSC color gamut over 100% was often observed in TOLEDs. Furthermore, the enhancement of device efficiency in the normal direction reduces power consumption of the display effectively. As a result, highly efficient RGB TOLEDs are one of the most promising technologies to meet the requirements of personal mobile display applications.
2. Optical simulation
The principal of microcavity is realized that the spontaneous emission resonates in a cavity composed of the total reflective mirror and semitransparent thin film, only certain wavelength is allowed cavity modes, and emits light in a given direction. Intensity enhancement and spectra narrowing are the most common phenomena caused by microcavity effect. Taking the advantage of microcavity effect, high efficiency devices with saturated color can be achieved easily. Outcoupling light int. @ specific wavelength
FIG. 1. Calculated luminance intensity of RGB microcavity devices as a function of NPB hole-transport-layer thickness.
Figure 1 shows calculated luminance intensity of RGB microcavity devices as a function of NPB hole-transport-layer thickness. As higher carrier mobility of HTL than that of ETL,tuning NPB thickness in a reasonable range won’t sacrifice device voltage. The modeled structure is glass/Ag(100nm)/NPB(vary)/EML(30nm)/Alq(30nm)/Ag(20nm). The curves show the luminance predicted by theoretical model for blue (460 nm), green(530 nm),and red (640 nm) emitting layers.
Thin hole-injecting or electron-injecting layers were neglected which are irrelevant from an optical point of view. The simulated results show the RGB devices with intense intensity when NPB thickness are 85, 55 and 45 nm, respectively. It is noted that intensity of the red device is stronger than those of green and blue one. This implicates that
light of shorter wavelength tends to be trapped in the device.
3. High efficiency TOLED devices
The device structures of three top-emitting devices were designed and fabricated as shown in Figure 2. In device A of the conventional top-emitting device, 200-nm-thick Ag as reflective anode coated with a polymerized fluorocarbon film (CFx) as holeinjection layer was used and Ca/Ag was used as semi-transparent cathode [2-3]. Compared with device A, Ca was replaced by an ndoped ETL in device B. Finally, in device C, two new holeinjection materials, HIM1 and HIM2 were used in place of CFx.
Optical length of all devices were optimized for RGB with proper thickness 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 which DCJTB, C545T and DSA-Ph are the dopants for RGB emissive
layers. The RGB devices were optimized with fixed EML and ETL and different thickness of NPB according to simulation results. RGB devices show the best efficiency as NPB thickness are 60, 50 and 10 nm, respectively. DCJTB doped in co-host [4] consisting of Alq and rubrene, a luminance yield of 17.4 cd/A with CIEx,y coordinates of (0.67, 0.33) has been achieved. Green and blue devices with luminance yields of 26.7 and 3.7 cd/A and the corresponding CIEx,y of (0.22, 0.72) and (0.12, 0.18) were demonstrated, respectively. High color gamut of 92% was also observed.
3.2 n-doing ETL/Ag system
However, in mass production consideration, evaporation of calcium metal is not compatible with existing manufacturing process. An n-doped ETL combined with metal cathode were employed instead in the second type TOLEDs, as device B in which we replaced Ca/Ag to Cs-doped ETL/Ag cathode. A phosphorescent red dopant, ER33, a deep blue dopant, EB512 and a deep blue host EB46 were introduced in the TOLEDs. The detailed EL performances of top and bottom-emitting RGB devices are compared in Table 2 and CIE colors and EL spectra of RGB TOLEDs are plotted in Figure 3 and Figure 4. Highly saturated color of CIEx,y (0.646, 0.353), (0.227, 0.721) and (0.135,0.056) for RGB, respectively were demonstrated and shown a NTSC color gamut of 102%. High efficiencies of the green and red TOLEDs of 26.2 and 31.5 cd/A which are more than 2 times higher 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 show relatively lower luminance efficiency than that of the bottomemitting device, stronger radiance of much deeper blue devices with CIEy exceeding NTSC blue color was demonstrated for the first time.
Table 2. Comparison of EL performance between bottom and top 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 using device structures B. It is also particularly noted that optimal thickness of NPB for RGB devices are 60, 40 and 160 nm,
respectively. Introducing n-doping ETL system does change position of emission dipole of the device. That is the reason why there is a slight different between device A and B. A thick NPB thickness of 160 nm was the second mode of optimal optical length 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 two
new hole-injection materials, HIM-1 and HIM-2, to develop the third type TOLEDs whose structure is shown as device C. We find these new HIMs not only improve hole-injection from metal anode, Ag, but also provide good hole and electron balances in the TOLEDs. Efficiency of RGB device were shown in Table 4. The same RGB dopants were used as in device B. We demonstrate that we can achieve similar chromaticity (NTSC 105%) but better luminance efficiency than that of device B. In particular, the
efficiency of the red device using HIM-1 and HIM-2 reached 37.5 cd/A and 39.3 cd/A, respectively. We believe that these red TOLEDs’ luminance efficiencies with more than 3 times enhancement were among the best ever reported in the literatures. Not only efficiency and color saturation are improved , but also driving voltage. Table 5 shows driving voltage for the green device at a luminance of 1000 nits with three different HIMs.Using HIM-1and HIM-2 as hole injection layer, a significantly improvement of driving voltage around 1.5-2 V was achieved.
Viewing angle preporty of top-emitting devices were always highly corcerned for a full color display. Compared with device B and C in table 3 and 5, there is also a large improvement from 10% to 30%. This can be rationalized that our new HIMs with different optical properties from organic layer results in better light coupling.(责任编辑：Lanyin)
