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Unformatted text preview: Home Search Collections Journals About Contact us My IOPscience White organic light-emitting devices employing phosphorescent iridium complex as RGB dopants This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 Semicond. Sci. Technol. 22 728 (http://iopscience.iop.org/0268-1242/22/7/009) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 134.129.205.137 The article was downloaded on 16/03/2011 at 21:28 Please note that terms and conditions apply. IOP PUBLISHING Semicond. Sci. Technol. 22 (2007) 728–731 SEMICONDUCTOR SCIENCE AND TECHNOLOGY doi:10.1088/0268-1242/22/7/009 White organic light-emitting devices employing phosphorescent iridium complex as RGB dopants Ruili Song1,2, Yu Duan1, Shufen Chen1, Yi Zhao1, Jingying Hou1 and Shiyong Liu1 1 State Key Laboratory of Integrated Optelectronics, Jilin University, Changchun 130012, People’s Republic of China 2 Physics Department, Northeast Dianli University, Jilin 132012, People’s Republic of China E-mail: jilinsongruili@163.com and syliu@mail.jlu.edu.cn Received 3 February 2007, in final form 9 May 2007 Published 31 May 2007 Online at stacks.iop.org/SST/22/728 Abstract Efficient phosphorescent white organic light-emitting diodes (WOLEDs) were realized by using a bright blue-emitting layer, iridium (III) bis-[(4,6di-fluoropheny)-pyridinato-N, C2 ] picolinate doped 4.4 -bis-(9-carbazolyl)-2, 2 -dimethyl–biphenyl doped, together with tris-(2-phenylpyridine) iridium and bis-(1-phenyl–isoquinoline) acetylacetonate iridium (III) are codoped into a 4,4 -N,N -dicarbazolebiphenyl layer to provide blue, green and red emission for colour mixing. The device emission colour is controlled by varying dopant concentrations and the thicknesses of blue and green–red layers as well as tuning the thickness of an exciton-blocking layer. The maximum luminance and power efficiency of the WOLED are 42700 cd m−2 at 17 V and 8.48 lm W−1 at 5 V, respectively. The Commission Internationale de 1’Eclairage (CIE) chromaticity coordinate changes from (0.41, 0.42) to (0.37, 0.39) when the luminance ranges from 1000 cd m−2 to 30000 cd m−2. White organic light-emitting diodes (WOLEDs) have been attracted much current interest because of their good potential for various applications, such as low driving, high efficiency and large area, which can be used as backlights in flat-panel display and eventually as illumination light sources [1–3]. Among the WOLEDs reported [4–15], efficiencies of those [7–15] based on phosphorescent materials are outstandingly high due to their demonstrated potential for achieving 100% internal emission efficiency. Recently, highly efficient WOLEDs using phosphorescent materials with incorporated heavy metal complexes have been reported. Since fluorescent OLEDs which utilize only the singlet excitons, phosphorescent OLEDs have proven to be potentially more efficient because they can harvest both singlet and triplet excitons and have the potential of reaching a maximum internal efficiency of 100%. In order to optimize device efficiency up to a theoretical limit, these heavy metal complex emitters have been doped into a layer of charge-transporting hosts such as 4,4 -N,N -dicarbazole-biphenyl (CBP) or other host such as p0268-1242/07/070728+04$30.00 © 2007 IOP Publishing Ltd bis(triphenylsilyly)benzene (UGH2), N, N -dicarbazolyl-3,5benzene (mCP), N,N -dicarbazolyl-1, 4-dimethene–benzene (DCB). Because of an efficient transfer of both singlet and triplet excited states in the host to dopant, there is an increase in the internal efficiency. In addition, diluting the molecules into the host matrix also results in a decrease of triplet–triplet (T–T) annihilation [14, 15]. Among all phosphorescent materials, iridium metal complexes are the most famous materials due to the relatively short lifetime of their triplet state. In this paper, we demonstrate and characterize WOLEDs based on three kinds of iridium complex, which emitting red–green and blue light, respectively. The WOLEDs with two different hosts, 4.4 bis-(9-carbazolyl)-2, 2 -dimethyl–biphenyl (CDBP) and CBP, were employed. The phosphorescent dye, iridium (III) bis[(4,6-di-fluoropheny)-pyridinato-N, C2 ] picolinate (FIrpic) was doped in CDBP as a blue emissive layer, while another two phosphorescent dyes, tris-(2-phenylpyridine) iridium [Ir(ppy)3] and bis-(1-phenyl-isoquinoline) acetylacetonate 728 Printed in the UK White organic light-emitting devices employing phosphorescent iridium complex as RGB dopants Figure 1. WOLED structure and molecular structure of materials. iridium (III) Ir(piq)2(acac) were codoped in CBP as the green–red emissive layer, respectively. N,N-diphenyl-N,Nbis1-naphthy l-1,1-biphenyl-4,4-diamine (NPB) as a hole transparent layer. A 4 nm thick CBP layer was inserted between the blue and green–red layer as the exciton-blocking layer. The TPBI layer was used as the hole blocking and electron-transport layer. Figure 1 shows the chemical structures of organic materials used in this work and the configuration of the WOLEDs. The indium tin oxide (ITO)-coated glass substrates used have a film thickness of 100 nm and sheet resistance of approximately 10 /square. Prior to the organic films being deposited, the substrates were cleaned by scrubbing, sonication and deionized water sequentially. They were then immersed sequentially in an ultrasonic bath of acetone, ethanol and deionized (DI) water for 10 min each, followed by being rinsed in DI water. Finally, the substrates were heated dry with an oven and then treated by UV ozone for 15 min prior to use. Organic materials were deposited by vacuum deposition at 10−4–10−5 Pa and at a rate of 0.1–0.2 nm s−1 using resistively heated quartz boats. The layer thickness of deposited material was monitored in situ using an oscillating quartz thickness monitor. Finally a LiF buffer layer and an Al cathode were deposited at a background pressure of 10−4 Pa onto the organic films. A shadow mask was used for the deposition of the cathode. The active area of the devices is 2 × 2 mm. EL spectra and Commission Internationale de l’Eclairage (CIE) coordination of the devices were measured by a PR650 spectra scan spectrometer and the current—voltage– brightness characteristics were simultaneously measured by a Keithley 2400 programmable voltage–current source. All measurements were carried out at room temperature under ambient conditions. In this work, we used CDBP instead of CBP as the host for blue dopant FIrpic. The triplet energy lever of CBP is lower than that of FIrpic, which indicates that higher efficiency cannot be attained from this host and guest combination [16]. The improved efficiency can be explained in terms of the triplet energies of organic materials used in the devices. The triplet energy lever of CDBP (3.0 eV) was much higher than those of CBP (2.6 eV) and FIrpic (2.7 eV) [17] indicating that there was a very efficient energy transfer from the CDBP triplet states to the FIrpic triplet states and excellent triplet energy confinement on the FIrpic molecules, leading to high blue emitting performance. In order to determine the optimal concentrations of FIrpic in the host material CDBP, a group of control devices with the structure ITO/NPB/CDBP:FIrpic (30 nm)/TPBI(50 nm)/LiF(0.8 nm)/Al with various FIrpic doping concentrations of between 0 and 12% were fabricated. As shown in figure 2, at doping concentrations of FIrpic of between 2 and 12%, the intensity of blue emission at a peak of approximately 476 nm (contributed by FIrpic) was observed to increase with the doping concentration of FIrpic up to 10%. However, at doping concentrations of FIrpic of over 10%, the intensity of the blue emission dropped as the doping concentration of FIrpic increased. This is due to triplet–triplet (T–T) annihilation between the FIrpic and host triplets. Therefore, the host material of CDBP doped with 10% FIrpic exhibits an energy transfer at the appropriate rate, optimally emitting blue light from FIrpic. Because Ir(ppy)3 and Ir(piq)2(acac) were adopted as the green and red guest phosphorescent iridium complexes, respectively, the optimal concentrations of these two dopants added to the host material CBP must be determined. The effect of doping CBP with Ir(ppy)3 and Ir(piq)2(acac) on the balance between green and red 729 R Song et al 25 0% 2% 4% 6% 8% 10% 12% 0.5 0.4 0.3 0.2 0.1 20 EL Intensity (a.u.) Efficiency (cd /A) 15 10 5 0 1% Ir(ppy)3 and 3% Ir(piq)2(acac) 3% Ir(ppy)3 and 5% Ir(piq)2(acac) 5% Ir(ppy)3 and 3% Ir(piq)2(acac) 7% Ir(ppy)3 and 5% Ir(piq)2(acac) 0.0 400 500 600 Wavelength (nm) 700 -5 0 20 40 60 80 100 120 140 160 180 200 Current density (mA /cm 2 ) Figure 2. Electroluminescent spectra of ITO/NPB (40 nm)/CDBP:FIrpic/TPBI (50 nm)/LiF(0.8 nm)/Al with 0, 2, 4, 6, 8, 10 and 12% FIrpic in a CDBP layer whose thickness is 30 nm and current density is 50 mA cm−2. 1% Ir(ppy)3 and 3% Ir(piq)2(acac) 3% Ir(ppy)3 and 5% Ir(piq)2(acac) 5% Ir(ppy)3 and 3% Ir(piq)2(acac) 7% Ir(ppy)3 and 3% Ir(piq)2(acac) 7% Ir(ppy)3 and 5% Ir(piq)2(acac) Figure 4. Current efficiency of devices with structure: ITO/NPB (40 nm)/CBP: Ir(ppy)3:Ir(piq)2(acac) (20 nm)/TPBI (50 nm)/LiF (0.8 nm)/Al with various doping concentrations of Ir(ppy)3:Ir(piq)2(acac). 1.4 Normalized Intensity (a.u.) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400 1.6 Normalized intensity (a.u.) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400 30000cd/m (0.37,0.39) 10000cd/m (0.38,0.39) 1000cd/m (0.41,0.42) 2 2 2 500 600 Wavelength (nm) 700 800 500 600 700 Figure 3. Normalized electroluminescent spectra with structure: ITO/NPB(40 nm)/CBP:Ir(ppy)3:Ir(piq)2(acac) (20 nm)/TPBI (50 nm)/LiF(0.8 nm)/Al with various doping concentrations of Ir(ppy)3:Ir(piq)2(acac). Wavelength (nm) Figure 5. Normalized electroluminescent spectra of the WOLEDs with the configuration: ITO/NPB(40 nm)/CDBP:10% FIrpic (10 nm)/TPBI (4 nm)/CBP:5% Ir(ppy)3:3% Ir(piq)2(acac) (20 nm)/TPBI (50 nm)/LiF(0.8 nm)/Al. emission intensities was studied. Figure 3 shows the EL spectra of ITO/NPB(40 nm)/CBP:Ir(ppy)3:Ir(piq)2(acac) (30 nm)/TPBI(50 nm)/LiF(0.8 nm)/Al with various doping concentrations of Ir(ppy)3 and Ir(piq)2(acac). The analytical results in figures 3 and 4 indicated that the device with 5% Ir(ppy)3 and 3% Ir(piq)2(acac) in the host material CBP exhibited not only balanced green and red emission intensities but also relatively high-energy transfer from CBP to Ir(ppy)3 and Ir(piq)2(acac). Based on these results, we fixed the concentrations of blue, green and red phosphorescent dyes for 10, 5 and 3%, respectively. The total thickness of emissive layers was fixed for 30 nm and then the thickness of each emissive layer was varied. The structure of devices is ITO/NPB (40 m)/ CDBP:FIrpic/CBP:Ir(ppy)3:Ir(piq)2(acac)/TPBI (50 nm)/ LiF(0.8 nm)/Al. For device A, the blue emissive layer and green–red emissive layer are both 15 nm, while for device B, 20 nm blue emission layer and 10 nm green–red emission layer and for device C, 25 nm blue emission layer and 5 nm green–red emission layer, respectively. However, the blue light emission of device A was almost suppressed, due to the energy export from Firpic to other phosphorescent dipants 730 [16]; furthermore, the blue light emission is still not up to other emissions even in device C. To obtain a bright and high purity WOLEDs, we tuned the balance of white emission employing a thin TPBI layer, which was inserted between excitons the blue and green–red layers as an excitons blocking layer. Figure 5 shows the EL spectra of the WOLEDs with configuration of ITO/NPB(40 nm)/CDBP:FIrpic (10 nm)/CBP (4 nm)/CBP:Ir(ppy)3:Ir(piq)2(acac) (20 nm)/ TPBI (50 nm)/LiF(0.8 nm)/Al under various operating voltages. The EL spectra of the devices are full-spectrum and cover the range 400–800 nm, with three major emissions at 476 nm (from FIrpic), 520 nm [from Ir(ppy)3] and 620 nm [from Ir(piq)2(acac)]. The emission colour of this device is fairly stable at high luminance: its CIE chromaticity coordinate changes from (0.41, 0.42) to (0.37, 0.39) when the luminance ranges from 1000 cd m−2 to 30 000 cd m−2. Moreover, this white OLED has a maximum luminance of 42 700 cd m−2 at 17 V and maximum power efficiency of 8.48 lm W−1 at 5 V, as shown in figure 6, as well as a colour rendering index (CRI) of 79. White organic light-emitting devices employing phosphorescent iridium complex as RGB dopants 50000 10 Luminance (cd/m2) Luminance(cd/m ) Power efficiency(lm/w) 2 and Ministry of Science and Technology of China (nos. 2003CB314703). Power efficiency (lm/w) 40000 30000 20000 10000 0 2 4 6 8 10 12 14 16 18 20 8 6 4 2 0 References [1] Kido J, Kimura M and Nagai K 1995 Science 267 1332 [2] Deshpande R S, Bulovic V and Forrest S R 1999 Appl. Phys. Lett. 75 888 [3] D’Andrade B W, Thompson M E and Forrest S R 2002 Adv. Mater. 14 147 [4] Tsuzuki T, Shirasawa N, Suzuki T and Tokito S 2003 Adv. Mater. (Weinheim, Ger.) 15 1455 [5] Baldo M A, Lamansky S, Burrows P E, Thompson M E and Forrest S R 1999 Appl. Phys. Lett. 75 4 [6] Adachi C, Baldo M A, Forrest S R, Lamansky S, Thompson M E and Kwong R C 2001 Appl. Phys. Lett. 78 1622 [7] D’Andrade B W, Thompson M E and Forrest S R 2002 Adv. Mater. 14 147 [8] D’Andrade B W, Holmes R J and Forrest S R 2004 Adv. Mater. 16 624 [9] D’Andrade B W, Brooks J, Adamovich V, Thompson M E and Forrest S R 2002 Adv. Mater. 14 1032 [10] Tokito S, Lijima T, Tsuzuki T and Sato F 2003 Appl. Phys. Lett. 83 2459 [11] Gong X, Ma W, Ostrowski J C, Bazan G C, Moses D and Heeger A J 2004 Adv. Mater. 16 615 [12] Suzuki M, Hatakeyama T, Tokito S and Sato F 2004 IEEE J. Sel. Top. Quantum Electron. 10 115 [13] Li F, Cheng G, Zhao Y, Feng J, Liu S Y, Zhang M, Ma Y G and Shen J C 2003 Appl. Phys. Lett. 83 4716 [14] Duan J P, Sun P P and Cheng C H 2003 Adv. Mater. 15 224 [15] Yang C H, Fang K H, Su W L, Wang S P, Su S K and Sun I W 2006 J. Organomet. Chem. 691 2767 [16] Cheng G, Zhang Y F, Zhao Y, Lin Y, Ruan C Y, Liu S Y, Fei T, Ma Y G and Cheng Y X 2006 Appl. Phys. Lett. 89 043504 [17] Tokito S, Iijima T, Suzuri Y, Kita H, Tsuzuki T and Sato F 2003 Appl. Phys. Lett. 83 569 Voltage (v) Figure 6. Power efficiency and luminance–voltage characteristics of a device with the structure: ITO/NPB(40 nm)/CDBP:10% FIrpic (10 nm)/TPBI (4 nm)/CBP:5% Ir(ppy)3:3% Ir(piq)2(acac) (20 nm)/TPBI (50 nm)/LiF(0.8 nm)/Al. In summary, we have fabricated high-bright electrophosphorescent white organic light-emitting devices with the structure of ITO/NPB(40 nm)/CDBP:10% FIrpic (10 nm)/TPBI (4 nm)/CBP:5% Ir(ppy)3:3% Ir(piq)2(acac) (20 nm)/TPBI (50 nm)/LiF(0.8 nm)/Al exhibiting a maximum luminance of 42 700 cd m−2 at 17 V and maximum power efficiency of 8.48 lm w−1 at 5 V; the CIE coordinates vary from (0.41, 0.42) at 1000 cd m−2 to (0.37, 0.39) 30 000 cd m−2 as well as CRI of 79. Acknowledgments This work is supported by National Natural Science Foundation of China (nos. 60376028 and 60207003) 731 ...
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