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Simplified Bilayer White Phosphorescent Organic Light-Emitting Diodes
Simplified Bilayer White Phosphorescent Organic Light-Emitting Diodes
ETRI Journal. 2016. Apr, 38(2): 260-264
Copyright © 2016, Electronics and Telecommunications Research Institute (ETRI)
  • Received : July 15, 2015
  • Accepted : September 07, 2015
  • Published : April 01, 2016
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About the Authors
Jonghee Lee
Woo Jin Sung
Chul Woong Joo
Hyunsu Cho
Namsung Cho
Ga-Won Lee
Do-Hoon Hwang
Jeong-Ik Lee

Abstract
We report on highly efficient blue, orange, and white phosphorescent organic light-emitting diodes consisting only two organic layers. Hole transporting 4, 4,’ 4”-tris (N-carbazolyl)triphenylamine (TcTa) and electron transporting 2-(diphenylphosphoryl) spirofluorene (SPPO1) are used as an emitting host for orange light-emitting bis(3-benzothiazol-2-yl-9-ethyl-9H-carbazolato) (acetoacetonate) iridium ((btc)2(acac)Ir) and blue light-emitting iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C2’) picolinate (FIrpic) dopant, respectively. Combining these two orange and blue light-emitting layers, we successfully demonstrate highly efficient white PHOLEDs while maintaining Commission internationale de l'éclairage coordinates of ( x = 0.373, y = 0.443). Accordingly, we achieve a maximum external quantum, current, and power efficiencies of 12.9%, 30.3 cd/A, and 30.0 lm/W without out-coupling enhancement.
Keywords
I. Introduction
Organic light-emitting diodes (OLEDs) are attracting widespread attention as next-generation low-cost, high-efficiency thin-film electroluminescent devices for both flat panel displays and lighting applications. While conventional fluorescent tubes and market-pioneering white inorganic light-emitting diodes are currently holding most of the market share in the lighting industry, white OLEDs (WOLEDs) could be a very competitive candidate due to their unique advantages, including low operating voltages, high power efficiencies, and suitability for processing on flexible substrates. WOLEDs for solid-state lighting applications require high efficiency, high color rendering index, high color stability, and appropriate color temperature [1] [5] .
There are various methods for highly efficient WOLEDs. In particular, to use phosphorescent materials is one effective way to get high efficiency in WOLEDs due to their ability to efficiently utilize both singlet and triplet excitons. However, phosphorescent OLEDs (PHOLEDs) require a complicated device structure with numerous organic layers to improve charge injection, transport, balance, and exciton confinement. Since the complicated structures increase the manufacturing cost, simplified device structures are desirable for OLED applications.
In this work, we have demonstrated a simplified white PHOLED consisting of only two organic layers; that is, a hole transporting–type orange emitting layer (EML) and an electron transporting–type blue EML. We believe that this simple design concept can provide a new avenue for achieving high-performance WOLEDs for lighting applications.
II. Experiments
As shown in Fig. 1 , three simplified PHOLEDs in the current study (Device A-C) were fabricated as follows:
  • Device A (blue) – indium tin oxde (ITO)/TcTa (50 nm)/SPPO1: FIrpic (50 nm, 10%)/LiF/Al
  • Device B (orange) – ITO/TcTa: (btc)2(acac)Ir (50 nm, 7%)/SPPO1 (50 nm)/LiF/Al
  • Device C (white) – ITO/TcTa: (btc)2(acac)Ir (50 nm, 7%)/SPPO1: FIrpic (50 nm, 10%)/LiF/Al
The hole-transporting material, TcTa, with a high triplet level (2.85 eV), and the high-lying lowest unoccupied molecular orbital (LUMO) energy level of 2.4 eV, which can efficiently confine the triplet energy of the orange phosphorescent emitter ((btc) 2 (acac)Ir) and electrons, is selected as the hole-transport/host of orange EML [6] , [7] . The electron-transporting material SPPO1 with a high triplet level (2.90 eV) and the low-lying highest occupied molecular orbital (HOMO) energy level of 6.5 eV, which can efficiently confine the triplet energy of the blue phosphorescent emitter (FIrpic) and holes, is selected as the hole-transport/host of blue EML. The chemical structures of the materials and energy level diagrams used in this study are shown in Figs. 2 and 3 (the energy levels are taken from the literature) [8] [10] , respectively.
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Device structures of blue (device A), orange (device B), and white (device C) PHOLEDs tested in this study.
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Chemical structures for materials tested in this study.
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Energy level diagrams for materials tested in this study.
ITO was cleaned using a standard oxygen plasma treatment. The OLED grade materials were purchased and used without further purification. All organic layers were deposited in a high vacuum chamber below 5 × 10 −7 Torr, and thin LiF and Al films were deposited as a cathode electrode. The OLEDs were transferred directly from a vacuum into an inert environment glove-box, where they were encapsulated using a UV-curable epoxy and a glass cap with a moisture getter. The electroluminescence spectrum was measured using a Minolta CS-1000. The current–voltage (J–V) and luminescence-voltage (L-V) characteristics were measured using a current/voltage source/measure unit (Keithley 238) and a Minolta CS-100, respectively.
III. Results and Discussion
To develop efficient PHOLEDs, the effective confinement of both the charge carriers and the triplet excitons is necessary. By stacking two organic layers with different charge-transporting properties, the charge carriers (holes and electrons) are accumulated at the interface of TcTa and SPPO1. Furthermore, the high-lying LUMO level of TcTa and the low-lying HOMO of SPPO1 level help to confine the charge carriers. Therefore, the effective hole/electron recombination could be achieved, as is shown in Fig. 3 .
Figure 4 shows the current density-voltage-luminance (J-V-L) curves of the PHOLEDs (devices A–C). To obtain more efficient WOLEDs, particularly to achieve high luminance efficacy for solid-state lighting application, it is very desirable that the driving voltage of the WOLEDs be lowered. The turn-on voltages of devices A, B, and C are approximately 3.0 V. In particular, the relatively low turn-on voltage (3.2 V at 1 cd/m 2 ) of the white PHOLED (device C) with only two layers is noticeable because it is almost similar in value to that of the complicated conventional white PHOLEDs. This could be explained by the following: (a) the acceptable low-lying HOMO energy level (5.7 eV) and the high hole mobility of TcTa for hole carrier and (b) the adequate high-lying LUMO energy level (2.8 eV) and the high electron mobility of SPPO1 for the hole carrier [8] , [11] . The driving voltages at 100 cd/m 2 of devices A, B, and C are 5.8 V, 4.1 V, and 6.0 V, respectively. The maximum luminance values of 7,108 cd/m 2 , 16,410 cd/m 2 , and 18,330 cd/m 2 were achieved for devices A–C.
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Current density-voltage-luminescence-voltage (J-V-L) characteristics of devices A–C.
The electroluminescence (EL) spectra at a driving current density of 10 mA/cm 2 for devices A–C are shown in Fig. 5 . The EL spectra of devices A and B exhibited peak wavelengths at 472 nm and 559 nm, respectively. These emissions correspond to the peak wavelengths of the EL spectra of the single-color OLEDs using the FIrpic and (btc) 2 (acac)Ir, respectively. For device C, we could easily find the white EL emission consisting of blue (FIrpic) and orange ((btc) 2 (acac)Ir) emissions, by which we mean that the simplified white PHOLED was successfully demonstrated with only two organic layers. The CIE coordinates and correlated color temperature (CCT) of WOLEDs (device C) at a driving current density of 10 mA/cm 2 are (0.373, 0.48) and 4,512 K, respectively.
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EL spectra of devices A–C at driving current of 10 mA/cm2.
The external quantum, current, and luminance efficiencies of the emitted light in the forward direction of the WOLEDs versus luminance are plotted in Fig. 6 . The quite high peak external quantum efficiency of 12.3% (device A), and 10.6% (device B) are indicating a successful effective confinement of both the charge carriers and the triplet excitons by stacking two organic layers with different charge-transporting properties. Finally, maximum external quantum, current, and power efficiencies of simplified white PHOLEDs (device C) were achieved without any out-coupling enhancement — 12.9%, 30.3 cd/A, and 30.0 lm/W, respectively. The reduced efficiency at a high luminance region (that is, roll-off phenomenon) for blue (A) and white (C) OLED comes from a poor electrical optimization (triplet-triplet annihilation (TTA) or triplet-polaron annihilation (TPA) and low charge carrier balance in the FIrpic-based blue EML [11] [13] . The external quantum, current, and power efficiencies of simplified white PHOLEDs (device C) at a luminance of 1,000 cd/m 2 was reduced to 8.6%, 20.3 cd/A, and 7.8 lm/W, respectively. This could be resolved by further device engineering such as an optimization of doping ratio or a use of a host mixing structure; especially, for the blue layer.
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External quantum, current, luminance efficiency vs. luminance characteristics of devices A–C.
IV. Conclusion
We have demonstrated bilayer phosphorescent white OLEDs with a configuration of ITO/TcTa: orange dopant/SPPO1: blue dopant/LiF/Al. These simplified structures are also applicable to orange and blue monochromic OLEDs with common phosphorescent dopants. Efficient performance of the simplified bilayer devices is attributed to direct hole/electron injection and transport on the triplet dopants. This concept indicates a possible new direction to fabricate simplified OLEDs with competitive performance.
This work was supported by the Industrial Strategic Technology Development Program (10045269, Development of Soluble TFT and Pixel Formation Materials/Process Technologies for AMOLED TV funded by MOTIE/KEIT).
BIO
jonghee.lee@etri.re.kr
Jonghee Lee received his BS, MS, and PhD degrees in chemistry from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 2002, 2004, and 2007, respectively. He joined ETRI in 2007 and worked on white organic light-emitting diodes for display and lighting applications. Then, he moved to the Institut für Angewandte Photophysik, Technische Universität, Dresden, Germany, in 2010, as a post-doctorate. After two years, he joined ETRI again, and has since been working on flexible solution process OLEDs.
westlifen@naver.com
Woo Jin Sung received his BS degree in information and communication engineering from Hanbat University, Daejeon, Rep. of Korea, in 2013 and his MS degree in electrical engineering from Chungnam University, Daejeon, Rep. of Korea, in 2015. Since 2013, he has been at ETRI working on developing processes and applications for OLEDs.
cwjoo@etri.re.kr
Chul Woong Joo received his BS and MS degrees in polymer science and engineering of organic electronics devices from Dankook University, Cheonan, Rep. of Korea, in 2008 and 2010, respectively. He joined ETRI in 2011, and his current research interests include device architectures in organic light-emitting devices.
hyunsucho@etri.re.kr
Hyunsu Cho received his BS and PhD degrees in electrical engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 2008 and 2014, respectively. He joined ETRI in 2014. His current research interests include graphene electrodes and optical designs of OLEDs.
kevinchons@etri.re.kr
Namsung Cho received his BS degree from Chung-Ang University, Seoul, Rep. of Korea, in 2000 and his MS and PhD degrees in chemistry from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 2002 and 2006, respectively. From 2006 to 2008, he was a postdoctoral associate at the University of California Santa Barbara, CA, USA. He worked in material development for OLEDs in LG Display R&D Center, Paju, Rep. of Korea, from 2008 to 2012. He joined ETRI in 2012. His current research interests include OLED structures, OLED materials, and white OLEDs.
gawon@cnu.ac.kr
Ga-Won Lee received her BS, MS, and PhD degrees in electrical engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 1994, 1996, and 1999, respectively. In 1999, she joined Hynix Semiconductor Ltd., Incheon, Rep. of Korea, as a senior research engineer, where she was involved in the development of DRAM technologies. Since 2005, she has been at Chungnam National University, Daejeon, Rep. of Korea, as a professor with the Department of Electronics Engineering. Her main research interests include flexible device technologies, including fabrication, electrical analysis, and modeling.
dohoonhwang@pnu.ac.kr
Do-Hoon Hwang received his BS degree in chemistry from Pusan National University, Busan, Rep. of Korea, in 1990 and his MS and PhD degrees in chemistry from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 1992 and 1995, respectively. From 1995 to 1996, he was a postdoctoral associate at the University of Cambridge, UK. He worked at ETRI as a senior researcher from 1997 to 2000. Since then, he has joined the Department of Applied Chemistry, Kumoh National Institute of Technology, Gumi, Rep. of Korea. In 2010, he moved to the Department of Chemistry, Pusan National University. His main research interests lie in the synthesis of new organic semiconducting materials for diverse organic electronic devices, such as organic light-emitting diodes, organic transistors, and organic solar cells.
Corresponding Author jiklee@etri.re.kr
Jeong-Ik Lee received his BS, MS, and PhD degrees in chemistry from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 1992, 1994, and 1997, respectively. Upon graduating, he joined the IBM Almaden Research Center, San Jose, CA, USA, as a post-doctorate, where he worked on organic light-emitting materials. He moved to ETRI in 1999 and has been continuing his research on organic light-emitting materials and devices. Since 2011, he has been in charge of OLED research at ETRI in his capacity as a director.
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