Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure

Organic Electronics 14 (2013) 1183–1188

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Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure Young Hoon Son a, Mi Jin Park a, Young Jae Kim a, Joong Hwan Yang b, Jung Soo Park b, Jang Hyuk Kwon a,⇑ a b

Department of Information Display, Kyung Hee University, Dongdaemoon-gu, Seoul 130-701, Republic of Korea Material R&D Team 3, LG Display R&D Center 1007 Deogeun-ri, Wollong-myeon, Paju-si Gyeonggi-do 413–811, Republic of Korea

a r t i c l e

i n f o

Article history: Received 4 December 2012 Received in revised form 18 January 2013 Accepted 19 January 2013 Available online 4 February 2013 Keywords: Organic light-emitting diode Phosphorescent White Color stable Double emission layer

a b s t r a c t In this paper, we report color stable phosphorescent white organic light-emitting diodes (OLEDs) based on a double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/ green EML could induce less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band-gap host material, 2,6-bis(3-(carbazol9-yl)phenyl)pyridine (26DCzPPy), was used for achieving such deep-trap generation. Fabricated phosphorescent white OLED shows a slight color coordinate change of (0.002, +0.002) from 1000 cd/m2 to 5000 cd/m2 with power efficiency of 38.7 lm/W and current efficiency of 46.4 cd/A at 1000 cd/m2. In addition, negligible color changes were observed by delaying red dopant saturation time using optimum red dopant concentration. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction White organic light-emitting diodes (WOLEDs) have shown strong potential possibilities as next generation solid-state light sources due to their promising properties such as spatially homogeneous white emission across the surface of large-area and are eco-friendly. There are several requirements for WOLEDs to be used as solid-state luminaire, such as high efficiency, high color rendering index (CRI), proper color coordinates, long lifetime, high color stability etc. [1,2]. There have been reports on various fabrication approaches of WOLEDs to fulfill all these requirements. Among of them, mainly three categories according to the architecture of emissive layers (EMLs) could be defined. The use of multiple emitters within a single EML is the first way [3–6]. A second is to use multiple adjacent EMLs and tandem stacks with two or three color combinations are the third one [7–12]. The first and second methods are more suitable for the mass production than the ⇑ Corresponding author. Tel.: +82 2 961 0948; fax: +82 2 961 9154. E-mail address: [email protected] (J.H. Kwon).

third one due to their simplicity of fabrication. However, these show a poor color stability problem as voltage variation because of the changes in emission zone and/or energy transfer situation. When two or three different phosphorescent emitters are close to each other within Dexter radius, sequential energy transfer from higher triplet to lower triplet state occurs until neighbor lower energy state of dopants are fully excited. After full excitation of lower energy state of dopants at the high current density, higher triplet state dopants contribute more for the light generation. Therefore, color shift from red to green or blue as the voltage increase is generally observed in such as single and multi-EML devices. Even in single EML structure, there is some color variation with the voltage increase if the dopant ratio among three dopants is not an optimum condition [3,13–16]. Normally, we could drastically reduce the voltage dependent spectral changes of single EML device by using the doping concentration of a lower band gap emitter as low as possible, about 0.1–1.0% [17,18]. In addition to this color change issue related by energy transfer, the change of exciton generation zone also gives rise to color shift as the voltage increase in many multi-EML cases.

1566-1199/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.01.019

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The different field dependency between hole mobility of hole transporting layer (HTL) and electron mobility of electron transporting layer (ETL) and/or different carrier trapping situation at the EML move exciton emission zone with increasing voltage in multi-EML structure. From the color stability perspective, blue/orange complementary colors WOLEDs are better than three primary colors (red, green, and blue) WOLEDs. The blue/orange complementary colors has only one energy transfer path from blue to orange dopant that is easier to determine the optimum values of doping concentration, while three color system has at least three energy transfer paths (blue to green dopant, blue to red dopant, and green to red dopant), therefore it is more difficult to control the emission balance and optimum concentrations. However, three color system is more favorable for high CRI [3,13]. To date, many efforts are under way to improve the color stability in multi-EML white devices. Fig. 1 shows several reported concept of device structures to have good color stability in multi-EML WOLEDs. The first concept has the low band gap emitter between high band gap emitters [19–21]. Quantum well type energy band could restrict carrier movement and exciton zone effectively at the center of emitting layer, which results in good color stability characteristics. This structure is quite good for two color WOLEDs, but not for three color WOLEDs because three color emitters do not have large energy difference. The second method is to insert the interlayer(s) between different emitting layers [22–25]. This interlayer could control balanced hole and electron movement regardless of voltage increase. It also helps to keep exciton formation zone without any variation by blocking energy transfer between different EMLs or effectively transferring triplet exciton energy to different EML layers. So far, there are so many papers based on this concept. Unfortunately, such additional use of the interlayer(s) is not good for the mass production. The third concept is to use adjacent two EMLs with deep charge trapping situation at the one of EMLs. As shown in Fig. 1, deep hole trapped carriers could restrict hole carrier and exciton movement. Hence it can achieve stable WOLEDs. There was a report of similar device structure, however, color stability is not so excellent because they did not fully utilize this concept [26].

In this paper, we demonstrated color stable phosphorescent WOLEDs with the deep hole trapped double emitting layer structure. The fabricated devices with this concept shows slight color coordinates change of (60.002, 60.002) from 1000 cd/m2 to 5000 cd/m2. Power efficiency of 38.7 lm/W and current efficiency of 46.4 cd/ A at 1000 cd/m2 are achieved in this optimized device. Additional negligible color changes were induced by delaying red dopant saturation time using optimum red dopant concentration.

2. Experimental The sublimated grade di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane (TAPC) and 2,6-bis(3-(carbazol-9yl)phenyl)pyridine (26DCzPPy) were purchased from Luminescence Technology and were used as the HTL and host material, respectively. Twice sublimated 1,3,5-tri[(3pyridyl)-phen-3-yl]benzene (TmPyPB) from Daejoo Electronic Materials were used as an ETL. Dipyrazino[2,3f:20 ,30 -h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) and bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic) from Luminescence Technology were used as the HTL and blue phosphorescent dopant material, respectively. (Tris(2-phenylpyridine)iridium) (Ir(ppy)3) from Gracel (now Dow Chemical company) was used as a green phosphorescent dopant material. A red phosphorescent dopant material iridium(III)bis(4-methyl2-(3,5-dimethylphenyl)quinolinato-N,C20 )acetylacetonate Ir(mphmq)2(acac)) was synthesized by our reporting method [27]. Fig. 2 shows the chemical structures of all organic materials used in the device fabrication. To fabricate WOLED devices, we used a clean glass substrate coated with a 150 nm thickness of ITO layer having a sheet resistance 10 X/square as an anode. The active patterns size of 2  2 mm2 were formed by the photolithography and wet etching processes. The ultrasonic cleaned glass substrate in an isopropyl alcohol, acetone, and methanol was rinsed in deionized water, and finally treated in ultraviolet (UV)ozone for 3 min. The ozone gas was generated using UV light to excite the oxygen in the air inside the chamber. Each organic layer was deposited under a pressure of 107 torr with total deposition rate of 0.5 Å/s. Subsequently, 0.5 nm thickness of lithium fluoride (LiF) and

Fig. 1. Emission zone configurations for color stable WOLEDs.

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Fig. 2. Chemical structures of used all organic materials for white phosphorescent OLEDs.

100 nm thickness of aluminum (Al) were deposited in a vacuum chamber without breaking the vacuum and used as a cathode. The current density–voltage (J–V) and luminance–voltage (L–V) data of white OLEDs were measured employing a Keithley SMU 238 and a Minolta CS-100A. The electroluminescence (EL) spectra and Commission Internationale de I’Eclairage (CIE) color coordinate were obtained using a Minolta CS-1000A.

3. Results and discussion In this work, we have designed a deep hole trapped double emitting layer structure as shown in Fig. 3. The 26DCzPPy was selected as a host material for the deep hole

trapping at the EML. This material was reported to have bipolar transporting characteristics and its highest occupied molecular orbital (HOMO) value was reported as 6.1 eV. It can move holes and electrons in the EMLs except for a deep trapped region. The triplet energy of 2.71 eV for 26DCzPPy is enough to prevent back energy transfer from FIrpic molecules whose triplet energy is 2.65 eV [28]. The energy differences between host and dopant are about 0.8 eV for green and 1.0 eV for red. Holes can flow from the anode to HTL and HTL to EML1 via HOMO levels of both host and dopant molecules due to the almost negligible HOMO energy difference. On the other hand, they can be easily captured by deep traps of HOMO levels of Ir(ppy)3 and Ir(mphmq)2(acac) in the EML2. Similarly, electrons could flow from cathode to ETL and ETL to EML2 via LUMO levels of both host and dopant molecules. They finally

Fig. 3. Designed color stable double EML WOLEDs with deep hole trapping of red and green dopant molecules. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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could be captured by shallow traps of LUMO of FIrpic. Hence, a good color stable performance is to be expected in this device structure. The fabricated device structure was ITO/TAPC (20 nm)/HATCN (10 nm)/TAPC (20 nm)/ 26DCzPPy:20% FIrpic (x nm, EML1)/26DCzPPy: 1% Ir(mphmq)2(acac): 3% Ir(ppy)3 (10x nm, EML2)/TmPyPB (40 nm)/LiF (0.5 nm)/Al (100 nm), where, x was chosen as 2, 3, and 3.5 nm (named as Devices A, B, and C, respectively). A trace amount of red and green dopant and rather high concentration of blue dopant was used to minimize color shift due to energy transfer from blue EML to red/ green EML. High triplet energies of TAPC (2.9 eV) and TmPyPB (2.8 eV) can effectively confine the triplet exciton within the EMLs [29,30]. The TAPC/ HATCN/ TAPC stack was used to improve the hole conduction [31]. For the evaluation of hole trapping effect, which significantly influences the change of the exciton generation interface, the same device structure except for deposition sequence of EML1 and EML2, Devcie D: ITO/TAPC/HATCN/TAPC/ 26DCzPPy: 1% Ir(mphmq)2(acac): 3% Ir(ppy)3 (8 nm)/ 26DCzPPy: 20% FIrpic (2 nm)/TmPyPB/LiF/Al, was fabricated. To further understand how the color stability changes according to different energy transfer situations, Devices E and F with higher red dopant concentration of 1.3% and 1.7% were fabricated without changing blue and green doping concentrations with the same configuration of Device C. Fig. 4a shows the EL spectra of Devices A, B, C and D at the 1000 and 5000 cd/m2 brightness values. All EL spectra exhibit three red, green, and blue peaks at 470, 505 and 602 nm. These are well matched with the peak wavelength of FIrpic, Ir(ppy)3, and Ir(mphmq)2(acac), respectively. The EL spectrum of Device C shows balanced red, green and blue emission peaks and color coordinates of (0.389, 0.394). Device B shows rather weak green and blue emission peaks than those of Device C and red shifted color coordinates of (0.427, 0.397). Device A shows more red shifted color coordinates of (0.454, 0.404). The fabricated Devices A, B, and C show the color coordinates within the quadrangle. As the thickness of red/green EML (EML2) increases, we could obtain more reddish-white spectrum. On the other hand, Device D shows dominant red emission as color coordinates of (0.559, 0.414) compared with other devices because the exciton generation zone is not located at the double EML interface but towards hole transporting side due to significant hole trapping situation at HOMO energies of red and green dopant molecules. Very small changes in spectral emission between the brightness of 1000 and 5000 cd/m2 are observed in Devices A, B, and C thanks to effective exciton confining at the interface of EMLs. Only a slight increment of green emission peak is observed while a blue emission peak remained almost unchanged. This implies that not only the exciton generation zone is unaffected during the voltage region of brightness of 1000–5000 cd/m2, but also color shifts are mainly caused by the energy transfer mechanism after red dopant saturation. The CIE 1931 color coordinates of Devices A, B, and C from 1000 cd/m2 to 5000 cd/m2 changed only (0.003, +0.003), (0.003, +0.003), (0.002, +0.002), respectively. On the other hand, Device D shows significant spectral variation of (0.015, +0.009) according

Fig. 4. (a) EL spectra of fabricated Devices A–D at 1000 and 5000 cd/m2. (b) EL spectra of fabricated Devices C, E, and F at 1000 and 5000 cd/m2. (c) Color coordinates of fabricated Devices A–F. Inside boxes represent a quadrangle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to the voltage increase as expected because excitons are not confined in double EML interface. The green emission intensities are increased to 4.4%, 3.3%, 2.2% and 5.1 % of brightness from 1000 to 5000 cd/m2 in Devices A, B, C and D, respectively. To further understand color stability according to energy transfer mechanism related to red dopant saturation,

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Fig. 5. J–V–L characteristics of fabricated Devices A–C.

Fig. 6. Current and power efficiency characteristics of fabricated Devices A–C.

we increased the red doping concentration to delay exciton saturation in red dopant molecules during 5000 cd/m2 brightness value region. Fig. 4b shows EL spectra of Devices E and F compared with Device C. As expected, color stability is improved with increasing red dopant concentrations. Especially, green emission peak shows a negligible change as the voltage increase with prolonging red dopant saturation time. The color shifts of Devices E and F between luminance of 1000 and 5000 cd/m2 are only (0.002, +0.002), (0.002, +0.001), respectively. Only 2.1% and 1.7% of green emission peak intensities are increased during the same luminance region. Fig. 4c shows CIE chromaticity coordinates of WOLEDs at the luminance of 1000 cd/m2. As shown in Fig. 4c, color coordinates of WOLEDs exhibit a clear tendency to depend on thickness of red/green EML and red dopant concentration. The color coordinates of Devices A, B, and C are lying on the same straight line. Similarly, Devices C, E, and F show collinear color coordinates. This collinearity of color coordinates indicates that energy transfer is linearly correlated to the amount of lower energy materials of red and green.

Fig. 5 is current density–voltage–luminance (J–V–L) characteristics of Devices A–C, while Fig. 6 shows current and power efficiencies as a function of luminance. The current density and luminance curves of all devices show almost similar behaviors because they have only slight differences in the thickness of EML, whereas total thickness of EMLs were fixed at 10 nm. The driving voltages at 1000 cd/m2 are 3.8–4.1 V for Devices A–F, which show substantial identity with insignificant differences. (see Table 1) Device C shows the highest current and power efficiencies of 46.4 cd/A, and 38.7 lm/W at the at 1000 cd/m2 brightness value, respectively. Devices A and B show lower efficiencies than those of Device C at the 1000 cd/m2 as current efficiencies of 41.2 cd/A and 43.6 cd/A and power efficiencies of 32.3 lm/W and 34.5 lm/W, respectively. There is a tendency of current and power efficiencies decrease with increasing red/green EML thickness because EML2 has relatively lower efficiency characteristics than blue EML1. Device D shows relatively lower efficiency attributed to unbalanced charges. The current and power efficiencies of Device D are 39.5 cd/A and 31.4 lm/W at

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Table 1 Performance summary of Devices A–F.

Turn-on voltagea (V) Operating voltageb (V) Efficiency b Efficiency (Max.) CIE (x, y)b Color shiftc a b c

Device A

Device B

Device C

Device D

Device E

Device F

3.3 4.1 41.2 cd/A 32.3 lm/W 43.0 cd/A 38.7 lm/W (0.454, 0.404) (0.003, +0.003)

3.1 3.9 43.6 cd/A 34.5 lm/W 43.8 cd/A 40.6 lm/W (0.427, 0.397) (0.003, +0.003)

3.1 3.8 46.4 cd/A 38.7 lm/W 46.8 cd/A 40.4 lm/W (0.389, 0.394) (0.002, +0.002)

3.1 4.0 39.5 cd/A 31.4 lm/W 41.7 cd/A 40.1 lm/W (0.559, 0.414) (0.015, +0.009)

3.1 3.9 38.8 cd/A 31.5 lm/W 40.8 cd/A 37.9 lm/W (0.404, 0.386) (0.002, +0.002)

3.1 3.9 36.0 cd/A 29.0 lm/W 37.4 cd/A 34.6 lm/W (0.433, 0.374) (0.002, +0.001)

Measured at 1 cd/m2. Measured at 1000 cd/m2. Measured from 1000 cd/m2 to 5000 cd/m2.

the 1000 cd/m2 brightness value, respectively. As maximum current and power efficiency values, 43.0 cd/A and 38.7 lm/W for Device A, 43.8 cd/A and 40.6 lm/W for Device B, 46.8 cd/A and 40.4 lm/W for Device C, 41.7 cd/A and 40.1 lm/W for Device D, 40.8 cd/A and 37.9 lm/W for Device E, and 37.4 cd/A and 34.6 lm/W for Device E were obtained, respectively. 4. Conclusion We have successfully demonstrated color stable WOLEDs with a double EML structure. Deep hole trapping of red and green dopant molecules could effectively confine the exciton generation zone at the double EML interface, resulting in good color stability characteristics of double EML white OLEDs. Only slight color coordinates change of (0.002, +0.002) from 1000 cd/m2 to 5000 cd/m2 with power efficiency of 38.7 lm/W and current efficiency 46.4 cd/A at 1000 cd/m2 are achieved. By increasing the red doping concentration by a small amount for delaying exciton saturation time at the red dopant molecules we could achieve better color stability of (0.002, +0.001), which is a negligible change for human eyes. Acknowledgments This work was supported by LG Display Co., Ltd. This work was also supported by the Industrial Strategic Technology Development Program (10028439) funded by the Ministry of Knowledge Economy (MKE, Korea). References [1] ENERGY STARÒ Program Requirements for Solid State Lighting Luminaires, Version 1.1, 2008. [2] G. Zhou, W. Wong, S. Suo, J. Photochem. Photobio. C: Photochem. Rev. 11 (2010) 133–156. [3] B.W. D’Andrade, R.J. Holmes, S.R. Forrest, Adv. Mater. 16 (2004) 624– 628. [4] H. Wu, G. Zhou, J. Zou, C.L. Wong, W.Y. Wong, W. Yang, J. Peng, Y. Cao, Adv. Mater. 21 (2009) 4181–4184.

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