当前位置:首页 > 显示光电 > 显示光电
[导读]Efficient deep blue emitters for organic electroluminescent devicesMeng-Huan Ho,a) Yao-Shan Wu, Shih-Wen Wen, and Teng-Ming ChenDepartment of Applied Chemistry, NationalChiaoTungUniversity, Hsinshu, T

Efficient deep blue emitters for organic electroluminescent devices

Meng-Huan Ho,a) Yao-Shan Wu, Shih-Wen Wen, and Teng-Ming Chen

Department of Applied Chemistry, NationalChiaoTungUniversity, Hsinshu, Taiwan 300, Republic of China

Chin H. Chen

Display Institute, National Chiao Tung University, Hsinshu, Taiwan 300, Republic of China and Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinshu, Taiwan 300, Republic of China

Abstract: Highly efficient deep blue organic light emitting devices have been fabricated with a 4-(styryl)biphenyl-core based fluorescent dopant (SK-1) in the wide band gap 2-methyl-9,10-di (1-naphthyl) anthracene (α,α-MADN) host system which achieved an electroluminescence efficiency of 5.0 cd/A and an external quantum efficiency of 4.2% at 20 mA/cm2 with a saturated blue Commission Internationale de l’Eclairage coordinates of (0.15, 0.14) and a half-decay lifetime of 8000 h at an initial brightness of 100 cd/m2. The current efficiency and electroluminescent color of SK-1 doped devices have been shown to be essentially immune to drive current density.

In recent years, there has been considerable interest in developing blue organic light-emitting diodes (OLEDs) with high efficiency, deep blue color, and long operational lifetime.1 Deep blue color is defined here as having a blue electroluminescent emission with a Commission Internationale d’Eclairage (CIEx,y) coordinates of x~0.15 and y <0.15. Such an emitter can effectively reduce the power consumption of a full-color OLED (Ref. 2) and also be utilized to generate light of other colors by energy cascade to a suitable emissive dopant.3

Unfortunately, literature with full disclosure on deep blue OLED dopant/host material structures is rather rare and sketchy. One notable example, a styrylamine-based dopant BD-3 to produce an electroluminescence (EL) efficiency of 7.2 cd/A and a blue color of (0.14, 0.16) (Ref. 4) was recently

presented by Idemitsu Kosan Co. and likewise, Kodak also talked about one of their best deep blue device performances with dopant BK-9 in host BH-3 which achieved an efficiency of 7.4 cd/A and a blue color of (0.14, 0.17)5 Neither of them disclosed in these presentations any useful structural information that was needed to substantiate their great device performances.

Recently, it was found that an [!--empirenews.page--]unsymmetrical mono(styryl)amine fluorescent dopant, diphenyl-[4-(2-[1,1’;4’,1’’]terphenyl-4-yl-vinyl)-phenyl]-amine (BD-1)upon doping in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) produced one of the best deep devices reported

then of 5.4 cd/A and a CIEx,y of (0.14, 0.13).6 This optimized efficiency and device stability could only be obtained by incorporating the composite hole transporting layer (c-HTL) of N,N’-bis(1-naphthyl)-N,N’-diphenyl-1,1’-biphenyl-4 ,4’-diamine (NPB):copper phthalocyanin

(CuPc)(1:1) to balance the charge carriers,7 however, which were not readily controlled during fabrication.

Later it was discovered that when BD-1 was doped in the modified wide band gap host of 2-methyl-9,10-di(1- naphthyl)anthracene (α,α-MADN), it could achieve an EL efficiency of 3.3 cd/A with a saturated blue CIEx,y of (0.15, 0.13) without the introduction of c-HTL.8 However, the device efficiency leaves much to be improved and the presence of long wavelength shoulder of these mono (styryl) aminebased emitters which tends to grow with increasing dopant concentration is problematic as it will increase the CIEy value of the blue devices, leading to an unsatisfactory blue

color.9 This phenomenon can be attributed to the aggregation propensity of these relatively flat dopant molecules at high concentration, which also results in low device efficiency due to concentration quenching.10 Therefore, how to suppress the molecular aggregation propensity of these blue doped emitters is critical for developing high-efficiency deep blue OLEDs and an adequate approach to molecular engineering is necessary.

In this letter, we report the development of a 4-(styryl)biphenyl-core based deep blue emitter,

4-N,N’diphenylamino-4’- [(4-N’ ,N’-diphenylamino)styryl] biphenyl (SK-1) by introducing a particular linkage of aryl group in between the diphenylamino and the styrene moieties, as shown in Fig. 1, which is designed to twist the planar structure of BD-1 and thus alleviate these aforementioned

issues related to molecular aggregation.11 Two types of SK-1 doped blue devices in two different host materials of MADN with lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) of 2.6/ 5.6 eV (device I) andα, α-MADN with LUMO/HOMO of 2.8/ 5.8 eV (device II) as emitting layers (EMLs) have been fabricated. The basic device structure was indium tin oxide/ CFx /NPB (50 nm)/EML (40 nm)/tris(8-quinolinolato)aluminum(Alq3) (10 nm)/LiF (1 nm)Al (200 nm), in which CFx, NPB, and Alq3 were used as the hole injection material,12 hole, and electron transport materials, respectively. The optimized doping concentration for SK-1 dopant has been determined to be at 7%.[!--empirenews.page--]

To investigate the energy transfer between the dopant/host material of SK-1 and MADN, the solid-state emission spectra of various concentrations of SK-1 doped MADN thin films [spin coated with poly(methyl methacrylate) (PMMA),excited with 400 nm uv source which is near the λex,max of MADN] have been measured, as shown in Fig. 1. The solidstate emission spectra of 5% SK-1 doped MADN feature a main emission peak at 449 nm and a shoulder at 471 nm with a full width at half maximum (FWHM) of 68 nm, we also note that the emission of MADN around 430 nm essentially quenched confirming that the Förster energy transfer from MADN to SK-1 is complete when the dopant concentration reaches 5%. It can also be demonstrated from Fig. 1 that the FWHM of solid-state emission spectra and intensity of long wavelength shoulder were not significantly affected with increasing doping concentration of SK-1 from 5% to 9%.

Detailed EL performances measured at 20 mA/cm2 are summarized in Table I. Device I shows an EL efficiency of 4.4 cd/A and an external quantum efficiency (EQE) of 3.5% at 7.0 V with a deep blue CIEx,y coordinates (0.15, 0.15) The SK-1/MADN emitter system shows a near flat EL efficiency versus current density response, as shown in Fig. 2. The EL efficiency is sustained at 4.3 cd/A even at 155 mA/cm2. It suffers essentially no current-induced quenching, and there is also no EL color shift with respect to varying drive currents as the CIEx,y coordinates only shift from (0.148, 0.156) at 2 mA/cm2 to (0.145, 0.148) at 155 mA/cm2 with △CIEx,y= ±(0.003,0.008). This apparent resistance to color change under various drive current densities suggests that the charge carriers for recombination are well balanced in this blue emitter and both excitation mechanism of charge trapping and Förster energy transfer may be prevalent in the SK-1 doped devices.

The EL spectra of type I devices with various doping concentrations at 20 mA/cm2 is depicted in the inset of Fig.2, which exhibits one main peak at 448 nm with a shoulder at 472 nm and a FWHM of 60 nm. It is noteworthy that the EL peaks are neither broadened nor enhanced particularly with respect to the intensity of long wavelength shoulder at various SK-1 concentrations of 3%, 5%, 7%, and 9%. As a result, the saturated blue color of these devices is essentially unchanged with CIEx,y coordinates maintained at (0.15, 0.15). Based on these results, we attribute this apparent resistance to color shift to the inserted steric aryl linking group, which effectively prevents the dyes from aggregation at high concentration.

In order to further enhance the efficiency of these deep devices, we turn to the modified blue host material of α, α-MADN (Ref. 8) which has a blueshifted fluorescence emission of about 17 nm with respect to that of MADN, as shown in Fig. 3. It is evident from Fig. 3 that the overlap between the hypsochromic-shifted emission peak ofα, α-MADN and the absorption peak of SK-1 is better than that of MADN, which is essential for efficient Förster energy transfer. Figure 3 also depicts the emission spectra of 5% SK-1 doped MADN andα, α-MADN thin films (spin coated with PMMA). The emissive intensity of SK-1/α, α-MADN film is 1.1 times higher than that of SK-1/MADN film confirming that the Förster energy transfer is indeed more efficient betweenα, α-MADN and SK-1. The EL efficiency of device II was found to boost up to 5.0 cd/A and EQE of 4.2% at 7.3 V with a deep blue CIEx,y of (0.15, 0.14). This performance is higher than that of device I with SK-1/ MADN system (4.4 cd/A). Furthermore, the low-lying HOMO ofα, α-MADN also restrains the hole injection from the hole-transport layer of NPB to EML, which makes the hole-electron recombination more confined in the EML ofα, α-MADN device than that of MADN.8We believe that the more balanced carriers for recombination inα, α-MADN device is another reason for the enhanced device efficiency in addition to the more effective Förster energy transfer. DeviceII also shows a near flat EL efficiency versus current density, as shown in Fig. 2, and is resistant to color shift with various doping concentration as well.[!--empirenews.page--]

Figure 4 shows the operational lifetime of these two blue devices at a constant current density of 20 mA/cm2 monitored in a glovebox under nitrogen atmosphere (H2O <3 ppm, O2<3 ppm). The t1/2 (the time for the luminance to drop to 50% of initial luminance)and initial luminance(L0) measured for devices I and II were 800 h at L0=872 cd/m2, and 800 h at L0=1000 cd/m2, respectively. Moreover, the drive voltage of both devices increased only 0.4 V with continuous operation after 600 h. Assuming scalable Coulombic degradation13 driving at a L0 value of 100 cd/m2, the half-lives(t1/2) of devices I (SK-1/MADN) and II (SK-1/α,α-MADN)are projected to be 7000 and 8000 h, respectively.

In summary, we have developed a 4-(styryl) biphenylcore based dopant, SK-1, as an effective emitter for the doped deep blue OLED device. The introduction of steric aryl linkage to SK-1 molecular structure effectively prevents the dyes from aggregation at high doping concentration while maximized device EL efficiency is desired. The current efficiency and CIEx,y color of SK-1 doped devices have been shown to be essentially immune to drive current density. In addition, the deep blue SK-1/α, α-MADN doped emitter with simple device structure can achieve one of the best deep blue EL efficiencies of 5.0 cd/A at 7.3 V with a deep blue CIEx,y color of (0.15, 0.14) and a long operational lifetime.

This work was supported by grants from Chunghwa Picture Tubes, Ltd. (CPT) of Taoyuan, Taiwan and National Science Council of Taiwan. The authors also thank e-Ray Optoelectronics Technology Co., Ltd., of Taiwan for generously supplying some of the OLED materials studied in this work.

1Y. Kijima, N. Asai, and S. Tamura, Jpn. J. Appl. Phys., Part 1 38, 5274 _1999_.

2Y. J. Tung, T. Ngo, M. Hack, J. Brown, N. Koide, Y. Nagara, Y. Kato, and H. Ito, Proceedings of the Society for Information Display, Seattle, Washington, 2004 _unpublished_, p. 48.

3C. W. Tang, S. A. Van Slyke, and C. H. Chen, J. Appl. Phys. 65, 3610 _1989_.

4T. Arakane, M. Funahashi, H. Kuma, K. Fukuoka, K. Ikada, H. Yamamoto, F. Moriwaki, and C. Hosokawa, Proceedings of the Society for Information Display, San Francisco, California, 2006 _unpublished_, p. 37.[!--empirenews.page--]

5L. S. Liao, K. P. Klubek, M. J. Helber, L. Cosimbescu, and D. L. Comfort, Proceedings of the Society For Information Display, San Francisco, California, 2006 _unpublished_, p. 1197.

6M. T. Lee, C. H. Liao, C. H. Tsai, and C. H. Chen, Adv. Mater. _Weinheim, Ger._ 17, 2493 _2005_.

7C. H. Liao, M. T. Lee, C. H. Tsai, and C. H. Chen, Appl. Phys. Lett. 86, 203507 _2005_.

8M. H. Ho, Y. S. Wu, S. W. Wen, M. T. Lee, T. M. Chen, C. H. Chen, K. C. Kwok, S. K. So, K. T. Yeung, and Z. Q. Gao, Appl. Phys. Lett. 89, 252903 _2006_.

9Z. Q. Gao, B. X. Mi, C. H. Chen, K. W. Cheah, Y. K. Cheng, and S. W. Wen, Appl. Phys. Lett. 90, 506 _2007_.

10C. W. Tang, S. A. Van Slyke, and C. H. Chen, J. Appl. Phys. 65, 3610 _1989_.

11M. H. Ho, Y. S. Wu, S. W. Wen, and C. H. Chen, Proceedings of the Society for Information Display, Long Bench, California, 2007 _unpublished_, p. 1768.[!--empirenews.page--]

12L. S. Hung, L. R. Zheng, and M. G. Mason, Appl. Phys. Lett. 78, 673 _2001_.

13S. A. Van Slyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett. 69, 2160 _1996_.


(该文章出自2008中国光电产业高层论坛论文集)

本站声明: 本文章由作者或相关机构授权发布,目的在于传递更多信息,并不代表本站赞同其观点,本站亦不保证或承诺内容真实性等。需要转载请联系该专栏作者,如若文章内容侵犯您的权益,请及时联系本站删除。
换一批
延伸阅读

9月2日消息,不造车的华为或将催生出更大的独角兽公司,随着阿维塔和赛力斯的入局,华为引望愈发显得引人瞩目。

关键字: 阿维塔 塞力斯 华为

加利福尼亚州圣克拉拉县2024年8月30日 /美通社/ -- 数字化转型技术解决方案公司Trianz今天宣布,该公司与Amazon Web Services (AWS)签订了...

关键字: AWS AN BSP 数字化

伦敦2024年8月29日 /美通社/ -- 英国汽车技术公司SODA.Auto推出其旗舰产品SODA V,这是全球首款涵盖汽车工程师从创意到认证的所有需求的工具,可用于创建软件定义汽车。 SODA V工具的开发耗时1.5...

关键字: 汽车 人工智能 智能驱动 BSP

北京2024年8月28日 /美通社/ -- 越来越多用户希望企业业务能7×24不间断运行,同时企业却面临越来越多业务中断的风险,如企业系统复杂性的增加,频繁的功能更新和发布等。如何确保业务连续性,提升韧性,成...

关键字: 亚马逊 解密 控制平面 BSP

8月30日消息,据媒体报道,腾讯和网易近期正在缩减他们对日本游戏市场的投资。

关键字: 腾讯 编码器 CPU

8月28日消息,今天上午,2024中国国际大数据产业博览会开幕式在贵阳举行,华为董事、质量流程IT总裁陶景文发表了演讲。

关键字: 华为 12nm EDA 半导体

8月28日消息,在2024中国国际大数据产业博览会上,华为常务董事、华为云CEO张平安发表演讲称,数字世界的话语权最终是由生态的繁荣决定的。

关键字: 华为 12nm 手机 卫星通信

要点: 有效应对环境变化,经营业绩稳中有升 落实提质增效举措,毛利润率延续升势 战略布局成效显著,战新业务引领增长 以科技创新为引领,提升企业核心竞争力 坚持高质量发展策略,塑强核心竞争优势...

关键字: 通信 BSP 电信运营商 数字经济

北京2024年8月27日 /美通社/ -- 8月21日,由中央广播电视总台与中国电影电视技术学会联合牵头组建的NVI技术创新联盟在BIRTV2024超高清全产业链发展研讨会上宣布正式成立。 活动现场 NVI技术创新联...

关键字: VI 传输协议 音频 BSP

北京2024年8月27日 /美通社/ -- 在8月23日举办的2024年长三角生态绿色一体化发展示范区联合招商会上,软通动力信息技术(集团)股份有限公司(以下简称"软通动力")与长三角投资(上海)有限...

关键字: BSP 信息技术
关闭
关闭