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,
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
Recently, it was found that an [!--empirenews.page--]unsymmetrical mono(styryl)amine fluorescent dopant, diphenyl-[4-(2-[1,
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,
(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
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
1Y. Kijima, N. Asai, and
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.
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--]
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_.
13S. A. Van Slyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett. 69, 2160 _1996_.
(該文章出自2008中國光電產(chǎn)業(yè)高層論壇論文集)
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