Display Institute, Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinchu, Taiwan 300
Abstract
A series of highly efficient blue dopants based on the iminodibenzyl-substituted distyrylarylene (IDB-DSA) fluorescent dye using the concept of steric compression of a rigidized and over-sized ring have been designed and synthesized. When doped in the stable 2-(methyl)-9,10-di(2-naphthyl)anthracene (MADN), the seven-membered DSA (7-DSA-Ph) doped device achieved a luminance efficiency of 9.1 cd/A at 20 mA/cm2 with a CIEx,y color coordinate of (0.16, 0.28) and a maximum external quantum efficiency of 4.8%. When used in a 2-element WOLED system, the 7-DSA-Ph doped device achieved a luminance efficiency of 11.0 cd/A at 20 mA/cm2 with a CIEx,y color coordinate of (0.29, 0.36).
1. Introduction
In recent years, there has been considerable interest in developing blue organic electroluminescence devices with high efficiency, deep blue color and long operational lifetime [1] for full color OLED applications. Recently, we had modified Kodak’s basic ADN structure and developed an improved host material, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) with a stable thin-film morphology and a wide energy bandgap [2]. When doped with p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), the luminance efficiency is 9.7 cd/A with CIEx,y color coordinate of (0.16, 0.32) [3]. But the y-value of CIEx,y color coordinate is still too high to be adequate for application in full color OLED display. To find a new stable blue dopant and its matching host which show better CIEx,y color coordinate value as well as high efficiency remains to be one of the most important research topics of today.
As presented in SID’04, if host materials of deeper blue color, such as 1,3,6,8-tetra(o-tolyl)pyrene (TOTP) was chosen, DSA-Ph doped emitter produced a luminance efficiency of 8.64 cd/A and a CIEx,y color coordinate of (0.15, 0.28) [4]. This result suggests that we can only modify y-value of the blue CIEx,y color coordinate slightly by selecting a wider bandgap host material.
But, further shift into the deep blue region will need to derive from additional molecular engineering of DSA-Ph dopant.
Recently in NCTU, we had been successful in the design and synthesis of a series of new blue dopants which contains a 7-membered N-heterocyclic amino-substituent (7-DSA-Ph) and found by high-speed computer molecular orbital simulation (DFT with B3LYP) that the emission wavelength could be shifted to deeper blue with increasing the number of phenyl moiety in the center core. We expect these new blue dopants will be potentially useful in producing a deep blue emission with a properly matched host and a 2-element white OLED system as well.
2. Experiment
Scheme 1 shows the synthesis of the 7-DSA-Ph which was prepared by coupling iminodibenzyl (IDB) and 4-bromobenzene with a palladium-catalyzed aromatic amination reaction [5]. After the reaction was completed, the intermediate (A) was purified by column chromatography then mixed with phosphoryl chloride in DMF at room temperature for 8 hr under nitrogen (Vilsmeier reaction) [6]. The mixture was quenched with sodium acetate and water to precipitate the grey solid which was purified by recrystallizing twice from ethanol to afford the key intermediate (B) as colorless crystal.
A mixture of p-xylylene dichride and triethyl phosphate was heated at 200 ℃ for 24 hr under nitrogen to produce the intermediate (C) (Arbuzov reaction) [7]. Finally, The novel blue dopant (7-DSA-Ph) is readily synthesized by the Horner–
Wadsworth–Emmons reaction according to a known procedure [8] from compounds (B) and (C) at room temperature in the presence of sodium tert-butoxide. The crude product was purified by chromatography to give pure 7-DSA-Ph as a yellow solid.
Scheme 1. Synthesis of blue dopant 7-DSA-Ph.
At the same time, we also want to study the structure-activity-relationship of materials by increasing the number of phenyl moiety in the molecular core. As a result, we designed and
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synthesized two IDB-DSA derivatives, biPh and 7-DSA-triPh. The chemical structures of DSA-Ph and new blue dopants are shown in Figure 1.
Figure 1. Structures of DSA-Ph, 7-DSA-Ph dopants and blue device.
All novel blue dpoants were further purified via train sublimation prior to spectroscopic measurement and device fabrication. UV-Vis and solution photoluminescence spectra were recorded in toluene by Hewlett Packard 8453 and Acton Research Spectra Pro-150, respectively. Electrochemical properties were studied by cyclic voltammetry using CHI 604A. The energy gap can be calculated from the edge of UV-Vis absorption peak. Melting points (Tm), glass transition temperatures (Tg), and crystallization temperatures (Tc) of the respective compounds were measured by differential scanning calorimetry (DSC) under nitrogen atmosphere using a SEIKO SSC 5200 DSC Computer/thermal analyzer.
Figure 1 also depicts the blue device structurein which CFx, 4,4’-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB), and tris(8-hydroxyquinolinolato)aluminum (Alq3) were used as the hole injection material [9], hole transport [1], and electron transport material, respectively. After a routine cleaning procedure, the indium-tin-oxide (ITO)-coated glass was loaded on the grounded electrode of a parallel-plate plasma reactor, pretreated by oxygen plasma, and then coated with a polymerized fluorocarbon film (CFx). Devices were fabricated under the base vacuum of about 10-6 Torr in a thin-film evaporation coater following a published protocol [10]. A multilayer structure of NPB/EML/Alq3/LiF/Al was deposited on the substrate by resistive heating with a thickness of 70, 40, 10, 1, and 200 nm for NPB, EML, Alq3, LiF, and Al, respectively. In the evaporation of EML, the fluorescent dopant was co-deposited at the designated optimal molar ratio. All devices were hermetically sealed prior to testing. The active area of the EL device, defined by the overlap of the ITO and the cathode electrodes, was 9 mm2. The current-voltage-luminance characteristics of the devices were measured with a diode array rapid scan system using a Photo Research PR650 spectrophotometer and a computer-controlled programmable dc source. The encapsulated device lifetime measurements were performed in a glove box at a constant drive current density of 20 mA/cm2.
3. Results and Discussion
The photo-physical, electrochemical and thermal properties of DSA-Ph and new blue dopants are summarized in Table 1. From our previous report [11], we found that the rigid 7-membered N-heterocycle iminodibenzyl (IDB) would increase the steric strain and cause the iminodibenzyl moiety to twist slightly out of the plane defined by the π-π conjugation of the distyryl benzene (see Figure 2). We found this steric-compression effect can cause the
Table 1. The photo-physical, electrochemical and thermal properties of DSA-Ph, Ph, biPh and 7-DSA-triPh.
Figure 2. Conformational Structures of DSA-Ph, DSA-Ph,
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blue emission wavelength of 7-DSA-Ph to hypsochromic-shift about 9 nm as compared to that of DSA-Ph. It was also found that the emission wavelength can be shifted to deeper blue with increasing number of phenyl moiety as the molecular core. The hypsochromic-shifted phenomenon can be rationalized from the conformational structures of the blue dopants by density functional theory (DFT) with B3LYP/6-31G(d) basis set of high-speed computer simulation shown in Figure 2 in which the added phenyl moiety appear to enlarge the twist angle and thus decrease the π-π conjugation. As a result, the effective conjugation length (chromophore) of the molecule is slightly shortened and shifts the emission wavelength to deeper blue. From the DSC result, the Tg of DSA-Ph, 7-DSA-Ph, 7-DSA-biPh and 7-DSA-triPh are found at 89 ℃, 119 ℃, 131 ℃ and 137 ℃, respectively, which indicates that the steric N-heterocycle aminosubstituent and the added phenyl moiety would also improve the material thermal stability.
The EL efficiency of the undoped MADN is 1.5 cd/A at 20 mA/cm2 with a CIEx,y color coordinate of (0.15, 0.10). When doped with 7-DSA-Ph, 7-DSA-biPh and 7-DSA-triPh, the EL efficiencies are increased to 9.1, 6.3, and 3.5 cd/A with CIEx,y color coordinate of (0.16, 0.28), (0.15, 0.24) and (0.16, 0.17) and EL spectra peak of 488 nm, 460 nm and 458 nm at 20 mA/cm2, respectively. The overall EL performances of the new blue dopants doped devices are summarized in Table 2. The maximum external quantum efficiency (E.Q.E) of 7-DSA-Ph doped device is close to the theoretical limit of 4.8% and the half-decay lifetime (t1/2) is 700 h with an initial brightness of 1976 cd/m2. Assuming the scalable law of Coulombic degradation [10] for driving at L0
of 100 cd/m2, the half-decay lifetime (t1/2) of the7-DSA-Ph doped device is projected to be over 13,000 h.
Table 2. EL performance of the blue doped devices (@ 20 mA/cm2).
In NCTU, we also developed white OLED structure incorporating a dual-layered emitting layer (EML) of blue and yellow to synthesize the white emission additively. The blue and yellow dopants used were DSA-Ph and 2,8-di(t-butyl)-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb) [12], respectively. The structures of MADN, TBRb and the 2-element WOLED device are depicted in Figure 3. The EL efficiency of the white device was 9.8 cd/A at 20 mA/cm2 and 7.9 V with a CIEx,y color coordinate of (0.31, 0.41). In this 2-element WOLED device, the sky-blue light emission was generated by doping the highly fluorescent DSA-Ph into the stable blue host molecule of MADN while the yellow emission was derived from the doping TBRb in NPB where the optimal concentration to obtain a white emission was 4%.
Figure 3. Structures of MADN, TBRb and 2-element WOLED device.
To improve the device performance, we replaced DSA-Ph for 7-DSA-Ph as the blue emitter in the 2-element WOLED system.
Figure 4 shows the EL spectra of the 7-DSA-Ph doped white device which achieved an EL efficiency of 11.0 cd/A at 20 mA/cm2 and 6.5 V with a CIEx,y color coordinate of (0.29, 0.36).
We attributed the improved result to the novel blue dopant, 7-DSA-Ph, which emits deeper blue light with higher efficiency then that of DSA-Ph. Therefore, when this new blue dopant was used in a 2-element WOLED system, it can achieve a high E.Q.E of 4.8% and generate a more balanced CIEx,y color coordinate of (0.29, 0.36) as well. The 2-element WOLED device performances are summarized in Table 3.
Figure 4. The EL spectra of 7-DSA-Ph doped 2-element WOLED device.
Table 3. EL performance of the DSA-Ph and 7-DSA-Ph doped 2-element WOLED devices (@ 20 mA/cm2).
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P-192 / M.-H. Ho 4. Conclusion
By molecular engineering of the di(styryl)amine-based structure, we have designed and synthesized a series of highly efficient blue dopants based on the iminodibenzyl-substituted distyrylarylene (IDB-DSA) compounds. The steric-compression effect and the added phenyl moiety in the core can shorten the effective conjugation length (chromophore) of the molecule and shift the emission wavelength to deeper blue. When Ph, 7-DSA-biPh and 7-DSA-triPh doped in the stable blue host material, MADN, the devices achieved a luminance efficiency of 9.1, 6.3, and 3.5 cd/A with CIEx,y color coordinate of (0.16, 0.28), (0.15, 0.24) and (0.16, 0.17) at 20 mA/cm2, respectively. The maximum external quantum efficiency (E.Q.E) of 7-DSA-Ph doped device is close to the theoretical limit of 4.8%. When 7-DSA-Ph was used in a 2-element WOLED system, the doped device achieved a luminance efficiency of 11.0 cd/A at 20 mA/cm2 with a CIEx,y
color coordinate of (0.29, 0.36).
5. Acknowledgements
This work was supported by the MOE Program for Promoting Academic Excellence of Universities under the grant (91-E-FA04-2-4-B) and National Science Council of Taiwan. We thank e-Ray Optoelectronics Technology Co., Ltd. of Taiwan for generously supplying some of the OLED materials studied in this work.
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