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Efficient Hole-Blocker with Electron Transporting Property and Its Applications in Blue Organic Light-Emitting Devices Silu Tao,*,†,‡ Shiu Lun Lai,‡ Junsheng Yu,† Yadong Jiang,† Yechun Zhou,‡ Chun-Sing Lee,‡ Xiaohong Zhang,*,§ and Shuit-Tong Lee*,‡ State Key Laboratory of Electronic Thin Films and Integrated DeVices, School of Optoelectronic Information, UniVersity of Electronic Science and Technology of China (UESTC), Chengdu 610054, China, Center of Super-Diamond and AdVanced Films (COSDAF), Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China, and Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: April 10, 2009; ReVised Manuscript ReceiVed: July 28, 2009
An aluminum complex, tri-(2-(2-hydroxyphenyl) benzoxazole) aluminum (AlLO3), has been synthesized and applied as an electron-transporting hole-blocker in organic light-emitting devices (OLEDs). Electron-transporting and hole-blocking properties of the compound have been investigated in detail. Performance of the blue OLEDs based on AlLO3 is considerably better than that of the devices based on tris-(8-quinolinolato) aluminum (Alq3), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and 2,2′,2′′-(Benzene-1,3,5-triyl)-tris(1-phenyl1H-benzimidazole) (TPBI) with a similar device structure. Introduction Since the first application of a multilayer thin-film structure in organic light-emitting devices (OLEDs) by Tang et al.,1 OLEDs have attracted a great deal of attention because of its wide applications in full-color flat-panel displays and solid-state lightings. Maintaining a balance between electron and hole currents in OLEDs is an important factor to achieve high performance.2,3 Because of the much higher mobility of holes than electrons in most organic materials, holes usually transport without hole-electron recombination through the hole transporting layer to the electron transporting layer (ETL) and even further to the cathode in OLEDs, causing decrease of device efficiency and lifetime.2,4 To confine and enhance hole-electron recombination in an emission layer (EML), application of a holeblocker between EML and ETL is beneficial for improving device efficiency and color purity.5-12 However, an extra holeblocking layer would inevitably lead to an increase in turn-on voltage and production cost. On the other hand, an efficient holeblocker with good electron-transporting properties would simultaneously combine the two functions and simplify the fabrication process.4,5 Until now, materials possessing both good electron-transporting and efficient hole-blocking properties were relatively rare.13-16 2,2′,2′′-(Benzene-1,3,5-triyl)-tris(1-phenyl1H-benzimidazole) (TPBI), 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP), and 4,7-diphenyl-1,10-phenanthroline (BPhen) are typical materials used both as electron-transporters and hole-blockers in OLEDs. One major drawback of the abovementioned materials is their inadequate thermal stability as indicated by their relatively low glass transition temperatures (Tg).4,7,11 Materials with low Tg used in OLEDs can crystallize easily under continuous operation or storage, which leads to * To whom correspondence should be addressed. Fax: (S.L.T.) +8628-83201745; (X.H.Z.) +86-10-62554670; (S.T.L.) +852-27844696. Email: (S.L.T.)
[email protected]; (X.H.Z.)
[email protected]; (S.T.L.)
[email protected]. † University of Electronic Science and Technology of China (UESTC). ‡ City University of Hong Kong. § Chinese Academy of Sciences.
decrease of device performance and lifetime.4 It is thus highly desirable to develop high-performance hole-blockers with good thermal stability and electron-transporting properties. In this work, we report the synthesis and characterization of a new aluminum complex, tri-(2-(2-hydroxyphenyl) benzoxazole) aluminum (AlLO3). Electron-transporting and holeblocking properties of AlLO3 have been investigated in detail. The performance of AlLO3 as an electron-transporting holeblocker in blue OLEDs were investigated and compared with several commonly used hole-blockers. Experimental Details Material Synthesis. Tri-(2-(2-hydroxyphenyl) benzoxazole) aluminum (AlLO3), aluminum isopropoxide (1.02 g, 5 mmol), and 2-(2-hydroxyphenyl) benzoxazole (3.38 g, 16 mmol) were added in ethanol (100 mL) and the mixture was refluxed for 10 h. Precipitate was collected and then recrystallized from chloroform twice. The title compound was obtained as a white powder with a yield of 67%. Ms: m/z 657 (M +). 1H NMR (CDCl3, 300 MHz) δ: 8.03-8.06 (m, 1H), 7.73-7.75 (m, 1H), 7.61-7.64 (m, 1H), 7.55-7.58 (m, 3H), 7.31-7.51 (m, 9H), 7.22-7.25 (m, 1H), 7.12-7.15 (m, 1H), 7.00-7.05 (m, 1H), 6.76-6.82 (m, 3H). 6.65-6.68 (m, 3H). Anal. Calcd for C 39H24N3O6Al: C, 71.23%; H, 3.68%; N, 6.39%. Found: C, 70.81%; H, 3.66%; N, 6.09%. Measurements and OLEDs Fabrication. Absorption and fluorescence spectra were recorded using a Perkin-Elmer Lambda 2S UV-vis spectrophotometer and a Perkin-Elmer LS50B Luminescence spectrophotometer, respectively. The highest occupied molecular orbital (HOMO) energy level was measured directly using ultraviolet photoelectron spectroscopy (UPS), while the lowest unoccupied molecular orbital (LUMO) value was estimated by subtracting from the HOMO value the lowest energy absorption edges of the UV absorption spectra. OLEDs were fabricated by vacuum deposition on ITO glass substrates with a sheet resistance of 30 Ω/square. Before deposition, the ITO substrate was carefully cleaned, dried in an oven at 120 °C for an hour, and finally treated with UV-
10.1021/jp903330z CCC: $40.75 2009 American Chemical Society Published on Web 09/01/2009
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Figure 1. Molecular structure of AlLO3. Figure 4. I-V-L curves of Device I and II. Inset shows the current efficiencies of Devices I and II.
Figure 2. Absorption and emission spectra of AlLO3 in solution and in film.
Figure 5. (a) Energy level diagram of Device III. (b) EL spectra of Devices III and PL spectra of NPB and AlLO3 film. (c) Energy level diagram of Device IV. (d) EL spectra of Device IV. Figure 3. EL spectra of Devices I and II.
ozone and then loaded into a deposition chamber. Devices were fabricated by evaporating organic layers onto the ITO substrate sequentially at an evaporation rate of 2-4 Å/s and a vacuum better than 5 × 10 -6 mbar. Mg:Ag alloy cathode was prepared by coevaporation of Mg and Ag at a volume ratio of 10:1. EL spectra and current-voltage-luminescence characteristics of OLEDs were measured with a Spectrascan PR650 photometer and a computer-controlled Keithley 236 Source-Meter under ambient conditions. Results and Discussion Figure 1 shows the molecular structure of the aluminum complex, AlLO3. The compound can be synthesized via a singlestep reaction with a good yield. The molecular structure was confirmed by 1H nuclear magnetic resonance, mass spectrometry, and element analysis, respectively. Figure 2 shows absorption and emission spectra of AlLO3 in solution and in film. AlLO3 shows a deep blue emission both
in dilute dichloromethane and in film with peaks centered at 412 and 421 nm respectively, and the emission peak position remains the same in n-hexane and tetrahydrofuran. The fluorescence quantum yield of AlLO3 in dilute dichloromethane solution is 0.14 as measured using 9,10-diphenylanthracene (Φ)0.9) as reference. HOMO level determined with UPS of AlLO3 is 6.2 eV. The LUMO level as determined from the HOMO level and the optical absorption edge is 3.2 eV. Considering its benzoxazole ligand and quite low HOMO level (6.2 eV), AlLO3 is expected to have both good electrontransporting and hole-blocking properties. To investigate the electron transporting properties of AlLO3, a device (Device I) with a configuration of ITO/NPB (50 nm)/ Alq3 (20 nm)/AlLO3 (30 nm)/LiF (0.5 nm)/MgAg was fabricated. In this device, indium tin oxide (ITO) and LiF/Mg:Ag are the anode and the cathode, respectively; 4,4′-bis[N-(1naphthyl)-N-phenyl amino] biphenyl (NPB) is the holetransporting layer (HTL). Tris-(8-quinolinolato) aluminum (Alq3) is the emitting layer. For comparison, a typical Alq3 device (Device II) with a configuration of ITO/NPB (50 nm)/ Alq3 (50 nm)/LiF (0.5 nm)/MgAg was fabricated with the same
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Figure 6. (a) Molecular structure of TBADN. (b) EL spectra of Alq3- and AlLO3-based blue emitting device (Device V).
total thickness as in Device I under the same conditions. As shown in Figure 3, EL spectra of the two devices show identical Alq3 emission centered at 520 nm. This confirms that AlLO3 can be used as an electron transporting material. Figure 4 shows the I-V-L curves of Devices I and II. The turn-on voltage (defined as the driving voltage to yield a brightness of 1 cd/m2) of Device I is 3.7 V. The current density of Device I is lower than that of Device II, which may be attributed to the lower HOMO level of AlLO3 (6.2 eV) than Alq3 (5.8 eV), leading to more hole blocking. Current efficiencies of the two devices are shown in the inset of Figure 4. The maximum current efficiency of Device I using AlLO3 as the ETL is 3.9 cd/A, which is comparable with that of Device II using Alq3 (3.8 cd/A). These results indicate that AlLO3 is an efficient electron-transporting material as Alq3. Due to the low HOMO level (6.2 eV) of AlLO3, which is close to those of common hole-blockers (TPBI 6.2 eV, BCP 6.4 eV and BPhen 6.4 eV), AlLO3 is expected to be an efficient hole-blocker. Device III with a configuration of ITO/NPB (60 nm)/ AlLO3 (40 nm)/LiF (0.5 nm)/Mg:Ag was fabricated to explore the application of AlLO3 as an electron-transporting hole-blocker. Figure 5a shows the energy level diagram of the device, while Figure 5b shows the EL spectrum of the device, in which the PL spectra of AlLO3 and NPB films are also shown for reference. The device gives a blue emission with a peak centered at 450 nm, which differs considerably from the PL spectrum of AlLO3 film (420 nm). Comparing the EL spectrum with the PL spectrum of NPB film, it suggests that the EL of Device III comes from NPB emission, and AlLO3 acts both as a hole-blocker and an ETL. To further confirm the hole-blocking property of AlLO3, Device IV with a configuration of ITO/NPB (60 nm)/rubrene: AlLO3 (1%, 40 nm)/LiF (0.5 nm)/Mg:Ag was fabricated. Figure 5c shows an energy level diagram of the device. It is known that rubrene is a good charge trapper, and the HOMO and the LUMO levels of rubrene are bracketed by those of AlLO3. If hole and electron recombine in the AlLO3 layer in Device IV, the emission should mainly come from rubrene emission, which is yellow. It can be seen that Devices IV has a blue emission with a maximum peak at 446 nm and CIE coordinates of x ) 0.16, y ) 0.11. It implies that the injected hole and electron mainly recombine in the NPB layer, and the AlLO3 layer acts as an efficient hole-blocker. To investigate the application of AlLO3 in OLEDs, blueemitting OLEDs (Device V) with a configuration of ITO/NPB (50 nm)/TBADN (20 nm)/ETL (30 nm)/LiF (0.5 nm)/Mg:Ag were fabricated, in which Alq3 and AlLO3 were used as ETL. 2-tert-Butyl-9,10-di(2-naphthyl) anthracene (TBADN) is the emitting material whose molecular structure is shown in Figure 6a. As shown in figure 6b, the EL spectrum of the AlLO3-based device exhibits a deep blue emission with a peak at 444 nm,
TABLE 1: Key Performance Parameters of Blue OLEDS Based on Different ETLs ETL
max current efficiency(cd/A)
EL λmax(nm)
Alq3 TPBIa BCPa AlLO3a
1.96 1.84 1.88 2.95
456 444 452 444
CIE (x,y) 20 mA/cm2 0.16, 0.15, 0.15, 0.15,
0.14 0.08 0.10 0.08
ref 17 this work this work this work
a Device structure: ITO/NPB(50 nm)/TBADN(20 nm)/ETL(30 nm)/LiF(0.5 nm)/Mg:Ag.
Figure 7. EL spectra of AlLO3-based blue device at different voltages.
which is almost the same as that of the PL spectrum of TBADN film (442 nm). However, the emission peak of the Alq3-based device centers at 456 nm, which is red shifted 14 nm from the PL peak of TBADN film. Moreover, the emission in longwavelength region coming from Alq3 emission was also observed in the Alq3-based device. As a result, CIE coordinates of the AlLO3-based device (x ) 0.15, y ) 0.08) are obviously better than those of the Alq3-based device (x ) 0.16, y ) 0.14) in terms of color purity. The key performance data of the devices are listed in Table 1. The EL spectra of AlLO3-based blue OLED at different voltages are shown in Figure 7, revealing that the EL spectra are virtually unchanged as the applied voltage increased from 5 to 10 V. The maximum current efficiency of the AlLO3-based device is 2.95 cd/A, which is much better than that of the Alq3-based device (1.96 cd/A).17 TPBI and BCP are widely employed both as electron transporter and hole blocker in OLEDs. For comparison, TPBI and BCP are used both as the ETL and the hole-blocker in device V. I-V-L properties of the blue OLEDs based on TPBI, BCP and AlLO3 are shown in Figure 8. Key performance parameters of the devices are also listed in Table 1. It shows the performance of the AlLO3-based device is considerably better than the devices based on TPBI and BCP. The results show that AlLO3 is a material with both good electrontransporting and hole-blocking properties.
Hole-Blocker with Electron Transporting Property
Figure 8. I-V-L properties of the blue OLEDs based on the ETLs.
J. Phys. Chem. C, Vol. 113, No. 38, 2009 16795
Figure 10. Efficiency-Current density of Device VI based on AlLO3 and BAlq.
Blue-emitting OLED based on AlLO3 was shown to perform much better than the devices based on Alq3, BCP, or TPBI. Acknowledgment. The work is supported by the Innovation and Technology Commission (Project No. ITP/011/08NP) of Hong Kong SAR and Natural Science Foundation of China (Grants 50773090, 50825304). The author Silu Tao thanks for the financial support from University of Electronic Science and Technology of China (UESTC). References and Notes
Figure 9. EL spectra of Device VI based on AlLO3 and BAlq.
To investigate the application of AlLO3 in phosphorescent OLEDs, blue phosphorescent OLEDs (Device VI) have been fabricated with the following structure: ITO/NPB (30 nm)/ 6%FIrpic:CBP (30 nm)/ETL (30 nm)/LiF (0.5 nm)/Al (80 nm). For comparison, the prototypical BAlq is used as both hole transporter and hole blocker in the device with the same device structure. Figure 9 gives the EL spectra of the two devices based on AlLO3 and BAlq. The El spectra of the two devices are almost identical, which show that AlLO3 works as the same function as BAlq in the phosphorescent OLED. Figure 10 gives the curves of the efficiency versus current density of the two devices. It shows the efficiency of AlLO3-based device is comparable with that of BAlq-based device. The results indicate that AlLO3 also can act as both an efficient electron transporter and a hole blocker in phosphorescent OLED. In summary, a new aluminum complex, tri-(2-(2-hydroxyphenyl) benzoxazole) aluminum has been synthesized and investigated. Electron-transporting and hole-blocking properties of AlLO3 have been investigated in detail. AlLO3 can act both as an efficient electron-transporter and an excellent hole-blocker. The applications of AlLO3 in blue OLEDs were investigated.
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