J. Phys. Chem. C 2008, 112, 14603–14606
14603
Highly Efficient Nondoped Blue Organic Light-Emitting Diodes Based on Anthracene-Triphenylamine Derivatives Silu Tao,†,‡ Yechun Zhou,† Chun-Sing Lee,† Shuit-Tong Lee,*,† Da Huang,§ and Xiaohong Zhang*,§ Center of Super-Diamond and AdVanced Films (COSDAF), Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China, School of Optoelectronic Information, UniVersity of Electronic Science and Technology of China (UESTC), Chengdu 610054, China, and Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: May 5, 2008; ReVised Manuscript ReceiVed: July 3, 2008
Blue light-emitting anthracene derivatives end-capped with triphenylamine for efficient hole transportation have been designed and synthesized using two-step Suzuki coupling reactions. The compounds possess high glass transition temperatures for good thermal stability and strong blue emission in solution. Typical threelayer organic light-emitting devices (OLEDs) made from these compounds show highly efficient blue emission, which are better than or comparable to state-of-the-art fluorescent OLEDs performance. For example, 9-pyrenyl10-(4-triphenylamine) anthrancene (PAA)-based nondoped device exhibits efficient blue emission with a maximum efficiency up to 7.9 cd/A (or 6.8 lm/W). Based on the good hole transport of the anthracenetriphenylamine derivatives, deep blue emitting devices with high efficiency were achieved by using the derivatives as both emitter and hole transporter. Introduction (OLEDs)1,2
have been attracting Organic light-emitting diodes much attention due to their huge application potential in flatpanel displays and solid-state lightings. To produce highly efficient OLEDs, a key approach is to develop high-performance materials including RGB (red, green, blue) emitters with desirable device properties. Blue-emitting materials can deliver not only blue emission but also white and other color emission via dopant emitters through energy conversion.3-5 There have been many reports on blue-emitting OLEDs, including those using blue host emitters and blue dopants.6-21 While dopant emitters can substantively improve electroluminescence (EL) efficiency, addition of dopants would invariably introduce extra complexity and cost for mass production.22 Moreover, heatinduced phase separation of dopant and host can be a serious degradation issue in some dopant-host systems.23 Thus, for some applications (e.g., operation at high temperatures), nondoped host-emitting OLED is desirable and can offer advantages over those doped OLEDs. Among the blue-emitting materials, anthracene derivatives are particularly attractive due to their excellent photoluminescence and electroluminescence properties.6-16 Additionally, triaylamines or their moieties have been incorporated into molecules, such as fluorene cores, to improve hole injection and carrier transport properties.24-26 In this Article, we report the synthesis and characterization of a new series of triphenylamine end-capped anthracence derivatives, 9-phenyl-10-(4-triphenylamine)anthrancene (PhAA), 9-naphthyl-10(4-triphenylamine)anthrancene (NAA), and 9-pyrenyl-10-(4* Corresponding author. Fax: +852-27844696 (S.-T.L.); +86-1062554670 (X.Z.). E-mail:
[email protected] (S.-T.L.); xhzhang@ mail.ipc.ac.cn (X.Z.). † City University of Hong Kong. ‡ UESTC. § Chinese Academy of Sciences.
triphenylamine)anthrancene (PAA). These compounds show blue emission with moderate fluorescence quantum yields in solutions and films. OLEDs based on these compounds have been fabricated in a nondoped device structure; PAA-based device exhibits highly efficient sky-blue emission with a maximum efficiency of 7.9 cd/A (6.8 lm/W). In addition, efficient hole-transport-layer-free OLEDs based on these compounds have been demonstrated; PhAA-based nondoped HTL-free device gives deep blue emission with a maximum efficiency of 3.0 cd/A (2.4 lm/W) and CIE coordinates of (0.14, 0.14). Experimental Details Material Synthesis. The anthracene derivatives were synthesized according to the procedures in Scheme 1 using twostep Suzuki coupling reactions. All solvents used in the reaction were purified by routine procedures. Other reagents in the scheme were used as received from commercial sources. 9-Bromo-10-phenylanthracene (1). 9,10-Dibromoanthrancene (5 mmol), phenyl boronic acid (5 mmol), Pd (PPh3)4 (0.5 mmol), aqueous Na2CO3 (2.0 M, 15 mL), ethanol (10 mL), and toluene (30 mL) were mixed in a flask. The mixture was degassed and refluxed for 24 h under a nitrogen atmosphere. After being cooled, the solvent was evaporated under vacuum and the product was extracted with dichloromethane (CH2Cl2). The CH2Cl2 solution was washed with water and dried with MgSO4. Evaporation of the solvent, followed by column chromatography on silica gel, gives a white yellow product. Yield: 85%. MS (m/z): 332 (M+). 9-Bromo-10-naphthylanthracene (2). The synthetic procedure is similar to that of compound 1 and gives a yellow product. Yield: 79%. MS (m/z): 382 (M+). 9-Bromo-10-pyrenylanthracene (3). The synthetic procedure is similar to that of compound 1 and gives a yellow powder product. Yield: 68%. MS (m/z): 456 (M+).
10.1021/jp803957p CCC: $40.75 2008 American Chemical Society Published on Web 08/22/2008
14604 J. Phys. Chem. C, Vol. 112, No. 37, 2008
Tao et al.
SCHEME 1: Synthesis Routes for PhAA, NAA, and PAA
9-Phenyl-10-(4-triphenylamine)anthrancene (PhAA). Compound 1 (2.5 mmol), triphenyl boronic acid (2.5 mmol), Pd (PPh3)4 (0.3 mmol), aqueous Na2CO3 (2.0 M, 10 mL), ethanol (5 mL), and toluene (15 mL) were mixed in a flask. The mixture was degassed and refluxed for 36 h under nitrogen. After being cooled, the solvent was evaporated under vacuum and the product was extracted with dichloromethane (CH2Cl2). Evaporation of the solvent, followed by column chromatography (hexane:CH2Cl2, 5:1) on silica gel, gives PhAA as a white product. Yield: 82%. 1H NMR (CDCl3, 300 MHz): δ 7.07-7.12 (t, 2H), 7.30-7.42 (m, 16H), 7.47-7.50 (d, 2H), 7.56-7.64 (m, 3H), 7.68-7.71 (d, 2H), 7.84-7.87 (d, 2H). MS (m/z): 497 (M+). Anal. Calcd for C38H27N: C, 91.72; H, 5.47; N, 2.81. Found: C, 91.48; H, 5.42; N, 2.81. 9-Naphthyl-10-(4-triphenylamine)anthrancene (NAA). The synthetic procedure is similar to that of PhAA and gives a yellow powder product. Yield: 78%. 1H NMR (CDCl3, 300 MHz): δ 7.07-7.11 (t, 2H), 7.25-7.42 (m, 16H), 7.59-7.62 (m, 3H), 7.70-7.73 (d, 2H), 7.87-7.93 (m, 3H), 7.98 (s, 1H), 8.01-8.09 (m, 2H). MS (m/z): 547 (M+). Anal. Calcd for C42H29N: C, 92.11; H, 5.34; N, 2.56. Found: C, 91.40; H, 5.56; N, 2.51. 9-Pyrenyl-10-(4-triphenylamine)anthrancene (PAA). The synthetic procedure is similar to that of PhAA and gives a yellow powder product. Yield: 75%. 1H NMR (CDCl3, 300 MHz): δ 7.08-7.13 (t, 3H), 7.19-7.24 (t, 3H), 7.30-7.49 (t, 15H), 7.82-7.85 (d, 1H), 7.93-7.96 (d, 2H), 8.01-8.16 (t, 3H), 8.20-8.28 (t, 3H), 8.38-8.42 (d, 1H). MS (m/z): 621 (M+). Anal. Calcd for C48H31N: C, 92.72; H, 5.03; N, 2.25. Found: C, 92.17; H, 4.82; N, 2.18. 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. Glass transition temperatures (Tg) were determined from differential scanning calorimeters (DSC) using a Perkin-Elmer DSC7 at a heating rate of 20 °C min-1. The highest occupied molecular orbital (HOMO) values were measured directly by using ultraviolet photoelectron spectroscopy (UPS), while the lowest
unoccupied molecular orbital (LUMO) values were determined from the HOMO and the lowest energy absorption edge of the UV absorption spectra. OLEDs were fabricated by vacuum deposition on ITO glass substrates with a sheet resistance of 30Ω cm-2. Before deposition, the ITO substrate was carefully cleaned, then dried in an oven at 120 °C, and finally treated with UV-ozone and then loaded into a deposition chamber. The devices were fabricated by evaporating organic layers onto the ITO substrate sequentially with an evaporation rate of 2-4 Å/s and a pressure better than 5 × 10-6 mbar. The Mg:Ag alloy cathode was prepared by coevaporation of Mg and Ag at a volume ratio of 10:1. EL spectra and CIE color coordinates were measured with a Spectrascan PR650 photometer, and the current-voltage-luminescence characteristics were measured with a computer-controlled Keithley 236 SourceMeter under ambient atmosphere. Results and Discussion Molecular structures of the triphenylamine end-capped anthracene derivatives and their synthetic routes are shown in Scheme 1. The compounds can be obtained with high yields using typical two-step Suzuki coupling reactions according to the reported procedure.27 The molecular structures of the compounds were confirmed by 1H nuclear magnetic resonance, mass spectrometry, and element analysis. Figure 1 shows the absorption and fluorescence spectra of the three compounds in dilute n-hexane solution. All absorption spectra have two major bands, with similar absorption peaks in the range from 350 to 400 nm. These characteristic vibronic patterns can be attributed to the π-π* transitions of the middle anthracene core of the compounds. The absorption bands at 300-340 nm come from the combination of the n-π* transition of triphenylamine moieties and the π-π* transitions of the substituted aryl groups on the anthracene core. All three compounds exhibit blue emission in solution with a maximum peak at 438-444 nm. Both the absorption and the photoluminescence (PL) spectra of these compounds in films show slight
Efficient Nondoped Blue Organic Light-Emitting Diodes
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14605
Figure 2. EL spectra of PhAA-, NAA-, and PAA-based devices I. Figure 1. Absorption and emission spectra of the compounds in solution.
TABLE 2: Summary of Device Data for PhAA-, NAA-, and PAA-Based Devicesa
TABLE 1: Summary of Physical Measurements of PhAA, NAA, and PAA
maximum maximum CIE EL current power peak efficiency efficiency coordinates turn-on (x, y) (cd/A) (lm/w) voltage (v) (nm)
λmaxabsa λmaxemb λmaxabsc λmaxemd HOMOf LUMOf Tgg nm nm nm nm ΦFLe eV eV °C PhAA NAA PAA
373 397 399
438 439 444
380 404 405
457 462 468
0.44 0.46 0.48
5.70 5.66 5.63
2.94 2.91 2.93
104 129 162
a Maximum absorption wavelength in solution. b Maximum emission wavelength in solution. c Maximum absorption wavelength in film. d Maximum emission wavelength in film. e Fluorescence quantum yields in solution, using 9,10-diphenyl-anthracence (Φ360 ) 0.9) as a standard. f Obtained from ultraviolet photoelectron spectroscopy (UPS) measurement and absorption spectra. g Obtained from DSC measurements.
red shifts relative to those in solution, indicating intermolecular interactions in their solid states. The compounds give moderate fluorescence quantum yields of 0.44-0.48 in dilute dichloromethane solution (using 9,10-diphenylanthracene (Φ ) 0.9) as the reference) going from PhAA to PAA. The photophysical and thermal properties of these compounds are summarized in Table 1. These compounds show high glasstransition temperature (Tg) in the range of 104-162 °C, as determined from DSC measurements. As the substituent changes from a phenyl group in PhAA to a pyrenyl group in PAA, Tg increases from 104 to 162 °C, suggesting increasing thermal stability of the compound. To investigate the potential applications of these blue-emitting compounds, nondoped devices I with the typical three-layer structure of ITO/NPB (50 nm)/EML (20 nm)/TPBI (30 nm)/ LiF/MgAg were fabricated. In these devices, ITO (indium-tinoxide) and LiF/Mg:Ag are the anode and the cathode, respectively, 4,4′-bis[N-(1-naphthyl)-N-phenyl amino] biphenyl (NPB) is the hole transporting layer (HTL), 2,2′,2′′-(benzene-1,3,5triyl)-tris(1-phenyl-1H-benzimidazole) (TPBI) is the electron transporting layer (ETL), and the new compounds are used as the emitting layer. Figure 2 shows the electroluminescence (EL) spectra of devices I. The devices exhibit blue to sky-blue emission with peak maxima at 470-480 nm and CIE coordinates of (0.14, 0.20) to (0.15, 0.30). The EL spectra of the devices have little difference except for about a 10 nm red-shift as compared to their PL spectra in films, indicating that the EL emission originates from the singlet excited states of the compounds. The performances of these devices are summarized in Table 2. Figure 3 shows current efficiency versus current density of the devices. PhAA-based device has a maximum luminescence efficiency of 4.0 cd/A (4.0 lm/W) with CIE coordinates of (0.14, 0.20). The maximum luminescence efficiency of PAA-based device is 7.9 cd/A (6.8 lm/W) and is much higher than that of the other devices. Significantly, we point out that the efficiency
PhAA I II NAA I II PAA I II
3.2 3.2 2.9 2.9 2.9 3.0
470 460 472 468 480 476
4.0 3.0 5.5 3.4 7.9 6.1
4.0 2.4 3.5 3.1 6.8 5.1
(0.14, 0.20) (0.14, 0.14) (0.14, 0.21) (0.14, 0.17) (0.15, 0.30) (0.15, 0.28)
a Device I: ITO/NPB (50 nm)/EML (20 nm)/TPBI (30 nm)/LiF/ MgAg. Device II: ITO/EML (50 nm)/TPBI (30 nm)/LiF/MgAg.
Figure 3. Current efficiency versus current density of devices I.
of the three devices is among the best values ever reported for nondoped blue fluorescence OLEDs. Furthermore, the devices show only very mild decreases in efficiency with increasing current density. This characteristic is useful for high-brightness applications. Figure 4 gives the current density-voltage-brightness plots of the three devices. The turn-on voltage (at a brightness of 1 cd/m2) is very low, ranging from 2.9 to 3.2 V. The low turn-on voltage is partially attributed to the efficient hole injection from HTL to EML due to the low energy barrier between the two layers. The incorporation of the triphenylamine group is expected to lead to efficient hole transportation in these compounds. The HOMO levels of the three compounds, as determined by UPS, are 5.70, 5.66, and 5.63 eV, respectively. The high HOMO levels suggest that these compounds may serve as both the hole transporter layer (HTL) and the light emitter layer (EML) in a two-layer hole-transporting-layer-free device architecture. This expectation is examined by fabricating two-layer devices II with a structure of ITO/compound (50 nm)/TPBI (30 nm)/LiF/MgAg, in which the compounds act as both the HTL and the EML. The two-layer devices show blue emission with peaks at 460, 468, and 476 nm for the PhAA-, NAA-, and PAA-based devices, respectively. Because of the differences in the device structure, EL spectra of device show a 10 nm blue-shift as compared to
14606 J. Phys. Chem. C, Vol. 112, No. 37, 2008
Tao et al. Conclusion Triphenylamine end-capped anthracene derivatives for efficient blue emission and hole transport have been synthesized and characterized. The compounds show high glass transition temperatures (Tg) of 104-162 °C for good thermal stability, and strong blue emission with quantum yields of 0.44-0.48. Typical three-layer OLED devices made from these compounds show excellent device performance; the PAA-based device exhibits highly efficient sky-blue emission with a maximum efficiency of 7.9 cd/A (6.8 lm/W) in a nondoped device structure. In addition, efficient deep-blue emissions have been achieved using a HTL-free two-layer device architecture based on these compounds. A maximum efficiency of 3.0 cd/A (2.4 lm/W) and CIE coordinates of (0.14, 0.14) have been achieved in a PhAA-nondoped HTL-free device. The efficiency of these devices is among the best results ever reported for nondoped fluorescent blue OLEDs. Acknowledgment. This work was supported by the Innovation and Technology Commission (No. GHP/023/05), KSAR, and National Natural Science Foundation of China (Grant 50773090), People’s Republic of China. References and Notes
Figure 4. Current density-voltage-brightness plots of (a) PhAA-, (b) NAA-, and (c) PAA-based devices I.
Figure 5. EL efficiency versus current density of devices II.
that of device I. The lack of the NPB layer in device II should let more exciton form at the interface of the EML/TPBI, and this will lead to more emission from TPBI. Thus, the efficiency of device II will be decreased, and the EL spectra will be blueshifted as compared to device I. Figure 5 gives the EL efficiency versus current density of the devices. Indeed, the PhAA-based device shows efficient deep-blue emission with a maximum EL efficiency of 3.0 cd/A (maximum power efficiency of 2.4 lm/ W) and CIE coordinates of (0.14, 0.14). Similarly, the NAAbased device gives a maximum efficiency of 3.4 cd/A (3.1 lm/ W) with CIE coordinates of (0.14, 0.17), while the PAA-based device yields 6.1 cd/A (5.1 lm/W) with CIE coordinates of (0.15, 0.28). Importantly, the PhAA-based device shows little shift in EL color with drive currents as CIE coordinates only change from (0.1385, 0.1436) at 1 mA/cm2 to (0.1403, 0.1408) at 190 mA/cm2. This stable emission color under different current densities suggests that charge carriers are efficiently recombined in the emitter.
(1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Burroughes, J. H.; Bradley, D. D. C.; Broun, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539. (3) Sun, Y.; Giebink, N. C.; Knanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908. (4) Chuen, C. H.; Tao, Y. T. Appl. Phys. Lett. 2004, 85, 4609. (5) Tao, S.; Zhang, X.; Lee, C.-S.; Lee, S.-T. Appl. Phys. Lett. 2007, 91, 013507. (6) Shen, W.-J.; Dodda, R.; Wu, C.-C.; Wu, F.-I.; Liu, T.-H.; Chen, C.-H.; Shu, C.-F. Chem. Mater. 2004, 16, 930. (7) Kim, Y.-H.; Shin, D.-C.; Kwon, S.-K. AdV. Mater. 2001, 13, 1690. (8) Shi, J.; Tang, C. W. Appl. Phys. Lett. 2002, 80, 3201. (9) Ho, M.-H.; Wu, Y.-S.; Wen, S.-W.; Chen, T.-M.; Chen, C.-H. Appl. Phys. Lett. 2007, 91, 083515. (10) Lee, M.-T.; Liao, C.-H.; Tsai, C.-H.; Chen, C.-H. AdV. Mater. 2005, 17, 2493. (11) Tao, S.; Xu, S.; Zhang, X. Chem. Phys. Lett. 2006, 429, 622. (12) Kan, Y.; Wang, L.; Duan, L.; Hu, Y.; Wu, G.; Qiu, Y. Appl. Phys. Lett. 2004, 84, 1513. (13) Peng, Z.-K.; Tao, S.-L.; Zhang, X.-H.; Tang, J.-X.; Lee, C.-S.; Lee, S.-T. J. Phys. Chem. C 2008, 112, 2165. (14) Gao, Z.-Q.; Mi, B.-X.; Chen, C.-H.; Cheah, K.-W.; Cheng, Y.-K.; Wen, W.-S. Appl. Phys. Lett. 2007, 90, 123506. (15) Danel, K.; Huang, T.-H.; Lin, J.-T.; Tao, Y.-T.; Chuen, C.-H. Chem. Mater. 2002, 14, 3860. (16) Shih, P.-I.; Chuang, C.-Y.; Chien, C.-H.; Diau, E. W. G.; Shu, C.F. AdV. Funct. Mater. 2007, 17, 3141. (17) Tao, S.; Peng, Z.; Zhang, X.; Wang, P.; Lee, C.-S.; Lee, S.-T. AdV. Funct. Mater. 2005, 15, 1716. (18) Luo, J.; Zhou, Y.; Niu, Z.-Q.; Zhou, Q. F.; Ma, Y.; Pei, J. J. Am. Chem. Soc. 2007, 129, 11314. (19) Xu, X.; Chen, S.; Yu, G.; Di, C.-A.; You, H.; Ma, D.; Liu, Y. AdV. Mater. 2007, 19, 1281. (20) Chiechi, R. C.; Tseng, R. J.; Marchioni, F.; Yang, Y.; Wudl, F. AdV. Mater. 2006, 18, 325. (21) Wei, Y.; Chen, C.-T. J. Am. Chem. Soc. 2007, 129, 7478. (22) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. (23) Gong, J.-R.; Wan, L.-J.; Lei, S.-B.; Bai, C.-L.; Zhang, X.-H.; Lee, S.-T. J. Phys. Chem. B 2005, 109, 1675. (24) Li, Z. H.; Wong, M. S.; Tao, Y.; Fukutani, H. Org. Lett. 2007, 9, 3660. (25) Huang, T.-H.; Lin, J.-T.; Chen, L.-Y.; Lin, Y.-T.; Wu, C.-C. AdV. Mater. 2006, 18, 602. (26) Liao, Y.-L.; Lin, C.-Y.; Wong, K.-T.; Hou, T.-H.; Huang, W.-Y. Org. Lett. 2007, 9, 4511. (27) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
JP803957P
