High-efficiency red organic light emitting diodes Incorporating 1,3,5-tris(1-pyrenyl)benzene as the host material

High-efficiency Red Organic Light Emitting Diodes

Incorporating 1,3,5-Tris(1-pyrenyl)benzene as the Host Material

Mei-Ying Chang,a,

*

*

Wen-Yao Huanga

a

Institute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan b

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan

We have developed high-efficiency red organic light emitting diodes共OLEDs兲 incorporating 1,3,5-tris共1-pyrenyl兲benzene 共TPB3兲 as the host material and 4-共dicyanomethylene兲-2-tert-butyl-6-共1,1,7,7-tetramethyljulolidyl-9-enyl兲-4H-pyran 共DCJTB兲 as the dopant. The highly efficient energy transfer, which arose as a result of 共i兲 perfect overlap between the photoluminescence spectrum of TPB3 and the absorption spectrum of DCJTB and 共ii兲 the high fluorescence quantum yield of TPB3, allowed us to fabricate red OLEDs exhibiting improved efficiency. A device having the configuration of indium-tin oxide/

N,N⬘-bis共1-naphthyl兲-N,N⬘-diphenyl-1,1⬘-biphenyl-4,4⬘-diamine_/TPB3:DCJTB共2%兲/LiF/Al exhibited a maximum luminance at 13.5 V of 70,600 cd/m2_{, ca. four times higher than that of the device incorporating Alq}

3 as the host material at the same

potential. The device’s current efficiency was 4.38 cd/A; its power efficiency was 2.12 lm/W at 20 mA/cm2_{. The current and}

power efficiencies were greater than 4 cd/A and 1 lm/W, respectively, over a large range of potentials 共3.5–13.5 V兲, with good Commission Internationale de l’Eclairage coordinates of共0.63, 0.37兲. These results indicate that searching for a suitable host material is a promising approach toward achieving high-efficiency red OLEDs.

© 2008 The Electrochemical Society. 关DOI: 10.1149/1.2981048兴 All rights reserved.

Manuscript submitted May 29, 2008; revised manuscript received August 19, 2008. Published October 6, 2008.

Although organic light emitting diodes 共OLEDs兲1 are used widely in flat-panel displays because of their superior brightness, high contrast, wide viewing angle, rapid response, and low produc-tion cost, the poor color purity and low efficiency of red OLEDs have limited the development of organic full-color displays. A doping system1-5is often required in red OLEDs to obtain higher efficiencies and to prevent concentration quenching.6 Several red fluorescent dyes, including 4- 共dicyanomethylene兲-2-methyl-6-p-共dimethylamino兲styryl-4H-pyran 共DCM兲, 4-dicyanomethylene-2-methyl6-共2-共2,3,6,7-tetrahydro-1H,5H-benzo兲 关ij兴-quinolizin-8-yl兲-4H-pyran 共DCM2兲, 4-共dicyanomethylene兲-2-methyl-6-共1, 1, 7 ,7-tetramethyljulolidyl9-enyl兲-4H-pyran共DCJT兲, and 4-共dicyanometh-ylene兲-2-tert-butyl-6-共1,1,7,7-tetramethyljulolidyl-9-enyl兲-4H-pyran 共DCJTB兲,6-11

have been studied as red dopants for OLED applica-tions, usually in conjunction with tris共8-hydroxyquinolinato兲 aluminum共Alq3兲 as the host material because of its high stability

and good carrier-transport properties. Nevertheless, energy transfer from the Alq₃host to, for example, DCJTB, is not optimal because of poor overlap between the photoluminescence共PL兲 spectrum of the former and the absorption spectrum of the latter. As a result, the efficiency of such a doping system is too low for practical applica-tions. Although the use of rubrene as an assistant dopant12-17aids the energy transfer from Alq₃to DCJTB and improves the optical and electrical properties of the resulting red OLEDs, the preparation of such devices requires the evaporation of all three materials simulta-neously, which can be difficult to control. Chen et al.18and Qui et al.19found that use of tris共8-hydroxyquinoline兲gallium 共Gaq₃兲 and bis共salicylidene-o-aminophenolato兲bis共8-quinolinoato兲bisgallium共III兲 关Ga2共saph兲2q2兴, respectively, as host materials improved the

effi-ciency relative to that of the doped Alq3device. Although the

de-velopment of these host materials is a promising approach toward improved performance, their current and power efficiencies at 20 mA/cm2_{are less than 3 cd/A and 1 lm/W, respectively, which,}

for red OLEDs, is unsatisfactory. The main issue in those systems is incomplete energy transfer from the host to the dopant.

There are, however, several red phosphorescent emitters, includ-ing iridium20,21and platinum porphyrin22,23complexes, that provide high efficiency and brightness, but they perform well only in devices

operated at low current densities. In most cases, the long lifetimes of triplet-state excitons lead to triplet–triplet annihilation. As a result, the emission is quenched even at medium current densities. Red OLEDs fabricated with such emitters usually fall short of the effi-ciency and chemical stability required for commercial applications, even though the current efficiency can reach as high as 29 cd/A.20 Recently, an osmium complex has been reported for use in red-emitting phosphorescent devices exhibiting a high current efficiency 共29.9 cd/A兲 with Commission Internationale de l’Eclairage 共CIE兲 coordinates of共0.64, 0.36兲.24The current efficiency of fluorescent red OLEDs based on Alq₃and 2-methyl-9,10-di共2-napthyl兲 anthra-cene共MADN兲 as cohost and DCJTB as the doped material reaches 5.42 cd/A at 20 mA/cm2_{with CIE coordinates of共0.63, 0.37兲;}25_this

device is one of the best fluorescent red OLEDs reported to date. There is still, therefore, much room for improvement in the effi-ciency of fluorescent red OLEDs. To do so, the host and doped materials must have good carrier-transport properties and undergo efficient energy transfer, which arises as a result of共i兲 perfect over-lap between the PL spectrum of the host material and the absorption spectrum of the doped material and共ii兲 a high fluorescence quantum yield of the host material.

In this study, we describe the use of 1,3,5-tris共1-pyrenyl兲benzene 共TPB3兲 as a host material that undergoes efficient energy transfer to the red dopant DCJTB. We fabricated a red OLED based on the single host, having the configuration of indium-tin oxide 共ITO兲/N,N

⬘

⬘

⬘

⬘

nm兲/LiF 共0.8 nm兲/Al 共200 nm兲 that exhibited a maximum lumi-nance at 13.5 V of 70,600 cd/m2_{, ca. four times higher than that of}

the device incorporating Alq3 as the host material at the same

po-tential. The doped TPB3 device displayed a high current efficiency of 4.38 cd/A and a power efficiency of 2.12 lm/W at 20 mA/cm2_;

these efficiencies remained higher than 4 cd/A and 1 lm/W, respec-tively, over a wide range of potentials共3.5–13.5 V兲, with good CIE coordinates of共0.63, 0.37兲.

Experimental

Figure 1 presents the molecular structures of TPB3, which was prepared according to published procedures,26and DCJTB. The de-vices were fabricated on glass substrates layered with 130 nm ITO having a sheet resistance of ca. 13⍀/䊐. The substrates were cleaned through ultrasonication in isopropyl alcohol and deionized water, followed by treatment with oxygen plasma. All of the organic

*Electrochemical Society Active Member.

z

E-mail: [email protected]

Journal of The Electrochemical Society, 155共12兲 J345-J349 共2008兲

0013-4651/2008/155共12兲/J345/5/$23.00 © The Electrochemical Society J345

layers and the LiF/Al layers were evaporated onto the substrates at 10−6_{Torr without breaking the vacuum. The rates of evaporation}

and thicknesses of the organic and metal layers were monitored using quartz oscillators. The device structure was ITO/ NPB 共65 nm兲/TPB3:DCJTB 共1,1.5,2,2.5,or 3 wt %, 40 nm兲/Alq3

共30 nm兲/LiF 共0.8 nm兲/Al 共200 nm兲. NPB and Alq3 were used as

hole and electron transporting layers, respectively. As a reference, a device was prepared incorporating Alq₃ as the host material; its configuration was ITO/NPB共65 nm兲/Alq₃:DCJTB 共2 wt %,40 nm兲/Alq₃共30 nm兲/LiF 共0.8 nm兲/Al 共200 nm兲. The emission layer was fabricated through coevaporation of DCJTB with the host ma-terial; the concentrations共wt %兲 of the dopant material were esti-mated from the ratio of the evaporation rates. ITO and LiF/Al were employed as the anode and cathode, respectively. The thickness was measured using a Dektak 150 surface profiler. The PL and absorp-tion spectra were measured using a Perkin–Elmer FL LS55 fluores-cence spectrophotometer and a Perkin–Elmer Lambda35 UV-visible spectrometer, respectively. The energy levels were determined using a Riken KeiKi AC-2 photoelectron spectrometer. The electrolumi-nescence共EL兲 spectra and CIE coordinates of the devices were mea-sured using a PR650 spectrometer. The fluorescence quantum yield was measured using a Hamamatsu C9920-02 absolute PL quantum-yield measurement system. The luminance–current density–voltage characteristics were recorded simultaneously with the EL spectra by combining the spectrometer with a Keithley 2400 programmable voltage–current共V-I兲 source after the devices had been sealed in a nitrogen-filled glove box. All measurements were recorded at room temperature under ambient conditions. The active area of the device was 3 mm2_.

Results and Discussion

Photophysical properties.— In this study, we used TPB3 as a host material for two reasons: 共i兲 most triarylbenzenes and tetraarylbenzenes26 have high glass transition temperatures 共Tg

⬎ 130°C兲; among them, TPB3 possesses particularly good thermal stability共T_g= 165°C兲; 共ii兲 the fluorescence quantum yield of TPB3 共0.8兲 is four times of that of Alq3 共0.2兲. In addition to these two

factors, another critical issue is the effect of energy transfer from the host material to the dopant.

One method of determining the effect of energy transfer is through calculating the Förster radius共R0兲, defined as the distance

between the host material and the dopant material at which the prob-ability of relaxation of the host material via energy transfer equals that through other processes. A large Förster radius indicates effi-cient energy transfer. The Förster radius can be calculated using the equation27 R₀6= 0.5291␬ 2_␾ D n4_N

冕

where the value of␬ depends on the relative orientation of the host and dopant materials’ dipoles共␬2_{= 2/3 for randomly oriented}

di-poles兲, n is the refractive index of the medium, N is Avogadro’s number,␾Dis the fluorescence quantum yield of the host material, F_D共␯兲 is the normalized emission spectrum of the host material, ␧A共␯兲 is the molar extinction coefficient of the dopant material, and

␯ is the energy, expressed in wavenumbers.

From Eq. 1, it is clear that good overlap between the emission spectrum of the host material and the absorption spectrum of the dopant material is an important factor affecting efficient energy transfer, as is a high fluorescence quantum yield of the host material. Figure 2 displays the optical absorption spectrum of DCJTB and the PL spectra of TPB3 and Alq₃. There is a much better overlap be-tween the PL spectrum of TPB3 and the absorption spectrum of DCJTB than that between the PL spectrum of Alq3and the

absorp-tion spectrum of DCJTB. The fluorescence quantum yield of TPB3 共0.8兲 is four times of that of Alq3 共0.2, Table I兲. The calculated

Förster radius between TPB3 and DCJTB共63 Å兲 is larger than that between Alq3and DCJTB共47 Å兲 because of the improved spectral

overlap of the former pair and the high quantum yield of

400 450 500 550 600 650 700 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Intensity (a.u. ) Wavelength (nm) PL of TPB3 PL of Alq3 abs of DCJTB

Figure 2. PL spectra of TPB3 and Alq3 and the absorption spectrum of

DCJTB. TPB3 Alq3 DCJTB N O CN NC

Figure 1. Chemical structures of TPB3,

Alq₃, and DCJTB.

J346 Journal of The Electrochemical Society, 155共12兲 J345-J349 共2008兲 J346

National Sun Yat-Sen University assisted in meeting the publication costs of this article.

References

1. C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett., 51, 913共1987兲. 2. Z. Y. Xie, L. S. Hung, and S. T. Lee, Appl. Phys. Lett., 79, 1048共2001兲. 3. E. J. W. List, S. Tasch, C. Hochfilzer, G. Leising, P. Schlichting, U. Rohr, Y.

Geerts, U. Scherf, and K. Müllen, Opt. Mater., 9, 183共1998兲.

4. Y. D. Jin, J. P. Yang, P. L. Heremans, M. V. Auweraer, E. Rousseau, H. J. Geise, and G. Borghs, Chem. Phys. Lett., 320, 387共2000兲.

5. G. Sakamoto, C. Adachi, T. Koyama, Y. Taniguchi, C. D. Merritt, H. Murata, and Z. H. Kafafi, Appl. Phys. Lett., 75, 766共1999兲.

6. C. W. Tang, S. A. Van Slyke, and C. H. Chen, J. Appl. Phys., 65, 3610共1989兲. 7. C. T. Chen, Chem. Mater., 16, 4389共2004兲.

8. V. Bulović, A. Shoustikov, M. A. Baldo, E. Bose, V. G. Kozlov, M. E. Thompson, and S. R. Forrest, Chem. Phys. Lett., 287, 455共1998兲.

9. M. A. Baldo, D. F. Brien, Y. You, S. S. Shoustikov, M. E. Thompson, and S. R. Forrest, Nature (London), 395, 151共1998兲.

10. V. Bulović, R. Deshpande, M. E. Thompson, and S. R. Forrest, Chem. Phys. Lett.,

308, 317共1999兲.

11. Y. Hamada, H. Kanno, T. Tsujioka, H. Takahashi, and T. Usuki, Appl. Phys. Lett.,

75, 1682共1999兲.

12. T. W. Lee, O. O. Park, H. N. Cho, and Y. C. Kim, Curr. Appl. Phys., 1, 363共2001兲. 13. B. W. D’Andrade, M. A. Baldo, C. Adachi, and J. Brooks, Appl. Phys. Lett., 79,

1045共2001兲.

14. Y. Hamada, H. Kanno, T. Tsujioka, and H. Takahashi, Appl. Phys. Lett., 75, 1682 共1999兲.

15. Y. Ohmori, H. Kajii, T. Sawatani, H. Ueta, and K. Yoshino, Thin Solid Films, 393, 407共2001兲.

16. J. Feng, F. Li, W. Gao, G. Cheng, W. Xie, and S. Liu, Thin Solid Films, 499, 343 共2006兲.

17. H. Tang, Y. Li, X. Wang, W. Wang, and R. Sun, Semicond. Sci. Technol., 22, 287 共2007兲.

18. B. Chen, X. Lin, L. Cheng, C. S. Lee, W. A. Gambling, and S. T. Lee, J. Phys. D,

34, 30共2001兲.

19. J. Qiao, Y. Wang, L. Duan, Y. Li, and D. Zhang, Appl. Phys. Lett., 81, 4913共2002兲. 20. X. Gong, S. H. Lim, J. C. Ostrowski, D. Moses, C. J. Bardeen, and G. C. Bazan, J.

Appl. Phys., 95, 948共2004兲.

21. X. R. Wang, H. You, H. Tang, G. H. Ding, D. G. Ma, H. Tian, and R. G. Sun, J.

Lumin., 128, 27共2008兲.

22. J. Kavitlha, S. Y. Chang, Y. Chi, J. K. Yu, Y. H. Hu, P. T. Chou, S. M. Peng, G. H. Lee, Y. T. Tao, C. H. Chien, et al., Adv. Funct. Mater., 15, 223共2005兲. 23. G. E. Jabbour, J. F. Wang, and N. Peyghambarian, Appl. Phys. Lett., 80, 2026

共2002兲.

24. C. H. Wu, P. I. Shih, C. F. Shu, and Y. Chi, Appl. Phys. Lett., 92, 233303共2008兲. 25. Y. G. Lee, S. K. Kang, T. S. Oh, H. N. Lee, S. Lee, and K. H. Koh, Org. Electron.,

9, 339共2008兲.

26. K. Suzuki, A. Seno, H. Tanabe, and K. Ueno, Synth. Met., 143, 89共2004兲. 27. R. G. Bennett, J. Chem. Phys., 41, 3037共1964兲.

28. C. C. Wang, M.Sc. Thesis, National Sun Yat-Sen University, Taiwan共2007兲.

J349 Journal of The Electrochemical Society, 155共12兲 J345-J349 共2008兲 J349