A morphologically stable host material for ef
ficient phosphorescent green and red
organic light emitting devices
Meng-Huan Ho
a,⁎
, Banumathy Balaganesan
a, Ta-Ya Chu
b, Teng-Ming Chen
a, Chin H. Chen
ca
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, 300, Taiwan, ROC
bDepartment of Electrophysics, National Chiao Tung University, Hsinchu, 300, Taiwan, ROC c
Display Institute, Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinchu, 300, Taiwan, ROC
a b s t r a c t
a r t i c l e i n f o
Article history: Received 26 June 2007
Received in revised form 5 June 2008 Accepted 13 July 2008
Available online 19 July 2008 Keywords:
Organic electroluminescent devices Phosphorescence
Host material Carbazole
A carbazole-based host material, 4,4′-N,N′-[di(3, 6-di(t-butyl)carbazole]biphenyl (ttbCBP), with four t-butyl steric spacers on peripheral carbazole moieties was effectively synthesized through Friedel–Crafts Alkylation. Owing to the presence of sterically hindered t-butyl groups, ttbCBP exhibits a high glass transition temperature (175 °C) and is morphologically stable. This host material also preserves the characteristics of wide band-gap of 3.2 eV and high triplet energy of 2.64 eV. In addition, ttbCBP has been shown to be effective hosts for green and red phosphorescent emitters in organic electrophosphorescent devices.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Recently, high-efficiency organic light emitting diodes (OLEDs) have been demonstrated due to the harvest of both singlet and triplet excitons of phosphorescent emitters [1–4]. The phosphorescence from the devices can be understood in terms of the energy transfer from both the singlet and triplet excited states of the host materials to the triplet excited states of the phosphorescent guest molecules and/or by direct excitation of the phosphorescent guest molecules by charge trapping. Both of these mechanisms are capable of produc-ing nearly 100% internal quantum efficiency. However, it is essen-tial for host materials to possess high triplet energy to prevent from back energy transfer and confine triplet exciton on guest molecules
[5,6].
For OLEDs, a prominent class of materials that fulfills the above-mentioned boundary conditions is the class of carbazoles. A common carbazole derivative that is often used as host for triplet emitters is N, N′-dicarbazolyl-4,4′-biphenyl (CBP), resulting in maximum internal quantum efficiencies of 60 to 80% for green and red triplet emitters
[3,7–11]. When doping with the green phosphor fac-tris(2-phenylpyr-idine)iridium (Ir(ppy)3)[7] and red phosphor tris(2-(2′benzo[4,5-a]
thienyl)pyridinato-N,C3′) iridium (acethylacetonate)[8], high external
quantum efficiency (E. Q. E.) of 7.5% and 7.0% can be achieved, respectively.
Although the triplet energy is an important factor for host materials applied in phosphorescent OLEDs, the morphological stability of host materials should also be considered[12,13]. To have a large energy gap, the extent of conjugation in the molecule must be limited, which in turn would usually impose considerable constraints in molecular size and weight. However, the reduced molecular size restricts the thermal stability of molecules and further makes molecules difficult to form morphologically stable and uniform thin films. Several studies have indicated that the thermal and morpho-logical instabilities of host materials may shorten the operational lifetime of devices[14]. Therefore, various studies have been focused on hybridizing carbazole moiety with some bulky and steric groups, like triphenylsilane[15],fluorene[16], or tert-butyl benzene[17,18], to improve the thermal and morphological stability of the carbazole-based host materials and applied in electrophosphorescent OLEDs.
Recently, Chen et al. reported the synthesis of a 3,6-di(t-butyl) CBP with high triplet energy via several synthetic steps[19]. In this paper, we describe a simple, convenient and efficient synthesis of a tetra(t-butyl)-substituted carbazole derivative directly from CBP, 4,4′-N,N′-[di (3, 6-di(t-butyl)carbazole]biphenyl, (abbreviated as ttbCBP), bearing the t-butyl groups at C-3, C-6, C-3′, and C-6′ carbon atoms on both carbazole moieties of CBP. This synthetic procedure furnished high yields of ttbCBP with high purity. We propose the sterically hindered t-butyl spacers can avoid the formation of polycrystallinefilms and improve the thermal properties. This host material also preserves the characteristics of wide band-gap and high triplet energy, both of which are important in the confinement of triplet exciton on the guest molecules, thus is suitable as a host material for green and red electrophosphorescent devices.
⁎ Corresponding author. Tel.: +886 3 5712121x52919; fax: +886 3 573 5601. E-mail address:[email protected](M.-H. Ho).
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Thin Solid Films
CBP (1.0 equiv) and tert-butyl chloride (4.2 equiv) were dissolved in dichloromethane and heated to reflux at 80 °C while anhydrous AlCl3
(20 mol%) was added slowly into the mixture and reflux was
continued. The reaction was monitored by thin-layer chromatography [2% ethyl acetate (EA) in n-hexane]. After 30 min of reflux, the mixture was poured into ice-water and extracted with EA. The organic layer was washed with water several times and dried with anhydrous Na2SO4. After rotary evaporation, the crude product was washed with
n-hexane to getN90% pure white–yellow solid (yield ∼75%). The crude product was further purified via train sublimation and its purity was thoroughly checked by high-performance liquid chromatography to beN99%. The molecular structure of ttbCBP was characterized by1H
and 13C nuclear magnetic resonance, mass spectra, and elemental
analyses. ttbCBP.1H NMR (300 MHz, CDCl 3):δ/ppm 1.47 (s, 36H), 7.41–7.50 (m, 8H), 7.66–7.69 (d, 4H), 7.86–7.89 (d, 4H), 8.14–8.15 (d, 4H). FAB-MS: m/z = 708 (M+). Anal. for C 50H40N2: Calcd: C, 88.09; H, 7.96; N, 3.95. Found: C, 88.37; H, 7.61; N, 3.76.
Differential scanning calorimetry (DSC) was performed using a Seiko SSC 5200 instrument operated at heating and cooling rates of 10 and 30 °C/min, respectively. Samples were scanned from 30 to 400 °C, cooled to 30 °C, and then scanned again from 30 to 400 °C; the glass transition temperatures (Tg) were determined from the second
heating scan. Thermogravimetric analysis (TGA) was undertaken on a Seiko TG/DTA 200 instrument under a nitrogen atmosphere, by measuring weight loss while heating at a rate of 10 °C/min. UV–Vis and photoluminescence spectra were measured by Hewlett Packard 8453 and Acton Research Spectra Pro-150, respectively. The ionization potential was determined by the UV photoelectron spectroscopy (Model AC-2). The calculated electron distribution of highest occupied molecular orbital/lowest occupied molecular orbital (HOMO/LUMO) levels were optimized by ab initio density functional theory (DFT) with the B3LYP/6-31G⁎ basis sets[21]in Gaussian 03 program[22].
To demonstrate the efficacy of ttbCBP, the following devices have been fabricated. Green devices: Indium tin oxide (ITO)/CFx/N,N
′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) (60 nm)/ 7% Ir(ppy)3doped in ttbCBP (Device A) or CBP (Device B) (30 nm)/
4,7-diphenyl-1,10-phenanthroline (BPhen) (30 nm)/tris(8-quinolinolato) aluminium (Alq3) (15 nm)/LiF (1 nm)/Al (200 nm); Red devices: ITO/
CFx/NPB (60 nm)/9% tris(1-phenylisoquinolinato-C2,N)iridium(III) (Ir
(piq)3)[11]doped in ttbCBP (Device C) or CBP (Device D) (50 nm)/
BPhen (10 nm)/Alq3(35 nm)/LiF (1 nm)/Al (200 nm). CFx, NPB, Alq3
and LiF were used as the hole injection material[23], hole and electron transport materials and electron injection material, respectively. A thin layer of BPhen was inserted between the EML and electron-transporting layer as the hole-blocking and exciton-blocking layer to provide the carrier and exciton confinement.
After a routine cleaning procedure, the 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. Devices were fabricated under the base vacuum of about 5 × 10− 4 Pa in a thin-film evaporation coater following a published protocol[24]. In the evaporation of EML, the phosphor-escent dopant was co-deposited at its 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 direct current source.
3. Results and discussion
3.1. Thermal properties and morphological stability
We used differential scanning calorimetry (DSC) and thermogravi-metric analysis (TGA) to investigate the thermal properties of CBP and ttbCBP as summarized inTable 1.Fig. 1displays the TGA thermogram and DSC curve of ttbCBP. TGA revealed that the onset decomposition temperature of ttbCBP was 404 °C, followed by 5% weight loss at 478 °C (Td, decomposition temperature). During DSC measurements,
ttbCBP underwent a glass transition at a relatively high temperature (Tg= 175 °C) and showed no further phase transition upon heating up
to 350 °C which means ttbCBP did not undergo recrystallization or melting. The higher value of Tdand Tgfor ttbCBP, with respect to those
of CBP (Td= 453 °C, Tg= 78 °C) is presumably the result of its bulky and
rigid molecular conformation, which is essential for morphological stability of thinfilms.
The thin-film morphological stability is very sensitive to pin-hole formation due to crystallization or any interfacial change that might
Scheme 1. Synthesis of ttbCBP.
CBP 78 284 453 294, 319 375 5.7 2.64 ttbCBP 175 – 478 299, 334 395 5.7 2.64
be induced by Joule heating[25]during device operation which may in turn impact adversely on the lifetime of the device. The results of atomic force microscopy (AFM) measurements as shown in Fig. 2
indicated that an evaporated film of ttbCBP possessed a uniform surface that did not undergo any morphological changes when heated at 95 °C for 1 h. The root-mean-square surface roughnesses of unheated/heated ttbCBPfilms were 0.55 nm/0.38 nm, respectively. On the other hand, the root-mean-square surface roughnesses of unheated CBP films was 0.70 nm, and the degradation of surface morphology can be clearly observed after heated at 95 °C for 1 h. Consequently, ttbCBP forms an amorphous glass that is more stable under anticipated Joule heating. Due to the high thermal properties and morphological stability of ttbCBP, we are able to form homo-genous and stable amorphous thinfilms of ttbCBP through vacuum evaporation.
3.2. Photo-physical properties and energy levels consideration
The photo-physical properties of CBP and ttbCBP are also
summarized in Table 1. Fig. 3 compares the absorption and
photoluminescence spectra of CBP and ttbCBP in CHCl3. The features
of the lowest absorption band andfluorescence of ttbCBP are very similar to those of CBP, except they are 15–20 nm red-shifted. Such a
red red-shift is similar to what has been observed in carbazoles moiety with quaternary carbon substitutions at C-3 and C-6 positions
[26,27].
Fig. 2. AFM topographic images of CBP and ttbCBP thinfilms deposited on silicon wafers (50 nm). (a) CBP before heating; (b) CBP after heating; (c) ttbCBP before heating; (d) ttbCBP after heating (scanned area: 3μm×3 μm).
Fig. 3. Normalized absorbance, photoluminescence and phosphorescence spectra of CBP and ttbCBP.
The calculated HOMO and LUMO electron distribution of CBP and ttbCBP as shown inFig. 4indicate that thefilled π orbitals (or HOMO level) of CBP are located on the peripheral carbazole moieties and the unfilled π⁎ orbitals (or LUMO level) are on the biphenyl core. It can be clearly observed that the electron distribution of ttbCBP is more delocalized with respect to that of CBP, especially the HOMO level, when introducing t-butyl substituents at C-3 and C-6 positions of both carbazole moieties. We believe this is primarily due to the electron-donating property of the quaternary carbon substitutions which could also be the reason for the red-shift of optical spectra. The ionization potential of CBP and ttbCBP are all 5.7 eV as determined by the UV photoelectron spectroscopy (Model AC-2). The absorption onset energy together with ionization energy provide the estimation of HOMO/LUMO energy levels of CBP and ttbCBP to be 5.7/2.4 eV and 5.7/ 2.5 eV, respectively.
3.3. Triplet energy
To evaluate the potential of ttbCBP as a host for electropho-sphorescent devices, phosphorescence spectrum of ttbCBP was mea-sured at 77 K in 2-methyl tetrahydrofuran glass as shown inFig. 3. A well-characterized emission from the triplet state of ttbCBP in the region of 440–600 nm is observed, in which the first vibronic transition (T1ν = 0→S1ν = 0) of the phosphorescence is assigned as the triplet energy
(ET) [28]. The ETof ttbCBP is estimated to be ca. 2.64 eV which is
identical to that of CBP[29], suggesting that ttbCBP can be used as host material for triplet green and red emitters. These photophysical results clearly indicate that despite the introduction of t-butyl substituents which result in a slightly narrower band-gap, the sterically hindered spacers give ttbCBP molecule with surprisingly robust morphological stability and most importantly it preserves the high triplet energy. 3.4. Device performance
The detailed electroluminescent (EL) performances measured at 20 mA/cm2are summarized inTable 2, the EL efficiency of ttbCBP-host
devices are comparable to those of CBP-host devices B and D. Devices A and C obtained a maximum E. Q. E. of 8.3% with 28.8 cd/A and 16.4 lm/W at 0.5 mA/cm2 and 9.8% with 6.7 cd/A and 3.0 lm/W at
2 mA/cm2, respectively, The high EL efficiency can be attributed to the
ET of ttbCBP (∼2.64 eV) which is higher than that of Ir(ppy)3
(∼2.42 eV) and Ir(piq)3 (∼2.0 eV), ensuring that the triplet energy
transfer from ttbCBP to Ir(ppy)3or Ir(piq)3is exothermic and that the
transferred excitons are well confined on the iridium dopants. As a result, devices A and C exhibit pure EL peaks at 512 nm from Ir(ppy)3
and 628 nm from Ir(piq)3, respectively. Furthermore, no emission
from ttbCBP is observed, indicative of complete energy transfer from ttbCBP to the green and red iridium phosphors in the emitting layer.
Fig. 5shows the E. Q. E. versus voltage characteristics of these electrophosphorescent devices. All devices show a gradual decrease in EL efficiency with voltage, which is often observed in phosphorescent OLEDs and is usually attributed to triplet–triplet annihilation[30]. Nevertheless, as the drive voltage was increased to 16 V, E. Q. E. of ttbCBP-host devices A and C remains above 5%, which are still substantially higher than that offluorescent green and red OLED device (3–5%). Furthermore, ttbCBP-host devices appear to less efficiency roll-off than that obtained with the CBP-host devices, which is presumably due to the high morphological stability of ttbCBP thin film and thus alleviate EL efficiency quenching under high voltage.
Fig. 4. The calculated HOMO/LUMO electron distribution of CBP and ttbCBP.
Table 2
EL performances of four electrophosphorescent devices driven at 20 mA/cm2
Device (host) Voltage (V) Luminance (cd/m2 ) Current Eff. (cd/A) E. Q. E. (%) Peak (nm) CIE (x, y) A (ttbCBP) 9.6 4983 24.9 7.3 512 (0.31, 0.62) B (CBP) 8.4 4935 24.6 7.2 512 (0.30, 0.62) C (ttbCBP) 11.7 1181 5.9 8.6 628 (0.68, 0.32) D (CBP) 10.0 1282 6.4 9.2 628 (0.68, 0.32)
4. Conclusion
We have discovered a host material for green and red triplet emitters — 4,4′-N,N′-[di(3,6-di(t-butyl)carbazole]biphenyl (ttbCBP) synthesized through a facile route, which is based on the design strategy of steric substitution on peripheral carbazole moieties of CBP. The introduction of the t-butyl substituents significantly improve the thermal and morphological stability of amorphous thin film and preserve the high triplet energy of the molecule. Green Ir(ppy)3and
red Ir(piq)3doped phosphorescent OLEDs with ttbCBP as host material
show remarkable external quantum efficiencies and less efficiency roll-off at high drive voltage.
Acknowledgements
This work was supported by grants from Chunghwa Picture Tubes, Ltd. (CPT) and National Science Council of Taiwan. We also thank e-Ray Optoelectronics Technology Co., Ltd. of Taiwan for supplying some of the OLED materials studied in this work.
References
[1] M.A. Baldo, D.F. O'Brien, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151.
[2] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750.
[3] S. Lamansky, P.I. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704.
[4] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.E. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4303. [5] C.F. Chen, G. He, Y. Yang, Appl. Phys. Lett. 82 (2003) 1006.
[6] M. Sudhakar, P.T. Djurovich, T.E. Hogen-Esch, M.E. Thompson, J. Am. Chem. Soc. 123 (2003) 7796.
[7] D.F. O'Brien, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 74 (1999) 442. [8] C. Adachi, M.A. Baldo, S.R. Forrest, S. Lamansky, M.E. Thompson, R.C. Kwong, Appl.
Phys. Lett. 78 (2001) 1622.
[9] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4.
[10] C. Adachi, R.C. Kwong, S.R. Forrest, Org. Electron. 2 (2001) 37.
[11] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971.
[12] S. Tokido, H. Tanaka, N. Noda, A. Okada, T. Taga, Appl. Phys. Lett. 70 (1997) 1929. [13] P. Fenter, F. Schreiber, V. Buloviæ, S.R. Forrest, Chem. Phys. Lett. 277 (1997) 521. [14] B.W. D'Andrade, S.R. Forrest, A.B. Chwang, Appl. Phys. Lett. 83 (2003) 3858. [15] S.J. Yeh, W.C. Wu, C.T. Chen, Y.H. Song, Y. Chi, M.H. Ho, S.F. Hsu, C.H. Chen, Adv.
Mater. 17 (2005) 285.
[16] P.I. Shih, C.L. Chiang, A.K. Dixit, C.K. Chen, M.C. Yuan, R.Y. Lee, C.T. Chen, E.W.G. Diau, C.F. Shu, Org. Lett. 8 (2006) 2799.
[17] M.H. Tsai, H.W. Lin, H.C. Su, T.H. Ke, C.C. Wu, F.C. Fang, Y.L. Liao, K.T. Wong, C.I. Wu, Adv. Mater. 18 (2006) 1216.
[18] K.T. Wong, Y.M. Chen, Y.T. Lin, H.C. Su, C.C. Wu, Org. Lett. 7 (2005) 5361. [19] Y.C. Chen, G.S. Huang, C.C. Hsiao, S.A. Chen, J. Am. Chem. Soc. 128 (2006) 8549. [20] C. Friedel, J.M. Crafts, Compt. Rend. 84 (1877) 1392.
[21] C. Lee, W. Yang, R.G. Parr, Phys. Rev., B 37 (1988) 785.
[22] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision A.1, Gaussian, Inc., Pittsburgh PA, 2003.
[23] L.S. Hung, L.R. Zheng, M.G. Mason, Appl. Phys. Lett. 78 (2001) 673. [24] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160.
[25] J.W. Choi, J.S. Kim, S.Y. Oh, H.W. Rhee, W.H. Lee, S.B. Lee, Thin Solid Films 363 (2000) 271.
[26] Z. Zhu, J.S. Moore, J. Org. Chem. 65 (2000) 116.
[27] N.D. McClenaghan, R. Passalacqua, F. Loiseau, S. Campagna, B. Verheyde, A. Hameurlaine, W. Dehaen, J. Am. Chem. Soc. 125 (2003) 5356.
[28] A.V. Dijken, J.J.A.M. Bastiaansen, N.M.M. Kiggen, B.M.W. Langeveld, C. Rothe, A.P. Monkman, I. Bach, P. Stössel, K. Brunner, J. Am. Chem. Soc. 126 (2004) 7718. [29] R.J. Holmes, S.R. Forrest, Y.J. Tung, R.C. Kwong, J.J. Brown, S. Garon, M.E. Thompson,
Appl. Phys. Lett. 82 (2003) 2422.
