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An ethylcarbazole based phosphine oxide derivative as a host for deep blue phosphorescent organic light-emitting diode
Journal of Luminescence 130 (2010) 2238–2241
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
An ethylcarbazole based phosphine oxide derivative as a host for deep blue phosphorescent organic light-emitting diode Soon Ok Jeon, Kyoung Soo Yook, Hyo Suk Son, Jun Yeob Lee n Department of Polymer Science and Engineering, Dankook University, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 July 2009 Received in revised form 23 June 2010 Accepted 29 June 2010 Available online 16 July 2010
Deep blue phosphorescent organic light-emitting diodes (PHOLEDs) were developed using a 9-ethylcarbazole based phosphine oxide material (EPO1). A high triplet energy of 3.01 eV was obtained from the EPO1 host material and efficient energy transfer from the host to the deep blue emitting phosphorescent dopant was observed. A high quantum efficiency of 7.9% with a color coordinate of (0.15, 0.17) was achieved in the deep blue PHOLED using the EPO1 host material. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ethylcarbazole Phosphine oxide Deep blue phosphorescent light-emitting diodes High triplet energy
1. Introduction The development of high efficiency deep blue phosphorescent organic light-emitting diodes (PHOLEDs) is important to reduce the power consumption of organic light-emitting diodes (OLEDs). Theoretically, the quantum efficiency of the deep blue PHOLEDs can be better than that of the fluorescent OLEDs by four times and the power consumption of the active matrix type OLEDs can be greatly reduced [1–3]. However, the device performances of the deep blue PHOLEDs are not good enough in spite of a lot of researches about the materials and device structures for deep blue PHOLEDs. There have been many studies to develop host and dopant materials for deep blue PHOLEDs [4–10]. Several Ir based dopant materials have been synthesized and a deep blue color could be obtained. A tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium (FCNIr) was developed as a blue dopant and a Commission International De L’Eclairage (CIE) of (0.15, 0.16) with a quantum efficiency of 9% was obtained from the blue PHOLEDs doped with the FCNIr [4,5]. Ir based dopant materials with carbene based ligand units were synthesized and a deep blue color was achieved [6–8]. Other than these, pyrazole based Ir dopants were reported to give deep blue color [9]. In addition to the dopant materials, a lot of researches were carried out to synthesize host materials for deep blue PHOLEDs.
n
Corresponding author. Tel./fax: + 82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.06.026
Carbazole based host materials were effective as host materials for deep blue PHOLEDs due to the high triplet energy of the carbazole unit [10]. Several Si based host materials could also play a role of high triplet energy host materials for deep blue PHOLEDs [11,12]. The combination of the carbazole core and Si units was proven to be a good approach to fabricate deep blue PHOLEDs [13,14]. However, more studies are required to develop high efficiency deep blue PHOLEDs. In this work, a 9-ethylcarbazole based phosphine oxide (EPO1) compound was synthesized and it was applied as a host material in the deep blue PHOLEDs. It was demonstrated that the 9-ethylcarbazole based phosphine oxide was effective as a high triplet energy host material for the deep blue PHOLEDs.
2. Experimental 2.1. Materials and measurements 9-ethylcarbazole, n-butyllithium, chlorodiphenyphosphine, Nbromosuccinimide (Aldrich Chem. Co.) and hydrogen peroxide (Duksan Sci. Co.) were used without further purification. Tetrahydrofuran was distilled over sodium and calcium hydride. The 1H nuclear magnetic resonance (NMR) was recorded on a Varian 200 (200 MHz) spectrometer. The photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (Jasco FP-6500) and the ultraviolet–visible (UV–vis) spectra were obtained using a UV–vis spectrophotometer (Shimadzu, UV-1601PC). The differential scanning calorimeter (DSC, Mettler
S.O. Jeon et al. / Journal of Luminescence 130 (2010) 2238–2241
DSC 822 e) was used to obtain melting temperature (MP) and glass transition temperature (Tg) at a heating rate of 10 1C/min under nitrogen. The low and high resolution mass spectra were obtained using a JEOL, JMS-AX505WA spectrometer in FAB mode. The energy levels were measured with cyclic voltametry (CV). The EPO1 host material was synthesized by the phosphonation reaction of the brominated 9-ethylcarbazole. The synthetic route of 1 is shown in Scheme 1. 2.2. Synthesis of 3-bromo-9-ethylcarbazole Bromine (30.0 mmol) was added in small portions to a solution of ethylcarbazole (30.0 mmol) in tetrahydrofuran at room temperature under nitrogen atmosphere. After stirring for 5 h, water and dichloromethane were added to the solution. The organic phase was separated, washed with water, brine solution, dried over anhydrous magnesium sulphate, filtered and dried to remove the solvents. Purification by column chromatography with a mixture of dichloromethane and n-hexane gave a white solid. 2.3. Synthesis of 3-(9-ethylcarbazolyl) diphenylphosphine oxide The 3-bromo-9-ethylcarbazole (1) (4.4 mmol) was dissolved in 20 mL of anhydrous tetrahydrofuran under argon and cooled to 78 1C. 1.2 equivalent of n-butyllithium (10.0 M in hexanes, 5.3 mmol) was added dropwise to give a bright yellow solution that thickened to a slurry. Stirring was continued for 3 h at 78 1C after which 0.99 mL (5.3 mmol) of chlorodiphenylphosphine was added to give a clear, pale yellow solution. The solution was stirred for additional 3 h at 78 1C before quenching with 2 mL of degassed methanol. Volatiles were removed under a reduced pressure to give an off-white solid that was digested in methanol, filtered, then digested in water and filtered. The crude material was purified by column chromatography. 3-(diphenylphosphino)-9-ethylcarbazole (1.8 mmol), 10 mL of methylene chloride and 2 mL of 30% hydrogen peroxide were stirred overnight at room temperature. The organic layer was separated and washed with water and then brine. The extract was evaporated to dryness affording a white solid. EPO1 yield 93%. MP 119 1C. Tg 62.4 1C. 1H NMR (200 MHz, CDCl3): d 8.50–8.44 (d, 1H, Ar–CH–carbazole), 8.05–8.01 (d, 1H, Ar–CH–carbazole), 7.77–7.62 (m, 6H, OP–CH–phenyl), 7.48–7.23 (m, 7H, Ar–CH–carabazole), 4.37–4.30 (q, 2H, CH2), 1.46–1.39 (t, 3H, CH3). MS (FAB) m/z 396 [(M+ 1)+ ].
2239
Keithley 2400 source measurement unit and CS-1000 spectroradiometer.
3. Results and discussion The EPO1 was effectively synthesized by the phosphonation reaction of the brominated 9-ethylcarbazole. The synthetic yield of the EPO1 was over 90% and the purity of the compound was over 99%. The EPO1 was designed as a host material which can have high triplet energy for efficient energy transfer from the EPO1 to the deep blue phosphorescent dopant. The ethylcarbazole group has high triplet energy of 3.02 eV from our measurements and it is suitable as a core structure of the triplet host material. However, the ethylcarbazole unit has strong hole transport properties and the diphenylphosphine oxide group was attached to the ethylcarbazole group to add electron transport properties to the molecule. The diphenyl phosphine oxide unit has strong electron withdrawing character due to the polar phosphine oxide group and it improves the electron transport properties of the EPO1 [15,16]. In addition, the phosphine oxide group isolates the ethylcarbazole core from the diphenyl unit and does not affect the triplet energy of the ethylcarbazole core. The conjugation of the ethylcarbazole group is not extended by the phosphine oxide and the triplet energy of the ethylcarbazole group can be maintained. In fact, the triplet energy of the EPO1 was the same as that of the carbazole unit. Molecular simulation of the EPO1 was performed to study the molecular orbital distribution of the EPO1. Fig. 1 shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of the EPO1. The HOMO orbitals of the EPO1 were mostly localized in the ethylcarbazole unit with some orbital distribution in the diphenylphosphine oxide. The LUMO distribution of the EPO1 was quite similar to the HOMO distribution of the EPO1. This indicates that the HOMO and LUMO levels of the EPO1 are dominated by the ethylcarbazole core structure and the diphenylphosphine oxide group slightly affects the HOMO and LUMO orbital distribution of the EPO1. The diphenylphosphine oxide group may shift the HOMO and LUMO levels of the ethylcarbazole group.
2.4. Device fabrication The device structure of the deep blue PHOLED was indium tin oxide (ITO, 150 nm)/N,N0 -diphenyl-N,N0 -bis-[4-(phenyl-mtolyl-amino)-phenyl]-biphenyl-4,40 -diamine (60 nm)/N,N0 -di(1naphthyl)-N,N0 -diphenylbenzidine (30 nm)/EPO1:tris((3,5-difluoro4-cyanophenyl)pyridine) iridium (FCNIr, 30 nm, x%)/4,7-diphenyl1,10-phenanthroline (Bphen, 25 nm)/LiF (1 nm)/Al (200 nm). The doping concentrations of the FCNIr were 10%, 15% and 20%. Current density–voltage–luminance characteristics and electroluminescence (EL) spectra of the devices were measured with
N
Br2 CHCl3
n-Butyllithium
N
Fig. 1. HOMO/LUMO distribution of EPO1.
N
Chlorodiphenylphosphine Br Scheme 1. Synthetic scheme of EPO1.
CH2Cl2 P
H2O2
N O P
S.O. Jeon et al. / Journal of Luminescence 130 (2010) 2238–2241
the energy level diagram (Fig. 4), there is a large LUMO level difference of 0.65 eV between the EPO1 and the FCNIr. In addition, the energy barrier for electron injection between the Bphen and the EPO1 is 0.45 eV. Therefore, electrons are directly trapped by the FCNIr dopant and electron hopping through the FCNIr dominates the charge transport in the emitting layer. The electron hopping is facilitated at high doping concentration. The quantum efficiency of the deep blue PHOLEDs with the EPO1 host is shown in Fig. 5. The quantum efficiency was gradually increased according to the doping concentration and the EPO1 device with 20% doping concentration showed the highest quantum efficiency value. The maximum quantum efficiency of the deep blue PHOLED was 7.9%. The high efficiency in the highly doped device can be explained by the improved charge balance in the emitting layer. The electron transport in the emitting layer is facilitated at high doping concentration as explained in Fig. 3 due to the decreased hopping distance between FCNIr dopant materials. The hole injection from the hole transport layer to the EPO1 emitting layer is better than the electron injection from the electron transport layer to the EPO1 layer. Therefore, the charge balance is improved in the highly doped device because the electron transport is facilitated at high doping concentration. The better electron injection leads to better charge balance in the emitting layer. However, the
80
1200 10% 15% 20%
60
1000 800 600
40
400 20 200 0
0 0 2
10
6 8 Voltage (V)
4
12
Fig. 3. Current density–voltage–luminance curves of the EPO1 devices according to the doping concentration of FCNIr.
2.1 2.4
2.35
UV-Vis (EPO1) PL (298 k)
2.8
Bphen
FCNIr
metal to ligand charge transfer absorption
EPO1
Intensity (a.u.)
UV-Vis (FCNIr)
3.0 NPB
DNTPD
PL (77 k)
5.1 5.5 250
300
350
400
450
500
550
Wavelength (nm) Fig. 2. UV–vis and photoluminescence spectra of the EPO1(in solution) and FCNIr.
Luminance (cd/m2)
The HOMO and LUMO levels of the EPO1 were measured by CV and UV–vis absorption spectroscopic measurements. The HOMO level was determined by the CV and it was 5.84 eV. The bandgap of the EPO1 was 3.49 eV from UV–vis measurement and the LUMO level was 2.35 eV from the difference between the HOMO and the bandgap of the EPO1. The ethylcarbazole core has the HOMO/LUMO of 5.57 eV/2.06 eV from CV and UV–vis measurements and the diphenylphosphine oxide group lowered the energy level of the ethylcarbazole core by about 0.3 eV. The electron withdrawing properties of the phosphine oxide unit shifted the LUMO level for better electron injection than the ethylcarbazole core. UV–vis and PL spectra of the EPO1 are shown in Fig. 2. The EPO1 showed strong absorption peaks at 267 and 292 nm that are assigned to the p p* absorption of the carbazole unit. This implies that the UV–vis absorption occurs in the ethylcarbazole core of the EPO1 molecule. The bandgap was calculated from the UV–vis absorption edge of the spectrum and it was 3.49 eV. The PL emission of the EPO1 was observed at 348 and 365 nm due to the wide bandgap of the EPO1. The triplet energy of the EPO1 was measured from the PL spectrum of the EPO1 at 77 K and it was 3.01 eV from the emission peak at 410 nm. The triplet energy and bandgap of the EPO1 were quite similar to those of the 9ethylcarbazole. The diphenylphosphine oxide group did not affect the triplet energy and bandgap of the EPO1. The UV–vis absorption spectrum of the FCNIr dopant was also added and it can be clearly seen that the metal to ligand charge transfer absorption peak of the FCNIr dopant is well overlapped with the PL emission of the EPO1, indicating that the energy transfer from the EPO1 host to the FCNIr dopant is efficient. The triplet energy of the EPO1 (3.01 eV) was also higher than that of the FCNIr (2.8 eV). Therefore, efficient energy transfer from the EPO1 host to the FCNIr dopant is expected. The EPO1 can be used as a host material for deep blue PHOLED due to the high triplet energy of 3.01 eV for an efficient energy transfer to the deep blue phosphorescent dopant. The deep blue emitting FCNIr was doped into the EPO1 host and the device performances of the deep blue PHOLED were investigated. Fig. 3 shows the current density–luminance–voltage curves of the blue PHOLEDs with the EPO1:FCNIr emitting layer. The doping concentration of the device was controlled from 10% to 20%. The current density and luminance of the EPO1 based blue PHOLEDs were not greatly affected by the doping concentration although the current density was slightly increased at high doping concentration. The slight increase of the current density at high voltage in the 15% and 20% doped devices is due to the decrease of hopping distance at high doping concentration. As can be seen in
Current density (mA/cm2)
2240
5.8
5.84
6.1
Fig. 4. Energy level diagram of the blue phosphorescent device.
S.O. Jeon et al. / Journal of Luminescence 130 (2010) 2238–2241
Fig. 6 shows the electroluminescence spectra of the EPO1 based deep blue PHOLEDs. The EPO1 based deep blue PHOLEDs showed a main emission peak at 454 nm and a shoulder peak at 480 nm. The shoulder peak was rather intensified in the highly doped blue PHOLEDs, which is due to polar–polar interaction of the FCNIr molecules at high doping concentration. The color coordinate of the deep blue PHOLEDs was (0.15, 0.16) at 10% doping concentration and it was red-shifted at 15% and 20% doping concentration. The color coordinate of the 15% and 20% doped deep blue PHOLEDs was (0.15, 0.17). The effective energy transfer from the EPO1 host to the deep blue emitting FCNIr resulted in the deep blue color and high efficiency in the EPO1 based deep blue PHOLEDs.
Quantum Efficiency (%)
10 10%
8
15% 20%
6
4
2
0 0.01
0.1
1 10 Current density (mA/cm2)
2241
100
Fig. 5. Quantum efficiency–current density curves of the EPO1 devices according to the doping concentration of FCNIr.
10%
4. Conclusions In summary, 9-ethylcarbazole based phosphine oxide was effectively synthesized by the phosphonation reaction of the brominated 9-ethylcarbazole intermediate and they could be used as a host material in the deep blue PHOLED. A high quantum efficiency of 7.9% with a color coordinate of (0.15, 0.17) was obtained from the deep blue PHOLED with the 9-ethylcarbazole based phosphine oxide host.
15% Intensity (arb. unit)
20%
400
500
600
700
References
800
Wavelength (nm) Fig. 6. Electroluminescence spectra of EPO1 devices according to the doping concentration of FCNIr.
quantum efficiency was reduced at high current density irrespective of doping concentration, which is due to triplet– triplet annihilation as reported in other works [17,18]. Triplet exciton density is increased at high current density and the probability for the triplet–triplet annihilation is also enhanced due to long lifetime of triplet excitons. The triplet–triplet annihilation leads to the quenching of triplet excitons, decreasing the quantum efficiency of the device.
[1] M.A. Baldo, D.F.O. Brian, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [2] M.A. Baldo, S. Lamansky, P.E. Burrows, Appl. Phys. Lett. 75 (1999) 4. [3] J.Y. Lee, J.H. Kwon, H.K. Chung, Org. Electron. 4 (2003) 143. [4] S.H. Kim, J. Jang, S.J. Lee, J.Y. Lee, Thin Solid Films 517 (2008) 722. [5] K.S. Yook, S.O. Jeon, C.W. Joo, J.Y. Lee, Org. Electron. 10 (2009) 170. [6] C.-F. Chang, Y.-M. Cheng, Y. Chi, Y.-C. Chiu, C.-C. Lin, G.-H. Lee, P.-T. Chou, C.-C. Chen, C.-H. Chang, C.-C. Wu, Angew. Chem. Int. Ed. 47 (2008) 4542. [7] T. Sajoto, P.I. Djurovich, A. Tamayo, M. Yousufuddin, R. Bau, M.E. Thompson, R.J. Holmes, S.R. Forrest, Inorg. Chem. 44 (2005) 7992. [8] P. Erk, M. Bold, M. Egen, E. Fuchs, T. Geßner, K. Kahle, C. Lennartz, O. Molt, S. Nord, H. Reichelt, C. Schildknecht, H.–H. Johannes, W. Kowalsky, SID Digest 37 (2006) 131. [9] Y.-C. Chiu, J.–Y. Hung, Y. Chi, C.–C. Chen, C.–H. Chang, C.–C. Wu, Y.–M. Cheng, Y. –C. Yu, G. –H. Lee, P. –T. Chou, Adv. Mater. (2009), 10.1002/ adma.200802546. [10] 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. [11] R.J. Holmes, B.W. D0 Andrade, S.R. Forrest, X. Ren, J. Li, M.E. Thompson, Appl. Phys. Lett. 83 (2003) 3818. [12] X. Ren, J. Li, R.J. Holmes, P.I. Djurovich, S.R. Forrest, M.E. Thompson, Chem. Mater. 16 (2004) 4743. [13] 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. [14] M.F. Wu, S.J. Yeh, C.T. Chen, H. Murayama, T. Tsuboi, W. Li, I. Chao, S. Liu, J.K. Wang, Adv. Funct. Mater. 17 (2007) 1887. [15] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E. Thompson, Appl. Phys. Lett. 88 (2006) 183503. [16] A.B. Padmaperuma, L.S. Sapochak, P.E. Burrows, Chem. Mater. 18 (2006) 2389. [17] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. [18] K.S. Yook, S.O. Jeon, C.W. Joo, J.Y. Lee, J. Ind. Eng. Chem. 15 (2009) 420.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
An ethylcarbazole based phosphine oxide derivative as a host for deep blue phosphorescent organic light-emitting diode Soon Ok Jeon, Kyoung Soo Yook, Hyo Suk Son, Jun Yeob Lee n Department of Polymer Science and Engineering, Dankook University, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 July 2009 Received in revised form 23 June 2010 Accepted 29 June 2010 Available online 16 July 2010
Deep blue phosphorescent organic light-emitting diodes (PHOLEDs) were developed using a 9-ethylcarbazole based phosphine oxide material (EPO1). A high triplet energy of 3.01 eV was obtained from the EPO1 host material and efficient energy transfer from the host to the deep blue emitting phosphorescent dopant was observed. A high quantum efficiency of 7.9% with a color coordinate of (0.15, 0.17) was achieved in the deep blue PHOLED using the EPO1 host material. & 2010 Elsevier B.V. All rights reserved.
Keywords: Ethylcarbazole Phosphine oxide Deep blue phosphorescent light-emitting diodes High triplet energy
1. Introduction The development of high efficiency deep blue phosphorescent organic light-emitting diodes (PHOLEDs) is important to reduce the power consumption of organic light-emitting diodes (OLEDs). Theoretically, the quantum efficiency of the deep blue PHOLEDs can be better than that of the fluorescent OLEDs by four times and the power consumption of the active matrix type OLEDs can be greatly reduced [1–3]. However, the device performances of the deep blue PHOLEDs are not good enough in spite of a lot of researches about the materials and device structures for deep blue PHOLEDs. There have been many studies to develop host and dopant materials for deep blue PHOLEDs [4–10]. Several Ir based dopant materials have been synthesized and a deep blue color could be obtained. A tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium (FCNIr) was developed as a blue dopant and a Commission International De L’Eclairage (CIE) of (0.15, 0.16) with a quantum efficiency of 9% was obtained from the blue PHOLEDs doped with the FCNIr [4,5]. Ir based dopant materials with carbene based ligand units were synthesized and a deep blue color was achieved [6–8]. Other than these, pyrazole based Ir dopants were reported to give deep blue color [9]. In addition to the dopant materials, a lot of researches were carried out to synthesize host materials for deep blue PHOLEDs.
n
Corresponding author. Tel./fax: + 82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.06.026
Carbazole based host materials were effective as host materials for deep blue PHOLEDs due to the high triplet energy of the carbazole unit [10]. Several Si based host materials could also play a role of high triplet energy host materials for deep blue PHOLEDs [11,12]. The combination of the carbazole core and Si units was proven to be a good approach to fabricate deep blue PHOLEDs [13,14]. However, more studies are required to develop high efficiency deep blue PHOLEDs. In this work, a 9-ethylcarbazole based phosphine oxide (EPO1) compound was synthesized and it was applied as a host material in the deep blue PHOLEDs. It was demonstrated that the 9-ethylcarbazole based phosphine oxide was effective as a high triplet energy host material for the deep blue PHOLEDs.
2. Experimental 2.1. Materials and measurements 9-ethylcarbazole, n-butyllithium, chlorodiphenyphosphine, Nbromosuccinimide (Aldrich Chem. Co.) and hydrogen peroxide (Duksan Sci. Co.) were used without further purification. Tetrahydrofuran was distilled over sodium and calcium hydride. The 1H nuclear magnetic resonance (NMR) was recorded on a Varian 200 (200 MHz) spectrometer. The photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (Jasco FP-6500) and the ultraviolet–visible (UV–vis) spectra were obtained using a UV–vis spectrophotometer (Shimadzu, UV-1601PC). The differential scanning calorimeter (DSC, Mettler
S.O. Jeon et al. / Journal of Luminescence 130 (2010) 2238–2241
DSC 822 e) was used to obtain melting temperature (MP) and glass transition temperature (Tg) at a heating rate of 10 1C/min under nitrogen. The low and high resolution mass spectra were obtained using a JEOL, JMS-AX505WA spectrometer in FAB mode. The energy levels were measured with cyclic voltametry (CV). The EPO1 host material was synthesized by the phosphonation reaction of the brominated 9-ethylcarbazole. The synthetic route of 1 is shown in Scheme 1. 2.2. Synthesis of 3-bromo-9-ethylcarbazole Bromine (30.0 mmol) was added in small portions to a solution of ethylcarbazole (30.0 mmol) in tetrahydrofuran at room temperature under nitrogen atmosphere. After stirring for 5 h, water and dichloromethane were added to the solution. The organic phase was separated, washed with water, brine solution, dried over anhydrous magnesium sulphate, filtered and dried to remove the solvents. Purification by column chromatography with a mixture of dichloromethane and n-hexane gave a white solid. 2.3. Synthesis of 3-(9-ethylcarbazolyl) diphenylphosphine oxide The 3-bromo-9-ethylcarbazole (1) (4.4 mmol) was dissolved in 20 mL of anhydrous tetrahydrofuran under argon and cooled to 78 1C. 1.2 equivalent of n-butyllithium (10.0 M in hexanes, 5.3 mmol) was added dropwise to give a bright yellow solution that thickened to a slurry. Stirring was continued for 3 h at 78 1C after which 0.99 mL (5.3 mmol) of chlorodiphenylphosphine was added to give a clear, pale yellow solution. The solution was stirred for additional 3 h at 78 1C before quenching with 2 mL of degassed methanol. Volatiles were removed under a reduced pressure to give an off-white solid that was digested in methanol, filtered, then digested in water and filtered. The crude material was purified by column chromatography. 3-(diphenylphosphino)-9-ethylcarbazole (1.8 mmol), 10 mL of methylene chloride and 2 mL of 30% hydrogen peroxide were stirred overnight at room temperature. The organic layer was separated and washed with water and then brine. The extract was evaporated to dryness affording a white solid. EPO1 yield 93%. MP 119 1C. Tg 62.4 1C. 1H NMR (200 MHz, CDCl3): d 8.50–8.44 (d, 1H, Ar–CH–carbazole), 8.05–8.01 (d, 1H, Ar–CH–carbazole), 7.77–7.62 (m, 6H, OP–CH–phenyl), 7.48–7.23 (m, 7H, Ar–CH–carabazole), 4.37–4.30 (q, 2H, CH2), 1.46–1.39 (t, 3H, CH3). MS (FAB) m/z 396 [(M+ 1)+ ].
2239
Keithley 2400 source measurement unit and CS-1000 spectroradiometer.
3. Results and discussion The EPO1 was effectively synthesized by the phosphonation reaction of the brominated 9-ethylcarbazole. The synthetic yield of the EPO1 was over 90% and the purity of the compound was over 99%. The EPO1 was designed as a host material which can have high triplet energy for efficient energy transfer from the EPO1 to the deep blue phosphorescent dopant. The ethylcarbazole group has high triplet energy of 3.02 eV from our measurements and it is suitable as a core structure of the triplet host material. However, the ethylcarbazole unit has strong hole transport properties and the diphenylphosphine oxide group was attached to the ethylcarbazole group to add electron transport properties to the molecule. The diphenyl phosphine oxide unit has strong electron withdrawing character due to the polar phosphine oxide group and it improves the electron transport properties of the EPO1 [15,16]. In addition, the phosphine oxide group isolates the ethylcarbazole core from the diphenyl unit and does not affect the triplet energy of the ethylcarbazole core. The conjugation of the ethylcarbazole group is not extended by the phosphine oxide and the triplet energy of the ethylcarbazole group can be maintained. In fact, the triplet energy of the EPO1 was the same as that of the carbazole unit. Molecular simulation of the EPO1 was performed to study the molecular orbital distribution of the EPO1. Fig. 1 shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of the EPO1. The HOMO orbitals of the EPO1 were mostly localized in the ethylcarbazole unit with some orbital distribution in the diphenylphosphine oxide. The LUMO distribution of the EPO1 was quite similar to the HOMO distribution of the EPO1. This indicates that the HOMO and LUMO levels of the EPO1 are dominated by the ethylcarbazole core structure and the diphenylphosphine oxide group slightly affects the HOMO and LUMO orbital distribution of the EPO1. The diphenylphosphine oxide group may shift the HOMO and LUMO levels of the ethylcarbazole group.
2.4. Device fabrication The device structure of the deep blue PHOLED was indium tin oxide (ITO, 150 nm)/N,N0 -diphenyl-N,N0 -bis-[4-(phenyl-mtolyl-amino)-phenyl]-biphenyl-4,40 -diamine (60 nm)/N,N0 -di(1naphthyl)-N,N0 -diphenylbenzidine (30 nm)/EPO1:tris((3,5-difluoro4-cyanophenyl)pyridine) iridium (FCNIr, 30 nm, x%)/4,7-diphenyl1,10-phenanthroline (Bphen, 25 nm)/LiF (1 nm)/Al (200 nm). The doping concentrations of the FCNIr were 10%, 15% and 20%. Current density–voltage–luminance characteristics and electroluminescence (EL) spectra of the devices were measured with
N
Br2 CHCl3
n-Butyllithium
N
Fig. 1. HOMO/LUMO distribution of EPO1.
N
Chlorodiphenylphosphine Br Scheme 1. Synthetic scheme of EPO1.
CH2Cl2 P
H2O2
N O P
S.O. Jeon et al. / Journal of Luminescence 130 (2010) 2238–2241
the energy level diagram (Fig. 4), there is a large LUMO level difference of 0.65 eV between the EPO1 and the FCNIr. In addition, the energy barrier for electron injection between the Bphen and the EPO1 is 0.45 eV. Therefore, electrons are directly trapped by the FCNIr dopant and electron hopping through the FCNIr dominates the charge transport in the emitting layer. The electron hopping is facilitated at high doping concentration. The quantum efficiency of the deep blue PHOLEDs with the EPO1 host is shown in Fig. 5. The quantum efficiency was gradually increased according to the doping concentration and the EPO1 device with 20% doping concentration showed the highest quantum efficiency value. The maximum quantum efficiency of the deep blue PHOLED was 7.9%. The high efficiency in the highly doped device can be explained by the improved charge balance in the emitting layer. The electron transport in the emitting layer is facilitated at high doping concentration as explained in Fig. 3 due to the decreased hopping distance between FCNIr dopant materials. The hole injection from the hole transport layer to the EPO1 emitting layer is better than the electron injection from the electron transport layer to the EPO1 layer. Therefore, the charge balance is improved in the highly doped device because the electron transport is facilitated at high doping concentration. The better electron injection leads to better charge balance in the emitting layer. However, the
80
1200 10% 15% 20%
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400 20 200 0
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6 8 Voltage (V)
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Fig. 3. Current density–voltage–luminance curves of the EPO1 devices according to the doping concentration of FCNIr.
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UV-Vis (EPO1) PL (298 k)
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Bphen
FCNIr
metal to ligand charge transfer absorption
EPO1
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Wavelength (nm) Fig. 2. UV–vis and photoluminescence spectra of the EPO1(in solution) and FCNIr.
Luminance (cd/m2)
The HOMO and LUMO levels of the EPO1 were measured by CV and UV–vis absorption spectroscopic measurements. The HOMO level was determined by the CV and it was 5.84 eV. The bandgap of the EPO1 was 3.49 eV from UV–vis measurement and the LUMO level was 2.35 eV from the difference between the HOMO and the bandgap of the EPO1. The ethylcarbazole core has the HOMO/LUMO of 5.57 eV/2.06 eV from CV and UV–vis measurements and the diphenylphosphine oxide group lowered the energy level of the ethylcarbazole core by about 0.3 eV. The electron withdrawing properties of the phosphine oxide unit shifted the LUMO level for better electron injection than the ethylcarbazole core. UV–vis and PL spectra of the EPO1 are shown in Fig. 2. The EPO1 showed strong absorption peaks at 267 and 292 nm that are assigned to the p p* absorption of the carbazole unit. This implies that the UV–vis absorption occurs in the ethylcarbazole core of the EPO1 molecule. The bandgap was calculated from the UV–vis absorption edge of the spectrum and it was 3.49 eV. The PL emission of the EPO1 was observed at 348 and 365 nm due to the wide bandgap of the EPO1. The triplet energy of the EPO1 was measured from the PL spectrum of the EPO1 at 77 K and it was 3.01 eV from the emission peak at 410 nm. The triplet energy and bandgap of the EPO1 were quite similar to those of the 9ethylcarbazole. The diphenylphosphine oxide group did not affect the triplet energy and bandgap of the EPO1. The UV–vis absorption spectrum of the FCNIr dopant was also added and it can be clearly seen that the metal to ligand charge transfer absorption peak of the FCNIr dopant is well overlapped with the PL emission of the EPO1, indicating that the energy transfer from the EPO1 host to the FCNIr dopant is efficient. The triplet energy of the EPO1 (3.01 eV) was also higher than that of the FCNIr (2.8 eV). Therefore, efficient energy transfer from the EPO1 host to the FCNIr dopant is expected. The EPO1 can be used as a host material for deep blue PHOLED due to the high triplet energy of 3.01 eV for an efficient energy transfer to the deep blue phosphorescent dopant. The deep blue emitting FCNIr was doped into the EPO1 host and the device performances of the deep blue PHOLED were investigated. Fig. 3 shows the current density–luminance–voltage curves of the blue PHOLEDs with the EPO1:FCNIr emitting layer. The doping concentration of the device was controlled from 10% to 20%. The current density and luminance of the EPO1 based blue PHOLEDs were not greatly affected by the doping concentration although the current density was slightly increased at high doping concentration. The slight increase of the current density at high voltage in the 15% and 20% doped devices is due to the decrease of hopping distance at high doping concentration. As can be seen in
Current density (mA/cm2)
2240
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Fig. 4. Energy level diagram of the blue phosphorescent device.
S.O. Jeon et al. / Journal of Luminescence 130 (2010) 2238–2241
Fig. 6 shows the electroluminescence spectra of the EPO1 based deep blue PHOLEDs. The EPO1 based deep blue PHOLEDs showed a main emission peak at 454 nm and a shoulder peak at 480 nm. The shoulder peak was rather intensified in the highly doped blue PHOLEDs, which is due to polar–polar interaction of the FCNIr molecules at high doping concentration. The color coordinate of the deep blue PHOLEDs was (0.15, 0.16) at 10% doping concentration and it was red-shifted at 15% and 20% doping concentration. The color coordinate of the 15% and 20% doped deep blue PHOLEDs was (0.15, 0.17). The effective energy transfer from the EPO1 host to the deep blue emitting FCNIr resulted in the deep blue color and high efficiency in the EPO1 based deep blue PHOLEDs.
Quantum Efficiency (%)
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Fig. 5. Quantum efficiency–current density curves of the EPO1 devices according to the doping concentration of FCNIr.
10%
4. Conclusions In summary, 9-ethylcarbazole based phosphine oxide was effectively synthesized by the phosphonation reaction of the brominated 9-ethylcarbazole intermediate and they could be used as a host material in the deep blue PHOLED. A high quantum efficiency of 7.9% with a color coordinate of (0.15, 0.17) was obtained from the deep blue PHOLED with the 9-ethylcarbazole based phosphine oxide host.
15% Intensity (arb. unit)
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References
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Wavelength (nm) Fig. 6. Electroluminescence spectra of EPO1 devices according to the doping concentration of FCNIr.
quantum efficiency was reduced at high current density irrespective of doping concentration, which is due to triplet– triplet annihilation as reported in other works [17,18]. Triplet exciton density is increased at high current density and the probability for the triplet–triplet annihilation is also enhanced due to long lifetime of triplet excitons. The triplet–triplet annihilation leads to the quenching of triplet excitons, decreasing the quantum efficiency of the device.
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