ARTICLE pubs.acs.org/JPCC
Tetraphenylsilane-Based High Triplet Energy Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes Yong Joo Cho and Jun Yeob Lee* Department of Polymer Science and Engineering, Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Korea ABSTRACT:
A high triplet energy host material based on a tetraphenylsilane core, (4-((4-(9H-carbazol-9-yl)phenyl)diphenylsilyl)phenyl)diphenylphosphine oxide (TSPC), was synthesized as the bipolar host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). A diphenylphosphine oxide group and a carbazole group were attached to the tetraphenylsilane core to improve the hole and electron transport properties while keeping the high triplet energy of the core structure. TSPC was effective as the host material for blue PHOLEDs, achieving a high external quantum efficiency of 22.0% with a color coordinate of (0.14, 0.18).
’ INTRODUCTION It is important to develop phosphorescent organic lightemitting diodes (PHOLEDs) to reduce the power consumption of organic light-emitting diode panels.1 In particular, high efficiency deep blue PHOLEDs have to be developed because the efficiency of deep blue PHOLEDs is currently less than that of red or green PHOLEDs.25 There are many ways to improve the efficiency of deep blue PHOLEDs. One efficient way is the development of high triplet energy host materials.612 High triplet energy dopant materials have to be used in deep blue PHOLEDs, and high triplet energy host materials are required for energy transfer to the deep blue dopants to maximize the efficiency of deep blue PHOLEDs. Several high triplet energy host materials have been synthesized; carbazole,612 arylsilane,1315 dibenzofuran, and dibenzothiophene16,17 are typical core structures for high triplet energy host materials. Among these core structures, arylsilane has a triplet energy higher than that of other core structures, and several derivatives of arylsilane have been reported.1315 Diphenyldi(o-tolyl)silane, p-bis(triphenylsilyl)benzene, and m-bis(triphenylsilyl)benzene were the first arylsilane-type host materials, and carbazole-modified arylsilane compounds were also developed. However, high external quantum efficiency could not be obtained in the deep blue PHOLEDs with arylsilane derivatives as the host materials because of poor bipolar charge transport properties. Therefore, it is mandatory to synthesize bipolar-type high triplet energy host materials with arylsilane core structure to enhance the external quantum efficiency of deep blue PHOLEDs. r 2011 American Chemical Society
In this work, a bipolar-type high triplet energy host material based on arylsilane core was synthesized and evaluated as the host material for blue PHOLEDs. The arylsilane core was modified with a hole transport-type carbazole and electron transport-type phosphine oxide to obtain bipolar charge transport properties. High external quantum efficiency of 22.0% with a color coordinate of (0.14, 0.18) was demonstrated from the blue PHOLEDs with the tetraphenylsilane-based host material.
’ EXPERIMENTAL SECTION Synthesis. The synthetic route to the TSPC host material is shown in Scheme 1. Compound 1 was synthesized according to the synthetic method reported previously18 9-(4-((4-Bromophenyl)diphenylsilyl)phenyl)-9H-carbazole (2). A mixture of compound 1 (1.00 g, 2.02 mmol), 9H-carbazole (0.33 g, 2.02 mmol), copper(I) iodide (0.38 g, 2.02 mmol), potassium carbonate (1.11 g, 8.09 mmol), and dichorobenzene (30 mL) was refluxed under argon for 30 h. After cooling, the reaction mixture was extracted with distilled water and dichloromethane and dried over anhydrous magnesium sulfate. After removal of the solvent, the residue was purified by column chromatography on silica gel using dichloromethane/n-hexane as the eluent to give a white powder. Yield 42%, 1H NMR (200 MHz, CDCl3): δ 8.168.12 (d, 2H, J = 8 Hz), 7.797.75 Received: February 25, 2011 Revised: April 19, 2011 Published: May 03, 2011 10272
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Scheme 1. Synthesis of TSPC
(d, 2H, J = 8 Hz), 7.627.37 (m, 20H), 7.327.29 (m, 2H) MS (FAB) 581 [(MþH)þ]. (4-((4-(9H-Carbazol-9-yl)phenyl)diphenylsilyl)phenyl)diphenylphosphine Oxide (TSPC). 2 (1.00 g, 1.72 mmol) was dissolved in anhydrous THF (20 mL) under argon atmosphere. The reaction flask was cooled to 78 °C, and n-BuLi (2.5 M in hexane, 1.17 mL) was added dropwise slowly. Stirring was continued for 3 h at 78 °C, followed by addition of chlorodiphenylphosphine (0.64 g, 2.92 mmol) under argon atmosphere. The resulting mixture was gradually warmed to ambient temperature and quenched by methanol (7 mL). The mixture was extracted with dichloromethane. The combined organic layers were dried over magnesium sulfate, filtered, and evaporated under reduced pressure. A white powdery product was obtained. It was dissolved in dichloromethane (20 mL) and hydrogen peroxide (4 mL), which was stirred overnight at room temperature. The organic layer was separated and washed with dichloromethane and distilled water. The extract was evaporated to dryness, affording a white solid that was further purified by column chromatography to yield 0.5 g of chemically pure TSPC. Yield 41%, 1H NMR (200 MHz, CDCl3): δ 8.178.13 (d, 2H, J = 8.0 Hz), 7.797.69 (m, 6H), 7.657.38 (m, 26H), 7.337.30 (d, 2H, J = 6.0 Hz). 13C NMR (200 MHz, CDCl3): δ 140.7, 139.5, 138.7, 137.7, 137.2, 136.2, 135.3, 133.2, 132.9, 131.8, 131.4, 131.0, 129.9, 129.5, 129.0, 128.5, 128.3, 128.1, 127.0, 126.8, 126.0, 125.8, 123.8, 121.3, 121.0, 120.0, 110.7, 109.7. MS (FAB) 702 [(M þ H)þ]. Anal. Calcd for C48H36NOPSi: C, 82.14; H, 5.17; N, 2.00. Found: C, 82.10; H, 5.25; N, 2.05. Device Fabrication and Measurements. Photophysical properties of TSPC were analyzed using ultravioletvisible (UV vis) and photoluminescence (PL) spectrometers. TSPC was dissolved in tetrahydrofuran at a concentration of 1.0 104 M for UVvis and PL measurements. Triplet energy analysis of TSPC was carried out using low temperature PL measurement in liquid nitrogen. Energy levels of TSPC were measured using cyclic voltametry (CV). Cyclic voltametry measurement of organic materials was carried out in acetonitrile solution with tetrabutylammonium perchlorate at 0.1 M concentration. Organic materials were coated on indium tin oxide substrate and were immersed in
electrolyte for analysis. Elemental analysis of the materials was carried out using EA1110 (CE instrument). The device structure used in this work was indium thin oxide (ITO, 50 nm)/N,N0 -diphenyl-N,N0 -bis[4-(phenyl-m-tolylamino)phenyl]biphenyl-4,40 -diamine (DNTPD) (60 nm)/N, N0 -di(1-naphthyl)-N,N0 -diphenylbenzidine (NPB) (20 nm)/N, N0 -dicarbazolyl-3,5-benzene (mCP) (10 nm)/TSPC: iridium(III) bis((3,5-difluoro-4-cyanophenyl)-pyridinato-N,C0 ) picolinate (FCNIrpic) (30 nm, x%)/diphenylphosphine oxide-4(triphenylsilyl)phenyl (TSPO1, 25 nm)/LiF (1 nm)/Al (200 nm). The total thickness of the organic layer was 145 nm. The doping concentration of the FCNIrpic was changed from 1% to 10%. A blue PHOLED with diphenyldi(4-(9-carbazolyl)phenyl)silane (TSDC) instead of TSPC was also fabricated for comparison.19 Electron only devices with a device structure of ITO/TSPO1(5 nm)/TSPC (30 nm) or TSDC (30 nm)/TSPO1 (25 nm)/ LiF/Al were prepared to study the electron transport properties of TSPC. A hole only device with a device structure of ITO/LiF (5 nm)/DNTPD (60 nm)/NPB (20 nm)/mCP (10 nm)/TSPC (30 nm)/DNTPD (5 nm)/Al was also fabricated. All organic materials except dopant materials were deposited at an evaporation rate of 1 Å/s. LiF was deposited at a rate of 0.1 Å/s, and Al was evaporated at a deposition rate of 5 Å/s. Devices were encapsulated with CaO getter and were sealed with a glass lid after device fabrication. Current densityvoltageluminance characteristics of the PHOLEDs were measured with a Keithley 2400 source measurement unit and a CS1000 spectroradiometer.
’ RESULTS AND DISCUSSION The high triplet energy host material, TSPC, was designed as a bipolar-type high triplet energy host material. High triplet energy can be obtained from the tetraphenylsilane core, while bipolar charge transport properties can be obtained from the carbazole and diphenylphosphine oxide units. The asymmetric TSPC can be effectively prepared from the reaction of the dibromotetraphenylsilane intermediate with 9H-carbazole followed by phosphorylation of the intermediate. 10273
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Table 1. Photophysical Properties of TSPC UV absortion PL solution Tg
TSPC
HOMO LUMO band gap ET
(nm)
(nm)
(°C)
(eV)
(eV)
(eV)
(eV)
324, 339
361
107
6.03
2.49
3.54
3.01
Figure 1. Molecular orbital simulation results of TSPC.
Figure 3. Current densityvoltage-luminance curves of TSPC blue devices according to doping concentration.
Figure 2. UVvis and PL spectra of TSPC.
Density functional theory (DFT) calculation for TSPC was carried out using a suite of Gaussian 09 program to study the molecular orbital distribution. The nonlocal density functional of Becke’s three-parameters employing LeeYangParr functional (B3LYP) with 6-31G* basis sets was used for the calculation. The molecular simulation result for TSPC is shown in Figure 1. The highest occupied molecular orbital (HOMO) of TSPC was localized on the carbazole unit, while the lowest unoccupied molecular orbital (LUMO) was distributed over the diphenylphosphine oxide-substituted phenyl unit. This indicates that the HOMO and LUMO are mostly affected by the carbazole and diphenylphosphine oxide of TSPC. As the tetraphenylsilane core does not extend the conjugation between the carbazole and diphenylphosphine oxide,13 the HOMO and LUMO were localized on the carbazole and diphenylphosphine oxide. Therefore, it is expected that TSPC would show both hole transport and electron transport properties. In addition, the band gap of TSPC can be reduced compared with that of the tetraphenylsilane core, as the molecular orbital is not dispersced over the tetraphenylsilane core. The photophysical properties of TSPC were analyzed using UVvis and PL spectra. Figure 2 shows the UVvis absorption and PL spectra of TSPC. TSPC exhibited strong absorption peaks of the carbazole and diphenylphosphine oxide at 339 nm and solution PL emission at 361 nm. The band gap was calculated from the edge of the UVvis absorption peak, and it was 3.54 eV. The band gap of TSPC was reduced compared with that of the tetraphenylsilane core with over 4.00 eV band gap because of the carbazole unit. The triplet energy was measured from the low temperature PL spectra of TSPC solution at 77 K, and it was 3.01 eV. The triplet energy of the solid film was shifted by 0.30 eV, and it was 2.71 eV. The solid PL spectrum of TSPC was red-shifted compared to the solution PL spectra because of intermolecular interaction between TSPC molecules.
The HOMO level of TSPC was measured using CV, and it was 6.03 eV. As the HOMO was localized on the phenylcarbazole unit of TSPC, a HOMO level corresponding to the phenylcarbazole unit was observed. The LUMO level was calculated from the HOMO and band gap from UVvis spectra, and it was 2.49 eV. The HOMO level of TSPC was suitable for hole injection from common hole transport material, while the LUMO level was suitable for electron injection from TSPO1 electron transport material. Detailed photophysical properties of TSPC are summarized in Table 1. TSPC was tested as the high triplet energy host material for deep blue emitting FCNIrpic dopant and was compared with TSDC. Figure 3 shows the current densityvoltage and luminancevoltage curves of FCNIrpic-doped TSPC according to the doping concentration. The current density decreased according to the doping concentration up to 5% and then increased above 5% doping concentration. The decrease of the current density up to 5% is due to a charge-trapping effect by the FCNIrpic dopant, while the increase of the current density at high doping concentration is due to a charge-hopping effect between dopants. The luminance dependency on doping concentration was similar to current density dependency, as the current density is critical to the luminance. The turn-on voltage of TSPC blue PHOLEDs was 3.5 V at all doping concentrations. The TSPC devices showed a current density and luminance much higher than that of the TSDC device with carbazole groups but without a diphenylphosphine oxide group. The turn-on voltage of the TSDC device was 4 V compared with 3.5 V of the TSPC device. The improved current density and luminance of the TSPC device is closely related with the better electron transport properties of TSPC. The TSPC has an electron-transporting diphenylphosphine oxide group in the molecular structure, which increases the electron density in the emitting layer. Hole only and electron only devices of TSPC and TSDC were fabricated to compare the hole and electron density in the two host materials. Figure 4 shows the hole and electron current density of TSPC and TSDC devices. As seen in the figure, the electron current density of the TSPC electron only device was 10274
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Figure 6. Energy level diagram of the blue device.
Figure 4. Current densityvoltage curves of the hole and electron only devices of TSPC and TSDC.
Figure 5. External quantum efficiencyluminance (a) and power efficiencyluminance curves of TSPC blue devices according to doping concentration.
much higher than that of the TSDC electron only device. The TSDC does not have any electron transport unit in the molecular structure, which hinders electron transport through the TSDC. However, TSPC has the electron-transporting diphenylphosphine oxide group, facilitating electron transport and injection from electron transport layer. Therefore, TSPC showed electron current density higher than that of TSDC. TSPC showed similar hole and electron current density, indicating bipolar charge transport properties. External quantum efficiencyluminance curves of the TSPC blue PHOLEDs are shown Figure 5. The external quantum efficiency of the TSPC device was optimized at 5% doping concentration, and the maximum external quantum efficiency of the optimized device was 22.0% with a color coordinate of (0.14, 0.18). Although there have been several papers reporting
over 20% external quantum efficiency in sky blue PHOLEDs, there have been few data reporting theoretical maximum external quantum efficiency in blue PHOLEDs.20 This efficiency value is one of the best reported for blue PHOLEDs. The external quantum efficiency at 1000 cd/m2 of the TSPC device was 15.3%. The external quantum efficiency of the TSPC blue PHOLEDs was a little degraded at 1% and 10% doping concentrations, but it was not significantly decreased. The maximum external quantum efficiency of the 1% FCNIrpic-doped TSPC device was 19.0%, while that of the 10% FCNIrpic-doped TSPC device was 21.8%. The external quantum efficiency change according to doping concentration was not significant in the TSPC device. The high efficiency of the TSPC device can be explained by the efficient charge injection, bipolar charge transport properties, and efficient energy transfer from TSPC to FCNIrpic dopant. There was little energy barrier for charge injection in the TSPC deep blue PHOLEDs. The energy barrier for electron injection was 0.03 eV, and no energy barrier existed for hole injection. Therefore, holes and electrons could be injected effectively from the charge transport layers to the emitting layer. The energy level diagram of blue PHOLEDs is shown in Figure 6. Bipolar charge transport properties of TSPC are also responsible for the high external quantum efficiency. TSPC has hole-transporting carbazole and electron-transporting diphenylphosphine oxide units, inducing bipolar charge transport properties in the emitting layer. In addition, efficient energy transfer from TSPC to FCNIrpic dopant contributed to the high external quantum efficiency. The triplet energy of 2.75 eV for TSPC was high enough for energy transfer to FCNIrpic dopant, enhancing the external quantum efficiency of the device. Compared with the TSDC blue PHOLED, TSPC showed higher external quantum efficiency at the same luminance. Power efficiencyluminance curves of the TSPC and TSDC blue PHOLEDs are also shown in Figure 5. The power efficiency of the TSPC device was higher than that of the TSDC device over the entire luminance range. It is generally known that the power efficiency is determined by the current efficiency and driving voltage. In the case of the TSPC device, both the efficiency and driving voltage were improved compared to those of TSDC device. Therefore, the power efficiency of the TSPC device was significantly higher than that of the TSDC device. Electroluminescence (EL) spectra of the TSPC blue PHOLEDs are shown in Figure 7. All devices showed a strong emission peak of FCNIrpic between 457 and 459 nm which is designated as the main emission peak. A shoulder peak also appeared in the EL spectra and was strong at high doping concentration because of intermolecular interaction. The color coordinate of the 1% FCNIrpic-doped device was (0.14, 0.17), and it was slightly red-shifted at 5% and 10% doping concentrations. The color coordinate of the 5% FCNIrpic-doped device 10275
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Figure 7. Electroluminescence spectra of TSPC blue devices according to doping concentration.
was (0.14, 0.18), while that of 10% FCNIrpic-doped device was (0.14, 0.20). Therefore, high efficiency blue PHOLEDs could be effectively fabricated using the TSPC host material.
ARTICLE
(11) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Funct. Mater. 2009, 19, 3644. (12) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Mater. 2010, 22, 1872. (13) Ren, X.; Li, J.; Homes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004, 16, 4743. (14) Lin, J.-J.; Liao, W.-S.; Huang, H.-J.; Wu, F.-I.; Cheng, C.-H. Adv. Funct. Mater. 2008, 18, 485. (15) Shih, P.-I.; Chien, C.-H.; Chuang, C.-Y.; Shu, C.-F.; Yang, C.-H.; Chen, J.-H.; Chi, Y. J. Mater. Chem. 2007, 17, 1692. (16) Vecch, P. A.; Padmaperuma, A. B.; Qiao, H.; Sapochak, L. S.; Burrows, P. E. Org. Lett. 2006, 8, 4211. (17) Kim, D.; Salman, S.; Coropceanu, V.; Saomon, E.; Padmaperuma, A B.; Sapochak, L. S.; Kahn, A.; Bredas, J.-L. Chem. Mater. 2010, 22, 247. (18) Jung, S. O.; Kim, Y.-H.; Kwon, S.-K.; Oh, H.-Y.; Yang, J.-H. Org. Electron. 2007, 8, 349. (19) Kim, S. H.; Jang, J.; Lee, S. J.; Lee, J. Y. Thin Solid Films 2008, 517, 722. (20) Jeon, S. O.; Jang, S. E.; Son, H. S.; Lee, J. Y. Adv. Mater. 2011, 23, 1436.
’ CONCLUSIONS A high triplet energy host material, TSPC, was developed as a host for blue PHOLEDs, and a high efficiency blue PHOLED with a external quantum efficiency of 22.0% was fabricated. The substitution of carbazole and diphenylphosphine oxide on the tetraphenylsilane backbone was effective to enhance the external quantum efficiency and power efficiency of blue PHOLEDs. This approach can be useful in future developments of high triplet energy host materials for high external quantum efficiency in blue PHOLEDs. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: 82-31-8005-3585. Fax: 82-31-8005-3585.
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