Highly Efficient Bipolar Host Materials with Indenocarbazole and

Nov 17, 2014 - Su , S. J.; Cai , C.; Kido , J. Three-Carbazole-Armed Host Materials with Various Cores for RGB Phosphorescent Organic Light-Emitting D...
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Highly Efficient Bipolar Host Materials with Indenocarbazole and Pyrimidine Moieties for Phosphorescent Green Light-Emitting Diodes Gyeong Heon Kim,† Raju Lampande,† Mi Jin Park,† Hyeong Woo Bae,† Ji Hoon Kong,† Jang Hyuk Kwon,*,† Jung Hwan Park,*,‡ Yong Wook Park,‡,§ and Choong Eui Song§ †

Department of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul, 130-701 Republic of Korea ‡ Duksan Hi-metal Co., Ltd., 21-32, Ssukgol-gil, Ipjang-myeon, Seobuk-gu, Cheonan-si, Chungcheongnam-do 331-821 Republic of Korea § Department of Chemistry, Sungkyungkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 440-746 Republic of Korea S Supporting Information *

ABSTRACT: In this paper, we report bipolar host materials, 6-(2,6-diphenylpyrimidin-4-yl)-12,12-dimethyl-6,12,12′-trihydroindeno[1,2-b]carbazole (DPICz1) and 6-(2,6-diphenyl-pyrimidin-4-yl)-12,12-dimethyl-6,12,12′trihydroindeno[1,2-b]carbazole (DPICz2), with indenocarbazole moiety as a hole-transporting unit and pyrimidine moiety as an electron-transporting unit for highly efficient green phosphorescent organic light-emitting diodes. The synthesized materials show excellent electro-optical properties. Similarly, it also exhibits outstanding thermal and morphological stability due to high glass transition temperature (Tg) of 161 °C and decomposition temperature (Td) of 327−401 °C. Fabricated green phosphorescent organic light-emitting diode shows excellent current and power efficiency of 77.8 cd/A and 62.8 lm/W as well as almost ideal external quantum efficiency of 22.3% at the brightness of 1000 cd/m2.

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INTRODUCTION Phosphorescence organic light emitting diode (PhOLED) materials have attracted a great interest for display and lighting applications due to their theoretical ideal 100% internal quantum efficiency and about 20−25% external quantum efficiency (EQE).1−5 In addition, efficient phosphorescent dopant materials, appropriate host materials with suitable triplet energy, proper charge (holes and electrons) balance at the emissive layer (EML) etc. are the vital requirements for high performance of OLED. The proper charge balance widens the recombination zone in the EML, which leads to a high efficiency and low roll-off characteristics with minimization of triplet−triplet annihilation.6−8 The energy gap (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) between host and dopant is also very essential parameter to achieve a good charge balance. If this energy gap is less than 0.3 eV then the electron and hole could easily flow without charge carrier trapping by the dopant molecules. This minimization of charge trapping emission by dopant molecules results in the enhancement of light emission by proper energy transfer from host to dopant molecules without dopant−dopant self-quenching.9−11 To achieve a good charge balance at the EML, bipolar characteristics of host materials are very crucial for efficient hole and electron © 2014 American Chemical Society

movement. Therefore, the development of host materials with bipolar characteristics, large triplet energy, high carrier mobility, and high glass transition temperature are the essential objective for the performance improvement of OLEDs. One of the important approaches to fulfill the above requirements is by incorporating an efficient hole and electron transporting chemical moieties in the host molecules to provide a strong bipolar characteristic. Over the past few years, several efforts have been made to develop efficient phosphorescent green host materials by introducing carbazole and arylamine moieties as hole-transporting units, and phosphine oxide, oxadiazole, benzimidazole, phenanthroimidazole, phenanthroline, pyridine, triazine, and fluorene as electron-transporting units.12−16 Zhang et al. have reported 21.2% external quantum efficiency (EQE) for their synthesized green bipolar host material using 1, 3, 5-triazine and carbazole moieties.17 Host materials synthesized using carbazole or arylamine and benzimidazole or oxadiazole chemical moieties for hole and electron transport characteristics have been reported about 20−22% EQE.18−22 Likewise, Received: July 15, 2014 Revised: November 10, 2014 Published: November 17, 2014 28757

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Scheme 1. Synthetic Route of DPICz1 and DPICz2

characteristics of materials were successfully examined by using UV−vis absorption, photoluminescence (PL), cyclic voltammetry (CV), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) respectively. Our synthesized host materials, DPICz1 (6-(2,6-diphenyl-pyrimidin-4-yl)12,12-dimethyl-6,12,12′-trihydroindeno[1,2-b]carbazole) and DPICz2 (6-(2,6-diphenyl-pyrimidin-4-yl)-12,12-dimethyl6,12,12′-trihydroindeno[1,2-b]carbazole) show excellent device performances.

carbazole and phenanthroimidazole moieties were successfully incorporated in bipolar green host materials and demonstrated around 21.0% EQE.23 In particular, the rigid phenanthroimidazole moiety in host material shows an excellent morphological stability, due to high decomposition temperature (Td) and high glass transition temperature (Tg) in the range of 394− 417 and 113−243 °C, respectively.24 There are several additional reports have been available on green host materials. Recently, Li et al. reported the modified structure of CBP (N,N′-carbazole-4,4′-biphenyl) host material by adding cyano (−CN) groups to carbazole unit for enhancing the electron transport property and high thermal stability (Tg: 162 °C). They demonstrated the current efficiency of 80.61 cd/ A and EQE value of 23.13%.25 Cheng et al. have also reported the similar concept of bipolar host material by modifying CBP. They incorporated a phosphine oxide moiety into the middle of biphenyl, which reduces the conjugation length and also lowers the LUMO level with higher T1.26 The maximum EQE value of 21.6% was reported for this type host materials. In this paper, we report our designed and synthesized bipolar host materials by incorporating indenocarbazole as the hole and biphenyl pyrimidine as the electron transporting chemical unit. The indenocarbazole moiety gives higher hole transporting property and expect to offer better hole stability owing to longer conjugation length as compare to carbazole moiety. Additionally, it is also appropriate for green host materials with such extension of conjugation length. Likewise, pyrimidine moiety also provides higher electron transportation. Such good electron and hole transporting moieties could be more suitable for the green host materials. The electro-optical and thermal

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EXPERIMENTAL DETAILS Synthesis. A synthetic route and chemical structure of bipolar host materials, DPICz1 and DPICz2, are represented in Scheme 1. The two intermediates of indenocarbazole, (12,12dimethyl-11,12-dihydroindeno[2,1-a]carbazole and 11,11-dimethyl-5,11-dihydroindeno[1,2-b]carbazole) were successfully synthesized from the reaction with 9,9-dimethyl-2-(2-nitrophenyl)-9H-fluorene and triphenylphosphine in 1,2-dichlorobenzene solvent. The mixture of indenocarbazole was easily divided by column chromatography (eluent = CH2Cl2/hexane, 1:7). The DPICz1 and DPICz2 were obtained from the indenocarbazole intermediates with 4-chloro-2,6-diphenylpyrimidine by using Suzuki-coupling reaction, and their yields were 60 and 40%, respectively. General Information. 1H and 13C NMR spectra were recorded using Bruker Avance 400 NMR spectrometer. The DSC was performed using a TA Instruments DSC Q2000. Elemental analyses were carried out by using Thermoscientific FLASH 2000. UV−vis and PL spectra were measured using SCINCO S-4100 spectrometer and JASCO FP6500 spectrom28758

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eter, respectively. All reagents and solvents were purchased from Aldrich Chemical Co. and Fluka and used as received. The CV was performed using EC epsilon electrochemical analysis equipment. Platinum wire, synthesized material on indium tin oxide (ITO)/glass substrate and Ag wire in 0.1 M AgNO3 were used as counter, working and reference electrodes, respectively. Similarly, tetrabutyl ammonium perchlorate (Bu4NClO4) was used as a supporting electrolyte. Using an internal ferrocene/ferrocenium (Fc/Fc−) standard, the potential values were converted to the saturated calomel electrode (SCE) scale. The optical band gap was determined by using the tailing edge of absorption spectra. The HOMO level was obtained from the CV characteristics. The LUMO level of each material was estimated from the optical band gap and HOMO value. Fabrication and Characterization of Devices. ITO (1500 Å) coated glass substrates (sheet resistance of 10 Ω/ □) were sequentially cleaned in ultrasonic bath with acetone, and isopropyl alcohol for 10 min each, and also rinsed with deionized water after each cleaning step. Finally, the substrates were dried using nitrogen followed by UV-ozone treatment for 10 min. All organic layers and metal cathode were deposited by thermal vacuum deposition technique under a vacuum pressure of ∼1 × 10−7 Torr without breaking the vacuum pressure. The deposition rate of 0.5 Å/s was maintained for all organic layers. Deposition rates of lithium fluoride (LiF) and aluminum (Al) were maintained at 0.1 Å/s and 5 Å/s, respectively. Finally all devices were encapsulated using glass cover and UV curable resin inside the nitrogen filled glovebox. Current density− voltage (J−V) and luminance−voltage (L−V) characteristics of fabricated OLED devices were measured by using Keithley 2635A Source Meter Unit (SMU) and Konica Minolta CS100A. Electroluminescence (EL) spectra and CIE (Comission Internationale de l’Eclairage) color coordinates were obtained using a Konica Minolta CS-2000 spectroradiometer. All measurements were performed in ambient condition. The EQE values of our devices were calculated based on assumption of Lambertian distribution by the reported method.27

Figure 1. Spatial distribution of the HOMO and LUMO from DFT calculation.

particular, rotated angles of these two moieties in DPICz1 and DPICz2 are 71.8°, 30.9° respectively. A strong steric hindrance by inner dimethyl groups of DPICz1 affects to the bigger rotated angle of DPICz1. This phenomenon seems to provide a little wider band gap and high T1 characteristics. The calculated T1 values from the molecular simulations of DPICz1 and DPICz2 are 2.76 and 2.72 eV, which is in valid agreement with the experimental results. The triplet energy of bipolar host materials is well suited to use as host in green OLED device. On the other hand, calculated HOMO values are 5.28 eV (DPICz1) and 5.45 eV (DPICz2), which is slightly less deep and correlate well with the experimental values. On the other hand, the calculated LUMO values of DPICz1 and DPICz2 are 1.85 (experimental: 2.66 eV) and 1.65 eV (experimental: 2.72 eV), respectively, which is quite higher than the experimental values. The calculated band gap of DPICz2 should be smaller than DPICz1 to consider better conjugation through the smaller rotation angles between two moieties; however, our calculation results always show bigger values. It seems to be the DFT calculation error. Hence calculated LUMO of DPICz2 has higher energy level. In the experiment, the LUMO of DPICz2 is lower than the DPICz1. According to our calculations, both materials have proper T1 and appropriate HOMO, LUMO values for good phosphorescent host. The complete physical properties are summarized in Table 1. Electrochemical and Photophysical Properties. The electrochemical properties of DPICz1 and DPICz2 were evaluated by CV measurements using tetrabutylammonium perchlorate 10−4 M in acetonitrile solution and their plots are shown in Figure S1 (See Supporting Information). The HOMO values of the host materials were measured from the onset point of oxidation potential and assuming the energy level of ferrocene/ferrocenium (Fc/Fc−) (i.e., 4.80 eV). Both host materials display similar HOMO level of 5.86 eV for DPICz1 and 5.89 eV for DPICz2. The experimental LUMO energy level was determined from HOMO level and the energy band gap values. The measured energy band gap of DPICz1 and DPICz2 from the tailing edge of UV−vis absorption spectrum are 3.20 and 3.17 eV, respectively. The PL spectra at both room and low (77 K) temperatures were investigated to confirm the emission properties of two host materials. Figure 2 shows the UV−vis absorption and PL spectra of DPICz1 and DPICz2 in dilute CH2Cl2. The maximum PL peak for DPICz1 and DPICz2 in CH2Cl2 solution state (1 × 10−4 M) are observed at 451, 445 nm, respectively. The strong absorption peaks at 257 nm (for DPICz1) and 259 nm (for DPICz2) are attributed to the π−π* transition from indenocarbazole to biphenyl pyrimidine, whereas the relatively weak absorption peak at 315−333 nm could be assigned to the intramolecular charge transfer (CT) transition from indenocarbazole to the

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RESULTS AND DISCUSSION Molecular Design and Simulation. To design phosphorescent bipolar host materials, indenocarbazole, and biphenylpyrimidine were utilized as hole and electron moieties. Two indenocarbazole chemical structures are considered, where dimethyl functional group and nitrogen atom position are located in the same and opposite directions. For electron transporting pyrimidine core, biphenyl units at 2,6-positions are considered for the enhancement of stability and to increase the conjugation length. These two moieties are connected by synthetic coupling reaction. In order to understand the structural property of our designed host materials, molecular simulations were performed by using density functional theory (DFT) calculation at B3LYP/63-1G level. Figure 1 depicts molecular orbital dispersion of bipolar hosts (DPICz1 and DPICz2) in the HOMO and LUMO states. It is clearly observed that HOMO and LUMO orbitals are well-separated. The HOMO orbitals are mainly located on the hole-transport indenocarbazole moiety, whereas, LUMO orbitals are situated on the electron-transport biphenylpyrimidine moiety. Such type of arrangements of HOMO and LUMO orbitals are expected to have high T1 values. The single C−C bond between indenocarbazole and biphenylpyrimidine is not easily rotated by the steric hindrance between both of these moieties. In 28759

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Table 1. Physical Properties of DPICz1 and DPICz2 host

Tga (°C)

Tmb (°C)

Td(0.5/5.0)c (°C)

λabsd (nm)

λPLd (nm)

Ege (eV)

HOMOf (calcdg)

LUMOh (calcdg)

T1d,i (calcdg)

DPICz1 DPICz2

116 NAj

227 223

287/328 304/342

314 319, 333

451 445

3.2 (3.43) 3.17 (3.8)

5.86 (5.28) 5.89 (5.45)

2.66 (1.85) 2.72 (1.65)

2.61 (2.76) 2.58 (2.72)

a

Measured from DSC. bMelting temperature. cMeasured from TGA. dMeasured in 2-methyl-THF. eMeasured by a onset of absorption spectra. Determined from the onset of the oxidation potentials. gValues from DFT calculation B3LYP/6-31G(d). hCalculated from the HOMO and Eg. i Determined from peak value of low temperature PL spectra (77 K). jNot detected. f

at 475 and 480 nm, respectively. The calculated triplet energy levels of DPICz1 and DPICz2 using PL peaks are 2.61, 2.58 eV, respectively. The calculated and measured triplet energy values of both these materials displayed virtually small changes. The triplet energy values of both host materials are about 0.2 eV higher than the Ir(ppy)3 (tris[2-phenylpyridinato-C2,N]iridium(III)) dopant; hence, both materials could be suitable for a green host in phosphorescent devices. These host materials also show similar HOMO and LUMO values of about 5.8 and 2.7 eV, which can easily minimize the charge trapping emission by Ir(ppy)3 dopant molecules, because the energy gap between HOMO and LUMO are within 0.4 eV. Reported HOMO and LUMO values of Ir(ppy)3 are 5.5 and 3.1 eV, respectively.28 Thermal Analysis. The thermal characteristics of host materials were evaluated by TGA and DSC measurements. These compounds exhibit high thermal stability (Table 1.). The Td of DPICz1 and DPICz2 (corresponding to 5% loss) are 328 and 342 °C, respectively. This is a very stable temperature range for host materials to fabricate OLED devices using thermal evaporation process. Additionally, Tg of DPICz1 was detected at 161 °C, which is extremely higher than the widely used host material CBP (4,4′-di(N-carbazolyl)biphenyl). The reported Tg value of CBP host material is 62 °C.29 On the other hand, Tg value of DPICz2 was not observed (no phase transition determined through DSC). We strongly believe that such a high Tg value of our host materials can provide a reasonably good morphological and thermal stability for the film. Electroluminescence Properties. To evaluate the performances of bipolar host materials, green PhOLEDs were fabricated using Ir(ppy)3 as a green dopant. The fabricated device configuration is ITO/di-[4-(N,N-ditolylamino)phenyl]cyclohexane (TAPC, 75 nm)/host: 5 wt % Ir(ppy)3 (15 nm)/ 1,3,5-tris[(3-pyridyl)phen-3-yl]benzene (TmPyPB, 40 nm)/LiF (1 nm)/Al (100 nm). In these devices, ITO and Al are used as anode and cathode, respectively. The fabricated devices also contain TAPC and TmPyPB as hole and electron transport layers. The triplet energies (T1) of both hole and electron transport materials are high enough to block the triplet exciton quenching from the emissive layer.30,31 The energy level diagram of the fabricated OLED structure using host materials are shown in Figure S2 (see Supporting Information). The J−V and L−V characteristics of the fabricated devices are shown in Figure 4a. Turn-on voltages of DPICz1 and DPICz2 based devices are identical of about 2.5 V at brightness of 1 cd/m2. In particular, such a low turn on voltages of both devices are attributed to the excellent bipolar characteristics of host materials and proper charge balance at the emissive layer. However, the driving voltages of DPICz1 and DPICz2 at brightness of 1000 cd/m2 are 4.3 and 3.9 V, respectively. The device based on DPICz2 host exhibits maximum current and power efficiencies of 79.2 cd/A and 78.7 lm/W (Figure 4c), respectively. On the other hand, DPICz1 host based device

Figure 2. UV−vis absorption and PL spectra of DPICz1 and DPICz2.

pyrimidine moiety. The UV−vis absorption peaks for DPICz1 and DPICz2 were observed at 314 and 333 nm, respectively. The absorption peak of DPICz2 was detected at relatively longer wavelength as compare to the DPICz1. Relatively longer conjugation length of DPICz2 due to their outer dimethyl group is well matched with the simulated lower torsional angle results. The maximum PL peak for DPICz1 and DPICz2 in CH2Cl2 solution state (1 × 10−4 M) at room temperature are observed at 451 and 445 nm, respectively. Although the absorption spectra of DPICz1 shows shorter wavelength as compare to DPICz2, the PL spectra shows a red-shift result. This big Stoke shift means that DPICz1 molecule has more geometrical changes at excited state than DPICz2 molecule. Detail photophysical properties of the two host materials are depicted in Table 1. Phosphorescence PL spectra of DPICz1 and DPICz2 were investigated at low temperature (77 K, in liquid nitrogen) in 2methyltetrahydrofuran (THF) solution and are shown in Figure 3. Initial bands for DPICz1 and DPICz2 were observed

Figure 3. Phosphorescence PL spectra of DPICz1 and DPICz2 in 2methyl THF solution (1 × 10−4 M) at 77 K. 28760

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Figure 4. Performances of bipolar host DPICz1, PDICz2 devices with CBP host. (a) J−V and L−V characteristics, (b) EQE−luminance characteristics, and (c) power efficiency−luminance characteristic (d) EL spectra.

Table 2. EL Properties of DPICz1 and DPICz2 efficiencyb (max.)

a

a

b

c

host

turn-on voltage [V]

driving voltage [V]

CE [cd/A]

PEd [lm/W]

EQE [%]

CIE 1931 (x, y)b

DPICz1 DPICz2 CBP

2.5 2.5 2.9

4.3 3.9 4.0

58.7 (73.2.) 77.8 (79.2) 65.5 (70.1)

42.2 (88.5) 62.8 (78.7) 51.4 (64.7)

16.8 (21.0) 22.3 (22.7) 18.7 (20.1)

(0.29, 0.62) (0.30, 0.63) (0.30, 0.62)

Measured at 1 cd/m2. bMeasured at 1,000 cd/m2, cCurrent efficiency. dPower efficiency.

uration of the HOD is ITO/TAPC (800 Å)/host (300 Å)/ TAPC (400 Å)/Al (1000 Å), and the EOD is ITO/TmPyPB (800 Å)/host (300 Å)/TmPyPB (400 Å)/LiF (15 Å)/Al (1000 Å). J−V characteristics of the HOD and EOD are shown in Figure 5. The devices with DPICz2 host shows high electron and slightly lower hole current densities than the CBP host. Such efficient electron and hole current densities could provide a proper charge balance at the emissive layer, whereas DP1Cz1 shows lower hole and electron current densities but also good charge balance. The appropriate hole and electron current densities in both devices provide the evidence of bipolar

shows maximum efficiencies of 73.2 cd/A and 88.5 lm/W (Figure 4c). The DPICz2 based device also shows an excellent EQE as high as 22.3% (Figure 4b), which is an almost ideal EQE value. Additionally, the maximum luminance of DPICz1 and DPICz2 devices are 67 540 and 46 250 cd/m2, respectively. The CIE color coordinates and EL peaks of DPICz1, DPICz2 devices are (0.29, 0.63), (0.30, 0.63) and 510, 512 nm (Figure 4d), respectively. Both devices emit clean green light at the wavelength of reported Ir(ppy)3 emission, indicating that there are almost complete energy transfer from host to dopant. In addition, for the valid comparison of the OLED device performances, reference device was fabricated with widely used and commercially available green host material (CBP). The fabricated devices with indenocarbazole/biphenyl pyrimidine host materials show an excellent device performance as compare to the reference device. The detail performances of fabricated green PhOLEDs are summarized in Table 2. We strongly believe that such outstanding performances of host materials can be attributed to their excellent hole and electron transport properties, appropriate HOMO and LUMO energy values, and proper charge balance at the emissive layer. Thus, charge carrier transport properties of bipolar host materials have been carried out to corroborate our argument. In order to investigate the bipolar charge carrier transport characteristics of host materials, a hole-only device (HOD) and an electron-only device (EOD) were fabricated. The config-

Figure 5. J−V characteristics of (a) HOD and (b) EOD. 28761

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(7) Baldo, M. A.; Adachi, C.; Forrest, S. R. Transient Analysis of Organic Electrophosphorescence. II. Transient Analysis of TripletTriplet Annihilation. Phys. Rev. B 2000, 62, 10967−10977. (8) Singh-Rachford, T. V.; Castellano, F. N. Photon Upconversion Based on Sensitized Triplet-Triplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560−2573. (9) Jeon, W. S.; Park, T. J.; Kim, S. Y.; Pode, R.; Jang, J.; Kwon, J. H. Ideal Host and Guest System in Phosphorescent OLEDs. Org. Elec. 2009, 10, 240−246. (10) Chen, F. C.; He, G.; Yang, Y. Triplet Exciton Confinement in Phosphorescent Polymer Light-Emitting Diodes. Appl. Phys. Lett. 2003, 82, 1006−1008. (11) Kim, Y. H.; Kim, W. Y.; Moon, C. B. Energy Transfer between Host and Dopant Molecules in Blue Organic Light-Emitting Devices. J. Appl. Phys. 2011, 110, 034501. (12) Liu, X. K.; Zheng, C. J.; Lo, M. F.; Xiao, J.; Lee, C. S.; Fung, M. K.; Zhang, X. H. A Multifunctional Phosphine Oxide-Diphenylamine Hybrid Compound as a High Performance Deep-Blue Fluorescent Emitter and Green Phosphorescent Host. Chem. Commun. 2014, 50, 2027−2029. (13) Lee, K. H.; Kang, H. J.; Kim, H. M.; Seo, J. H.; Kim, Y. K.; Yoon, S. S. Pyridine/Isoquinoline-Carbazole Containing Bipolar Host Materials for Green Phosphorescent Organic Light-Emitting Diodes. J. Nanosci. Nanotechnol. 2011, 11, 1499−1502. (14) Chen, H. F.; Wang, T. C.; Hung, W. Y.; Chiu, H. C.; Yun, C.; Wong, K. T. Spiro-Configured Bipolar Hosts Incorporating 4,5Diazafluroene as the Electron Transport Moiety for Highly Efficient Red and Green Phosphorescent OLEDs. J. Mater. Chem. 2012, 22, 9658−9664. (15) Su, S. J.; Cai, C.; Kido, J. Three-Carbazole-Armed Host Materials with Various Cores for RGB Phosphorescent Organic LightEmitting Diodes. J. Mater. Chem. 2012, 22, 3447−3456. (16) Park, J. H.; Kim, E. K.; El-Deeb, I. M.; Jung, S. J.; Choi, D. H.; Kim, D. H.; Yoo, K. H.; Kwon, J. H.; Lee, S. H. New Bipolar Green Host Materials Containing Benzimidazole-Carbazole Moiety in Phosphorescent OLEDs. Bull. Korean Chem. Soc. 2011, 32, 841−846. (17) Liu, X. K.; Zheng, C. J.; Xiao, J.; Ye, J.; Liu, C. L.; Wang, S. D.; Zhao, W. M.; Zhang, X. H. Novel Bipolar Host Materials Based on 1,3,5-Triazine Derivatives for Highly Efficient Phosphorescent OLEDs with Extremely Low Efficiency Roll-Off. Phys. Chem. Chem. Phys. 2012, 14, 14255−14261. (18) Chen, Y. M.; Hung, W. Y.; You, H. W.; Chaskar, A.; Ting, H. C.; Chen, H. F.; Wong, K. T.; Liu, Y. H. Carbazole−Benzimidazole Hybrid Bipolar Host Materials for Highly Efficient Green and Blue Phosphorescent OLEDs. J. Mater. Chem. 2011, 21, 14971−14978. (19) Gong, S.; Chen, Y.; Luo, J.; Yang, C.; Zhong, C.; Qin, J.; Ma, D. Bipolar Tetraarylsilanes as Universal Hosts for Blue, Green, Orange, and White Electrophosphorescence with High Efficiency and Low Efficiency Roll-Off. Adv. Funct. Mater. 2011, 21, 1168−1178. (20) Tao, Y.; Wang, Q.; Yang, C.; Zhong, C.; Zhang, K.; Qin, J.; Ma, D. Tuning the Optoelectronic Properties of Carbazole/Oxadiazole Hybrids through Linkage Modes: Hosts for Highly Efficient Green Electrophosphorescence. Adv. Funct. Mater. 2010, 20, 304−311. (21) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin, J.; Ma, D. A Simple Carbazole/Oxadiazole Hybrid Molecule: An Excellent Bipolar Host for Green and Red Phosphorescent OLEDs. Angew. Chem., Int. Ed. 2008, 47, 8104−8107. (22) Mondal, E.; Hung, W. Y.; Chen, Y. H.; Cheng, M. H.; Wong, K. T. Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene-Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence. Chem.Eur. J. 2013, 19, 10563−10572. (23) Zhang, X.; Lin, J.; Ouyang, X.; Liu, Y.; Liu, X.; Ge, Z. Novel Host Materials Based on Phenanthroimidazole Derivatives for Highly Efficient Green Phosphorescent OLEDs. J. Photochem. Photobiol. A 2013, 628, 37−43. (24) Huang, H.; Wang, Y.; Zhuang, S.; Yang, X.; Wang, L.; Yang, C. Simple Phenanthroimidazole/Carbazole Hybrid Bipolar Host Materials for Highly Efficient Green and Yellow Phosphorescent Organic Light-Emitting Diodes. J. Phys. Chem. C 2012, 116, 19458−19466.

characteristics of indenocarbazole and biphenyl pyrimidine moiety based host materials. On the other hand, CBP shows slightly higher hole current density but poor electron density, which clearly indicate the improper charge balance.

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CONCLUSION In this work, we demonstrate bipolar green host materials, DPICz1 and DPICz2, with indenocarbazole moiety as a hole and pyrimidine moiety as electron-transporting unit for highly efficient phosphorescent green organic light-emitting diodes. The host materials have sufficient T1 values of ∼2.6 eV for good energy transfer to Ir(ppy)3 and proper HOMO and LUMO values to minimize charge trapping emission. Fabricated green PhOLEDs show excellent EQE of 22.3% at brightness of 1,000 cd/m2. Similarly both materials have excellent thermal and morphological stability to improve device lifetime by having very high Tg of 161 °C and Td of 327−401 °C. We strongly believe that our host materials will be suitable for high performance of OLED display and lighting applications.

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ASSOCIATED CONTENT

S Supporting Information *

Cyclic voltammetry characteristics, energy diagram of fabricated devices, and synthetic processes of DPICz1 and DPICz2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(J.H.K.) E-mail: [email protected]. Telephone: +82-2-9610948. Fax: +82-2-968-6924. *(J.H.P.) E-mail: [email protected]. Telephone: +82-41-5905432, Fax: +82-41-590-5498. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Human Resources Development program (No. 20134010200490) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and Industrial Strategic Technology Development Program (10041556) grants funded by the Korea government Ministry of Trade, Industry and Energy.

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REFERENCES

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