Dyes and Pigments 97 (2013) 286e290
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A carbazole-based dendritic host material for efﬁcient solutionprocessed blue phosphorescent OLEDs Wen Yang, Yuansheng Chen, Wei Jiang*, Xinxin Ban, Bin Huang, Yunqian Dai, Yueming Sun* School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2012 Received in revised form 14 December 2012 Accepted 17 December 2012 Available online 23 January 2013
A dendritic host material 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazol-9-yl]-biphenyl for solution-processed blue phosphorescent organic light-emitting devices was designed and synthesized. Owing to the decrease of the p conjugation length of biphenyl moiety in molecule structure, this carbazole derived derivative shows high triplet energy. Furthermore, the thermal, photophysical and electrochemical properties of 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazol-9-yl]-biphenyl were investigated. The high triplet energy of 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazol-9yl]-biphenyl ensures efﬁcient energy transfer from the host to the triplet emitter iridium(III) bis(4,6diﬂuorophenylpyridinato)picolinate. The single layer device using the carbazole derivatives as the host for iridium(III) bis(4,6-diﬂuorophenylpyridinato)picolinate showed the maximum luminance efﬁciencies of 5.8 cd A1, and a maximum external quantum efﬁciency 2.8%. The efﬁciency of the carbazole derivative based device was almost 3 times higher than the corresponding 1,3-bis(9-carbazolyl)benzene based device. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: OLEDs Blue phosphorescence Dendrimer Host materials Carbazole CBP
1. Introduction In contrast to conventional ﬂuorescent organic light-emitting diodes (OLEDs), the phosphorescent OLEDs (PhOLEDs) can reach to 100% of theoretical internal quantum efﬁciency by harvesting both singlet and triplet excitons, PhOLEDs are considered as promising candidates for the next generation large-size displays and solid-state lighting panels [1e5]. Usually, the phosphorescent emitters are doped into a suitable host to reduce self-aggregation quenching and tripletetriplet annihilation. It is essential that the triplet energy of the host material should higher than that of the phosphorescent emitter to prevent energy back transfer and to conﬁne the electro-generated triplet excitons on the dopant molecules. Therefore, to achieve the high performance PhOLEDs, the development of efﬁcient host materials is of great importance. For example, 4,40 -bis(9-carbazolyl)- 2,20 -biphenyl (CBP) is commonly used as a host material, but not for blue triplet emitters because of its lower triplet energy gap of 2.56 eV relative to those of blue triplet emitters such as iridium(III) bis(4,6-diﬂuorophenylpyridinato)picolinate (FIrpic) (ET ¼ 2.62 eV) . Recently, a series of structurally
* Corresponding authors. Tel./fax: þ86 25 52090621. E-mail addresses: [email protected]
n, [email protected]
(W. Jiang), [email protected]
(Y. Sun). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2012.12.030
modiﬁed CBP-derivatives with high triplet energies for highly efﬁcient blue phosphorescent OLEDs have been developed [7e13]. For example, 1,3-bis(9-carbazolyl)benzene (mCP) with a high triplet energy of 2.90 eV has been employed as a host material for blue phosphorescent OLEDs through replacing the biphenyl unit in CBP by a single benzene group in combination with a meta-relationship instead of a para-relationship between the N-position of carbazole and phenyl . Gong et al. has recently reported two CBP isomers with high triplet energies (3.00 and 2.84 eV) by ﬁnely tuning the linking topology between the two carbazole units and biphenyl group from di-para-position to di-meta- or di-orhto-positions . However, those CBP-derivatives also suffer from poor thermal and morphological stability due to the low molecular weight and may exhibit phase segregation and aggregation in the devices. During the search for host materials for blue PhOLEDs, a great deal of attention has been focused on carbazole-based molecules, owing to their high triplet energy and excellent hole transporting properties [14e32]. In our recent work, we proposed to modify CBP by the linking of two 3,6-di-tert-butylcarbazole moieties into the 3, 6 positions of the carbazole units to build a dendritic host material CzCBP. As a result, the new host exhibits excellent thermal and morphological stability. The HOMO energy level of Cz-CBP is raised and approaches the work function of poly(3,4-ethylenedioxythiophene) (PEDOT, 5.2 eV), which is believed to facilitate the hole injecting. Furthermore, the triplet energy of Cz-CBP is increased to 2.68 eV,
W. Yang et al. / Dyes and Pigments 97 (2013) 286e290
higher than the value of the commonly used triplet blue-emitter FIrpic, which enables its application as a host for blue phosphorescent dopants. The inﬂuence of the carbazole moieties on the thermal stability, electrochemical properties, photophysical behavior, and EL performance of Cz-CBP host material is investigated.
collected with a Photo Research PR705 Spectrophotometer. All measurements of the devices were carried out in ambient atmosphere without further encapsulations.
The geometrical and electronic properties of CBP and Cz-CBP were performed with the Gaussian 03 program package. The calculation was optimized at the B3LYP/6-31G(d) level of theory. The molecular orbitals were visualized using Gaussview.
2.1. General information All reactants and solvents, unless otherwise stated, were purchased from commercial sources and used as received. 1H NMR and 13 C HMR spectra were measured on a Bruker ARX300 NMR spectrometer with tetramethylsilane as the internal standard. Elemental analysis was performed on an Elementar Vario EL CHN elemental analyzer. Mass spectrometry was performed with a Thermo Electron Corporation Finnigan LTQ mass spectrometer. Absorption spectra were recorded with a UVevis spectrophotometer (Agilent 8453) and PL spectra were recorded with a ﬂuorospectrophotometer (Jobin Yvon, FluoroMax-3). TGA was recorded with a Netzsch simultaneous thermal analyzer (STA) system (STA 409PC) under a dry nitrogen gas ﬂow at a heating rate of 10 C min1. Glass-transition temperature was recorded by DSC at a heating rate of 10 C min1 with a thermal analysis instrument (DSC 2910 modulated calorimeter). Cyclic voltammetry was performed on a Princeton Applied Research potentiostat/galvanostat model 283 voltammetric analyzer in CH2Cl2 solutions (103 M) at a scan rate of 100 mV s1 with a platinum plate as the working electrode, a silver wire as the pseudo-reference electrode, and a platinum wire as the counter electrode. The supporting electrolyte was tetrabutylammonium hexaﬂuorophosphate (0.1 M) and ferrocene was selected as the internal standard. The solutions were bubbled with a constant argon ﬂow for 10 min before measurements.
2.3. Quantum chemical calculations
2.4. Materials Compound 3,6-Bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole was prepared according to published procedures . Synthesis of 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)carbazol-9-yl]-biphenyl (Cz-CBP). 4,4 - Dibromobiphenyl (1.0 g, 3.2 mmol), 3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole (5.4 g, 7 mmol), copper(I) iodide (0.06 g, 0.3 mol), 18-crown-6 (0.08 g, 0.3 mmol), K2CO3 (3.18 g, 30 mmol), and DMPU (10 mL) were added sequentially to a sealed tube under nitrogen and heated at 180 C in oil bath for 48 h. Then the mixture was cooled to room temperature and ﬁltered. The ﬁltrate was extracted with dichloromethane and water. After the organic layer was dried by anhydrous MgSO4 and ﬁltered, the product was puriﬁed by chromatography (silica gel, nhexane/CH2Cl2, 10:1) to give 1.79 g (35.2%) of a white solid which was further recrystallized three times from n-hexane/CH2Cl2 to give 0.51 g (10.6%) of Cz-CBP. 1H NMR (500 MHz, CDCl3, d): 8.28 (s, 4H), 8.18 (s, 8H), 8.11-7.96 (m, 4H), 7.92 (d, J ¼ 8.0 Hz, 4H), 7.81-7.47 (m, 8H), 7.49 (d, J ¼ 7.2 Hz, 8H), 7.37 (d, J ¼ 7.5 Hz, 8H), 1.48 (s, 72H). 13 C NMR (500 MHz, CDCl3, d): 142.7, 140.2, 131.2, 129.0, 128.9, 127.7, 127.6, 126.1, 126.0, 124.2, 123.6, 123.2, 119.4, 116.3, 111.2, 111.1, 109.1, 34.8, 32.1. MS (MALDI-TOF) [m/z]: calcd for C116H1116N6, 1594.2; found, 1593.9. Anal. Calcd. for C116H1116N6 (%): C, 87.39; H, 7.33; N 5.27. Found: C, 87.50; H, 7.26; N 5.20.
2.2. Device fabrication and performance measurements 3. Results and discussion In a general procedure, indium-tin oxide (ITO)-coated glass substrates were pre-cleaned carefully and treated by UV ozone for 4 min. A 40 nm poly(3,4-ethylenedioxythiophene) doped with poly(styrene-4-sulfonate)(PEDOT:PSS) aqueous solution was spin coated onto the ITO substrate and baked at 210 C for 10 min. The substrates were then taken into a nitrogen glove box, where FIrpicdoped host:1,3-bis[4-tert-butylphenyl0-1,3,4-oxidiazolyl]phenylene (OXD-7) layer was spin coated onto the PEDOT:PSS layer from 1,2-dichloroethane solution and annealed at 120 C for 30 min. The substrate was then transferred into an evaporation chamber, where the Cs2CO3/Al bilayer cathode was evaporated at evaporation rates of 0.2 and 10 A/s for Cs2CO3 and Al, respectively, under a pressure of 1 103 Pa. The current-voltage-brightness characteristics of the devices were characterized with Keithley 4200 semiconductor characterization system. The electroluminescent spectra were
3.1. Synthesis and characterization Scheme 1 shows the synthetic routes and structure of the newly synthesized dendrimer Cz-CBP. The intermediate 3,6-bis(3,6-ditert-butylcarbazol-9-yl)-carbazole was obtained according to previously methods . Subsequently, the aromatic CeN coupling reactions of 4,4-dibromobiphenyl with 3,6-Bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole led to Cz-CBP with a yields of 35.2%. Finally, the dendrimer was puriﬁed by the silica column method and recrystallization three times from n-hexane/CH2Cl2, yielding a very pure white powder. 1H NMR, 13C NMR, matrix-assisted laser desorption ionization time-of-ﬂight (MALDI-TOF) mass spectrometry, and elemental analysis were employed to conﬁrm the chemical structures of Cz-CBP.
Scheme 1. Synthesis and molecular structure of Cz-CBP.
W. Yang et al. / Dyes and Pigments 97 (2013) 286e290
3.2. Thermal analysis The thermal stabilities of Cz-CBP was investigated by thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere at a heating rate of 10 C min1. As shown in Fig. 1, TGA measurement reveals its high thermal decomposition temperature (Td), corresponding to 5% weight-loss) of 475 C. This value is about 100 C above the CBP  which is attributed to the high molecular weight of Cz-CBP. The DSC trace exhibits a clear glass transition temperature (Tg) of 287 C during the second heating scans, which is much higher than the value of CBP (62 C) . The thermal analysis results clearly demonstrate the additional 3,6-di-tert-butylcarbazole moieties greatly enhances the thermal stability of Cz-CBP, which facilitates the forming of amorphous ﬁlms through solution processing. 3.3. Photophysical properties The photophysical properties of Cz-CBP in CH2Cl2 were analyzed using UVevis absorption and photoluminescence (PL) spectra. As shown in Fig. 2, the absorption spectra of Cz-CBP exhibits three absorption bands centered at 245e350 nm, which are nearly identical to those of the outer-layer carbazole monomer and transitions between the inner-layer carbazole and biphenyl units, thus can be attributed to the pep* transitions. Upon UV excitation, the PL spectrum of Cz-CBP has lost vibronic structure and the emission peaks red-shifted to 402 nm. The band gap was calculated from the edge of the Uvevis absorption peak, giving a value of 3.42 eV for CzCBP. The phosphorescence spectra measured from a frozen 2methyltetrahydrofuran matrix at 77 K are also shown in Fig. 3. The triplet energy of Cz-CBP was determined to be 2.68 eV by the highest energy 0e0 phosphorescent emission in 2methyltetrahydrofuran at 77 K, which is sufﬁciency high enough to serve as the appropriate hosts for FIrpic (2.62 eV). 3.4. Electrochemical analysis and theoretical calculations Cyclic voltammetry (CV) was performed to investigate the electrochemical properties of the compounds. As shown in Fig. 3, during the anodic scan in dichloromethane, Cz-CBP showed reversible oxidation behavior, which can be attributed to the introduction of two tert-butyl groups at the 3,6 positions of outlayer carbazole units . In contrast, when the electrochemically
Fig. 1. TGA and DSC traces of Cz-CBP.
Fig. 2. Normalized absorption, emission and phosphorescence (77 K) spectra of CzCBP.
active sites (3,6 positions of carbazole) are left unblocked, the oxidation processes of CBP are not reversible, with the oxidation potential gradually shifting to lower potentials and the current increasing during repeated CV scans (inset of Fig. 3) . No reduction waves were detected. On the basis of the onset potentials for oxidation, the HOMO energy of Cz-CBP was estimated to be 5.30. Compared with CBP (HOMO ¼ 5.51 eV), it is believed that the holes are easily injected from the PEDOT:PSS to the emitting layer. The LUMO level was calculated from the HOMO and bandgap from UVevis, giving a value 1.98 eV for Cz-CBP. The effect of the substitution on CBP of the carbazole units was also investigated with the density functional theory (DFT) calculations. The calculated geometries of Cz-CBP and CBP in Fig. 4 show that the outer-layer carbazole units are signiﬁcantly twisted against the biphenyl groups. The geometrical characteristic of Cz-CBP can effectively prevent intermolecular interactions between pe systems and thus suppress molecular recrystallization, which improves the morphological stability of the thin ﬁlm. In the case of CBP, the dihedral angle of carbazole and biphenyl unit was 37.7, while the dihedral angle of Cz-CBP increased to a value of 41.0 . The
Fig. 3. Oxidation part of the CV curves of CBP and Cz-CBP in CH2Cl2 solutions.
W. Yang et al. / Dyes and Pigments 97 (2013) 286e290
Fig. 4. Optimized geometries and calculated HOMO and LUMO density maps for CBP and Cz-CBP.
extent of conjugation between the central biphenyl units was shortened by the increase dihedral angles of Cz-CBP, which leads to improvement of the triplet energy gap. This calculated result is in agreement with the values measured by the low temperature phosphorescence spectra. In addition, the HOMO and LUMO level of CBP is localized on the carbazole units and the biphenyl core, respectively. However, for Cz-CBP, its LUMO level is similarly localized on the biphenyl core, while the HOMO is mostly distributed over the three outer-layer carbazole units. It was conﬁrmed by CV results that the introduction of carbazole units linked
through the 3, 6 positions of the carbazole unit can lead to the increasing of the HOMO levels. This design strategy should be extended to other will-designed host materials of low-lying HOMO levels . 3.5. Electroluminescent properties The single-layer blue electrophosphorescent device with the conﬁguration of ITO/PEDOT:PSS/Host:OXD-7(30 wt%):FIrpic(10 wt %)/Cs2CO3/Al was fabricated by solution-processing methods. In this device, PEDOT:PSS was used as the hole-injection layer; the electron-transporting material OXD-7 was mixed into the host materials to facilitate the electron transport in the emitting layer [35,36]; FIrpic with an optimized concentration of 10 wt% was used as a dopant emitter; and Cs2CO3 was used as the electron-injection layer. Current density-voltage-luminance and luminous efﬁciencycurrent density characteristics are shown in Fig. 5. The device with Cz-CBP as host displays a turn-on voltages of 5.6 V and a maximum luminance of 8600 cd m2 (13.5 V), achieves a maximum current efﬁciency of 5.8 cd A1, and the corresponding maximum external quantum efﬁciency of 2.8%. The efﬁciencies of Cz-CBP showed nearly 3 times higher than the reported values of mCP with the maximum current efﬁciency of 1.8 cd A1 and maximum external quantum efﬁciency of 1.0% with the same device conﬁguration . This performance is probably due to the excellent thermal stability of the dendritic host Cz-CBP, which signiﬁcantly enhances the capability of forming stable amorphous thin ﬁlms. The normalized electroluminescence (EL) spectrum of device was shown in Fig. 5(b). As clearly seen, the spectra peak at 474 nm, and the EL bands are nearly identical with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.36) for a pure blue shade. Hence, the EL originates only from the FIrpic, indicating a complete energy transfer from the blend hosts to the FIrpic. 4. Conclusions
Fig. 5. (a) Current density-voltage-luminance characteristics, (b) luminance efﬁciency versus current density plots and EL spectra for the Cz-CBP devices.
In conclusion, we have designed and synthesized a solutionprocessable dendritic host material based on the CBP molecule. The molecular structure of Cz-CBP shows a more twist conﬁguration and decreases the p conjugation of biphenyl moiety. As a result, Cz-CBP possesses high triplet energy of 2.68 eV and excellent thermal and morphological stability with high glass transition temperature of 287 C. Utilizing the dendrimer as host material, high-performance single-layer blue PhOLEDs has been successfully fabricated by spin coating, which is almost 3 times higher than the corresponding mCP-based devices.
W. Yang et al. / Dyes and Pigments 97 (2013) 286e290
Acknowledgments The authors thank Dr. Zhao Yan for OLEDs fabrication and measurement. This work was supported by the National Natural Science Foundation of China (Grant No.51103023, 21173042 and 21201034) and the National Basic Research Program of China (Grant No.2013CB932900).
References  Tang CW, Vanslyke SA. Organic electroluminescent diodes. Applied Physics Letters 1987;51:913e5.  Forrest SR. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 2004;428:911e8.  D’Andrade BW, Forrest SR. White organic light-emitting devices for solid-state lighting. Advanced Materials 2004;16:1585e95.  Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al. Highly efﬁcient phosphorescent emission from organic electroluminescent devices. Nature 1998;395:151e4.  Baldo MA, O’Brien DF. Excitonic singlet-triplet ratio in a semiconducting organic thin ﬁlm. Physical Review B 1999;60:14422e8.  Holmes RJ, D’Andrade BW, Forrest SR, Ren X, Li J, Thompson ME. Efﬁcient, deep-blue organic electrophosphorescence by guest charge trapping. Applied Physics Letters 2003;83:3818e20.  Gong SL, He X, Chen YH, Jiang ZQ, Zhong C, Ma DG, et al. Simple CBP isomer with high triplet energies for highly efﬁcient blue electrophosphorescence. Journal of Materials Chemistry 2011;22:2894e9.  Holmes RJ, Forrest SR, Tung YJ, Kwong RC, Brown JJ, Garon S, et al. Blue organic electrophosphorescence using exothermic host-guest energy transfer. Applied Physics Letters 2003;82:2422e4.  Shirota Y. Organic materials for electronic and optoelectronic devices. Journal of Materials Chemistry 2000;10:1e25.  Hu DH, Lu P, Wang CL, Liu H, Wang H, Wang ZM, et al. Silane coupling dicarbazoles with high triplet energy as host materials for highly efﬁcient blue phosphorescent devices. Journal of Materials Chemistry 2009;19:6143e8.  Tokito S, Lijima T, Suzuri Y, Kita H, Tsuzuki T, Sato F. Conﬁnement of triplet energy on phosphorescent molecules for highly-efﬁcient organic blue-lightemitting devices. Applied Physics Letters 2003;83:569e71.  Lee N, Lee D, Kim D, Lee J, Cho SH, Jeon WS, et al. Highly efﬁcient soluble materials for blue phosphorescent organic light-emitting diode. Dyes and Pigments 2012;95:221e8.  Schrogel P, Tomkeviciene A, Strohriegl P, Hoffmann ST, Kohler A, Lennartz C. A series of CBP-derivatives as host materials for blue phosphorescent organic light-emitting diodes. Journal of Materials Chemistry 2011;21:2266e73.  Ding JQ, Zhang BH, LÜ JH, Xie ZY, Wang LX, Jing XB, et al. Solution-processable carbazole-based conjugated dendritic hosts for power-efﬁcient blue-electrophosphorescent devices. Advanced Materials 2009;21:4983e6.  Ding JQ, Wang Q, Zhao L, Ma DG, Wang LX, Jing XB, et al. Design of star-shaped molecular architectures based on carbazole and phosphine oxide moieties: towards amorphous bipolar hosts with high triplet energy for efﬁcient blue electrophosphorescent devices. Journal of Materials Chemistry 2010;20: 8126e33.  Hou LD, Duan L, Qiao J, Li W, Zhang DQ, Qiu Y. Efﬁcient single layer solutionprocessed blue-emitting electrophosphorescent devices based on a smallmolecule host. Applied Physics Letters 2008;92:263301.  Duan L, Qiao J, Sun YD, Qiu Y. Strategies to design bipolar small molecules for OLEDs: donor-acceptor structure and non-donor-acceptor structure. Advanced Materials 2011;23:1137e44.
 Jiang W, Duan L, Qiao J, Wang LD, Zhang DQ, Qiu Y. Novel star-shaped host materials for highly efﬁcient solution-processed phosphorescent organic light-emitting diodes. Journal of Materials Chemistry 2010;20:6131e7.  Jeon YM, Lee IH, Lee HS, Gong MS. Orange phosphorescent organic lightemitting diodes based on spirobenzoﬂuorene type carbazole derivatives as a host material. Dyes and Pigments 2011;89:29e36.  Chen YM, Huang WY, You HW, Chaskar A, Ting HC, Chen HF, et al. Carbazole-benzimidazole hybrid bipolar host materials for highly efﬁcient green and blue phosphorescent OLEDs. Journal of Materials Chemistry 2011;21:14971e8.  Zhang FF, Zhou CH, Yan JP. New progress of researches in carbazole compounds. Chinese Journal of Organic Chemistry 2010;30:783e96.  Tao YT, Wang Q, Yang CL, Zhong C, Zhang K, Qin JG, et al. Tuning the optoelectronic properties of carbazole/oxadiazole hybrids through linkage modes: hosts for highly efﬁcient green electrophosphorescence. Advanced Functional Materials 2010;20:304e11.  Jiang W, Duan L, Qiao J, Wang LD, Qiu Y. Tuning of charge balance in bipolar host materials for highly efﬁcient solution-processed phosphorescent devices. Organic Letters 2011;13:3146e9.  Jiang W, Duan L, Qiao J, Wang LD, Zhang DQ, Qiu Y. High-triplet-energy tricarbazole derivatives as host materials for efﬁcient solution-processed blue phosphorescent devices. Journal of Materials Chemistry 2011;21:4918e26.  Su SJ, Cai C, Kido J. Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes. Journal of Materials Chemistry 2012;22:3447e56.  Chou HH, Cheng CH. A highly efﬁcient universal bipolar host for blue, green, and red phosphorescent OLEDs. Advanced Materials 2010;22:2468e71.  Tsai MH, Lin HW, Su HC, Ke TH, Wu CC, Fang FC, et al. Highly efﬁcient organic blue electrophosphorescent devices based on 3,6-bis(triphenylsilyl)carbazole as the host material. Advanced Materials 2006;18:1216e20.  Sapochak LS, Padmaperuma AB, Cai X, Male JL, Burrows PE. Inductive effects of diphenylphosphoryl moieties on carbazole host materials: design rules for blue electrophosphorescent organic light-emitting devices. Journal Physical Chemistry C 2008;112:7989e96.  Vezzu DAK, Deaton JC, Shayeghi M, Li YM, Huo SQ. Acridinone/amine(carbazole)-based bipolar molecules: efﬁcient hosts for ﬂuorescent and phosphorescent emitters. Organic Letters 2009;11:4310e3.  Jiang W, Duan L, Qiao J, Dong GF, Zhang DQ, Wang LD, et al. Novel carbazole/ pyridine-based host material for solution-processed blue phosphorescent organic light-emitting devices. Dyes and Pigments 2012;92:891e6.  Kim D, Coropceanu V, Bredas JL. Design of efﬁcient ambipolar host materials for organic blue electrophosphorescence: theoretical characterization of hosts based on carbazole derivatives. Journal of American Chemical Socity 2011; 133:17895e900.  Hsu FM, Chien CH, Shu CF, Lai CH, Hsieh CC, Wang KW, et al. A bipolar host material containing triphenylamine and diphenylphosphoryl-substituted ﬂuorene units for highly efﬁcient blue electrophosphorescence. Advanced Functional Materials 2009;19:2834e43.  McClenaghan ND, Passalacqua R, Loiseau F. Ruthenium(II) dendrimers containing carbazole-based chromophores as branches. Journal of the American Chemical Society 2003;125:5356e65.  Brunner K, van Dijken A, Borner H, Bastiaansen JJAM, Kiggen NMM, Langeveld BMW. Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: tuning the HOMO level without inﬂuencing the triplet energy in small molecules. Journal of the American Chemical Society 2004;126:6035e42.  Nakamura A, Tada T, Mizukami M, Yagyu S. Efﬁcient electrophosphorescent polymer light-emitting devices using a Cs/Al cathode. Applied Physical Letters 2004;84:130e2.  Mathai MK, Choong VE, Choulis SA, Krummacher B, So F. Highly efﬁcient solution processed blue organic electrophosphorescence with 14 lm/W luminous efﬁcacy. Applied Physical Letters 2006;88:243512.