A promising phosphorescent heteroleptic iridium complex with carbazole-functionalized substituent: Synthesis, photophysical and electroluminescent performances

Optical Materials 35 (2012) 300–306

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A promising phosphorescent heteroleptic iridium complex with carbazole-functionalized substituent: Synthesis, photophysical and electroluminescent performances Xiao Li a,⇑, Tienan Zang a, Lianshui Yu a, Dongyu Zhang b, Gonghao Lu a, Haijun Chi a, Guoyong Xiao a, Yan Dong a, Zheng Cui b, Zhiqiang Zhang a, Zhizhi Hu a a b

School of Chemical Engineering, University of Science and Technology Liaoning (USTL), Anshan 114051, People’s Republic of China Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 31 March 2012 Received in revised form 30 August 2012 Accepted 4 September 2012 Available online 11 October 2012 Keywords: Iridium complex Optical material Synthesis Luminescence

a b s t r a c t A promising heteroleptic iridium complex with carbazole-functionalized substituent, i.e., (cbbt)2Ir(acac), in which cbbt was 2-(4-(4-(9H-carbazol-9-yl)butoxy)phenyl)benzo[d]thiazole and acac acetylacetonate, was designed, synthesized and structurally characterized. The complex emitted intensely orange phosphorescence upon photoexcitation with high luminescent quantum efficiency (0.30), short triplet lifetime (0.15 ls) and high decomposition temperature (310 °C). The electronic properties of (cbbt)2Ir(acac) were also examined according to the density functional theory calculations. Organic light-emitting diodes using (cbbt)2Ir(acac) as doped emitter exhibited orange electrophosphorescence with prominent performances. An extremely high brightness of 17,910 cd m2 at a high voltage (17 V) and a maximum luminance efficiency of 25.5 cd A1 were achieved. More importantly, those devices exhibited low efficiency roll-off of 30–35% from 10 to 100 mA cm2, which was probably attributed to the short triplet lifetime of (cbbt)2Ir(acac). Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Since the first example of electrophosphorescence at room temperature was reported, phosphorescent materials based on heavy metal complexes have attracted much attention due to their high external quantum efficiency in organic light-emitting diodes (OLEDs) [1–6]. Extensive investigations of phosphorescent materials have focused on heavy metal complexes such as iridium [7–13], platinum [14–18] and rhenium [19,20]. OLEDs based on those phosphorescent materials can significantly improve electroluminescent (EL) performances because both singlet and triplet excitons can be harvested for light emission by strong spin-orbit coupling, so the internal quantum efficiency of phosphorescent emitters can theoretically approach 100%. Among these transition metal complexes, iridium (III) [Ir(III)] complexes have been regarded as excellent phosphorescent materials because of their favorable short phosphorescent lifetime, high phosphorescent efficiency, easy-tuning emission, thermal stability, and environmental inertness. Based on these considerations, a number of researchers are devoted to the development of cyclometalated (C^N) Ir(III) complexes. For full-color display applications, although three primary colors (red, green and blue) have been extensively explored, ⇑ Corresponding author. Tel.: +86 412 5929881; fax: +86 412 5929627. E-mail address: [email protected] (X. Li). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.09.002

there is still a great demand for emitters which give a bright color, such as orange for multiple color display purposes. Moreover, the development of a highly efficient phosphorescent orange emitter is of great importance due to its wide application on the simple white OLEDs combined with both blue fluorescent and orange phosphorescent emitters [21–30]. 0 Bis(2-phenylbenzothiazolato-N,C 2 )iridium(acetylacetonate) [(bt)2Ir(acac)] is one typical orange phosphorescent emitter [31]. Although some derivatives of (bt)2Ir(acac) have been developed for OLEDs applications [22,32–36], the electroluminescent properties of those complexes are still not very satisfactory when they are used as the orange-emitting dopant in OLEDs. Therefore, further investigation of phosphorescent materials is still required for the research and development of phosphorescent EL devices. Some well-known hole-transporting groups such as carbazole have been introduced into Ir(III) complexes [37–49], and the performances of OLEDs based on those Ir(III) complexes have been encouragingly improved. Therefore, the modification of (bt)2Ir(acac) with functionalized carbazole group on ligands seems to be a wise choice to obtain orange OLEDs with improved device performances. In this work, we report a new heteroleptic cyclometalated iridium triplet orange emitter based on 2-phenylbenzo[d]thiazole ligand functionalized with carbazole group, i.e., bis[2-(4-(4-(9H0 carbazol-9-yl)butoxy)phenyl)benzo[d]thiazole-N,C2 ]iridium(acetylacetonate) [(cbbt)2Ir(acac)]. The complex shows good thermal

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stability, strong orange phosphorescence and short triplet lifetime. OLEDs using (cbbt)2Ir(acac) as orange dopant exhibit outstanding electrophosphorescent performances. 2. Experimental 2.1. Reagents and physical measurements Commercially available reagents and starting materials were used for synthesis of (cbbt)2Ir(acac) without further purification. Solvents were purified and dried by standard procedures prior to use. 1H NMR spectra were recorded on a Bruker AC 500 spectrometer with tetramethylsilane (TMS) as an internal reference. Mass spectroscopy (MS) was performed on an Agilent 1100 LC/MSD Trap VL spectrometer with APCI ion source. Elemental analysis was carried out on Vario EL III CHNS instrument. Infrared spectra were measured with samples as KBr pellets using WQF 200 FTIR spectrophotometer. Thermogravimetric analysis (TGA) was undertaken under nitrogen atmosphere at a heating rate of 10 °C min1 on a PerkinElmer Diamond TG-DTA 6300 thermal analyzer. UV–vis absorption spectrum of the complex in CH2Cl2 solution of 1.0  105 mol L1 was completed on a PerkinElmer Lambda 900 spectrophotometer. PL spectrum of (cbbt)2Ir(acac) from the same solution was obtained using a PerkinElmer LS 55 fluorescence spectrophotometer with excitation at 360 nm. The luminescent lifetime of (cbbt)2Ir(acac) in CH2Cl2 was detected by a system equipped with a TDS 3052 digital phosphor oscilloscope pulsed Nd:YAG laser with a Third-Harmonic-Generator (THG) 355 nm output. All measurements were carried out at room temperature (RT).

301

24 h. The reaction was quenched by water and the resulting solid was filtered off. The obtained solid was purified by recrystallization with ethyl acetate. A light yellow solid product was obtained with the yield of 75%. 1H NMR (500 MHz, CDCl3, TMS): 8.11 (d, J = 7.71 Hz, 2H), 8.03–7.86 (m, 4H), 7.48–7.33 (m, 6H), 7.23 (d, J = 7.22 Hz, 2H), 6.93 (d, J = 8.71 Hz, 2H), 4.42 (t, J = 6.97 Hz, 2H), 3.99 (t, J = 6.07 Hz, 2H), 2.13–2.08 (m, 2H), 1.90–1.85 (m, 2H). IR (KBr, cm1):1581, 1250, 758. MS (APCI): m/z 449.56 [M + H+]. 2.2.4. Synthesis of (cbbt)2Ir(acac) The cbbt (0.98 g, 2.2 mmol) was dissolved in 2-ethoxyethanol (10 mL) in 100 mL flask. Iridium trichloride hydrate (0.35 g, 1.0 mmol) and deionized water (3.0 mL) were then added into the flask. The mixture was stirred under nitrogen at 120 °C for 24 h and was cooled to RT. The precipitate was collected and dried in vacuum to give the corresponding chloride-bridged dimer. Then in a 50 mL flask, the above dimer, acetylacetone (0.55 mL, 5.0 mmol) and sodium carbonate (1.06 g, 10.0 mmol) were mixed with 2-ethoxyethanol (15 mL) and the mixture was refluxed at 120 °C under nitrogen atmosphere for 20 h. After cooling to RT, the precipitate was filtered off and washed with water and ethanol. The crude product was purified on a silica–gel column with ethyl acetate and n-hexane as eluent to give the desired bright yellow powder (cbbt)2Ir(acac). Yield: 45%. 1H NMR (500 MHz, CDCl3, TMS): 8.08 (d, J = 7.72 Hz, 4H), 8.00 (d, J = 8.21 Hz, 4H), 7.71 (d, J = 7.84 Hz, 4H), 7.51 (d, J = 8.40 Hz, 4H), 7.48–7.42 (m, 6H), 7.34– 7.22 (m, 4H), 6.92 (d, J = 8.71 Hz, 4H), 6.46 (s, 1H), 4.18(t, J = 6.92 Hz, 4H), 4.07 (t, J = 5.98 Hz, 4H), 2.11–2.04 (m, 4H), 1.80– 1.77 (m, 4H), 1.63 (s, 6H). MS (APCI): m/z 1187.3 [M + H+]. Elemental analysis for C63H53IrN4O4S2. Calcd: C 63.78, H 4.50, N 4.72; Found: C 63.52, H 4.61, N 4.79.

2.2. Synthesis of (cbbt)2Ir(acac) 2.3. Device fabrication and EL measurements 2.2.1. Synthesis of 9-(4-bromobutyl)-9H-carbazole The 9-(4-bromobutyl)-9H-carbazole was synthesized according to generally modified procedures [50]. Yield: 32%, 1H NMR (500 MHz, CDCl3, TMS): d 8.09 (d, J = 7.75 Hz, 2H), 7.46 (d, J = 7.67 Hz, 2H), 7.38 (d, J = 8.15 Hz, 2H), 7.218 (d, J = 7.6 Hz, 2H), 4.34 (t, J = 6.98 Hz, 2H), 3.68 (t, J = 6.49 Hz, 2H), 2.07–2.02 (m, 2H), 1.89–1.94 (m, 2H). IR (KBr, cm1): 2927, 1458, 650. MS (APCI): m/z 302.9 [M + H+]. 2.2.2. Synthesis of 4-(4-(9H-carbazol-9-yl)butoxy)benzaldehyde 4-Hydroxybenzaldehyde (2.5 g, 0.02 mol) and potassium carbonate (3.4 g, 0.025 mol) were dissolved in 50 mL of N,NDimethylformamide (DMF) and stirred at 90 °C for 30 min. The 9-(4-bromobutyl)-9H-carbazole (4.5 g, 0.015 mol) in DMF was added dropwise into the above solution and then the reaction mixture was stirred at 120 °C for 20 h. The reaction was quenched by adding saturated salty water and the resulting solid was filtered off. The resulting crude solid was recrystallized with ethanol. Light white solid product was obtained with the yield of 83%. 1H NMR (500 MHz, CDCl3, TMS): d 9.87 (s, 1H), 8.11 (d, J = 7.73 Hz, 2H), 7.79 (d, J = 8.63 Hz, 2H), 7.46 (t, J = 7.97 Hz, 2H), 7.41 (d, J = 8.07 Hz, 2H), 7.23 (d, J = 7.29 Hz, 2H), 6.92 (d, J = 8.62 Hz, 2H), 4.41 (t, J = 6.97 Hz, 2H), 3.99 (t, J = 6.10 Hz, 2H), 2.12–2.09 (m, 2H), 1.89–1.86 (m, 2H). IR (KBr, cm1): 2821, 2720, 1725, 1255. MS (APCI): m/z 344.16 [M + H+]. 2.2.3. Synthesis of 2-(4-(4-(9H-carbazol-9-yl)butoxy)phenyl)benzo[d] thiazole (cbbt) 4-(4-(9H-carbazol-9-yl)butoxy)benzaldehyde (3.4 g, 0.01 mol) and 2-aminothiophenol (1.12 mL, 0.011 mol) were dissolved in 10 mL dry dimethylsulfoxide (DMSO) and stirred at 140 °C for

OLEDs were fabricated through vacuum deposition of the materials at about 1  106 Torr onto ITO-coated glass substrates with a sheet resistance of 25 X sq1. The ITO-coated substrates were routinely cleaned by ultrasonic treatment in solvents and then cleaned by exposure to a UV-ozone ambient. All organic layers were deposited in succession without breaking vacuum. The devices were prepared with the following structures of ITO/NPB (30 nm)/CBP: x% (cbbt)2Ir(acac) (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm), in which NPB (4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl), CBP (4,40 -N,N0 -dicarbazolebiphenyl), BCP (2,9dimethyl-4,7-diphenyl-1,10-phenanthroline), and Alq3 (tris (8-hydroxyquinoline) aluminum) were used as hole transporting layer, host, exciton blocking layer and electron transporting layer, respectively, and LiF/Al as the composite cathode. Deposition rates and thicknesses of the layers were monitored in situ using oscillating quartz monitors. Thermal deposition rates for organic materials, LiF, and Al were 1, 1, and 10 Å/s, respectively. The active area of the fabricated devices is 2.0  3.0 mm2. EL spectra were measured by a PR655 spectra scan spectrometer. The luminance– current–voltage (L–I–V) characteristics were recorded simultaneously with the measurement of EL spectra by combining the spectrometer with a Keithley 2400 source meter. All measurements were carried out at RT under ambient conditions. 3. Results and discussion 3.1. Synthesis, characterization and thermal stability of (cbbt)2Ir(acac) The synthetic routes of (cbbt)2Ir(acac) are shown in Scheme 1. 4-(4-(9H-carbazol-9-yl)butoxy)benzaldehyde was prepared by a base-catalyzed Williamson synthetic method. Condensation of

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2-aminothiophenol with 4-(4-(9H-carbazol-9-yl)butoxy)benzaldehyde gave the desired ligand cbbt in 75% yield. The synthesis of (cbbt)2Ir(acac) was accomplished in two step sequence, in which a chloride-bridged dimer, formed by the reaction of excess cbbt with IrCl33H2O in the first step, could be readily converted to the monomeric complex by replacing the bridging chlorides with acetylacetone in the presence of sodium carbonate. The complex (cbbt)2Ir(acac) is stable in air as bright orange solid and further confirmed by 1H NMR and MS. As one of the key parameters characterizing its possibility for device application, good thermal stability of a material is usually desired, especially for fabricating OLEDs by the conventional high vacuum deposition method. During the vacuum deposition process, materials for OLEDs should be stable even at high temperatures (300–400 °C), because the decomposition products may contaminate the OLEDs and lead to poor device performance. The thermal property of (cbbt)2Ir(acac) was determined by TGA under a nitrogen stream with a scanning rate of 10 °C min1. The TGA trace of (cbbt)2Ir(acac) was presented in Fig. 1. The 5% weight loss temperature of (cbbt)2Ir(acac) is about 310 °C, which is slightly lower than that of (bt)2Ir(acac) (ca. 338 °C) [51]. It’s known that the long alkyl chain can improve the good solubility, but it may significantly also decrease the thermal performance. However, the decomposition temperature of this complex is not remarkably changed compared with that of (bt)2Ir(acac). That might be attributed to the carbazole group which increases the thermal stability of this complex. From the TGA curve, no obvious decomposition temperature from alkoxyl chain was observed, which was similar to those from the references [20,49]. However, the decomposition temperature remains much higher than 300 °C, indicating its relative high thermal stability to be suitable for being used in OLEDs by vacuum deposition technique.

100

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Temperature (ºC) Fig. 1. TGA trace of (cbbt)2Ir(acac) measured at a heating rate of 10 °C min1 under nitrogen atmosphere.

3.2. Photophysical properties The UV-vis absorption and PL spectra of (cbbt)2Ir(acac) in CH2Cl2 are shown in Fig. 2a. By comparison to the absorption of the free ligand, the absorption band located at 260–330 nm with intense multiple absorption bands can be assigned to spin-allowed 1 p–p transitions on the cyclometalating ligand. Somewhat weaker bands are observed at lower energies among 400–550 nm in the visible region which correspond to electronic excitations to the metal–ligand charge transfer of the spin-allowed transition (1MLCT) and a mixture of spin-forbidden bands of 3MLCT with ligand-centered 3p–p transitions, respectively. These assignments

Scheme 1. The synthetic routes of (cbbt)2Ir(acac).

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(b)1.0

(a)

Abs PL

0.8

Emission intensity (a.u.)

Normal Intensity (a.u.)

1.0

0.6 0.4 0.2

0.8 Equation: y = A1*exp(-x/t1) + y0 R2 = 0.99766 y0 -0.00483 A1 1.38254 t1 0.14697

0.6 0.4 0.2 0.0

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Fig. 2. (a) UV–vis absorption and PL spectra of (cbbt)2Ir(acac) in CH2Cl2. (b) The emission decay curve of (cbbt)2Ir(acac) in CH2Cl2.

are supported by subsequent density functional theory (DFT) calculations on (cbbt)2Ir(acac). The strong spin-orbital coupling induced by the heavy-atom effect of the iridium center between the singlet and triplet manifolds can give rise to the 3MLCT and 3 p–p bands. Upon photoexcitation at 360 nm, (cbbt)2Ir(acac) emits strong orange phosphorescence with emission peak at 546 nm and shoulder peak at 582 nm at the ambient temperature, which can be assigned to a predominantly 3MLCT state radiative transition [52]. Using (ppy)2Ir(acac) as a standard reference [53], the phosphorescent quantum yield for (cbbt)2Ir(acac) in solution is determined to be as high as 0.30. Additionally, its quantum yield in neat film is found to be 0.11, which is a usual phenomenon that in most complexes intermolecular interaction in neat films results in quenching with respect to the solution. The emission decay curve of (cbbt)2Ir(acac) in CH2Cl2 is shown in Fig. 2b. Phosphorescence decay follows the single exponential equation: y = A1  exp(x/t1) + y0, where A1 is the initial intensity of phosphorescence decay, t1 the phosphorescence decay lifetime and y0 the random noise. The observed emissive lifetime of the complex in degassed CH2Cl2 at RT is as short as 0.15 ls, which is much shorter than those of popular fac-[Ir(ppy)3] (ca. 4.75 ls) [54] and (bt)2Ir(acac) (ca. 1.8 ls) [31]. As we all know, the long lifetime would increase the possibility of intrinsic triplet–triplet annihilation in the phosphorescent OLEDs. Therefore, the short phosphorescent lifetime of (cbbt)2Ir(acac) could effectively restrain the phosphorescence self-quenching to show high quantum yield.

3.3. Density functional theory calculations on Ir(III) complex DFT calculations have been proven to be very helpful in understanding the photophysical properties of Ir(III) complexes [55,56], so the ground state geometries and electronic structures of (cbbt)2Ir(acac) are calculated according to DFT calculations using the GAUSSIAN-03 software package (Gaussian, Inc.) at the B3LYP/ LANL2DZ level. It will be instructive to examine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of this complex. As shown in Fig. 3, the HOMOs of (cbbt)2Ir(acac) mainly consist of a predominant ‘‘t2g’’ d-orbitals from the metal Ir center, admixed with large contributions from the p-orbitals of the phenyl rings that are directly bonded to the metal Ir center. While, its LUMOs are essentially p orbital localized on the electron-rich benzothiazole ring, with very slim contributions from Ir atom. Those results mean that the lowest excited state of (cbbt)2Ir(acac) is a mixture of MLCT (Ir–C^N) and p–p transition of cbbt. Therefore, the lowest excited state is mainly determined by the C^N ligand, but not the

b-diketonate (acac). It can be observed that carbazole group moiety does not dominate its excited-state, which explains that the introduction of carbazole group by alkoxyl chain in the ligand (bt) almost does not influence its emission spectrum. The energy of band gap of (cbbt)2Ir(acac) obtained from DFT calculations is 2.55 eV, which is close to the energy band gap (2.46 eV) obtained from the optical absorption of (cbbt)2Ir(acac). Our calculated results are well in agreement with previous reports [57,58].

3.4. Electroluminescent properties In order to evaluate EL performances of (cbbt)2Ir(acac) as orange emitter, multilayer OLEDs with the structure of ITO/NPB (30 nm)/CBP: x% (cbbt)2Ir(acac) (30 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) were fabricated. Doping levels of this complex in the host of CBP were varied from 4% to 8%. Fig. 4 shows EL spectra of 6% (cbbt)2Ir(acac) doped device under different voltages. It can be found that EL spectra are located in the same wavelength region from 10 V to 16 V and almost identical to the PL spectrum, indicating that the EL emissions originate from the triplet excited state of the complex. The EL spectra of all devices show good color stability with the CIE coordinate of (0.49 ± 0.02, 0.51 ± 0.02). A bathochromic shift of about 12 nm is observed in the EL spectra compared with the PL spectrum, which is a frequently observed phenomenon and probably caused by additional effect of the electric field on the excited states of the emitting species in OLEDs. Additionally, there is no residual emission from CBP or the adjacent layer even at high drive voltages, implying a complete energy transfer from the CBP host to the Ir(III) complex in the emissive layer. The important EL performances of the devices based on different ratios of (cbbt)2Ir(acac) are summarized in Table 1, and Fig. 5a depicts L–I–V characteristics of 6% (cbbt)2Ir(acac) doped device. The luminance increases with increasing injection current as well as voltage in the device. The device exhibits a peak luminance of 17,910 cd m2 at 17 V. The turn-on voltage is found to be less than 5 V. A maximum current efficiency of 25.5 cd A1 is obtained at current density of 3.2 mA cm2, corresponding to maximum external quantum efficiency of 10.3%. As illustrated by the current efficiency–current density curves in Fig. 5b, all those devices witness gradual efficiency decay with increasing current density, which are typically attributed to a combination of triplet–triplet annihilation and field-induced quenching effects. Such efficiency roll-off is frequently observed especially for those phosphorescent OLEDs. However, at a high current density of 100 mA cm2, the efficiencies remain high at 50% of the peak efficiency for different

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Fig. 3. Contour plots of HOMO and LUMO for (cbbt)2Ir(acac) as determined by density functional theory calculations.

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EL Intensity (a.u.)

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doped devices. The present orange OLEDs based on the Ir(III) complex are very advantageous in the relatively slow efficiency decay and have the low efficiency losses of 30–35% from 10 to 100 mA cm2 [59]. It is well noted that the overall performances of (cbbt)2Ir(acac) are much better than those of the parent complex (bt)2Ir(acac) under the same device configuration such as the maximum external quantum efficiency of 9.7% [31] and the peak luminance of 8791 cd m2, current efficiency of 9.6 cd A1 [36], which are probably attributed to more balanced electron and hole recombination in the emissive layer. To further improve the device performances, TPBI (1,3,5-tris [N-(phenyl)benzimidazole]benzene) as the hole/exciton blocking layer by replacing BCP layer could be utilized to confine all excitons in the emissive layer. Besides, the complex (cbbt)2Ir(acac) exhibits good solubility in a wide range of organic solvents such as toluene, ethyl acetate, dichlorobenzene, DMF and so on, and can be processed as emissive layer by solution-processable technique which

Fig. 4. EL spectra of 6% (cbbt)2Ir(acac) doped device under different voltage.

Table 1 EL performances of (cbbt)2Ir(acac) doped electrophosphorescent devices.

a b c d e f g

Ratio (wt.%)

kmaxa (nm)

Vonb

gLc (cd A1)

gLd (cd A1)

gL e (cd A1)

Lmaxf (cd A1)

EQEmaxg (%)

4 6 8

557 (600) 558 (602) 558 (601)

5.1 4.5 5.2

5.3 25.5 13.6

5.1 19.2 9.1

4.5 12.5 6.4

8390 17910 11600

6.7 10.3 8.5

Maximum emission wavelength. Values in parentheses are shoulder peaks. Turn-on voltage. Maximum current efficiency. Current efficiency at 10 mA cm2. Current efficiency at 100 mA cm2. Maximum luminance. Maximum external quantum efficiency.

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Luminance Current density

16000

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Fig. 5. (a) L–I–V characteristics of 6% (cbbt)2Ir(acac) doped device; (b) current efficiency–current density curves of different doped devices.

could be an attractive alternative mainly due to its significantly reduced production cost. 4. Conclusions In conclusion, a heteroleptic bis-cyclometalated iridium complex with carbazole- functionalized substituent was designed, synthesized and characterized. Its photophysical properties were investigated in detail. By using the orange-emitting Ir(III) complex as dopant, the OLEDs exhibited high luminance of 17,910 cd m2 and efficiency of 25.5 cd A1, and more importantly, very slow efficiency roll-off. Further works on optimizing the device configuration and adopting solution-processable technique would provide many more satisfactory results. Acknowledgments Authors gratefully acknowledge the supports from the National Natural Science Foundation of China (Grant No. 61204021), the Project from Department of education of Liaoning Province (Grant No. L2012094), Open Fund of Key Laboratory of Functional Materials, Colleges and Universities in Liaoning Province (Grant No. USTLKL 2012-02) and Research Project for Young Teachers of USTL (Grant No. 2010Y08). References [1] M.A. Baldo, D.F. O’Brian, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151–154. [2] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750–753. [3] B.M.J.S. Paulose, D.K. Rayabarapu, J.-P. Duan, C.-H. Cheng, Adv. Mater. 16 (2004) 2003–2007. [4] W.-Y. Wong, C.-L. Ho, Coord. Chem. Rev. 253 (2009) 1709–1758. [5] W.-Y. Wong, C.-L. Ho, J. Mater. Chem. 19 (2009) 4457–4482. [6] G. Zhou, W.-Y. Wong, X. Yang, Chem. Asian J. 6 (2011) 1706–1727. [7] H.Z. Xie, M.W. Liu, O.Y. Wang, X.H. Zhang, C.S. Lee, L.S. Hung, S.T. Lee, P.F. Teng, H.L. Kwong, H. Zheng, C.M. Che, Adv. Mater. 13 (2001) 1245–1248. [8] Y. You, C.-G. An, D.-S. Lee, J.-J. Kim, S.Y. Park, J. Mater. Chem. 16 (2006) 4706– 4713. [9] G.-J. Zhou, W.-Y. Wong, B. Yao, Z.-Y. Xie, L.-X. Wang, Angew. Chem. Int. Ed. 46 (2007) 1149–1151. [10] G. Zhou, C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang, Z. Lin, T.B. Marder, A. Beeby, Adv. Funct. Mater. 18 (2008) 499–511. [11] B. Zhang, G. Tan, C.-S. Lam, B. Yao, C.-L. Ho, L. Liu, Z. Xie, W.-Y. Wong, J. Ding, L. Wang, Adv. Mater. 24 (2012) 1873–1877. [12] J. Zou, H. Wu, C.-S. Lam, C. Wang, J. Zhu, C. Zhong, S. Hu, C.-L. Ho, G.-J. Zhou, H. Wu, W.C.H. Choy, J. Peng, Y. Cao, W.-Y. Wong, Adv. Mater. 23 (2011) 2976– 2980. [13] H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng, Y. Cao, Adv. Mater. 21 (2009) 4181–4184. [14] G.-J. Zhou, X.-Z. Wang, W.-Y. Wong, X.-M. Yu, H.-S. Kwok, Z. Lin, J. Organomet. Chem. 692 (2007) 3461–3473. [15] G. Zhou, Q. Wang, C.-L. Ho, W.-Y. Wong, D. Ma, L. Wang, Chem. Commun. (2009) 3574–3576.

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