High Power Efficiency Yellow Phosphorescent OLEDs by Using New

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High Power Efficiency Yellow Phosphorescent OLEDs by Using New Iridium Complexes with Halogen-Substituted 2‑Phenylbenzo[d]thiazole Ligands Cong Fan,†,§ Liping Zhu,‡,§ Bei Jiang,† Yifan Li,† Fangchao Zhao,‡ Dongge Ma,*,‡ Jingui Qin,† and Chuluo Yang*,† †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ABSTRACT: On the basis of the yellow iridium phosphor, bis(2-phenylbenzothiozolato-N,C2′)iridium(acetylacetonate) [(bt)2Ir(acac)], the three halogen-substituted analogues were designed and synthesized by introducing the F, Cl, and Br atoms to the 4-position of phenyl ring in the ligand of 2phenylbenzo[d]thiazole. The optoelectronic properties of all the four iridium complexes were fully investigated. Compared to the 559 nm peak emission of (bt)2Ir(acac) in CH2Cl2 solution, adding F atom caused the peak emission of (4-F-bt)2Ir(acac) blue shift to 540 nm, while adding Cl and Br atoms made the peak emissions of (4-Cl-bt)2Ir(acac) and (4-Br-bt)2Ir(acac) slightly blue shift to 554 and 555 nm, respectively. The PhOLEDs using the four iridium complexes as dopants were initially fabricated in the conventional device structure (device I): ITO/MoO3/NPB/CBP/CBP:dopants/TPBi/LiF/Al. The three halogen-substituted analogues exhibited turn-on voltages of 3.5−3.9 V, maximum current efficiencies of 35.5−52.4 cd A−1, maximum power efficiencies of 18.3−29.4 lm W−1 and maximum external quantum efficiencies (EQE) of 12.1−17.3%, which were superior than the (bt)2Ir(acac)-based device (28.4 cd A−1, 19.9 lm W−1, 9.8%). After reducing the hole-injecting barrier and using better carrier-transporting materials in the optimized device II, ITO/ MoO3/TAPC/TCTA/CBP:dopants/TmPyPB/LiF/Al, all the four devices exhibited lower turn-on voltages of 2.9−3.1 V and excellent performance with maximum EQE over 20%. As a result, they showed high power efficiencies in the range of 55.9−83.2 lm W−1. Among the four optimized devices, the (4-F-bt)2Ir(acac)-based device achieved the highest power efficiency of 83.2 lm W−1. Remarkably, the (bt)2Ir(acac)-based device still possessed high current efficiency of 53.5 cd A−1, power efficiency of 23 lm W−1, and EQE of 19.6% at extremely high luminance of 10 000 cd m−2.

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yields and proper lifetime.16−18 The iridium complex, bis(2phenylbenzothiozolato-N,C 2 ′ )iridium(acetylacetonate) [(bt)2Ir(acac)] is a widely used yellow phosphor. Some functional groups, such as −OMe, −Me, and −CF3, have been introduced to the frame of (bt)2Ir(acac) molecule to develop the new yellow phosphors.19 For example, Li et al. reported a new iridium compound of (CF3-bt)2Ir(acac) by introducing −CF3 group to the benzo[d]thiazole ring, and the corresponding PhOLEDs achieved a current efficiency of 76 cd A−1, a power efficiency of 39.6 lm W−1, and an external quantum efficiency (EQE) of 27.2%.20 Interestingly, few reports studied the influence of halogen atoms (F, Cl, and Br) on the electronic states and optoelectronic properties of yellow iridium complexes.21 In this article, we designed and synthesized new yellow-emissive iridium complexes by

INTRODUCTION White organic light-emitting diodes (WOLEDs) have shown bright future for wide applications in flat-panel displays and solid-state lighting.1−3 WOLEDs can be achieved by incorporating either three primary colors (red, green, and blue) or two complementary colors (blue and yellow/orange) into a multior single-emissive layer.4−7 By using phosphorescent heavymetal complexes as the emissive dyes, the OLEDs can harvest both electro-generated singlet and triplet excitons and consequently elevate the internal quantum efficiency up to 100%. Therefore, among the WOLEDs, two-color, all-phosphor WOLEDs have been attracting considerable attentions owing to their high efficiencies and simple device structures.8−15 In this context, the yellow phosphors play the indispensible role to achieve high efficiencies in two-color, all-phosphor WOLEDs. New yellow phosphors for high-efficiency phosphorescent OLEDs (PhOLEDs) remain to be developed. To the present, cyclometalated iridium(III) complexes are still the most promising phosphors due to their high quantum © 2013 American Chemical Society

Received: June 24, 2013 Revised: August 21, 2013 Published: August 22, 2013 19134

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employed. The scan rate is 0.1 V s−1. At the end of the experiment, the ferrocene/ferricenium (Fc/Fc+) couple was used as the internal standard. The HOMO energy levels (eV) of the four complexes are calculated according to the formula: −[4.8 eV + (E1/2(reversible) − E1/2(Fc/Fc+))]. Device Fabrication and Measurement. The yellow iridium compound, (bt)2Ir(acac) was commercially available. The hole-injection material MoO3, hole-transporting material: 1,4-bis(1-naphthylphenylamino)-biphenyl (NPB), 4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA), and 1,1-bis[(di-4tolylamino)phenyl]cyclohexane (TAPC), host material 4,4′N,N′-dicarbazolbiphenyl (CBP), and electron-transporting material 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) and 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB) were commercially available. Commercial indium tin oxide (ITO) coated glass with sheet resistance of 10 Ω/square was used as the starting substrates. Before device fabrication, the ITO glass substrates were precleaned carefully and treated by UV/O3 for 2 min. Then the sample was transferred to the deposition system: (i) in the conventional device configuration, 10 nm of MoO3 was first deposited to ITO substrates, followed by 70 nm NPB, 5 nm CBP, 20 nm emissive layer, and 35 nm TPBi; (ii) in the modified device configuration, 10 nm of MoO3 was first deposited to ITO substrates, followed by 60 nm TAPC, 5 nm TCTA, 20 nm emissive layer, and 35 nm TmPyPB. Finally, a cathode composed of 1 nm of lithium fluoride and 120 nm of aluminum was sequentially deposited onto the substrates in the vacuum of 10−6 Torr to construct the devices. In the deposition of the emissive layer, the host and guest were placed into different evaporator sources. The deposition rates of both host and guest were controlled with their correspondingly independent quartz crystal oscillators. The evaporation rates were monitored by a frequency counter and calibrated by a Dektak 6 M profiler (Veeco). The I−V−B of all devices was measured with a Keithey 2400 Source meter and a Keithey 2000 Source multimeter equipped with a calibrated silicon photodiode. The electroluminance (EL) spectra were measured by JY SPEX CCD3000 spectrometer. All measurements were carried out at room temperature under ambient conditions. Synthesis. 2-(4-Fluorophenyl)benzo[d]thiazole (4-F-bt) and (4-F-bt)2Ir(acac) were synthesized according to the ref 22. 2-(4-Chlorophenyl)benzo[d]thiazole (4-Cl-bt): A mixture of 4-chlorobenzaldehyde (1.41 g, 10 mmol) and 2-aminobenzenethiol (1.25 g, 10 mmol) in 10 mL of dimethyl sulfoxide (DMSO) was refluxed at 200 °C for 1 h under argon atmosphere. After being cooled to room temperature, the mixture was poured into water, and the formed precipitate was filtered and dried. The product, 2-(4′-chlorophenyl)benzo[d]thiazole (4-Cl-bt), was obtained by column chromatography on silica gel using 1:10 (v/v) ethyl acetate/petroleum as eluent to give a white solid. The yield was 76%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.08−8.02 (m, 3H), 7.92−7.89 (d, J = 9 Hz, 1H), 7.53−7.38 (m, 4H). 13C NMR (75 MHz, CDCl3) δ [ppm]: 166.19, 153.75, 136.65, 134.76, 131.72, 128.88, 128.35, 126.15, 125.08, 123.03, 121.34. MS (EI, m/z): [M]+ calcd for C13H8ClNS. 245.01; found, 244.98. Anal. Calcd for C13H8ClNS (%), C 63.54, H 3.28, N 5.70; found, C 63.47, H 3.54, N 5.40. 2-(4-Bromophenyl)benzo[d]thiazole (4-Br-bt) was synthesized according to the same procedure as (4-Cl-bt).23 (4-Cl-bt)2Ir(acac): A mixture of 2.5 equiv of 2-(4chlorophenyl)benzo[d]thiazole (0.61 g, 2.5 mmol) and 1 equiv of IrCl3·3H2O (0.35 g, 1 mmol), 15 mL of 2ethoxyethanol, and 5 mL of H2O was refluxed at 120 °C for

introducing the F, Cl, and Br atoms to the 4-position of phenyl ring in the ligand of 2-phenylbenzo[d]thiazole (bt). We systematically investigated the influence of halogen atoms on the photophysical and electrochemical properties of (bt)2Ir(acac) molecule. We initially fabricated the yellow PhOLEDs using the four iridium complexes as dopants in the conventional device structure (device I): ITO/MoO3/NPB/CBP/ CBP:dopants/TPBi/LiF/Al. The three halogen-substituted analogues exhibited superior performance compared with the (bt)2Ir(acac)-based device (28.4 cd A−1, 19.9 lm W−1, 9.8%). We further optimized the device structure (device II): ITO/ MoO3/TAPC/TCTA/CBP:dopants/TmPyPB/LiF/Al) by reducing the hole-injecting barrier and applying better carriertransporting materials. As a result, the optimized devices showed lower turn-on voltages of 2.9−3.1 V, much higher power efficiencies of 55.9−83.2 lm W−1 and EQE of 20.2− 25.7% than those of device I. Among them, the (4-Fbt)2Ir(acac)-based device displayed a maximum current efficiency of 76.8 cd A−1, a maximum power efficiency of 83.2 lm W−1, and a maximum EQE of 24.3%. To the best of our knowledge, these efficiencies were among the highest ever reported for vacuum-deposited yellow PhOLEDs.

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EXPERIMENTAL SECTION General Information. All reactions were carried out using Schlenk tube in an argon atmosphere. All reagents commercially available were used as received unless otherwise stated. 1H NMR spectra were measured on Varian Unity 300 MHz spectrometer using CDCl3 as solvent and tetramethylsilane as an internal reference. 13C NMR (75 MHz) spectra were measured on Varian Unity 300 MHz spectrometer using CDCl3 as solvent. Elemental analysis of carbon, hydrogen, and nitrogen was performed on Vario EL-III microanalyzer. EIMS spectra were recorded on VJ-ZAB-3F-Mass spectrometer. Thermogravimetric analysis (TGA) was undertaken with a NETZSCH STA 449C instrument, and the thermal stability of the samples under nitrogen atmosphere was determined by measuring their weight loss, heated at a rate of 10 °C min−1 from 25 to 800 °C. Photophysical Characterization. UV−vis absorption spectra were recorded on Shimadzu UV-2550 spectrophotometer with baseline corrected. Steady-state emission spectra were recorded on Hitachi F-4500 fluorescence spectrophotometer. Absolute photoluminescence quantum yields measured in CH2Cl2 were recorded on FLS920 spectrometer with Xenon light source (450 W) through the Edinburgh Instruments integrating sphere. The integrating sphere is 150 mm in diameter and has its inner surface coated with barium sulfate (BaSO4). Time-resolved measurements in CH2Cl2 solution were performed on FLS920 spectrometer using the timecorrelated single-photon counting (TCSPC) option and the Edinburgh Instruments picoseconds pulsed diode laser (Model: EPL-375) as the light source. The excited state lifetime data were analyzed using F900 software by minimizing the reduced chi squared function (χ2) and visual inspection of the weighted residuals. All the samples were fresh and carefully prepared. Deaerated samples were prepared by purging argon for 60 min. Cyclic Voltammetry. Cyclic voltammetric measurements were carried out by using a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. The conventional three-electrode cell with a Pt work electrode of 2 mm diameter, a platinum-wire counter electrode, and a Ag/AgCl reference electrode was 19135

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Scheme 1. Synthesis of the Ligands and the Iridium Complexes

and elemental analysis (see details in the Experimental Section). As shown in Figure 1, the thermal decomposition

24 h under argon atmosphere. After cooling back down to room temperature, the formed precipitate was filtered and washed by water, ethanol, and diethyl ether to obtain the assumed chloride-bridged dimer, (4-Cl-bt)2Ir(μ-Cl)2Ir(4-Clbt)2. Without further purification, a mixture of the dimer (4-Clbt)2Ir(μ-Cl)2Ir(4-Cl-bt)2 (1.15 g, 0.8 mmol), Na2CO3 (0.21 g, 2 mmol), and pentane-2,4-dione (0.2 g, 2 mmol) was added into a solution of 20 mL of 2-ethoxyethanol. The reaction was refluxed at 120 °C for 12 h under argon atmosphere. After back to room temperature, the raw product was filtered and washed by water, ethanol, and diethyl ether. The product, (4-Clbt)2Ir(acac), was purified by column chromatography on silica gel using CH2Cl2 as eluent to give a yellow solid. Overall yields: 67%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.03−8.01 (m, 2H), 7.93−7.90 (m, 2H), 7.60−7.57 (d, J = 9 Hz, 2H), 7.47− 7.45 (m, 4H), 6.90−6.88 (d, J = 6 Hz, 2H), 6.31 (s, 2H), 5.13 (s, 1H), 1.76 (s, 6H). MS (EI, m/z): [M]+ calcd for C31H21Cl2IrN2O2S2, 780.01; found, 780.34. Anal. Calcd for C31H21Cl2IrN2O2S2 (%), C 47.69, H 2.71, N 3.59; found, C 47.55, H 2.88, N 3.42. (4-Br-bt)2Ir(acac): The procedure was the same as the synthesis of (4-Cl-bt)2Ir(acac) by using 2-(4′-bromophenyl)benzo[d]thiazole to replace 2-(4′-chlorophenyl)benzo[d]thiazole. Overall yields: 70%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.03−8.01 (m, 2H), 7.93−7.91 (m, 2H), 7.54−7.45 (m, 6H), 7.08−7.05 (d, J = 9 Hz, 2H), 6.48 (s, 2H), 5.12 (s, 1H), 1.76 (s, 6H). MS (EI, m/z): [M]+ calcd for C31H21Br2IrN2O2S2, 867.90; found, 869.42; Anal. Calcd for C31H21Br2IrN2O2S2 (%), C 42.81, H 2.43, N 3.22; found, C 42.70, H 2.71, N 3.01.

Figure 1. Thermogravimetric analysis of the three iridium complexes.

temperatures of the three iridium complexes (Td, corresponding to 5% weight loss in the thermogravimetric analysis) were in the range of 350−370 °C, indicating their satisfactory thermal stabilities. Photophysical Properties. The UV−vis absorption and photoluminescence (PL) spectra of (bt)2Ir(acac), (4-F-bt)2Ir(acac), (4-Cl-bt)2Ir(acac), and (4-Br-bt)2Ir(acac) in CH2Cl2 solution were shown in Figure 2. The intense short wavelength absorptions below 400 nm were due primarily to the spinallowed π−π* transitions of the ligands, while the long wavelength absorptions between 400 and 550 nm were assigned to the admixture of 1MLCT and 3MLCT transitions.27 The four iridium complexes showed intense photoluminescence emission in CH2Cl2 solution at room temperature. Their peak emissions were 559 nm for (bt)2Ir(acac), 540 nm for (4F-bt)2Ir(acac), 554 nm for (4-Cl-bt)2Ir(acac), and 555 nm for (4-Br-bt)2Ir(acac), respectively. Adding F atom caused the emission of (bt)2Ir(acac) to notably blue shift by 19 nm, while Cl and Br atoms caused the emission of (bt)2Ir(acac) to slightly blue shift by 4−5 nm. Their absolute photoluminescence quantum yields (PLQYs) measured in deaerated CH2Cl2 solution were 65% for (bt)2Ir(acac), 56% for (4-F-bt)2Ir(acac), 59% for (4-Cl-bt)2Ir(acac), and 46% for (4-Br-bt)2Ir(acac).

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 depicted the synthesis of the ligands and the corresponding iridium complexes. The ligands of the halogen-substituted 2phenylbenzo[d]thiazole could be obtained in a one-step condensation reaction by the combination of halogensubstituted benzaldehyde and 2-aminobenzenethiol at high temperature. The three iridium complexes were prepared in a conventional two-step procedure.24−26 All the complexes were fully characterized by 1H NMR spectroscopy, mass spectrum, 19136

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energy levels determined from the half-wave oxidation potentials were −5.29 eV [(bt)2Ir(acac)], −5.44 eV [(4-Fbt)2Ir(acac)], −5.45 eV [(4-Cl-bt)2Ir(acac)], and −5.45 eV [(4-Br-bt)2Ir(acac)], respectively. Deduced from the energy gaps and HOMO levels, the LUMO energy levels of the four iridium complexes were −2.95 eV [(bt)2Ir(acac)], −2.96 eV [(4-F-bt)2Ir(acac)], −3.05 eV [(4-Cl-bt)2Ir(acac)], and −3.04 eV [(4-Br-bt)2Ir(acac)], respectively. We note that the introduction of F, Cl, and Br atoms all lowered the HOMO level of (bt)2Ir(acac). However, adding F atom made LUMO level of (bt)2Ir(acac) nearly invariant, but adding Cl and Br atoms both lowered the LUMO level of (bt)2Ir(acac). All the data were listed in Table 1. Phosphorescent OLEDs. To evaluate the three halogensubstituted iridium complexes as the yellow emissive dyes in phosphorescent OLEDs, we designed the following conventional device structure (device-I): ITO/MoO3 (10 nm)/NPB (70 nm)/CBP (5 nm)/CBP:dopants (20 nm)/TPBi (35 nm)/ LiF (1 nm)/Al (120 nm). For comparison, the device based on (bt)2Ir(acac) was also fabricated. The device I configuration was shown in Figure 5. In these devices, MoO3 and LiF served as hole- and electron-injecting materials; NPB was used as the hole-transporting material; CBP was employed as host material as well as exciton-blocking material; the four iridium complexes were used as the guest emitters with optimized doping level at 8%. The current density−voltage−luminance (J−V−L) characteristics, current efficiency-power efficiency/EQE versus current density, and EL of all these devices were shown in Figure 6. All the data were collected in Table 2. As shown in Figure 6, the (bt)2Ir(acac)-based device I displayed fair efficiencies, which showed a turn-on voltage of 3.5 V, a maximum current efficiency of 28.4 cd A−1, a maximum power efficiency of 19.9 lm W−1, and EQE of 9.8%, which were comparable to the efficiencies reported by the literature with the similar device configuration.22 Noticeably, the three devices based on these halogen-substituted analogues displayed superior efficiencies to the (bt)2Ir(acac)-based device. The (4-Br-bt)2Ir(acac)-based device I displayed a turn-on voltage of 3.9 V, a maximum current efficiency of 35.5 cd A−1, a maximum power efficiency of 18.3 lm W−1, and EQE of 12.1%; the (4-Fbt)2Ir(acac)-based device I exhibited a turn-on voltage of 3.7 V, a maximum current efficiency of 52.4 cd A−1, a maximum power efficiency of 29.4 lm W−1, and EQE of 17.2%; The (4Cl-bt)2Ir(acac)-based device I showed a turn-on voltage of 3.5 V, a maximum current efficiency of 50.7 cd A−1, a maximum power efficiency of 28.4 lm W−1, and EQE of 17.3%. Moreover, the (4-Cl-bt)2Ir(acac)-based device I displayed very good device stability. At extremely high luminance of 10 000 cd m−2 (26.8 mA cm−2, 9.9 V), the (4-Cl-bt)2Ir(acac)-based device I still possessed a high current efficiency of 39.7 cd A−1, power efficiency of 12.6 lm W−1, and EQE of 13.5%. These efficiencies were comparable with the (bt)2Ir(acac)-based devices hosted by bipolar materials.29,30 Although these halogen-substituted iridium complexes achieved good device performance, the yellow devices showed high turn-on voltages (>3.4 V) and unsatisfactory power efficiencies, which were probably due to the large hole-injecting barrier (ΔE = 0.9 eV) between the hole-transporting NPB layer (HOMO: −5.4 eV) and the CBP host (HOMO: −6.3 eV). Meanwhile, to pursue higher power efficiencies, it was also important to employ transporting materials with better carrier mobility. Therefore, in the improved device structure (device II in Figure 5), we employed TAPC (hole mobility ≈ 10−2 cm2/V

Figure 2. UV−vis absorption and PL spectra of the iridium complexes in CH2Cl2 solution at room temperature.

The photoexcited decay curves of all the four iridium complexes were shown in Figure 3. Their fitting lifetimes

Figure 3. Photoexcited decay curves of the iridium complexes in deaerated CH2Cl2 solution at room temperature.

were calculated to be 0.62 μs [(bt)2Ir(acac)], 0.92 μs [(4-Fbt)2Ir(acac)], 1.11 μs [(4-Cl-bt)2Ir(acac)], and 0.92 μs [(4-Brbt)2Ir(acac)], respectively, which were almost monoexponential and indicated the triplet nature of the excited states.28 Electrochemical Properties. The electrochemical behaviors of (bt)2Ir(acac), (4-F-bt)2Ir(acac), (4-Cl-bt)2Ir(acac), and (4-Br-bt)2Ir(acac) were examined by the cyclic voltammetry (CV) using Fc/Fc+ couple as the internal standard. As shown in Figure 4, all the four iridium complexes exhibited a reversible oxidation process in CH2Cl2 solution. The half-wave oxidation potentials (Eox vs Fc/Fc+) were 0.49 V [(bt)2Ir(acac)], 0.64 V [(4-F-bt)2Ir(acac)], 0.65 V [(4-Cl-bt)2Ir(acac)], and 0.65 V [(4-Br-bt)2Ir(acac)], respectively. Consequently, their HOMO

Figure 4. Oxidation behaviors of the iridium complexes in cyclic voltammogram (CV). 19137

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Table 1. Thermal and Photophysical Properties of the Iridium Complexes complexes

Td (°C)

(bt)2Ir(acac) (4-F-bt)2Ir(acac) (4-Cl-bt)2Ir(acac) (4-Br-bt)2Ir(acac)

356 365 356

λabs, maxa (nm) (ε × 104) 404 392 402 403

(0.74), 442 (0.69), 488 (0.47) (0.8), 425 (0.7), 460 (0.48) (1.1), 436 (0.97), 472 (0.64) (1.0), 438 (0.84), 474 (0.54)

λPL,maxa (nm) 559, 540, 554, 555,

595 574 591 592

Φb

τobsb (μs)

energy gapc (eV)

HOMOd (eV)

LUMOe (eV)

65% 56% 59% 46%

0.62 0.92 1.11 0.92

2.34 2.48 2.40 2.41

−5.29 −5.44 −5.45 −5.45

−2.95 −2.96 −3.05 −3.04

Measured in CH2Cl2 solution (extinction coefficient in parentheses). bΦ = absolute photoluminescence quantum yields in deaerated CH2Cl2; τobs = excited-state lifetimes. cCalculated from the onsets of absorption spectra. dEstimated from the CV. eEstimated from the HOMO and energy gaps. a

Figure 5. Energy level of the device configurations and the chemical structures of organic materials used in devices I and II.

s vs NPB; hole mobility ≈ 10−3 cm2/V s)31,32 and TmPyPB (electron mobility ≈ 10−3 cm2/V s vs TPBi; electron mobility ≈ 10−5 cm2/V s)33,34 as the hole-transporting and electrontransporting materials to replace NPB and TPBi, respectively. Simultaneously, to reduce the hole-injecting barrier between the TAPC (HOMO: −5.5 eV) and CBP (HOMO: −6.3 eV), we used TCTA as the buffer layer (HOMO: −5.7 eV) (see

Figure 5). TCTA can also serve as an exciton-blocking layer for its high triplet energy of 2.85 eV.35 The optimized device configuration was as follows: ITO/MoO3 (10 nm)/TAPC (60 nm)/TCTA (5 nm)/CBP:dopants (20 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al (120 nm). The current density−voltageluminance (J−V−L) characteristics and current efficiency− power efficiency/EQE versus current density and EL of all the 19138

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Figure 6. (a) J−V−L characteristics of device I; (b) current efficiency and power efficiency versus current density of device I; (c) EQE versus current density of device I; (d) EL spectra of device I at 6 V.

Table 2. Characteristic Data of Device I dopant

Vturn‑on (V)a

Lmax (cd/m2)b

EQE (%)c

LE (cd A−1)c

PE (lm W−1)c

(bt)2Ir(acac) (4-F-bt)2Ir(acac) (4-Cl-bt)2Ir(acac) (4-Br-bt)2Ir(acac)

3.5 3.7 3.5 3.9

48649 47592 46005 22396

9.8/9.4/8.9/8.9 17.2/15.5/15.1/12.3 17.3/14.6/17.0/13.5 12.1/10.9/11.1/5.9

28.4/27.3/25.7/25.7 52.4/47.7/46.3/37.8 50.7/42.8/50.0/39.7 35.5/32.0/32.5/17.0

19.9/16.8/11.1/7.8 29.4/29.4/21.1/11.3 28.4/27.5/23.4/12.6 18.3/18.3/13.2/4.5

CIE (x,y)d (0.49, (0.48, (0.49, (0.48,

0.50) 0.51) 0.50) 0.50)

Turn-on voltages at 1 cd m−2. bMaximum luminance. cOrder of measured efficiency values: maximum, then values at 100/1000/10000 cd m−2 for device I. dCommission International de I’Eclairage (CIE) coordinate measured at 6 V.

a

Figure 7. (a) J−V−L characteristics of device II; (b) current efficiency and power efficiency versus current density of device II; (c) EQE versus current density of device II; (d) EL spectra of device II at 6 V.

modified devices were shown in Figure 7. All the data were collected in Table 3.

As shown in Figure 7, all the devices II displayed lower turnon voltages (2.9−3.1 V) than those in device I (3.5−3.9 V), 19139

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Table 3. Characteristic Data of Device II dopant

Vturn‑on (V)a

Lmax (cd/m2)b

EQE (%)c

LE (cd A−1)c

PE (lm W−1)c

(bt)2Ir(acac) (4-F-bt)2Ir(acac) (4-Cl-bt)2Ir(acac) (4-Br-bt)2Ir(acac)

2.9 2.9 2.9 3.1

47711 47777 27613 23415

25.7/25.5/24.4/19.6 24.3/22.1/19.4/10 20.2/19.6/17.8/8.2 21.6/21.2/14.6/5

69.7/69.1/66.5/53.5 76.8/69.9/61.6/31.8 55.9/54.3/49.5/22.9 60.3/59.2/40.8/13.8

69/62/42.6/23 83.2/62.8/41.2/13.7 55.9/46.7/33/9.3 56.7/47.6/23.3/4.3

CIE (x,y)d (0.51, (0.45, (0.50, (0.50,

0.48) 0.53) 0.49) 0.49)

Turn-on voltages at 1 cd m−2. bMaximum luminance. cOrder of measured efficiency values: maximum, then values at 100/1000/10000 cd m−2 for device II. dCommission International de I’Eclairage (CIE) coordinate measured at 6 V.

a

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ACKNOWLEDGMENTS We are grateful to the National Basic Research Program of China (973 Program 2013CB834805 and 2009CB623602) and the National Science Fund for Distinguished Young Scholars of China (No. 51125013).

which was most likely due to the reduced hole-injecting barrier. All the four devices II achieved extremely high efficiencies. The (4-F-bt)2Ir(acac)-based device II exhibited a maximum current efficiency of 76.8 cd A−1, a maximum power efficiency of 83.2 lm W−1, and EQE of 24.3%. The (4-Cl-bt)2Ir(acac)-based device II exhibited a maximum current efficiency of 55.9 cd A−1, a maximum power efficiency of 55.9 lm W−1, and EQE of 20.2%. The (4-Br-bt)2Ir(acac)-based device II exhibited a maximum current efficiency of 60.3 cd A−1, a maximum power efficiency of 56.7 lm W−1, and EQE of 21.6%. The increased power efficiencies from 18.3−29.4 lm W−1 (device I) to 55.9− 83.2 lm W−1 (device II) should be attributed to the better carrier-transporting materials and reduced turn-on voltages. Remarkably, the (bt)2Ir(acac)-based device II achieved the best device efficiencies and stability, with a maximum current efficiency of 69.7 cd A−1, a maximum power efficiency of 69 lm W−1, and EQE of 25.7%. At high luminance of 1000 cd m−2 (1.7 mA cm−2, 4.9 V), the (bt)2Ir(acac)-based device II showed very high current efficiency of 66.5 cd A−1, power efficiency of 42.6 lm W−1, and EQE of 24.4%. At extremely high luminance of 10 000 cd m−2 (21 mA cm−2, 7.5 V), the (bt)2Ir(acac)-based device II still possessed current efficiency of 53.5 cd A−1, power efficiency of 23 lm W−1, and EQE of 19.6%.

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CONCLUSIONS In summary, we designed and syntheszied new iridium complexes by introducing the halogen atoms on the frame of (bt)2Ir(acac) molecule to develop highly efficient yellow phosphors. The influence of the halogen atoms on the optoelectronic properties of the iridium compounds was systematically investigated. The yellow PhOLEDs by using the iridium complexes as triplet emitters achieved very good device performance with the external quantum efficiencies over 20%. Remarkably, they all showed high power efficiencies in the range of 55.9−83.2 lm W−1. The (4-F-bt)2Ir(acac)-based device II achieved a maximum power efficiency of 83.2 lm W−1. In addition, the (bt)2Ir(acac)-based device II still possessed high current efficiency of 53.5 cd A−1, power efficiency of 23 lm W−1, and EQE of 19.6% at extremely high luminance of 10 000 cd m−2. To the best of our knowledge, these efficiencies were among the highest ever reported for vacuum-deposited yellow PhOLEDs.20

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AUTHOR INFORMATION

Corresponding Authors

*(D.M.) E-mail: [email protected]. *(C.Y.) E-mail: [email protected]. Author Contributions §

C.F. and L.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest. 19140

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