White polymer phosphorescent light-emitting devices with a new yellow-emitting iridium complex doped into polyfluorene

Synthetic Metals 159 (2009) 1876–1879

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White polymer phosphorescent light-emitting devices with a new yellow-emitting iridium complex doped into polyfluorene Bo Liang a,b,∗ , Yunhua Xu a , Zhao Chen a , Junbiao Peng a , Yong Cao a,∗ a Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Specially Functional Materials of the Ministry of Education, South China University of Technology, Guangzhou 510640, China b School of Automobile and Mechanic Engineering, Changsha University of Science & Technology, Changsha 410076, China

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Article history: Received 28 February 2009 Received in revised form 27 May 2009 Accepted 18 June 2009 Available online 18 July 2009 Keywords: Iridium complexes Phosphorescence White polymer light-emitting devices

a b s t r a c t A new yellow-emitting iridium complex Ir(3-piq)2 pt with 3-phenylisoquinoline(3-piq) as cyclometalated ligand by introducing 2-(2H-1,2,4-triazol-3-yl)pyridine (pt) as ancillary ligand was synthesized. Efficient yellow polymer light-emitting devices (PLEDs) with the new iridium complex Ir(3-piq)2 pt in device structure ITO/PEDOT/PVK/Ir-complex (x%):PFO (or +PBD (30%))/Ba/Al (with or without PBD electron transports additive) were fabricated. The device doped with 4% Ir(3-piq)2 pt displayed a quantum efficiency of 9.4% (16.2 cd/A) at 5.06 mA/cm2 with PBD additive. A white emission was also obtained at a doping concentration of 0.5% Ir(3-piq)2 pt with no PBD added. CIE coordinate (0.34 and 0.31) close to National Television Standards Committee (NTSC) white standard, external quantum efficiency of 2.25%, and luminance of 2250 cd/m2 at applied voltage of 15 V were obtained. The results indicated that introducing triazole group based ancillary ligand into iridium complexes could enhance the electron-transporting ability of the iridium complexes. © 2009 Elsevier B.V. All rights reserved.

1. Introduction White organic/polymer light-emitting diodes (WO/PLEDs) have received significant attention over the past several years, due to their great potential for LCD display backlight and general lighting sources [1,2]. The promise of cost-effective fabrication and high efficiency provides an incentive to further develop these devices. Such approaches for creating white light rely on emission from primary colors (red, green, and blue) [3,4] or complementary colors (i.e., blue and yellow) [5,6]. Highly efficient and color stability WO/PLEDs using iridium complexes have been reported using blue fluorescence and green and red phosphorescent dyes [3,7]. Many studies were proposed in designing new materials [8] and device structures [9]. Significant progress has been made for WOLEDs with a phosphor doped into polymer host [10]; however, it needs further improvement in order to meet commercial application. Recently, we have reported a new red iridium complex by introducing the electron-transfer ancillary ligand 2-(2H-1,2,4triazol-3-yl)pyridine (pt) into Ir(1-piq)2 pt complex to alter the electron-transporting property [11]. High-efficiency PLED was

∗ Corresponding author at: Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Specially Functional Materials of the Ministry of Education, South China University of Technology, Guangzhou 510640, China. Tel.: +86 20 87114609; fax: +86 20 87110606. E-mail addresses: [email protected] (B. Liang), [email protected] (Y. Cao). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.06.013

achieved with PFO-poss doped with 2% Ir(1-piq)2 pt with an external quantum efficiency of 10.4% and a luminous efficiency of 9.4 cd/A at 10.8 mA/cm2 , while Ir(1-piq)3 performed the highest efficiency of 3.9% at 11 mA/cm2 with the same device structure. In this paper, a new yellow-emitting iridium complex, Ir(3piq)2 pt with 3-phenylisoquinoline as cyclometalated ligand, by introducing pt as ancillary ligand was synthesized. Introducing the electron-transfer ancillary ligand pt into Ir(3-piq)2 pt complex may be anticipated to alter the electron-transporting property. By using this complex as dopant and polyfluorene (PF) as the host as well as the blue emitter, efficient white-emitting device was obtained. The devices with yellow-emitting and white-emitting were obtained, and the device performance of these two systems was discussed. This information could be useful for the design of an efficient white phosphor.

2. Experiments 2.1. Measurement and characterization 1 H NMR spectra were collected on a Bruker DRX 400 with tetramethylsilane as reference. EI-MS was recorded on a LCQ DECA XP liquid chromatography–mass spectrometry (Thremo Group). UV–vis absorption spectra were recorded on a HP 8453 UV–vis spectrophotometer. Cyclic voltammetry was carried out on a CHI660A electrochemical workstation in a solution of

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tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) (0.1 M) in dichloromethane at a scan rate of 10 mV/s at room temperature under the argon atmosphere. A platinum electrode was used as the working electrode. A Pt wire was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. All reactions and manipulations were carried out under N2 atmosphere. All chemicals were purchased from commercial sources and used without further purification. 2.2. Synthesis 2.2.1. 2-(2-Phenylethynyl)benzaldehyde (1) To a solution of 2-bromobenzaldehyde (19.23 g, 104 mmol) and phenylacetylene (12.70 g, 124 mmol) in Et3 N (40 mL) was added PdCl2 (PPh3 )2 (1.45 g, 2 mmol). The mixture was stirred for 5 min, and CuI (0.196 mg, 1 mmol) was added. The resulting mixture was then heated under a nitrogen atmosphere at 50 ◦ C for 4 h. TLC to establish completion monitored the reaction. The reaction mixture was cooled to room temperature, and the ammonium salt was removed by filtration. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel using 20:1 hexanes/EtOAc to afford 13.20 g (61.6%) of the compound as a yellow oil [12], GC–MS: m/z 206 (M+ ). 2.2.2. N-(2-(2-phenylethynyl)benzylidene)-tert-butylamine (2) To a mixture of 1 (13.2 g, 64 mmol) and H2 O (16 mL) was added tert-butylamine (15.6 g, 213 mmol). The mixture was stirred under nitrogen atmosphere at room temperature for 12 h. The excess tertbutylamine was removed under reduced pressure, and the resulting mixture was extracted with ether. The combined organic layers were dried (Na2 SO4 ) and filtered. Removal of the solvent afforded 15.1 g (90.5%) of the compound as a yellow oil [12]. GC–MS: m/z 261 (M+ ), 1 H NMR (CDCl3 , 300 MHz), ı (ppm): 8.92 (s, 1H), 8.07–8.05 (m, 1H), 7.56–7.50 (m, 3H), 7.39–7.33 (m, 5H), 1.33 (s, 9H). 2.2.3. 3-Phenylisoquinoline (3) 2 (1.83 g, 7 mmol), and CuI (0.134 g, 0.7 mmol) were mixed in DMF (20 mL). The mixture was heated in an oil bath at 100 ◦ C under a nitrogen atmosphere. TLC monitored the reaction. The total reaction time is 3 h. The reaction mixture was cooled, diluted with 125 mL of ether, washed with 30 mL of saturated NH4 Cl solution, dried (Na2 SO4 ), and filtered. The solvent was evaporated under reduced pressure, and the crude product was chromatographed using 15:1 hexane/EtOAc as the eluent to obtain yellow powder [12] (1.43 g, 99%). GC–MS: m/z 204 (M+ ), 1 H NMR (CDCl3 , 400 MHz), ı (ppm): 9.33 (s, 1H), 8.12–8.10 (d, 2H), 8.05 (s, 1H), 7.99–7.97 (d, 1H), 7.87–7.85 (d, 1H), 7.74–7.68 (m, 1H), 7.61–7.57 (m, 1H), 7.51–7.48 (m, 1H), 7.42–7.38 (m, 2H). 2.2.4. [Ir(3-piq)2 Cl]2 Tetrakis(3-phenylisoquinoline-C2,N )(␮-chlorobridged)diiridium(III) Iridium trichloride hydrate (2.07 g, 5.9 mmol) was combined with 3 (3.48 g, 17 mmol), dissolved in a mixture of 60 mL 2ethoxyethanol and water (3:1), and refluxed for 24 h in N2. The solution was cooled to a room temperature, and the deep-red precipitate was collected on a glass filter frit. The precipitate was washed with 95% of ethanol and ethyl ether to form dark-yellow power (3.24 g, 86.9%), which was used directly for the next step without purification. 2.2.5. Ir(3-piq)2 pt Bis(3-phenylisoquinoline-C2,N )iridium(III)(2-(1,2,4-triazol-3yl)pyridinate) The solution of pt (0.206 g, 1.4 mmol) and sodium methoxide (0.088 g, 1.6 mmol) in anhydrous ethanol was heated to 50 ◦ C

Scheme 1. Synthetic route for the ligands and iridium complex. Reagents and conditions: (i) 2% PdCl2 (PPh3 )2 , 1% CuI, Et3 N, 50 ◦ C, N2 , 4 h; (ii) t-BuNH2 , N2 , 12 h; (iii) DMF, CuI, 100 ◦ C, N2 , 3 h; (iv) 3, 2-ethoxylethanol/H2 O (3/1), 120 ◦ C, N2 , 24 h; and (v) NaOCH3 , ethanol, 50 ◦ C, pt, N2 , 3 h.

for 1 h. A mixture of [Ir(3-piq)2 Cl]2 0.502 g (0.4mmol) in 10 mL dichloromethane was dropped into the reaction solution. Then the reacting mixture was refluxed for 3 h and cooled down to a room temperature. 50 mL of water and 30 mL of dichloromethane were added. The organic phase was separated and washed with water, and dried with anhydrous magnesium sulfate. Further purification by silica gel column using acetone/dichloromethane (1:1) as an eluent gave yellow power (0.302 g, 52%). ESI-MS: 747.07 (M+1)+ . 1 H NMR (400 MHz, CDCl3 ), ı (ppm): 8.48 (s, 1H), 8.23–8.15 (m, 4H), 7.88–7.84 (m, 4H), 7.76–7.73 (m, 2H), 7.66–7.64 (q, 3H), 7.49–7.43 (m, 4H), 7.18–7.15 (m, 1H), 6.99–6.91 (m, 2H), 6.80–6.75 (m, 2H), 6.32–6.26 (m, 2H). Anal. Calcd. for C37 H25 IrN6 : C, 59.58, H, 3.39, N, 11.27, Found: C, 59.34, H, 3.76, N, 11.01. 3. Results and discussion 3.1. Synthesis and physical properties The ligands 3-phenylisoquinoline (3-piq) [12] and 2-(2H-1,2,4triazol-3-yl)pyridine (pt) [13] were synthesized according to the literatures. The synthesis route of the yellow iridium complex is outlined in Scheme 1. The heteroleptic Ir-complex was synthesized by the direct reaction of the dimer with pt in dichloromethane under 50 ◦ C [11]. Purification of the mixture by silica chromatography with acetone/dichloromethane (1:1) produced Ir(3-piq)2 pt a yellow powder in high purity. Fig. 1 shows the absorption of Ir-complex and PL spectra of neat PFO film and PFO + PBD (30%). The bands around 290, 320 nm are the absorption of ligand center, while the peak at 400–420 nm is the absorption of metal to ligand charge transfer (MLCT) absorption band [14]. The emission spectra of PFO and PFO + PBD (30%) showed a large spectral overlap with the MLCT band of Ir(3-piq)2 pt, which indicate the Forster energy transfer from host to Ir(3-piq)2 pt dopant is efficient. The inset of Fig. 1 shows the normalized PL spectra of Ir(3-piq)2 pt in dichloromethane, with a peak at 550 nm and a shoulder at 582 nm, which is blue-shifted about 50 nm compared with Ir(1-piq)2 pt, and 12 nm blue-shifted compared with Ir(3-piq)2 acac. The data indicated color tuning of cyclometalated iridium complexes could be realized by the modification of the cyclometalated ligand and/or ancillary ligand. Cyclic voltammetry was used to investigate the electrochemical behavior of the iridium complex. The complex showed one irreversible reduction process with onset at −1.54 V, and one

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Fig. 3. EL spectra of the device at different driving voltages (device structure: ITO/PEDOT/PVK/Ir-complex (5%):PFO/Ba/Al). Fig. 1. The normalized UV–vis absorption of Ir(3-piq)2 pt (in CH2 Cl2 ) and PL spectra of PFO, PFO + 30% PBD (in films), inset shows the PL spectra of Ir(3-piq)2 pt in CH2 Cl2 .

quasireversible oxidation with onset at 1.16 V. Based on the data of oxidation and reduction, the HOMO and LUMO energy levels of the iridium complex could be calculated by the empiric formula as −5.56 eV, −2.86 eV relative to the energy level of vacuum level [15]. 3.2. Electroluminescence Devices employing blends as emissive layer were fabricated. The device architectures were as follows: device 1: ITO/PEDOT/PVK/Ir-complex (x%):PFO + PBD (30%)/Ba/Al; device 2: ITO/PEDOT/PVK/Ir-complex (x%):PFO/Ba/Al (x = 0.5, 1, 2, 4, 8). The EL emissions of the two devices are shown in Fig. 2(a and b). The emission of the host decreased with the increase of guest doping concentration, which is attributed to the efficient Forster energy transfer. Meanwhile, PFO + PBD (30%) appeared to be a better host compared with PFO (100%). EL emission is dominated by Ir-complex emission peaked at 554 and 594 nm. No host (PFO + PBD (30%)) emission was observed even at dopant concentration as low as 2% (w/w). The EL performance was also studied at different Ir-complex doped concentration. As compared to device 2, device 1 showed higher efficiencies. The maximum quantum efficiency was 9.4% (16.2 cd/A) at 5.06 mA/cm2 for device 1 (4% Ir-complex doped). The maximum brightness is 3841 cd/m2 at 177 mA/cm2 . The best device performance for device 2 (4% Ir-complex doped) is 4.1% (7.2 cd/A) at 3.42 mA/cm2 . Adding PBD (30%) into PFO significantly enhanced device performance due to the improved electron transport in the presence of PBD resulting in more balanced ratio of holes and electrons. White emission can be obtained by further reducing dopant concentration purposely allowing incomplete energy transfer. A WPLED with 0.5% Ir-complex doped was studied with device 2, without electron-transporting material PBD. The CIE coordinates under driving voltages of the white-light-emitting device are summarized in Table 1. EL spectra of WPLED investigated in this work show color stability with the increase of the voltage from 10 V to 15 V (Fig. 3). Obviously, the device emitted blue fluorescent and yellow phosphorescent emissions. Moreover, the EL spectra of the device showed an apparent dependence on the operating voltage: with the increasing of the voltage, the intensity of yellow decreased gradually relative to the blue emission, but all the

Fig. 2. EL spectra of the PLEDs with various concentrations of Ir(3-piq)2 pt doped in PFO (a) and PFO + PBD (30%) (b).

Table 1 Chromaticity and volt values for WOLEDs. Bias (V) 2

Luminance (cd/m ) CIE (x, y)

15

14

13

12

11

10

2250 0.34, 0.31

2050 0.34, 0.30

1580 0.34, 0.30

1010 0.36, 0.31

537 0.38, 0.33

213 0.40, 0.35

Device structure: ITO/PEDOT/PVK/PFO: 0.5% Ir-complex/Ba/Al.

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CIE coordinates of the device were in the white light region. The balanced emission of the device is due to the combined effects of efficient energy transfer from the blue fluorescent and exciton formation by charge trapping on the yellow Ir-complex dopant. The CIE coordinate (0.34 and 0.31) was obtained at 15 V, which is close to the ideal white point of (0.33 and 0.33). The external quantum efficiency is 2.25% and luminance is 2250 cd/m2 at this voltage. 4. Conclusion We synthesized a new yellow-emitting iridium complex Ir(3piq)pt. Two types of device architecture were fabricated by spin casting with PFO or PFO + PBD (30%) as host materials. The best device performance with PFO as host material (4% Ir-complex doped) is 4.1% (7.2 cd/A) at 3.42 mA/cm2 . Nearly pure white light emission with CIE coordinate (0.34 and 0.31), luminescence of 1240 cd/cm2 , and external quantum efficiency of 2.25% was obtained. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Nos. 50433030, 50803008, and U0634003), the MOST National Research Project (No. 2009CB623602), Provincial

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