Improved performance of solution-processable OLEDs by silyl substitution to phosphorescent iridium complexes

Synthetic Metals 162 (2012) 1961–1967

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Improved performance of solution-processable OLEDs by silyl substitution to phosphorescent iridium complexes Jaemin Lee a,∗ , Chan Hyuk Park a , Jiyoung Kwon a , Sung Cheol Yoon a , Lee-Mi Do b , Changjin Lee a,∗∗ a b

Advanced Materials Division, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea

a r t i c l e

i n f o

Article history: Received 30 April 2012 Received in revised form 14 August 2012 Accepted 17 August 2012 Available online 13 October 2012 Keywords: Ir(ppy)3 Organic light-emitting diode Phosphorescence Structure–property relations

a b s t r a c t Development of new materials for solution-processable organic light-emitting diodes (OLEDs) has been gaining much attention recently. Although many results have been reported about new OLED materials, most of them are aiming for vacuum-deposition. The OLED materials developed for vacuum-deposition cannot be easily applied to solution processing due to their limited solubility and processability. We therefore designed and synthesized two kinds of iridium complexes, Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac), with similar chemical structures but with or without silyl substitution. From the optical and electrochemical characterization, Ir(Si-bppy)2 (acac) showed an emission maximum at 537 nm (ET1 = 2.33 eV) and HOMO energy level of −5.13 eV, while Ir(bppy)2 (acac) showed an emission maximum at 531 nm (ET1 = 2.31 eV) and HOMO energy level of −5.14 eV. While the optical and electrochemical properties are almost the same, we found out that OLED device performance can differ pretty fair. The emitting layer of our OLEDs consists of PVK, TPD, PBD and the iridium complexes. With the configuration of [ITO/PEDOT:PSS/emitting layer/CsF/Al], the maximum current efficiency of Ir(Si-bppy)2 (acac) reached 24.5 cd/A, while that of Ir(bppy)2 (acac) was 18.0 cd/A. This enhancement is attributed to the existence of the trimethylsilyl substituents at the pyridine ring of the ligand, which increased solubility and processability of the compounds, and reduced intermolecular interaction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The discovery of organic electrophosphorescence marked the beginning of new era in organic light-emitting diodes (OLEDs) research fields [1]. Theoretically, almost 100% of internal efficiency has been made possible by electrophosphorescence and this further promoted the development and commercialization of OLEDs which are not unfamiliar to the general public any more nowadays. However, all of the current OLED products are fabricated by vacuum-deposition methods which have limitations in lowering manufacturing cost, processing with large-sized substrates etc. So, solution-processable OLEDs are gaining much research interest as alternatives to the current vacuum-deposited OLEDs. There have been much research efforts to accomplish solutionprocessable OLEDs since the first report from the Cambridge research group [2]. During the 1990s “solution-processable OLEDs” was equated with “polymer-based OLEDs”, where the emitting materials were conjugated polymers. Despite of various advantages of such polymer-based OLEDs, their drawbacks, e.g. low

∗ Corresponding author. Tel.: +82 42 860 7212; fax: +82 42 860 7200. ∗∗ Corresponding author. Tel.: +82 42 860 7208; fax: +82 42 860 7200. E-mail addresses: [email protected] (J. Lee), [email protected] (C. Lee). 0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.08.011

efficiency and short life-time were also obvious. Recently, new solution-processable OLEDs have been reported to show good performances comparable to the vacuum-deposited ones, which are based on phosphorescent small molecular emitters [3]. The typical example of such phosphorescent emitters is Ir(ppy)3 (facialtris(2-phenylpyridine)iridium), [4] but it is not an ideal emitter in solution-processable OLEDs because of its limited solubility in common organic solvents. So some results using chemically modified Ir(ppy)3 for solution-processing have also been reported and showed better performances. For example, by using methyl [5] or butyl [6] attached Ir(ppy)3 derivatives, the solution-processed OLED device performance was improved. Sterically demanding xylyl substituent was introduced to the phenylpyridine ligand to improve solubility and to reduce aggregation tendency [7]. Substitution with tetraphenylsilane substituent also showed better device performance than Ir(ppy)3 in the case of solution-processing [8]. However, most of the substituents were introduced to the phenyl ring of the ligand and modification of pyridine ring is relatively uncommon. In addition to the iridium complexes, development of solutionprocessable host materials is also very important. For example, modification of mCP (1,3-bis(9-carbazolyl)benzene), which is a widely used host material in vacuum-deposited blue phosphorescent OLEDs, by triphenylsilyl, [9] meta-terphenyl [10] or phosphine

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oxide[11] moieties has shown enhanced device performance by solution-processing. Furthermore, a certain type of phosphine oxide host material has been utilized to achieve multi-layer structure by solution-processing due to its unique solubility in isopropyl alcohol [12]. In this work, we focused on introducing silyl substituents to the pyridine ring of Ir(ppy)3 -based emitting materials. The silyl substituents can improve the solubility of the compounds, and also hinder intermolecular aggregation, which lowers the device efficiency. Two kinds of iridium complexes, Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac), with similar chemical structures but with or without substitution have been designed and synthesized for the purpose of structure–property relations comparison. The optical and electrochemical properties of the newly synthesized materials were characterized by UV–vis spectroscopy, photoluminescence (PL) spectroscopy and cyclic voltammetry. OLED devices were also fabricated from spin-coating the emitting materials and their electrical characteristics were investigated. While the optical and electrochemical properties were very similar for the two iridium complexes, there was large difference in OLED device performance. Based on these results, we could verify the effectiveness of silyl substitution for solution-processable materials development.

Heraeus. All other chemicals were commercially available and used without further purification. 2.2. Instrumentation and characterization The nuclear magnetic resonance (NMR) spectra were recorded at room temperature using a Bruker 300 and Bruker 500 NMR spectrometer. High resolution mass (HRMS) analysis was performed with a JEOL JMS-700 mass spectrometer. Elemental analysis was performed with a Thermo Scientific FLASH 2000 organic elemental analyzer. Absorption spectra were measured using a SHIMADZU UV-2550 UV–Visible spectrophotometer. Photoluminescence spectra were measured using PerkinElmer LS-55 Fluorescence spectrometer. Cyclic voltammetric measurements were performed on a BAS 100 voltammetric system with a three-electrode cell in a solution of 0.10 M tetrabutylammonium tetrafluoroborate in 1,2-dichloroethane at a scanning rate of 50 mV/s. An Ag/Ag+ electrode (0.01 M AgNO3 in acetonitrile) was used as a reference electrode and platinum wires were used as a counter electrode and a working electrode, respectively. Atomic force microscopy (AFM) measurements were performed with Digital Instruments Nanoscope IVa. 2.3. OLED device fabrication and characterization

2. Experimental 2.1. Materials 2-Bromo-5-(trimethylsilyl)pyridine was prepared according to the previous literature [13]. 3-Biphenylboronic acid, 2-bromopyridine, iridium(III) chloride hydrate, 2ethoxyethanol, acetylacetone, poly(N-vinylcarbazole) (PVK), N,N -bis(3-methylphenyl)-N,N -diphenylbenzidine (TPD) and 2(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) were purchased from Aldrich Co. Tetrakis(triphenylphosphine)palladium(0) was purchased from Strem Chemicals, Inc. Potassium carbonate and sodium carbonate were purchased from Daejung Chemicals and Materials Co., Ltd. PEDOT:PSS (AI4083) was purchased from

Pre-patterned ITO substrates were first cleaned by conventional wet cleaning method (ultrasonication in acetone, isopropanol and water) and further cleaned by UV-ozone treatment. PEDOT/PSS hole-injection layer (35 nm) was spin-coated and annealed at 120 ◦ C for 30 min. The emitting layer (77 nm) was then successively formed by spin-coating onto the PEDOT/PSS layer, and annealed at 55 ◦ C for 30 min in a nitrogen atmosphere. The emitting layer consisted of a host polymer (PVK), a hole-transporting material (TPD), an electron-transporting material (PBD), and an iridium complex as a dopant. The composition ratio of PVK:TPD:PBD:Ircomplex was 51:10:32:7. The substrates were then transferred to a vacuum evaporation chamber and CsF (1 nm) and Al (120 nm) were thermally evaporated on the emissive layer as a cathode at

Scheme 1. Synthetic scheme of the iridium complexes.

J. Lee et al. / Synthetic Metals 162 (2012) 1961–1967

Fig. 1.

1

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H NMR spectra of the iridium complexes.

a pressure of 1 × 10−6 Torr. The emissive area of the devices was 2 mm × 2 mm. The fabricated OLED devices were encapsulated by cover glass with photo-curable epoxy sealant in a nitrogen atmosphere. Current and voltage of the OLED device was controlled and monitored by Keithley 2400 SourceMeter and PR-650 SpectraCalorimeter was used to measure OLED device characteristics, such as emission spectrum, CIE color coordinates and luminance.

2.4. Synthesis of the ligands and iridium complexes 2.4.1. 2-(Biphenyl-3-yl)-5-(trimethylsilyl)pyridine (1) 2-Bromo-5-(trimethylsilyl)pyridine (3.00 g, 13.0 mmol), 3-biphenylboronic acid (2.15 g, 10.9 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.251 g, 0.217 mmol) were dissolved in toulene (20 mL) and tetrahydrofuran (5 mL) under nitrogen atmosphere. Into this solution, an aqueous solution of potassium carbonate (2 M, 27.3 mL) was further added, and then refluxed at 100 ◦ C for 12 h under nitrogen atmosphere while stirring. After cooling to room temperature, the mixture was poured into water and extracted with ethyl acetate. The organic layer was washed three times with water and dried over magnesium sulfate. The solvent was evaporated and the residue was purified by column chromatography on silica gel with ethyl acetate/hexane (1/4) to give the compound 1 (2.14 g, yield = 65%). 1 H NMR (300 MHz, CDCl ): ı = 8.80 (m, 1H), 8.24 (t, 1H), 7.96 (m, 3 1H), 7.87 (dd, 1H), 7.77 (dd, 1H), 7.71–7.63 (m, 3H), 7.76–7.52 (m, 1H), 7.49–7.43 (m, 1H), 7.41–7.33 (m, 1H), 0.34 (s, 9H). 13 C NMR (75 MHz, CDCl ): ı = 157.29, 153.83, 141.89, 141.68, 3 140.98, 139.88, 133.26, 129.11, 128.69, 127.74, 127.33, 127.21, 125.75, 119.95, −1.32.

2.4.2. 2-(Biphenyl-3-yl)pyridine (2) The compound 2 was synthesized using the same method as for the compound 1 with 2-bromopyridine (0.957 g, 6.06 mmol), 3-biphenylboronic acid (1.00 g, 5.05 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.117 g, 0.101 mmol) in toluene (10 mL) and tetrahydrofuran (2.5 mL). The product was purified by column chromatography on silica gel with ethyl acetate/hexane (1/10) to give the compound 2 (0.700 g, yield = 60%).

1H

NMR (300 MHz, CDCl3 ): ı = 8.71 (d, 1H), 8.23 (s, 1H), 7.94 (d, 1H), 7.77 (m, 2H), 7.66 (t, 3H), 7.54 (t, 1H), 7.45 (t, 2H), 7.36 (t, 1H), 7.22 (m, 1H). 2.4.3. Ir(Si-bppy)2 (acac), Iridium (III)  bis(2-(biphenyl-3-yl)-5-(trimethylsilyl)pyridinato-N, C 4 ) (acetylacetonate) The compound 1 (0.500 g, 1.65 mmol) and iridium(III) chloride hydrate (0.246 g, 0.824 mmol) were dissolved in 2-ethoxyethanol (15 mL) and water (5 mL) under nitrogen atmosphere. The mixture was then refluxed at 135 ◦ C for 12 h under nitrogen atmosphere while stirring. After cooling to room temperature, the mixture was poured into water and the precipitate was filtered off to give the cyclometalated Ir(III) ␮-chloro-bridged dimer (0.590 g). The dimer was not further purified, and used as is in the next reaction. The crude dimer (0.500 g, 0.300 mmol), acetylacetone (0.075 g, 0.75 mmol) and sodium carbonate (0.318 g, 3.00 mmol) were dissolved in 2-ethoxyethanol (25 mL), and then refluxed at 135 ◦ C for 15 h under nitrogen atmosphere while stirring. After cooling to room temperature, a colored precipitate was filtered off and washed with water, hexane, and ether. The product was purified by column chromatography on silica gel with dichloromethane to give the Ir(Si-bppy)2 (acac) (0.188 g, overall yield = 30%). 1 H NMR (300 MHz, acetone-d ): ı = 8.64 (s, 2H), 8.23 (d, 2H), 6 8.05 (d, 2H), 7.96 (s, 2H), 7.54 (d, 4H), 7.33 (t, 4H), 7.20 (t, 2H), 6.89 (d, 2H), 6.36 (d, 2H), 5.33 (s, 1H), 1.73 (s, 6H), 0.35 (s, 18H). 13 C NMR (125 MHz, CDCl ): ı = 184.43, 168.20, 151.97, 147.48, 3 145.49, 142.12, 141.79, 133.72, 133.62, 132.99, 128.50, 128.24, 126.51, 126.04, 122.41, 117.84, 100.35, 28.65, −1.37. HRMS Calcd. for C45 H47 IrN2 O2 Si2 : 896.2805. Found: 896.2803. Elem. Anal. Calcd. for C45 H47 IrN2 O2 Si2 : C, 60.30%; H, 5.29%; N, 3.13%. Found: C, 59.20%; H, 5.26%; N, 2.92%. 2.4.4. Ir(bppy)2 (acac), Iridium (III) bis(2-(biphenyl-3-yl)  pyridinato-N, C 4 ) (acetylacetonate) Ir(bppy)2 (acac) was synthesized using the same method as for Ir(Si-bppy)2 (acac) with the compound 2 (1.00 g, 4.32 mmol) and iridium(III) chloride hydrate (0.645 g, 2.16 mmol). 1.23 g of the crude dimer was obtained after the reaction, and of these, 0.500 g of the crude dimer was used in the next reaction with acetylacetonate

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(0.089 g, 0.89 mmol) and sodium carbonate (0.377 g, 3.55 mmol). The overall yield was 36% (0.255 g). 1 H NMR (300 MHz, CDCl ): ı = 8.56 (d, 2H), 7.96 (d, 2H), 7.77 (m, 3 4H), 7.50 (d, 4H), 7.35 (t, 4H), 7.23 (m, 4H), 6.96 (d, 2H), 6.40 (d, 2H), 5.24 (s, 1H), 1.82 (s, 6H). 13 C NMR (125 MHz, CDCl ): ı = 184.73, 168.43, 148.28, 45.35, 3 142.10, 136.96, 133.85, 133.43, 128.54, 128.24, 126.53, 126.10, 122.53, 121.69, 118.57, 100.52, 28.81. HRMS calcd. for C39 H31 IrN2 O2 752.2015, found 752.1998. Elem. Anal. Calcd. for C39 H31 IrN2 O2 : C, 62.30%; H, 4.16%; N, 3.73%. Found: C, 60.36%; H, 3.99%; N, 3.52%. 3. Results and discussion 3.1. Design, synthesis and characterization of the iridium complexes Scheme 1 shows the synthetic routes and chemical structures of the iridium complexes used for this study. The iridium complexes consist of two cyclometalated (C∧ N) ligands and one acetylacetonate ancillary ligand. It has been reported that the light-emitting characteristics of bis-cyclometalated iridium complexes are almost the same as those of tris-chelate iridium complexes [14]. We could therefore easily investigate the influence of silyl substitution at the pyridine ring of the ligands on the characteristics of the iridium complexes. We also attached additional phenyl substituents to the phenyl ring of the ligands to improve the processability in the solution state [7,15]. For the syntheses of the ligands, 3-biphenylboronic acid was coupled with 2-bromo-5-(trimethylsilyl)pyridine[13] or 2bromopyridine to give the respective phenypyridine-type ligands, compound 1 or 2, via palladium-catalyzed Suzuki reaction. They were then subjected to Nonoyama reaction [16] with IrCl3 ·H2 O to yield chloride-bridged dimers followed by the substitution of the chlorides with acetylacetonate ancillary ligands to give the final iridium complexes, Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac). While the structural difference between Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac) is the existence of trimethylsilyl substituents at the pyridine ring of the ligands, the better solubility of the silylcontaining Ir(Si-bppy)2 (acac) was easily verified. Ir(Si-bppy)2 (acac) was soluble enough to NMR characterization in acetone-d6 , while Ir(bppy)2 (acac) was not, so NMR spectrum of Ir(bppy)2 (acac) was measured in chloroform-d. The 1 H NMR spectra of both of the compounds are shown in Fig. 1. The trimethylsilyl proton of Ir(Sibppy)2 (acac) was clearly observed at 0.35 ppm. The 13 C NMR spectra of both of the compounds are also shown in the supporting information (Figs. S1 and S2). The relatively low signal-to-noise of the 13 C NMR spectrum of Ir(bppy)2 (acac) is caused by the limited solubility of Ir(bppy)2 (acac) in chloroform. The better solubility of silyl-containing Ir(Si-bppy)2 (acac) implies that processability of the iridium complexes in the solution state can be improved by the silyl-substitution strategy. 3.2. Optical properties of the iridium complexes To investigate the optical properties of the iridium complexes, UV–vis absorption and photoluminescence (PL) emission spectra of the iridium complexes were measured. Fig. 2(a) shows the absorption spectra of Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac) in 1,2-dichloroethane solution. Both of the two compounds showed very similar absorption features. The intense absorption around ˆ ligand singlet 1 (␲–␲*) transition. As 270 nm originates from CN a result of silyl substitution, the maximum absorption wavelength was slightly shifted by 4 nm, from 271 nm for Ir(bppy)2 (acac) to 275 nm for Ir(Si-bppy)2 (acac). The less intense but still clearly

Fig. 2. (a) UV–vis absorption spectra and (b) photoluminescence emission spectra of the iridium complexes.

seen two additional peaks in the range of 400–500 nm can be assigned to metal-to-ligand charge-transfer (MLCT) transition. The peak around 410 nm originates from spin-allowed singlet MLCT (1 MLCT) transition and the lower energy peak around 470 nm originates from formally spin-forbidden triplet MLCT (3 MLCT) transition. These MLCT absorption regions are shown enlarged as an inset of Fig. 2(a). Similar to 1 (␲–␲*) transition, the 1 MLCT and the 3 MLCT peaks were also slightly shifted by 5–6 nm as a result of silyl substitution. In the PL emission measurement, the two compounds showed intense green luminescence upon photoexcitation at 360 nm (Fig. 2(b)). The shapes of emission spectra were almost the same for the two compounds. The emission peak wavelength differs by 6 nm, and the full-width at half-maximum (FWHM) of emission spectra was the same, 62 nm. The emission maximum wavelength for Ir(Sibppy)2 (acac) was 537 nm, and Ir(bppy)2 (acac) showed 531 nm. In general, there can be two phosphorescence emission paths possible for most of Ir-based complexes, one from 3 MLCT states, and the other from ligand-centered triplet 3 (␲–␲*) states. It has been reported that phosphorescence from 3 MLCT shows broad and structureless spectrum, while phosphorescence from 3 (␲–␲*) usually shows highly structured spectrum [14]. In addition to the shape of the spectra, the Stokes shift between 3 MLCT absorption and phosphorescent emission is known to be relatively small for the phosphorescence from 3 MLCT, while it is relatively large

J. Lee et al. / Synthetic Metals 162 (2012) 1961–1967 Table 2 Electrochemical characteristics of iridium complexes.

Table 1 Optical characteristics of iridium complexes. UV–vis absorption [nm]

Ir(Si-bppy)2 (acac) Ir(bppy)2 (acac)

275 1 (␲–␲*), 417 (1 MLCT), 476 (3 MLCT) 271 1 (␲–␲*), 412 (1 MLCT), 470 (3 MLCT)

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PL emission [nm] max

FWHM

537

62

531

62

Ir(Si-bppy)2 (acac) Ir(bppy)2 (acac)

Eox,pa [mV]

Eox,pc [mV]

Eox,1/2 [mV]

HOMO [eV]

381 388

286 298

334 343

−5.13 −5.14

3.4. OLED device characteristics of the iridium complexes

From the optical measurements of the two compounds, it was found that silyl substitution at the pyridine ring of the ligand had a minor effect on the absorption and emission spectra. The spectra were almost the same, but the absorption and emission maxima were slightly shifted to a longer wavelength, about 4–6 nm. To discover the influence of silyl substitution on the electrochemical properties of the iridium complexes, cyclic voltammetry (CV) measurements were performed. The CV measurements were done in the solution state, where the iridium complexes were dissolved in the supporting electrolyte solution. Fig. 3 shows the cyclic voltammogram of Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac) during oxidative scan. After the measurement, the potential was re-calculated and plotted against the first oxidation potential of ferrocene, an internal reference. As are shown in Fig. 3, both of the two iridium complexes showed almost the same electrochemical characteristics. The HOMO (highest occupied molecular orbital) energy levels of Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac) were calculated to be −5.13 eV and −5.14 eV, respectively, from their Eox,1/2 by regarding Eox,1/2 of ferrocene as −4.80 eV [17]. In general, 0.01 eV of difference in energy level is such a small value that this cannot lead to serious difference in charge injection or transport in OLED devices. The detailed electrochemical measurement results are summarized in Table 2.

In the final step, the OLED test devices were fabricated and characterized using the iridium complexes to explore the influence of silyl substitution. Usually the emitting layer of phosphorescent OLED consists of mixture of host and emitting dopant. However, unlike vacuum-deposited multi-layer devices, whose basic structure is [hole-transporting layer (HTL)/emitting layer (EML)/electron-transporting layer (ETL)], it is difficult to obtain multi-layer structure with separate layers in the solutionprocessed OLEDs. So single-layer structure with one organic emitting layer is generally adopted for solution-processed OLEDs. In that case, an organic emitting layer contains additional chargetransporting materials as well as host and dopant [3]. In our experiments, we used PVK as a host polymer, TPD as a holetransporting material, PBD as an electron-transporting material, and Ir(Si-bppy)2 (acac) or Ir(bppy)2 (acac) as an emitting dopant. The composition ratio of PVK:TPD:PBD:Ir-complex was 51:10:32:7. Fig. 4 shows the electroluminescence (EL) spectra of OLED devices containing Ir(Si-bppy)2 (acac) or Ir(bppy)2 (acac). Similarly to the PL emission spectra, Ir(Si-bppy)2 (acac) showed a slightly red-shifted EL emission (max = 536 nm) compared with Ir(bppy)2 (acac) (max = 528 nm). The CIE 1931 color coordinates of Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac) were (0.35, 0.61) and (0.38, 0.59), respectively. Fig. 5 shows the current density–voltage–luminance (J–V–L) characteristics of the OLED devices. Both of the devices showed very low turn-on voltage, that is, the turn-on voltage at 1 cd/m2 was 2.8 V and 3.0 V for Ir(Sibppy)2 (acac) and Ir(bppy)2 (acac), respectively. This low turn-on voltage implies a good charge balance inside the emitting layer during operation. The overall shape of the curves seems to be similar between the two devices, but if we look closer, we can find out that Ir(Si-bppy)2 (acac) shows the higher luminance and the lower current than Ir(bppy)2 (acac) at the same applied voltage. This difference becomes more pronounced in the efficiency-related curve. In Fig. 6, the current efficiency was plotted against the current density. The maximum current efficiency of Ir(Si-bppy)2 (acac) reached almost 25 cd/A, while that of Ir(bppy)2 (acac) was around 18 cd/A.

Fig. 3. Cyclic voltammograms of the iridium complexes.

Fig. 4. Electroluminescence spectra of the iridium complexes.

for the phosphorescence from 3 (␲–␲*). By considering the Stokes shift value (61 nm for both of the two compounds) and the line shape of the emission spectra, for both of the two compounds, it is evident that the phosphorescence emission predominantly arises from 3 MLCT state, and the 3 MLCT becomes the lowest energy excited state. The triplet (T1 ) energy levels of Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac) were thus calculated to be 2.31 eV and 2.33 eV, respectively. The optical characteristics of the iridium complexes are summarized in Table 1. 3.3. Electrochemical properties of the iridium complexes

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Fig. 5. Current density–voltage–luminance characteristics of OLED devices.

Fig. 6. Current efficiency–current density characteristics of OLED devices.

Furthermore, as is shown in the inset of Fig. 6, it is noteworthy that the maximum efficiency was achieved at around 1000 cd/m2 of luminance, because obtaining high efficiency at high luminance is important for practical application. The OLED device characteristics are summarized in Table 3. The film morphologies of the emitting layers were investigated by AFM measurements to understand the relationship between the film morphology and the OLED device characteristics. Fig. 7 shows the comparison of the AFM images of the emitting layers containing Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac), respectively. The analyzed emitting layer samples were prepared in the same way as the real OLED devices have been prepared. Both of the films showed quite uniform surfaces, but root-mean-square (RMS) roughness of the Ir(Si-bppy)2 (acac)-based film, 0.392 nm, was slightly lower than that of Ir(bppy)2 (acac)-based film, 0.430 nm. However, these RMS roughness values are comparable to

Fig. 7. AFM images of the surface of the emitting layers onto the [ITO/PEDOT:PSS] substrates. (a) Ir(Si-bppy)2 (acac)-based emitting layer and (b) Ir(bppy)2 (acac)-based emitting layer.

previously reported other solution-processed OLEDs [9–11]. We also further investigated the film morphology after thin aluminum deposition (30 nm) on the emitting layer to estimate the change of surface roughness during cathode deposition. Fig. S6 shows the comparison of the AFM images of the aluminum surfaces containing Ir(Si-bppy)2 (acac)-based emitting layer and Ir(bppy)2 (acac)-based emitting layer, respectively. While the RMS roughness values were slightly increased after aluminum deposition, Ir(Si-bppy)2 (acac)based one again showed the lower roughness (1.85 nm) than Ir(bppy)2 (acac)-based one (2.10 nm). From these AFM results, it is plausible that Ir(Si-bppy)2 (acac) is more advantageous to form uniform film by solution-processing than Ir(bppy)2 (acac), even though

Table 3 OLED device characteristics of iridium complexes.

2

Turn-on voltage @ 1 cd/m [V] Maximum current efficiency [cd/A] Maximum power efficiency [lm/W] At 1000 cd/m2

Applied voltage [V] Current efficiency [cd/A] Power efficiency [lm/W]

Ir(Si-bppy)2 (acac)

Ir(bppy)2 (acac)

3.0 24.5 (@ 6.0 V) 16.5 (@ 4.5 V)

2.8 18.0 (@ 6.5 V) 11.1 (@ 5.0 V)

5.5 24.4 15.3

5.7 17.4 10.7

J. Lee et al. / Synthetic Metals 162 (2012) 1961–1967

the film morphology cannot fully explain the OLED device performance differences alone. The advantage of Ir(Si-bppy)2 (acac) can therefore be understood as follows. By introducing silyl substituents, the solubility of the iridium complex becomes increased, and the increased solubility of Ir(Si-bppy)2 (acac) is advantageous to form uniform film after spincoating. Furthermore, the bulky silyl substituents would prevent from crystallization or aggregation of the iridium complexes inside the film. Because the microscopic non-uniform sites inside the organic film can act as traps in OLEDs, it is especially important to get uniform film in small-molecule-based OLEDs. In addition to the film-forming properties, the bulky silyl substituents can also prevent exciton quenching by increasing the dopant–dopant distances. Silyl substitution at the pyridine ring of the ligand is therefore a simple but useful strategy to the development of solution-processable iridium-based emitting materials. 4. Conclusions In this paper, we investigated the effect of silyl substitution to iridium-based green emitting phosphorescent materials for the purpose of enhancing solution-processable OLED device performances. Two kinds of iridium complexes, Ir(Si-bppy)2 (acac) and Ir(bppy)2 (acac), with similar chemical structures but with or without substitution have been designed and synthesized. The optical and electrochemical properties of the two iridium complexes were characterized by UV–vis spectroscopy, PL emission spectroscopy and cyclic voltammetry. The two compounds showed very similar optical and electrochemical properties which are believed not to lead to serious difference in charge injection or transport in OLED devices. Contrary to these slight differences in optical and electrochemical characteristics, there was a large difference between the two compounds when applied to solution-processed OLEDs. By combining host, dopant and charge-transporting additives in the single emitting layer, both of the two OLED devices showed low turn-on voltage (3.0 V or less) and high luminance (up to around 30,000 cd/m2 ). However, the maximum current efficiency of OLED device was increased from 18.0 cd/A to 24.5 cd/A as a result of silyl substitution. This enhancement of OLED device performance can be attributed to the increased solubility and processability of Ir(Sibppy)2 (acac). Furthermore, the existence of bulky silyl substituents can also reduce intermolecular interaction of the compounds and prevent exciton quenching during OLED operation. Combining all

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these results, we suggest silyl-substitution at the pyridine ring of the ligand as a useful strategy to improve the OLED device performance of iridium complexes, especially in solution-processing. Acknowledgements This work was supported by the Industrial Strategic Technology Development Program (10042590, “Materials Development for 50-inches UD OLED TV Using Super Hybrid Process”) funded by the Ministry of Knowledge Economy (MKE) of Korea, and also supported by the Joint Research Program (B551179-09-07) funded by the Korea Research Council for Industrial Science & Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2012.08.011. References [1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. MacKay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [3] X.H. Yang, D. Neher, Applied Physics Letters 84 (2004) 2476. [4] Y. You, S.Y. Park, Dalton Transactions (2009) 1267. [5] X. Yang, D.C. Müller, D. Neher, K. Meerholz, Advanced Materials 18 (2006) 948. [6] W. Zhu, Y. Mo, M. Yuan, W. Yang, Y. Cao, Applied Physics Letters 80 (2002) 2045. [7] N. Rehmann, D. Hertel, K. Meerholz, H. Becker, S. Heun, Applied Physics Letters 91 (2007) 103507. [8] Y. You, C.G. An, D.S. Lee, J.J. Kim, S.Y. Park, Journal of Materials Chemistry 16 (2006) 4706. [9] J.H. Jou, W.B. Wang, S.Z. Chen, J.J. Shyue, M.F. Hsu, C.W. Lin, S.M. Shen, C.J. Wang, C.P. Liu, C.T. Chen, M.F. Wu, S.W. Liu, Journal of Materials Chemistry 20 (2010) 8411. [10] W. Jiang, L. Duan, J. Qiao, D. Zhang, G. Dong, L. Wang, Y. Qiu, Journal of Materials Chemistry 20 (2010) 6131. [11] K.S. Yook, J.Y. Lee, Organic Electronics 12 (2011) 1711. [12] K.S. Yook, S.E. Jang, S.O. Jeon, J.Y. Lee, Advanced Materials 22 (2010) 4479. [13] S.O. Jung, Q. Zhao, J.W. Park, S.O. Kim, Y.H. Kim, H.Y. Oh, J. Kim, S.K. Kwon, Y. Kang, Organic Electronics 10 (2009) 1066. [14] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.E. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, Journal of the American Chemical Society 123 (2001) 4304. [15] D.H. Hwang, M.J. Park, J.H. Eom, H.K. Shim, S. Lee, N.C. Yang, D. Lian, M.C. Suh, B.D. Chin, Journal of Nanoscience and Nanotechnology 8 (2008) 4649. [16] Nonoyama, Bulletin of the Chemical Society of Japan 47 (1974) 767. [17] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bässler, M. Porsch, J. Daub, Advanced Materials 7 (1995) 551.