Blue Light-Emitting Bisorthometalated Ir(III) Complex: Origin of Blue

Blue light-emitting bisorthometalated Ir(III) complex with substitutional pyridine and phosphine ligands has been studied. From quantum chemical calcu...
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J. Phys. Chem. C 2008, 112, 4743-4747

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Blue Light-Emitting Bisorthometalated Ir(III) Complex: Origin of Blue Emission and Application in Electrophosphorescent Devices Yuan-Min Wang,† Feng Teng,*,† Li-hua Gan,§ Hong-Mei Liu,‡ Xiao-Hong Zhang,*,‡ Wen-Fu Fu,*,‡ Yong-Sheng Wang,† and Xu-Rong Xu† Institute of Optoelectronic Technology, Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong UniVersity, Beijing, 100044, China, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing, 100080, China, and College of Chemistry & Chemical Engineering, Southwest UniVersity, Chongqing, 400715, China ReceiVed: August 19, 2007; In Final Form: January 15, 2008

Blue light-emitting bisorthometalated Ir(III) complex with substitutional pyridine and phosphine ligands has been studied. From quantum chemical calculation we suggest that the blue emission can be ascribed to the redistribution of the molecular orbitals after the phosphine ligand has been introduced. This result is not only helpful to investigate the emission mechanism of the Ir(III) complex but also is meaningful for the design of new luminescent materials based on organometallics. The electrophosphorescent polymer light-emitting diodes based on the Ir(III) complex have low turn-on voltages less than 4 V. The brightness and external quantum efficiency (EQE) for the single-layer device reaches 1190 cd/m2 at 12 V and 2.1% at a current density of 9.85 mA/ cm2, respectively. For the multilayer device, because of more balanced carrier injection, the external quantum efficiency reaches 4.0% at a current density of 3.3 mA/cm2.

Introduction Polymer light-emitting diodes (PLEDs) have been under active investigation because of the potential applications in flat panel displays and the convenient fabrication methods such as spin-casting, screen-printing, or inkjet-printing.1-3 Phosphorescent materials and devices are the prime focus of PLED research because of their ability to utilize both singlet and triplet excitons.4 The reported phosphorescent materials are mainly based on heavy metals, such as Ir(III),5-7 Pt(II),8-10 Os(II),11-13 Re(I),14-16 Au(I),17 and Cu(I)18-21 (The latest comments on phosphorescent dyes for organic light-emitting diodes can be found in ref 7.) However, cyclometalated Ir(III) materials have shown the most promising applications because of their higher stability, higher photoluminescence (PL) efficiency, and relatively shorter excited-state lifetime. Recently, green or red emitting Ir(III) compounds have been reported extensively while it is rare for the blue light-emitting materials.22-25 To design and synthesize an excellent light-emitting material, the synthetic strategy is the most important. Lee et al.22-23 have reported the blue light-emitting Ir(III) complex based on mixed pyridine and phosphine ligands. About the origin of the blue emission, they suggested that the mechanism is π-backbonding. However, there is no evidence supporting this assumption. Moreover, though they have applied the chloro Ir(III) complex to fabricate the light-emitting devices, the performance is not satisfied. For device ITO/PEDOT/PVK: iridium complexes (8 wt %)/Mg:Ag/Ag, the peak brightness is only 22 cd/m2 at 18 V. Recently, we also studied the blue light-emitting Ir(III) * Corresponding author. Prof. Feng Teng: e-mail: advanced9898@ yahoo.com.cn, tel: 86-10-51688605, fax: 86-10-51688018. Prof. Xiaohong Zhang: e-mail: [email protected], tel: 86-10-82543510. Prof. Wenfu Fu: e-mail: [email protected], tel: 86-10-82543519. † Beijing Jiaotong University. ‡ Chinese Academy of Science. § Southwest University.

complex based on pyridine and phosphine ligands. For the chloro complex Ir(ppy)2(PPh3)Cl, the detailed structure has been first examined by X-ray crystallography.26 In this paper, the origin of the blue emission of complex Ir(ppy)2(PPh3)Cl has been investigated by quantum chemical calculation and efficient light-emitting devices have been obtained by using a co-host device structure and CsF/Mg:Ag as the complex cathode. Experimental Section Materials. Figure 1a shows the synthesis procedure of compex Ir(ppy)2(PPh3)Cl. The molecular structure has been characterized by X-ray crystallography in our previous report.26 The synthesis of dimmer Ir2(ppy)4Cl2 is according to the literature method.27 PLEDs Fabrication and Measurements. ITO-coated glass substrates with a sheet resistive of 60 Ω/0 were cleaned consecutively in ultrasonic baths containing deion water, ethanol, and acetone. PEDOT:PSS and (PVK:PBD:Ir complex) film were fabricated by the spin-coating method. The PEDOT:PSS layer has been heated at 90 °C for 30 min before the following procedure. The CsF/Mg:Ag cathodes were fabricated by vapor deposition. The brightness was measured by a PR-650 spectroscan spectrometer. The current-voltage curve was measured by Keithley 2400 programmable voltage-current source. All of the measurements were carried out in air and under room temperature without encapsulation. Results and Discussion Figure 2 shows the absorption and PL spectra of Ir2(ppy)4Cl2 and Ir(ppy)2(PPh3)Cl in CH2Cl2 solution. Both the complexes have low-energy absorption ranging from 400 to 500 nm, which is evidence of metal to ligand charge transfer (MLCT).28 For Ir2(ppy)4Cl2, the absorption bands at 400 nm and 450 nm can be ascribed to the 1MLCT and 3MLCT excited

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Figure 1. (a) Synthetic procedure of compex Ir(ppy)2(PPh3)Cl. (b) Structure of the co-host light-emitting device.

Figure 3. Quantum chemical calculation results: (a) HOMO of Ir2(ppy)4Cl2, (b) LUMO of Ir2(ppy)4Cl2, (c) HOMO of Ir(ppy)2(PPh3)Cl, and (d) LUMO of Ir(ppy)2(PPh3)Cl.

Figure 2. Absorption and photoluminescence (PL) spectra of Ir2(ppy)4Cl2 and Ir(ppy)2(PPh3)Cl in CH2Cl2 (the inserted figure is the emission decay of Ir(ppy)2(PPh3)Cl in powder).

state, respectively. However, compared with Ir2(ppy)4Cl2, the absorption band of the 3MLCT excited state of Ir(ppy)2(PPh3)Cl shifts to about 400 nm. Accordingly, the emission peak also shifts from 512 to 485 nm. The blue-shifted absorption and emission imply an increased HOMO-LUMO energy gap of the 3MLCT excited states, which is described by following quantum chemical calculation. The quantum efficiency of Ir(ppy)2(PPh3)Cl in CH2Cl2 at room temperature is about 0.014 by using [Ru(bpy)3]2+ (φem ) 0.042) as the standard sample. The lifetime of Ir(ppy)2(PPh3)Cl (powder) shows double exponentials as 0.123 and 0.343 µs, respectively. The PL decay curve is inserted in Figure 2. Because long lifetime will give rise to the emission saturation and then decrease the efficiency, suitable lifetime of Ir(III) complexes (∼0.1-14 µs) make them ideal candidates for the OLEDs application. For the blue emission of the chloro Ir(III) complex, Lee et al.22 suggested that it can be ascribed to the π-backbonding from the dπ orbital to the phosphine ligands that moves up the energy of the MLCT states. However, they have not given out the proof of the explanation. Here, we investigated the origin of the blue emission by quantum chemical calculation and suggested that the blue emission is due to the redistribution of the molecular orbitals (HOMO and LUMO) after the phosphine ligand has been introduced. When we carried out the quantum chemical calculation, the Gaussian 03 program was used in all of these calculations.29 MO analysis were performed by using the ab initio HartreeFock method at HF/3-21G*level of theory with the basis sets of LANL2DZ30 for iridium atoms. Figure 3 shows the quantum chemical calculation results. Figure 3a and b are the HOMO and LUMO for complex Ir2(ppy)4Cl2. (The experimental data can be found in ref 31.) From the figure we can see that the HOMO and LUMO localize on the iridium atom (mainly on dπ orbitals) and the pyridine rings, respectively. This is consistent with the property of the excited state of complex Ir2(ppy)4Cl2 because the luminescence mechanism is the metal to ligand charge transfer (dπ of iridium to π* of the pyridine).

However, when the phosphine ligand is introduced in the molecule, the distribution of the molecular orbitals has changed. Figure 3c and d shows the HOMO and LUMO of complex Ir(ppy)2(PPh3)Cl. It is obvious that the HOMO is localized mainly on the iridium atom and pyridine rings and much more delocalized compared with the HOMO of complex Ir2(ppy)4Cl2. Moreover, the LUMO partially localizes on the phosphine ligand besides the pyridine ring and it looks also be delocalized compared with the LUMO of complex Ir2(ppy)4Cl2. The delocalization of the orbitals implies that the energy level of both HOMO and LUMO has decreased. Though it is, our calculation result unambiguously indicates the energy gap (∆E ) LUMO - HOMO) has increased after the phosphine ligand has been introduced, that is, ∆E[Ir(ppy)2(PPh3)Cl] > ∆E[Ir2(ppy)4Cl2], which is consistent with our spectral measurements such as above absorption and emission spectra in Figure 2. A previous report22 has proposed that the origin of the blue emission maybe due to the π-backbonding between the iridium atom and the phosphine ligand. π-backbonding, also called π-backdonation, refers to the flow of electrons from an atomic orbital on one atom to a π antibonding orbital that belongs to other atoms. It is especially common in the coordination of multiatomic ligands to metal centers, where electrons from the metal flow toward the bond between the ligand atoms. If the π-backbonding between the Ir(III) dπ orbital to the phosphine ligand exists, it may decrease the HOMO level and move up the energy of the MLCT states. However, our statical calculation shows the delocalization of the HOMO but it does not show some contribution from π-backbonding. On the basis of our calculation, we conclude that the blue emission of Ir(ppy)2(PPh3)Cl is ascribed to the redistribution of the molecular orbitals after the phorsphine ligand is introduced. In order to get more fundamental and critical understanding of the origin of the blue emission, a time-dependent DFT calculation is in progress. Because Ir(ppy)2(PPh3)Cl can gives out bright-blue emission both in solution and in the solid state, a series of EL devices with different dopant concentrations have been fabricated: indium-tin oxide (ITO)/poly(3, 4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) (40 nm)/ poly(Nvinylcarbazole) (PVK):2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD) (10:4 w/w):Ir(III) complex(x%)(80 nm)/ CsF(1 nm)/Mg:Ag(200 nm), x ) 1% (device A), 3%(device B), 10%(device C), ITO/PEDOT:PSS(40 nm)/ PVK:PBD(10:4

Blue Light-Emitting Bisorthometalated Ir(III)

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Figure 4. (a) Normalized EL spectra of devices A, B, C, and D. (b) Normalized EL spectra of device A with different applied voltages.

Figure 5. Brightness (a), current density (b), power efficiency (c), and external quantum efficiency (d) of devices A, B, C, and D.

w/w):Ir(III) complex(16%)(80 nm)/BCP(10 nm)/AlQ3(30 nm)/ CsF(1 nm)/Mg:Ag(200 nm)(device D). In these devices, we use (PVK:PBD (10:4 w/w):Ir(III) complex) as the co-host emitting layer and CsF/Mg:Ag as a complex cathode. PEDOT:PSS is used to increase the anode work function to improve the hole transport by facilitating hole-injection. Figure 4a shows the normalized EL spectra of devices A, B, C and D. It can be seen that the emission from PVK has not been completely quenched until the concentration of the Ir(III) complex reaches 10%. Moreover, we have observed slight red shift of the EL spectra for all of the devices under high applied voltage. The red shift of the EL spectra can be observed from Figure 4a. On the basis of the understanding of interactions between carriers and excitons under high electric field, we suggest that this kind of red shift is probably due to the polarization of the molecular orbitals under high electric field. Figure 4b shows the normalized EL spectra of device A with different applied voltages. When the emission from the Ir(III) complex was normalized, the emission from PVK host increased with the applied voltage. It is due to the saturation of the luminescent sites under low concentration (1%) conditions as suggested by Lee et al.23 that the energy transfer mechanism had occurred in the luminescent process of the Ir(III) complex with the phosphine ligand.

All of the devices show very low threshold voltages. For single-layer devices A, B, and C, the threshold voltage is less than 4 V. It is about 7 V for multilayer device D. Figure 5a shows the brightness-voltage relationship of the devices. The peak brightness of device B reaches 1190 cd/m2 at 12 V. For devices A, C, and D, the peak brightness reaches 966 (11 V), 1100 (11 V), and 725 (17 V) cd/m2, respectively. The performance of high brightness with low voltage is far better than that in previous reports.23,32 In Lee’s report,23 the singlelayer device ITO/PEDOT/PVK:phosphine complex (8 wt %)/ Mg:Ag/Ag is also based on the chloro Ir(III) complex and the peak brightness is only 22 cd/m2 at 18 V. Gong et al.32 were the first to publish the high-efficiency phosphorescent LEDs based on a single-layer device. However, for their device with 1% concentration of the Ir(III) complex, a voltage of more than 20 V was needed to drive the device at a brightness of 1000 cd/m2. Even though the formation of the emitter triplet excited state via carrier trapping and then directly carrier recombination on the emitter molecule is an elegant way to achieve good color purity and high efficiency.33,34 A high driven voltage is often needed in order to build up the space-charge field. However, in our experiment, the luminescence mechanism is deduced to include both energy transfer and trap effect. Especially, the trap

4746 J. Phys. Chem. C, Vol. 112, No. 12, 2008 effect can be concluded from Figure 5b, in which the currentvoltage (I-V) characteristics shift to high voltage with increasing the concentration of the Ir(III) complex and it is consistent with literatures that hole traps are often formed when the Ir(III) complex is doped in a PVK host.34-36 In addition, devices based on the blue light-emitting Ir(III) complex show relative high efficiency. From Figure 5c and d one can see the power efficiency of devices C and D reach 0.80 lm/W at 6 V and 0.86 lm/W at 11 V, respectively. The maximum of external quantum efficiency (EQE) reaches 2.1% at 9.85 mA/cm2 for device C and 4.0% at 3.3 mA/cm2 for device D. Because of more balanced carrier injection, multilayer device D has higher external quantum efficiency. This result is better than Laskar’s report25 in which four blue-emitting Ir(III) complexes have been synthesized and the peak power efficiency of the device is only 0.26 lm/W. Compared with an extensively studied blue light-emitting complex FIrpic, the maximum external quantum efficiency of 1.3% and power efficiency of 0.8 lm/W were achieved when it was doped in a PVK host.37 Recently,38 a series of new blue-phosphorescent iridium(III) complexes with ligand 2-phenylimidazo pyridine (pip) derivatives were prepared successfully, and their electroluminescent (EL) properties were investigated. However, the best external quantum efficiencies for the devices are 3.6%, 2.9%, and 1.7%, respectively. In addition, we found that, though device D has higher EQE, it shows the quenching effect under high current density because of the high doping concentration. Another noticeable property of our devices is the higher current density than that in previous reports. From Figure 5d we can see that the EQE of all of the devices reaches the maximum at a more reasonable current density ranging from 0.1 to 10 mA/cm2.37 However, it is less than 0.1mA/cm2 for most of the previous reports on the blue or near-blue light-emitting phosphorescent PLEDs to reach the peak efficiencies.39-41 In conclusion, all of the satisfied performance of the PLEDs can be ascribed to the integrated device structure optimizing strategy using a more efficient complex cathode CsF/Mg:Ag to improve electon injection and using a co-host emitting layer to confine the holes in the emission area. Conclusions In summary, we have studied the blue light-emitting bisorthometalated Ir(III) complex with substitutional pyridine and phosphine ligands. The quantum chemical calculation suggests the blue emission may be due to the redistribution of the molecular orbitals after the phosphine ligand has been introduced. This result is valuable for interpreting the emission mechanism and future materials design for new luminescent complexes. High-efficiency blue electrophosphorescent polymer light-emitting diodes have been fabricated based on the Ir(III) complex. The devices show high brightness under low voltage. The brightness and external quantum efficiency for the single-layer device reach 1190 cd/m2 at 12 V and 2.1% at 9.85 mA/cm2. For the multilayer device, the external quantum efficiency reaches 4.0% due to more balanced carrier injection. In order to get more understanding of the blue emission mechanism and highly efficient electrophosphorescent devices, further investigations, both theoretical and experimental, are in progress. Acknowledgment. This work was supported by National Basic Research Program of China (no. 2003CB314707), State Key Project (no. 2005CCA06800), National Natural Science Foundation of China (nos. 90301004, 60677007, 10434030, and

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