NHK Laboratories Note No. 484


Color tunable phosphorescent organic light-emitting diodes using pentafluorophenyl-substituted iridium complexes


Toshimitsu TSUZUKI and Shizuo TOKITO

Display and Optical Devices


Abstract

  We developed novel cyclometalated iridium complexes which possess perfluorophenyl-substituted phenylpyridine ligands for the phosphorescent emitting dopant in organic light-emitting diodes (OLEDs). By introducing a perfluorophenyl substituent in phenylpyridine ligands of iridium(III)bis(2-phenylpyridinato-N,C2')acetylacetonate [(ppy)2Ir(acac)] and by changing the position of the substitution, photoluminescence and electroluminescence spectra could be tuned to either a shorter or longer wavelength. The emission color changed; the changes ranged from green to orange. The OLEDs using developed complexes as an emitting dopant exhibited good performance characteristics. The turn-on voltages for light emission were 3-4 V, and the luminance reached 80,000 cd/m2 at 11 - 12 V. External quantum efficiencies were 10-17 % at a luminance of 100 cd/m2.

1. Introduction
   Organic light-emitting diodes (OLEDs) have attracted great attention because they can be applied to full-color flat-panel displays. Recently, highly efficient OLEDs using phosphorescent dyes such as 2, 3, 7, 8, 12, 13, 17, 18-octaethyl-21H, 23H-porphine platinum, iridium(III)fac-tris(2-phenylpyridinato-N,C2') [Ir(ppy)3], iridium(III)bis(2-phenylpyridinato-N,C2')acetyl-acetonate [(ppy)2Ir(acac)], and their derivatives have been reported.[1-12,14-19] In the OLEDs using phosphorescent dyes, the triplet exciton contributes to light emission and emission efficiency has greatly improved compared with that of conventional fluorescent OLEDs, which utilize the emission from only a singlet exciton. Ir(ppy)3 and (ppy)2Ir(acac) have brought about green emission with an extremely high external quantum efficiency even though its molecular structure is simple.[8,9] There are several reports about color tuning in these materials.[7,11,12,13,14]
   In this paper, we report on the color tuning of the phosphorescent OLEDs by simple way, only by controlling the position of a perfluorophenyl substituent in (ppy)2Ir(acac)[18,20]. The perfluorophenyl substituent should greatly affect the electronic structure of the (ppy)2Ir(acac) because the substituent has an electron accepting nature. By introducing a perfluorophenyl substituent in phenylpyridine ligands of (ppy)2Ir(acac) and by changing the position of the substitution, photoluminescence and electroluminescence spectra could be tuned to either a shorter or longer wavelength, which resulted from the change of energy levels of 3MLCT in the complexes. The OLEDs with the perfluorophenyl substituted (ppy)2Ir(acac) as an emitting material exhibited a high quantum efficiency of 10-17 %.

2. Development of Novel Iridium Complexes
  Figure 1 shows chemical structures of the developed perfluorophenyl-substituted complexes, m-PF-ph, p-PF-ph, p-PF-py, and m-PF-py as well as that of (ppy)2Ir(acac). These complexes possess 2-phenylpyridine ligands substituted with a pentafluorophenyl group at either the meta(m-)/para(p-) position of the phenyl or pyridyl ring. We expected a bulky and fluorinated substituent, such as a pentafluorophenyl group, to reduce the intermolecular interaction of the complexes and to enhance the photoluminescence by preventing self-quenching.[10,15,17] In addition to that, we are interested in examining how the position of the substitution with the -electron substituent such as the pentafluorophenyl group affects the photoluminescence of the complex from the viewpoint of color tuning the OLED. These novel complexes were synthesized as follows.[18,19] Na3IrCl6(2H2O) (280 mg, 0.55 mmol) and corresponding pentafluorophenyl substituted 2-phenylpyridine ligand[18] (441 mg, 1.37 mmol) were placed in a Shlenk tube under argon atmosphere. 2-ethoxyethanol (40 mL) was added and the slurry was heated at 105 for 10 h. After cooling to room temperature, solvent was removed by rotary evaporator and the resulting orange solid was dissolved in toluene and filtered. The filtrate was evaporated to dryness and the residue was washed with hexane. The obtained orange solid was placed in a Shlenk tube under argon and dissolved in ethanol (30 mL). Acetylacetone (125 mg, 1.23 mmol) and sodium carbonate (130 mg, 1.23 mmol) were added to the solution and the mixture was heated at 50 for 2-6 h. After cooling to room temperature, the solvent was removed in vacuo and the product was flash chromatographed over silica gel (CHCl3) to give pentafluorophenyl substituted iridium complexes. Analytically pure sample was obtained by train sublimation.







Fig. 1. Chemical structures of (ppy)2Ir(acac) and developed Iridium
complexes, m-PF-ph, p-PF-ph, p-PF-py, m-PF-py.




3. Fabrication of Organic Light-Emitting Diodes (OLEDs) using Developed Iridium Complexes



  We fabricated multilayered OLEDs such as indium-tin-oxide (ITO)/ N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (-NPD)(40 nm)/a developed complex doped in 4,4'-di(carbazole-9-yl)biphenyl (CBP)(35 nm)/ bathocuproine (BCP)(10 nm)/ tris(8-quinolinolato)aluminum (Alq3)(35 nm)/LiF(0.5 nm)/Al(100 nm) as shown in Fig.2. An OLED with (ppy)2Ir(acac) was also fabricated as a reference device. Organic layers and a metal cathode were deposited by high vacuum (10-5 Pa) thermal evaporation onto ITO-coated glass substrate. Prior to use, the ITO glass substrates were rinsed and degreased by sonication in a detergent solution, pure water, acetone, and 2-propanol. The substrates were cleaned in a UV-ozone chamber just before they were loaded into a vacuum evaporator. A 40-nm-thick layer of -NPD that acted as a hole transport layer, a 35-nm-thick emitting layer, a 10-nm-thick layer of BCP that acted as a hole-blocking layer, a 35-nm-thick layer of Alq3 that acted as an electron transport layer, and a 0.5-nm lithium fluoride/100-nm aluminum cathode were deposited successively without breaking the vacuum. As the emitting layer, a doped film that consisted of a 6wt%-doped iridium complex into a host material of CBP was co-deposited. CBP was chosen as a host material of emitting layer because it acts as a good host material for emissive guest such as Ir(ppy)3 or (ppy)2Ir(acac)[4,5,7]. After a cathode deposition, the devices were encapsulated with a resin polymerized by UV irradiation under a nitrogen atmosphere. Luminance was measured with a MINOLTA LS-110. Electroluminescence (EL) spectra were measured with a MINOLTA CS-1000. External quantum efficiency was calculated from the luminance, current density, and EL spectrum under an assumption of Lambertian emission.





Fig 2 .Side view of the multilayered OLEDs and chemical structures of materials used.


4. Results and Discussions

  Figure 3 shows photoluminescence (PL) spectra for the complexes doped in a host material of CBP film. The PL spectrum in a doped film is coincident with that in chloroform solution for each complex. We found that the peak of the PL spectra shifts to a shorter or longer wavelength by introducing a pentafluorophenyl group, indicating that the emission color is tunable according to the position of the substitution. The peak of the PL spectra for the complexes ranged in the wide region from 514 to 576 nm. The shift to a shorter wavelength was observed only in the complex substituted at a meta position of the phenyl ring (m-PF-ph). This result is interesting and useful in designing blue phosphorescent materials. The transient photoluminescence decay at room temperature for each complex doped in the CBP film was mono-exponential, and the estimated exciton lifetime of the complex ranged from 0.85 to 2.2 s. These lifetimes in the microsecond regime suggest that triplet excited states contributed to light emission, that is, the complexes emit phosphorescence.[7,13] The lowest energy (emissive) excited state in bis-cyclometalated iridium complexes has been reported to be predominantly 3(-*) or 3MLCT, and (ppy)2Ir(acac) reportedly emits primarily from a 3MLCT state.[7] We could observe the optical absorption bands of the 3MLCT in chloroform solution for the developed complexes and (ppy)2Ir(acac) as shown in Fig. 4. Although 3MLCT transition is essentially spin-forbidden, strong spin-orbit coupling on Ir gives a weak absorption of 3MLCT in such iridium complexes.[7,13] of 3MLCT band in (ppy)2Ir(acac) was estimated to 459 nm, which is similar to reported value.[7,13] The of 3MLCT band blueshifted in m-PF-ph, and redshifted in p-PF-ph, p-PF-py, and m-PF-py, compared to (ppy)2Ir(acac). A direction and magnitude of the shift of the 3MLCT absorption band is well coincident with that of the PL peak. From these results, we can say that the substitution with the perfluorophenyl group in the phenyl or pyridyl ring is useful in controlling the energy level of the 3MLCT state.
  All the fabricated OLEDs emitted brightly when positive bias was applied to the ITO electrode. The peak wavelength of the EL spectrum for the devices using m-PF-ph, (ppy)2Ir(acac), p-PF-ph, p-PF-py, and m-PF-py, was 513, 522, 544, 559, and 578 nm, respectively. The EL spectrum for each device was almost coincident with the PL spectrum of the corresponding complex doped in CBP film. The emission color changed; the changes ranged from green to orange. Thus, we could control the emission color of OLEDs by only changing the substitution position of the perfluorophenyl group. Figure 5 shows the applied voltage - luminance and applied voltage - current density characteristics of the fabricated OLEDs. The turn-on voltages of the OLEDs for light emission were 3-4 V, and the luminance reached 80,000 cd/m2 at 11 - 12 V. Figure 6 shows external quantum efficiency as a function of current density for the fabricated OLEDs. A maximum external quantum efficiency was 11.2 % for m-PF-ph, 12.3 % for p-PF-ph, 14.9 % for p-PF-py, and 16.9 % for m-PF-py. A gradual decrease of the external quantum efficiency with an increase of current density was observed for each device, which has been attributed to triplet-triplet annihilation.[6] The performance of the fabricated OLEDs is summarized in Table I. The devices showed very high efficiency. External quantum efficiency at a luminance of 100 cd/m2 was 10.0 % for m-PF-ph, 11.5 % for (ppy)2Ir(acac), 12.1 % for p-PF-ph, 14.7 % for p-PF-py, and 16.8 % for m-PF-py. The quantum efficiencies of the device using (ppy)2Ir(acac) is comparable with the reported OLED using (ppy)2Ir(acac) as an emitting dopant and CBP as a host.[7] There is an upward tendency in quantum efficiencies with redshift of the emission peak. That phenomenon can be considered as follows. An energy difference between the 3MLCT of the complex and triplet energy of host material becomes larger when the emission peak shifts longer wavelength region, that is, the 3MLCT energy level falls lower. Then, energy transfer from the host to the iridium complex occurs more efficiently or a triplet energy is confined more efficiently on the complex. As a result, high external quantum efficiency was achieved. Luminous efficiency (cd/A) and power efficiency (lm/W) were 41-47 cd/A and 19-29 lm/W, respectively - very high by all accounts.





Fig 3 .Photoluminescence spectra for films of the complexes 6wt% doped in CBP.






Fig 4 .Absorption spectra for developed complexes in chloroform solution.
The spectra are offset for clarity. Extinction coefficients at 600 nm of all the complexes are almost 0.
Each arrow in absorption spectra shows a peak of an absorption band of 3MLCT.







Fig 5 .Applied voltage - Luminance and Applied voltage - current density
characteristics for OLEDs using developed complexes as emitting material.







Fig 6 .External quantum efficiency as a function of current density for
OLEDs using developed complexes as emitting material.



Iridium Complex as

of EL
spectrum
(nm)
External quantum
efficiency (%)
Luminous
efficiency
at 100
cd/m2
(cd/A)
Power
efficiency
at 100
cd/m2
(lm/W)
emitting dopant
at 100
cd/m2
at 1000
cd/m2
m-PF-ph
513
10.0
8.1
45
19
(ppy)2Ir(acac)
522
11.5
9.5
42
23
p-PF-ph
544
12.1
8.1
41
20
p-PF-py
559
14.7
14.1
47
29
m-PF-py
578
16.8
16.0
43
28


Table 1 .Performance characteristics of fabricated OLEDs,
ITO/-NPD (40 nm)/CBP:6wt% Ir complex (35 nm)/BCP (10nm)/Alq3 (35 nm)/LiF (0.5 nm)/Al (100 nm).



5. Conclusion



  In conclusion, we developed novel iridium complexes with perfluorophenyl-substituted phenylpyridine ligands. By changing the position of substitution, the peaks of electroluminescence spectra was tuned in the wavelength region from 513 to 578 nm. OLEDs with developed perfluorophenyl-substituted complexes as an emitting material showed an external quantum efficiency of 10-17 %. We believe the efficiency can be further improved by using a more suitable host material for the emitting layer[4,8,19] or a more suitable material for the hole-blocking layer.[9]

Acknowledgment

  The authors would like to acknowledge Dr. Toshiyasu Suzuki and Dr. Nobuhiko Shirasawa in Institute of Molecular Science for synthesis of novel developed iridium complexes and helpful discussions.







Reference


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Dr. Toshimitsu Tsuzuki Dr. Toshimitsu Tsuzuki
He received the Ph.D. degree in materials chemistry from Osaka University, Osaka, Japan, in 1999. In the same year, he joined Toyota Motor Corporation, where he worked on silicon power devices for hybrid vehicles and polymer electrolyte for fuel cells. He joined NHK in 2002. Since 2002, he has been with NHK Science and Technical Research Laboratories, where he has been engaged in research on organic light-emitting displays based on phosphorescent materials.
Dr. Shizuo Tokito Dr. Shizuo Tokito
He received the M.S. degree in 1984 and Ph.D. degree in 1987 from the Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan, then, he became a Research Associate. From 1988 to 1989, he worked with Prof. Alan J. Heeger and Prof. Paul Smith as a Postdoctoral Researcher at University of California, Santa Barbra. In 1990, he moved to Toyota Central Research Laboratories, Inc. In 2001, he jointed Science and Technical Research Laboratories of the NHK (Japan Broadcasting Corporation) as a Senior Research Scientist. He is currently working on the research and development of flexible organic light-emitting display based on phosphorescent materials. His research interests cover optical and electrical properties of organic materials, and their applications.
 



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