Structure properties of a highly luminescent yellow emitting material for OLED and its application

Abstract

A tris-cyclometalated iridium(III) complex [Ir(DMP)₃] containing 2,6-dimethoxy phenol and an ancillary ligand was successfully prepared and used in the fabrication of organic light-emitting diodes (OLEDs). The absorption, emission, cyclic voltammetry and thermostability of the complex were systematically investigated. The structure of this complex was also characterized using single crystal X-ray diffraction analysis. Its crystal shows a cubic structure. Our device exhibits a yellow emission at 576 nm with a maximum luminescence efficiency of 10 564 cd m⁻¹2 at a voltage of 7 V and a current density of 118 mA cm⁻² respectively. The maximum quantum efficiency is 8.7% at 5.93 mA cm⁻². The Commission Internationale de l′Eclairage (CIE) coordinates were (0.49, 0.50) at a 2 wt% doping concentration and show typical rectifying diode characteristics in the ITO/PEDOS:PSS/PVK:PBD:Ir(DMP)₃/TPBI/Ba/Al device.

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1. Introduction

Organic light emitting diodes (OLEDs) have drawn tremendous research interest from both academia and industry over the last two decades because of their unique features and potential applications in full-color flat-panel displays, solid-state lighting, etc.^1,2 The basic technological requirements for both display and lighting applications include high efficiency, long operational stability, etc. Recently, phosphorescent OLEDs based on heavy metal complexes have been the prime focus of OLED research due their external quantum efficiency and power efficiency.^3,4 Owing to the strong spin-orbital mixing of the heavy metal ion, both the singlet and triplet excitons can be fully utilized in phosphorescent OLEDs and theoretically the internal efficiency can rich as high as 100%.⁵ Amongst these heavy metal complexes bearing transition metals, cyclo-metallated iridium complexes are regarded as the most successful family of phosphors due to their relatively short-excited state lifetime, high phosphorescence efficiency and excellent color tunability.⁶ Generally to maximize the device performance, it is necessary to introduce an iridium guest in a suitable host material to reduce triplet–triplet annihilation and concentration. These highly efficient phosphorescent OLEDs, formed by evaporation techniques of small molecules and also small molecules, can be easily purified and are readily soluble in common organic solvents.^7–10 To date, most of the dopant work has been driven by the need to develop highly efficient, stable and saturated red, green and blue fluorescent emitters for the rapidly increasing commercialization activities of low temperature poly silicon active matrix full color displays.^11–13 Relatively little work^14,15 appears to have been directed toward the design and synthesis of yellow dopants. Yet, according to a recent survey in the area-color display market, particularly of passive matrix OLEDs, the combination of blue and its complimentary color – yellow, is still preferred by most of the customers. In addition, yellow in combination with sky blue emissions is also one of the key components of the construction of white OLEDs. Therefore, there exists a continuing need and innovative opportunity for the development of highly efficient yellow dopant materials for various OLED display applications.^16,17

In this paper, we report the synthesis and improvement of the luminescent properties of an iridium complex based on 2,6-dimethoxyphenol. The structures of the ligand and iridium complex [Ir(DMP)₃] were characterized by UV-vis spectroscopy, ESI-MS, elemental analysis, ¹H NMR and single crystal structure analysis methodologies. The photoluminescent properties and thermal stability were investigated by photoluminescent and thermogravimetric analysis (TGA), respectively. The synthesized yellow dopant material was used to fabricate test organic light emitting devices.

2. Experimental

2.1. General information

All commercially available starting materials were purchased from Aldrich and Alfa Aesar, and used without further purification, unless otherwise stated. All solvents were purified using conventional methods before use. High performance liquid chromatography (HPLC) grade dimetheyl formamide was distilled from CaH₂ immediately before use. Hydrogen nuclear magnetic resonance (¹H NMR) spectra were measured on a Varian Inova 500 NB spectrometer. Elemental analysis of carbon, hydrogen and nitrogen was performed on a Vario EL analyzer. UV-vis absorption spectra were recorded on a SIMADZU UV 2501 PC spectrometer, while the luminescence lifetime was determined on an Edinburgh FL1920 time correlated pulsed single photon counting instrument. The electron ionization mass spectrum was recorded on a SIMADZU GC-MS-QP2010 plus mass spectrometer and electron spray ionization mass spectra on a thermo LCQ DECA XP mass spectrometer. Cyclic voltammetry was carried out on a solartron SI 1287 voltametric analyzer at room temperature in nitrogen purged anhydrous CH₂Cl₂ with tetrabutylammonium hexafluorophosphate as a supporting electrolyte at a scanning rate of 100 mV s⁻¹. A Pt disk, Pt wire and SCE (saturated calomel electrode) were used as the working, counter, and reference electrodes, respectively. The oxidation potential was calibrated with ferrocene. The energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) was estimated from the UV-vis absorption spectrum edge.

2.2. Synthesis of 3-(2,6-dimethylphenoxy)-6-phenylpyridazine (DMP)

A mixture of 3-chloro-6-phenylpyridazine (5.7 g, 30 mmol), 2,6-dimethylphenol (4.9 g, 40 mmol) and potassium carbonate (13.8 g, 100 mmol) in N,N-dimethylformamide (80 mL) was stirred at 110 °C for 5 h under a N₂ atmosphere. Then the reaction mixture was cooled to room temperature and 250 mL of water was added. The precipitates were collected by filtration, washed with water and dried. The crude product was chromatographed on a silica column using dichloromethane as the eluent to give 3-(2,6-dimethylphenoxy)-6-phenylpyridazine (DMP) as a white solid. Yield: 80%. M.P.: 172–173 °C.¹H NMR (CDCl₃, 500 MHz) (ppm): 8.2–8.0 (dd, 2H), 7.9 (d, 1H), 7.52–7.42 (m, 3H), 7.22 (d, 1H), 7.14–7.08 (m, 3H), 2.17 (s, 6H). EI-MS (m/z): 276 (M⁺).

2.3. Synthesis of tris(3-(2,6-dimethylphenoxy)-6-phenylpyridazine) iridium [Ir(DMP)₃]

3-(2,6-Dimethylphenoxy)-6-phenylpyridazine (DMP) (0.28 g, 1.02 mmol) and hydrated iridium(III) chloride (0.1 g, 0.284 mmol) were added to a mixture of 2-ethoxyethanol (12 mL) and distilled water (4 mL). The mixture was stirred at 100 °C for 20 h under a N₂ atmosphere. After cooling to room temperature, the reaction mixture was filtered, washed with ethanol 3 times, and the crude product was purified by column chromatography over aluminum oxide using dichloromethane as the eluent. Yield: 40%. ¹H NMR (CDCl₃, 500 MHz) (ppm): 7.20 (d,1H), 7.10 (d, 1H), 6.90–6.83 (m, 3H), 6.67 (d, 1H), 6.44 (d,1H), 1.71 (s, 6H). FAB (m/z): 1018.

Anal. calculation for C₅₄H₄₅N₆O₃Ir: C-63.49; H-4.48; N-8.31%. Observed: C-63.44; H-4.51; N-8.26%.

2.4 Spectroscopy and dynamic measurements

UV-vis absorption spectra were recorded on a Shimadzu UV-2501 PC spectrophotometer. Photoluminescence spectra were determined on a Shimadzu RF-5301PC fluorescence spectrophotometer.

Luminescence lifetime was measured by an Edinburgh FL920 time-correlated pulsed single-photon counting instrument. Cyclic voltammetry (CV) was carried out on a Solartron SI 1287 voltammetric analyzer at room temperature in nitrogen-purged anhydrous CH₂Cl₂ with tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte at a scanning rate of 50 mV s⁻¹. A Pt disk, Pt wire, and SCE (saturated calomel electrode) were used as the working, counter, and reference electrodes, respectively. Ferrocene was selected as the internal standard.

2.5 X-ray structural analysis

The single crystal X-ray diffraction data of the complexes were measured on a Bruker Smart CCD diffractometer using (Mo-Kα) radiation (λ = 0.71073 Å). The data collection was executed using the SMART program. Cell refinement and data reduction were made with the SAINT program. The structure was determined using the SHELXTL/PC program and refined using full-matrix least squares. All non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed at the calculated positions and included in the final stage of refinements with fixed parameters.

2.6 OLED fabrication and measurement

The pre-cleaned indium tin oxide (ITO) glass substrate was treated with plasma for 20 min. Then a 45 nm thick poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulfonate) (PSS) film was spin coated on to the ITO glass and dried at 200 °C for 20 min. A film of PVK and PBD, containing different concentrations of [Ir(DMP)₃], of about 70 nm thick was spin coated on the top of the PEDOT:PSS using CH₂Cl₂ as a solvent. The TBPI layer was grown by thermal sublimation under vacuum (3 × 10⁶ Torr). Subsequently, a layer of Ba (3 nm) and a layer of Al (200 nm) were vacuum evaporated on top of the EL polymer layer. Current–voltage characteristics were recorded with a Keith-ley 236 source meter. The electroluminescence spectra were collected by a PR 705 photometer. Luminescence was measured by a calibrated silicon diode and was calibrated by a PR 705 photometer. The external quantum efficiencies were determined by a Si photodiode with calibration in an integrating sphere (ISO80, Labsphere).

3. Results and discussion

3.1 Synthesis and structural characterization

The starting material of the reaction (3-chloro-6-phenylpyridazine) was synthesized by a Friedel–Crafts alkylation reaction.^18,19 The reaction was carried out using 2,6-dimethoxy phenol and K₂CO₃ in DMF solvent at 80 °C for 5 h in a N₂ atmosphere. The reaction of 3-(2,6-dimethylphenoxy)-6-phenylpyridazine (DMP) and IrCl₃·3H₂O give the tris-cyclometallated iridium complex [Ir(DMP)₃] at 100 °C for 20 h is shown as Fig. 1.^20,21 The ¹H NMR data indicates that [Ir(DMP)₃] is formed exclusively as a facial isomer, because the three ligands surrounding the iridium atom are magnetically equivalent.^22,23 The iridium complex was purified using column chromatography with CH₂Cl₂ as solvent. The yield of this compound was 40% and it is very stable in air. After purification, the compound was characterized with a mass spectrometer. A signal was observed at 1018 m/z, corresponding to the molecular weight formulated as [Ir(DMP)₃], as shown Fig. 2. Moreover, the complex is also characterized by elemental analysis. In order to further confirm the structure, single crystal X-ray diffraction analysis was carried out. The single crystals of [Ir(DMP)₃] were successfully prepared by slow evaporation of n-hexane and dichloromethane. The molecular structure obtained from X-ray analysis is a cubic structure, as shown in Fig. 3.

Fig. 1 Synthesis of tris(3-(2,6-dimethylphenoxy)-6-phenylpyridazine) iridium [Ir(DMP)₃].

Fig. 2 ESI mass spectra of [Ir(DMP)₃].

Fig. 3 ORTEP plot of [Ir(DMP)₃]. The hydrogen atoms have been omitted for clarity.

3.2 Thermal properties of the iridium complex

The thermal stability of a complex is very important for OLED applications because decomposition may lead to a decrease in the device performance. The thermal stability of the iridium complex was determined using thermogravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹ using two different atmospheres nitrogen and air, and vice versa.²⁴ No weight loss was observed in the range below 400 °C and the decomposition temperature, which is defined as a 5 wt% weight loss, appeared at 402 °C, as shown in Fig. 4(a). The decomposition temperature of the complex indicates high thermal stability; it shows that the benzene ring-based ligands have a better thermal stability.

Fig. 4 (a) TGA of [Ir(DMP)₃]. (b) UV-absorption spectra of (a) DMP (b) [Ir(DMP)₃], and photoluminance spectra of [Ir(DMP)₃] (c) in CH₂Cl₂ at 298 K.

3.3 Photo physical and electrochemical properties of the iridium complex

The absorption and photoluminescence spectra of [Ir(DMP)₃] in dichloromethane at room temperature are shown in Fig. 4(b). The absorption features of the higher energies (250–350 nm) are assigned mostly to the π → π* transitions of the ligand centered states.^25,26 The moderate intensity of the absorption peaks in the range of 350–500 nm can be attributed to the spin-allowed metal charge transfer (¹MLCT) transition from the Ir atom to the ligands (C⁁N) [dπ(Ir) → π* (C⁁N)].²⁷ The PL spectra of [Ir(DMP)₃] exhibits its maximum emission peak at 552 nm, which emits yellow light. The correlation between photoluminescence and temperature is shown in Fig. 5, which indicates that as the temperature increases, the photoluminance remains constant. From Fig. 5 it can be concluded that the photoluminescence is parallel with the temperature. The electrochemical properties of the iridium complex were measured by a cyclic voltammetric method. A saturated calomel electrode was used as a reference electrode; the oxidation potential of [Ir(DMP)₃] and ferrocene were used as internal standards of 0.66 V and 0.405 V respectively. This is owing to the oxidation of the trivalent iridium ion.²⁸ The HOMO energy level is −5.05 eV and the LUMO energy level is −2.74 eV. The LUMO energy level of the complex is higher than the HOMO energy level. It proves that the complex can function as a trap for electrons and holes.²⁹

Fig. 5 Photoluminescence spectra at various temperatures.

3.4 Electroluminescent OLED characterization

To illustrate the electroluminescent properties of the iridium complex, devices using [Ir(DMP)₃] as a dopant were fabricated with the following structure ITO/PEDOS:PSS(45 nm)/PVK:PBD:Ir(DMP)₃:66 : 30:xwt%(70 nm)/TPBI(30 nm)/Ba(3 nm)/Al(200 nm). PEDOT doped with PSS was used as a hole injecting material.³⁰ As a good hole-transporter, PVK was blended with PBD, which is an electron transporter material, to enable the host to transport both electron and holes.^31,32 To optimize the device efficiency, the thickness of each layer was fixed (HTL, EML and HBL/ETL), while the doping concentration of [Ir(DMP)₃] was varied from 2 to 8 wt% The electroluminescence (EL) spectra of the as-made OLEDs at different concentrations are shown in Fig. 6(a). Due to molecular aggregation ,peaks observed at different places. The device emits wavelength yellow light with a maximum of 584 nm.

Fig. 6 (a) Electroluminance spectra of devices containing [Ir(DMP)₃]. (b) Current density–voltage–luminance characteristics of the devices. (c) Luminance vs. current density at different doping concentrations. (d) External quantum efficiency vs. current density curves.

The current density and luminescence of the devices are plotted versus the operating voltage for the [Ir(DMP)₃]-doped OLEDs at 1 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%-doping concentrations in Fig. 6(b). Different voltages, generating different electric fields, result in different mobilities of the hole and electron carrier. The device at 2 wt% shows a maximum quantum efficiency of 8.7%, corresponding to the luminous efficiency of 14.1 cd A⁻¹ at a current density of 5.93 mA cm⁻², and exhibits maximum luminance of 10 564 cd m⁻² at 118 mA cm⁻². Fig. 6(c) shows the graph of luminous efficiency against current density, it shows 14.1 cd A⁻¹ at a current density of 219 mA cm⁻² for the 2 wt% doping concentration. The quantum efficiency decreases with increasing current density as shown in Fig. 6(d), which can be attributed to the increasing triplet–triplet annihilation of the phosphor-bound excitons and the field induced quenching effects.^33,34 The comprehensive performance of the device is summarized in Table 1. It indicates that the device performance is intimately related to the doping concentration. The performance of our device was also compared with other yellow emission devices, results are shown in Table 2. Its shows that our device has a high luminance efficiency (LE), including quantum efficiency (QE) and CIE parameters, but if we look at some of the others,^35,36 the quantum efficiency is high but the luminance efficiency is not. The devices could meet not only the emission efficiency requirement but also exhibit some unique features necessary for practical applications. For example, our material not only have quantum efficiency is good but also luminous efficiency. From the table we can clearly see that our material is good with regard to luminance but also in quantum efficiency, CIE parameters, etc. All these electroluminescent results demonstrate that the [Ir(DMP)₃] complex is a potential candidate for use as a phosphorescent emitter in OLEDs. It also shows that incorporation of the 2,6-dimethoxy phenol ligand is an effective way to synthesize the iridium complex. This information is useful for the design of an efficient phosphor.

Table 1 Device performances

Blend ratio	V on (V)	L max (cd m^−2)	LEmax (cd A^−1)	QEmax (%)	Wavelength (nm)	CIE (x,y)
60 : 30 : 1	6.75	9276	14.2	6.3	574	0.49,0.51
60 : 30 : 2	7	10 564	14.1	8.7	576	0.49,0.50
60 : 30 : 4	7	8419	12.8	5.9	580	0.50,0.50
60 : 30 : 6	6.5	7338	14.1	7.4	580	0.51,0.49
60 : 30 : 8	6.5	8315	14.5	8.0	584	0.51,0.48

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Table 2 Comparison with other yellow emission devices

Sample	V on (V)	L max (cd m^−2)	QEmax (%)	Wavelength (nm)	CIE (x,y)
1. Our sample	7	10 564	8.7	576	0.49,0.50
2. Ref. 35 sample	5	8180	9.9	545	0.44,0.55
3. Ref. 36 sample	7.1	1883	6.5	562	0.46,0.54
4. Ref. 36 sample	9.08	9117	2.5	582	0.46,0.53

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4. Conclusions

In summary, the tris-cyclometallated iridium complex can be readily synthesized from iridium chloride and the ligand 2,6-dimethoxy phenol in one step at 100 °C for 20 h. The device exhibits a peak electro phosphorescence wavelength of 576 nm, which shows in yellow light range. At different doping concentrations, for example at a 2 wt% concentration, a maximum external quantum of 8.7% at a current density of 5.93 mA cm⁻¹ and a maximum luminescence of 10 564 cd m⁻² can be obtained at 118 mA cm⁻². It is believed that the synthesized iridium complex provides an effective and potential applicable yellow material for OLED devices due to the high luminance quality and outstanding efficiencies. Such a good device performance suggests the potential of using [Ir(DMP)₃] as a yellow phosphor dye in fabricating high-efficiency WOLEDs.

Acknowledgements

The authors would like to thank the Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2010), the Guangdong Province Sci & Tech Bureau (Key Strategic Project Grant No. 10151027501000096, S2012010010545), and the Chinese Universities Basic Research Founding for financial support of work.

References

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. MacKay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature, 1990, 347, 539.
3. M. A. Baldo, D. F. O'Brien, A. Shoustikov, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
4. Y. Ma, H. Zhang, J. Shen and C. Che, Synth. Met., 1998, 94, 245.
5. M. A. Baldo, S. Lamansky, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4.
6. M. E. Thompson, J. Li, A. Tamayo, T. Sajoto, P. Djurovich, S. R. Forrest, R. J. Holmes, J. Brown and J. Brooks, Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp., 2005, 36, 1058.
7. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl. Phys., 2001, 90, 5048.
8. C. Ulbricht, B. Beyer, C. Friebe, A. Winter and U. S. Schubert, Adv. Mater., 2009, 21, 4418.
9. Z. Y. Ge, T. Hayakawa, H. Miyamoto, T. Kajita and M. Kakimoto, Adv. Funct. Mater., 2008, 18, 584.
10. A. Y. Li, Y. Y. Li, H. B. Wu, W. Y. Wong, W. Yang, J. B. Peng and Y. Cao, Org. Electron., 2010, 11, 529.
11. C. H. Chen and C. W. Tang, Appl. Phys. Lett., 2001, 79, 3711.
12. J. Shi and C. W. Tang, Appl. Phys. Lett., 2002, 80, 3201.
13. G. Rajeswaran, M. Itoh, M. Boroson, S. Barry, T. K. Hatwar, K. B. Kahen, K. Yoneda, R. Yokoyama, T. Yamada, N. Komiya and H. Takahashi, Proc. Soc. Inf. Dis., 2000, 14, 1.
14. X. Q. Lin, B. J. Chen, X. H. Zhang, C. S. Lee, H. L. Kwong and S. T. Lee, Chem. Mater., 2001, 13, 456.
15. B. W. D'Andrade, M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 2001, 79, 1045.
16. Y. Sato, T. Ogata, S. Ichinosawa and Y. Murata, Synth. Met., 1997, 91, 103.
17. Y.-S. Wu, T.-H. Liu and C. H. Chen, Thin Solid Films, 2006, 496, 626.
18. Bihai Tong, Q. Mei, Yuezhong Meng and B. Wang, J. Mater. Chem., 2008, 18, 1636.
19. Yuan Fang, Sujun Hu, Yuezhong Meng, Junbiao Peng and Biao Wang, Inorg. Chim. Acta, 2009, 362, 4985.
20. A. B. Tamayo, B. D. Alleyne, P. I. Djurovich and M. E. Thompson, J. Am. Chem. Soc., 2003, 125, 7377.
21. V. V. Grashin, N. Herron, D. D. Lecloux and W. J. Marshall, Chem. Commun., 2001, 1494.
22. M. G. Colombo, T. C. Brunold, T. Riedener, H. U. Güdel and M. H. Bürgi, Inorg. Chem., 1994, 33, 545.
23. A. B. Tamayo, B. D. Alleyne, P. I. Djurovich and M. E. Thompson, J. Am. Chem. Soc., 2003, 125, 7377.
24. Chaolong Yang, Jianxin Luo, Liyan Liang and Bihai Tong, Dyes Pigm., 2012, 92, 696.
25. Y. Fang, B. Tong, S. Hu, S. Wang, Y. Meng, J. Peng and B. Wang, Org. Electron., 2009, 106, 18.
26. Wu-Sian Sie, Gene-Hsiang Lee, I-Jy chang and Bei-Shiu Kom, J. Mol. Struct., 2008, 890, 198.
27. A. Tsuboyama, H. Iwawaki, S. Okada, M. Hoshino and K. Ueno, J. Am. Chem. Soc., 2003, 125, 12971.
28. M. K. Nazeeruddin, R. T. Wegh, F. D. Angelis, S. Fantacci and M. Grtzel, Inorg. Chem., 2006, 45, 9245.
29. D. R. Whang, Y. You, S. H. Kim, J.-J. Kim and S. Y. Park, Appl. Phys. Lett., 2007, 91, 233501.
30. T. M. Brown, J. S. Kim, R. H. Friend, F. Cacially, R. Daik and W. J. Feast, Appl. Phys. Lett., 1999, 75, 1679.
31. C. H. Kim and J. Sinnar, Appl. Phys. Lett., 2002, 80, 2201.
32. X. Yang, D. Neher, D. Hertel and T. K. Daubler, Adv. Mater., 2004, 16, 161.
33. M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B: Condens. Matter, 2000, 62, 10967.
34. J. Kalinowski, W. Stampor, J. Mezyk, M. Cocchi, D. Virgili, V. Fattori and P. D. Marco, Phys. Rev. B: Condens. Matter, 2002, 66, 235321.
35. X.-M. Yu, G.-J. Zhou and H.-S. Kwok, J. Organomet. Chem., 2008, 693, 1518.
36. C. L. Li, Y. T. Tao, C. H. Chien and R.-S. Liu, Adv. Funct. Mater., 2005, 15, 387.

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