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Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes
Thin Solid Films 517 (2009) 1807–1810
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes Ji Hyun Seo, Young Kwan Kim, Yunkyoung Ha ⁎ Department of Information Display, Hongik University, Seoul 121-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 November 2007 Received in revised form 5 August 2008 Accepted 18 September 2008 Available online 25 September 2008 Keywords: Blue-green organic light-emitting diodes Heteroleptic iridium complexes Ligand 2,4-difluorophenylpyridine 2-phenylpyridine
a b s t r a c t The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F2-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)3 has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F2ppy)3 was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)2(F2-ppy) and Ir(ppy)(F2-ppy)2, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)2(F2-ppy) and Ir(ppy)(F2-ppy)2 were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F2-ppy)2 was 3.63% at J = 10 mA/cm2. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the first great discovery that the organic material emits visible light in multilayered structure by Tang and coworkers [1,2], organic light emitting diodes (OLEDs) and luminescent materials [3,4] have been developed extensively. Luminescent materials for OLED devices are generally classified into two groups; fluorescence and phosphorescence. The OLED based on phosphorescent materials can significantly improve electroluminescence (EL) performance. The heavy metal, such as iridium or platinum, in the complexes can induce the intersystem crossing by strong spin-orbit coupling, and thus lead to mixing of the singlet and triplet excited states [5,6]. However, most of phosphorescent emitters have a long lifetime, which yield the dominant triplet-triplet (T-T) annihilation at high currents. The T-T annihilation makes the device containing a phosphorescent material performed badly, particularly in its maximum brightness and luminescence efficiency at high currents [7–12]. To improve the luminescence efficiency by avoiding T-T annihilation and to design efficient blue phosphors, the metal complex having a different species of plural ligands has been proposed [13]. Two kinds of ligands are introduced in these heteroleptic complexes; one is for energy absorbing and the other is for light-emitting. In other words, when a metal complex having a luminescent ligand among three chelating ligands is placed in the lowest excited state, the excited energy is transferred from the other two ligands to the luminescent ligand. Previously, Ir(ppy)3 (ppy = 2-phenylpyridine) was known to have high phosphorescence efficiencies in electroluminescent (EL) emis⁎ Corresponding author. Tel.: +82 2 320 1490; fax: +82 2 3142 0335. E-mail address: [email protected] (Y. Ha). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.075
sions near 516 nm with its metal-to-ligand charge transfer (MLCT) excited state [14]. Meanwhile, Ir(F2-ppy)3 was found to be a blue phosphor. Thus, we employed two ligands, ppy for energy harvesting and F2-ppy for light-emitting, in the iridium complex. Herein, we report two efficient heteroleptic phosphorescent blue-green emitters, Ir(ppy)2(F2-ppy) and Ir(ppy)(F2-ppy)2. The light-emitting properties of the Ir(III) complexes having two different ligands are investigated in comparison with those homoleptic Ir(III) complexes having the same ligand species. These heteroleptic complexes can be candidates in a component of white OLEDs with a suitable color complement. 2. Experiment 2.1. Synthesis of ligands and iridium complexes The 2,4-difluorophenylpyridine ligand was obtained from a reaction of 1-chloro-pyridine with 2,4-difluorophenylboronic acid by Suzuki coupling. The cyclometalated Ir (III) μ-chloro-bridged dimer, (CˆN)2Ir(μ-Cl)2Ir(CˆN), was synthesized by the method reported by Nonoyama with a slight modification [15]. Further reaction of the dimer with 2,4-pentadione followed by replacement of acac with ppy or F2-ppy gave Ir(ppy)x(F2-ppy)y as reported elsewhere [16] (yield: 16–23%). 2.2. Optical measurement UV–Vis absorption spectra were measured on a Hewlett Packard 8425A spectrometer. PL spectra were measured on a Perkin Elmer LS 50B spectrometer. UV–Vis and PL spectra of the iridium complexes were measured in a 10− 5M dilute CH2Cl2 solution.
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J.H. Seo et al. / Thin Solid Films 517 (2009) 1807–1810
methanol, distillated water and isopropyl alcohol [17]. The organic materials were deposited in the following sequence: Onto the ITO Glass, 60 nm of 4,4′,4′′-tris[2-naphthylphenylamino] triphenylamine and 20 nm of 4,4′-bis[N-(naphthyl)-N-phenylamino] biphenyl were applied as a hole injection layer and a hole transporting layer, respectively. Then, a 30 nm thick emissive layer of the iridium complex doped in 4,4,N,N'-dicarbazolebiphenyl was deposited. The doping ratio of the phosphor was 10%. 10 nm thick bathocuproine, 20 nm thick tris-(8-hydroxyquinoline) aluminum and 2 nm thick lithium quinolate were successively deposited as an exciton blocking layer, an electron transporting layer and an electron injection layer, respectively. The typical organic deposition rate was 0.1 nm/s. Finally, 100 nm of Al was deposited as a cathode. The active area of the OLEDs was 0.09 cm2. After the fabrication, the current density– voltage characteristics of the OLED were measured with a source measure unit (Kiethley 236) and the luminance and CIE (The Commission Internationale de L'Eclairage) chromaticity coordinates of the fabricated devices were measured using a chromameter (MINOLTA CS-100A). All measurements were performed in ambient conditions under direct current voltage bias. 3. Results and discussion
Fig. 1. Molecular structures of Ir(F2-ppy)2(ppy) and Ir(ppy)2(F2-ppy) and their device structure.
2.3. Device fabrication Fig. 1 shows the chemical structures of the iridium complexes, Ir (F2-ppy)2(ppy) and Ir(ppy)2(F2-ppy), investigated in this study, respectively. The device configuration used in this study is also shown in Fig. 1. The organic materials used as a carrier transport, a carrier injection and host materials were supplied by Gracel Display Incorporation in Korea. OLEDs were fabricated by high vacuum (5 × 10− 7Torr) thermal deposition of organic materials onto the surface of an indium tin oxide (ITO, 30 Ω/□, 80 nm) coated glass substrate. The ITO glass was chemically cleaned using acetone,
Fig. 2. Normalized UV–Vis absorption and PL spectra in 10− 5 M CH2Cl2 at room temperature for Ir(ppy)3, Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2(ppy).
The heteroleptic and homoleptic iridium complexes were synthesized according to the reported procedure [16]. The UV–Vis absorption and the photoluminescence spectra of the heteroleptic and homoleptic complexes were compared, shown in Fig. 2. The absorption spectra of these compounds have intense bands appearing in the ultraviolet part of the spectrum between 230 and 320 nm. These bands have been assigned to the spin-allowed 1(π → π⁎) transitions of the ligands. The 1(π → π⁎) bands are accompanied by weaker and lower energy features extending into the visible region from 320 to 480 nm that have been assigned to both allowed and spin-forbidden MLCT transitions. The high intensity of these MLCT bands has been attributed to effective mixing of these charge-transfer transitions with higher lying spin-allowed transitions on the cyclometalating ligand [10]. Ir(ppy)3 has clear and prominent absorption peaks at 380, 410 and 460 nm due to efficient intersystem crossing between the singlet and triplet excited states by the strong spin-orbit coupling of the Ir(III) metal center. On the other hand, Ir(F2-ppy)3 exhibits a poor metal-toligand charge-transfer character with their weak energy absorption feature at 350 and 390 nm. The absorption spectra show the
Fig. 3. Luminous efficiencies (inset: quantum efficiencies) of Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2(ppy).
J.H. Seo et al. / Thin Solid Films 517 (2009) 1807–1810
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Table 1 Characteristics of OLED devices with Ir(ppy)3, Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2(ppy) complexes Device
Dopant
Voltage at J = 10 mA/cm2[V]
Max. luminance [cd/m2]
Luminous efficiency at J = 10 mA/cm2[cd/A]
Quantum efficiency at J = 10 mA/cm2[%]
CIE coordinates at J = 10mA/cm2
A B C D
Ir(F2-ppy)3 Ir(F2-ppy)2(ppy) Ir(ppy) 2 (F2-ppy) Ir(ppy)3
8.0 V 7.5 V 7.5 V 7.0 V
1820 14,200 9038 73,180
1.0 11.7 8.8 41.8
0.60 3.63 0.64 5.69
(0.27, 0.38) (0.35, 0.54) (0.26, 0.55) (0.28, 0.63)
hypsochromic shift of 10–12 nm upon replacing a ppy ligand with a F2ppy ligand in the complex. The heteroleptic complexes Ir(F2-ppy)2 (ppy) and Ir(ppy)2(F2-ppy), show absorbance peaks at 372 and 360 nm, respectively, with similar absorption patterns of both homoleptic complexes. The MLCT transitions of the Ir(F2-ppy)2(ppy) are hypsochromically shifted relative to those of Ir(ppy)2(F2-ppy) due to involvement of one more F2-ppy in the complex. The photoluminescence (PL) spectra properties of Ir(ppy)3, Ir(ppy)2 (F2-ppy), Ir(F2-ppy)2(ppy) and Ir(F2-ppy)3 show the maximum intensity at 514, 502, 495 and 474 nm, respectively, as shown in Fig. 2. Substitution of the phenyl hydrogens with electron-withdrawing fluorinated atoms, particularly on the 4′- and 6′-positions, stabilizes the HOMO (the highest occupied molecular orbital) more than the LUMO (the lowest unoccupied molecular orbital), thus leading to increase of the energy gap. Therefore, by replacing ppy with F2-ppy as a ligand, hypsochromic shifted emissions are observed. The luminous efficiencies of compounds are shown in Fig. 3. The corresponding devices and their performances are summarized in Table 1. The maximum efficiencies for Ir(ppy)2(F2-ppy), Ir(F2-ppy)2 (ppy) and Ir(F2-ppy)3 are 8.8, 11.7 and 1.0 cd/A, respectively, at a current density of J = 10 mA/cm2and their power efficiencies are 3.7, 4.8 and 0.4 lm/W, respectively. The luminous efficiency of the device containing Ir(F2-ppy)2(ppy) is higher than those of Ir(ppy)2(F2-ppy) and Ir(F2-ppy)3. These trends in efficiencies can be expected from the UV–Vis absorption characteristics: The absorption spectrum of Ir(F2ppy)2(ppy) exhibits higher MLCTs than that of Ir(ppy)2(F2-ppy). Inset of Fig. 3 compares quantum efficiency versus current density characteristics of the compounds. The quantum efficiencies of Ir (ppy)2(F2-ppy), Ir(F2-ppy)2(ppy) and Ir(F2-ppy)3 are 0.64, 3.63 and 0.60%, respectively, at current density of J = 10 mA/cm2. It is found that Ir(F2-ppy)2(ppy) has better luminous efficiency and quantum efficiency than Ir(ppy)2(F2-ppy). Thus, we suggest that one energy absorbing ligand is enough for efficiency increase in the heteroleptic complex.
Our experimental results provide two possible emitting mechanisms. The first mechanism is that absorption occurs mainly in ppy ligand, and then both ppy and F2-ppy ligands in the complex participate in luminescence. However, it is less likely since only one emission peak is observed in each complex. The second theory is that replacement of ppy with F2-ppy ligand just increases the energy band gap. The hypsochromic shift in emission upon involvement of more F2-ppy ligands in the complex indicates that the second theory is more plausible. Fig. 4 shows the EL spectra of compounds at J = 10 mA/cm2. Upon replacement of ppy with F2-ppy in the complex one by one, hypsochromic shifted EL peaks are observed, similar to the case of PL spectra, but not significantly. The EL maxima of Ir(ppy)3, Ir(ppy)2(F2ppy) and Ir(F2-ppy)2(ppy) occur at 510 nm, 508 nm and 504 nm, respectively. The EL spectra of the complexes did not reflect ligand replacement effects much, compared with their PL spectra. However, Commission Internationale de L'Eclairage (CIE) chromaticity coordinates for each device show more distinctive characteristics. The CIE coordinates of the devices containing Ir(ppy)3, Ir(ppy)2(F2-ppy), Ir(F2ppy)2(ppy) and Ir(F2-ppy)3 are (0.28, 0.63), (0.26, 0.55), (0.35, 0.54) and (0.27, 0.38), respectively, at J = 10 mA/cm2 as shown in Table 1. As expected, the device with Ir(F2-ppy)3 shows the most hypsochromic shift toward blue emission. In summary, two heteroleptic iridium complexes containing both ppy and F2-ppy ligands were prepared in this study and were found to exhibit efficient blue-treen phosphorescence. It is expected that blue-green phosphors can be applied as perspective components for white OLEDs with suitable color complements. 4. Conclusion We have studied the electrical and optical characteristics of the heteroleptic iridium complexes, such Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2 (ppy), for the application in OLEDs. We found that replacement of ppy with F2-ppy in the complex leads to increase of the HOMO-LUMO gap and thus shows the hypsochromic shift in emission. Ir(F2-ppy)2(ppy) is found to have a higher luminance and luminous efficiency of 14200 cd/m2 and 11.7 cd/A than Ir(F2-ppy)(ppy)2 of 9038 cd/m2 and 8.8 cd/A in the devices. Both devices containing these heteroleptic complexes show efficient bluish-green luminescence in PL and EL. Acknowledgment This work was supported by Korea Research Foundation (KRF2007-531-C00031). References [1] [2] [3] [4] [5] [6]
Fig. 4. Normalized EL spectra of Ir(ppy)3, Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2 (ppy) at a current density of J = 10 mA/cm2.
[7] [8]
C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750. G.Y. Park, Y. Ha, Synth. Met. 158 (2008) 120. B. Hirani, J. Li, P.I. Djurovich, M. Yousufuddin, J. Oxgaard, P. Persson, S.R. Wilson, R. Bau, W.A. Goddard III, M.E. Thompson, Inorg. Chem. 46 (2007) 3865. M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl. Chem. 71 (1999) 2095. J. Brooks, T. Babaya, S. Lamansky, P.I. Djurovich, I. Tsyba, R. Bau, M.E. Thompson, Inorg. Chem. 41 (2002) 3055. A. Kohler, J.S. Wilson, R.H. Friend, Adv. Mater. 14 (2002) 701. A.B. T amayo, B.D. Alleyne, P.I. Djurovich, S. Lamansky, I. Tsyba, N.N. Ho, R. Bau, M.E. Thompson, J. Am. Chem. Soc. 125 (2003) 7377.
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[9] I.R. Lsakar, S.-F. Hsu, T.-M. Chen, Polyhedron 24 (2005) 189. [10] R.J. Holmes, B.W. D'Andrade, S.R. Forrest, X. Ren, J. Li, M.E. Thompson, Appl. Phys. Lett. 83 (2003) 3818. [11] J. Li, P.I. Djurovich, B.D. Alleyne, I. Tsyba, N.N. Ho, R. Bau, M.E. Thompson, Polyhedron 23 (2004) 419. [12] P. Coppo, E.A. Plummer, L. De Cola, Chem. Commun. (2004) 1774. [13] H.H. Rho, Y.H. Lee, Y.H. Park, G.Y. Park, Y.K. Ha, Y.S. Kim, Jpn. J. Appl. Phys. 45 (2005) 568.
[14] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971. [15] M. Nomoyama, J. Organomet. Chem. 86 (1975) 263. [16] G.Y. Park, J.H. Seo, Y.K. Kim, Y.S. Kim, Y. Ha, J. Korean Phys. Soc. 50 (2007) 1729. [17] T.P. Nguyen, L.P. Rendu, N.N. Dinh, Synth. Met. 138 (2003) 229.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes Ji Hyun Seo, Young Kwan Kim, Yunkyoung Ha ⁎ Department of Information Display, Hongik University, Seoul 121-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 November 2007 Received in revised form 5 August 2008 Accepted 18 September 2008 Available online 25 September 2008 Keywords: Blue-green organic light-emitting diodes Heteroleptic iridium complexes Ligand 2,4-difluorophenylpyridine 2-phenylpyridine
a b s t r a c t The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F2-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)3 has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F2ppy)3 was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)2(F2-ppy) and Ir(ppy)(F2-ppy)2, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)2(F2-ppy) and Ir(ppy)(F2-ppy)2 were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F2-ppy)2 was 3.63% at J = 10 mA/cm2. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the first great discovery that the organic material emits visible light in multilayered structure by Tang and coworkers [1,2], organic light emitting diodes (OLEDs) and luminescent materials [3,4] have been developed extensively. Luminescent materials for OLED devices are generally classified into two groups; fluorescence and phosphorescence. The OLED based on phosphorescent materials can significantly improve electroluminescence (EL) performance. The heavy metal, such as iridium or platinum, in the complexes can induce the intersystem crossing by strong spin-orbit coupling, and thus lead to mixing of the singlet and triplet excited states [5,6]. However, most of phosphorescent emitters have a long lifetime, which yield the dominant triplet-triplet (T-T) annihilation at high currents. The T-T annihilation makes the device containing a phosphorescent material performed badly, particularly in its maximum brightness and luminescence efficiency at high currents [7–12]. To improve the luminescence efficiency by avoiding T-T annihilation and to design efficient blue phosphors, the metal complex having a different species of plural ligands has been proposed [13]. Two kinds of ligands are introduced in these heteroleptic complexes; one is for energy absorbing and the other is for light-emitting. In other words, when a metal complex having a luminescent ligand among three chelating ligands is placed in the lowest excited state, the excited energy is transferred from the other two ligands to the luminescent ligand. Previously, Ir(ppy)3 (ppy = 2-phenylpyridine) was known to have high phosphorescence efficiencies in electroluminescent (EL) emis⁎ Corresponding author. Tel.: +82 2 320 1490; fax: +82 2 3142 0335. E-mail address: [email protected] (Y. Ha). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.075
sions near 516 nm with its metal-to-ligand charge transfer (MLCT) excited state [14]. Meanwhile, Ir(F2-ppy)3 was found to be a blue phosphor. Thus, we employed two ligands, ppy for energy harvesting and F2-ppy for light-emitting, in the iridium complex. Herein, we report two efficient heteroleptic phosphorescent blue-green emitters, Ir(ppy)2(F2-ppy) and Ir(ppy)(F2-ppy)2. The light-emitting properties of the Ir(III) complexes having two different ligands are investigated in comparison with those homoleptic Ir(III) complexes having the same ligand species. These heteroleptic complexes can be candidates in a component of white OLEDs with a suitable color complement. 2. Experiment 2.1. Synthesis of ligands and iridium complexes The 2,4-difluorophenylpyridine ligand was obtained from a reaction of 1-chloro-pyridine with 2,4-difluorophenylboronic acid by Suzuki coupling. The cyclometalated Ir (III) μ-chloro-bridged dimer, (CˆN)2Ir(μ-Cl)2Ir(CˆN), was synthesized by the method reported by Nonoyama with a slight modification [15]. Further reaction of the dimer with 2,4-pentadione followed by replacement of acac with ppy or F2-ppy gave Ir(ppy)x(F2-ppy)y as reported elsewhere [16] (yield: 16–23%). 2.2. Optical measurement UV–Vis absorption spectra were measured on a Hewlett Packard 8425A spectrometer. PL spectra were measured on a Perkin Elmer LS 50B spectrometer. UV–Vis and PL spectra of the iridium complexes were measured in a 10− 5M dilute CH2Cl2 solution.
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methanol, distillated water and isopropyl alcohol [17]. The organic materials were deposited in the following sequence: Onto the ITO Glass, 60 nm of 4,4′,4′′-tris[2-naphthylphenylamino] triphenylamine and 20 nm of 4,4′-bis[N-(naphthyl)-N-phenylamino] biphenyl were applied as a hole injection layer and a hole transporting layer, respectively. Then, a 30 nm thick emissive layer of the iridium complex doped in 4,4,N,N'-dicarbazolebiphenyl was deposited. The doping ratio of the phosphor was 10%. 10 nm thick bathocuproine, 20 nm thick tris-(8-hydroxyquinoline) aluminum and 2 nm thick lithium quinolate were successively deposited as an exciton blocking layer, an electron transporting layer and an electron injection layer, respectively. The typical organic deposition rate was 0.1 nm/s. Finally, 100 nm of Al was deposited as a cathode. The active area of the OLEDs was 0.09 cm2. After the fabrication, the current density– voltage characteristics of the OLED were measured with a source measure unit (Kiethley 236) and the luminance and CIE (The Commission Internationale de L'Eclairage) chromaticity coordinates of the fabricated devices were measured using a chromameter (MINOLTA CS-100A). All measurements were performed in ambient conditions under direct current voltage bias. 3. Results and discussion
Fig. 1. Molecular structures of Ir(F2-ppy)2(ppy) and Ir(ppy)2(F2-ppy) and their device structure.
2.3. Device fabrication Fig. 1 shows the chemical structures of the iridium complexes, Ir (F2-ppy)2(ppy) and Ir(ppy)2(F2-ppy), investigated in this study, respectively. The device configuration used in this study is also shown in Fig. 1. The organic materials used as a carrier transport, a carrier injection and host materials were supplied by Gracel Display Incorporation in Korea. OLEDs were fabricated by high vacuum (5 × 10− 7Torr) thermal deposition of organic materials onto the surface of an indium tin oxide (ITO, 30 Ω/□, 80 nm) coated glass substrate. The ITO glass was chemically cleaned using acetone,
Fig. 2. Normalized UV–Vis absorption and PL spectra in 10− 5 M CH2Cl2 at room temperature for Ir(ppy)3, Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2(ppy).
The heteroleptic and homoleptic iridium complexes were synthesized according to the reported procedure [16]. The UV–Vis absorption and the photoluminescence spectra of the heteroleptic and homoleptic complexes were compared, shown in Fig. 2. The absorption spectra of these compounds have intense bands appearing in the ultraviolet part of the spectrum between 230 and 320 nm. These bands have been assigned to the spin-allowed 1(π → π⁎) transitions of the ligands. The 1(π → π⁎) bands are accompanied by weaker and lower energy features extending into the visible region from 320 to 480 nm that have been assigned to both allowed and spin-forbidden MLCT transitions. The high intensity of these MLCT bands has been attributed to effective mixing of these charge-transfer transitions with higher lying spin-allowed transitions on the cyclometalating ligand [10]. Ir(ppy)3 has clear and prominent absorption peaks at 380, 410 and 460 nm due to efficient intersystem crossing between the singlet and triplet excited states by the strong spin-orbit coupling of the Ir(III) metal center. On the other hand, Ir(F2-ppy)3 exhibits a poor metal-toligand charge-transfer character with their weak energy absorption feature at 350 and 390 nm. The absorption spectra show the
Fig. 3. Luminous efficiencies (inset: quantum efficiencies) of Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2(ppy).
J.H. Seo et al. / Thin Solid Films 517 (2009) 1807–1810
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Table 1 Characteristics of OLED devices with Ir(ppy)3, Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2(ppy) complexes Device
Dopant
Voltage at J = 10 mA/cm2[V]
Max. luminance [cd/m2]
Luminous efficiency at J = 10 mA/cm2[cd/A]
Quantum efficiency at J = 10 mA/cm2[%]
CIE coordinates at J = 10mA/cm2
A B C D
Ir(F2-ppy)3 Ir(F2-ppy)2(ppy) Ir(ppy) 2 (F2-ppy) Ir(ppy)3
8.0 V 7.5 V 7.5 V 7.0 V
1820 14,200 9038 73,180
1.0 11.7 8.8 41.8
0.60 3.63 0.64 5.69
(0.27, 0.38) (0.35, 0.54) (0.26, 0.55) (0.28, 0.63)
hypsochromic shift of 10–12 nm upon replacing a ppy ligand with a F2ppy ligand in the complex. The heteroleptic complexes Ir(F2-ppy)2 (ppy) and Ir(ppy)2(F2-ppy), show absorbance peaks at 372 and 360 nm, respectively, with similar absorption patterns of both homoleptic complexes. The MLCT transitions of the Ir(F2-ppy)2(ppy) are hypsochromically shifted relative to those of Ir(ppy)2(F2-ppy) due to involvement of one more F2-ppy in the complex. The photoluminescence (PL) spectra properties of Ir(ppy)3, Ir(ppy)2 (F2-ppy), Ir(F2-ppy)2(ppy) and Ir(F2-ppy)3 show the maximum intensity at 514, 502, 495 and 474 nm, respectively, as shown in Fig. 2. Substitution of the phenyl hydrogens with electron-withdrawing fluorinated atoms, particularly on the 4′- and 6′-positions, stabilizes the HOMO (the highest occupied molecular orbital) more than the LUMO (the lowest unoccupied molecular orbital), thus leading to increase of the energy gap. Therefore, by replacing ppy with F2-ppy as a ligand, hypsochromic shifted emissions are observed. The luminous efficiencies of compounds are shown in Fig. 3. The corresponding devices and their performances are summarized in Table 1. The maximum efficiencies for Ir(ppy)2(F2-ppy), Ir(F2-ppy)2 (ppy) and Ir(F2-ppy)3 are 8.8, 11.7 and 1.0 cd/A, respectively, at a current density of J = 10 mA/cm2and their power efficiencies are 3.7, 4.8 and 0.4 lm/W, respectively. The luminous efficiency of the device containing Ir(F2-ppy)2(ppy) is higher than those of Ir(ppy)2(F2-ppy) and Ir(F2-ppy)3. These trends in efficiencies can be expected from the UV–Vis absorption characteristics: The absorption spectrum of Ir(F2ppy)2(ppy) exhibits higher MLCTs than that of Ir(ppy)2(F2-ppy). Inset of Fig. 3 compares quantum efficiency versus current density characteristics of the compounds. The quantum efficiencies of Ir (ppy)2(F2-ppy), Ir(F2-ppy)2(ppy) and Ir(F2-ppy)3 are 0.64, 3.63 and 0.60%, respectively, at current density of J = 10 mA/cm2. It is found that Ir(F2-ppy)2(ppy) has better luminous efficiency and quantum efficiency than Ir(ppy)2(F2-ppy). Thus, we suggest that one energy absorbing ligand is enough for efficiency increase in the heteroleptic complex.
Our experimental results provide two possible emitting mechanisms. The first mechanism is that absorption occurs mainly in ppy ligand, and then both ppy and F2-ppy ligands in the complex participate in luminescence. However, it is less likely since only one emission peak is observed in each complex. The second theory is that replacement of ppy with F2-ppy ligand just increases the energy band gap. The hypsochromic shift in emission upon involvement of more F2-ppy ligands in the complex indicates that the second theory is more plausible. Fig. 4 shows the EL spectra of compounds at J = 10 mA/cm2. Upon replacement of ppy with F2-ppy in the complex one by one, hypsochromic shifted EL peaks are observed, similar to the case of PL spectra, but not significantly. The EL maxima of Ir(ppy)3, Ir(ppy)2(F2ppy) and Ir(F2-ppy)2(ppy) occur at 510 nm, 508 nm and 504 nm, respectively. The EL spectra of the complexes did not reflect ligand replacement effects much, compared with their PL spectra. However, Commission Internationale de L'Eclairage (CIE) chromaticity coordinates for each device show more distinctive characteristics. The CIE coordinates of the devices containing Ir(ppy)3, Ir(ppy)2(F2-ppy), Ir(F2ppy)2(ppy) and Ir(F2-ppy)3 are (0.28, 0.63), (0.26, 0.55), (0.35, 0.54) and (0.27, 0.38), respectively, at J = 10 mA/cm2 as shown in Table 1. As expected, the device with Ir(F2-ppy)3 shows the most hypsochromic shift toward blue emission. In summary, two heteroleptic iridium complexes containing both ppy and F2-ppy ligands were prepared in this study and were found to exhibit efficient blue-treen phosphorescence. It is expected that blue-green phosphors can be applied as perspective components for white OLEDs with suitable color complements. 4. Conclusion We have studied the electrical and optical characteristics of the heteroleptic iridium complexes, such Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2 (ppy), for the application in OLEDs. We found that replacement of ppy with F2-ppy in the complex leads to increase of the HOMO-LUMO gap and thus shows the hypsochromic shift in emission. Ir(F2-ppy)2(ppy) is found to have a higher luminance and luminous efficiency of 14200 cd/m2 and 11.7 cd/A than Ir(F2-ppy)(ppy)2 of 9038 cd/m2 and 8.8 cd/A in the devices. Both devices containing these heteroleptic complexes show efficient bluish-green luminescence in PL and EL. Acknowledgment This work was supported by Korea Research Foundation (KRF2007-531-C00031). References [1] [2] [3] [4] [5] [6]
Fig. 4. Normalized EL spectra of Ir(ppy)3, Ir(F2-ppy)3, Ir(ppy)2(F2-ppy) and Ir(F2-ppy)2 (ppy) at a current density of J = 10 mA/cm2.
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