Copyright © 2008 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 8, 1–6, 2008
Solution-Processed High-Efficiency
Organic Phosphorescent Devices
Utilizing a Blue Ir(III) Complex
Eunsil Han, Yi-Yeol Lyu, Tae-Woo Lee∗ , Younghun Byun, Ohyun Kwon,
Youngnam Kwon, Gyeong-Su Park, and Rupasree Ragini Das∗
Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-Ri, Kiheung-Eup, Youngin-Si, Kyunggi-Do, 449-712, Korea
We report high-efficiency blue phosphorescence organic light-emitting devices by solution process
utilizing a blue Ir(III) complex [(F2 ppy)2 Ir(ph-imz)CN] (F2 ppy = 2 ,6 -difluoro-2-phenyl pyridine and
ph-imz = N-phenyl imidazole) blended with the host mCP (N N -dicarbazolyl-3,5-benzene), and the
inert polymers polystyrene (PS) and polymethylmethacrylate (PMMA). The effects of the dopant
confinement in the PS and PMMA matrix on the device performance are studied by field emission transmission electron microscopy (FE-TEM) and atomic force microscopy (AFM). The complex
shows photoluminescence peaked at 458 nm with CIE color coordinates (0.14, 0.21) in solution
and (0.14, 0.18) in doped PMMA film. The PS based device showed better device performance
than the PMMA based device with a maximum luminous efficiency of (L ) 5.11 cd/A with CIE color
coordinates (0.17, 0.29) (at 10 mA/cm2 ) and a maximum luminance of 9765 cd/m2 .
Keywords: Phosphorescent, Blue Emitting Iridium(III) Complex, Dipolar Cloud.
The cyclometalated complexes of heavy transition metals with large spin-orbit coupling constant have offered a
wide range of phosphors emitting colors from red to blue
from the triplet metal-to-ligand-charge-transfer (3 MLCT)
states1–3 that are suitable for use in organic light emitting devices (OLED) as triplet emitters.4–6 Although green
and red emitting OLEDs using these complexes furnish
very high efficiency and incredible lifetime,7 there have
been a less number of reports on blue emitting phosphors with high efficiency and longer lifetime.8–10 Blue
phosphorescence can be achieved either by the structural
tuning of the cyclometalating ligand, or by a proper selection of the ancillary ligands,11 or both. The availability
of a wide gap host for efficient energy transfer to the
triplet MLCT state of the phosphorescent metal complex
is one of the most critical issues for the blue phosphorescent OLEDs. Sato,9 Forrest, and Thompson4 8 groups have
obtained high efficiencies by using hosts with high triplet
energies. Their devices are fabricated by vapor evaporation method using small organic molecules as the hosts.
So far there have been only a few reports12–14 on blue
emitting phosphorescent polymer OLEDs. The emissive
∗
Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 6
layers can be spin-coated utilizing such complexes blended
with emitting or non-emitting polymers.15–17 Unfortunately
polymeric hosts with high triplet energies are not yet
known to fabricate devices utilizing deep blue phosphors.
The use of low triplet energy polymeric hosts will not
give desired efficiency, rather this will lead to phosphorescence quenching due to the back transfer of energy
from the 3 MLCT state of the phosphor to the triplet
energy level of the host. A blend of inert polymers
like polymethyl methacrylate (PMMA) and polystyrene
(PS) and a high triplet energy organic molecule like
mCP (N ,N -dicarbazolyl-3,5-benzene) can be used as the
emitting layer in the solution-processed device. Effective exciton confinement in a nanometer-order confined
geometry by mixing the emitting dyes with an inert host
matrix (e.g., PS or Clay) can lead to enhanced photoluminescence and electroluminescence quantum yield.18 19
Therefore, the inert host matrices (PMMA and PS) play
two important roles in increasing exciton confinement in
the matrix as well as in making solution process possible. Here, we report a new blue emitting phosphorescent Ir(III) complex, [(F2 ppy)2 Ir(ph-imz)CN], where
F2 ppy = 2 ,6 -difluoro-2-phenyl pyridine and ph-imz =
N -phenyl imidazole, and the solution-processed electroluminescent device with high efficiency. The complex is
doped in a blend of mCP and PS or PMMA at different
1533-4880/2008/8/001/006
doi:10.1166/jnn.2008.153
1
RESEARCH ARTICLE
1. INTRODUCTION
Solution-Processed High-Efficiency Organic Phosphorescent Devices Utilizing a Blue Ir(III) Complex
concentrations and the effect of the matrix on the device
performance is studied.
2. EXPERIMENTAL DETAILS
2.1. Synthesis of [(F2 ppy)2 Ir(ph-imz)CN]
[(F2 ppy)2 Ir(ph-imz)CN] as shown in Figure 1(a) is synthesized in two steps from the dimer [(F2 ppy)2 IrCl]2 . The
dimer20 and N -phenyl imidazole mixture in the molar ratio
of 1:2.5 in chloroform was stirred for 24 h at 50 C. The
resulting product [(F2 ppy)2 Ir(ph-imz)Cl] is purified by silica column using a solution of CH2 Cl2 and acetone (1:2)
as the eluent. The chlorine atom in [(F2 ppy)2 Ir(ph-imz)Cl]
is then replaced by CN ligand by treating the chloroform solution of [(F2 ppy)2 Ir(ph-imz)Cl] with 5 equivalent
of KCN in methanol at 50 C for 5 minutes. The final
product is obtained in the pure state as pale yellow powder after column separation and subsequent evaporation of
the solvent. The complex [(F2 ppy)2 Ir(ph-imz)CN] is further purified by repeated reprecipitation process from a
solution of chloroform and n-hexane. Elemental analysis,
1H NMR, and mass spectra confirmed the synthesis of
the complex. Anal. Calcd: C, 51.68; H, 2.69; N, 9.42.
Found: C, 51.33; H 2.60; N, 9.32. 1 H NMR (300 MHz,
CD2 Cl2 ) (ppm): 9.80 (d, 1H), 8.33 (d, 2H), 8.22 (d, 1H),
RESEARCH ARTICLE
(a)
F
N
N
N
F
Ir
F
CN
N
F
mCP LUMO
2.3 eV
(b)
Dopant LUMO
2.6 eV
PEDOT
2.8 eV
LiF/Al
3.2 eV
MLCT
3
BAlq
ITO
5.2 eV
mCP
HOMO
5.8 eV
Dopant
HOMO
5.9 eV
mCP triplet
2.9 eV
Endothermic
backward
energy
transfer
6.5 eV
Exothermic
forward
energy
transfer
Dopant triplet
2.7 eV
Fig. 1. (a) The chemical structure of [(F2 ppy)2 Ir(ph-imz)CN],
(b) energy levels of the materials used in the devices. A difference of
0.2 eV between the triplets of the host and the dopant could permit the
endothermic backward energy transfer from the dopant to the host in
addition to the forward exothermic energy transfer from the host to the
dopant.
2
Han et al.
8.14 (d, 1H), 7.83 (t, 2H), 7.56–7.40 (m, 3H), 7.37 (d,
2H), 7.22 (d, 2H), 7.12 (t, 1H), 7.03 (s, 1H), 6.53–6.35 (m,
2H), 5.79 (t, 1H). The mass spectra of the complex was
monitored by electrospray ionization (ESI) method using
95% tetrahydrofuran, 4.9% water and 0.1% ceasium iodide
as the eluent. Calculated molecular mass, 743.1. Found:
m/e = 875.9 [(F2 ppy)2 Ir(ph-imz)CN + Cs]+ (100%) and
803.9 [(F2 ppy)2 IrCN + Cs + THF]+ (15%).
2.2. Device Fabrication
For the electroluminescence (EL) studies, devices were
fabricated as below. Pre-cleaned ITO from Samsung
Corning with a sheet resistance of 10 / was treated
with UV-ozone plasma. A 50 nm thick hole-injecting
PEDOT-PSS (AI4083) from H. C. Stark was spin coated
on the ITO surface and dried at 120 C for 5 minutes in
the globe box. The emissive layers were prepared from the
solution composed of inert matrix (PMMA or PS), mCP
and the phosphor. We fabricated four different devices by
changing the compositions and the inert matrices as specified below:
Device 1: ITO/PEDOT-PSS (50 nm)/PMMA-mCP-phosphor (18.8:75.2:6)/BAlq (40 nm)/LiF (0.9 nm)/Al
(200 nm) (PMMA:mCP = 20:80 and dopant is 8 weight%
of the host mCP).
Device 2: ITO/PEDOT-PSS (50 nm)/PMMA-mCP-phosphor (37.6:56.4:6)/BAlq (40 nm)/LiF (0.9 nm)/Al
(200 nm) (PMMA:mCP = 40:60 and dopant is 10.5
weight% of the host mCP).
Device 3: ITO/PEDOT-PSS (50 nm)/PMMA-mCP-phosphor (36:54:10)/BAlq (40 nm)/LiF (0.9 nm)/Al
(200 nm) (PMMA:mCP = 40:60 and dopant is 18.5
weight% of the host mCP).
Device 4: ITO/PEDOT-PSS (50 nm)/PS-mCP-phosphor
(18.8:75.2:6)/BAlq (40 nm)/LiF (0.9 nm)/Al (200 nm)
(PS:mCP = 20:80 and dopant is 8 weight% of the host
mCP).
The emissive layer was spin-coated from toluene solution on the PEDOT-PSS layer to make a film of 50 nm. It
was then kept at 100 C in vacuum for an hour to remove
the solvent. A 40 nm thick hole-blocking and electroninjecting BAlq (aluminium(III)bis(2-methyl-8-quinolinato)
4-phenylphenolate) layer was vapor deposited on the emissive layer followed by 0.9 nm thick LiF and 200 nm Al
deposition. The devices were then encapsulated by a curing resin. The light output was collected by a calibrated
silicon photodiode and the current voltage characteristics
were measured by using a Keitheley 237 source measurement unit. All the measurements were performed at room
temperature in air.
3. RESULTS AND DISCUSSION
The thermogravimetric analysis reveals that the complex [(F2 ppy)2 Ir(ph-imz)CN] is thermally unstable, the
J. Nanosci. Nanotechnol. 8, 1–6, 2008
Solution-Processed High-Efficiency Organic Phosphorescent Devices Utilizing a Blue Ir(III) Complex
J. Nanosci. Nanotechnol. 8, 1–6, 2008
Abs
PLE at 454 nm
Film PL λex = 368 nm
Normalized intensity (a.u.)
1.0
Soln PL λex = 368 nm
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 2. Absorption, photoluminescence excitation (PLE), and photolumenscence (PL) spectra of [(F2 ppy)2 Ir(ph-imz)CN] in dichloroethane
solution and emission spectra of 5% doped PMMA film.
above blends were prepared to measure the absolute quantum efficiencies (QE) and the transient PL lifetimes (
PL )
of the dopant emission at 458 nm in different matrices.
The QE of the 6% doped PMMA and PS films are less
than 10% where as the PL values are 2.68 and 1.62 s,
respectively. On the other hand, the QE and PL values of
the 6% doped PMMA-mCP and PS-mCP are 75%, 2.2 s
and 73%, 2.1 s, respectively. It indicates a very highly
efficient energy transfer between the host mCP and the
dopant. The QE and PL values are almost similar in both
the PMMA-mCP and PS-mCP matrices, being independent of PMMA or PS. However, the effect of the PMMA
and PS matrices are well pronounced in the observed electroluminescence properties of the devices.
The EL spectra of the devices at 10 mA/cm2 are shown
in Figure 3. In all of the three PMMA based devices
(device 1, 2 and 3), the emission intensity is almost similar, where as the emission intensity is almost double in
the PS based device (device 4). Again, the device 1 and
0.020
Device 1
Device 2
Device 3
Device 4
0.015
@ 10 mA/cm2
0.010
0.005
0.000
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. EL spectra of the devices 1, 2, 3 and 4 at 10 mA/cm2 .
3
RESEARCH ARTICLE
dissociation temperature being observed at 190 C.
However the electrochemical studies in acetonitrile solution
using a platinum disc, a platinum wire, Ag | AgNO3 and
ferrocene/ferrocenium as the working, counter, reference
and internal reference electrodes and tetrabutylammonium
tetraborofluoroborate as the electrolyte demonstrated
quasireversible oxidative nature of the complex. The
E1/2(oxidation) was observed at 1 V relative to the ferrocene/ferrocenium redox couple with a difference of
0.08 V between the peaks of oxidation and reduction half
waves.
The device configuration, energy levels of the materials
used in the device are shown in Figure 1(b). The HOMO
level of Ir(III) complex is determined by ultraviolet photoelectron spectroscopy. The LUMO and 3 MLCT levels
of the complex are calculated from the HOMO, and optical absorption, and emission energies, respectively. The
ideal condition for the energy transfer from the host to
the dopant is to nest the HOMO and 3 MLCT level of
the dopant within the HOMO and LUMO of the host.
However, in the present case the HOMO of the dopant
is 0.1 eV deeper than that of the host. This could allow
tunneling of the holes into the dopant. Since the triplet
energy of the dopant (2.7 eV) is lower than that of the
host (2.9 eV), the triplet exciton transfer from the host to
the dopant is exothermic and can be considered as an efficient process. But, a small difference (0.2 eV) between the
host-dopant triplet energies could not rule out an endothermic backward triplet transfer from the dopant to the
host.
The absorption, photoluminescence excitation (PLE),
and photolumenscence (PL) spectra of the complex in
dichloroethane solution and the emission spectra of a 6%
doped PMMA film are shown in Figure 2. The absorption spectra of the compound shows intense absorption
peaks below 380 nm, which are the characteristics of
the – ∗ transitions of the aromatic ligands. The shoulder around 306 nm indicates absorption corresponding to
MLCT state perturbed by ligands. The absorption peak
at 368 nm can be attributed to the electronic transition
from the ground state to an admixture of the ligand
– ∗ and MLCT state. A large Stokes shift of 5413 cm−1
between the MLCT absorption at 368 nm and emission
at 458 nm indicates more – ∗ ligand character21 in the
charge transfer state. The complex shows strong luminescence both in the solution and film state. The two emission
peaks at 458 and 485 nm are the vibronic structures of
the emission spectra as confirmed from the similar excitation (PLE) spectra at the emission wavelength of 458
and 485 nm.22 The CIE color coordinates of the solution of the complex in dichloromethane and 6% doped
PMMA film are (0.14, 0.21) and (0.14, 0.18), respectively.
The complex was 6% doped in PMMA and PS separately,
and 6% in PMMA-mCP blend as in the device 1 and in
PS-mCP blend as in the device 4. Thin films from the
EL intensity (a.u.)
Han et al.
Solution-Processed High-Efficiency Organic Phosphorescent Devices Utilizing a Blue Ir(III) Complex
102
101
10000
100
8000
10–1
6000
10
–2
4000
10–3
2000
10–4
10–5
0
2
4
6
8
10
12
14
16
18
Voltage (V)
RESEARCH ARTICLE
Fig. 4.
1
0.1
0.01
0.01
Device 1
Device 2
Device 3
Device 4
0.1
1
10
100
Current density (mA/cm2)
(a) I–V –L characteristics of the devices 1, 2, 3, and 4. (b) Luminescence efficiency of the devices 1, 2, 3, and 4 versus current density.
device 4 demonstrate red shift of the emission peaks and
change of spectral shape compared to the solution PL
spectra whereas devices 2 and 3 have peaks at 458 and
480 nm, almost similar to that of solution. The CIE color
coordinates obtained from the device 2 and device 4 are
(0.17, 0.25) and (0.17, 0.29) respectively. We attribute such
change to the effect of the matrix on the excited state of the
phosphor. In such cases the electronic charge distribution
of the phosphor in the excited state can be affected by the
positive and negative polarons surrounding the phosphor.23
As PMMA, PS24 and mCP have different polar characteristics, under electrical excitation different combinations
of these materials could generate different local electric
fields inside the organic films and subsequently influence
the excited state charge distribution and nuclear distribution of the phosphor.
Figure 4 shows the I–V –L characteristics and the luminance efficiencies (L ) of the devices. The PMMA based
devices show a gradual increase in the efficiencies with
the increase in the doping concentration, the efficiency
being maximum for the 18.5% doping concentration. The
18.5% doped device furnishes a luminous efficiency (L )
of 4.01 cd/A at 1.53 mA and 9.4 V. The 18.5% doping concentration seems to be quite a high concentration
for the phosphorescent devices. However in the present
case the matrix PMMA dilutes the host and the phosphor
in the film. It increases the distance between the host and
the phosphor and consequently the energy transfer of the
three-component blend is not so effective as the host-guest
blend without the inert matrix. Upon increasing the phosphor concentration the distance of transfer between the
host and phosphor decreases and the effective energy transfer is favored.25 It is also noticed that the turn on voltage
decreases and the current density increases upon increasing the doping concentration in the PMMA based device.
This indicates that at higher dopant concentration, direct
hopping via dopant sites is facilitated.
When the matrix is changed to PS, the device performance is enhanced. The L shows a maximum value
4
(b) 10
12000
Device 1
Device 2
Device 3
Device 4
ηl (cd/A)
103
Luminance (cd/m2)
Current density (mA/cm2)
(a)
Han et al.
of 5.11 cd/A at 0.08 mA/cm2 and 7.6 V. The maximum luminance furnished is 9765 cd/m2 at 14 V and
424 mA/cm2 . Similar PMMA based device with 8% doping concentration furnished a maximum luminous efficiency of 2.68 cd/A at 1.86 mA/cm2 and 11.4 V. Such
improvement of the device performance with the PS
matrix can be discussed from the following angles. In
the first place, there could be energy transfer from PS
to the dopant leading to higher efficiency. Polystyrene
emits fluorescence around 318 nm in the solid state.
51% Forster energy transfer from polystyrene to a fluoroscent dye BODIPY® (4,4-difluoro-1,3,5,7-tetraphenyl-4bora-3a,4a-diaza-s-indacene is documented in literature.26
Hence in the present case the Förster energy transfer from
PS to the phosphor could not be ruled out. Secondly, the
PMMA matrix has been reported to quench phosphorescence of benzophenon presumably because of the carbonyl
(C O) group.27 We apprehended similar phenomena in
the present case and checked the PL efficiency of 6%
doped PMMA and PS films without mCP. We did not
observe quenching of dopant luminescence by PMMA.
Both the doped systems revealed almost similar results
thus negating the possibilities of quenching by the PMMA
matrix and the Förster energy transfer from the PS matrix
to the dopant. Therefore we attribute the enhanced device
efficiency more likely to the following two reasons.
The surface topographies of the PMMA and PS blended
films obtained by the FE-TEM and AFM microscopies
are shown in Figures 5 and 6, respectively. The FE-TEM
images of both the PMMA and PS matrices show crystallization of the complex [(F2 ppy)2 Ir(ph-imz)CN] in both
the blends as confirmed by the EDS (energy dispersive
spectroscopy) and EELS28 (electron energy loss spectrum)
but the AFM images show a greater phase separation is
observed in the PMMA blend than in the PS blend. The
root-mean-square roughness range of the PMMA blend is
0.802 nm whereas the same for the PS blend is 0.301 nm.
The greater phase separation in the PMMA matrix suggests a lesser mixing of the host and dopants and the
J. Nanosci. Nanotechnol. 8, 1–6, 2008
Han et al.
Solution-Processed High-Efficiency Organic Phosphorescent Devices Utilizing a Blue Ir(III) Complex
(a)
(b)
(a)
(b)
Fig. 6. AFM images of the surfaces of (a) PMMA-mCP-phosphor (18.8:75.2:6) film. (b) PS-mCP-phosphor (18.8:75.2:6) film. The Z-heights are
shown in the right hand sides of the respective figures.
J. Nanosci. Nanotechnol. 8, 1–6, 2008
5
RESEARCH ARTICLE
Fig. 5. FE-TEM images of the surfaces of (a) PMMA-mCP-phosphor (18.8:75.2:6) film. (b) PS-mCP-phosphor (18.8:75.2:6) film casted on the
carbon-coated copper grid from the respective solutions.
Solution-Processed High-Efficiency Organic Phosphorescent Devices Utilizing a Blue Ir(III) Complex
RESEARCH ARTICLE
consequent lesser energy and charge transfer from the
host to the dopant site. Secondly, the charge transport in
PMMA and PS could have some impact on the device performances. The charge transport in PMMA and PS doped
with small organic molecules like p-diarylaminostilbene29
is found to occur by hopping through localized states
associated with the doped dyes. The hopping creates a
charge-induced dipole moment on each of the neighboring
molecules. The occupied hopping site is thus surrounded
by a dipolar cloud and can interact with the matrix. Again,
the mobility of the charge in the doped matrix is inversely
proportional to the dipole moment and dielectric constant
of the dopant and matrix. The dipole moment of the PS
repeat unit is near zero (0.13 D); so any dipole contribution
from the polymer can be neglected. Since PMMA repeat
unit has a higher dipole moment than PS repeat unit, its
dipole moment contribution is not negligible (1.6–1.97 D).
It contributes to the dipolar cloud that surrounds the
hopping site and has presumably affected the charge
mobility.
The lifetime of the device 4 was found to be 60 min at
10 mA/cm2 and 379 cd/m2 as the initial luminance. But
the similar PMMA based device (device 1) lasted only for
a few seconds. Even if these devices can not be used for
commercial purpose, but a better device performance of
the PS matrix is indicated.
4. SUMMARY
In summary, we have synthesized a deep blue emitting biscyclometalated Ir(III) complex coordinated to two monodentate ligands and fabricated devices using inert polymer
matrices of PMMA and PS and mCP as the host for the
blue phosphorescent complex. The two matrices demonstrate different device characteristics as regards to the
color coordinates, efficiencies, I–V characteristics and
device lifetimes. Even if the PMMA based device shows
good color coordinates, the device performance of PS
based device is better than the PMMA based ones. Higher
efficiency and less roll off with current is observed for
the PS based device. PS based device furnishes a luminous efficiency of 5.11 cd/A. Since the charge hopping can
be more facilitated in the inert matrix with lower dipole
moment, the higher luminous efficiency can be attributed
to the better charge transport in the PS matrix. The lesser
phase separation effect in the PS matrix also favors better
energy and charge transfer from the host to the dopant.
The device lifetime of PS based device is better than its
PMMA counterpart.
Han et al.
References and Notes
1. R. Laskar and T.-M. Chen, Chem. Mater. 16, 111 (2004).
2. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani,
S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada,
M. Hoshini, and K. Ueno, J. Am. Chem. Soc. 125, 12971 (2004).
3. L. Jian, P. I. Djurovich, B. D. Alleyne, M. Yousufuddin, N. N. Ho,
J. C. Thomas, J. C. Peters, R. Bau, and M. E. Thompson, Inorg.
Chem. 44, 1713 (2005).
4. Adachi, M. A. Baldo, S. R. Forrest, and M. E. Thompson, Appl.
Phy. Lett. 77, 904 (2000).
5. Jiang, W. Yang, J. Peng, S. Xiao, and Y. Cao, Adv. Mater. 16, 537
(2004).
6. R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, J. J. Brown,
S. Garon, and M. E. Thompson, Appl. Phy. Lett. 82, 2422 (2003).
7. R. C. Kwong, M. S. Weaver, M.-H. M. Lu, Y.-J. Tung, A. B. Chwang,
T. X. Zhou, M. Hack, and J. J. Brown, Org. Elec. 4, 155 (2003).
8. R. J. Holmes, B. W. D. Andrade, S. R. Forrest, X. Ren, J. Li, and
M. E. Thompson, Appl. Phy. Lett. 83, 3818 (2003).
9. S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, and F. Sato, Appl.
Phy. Lett. 82, 569 (2003).
10. S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu, and C. H. Chen, Adv. Mater. 17, 285 (2005).
11. J. Li, P. I. Djurovitch, B. D. Alleyene, I. Tsyba, N. H. Ho, R. Bau,
and M. E. Thompson, Polyhedon 23, 419 (2004).
12. S. Tokito, M. Suzuki, F. Sato, M. Kamachi, and K. Shrane, Org.
Elec. 4, 105 (2003).
13. Y. Kawamura, S. Yanagida, and S. R. Forrest, J. Appl. Phys. 92, 87
(2002).
14. X. Zhang, C. Jiang, Y. Mo, Y. Xu, H. Shi, and Y. Cao, Appl. Phy.
Lett. 88, 05116 (2006).
15. H. Rudmann and M. F. Rubner, J. Appl. Phys. 90, 4338 (2001).
16. G. He, Y. Li, J. Liu, and Y. Yang, Appl. Phy. Lett. 80, 4247 (2002).
17. W. Mingliang, Z. Junxiang, and L. Zuzheng, J. Solid State Chem.
153, 192 (2000).
18. T.-W. Lee, O. O. Park, J. Yoon, and J.-J. Kim, Adv. Mater. 13, 211
(2001); T.-W. Lee, O. O. Park, J.-J. Kim, J.-M. Hong, and Y. C.
Kim, Chem. Mater. 13, 2217 (2001); T.-W. Lee, O. O. Park, J. Yoon,
and J.-J. Kim, Synth. Met. 121, 1737 (2001).
19. H. Rudmann and M. F. Rubner, J. Appl. Phys. 90, 4338 (2001).
20. M. Nanoyama, Bull. Chem. Soc. Jpn. 47, 767 (1974).
21. S. Lamansky, P. Djurovitch, D. Murphy, F. A. Razzaq, H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest, and M. E. Thompson, J. Am.
Chem. Soc. 123, 4304 (2001).
22. J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba,
R. Bau, and M. E. Thompson, Inorg. Chem. 41, 3055 (2002).
23. P. M. Borsenberger, J. R. Cowdery-Corvan, E. H. Magin, and J. A.
Sinicropi, Thin Solid Films 307, 21 (1997).
24. J. F. Rabeck, Mechanisms of Photophysical Processes and Photochemical Reactions in Polymers: Theory and Applications, Wiley,
Chichester (1987).
25. S. Lamansky, R. C. Kwong, M. Nugent, and P. I. Djurovich, Org.
Elec. 2, 53 (2001).
26. B. P. Wittmershaus, T. T. Baseler, G. T. Beaumont, and Y.-Z. Zhang,
J. Lum. 96, 107 (2002).
27. J. R. Ebdon, D. M. Lucas, I. Soutar, A. R. Lane, and L. Swanson,
Polymer 36, 1577 (1995).
28. We will discuss the detailed results in a separate communication.
29. J. H. Park, O. O. Park, J.-W. Yu, J. K. Kim, and Y. C. Kim, Appl.
Phy. Lett. 84, 1783 (2004).
Received: 7 August 2007. Accepted: 14 September 2007.
6
J. Nanosci. Nanotechnol. 8, 1–6, 2008