External quantum efficiency above 20% in deep blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/adma.201004372

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) with external quantum efficiency above 20% are developed using a bipolar-type high-triplet-energy host material and a high-triplet-energy exciton blocking material. Maximum quantum efficiency of 25.1% and low roll-off (still 23.1% at 1000 cd m^-2) are achieved in these deep blue PHOLEDs. Copyright

Full text of DOI:10.1002/adma.201004372

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External Quantum Efficiency Above 20% in Deep Blue
Phosphorescent Organic Light-Emitting Diodes
Soon Ok Jeon, Sang Eok Jang, Hyo Suk Son, and Jun Yeob Lee*
The development of high-efficiency blue-light-emitting phos-
phorescent organic light-emitting diodes (PHOLEDs) is impor-
tant in order to reduce the power consumption of organic
light-emitting diodes (OLEDs) in display and lighting applica-
tions.[1–10] Iridium(III) bis(4,6-(difluorophenyl)-pyridinato-N,C′)
picolinate (FIrpic) has typically been used as the dopant mate-
rial for blue PHOLEDs, with a theoretical maximum quantum
efficiency of 20% already reported in sky blue PHOLEDs using
various host and exciton blocking materials.[11–14] Although
20% external quantum efficiency was achieved in sky blue
PHOLEDs, it could not be achieved in deep blue PHOLEDs
owing to the requirement of high triplet energy of the host and
deep blue dopant material. Our group has reported a 19.2%
maximum external quantum efficiency in deep blue PHOLEDs,
but the efficiency could not be further improved.[8] Moreover,
the quantum efficiency dropped sharply at high luminance, and
therefore high quantum efficiency could not be achieved above
100cdm−2 .[2] Inparticular,thequantumefficienciesat1000cdm−2
were significantly reduced in all deep blue PHOLEDs. In addi-
tion, the optimum doping concentration of blue PHOLEDs
was generally 10%–20%, which needs to be decreased in order
to save materials. It is thus urgently required to develop deep
blue PHOLEDs with external quantum efficiency above 20%
at 1000 cd m−2 and optimum doping concentration less than
5%. However, there has been no report of quantum efficien-
cies exceeding 20% even at a low luminance in deep-blue
PHOLEDs.
oxide to improve the electron transport properties. The high tri-
plet energy of mCP can be maintained as the conjugation of the
mCP cannot be extended through the phosphine oxide while
improving the electron transport properties.[15,16] The motiva-
tion for synthesizing the asymmetric mCP-based phosphine
oxide compound with only one diphenylphosphine oxide group
was to obtain a highest occupied molecular orbital (HOMO)
level similar to that of the hole transport layer (mCP) through
the unsubstituted phenylcarbazole unit. The electron transport
properties can be improved through the diphenylphosphine
oxide-substituted phenylcarbazole unit. Therefore, a high tri-
plet energy host material with a similar HOMO level to the
hole transport layer and bipolar charge transport properties can
be developed. The HOMO level of mCPPO1 was 6.13 eV and
6.21 eV from cyclic voltammetry (CV) measurement and the
lowest unoccupied molecular orbital (LUMO) level was 2.64 eV
from the bandgap and HOMO of mCPPO1. The triplet energy
of mCPPO1 from low temperature photoluminescence (PL)
spectra was 3.00 eV. Detailed data are summarized in Table S1
and Figures S1 and S2 in the Supporting Information.
The molecular orbital distribution of mCPPO1 is shown in
Figure 1c. As can be expected from the molecular structure, the
HOMO was localized on the carbazole unit, while the LUMO
was dispersed over the diphenylphosphine oxide–substituted
carbazole unit. Therefore, the HOMO is dominated by the car-
bazole unit and the LUMO is determined by the diphenylphos-
phine oxide-modified carbazole unit.
In this Communication, we report novel deep blue PHOLEDs
with external quantum efficiency above 20% at up to 2500 cd
m−2 , 3% optimum doping concentration, and quantum effi-
ciency above 20% for 1%–20% doping concentration, developed
using a novel high triplet energy bipolar host material and a
high triplet energy exciton blocking material. The origin of the
high quantum efficiency, low optimum doping concentration,
and suppressed efficiency roll-off was investigated.
The synthetic schemes of the 9-(3-(9H-carbazole-9-yl)
phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole (mCPPO1)
host and diphenylphosphine oxide-4-(triphenylsilyl)phenyl
(TSPO1) exciton blocking material is shown in Scheme 1.
mCPPO1 was designed as a high triplet energy host material
with bipolar charge transport properties. N,N′-dicarbazolyl-
3,5-benzene (mCP) was modified with a diphenylphosphine
TSPO1 was designed as a high triplet energy exciton
blocking material with electron transport properties. The tetra-
phenylsilane core structure has been reported to have a high
triplet energy exceeding 3.0 eV for exciton blocking, although
the tetraphenylsilane core did not show good electron trans-
port properties.[17,18] The electron transport properties can be
improved by modifying the tetraphenylsilane core with a diphe-
nylphosphine oxide group. Therefore, TSPO1 can be used as
an electron- transport-type high-triplet-energy exciton blocking
material for deep blue PHOLEDs. In addition, the LUMO of
the TSPO1 can be controlled for efficient electron injection,
owing to the wide bandgap of the tetraphenylsilane core struc-
ture. The HOMO and LUMO levels of TSPO1 were 6.79 eV
and 2.52 eV, respectively. The HOMO of TSPO1 was suitable
for hole blocking, while the LUMO level was suitable for elec-
tron injection into the mCPPO1 host material. There was a
small energy barrier for electron injection from TSPO1 to the
mCPPO1 host. The triplet energy of TSPO1 was 3.36 eV from
the low temperature PL spectra, high enough to suppress the
triplet exciton quenching of bis((3,5-difluoro-4-cyanophenyl)
pyridine) iridium picolinate (FCNIrpic).
S. O. Jeon, S. E. Jang, H. S. Son, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook University
126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Republic of Korea
E-mail: leej17@dankook.ac.kr
A detailed energy level diagram of the materials is shown
in Figure 1b. Therefore, mCPPO1 can be effective as the host
DOI: 10.1002/adma.201004372
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H
N
H
N
O
P
Br
Br
Br
N
1
n-BuLi
P Cl
N
N
N
N
Br
I
Cu-catalyst, DCB
Ullmann reaction
Cu-catalyst, DCB
Ullmann reaction
H2O2
CH2Cl2
2
mCPPO1
P
Si
Cl
O
Cl
H2O2
Br
Br
Si
P
Si
Br
Si
P
n-BuLi, THF
n-BuLi, THF
CH2Cl2
4
3
TSPO1
Scheme 1. Synthetic schemes of mCPPO1 and TSPO1. DCB: dichlorobenzene; n-BuLi: n-butyllithium; THF: tetrahydrofuran.
material and TSPO1 can be efficient as the electron transport
and exciton blocking material for deep blue PHOLEDs. From
the energy level diagram, there was only a 0.03 eV energy bar-
rier for hole injection from mCP into mCPPO1, with no energy
barrier for electron injection from TSPO1 into mCPPO1. The
small energy barrier makes it easy to balance holes and elec-
trons inside the emitting layer. In particular, charge accumu-
lation at the interface can be reduced as there is a minute
energy barrier for charge injection, which can reduce efficiency
roll-off within the device. It was difficult to fabricate the energy-
barrier-free device structure in deep blue PHOLEDs because of
the wide bandgap of the deep blue host material, but it could
be realized in the presented device structure using the TSPO1
exciton blocking layer and mCPPO1 host material. In addition,
the energy level difference between mCPPO1 and FCNIrpic
was suitable for balanced hole and electron injection. The
HOMO and LUMO level differences between mCPPO1 and
FCNIrpic were 0.41 and 0.34 eV, respectively. A similar HOMO
Figure 1. Device structure (a) and energy level diagram (b) of the blue-light-emitting PHOLED. FCNIrpic: bis((3,5-difluoro-4-cyanophenyl)pyri-
dine) iridium picolinate; NPB: N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine; DNTPD: N,N′-diphenyl-N,N′-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-
biphenyl-4,4′-diamine; ITO: indium tin oxide. c) HOMO/LUMO molecular orbital distribution of mCPPO1. d) Hole only/electron only device data of
mCP and mCPPO1.
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and LUMO level difference in the host and dopant may balance
holes and electrons inside the emitting layer.
blue dopant, FCNIrpic, was a trap site in the emitting layer; the
current density reduced as the dopant concentration increased.
Above 10% doping concentration, the dopant transports holes
and electrons by hopping between dopant sites; the current
density was slightly increased. In the case of the luminance, the
PHOLEDs with high current density showed high luminance.
The quantum efficiency of the blue PHOLEDs is plotted
against luminance in Figure 2c. The quantum efficiency of
the blue PHOLEDs was optimized at a doping concentra-
tion of 3%, but the quantum efficiency of the blue PHOLEDs
at other doping concentrations was also high. The maximum
quantum efficiency of the device with 3% FCNIrpic doping was
22.3%, and the quantum efficiencies at 100 and 500 cd m−2
were 21.7% and 20.6%, respectively. The color coordinate
of the 3% FCNIrpic-doped device was (0.14, 0.17). There has
been no work reporting external quantum efficiencies above
20% in deep blue PHOLEDs with y coordinate less than 0.20.
In addition, the efficiency roll-off was significant in other deep
blue PHOLEDs and the best quantum efficiency at 500 cd m−2
was less than 13%. The quantum efficiency was significantly
improved using the mCPPO1 host and TSPO1 exciton blocking
layer, with a quantum efficiency above 20% being achieved up
to 860 cd m−2 in deep blue PHOLEDs for the first time. The
Hole-only and electron-only devices of mCPPO1 were fab-
ricated to compare hole and electron densities within the
mCPPO1 emitting layer. Figure 1d shows hole-only and elec-
tron-only device data of mCPPO1 compared with mCP. There
was a large difference of the current densities in the hole-only
and electron-only devices of mCP, but only a slight difference
between the hole and electron current densities in the mCPPO1
device; the electron current density was slightly higher than the
hole current density. This indicates that mCPPO1 has bipolar
charge transport properties and may balance holes and electrons
inside the emitting layer. The electron density of mCPPO1 was
higher than that of mCP, while the hole density of mCPPO1
was lower than that of mCP. This proves that the electron trans-
port properties were improved and that the hole transport prop-
erties were reduced by the diphenylphosphine oxide unit.
Current density–voltage–luminance and quantum efficiency–
current density curves of the deep blue PHOLEDs with
mCPPO1 host and TSPO1 exciton blocking layer are shown in
Figure 2. The device structure is shown in Figure 1a. The cur-
rent density decreased with increasing doping concentration
from 0.5% to 10%, and then increased slightly above 10%. The
(a)
(b)
10000
1000
100
10
1000
10
0.1
0.001
0.5%
1%
3%
10%
15%
20%
30%
0.5%
1%
3%
10%
15%
20%
30%
1
0.00001
0.0000001
0.1
0.01
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Vo l t a g e ( V )
(c)
(d)
25
20
15
10
5
0.5%
1%
3%
10%
15%
20%
30%
0.5%
1%
3%
10%
15%
20%
30%
0
1
10
100
1000
10000
400
500
600
700
Luminance (cd/m2)
Wavelength (nm)
Figure 2. Current density–voltage (a), luminance–voltage (b), quantum efficiency–luminance (c), and electroluminescence (d) curves of mCPPO1-
based blue PHOLEDs for different doping concentrations.
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T a b le 1 . Device performances of deep blue PHOLEDs.
Color indexa)
Quantum efficiency [%]
Power efficiency [lm W−1
]
1000 [cd m−2
8.68
100 [cd m−2
]
500 [cd m−2
]
1000 [cd m−2
]
100 [cd m−2
]
500 [cd m−2
]
]
Max
DNTPD 0.5%
DNTPD 1%
DNTPD 3%
DNTPD 10%
DNTPD 15%
DNTPD 20%
DNTPD 30%
PEDOT 3%
0.14, 0.17
0.14, 0.17
0.14, 0.17
0.14, 0.18
0.14, 0.19
0.14, 0.20
0.14, 0.21
0.14, 0.18
0.14, 0.20
19.2
16.3
13.4
11.3
12.9
6.5
10.1
10.9
9.5
20.0
19.4
18.5
17.4
15.5
11.9
22.3
21.7
20.6
19.7
16.3
12.6
22.0
20.5
18.9
17.7
14.7
11.2
21.9
19.5
18.1
17.1
15.0
11.4
9.8
20.5
19.8
18.4
17.5
14.6
11.9
10.2
10.1
15.1
13.6
18.9
18.5
17.7
16.7
14.9
11.8
25.1
24.8
24.0
23.1
21.5
17.2
PEDOT 15%
24.2
23.7
22.4
21.4
19.8
15.6
a)Color index was obtained at 1000 cd m−2
.
critical problem of efficiency roll-off in deep blue PHOLEDs
was also significantly reduced in this work.
the 0.66 eV energy barrier for hole leakage from the mCPPO1
layer into the TSPO1 layer. Therefore, the efficiency roll-off was
significantly reduced in the deep blue PHOLEDs with mCPPO1
and TSPO1 layers. Hole and exciton blocking by TSPO1 was
also confirmed in other devices, as shown in Figure S3 in the
Supporting Information.
Although the quantum efficiency at 0.5% doping concentra-
tion was rather low owing to incomplete energy transfer, the
deep blue PHOLEDs showed maximum quantum efficiency
above 20% at doping concentrations of 1%–20%, the quantum
efficiency at 500 cd/m2 being 18.0%–21.0% irrespective of
the doping concentration. The color coordinate changed from
(0.14,0.17) to (0.14,0.20), while the quantum efficiency of the
blue PHOLEDs was maintained. Detailed device data are sum-
marized in Table 1. The low doping concentration dependence
was due to high PL intensity up to 20% doping concentration,
as shown in the PL spectra of Figure S4 in the Supporting
Information. The low PL intensities at 0.5% and 30% are due
to incomplete energy transfer and the concentration quenching
effect, respectively.
The device performances of the deep blue PHOLEDs were
further optimized by changing the hole-injecting layer from
N,N′-diphenyl-N,N′-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-
biphenyl-4,4′-diamine (DNTPD) to poly(3,4-ethylenedioxythio
phene):polystyrenesulfonate (PEDOT:PSS), because the elec-
tron current density was slightly higher than the hole current
density. Current density–voltage–luminance and quantum
efficiency–luminance curves of the deep blue PHOLEDs with
PEDOT:PSS are shown in Figure 3. The quantum efficiency
was further improved using PEDOT:PSS instead of DNTPD.
A maximum quantum efficiency of 25.1% was obtained at 3%
doping concentration, and the quantum efficiencies at 500
and 1000 cd m−2 were 24.0% and 23.1%, respectively. In par-
ticular, quantum efficiency above 20% could be maintained up
to 2500 cd m−2 . Additionally, the power efficiency of the deep
blue PHOLEDs was significantly enhanced due to the reduced
driving voltage of PEDOT:PSS. The maximum power efficiency
was 29.8 lm W−1 , and the power efficiencies at 100, 500, and
1000 cd m−2 were 21.5, 17.2, and 15.1 lm W−1 , respectively. The
current efficiencies at 100, 500, and 1000 cd m−2 were 31.0, 30.0,
and 28.9 cd A−1 , respectively. These values are much better than
Electroluminescence (EL) spectra are shown in Figure 2d.
There was a red shift in the EL spectra as a function of doping
concentration; the color coordinate changed from (0.14,0.17)
to (0.14,0.20) over the doping concentration range, with an
external quantum efficiency that exceeded 20%. The color was
red shifted at high doping concentration owing to intermo-
lecular interactions of the FCNIrpic dopant materials.
The high quantum efficiency and low efficiency roll-off can
be explained by the bipolar charge transport properties of the
host, balanced charge injection, and triplet exciton confine-
ment in the emitting layer. As explained in the hole-only and
electron-only data of mCPPO1, the hole and electron densi-
ties of the mCPPO1 device were similar because of the bipolar
charge transport properties of mCPPO1; this aids the charge
balance in the emitting layer. The small energy barrier for hole
and electron injection contributed to the high efficiency and
low efficiency roll-off. As there was only a small energy barrier
for charge injection, the relative hole and electron injection was
not greatly affected by the electric field, leading to similar hole
and electron densities. Hole and electron accumulation at the
interface between the emitting layer and charge transport layers
was suppressed because of the small energy barrier, which led
to similar relative hole and electron densities in the emitting
layer, even at high current density. The small energy barrier and
bipolar charge transport properties of mCPPO1 can keep the
relative hole and electron densities in the emitting layer con-
stant. The similar HOMO–LUMO difference between mCPPO1
and FCNIrpic may also balance holes and electrons because the
degree of charge trapping by the dopant may be similar at low
and high current densities.
Triplet exciton confinement by the TSPO1 exciton blocking
layer is also important for high quantum efficiency and effi-
ciency roll-off.[19,20] TSPO1 has triplet energy higher than
FCNIrpic and can subsequently suppress the triplet exciton
quenching. Therefore, high efficiency could be obtained using
the TSPO1 exciton blocking layer. The hole blocking properties
of TSPO1 were also critical for efficiency roll-off. It has been
reported that charge leakage is a critical factor for efficiency
roll-off.[21] The TSPO1 layer blocked the hole leakage owing to
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(a)
(b)
1000
10
5000
4000
3000
2000
1000
0
30
25
20
15
10
5
3%
15%
0.1
3%
0.001
15%
0.00001
0.0000001
0
1
10
100
1000
10000
0
2
4
6
8
10
Luminance(cd/m2)
Vo l t a g e ( V )
Figure 3. Current density–voltage–luminance curves (a) and quantum efficiency–luminance curves (b) of mCPPO1-based blue-light-emitting PHOLEDs
with PEDOT:PSS as the hole injection layer instead of DNTPD.
source measurement unit and a CS 1000 spectroradiometer from Konica
Minolta.
other data reported in the literature in deep blue PHOLEDs.
Detailed data are summarized in Table 1. Other blue dopant
materials such as iridium(III) bis(4,6-difluorophenylpyridinato)
picolinate (FIrpic) and iridium(III) bis(4,6-difluorophenylpy-
ridinato) tetrakis(1-pyrazolyl)borate (Fir6) were also evaluated
in the mCPPO1 host, and high quantum efficiency over 20%
was achieved (Fig. S5). Therefore, it can be concluded that the
mCPPO1 host can be effectively used in PHOLEDs ranging
from sky blue (FIrpic and Fir6) to deep blue (FCNIrpic).
In summary, deep blue PHOLEDs with an external
quantum efficiency above 20% at up to 2500 cd m−2 , at a
low doping concentration of 3%, have been successfully
developed using a bipolar host material and a high triplet
energy exciton-blocking material. The color coordinate could
be controlled from (0.14,0.17) to (0.14,0.20), while keeping
the external quantum efficiency at 20% or above for doping
concentrations from 1% to 20%. Management of the charge
transport properties and energy levels of the organic mate-
rials were important for the development of high-efficiency
color-tunable deep blue PHOLEDs. This approach could
prove useful in future PHOLED applications in displays and
lighting.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by RTI04-01-02 from the Rigional Technology
Innovation Program, MKE/ITEP(10028439-2009-22), IT R&D program
of MKE/KEIT[KI002104-2010-02], grant (M2009010025) from the Fund-
amental R&D Program for Core Technology of Materials funded by MKE,
and GRRC program of Gyeonggi Province (GRRC Dankook2010-B01,
Materials development for high efficiency solid state lighting).
Received: November 26, 2010
Published online: February 10, 2011
[1] C.-F. Chang, Y.-M. Cheng, Y. Chi, Y.-C. Chiu, C.-C. Lin, G.-H. Lee,
P.-T. Chou, C.-C. Chen, C.-H. Chang, C.-C. Wu, Angew. Chem. Int.
Ed. 2008, 47, 4542.
[2] Y.-C. Chiu, J.-Y. Hung, Y. Chi, C.-C. Chen, C.-H. Chang, C.-C. Wu,
Y.-M. Cheng, Y.-C. Yu, G.-H. Lee, P.-T. Chou, Adv. Mater. 2009, 10,
2221.
[3] T. Sajoto, P. I. Djurovich, A. Tamayo, M. Yousufuddin, R. Bau,
M. E. Thompson, R. J. Holmes, S. R. Forrest, Inorg. Chem. 2005, 44,
7992.
[4] P. Erk, M. Bold, M. Egen, E. Fuchs, T. Geßner, K. Kahle, C. Lennartz,
O. Molt, S. Nord, H. Reichelt, C. Schildknecht, H.-H. Johannes,
W. Kowalsky, SID Digest 2006, 37, 131.
[5] S. H. Kim, J. Jang, S. J. Lee, J. Y. Lee, Thin Solid Films 2008, 517,
722.
[6] K. S. Yook, S. O. Jeon, C. W. Joo, J. Y. Lee, Org. Electron. 2009, 10,
170.
[7] R. J. Holmes, S. R. Forrest, T. Sajoto, A. Tamayo, P. I. Djurovich,
M. E. Thompson, J. Brooks, H. J. Tung, B. W. D’Andrade,
M. S. Weaver, R. C. Kwong, J. J. Brown, Appl. Phys. Lett. 2005, 87,
243507.
Experimental Section
The synthesis of mCPPO1 and TSPO1 is described in the Supporting
Information.
The deep blue phosphorescent dopant for the device was FCNIrpic.
The device structure of the blue PHOLED was indium tin oxide (ITO,
150 nm)/DNTPD (60 nm)/N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine
(NPB, 20 nm)/mCP (10 nm)/mCPPO1:FCNIrpic (30 nm, x%)/TSPO1
(20 nm)/LiF (1 nm)/Al (200 nm). The doping concentration of the
FCNIrpic was controlled from 0.5% to 30%. Deep blue PHOLEDs with
PEDOT:PSS, instead of DNTPD, were also fabricated to further optimize
device performance. Hole-only and electron-only devices of mCPPO1
were prepared to study the hole and electron density in the emitting
layer. The device structure of the hole-only device was ITO/DNTPD/
NPB/mCP/mCPPO1/Au, and that of the electron-only device was ITO/
TSPO1/mCPPO1/TSPO1/LiF/Al. Blue PHOLEDs with FIrpic and Fir6
dopants were also prepared. Solid PL measurements of the FCNIrpic-
doped mCPPO1 film were carried out for various doping concentrations
to study the energy transfer from host to dopant. The device
performances of the blue PHOLEDs were measured with a Keithley 2400
[8] S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, Adv. Mater. 2010, 22,
1872.
©
1440 wileyonlinelibrary.com
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2011, 23, 1436–1441

www.advmat.de
www.MaterialsViews.com
[9] S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, Adv. Funct. Mater. 2009,
19, 3644.
[10] Y.-C. Chiu, Y. Chi, J.-Y. Hung, Y.-M. Cheng, Y.-C. Yu, M.-W. Chung,
G.-H. Lee, P.-T. Chou, C.-C. Chen, C.-C. Wu, H.-Y. Hsieh, ACS Appl.
Mater. Interfaces 2009, 1, 433.
[11] L. Xiao, S.-J. Su, Y. Agata, H. Lan, J. Kido, Adv. Mater. 2009, 21,
1271.
[12] S.-J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 2008, 20, 2125.
[13] H. Sasabe, E. Gonmori, T. Chiba, Y.-J. Li, D. Tanaka, S.-J. Su,
T. Takeda, Y.-J. Pu, K. Nakayama, J. Kido, Chem. Mater. 2008, 20,
5951.
[17] R. J. Holmes, B. W. D’Andrade, S. R. Forrest, X. Ren, J. Li,
M. E. Thompson, Appl. Phys. Lett. 2003, 83, 3818.
[18] X. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest,
M. E. Thompson, Chem. Mater. 2004, 16, 4743.
[19] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett.
2001, 79, 156.
[20] V. I. Adamovich, S. R. Cordero, P. I. Djurovich, A. Tamayo,
M. E. Thompson, B. W. D’Andrade, S. R. Forrest, Org. Electron.
2003, 4, 77.
[21] S. H. Kim, J. Jang, K. S. Yook, J. Y. Lee, Appl. Phys. Lett. 2008, 92,
023513.
[14] N. Chopra, J. Lee, Y. Zheng, S.-H. Eom, J. Xue, F. So, Appl. Phys.
Lett. 2008, 93, 143307.
[15] A. B. Padmaperuma, L. S. Sapochak, P. E. Burrows, Chem. Mater.
2006, 18, 2389.
[22] M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151.
[23] M. A. Baldo, C. Adachi, S. R. Forrest, Phys. Rev. B 2000, 62,
10967.
[16] P. E. Burrows, A. B. Padmaperuma, L. S. Sapochak, P. Djurovich,
M. E. Thompson, Appl. Phys. Lett. 2006, 88, 183503.
[24] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem,
K. Leo, Nature 2009, 459, 234.
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