New carbazole-based host material for low-voltage and highly efficient red phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm15138c

A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red PHOLEDs employing a conventional dopant material, bis(1-phenylisoquinoline)(acetylacetonate)iridium(iii) ((piq) ₂Ir(acac)), in the emissive layer showed nearly 100% internal quantum efficiency (corresponding to the external quantum efficiency of 19.3%) with reduced efficiency roll-off. We attribute these results to the good electron-hole balance resulting from the good hole mobility of BBTC and low hole injection barrier from the hole transport layer to BBTC host in the emission layer. In addition, owing to its good hole transporting property, it can be utilized as a hole transport layer in organic light-emitting devices enabling low voltage operation of the devices. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm15138c

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PAPER
New carbazole-based host material for low-voltage and highly efficient red
phosphorescent organic light-emitting diodes†‡
a
b
c
d
e
Jeonghun Kwak, Yi-Yeol Lyu, Hyunkoo Lee, Bonggoo Choi, Kookheon Char and Changhee Lee
c
*
*
Received 11th October 2011, Accepted 16th January 2012
DOI: 10.1039/c2jm15138c
A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and
characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular
design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole
while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole
are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red
PHOLEDs employing a conventional dopant material, bis(1-phenylisoquinoline)(acetylacetonate)
iridium(III) ((piq)2Ir(acac)), in the emissive layer showed nearly 100% internal quantum efficiency
(corresponding to the external quantum efficiency of 19.3%) with reduced efficiency roll-off. We
attribute these results to the good electron–hole balance resulting from the good hole mobility of BBTC
and low hole injection barrier from the hole transport layer to BBTC host in the emission layer. In
addition, owing to its good hole transporting property, it can be utilized as a hole transport layer in
organic light-emitting devices enabling low voltage operation of the devices.
These approaches, however, often require detailed optimiza-
tion of various layers such as carrier injection and transport
layers, emissive layers, and electron/hole/exciton blocking layers,
resulting in a complicated device structure. Thus, it is desirable to
develop suitable host materials for PHOLEDs that have high
carrier mobility, higher triplet energy (ET) and appropriate
energy levels of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) in order
to align the energy levels with proximal layers and obtain high
device performances.
Introduction
Phosphorescent organic light-emitting diodes (PHOLEDs) have
been intensively investigated because their internal quantum
efficiency can be increased up to 100% in principle, corre-
sponding to an external quantum efficiency of about 20%.1
Owing to their high efficiency, PHOLEDs are some of the most
noticeable candidates for full-color, large-area flat-panel displays
and solid-state lightings.2,3 However, efficiency roll-off at high
current densities or high operating voltages due to triplet
exciton–triplet exciton annihilation (TTA) or triplet exciton–
polaron quenching (TPQ) are significant issues of PHOLEDs.4,5
Improving the charge carrier balance and broadening the
recombination zone using double-emission layers or mixed host
structure of electron- and hole-transporting materials can reduce
the efficiency roll-off phenomenon.6–8
Various organic compounds especially carbazole derivatives
have been synthesized recently to satisfy these properties for
PHOLEDs. They usually have a good carrier transporting ability
to be used as a hole transporting material,9 and high triplet
energy suitable for the host materials of red,10,11 green,11,12 and
even blue PHOLEDs.13,14 For example, 4,40-N,N0-dicarbazole-
biphenyl (CBP) is a representative carbazole derivative as a host
for red and green phosphorescent emitters, which has a high
triplet energy (ET z 2.56 eV)15 and ambipolar conducting
properties. When we adopt CBP for PHOLEDs, however, high
driving voltage was required due to the large energy barrier
between CBP and the adjacent transporting layers because CBP
has low HOMO energy level (6.0 eV).16 Since many of the
carbazole-based organic materials have low glass transition
temperatures (Tg) (e.g., CBP: 62 ꢀC,17 1,3-bis(carbazol-9-yl)
benzene (mCP): 55 ꢀC18), the morphological and device stabilities
were poor when we utilized those materials. In addition, CBP
film can easily crystallize,16 causing short operational lifetime in
PHOLEDs. In order to enhance the thermal properties, a lot of
carbazole derivatives have been newly synthesized so far such as
aDepartment of Electronic Engineering, Dong-A University, Busan,
604-714, Republic of Korea
bUnitech Co., Ltd., Ansan-city, Gyeonggi-do, 425-100, Republic of Korea
cSchool of Electrical and Computer Engineering, Inter-University
Semiconductor Research Center, Seoul National University, Seoul, 151-
744, Republic of Korea. E-mail: chlee7@snu.ac.kr; Fax: +82-2-877-6668
dDepartment of Chemical and Biomolecular Engineering, Electronic
Material Lab, Yonsei University, Seoul, 120-749, Republic of Korea
eSchool of Chemical and Biological Engineering, Intelligent Hybrids
Research Center, Seoul National University, Seoul, 151-744, Republic of
Korea. E-mail: khchar@snu.ac.kr
† Electronic supplementary information (ESI) available: The DSC and
TGA data, current density–voltage curves of the hole-only devices, and
the operational lifetime of the devices. See DOI: 10.1039/c2jm15138c
‡ J. Kwak and Y.-Y. Lyu contributed equally to this work.
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2,20-bis(4-carbazolyl-phenyl)-1,10-biphenyl (BCBP),19 2,6-bis(3-
(carbazol-9-yl)phenyl) pyridine (26DCzPPy) and 3,5-bis(3-(car-
bazol-9-yl)phenyl) pyridine (35DCzPPy)20 with improved
performances.
In this study, we report a novel carbazole derivative, 3,6-bis-
biphenyl-4-yl-9-[1,10,40,100]terphenyl-4-yl-9H-carbazole (BBTC),
which possesses high Tg and good hole transporting property.
Red PHOLEDs using this material as a host exhibited nearly
100% internal quantum efficiency. We also demonstrated PHO-
LEDs using BBTC as the hole transporting layer in this work.
Fig. 1 (a) TGA and (b) DSC curves of BBTC.
which means that BBTC has excellent thermal and morpholog-
ical stabilities. Thus, BBTC can form homogeneous and amor-
phous thin film through thermal evaporation for the high
performance PHOLEDs.
Results and discussion
Design and synthesis
The chemical structure and synthetic routes of the targeted
compound, BBTC, are shown in Scheme 1. With the molecular
design strategy of maintaining the large triplet energy and a good
hole transporting ability of carbazole while increasing the
morphological and electrochemical stability, the C3 and C6
positions of carbazole are blocked with biphenyl groups and the
C9 position is terminated with terphenyl group. Compound 1 was
synthesized from 1,4-dibromobenzene and 9-H carbazole through
copper-catalyzed Ullmann condensations. Bromination of
compound 1 with N-bromosuccinimide (NBS) leads to the ter-
bromo compound 2 in reasonable yield (61%). Subsequently,
Suzuki coupling reactions of 3,6-dibromo-9-(4-bromophenyl)-
9H-carbazole with 4-biphenylboronic acid, in the presence of
a catalytic amount of Pd(PPh3)4 and 2 M K2CO3, yielded the
BBTC, which was purified by multiple sublimation in a tube
furnace to give the final product and then characterized by
Optical properties
Fig. 2 plots the optical absorption and photoluminescence (PL)
spectra at room temperature (293 K) and the phosphorescence
spectrum at 77 K of BBTC dissolved in toluene, normalized at
the peak of each spectrum. The PL spectrum shows deep blue
emission with vibronic peaks at 375, 393 and 414 nm. The optical
absorption peak is at 315 nm and the energy gap is estimated at
3.35 eV from the absorption edge at around 370 nm. The phos-
phorescence spectrum of BBTC at 77 K shows the highest energy
peak at 495 nm, which corresponds to the triplet energy level of
2.51 eV. In general, red phosphorescent dopants such as [Ir(piq)3]
(piq ¼ 1-phenylisoquinolinato) or [(piq)2Ir(acac)] (acac ¼ ace-
tylacetonate) have the triplet energy level of about 2.0 eV,22
1
elemental analysis (EA), H NMR, 13C NMR, and mass spec-
trometry. The detailed data are shown in the Experimental section.
Thermal properties
The thermal properties of BBTC are investigated by thermog-
ravimetric analysis (TGA) and differential scanning calorimetry
(DSC). As shown in Fig. 1(a), TGA indicates that BBTC exhibits
a high decomposition temperature (Td, which corresponds to
ꢀ
a 5% weight loss) of 482 C. The melting temperature (Tm) was
ꢀ
observed at 270 C with DSC in the first heating scan, and the
glass-transition temperature (Tg) was measured at 148 ꢀC in the
second, third, and fourth heating scans, which are depicted in
Fig. 1(b). The values of Td and Tg are very high among the
reported carbazole derivative compounds as host materials,10–14
Fig. 2 The optical absorption (dashed line), the PL spectrum (solid line)
at room temperature (293 K) and the phosphorescent (Ph) spectrum at
77 K of BBTC dissolved in toluene in normalized values.
Scheme 1 Synthesis procedure of BBTC.
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indicating that BBTC can be a suitable host material for red
phosphorescent emitters. The HOMO energy level of BBTC was
5.68 eV, which can reduce the hole injection barrier from the hole
transporting layer (HTL) to BBTC, compared with commercial
host materials such as CBP (6.0 eV). The LUMO energy level
which was calculated by the difference between the HOMO level
and optical energy band gap was 2.33 eV.
Electroluminescence properties
To investigate the characteristics of BBTC as a host material for
red phosphorescent emitters, we fabricated PHOLEDs with the
device structure of ITO/MoO3 (10 nm)/4,40-bis[N-(1-naphthyl)-
N-phenyl-amino]biphenyl (a-NPD, 50 nm)/BBTC doped with [Ir
(piq)3] or [(piq)2Ir(acac)] (8 wt%, 30 nm)/BCP (10 nm)/Bebq2
(40 nm)/LiF (0.5 nm)/Al (100 nm), as can be seen in Fig. 3. Two
different red phosphorescent dopants, [Ir(piq)3] for device A and
[(piq)2Ir(acac)] for device B, were co-evaporated to examine the
performance of BBTC as a host with a concentration of 8 wt% in
BBTC. We employed MoO3 as a hole injecting layer, a-NPD as
a HTL, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as
an efficient hole blocking layer, and bis(10-hydroxybenzo[h]
quinolinato)beryllium (Bebq2) as an electron transporting layer
(ETL). Since the electron mobility of Bebq2 is about one order of
magnitude higher than that of tris(8-hydroxyquinoline)
aluminium (Alq3),21 we can lower the operating voltage as well as
enhance the hole–electron balance.
Fig. 4 (a) Current density–voltage–luminance curves and (b) the
external quantum efficiency and current efficiency curves of the devices
A–C.
The current density (J)–voltage (V) characteristics, and the
efficiency curves of devices A and B are shown in Fig. 4(a) and
(b), respectively. The device with [Ir(piq)3] dopant (device A)
showed a maximum brightness of 49 370 cd mꢁ2 at 15.4 V,
a maximum external quantum efficiency (hEQE) of 11.8%, cor-
BBTC as a host layer can be attributed to the facilitated hole
injection owing to the high HOMO energy level of BBTC
compared with the conventional host materials (e.g. CBP) and
the suppression of the triplet exciton quenching processes (TTA
and TPQ) through the enhanced and balanced electron and hole
injection by using the materials possessing high electron and hole
mobility (the hole transport characteristics of BBTC is described
in the following paragraph and in Fig. S1†). The turn-on voltage
(corresponding to the voltage at the luminance of >1 cd mꢁ2) of
the device B was 2.9 V, which is lower than the device A by 0.3 V,
and the driving voltage at 1000 cd mꢁ2 for the device B was 7.0 V,
while the device A showed the same brightness at 8.7 V. The
current density of device B is also higher than device A at the
same voltage. We attribute these phenomena to the difference of
charge trap depth between two dopants, [(piq)2Ir(acac)] and [Ir
(piq)3]. As depicted in Fig. 3, [(piq)2Ir(acac)] has 0.38 eV higher
HOMO level and 0.47 eV lower LUMO level than the BBTC
host. In contrast, [Ir(piq)3] has much larger energy gaps, 0.48 eV
higher and 0.67 eV lower than the HOMO and LUMO levels of
BBTC, respectively, which indicates that [Ir(piq)3] makes deeper
traps in BBTC to both holes and electrons, causing the higher
turn-on and operating voltages.
responding to the current efficiency (hCE) of 10.9 cd Aꢁ1
.
Meanwhile, the device B using [(piq)2Ir(acac)] as a dopant
exhibited much higher performances: the maximum external
quantum efficiency of 19.3% and current efficiency of 16.4 cd Aꢁ1
at the luminance of 98 cd mꢁ2, and the maximum luminance of
82 625 cd mꢁ2 at 14.3 V. Both efficiencies and maximum lumi-
nance are excellent performances among recently reported red
PHOLEDs.22–25 This device also showed low efficiency roll-off at
high current densities. The current efficiencies at 1000 cd mꢁ2 and
at 10 000 cd mꢁ2 were still high as 15.9 cd Aꢁ1 (hEQE ¼ 18.7%)
and 13.1 cd Aꢁ1 (hEQE ¼ 15.4%), respectively, compared with the
efficiencies of 16.4 cd Aꢁ1 (hEQE ¼ 19.3%) at 100 cd mꢁ2. The high
efficiency and reduced efficiency roll-off for the device using
To evaluate the hole transporting characteristics of the BBTC,
we fabricated and compared the hole-only devices of BBTC with
a typical hole transporting materials, i.e., a-NPD and CBP,
in the structures of ITO/MoO3 (10 nm)/organic semiconductor
(100 nm)/MoO3 (10 nm)/Al (100 nm). As shown in Fig. S1 in
ESI†, BBTC conducted higher current (two times or more) than
a-NPD or CBP. Considering the hole mobilities of a-NPD
Fig. 3 Energy level diagram of the materials and the chemical structures
of the phosphorescent dyes.
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(7.64 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1
)
26 and CBP (1 ꢂ 10ꢁ3 cm2 Vꢁ1 sꢁ1),27 the
lowered due to high hole mobility and low hole injection barrier
from HTL to the BBTC host compared to the conventional host.
These excellent performances are attributed to the suppression of
the triplet exciton quenching processes through the enhanced
electron and hole balance. In addition, BBTC possesses high hole
mobility comparable with a-NPD and CBP, so that BBTC can
extend its role into HTL in PHOLEDs requiring high hole
mobility and low energy barrier from/to the adjacent layers.
hole mobility of BBTC may be similar with or higher than CBP
or a-NPD. All these results explain that BBTC can be used in the
HTL with high hole mobility as well as the phosphorescent host.
In order to utilize these good hole transporting properties of
BBTC, we substituted a-NPD with BBTC in the device B. The
device structure is ITO/MoO3 (10 nm)/BBTC (50 nm)/BBTC:
[(piq)2Ir(acac)] (8 wt%, 30 nm)/BCP (10 nm)/Bebq2 (40 nm)/LiF
(0.5 nm)/Al (100 nm) (device C). The J–V characteristics and
efficiency curves of device C is also plotted in Fig. 4(a) and (b),
respectively. The device C showed a maximum current efficiency
of 14.1 cd Aꢁ1 (hEQE ¼ 16.4%), which is a bit lower than that of
device B, but its power efficiency (12 lm Wꢁ1) is still comparable
to that of device B (13 lm Wꢁ1). The higher hole conducting
property of BBTC compared to a-NPD causes excess hole
injection and transport into the emissive layer. This deteriorates
the electron–hole balance, resulting in slightly lower efficiencies
in the device C. Nevertheless, in device C, turn-on voltage and
the driving voltage at 1000 cd mꢁ2 are improved as 2.8 V and
5.9 V, respectively. It is an encouraging fact that BBTC lowers
the operating voltages by 1.1 V at 1000 cd mꢁ2 as compared to
device B (a-NPD as the HTL). Therefore, BBTC can be a good
hole transporting material. The performances of devices A–C are
summarized in Table 1.
Experimental section
Instrumentation
The 1H and 13C NMR spectra were recorded at 25 ꢀC on a Bruker
DPX-300 spectrometer. Mass spectra were recorded on a Shi-
madzu LCMS-IT-TOF. Elemental analysis was performed with
a CE Instrument EA 1110 Elemental Analyzer. Differential
scanning calorimetry (DSC) was performed on a TA Instruments
DSC 2020 instrument with a heating rate of 10 ꢀC minꢁ1 and
a cooling rate of 20 ꢀC minꢁ1. Thermogravimetric analysis (TGA)
was conducted on a TA Instruments TGA 2050 unit under
a heating rate of 10 ꢀC minꢁ1 and a N2 flow rate of 90 ml minꢁ1
.
UV-Visible spectra were measured with an HP 8453 spectro-
photometer. Fluorescence spectra of BBTC in both solution and
film were obtained at room temperature and phosphorescence
spectrum of BBTC in a dilute 2-methyltetrahydrofuran solution
was taken at 77 K using a Hitachi F-4500 fluorescence spec-
trometer. The HOMO energy levels were measured with an AC-2
photoelectron spectrometer (Riken Keiki Co.)
Considering these results, BBTC has various advantages as the
host material for red phosphorescent emitters and as the HTL: as
a host, it is a very efficient host material for red phosphorescent
emitters, and also has a small hole injection barrier from HTL
compared to common host materials like CBP (HOMO: 6.0 eV).
As the HTL, it lowers operating voltage owing to superior hole
transporting property. Comparing with a-NPD, rather low
HOMO energy (5.68 eV) facilitates hole injection into wide band
gap host materials which usually have HOMO levels from 6.0 eV
(CBP) to 7.1 eV (p-bis(triphenylsilyl)benzene; UGH2).28 There-
fore, BBTC can contribute to lowering the operating voltages,
increasing the efficiencies, and simplifying the device structures
as both the HTL and host layer, which are all important issues in
this field.
Synthesis
3,6-Bis-biphenyl-4-yl-9-[1,10,40,100]terphenyl-4-yl-9H-carbazole
(BBTC) was synthesized according to the procedure shown in
Scheme 1. Unless stated otherwise, all the reagents were used as
received from commercial sources. The solvents were dried using
the standard procedure. All the reactions were performed under
purified nitrogen atmosphere using the standard Schlenk
technique.
9-(4-Bromophenyl)-9H-carbazole (1). 1,4-Dibromobenzene
(24.00 g, 101.74 mmol), copper (0.30 g, 4.72 mmol) and potas-
sium carbonate (20.00 g, 144.70 mmol) were added to a solution
of 9-H carbazole (7.00 g, 41.86 mmol) in dry 1,2-dichlorobenzene
(200 ml) under N2 atmosphere. The solution was then heated
under reflux for 24 h under N2. After the reaction mixture was
cooled, the solvent was evaporated and the product was extrac-
ted with ethyl acetate. The organic extracts were washed with
brine and H2O, and then dried over MgSO4. After the solvent
was evaporated, the crude product was crystallized from ethyl
Conclusions
In summary, a new carbazole derivative, BBTC, which shows
great properties as the host and hole transporting material in
PHOLED, was synthesized and investigated. Red PHOLEDs
employing [(piq)2Ir(acac)] doped BBTC in the emissive layer
showed nearly 100% internal quantum efficiency (hEQE ¼ 19.3%)
with reduced efficiency roll-off. The driving voltage was also
Table 1 The performances of devices A–C.
1
acetate. Yield: 75% (10.10 g). H NMR (300 MHz, CD2Cl2, d):
c
hEQE hCE
d
e
hPE /
/
8.21 (d, J ¼ 7.5 Hz, 2H), 7.77–7.81 (m, 2H), 7.42–7.53 (m, 6H),
7.32–7.38 (m, 2H). 13C NMR (75 MHz, CD2Cl2, d): 141.2, 137.3,
133.6, 129.2, 126.6, 124.0, 121.3, 120.8, 120.7, 110.1. LCMS-IT-
TOF [M]+: calcd 321.0153. Found: 321.0162.
Device HTL
Dopant
Vona/V V1000b/V (%) cd Aꢁ1 lm Wꢁ1
A
B
C
a-NPD Ir(piq)3
a-NPD (piq)2Ir(acac) 2.9
BBTC (piq)2Ir(acac) 2.8
3.2
8.7
7.0
5.9
11.8 10.9
19.3 16.4
16.4 14.1
7.2
13.0
12.0
a
Turn-on voltage defined as the lowest voltage at the luminance of >1 cd
3,6-Dibromo-9-(4-bromophenyl)-9H-carbazole (2). Acetic acid
(100 ml) was added to a solution of 9-(4-bromophenyl)-9H-carbazole
(10.00 g, 31.15 mmol) in dry chloroform (100 ml). The mixture was
stirred at room temperature for 1 h. After the mixture was cooled to
b
. . The maximum external quantum
c
mꢁ2
Driving voltage at 1000 cd mꢁ2
d
e
efficiency. The maximum current efficiency. The maximum power
efficiency.
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0 ꢀC, N-bromosuccinimide (NBS) (11.60 g, 65.17 mmol) was added
to the mixture. The solution was slowly warmed to room temperature
and then stirred for 3 h. The reaction mixture was quenched with
H2O, and extracted with chloroform. The organic extracts were
washed with brine and H2O, and then dried over MgSO4. After the
solvent was evaporated, the crude product was crystallized from
toluene. Yield: 61% (9.10 g). 1H NMR (300 MHz, CD2Cl2, d): 8.21 (d,
J ¼ 1.8 Hz, 2H), 7.73–7.78 (m, 2H), 7.54 (d, J ¼ 1.8 Hz, 1H), 7.51 (d,
Acknowledgements
This work was supported by the BK21 Program, the Creative
Research Initiative Program for ‘‘Intelligent Hybrids Research
Center’’ (2010-0018290), and Basic Science Research Program
(2011-0022716) of the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science, and
Technology (MEST), and by the grant from the Industrial
Source Technology Development Program (KI002110) of the
Ministry of Knowledge Economy (MKE) of Korea.
J ¼ 1.8 Hz, 1H), 7.38–7.43 (m, 2H), 7.26 (d, J ¼ 8.7 Hz, 2H). 13
C
NMR (75 MHz, CD2Cl2, d): 140.2, 136.4, 133.9, 130.0, 129.1, 124.6,
123.7, 122.1, 113.7, 112.0. LCMS-IT-TOF [M]+: calcd 476.8363.
Found: 476.8373.
Notes and references
3,6-Bis-biphenyl-4-yl-9-[1,10,40,100]terphenyl-4-yl-9H-carbazole
(BBTC). 3,6-Dibromo-9-(4-bromophenyl)-9H-carbazole (1.00 g,
2.10 mmol) and 4-biphenylboronic acid (1.37 g, 6.92 mmol)
were mixed in dry toluene. K2CO3 (2.0 M, 12 ml) was added, and
the mixture was stirred. The mixture was degassed and tetrakis
(triphenylphosphine)palladium (0.36 g, 0.31 mmol) was added in
one portion under N2 atmosphere. The solution was then heated
under reflux for 36 h under N2. After the reaction mixture was
cooled, the solvent was evaporated and the product was extrac-
ted with chloroform. The organic extracts were washed with
brine and H2O, and then dried over MgSO4. After the solvent
was evaporated, the crude product was purified by column
chromatography (eluent ¼ hexane/chloroform; 8 : 2 v/v), the
product was crystallized from chloroform/methanol. Yield: 62%
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(0.92 g). H NMR (300 MHz, CD2Cl2, d): 8.57 (d, J ¼ 1.2 Hz,
2H), 7.98–8.01 (m, 2H), 7.85–7.92 (m, 7H), 7.78–7.83 (m, 9H),
7.73–7.76 (m, 6H), 7.67 (d, J ¼ 8.7 Hz, 2H), 7.49–7.54 (m, 6H),
7.41–7.43 (m, 3H). 13C NMR (75 MHz, CD2Cl2, d): 141.4, 141.2,
141.0, 139.9, 133.5, 129.5, 129.4, 129.0, 128.1, 128.0, 127.8, 127.7,
127.5, 127.4, 126.1, 124.7, 119.2, 110.9. LCMS-IT-TOF [M]+:
calcd 699.2926. Found: 699.2917. Anal. calcd for C54H37N: C,
92.67; H, 5.33; N, 2.00%. Found: C, 92.65; H, 5.38; N, 2.01%.
14 P.-I. Shih, C.-L. Chiang, A. K. Dixit, C.-K. Chen, M.-C. Yuan,
R.-Y. Lee, C.-T. Chen, E. W.-G. Diau and C.-F. Shu, Org. Lett.,
2006, 8, 2799.
Device fabrication and characterization
15 K. Goushi, R. Kwong, J. J. Brown, H. Sasabe and C. Adachi, J. Appl.
Phys., 2004, 95, 7798.
16 C.-L. Ho, W.-Y. Wong, Z.-Q. Gao, C.-H. Chen, K.-W. Cheah,
B. Yao, Z. Xie, Q. Wang, D. Ma, L. Wang, X.-M. Yu, H.-S. Kwok
and Z. Lin, Adv. Funct. Mater., 2008, 18, 319.
Phosphorescence organic light-emitting diodes were fabricated
according to the following procedure: the ITO substrates were
cleaned ultrasonically in organic solvents (isopropyl alcohol,
acetone, and methanol), rinsed in deionized water, and dried in
an oven at 120 ꢀC for more than 30 minutes. Subsequently,
MoO3, a-NPD or BBTC as a hole transporting layer, BBTC
doped with phosphorescent guest ([(piq)3Ir] or [(piq)2Ir(acac)]),
BCP as a hole blocking layer, Bebq2 as an electron transporting
layer, LiF and Al electrodes were deposited on the substrates
under the vacuum of about 3 ꢂ 10ꢁ6 Torr. The deposition rate
17 M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu,
A.
Matoliukstyte,
J.
Simokaitiene,
S.
Grigalevicius,
J. V. Grazulevicius and C.-P. Hsu, Adv. Mater., 2007, 19, 862.
18 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., 2005, 17, 285.
19 Y. Agata, H. Shimizu and J. Kido, Chem. Lett., 2007, 316.
20 S.-J. Su, H. Sasabe, T. Takeda and J. Kido, Chem. Mater., 2008, 20,
1691.
21 J. H. Lee, C. I. Wu, S. W. Liu, C. A. Huang and Y. Chang, Appl.
Phys. Lett., 2005, 86, 103506.
22 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and
D. Ma, Angew. Chem., Int. Ed., 2008, 47, 8104.
23 Y.-Y. Lyu, J. Kwak, W. S. Jeon, Y. Byun, H. S. Lee, D. Kim, C. Lee
and K. Char, Adv. Funct. Mater., 2009, 19, 420.
24 C.-H. Chien, F.-M. Hsu, C.-F. Shu and Y. Chi, Org. Electron., 2009,
10, 871.
25 Y. Tao, Q. Wang, L. Ao, C. Zhong, J. Qin, C. Yang and D. Ma,
J. Mater. Chem., 2010, 20, 1759.
26 T.-Y. Chu and O.-K. Song, Appl. Phys. Lett., 2007, 90, 203512.
27 H.-I. Baek and C. Lee, J. Appl. Phys., 2008, 103, 054510.
28 X. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest and
M. E. Thompson, Chem. Mater., 2004, 16, 4743.
ꢁ1
was about 1–2 A s for organic materials, and 4–6 A sꢁ1 for Al
electrodes. The active area of the devices, defined by the overlap
of ITO and Al cathode, was 1.96 mm2. The current–voltage–
luminance characteristics were measured at room temperature
using a Keithley 236 source-measure unit and a Keithley 2000
multimeter equipped with a calibrated Si photodiode and
Konica-Minolta CS-1000 spectroradiometer, and the EL
spectra were measured by a photomultiplier tube (PMT)
through an ARC 275 monochromator. For the lifetime test
(Fig. S2†), the devices were encapsulated with a cover glass and
UV-epoxy resin.
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 6351–6355 | 6355