Fused indole derivatives as high triplet energy hole transport materials for deep blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm15204e

High triplet energy hole transport materials based on novel fused indole core structure were synthesized and their device performances in deep blue phosphorescent organic light emitting diodes (PHOLEDs) were investigated. The fused indole based hole trans

Full text of DOI:10.1039/c2jm15204e

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PAPER
Fused indole derivatives as high triplet energy hole transport materials for
deep blue phosphorescent organic light-emitting diodes
a
Min Su Park, Dae Hyuk Choi, Bum Sung Lee and Jun Yeob Lee
b
b
a
*
Received 13th October 2011, Accepted 3rd December 2011
DOI: 10.1039/c2jm15204e
High triplet energy hole transport materials based on novel fused indole core structure were synthesized
and their device performances in deep blue phosphorescent organic light emitting diodes (PHOLEDs)
were investigated. The fused indole based hole transport materials showed a high triplet energy over
ꢀ
2.90 eV and high glass transition temperature over 120 C. The high triplet energy hole transport
materials were used in deep blue PHOLEDs and gave a high external quantum efficiency of 24.7% with
a color coordinate of (0.137,0.182).
mCP has a problem of poor thermal and morphological stability,
Introduction
while TAPC suffers from poor hole injection into the emitting
layer. Other than these, bis[4-(p,p0-ditolylamino)-phenyl]diphe-
nylsilane,24 3,5-di(9H-carbazol-9-yl)-N,N-diphenylaniline,25 and
acridine based hole transport materials26 were also used as the
high triplet energy hole transport materials. However, only
a limited number of hole transport materials with a high triplet
energy of 2.90 eV are available and further development of the
thermally stable high triplet energy hole transport materials for
use in deep blue PHOLEDs is strongly required.
Phosphorescent organic light-emitting diodes (PHOLEDs) are
critical to reduce the power consumption of full color organic
light-emitting diode (OLED) panel as the quantum efficiency of
PHOLEDs can be higher than that of fluorescent OLEDs by
three or four times.1–3 External quantum efficiency between 25%
and 30% has already been achieved in red, green and blue
PHOLEDs,4–7 but the quantum efficiency of deep blue PHO-
LEDs needs to be improved further although a few papers
reported above 20% external quantum efficiency in deep blue
PHOLEDs.8,9
In this work, novel fused indole type high triplet energy hole
transport materials were synthesized and their device perfor-
mances were investigated. The fused indole derivꢀatives showed
a high glass transition temperature (Tg) over 120 C and a high
quantum efficiency of 24.7% was achieved in the deep blue
PHOLEDs.
The most popular approach to improve the quantum efficiency
of deep blue PHOLEDs was to develop new host and dopant
materials.10–16 Various high triplet energy host and dopant
materials were proven to be effective to enhance the quantum
efficiency of deep blue PHOLEDs.
In addition to the host and dopant materials, high triplet
energy charge transport layers have to be developed for high
quantum efficiency in deep blue PHOLEDs. The high triplet
energy charge transport materials played a role of confining
charges and triplet excitons inside the emitting layer, improving
the quantum efficiency. Several high triplet energy electron
transport materials were reported to be effective in blue PHO-
LEDs,17–21 but only a few high triplet energy hole transport
materials were synthesized.7,22–25 Typically, N,N0-dicarbazolyl-
3,5-benzene (mCP)22,23 and 1,1-bis[(di-4-tolylamino)phenyl]
cyclohexane (TAPC)7 have been applied as high triplet energy
hole transport materials in deep blue PHOLEDs. However, the
Experimental section
Synthesis
The synthesis of the fused indole type hole transport materials is
shown in Scheme 1.
Synthesis of 6,6-bis(4-bromophenyl)-6H-pyrrolo[3,2,1-de]acri-
dine. 1-(2-Bromophenyl)-1H-indole (5 g, 18.37 mmol) was dis-
solved in 100 mL of anhydrous tetrahydrofuran (THF) under
argon and cooled to ꢁ78 ꢀC and n-butyllithium (1.3 eq) was
added dropwise slowly. Stirring was continued for 2 h at ꢁ78 ꢀC,
followed by the addition of a solution of 4,40-dibromobenzo-
phenone (6.23 g, 23.88 mmol) in THF (100 mL) under an argon
atmosphere. The resulting mixture was gradually warmed to
ambient temperature and quenched by adding saturated,
aqueous NaHCO3 (50 mL). The mixture was extracted with
dichloromethane. The combined organic layers were dried over
MgSO4, filtered, and evaporated under reduced pressure. A
yellow powdery product was obtained. The crude residue was
aDepartment 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; Fax: +82 31 8005 3585; Tel: +82
31 8005 3585
bCorporate R&D center, Duksan Hi-Metal Co., Ltd., Floor #11, Heung-
Duk U-Tower, Youngduk-dong, Kiheung-Gu, Yongin-City, Geonggi-Do,
Korea
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J. Mater. Chem., 2012, 22, 3099–3104 | 3099

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times. The organic layer was dried over anhydrous MgSO4 and
evaporated in vacuo to give the crude product. The extract was
evaporated to dryness affording a white solid, which was further
purified by column chromatography using dichloromethane/n-
hexane to give a white powder (0.87 g). Yield: 65%. Tg 125 ꢀC. 1H
NMR (200 MHz, CDCl3): d 8.13 (d, 4H, J ¼ 8.0 Hz), 7.88 (d, 1H,
J ¼ 8.0 Hz), 7.44–7.26 (m, 27H), 6.69 (s, 1H). 13C NMR (50
MHz, CDCl3): d 148.6, 143.5, 140.1, 140.5, 136.1, 132.7, 130.3,
129.9, 129.5, 128.8, 128.4, 127.0, 125.8, 124.7, 122.8, 121.5, 120.9,
120.3, 119.8, 119.1, 118.9, 111.0, 109.7, 108.6, 58.2. MS (FAB)
m/z 688 [(M + H)+]. Anal. calcd for C51H33N3: C, 89.06; H, 4.84;
N, 6.11%. Found: C, 89.25; H, 4.78; N, 6.08%.
Device preparation and measurements
Scheme 1 Synthetic scheme of hole transport materials.
1
Synthesized materials were analyzed using H nuclear magnetic
resonance (NMR), 13C NMR, mass spectrometer, elemental
analyzer, ultraviolet-visible (UV-Vis) and photoluminescence
(PL) spectrometers. UV-Vis and PL measurements were carried
out using 1.0 ꢂ 10ꢁ4 M THF solution. The energy levels of
BIPPA and BCPPA were measured with a cyclic voltammetry
(CV) using ferrocene as the standard material. Tg measurement
of the hole transport materials was carried out using differential
scaꢀnning calorimeter (Mettler DSC 822) at a heating rate of
10 C minꢁ1 under nitrogen atmosphere.
placed in another two-necked flask and was dissolved in acetic
acid (40 mL). A catalytic amount of aqueous HCl (5 mol%, 12 N)
was then added and the whole solution was refluxed for 12 h.
After cooling to ambient temperature, purification by silica gel
chromatography using dichloromethane/n-hexane gave a white
powder (5.7 g). Yield: 60%. 1H NMR (200 MHz, CDCl3): d 7.78
(d, 1H, J ¼ 8.0 Hz), 7.64–7.61 (m, 2H), 7.40–7.31 (m, 7H), 7.16–
7.12 (m, 6H), 6.43 (s, 1H). 13C NMR (50 MHz, CDCl3): d 149.0,
144.4, 140.3, 133.6, 132.6, 131.4, 130.6, 129.5, 126.5, 123.9, 123.5,
122.9, 122.5, 121.6, 111.8, 110.7, 99.3, 98.4, 58.6. MS (FAB) m/z
515 [(M + H)+]. Anal. calcd for C27H17Br2N: C, 62.94; H, 3.33;
N, 2.72%. Found: C, 63.03; H, 3.34; N, 2.61%.
The device structure of the deep blue PHOLEDs was indium
tin oxide (ITO, 150 nm)/poly(3,4-ethylenedioxythiophene):poly-
styrenesulfonate (PEDOT:PSS, 60 nm)/N,N0-di(1-naphthyl)-N,
N0-diphenylbenzidine (NPB, 5 nm)/BIPPA or BCPPA (10 nm)/9-
(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-
9H-carbazole (mCPPO1):bis((3,5-difluoro-4-cyanophenyl)pyri-
dine) iridium picolinate (FCNIrpic)(30 nm, 3%)/diphenylphos-
phine oxide-4-(triphenylsilyl)phenyl (TSPO1, 25 nm)/LiF(1 nm)/
Al(200 nm). Deep blue PHOLEDs with common TAPC and
NPB hole transport layers instead of BIPPA and BCPPA were
also fabricated for comparison. All devices were encapsulated
with a glass lid and CaO getter after device fabrication. Device
performances were measured using Keithley 2400 source
measurement unit and CS 1000 spectroradiometer.
Synthesis of 6,6-bis(4-(1H-indol-1-yl)phenyl)-6H-pyrrolo[3,2,1-
de]acridine (BIPPA). 1H-Indole (0.52 g, 4.463 mmol), 6,6-bis(4-
bromophenyl)-6H-pyrrolo[3,2,1-de]acridine (1 g, 1.94 mmol),
K2CO3 (1.09 g, 7.95 mmol), CuI(I) (0.18 g, 0.97 mmol) and
dibenzo-18-crown-6 (0.10 g, 0.388 mmol) were dissolved in
dimethylformamide under nitrogen atmosphere. The reaction
mixture was stirred for 12 h at 150 ꢀC. The mixture was diluted
with dichloromethane and washed with distilled water three
times. The organic layer was dried over anhydrous MgSO4 and
evaporated in vacuo to give the crude product. The extract was
evaporated to dryness affording a white solid, which was further
purified by column chromatography using dichloromethane/n-
hexane to give a white powder (0.8 g). Yield: 65%. Tg 123 ꢀC. 1H
NMR (200 MHz, CDCl3): d 8.19–8.09 (m, 4H), 7.98 (d, 1H, J ¼
8.0 Hz), 7.62–7.10 (m, 21H), 6.65–6.61 (m, 3H). 13C NMR (50
MHz, CDCl3): d 143.5, 137.7, 136.9, 136.4, 135.1, 131.2, 130.7,
130.2, 128.8, 127.8, 126.8, 126.5, 126.1, 123.4, 122.2, 121.3, 120.3,
119.3, 114.5, 113.7, 113.2, 112.5, 110.5, 109.4, 103.7, 102.7, 56.0.
MS (FAB) m/z 588 [(M + H)+]. Anal. calcd for C43H29N3: C,
87.88; H, 4.97; N, 7.15%. Found: C, 87.87; H, 5.08; N, 7.03%.
Results and discussion
The new core structure, fused indole, was designed as a rigid core
structure with a high triplet energy and high Tg. The phenyl unit
in the N-phenylindole can freely rotate through the chemical
bond between the indole and phenyl unit, but the rotation of the
phenyl group is hindered in the fused indole compounds due to
the fused ring structure through the sp3 carbon atom. The rigid
fused ring formation makes the core structure stable even at high
temperature. In addition, the fused indole core is connected to
the carbazole and indole units through the sp3 carbon, which
does not extend the conjugation of the fused indole core and
keeps the high triplet energy of the fused indole core.
Synthesis of 6,6-bis(4-(9H-carbazol-9-yl)phenyl)-6H-pyrrolo
[3,2,1-de]acridine (BCPPA). 9H-Carbazole (0.75 g, 4.463 mmol),
6,6-bis(4-bromophenyl)-6H-pyrrolo[3,2,1-de]acridine (1 g, 1.94
mmol), K2CO3 (1.09 g, 7.95 mmol), CuI(I) (0.18 g, 0.97 mmol)
and dibenzo-18-crown-6 (0.10 g, 0.388 mmol) were dissolved in
dimethylformamide under nitrogen atmosphere. The reaction
mixture was stirred for 16 h at 150 ꢀC. The mixture was diluted
with dichloromethane and washed with distilled water three
The fused indole core was synthesized by the ring closing
reaction of the 1-(2-bromophenyl)-1H-indole using 4,40-dibro-
mobenzophenone. The brominated fused indole intermediate
was funtionalized with an indole and a carbazole to improve the
hole transport properties of the fused indole derivatives. The
synthetic yield of BIPPA and BCPPA was 65%. BIPPA and
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ꢀ
ꢀ
BCPPA showed high Tg of 123 C and 125 C due to the rigid
molecular backbone structure.
Photophysical properties of the synthesized compounds were
analyzed using UV-Vis and PL spectrometer. UV-Vis and PL
spectra of BIPPA and BCPPA are shown in Fig. 1. BIPPA
showed strong absorption of the fused indole core and N-phe-
nylindole at 265 nm and 301 nm,27 while BCPPA exhibited
additional absorption peak of the phenylcarbazole at 340 nm in
addition to the absorption peaks of fused indole core.28 The
bandgaps of BIPPA and BCPPA were calculated from the
absorption edge of the UV-Vis absorption spectra, which were
3.83 eV and 3.57 eV. The bandgap of BIPPA was larger than that
of BCPPA because of long wavelength absorption of carbazole in
BCPPA. Solution PL spectra of the BIPPA was blue shifted as
compared with that of BCPPA, which is due to the rather large
bandgap of BIPPA. The PL emission peak of BIPPA was
observed at 336 nm, while that of BCPPA was observed at 350
nm. The PL emission of BCPPA was red-shifted due to the
carbazole unit.
Fig. 2 HOMO and LUMO distribution of BIPPA and BCPPA.
HOMO level of the BIPPA was shifted by 0.14 eV because of the
indole unit. As explained above, the strong electron donating
character of the indole shifted the HOMO level by 0.14 eV. The
LUMO level was calculated from the HOMO level and bandgap.
The LUMO levels of the BIPPA and BCPPA were 2.09 eV and
2.24 eV, respectively. The LUMO levels of the BIPPA and
BCPPA were suitable for electron blocking from emitting layer.
The triplet energies of the BIPPA and BCPPA were calculated
from the low temperature PL spectra and they were 2.95 eV and
2.97 eV, respectively. The triplet energy of the BIPPA and
BCPPA was higher than that of common deep blue emitting
phosphorescent dopants with a triplet energy of 2.70–2.80 eV
and can block the triplet exciton quenching by the hole transport
layers in deep blue PHOLEDs.8,22 Therefore, the BIPPA and
BCPPA had the HOMO levels for hole injection into wide
bandgap deep blue host material, LUMO levels for electron
blocking and triplet energies for triplet exciton blocking, which
may increase the quantum efficiency of deep blue PHOLEDs.
Hole only devices of BIPPA and BCPPA were fabricated to
compare the hole injection and transport properties. Fig. 3 shows
the current density–voltage curves of the hole only devices of
BIPPA and BCPPA. Device structure of the hole only device was
ITO/DNTPD (60 nm)/NPB (5 nm)/BIPPA or BCPPA (20 nm)/
Al and the hole current density of the BIPPA device was higher
than that of the BCPPA device. In general, the hole current
density is decided by the energy barrier for hole injection and
hole transport properties. The energy barrier for hole injection
between NPB (5.50 eV) and BIPPA (5.90 eV) was 0.40 eV, while
Molecular simulation of the two hole transport materials was
carried out using density functional theory calculations. A suite
of Gaussian 03 program and the nonlocal density functional of
Becke’s 3-parameters employing Lee–Yang–Parr functional
(B3LYP) with 6-31G* basis sets were used for the calculation.29
Fig. 2 shows the molecular orbital simulation results of BIPPA
and BCPPA. The highest occupied molecular orbital (HOMO) of
the BIPPA was mostly localized on the indole unit of the BIPPA
and the lowest unoccupied molecular orbital (LUMO) of the
BIPPA was localized on the fused indole core. Compared with
the HOMO and LUMO distribution of BIPPA, the HOMO and
LUMO of BCPPA were rather dispersed over the molecule. The
HOMO of BCPPA was mostly distributed over the two carba-
zole units and was slightly expanded to the fused indole core
structure. The LUMO distribution of the BCPPA was similar to
that of the BIPPA. The difference of the HOMO distribution
between BIPPA and BCPPA is due to the different electron
donating properties between the indole and carbazole groups. As
indole is stronger than carbazole in terms of electron donating
properties, the HOMO was localized on the indole unit.
However, the carbazole is not as strong as the indole in terms of
electron donating character, resulting in rather extensive
dispersion of the HOMO.
The HOMO levels of the BIPPA and BCPPA were measured
using CV and were 5.90 eV and 6.04 eV, respectively. The
Fig. 1 UV-Vis and PL spectra of BIPPA and BCPPA.
This journal is ª The Royal Society of Chemistry 2012
Fig. 3 Hole only device data of BIPPA and BCPPA.
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it was 0.54 eV between NPB (5.50 eV) and BCPPA (6.04 eV) as
shown in energy level diagram in Fig. 4. Therefore, the low
energy barrier for hole injection enhanced the hole current
density of the BIPPA device. In addition to the low energy
barrier for hole injection, better hole transport properties of the
BIPPA than BCPPA also contributed to the high hole current
density as the indole group is better than the carbazole group in
terms of hole transport.
Deep blue PHOLEDs with mCPPO1:FCNIrpic emitting layer
were fabricated using the BIPPA and BCPPA as the hole
transport layers. Fig. 5 shows the current density–voltage–
luminance curves of the deep blue PHOLEDs. As expected from
the hole current density data of the hole only device, the BIPPA
device showed higher current density than the BCPPA device. In
addition, both BIPPA and BCPPA devices showed much higher
current density than common TAPC and NPB devices due to the
reduced energy barrier for hole injection into the emitting layer.
The luminance showed the same tendency as the current density
as the luminance is affected by the current density of the device.
The turn-on voltage of the BIPPA and BCPPA deep blue
PHOLEDs was 3.5 V and the driving voltages at 1000 cd mꢁ2
were 6.9 V and 7.4 V for BIPPA and BCPPA, respectively.
Quantum efficiency–luminance curves of the deep blue PHO-
LEDs are shown in Fig. 6. Both BIPPA and BCPPA devices
showed high quantum efficiency although the quantum efficiency
of the BCPPA device was a little degraded compared with that of
the BIPPA device. The maximum external quantum efficiencies
of the BIPPA and BCPPA devices were 24.7% and 23.9%, while
the quantum efficiencies at 1000 cd mꢁ2 were 22.3% and 21.7%.
The quantum efficiency of the mCPPO1 device is known to be
improved by enhancing the hole density in the emitting layer.8
The hole current density of the BIPPA device was higher than
that of the BCPPA device, leading to improved quantum effi-
ciency in the BIPPA device. The very high quantum efficiency of
the BIPPA and BCPPA devices can be explained by the efficient
hole injection, exciton blocking and electron blocking properties
of the BIPPA and BCPPA. Strong hole transport properties and
appropriate HOMO level for hole injection into the mCPPO1
emitting layer balanced the holes and electrons in the emitting
layer, enhancing the quantum efficiency of the deep blue PHO-
LEDs. The high triplet energy over 2.90 eV of the BIPPA and
BCPPA also contributed to the high quantum efficiency of the
device through triplet exciton blocking. The triplet energy of the
triplet emitter was 2.75 eV and both the BIPPA and BCPPA
Fig. 5 Current density–voltage–luminance curves of BIPPA, BCPPA,
TAPC and NPB blue devices.
could suppress the triplet exciton quenching by the hole trans-
port layer due to the high triplet energy of hole transport
materials. The electron blocking effect of the BIPPA and BCPPA
is also responsible for the high quantum efficiency of the deep
blue PHOLEDs. The LUMO level of the mCPPO1 was 2.64 eV
and electron leakage from the mCPPO1 layer to the hole trans-
port layer could be effectively blocked by the BIPPA and BCPPA
layer due to the high energy barrier for electron leakage over 0.40
eV. Compared with the acridine based hole transport materials
reported earlier,26 the LUMO level of BIPPA was more suitable
for electron blocking, resulting in improved quantum efficiency.
The bandgap of the fused indole based hole transport materials
was much larger than that of the acridine based materials, which
induced the shift of LUMO level for electron blocking. All three
parameters affected the high quantum efficiency of the BIPPA
and BCPPA devices. Standard devices with NPB and TAPC hole
transport layers instead of BIPPA and BCPPA showed much
lower quantum efficiency than BIPPA and BCPPA devices. The
quantum efficiency of NPB and TAPC devices was low due to the
large energy barrier for hole injection between TAPC and
mCPPO1 as can be seen in the current density–voltage curves.
Poor hole injection from NPB and TAPC to the emitting layer
degraded the charge balance in the emitting layer, resulting in
low quantum efficiency. In the case of NPB, low triplet energy of
NPB was also responsible for the low quantum efficiency, but it
was not so significant.
Fig. 4 Energy level diagram of deep blue PHOLEDs with triplet ener-
Fig. 6 Quantum efficiency–luminance curves of BIPPA, BCPPA, NPB
gies of each material.
and TAPC blue devices.
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the slight increase of the emission intensity of the vibrational
peak at 486 nm.
Conclusions
In conclusion, highly efficient deep blue PHOLEDs could be
developed using novel high triplet energy hole transport mate-
rials based on the fused indole core. BIPPA with indole unit was
better than BCPPA with carbazole unit in terms of hole injection/
transport and quantum efficiency. A high quantum efficiency of
24.7% could be obtained from BIPPA based deep blue
PHOLED. The fused indole core was effective as the thermally
stable high triplet energy core structure for hole transport layer
and can be useful in developing other hole transport layers and
host materials for deep blue PHOLEDs.
Fig. 7 Quantum efficiency–luminance curves of BIPPA and BCPPA
blue devices.
Notes and references
The maximum current efficiency and current efficiency at 1000
cd mꢁ2 of the BIPPA device were 30.7 cd Aꢁ1 and 27.6 cd Aꢁ1
,
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EL spectra of the BIPPA and BCPPA devices are shown in
Fig. 8. Both devices exhibited similar EL spectra with
a maximum emission peak at 458 nm and a shoulder at 486 nm.
Other than the main emission peak of the FCNIrpic dopant, no
emission peak was observed, indicating efficient charge confine-
ment inside the emitting layer without any charge leakage to the
charge transport layer. The color coordinates of the BIPPA and
BCPPA device at 1000 cd mꢁ2 were (0.137,0.182) and
(0.137,0.185), which correspond to the real deep blue color. The
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Fig. 8 Electroluminescence spectra of BIPPA and BCPPA devices.
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