Efficient phosphorescent green and red organic light-emitting diodes based on the novel carbazole-type host material

Article abstract of DOI:10.1166/jnn.2011.3370

We developed a novel carbazole-type material, 9-phenyl-3,6-bis(4-(1-phenyl- 1H-benzo[d]imidazol-2-yl)phenyl)-9H-carbazole (LPGH 153), and fabricated the green and red phosphorescent organic light-emitting diodes (OLEDs) using LPGH 153 as host. The red and green devices have the max. luminous efficiencies of 22.2 cd/A and 32.2 cd/A, respectively. Copyright

Full text of DOI:10.1166/jnn.2011.3370

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Journal of
Nanoscience and Nanotechnology
Vol. 11, 1373–1376, 2011
Efficient Phosphorescent Green and Red Organic
Light-Emitting Diodes Based on the Novel
Carbazole-Type Host Material
Ji Hyun Seo1, Hoe Min Kim1, Eun Young Choi1, Dae Hyuk Choi2, Jung Hwan Park2,
Kum Hee Lee3, Seung Soo Yoon3ꢀ∗, and Young Kwan Kim1ꢀ∗
1Department of Information Display, Hongik University, Seoul 121-791, Korea
2Corporate R&D center, Duksan Hi-Metal Co., Ltd., Seongnam 463-420, Korea
3Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
We developed a novel carbazole-type material, 9-phenyl-3,6-bis(4-(1-phenyl-1H-benzo[d]imidazol-
2-yl)phenyl)-9H-carbazole (LPGH 153), and fabricated the green and red phosphorescent organic
light-emitting diodes (OLEDs) using LPGH 153 as host. The red and green devices have the max.
luminous efficiencies of 22.2 cd/A and 32.2 cd/A, respectively.
Keywords: Carbazole-Type Host, Organic Light-Emitting Diode, Green, Red.
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was used as a host for phosphorescent device and high
1. INTRODUCTION
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efficiency, stable lifetime and low driving voltage were
Copyright: American Scientific Publishers
observed.10 Recently, Kanno et al. reported Zn based
organometallic complexes as host materials to reduce
the driving voltage of OLEDs.11 In this work, a novel
carbazole-type compound, 9-phenyl-3,6-bis(4-(1-phenyl-
1H-benzo[d]imidazol-2-yl)phenyl)-9H-carbazole (LPGH
153), was developed and the device performances of the
red and green phosphorescent OLED with the LPGH 153
was investigated.
In the last decade, efforts have been devoted to the
development of materials useful for the fabrication of
organic light-emitting diodes (OLEDs).1–2 The OLED
performance was found to depend not only on the lumi-
nescence efficiency of the emissive materials but equally
importantly also on the optical and semiconductor char-
acteristics of hosts and charge-transporting materials.
Because of the multiplicity of factors influencing these
features, molecular design of such materials is fairly com-
plicated as it requires optimizing properties that include
the highest occupied molecular orbital (HOMO)—the
lowest unoccupied molecular orbital (LUMO) levels,
triplet excited states energy levels, film forming behavior,
thermal stability, and suitable emission wavelength. Phos-
phorescent OLEDs employing phosphorescent emitters
doped into a proper charge transporting host material
have attracted significant attention because of their high
efficiencies as a consequence of the utilization of both
singlet and triplet excitons.3 In most studies, the phos-
phorescent dopant materials were doped into a common
4,4ꢀ-bis(N-carbazolyl)biphenyl (CBP) host material.4–9
However, the driving voltage of the CBP based device was
rather high and the quantum efficiency was not improved
in the CBP based devices. In addition to CBP host,
bis(2-methyl-8-quinolinolato)(biphenyl-4-olato)aluminium
2. EXPERIMENTAL DETAILS
2.1. Synthesis of LPGH 153
2.1.1. Synthesis of 3,6-Dibromo-9H-Carbazole (1)
Carbazole (15 g, 90 mmol) was dissolved in anhydrous
CH2Cl2 (250 mL) and added of N-bromosuccinimide
(NBS, 48 g, 270 mmol) at room temperature under
nitrogen atmosphere. The mixture was stirred at room tem-
perature for 6 h. After the reaction was done, the reac-
tion mixture was quenched by addition of water. The
crude was extraction with CH2Cl2 and washed with water.
The combined organic layer was dried over anhydrous
MgSO4, filtered and concentrated under reduced pressure.
The residue was purified by recrystallization with CH2Cl2
and hexane to obtain pure 1 (24.7 g, 85%) shown in
1
Scheme 1. H NMR (400 MHz, CDCl3ꢀ: ꢁ (ppm) 7.38 (d,
∗Authors to whom correspondence should be addressed.
2H), 7.57 (dd, 2H), 8.12 (d, 2H), 8.31 (s, 1H, NH).
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 2
1533-4880/2011/11/1373/004
doi:10.1166/jnn.2011.3370
1373

Efficient Phosphorescent Green and Red OLEDs Based on the Novel Carbazole-Type Host Material
Seo et al.
Br
2.2. Device Fabrication
Br
Br
Br
I
NBS
N
OLEDs were fabricated by high vacuum (5 × 10−7 torr)
thermal evaporation of organic materials onto the sur-
face of an indium tin oxide (ITO) coated glass sub-
strate with resistance of 30 ꢂ/sq. The ITO glass
was cleaned with acetone, methanol, distillated water,
and isopropyl alcohol. And then the ITO was treated
by O2-plasma with power of 125 W for 2 minutes.
The device configuration used in this work was ITO
(150 nm)/4,4ꢀ,4ꢀꢀ-tris[2-naphthylphenylamino]triphenyla-
mine (2-TNATA) (60 nm)/4,4ꢀ-bis[N-(naphthyl)-N-phenyl-
amino]biphenyl (NPB) (50 nm)/10% Ir complex doped
LPGH 153 (30 nm)/bathocuproine (BCP) (20 nm)/tris-
(8-hydroxyquinoline) aluminum (Alq3ꢀ (30 nm)/lithium
quinolate (Liq) (2 nm)/aluminum (Al) (100 nm). The irid-
ium(III) tris(2-phenylpyridine) [Ir(ppy)3] and iridium(III)
bis(2-phenylquinoline) acetylacetonate [Ir(pq)2(acac)]
were used as phosphorescent green and red emitter,
respectively. After the devices fabrication, the current
density–voltage (J–V ) characteristics of the OLEDs were
measured with a source measure unit (Kiethley 236).
The luminance and CIE chromaticity coordinates of the
fabricated devices were measured using a chromameter
(MINOLTA CS-1000).
N
H
N
H
CH2Cl2
Pd2(dba)3/P(t-Bu)3
NaOtBu
1
THF/H2O
2
N
N
N
N
N
N
(HO)2B
Pd(PPh3)4/K2CO3
Toluene
N
3
Scheme 1. Synthesis process of the LPGH 153.
2.1.2. Synthesis of 3,6-Dibromo-9-Phenyl-
9H-Carbazole (2)
Iodobenzene (13.8 g, 67.7 mmol), Pd2(dba)3 (2.8 g,
3.1 mmol) and P(t-Bu)3 (1.5 mL, 6.2 mmol) were added
in THF (150 mL). Then, NaOtBu (14.8 g, 153.8 mmol)
was dissolved in THF (50 mL) and water (50 mL), which
was added as well. A solution of 1 (20 g, 61.5 mmol)
in THF (50 mL) was slowly dropped into the reaction
mixture and then refluxed for 12 h. After cooled to
room temperature, the reaction mixture was filtered by
Celite and extracted with CH2Cl2 and washed with water.
The combined organic layer was dried over anhydrous
MgSO , filtered and concentrated under reduced pressure.
The residue was purified by column chromatography on
CH2Cl2: hexane/3:1 as eluent solvents to obtain pure 2
3. RESULTS AND DISCUSSION
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HOMO level of LPGH 153 was measured from cyclic
voltammetry (CV) and the oxidation-reduction reaction as
potential variation is shown in Figure 1. The HOMO level
can be calculated from first oxidation peak in CV graph
and electron energy level of reference electrode.12 As a
result, HOMO level of 5.9 eV was obtained. On the other
hand, the band gap (Egꢀ of LPGH 153 was measured from
absorption edge of UV/Vis. spectrum shown in inset of
Figure 1 and it was 3.2 eV. Therefore, the LUMO level
of 2.7 eV was obtained from difference between HOMO
and Eg of LGPH 153. Figure 2 shows the energy band
diagram of device with LPGH 153. The LPGH 153 is suit-
able for exciton energy transfer from the LPGH 153 to red
and green emitting Ir(pq)2(acac) and Ir(ppy)3 with the Eg
Copyright: American Scientific Publishers
4
1
(21 g, 75%) shown in Scheme 1. H NMR (400 MHz,
CDCl3ꢀ: ꢁ (ppm) 7.30 (d, 2H), 7.45 (m, 1H), 7.50 (d, 2H),
7.51 (dd, 2H), 7.58 (m, 2H), 8.07 (d, 2H).
2.1.3. Synthesis of 9-Phenyl-3,6-Bis(4-(1-Phenyl-1H-
Benzo[d]imidazol-2-yl)Phenyl)-9H-Carbazole (3)
Iodobenzene (13.8 g, 67.7 mmol), Pd2(dba)3 (2.8 g,
3.1 mmol) and P(t-Bu 4-(1-Phenyl-1H-benzo[d]imidazol-
2-yl)phenyl boronic acid (26.1 g, 83.1 mmol), Pd(PPh3ꢀ4
(1.9 g, 1.7 mmol), K2CO3 (11.5 g, 83.1 mmol) and 2
(25.9 g, 33.2 mmol) were successively added in toluene
(200 mL) and then refluxed for 12 h. After cooled to room
temperature, the reaction mixture was filtered by Celite
and washed with hot toluene. After removed of organic
solvent, the residue was purified by recrystallization with
CH2Cl2 and hexane to obtain pure 3 (14.6 g, 53%) shown
in Scheme 1. 1H NMR (400 MHz, CDCl3ꢀ: ꢁ (ppm) 7.25–
7.27 (m, 5H), 7.33–7.40 (m, 5H), 7.48–7.61 (m, 7H), 7.65
(d, 1H), 7.67 (d, 1H), 7.69 (m, 8H), 7.92 (m, 3H), 7.97–
8.00 (m, 1H), 8.05 (d, 1H), 8.11 (d, 1H), 8.41 (d, 2H). 13C
NMR (100 MHz, CDCl3ꢀ: ꢁ (ppm) 110.35, 110.42, 118.88,
119.78, 123.00, 123.27, 124.08, 125.62, 126.92, 127.01,
127.54, 127.98, 128.08, 128.63, 129.87, 129.96, 130.05,
132.57, 137.17, 137.39, 141.23, 143.10, 152.25.
Fig. 1. Cyclic voltammetry (inset; UV/Vis. spectrum) of LPGH 153.
J. Nanosci. Nanotechnol. 11, 1373–1376, 2011
1374

Seo et al.
Efficient Phosphorescent Green and Red OLEDs Based on the Novel Carbazole-Type Host Material
is relatively high as 0.4 eV. In this case, holes prefer to be
directly injected into HOMO of dopant rather than host.
The hole injection from NPB into Ir(ppy)3 with hole injec-
tion barrier of 0.1 eV is easier than that from NPB into
Ir(pq)2(acac) with hole injection barrier of 0.3 eV. There-
fore, operating voltage of green device is slightly lower
than that of red device. Moreover, we compared the current
density property of CBP and LPGH 153 in green device,
as shown in Figure 3 and inset of Figure 3. The current
density of green device with LPGH 153 is very high com-
pared with that with CBP because of the efficient electron
Fig. 2. Energy level diagram of the LPGH 153 device.
transport ability of LPGH 153 as confirmed in electron
only device. It can improve the charge balance of hole and
electron in organic semiconductor that electron mobility is
lower than hole mobility. The leakage current of device
with LPGH 153 is very low as shown in inset of Figure 3.
The leakage current of green device with CBP was located
between 10−5 mA/cm2 and 10−6 mA/cm2. On the other
hand, the leakage current of green device with LPGH 153
was located between 10−6 mA/cm2 and 10−7 mA/cm2,
implying the amorphous LPGH 153 film with a surface
roughness less than 1 nm is relatively free from the for-
mation of pin hole as comparison with CBP film under the
driving of device.
of 2.2 eV and 2.6 eV, respectively. Moreover, the HOMO
and LUMO levels of LPGH 153 are suitable for injection
of hole and electron and the carbazole unit is appropri-
ate for balancing holes and electrons in the emitting layer.
The electron transport property of the LPGH 153 was con-
firmed by the electron only devices and the LPGH 153
electron only device showed higher current density than
the common CBP electron only device. In addition, the
LPGH 153 host material forms an amorphous film with a
surface roughness less than 1 nm due to its bulky molec-
ular structure. Therefore, we expected that the LPGH 153
can be used as a host material for red and green phospho-
rescent OLEDs.
The luminous efficiency and external quantum efficiency
of red and green devices with LPGH 153 are shown in
Figure 4 and inset of Figure 4, respectively. The maxi-
Figure 3 shows the current density–voltage curves of
the red and green phosphorescent OLEDs with LPGH 153
host. The current density flow of red and green devices is
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mum luminous efficiencies are obtained as 22.2 cd/A and
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Copyright: American S3c2ie.2nticfidc/APufbolirshtheersred and green devices, respectively.
And the red and green devices have the maximum exter-
nal quantum efficiencies of 14.1% and 9.3%, respectively.
The different tendency between luminous efficiency and
external quantum efficiency results is because that the
luminous efficiency is regardless of the eye sensitivity of
human, while the external quantum efficiency is associated
with that. As a result, the absolute efficiency of LPGH
153 regardless of human eye sensitivity is superior in red
similar, but the current density of green device is slightly
higher than that of red device. It is because that the HOMO
energy levels of Ir(pq)2(acac) and Ir(ppy)3 are different,
although device structure of red and green devices and the
LUMO energy level of Ir(ppy)3 and Ir(pq)2(acac) are same.
The hole injection barrier from NPB into emission layer
125
100
10
1
0.1
Green device
100
75
50
25
0
0.01
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
LPGH 153 host device
CBP
LPGH153
10
LPGH 153 host device
–6 –4 –2
0
2
4
6
8 10 12 14 16 18
10
Voltage [v]
Green device
Red device
CBP in green device
LPGH 153 in green device
LPGH 153 in red device
Green device
Red device
1
0
500 100015002000250030003500400045005000
Luminance [cd/m2]
0
2
4
6
8
10
12
14
16
Voltage [v]
1
0
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Luminance [cd/m2]
Fig. 3. Current density–voltage characteristics of red and green phos-
phorescent OLEDs with LPGH 153. The current density of green device
with LPGH 153 was also compared with that of the green device with
CBP host. (inset; log-scale).
Fig. 4. Luminous efficiency (inset; external quantum efficiency) of red
and green phosphorescent OLEDs with LPGH 153.
J. Nanosci. Nanotechnol. 11, 1373–1376, 2011
1375

Efficient Phosphorescent Green and Red OLEDs Based on the Novel Carbazole-Type Host Material
Seo et al.
1.2
efficiencies 22.2 cd/A and 32.2 cd/A were obtained in red
and green devices, respectively, due to a wide HOMO-
LUMO bandgap of 3.2 eV for efficient energy transfer and
a charge balance in the light-emitting layer. The LPGH
153 also showed a smooth surface morphology with an
average surface roughness less than 1 nm and leaded to
few leakage current compared with CBP. Therefore, the
LPGH 153 is promising as a host in red and green phos-
phorescent OLEDs.
Green device
Red device
LPGH 153 host device
1.0
0.8
0.6
0.4
0.2
0.0
Acknowledgment: This work was supported by Energy
Resources Technology Development program (No. 2007-
E-CM11-P-07) and Strategy Technology Development
program (No. 10030834) from Ministry of Knowledge
Economy (MKE). Moreover, this research was also sup-
ported by the ERC program of the Korea Science and
Engineering Foundation (KOSEF) grant funded by the
Korea Ministry of Education, Science and Technology
(MEST) (No. R11-2007-045-03001-0).
400
500
600
700
Wavelength [nm]
Fig. 5. Normalized EL spectra of red and green phosphorescent OLEDs
with LPGH 153 as a host at applied voltage of 8 V.
device rather than in green device. It is because the energy
band of Ir(pq)2(acac) is relatively more narrow than that
of Ir(ppy)3. Therefore, the energy transfer from LPGH 153
into Ir(pq)2(acac) is more efficient than that from LPGH
153 into Ir(ppy)3 and the leakage of excitons formed in
Ir(pq)2(acac) can be relatively suppressed as comparison
with that in Ir(ppy)3.
The electroluminescence (EL) spectra of red and green
devices with LPGH 153 are shown in Figure 5. The
red and green devices showed typical EL spectra of
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Copyright: American Scientific Publishers
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and 523 nm for the red and green devices, respectively.
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4. CONCLUSION
In summary, a novel carbazole-type LPGH 153 is effective
as red and green phosphorescent host. A high luminous
Received: 1 November 2009. Accepted: 1 April 2010.
1376
J. Nanosci. Nanotechnol. 11, 1373–1376, 2011