Comparison of symmetric and asymmetric bipolar type high triplet energy host materials for deep blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm30742a

Symmetric and asymmetric bipolar host materials for deep blue phosphorescent organic light-emitting diodes were developed and the chemical structure of the host materials was correlated with the device performances. The bipolar host with the asymmetric molecular structure was better than the host with symmetric molecular structure in terms of driving voltage and quantum efficiency due to better charge transport properties. A high quantum efficiency of 24.5% and a high power efficiency of 31.0 lm W^-1 were achieved in the deep blue device using the asymmetric bipolar host due to balanced charge transport and low driving voltage.

Full text of DOI:10.1039/c2jm30742a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 7239
www.rsc.org/materials
PAPER
Comparison of symmetric and asymmetric bipolar type high triplet energy
host materials for deep blue phosphorescent organic light-emitting diodes
*
Soon Ok Jeon and Jun Yeob Lee
Received 7th February 2012, Accepted 9th February 2012
DOI: 10.1039/c2jm30742a
Symmetric and asymmetric bipolar host materials for deep blue phosphorescent organic light-emitting
diodes were developed and the chemical structure of the host materials was correlated with the device
performances. The bipolar host with the asymmetric molecular structure was better than the host with
symmetric molecular structure in terms of driving voltage and quantum efficiency due to better charge
transport properties. A high quantum efficiency of 24.5% and a high power efficiency of 31.0 lm Wꢀ1
were achieved in the deep blue device using the asymmetric bipolar host due to balanced charge
transport and low driving voltage.
high triplet energy host materials were also reported and high
Introduction
efficiency was obtained.18–22 Although several high triplet energy
bipolar host materials have been developed for deep blue PHO-
LEDs, there have been few systematic studies to correlate the
molecular structure of the host materials with the device perfor-
mances of deep blue PHOLEDs. The correlation of the molecular
structure with the device performances is important to design high
triplet energy host materials for high quantum efficiency in deep
blue PHOLEDs. In particular, the driving voltage of the deep blue
PHOLEDs was high due to the wide bandgap of the host material
and large energy barrier for charge injection. Therefore, it is
strongly required to develop new host materials which can lower
the driving voltage and enhance the quantum efficiency over 20%
through novel molecular design of the host materials.
The development of high efficiency deep blue phosphorescent
organic light-emitting diodes (PHOLEDs) is important because
the power consumption of organic light-emitting diodes
(OLEDs) can be greatly reduced by using the deep blue PHO-
LEDs instead of current deep blue fluorescent OLEDs. However,
the quantum efficiency of the deep blue PHOLEDs should be
improved further and the development of the materials and
device architecture is required.
There have been many studies to improve the quantum effi-
ciency of deep blue PHOLEDs and most research was focused on
developing host and dopant materials. Several deep blue dopant
materials based on carbene,1–3 pyridine triazolate4 and halogen
substituted phenylpyridine5,6 were synthesized and a deep blue
color coordinate could be obtained from those deep blue dopant
materials. Various host materials were also synthesized, which
include carbazole derivatives,7–9 silane derivatives,10,11 and phos-
phine oxide derivatives.12–17 Carbazole derivatives were generally
used as the host material for deep blue PHOLEDs, but they suffer
from poor electron injection due to strong hole transport prop-
erties of the carbzole.7 Several silane type materials were also
synthesized, but they had a problem of poor hole injection in spite
of high triplet energy.10,11 Phosphine oxide type compounds were
also effective as high triplet energy hosts for deep blue PHO-
LEDs.13–15 The merit of the phosphine oxide materials was to
improve the electron transport properties of the core structure.
Bipolar type phosphine oxide derivatives with carbazole based
core structures could improve the maximum quantum efficiency
of deep blue PHOLEDs up to the theoretical limit of quantum
efficiency.14,15 Other than these materials, several bipolar type
In this work, novel bipolar type high triplet energy host
materials with symmetric and asymmetric molecular structures
were developed and their device performances were investigated.
The symmetric and asymmetric host materials were compared in
terms of photophysical properties and device performances. It
was demonstrated that the asymmetric molecular structure is
better than the symmetric molecular structure to improve the
driving voltage and quantum efficiency of the deep blue PHO-
LEDs. A high maximum quantum efficiency of 24.5% and low
driving voltage of 6.5 V at 1000 cd mꢀ2 were achieved using the
asymmetric host material.
Experimental section
Synthesis
The synthetic scheme of the host material is shown in Scheme 1.
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; Fax: +82 31 8005 3585; Tel: +82
31 8005 3585
Synthesis of 1,3-bis(3-bromo-9H-carbazole-9-yl)benzene. 1,3-
Diiodobenzene (4.00 g, 12.1 mmol), 3-bromo-9H-carbazole
(6.56 g, 26.6 mmol), potassium carbonate (13.4 g, 96.9 mmol), Cu
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7239–7244 | 7239

View Article Online
Synthesis of 9-(3-(9H-carbazole-9-yl)phenyl-3,6-dibromo-9H-
carbazole. 9-(3-Bromophenyl)-9H-carbazole (4.75 g, 14.7 mmol),
3.6-dibromo-9H-carbazole (4.00 g, 12.3 mmol), potassium
carbonate (6.80 g, 49.2 mmol), Cu powder (1.56 g, 24.6 mmol)
and dibenzo 18-crown-6 (0.97 g, 3.69 mmol) were dissolved in
anhydrous o-dichlorobenzene under nitrogen atmosphere. The
reaction mixture was stirred for 12 h at 100 ꢁC. The mixture was
diluted with dichloromethane and washed with distilled water
(100 mL) three times. The organic layer was dried over anhy-
drous MgSO4 and evaporated in vacuo to give the crude product,
which was purified by column chromatography by n-hexane. The
final white powdery product was obtained in 90% yield.
Scheme 1 Synthetic scheme of BCPCB and CPBDC.
1
powder (3.08 g, 48.4 mmol) and dibenzo 18-crown-6 (0.64 g, 2.42
mmol) were dissolved in anhydrous o-dichlorobenzene under
nitrogen atmosphere. The reaction mixture was stirred for 12 h at
100 ꢁC. The mixture was diluted with dichloromethane and
washed with distilled water (100 mL) three times. The organic
layer was dried over anhydrous magnesium sulfate and evapo-
rated in vacuo to give the crude product, which was purified by
column chromatography by n-hexane. The final white powdery
Yield 90%. H NMR (500 MHz, CDCl3): d 8.16 (s, 2H), 8.13
(d, 2H, J ¼ 7.7 Hz), 7.82 (t, 1H, J ¼ 7.9 Hz), 7.73–7.69 (m, 2H),
7.58 (d, 1H), 7.51–7.48 (m, 4H), 7.44–7.41 (m, 2H), 7.35–7.29
(m, 4H), 13C NMR (125 MHz, CDCl3) d 140.7, 139.9, 139.8,
128.6, 131.7, 129.9, 126.7, 126.4, 125.8, 125.4, 124.4, 123.9, 123.6,
120.7, 113.7, 111.6, 109.7. MS (FAB) m/z 566 [(M + H)+]. Anal.
calcd for C18H30Br2N2: C, 63.63; H, 3.20; N, 4.95. Found: C,
63.70; H, 3.32; N, 4.94.
1
product was obtained in 90% yield. Yield: 90%. H NMR (500
MHz, CDCl3): d 8.22 (s, 2H), 8.12 (d, 2H, J ¼ 7.7 Hz), 8.05
(d, 2H, J ¼ 8.0 Hz), 7.82 (t, 2H, J ¼ 7.9 Hz), 7.68–7.62 (m, 3H),
7.53–7.22 (m, 7H). 13C NMR (125 MHz, CDCl3): d 141.1, 139.2,
131.7, 129.1, 127.1, 126.4, 126.2, 125.3, 123.5, 122.7, 121.0, 120.9,
120.6, 113.4, 110.0, 109.8. MS (FAB) m/z 566 [(M + H)+]. Anal.
calcd for C18H30Br2N2: C, 63.63; H, 3.20; N, 4.95. Found: C,
63.32; H, 3.53; N, 4.94.
Synthesis of 9-(3-(9H-carbazole-9-yl)phenyl-3,6-bis(diphenyl-
phosphoryl)-9H-carbazole (CPBDC). Into a 100 mL, two-neck
flask was placed 9-(3-(9H-carbazole-9-yl)phenyl-3,6-dibromo-
9H-carbazole (4) (2.06 g, 3.63 mmol) in tetrahydrofuran (30 mL).
The reaction flask was cooled to ꢀ78 ꢁC and n-butyllithium (2.5 M
in hexane, 3.63 mL) was added dropwise slowly. The whole solu-
tion was stirred at this temperature for 3 h, followed by addition of
a solution of chlorodiphenylphophine (2.00 g, 9.09 mmol) under
argonatmosphere. Theresultingmixturewasgraduallywarmed to
ambient temperature and quenched by methanol (10 mL). The
mixture was extracted with dichloromethane. The organic layers
were dried over magnesium sulfate, filtered, and evaporated under
reduced pressure. The white powdery product was obtained and
then a total of 9-(3-(9H-carbazol-9-yl)phenyl)-3,6-bis(diphenyl-
phosphino)-9H-carbazole, dichloromethane (20 mL), and
hydrogen peroxide (4 mL) were stirred overnight at room
temperature. The organic layer was separated and washed with
dichloromethane and water. The extract was evaporated to
dryness affording a white solid. The final product was purified by
sublimation.
Overall yield 60%. 1H NMR (500 MHz, CDCl3): d 8.50 (d 2H,
J ¼ 12.2 Hz), 8.13 (d, 2H, J ¼ 7.7 Hz), 7.87 (t, 1H, J ¼ 7.9 Hz),
7.79–7.75 (m, 4H), 7.72–7.66 (m, 8H), 7.60–7.58 (m, 2H), 7.55–
7.52 (m, 4H), 7.49–7.40 (m, 13H), 7.29 (t, 2H, J ¼ 7.4 Hz). 13C
NMR (500 MHz, CDCl3) d 143.5, 140.9, 140.2, 138.3, 133.7,
132.9, 132.6, 132.1, 130.8, 129.1, 127.5, 126.7, 126.2, 125.8, 125.0,
124.1, 123.5, 121.0, 110.6, 109.9. MS (FAB) m/z 808 [(M + H)+].
Anal. calcd for C54H38N2O2P2: C, 80.19; H, 4.74; N, 3.46.
Found: C, 80.13; H, 4.73; N, 3.40.
Synthesis of 1,3-bis(3-(diphenylphosphoryl)-9H-carbazole-9-yl)
benzene (BCPCB). Into a 100 mL, two-neck flask was placed 1,3-
bis(3-bromo-9H-carbazole-9-yl)benzene (2.00 g, 3.53 mmol) in
tetrahydrofuran (30 mL). The reaction flask was cooled to ꢀ78
ꢁC and n-butyllithium (2.5 M in hexane, 3.53 mL) was added
dropwise slowly. The whole solution was stirred at this temper-
ature for 3 h, followed by addition of a solution of chlor-
odiphenylphophine (1.94 g, 8.82 mmol) under argon atmosphere.
The resulting mixture was gradually warmed to ambient
temperature and quenched by methanol (10 mL). The mixture
was extracted with dichloromethane. The organic layers were
dried over magnesium sulfate, filtered, and evaporated under
reduced pressure. The white powdery product was obtained to
1.14 g.
A mixture of 1,3-bis(3-(diphenylphosphino)-9H-carbazol-9-yl)-
benzene, dichloromethane (20 mL), and hydrogen peroxide
(4 mL) was stirred overnight at room temperature. The organic
layer was separated and washed with dichloromethane and water.
The extract was evaporated to dryness affording a white solid.
The final product was purified by sublimation. Overall yield: 40%.
Tg 146 ꢁC. 1H NMR (500 MHz, CDCl3): d 8.65 (d, 2H,
J ¼ 12.2 Hz), 8.11 (d, 2H, J ¼ 7.8 Hz), 7.86 (t, 1H, J ¼ 7.9 Hz),
7.81–7.75 (m, 10H), 7.70–7.66 (m, 3H), 7.59–7.57 (m, 2H), 7.52–
7.50 (m, 6H), 7.46–7.43 (m, 10H), 7.31–7.28 (t, 2H, J ¼ 7.4 Hz), 13C
NMR (125 MHz, CDCl3): d 142.3, 141.0, 138.6, 133.5, 132.7,
132.0, 131.9, 131.8, 129.5, 129.4, 128.4, 128.3, 127.0, 126.4,
125.3, 125.1, 123.7, 123.5, 122.9, 122.6, 121.2, 120.8, 109.8, 109.7,
109.6. MS (FAB) m/z 808 [(M + H)+]. Anal. calcd for
C54H38N2O2P2: C, 80.19; H, 4.74; N, 3.46. Found: C, 80.25; H,
4.70; N, 3.28.
Device preparation and measurements
The device structure of blue PHOLEDs was indium tin
oxide (ITO, 150 nm)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-
amino)-phenyl]-biphenyl-4,40-diamine (DNTPD, 60 nm)/N,
N0-di(1-naphthyl)-N,N0-diphenylbenzidine (NPB, 10 nm)/N,N0-
dicarbazolyl-3,5-benzene (mCP, 10 nm)/BCPCB or CPBDC:
bis((3,5-difluoro-4-cyanophenyl)pyridine) iridium picolinate
7240 | J. Mater. Chem., 2012, 22, 7239–7244
This journal is ª The Royal Society of Chemistry 2012

View Article Online
(FCNIrpic) (30 nm, 10%)/diphenylphosphine oxide-4-(triphe-
nylsilyl)phenyl (TSPO1, 15 nm)/LiF (1 nm)/Al (200 nm). The
device was further optimized and the blue PHOLED with
poly(3,4-ethylenedioxythiophen):polystyrenesulfonate (PEDOT:
PSS) and 25 nm thick TSPO1 layer was also fabricated. The
device configurations of the hole only and electron only devices
were ITO/DNTPD (60 nm)/NPB (10 nm)/mCP (10 nm)/BCPCB
or CPBDC (30 nm)/Au (100 nm) and ITO/TSPO1 (10 nm)/
BCPCB or CPBDC (30 nm)/TSPO1 (25 nm)/LiF (1 nm)/Al (100
nm). The device performances of the blue PHOLEDs were
measured with a Keithley 2400 source measurement unit and
a CS1000 spectroradiometer.
1
The H nuclear magnetic resonance (NMR) and 13C NMR
spectra were recorded on a Varian 500 (500 MHz) spectrometer.
The photoluminescence (PL) spectra were recorded on a fluores-
cence spectrophotometer (HITACHI, F-7000) and the ultravi-
olet-visible (UV-Vis) spectra were obtained by means of a UV-Vis
spectrophotometer (Shimadzu, UV-2501PC). The differential
scanning calorimetry (DSC) measurements were performed on
Fig. 1 UV-Vis and PL spectra of BCPCB and CPBDC.
host materials were 3.00 eV and 3.01 eV from low temperature
PL spectra. The UV-Vis light is mostly absorbed by the carbazole
unit and the diphenylphosphine oxide unit did not affect the UV-
Vis absorption of the carbazole moiety. Therefore, similar UV-
Vis spectra were obtained in BCPCB and CPBDC. The PL
spectra were also similar because the PL emission was originated
from the carbazole unit. The UV-Vis and PL emissions of
BDPCB and CPBDC were quite similar to that of mCP. The
bandgap of mCP from the absorption edge of UV-Vis spectra
was 3.60 eV.
a Mettler DSC 822 under nitrogen at a heating rate of 10 ꢁC minꢀ1
.
The mass analysis was performed using a JEOL, JMS-AX505WA
spectrometer in fast atom bombardment mode. Photophysical
properties of the material were analyzed using UV-Vis and PL
spectrometer. BCPCB was dissolved in tetrahydrofuran at
a concentration of 1.0 ꢂ 10ꢀ4 M for UV-Vis and PL measure-
ments. Triplet energy analysis was carried out using the low
temperature PL measurement in liquid nitrogen. Energy levels
were measured using cyclic voltammetry (CV). The CV
measurement of organic materials was carried out in acetonitrile
The highest occupied molecular orbital (HOMO) of host
materials was measured using CV. CV curves of BCPCB and
CPBDC are shown in Fig. 2. BCPCB showed an oxidation peak
by the diphenylphosphine oxide substituted carbazole unit, while
CPBDC showed an oxidation peak by the carbazole unit without
any substituent among two carbazole groups. The introduction
of the diphenylphosphine oxide group shifted the oxidation peak
to high voltage due to strong electron withdrawing properties of
the phosphine oxide group. The oxidation peak of CPBDC was
similar to that of mCP, indicating that the oxidation is originated
from the carbazole group without phosphine oxide units. The
HOMO level of the BDPCB calculated from the onset of
oxidation peak was ꢀ6.25 eV, while that of CPBDC was
ꢀ6.13 eV. The LUMO level of the BDPCB calculated from the
HOMO and the bandgap from UV-Vis was ꢀ2.73 eV, while the
LUMO levels of CPBDC were ꢀ2.59 eV. The HOMO and
LUMO of BDPCB were lowered compared to those of mCP by
the strong electron withdrawing phosphine oxide unit. In the case
solution with tetrabutylammonium perchlorate at 0.1
M
concentration. Ag was used as the reference electrode and Pt was
the counter electrode. Organic materials were coatedon anindium
tin oxide substrate and were immersed in an electrolyte for anal-
ysis. Ferrocene was used as the standard material for the CV
measurement. Elemental analysis of the materials was carried out
using an EA1110 (CE instrument).
Results and discussion
BCPCB and CPBDC were synthesized by the reaction of
brominated carbazole with chlorodiphenylphosphine followed
by the oxidation with hydrogen peroxide (Scheme 1). BCPCB
had a symmetric molecular structure with two carbazole units
substituted with a diphenylphosphine oxide, while CPBDC had
an asymmetric molecular structure with one carbazole and one
carbazole substituted with two diphenylphosphine oxide units.
Both materials had the same molecular weight and were designed
as the bipolar host material because they have hole transporting
carbazole and electron transporting diphenylphosphine oxide
groups. However, the difference in the molecular structure may
affect the physical properties of BCPCB and CPBDC. Therefore,
the photophysical properties of BCPCB and CPBDC were
compared.
Physical properties of BCPCB and CPBDC are summarized in
Fig. 1. UV-Vis absorption, PL and energy levels were measured.
There was little difference in the UV-Vis and PL spectra between
the two materials. The bandgaps calculated from the absorption
edge of the UV-Vis absorption were 3.52 eV and 3.54 eV for
BCPCB and CPBDC, respectively. The triplet energies of the two
Fig. 2 Cyclic voltammetry curves of BCPCB and CPBDC.
J. Mater. Chem., 2012, 22, 7239–7244 | 7241
This journal is ª The Royal Society of Chemistry 2012

View Article Online
of CPBDC, the HOMO was similar to that of mCP due to
molecular orbital distribution.
Molecular orbital simulation of BCPCB and CPBDC was
carried out to correlate the molecular orbital distribution with
the HOMO and LUMO levels of BCPCB and CPBDC. The ab
initio calculations were performed using a suite of Gaussian
programs and the chemical structures of the host materials were
fully optimized by density functional theory (DFT) using Becke’s
three parameterized Lee–Yang–Parr exchange functional
(B3LYP) with 6-31G* basis sets.23 Fig. 3 shows HOMO and
LUMO distribution of BCPCB and CPBDC. The HOMO of
CPBDC was mostly distributed over the unsubstituted carba-
zole, while the LUMO was concentrated on the carbazole
substituted with two diphenylphosphine oxide groups. This
Fig. 3 Molecular orbital distribution of BCPCB and CPBDC.
Fig. 6 Current density–voltage–luminance (a), quantum efficiency–
luminance (b) and power efficiency–luminance (c) curves of BCPCB and
CPBDC devices.
Fig. 4 Current density–voltage curves of the hole only and electron only
devices.
Fig. 5 Energy level diagram of the blue devices.
7242 | J. Mater. Chem., 2012, 22, 7239–7244
Fig. 7 Electroluminescence spectra of BCPCB and CPBDC devices.
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Table 1 Device performances of BCPCB and CPBDC blue devices
Driving
voltagea/nm
Maximum quantum
efficiency (%)
Quantum
efficiencya (%)
Maximum power
efficiency/lm Wꢀ1
Power
efficiencya/lm Wꢀ1
DNTPD–BCPCB
DNTPD–CPBDC
PEDOT–CPBDC
DNTPD–mCP
8.0
7.2
6.5
6.7
19.7
17.1
24.5
9.5
14.3
10.3
19.4
8.5
23.4
21.6
31.0
10.0
8.3
6.1
13.4
6.1
a
Data were measured at 1000 cd mꢀ2
.
indicates that the HOMO is dominated by the carbazole, while
the LUMO is dominated by the diphenylphosphine oxide
substituted carbazole. Compared with CPBDC, BCPCB showed
simple orbital distribution as it has symmetric molecular struc-
ture. Both HOMO and LUMO were dispersed over the phos-
phine oxide substituted carbazole. The HOMO distribution of
CPBDC explains the similar HOMO level to that of mCP. As the
HOMO was localized on the same carbazole unit, there was little
difference in HOMO levels between CPBDC and mCP.
Deep blue PHOLEDs were fabricated using BCPCB and
CPBDC as the host materials. Fig. 6 shows device performances
of the deep blue PHOLED with BCPCB and CPBDC host
materials. The current density of the CPBDC device was higher
than that of the BCPCB device. As shown in the hole only and
electron only device data, the high hole and electron current
density of the CPBDC device enhanced the current density of the
device because of efficient hole and electron injection. The
luminance was also increased due to more charge injection.
However, the quantum efficiency was high in the BCPCB device.
The efficiency is determined by the charge balance in the emitting
layer and the BCPCB device was better than CPBDC in terms of
recombination efficiency. Although CPBDC showed high
current density, the electron density of the CPBDC device was
much higher than the hole density, resulting in rather poor
charge balance in the emitting layer.
The difference in the HOMO and LUMO levels between
BCPCB and CPBDC may affect the hole and electron injection
in the device. Therefore, hole only and electron only devices of
blue PHOLEDs were fabricated. Fig. 4 shows current density–
voltage curves of the hole only and electron only devices of
BCPCB and CPBDC. The two host materials were designed as
the bipolar host materials with two diphenylphosphine oxide
groups in the molecular structure. However, both hole and
electron current densities of CPBDC were higher than those of
BCPCB. The high hole and electron current density of CPBDC
can be explained by the HOMO and LUMO distribution of
CPBDC. The hole current density is related to the HOMO level
of the material. The HOMO level of CPBDC is more suitable for
hole injection than that of BCPCB due to the HOMO level
difference of 0.12 eV between BCPCB and CPBDC. In addition,
the HOMO is mostly dispersed over the carbazole unit with good
hole transport properties. The hole transport of BCPCB is
dominated by the phosphine oxide substituted carbazole unit
which is worse than the carbazole in terms of hole transport.
Therefore, CPBDC showed higher hole current density. Simi-
larly, the LUMO level and electron transport of CPBDC are
determined by the carbazole unit substituted with two phosphine
oxide groups compared with the carbazole unit substituted with
one phosphine oxide group of BCPCB. The additional phos-
phine oxide group of CPBDC enhances the electron current
density of CPBDC. The asymmetrically designed CPBDC
showed good hole transport properties due to the carbazole
group and good electron transport properties owing to the
carbazole group substituted with two diphenylphosphine oxide
units. The asymmetric design was better than the symmetric
design in terms of hole and electron transport properties.
Compared with mCP with good hole transport and poor electron
transport properties, the two host materials showed higher
electron current density and lower hole current density. This
proves that the phosphine oxide substituents improved the
electron transport properties of BDPCB and CPBDC. Therefore,
BDPCB and CPBDC were more suitable as the bipolar host
material than mCP. The energy level diagram of the materials is
shown in Fig. 5.
Therefore, the device structure of the CPBDC device was
further optimized. PEDOT:PSS was used instead of DNTPD
and the thickness of the electron transport layer was changed
from 15 nm to 25 nm. The quantum efficiency was greatly
improved by optimizing the device structure and a maximum
quantum efficiency of 24.5% was obtained from the CPBDC
device. The quantum efficiency was not degraded even at high
luminance and a quantum efficiency of 19.4% was achieved at
1000 cd mꢀ2. The power efficiency was also high in the CPBDC
device and the maximum power efficiency and power efficiency at
1000 cd mꢀ2 were 31.0 lm Wꢀ1 and 13.4 lm Wꢀ1. The low driving
voltage and high quantum efficiency yielded high power effi-
ciency in the CPBDC device with the PEDOT:PSS hole injection
layer. The quantum and power efficiencies of CPBDC and
BDPCB devices were much higher than those of the mCP device.
This is due to bipolar charge transport properties of CPBDC and
BDPCB.
The electroluminescence spectra are shown in Fig. 7 and the
peak maximum of the spectra was 462 nm. The color coordinate
of the deep blue PHOLEDs was (0.14, 0.19). All device data of
deep blue PHOLEDs are summarized in Table 1.
Conclusions
In conclusion, symmetric and asymmetric bipolar host materials
were systematically compared as the host materials for deep blue
PHOLEDs. The asymmetric design was better than the
symmetric design in terms of device performances. The driving
voltage could be lowered due to efficient hole and electron
injection and high quantum efficiency could be achieved. A high
quantum efficiency of 24.5% and power efficiency of 31.0 lm Wꢀ1
could be obtained from deep blue PHOLEDs with the CPBDC
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7239–7244 | 7243

View Article Online
13 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Funct. Mater.,
2009, 19, 3644.
14 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Mater., 2010,
22, 1872.
high triplet energy host material. The asymmetric molecular
design can be useful to improve the driving voltage and quantum
efficiency in phosphorescent devices.
15 S. O. Jeon, S. E. Jang, H. S. Son and J. Y. Lee, Adv. Mater., 2011, 23,
1436.
16 L. S. Sapochak, A. B. Padmaperuma, X. Cai, J. L. Male and
P. E. Burrows, J. Phys. Chem. C, 2008, 112, 7989.
17 P. K. Koech, E. Polikarpov, J. E. Rainbolt, L. Cosimbescu,
J. S. Swensen, A. L. Von Ruden and A. B. Padmaperuma, Org.
Lett., 2010, 12, 5534.
18 M. Rothmann, S. Haneder, E. D. Como, C. Lennartz,
C. Schildknecht and P. Strohriegl, Chem. Mater., 2010, 22, 2403.
19 K. Son, M. Yahiro, T. Imai, H. Yoshizaki and C. Adachi, Chem.
Mater., 2008, 20, 4439.
20 S.-J. Su, H. Sasabe, T. Takeda and J. Kido, Chem. Mater., 2008, 20,
1691.
21 J. Ding, Q. Wang, L. Zhao, D. Ma, L. Wang, X. Jing and F. Wang, J.
Mater. Chem., 2010, 20, 8126.
22 F.-M. Hsu, C.-H. Chien, P.-I. Shih and C.-F. Shu, Chem. Mater.,
2009, 21, 1017.
Notes and references
1 T. Sajoto, P. I. Djurovich, A. Tamayo, M. Yousufuddin, R. Bau,
M. E. Thompson, R. J. Holmes and S. R. Forrest, Inorg. Chem.,
2005, 44, 7992–8003.
2 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 and W. Kowalsky, Dig. Tech. Pap. - Soc. Inf. Disp.
Int. Symp., 2006, 37, 131–134.
3 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 and J. J. Brown, Appl. Phys. Lett.,
2005, 87, 243507.
4 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 and C.-C. Wu, Angew. Chem.,
Int. Ed., 2008, 47, 4542–4545.
23 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven, Jr,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,
Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03,
revision B05, Gaussian, Inc., Pittsburgh, PA, 2003.
5 S. H. Kim, J. Jang, S. J. Lee and J. Y. Lee, Thin Solid Films, 2008, 517,
722–726.
6 K. S. Yook, S. O. Jeon, C. W. Joo and J. Y. Lee, Org. Electron., 2009,
10, 170–173.
7 R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, J. J. Brown,
S. Garon and M. E. Thompson, Appl. Phys. Lett., 2003, 82, 2422–
2424.
8 M. H. Tsai, H. W. Lin, H. C. Su, T. H. Ke, C. C. Wu, F. C. Fang,
Y. L. Liao, K. T. Wong and C. I. Wu, Adv. Mater., 2006, 18, 1216–
1220.
9 M. F. Wu, S. J. Yeh, C. T. Chen, H. Murayama, T. Tsuboi, W. Li,
I. Chao, S. Liu and J. K. Wang, Adv. Funct. Mater., 2007, 17,
1887–1895.
10 R. J. Holmes, B. W. D0Andrade, S. R. Forrest, X. Ren, J. Li and
M. E. Thompson, Appl. Phys. Lett., 2003, 83, 3818–3820.
11 X. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest and
M. E. Thompson, Chem. Mater., 2004, 16, 4743–4747.
12 A. B. Padmaperuma, L. S. Sapochak and P. E. Burrows, Chem.
Mater., 2006, 18, 2389.
7244 | J. Mater. Chem., 2012, 22, 7239–7244
This journal is ª The Royal Society of Chemistry 2012