Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode

Article abstract of DOI:10.1016/j.dyepig.2013.09.046

We report series of triphenylene derivatives with good electronic properties for the use as host materials for green phosphorescent organic light emitting diodes (PHOLEDs). We applied highly planar triphenylene moieties which could provide good electron t

Full text of DOI:10.1016/j.dyepig.2013.09.046

Dyes and Pigments 101 (2014) 221e228
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Triphenylene containing host materials with high thermal stability for
green phosphorescent organic light emitting diode
,
,
,
a,
Nam-Jin Lee b 1, Joon Ho Jeon a 1, Insik In b, Ji-Hoon Lee b , Min Chul Suh
*
**
a Department of Information Display and Advanced Display Research Center, Kyung Hee University, Dongdaemoon-Gu, Seoul 130-701, Republic of Korea
b Department of Polymer Science & Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju-Si, Chungbuk 380-702,
Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We report series of triphenylene derivatives with good electronic properties for the use as host materials
for green phosphorescent organic light emitting diodes (PHOLEDs). We applied highly planar triphe-
nylene moieties which could provide good electron transport ability and the carbazole or dibenzothio-
phene moieties which could provide good hole transport ability to obtain a bipolar host materials for
green PHOLEDs. From this approach we achieved relatively good current efficiency up to 64 cd/A and
external quantum efficiency up to 20.3%, respectively. This was the much more improved value (by
w22.3%) compare to that obtained from the 4,40-N,N0-dicarbazolebiphenyl system as a reference.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 21 June 2013
Received in revised form
23 August 2013
Accepted 30 September 2013
Available online 12 October 2013
Keywords:
Charge balance
Host materials
Green phosphorescent OLED
Triphenylene
Carbazole
Dibenzothiophene
1. Introduction
type host materials could change the resultant device characteris-
tics. For this purpose, manyachievements have been reported in this
Phosphorescent organic light-emitting diodes (PHOLEDs) have
attracted intense interest because of their merit of high quantum
efficiency as compared to conventional fluorescent OLEDs through
utilizing both singlet and triplet excitons for emission [1e4]. In fact,
red phosphorescent materials have already been applied in the
main-display of commercial mobile phones since 2007. Recently,
green emission is almost approaching tothe theoretical limitation of
efficiency (>20% of external quantum efficiency, EQE) by utilizing
various dopants such as fac-tris(2-phenylpyridinato)iridium(III)
[Ir(ppy)3] as the phosphorescent emitter, and the commercialization
of which has been successfully started [5e9]. However, compared
with the great achievements in red phosphorescent materials, green
component has many obstacles against commercialization
including its relatively short lifetime issue. To overcome such issue,
the mixed host system is widely utilized [10e12]. However, many
research groups are concentrating on preparation of the host ma-
terials which possess bipolar characteristics because the minute
change of mixing ratio of hole transport type and electron transport
field in recent years [2,13,14]. Especially, the host materials with hole
transport type functional groups such as carbazoles or triarylamine
moieties combined with electron transport type functional groups
such as heterocyclic moieties such as pyridine, triazine, benzimid-
azole showed great progress in the device performances [15,16]. Aryl
silanes [17,18] and phosphine oxides [19e21] have also been utilized
to obtain bipolar properties as electron transport type functional
groups [22]. As a part of such an effort, the efficiency greater than
20% (EQE) was frequently reported from the green PHOLEDs with
such kinds of bipolar host materials [23,24].
In this study, we report three different types of new host ma-
terials for green PHOLEDs having triplet energy greater than 2.5 eV
with highly planar triphenylene moieties to improve their electron
transport ability. The resultant PHOLEDs containing such moieties
showed moderate EQE up to 20.3%.
2. Results and discussion
2.1. Materials
* Corresponding author. Tel.: þ82 43 841 5427; fax: þ82 43 841 5420.
** Corresponding author. Tel.: þ82 2 961 0694; fax: þ82 2 968 6924.
E-mail addresses: jihoonli@ut.ac.kr (J.-H. Lee), mcsuh@khu.ac.kr (M.C. Suh).
We prepared three host materials to realize a highly efficient
green PHOLED. We selected triphenylene moiety as a basic
1
Both authors were equally contributed as a first author to this work.
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.046

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N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
functionality because they are renowned as discotic liquid crys-
talline molecules for their potential in one-dimensional charge
transporting properties. Conductivity along the columns in
columnar mesophases has been reported to be several orders of
magnitude greater than that in the perpendicular direction. The
triphenylene and its derivatives have been known as hole trans-
porting materials although it could also be used as an electron
transporting functionality [25] Their photoinduced charge-carrier
mobilities ranged from 10ꢀ5 cm2 Vꢀ1 sꢀ1 in the isotropic phase
[26] to 10ꢀ2 cm2 Vꢀ1 sꢀ1 in highly ordered discotic mesophases [27]
Meanwhile, we changed the numbers of triphenylene moieties
which are connected to the central core unit (1,3,5-trisubstituted
benzene) which also contains a hole transporting functional
groups such as carbazole and/or dibenzothiophene units. In prin-
ciple, carbazole containing host materials exhibit high triplet en-
energy level as a green host might be the disruption of p-conju-
gation just as in the case of 9,90-(1,3-Phenylene)bis-9H-carbazole
which has two carbazole units connected meta position of a single
benzene ring. Scheme 1 shows the schematic diagram of synthetic
route to prepare such compounds. Fig. 1 shows the geometries of
core parts of those three resultant host materials obtained from the
simulation at a DNP/GGA(PBE) level of theory using Dmol3 module
(Material Studio 6.1, Accelrys software). The triphenylene moieties
were rather twisted to the core benzene unit which could arouse a
broken conjugation as we expected and we could utilize those
materials as a green host materials (T1 > 2.6 eV). Besides, the mo-
lecular orbital distribution of 9,90-(5-(triphenylen-2-yl)-1,3-
phenylene)bis(9H-carbazole) (TP-mCP), 9-(3,5-di(triphenylen-2-
yl)phenyl)-9H-carbazole (DTP-mCP), and 4-(3-(triphenylen-2-yl)
phenyl)dibenzo[b,d]thiophene (TP-PDBTh) are also shown in Fig. 1.
As we expected from the molecular structure, the electrons in
highest occupied molecular orbital (HOMO) was localized on the
ergies if they have no extended
p-conjugation length [28,29]. In
other words, the most common approach to increase a triplet
Scheme 1. Synthesis of green host materials.

N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
223
Fig. 1. Geometries of three host materials having twisted core structures.
carbazole and/or dibenzothiophene units, while the electrons in
2.2. Device characteristics
lowest unoccupied molecular orbital (LUMO) was dispersed over
the triphenylene and benzene core units. In other words, the hole
carriers injected from the hole transport layer (HTL) could move
through the carbazole and/or dibenzothiophene units, while the
electrons injected from the electron transport layer (ETL) could be
transported through the triphenylene and/or central benzene
moieties.
Fig. 2(a) shows the ultravioletevisible (UVeVis) absorption and
photoluminescence spectra of new host materials in chloroform
(CHCl3). The band gap (Eg) of TP-PDBTh was slightly larger than
those of TP-mCP and DTP-mCP (TP-PDBTh: 3.63 eV, TP-mCP:
3.57 eV, DTP-mCP: 3.49 eV). All three compounds showed
vibronic fine structures in their emission spectra, especially at the
low temperature (77 K) as shown in Fig. 2(b). The triplet energies
(T1) of new host materials which were determined by the first
phosphorescence peak from the shorter wavelength region were
2.63, 2.63, 2.64 eV for TP-mCP, DTP-mCP, and TP-PDBTh, respec-
tively, as marked in the spectra as shown in Fig. 2(b). The repre-
sentative results are summarized in Table 1. The energy levels of
HOMO and LUMO were collected from cyclic voltammetry and
band edges of UVevisible absorption spectra and they were also
summarized in Table 1.
Bipolar character of host materials with proper triplet energy
level is very important for the remarkable enhancement of effi-
ciency upon doping. To investigate the bipolar characteristics, we
prepared the hole only devices (HODs) as well as electron only
devices (EODs) [30e32] of 4,4-N,N0-Dicarbazole-1,10-biphenyl (CBP)
and newly synthesized host materials by using molybdenum oxide
(MoO3) and lithium quinolate (LiQ) as charge carrier injection
layers from indium tin oxide (ITO) and aluminum (Al) as follows:
HOD A: ITO/MoO3 (0.75 nm)/CBP (200 nm)/MoO3 (10 nm)/Al
(100 nm)
HOD B: ITO/MoO3 (0.75 nm)/TP-mCP(200 nm)/MoO3 (10 nm)/Al
(100 nm)
HOD C: ITO/MoO3 (0.75 nm)/DTP-mCP (200 nm)/MoO3 (10 nm)/
Al (100 nm)
HOD D: ITO/MoO3 (0.75 nm)/TP-PDBTh(200 nm)/MoO3
(10 nm)/Al (100 nm)
EOD A: Al (50 nm)/LiQ (1.5 nm)/CBP(200 nm)/LiQ (1.5 nm)/Al
(100 nm)
EOD B: Al (50 nm)/LiQ (1.5 nm)/TP-mCP (200 nm)/LiQ (1.5 nm)/
Al (100 nm)
Fig. 3 shows the thermal stability behavior of the materials
synthesized in this study. All of the compounds having tripheny-
lene moieties showed outstanding thermal stability up to 430 ꢁC.
The decomposition temperature (Td) of TP-mCP, DTP-mCP, and TP-
PDBTh were 436, 488, and 445 ꢁC, respectively. Very interestingly,
the glass transition temperatures (Tg) of TP-mCP and DTP-mCP with
carbazole moieties were higher (159 and 179 ꢁC) than that of TP-
PDBTh with dibenzothiophene moiety (108 ꢁC). Besides, the
melting temperature (Tm) and crystallization temperature (Tc) of
the materials were also summarized in Table 1.
EOD C: Al (50 nm)/LiQ (1.5 nm)/DTP-mCP(200 nm)/LiQ (1.5 nm)/
Al (100 nm)
EOD D: Al (50 nm)/LiQ (1.5 nm)/TP-PDBTh (200 nm)/LiQ
(1.5 nm)/Al (100 nm)
Fig. 4(a) and (b) shows the energy band diagram of HODs and
EODs of host materials. For the effective ohmic injection of hole
carriers into the HOMO of new host materials, we utilized 0.75 nm
of MoO3 as a charge injection layer [30]. Meanwhile, we used LiQ/Al
to inject the electrons into the LUMO of new host materials

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N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
Fig. 3. TGA of newly synthesized host materials.
could expect that TP-mCP might show best efficiency value. Then
the efficiency could be decreased in order of increasing the relative
charge density value which may deviated more from the unit value
(1) (e.g. TP-PDBTh > CBP > DTP-mCP).
To verify the final characteristics as a full devices with new
synthetic host materials, we choose well-known standard refer-
ence device structure with CBP because it has similar T1 energy
value (w2.6 eV) to those obtained from new synthetic host ma-
terials prepared in this study [14,34]. Fig. 7 shows the perspective
images of the OLED devices fabricated in this study. We used ITO
as an anode, N,N0-Bis(naphthalen-1-yl)-N,N0-bis(phenyl)benzidine
(NPB) as a hole transport layer, 4,40,400-tris(carbazol-9-yl)-triphe-
nylamine (TCTA) as an electron blocking layer, CBP and new ma-
terials synthesized in this study as host materials for emission
layer (EML) and Ir(ppy)3 as a dopant material for EML, 4,7-
diphenyl-1,10-phenanthroline (Bphen) as an electron transport
layer as well as a hole blocking layer (HBL), lithium fluoride (LiF)
as an electron injection layer (EIL), Al as a cathode. The exact
device configuration used in this work was as follows (See also
Fig. 7):
Fig. 2. (a) UVevisible absorption and photoluminescence spectra (at 298 K), (b)
photoluminescence spectra (at 77 K) of three host materials in 2-methyl
tetrahydrofuran.
although newly synthesized materials showed more or less higher
lying LUMOs because it has been reported as an effective cathode
toward tris(8-hydroxyquinolinato)aluminum [33].
Device A: ITO/NPB (30 nm)/TCTA (10 nm)/CBP: Ir(ppy)3 (8%,
30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm)
Device B: ITO/NPB (30 nm)/TCTA (10 nm)/TP-mCP: Ir(ppy)3 (8%,
30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm)
Device C: ITO/NPB (30 nm)/TCTA (10 nm)/DTP-mCP: Ir(ppy)3
(8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm)
Device D: ITO/NPB (30 nm)/TCTA (10 nm)/TP-PDBTh: Ir(ppy)3
(8%, 30 nm)/Bphen (25 nm)/LiF (0.5 nm)/Al (100 nm)
Fig. 5(a) and (b) shows the hole and electron current charac-
teristics obtained from the HODs and EODs prepared with newly
synthesized host materials (HOD A w D; EOD A w D). The hole
current density was significantly reduced in the order of HOD C
(DTP-mCP) > HOD A (CBP) > HOD D (TP-PDBTh) > HOD B (TP-mCP)
while the electron current density was slightly reduced in the order
of EOD D (TP-PDBTh) > EOD A (CBP) > EOD C (DTP-mCP) > EOD B
(TP-mCP). To estimate the charge balance if we use those materials
as an emission layer, we calculate the relative charge density by
dividing the hole current density by electron current density [e.g.
J(hole)/J(electron) where J: current density, mA/cm2] as shown in
Fig. 6. From this calculation, we could expect that TP-mCP could
give the most desirable device performance because the relative
charge density value was very close to unity. In other words, we
Fig. 8(a) shows the current densityevoltage (JeV) and lumi-
nanceevoltage (LeV) characteristics of fabricated green devices
and the representative results are summarized in Table 2. At a
given constant voltage of 5.0 V, current density values of 1.5, 2.1,
5.5 and 2.7 mA/cm2 were observed in the fabricated Devices A, B,
C, and D, respectively. The reason that the DTP-mCP showed the
Table 1
Summary of physical properties of new host materials.
Compound labs [nm]
lpl. [nm]
T1 (eV) (Exp) T1 (eV) (Cal) HOMO [eV] (UPS) HOMO [eV] (CV) LUMO [eV] Eg [eV] Tg [ꢁC] Tm[ꢁC] Tc [ꢁC] Td [ꢁC]
Solution Film
TP-mCP
DTP-mCP
TP-PDBTh 246, 271
245, 273, 293 375
391 2.63
391 2.63
385 2.64
2.81
2.92
2.97
5.91
5.93
5.95
5.68
5.98
5.84
2.11
2.39
2.21
3.57
3.59
3.63
159
179
108
273
354
436
488
445
273
377
369
267

N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
225
Fig. 4. Energy band diagrams of used materials in this work.
greatest current density value could be due to the columnar
packing of triphenylene moieties with the neighboring molecules
which also makes the distances between carbazole moieties very
short. This is well consistent with the result of HOD characteristics
that the hole current level in the HOD C was the greatest as shown
in Fig. 5(a). On the other hand, the electron current level of EOD C
with DTP-mCP was about the same as those of other devices such
as EOD A (with CBP) and D (with TP-PDBTh), which means that the
triphenylene moiety could be acted as a hole transporter although
we designed it as an electron transporting functionality. The
driving voltages to reach 1000 cd/m2 were 5.2, 4.8, 4.5, and 4.8 V
for the Devices A, B, C, and D, respectively (Table 2). The turn-on
voltages of 3.0, 3.0, 3.0, and 3.0 V were observed for the Devices A,
B, C, and D, respectively.
The current and power efficiency characteristics of fabricated
devices are shown in Fig. 8(b) and also summarized in Table 2. At
a given constant luminance of 1000 cd/m2, the current and power
efficiencies were 46 cd/A and 28 lm/W for the Device A, 64 cd/A
and 42 lm/W for the Device B, 48 cd/A and 34 lm/W for the
Device C, 65 cd/A and 40 lm/W for the Device D, respectively.
These efficiency data correspond to 16.6, 20.3, 15.6, and 18.7%
external quantum efficiencies of Devices A, B, C and D, respec-
tively. The maximum current and power efficiencies were 50 cd/A
and 28 lm/W for the Device A, 64 cd/A and 49 lm/W for the
Device B, 52 cd/A and 51 lm/W for the Device C, 66 cd/A and
40 lm/W for the Device D, respectively. These results are slightly
different with our expectation from the calculation of relative
current density aforementioned. However, those are well
consistent with those of EQE. Thus, TP-mCP and TP-PDBTh have
emerged as good candidates for excellent host materials for green
PHOLEDs.
The normalized EL spectra at a brightness of 1000 cd/m2 of
fabricated green devices are shown in Fig. 9. The EL spectra have
clear Ir(ppy)3 emission with peak wavelength at 512 nm, 515 nm,
518 nm, 514 nm for the Devices A, B, C, and D, respectively.
3. Conclusion
In conclusion, we prepared new green PHOLEDs with newly
synthesized host materials which have bipolar characteristics. The
host materials with more triphenylene moieties showed higher
hole carrier characteristics although we expected those moiety
could have high electron transporting ability. Materials with rela-
tive charge density close to unity showed fairly good current effi-
ciency as well as EQE values when it was fabricated as green
PHOLEDs. Finally, we found that the TP-mCP and TP-PDBTh could
be emerged as good candidates for excellent host materials for
green PHOLEDs which could give high EQE up to 20.3% and 18.7%,
respectively.
Fig. 5. (a) Hole current characteristics of HOD A w D and (b) electron current char-
acteristics of EOD A w D.

226
N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
Fig. 6. Relative charge density in host materials.
4. Experimental
4.1. Instruments
1H NMR and 13C NMR spectra were recorded on a Bruker Avance
400 MHz NMR spectrometer and chemical shifts were referenced
to chloroform (7.26 ppm). UVeVis spectra were recorded with a
HEWLETT PACKARD 8453 UVeVis spectrophotometer. Photo-
luminescence Spectrometer was used PERKIN ELMER LS 50B
model. The electrospray ionization (ESI) source was coupled to a
hybrid quadrupole orthogonal time-of-flight (Q-TOF) mass spec-
trometer (SYNAPT G2, Waters, MS Technologies, Manchester, U.K.)
was used to mass spectra acquirement in positive- and negative-ion
mode. A capillary and cone voltage of ꢂ3.0 kV and 30 V, and
capillary temperature of 120 ꢁC were used for both polarities,
respectively. The desolvation source conditions were as follows; for
the desolvation gas 800 L/h was used and the desolvation tem-
perature was kept at 600 ꢁC. Data acquisition took place over the
mass range of m/z 50 to m/z 1200 for MS modes. The sample was
Fig. 8. (a) JeVeL characteristics of fabricated blue devices. (b) Luminance vs current
efficiency and power efficiency characteristics of fabricated blue PHOLEDs.
an UltrafleXtreme MALDI time-of-flight (TOF) mass spectrometer
equipped with a pulsed smartbeam II (355 nm Nd:YAG laser,
repetition rate 1 kHz) in reflector mode (Bruker Daltonics, Billerica,
MA) Thermal properties were measured by differential scanning
calorimetry (DSC, TA instruments) under nitrogen atmosphere. We
also utilized thermogravimetric analysis (TGA) using TGA-1000
(SINCO) for investigation of thermal stability of the materials. Cy-
clic voltammetry (CV) [35,36] were measured by a Autolab/PGSTAT
introduced into the ESI source at a constant flow rate of 20 ml/min
by using an external syringe pump (HARVARD 11Plus, Holliston,
MA, USA). Also, another mass measurement was performed using
2
model at room temperature in
a
solution of tetra-n-
n-Bu4NPF6) in
buthylammonium hexafluorophosphate (0.1
N
acetonitrile under nitrogen gas protection at a scan rate of 100 or
50 mV/s. The working electrode was platinum (Pt) disk type and
reference electrode was Ag/0.1 M AgNO3. The HOMO energies were
also measured with Ultraviolet Photoelectron Spectrometer (UPS)
in a Hitachi High Tech AC-2 surface analysis system [37].
4.2. Synthesis of new host materials
4.2.1. Synthesis of 4,4,5,5-tetramethyl-2-(triphenylen-2-yl)-1,3,2-
dioxaborolane (1)
2-bromotriphenylene (10 g, 32.6 mmol), bis(pinacolato) dibo-
rate (9.9 g, 39.1 mmol), [1,10-bis(diphenylphosphino)ferrocene]
dichloropalladium(II) (0.5 g, 0.7 mmol), and potassium acetate
(16.2 g, 162.8 mmol) were dissolved in anhydrous 1,4-dioxane
(180 ml) and refluxed under nitrogen for 24 h. After the reaction
had finished, the mixture was washed three times with distilled
water and extracted with chloroform. The organic layer was sepa-
rated, dried over anhydrous magnesium sulfate, and evaporated
under reduced pressure. The crude product was purified by silica
gel column chromatography using n-hexane-tetrahydrofuran (5:1)
Fig. 7. Schematic diagram of device architecture of green devices fabricated in this
study.

N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
227
Table 2
over anhydrous magnesium sulfate, and evaporated under reduced
pressure. The crude product was purified by silica gel column
chromatography using n-hexane eluent to afford (3) a white solid,
yield 27%. 1H NMR (400 MHz, CDCl3) 8.14e8.12 (d, J ¼ 7.6 Hz, 2H),
7.78 (s, 1H), 7.45 (s, 2H), 7.47e7.41 (m, 4H), 7.34e7.3 (t, J ¼ 8 Hz, 2H)
ppm.
Device characteristics of phosphorescent green OLEDs.
Device A
3 V
Device B
3 V
Device C
3 V
Device D
3 V
Turn-on voltage
(1 cd/m2)
Operating voltage 5.2 V
4.8 V
4.5 V
4.8 V
(1000 cd/m2)
Efficiency
(1000 cd/m2)
Efficiency (Max)
46 cd/A
28 lm/W
50 cd/A
64 cd/A
42 lm/W
64 cd/A
49 lm/W
48 cd/A
34 lm/W
52 cd/A
51 lm/W
65 cd/A
40 lm/W
66 cd/A
40 lm/W
4.2.4. Synthesis of 9,90-(5-(triphenylen-2-yl)-1,3-phenylene)bis(9H-
carbazole) (TP-mCP)
4,4,5,5-tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane
(0.65 g, 1.84 mmol), 9,90-(5-bromo-1,3-phenylene)bis(9H-carba-
zole) (0.91 g, 1.87 mmol), tetrakis(triphenylphosphine) palla-
dium(0) (0.06 g, 0.06 mmol), and 1 M Na2CO3 (7.3 ml) in 40 ml of
Toluene was stirred at 130 ꢁC for 48 h under nitrogen atmosphere.
After the reaction had finished, the mixture was washed three
times with distilled water and extracted with chloroform. The
organic layer was separated, dried over anhydrous magnesium
sulfate, and evaporated under reduced pressure. The crude product
was purified by silica gel column chromatography using n-hexane-
tetrahydrofuran (5:1). The final product was obtained as a white
powder after purification by vacuum sublimation at a synthetic
yield of 38%. 1H NMR (400 MHz, CDCl3) 8.97 (s, 1H), 8.79e8.77 (d,
J ¼ 8.8 Hz, 1H), 8.71e8.67 (m, 4H), 8.22e8.20 (d, J ¼ 7.6 Hz, 4H), 8.16
(s, 2H), 8.02e8.00 (d, J ¼ 8.8 Hz, 1H), 7.88 (s, 1H), 7.72e7.62 (m, 8H),
28 lm/W
CIE (x,y)
0.265, 0.631 0.279, 0.635 0.292, 0.634 0.273, 0.631
(1000 cd/m2)
E.Q.E.
16.6% 20.3% 15.6% 18.7%
(1000 cd/m2)
eluent to afford (1) a white solid, yield 80%. 1H NMR (400 MHz,
CDCl3) 9.15 (s, 1H), 8.84e8.82 (m,1H), 8.71e8.64 (m, 4H), 8.07e8.05
(d, J ¼ 8.0 Hz, 1H), 7.70e7.66 (m, 4H), 1.43 (s, 12H) ppm.
4.2.2. Synthesis of 9,90-(5-bromo-1,3-phenylene)bis(9H-carbazole)
(2)
9H-carbazole (40 g, 240 mmol), 1,3,5-tribromobenzene (39.2 g,
120 mmol), copper(I) iodide (6.83 g, 36 mmol), potassium phos-
phate (203 g, 96 mmol) and trans-1,2-diaminocyclohexane (5.8 ml,
48 mmol) in 200 ml of anhydrous toluene was stirred at 120 ꢁC for
48 h under nitrogen atmosphere. After the reaction had finished,
the mixture was washed three times with distilled water and
extracted with chloroform. The organic layer was separated, dried
over anhydrous magnesium sulfate, and evaporated under reduced
pressure. The crude product was purified by silica gel column
chromatography using n-hexane eluent to afford (2) a white solid,
yield 34%. 1H NMR (400 MHz, CDCl3) 8.16e8.14 (d, J ¼ 7.6 Hz, 4H),
7.87 (s, 2H), 7.8 (s, 1H), 7.57e7.54 (d, J ¼ 8.4 Hz, 4H), 7.49e7.45 (t,
J ¼ 7.2 Hz, 4H), 7.36e7.32 (t, J ¼ 7.6 Hz, 4H) ppm.
7.52e7.48 (t, J ¼ 8.4 Hz, 4H), 7.37e7.33 (t, J ¼ 6.8 Hz, 4H) ppm; 13
C
NMR (CDCl3, 400 MHz) 142.5, 138.6, 137.8, 135.5, 128.2, 128, 127.9,
127.7, 127.3, 127.2, 127, 126.2, 125.5, 125.3, 124.2, 123.8, 122.5, 122.2,
122, 121.6, 121.4, 121.3, 121.2, 119.8, 118.5, 118.4 ppm. ESI-MS
[M þ H]þ: m/z calcd. 635.2487; found 635.2460.
4.2.5. Synthesis of 9-(3,5-di(triphenylen-2-yl)phenyl)-9H-carbazole
(DTP-mCP)
9-(3,5-dibromophenyl)-9H-carbazole (1.8 g, 4.49 mmol), 4,4,5,5-
tetramethyl-2-(triphenylen-2-yl)-1,3,2-dioxaborolane (3.34 g,
9.42 mmol), tetrakis(triphenylphosphine) palladium(0) (0.26 g,
0.22 mmol), and 1 M Na2CO3 (19.1 ml) in 140 ml of Toluene was
stirred at 130 ꢁC for 48 h under nitrogen atmosphere. After the re-
action had finished, the mixture was washed three times with
distilled water and extracted with chloroform. The organic layer was
separated, dried overanhydrous magnesiumsulfate, and evaporated
under reduce pressure. The crude product was purified by silica gel
column chromatography using n-hexane-tetrahydrofuran (5:1). The
final product was obtained as a white powder after purification by
vacuum sublimation at a synthetic yield of 74%. 1H NMR (400 MHz,
CDCl3) 9.02 (s, 2H), 8.83e8.81 (d, J ¼ 8.8 Hz, 2H), 8.78e8.77 (d,
J ¼ 7.2 Hz, 2H), 8.74e8.69 (m, 6H), 8.34 (s, 1H), 8.25e8.23 (d,
J ¼ 7.6 Hz, 2H), 8.10e8.08 (d, J ¼ 5.6 Hz, 4H), 7.72e7.65 (m, 10H),
7.52e7.48 (t, J ¼ 7.2 Hz, 2H), 7.38-7.34 (t, J ¼ 7.6 Hz, 2H) ppm. MALDI-
TOF [M þ H]þ: 695.329.
4.2.3. Synthesis of 9-(3,5-dibromophenyl)-9H-carbazole (3)
9H-carbazole (40 g, 240 mmol), 1,3,5-tribromobenzene (98 g,
300 mmol), copper(I) iodide (6.83 g, 36 mmol), potassium phos-
phate (203 g, 96 mmol) and trans-1,2-diaminocyclohexane (5.8 ml,
48 mmol) in 300 ml of anhydrous toluene was stirred at 120 ꢁC for
48 h under nitrogen atmosphere. After the reaction had finished,
the mixture was washed three times with distilled water and
extracted with chloroform. The organic layer was separated, dried
4.2.6. Synthesis of 4-(3-(triphenylen-2-yl)phenyl)dibenzo[b,d]
thiophene (TP-PDBTh)
1-bromo-3-iodobenzene (2.0 g, 7.07 mmol), 4,4,5,5-tetramethyl-
2-(triphenylen-2-yl)-1,3,2-dioxaborolane (2.5 g, 7.07 mmol), tetra-
kis(triphenylphosphine)palladium(0) (0.25 g, 0.21 mmol), and 1 M
Na2CO3 (35.4 ml) in 100 ml of toluene was stirred at 90 ꢁC for 48 h
under nitrogen atmosphere. To the reaction mixture was added a
dibenzo[b,d]thiophene-4-ylboronic acid (1.61 g, 7.07 mmol) and
then stirred at 120 ꢁC for 24 h under nitrogen atmosphere. After the
reaction had finished, the mixture was washed three times with
distilled water and extracted with chloroform. The organic layer was
separated, dried overanhydrous magnesiumsulfate, and evaporated
under reduced pressure. The crude product was purified by silica gel
column chromatography using n-hexane-tetrahydrofuran (5:1). The
final product was obtained as a white powder after purification by
Fig. 9. The normalized EL spectra at a brightness of 1000 cd/m2 of fabricated green
devices.

228
N.-J. Lee et al. / Dyes and Pigments 101 (2014) 221e228
vacuum sublimation at a synthetic yield of 46%. 1H NMR (400 MHz,
CDCl3) 8.96 (s,1H), 8.79e8.75 (m, 2H), 8.71e8.67 (m, 3H), 8.24e8.21
(m, 3H), 8.01e8.00 (d, J ¼ 8.8 Hz,1H), 7.91e7.86 (m, 2H), 7.82e7.80 (d,
J ¼ 7.6 Hz, 1H), 7.70e7.68 (m, 5H), 7.63e7.62 (d, J ¼ 4 Hz, 2H), 7.52e
7.49 (m, 2H) ppm. ESI-MS [M þ H]þ: m/z 487.1539.
[10] Lee JH, Wu CI, Liu SW, Huang CA, Chang Y. Mixed host organic light-emitting devices
with low driving voltage and long lifetime. Appl Phys Lett 2005;86:103506.
[11] Tsai CY, Jou JH. Long-lifetime, high-efficiency white organic light-emitting
diodes with mixed host composing double emission layers. Appl Phys Lett
2006;89:243521.
[12] Chopra N, Swensen JS, Polikarpov E, Cosimbescu L, Franky. High efficiency and
low roll-off blue phosphorescent organic light-emitting devices using mixed
host architecture. Appl Phys Lett 2010;97:033304.
[13] Hung WY, Chi LC, Chen WJ, Chen YM, Chou SH, Wong KT. A new benzimid-
azole/carbazole hybrid bipolar material for highly efficient deep-blue elec-
trofluorescence, yellowegreen electrophosphorescence, and two-color-based
white OLEDs. J Mater Chem 2010;20:10113e9.
[14] Tao Y, Wang Q, Yang C, Wang Q, Zhang Z, Zou T, et al. A simple carbazole/
oxadiazole hybrid molecule: an excellent bipolar host for green and red
phosphorescent OLEDs. Angew Chem 2008;120:8224e7.
[15] Wong KT, Chen YM, Lin YT, Su HC, Wu CC. Nonconjugated hybrid of carbazole
and fluorene: a novel host material for highly efficient green and red phos-
phorescent OLEDs. Org Lett 2005;7(24):5361e4.
4.3. Fabrication of PHOLEDs
4.3.1. Materials
NPB as a hole injection layer, TCTA as an HTL, Ir(ppy)3 as a green
dopant, Bphen as an HBL as well as an ETL, LiF, and LiQ as an EIL and
Al as a cathode were purchased from commercial suppliers and
were used without purification.
[16] Ding J, Gao Jia, Cheng Y, Xie Z, Wang L, Ma D, et al. Highly efficient green-
emitting phosphorescent iridium dendrimers based on carbazole dendrons.
Adv Funct Mater 2006;16:575e81.
[17] Holmes RJ, D’Andrade BW, Forrest SR, Ren X, Li J, Thompson ME. Efficient,
deep-blue organic electrophosphorescence by guest charge trapping. Appl
Phys Lett 2003;83:3818e20.
[18] Ren X, Li J, Holmes RJ, Djurovich PI, Forrest SR, Thompson ME. Ultrahigh
energy gap hosts in deep blue organic electrophosphorescent devices.
Chemitry Mater 2004;16:4743e7.
[19] Lee J, Lee JI, Lee JY, Chu HY. Improved performance of blue phosphorescent
organic light-emitting diodes with a mixed host system. Appl Phys Lett
2009;95:253304e13.
[20] Chou HH, Cheng CH. A highly efficient universal bipolar host for blue, green,
and red phosphorescent OLEDs. Adv Mater 2010;22:2468e71.
[21] Jeon SO, Yook KS, Joo CW, Lee JY. High-efficiency deep-blue-phosphorescent
organic light-emitting diodes using a phosphine oxide and a phosphine sul-
fide high-triplet-energy host material with bipolar charge-transport proper-
ties. Adv Mater 2010;22:1872e6.
4.3.2. Device fabrication
To fabricate OLED devices, clean glass substrates pre-coated
with an 150-nm-thick ITO layer with a sheet resistance of w12 U/
sq were used. Line patterns of ITO were formed on glass by
photolithography process. The ITO glass was cleaned by sonifica-
tion in an isopropylalcohol and acetone, rinsed in deionized water,
and finally irradiated in a UV-ozone chamber. All organic materials
were deposited by the vacuum evaporation technique under a
pressure of w1 ꢃ 10ꢀ7 Torr. The deposition rate of organic layers
ꢀ
ꢀ
was about 0.5 A/s. Deposition rates of LiF and Al were 0.1 A/s and
ꢀ
3 A/s, respectively.
4.3.3. Measurements
The JeV and LeV data of OLEDs were measured by Keithley
2635A and Minolta CS-100A, respectively. The OLED area was
4 mm2 for all the samples studied in this work. Electrolumines-
cence spectra and CIE coordinate were obtained using a Minolta CS-
2000A spectroradiometer.
[22] Hsu FM, Chien CH, Shih PI, Shu CF. Phosphine-oxide-containing bipolar host
material for blue electrophosphorescent devices. Chemitry Mater 2009;21:
1017e22.
[23] Chen HF, Yang SJ, Tsai ZH, Hung WY, Wang TC, Wong KT. 1,3,5-Triazine de-
rivatives as new electron transport etype host materials for highly efficient
green phosphorescent OLEDs. J Mater Chem 2009;19:8112e8.
[24] Jeon WS, Park TJ, Kim SY, Pode R, Jang J. Low roll-off efficiency green phos-
phorescent organic light-emitting devices with simple double emissive layer
structure. Appl Phys Lett 2008;93:063303.
[25] Togashi K, Nomura S, Yokoyama N, Yasuda T, Adachi C. Low driving voltage
characteristics of triphenylene derivatives as electron transport materials in
organic light-emitting diodes. J Mater Chem 2012;22:20689e95.
[26] Adam D, Schuhmacher P, Simmerer J, Maubling L, Paulus W, Siemensmeyer K,
et al. Photoconductivity in the columnar phases of a glassy discotic twin. Adv
Mater 1995;7:276.
Acknowledgments
Prof. M. C. Suh was supported by the National Research Foun-
dation of Korea (NRF) (NRF-2011-00006847). Prof. J.-H. Lee was
supported by a grant (Catholic Univ.) from the Fundamental R&D
program for Core Technology of Materials funded by the Ministry of
Knowledge Economy (MKE), Republic of Korea. High resolution
mass spectroscopy and MALDI-TOF was carried out at Korea Basic
Science Institute (Mass Spectrometry Research Team).
[27] Simmerer J, Glusen B, Paulus W, Kettner A, Schuhmacher P, Adam D, et al.
Transient photoconductivity in a discotic hexagonal plastic crystal. Adv Mater
1996;8:815.
ꢁ
_
[28] Schrögel P, Tomkeviciene A, Strohriegl P, Hoffmann ST, Köhler A, Lennartz C.
A series of CBP-derivatives as host materials for blue phosphorescent organic
light-emitting diodes. J Mater Chem 2011;21:2266e73.
[29] Gong S, He X, Chen Y, Jiang Z, Zhong C, Ma D, et al. Simple CBP isomers with
high triplet energies for highly efficient blue electrophosphorescence. J Mater
Chem 2012;22:2894e9.
References
[30] Matsushima T, Kinoshita Y, Murata H. Formation of ohmic hole injection by
inserting an ultrathin layer of molybdenum trioxide between indium tin
oxide and organic holetransporting layers. Appl Phys Lett 2007;91:253504.
[31] Matsushima T, Murata H. Observation of space-charge-limited current due to
charge generation at interface of molybdenum dioxide and organic layer. Appl
Phys Lett 2009;95:203306.
[1] Kawamura Y, Goushi K, Brooks J, Brown JJ, Sasabe H, Adachi C. 100% phos-
phorescence quantum efficiency of Ir(III) complexes in organic semiconductor
films. Appl Phys Lett 2005;86:071104.
[2] Sasabe H, Pu YJ, Nakayama K, Kido J. m-Terphenyl-modified carbazole host
material for highly efficient blue and green PHOLEDS. Chem Commun 2009:
6655e7.
[32] Matsushima T, Jin GH, Kanai Y, Yokota T, Kitada S, Kishi T, et al. Interfacial
charge transfer and charge generation in organic electronic devices. Org
Electron 2011;12:520e8.
[3] Kim SY, Jang JS, Lee JY. High efficiency phosphorescent organic light-emitting
diodes using carbazole-type triplet exciton blocking layer. Appl Phys Lett
2007;90:223505.
[33] Liu Z, Salata OV, Male N. Improved electron injection in organic LED with
lithium quinolate/aluminium cathode. Synth Met 2002;128:211e4.
[34] Jeon WS, Choi JW, Park JS, Yu JH, Suh MC, Kwon JH. The ideal doping con-
centration in phosphorescent organic light emitting devices. Jpn J Appl Phys
2011;50:061603.
[4] Krummacher BC, Choong VE, Mathai MK, Choulis SA, Franky. Highly efficient
white organic light-emitting diode. Appl Phys Lett 2006;88:113506.
[5] King KA, Spellane PJ, Watts RJ. Excited-state properties of a triply ortho-
metalated iridium(III) complex. Am Chem Soc 1985;107:1431.
[6] Dedeian K, Djurovich PI, Garces FO, Carlson G, Watts RJ. A new synthetic route
to the preparation of a series of strong photoreducing agents: fac tris-ortho-
metalated complexes of iridium(III) with substituted 2-phenylpyridines.
Inorg Chem 1991;30:1685.
[7] Colombo MG, Brunold TC, Riedener T, Gudel HU, Fortsch M, Burgi HB. Facial
tris cyclometalated Rh3þ and Ir3þ complexes: their synthesis, structure, and
optical spectroscopic properties. Inorg Chem 1944;33:545.
[8] Tokito S, Iijima T, Suzuri Y, Kita H, Tsuzuki T, Sato F. Confinement of triplet
energy on phosphorescent molecules for highly-efficient organic blue-
lightemitting devices. Appl Phys Lett 2003;83:569e71.
[35] Tonzola CJ, Alam MM, Kaminsky W, Jenekhe SA. New n-type organic semi-
conductors: synthesis, single crystal structures, cyclic voltammetry, photo-
physics, electron transport, and electroluminescence of
a series of
diphenylanthrazolines. J Am Chem Soc 2003;125(44):13548e58.
[36] Shafiee A, Salleh MM, Yahaya H. Determination of HOMO and LUMO of [6,6]-
phenyl C61-butyric acid 3-ethylthiophene ester and poly (3-octyl-thiophene-
2, 5-diyl) through voltametry characterization. Sains Malaysiana 2011;40(2):
173e6.
[37] Hwang JH, Kim EG, Liu J, Brédas JL, Duggal A, Kahn A. Photoelectron spec-
troscopics of the electronic band structure of polyfluorene and fluorene-
arylamine copolymers at interfaces. J Phys Chem C 2007;111(3):1378e84.
[9] Watanabe S, Ide N, Kido J. High-efficiency green phosphorescent organic light-
emitting devices with chemically doped layers. Jpn J Appl Phys 2007;46:1186.