Carbazole modified terphenyl based high triplet energy host materials for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2013.10.003

Carbazole modified terphenyl based high triplet energy host materials were developed for application as host materials for blue phosphorescent organic light-emitting diodes. Two terphenyl based materials, 9-(5″-phenyl-[1, 1′:2′,1″:3″,1′′′-quaterphenyl]-3-

Full text of DOI:10.1016/j.dyepig.2013.10.003

Dyes and Pigments 101 (2014) 150e155
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Carbazole modified terphenyl based high triplet energy host materials
for blue phosphorescent organic light-emitting diodes
*
Chil Won Lee, Jun Yeob Lee
Department of Polymer Science and Engineering, Dankook University, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbazole modified terphenyl based high triplet energy host materials were developed for application as
host materials for blue phosphorescent organic light-emitting diodes. Two terphenyl based materials, 9-
(500-phenyl-[1,10:20,100:300,1000-quaterphenyl]-3-yl)-9H-carbazole (CzTPPh) and 9-(300,500-di(pyridin-3-yl)-
[1,1:20,100-terphenyl]-3-yl)-9H-carbazole (CzTPPy), were synthesized as the host materials with high
triplet energies of 2.75 eV and 2.73 eV, respectively. The two host materials were evaluated as the host
materials for blue phosphorescent organic light-emitting diodes and high quantum efficiencies of 20.2%
and 15.7% were obtained in the CzTPPh and CzTPPy devices, respectively.
Received 21 June 2013
Received in revised form
27 September 2013
Accepted 2 October 2013
Available online 11 October 2013
Keywords:
Carbazole
Ó 2013 Elsevier Ltd. All rights reserved.
Terphenyl
Blue device
High triplet energy
High efficiency
Host material
1. Introduction
A few high triplet energy host materials possessing the kinked
terphenyl core structure were reported as the host materials for
The development of high triplet energy host materials is
important to improve the external quantum efficiency of blue
phosphorescent organic light-emitting diodes (PHOLEDs) because
the efficiency of the blue PHOLEDs mainly depends on energy
transfer from host to blue triplet emitter. The triplet energy of the
host material should be higher than that of dopant material for
efficient energy transfer from host to dopant.
The high triplet energy host materials should be designed to
include high triplet energy moieties in the molecular structure [1e
20]. Common high triplet energy host materials have carbazole [1e
10], fluorene [11e13] and dibenzofuran [14e17] as the high triplet
energy moieties in the backbone structure, which was combined
with other aromatic units to produce the high triplet energy host
materials. In general, aromatic units derived from phenyl or
biphenyl backbone structure were used to realize high triplet en-
ergy because a terphenyl unit reduced the triplet energy of the host
materials. However, the terphenyl unit can be used to develop high
triplet energy host materials by decreasing the degree of conjuga-
tion through a kink structure [18]. The merits of using the kinked
terphenyl core were high triplet energy and high thermal stability.
blue PHOLEDs [19e23].
In this work, high triplet energy host materials derived from the
carbazole modified kinked terphenyl core structure, 9-(500-phenyl-
[1,10:20,100:300,1000-quaterphenyl]-3-yl)-9H-carbazole (CzTPPh) and 9-
(300,500-di(pyridin-3-yl)-[1,10:20,100-terphenyl]-3-yl)-9H-carbazole
(CzTPPy), were synthesized as the host materials for blue PHOLEDs.
A high triplet energy over 2.70 eV was obtained using the CzTPPh
and CzTPPy host materials and a high quantum efficiency of 20.2%
was achieved using the CzTPPh host material.
2. Experimental section
2.1. General information
All chemicals and reagents were purchased from Aldrich and TCI
Chem, and were used without further purification. 3-(9H-carbazol-
9-yl)phenylboronic acid was prepared according to the synthetic
method in literature [24] and general chemical analysis were
described in our previous work [25].
2.1.1. Synthesis of 9-(20-bromo-[1,10-biphenyl]-3-yl)-9H-carbazole
3-(9H-carbazol-9-yl)phenylboronic acid (12.0 g, 42.1 mmol),1,2-
dibromobenzene (50.0 g, 211 mmol) and tetrahydrofuran (480 ml)
were added to a two-necked flask equipped with a magnetic
* Corresponding author. Tel./fax: þ82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.10.003

C.W. Lee, J.Y. Lee / Dyes and Pigments 101 (2014) 150e155
151
stirring bar, a septum and a reflux condenser attached to a gas-flow
2.1.3. 9-(500-Phenyl-[1,10:20,100:300,1000-quaterphenyl]-3-yl)-9H-
carbazole (CzTPPh)
adapter with a stopcock. The solution was bubbled with high purity
nitrogen gas for 30 min to remove oxygen and potassium carbonate
(11.6 g, 84.3 mmol) dissolved in oxygen free distilled water (160 ml)
was added to the solution. Tetrakis(triphenylphosphine)palla-
dium(0) (Pd(PPh3)4, 2.43 g, 2.11 mmol) was added and the resulting
suspension was refluxed for 24 h under nitrogen. The solution was
allowed to cool and was transferred to a separatory funnel. The
solution was extracted with ethyl acetate and the organic layer was
combined, washed with water saturated with sodium chloride, and
dried over magnesium sulfate. Solvent was removed with a rotary
evaporator to yield a yellow oil, which was purified by column
chromatography(dichloromethane/n-hexane) to give 14.0 g of 9-
(20-bromo-[1,10-biphenyl]-3-yl)-9H-carbazole.
A solution of 9-(300,500-dichloro-[1,10:20,100-terphenyl]-3-yl)-9H-
carbazole (3.00 g, 6.46 mmol), phenylboronic acid (3.15 g,
25.8 mmol), K3PO4 (8.23 g, 38.8 mmol), 2-dicyclohexylphosphino-
20,60-dimethoxybiphenyl (S-Phos, 0.53 g, 1.29 mmol), tris(di-
benzylideneacetone)dipalladium (Pd2(dab)3, 0.30 g, 0.34 mmol),
toluene (210 ml) and distilled water (21 ml) was refluxed for 48 h.
The solution was cooled down to room temperature, diluted with
100 ml water and extracted with ethyl acetate. The organic extracts
were dried over anhydrous magnesium sulfate, filtered, and evap-
orated to yield a brown solid. The crude product was purified by
column chromatography on silica gel using dichloromethane/n-
hexane as an eluent. Additional purification by sublimation (250 ꢀC.
at 10ꢁ5 mmHg) resulted in 1.5 g of pure glassy solid.
Yield: 84%. 1H NMR (500 MHz, CDCl3):
d
7.23 (t, 1H, J ¼ 6.0 Hz),
7.29 (t, 2H, J ¼ 4.8 Hz), 7.37e7.44 (m, 4H), 7.49 (d, 1H, J ¼ 4.0 Hz),
7.52 (d, 2H, J ¼ 4.0 Hz), 7.60 (d, 1H, J ¼ 4.0 Hz), 7.63 (s, 1H), 7.66 (t,
1H, J ¼ 5.2 Hz), 7.70 (d, 1H, J ¼ 4.0 Hz), 8.15 (d, 2H, J ¼ 4.0 Hz), Mass
(FAB): m/z 397 [Mþ].
Yield: 42%. Tg: 90 ꢀC, Tm: 176 ꢀC. FT-IR (ATR): 3053, 1592, 1476,
1450, 1412, 1312, 1228, 1164, 1081, 1028, 880, 802, 749, 697 cmꢁ1. 1H
NMR (500 MHz, CDCl3):
d
7.03 (d, 2H, J ¼ 4.3 Hz), 7.18 (t, 4H,
J ¼ 8.3 Hz), 7.32 (t, 2H, J ¼ 4.8 Hz), 7.39 (t, 4H, J ¼ 5.0 Hz), 7.42e7.50
(m, 11H), 7.54e7.61 (m, 3H), 7.78 (s, 1H), 8.04 (d, 2H, J ¼ 4.3 Hz). 13
C
2.1.2. Synthesis of 9-(300,500-dichloro-[1,10:20,100-terphenyl]-3-yl)-9H-
carbazole
NMR (125 MHz, CDCl3): d 109.6, 119.8, 120.1, 123.3, 124.4, 125.0,
125.9, 127.3, 127.5, 127.8, 128.0, 128.1, 128.6, 128.8, 129.7, 130.6,
137.5, 140.0, 140.3, 140.7, 140.9, 141.9, 142.0, 143.9. Mass (FAB): m/z
547 [(M)þ]. Elemental analysis Calcd. for C42H29N: C, 92.11%; H,
5.34%; N, 2.56%. Found: C, 92.11%; H, 5.33%; N, 2.58%.
9-(20-Bromo-[1,10-biphenyl]-3-yl)-9H-carbazole
(10.0
g,
25.1 mmol), 3,5-dichlorophenylboronic acid (6.23 g, 32.6 mmol)
and tetrahydrofuran (240 ml) were added to a two-necked flask
and was bubbled with nitrogen for 30 min. Potassium carbonate
(8.68 g, 62.8 mmol) dissolved in oxygen free distilled water (80 ml)
was added to the solution followed by adding Pd(PPh3)4 (1.45 g,
1.26 mmol). The resulting suspension was refluxed for 24 h under
nitrogen. The reaction was allowed to cool and was extracted with
ethyl acetate. The organic layer was combined, washed with water
saturated with sodium chloride, and dried over magnesium sulfate.
Solvent was removed with a rotary evaporator to yield a brown
powder, which was purified by column chromatography using a
dichloromethane/n-hexane eluent to give 9-(300,500-dichloro-
[1,10:20,100-terphenyl]-3-yl)-9H-carbazole (9.0 g) as a white powder.
2.1.4. 9-(300,500-di(pyridin-3-yl)-[1,10:20,100-terphenyl]-3-yl)-9H-
carbazole (CzTPPy)
A solution of 9-(300,500-dichloro-[1,10:20,100-terphenyl]-3-yl)-9H-
carbazole (3.00 g, 6.46 mmol), 3-pyridinylboronic acid (3.18 g,
25.8 mmol), K3PO4 (8.23 g, 38.8 mmol), S-Phos (0.53 g, 1.29 mmol),
Pd2(dab)3 (0.30 g, 0.34 mmol), toluene (210 ml) and distilled water
(21 ml) was refluxed for 48 h. The solution was cooled down to room
temperature, diluted with 100 ml water and extracted with ethyl
acetate. The organic extract was dried over anhydrous magnesium
sulfate, filtered, and evaporated to yield a brown solid. The crude
product was purified by column chromatography on silica gel using
dichloromethane/n-hexane as an eluent. Additional purification by
sublimation (250ꢀC. at10ꢁ5 mmHg) resulted 1.7gofpure glassysolid.
Yield:77%.1HNMR (500 MHz, CDCl3):
d
7.01(d, 2H,J¼ 4.0Hz), 7.13
(s, 2H), 7.23 (s,1H), 7.26 (t,1H, J ¼ 4.8 Hz), 7.33e7.51 (m,10H), 7.59 (t,
1H, J ¼ 5.2 Hz), 8.10 (d, 2H, J ¼ 4.0 Hz). Mass (FAB): m/z 463 [Mþ].
Scheme 1. Synthetic scheme of CzTPPh and CzTPPy.

152
C.W. Lee, J.Y. Lee / Dyes and Pigments 101 (2014) 150e155
Yield: 48%. Tg: 98 ꢀC, Tm: N.D. FT-IR (ATR): 3046,1594,1477,1450,
1393, 1314, 1228, 1166, 1118, 1022, 885, 801, 750, 710 cmꢁ1. 1H NMR
(500 MHz, CDCl3):
d
6.95 (d, 2H, J ¼ 1.5 Hz), 7.16 (t, 4H, J ¼ 8.7 Hz),
7.30e7.34 (m, 3H), 7.49e7.52 (m, 6H), 7.58 (t, 2H, J ¼ 5.2 Hz), 7.65 (t,
1H, J ¼ 5.2 Hz), 7.73e7.75 (m, 3H), 8.04 (d, 2H, J ¼ 4.5 Hz), 8.60 (d,
2H, J ¼ 3.0 Hz), 8.75 (s, 2H). 13C NMR (125 MHz, CDCl3):
d 109.4,
119.9, 120.2, 123.3, 123.6, 124.2, 125.3, 125.8, 128.2, 128.3, 128.6,
128.7, 128.7, 129.9, 130.5, 130.8, 134.4, 136.0, 137.5, 139.0, 139.4,
139.9, 140.6, 142.9, 143.6, 148.3, 149.0. Mass (FAB): m/z 550
[(M þ H)þ]. Elemental analysis Calcd. for C40H27N3: C, 87.40%; H,
4.95%; N, 7.64%. Found: C,87.24%; H, 4.98%; N, 7.67%.
3. Device fabrication and measurement
The device structure of the blue PHOLEDs fabricated in this work
was indium tin oxide (ITO, 50 nm)/poly(3,4-ethylenedioxy
thiophene): poly(styrenesulfonate) (PEDOT:PSS, 60 nm)/4,40-cyclo-
hexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC, 20 nm)/1,3-
bis(N-carbazolyl)benzene (mCP, 10 nm)/CzTPPh or CzTPPy:ir-
idium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate (FIrp
ic) (25 nm, 10%)/diphenylphosphine oxide-4-(triphenylsilyl)phenyl
(TSPO1, 35 nm)/LiF (1 nm)/Al (200 nm). Doping concentration of
FIrpic were 5% and 10%. PEDOT:PSS and TAPC were used as a hole
injection layer and hole transport layer, respectively. TSPO1 and mCP
were exciton blocking layers. A control device with mCP host instead
of CzTPPh and CzTPPy was also fabricated. Device structure of elec-
tron only device was ITO (50 nm)/Ca (5 nm)/CzTPPh or CzTPPy
(25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm). Device evaluation
was carried out using CS 1000 spectroradiometer and Keithley 2400
source measurement unit.
Fig. 2. UVeVis, solution PL, solid PL and low temperature PL spectra of CzTPPh and
CzTPPy.
dichlorophenylboronic acid to prepare functionalized carbazole
modified terphenyl intermediate. The Cl functionalized terphenyl
core was substituted with phenyl and pyridine using phenylboronic
acid and 3-pyridinylboronic acid.
Molecular orbital simulation of the CzTPPh and CzTPPy host
materials was performed to study the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) distribution. The HOMO and LUMO distribution calculated
from Gaussian 09 program using density functional of B3LYP with
6-31G* basis sets is shown in Fig. 1. The HOMO of the two host
materials was localized on the carbazole unit of the host materials
due to electron donating character of the amine unit of carbazole.
There was little difference of the HOMO distribution between the
two host materials. However, the LUMO distribution was different
depending on the substituent of the carbazole modified terphenyl
core. The LUMO of CzTPPh was uniformly distributed over the
terphenyl core and the phenyl substituent because of relatively
weak electron donating character of the aromatic unit compared to
the carbazole unit. However, the LUMO of CzTPPy was localized on
the pyridine unit due to strong electron deficiency of the pyridine
unit. The different electron deficiency of the phenyl and pyridinyl
substituents induced the different LUMO distribution. The
4. Results and discussion
The two carbazole modified terphenyl compounds, CzTPPh and
CzTPPy, were designed to have high triplet energy by combining
high triplet energy carbazole and ortho connected terphenyl unit.
The carbazole modified terphenyl moiety was further substituted
with phenyl or pyridinyl unit to increase the glass transition tem-
perature and to manage charge transport properties of the host
materials. The phenyl and pyridyl units were attached to meta
position of the carbazole modified terphenyl core to suppress the
increase of the degree of conjugation.
Synthetic scheme of CzTPPh and CzTPPy is shown in Scheme 1.
The carbazole modified terphenyl core was prepared from 3-(9H-
carbazol-9-yl)phenylboronic acid. The 3-(9H-carbazol-9-yl)phe-
nylboronic acid intermediate was reacted with 1,2-
dibromobenzene to produce the 9-(20-bromo-[1,10-biphenyl]-3-
yl)-9H-carbazole intermediate, which was combined with 3,5-
Fig. 1. HOMO and LUMO distribution of CzTPPh and CzTPPy.
Fig. 3. Oxidation and reduction cyclic voltammetry curves of CzTPPh and CzTPPy.

C.W. Lee, J.Y. Lee / Dyes and Pigments 101 (2014) 150e155
153
Table 1
Physical properties of materials.
Material
UVeVis
Solution PL
(nm)a
Solid PL
(nm)
HOMO
(eV)b
LUMO
(eV)b
Bandgap
(eV)c
Triplet energy
(nm)a
(eV)d
CzTPPh
CzTPPy
241
239
366
364
370
373
ꢁ6.04
ꢁ6.03
ꢁ2.51
ꢁ2.68
3.53
3.35
2.75
2.73
a
Data were measured in tetrahydrofuran solution.
HOMO and LUMO were measured by CV.
Bandgap was calculated from the difference of HOMO and LUMO.
b
c
d
Triplet energy was calculated from the first phosphorescent emission peak of low temperature delayed PL spectra.
calculated HOMO/LUMO levels of CzTPPh and CzTPPy were ꢁ5.28/
ꢁ1.03 eV and ꢁ5.24 eV/ꢁ1.30 eV, respectively. As the HOMO was
localized on the same carbazole unit, the calculated HOMO levels
were similar.
Photophysical properties of the CzTPPh and CzTPPy host mate-
rials were measured using ultravioletevisible (UVeVis) and pho-
toluminescence (PL) spectrometers. Fig. 2 shows UVeVis and PL
spectra of CzTPPh and CzTPPy host materials. The two host mate-
rials showed similar UVeVis absorption spectra irrespective of the
The HOMO and LUMO levels of the host materials were
measured by cyclic voltammetry from oxidation and reduction
curves of CzTPPh and CzTPPy (Fig. 3). The HOMO/LUMO levels of
CzTPPh and CzTPPy were ꢁ6.04/ꢁ2.51 eV and ꢁ6.03/ꢁ2.68 eV,
respectively. As shown in the molecular orbital distribution, the
HOMO of CzTPPh and CzTPPy was localized on the same phenyl-
carbazole unit, which resulted in similar HOMO levels in CzTPPh
and CzTPPy. However, the LUMO level was stabilized in the CzTPPy
host because of electron deficient pyridine substituent. All energy
levels of the host materials are summarized in Table 1.
substituents. Weak n-p* absorption of the carbazole moiety was
observed between 310 nm and 350 nm and strong
p
e
p
* absorption
Thermal properties of the host materials were analyzed with
differential scanning calorimeter (DSC). Fig. 4 shows DSC thermo-
grams of CzTPPh and CzTPPy. Glass transition temperature (Tg) of
CzTPPy was higher than that of CzTPPh by 8 ꢀC due to strong
intermolecular interaction by the pyridine substituent. The pyri-
dine unit was effective to improve the Tg of carbazole modified
terphenyl derivatives.
As the CzTPPh and CzTPPy had different electron transport
moieties in the molecular structure, electron current density of the
device was compared. Fig. 5 shows current densityevoltage curves
of CzTPPh and CzTPPy devices. The electron current density of
CzTPPy was higher than that of CzTPPh and the voltage at the same
current density was reduced by 1.8 V. The high electron current
density of the CzTPPy device is due to the pyridine substituent.
Electron deficiency of the pyridine unit improved electron trans-
port properties of the CzTPPy device, resulting in high electron
current density.
Blue PHOLEDs were fabricated based on the high triplet energy
of CzTPPh and CzTPPy host materials. FIrpic was doped as a blue
triplet emitter and doping concentrations of FIrpic were 5% and
10%. Current densityevoltageeluminance curves of CzTPPh and
CzTPPy blue PHOLEDs are shown in Fig. 6. The current density of
the blue PHOLED was high in the CzTPPy device because of high
electron current density as shown in electron only device data. The
driving voltages at 1000 cd/m2 were 6.7 V and 6.2 V for CzTPPh and
CzTPPy devices. The doping concentration of FIrpic also affected the
of the carbazole modified terphenyl was detected below 300 nm.
The substituent had little influence on the UVeVis absorption of the
two host materials. Solution PL emission of CzTPPh and CzTPPy was
observed at 366 nm and 364 nm, respectively. There was little
difference of solution PL emission between the two host materials.
Solid PL peak positions of CzTPPh (370 nm) and CzTPPy (373 nm)
were also similar, but the solid PL emission above 400 nm was quite
different. The long wavelength emission was significant in CzTPPy,
while it was weak in CzTPPh. This indicates that strong intermo-
lecular interaction between CzTPPy molecules induces excimer
formation or charge transfer complex formation.
Phosphorescent emission of the two host materials was
observed in low temperature PL spectra and triplet energies
calculated from the first phosphorescent emission peak were
2.75 eV and 2.73 eV for CzTPPh and CzTPPy, respectively. There was
little difference of the triplet energy between CzTPPh and CzTPPy
because the phenyl and pyridinyl substituents did not affect the
triplet energy of the carbazole modified terphenyl core. As both
phenyl and pyridinyl substituents were attached to the meta-
position of the terphenyl unit, the triplet energy of the carbazole
modified terphenyl was not degraded. The triplet energy of the
CzTPPh and CzTPPy was higher than that of common FIrpic dopant
and efficient energy transfer from the host materials to the FIrpic
dopant is expected.
100
CzTPPh
CzTPPy
CzTPPh
10
CzTPPy
1
0.1
0.01
0.001
0.0001
0.00001
endothermic
0
2
4
6
8
10
50
100
150
200
250
300
Temperature (o
C)
Voltage (V)
Fig. 4. DSC thermograms of CzTPPh and CzTPPy.
Fig. 5. Current densityevoltage curves of electron only device of CzTPPh and CzTPPy.

154
C.W. Lee, J.Y. Lee / Dyes and Pigments 101 (2014) 150e155
100
10
100000
CzTPPh (5%)
CzTPPh (10%)
CzTPPy (5%)
CzTPPy (10%)
CzTPPh
CzTPPy
10000
1000
100
1
0.1
0.01
0.001
0.0001
0.00001
10
1
0
2
4
6
8
10
Voltage (V)
Fig. 6. Current densityevoltageeluminance curves of CzTPPh and CzTPPy blue
PHOLEDs.
400
500
600
700
800
Wavelength (nm)
2.40
2.52
2.68
CzTPPy
3.16
Fig. 9. EL spectra of CzTPPh and CzTPPy blue PHOLEDs.
2.51
CzTPPh
of the blue PHOLEDs was optimized at high doping concentration
and high quantum efficiency was achieved at 10% doping concen-
tration in both devices. Maximum quantum efficiencies of the
CzTPPh and CzTPPy blue PHOLEDs were 20.2% and 15.7%, respec-
tively. The high quantum efficiency was maintained even at high
luminance and high quantum efficiencies of 19.0% and 15.3% were
obtained at 1000 cd/m2 in the CzTPPh and CzTPPy blue PHOLEDs,
respectively. The quantum efficiency of the CzTPPy host was lower
than that of the CzTPPh host in spite of high electron current
density. The relatively low quantum efficiency of the CzTPPy host
can be explained by long wavelength solid PL emission of the
CzTPPy host (Fig. 2). The long wavelength emission of the CzTPPy
host could not contribute to the energy transfer to FIrpic, which
degraded the quantum efficiency of the CzTPPy blue PHOLEDs. The
CzTPPh host showed better quantum efficiency than common mCP
host.
Electroluminescence (EL) spectra of the CzTPPh and CzTPPy blue
PHOLEDs are shown in Fig. 9. The two devices exhibited typical
FIrpic emission with a peak maximum at 469 nm. The EL spectra
were maintained stable irrespective of the luminance of the device.
The color coordinates of CzTPPh and CzTPPy blue devices were
(0.14, 0.28) and (0.14, 0.30) at all luminance range from 10 cd/m2 to
2000 cd/m2.
mCP
FIrpic
TSPO1
6.11
6.03
6.10
6.04
6.79
Fig. 7. Energy level diagram of CzTPPh and CzTPPy blue PHOLEDs.
current density of the blue PHOLEDs and high current density was
obtained at 10% doping concentration. As shown in the energy level
diagram in Fig. 7, there was large difference of LUMO levels be-
tween host materials and FIrpic. The large LUMO level difference
induces electron trapping by FIrpic, which dominates electron
transport of the blue PHOLEDs. In the case of the electron trapped
devices, electron transport properties are improved at high doping
concentration due to electron hopping between dopant materials.
Therefore, high current density was obtained at high doping con-
centration. The luminance followed the same tendency as the
current density.
5. Conclusions
Quantum efficiencyeluminance curves of the CzTPPh and
CzTPPy blue PHOLEDs are shown in Fig. 8. The quantum efficiency
In conclusion, carbazole modified terphenyl based host mate-
rials, CzTPPh and CzTPPy, were synthesized as high triplet energy
host materials and showed high quantum efficiency in blue PHO-
LEDs. The CzTPPh host was better than CzTPPy host in terms of
quantum efficiency and showed high quantum efficiency of 20.2%.
Therefore, the carbazole modified terphenyl core can be effectively
used as the core structure of blue triplet host materials in future
development.
25
20
15
10
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1
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Luminance (cd/m2)
Fig. 8. Quantum efficiencyeluminance curves of CzTPPh and CzTPPy blue PHOLEDs.

C.W. Lee, J.Y. Lee / Dyes and Pigments 101 (2014) 150e155
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