Bipolar host materials with carbazole and dipyridylamine groups showing high triplet energy for blue phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2017.02.014

A series of new bipolar host materials with carbazole and different number of dipyridyl amine groups were synthesized for blue phosphorescent organic light-emitting diodes (PHOLEDs). Especially, we applied dipyridyl amine to control the polarity as well a

Full text of DOI:10.1016/j.dyepig.2017.02.014

Dyes and Pigments 141 (2017) 217e224
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Bipolar host materials with carbazole and dipyridylamine groups
showing high triplet energy for blue phosphorescent organic light
emitting diodes
So-Ra Park 1, Su-Mi Kim b 1, Ji-Ho Kang , Ji-Hoon Lee , Min Chul Suh a
a
,
,
a
b
,
*
, **
a
Organic Electronic Materials Laboratory, Department of Information Display, Kyung Hee University, Dongdaemoon-Gu, Seoul 02447, Republic of Korea
Department of Polymer Science and Engineering & Department of IT Convergence (BK21 PLUS), Korea National University of Transportation, 50 Daehak-
b
ro, Chungju 27469, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 January 2017
Received in revised form
A series of new bipolar host materials with carbazole and different number of dipyridyl amine groups
were synthesized for blue phosphorescent organic light-emitting diodes (PHOLEDs). Especially, we
applied dipyridyl amine to control the polarity as well as electron transporting property of the carbazole
based host materials for comparison with the properties of 1,3-di(9H-carbazol-9-yl)benzene (mCP). As a
result, all the carbazole based host materials with dipyridyl amine moieties showed very high triplet
energies (>2.9 eV) as well as wide band gap (>3.5 eV). Thus, the blue phosphorescent devices employing
those materials with bis(4,6-difluorophenyl pyridinato-N,C2)picolinato-iridium (III) (FIrpic) as a dopant
and we obtained moderately high external quantum efficiency (EQE) up to 21.6% with N-(3,5-di(9H-
carbazol-9-yl)phenyl)-N-(pyridin-2-yl)pyridin-2-amine (DCPPy).
10 February 2017
Accepted 10 February 2017
Available online 14 February 2017
Keywords:
Organic light emitting diodes
Blue host materials
Highly efficient blue PHOLED
Carbazole based host materials
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
[11,12].
In principle, blue host materials are required to have a large
Organic light emitting diodes (OLEDs) have received consider-
able attention due to their superior device properties which are
perfectly suitable for large area displays and solid-state lightings.
However, OLEDs are still struggling against liquid crystal displays
band gap (E
greater than 2.9 eV to prevent the back energy transfer from dopant
to host materials (T of dopant > 2.7 eV) [13,14]. Besides, they
should have moderate hole as well as electron charge transport
behavior and thermal stability for stable device performance
[15e20]. The most common approach is to use a carbazole core
moiety and an electron accepting heteroaromatic ring moiety with
g 1
) above 3.5 eV as well as a large triplet energy (T ) level
1
(
LCDs) due to a short lifetime as well as a low efficiency of blue
emitting materials. Hence, materials for blue phosphorescent
organic light-emitting diodes (PHOLEDs) has become very impor-
tant to overcome such critical issues [1e10]. Nevertheless, it is still
being required to understand premise condition for materials
design of blue phosphorescent emitters because there has been no
great success to synthesize the materials showing moderately long
lifetime behavior for last decade. Indeed many research groups are
focusing on the development into a suitable host materials for blue
PHOLEDs. Especially, they are trying their best to avoid side effects
such as triplet-triplet annihilation (TTA), triplet-polaron annihila-
tion (TPA), and many other types of self-quenching processes
limited conjugation length to increase the band gap as well as T
1
level for blue phosphorescent devices [21,22]. At the beginning, the
most representative material with carbazole groups was 1,3-di(9H-
carbazol-9-yl)benzene (mCP) reported by Forrest [23]. Especially, a
mCP was a very suitable host material for blue PHOLEDs because it
has moderately high triplet energy and large band gap about 2.9 eV
and 3.5 eV, respectively. Unfortunately, the devices prepared by
using this material show very short lifetime presumably due to its
ꢀ
low glass transition temperature (T
g
) (about 60 C) which may
cause molecular aggregation or crystallization from joule heating of
the devices [24]. To overcome such an issue, it is necessary to
synthesize new host materials with increased T
phorescent devices.
In this paper, we report three different types of bipolar host
g
for blue phos-
*
Corresponding author.
** Corresponding author.
E-mail addresses: jihoonli@ut.ac.kr (J.-H. Lee), mcsuh@khu.ac.kr (M.C. Suh).
Both authors have equally contributed to this work.
1
http://dx.doi.org/10.1016/j.dyepig.2017.02.014
143-7208/© 2017 Elsevier Ltd. All rights reserved.
0

218
S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
0
materials with high triplet energy (T
1
> 2.9 eV) by connecting
9H-carbazole (5 g, 10.3 mmol), 2,2 -dipyridylamine (1.9 g,
11.3 mmol), copper(I) iodide (0.2 g, 1.0 mmol), 18-Crown-6 (0.09 g,
0.34 mmol), cesium carbonate (7.2 g, 20.5 mmol) was dissolved in
1,2-dichlorobenzene (72 ml). And 1,3-Dimethyl-3,4,5,6-tetrahydro-
2(1H)-pyrimidinone (DMPU) (0.25 mL) was put into reactor, then
carbazole and different number of dipyridyl amine groups to the
core phenyl linker for improvement of their hole as well as electron
transport properties. As a result, we obtained moderately high
external quantum efficiency (EQE) up to 21.6% with N-(3,5-di(9H-
carbazol-9-yl)phenyl)-N-(pyridin-2-yl)pyridin-2-amine (DCPPy)
device.
ꢀ
keep heating at 170 C for 18 h under nitrogen. After cooling to
room temperature, the mixture was poured into a cold hexane. The
precipitate was filtered and washed with hexane. The solid
was purified with column chromatography [n-Hexane:
Ethylacetate ¼ 1: 5 (vol.: vol.)] to give yellowish powder in 38% of
yield. After primary purification, N-(3-(9H-carbazol-9-yl)phenyl)-
N-(pyridin-2-yl)pyridin-2-amine (mCPPy) was obtained as a white
2
2
2
. Experimental
.1. Synthesis of new host material
ꢀ
1
powder by train sublimation. (m.p. ¼ 132 C) H NMR (400 MHz,
CD Cl )
(ppm) 8.35 (s, 2H) 8.10e8.12 (d, J ¼ 8.0, 2H) 7.62e7.65 (t,
J ¼ 8.0, 2H) 7.56e7.60 (t, J ¼ 8.0, 1H) 7.50e7.52 (d, J ¼ 8.0, 2H)
.39e7.43 (t, J ¼ 8.0, 3H) 7.36 (s, 1H) 7.25e7.29 (t, J ¼ 8.0, 2H)
.21e7.23 (d, J ¼ 8.0, 1H) 7.11e7.12 (d, J ¼ 8.0, 2H) 7.00e7.02 (d,
.1.1. Instruments
1H NMR and 13C NMR spectra were recorded on a Bruker
2
d
2
AVANCE 400FT-NMR(400 Hz). The absorption spectra were ob-
tained by ultravioletevisible (UVevis) spectrophotometer (HEW-
7
7
LETT
PACKARD
8453
UVevis
spectrophotometer).
13
J ¼ 8.0, 1H) 6.99e7.00 (d, J ¼ 8.0, 1H) i C NMR (400 MHz, CD
2 2
Cl )
Photoluminescence (PL) spectra were collected by PERKIN ELMER
LS 50B luminescence spectrometer. The differential scanning
calorimetry (DSC) was performed on TA instruments and ther-
mogravimetric analysis (TGA) using TA INSTRUMENT TA4100. High
resolution mass spectra were recorded using a HR-ESI MS SYNAPU
G2 (Waters, U.K.) HOMO (Highest Occupied Molecular Orbital) was
measured with a photoelectron spectrometer (AC2, Hitachi High
Tech).
1
1
17.7, 119.1, 120.4, 120.5, 123.1, 123.7, 125.21, 125.25, 126.3, 130.8,
þ
38.0, 138.8, 140.8, 147.1, 148.9, 158.3 ppm [MþH] : m/z calcd for
C
28
H21 N4, 413.1766 found 413.1768.
0
2
.1.2.2. Synthesis of 9,9 -(5-bromo-1,3-phenylene)bis(9H-carbazole)
1). In a flask a mixture of 1,3,5-tribromobenzene (10 g, 31.8 mmol),
carbazole (10.7 g, 64.2 mmol), copper(I) iodide (2.4 g, 12.7 mmol),
Cesium carbonate (16.8 g, 47.7 mmol) in Toluene (96 ml) was stir-
red. And trans-1,2-diaminocyclohexane (1.9 ml,15.9 mmol) was put
(
2
2
.1.2. Synthetic details
ꢀ
.1.2.1. Synthesis of N-(3-(9H-carbazol-9-yl)phenyl)-N-(pyridin-2-yl)
into reactor, then keep it heating at 80 C under N
2
atmosphere for
pyridin-2-amine (mCPPy). In a Schlenk flask, 9-(3-bromophenyl)-
24 h. The resulting mixture was cooled to room temperature and
Scheme 1. Synthesis of new blue host materials.

S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
219
Fig. 1. Optimized molecular geometry of (a) mCPPy (b) CPDPy and (c) DCPPy molecules showing HOMO (left), LUMO (middle), and side view of molecules (right).
then filtered and washed CH
rotary evaporation, the residue was purified by silica gel column
chromatography (n-hexane:CH Cl 7:1, white powder, yield 28%).
(ppm) 8.14e8.16 (d, J ¼ 8.0 Hz, 4H)
2
Cl
2
. After removal of the solvent by
powder in 28% of yield. 1H NMR (400 MHz, CDCl
) d (ppm)
3
8.14e8.16 (d, J ¼ 8.0 Hz, 4H) 7.86e7.87 (s, 2H) 7.79e7.80 (s, 1H)
7.54e7.56 (d, J ¼ 8.0, 4H) 7.44e7.48 (t, J ¼ 8.0, 4H) 7.31e7.35 (t,
J ¼ 8.0, 4H).
2
2
1
3
H NMR (400 MHz, CDCl ) d
7
.86e7.87 (s, 2H) 7.79e7.80 (s, 1H) 7.54e7.56 (d, J ¼ 8.0, 4H)
7.44e7.48 (t, J ¼ 8.0, 4H) 7.31e7.35 (t, J ¼ 8.0, 4H).
2.1.2.5. Synthesis of N-(3,5-di(9H-carbazol-9-yl)phenyl)-N-(pyridin-
2
-yl)pyridin-2-amine (DCPPy). In a schlenk flask Compound (2)
1
1
3
3
0
2
.1.2.3. Synthesis of 5-(9H-carbazol-9-yl)-N ,N ,N ,N -tetra(pyridin-
(5 g, 10.3 mmol), 2,2 -dipyridylamine (1.9 g, 11.3 mmol), copper(I)
iodide (0.2 g, 1.0 mmol), 18-Crown-6 (0.1 g, 0.4 mmol), cesium
carbonate (7.4 g, 20.5 mmol) was dissolved in 1,2-dichlorobenzene.
And DMPU (3 mL) was put into reactor, then keep it heating at
2
-yl)benzene-1,3-diamine (CPDPy). In a schlenk flask Compound
0
(
1) (5 g, 12.5 mmol), 2,2 -dipyridylamine (4.9 g, 28.7 mmol), cop-
per(I) iodide (0.95 g, 5.0 mmol), 18-Crown-6 (0.33 g, 1.3 mmol),
cesium carbonate (15.4 g, 43.6 mmol) was dissolved in dime-
thylsulfoxide (52 ml). And DMPU (1.3 mL) was put into reactor, then
ꢀ
180 C for 13 h under nitrogen. After cooling to room temperature,
the mixture was pour cold hexane. The precipitate was filtered and
washed hexane. The solid was purified with column chromatog-
raphy [n-Hexane:Ethylacetate 7: 5 (vol.: vol)] to give yellowish
powder in 41% of yield. N-(3,5-di(9H-carbazol-9-yl)phenyl)-N-
(pyridin-2-yl)pyridin-2-amine (DCPPy) was obtained as a white
ꢀ
keep it heating at 170 C for 18 h under nitrogen. After cooling to
room temperature, the mixture was pour cold hexane. The pre-
cipitate was filtered and washed hexane. The solid was purified
with column chromatography [n-Hexane: Ethylacetate ¼ 1:1 (vol.:
vol.)] to give yellowish powder in 37% of yield. 5-(9H-carbazol-9-
ꢀ
1
powder by train sublimation. (m.p. ¼ 263 C) H NMR (400 MHz,
1
1
3
3
yl)-N ,N ,N ,N -tetra(pyridin-2-yl)benzene-1,3-diamine (CPDPy)
was obtained as white powder by train sublimation.
m.p. ¼ 263 C) H NMR (400 MHz, CD Cl 8.33e8.35 (d, 4H)
.06e8.08 (d, 2H) 7.59e7.63 (t, 4H) 7.55e7.58 (d, 2H) 7.37e7.41 (t,
H) 7.22e7.26 (t, 2H) 7.13e7.15 (d, 3H) 7.13 (s, 3H) 6.96e7.00 (m,
H) ppm 13C NMR (400 MHz, CD
Cl ) 117.9, 119.2, 120.4, 120.5,
CD Cl
2
2
)
d
(ppm) 8.45e8.47 (s, 2H) 8.12e8.13 (d, J ¼ 8.0, 4H)
a
7.67e7.71 (t, J ¼ 8.0, 2H) 7.64e7.66 (m, J ¼ 4.0, 5H) 7.41e7.45 (t,
J ¼ 8.0, 6H) 7.27e7.30 (t, J ¼ 8.0, 4H) 7.20e7.22 (d, 2H) 7.05e7.09 (t,
ꢀ
1
(
2
2
) d
13
8
2
5
J ¼ 8.0, 2H) C NMR (400 MHz, CD
2 2
Cl ) 110.4, 118.2, 119.8, 122.4,
123.8, 126.4, 138.4, 139.8, 140.6, 148.1, 149.2, 157.9 ppm ESI-MS
þ
2
2
[MþH] : m/z calcd for C40
28 5
H N , 578.2345; found 578.2345.
1
22.2, 123.7, 126.2, 138.0, 139.2, 140.5, 147.4, 148.9, 158.0 ppm ESI-
þ
MS [MþH] : m/z calcd for C38
H
28
7
N , 582.2406; found 582.2414.
2.2. Fabrication of blue PHOLEDs
0
2
.1.2.4. Synthesis of 9,9 -(5-bromo-1,3-phenylene)bis(9H-carbazole)
2.2.1. Materials
(
2). In a flask a mixture of 1,3,5-tribromobenzene (10 g, 31.8 mmol),
We purchased or synthesized the following materials. Poly(3,4-
ethylene dioxythiophene) (PEDOT) doped with poly(-
styrenesulfonate)anions (PSS) (PEDOT:PSS) (PEDOT:PSS) was pur-
chased from Heraeus (Heraeus Clevios™ P VP CH 8000) as a hole
carbazole (10.7 g, 64.2 mmol), copper(I) iodide (2.4 g, 12.7 mmol),
Cesium carbonate (16.8 g, 47.7 mmol) in Toluene (96 ml) was stir-
red. And trans-1,2-diaminocyclohexane (1.9 ml,15.9 mmol) was put
ꢀ
0
into reactor, then keep it heating at 80 C under N
2
atmosphere for
injection layer (HIL). 1,1 -bis(di-4-tolylaminophenyl) cyclohexane
2
4 h. The resulting mixture was cooled to room temperature and
then filtered and washed CH Cl . After removal of the solvent by
rotary evaporation, the residue was purified by silica gel column
chromatography [n-hexane: CH Cl
¼ 7: 1 (vol.: vol)], to give white
(TAPC) and mCP were purchased from Lumtec Corp. and used as a
hole transporting materials (HTL) as well as an exciton blocking
layer (EBL), respectively. mCPPy, CPDPy and DCPPy were synthe-
sized as aforementioned and utilized as host materials for emitting
2
2
2
2

220
S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
Fig. 3. (a) DSC and (b) TGA results of mCPPy, CPDPy and DCPPy, respectively.
2
oxide (ITO) glasses with an open emission area of 4 mm were used.
Fig. 2. UVevisible absorption and photoluminescence spectra of (a) mCPPy (b) CPDPy
and (c) DCPPy, respectively.
The ITO glasses were cleaned in acetone and isopropyl alcohol with
sonication process and rinsed in deionized water. Then, ITO glass
substrates were treated in UV-ozone to eliminate all the organic
impurity remained during previous fabrication processes.
PEDOT:PSS was spin-coated on ITO glass in ambient condition and
layer (EML). In addition, bis(4,6-difluorophenyl pyridinato-N,C2)
picolinato-iridium (III) (FIrpic) was purchased from Lumtec Corp.
and used as a phosphorescent blue dopant for EML. 1,3,5-tri(m-
pyrid-3-yl-phenyl)benzene (TmPyPB) as an electron transporting
material (ETL), lithium fluoride (LiF) as the materials for an electron
injection layer (EIL), and aluminum (Al) as a cathode were also
purchased from the commercial suppliers and were used without
purification.
ꢀ
annealed at 120 C for 15min in nitrogen atmosphere. Subse-
quently, All the organic materials were deposited on PEDOT:PSS by
the vacuum evaporation technique under
a
pressure of
ꢂ6
~1 ꢁ 10 Torr. The deposition rate of organic layers was about
0.5 Å/s. LiF and Al were deposited with rates of 0.1 Å/s and 3 Å/s,
respectively.
2.2.2. Device fabrication
2.2.3. Measurements
To fabricate OLED devices, a 150 nm thick patterned indium-tin
The current density-voltage (J-V) and luminance-voltage (L-V)
Table 1
Summary of physical properties of blue host materials.
a
b
c
HOMOd
[eV]
Compound
l
[
abs
l
pl
T
[eV]
1
T
1
LUMO
[eV]
E
[eV]
g
T
[ C]
g
T
[ C]
m
T
[ C]
d
ꢀ
ꢀ
ꢀ
nm]
[nm]
[eV]
(Exp)
(Cal)
(AC2)
Solution
Film
mCPPy
CPDPy
DCPPy
294, 341
294
294, 340
366
368
362
377
375
372
2.94
2.95
2.94
3.25
3.12
3.22
ꢂ5.86
ꢂ5.78
ꢂ5.86
ꢂ2.30
ꢂ2.24
ꢂ2.30
3.56
3.54
3.57
63
97
108
132
264
263
299
382
383
a
l
l
T
abs: peak wavelength observed from UVevisible absorption spectroscopy (at 297 K).
pl: peak wavelength measured from photoluminescence (PL) spectra (at 297 K and 77 K).
b
c
1
was calculated from DMol3.
d
HOMO values were determined by photoelectron spectroscopy (AC2, RIKEN KEIKEI).

S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
221
Fig. 4. Energy diagrams of (a) blue PHOLEDs (b) hole only devices, and (c) electron only devices in this study.
data of OLEDs were collected by Keithley SMU 2635A and Minolta
CS-100A, respectively. The OLED area was 4 mm for all the samples
studied in this work. Electroluminescence (EL) spectra and the
Commission Internationale De'Eclairage (CIE) coordinate were ob-
tained using a Minolta CS-2000A spectroradiometer.
(LUMO) was mostly localized over dipyridyl amine and benzene
core moieties in mCPPy, CPDPy and DCPPy molecules. In other
words, the hole carriers could be easily transported through the
carbazole, while the electron carriers could be transported through
the dipyridyl amine and benzene moieties after charge injection.
Fig. 2 shows the UVevis absorption and PL spectra for mCPPy,
CPDPy and DCPPy of Fig. 2(a), (b) and (c), respectively. The band
gap determined from the band edge of UVevis spectra were 3.56,
2
3
. Results and discussion
3
.54, 3.57 eV for mCPPy, CPDPy and DCPPy, respectively. The
3
.1. Analysis of newly synthesized blue host materials
HOMO energy level was measured by AC2 analyzer and LUMO
energy level of materials were assigned from subtraction of band
gap from those HOMO energy levels. All of those compounds
showed featureless emission spectral patterns, while we could
found some vibronic fine emission peaks from the emission spectra
We prepared three different types of bipolar host materials by
moderately simple procedure. Especially, we selected carbazole and
dipyridyl amine moieties to obtain high triplet energy. Those
moieties were selected to improve electron transport as well as
hole transport ability. Especially, we used benzene ring as a central
1
obtained at low temperature condition (77 K). Meanwhile, T of
those materials which were determined from such low tempera-
ture PL spectra were 2.94, 2.95, 2.94 eV for mCPPy, CPDPy and
DCPPy, respectively. Hence, they are suitable for host materials
which can be used in blue PHOLEDs. The representative results are
also summarized in Table 1.
linker unit of host materials which could appropriately limit the p-
conjugation to increase in triplet energy due to a steric hindrance of
substituents [25,26]. The detailed synthetic route for preparation of
such materials are shown in Scheme 1.
Fig. 1 shows the molecular geometries of the ground state of
new host materials obtained from the molecular simulation using
simulation at Lee-Yang-Parr correlation (B3LYP) functional with the
double numerical plus d-functions (DND) basis set in the DMol3
module of Materials Studio 2017. From this simulation, we found
that the electron density in the HOMO was mostly located on the
carbazole units, while that of lowest unoccupied molecular orbital
We investigated the thermal stability of the materials synthe-
sized in this study because this is very critical for the materials with
highly twisted geometry to result in high triplet energy as well as
wide band gaps as shown in Fig. 3. From the measurement of DSC,
we found that CPDPy and DCPPy showed higher T
g
of 97 and
ꢀ
ꢀ
108 C, respectively, than that of the mCPPy (63 C) as shown in

222
S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
Table 2
Summary of device characteristics of the blue PHOLEDs.
ona/Vop [V]
b
CE /PE /EQE [cdA /lmW /%]
c
d
e
ꢂ1
ꢂ1
CIE (x,y)
f
Device
V
ꢂ2
At 1000 cd m
Maximum
Reference
Device A
Device B
Device C
4.0/6.2
4.0/6.3
4.0/6.3
4.1/6.5
36.3/27.2/18.3
31.9/22.3/16.3
37.9/26.5/18.9
43.3/24.4/21.6
33.6/16.1/17.0
26.0/12.3/13.4
33.9/16.4/17.2
43.2/19.4/21.6
(0.16, 0.31)
(0.15, 0.30)
(0.16, 0.31)
(0.15, 0.32)
a
ꢂ2
.
V
V
on: turn-on voltage, measured at 1 cd m
op: operating voltage, measured at 1000 cd m
b
c
ꢂ2
.
CE: current efficiency.
PE: power efficiency.
d
e
EQE: external quantum efficiency, the EQE was obtained from the calculation
based on the assumption of a Lambertian distribution.
f
CIE: Commission International de L'Eclairage.
3.2. Device characteristics
To investigate the basic properties of those host materials in full
devices, we prepared the blue PHOLEDs having following device
configurations (see also Fig. 4(a)). We also fabricated the reference
devices with common host materials (mCP) to compare the per-
formance level of blue PHOLEDs applied new synthetic host
materials.
Reference: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP
(
(
10 nm)/mCP: FIrpic (10%, 25 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al
100 nm).
Device A: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP (10 nm)/
mCPPy: FIrpic (10%, 25 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al
100 nm).
(
Fig. 5. (a) J-V-L, (b) current efficiency e L, and (c) external quantum efficiency e L
characteristics of blue PHOLEDs.
Fig. 3(a). Meanwhile, the T
g
of mCPPy is very similar to that of mCP
ꢀ
(
60 C), which might be due to an increased free volume compared
to those of other materials having more substituents which may
reduce the free volume inside of bulk film. However, compared
g
with mCP with two carbarzole units on benzene core, T 's of CPDPy
ꢀ
and DCPPy were highly increased about 40 C or more due to
additional substituents such as dipyridyl amine or carbazole on the
same core. Generally, the T
lecular weight and molecular size. Besides, the crystallization
temperature (T ) and melting points (T ) of three host materials
g
values increase with increasing mo-
c
m
were not observed in DSC 2nd scans. It means that all synthesized
materials is amorphous. Therefore, it is expected that it will prevent
the decrease in the efficiency of the devices caused by the recrys-
tallization in the fabrication and the driving of device [27]. In
d
contrast, the thermal decomposition temperature (T ) determined
ꢀ
by TGA were pretty high up to 299, 382 and 383 C for mCPPy,
CPDPy and DCPPy, respectively, as shown in Fig. 3(b) although their
T
g
's were not that high. The representative results including T
m
's
Fig. 6. J-V characteristics of (a) hole only devices (blue host: FIrpic 10%) and electron
only devices (blue host: FIrpic 10%), (b) relative current density (J(hole)/J(electron))
with newly synthesized blue host materials in this study.
are also summarized in Table 1.

S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
223
To understand the reason of the difference in device perfor-
mances, we fabricated hole only devices (HODs) as well as electron
only devices (EODs) as follow (see also Fig. 4(b) and (c)):
HOD A: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP (10 nm)/
mCPPy: FIrpic (10%, 25 nm)/MoO
HOD B: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP (10 nm)/
CPDPy: FIrpic (10%, 25 nm) MoO (10 nm)/Al (100 nm).
3
(10 nm)/Al (100 nm).
3
HOD C: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP (10 nm)/
DCPPy: FIrpic (10%, 25 nm)/MoO3 (10 nm)/Al (100 nm).
EOD A: ITO/TmPyPB (10 nm)/mCPPy: FIrpic (10%, 25 nm)/
TmPyPB (35 nm)/LiF (1 nm)/Al (100 nm).
EOD B: ITO/TmPyPB (10 nm)/CPDPy: FIrpic (10%, 25 nm)/
TmPyPB (35 nm)/LiF (1 nm)/Al (100 nm).
EOD C: ITO/TmPyPB (10 nm)/DCPPy: FIrpic (10%, 25 nm)/
TmPyPB (35 nm)/LiF (1 nm)/Al (100 nm).
3
Here, the purpose of molybdenum oxide (MoO ) and TmPyPB in
HOD and EOD devices is to block the injection of electron and hole
carriers from the Al (~ꢂ4.30 eV) and ITO (~ꢂ2.90 eV) electrode,
respectively. Actually, we used CH8000 (PEDOT:PSS) as a HIL to
reduce the leakage current level mentioned above (see also
Fig. 4(a). Commonly, CH8000 based devices exhibit maximum
current efficiency at low current density, while the efficiency drops
sharply at high current density which might be due to the seriously
increased amount of PSS acting as an insulator [28,29]. As a result,
exciton may be formed at the interface close to the anode side since
the hole injection into the EML is pretty limited even at the high
current density condition. Hence, it is very important to inject the
electron carriers deeply into an anode side to increase the proba-
bility of charge recombination for outstanding device perfor-
mances. To estimate the relative position for charge recombination,
we prepared the half devices with EML doped by 10 wt% of FIrpic.
Particularly, we plotted the relative current density of those devices
by dividing the hole current density of HODs by the electron cur-
rent density of the EODs [e.g., J(hole)/J(electron) where J: current
Fig. 7. Normalized EL spectra of fabricated blue PHOLEDs (at a brightness of 1000 cd/
m ).
2
Device B: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP (10 nm)/
CPDPy: FIrpic (10%, 25 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al
(
100 nm).
Device C: ITO/PEDOT:PSS (55 nm)/TAPC (20 nm)/mCP (10 nm)/
DCPPy: FIrpic (10%, 25 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al
100 nm).
(
We used CH8000 (PEDOT:PSS) as a HIL to reduce the leakage
current level of the devices. Meanwhile, we used TAPC and TmPyPB
as a HTL and an ETL to suppress the exciton energy quenching of
EML because they show higher triplet energy levels [T
1
(
(
TAPC) ¼ 2.9 eV; T
¼ 2.7 eV).
Fig. 5(a) shows the current density - voltage - luminance (J e V e
1
(TmPyPB) ¼ 2.8 eV] than that of FIrpic
T
1
2
density, mA/cm ] as shown in Fig. 6(b), where the J - V character-
istics shown in Fig. 6(a) were collected from the HODs and EODs,
respectively. From this plot, we could expect that the DCPPy could
give the most desirable device performance because the relative
charge density value of the device fabricated with DCPPy showed
fastest electron transporting behavior. Very interestingly, the de-
vices fabricated with CPDPy also showed fast electron transporting
behavior so that the full device also show relatively high efficiency
values. In contrast, the devices containing EML with mCPPy
showed poor electron transporting behavior which cannot gives
high performances when it was applied to the full devices.
L) characteristics of the devices fabricated in this study. At a given
constant voltage of 5.0 V, current density values were recorded to
2
0
.39, 0.49, 0.26, and 0.12 mA/cm from Reference, Device A, Device
B and Device C, respectively. The turn-on voltages (Von) of 4.0, 4.0,
.0, and 4.1 V were obtained from Reference, Device A, Device B
and Device C, respectively. Meanwhile, the operation voltages (Vop
4
)
2
to reach 1000 cd/m were 6.2, 6.3, 6.3, and 6.5 V for Reference,
Device A, Device B, and Device C, respectively (see Table 2). At a
given constant luminance of 1000 cd/m , the current and power
2
efficiencies were 33.6 cd/A and 16.1 lm/W for Reference, 26.0 cd/A
and 12.3 lm/W for Device A, 33.9 cd/A and 16.4 lm/W for Device B
and 43.2 cd/A and 19.4 lm/W for Device C, respectively, as shown in
Fig. 5(b) And, those are also summarized in Table 2. Those efficiency
data correspond to 17.0, 13.4, 17.2 and 21.6% of EQEs for Reference
and Device A, B, and C, respectively. Very interestingly, the EQE
values increased the order of Device A < Reference < Device
Fig. 7 shows the normalized EL spectra of Reference and Device
2
A-C at the brightness of 1000 cd/m . All spectra have FIrpic emis-
sion with a peak wavelength (lmax) at 472 nm for Reference and
Device A-C, respectively.
2
4. Conclusions
B < Device C at the given brightness of 1000 cd/m as shown in
Fig. 5(c) Likewise, the maximum current and power efficiencies
were 36.3 cd/A and 27.2 lm/W for Reference, 31.9 cd/A and 22.3 lm/
W for Device A, 37.9 cd/A and 26.5 lm/W for Device B, 43.3 cd/A
and 24.4 lm/W for Device C, respectively, as summarized in Table 2.
In particular, maximum EQE values also increased in the order of
Device A < Reference < Device B < Device C (e.g., Reference:
In this study, we found that the introduction of carbazole moi-
eties and dipyridyl amine moieties is very useful to prepare the host
materials for highly efficient blue PHOLEDs. We could control the
electron transporting properties by changing the numbers of
dipyridyl amine moieties. New host materials showed moderate
current efficiency as well as EQE values when it was applied to the
blue PHOLEDs. Especially, we found that the selection of fast elec-
tron moving EML is profitable to prepare highly efficient blue
PHOLEDs. In this study, DCPPy was selected as the best candidate
for highly efficient blue PHOLEDs which could give high EQE up to
18.3%; Device A: 16.3%; Device B: 18.9%; Device C: 21.6%). As a
result, we found that Device C (DCPPy) showed a better efficiency
behavior than that of the other materials. Meanwhile, the device
performance of Device A (mCPPy) and Device B (CPDPy) is very
lower or similar to that of Reference (mCP).
21.6%.

2
24
S.-R. Park et al. / Dyes and Pigments 141 (2017) 217e224
[13] Tokito S, Lijima T, Suzuri Y, Kita H, Tsuzuki T, Sato F. Confinement of triplet
Acknowledgments
energy on phosphorescent molecules for highly-efficient organic blue-light-
emitting devices. Appl Phys Lett 2003;83:569.
This work was supported by the National Research Foundation
of Korea (NRF) funded by the Korea government (MSIP) (NRF-
2
Science Research Program through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Education (NRF-
2
[
[
[
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