Improved power efficiency in deep blue phosphorescent organic light-emitting diodes using an acridine core based hole transport material

Article abstract of DOI:10.1016/j.orgel.2012.03.040

A high triplet energy hole transport material with an acridine core was synthesized as the hole transport material for deep blue phosphorescent organic light emitting diodes. The acridine core based hole transport material showed a high triplet energy of 2.89 eV for efficient triplet exciton blocking and highest occupied molecular orbital of 5.96 eV for efficient hole injection in deep blue phosphorescent organic light-emitting diodes. The acridine core based hole transport material showed low driving voltage than common high triplet energy hole transport material and power efficiency of deep blue phosphorescent organic light-emitting diodes was improved by more than 50%.

Full text of DOI:10.1016/j.orgel.2012.03.040

Organic Electronics 13 (2012) 1245–1249
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Improved power efficiency in deep blue phosphorescent organic
light-emitting diodes using an acridine core based hole transport material
⇑
Mounggon Kim, 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:
A high triplet energy hole transport material with an acridine core was synthesized as the
hole transport material for deep blue phosphorescent organic light emitting diodes. The
acridine core based hole transport material showed a high triplet energy of 2.89 eV for effi-
cient triplet exciton blocking and highest occupied molecular orbital of 5.96 eV for efficient
hole injection in deep blue phosphorescent organic light-emitting diodes. The acridine core
based hole transport material showed low driving voltage than common high triplet
energy hole transport material and power efficiency of deep blue phosphorescent organic
light-emitting diodes was improved by more than 50%.
Received 2 February 2012
Received in revised form 19 March 2012
Accepted 24 March 2012
Available online 17 April 2012
Keywords:
Acridine core
Hole transport material
High efficiency
High triplet energy
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
its use as the high triplet energy HTM. Other than mCP, var-
ious carbazole or aromatic amine based HTMs have been
The device performances of deep blue phosphorescent
organic light-emitting diodes (PHOLEDs) depends on
charge transport materials and emitting materials.
Although the emitting materials mostly determine light
emitting performances of PHOLEDs, the charge transport
materials are also critical to the device performances of
PHOLEDs.
Hole transport materials (HTMs) in PHOLEDs are re-
quired to possess high triplet energy for triplet exciton
blocking, highest occupied molecular orbital (HOMO) level
for hole injection and lowest unoccupied molecular orbital
(LUMO) level for electron blocking. In particular, the triplet
energy of HTM should be higher than that of phosphores-
cent emitting material. Therefore, many high triplet energy
HTMs have been synthesized. The most well known high
triplet energy HTM is 1,3-di(9H-carbazol-9-yl)benzene
(mCP) [1,2]. It has a high triplet energy of 2.9 eV and HOMO
level of 6.1 eV for hole injection. However, poor thermal
stability caused by low glass transition temperature limited
reported [3–9]. However, there are only several high triplet
energy HTMs which can be used in deep blue PHOLEDs.
Therefore, further development of HTMs with high triplet
energy above 2.80 eV is necessary to develop high effi-
ciency deep blue PHOLEDs.
In this work, a novel high triplet energy HTM with an
appropriate HOMO level, 4,40-(10-phenyl-9,10-dihydroac-
ridine-9,9-diyl)bis(N,N-diphenylaniline) (PADPA), was de-
signed and synthesized as a HTM to obtain low driving
voltage and high power efficiency for deep blue PHOLEDs.
The synthesis and photophysical properties of PADPA are
described and the device performances of deep blue PHOL-
EDs with PADPA HTM were investigated. It was demon-
strated that PADPA can be effectively used as the HTM
for low driving voltage and high power efficiency in deep
blue PHOLEDs.
2. Experimental
The synthesis of intermediate compound and general
analysis of synthesized compound are described in our
previous work [10].
⇑
Corresponding author. Tel./fax: +82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.03.040

1246
M. Kim, J.Y. Lee / Organic Electronics 13 (2012) 1245–1249
2.1. Synthesis of 4,40-(10-phenyl-9,10-dihydroacridine-9,9-
diyl)bis(N,N-diphenylaniline) (PADPA)
nm)/LiF(1 nm)/Al(200 nm). All organic materials were
deposited by vacuum thermal evaporation. All devices
were encapsulated with a CaO getter and a glass lid after
device fabrication. The device performances of the blue
PHOLEDs were measured with Keithley 2400 source
measurement unit and CS1000 spectroradiometer.
A 100 ml flask was charged with 9,9-bis(4-bromophe-
nyl)-10-phenyl-9,10-dihydroacridine (0.6 g, 0.001 mol),
diphenylamine (0.44 g, 0.002 mol), palladium(II) acetate
(0.012 g, 0.00005 mol), and toluene (10 ml) under an argon
atmosphere.
A solution of sodium-t-butoxide (0.61 g,
3. Results and discussion
0.0064 mol) in toluene (5 ml) and tri-tert-butylphosphine
(1.27 ml, 1 M) were added slowly. The mixture was re-
fluxed under an argon atmosphere for 10 h. After cooling
the solution, the reaction mixture was extracted with di-
luted water and chloroform. The combined organic solution
was dried over magnesium sulfate and concentrated. Puri-
fication by silicagel chromatography using chloroform/n-
hexane gave a white powder. The product was obtained
to 0.48 g (yield 61%). Tg 125 °C. 1H-NMR (600 MHz, CDCl3):
d7.55 (t, 2H, J = 18.0 Hz), 7.47(t, 1H, J = 12.0 Hz), 7.26–
7.23(m, 8H), 7.12–7.09(m, 10H), 7.04(t, 2H, J = 12.0 Hz),
7.00–6.95(m, 10H), 6.90(t, 2H, J = 12.0 Hz), 6.85(d, 2H,
J = 6.0 Hz), 6.44 (d, 2H, J = 12.0 Hz).13C-NMR (600 MHz,
CDCl3): d 148.1, 146.0, 142.4, 140.8, 131.8, 130.9, 130.4,
130.1, 128.9, 127.5, 125.1, 124.0, 122.4, 56.1. MS (FAB)
miz 744 [(M+H)+]. Anal. Calcd for C55H41N3: C, 88.80; H,
5.55; N, 5.65. Found: C, 88.53; H, 5.46; N, 5.55.
PADPA was synthesized by the reaction of brominated
triphenylamine with 4,40-dibromobenzophenone followed
by ring closing reaction using a strong acid and amination
with diphenylamine [10]. Synthetic scheme of PADPA is
shown in Scheme 1. PADPA was purified by a column chro-
matography and a high purity over 99% was obtained from
a high performance liquid chromatography analysis. PAD-
PA compound was confirmed by 1H and 13C nuclear mag-
netic resonance, mass and elemental analysis.
Photophysical properties of PADPA were studied using
ultraviolet–visible (UV–Vis) and photoluminescence (PL)
measurements. Low temperature PL measurement of PAD-
PA was carried out in liquid nitrogen using tetrahydrofuran
solution of PADPA. The solution PL sample was exposed to
Xe-lamp at low temperature to obtain low temperature PL
spectrum. UV–Vis and PL spectra of PADPA are shown in
Fig. 1. PADPA showed strong
p–
p⁄ absorption peak of tri-
2.2. Device fabrication
phenylamine and acridine core at 300 nm. As can be seen
in the molecular structure, PADPA has three triphenyl-
amine units which are separated by sp3 carbon. Conjuga-
tion of three triphenylamine units is not extended due to
sp3 carbon and only the absorption of triphenylamine
was observed. The bandgap of PADPA could be calculated
from the absorption edge of UV–Vis spectrum and was
3.62 eV. PL emission of PADPA was observed at 364 nm.
Low temperature PL spectrum of PADPA was also analyzed
to measure the triplet energy of PADPA. The triplet energy
of PADPA was calculated from the first phosphorescent
emission peak at 428 nm and was 2.89 eV, which was high
enough for exciton blocking in deep blue PHOLEDs.
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,
5 nm)/PADPA or 1,1-bis[(di-4-tolylamino)phenyl]cyclo-
hexane (TAPC) or 4,40-(8H-indolo[3,2,1-de]acridine-8,8-
diyl)bis(N,N-diphenylaniline) (FPCA) (10 nm)/9-(3-(9H-
carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-
9H-carbazole (mCPPO1):bis((3,5-difluoro-4-cyanophenyl)-
pyridine) iridium picolinate (FCNIrpic)(30 nm, 3%)/diphen-
ylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 25
Scheme 1. Synthetic scheme of PADPA.

M. Kim, J.Y. Lee / Organic Electronics 13 (2012) 1245–1249
1247
1.4
1.2
1
100
PADPA UV
90
80
70
60
50
40
30
20
10
0
TAPC
mCP
PADPA
PADPA PL
PADPA Low T PL
0.8
0.6
0.4
0.2
0
225
275
325
375
425
475
525
575
0
3
6
9
12
Wavelength (nm)
Voltage (V)
Fig. 1. UV–Vis and PL spectra of PADPA. Data were obtained in tetrahy-
drofuran solution at a concentration of 1.0 ꢁ 10ꢀ4 M. Low temperature PL
measurement was carried out at 77 k.
Fig. 3. Current density–voltage curves of PADPA, TAPC and mCP hole only
devices.
molecular structure could be solved by designing a phenyl
substituted acridine core because the phenyl unit attached
to the acridine core is out of the acridine plane. Therefore,
excimer or exciplex formation at the interface can be sup-
pressed in PADPA device. This can be confirmed in the geo-
metrical structure of PADPA in Fig. 2.
One merit of the acridine core is the rigidity because the
molecular motion of the acridine unit is hindered by six-
membered ring structure. The rigid fused ring structure
can improve the glass transition temperature of PADPA
and a high glass transition temperature of 125 °C was ob-
tained from differential scanning calorimeter. Therefore,
the poor thermal stability problem of mCP and other high
triplet energy HTMs was solved by using PADPA as high
triplet energy HTM.
Hole only device of PADPA was fabricated to study the
hole transport properties of PADPA. TAPC and mCP were
used as standard materials. Fig. 3 shows current density–
voltage curves of hole only devices. Hole current density
of PADPA device was higher than that of mCP device, while
it was lower than that of TAPC device. It was reported that
TAPC and mCP show hole mobilities of 1.0 ꢁ 10ꢀ2 cm2/V s
and 5 ꢁ 10ꢀ4 cm2/V s, respectively [12,13]. Therefore, hole
only device result indicates that PADPA shows a hole
mobility between mCP mobility and TAPC mobility.
Deep blue PHOLEDs with PADPA as a HTM were fabri-
cated to investigate the device performances of PADPA.
Cyclic voltametry (CV) measurement of PADPA was car-
ried out to measure the HOMO level. The HOMO level of
PADPA was calculated to be ꢀ5.96 eV from the oxidation
curve of CV measurement. The LUMO level of PADPA was
ꢀ2.34 eV from the HOMO level and bandgap. The HOMO
and LUMO levels of PADPA were suitable for hole injection
into deep blue emitting layer and electron blocking from
deep blue emitting layer, respectively. Therefore, PADPA
can be effective as the HTM for deep blue PHOLEDs in
terms of triplet exciton blocking, hole injection and elec-
tron blocking.
Molecular simulation of PADPA was performed to study
the HOMO and LUMO distribution. Density functional the-
ory calculation was carried out using a suite of Gaussian 03
program and the nonlocal density functional of Becke’s 3-
parameters employing Lee-Yang-Parr functional (B3LYP)
with 6–31G⁄ basis sets [11]. HOMO and LUMO distribution
of PADPA is shown in Fig. 2. HOMO of PADPA was localized
on the triphenylamine unit, while LUMO of PADPA was dis-
persed mostly over the acridine core due to strong electron
donating character of the triphenylamine unit. We re-
ported two acridine type core structures in our previous
works [4,5]. Acridine based hole transport materials
showed good device performances, but the problem of
two acridine cores was the planarity of the core. As the
whole core structure was in the same plane, excimer for-
mation could be easily induced. The problem of planar
Fig. 2. HOMO and LUMO distribution of PADPA.

1248
M. Kim, J.Y. Lee / Organic Electronics 13 (2012) 1245–1249
30
25
20
15
10
5
10000
1000
100
10
PADPA
TAPC
FPCA
1
0.1
0
0.01
0
2
4
6
8
Voltage (V)
Fig. 4. Current density–voltage-luminance curves of blue PHOLEDs with TAPC, FPCA and PADPA hole transport layer.
(a)
(b)
2.0
5.5
2.1
5.1
2.4 2.34
2.42
5.78
2.52
3.00
2.87 2.90
2.89
2.98
5.72
2.64
2.74
5.5
6.13
6.21
5.96
6.79
Fig. 5. Energy level diagram of deep blue PHOLEDs (a) and triplet energy of materials (b).
TAPC and FPCA were used as standard materials. Fig. 4
shows current density–voltage and luminance-voltage
curves of PADPA, TAPC and FPCA based deep blue PHOL-
EDs. The current density and luminance of PADPA device
were much higher than those of TAPC device and similar
to those of FPCA at the same driving voltage. This indicates
that PADPA is much better than TAPC and similar to FPCA
to inject holes into mCPPO1:FCNIrpic emitting layer. In
general, hole transport and injection properties of HTM af-
fect the current density of the device. In the case of PADPA,
the high current density is mostly due to improved hole
injection properties. As can be seen in the energy level dia-
gram of the device in Fig. 5, the energy barriers for hole
injection from HTM to emitting layer are 0.17 and
0.63 eV for PADPA and TAPC, respectively. The large energy
barrier for hole injection from TAPC to mCPPO1:FCNIrpic
emitting layer limits hole current density in the emitting
layer, resulting in low current density. Compared with
TAPC device, the energy barrier for hole injection of PADPA
device is only 0.17 eV, facilitating hole injection from PAD-
PA to emitting layer. Therefore, high current density and
low driving voltage were obtained in PADPA device.
TAPC device and similar to that of FPCA device at the same
luminance. The maximum quantum efficiency of PADPA
device was 17.3% and the quantum efficiency at 1000 cd/
m2 was 15.3%. The high quantum efficiency of PADPA de-
vice is due to efficient hole injection effect. Efficient hole
injection balanced holes and electrons in the emitting
layer, resulting in high quantum efficiency in PADPA de-
vice. The high triplet energy of 2.89 eV of PADPA and
2.87 eV of TAPC similarly suppressed triplet exciton
quenching of the emitting layer as reported in other works
[14,15]. The quantum efficiency of PADPA device was com-
parable to that of FPCA device, indicating that PADPA is
suitable as high triplet energy HTMs for deep blue PHOLED.
The quantum efficiency of FPCA device was lower than that
we reported in our earlier work because of low quantum
efficiency of dopant material we used [4]. The quantum
efficiency of TAPC device was also high due to high triplet
energy of TAPC and electron blocking although the quan-
tum efficiency was slightly lower than that of PADPA due
to poor hole injection.
Power efficiency of PADPA device was compared with
that of TAPC and FPCA devices (Fig. 7). The power efficiency
of PADPA device was much higher than that of TAPC device
and similar to that of FPCA device over all luminance range.
The maximum power efficiency and power efficiency at
Quantum efficiency–luminance curves of PADPA, TAPC
and FPCA devices are shown in Fig. 6. The quantum effi-
ciency of PADPA device was slightly higher than that of

M. Kim, J.Y. Lee / Organic Electronics 13 (2012) 1245–1249
1249
at 1000 cd/m2 of 6.8 lm/W. There was more than 50%
improvement of power efficiency by replacing TAPC with
PADPA. The improved power efficiency of PADPA device is
mostly originated from the low driving voltage as shown
in Fig. 4 because there was only small difference of quan-
tum efficiency between PADPA and TAPC.
Electroluminescence (EL) spectrum of PADPA device are
shown in Fig. 8. EL spectrum of TAPC device is also shown
in this figure. Both devices showed deep blue emission
spectrum with a emission peak at 458 nm. Color coordi-
nates of PADPA and TAPC devices were (0.14, 0.19) and
(0.14, 0.18), respectively.
20
16
12
8
PADPA
TAPC
FPCA
4
0
0
1
100
10000
Luminance(cd/m2
)
4. Conclusions
Fig. 6. Quantum efficiency–luminance curves of blue PHOLEDs with
TAPC, FPCA and PADPA hole transport layer.
In conclusion, an acridine based HTM, PADPA, was
effectively synthesized as a HTM for deep blue PHOLEDs
and improved the driving voltage and power efficiency of
deep blue PHOLEDs. In particular, the power efficiency of
deep blue PHOLEDs was enhanced by more than 50% using
PADPA HTM. Therefore, the acridine core can be useful as
the core of organic charge transport materials.
30
PADPA
TAPC
25
FPCA
20
References
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1000 cd/m2 of PADPA device were 21.1 lm/W and 11.9 lm/
W, respectively. However, TAPC device showed only maxi-
mum power efficiency of 14.4 lm/W and power efficiency