Highly efficient soluble materials for blue phosphorescent organic light-emitting diode

Article abstract of DOI:10.1016/j.dyepig.2012.04.011

A series of new carbazole derivatives with good solubility and electronic properties for use as host materials for blue phosphorescent organic light-emitting diodes is reported. A twisted biphenyl moiety at the center of the host materials was applied to

Full text of DOI:10.1016/j.dyepig.2012.04.011

Dyes and Pigments 95 (2012) 221e228
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Dyes and Pigments
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Highly efficient soluble materials for blue phosphorescent organic lighteemitting
diode
,
b
b
Nam-Jin Lee a, Dae-Hee Lee a, Dong-Won Kim a, Ji-Hoon Lee a , Sang Hee Cho , Woo Sik Jeon ,
**
,
Jang Hyuk Kwon b, Min Chul Suh b
*
a Department of Polymer Science & Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju-Si, Chungbuk 380-702, Republic of Korea
b Department of Information Display, Kyung Hee University, Dongdaemoon-Gu, Seoul 130-701, 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 series of new carbazole derivatives with good solubility and electronic properties for use as host
materials for blue phosphorescent organic light-emitting diodes is reported. A twisted biphenyl moiety
at the center of the host materials was applied to enhance the solubility as well as triplet energies. Hence
all the molecules showed good solubility as well as a fairly large triplet energy up to 3.01 eV. Blue
phosphorescent devices employing bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (III) as the
guest and the three new twisted carbazole derivatives as co-hosts with an electron type host material
exhibited high efficiencies, up to 20.7 cd/A as a maximum current efficiency and 12.5% of external
quantum efficiency. This performance was much improved by w27.6%, compared to that measured for
the 1,3-di(9H-carbazol-9-yl)benzene: 2,6-bis(3-(9H-carbozol-9-yl)phenyl)pyridine reference system.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 27 February 2012
Received in revised form
7 April 2012
Accepted 23 April 2012
Available online 28 April 2012
Keywords:
Soluble host materials
Blue phosphorescent OLED
Carbazole
Sterically bulky side groups
High triplet energy
1. Introduction
achievements in red and green phosphorescent materials, the blue
component has many obstacles against commercialization
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. In fact, red
phosphorescent materials have already been applied in the main-
display of commercial mobile phones since 2007 by Universal
Display Corporation (UDC) and Samsung Mobile Display. Recently,
green emission is almost approaching to the theoretical limitation
of efficiency (over 20% of external quantum efficiency, EQE) by
utilizing fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3] as the
phosphorescent emitter, and the commercialization of which is
underway. Nevertheless, the development of much more efficient
PHOLEDs including blue components is still required to differen-
tiate their value from the competition with thin film transistor e
liquid crystal displays (TFT-LCD) because the power consumption of
TFT-LCD has been reduced significantly with the help of high
performance LED backlight. However, compared with the great
including its short lifetime issue. Thus, many research groups
concentrate on preparation of the host materials which possess: i)
higher triplet energies than those of the guest molecules to realize
exothermic energy transfer [1e3] to effectively confine triplet
excitons within the emitting layer (EML) [4e6], ii) appropriate
energy levels which can be easily aligned with those of the adjacent
transporting layers for efficient charge carrier injection to result in
a low driving voltage [7] and iii) moderate charge carrier mobility
to increase the opportunity for electron and hole recombination
within the EML [8]. For this purpose, many achievements have been
reported in this field in recent years. Especially, the host materials
with carbazoles [9,10], aryl silanes [11,12] and phosphine oxides
[13e15] showed great progress in the device performances. Among
them carbazole derivatives such as 1,3-di(9H-carbazol-9-yl)
benzene (mCP) which is a sort of hole transport type host material
with high triplet energy (T1 ¼ 2.9 eV) are still the best candidates
for blue electrophosphorescent devices [16]. Meanwhile, there has
been much interest in soluble OLED materials to realize a solution
process such as ink-jet printing and nozzle printing because they
are much more compatible with large area processing than
common evaporation process. However, the greater part of mate-
rials showing high device performances (e.g. efficiency, operating
* Corresponding author. Tel.: þ82 2 961 0694; fax: þ82 2 968 6924.
** Corresponding author. Tel.: þ82 43 841 5427; fax: þ82 43 841 5420.
E-mail addresses: jihoonli@ut.ac.kr (J.-H. Lee), mcsuh@khu.ac.kr (M.C. Suh).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.04.011

222
N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
voltage, lifetime, etc.) are normally not suitable in solution pro-
cessed OLEDs as itself due to an insufficient solubility, a high
crystallization and phase separation tendency, and a glass transi-
tion temperature lower than that needed for solvent removal. To
overcome those problems, many research groups have tried to use
polyvinylcarbazole (PVK) mixed with 1,3-bis[(4-tert-butylphenyl)-
1,3,4-oxidiazolyl]phenylene (OXD-7) as a basic bipolar host system
because they give good amorphous film without any crystallization
defects during processing [17e22]. As part of such efforts, the high
efficiency, over 15% EQE, was recently reported from the blue
PHOLEDs [19,20]. However, PVK based host system generally gives
a high driving voltage plausibly due to its low hole mobility char-
acteristics although it has a high triplet energy as well as a good
film quality [23]. Thus, a development of solution based PHOLEDs
with other soluble small molecular host systems with both hole
and electron carrier transporting characteristics is required to
overcome such problems. Nevertheless, most of blue PHOLEDs
showed relatively high operating voltage greater than 6.5 V (at
1000 cd/m2) until now.
separated, dried over anhydrous magnesium sulfate, and evapo-
rated under reduced pressure to afford (2) as a white solid, yield
74%. 1H NMR (CDCl3, 300 MHz) 7.82 (d, J ¼ 10.0 Hz, 2 H), 7.46 (d,
J ¼ 8.0 Hz, 2 H), 1.37 (s, 12 H), 1.36 (s. 9 H) ppm.
2.2.3. Synthesis of 3,6-bis(4-tert-butylphenyl)-9H-carbazole (3)
tert-Butyl 3,6-dibromo-9H-carbazole-9-carboxylate (24 g,
55
mmol),
2-(4-tert-butylphenyl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (49 g, 188 mmol) dissolved in anhydrous toluene
(300 mL) under nitrogen and stirred for 10 min at room tempera-
ture. Palladium(II) acetate (1.2 g, 10 mol%) and potassium carbonate
(200 mL, 2 M aqueous solution) was added to the reaction mixture.
The mixture was stirred at 70 ꢁC for 16 h. The mixture was extracted
with ethyl acetate (400 mL), and evaporated under reduced pres-
sure. The crude compound was added in trifluoroacetic acid (TFA,
30 mL)/DMF (100 mL). The reaction mixture was stirred under
nitrogen at 100 ꢁC for 48 h. After the reaction was finished, the
mixture was evaporated under reduced pressure to remove tri-
fluoroacetic acid. The crude compound was added in NaOH
(200 mL, 1N aqueous solution). White solid product was filtered
and washed with ethyl acetate. The product (3) was isolated as
a white solid, yield 80%. FT-IR (KBr, cmꢀ1) 3430, 3024, 2957, 2901,
2865, 1625, 1609, 1520, 1484, 1461, 1393, 1363, 1315, 1288, 1266,
1237, 1139, 1111, 1028, 1007, 888, 838, 813, 736, 565. 1H NMR
(DMSO-d6, 300 MHz) 11.35 (s, 1 H), 8.50 (s, 2 H), 7.71 (d, J ¼ 9 Hz,
6 H), 7.56 (m, 6 H), 1.54 (s, 18 H) ppm.
In this study, we report three different types of host materials
for blue PHOLEDs having twisted core structures to increase their
triplet energies as well as solubility. The PHOLEDs containing those
materials
with
2,6-bis(3-(9H-carbozol-9-yl)phenyl)pyridine
(26DCzPPy) showed moderate EQE up to 12.5%.
2. Experimental
2.1. Instruments
2.2.4. Synthesis of 5,50-dibromo-2,20-dimethoxybiphenyl (4)
2,20-dimethoxybiphenyl (12 g, 56 mmol) was dissolved in CHCl3
(180 mL). The resulting solution was cooled down to 0 ꢁC and
bromine (6.35 mL, 123 mmol) was added. The mixture was stirred
for 12 h. The reaction mixture was poured into excess water and
extracted with chloroform. The organic layer was separated, dried
over anhydrous magnesium sulfate, and evaporated under reduced
1H and 13C-NMR spectra were recorded on a Bruker Avance 300
and 400 NMR spectrometer and chemical shifts were referenced to
chloroform (7.26 ppm). All mass spectra were acquired on a hybrid
ion-trap time-of-flight mass spectrometer (Shimadzu LCMS-IT-TOF,
Kyoto, Japan) equipped with an ESI source (ESI-IT-TOFMS) in
negative ion mode at a mass resolution of 10,000 full-width at half
maximum (FWHM). Accurate masses were corrected by calibration
using the sodium trifloroacetate clusters as internal references.
pressure to afford (4) as a white solid, yield 94%. FT-IR (KBr, cmꢀ1
)
3079, 3007, 2941, 2904, 2835, 1585, 1434, 1412, 1243, 1136, 1029,
873, 794, 614. 1H NMR (CDCl3, 400 MHz) 7.43 (dd, J ¼ 2.5 Hz, 2 H),
7.32 (d, J ¼ 2.5 Hz, 2 H), 6.84 (d, J ¼ 8.8 Hz, 2 H), 3.75 (s, 6 H) ppm.
2.2. Synthesis of new host materials
2.2.5. Synthesis of 3,30,5,50-tetrabromobiphenyl (5)
2.2.1. Synthesis of tert-butyl 3,6-dibromo-9H-carbazole-9-
carboxylate (1)
n-Butyllithium (34.4 mL, 55 mmol) was added to a solution of
1,3,5-tribromobenzene (15.74 g, 50 mmol) in anhydrous THF
(100 mL) at ꢀ78 ꢁC under nitrogen atmosphere. The mixture was
stirred for 1 h at the same temperature. And then, copper(II)
chloride (7.39 g, 55 mmol) was added to the reaction mixture. The
mixture was stirred for 12 h. After the reaction was finished, the
mixture was washed three times with distilled water and extracted
with ethyl acetate. The organic layer was separated, dried over
magnesium sulfate, and evaporated under reduced pressure, then
recrystallized from dichloromethane-acetone. The product (5) was
isolated as a white solid, yield 90%. FT-IR (KBr, cmꢀ1) 3101, 3067,
1577, 1539, 1406, 1382, 1106, 1095, 1064, 847, 744, 670, 648. 1H NMR
(CDCl3, 400 MHz) 7.69 (t, J ¼ 1.7 Hz, 2 H), 7.59 (d, J ¼ 1.7, 4 H) ppm.
3,6-dibromo-9H-carbazole (70 g, 0.22 mol) was dissolved in
anhydrous THF (1 L) under nitrogen and stirred for 10 min at room
temperature. Di-tert-butyldicarbonate (56.7 g, 0.26 mol) and 4-
dimethylaminopyridine (3.2 g, 0.03 mol) were added to the reac-
tion mixture. The mixture was stirred at room temperature for 12 h.
After the reaction was finished, the mixture was washed three
times with distilled water and extracted with ethyl acetate. The
organic layer was separated, dried over anhydrous magnesium
sulfate, and evaporated under reduced pressure to afford (1) as
a white solid, yield 82%. 1H NMR (DMSO-d6, 300 MHz) 8.52 (d,
J ¼ 2.7 Hz, 2 H), 8.14 (d, J ¼ 7.3 Hz, 2 H), 7.71 (dd, J ¼ 8.9 Hz, 2 H),1.69
(s, 9 H) ppm.
2.2.6. Synthesis of 9,90-(6,60-dimethoxybiphenyl-3,30-diyl)bis(9H-
carbazole) (DMBC)
2.2.2. Synthesis of 2-(4-tert-Butylphenyl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (2)
5,5’-dibromo-2,20-dimethoxybiphenyl (1.5 g, 4.0 mmol), 9H-
carbazole (1.42 g, 8.5 mmol), dissolved in anhydrous toluene
(80 mL) under nitrogen. Sodium tert-butoxide (t-BuONa, 5.03 g,
50.8 mmol), tri-tert-butylphosphine ((t-Bu)3P, 10 wt% in n-hexane,
1.2 mL, 0.8 mmol) and tris(dibenzylideneaceton)dipalladium(0)
(Pd2(dba)3, 0.22 g, 5 mol%)] was added to the reaction mixture. The
mixture was stirred at 120 ꢁC for 48 h. After the reaction was
finished, the mixture was washed three times with distilled water
and extracted with ethyl acetate. The organic layer was separated,
n-Butyllithium (127.4 mL, 0.32 mol) was added to a solution of
1-bromo-4-tert-butylbenzene (47.5 g, 0.22 mol) in anhydrous
THF (500 mL) at ꢀ78 ꢁC under nitrogen atmosphere. The mixture
was stirred for 30 min at the same temperature. And then,
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (50 mL,
0.25 mol) was added dropwise to the reaction mixture and stirred
for 12 h at room temperature. The reaction mixture was poured into
water and extracted with chloroform. The organic layer was

N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
223
dried over anhydrous magnesium sulfate, and evaporated under
reduced pressure. The crude product was purified by silica gel
column chromatography using n-hexane-THF (5:1) eluent to afford
DMBC a white solid, yield 66%. Tm: 292 ꢁC, FT-IR (KBr, cmꢀ1) 3049,
2987, 2926, 2829, 1595, 1494, 1453, 1336, 1312, 1244, 1231, 1180,
1145, 1044, 1025, 922, 818, 749, 724. 1H NMR (CDCl3, 400 MHz) 8.14
(d, J ¼ 7.6 Hz, 4 H), 7.62 (d, J ¼ 2.7, 2 H), 7.52 (dd, J ¼ 2.7, 4 H), 7.46 (s,
2 H), 7.42 (td, J ¼ 7.1, 1.2 Hz, 4 H), 7.28 (td, J ¼ 7.9, 1.0 Hz, 4 H), 7.18 (d,
J ¼ 8.7 Hz, 2 H), 3.92 (s, 6 H) ppm; 13C NMR (CDCl3, 400 MHz) 155.8,
141.2, 130.8, 129.8, 12734, 127.3, 125.8, 123.2, 120.3, 119.7, 112.4,
109.9, 56.0 ppm. HRMS [M þ H]þ: m/z calcd. 545.2224; found
545.2226. Anal. calcd. for C38H28N2O2: C, 83.80; H, 5.18; N, 5.14
found: C, 83.83; H, 5.15; N, 5.14.
26DCzPPy as an electron transporting co-host in EML, 1,3,5-tri(m-
pyrid-3-yl-phenyl)benzene (TmPyPB) as an electron transporting
material were purchased from H. C. Starck GmbH. and Daejoo
Electronic Material Co. and were used without purification. To
fabricate PHOLEDs, a patterned indium-tin oxide (ITO) glass with
a 150 nm thickness having an emission area of 4 mm2 with a sheet
resistance of 10e12
U/, were used. A line pattern of ITO and an
insulating layer to make active areas were formed by the photoli-
thography process. The ITO glass substrate was cleaned by soni-
cation in isopropyl alcohol (IPA) and then rinsed in deionized water.
After a series of wet cleaning process, the ITO glass substrate was
treated in a UV-ozone chamber prior to device fabrication. The
PEDOT:PSS [CLEVIOSTM AI4083 (H.C. Stack)] as a hole injection
layer was spin-coated on the ITO substrates and dried on a hot plate
at 120 ꢁC for 20 min to remove the solvent. The materials for the
emission layer were dissolved in chlorobenzene and were spin and
dried at 120 ꢁC for 10 min. In case of mixed host, a blending ratio
was fixed to 1:1. All solution process was performed in a dry
nitrogen filled glove box at room temperature. After coating of EML,
TmPyPB as an electron transporting layer was thermally deposited
at a base pressure of 10ꢀ7 torr and then lithium fluoride (LiF) and
Aluminum (Al) were deposited successively. The deposition rates of
TmPyPB, LiF, and Al were 1, 0.5, and 5 w 10 Å/s, respectively.
The current density-voltage (JeV) and luminance-voltage (LeV)
data of OLEDs were collected by Keithley 2635A and Minolta CS-
100A, respectively. The area for measurement of OLED emission
was 4 mm2 for all the samples studied in this work. Electrolumi-
nescence (EL) spectra and CIE coordinate were measured by using
a Minolta CS-1000 spectroradiometer.
Cyclic voltammetry experiments were performed using BASi
(Bioanalytical Systems, Inc.) analysis equipment (C-3 standard). A
platinum wire electrode and 150-nm ITO film on glass were used as
counter- and working electrodes, respectively. Silver/silver ion (Ag
wire in a 0.1 M AgNO3 solution) was used as a reference electrode.
The Ag/Agþ (AgNO3) reference electrode was calibrated at the
beginning of the experiments by running cyclic voltammetry on
ferrocene as the internal standard. By means of the internal ferro-
cenium/ferrocene (Fcþ/Fc) standard, the potential values were
converted to the saturated calomel electrode scale. Experiments
were run on the film states of the materials that were used. The
films that had been coated with these materials were made on ITO
glass through the solution drop coating. A 0.1 M Bu4NClO4 (tetra-
butylammonium perchlorate) electrolyte solution in acetonitrile
was used in all the experiments.
2.2.7. Synthesis of 9,90-(6,60-dimethoxybiphenyl-3,30-diyl)bis(3,6-
bis(4-tert-butylphenyl)-9H-carbazole) (B-DMBC)
5,50-dibromo-2,20-dimethoxybiphenyl (0.6 g, 1.6 mmol), 3,6-
bis(4-tert-butylphenyl)-9H-carbazole (1.46 g, 3.4 mmol), dis-
solved in anhydrous toluene (50 mL) under nitrogen. Sodium tert-
butoxide (2.01 g, 20.3 mmol), tri-tert-butylphosphine (10 wt% in n-
hexane, 0.5 mL, 0.2 mmol) and tris(dibenzylideneaceton)dipalla-
dium(0) (0.09 g, 6 mol%) was added to the reaction mixture. The
mixture was stirred at 120 ꢁC for 48 h. After the reaction was
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-THF (4:1) eluent to afford
B-DMBC a white solid, yield 58%. Tm: 324 ꢁC, FT-IR (KBr, cmꢀ1) 3027,
2960, 2903, 2867, 1605, 1480, 1363, 1267, 1245, 1114, 1030, 883, 837,
809, 651, 604, 557. 1H NMR (CDCl3, 400 MHz) 8.4 (d, J ¼ 1.5 Hz, 4 H),
7.71 (d J ¼ 3.0 Hz, 2 H), 7.70 (s, 4 H), 7.68 (d, J ¼ 1.8 Hz, 8 H), 7.59 (m
J ¼ 2.7 Hz, 4 H), 7.55 (s, 2 H), 7.53 (d, J ¼ 8.4 Hz, 8 H), 7.24 (d,
J ¼ 8.8 Hz, 2 H), 3.97 (s, 6 H), 1.40 (s, 36 H) ppm; 13C NMR (CDCl3,
400 MHz) 155.9, 149.5, 141.0, 139.1, 133.3, 130.7, 129.8, 127.4, 127.2,
127.0, 125.8, 125.5, 123.9, 118.7, 56.1, 34.5, 31.5 ppm. Anal. calcd. for
C
78H76N2O2: C, 87.27; H, 7.14; N, 2.61 found: C, 87.31; H, 7.10; N,
2.58.
2.2.8. Synthesis of 3,30,5,50-tetra(9H-carbazole-yl)biphenyl (TCBP)
3,30,5,50-tetrabromobiphenyl (2.5 g, 5.3 mmol), 9H-carbazole
(3.6 g, 21.4 mmol), dissolved in anhydrous toluene (60 mL) under
nitrogen. Sodium tert-butoxide (4.13 g, 42.8 mmol), tri-tert-butyl-
phosphine (10 wt% in n-hexane, 3.9 mL, 1.6 mmol) and tris(di-
benzylideneaceton)dipalladium(0) (1.169 g, 25 mol%) was added to
the reaction mixture. The mixture was stirred at 120 ꢁC for 48 h.
After the reaction was finished, the mixture was washed three
times with distilled water and extracted with ethyl acetate. The
organic layer was separated, dried over magnesium sulfate, and
evaporated under reduced pressure. The crude product was puri-
fied by silica gel column chromatography using n-hexane-ethyl
acetate (10:1) eluent to afford TCBP a white solid, yield 60%. Tm:
363 ꢁC, FT-IR (KBr, cm-1) 3050, 1587, 1450, 1333, 1311, 1227, 1156,
1027, 922, 746, 772, 707, 648. 1H NMR (CDCl3, 400 MHz) 8.16 (d,
J ¼ 7.7 Hz, 8 H), 8.07 (d, J ¼ 1.9 Hz, 4 H), 7.92 (t, J ¼ 1.8 Hz, 2 H), 7.64
(d, J ¼ 8.2, 8 H), 7.48 (td, J ¼ 8.2, 1.0 Hz, 8 H), 7.35 (t, J ¼ 7.6 Hz, 8 H)
ppm; 13C NMR (CDCl3, 400 MHz) 142.9, 140.5, 140.4, 126.4, 125.0,
124.4, 123.8, 120.6, 120.5, 109.7 ppm. HRMS [M þ H]þ: m/z calcd.
815.3169; found 815.3161. Anal. calcd. for C60H38N4: C, 88.43; H,
4.70; N, 6.87 found: C, 88.37; H, 4.71; N, 6.88.
3. Results and discussion
3.1. Materials
Three host materials were prepared to realize a highly efficient
blue PHOLED. Carbazole moieties were selected as a key function-
ality because they have proved to afford immensely successful
results as blue PHOLEDs when utilized with bis(4,6-
difluorophenylpyridinato-N,C2)picolinatoiridium (III) (FIrpic) [24].
In principle, carbazole containing host materials exhibit high triplet
energies if they have no extended
other words, the most common approach to increase a triplet
energy level as a blue host might be the disruption of -conjugation
p-conjugation length [25,26]. In
p
just as in the case of mCP which has two carbazole units connected
by the meta position of a single benzene ring and shows high triplet
energy level up to 2.9 eV. Thus, we used the mCP units as a main
functional moiety and connected to the biphenyl core structure
with sterically hindered methoxy groups in the 6,60-position, which
result in steric crowding such as in 4,40-Bis-(9-carbazolyl)-biphenyl
(CBP) derivatives with methyl or trifluoromethyl groups in the 2,20-
2.3. Fabrication of PHOLEDs
Poly(3,4-ethylene dioxythiophene) (PEDOT) doped with poly(-
styrenesulfonate) (PSS) (PEDOT:PSS) as a hole injection layer,

224
N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
position of the biphenyl group which are prepared by Schrögel et al.
[25]. This approach gives higher molecular weight species which
could be associated with an improved thermal stability of the host
materials for blue PHOLED. In addition, a new host material simply
having sterically hindered four carbazole moieties in its 3,30,5,50-
position was also prepared as shown in Scheme 1. 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, Accelrys software). The
biphenyl moieties were significantly twisted which results in
diminished conjugation as we expected. This approach could also
be helpful to improve the solubility of the materials because these
materials do not tend to stack due to a larger vacant volume which
results from the steric distortions.
Scheme 1. Synthesis of carbazole derivatives for blue PHOLEDs.

N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
225
Fig. 1. Geometries of three host materials having twisted core structures.
Fig. 2 shows the UVevis absorption and photoluminescence
spectra of new host materials in 2-methyl tetrahydrofuran. DMBC
and TCBP showed similar absorption behavior, in which the two
lower-lying bands that appear in the region of w 320e335 and
335e350 nm which are tentatively assigned to the n / p* transi-
tions as shown Fig. 2(a). Very interestingly, TCBP showed a slightly
larger band gap (3.57 eV) than DMBC molecule (3.49 eV) presum-
ably due to a serious disruption of -conjugation from the much
p
more hindered stereo-interaction among carbazole moieties. On
the contrary to those materials, B-DMBC showed much smaller
band gap (w3.30 eV) due to its extended conjugation length
presumably due to its relatively less twisted core geometry as
shown in Fig. 1. All three compounds showed vibronic fine struc-
tures 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 from the phosphorescence peak
wavelength of the low temperature PL spectra were 3.02, 2.75,
2.79 eV for DMBC, B-DMBC, and TCBP respectively as shown in
Fig. 2(b) and the representative results are summarized in Table 1.
The energy levels of Highest Occupied Molecular Orbital (HOMO)
and Lowest Unoccupied Molecular Orbital (LUMO) were collected
from cyclic voltammetry and band edges of UVevis absorption
spectra and they were also summarized in Table 1.
Besides, the molecular orbitals of DMBC, B-DMBC, and TCBP are
shown in Fig. 3. As we expected from the molecular structure, the
electrons in HOMO was localized on the carbazole unit, while the
electrons in LUMO was dispersed over the biphenyl core unit.
3.2. Device characteristics
The chemical structures of newly synthesized host materials
and other materials for blue PHOLEDs in this study are shown in
Fig. 4.
For the remarkable enhancement of efficiency upon doping, the
bipolar character of host materials with an appropriate triplet
energy level is very important. To satisfy this requirement, we
utilized 1:1 mixed host system of those new host materials with
26DCzPPy having relatively high electron transporting property as
well as high triplet energy [electron mobility (me) of 26DCzPPy :
2 ꢂ 10ꢀ5 cm2
V
ꢀ1sꢀ1; T1 ¼ 2.71 eV] [27]. In addition, we selected
TmPyPB as an electron transport layer (ETL) due to its high triplet
Table 1
Summary of physical properties of new host materials.
Absorption
(nm)
6
Eg (eV)
PL
(nm)
T1 (eV)
HOMO LUMO
DMBC
B-DMBC 297, 346, 362 3.30 (375 nm) 380, 397 2.75 (451 nm) 5.51
TCBP 293, 325, 339 3.57 (347 nm) 406 2.79 (445 nm) 5.69
293, 629, 343 3.49 (355 nm) 360
3.02 (411 nm) 5.61
2.12
2.21
2.12
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.

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N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
Fig. 5. Schematic diagram of device architecture of blue PHOLEDs fabricated in this
study.
Device A: ITO/PEDOT:PSS (60 nm)/mCP:26DCzPPy:FIrpic (10%,
60 nm)/TmPyPB (30 nm)/LiF (0.5 nm)/Al (100 nm)
Device B: ITO/PEDOT:PSS (60 nm)/DMBC:26DCzPPy:FIrpic (10%,
60 nm)/TmPyPB (30 nm)/LiF (0.5 nm)/Al (100 nm)
Device C: ITO/PEDOT:PSS (60 nm)/B-DMBC:26DCzPPy:FIrpic
(10%, 60 nm)/TmPyPB (30 nm)/LiF (0.5 nm)/Al (100 nm)
Device D: ITO/PEDOT:PSS (60 nm)/TCBP:26DCzPPy:FIrpic (10%,
60 nm)/TmPyPB (30 nm)/LiF (0.5 nm)/Al (100 nm)
Fig. 3. HOMO and LUMO shapes of three host materials.
Fig. 6(a) shows the JeV and LeV characteristics of fabricated blue
PHOLEDs and the representative results are summarized in Table 2.
At a given constant voltage of 7.0 V, current density values of
5.3, 8.9, 5.8, and 4.8 mA/cm2 were observed in the fabricated
PHOLEDs A, B, C, and D, respectively. The driving voltages to reach
1000 cd/m2 were 7.0, 6.6, 7.4, and 8.6 V for the PHOLEDs A, B, C, and
energy (T1 ¼ 2.7 eV) which gives a favorable energy transfer inside
EML without exciton quenching at the neighboring non-emissive
layers. The device structures tested in this study are shown in
Fig. 5. The exact device configuration used in this work was as
follows:
Fig. 4. Chemical structures of the materials for blue PHOLEDs fabricated in this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)

N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
227
Fig. 7. Energy band diagrams of used materials in this work.
the electron hopping distances are dependent upon the sizes of the
neighboring substituents as shown in Fig. 3. On the other hand, the
HOMO in DMBC, B-DMBC, and TCBP are localized on the carbazoles
at the ends of the molecules (See also Fig. 3). Thus, 4-tert-butyl-
phenyl moieties which are utilized as solubilizing groups in B-
DMBC molecule may increase the hopping distances in between
HOMOs or LUMOs compared to those in DMBC molecule. Thus, the
charge-transfer integral of B-DMBC might be smaller than that of
DMBC which causes an increased operating voltage [28]. Similarly,
the HOMOs in TCBP are localized in four neighboring carbazole
moieties. However, those moieties may disturb the molecular
stacking due to a totally distorted steric interaction between the
four carbazole units around the biphenyl moiety which causes
a disturbed charge carrier hopping process (through p-orbital
interaction). Hence the operating voltage of PHOLED D with TCBP
was even higher (w8.6 V) than that from the PHOLED C with B-
DMBC.
The current and power efficiency characteristics of the fabri-
cated PHOLEDs are shown in Fig. 6(b) and also summarized in
Table 2. At a given constant luminance of 1000 cd/m2, the current
and power efficiencies were 19.3 cd/A and 8.7 lm/W for the
PHOLED A, 20.0 cd/A and 9.5 lm/W for the PHOLED B, 11.8 cd/A and
5.0 lm/W for the PHOLED C, 4.6 cd/A and 1.7 lm/W for the PHOLED
D, respectively. These efficiency data correspond to 9.8, 12.5, 5.3,
and 2.0% external quantum efficiencies of PHOLEDs A, B, C and D,
respectively. The maximum current and power efficiencies were
19.8 cd/A and 9.4 lm/W for the PHOLED A, 20.7 cd/A and 10.5 lm/W
for the PHOLED B, 15.7 cd/A and 8.2 lm/W for the PHOLED C, 7.0 cd/
A and 3.0 lm/W for the PHOLED D, respectively.
Fig. 6. a) J e V e L characteristics of fabricated blue PHOLEDs. b) Luminance vs current
efficiency and power efficiency characteristics of fabricated blue PHOLEDs. (For inter-
pretation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
D, respectively (Table 2). The relatively high turn-on voltages of 5.4,
5.0, 5.2, and 6.2 V were observed for the PHOLEDs A, B, C, and D,
respectively. In the described PHOLED configuration, relatively high
driving voltage performances might be attributed to the relatively
high energy barrier in between PEDOT/PSS and the new synthetic
host materials as shown in Fig. 7. Meanwhile, the PHOLED B with
DMBC showed the greatest current density at the observed driving
voltage region and the operating voltage at 1000 cd/m2 of bright-
ness was observed to be w6.6 V which is the lowest among the
devices fabricated in this study presumably due to a relatively
higher lying HOMO level (w5.61 eV) of DMBC (cf. PHOLED A with
mCP: 7.0 V at 1000 cd/m2, HOMO: 5.90 eV). However, the PHOLED C
with B-DMBC which is a very bulky molecule and was prepared as
a highly soluble host material in common organic solvents, showed
a relatively high operating voltage of w7.4 V. From the density
functional theory (DFT) calculations, the LUMOs in DMBC, B-DMBC,
and TCBP are more concentrated on the center of the molecules and
Fig. 8 shows the EL spectra at a brightness of 1000 cd/m2 of
different fabricated blue PHOLEDs. The PHOLEDs A, B, C showed
similar EL spectra. However, PHOLED D with TCBP showed
a significant red shift in the EL spectra, that is, the color coordinate
was changed from (0.17, 0.28) to (0.18, 0.31). From this phenomena,
we could presume that the recombination zone of PHOLED
A, B,
C with mCP:26DCzPPy:FIrpic, DMBC:26DCzPPy:FIrpic,
B-DMBC:26DCzPPy:FIrpic systems are approximately in the center
through ETL side of EML due to preferred charge (hole) carrier
Table 2
Device characteristics of phosphorescent blue OLEDs.
Device A
Device B
Device C
Device D
Turn-on voltage (1 cd/m2)
Operating voltage (1000 cd/m2)
Efficiency (1000 cd/m2)
Efficiency (Max)
5.4 V
7.0 V
19.3 cd/A 8.7 lm/W
19.8 cd/A 9.4 lm/W
0.169, 0.284
9.8%
5.0 V
6.6 V
5.2 V
7.4 V
11.8 cd/A 5.0 lm/W
15.7 cd/A 8.2 lm/W
0.169, 0.288
5.3%
6.2 V
8.6 V
4.6 cd/A 1.7 lm/W
7.0 cd/A 3.0 lm/W
0.184, 0.313
2.0%
20.0 cd/A 9.5 lm/W
20.7 cd/A 10.5 lm/W
0.167, 0.278
12.5%
CIE (x,y) (1000 cd/m2)
E.Q.E. (1000 cd/m2)

228
N.-J. Lee et al. / Dyes and Pigments 95 (2012) 221e228
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conductivity due to a lower order of molecular stacking order.
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we observed very low efficiency (w2%) when we use a TCBP as
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balanced exciton recombination in the EML could be achieved
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26DCzPPy and DMBC is the most desirable material for the blue
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twisted biphenyl core unit to realize a highly efficient blue PHOLED.
Every host materials having carbazole moieties showed improved
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Acknowledgments
Prof. J.-H. Lee acknowledges to Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (Grant No.
NRF-2010-0012463). Prof. M. C. Suh gratefully acknowledges to
Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education,
Science and Technology (Grant No. NRF-2011-0006847). High
resolution mass spectroscopy was carried out at Korea Basic
Science Institute (Mass Spectrometry Research Team).
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ꢀ
_
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