Structure-property relationship of pyridoindole-type host materials for high-efficiency blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1021/cm403750p

The structure-property relationship of pyridoindole-type host materials for blue phosphorescent organic light-emitting diodes (PHOLEDs) was investigated by synthesizing three pyridoindole derivatives modified with dibenzofuran, dibenzothiophene, and 9-phenylcarbazole. The 9-phenylcarbazole moiety was better than dibenzofuran or dibenzothiophene to manage the energy levels and charge-transport properties of the pyridoindole-derived host materials. The PCb-PCz host with the 9-phenylcarbazole moiety showed balanced charge density in the emitting layer and exhibited a high external quantum efficiency of 29.7% in blue PHOLEDs.

Full text of DOI:10.1021/cm403750p

Article
pubs.acs.org/cm
Structure−Property Relationship of Pyridoindole-Type Host
Materials for High-Efficiency Blue Phosphorescent Organic Light-
Emitting Diodes
Chil Won Lee and Jun Yeob Lee*
Department of Polymer Science and Engineering, Dankook University, 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, Korea
ABSTRACT: The structure−property relationship of pyridoindole-type
host materials for blue phosphorescent organic light-emitting diodes
(PHOLEDs) was investigated by synthesizing three pyridoindole
derivatives modified with dibenzofuran, dibenzothiophene, and 9-phenyl-
carbazole. The 9-phenylcarbazole moiety was better than dibenzofuran or
dibenzothiophene to manage the energy levels and charge-transport
properties of the pyridoindole-derived host materials. The PCb-PCz host
with the 9-phenylcarbazole moiety showed balanced charge density in the
emitting layer and exhibited a high external quantum efficiency of 29.7% in
blue PHOLEDs.
derivatives were developed and showed high quantum
efficiency in blue PHOLEDs and white tandem PHOLEDs.14
Although several studies proved that the pyridoindole moiety
would be effective as the building block of high-triplet-energy
host materials for blue PHOLEDs, further systematic study is
required to develop pyridoindole derivatives as the host
materials for blue PHOLEDs.
In this work, three pyridoindole derivatives, 9-(3-(dibenzo-
[b,d]furan-2-yl)phenyl)-α-carboline (PCb-DBF), 9-(3-(diben-
zothiophen-2-yl)phenyl)-α-carboline (PCb-DBT), and 9-(3-(9-
phenylcarbazol-3-yl)phenyl)-α-carboline (PCb-PCz), were syn-
thesized as the triplet host materials for blue PHOLEDs, and
the effect of dibenzofuran, dibenzothiophene and 9-phenyl-
carbazole on the photophysical properties and device perform-
ance of the pyridoindole derivatives was systematically studied.
It was demonstrated that the 9-phenylcarbazole moiety is better
than other substituents for improving the device performance
of the pyridoindole derivatives. A high quantum efficiency of
29.7% was achieved in FIrpic-doped blue PHOLEDs using
PCb-PCz as the host material, which is one of the best
efficiencies reported for blue PHOLEDs.
INTRODUCTION
■
Quantum efficiency is one of the most important device
performance characteristics of phosphorescent organic light-
emitting diodes (PHOLEDs), and host materials play a key role
in improving the quantum efficiency of PHOLEDs through
charge balance and energy transfer. In particular, the host
materials are critical to the quantum efficiency of blue
PHOLEDs because high triplet energy is required for the
host materials of blue PHOLEDs.
Many high-triplet-energy host materials have been developed
to enhance the quantum efficiency of blue PHOLEDs, and
most host materials were derived from high-triplet-energy
moieties. Carbazole has been the most popular building block
to obtain high triplet energy and high quantum efficiency in
blue PHOLEDs.1−10 However, the carbazole moiety showed
poor electron-accepting properties in spite of its good electron-
donating properties for stable cation radical formation.
Therefore, pyridoindole was developed as a building block to
improve the poor electron-accepting properties of the carbazole
moiety.11−15 The pyridoindole unit exhibited high triplet
energy and better electron-accepting properties than carbazole.
Several pyridoindole derivatives have been developed as the
host materials for blue PHOLEDs and were effective at
enhancing the quantum efficiency of blue PHOLEDs. Kido et
al. reported a pyridoindole derivative as a host material for blue
PHOLEDs that had a high quantum efficiency using iridium-
(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2 ]picolinate
(FIrpic) as a blue triplet emitter.11 A high-triplet-energy host
material derived from an adamantane core and pyridoindole
was reported by Tokito et al.,12 and the effect of nitrogen
orientation on the photophysical properties and device
performance of pyridoindole-modified triplet host materials
was studied by our group.13 Recently, bipolar-type pyridoindole
EXPERIMENTAL SECTION
■
General Information. All chemicals and reagents were used
without further purification. 9-(3-Bromophenyl)-α-carboline was
synthesized according to a method reported in the literature.16 The
1H and 13C nuclear magnetic resonance (NMR) data were recorded
on an Avance 500 (Bruker) NMR spectrometer, and the Fourier-
transform infrared (FT-IR) spectra for all of the samples were
Received: November 12, 2013
Revised: February 4, 2014
© XXXX American Chemical Society
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obtained using a Nicolet 380 FT-IR spectrometer. The mass spectra
were recorded using a JEOL JMS-600W spectrometer in fast-atom-
bombardment mode. Elemental analysis of the materials was carried
out using a Flash2000 (ThermoFisher). The differential scanning
calorimetric (DSC) measurements were performed on a Mettler
DSC822e under nitrogen at a heating rate of 10 °C/min. The PL and
UV−vis spectra were obtained using a fluorescence spectrophotometer
(HITACHI, F-7000) and a UV−vis spectrophotometer (Shimadzu,
UV-2501PC), respectively. Low-temperature PL measurement for
triplet-energy analysis was carried out using a PerkinElmer LS-55 in
liquid nitrogen. The oxidation and reduction potentials of compounds
were measured with a cyclic voltammetry. Cyclic voltammetry
measurement of organic materials was carried out in an acetonitrile
solution with tetrabutylammonium perchlorate at 0.1 M concentration.
Ag was used as the reference electrode, and Pt was the counter
electrode. The oxidation potential was converted to the highest
occupied molecular orbital by a method reported previously.17
Synthesis of 9-(3-(Dibenzo[b,d]furan-2-yl)phenyl)-α-carbo-
line (PCb-DBF). 9-(3-Bromophenyl)-α-carboline (1.70 g, 5.21 mmol),
dibenzofuran-2-ylboronic acid (1.45 g, 6.77 mmol), potassium
carbonate (1.94 g, 14.1 mmol), THF (90 mL), and 30 mL of distilled
water were placed in a two-necked 250 mL round-bottomed flask
equipped with a magnetic stirrer bar, a reflux condenser, and a
nitrogen inlet. The solution was stirred and bubbled with nitrogen gas
for 30 min. Tetrakis(triphenylphosphine)palladium(0) (0.27 g, 0.23
mmol) was added to the above reaction mixture, and the resulting
solution was refluxed for 24 h under nitrogen. The reaction mixture
was cooled to room temperature and extracted with ethyl acetate and
distilled water. The ethyl acetate phase was dried over magnesium
sulfate, filtered, rotary-evaporated to remove solvent, and dried in a
vacuum. The crude product was purified by column chromatography
on silica gel using n-hexane/dichloromethane as eluent. Additional
purification by vacuum train sublimation resulted in 1.5 g of PCb-DBF.
9-(3-(Dibenzothiophen-2-yl)phenyl)-α-carboline (PCb-DBT) and 9-
(3-(9-phenylcarbazol-3-yl)phenyl)-α-carboline (PCb-PCz) were syn-
thesized using a similar procedure as that for PCb-DBF.
(d, 1H, J = 3.2 Hz). 13C NMR (125 MHz, CDCl3): δ 109.9, 110.1,
110.5, 116.0, 116.3, 118.9, 120.1, 120.4, 120.7, 120.9, 120.9, 123.4,
123.9, 125.5, 126.2, 126.5, 126.9, 127.0, 127.5, 128.2, 129.9, 130.0,
132.5, 136.7, 137.6, 140.2, 140.6, 141.4, 143.6, 146.5, 152.1. Mass
(FAB) m/z 486 (M + H)+. Anal. Calcd for C35H23N3: C, 86.57; H,
4.77; N, 8.65. Found: C, 86.57; H, 4.71; N, 8.76.
Device Fabrication and Measurements. The basic device
structure of blue PHOLEDs was indium tin oxide (ITO, 50 nm)/
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE-
DOT:PSS, 60 nm)/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)-
aniline] (TAPC, 20 nm)/1,3-bis(N-carbazolyl)benzene (mCP, 10
nm)/PCb-DBF:Firpic or PCb-DBT:Firpic or PCb-PCz:FIrpic (25
nm)/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 35
nm)/LiF (1 nm)/Al (200 nm). Device structures of hole and electron
devices were ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/
mCP (10 nm)/PCb-PCz or PCb-DBT or PCb-DBF (25 nm)/TAPC
(5 nm)/Al (100 nm) and ITO (50 nm)/Ca (10 nm)/PCb-PCz or
PCb-DBT or PCb-DBF (25 nm)/LiF (1 nm)/Al (100 nm),
respectively. All devices were fabricated by a vacuum thermal-
evaporation process. Only PEDOT:PSS was spin-coated on the ITO
substrate. Current density−voltage−luminance characteristics of blue
PHOLEDs were measured using a CS1000 spectroradiometer and a
Keithley 2400 source measurement unit.
RESULTS AND DISCUSSION
■
The three host materials synthesized in this work were designed
to study the effect of dibenzofuran, dibenzothiophene, and 9-
phenylcarbazole with different heteroatoms in the backbone
structure on the photophysical properties and device perform-
ance of pyridoindole-based triplet host materials. The three
moieties have a high triplet energy above 3.00 eV and are
suitable as high-triplet-energy units for blue triplet host
materials. In addition, the three moieties have different
electron-donating character, and the comparison of the three
host materials can be used to guide the design of host materials
derived from pyridoindole to achieve high quantum efficiency
in blue PHOLEDs.
PCb-DBF. Yield 70%. Tg 69 °C, Tm 150 °C. FT-IR: 3052, 1925,
1859, 1774, 1591, 1451, 1409, 1337, 1289, 1222, 1195, 1119, 1022,
1
964, 931, 873, 841, 807, 770, 736, 692 cm−1. H NMR (500 MHz,
CDCl3): δ 7.23 (t, 1H, J = 4.2 Hz), 7.30−7.35 (m, 2H), 7.42−7.49 (m,
2H), 7.55−7.56 (m, 2H), 7.59 (d, 1H, J = 4.3 Hz), 7.64 (d, 1H, J = 5.2
Hz), 7.68−7.76 (m, 3H), 7.93−7.94 (m, 2H), 8.12 (d, 1H, J = 3.8 Hz),
8.17 (s, 1H), 8.37 (d, 1H, J = 4.8 Hz), 8.50 (d, 1H, J = 3.0 Hz). 13C
NMR (125 MHz, CDCl3): δ 110.4, 111.7, 111.8, 116.1, 116.4, 119.3,
120.7, 120.8, 120.9, 120.9, 122.8, 124.1, 124.8, 126.0, 126.3, 126.6,
126.7, 127.0, 127.3, 128.3, 130.1, 135.5, 136.8, 140.1, 142.9, 146.5,
152.0, 155.9, 156.7. Mass (FAB) m/z 411 (M + H)+. Anal. Calcd for
C29H18N2O: C, 84.86; H, 4.42; N, 6.82; O, 3.90. Found: C, 84.83; H,
4.45; N, 6.91; O, 3.83.
The three host materials were synthesized by a simple Suzuki
coupling reaction between 9-(3-bromophenyl)-α-carboline and
the corresponding boronic acid of dibenzofuran, dibenzothio-
phene, and 9-phenylcarbazole. The boronic acid functional
group was substituted at the para position of the heteroatom to
obtain high triplet energy from the synthesized host materials.
The synthetic process for the three host materials is shown in
Scheme 1. All three host materials were purified by vacuum
train sublimation after purification by column chromatography.
Synthetic yields of PCb-DBF, PCb-DBT, and PCb-PCz were
70, 68, and 75%, respectively. Chemical structures of the host
materials were confirmed by chemical analysis.
PCb-DBT. Yield 68%. Tg 79 °C, Tm 188 °C. FT-IR: 3051, 1862,
1783, 1727, 1673, 1591, 1451, 1408, 1335, 1290, 1223, 1167, 1120,
1
1086, 1020, 933, 883, 846, 766, 729, 691 cm−1. H NMR (500 MHz,
CDCl3): δ 7.24 (t, 1H, J = 4.0 Hz), 7.34 (t, 1H, J = 5.2 Hz), 7.42−7.44
(m, 2H), 7.48 (t, 1H, J = 5.3 Hz), 7.56 (d, 1H, J = 4.0 Hz), 7.66 (d,
1H, J = 4.0 Hz), 7.70−7.74 (m, 2H), 7.79 (d, 1H, J = 4.5 Hz), 7.83 (t,
1H, J = 3.0 Hz), 7.88 (d, 1H, J = 4.3 Hz), 7.98 (s, 1H), 8.13 (d, 1H, J =
3.8 Hz), 8.16 (t, 1H, J = 3.0 Hz), 8.37−8.39 (m, 2H), 8.51 (d, 1H, J =
3.2 Hz). 13C NMR (125 MHz, CDCl3): δ 110.4, 116.1, 116.4, 120.1,
120.8, 120.9, 121.0, 121.7, 122.9, 123.1, 124.4, 126.1, 126.3, 126.3,
126.6, 126.9, 127.0, 128.3, 130.1, 135.4, 136.1, 136.9, 137.0, 138.9,
139.9, 140.2, 142.8, 146.6, 152.0. Mass (FAB) m/z 427 (M + H)+.
Anal. Calcd for C29H18N2S: C, 81.66; H, 4.25; N, 6.57; S, 7.52;.
Found: C, 81.65; H, 4.25; N, 6.59; S, 7.51.
PCb-PCz. Yield 75%. Tg 97 °C, Tm 215 °C. FT-IR: 3051, 1952,
1592, 1496, 1458, 1411, 1334, 1289, 1226, 1171, 1122, 997, 932, 891,
847, 760, 737, 695 cm−1. 1H NMR (500 MHz, CDCl3): δ 7.21 (t, 1H,
J = 4.2 Hz), 7.27(t, 1H, J = 4.2 Hz), 7.32 (t, 1H, J = 4.8 Hz), 7.39−
7.39 (m, 2H), 7.43−7.48 (m, 3H), 7.53−7.61 (m, 6H), 7.67−7.70 (m,
2H), 7.80 (d, 1H, J = 4.0 Hz), 7.98 (s, 1H), 8.11 (d, 1H, J = 4.0 Hz),
8.15 (d, 1H, J = 3.8 Hz), 8.36 (d, 1H, J = 4.5 Hz), 8.40 (s, 1H), 8.50
Molecular simulation of PCb-DBF, PCb-DBT, and PCb-PCz
was carried out to investigate the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of the host materials. Gaussian 09 using the density
functional of B3LYP with the 6-31G* basis set was used for the
molecular simulation. Figure 1 shows the HOMO and LUMO
distribution of the host materials. The LUMO distribution was
similar among the host materials, but the HOMO distribution
was different in the three host materials. The LUMO was
localized on the pyridoindole unit of the host materials because
of the electron-accepting pyridine unit of the pyridoindole
moiety. The pyridine unit of pyridoindole strengthened the
electron-accepting properties of the pyridoindole unit, resulting
in the localization of the LUMO on the pyridoindole unit. The
HOMO was dispersed over 9-phenylcarbazole in PCb-PCz
because of the strong electron-donating character of the
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Scheme 1. Synthetic Scheme of PCb-DBF, PCb-DBT, and
PCb-PCz
Figure 2. CV curves of PCb-DBF, PCb-DBT, and PCb-PCz.
potential. As explained in the molecular simulation result,
pyridoindole is an electron-accepting moiety in the three host
materials, which resulted in similar reduction potentials in the
three host materials. Therefore, the difference between the
oxidation and reduction potential was 3.44 V in the PCb-PCz
host with electron donor−acceptor structure compared with
3.68 and 3.63 V for PCb-DBF and PCb-DBT, respectively.
The ionization potentials (IPs) calculated from the oxidation
potential of PCb-DBF, PCb-DBT, and PCb-PCz according to
the conversion factor proposed by Forrest et al.17 were −6.08,
−6.06, and −5.75 eV, respectively. As expected from the
HOMO distribution, the IPs of PCb-DBF and PCb-DBT were
stabilized compared with that of PCb-PCz because of the weak
electron-donating property of pyridonindole that dominates the
HOMO distribution of PCb-DBF and PCb-DBT. The strong
electron-donating character of 9-phenylcarbazole shifted the IP
of PCb-PCz to −5.75 eV. The electron affinities (EAs) of PCb-
DBF, PCb-DBT, and PCb-PCz from the reduction potentials
were −2.40, −2.43, and −2.31 eV, respectively. The EA was
similar in the three host materials because of the LUMO
localization on the pyridoindole unit.
Figure 1. Molecular orbital distribution of PCb-DBF, PCb-DBT, and
PCb-PCz.
Photophysical properties of the host materials were analyzed
using ultraviolet−visible (UV−vis) and photoluminescence
(PL) spectrometers. Figure 3 shows UV−vis absorption,
solution PL, and low-temperature PL spectra of PCb-DBF,
PCb-DBT, and PCb-PCz. Weak broad UV−vis absorption
between 310 and 360 nm was observed in all three host
materials because of the n−π* transition of pyridoindole. The
n−π* transition of PCb-PCz was stronger than that of other
host materials because of the additional n−π* transition of 9-
phenylcarbazole. Strong absorption peaks below 300 nm are
assigned to the π−π* transition of the conjugated backbone
structure of the host materials. Optical band gaps calculated
from the absorption edge of the UV−vis absorption spectra
were 3.45, 3.46, and 3.44 eV for PCb-DBF, PCb-DBT, and
PCb-PCz, respectively. There was little difference in the band
gap from the UV−vis absorption spectra because the
absorption edge was dominated by the n−π* transition of
the pyridoindole unit. Solution PL emission spectra were also
similar, and the PL emission peak was observed at 382 nm in all
three host materials. Phosphorescent emission spectra of the
host materials were measured in liquid nitrogen with a delay
time of 0.1 ms to remove fluorescent emission. Triplet energies
of the host materials were determined from the first
carbazole unit, whereas it was dispersed over 9-phenyl-
pyridoindole in PCb-DBF and PCb-DBT. The dibenzofuran
and disbenzothiophene possess very weak electron-donating
character, which leads to the localization of the HOMO on the
pyridoindole unit. The pyridineindole unit plays the role of the
electron-accepting unit because of pyridine and the electron-
donating unit because of indole. Therefore, it is presumed that
the HOMO and LUMO levels of PCb-DBF and PCb-DBT
would be similar, whereas the HOMO level of PCb-PCz would
be destabilized compared to that of PCb-DBF and PCb-DBT.
Oxidation and reduction potentials of the three host
materials were measured by cyclic voltammetry (CV). The
oxidation and reduction curves of the host materials are shown
in Figure 2. The oxidation potentials of PCb-DBF and PCb-
DBT were 1.06 and 1.04 V, whereas the oxidation potential of
PCb-PCz was 0.82 V. As the oxidation of PCb-PCz occurs in
the electron-donating carbazole unit compared to carboline
unit of PCb-DBF and PCb-DBT, the oxidation potential was
low in PCb-PCz. The reduction potentials of PCb-DBF, PCb-
DBT, and PCb-PCz were −2.62, −2.59, and −2.62 V,
respectively, and there was little difference of the reduction
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Figure 3. UV−vis, solution PL, and low-temperature PL spectra of PCb-DBF (a), PCb-DBT (b), and PCb-PCz (c).
Table 1. Basic Material Properties of PCb-DBF, PCb-DBT, and PCb-PCz
solution PL
IP
(eV)
EA
(eV)
band gap triplet energy
hole mobility
electron mobility
a
a
b
c
d
e
f
f
material
UV−vis (nm)
(nm)
(eV)
(eV)
Tg (°C) Tm (°C)
(cm2/V s)
(cm2/V s)
PCb-DBF 243, 254, 291, 298,
320, 338
382
382
383
−6.08 −2.40
3.68
3.63
3.44
2.82
2.72
2.74
69
79
97
150
188
215
8.1 × 10−4
4.2 × 10−5
4.3 × 10−5
4.1 × 10−5
PCb-DBT 242, 251, 267, 287,
337
−6.06 −2.43
9.2 × 10−5
PCb-PCz
238, 254, 294, 339,
345
−5.75 −2.31
1.3 × 10−4
a
b
c
Data were measured in tetrahydrofuran solution. IP was calculated from oxidation potential by cyclic voltammetry. EA was calculated from
d
e
reduction potential by cyclic voltammetry. Band gap was calculated from the IP and EA. Triplet energy was calculated from the first emission peak
f
of low-temperature PL spectra. Mobility value at 600 (V/cm)1/2
.
phosphorescent emission peak of the low-temperature PL
spectra, which were 2.82, 2.72, and 2.74 eV for PCb-DBF, PCb-
DBT, and PCb-PCz, respectively. The extension of conjugation
through cental phenyl linkage resulted in relatively low triplet
energy compared to that of dibenzofuran, dibenzothiphene, and
9-phenylcarbazole. However, the triplet energy of the host
materials was high enough for application as the host materials
for Firpic (2.65 eV)-doped blue PHOLEDs. Basic material
parameters are summarized in Table 1.
Single-carrier devices of the host materials were fabricated to
compare hole and electron density in the emitting layer. Figure
4 shows current density−voltage curves of hole-only and
electron-only devices of PCb-DBF, PCb-DBT, and PCb-PCz
host materials. The order of hole current density was PCb-PCz
> PCb-DBT > PCb-DBF, and PCb-PCz showed a high hole
current density. The high hole current density of the PCb-PCz
device is related to the HOMO distribution of the host
materials. The HOMO of PCb-PCz is localized on the
carbazole unit, which has stronger hole-transport character
Figure 4. Current density−voltage curves of hole-only and electron-
only devices of PCb-DBF, PCb-DBT, and PCb-PCz.
than the carboline unit, resulting in high hole current density in
the PCb-PCz device. There was little energy barrier for hole
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injection, as shown in the energy-level diagram in Figure 5. The
electron current density was similar in the three host materials
Figure 5. Energy-level diagram of PCb-DBF, PCb-DBT, and PCb-PCz
devices.
Figure 7. External quantum efficiency−luminance curves of PCb-DBF,
PCb-DBT, and PCb-PCz devices.
because the electron-transport properties are dominated by the
pyridoindole unit, which is present in all three host materials.
The hole current density and the electron current density were
well-balanced in the PCb-PCz device compared to the PCb-
DBF and PCb-DBT devices. Hole and electron mobilities of
the host materials were further studied to correlate the
molecular structure of the host materials with charge-transport
properties, which are summarized in Table 1. The order of hole
mobility of the host materials was similar to that of the hole
current density of the host materials, and the electron mobility
of the host materials was similar in all host materials.
Blue PHOLEDs were fabricated to compare the device
performance of the three host materials. The device structure
was ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/
mCP (10 nm)/PCb-DBF:Firpic or PCb-DBT:Firpic or PCb-
PCz:FIrpic (25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200
nm). The doping concentration of FIrpic was 3% in all host
materials. Current density−voltage−luminance curves of the
blue PHOLEDs are shown in Figure 6. The current density and
exhibited a higher quantum efficiency than the PCb-DBT and
PCb-DBF devices, and the maximum external quantum
efficiency of the PCb-PCz device was 29.6 0.1%. The high
quantum efficiency of PCb-PCz was maintained at high
luminance, and the quantum efficiency at 1000 cd/m2 was
27.1
0.2%. There was less than a 10% decrease of the
quantum efficiency from the maximum quantum efficiency. The
maximum quantum efficiencies of the PCb-DBT and PCb-DBF
devices were 25.9 0.1 and 24.0 0.1%, respectively. The high
quantum efficiency of the PCb-PCz blue PHOLEDs is related
to the charge balance in the emitting layer and the high PL
quantum yield of PCb-PCz film. The hole and electron current
densities of the single-carrier devices were well-balanced in the
PCb-PCz devices compared to the PCb-DBT and PCb-DBF
devices, which improved the quantum efficiency of the PCb-
PCz blue PHOLEDs. The high PL quantum efficiency of PCb-
PCz:Firpic also enhanced the quantum efficiency of the PCb-
PCz blue PHOLEDs, as the PL quantum efficiencies of PCb-
PCz:FIrpic, PCb-DBF:FIrpic, and PCb-DBT:FIrpic were 98,
82, and 85%, respectively. Other parameters, such as triplet
energy, charge confinement, and optical outcoupling effect by
thin ITO, were similar in the three host materials, and they
enhanced the quantum efficiency of the blue PHOLEDs
fabricated in this work. Outcoupling improvement by thin ITO
(50 nm) used in this work was 2% compared to common ITO
with a thickness of 150 nm. The quantum efficiency achieved in
this work is close to the state-of-the-art efficiency data reported
in the literature. The maximum power efficiencies of PCb-DBF,
PCb-DBT, and PCb-PCz devices were 30.3 0.4, 36.2 1.6,
44.2
0.2 lm/W, respectively. All device performance
Figure 6. Current density−voltage−luminance curves of PCb-DBF,
PCb-DBT, and PCb-PCz devices.
characteristics are summarized in Table 2.
Electroluminescence (EL) spectra of the blue PHOLEDs are
shown in Figure 8. All devices showed maximum emission
peaks at 470 nm and a shoulder peak at 492 nm without any
emission from mCP or TSPO1, indicating that there was little
charge leakage in the device. The color coordinates of the PCb-
DBF, PCb-DBT, and PCb-PCz devices were (0.14,0.29),
(0.14,0.29), and (0.14,0.28), respectively. The rather strong
shoulder peak in the PCb-DBF and PCb-DBT blue PHOLEDs
is due to recombination zone shift compared to that of PCb-
PCz blue PHOLED. The strong hole-transport properties of
PCb-PCz shift the recombination zone from the hole-transport
layer side to electron-transport layer side, which induces an
optical effect in the device and changes the EL spectra.
luminance of the PCb-PCz device were higher than those of the
PCb-DBF and PCb-DBT devices. The high current density of
the PCb-PCz device is related to the high hole current density
of the PCb-PCz hole-only device. Because the hole current
density of the PCb-PCz hole-only device was higher than that
of the PCb-DBF and PCb-DBT devices while maintaining a
similar electron current density, a high current density was
obtained in the PCb-PCz device. The turn-on voltage of the
PCb-PCz device was also lower than that of the PCb-DBF and
PCb-DBT devices.
External quantum efficiency−luminance curves of the blue
PHOLEDs are shown in Figure 7. The PCb-PCz device
E
dx.doi.org/10.1021/cm403750p | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Table 2. Performance of PCb-DBF, PCb-DBT, and PCb-PCz Devices
host
material
maximum quantum
efficiency (%)
quantum efficiency
maximum power efficiency
power efficiency at 1000 cd/m2
color
coordinate
at 1000 cd/m2 (%)
(lm/W)
(lm/W)c
PCb-DBF
PCb-DBT
PCb-PCz
24.0 0.1
25.9 0.1
29.6 0.1
20.4 0.2
21.7 0.1
27.1 0.2
30.3 0.4
36.2 1.6
44.2 0.2
18.0 0.1
19.0 0.1
25.1 0.1
(0.14,0.29)
(0.14,0.29)
(0.14,0.28)
(6) Ding, J.; Wang, Q.; Zhao, L.; Ma, D.; Wang, L.; Jing, X.; Wang, F.
J. Mater. Chem. 2010, 20, 8126.
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2007, 19, 197.
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C.; Liao, Y. L.; Wong, K. T.; Wu, C. I. Adv. Mater. 2006, 18, 1216.
(9) Lee, C. W.; Lee, J. Y. Adv. Mater. 2013, 25, 596.
(10) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin,
J.; Ma, D. Angew. Chem., Int. Ed. 2008, 47, 8104.
(11) Motoyama, T.; Sasabe, H.; Seino, Y.; Takamatsu, J.; Kido, J.
Chem. Lett. 2011, 40, 306.
(12) Fukagawa, H.; Yokoyama, N.; Irisa, S.; Tokito, S. Adv. Mater.
2010, 22, 4774.
(13) Im, Y.; Lee, J. Y. Chem. Commun. 2013, 49, 5948.
(14) Kim, S. J.; Kim, Y. J.; Son, Y. H.; Hur, J. A.; Um, H. A.; Shin, J.;
Lee, M.; Cho, J.; Kim, J. K.; Joo, S.; Yang, J. H.; Chae, G. S.; Choi, K.;
Kwon, J. H.; Choi, D. H. Chem. Commun. 2013, 49, 6788.
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2687.
Figure 8. Electroluminescence spectra of PCb-DBF, PCb-DBT, and
PCb-PCz devices.
(16) Lee, C. W.; Lee, J. Y. Adv. Mater. 2013, 25, 5450.
(17) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.;
Polikarpov, E.; Thompson, M. E. Org. Electron. 2005, 6, 11.
CONCLUSIONS
■
Three host materials derived from a pyridoindole moiety, PCb-
PCz, PCb-DBT, and PCb-DBF, were synthesized as high-
triplet-energy host materials for blue PHOLEDs, and the
molecular structure could be well-correlated with the photo-
physical properties and device performance of the host
materials. The 9-phenylcarbazole moiety was better than
dibenzofuran or dibenzothiophene to manage the energy levels
and charge-transport properties of the pyridoindole-derived
host materials. The PCb-PCz host with the 9-phenylcarbazole
moiety showed balanced charge density in the emitting layer
and exhibited a high external quantum efficiency of 29.6%
0.1% in blue PHOLEDs. Therefore, the combination of
pyridoindole and 9-phenylcarbazole is one effective way of
developing high-triplet-energy host materials for blue PHO-
LEDs.
AUTHOR INFORMATION
Corresponding Author
*E-mail: leej17@dankook.ac.kr. Tel: 82-31-8005-3585. Fax: 82-
31-8005-3585.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The present research was conducted by the research fund of
Dankook University in 2013.
■
REFERENCES
■
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dx.doi.org/10.1021/cm403750p | Chem. Mater. XXXX, XXX, XXX−XXX