9,10-bis(bipyridyl, pyridylphenyl, phenylpyridyl, and biphenyl)anthracenes combining high electron transport and injection, efficiency and stability in fluorescent organic light-emitting devices

Article abstract of DOI:10.1246/cl.2011.1092

A series of anthracene derivatives, 9,10-bis(bipyridyl, pyridylphenyl, phenylpyridyl, and biphenyl)anthracenes, were synthesized. They exhibited properties of small electron injection barrier from cathode in electron-only devices, high electron mobility f

Full text of DOI:10.1246/cl.2011.1092

doi:10.1246/cl.2011.1092
1092
Published on the web September 23, 2011
9,10-Bis(bipyridyl, pyridylphenyl, phenylpyridyl, and biphenyl)anthracenes
Combining High Electron Transport and Injection, Efficiency and Stability
in Fluorescent Organic Light-emitting Devices
Jae-Jin Oh,1 Yong-Jin Pu,*1,2 Hisahiro Sasabe,1,2 Ken-ichi Nakayama,1,2 and Junji Kido*1,2
1Department of Organic Device Engineering, Yamagata University, 4-3-16 Johnan, Yonezawa, Yamagata 992-8510
2Research Center for Organic Electronics, Yamagata University, 4-3-16 Johnan, Yonezawa, Yamagata 992-8510
(Received July 6, 2011; CL-110571; E-mail: pu@yz.yamagata-u.ac.jp, kid@yz.yamagata-u.ac.jp)
A series of anthracene derivatives, 9,10-bis(bipyridyl,
pyridylphenyl, phenylpyridyl, and biphenyl)anthracenes, were
synthesized. They exhibited properties of small electron
injection barrier from cathode in electron-only devices, high
electron mobility from time-of-flight measurement, and high
efficiency and stability of OLEDs.
Organic light-emitting devices (OLEDs) are charge injection
devices, requiring the simultaneous supply of both electrons and
holes to a light-emitting material sandwiched between two
electrodes.1 One of the issues in achieving high quantum
efficiency is to balance the number of holes and electrons.
Because the hole mobility in the OLED is usually much higher
than the electron mobility under the same electric field,2
development of electron-transporting materials with high mobi-
lity is necessary. The hole- and electron-transporting molecules,
4,4¤-bis[1-naphthyl(phenyl)amino]-1,1¤-biphenyl (NPD) and tris-
(8-quinolinolato)aluminum (Alq3), commonly used in OLEDs,
have hole and electron mobilities that differ by a factor of ca.
1000 (ca. 10¹3 and ca. 10¹6 cm2 V¹1 s¹1 for holes and electrons,
respectively). The electron-transport materials (ETMs) should
have the abilities of electron transport and injection, hole block,
and electrochemical stability. In addition to the widely used
Alq3, various derivatives of siloles,3,4 oxadiazoles,5 oligopyr-
idine,6,7 etc. have been reported to have high electron mobility
and performance in OLEDs as an ETM. 9,10-Diphenylanthra-
cene-based derivatives have been used as a highly efficient host
material in fluorescent OLEDs8 and have high electrochemical
stability, which is an advantage for device lifetime in OLED
application.9 However, there are few reports of ETM based on
9,10-diphenylanthracene structure, except a very recent report by
Qiu et al.10 In this work, we synthesized four kinds of 9,10-
bis(biaryl)anthracene derivatives containing pyridyl group, 9,10-
bis[3-(pyridin-4-yl)phenyl]anthracene (PyPhAnt), 9,10-bis(5-
phenylpyridin-3-yl)anthracene (PhPyAnt), 9,10-bis(3,4¤-bipyri-
din-5-yl)anthracene (PyPyAnt), and 9,10-bis(biphenyl-3-yl)-
anthracene (PhPhAnt), shown in Figure 1. They exhibited high
electron mobility in time-of-flight measurement, good electron
injection properties from cathode in an electron-only device,
and high efficiencies in fluorescent OLEDs. In addition to this
improvement, the device lifetime was much improved with
PyPhAnt, compared with a device with Alq3 as an ETM.
PyPhAnt showed no trade-off performance between the
efficiency and lifetime of the device.
Figure 1. Chemical structures of the compounds.
Table 1. Thermal and optoelectrochemical properties
a
c
Tg
Tc
Tm Td5% Eg
PLb
Ip
Ea
/°C /°C /°C /°C /eV /nm /eV /eV
PyPhAnt 92 200 284 381 2.95 425 5.97 3.02
PhPyAnt 93 190 277 380 2.95 440 6.05 3.10
PyPyAnt 93 215 320 411 2.95 420 6.08 3.13
PhPhAnt 90 185 267 378 2.95 440 5.97 3.02
aAbsorption edge of the film. bFilm. cThe difference between Ip
and Eg.
obtained from 3,5-dibromopyridine and 1,3-dibromobenzene,
respectively, by Suzuki coupling of 9,10-bis(4,4,5,5-tetrameth-
yl-1,3,2-dioxaborolan-2-yl)anthracene. Another Suzuki coupling
with the obtained bromophenyl- or bromopyridylanthracene and
4-pyridyl- or phenylboronic acid pinacol ester gave the target
anthracene derivatives. The compounds were purified by
temperature-gradient sublimation under flow of N2 gas and
characterized by 1H NMR, mass spectrometry, and elemental
analysis.
Glass-transition temperatures (Tg) and thermal decomposi-
tion temperatures with loss of 5 wt % (Td5%) were measured by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA), respectively. Td’s of the compounds were
around 400 °C, and Tg’s of the compounds were higher than
90 °C (Table 1). Although the Tg’s are not different among the
compounds, the melting point of only PyPyAnt was much
higher than the others, probably because the four pyridine
groups caused intermolecular hydrogen-bonding interaction.
The UV absorption and PL spectrum of the films on a quartz
substrate were measured (Figure S1).17 All of the compounds
exhibited similar absorption spectra and edges, so that their
optical energy gaps were almost the same. The PL spectra were
blue emission with wavelength maxima located around 425-
440 nm. Ionization potentials (Ip’s) were measured by atmos-
The anthracene derivatives were synthesized according to
the procedures in Scheme S1.17 9,10-Bis(3-bromophenyl)-
anthracene and 9,10-bis(5-bromopyridin-3-yl)anthracene were
Chem. Lett. 2011, 40, 1092-1094
© 2011 The Chemical Society of Japan
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1093
10-3
10-4
10-5
10-6
200
150
100
50
104
103
102
101
100
10-1
140
120
100
80
60
40
20
0
0
650 700 750 800 850 900
Electric field E1/2/V cm–1/2
0
2
4
6
8
10
0
2
4
6
8
0
5
10
Voltage/V
15
20
Voltage/V
Voltage/V
Figure 3. Current density-voltage (left) and luminance-volt-
age (right) plots of the device: ITO/NPD/Alq3/ETM/LiF/Al.
ETM: PyPhAnt (closed circle), PhPyAnt (open circle),
PyPyAnt (closed triangle), PhPhAnt (open triangle), and Alq3
(cross).
Figure 2. Left: current density-voltage plots of the electron-
only decvices. Right: Electron mobility of the compounds.
PyPhAnt: closed circle, PhPyAnt: open circle, PyPyAnt:
closed triangle, PhPhAnt: open triangle, and Alq3: cross.
pheric ultraviolet photoelectron yield spectroscopy. The Ip’s of
the compounds with attached pyridine groups directly on
anthracene, PhPyAnt and PyPyAnt, were higher than that of
the compounds with attached phenyl groups on anthracene,
PyPhAnt and PhPhAnt, because of the electrophilicity of the
pyridine group.
100
90
80
70
60
50
Figure 2 (left) shows the current density-voltage character-
istics of the electron-only devices, of which structure was ITO/
4,6-bis[3,5-(dipyrid-4-yl)phenyl]-2-methylpyrimidine (B4PYMPM)6
(20 nm)/PyPhAnt, PhPyAnt, PyPyAnt, PhPhAnt, or Alq3
(150 nm)/LiF (0.5 nm)/Al, where B4PYMPM was used as a
hole-blocking layer. The devices of PyPhAnt, PhPyAnt, and
PyPyAnt having pyridine group showed much lower driving
voltage than the device with Alq3. On the other hand, the device
of PhPhAnt having no pyridine group showed no current
response. This demonstrates that the pyridine group of the
compounds enhanced the electron injection from LiF/Al
cathode. Electron affinities, Ea’s, of the compounds were almost
the same around 3.0-3.1 eV, and there was no correlation
between the order of these Ea’s and the order of electron
injection barrier in the electron only device. The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) of the compounds were calculated by
density functional theory. The frontier orbitals of the compounds
are shown in Figure S2.17 Both the HOMO and LUMO of the
compounds are located only on the anthracene moiety. Their
LUMO energy levels are similar, and their order also cannot
explain the order of the electron injection barrier. In the electron
injection process, Li ion in LiF is expected to be reduced to
Li metal by the thermally activated Al during vacuum
deposition11-13 and interact with the pyridine group of elec-
tron-transporting materials as an electron donor.14 PyPhAnt
bearing a pyridine group at the outside of the molecule showed
the lowest turn-on voltage, suggesting that the positions of the
pyridine ring affect the specific affinity with cathode metal or
metal halide to determine the barrier height for the electron
injection.
0
500 1000 1500 2000 2500 3000
Time/h
Figure 4. Stability of the devices: ITO/NPD/Alq3/ETM/
LiF/Al. ETM: PyPhAnt (solid line) and Alq3 (dashed line).
films as a function of the square root of the electric field. The
electron mobility of the compounds was much higher than that
of Alq3: 1.3-2.9 © 10¹5 cm2 V¹1 s for PyPyAnt, 6.4 © 10
-
¹1
¹6
1.7 © 10¹5 cm2 V¹1 s
for PhPyAnt, and 2.5-4.5 © 10
¹1
¹5
cm2 V¹1 s for PhPhAnt. Particularly, PyPhAnt shows the
highest electron mobility of 1.6-2.8 © 10¹4 cm2 V¹1 s¹1. This
value is higher than that of well-known electron-transporting
materials, 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]-
benzene (OXD-7)5 and 2,2¤,2¤¤-(1,3,5-benzenetriyl)tris(1-phen-
yl-1H-benzimidazole) (TPBI)15 and comparable to that of high
electron-transporting silole compound, 2,5-bis(2,2¤-bipyridin-
6-yl)-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy)3 and oligo-
¹1
pyridine
compound,
1,3,5-tris(2,2¤-bipyridin-6-yl)benzene
(BPyB).16
The devices with the following structure: ITO/NPD (40
nm)/Alq3 (40 nm)/PyPhAnt, PhPyAnt, PyPyAnt, PhPhAnt,
or Alq3 (20 nm)/LiF (0.5 nm)/Al (80 nm) were fabricated.
Figure 3 shows current density-voltage-luminance character-
istics. The device with PyPhAnt, PhPyAnt, and PyPyAnt with
a pyridine group in an ETL showed lower driving voltage than
that of the device with Alq3, resulting in higher power efficiency
(lm W¹1) (Table S1).17 PyPhAnt also improved current effi-
ciency compared with the Alq3 device. On the other hand, the
device with PhPhAnt with no pyridine group showed highest
driving voltage and lowest efficiency, because of the poor
Electron transport properties of the compounds were
investigated by time-of-flight (TOF) measurement. The device
structure was ITO/anthracene derivatives (5.0 ¯m)/Al (80
nm), and the transient photocurrent signals were dispersive
(Figure S3).17 Figure 2 (right) shows the electron mobility of the
Chem. Lett. 2011, 40, 1092-1094
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1094
400
the 9,10-bis(pyridylphenyl)anthracenes can be useful as an ETM
for the anthracene host in a fluorescent OLED to improve power
efficiency and current efficiency.
In summary, new 9,10-bis(biaryl)anthracene-based electron-
transporting materials were synthesized. The compounds having
pyridine groups have both small electron injection barrier from
cathode and high electron mobility. The compound PyPhAnt
with outer pyridine showed longer lifetime of the device as an
ETM, even with higher efficiency than the device with Alq3.
104
103
102
101
100
10-1
350
300
250
200
150
100
50
0
0
We would like to thank the support by Japan Regional
Innovation Strategy Program by the Excellence (J-RISE)
(Creating International Research Hub for Advanced Organic
Electronics) of Japan Science and Technology Agency (JST).
Y.-J. Pu is grateful for support from Industrial Technology
Research Grant Program from New Energy and Industrial
Technology Development Organization (NEDO) of Japan.
0
2
4
6
8
2
4
6
8
Voltage/V
Voltage/V
Figure 5. Current density-voltage and luminance-voltage
caracteristics of the device: ITO/NPD/MADN: 8 wt % ru-
brene/ETM/LiF/Al. ETM: PyPhAnt (closed circle), PyPyAnt
(closed triangle), and Alq3 (cross).
Table 2. Efficiencies of the devices with MADN host and
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¹2
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)
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9
¹2
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¹2
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with Alq3. The device with the anthracene compound PyPhAnt
as an ETM exhibited 2880 h of much higher half-life (LT50) than
660 h of the device with Alq3 ETM. This anthracene compound,
PyPhAnt, is thought to achieve the high efficiency and the long
lifetime of the device simultaneously.
Yellow fluorescent devices with rubrene dopant and
2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) host
were fabricated with the following structure: ITO/NPD (40
nm)/MADN: 8 wt % rubrene (20 nm)/PyPhAnt, PyPyAnt, or
Alq3 (40 nm)/LiF (0.5 nm)/Al (80 nm). Figure 5 shows the
current density-voltage-luminance characteristics of the de-
vices. The efficiencies of the device with PyPhAnt and
PyPyAnt, summarized in Table 2, were much improved over
the device with Alq3. The EL spectrum of the device with Alq3
(Figure S4)17 showed a shoulder peak around 520 nm derived
from Alq3 emission, indicating the poor electron-transporting
and injection properties of Alq3. These results demonstrated that
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17 Supporting Information is available electronically on the
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Chem. Lett. 2011, 40, 1092-1094
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/