Phenanthroline derivatives for electron-transport layer in organic light-emitting devices

Article abstract of DOI:10.1246/cl.2009.712

A series of novel phenanthroline derivatives (Phens) have been synthesized and their application to organic light-emitting devices (OLEDs) as an electron-transport layer was investigated. The OLEDs with a structure of ITO/α-NPD/Alq₃/Phen/LiF/Al exhibited remarkable performances and lower operating voltages compared to Alq₃-based device, indicating that Phens posses favorable electron-transport properties. Copyright

Full text of DOI:10.1246/cl.2009.712

712
Chemistry Letters Vol.38, No.7 (2009)
Phenanthroline Derivatives for Electron-transport Layer
in Organic Light-emitting Devices
Yan-Jun Li,1 Hisahiro Sasabe,1;2 Shi-Jian Su,1;2 Daisaku Tanaka,1
Takashi Takeda,1 Yong-Jin Pu,2 and Junji Kidoꢀ1;2
1Optoelectronic Industry and Technology Development Association (OITDA),
1-20-10 Sekiguchi, Bunkyo-ku, Tokyo 112-0014
2Department of Organic Device Engineering, Yamagata University,
4-3-16 Johnan, Yonezawa 992-8510
(Received April 27, 2009; CL-090420; E-mail: kid@yz.yamagata-u.ac.jp)
A series of novel phenanthroline derivatives (Phens) have
1a: X=CH
1b: X=N
been synthesized and their application to organic light-emitting
devices (OLEDs) as an electron-transport layer was investigated.
The OLEDs with a structure of ITO/ꢀ-NPD/Alq3/Phen/LiF/Al
exhibited remarkable performances and lower operating voltag-
es compared to Alq3-based device, indicating that Phens posses
favorable electron-transport properties.
Br
X
Br
MeOC
MeOC
B(OH)
2
Pd(PPh ) , Na CO aq.
3 4
2
3
Route 1
X
Y
Y
COMe
COMe
pinB
Bpin
2a: X=Y=CH
N
Br
2b: X=N, Y=CH
2c: X=CH, Y=N
Pd(PPh ) , Na CO aq.
3 4
2
3
Since Tang et al. reported an organic light-emitting device
(OLED) using tris(8-quinolinolato)aluminum(III) (Alq3) as an
electron-transport material (ETM) in 1987,1 Alq3 has been wide-
ly used. However, devices with Alq3 generally show higher driv-
ing voltage and low efficiency due to its low electron mobility
and injection properties.2 Therefore, the development of ETMs
is still necessary for the development of high performance
OLEDs. To date, a few materials have been known as elec-
tron-transport and injection layers (ETL and EIL) in OLEDs.
Among them, bathocuproine (BCP),3 bathophenanthroline,4
Route 2
X
N
CHO
NH
Y
Y
2
Phen1: X=Y=CH
Phen2: X=N, Y=CH
Phen3: X=CH, Y=N
3
2
N
N
N
N
Phens
Figure 1. Synthesis of phenanthroline derivatives (Phens).
tetra(2-pyridinyl)pyrazine,5
1,3,5-tris(3-methylpyrid-5-yl)tri-
azine,6 3,5,30,50-tetra(p-pyrid-3-yl)phenyl[1,10]biphenyl7 have
been reported and showed electron-transport and electron-injec-
tion properties in OLEDs.
Table 1. Thermal and electrochemical data of Phens
Compound Tm/ꢁCa Td/ꢁCb Ip/eVc Eg/eVd Ea/eVe
Phen1
Phen2
Phen3
n.d.
n.d.
n.d.
489
493
475
6.27
6.25
5.68
3.45
3.39
3.46
2.82
2.86
2.22
Here, we successfully developed three novel 2-substituted
phenanthroline derivatives (Phens) and NPD/Alq3-based
OLEDs using Phens as an ETL. The synthetic route of Phens
is shown in Figure 1. First the precursors 2a and 2b were pre-
pared via the Suzuki coupling8 of 3-acetylbenzeneboronic acid
and dibromide 1a and 1b. Similary, 2c was prepared from the
reaction of 2-acetyl-6-bromopyridine with 1,3-di(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)benzene. 8-Amino-7-quinoline-
carbaldehyde (3) was obtained in several steps from 7-methyl-
quinoline. The target material Phens were synthesized in 46%
yield for Phen1, 64% for Phen2, and 73% for Phen3 respectively
according to the literature.9,10 The compounds were character-
aDetermined by DSC measurement. bObtained from TGA
analysis. Measured by AC-3 UV photoelectron spectrome-
c
ter. dTaken as the point of intersection of the normalized ab-
sorption spectra. Calculated using Ip and Eg values.
e
sorption edge (ꢁabs) of Phen1, Phen2, and Phen3 are at 359, 367,
and 359 nm and the first PL peaks are at 358, 360, and 355 nm,
respectively. According to the absorption edges, the Eg of Phens
were estimated to be 3.45, 3.39, and 3.46 eV, respectively.
The electrochemical properties of Phens are very similar to
that of BCP. The Ip values of Phen1 and Phen2 were observed
at ca. 6.3 eV, which are 0.4 eV deeper than that of Alq3
(5.9 eV). Thus, hole-blocking properties are expected in OLEDs.
Although Phen2 has a pyridine ring in the center of the molecule,
the Ip and Ea values of Phen1 and Phen2 are almost the same. In-
troduction of the pyridine ring does not affect the Ip and Ea values
of Phen2. On the other hand, in the case that the pyridine rings are
substituted directly to phenanthroline group such as Phen3, shal-
lower Ip and Ea values were observed than that of Phen1 and
Phen2. These results clearly show that the position of nitrogen
is critically important for tuning the Ip and Ea values of materials.
1
ized by H NMR, 13C NMR, and mass spectrometry, and were
purified repeatedly by temperature gradient vacuum sublimation
before device fabrication.
The melting points (Tm), decomposition temperatures (Td),
ionization potentials (Ip), electron affinities (Ea), and energy gaps
(Eg) of Phens are summarized in Table 1. The glass-transition
temperatures (Tg) and Tm of Phens could not be observed, indi-
cating that they form a stable amorphous state. The results of
Td (475–493 ꢁC) exhibit that these phenanthroline derivatives
present constant thermal stability. The room-temperature UV–vis
absorption (abs) and photoluminescent (PL) spectra of Phens-
deposited film on quartz substrates are shown in Figure 2. The ab-
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.7 (2009)
713
400
300
200
100
0
104
103
102
101
100
10-1
Phen1
Phen2
Phen3
0
2
4
6
8
10
250
300
350
400
450
Voltage/V
Wavelength/nm
Figure 3. Current density–voltage–luminance characteristics of
ITO/ꢀ-NPD(50nm)/Alq3(40 nm)/ETL(30nm)/LiF/Al, ETL =
Alq3 (open circle), Phen1 (open square), Phen2 (closed circle),
and Phen3 (closed square).
Figure 2. Absorption and photoluminescent spectra of Phens
films (50 nm) vacuum deposited on quartz substrate.
To examine the electron-transport properties of Phens,
OLEDs with a structure of indium-tin oxide (ITO)/4,40-N,N0-
bis[N-(1-naphthyl)-N-phenylamino]biphenyl (ꢀ-NPD)/Alq3/
Phen/LiF/Al were fabricated. Prior to the fabrication of the or-
ganic layers, the ITO substrate was cleaned with ultra-purified
water and organic solvents, and treated with an UV–ozone am-
bient. The hole-transport layer (ꢀ-NPD), emission layer (Alq3),
and ETL (Alq3 or Phens) were continuously formed on the sub-
strate by vacuum deposition. Finally, a 1-nm-thick LiF layer
and 100-nm-thick Al were deposited as the cathode. Current
density (J), voltage (V), and luminance (L) characteristics were
measured by using a Keithley source meter 2400 and a TOPCON
BM-8, respectively.
performance compared to Alq3 due to favorable electron-trans-
port properties. Further study of Phens for the application to
OLEDs is currently in progress.
This work was financially supported in part by the New
Energy and Industrial Technology Development Organization
(NEDO) through the ‘‘Advanced Organic Device Project.’’
References and Notes
1
2
3
C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913.
G. Hughes, M. R. Bryce, J. Mater. Chem. 2005, 15, 94.
G. Parthasarathy, C. Adachi, P. E. Burrows, S. R. Forrest,
Appl. Phys. Lett. 2000, 76, 2128.
Figure 3 shows J–V–L characteristics of the devices with
Phens or Alq3 as an ETL. The turn-on voltage of devices using
Phen1 at 2.7 V and Phen2 at 2.8 V were obviously lower than
that with Alq3 at 2.9 V, and Phen1 showed the best performance
among these devices. The current densities of devices based on
Phen1–3 presented 5.8, 4.0, and 2.3 mA cmꢂ2 at 5.0 V, and 424,
324, and 238 mA cmꢂ2 at 10.0 V respectively. The increase of
nitrogen in Phens tends to reduce the current densities of the de-
vice. The Alq3-based device provided current densities of
1.5 mA cmꢂ2 at 5.0 V, and 87 mA cmꢂ2 at 10.0 V, respectively.
High luminance of more than 20000 cd mꢂ2 was achieved utiliz-
ing Phen1 and Phen2 as an ETL at an applied voltage of 10.0 V.
At the same voltage, the Alq3-based device showed about 1/4
lower luminance of 4880 cd mꢂ2 than that of Phens. The external
quantum efficiency (and the power efficiency) of four devices
were recorded at 100 cd mꢂ2 to be 1.48% (2.98 lm Wꢂ1) for
4
5
J. Kido, T. Matsumoto, Appl. Phys. Lett. 1998, 73, 2866.
T. Oyamada, C. Maeda, H. Sasabe, C. Adachi, Chem. Lett.
2003, 32, 388.
6
7
8
T. Oyamada, H. Yoshizaki, H. Sasabe, C. Adachi, Chem.
Lett. 2004, 33, 1034.
S.-J. Su, D. Tanaka, Y.-J. Li, H. Sasabe, T. Takeda, J. Kido,
Org. Lett. 2008, 10, 941.
A. Suzuki, H. C. Brown, Organic Synthesis via Boranes,
vol. 3, Suzuki coupling, Aldrich Chemical Company,
Milwaukee, 2003.
9
E. C. Riesgo, X. Jin, R. P. Thummel, J. Org. Chem. 1996, 61,
3017.
10 A mixture of 2a (2.1 g, 6.67 mmol), 3 (2.3 g, 13.3 mmol), and
saturated ethanolic KOH (1.0 g) in absolute ethanol (20 mL)
was heated, and refluxed under nitrogen gas for 24 h. The
solvent was evaporated, and the residue was purified by
chromatography on silica gel, eluting with CHCl3/EtOAc
(1:1), to provide 1.8 g (46%) of Phen1. 1H NMR (CDCl3,
500 MHz), ꢂ 7.62–7.70 (m, 5H), 7.77–7.87 (m, 8H), 8.09
(s, 1H), 8.21 (d, J ¼ 8:0 Hz, 2H), 8.27 (d, J ¼ 8:5 Hz, 2H),
8.35 (d, J ¼ 8:0 Hz, 2H), 8.42 (d, J ¼ 8:0 Hz, 2H), 8.55 (s,
2H), 9.22 (d, J ¼ 4:5 Hz, 2H); 13C NMR (CDCl3, 125
MHz): ꢂ 120.98, 122.92, 126.35, 126.39, 126.60, 126.86,
127.41, 127.65, 128.41, 129.09, 129.19, 129.35, 136.06,
136.91, 140.39, 141.79, 141.83, 146.19, 146.46, 150.44,
157.70. MS(EI): m=z 587 [M]þ.
Alq3, 1.47% (3.65 lm Wꢂ1) for Phen1, 1.54% (3.66 lm Wꢂ1
)
for Phen2, and 1.29% (2.75 lm Wꢂ1) for Phen3, respectively.
At 1000 cd mꢂ2, the efficiencies were also recorded to be
1.63% (2.20 lm Wꢂ1) for Alq3, 1.65% (2.81 lm Wꢂ1) for Phen1,
1.78% (2.90 lm Wꢂ1) for Phen2, and 1.53% (2.19 lm Wꢂ1) for
Phen3, respectively. These results apparently show that elec-
tron-transport properties of Phens are much greater than that
of traditional electron-transport material, Alq3.
In summary, a series of novel phenanthroline derivatives
were synthesized and their OLED performances as an ETL have
been investigated. The Phen-based OLEDs exhibited remarkable
Published on the web (Advance View) June 13, 2009; doi:10.1246/cl.2009.712