Organic light-emitting devices consisting of N-triarylamine-based hole injecting/transporting materials

Article abstract of DOI:10.1080/15421400802619735

A series of N-triarylamine-based derivatives such as 4,4'-bis[N-(1- naphthyl)-N-phenyl-amino]biphenyl (-NPD), 1,3,5-tris[4-(3-methylphenyl(phenyl) amino)phenyl]benzene (m-MTDAPB) and 1,3,5-tris-{4-[1-naphthyl(phenyl)amino] phenyl}benzene (1-TNAPB) were sy

Full text of DOI:10.1080/15421400802619735

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Organic Light-Emitting
Devices Consisting of N-
Triarylamine-Based Hole
Injecting/Transporting Materials
Ho Young Cho a , Lee Soon Park a , Yoon Soo Han b ,
Younghwan Kwon c & Jae-Yong Ham c
a Department of Polymer Science, Kyungpook
National University, Daegu, Korea
b Display & Nano Devices Laboratory, DGIST, Daegu,
Korea
c Department of Chemical Engineering, Daegu
University, Gyeongsan, Gyeongbuk, Korea
Version of record first published: 22 Jul 2010.
To cite this article: Ho Young Cho , Lee Soon Park , Yoon Soo Han , Younghwan Kwon
& Jae-Yong Ham (2009): Organic Light-Emitting Devices Consisting of N-Triarylamine-
Based Hole Injecting/Transporting Materials, Molecular Crystals and Liquid Crystals,
498:1, 314-322
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Mol. Cryst. Liq. Cryst., Vol. 498, pp. 314–322, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802619735
Organic Light-Emitting Devices Consisting
of N-Triarylamine-Based Hole Injecting=Transporting
Materials
Ho Young Cho1, Lee Soon Park1, Yoon Soo Han2,
Younghwan Kwon3, and Jae-Yong Ham3
1Department of Polymer Science, Kyungpook National University,
Daegu, Korea
2Display & Nano Devices Laboratory, DGIST, Daegu, Korea
3Department of Chemical Engineering, Daegu University, Gyeongsan,
Gyeongbuk, Korea
A series of N-triarylamine-based derivatives such as 4,40-bis[N-(1-naphthyl)-
N-phenyl-amino]biphenyl (a-NPD), 1,3,5-tris[4-(3-methylphenyl(phenyl)amino)
phenyl]benzene (m-MTDAPB) and 1,3,5-tris-{4-[1-naphthyl(phenyl)amino]phenyl}-
benzene (1-TNAPB) were synthesized and characterized. Specially, 1-TNAPB was
synthesized with high yield in 1 hr. The hole injecting=transporting properties of
these materials were investigated by using double-layered organic light-emitting
diodes (OLEDs) with the configuration of ITO=N-triarylamine=Alq3=LiF=Al, in
which Alq3 was used as an emitting and electron transporting layer. A compara-
tive study on EL performance of OLEDs fabricated with N-triarylamines as the
hole injecting=transporting layer revealed that better results were observed in
OLED with m-MTDAPB and a-NPD, respectively. OLED with m-MTDAPB
(20 nm thickness) showed maximum luminescence of 15,970 cd=m2 at 10 V and
turn-on voltage of 4 V. On the other hand, a-NPD (50 nm thickness) exhibited
maximum luminescence of 17,650 cd=m2 at 17 V and turn-on voltage of 7 V.
Keywords: hole injection; hole transport; N-triarylamine; OLED
INTRODUCTION
Organic light-emitting diodes (OLEDs) are promising for the next
generation of flat panel display [1]. Typically, OLEDs are fabricated
This research has been supported by 2004 Daegu University Research Grant.
Address correspondence to Prof. Jae-Yong Ham, Department of Chemical Engineering,
Daegu University, Gyeongsan, Gyeongbuk 712-714, Korea. E-mail: hjy6494@daegu.ac.kr
314

Organic Light-Emitting Devices Consisting
315
using organic thin films derived either from vapor deposited small
molecules or spin cast polymers. It is well known that the charge
injection and transport can be facilitated by sandwiching the emissive
layer between a hole injection=transporting layer (HIL=HTL) above
the anode and an electron injection=transporting layer (EIL=ETL)
under the cathode. The use of multilayer structure with appropriate
HTL and ETL can not only lower the driving voltage but also balance
the charge transporting, hence improving the performance of the
devices with long lifetime and high quantum efficiency [2,3].
It has been known that there are three key requirements for hole
injecting=transporting materials for use in OLEDs. These materials
must have good hole injection efficiency from a certain anode, demon-
strate high hole mobility, and possess suitable band gap offsets
between an anode and an emitting layer within OLEDs [4,5]. Varying
the chemical structure of materials can influence hole injecting=
transporting capabilities in OLEDs. One of the most common classes
of these materials is the arylamines, exhibiting extremely high hole
mobility. The other one is carbazole that has interesting optical and
electronic properties. OLEDs based on carbazole derivatives have been
investigated, where carbazole derivatives are often used as hole trans-
porting layer, utilizing the high hole mobility [6,7].
In this paper, we report the synthesis of a series of N-triarylamine
derivatives such as 4,40-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl
(a-NPD), 1,3,5-tris[4-(3-methylphenyl(phenyl)amino)phenyl]benzene
(m-MTDAPB) and 1,3,5-tris-{4-[1-naphthyl(phenyl)amino]phenyl}
benzene (1-TNAPB). We chose N-triarylamine group as a building
block because of its well-known hole transporting property, feasibility
on modifying chemical structure, and wide application in OLEDs.
Physical, thermal, optical and electrochemical properties of these
compounds were measured, and their application as a hole injecting=
transporting layer in OLEDs was presented.
EXPERIMENTAL
Materials
N-phenyl-1-naphthylamine, 4,40-diiodobiphenyl, 1,10-phenanthroline,
3-methyldiphenylamine, and 1,3,5-tris(p-iodiphenyl)benzene were
purchased from Aldrich Chemical Co. Tri(8-hydroxyquinolate)alumi-
num (Alq3) and lithium fluoride (LiF) were obtained from Tokyo Kasei
Co. Aluminum was purchased from CERAC, USA. Indium-tin oxide
(ITO) coated glass with a sheet resistance of 50X=n was obtained from
Sin’an SNP Co.

316
H. Y. Cho et al.
Synthesis
a-NPD and m-MTDAPB were prepared by using Ullmann conden-
sation reaction, as shown in Scheme 1.
1,3,5-Tris[4-{1-naphthyl(phenyl)amino}phenyl]benzene
(1-TNAPB)
Into a three neck round bottomed flask was added a mixture of N-
phenyl-1-naphthylamine (19.2 g, 8.8 ꢀ 10ꢁ2 Mole), 1,3,5-tris(p-iodiphenyl)
SCHEME 1 Synthetic route to (a) a-NPD, (b) m-MTDAPB, and (c) 1-TNAPB.

Organic Light-Emitting Devices Consisting
317
benzene (10.0 g, 1.5 ꢀ 10ꢁ2 mole), 1,10-phenanthroline (2.0 g, 1.1 ꢀ
10ꢁ2 mole), CuCl (1.2 g, 1.2 ꢀ 10ꢁ2 mole) in 30 mL of p-xylene.
Temperature was increased to 100ꢂC, followed by addition of KOH
(16.02 g, 28.6 ꢀ 10ꢁ2 mole). The mixture was refluxed with stirring
for 1 h. After reaction, the mixture was cooled down to room tempera-
ture, and then 50 mL of toluene was poured into the mixture. The
mixture was filtered after stirring, followed by recrystallization from
chloroform=methanol. The product was purified by sublimation with
high vacuum (Yield ¼ 75.6%).
Characterization
Differential scanning calorimetry (DSC) was performed with a 2010
DSC (TA Instruments) using a heating scan rate of 10ꢂC=min in the
temperature range from 30ꢂC to 350ꢂC. UV-Visible absorption spectra
were taken with Shimadzu UV-2100. Photoluminescence (PL) spectra
excited by He-Cd laser at 325 nm were monitored by Optical
Multichannel Analyzer (Laser Photonics, OMA system). The ioniza-
tion potential (IP) was measured by a photoelectron spectroscopy
(Riken Keiki AC-2). Electroluminescence (EL) spectra were measured
by using Spectroscan PR 704 (Photoresearch Inc). Current-
luminescence-voltage (I-V-L) profiles of devices were obtained by using
dc power supply connected Model 8092 A Digital Multimeter and
luminance meter (Minolta LS-100).
Fabrication of Oleds
ITO coated glass was cut into 5.0 cm ꢀ 5.0 cm, and electrode area was
prepared by photo-etching technique. It was sequentially cleaned
in an ultrasonic bath of acetone, methanol, and mixture of isopropyl
alcohol and water solution. For the device with the configuration of
ITO=HTL=Alq3 (40 nm)=LiF (2 nm)=Al (140 nm), HTL material such
as a-NPD, m-MTDAPB, or 1-TNAPB was first vacuum deposited,
and then an emissive layer of Alq3 was deposited sequentially. Finally
LiF and Al were successively deposited under pressure < 10ꢁ6 torr.
The active area of the OLEDs was 2 mm ꢀ 2 mm.
RESULTS AND DISCUSSION
The N-triarylamine derivatives such as a-NPD, m-MTDAPB, and
1-TNAPB (Scheme 1) required for the present study were successfully
synthesized generally by using Cu-catalyzed Ullmann reaction of an
arylhalide and an arylamine. a-NPD was synthesized by the reaction

318
H. Y. Cho et al.
of 4,40-diiodobiphenyl and N-phenyl-1-naphthylamine, and m-MTDAPB
and 1-TNAPB were prepared by the reaction of 1,3,5-tris(p-iodiphe-
nyl)benzene and the corresponding arylamines, respectively. Synthe-
sized N-triarylamine derivatives were sublimed twice prior to use.
Table 1 summarized their thermal, optical and electrochemical
properties of these compounds. These compounds are amorphous in
nature with obvious glass transition temperature (Tg). It was observed
that 1-TNAPB had higher Tg than a-NPD and m-MTDAPB, showing
100.4ꢂC for a-NPD, 107.7ꢂC for m-MTDAPB, and 149.4ꢂC for 1-TNAPB,
respectively.
UV-Visible absorption spectra of these compounds diluted in
chloroform are presented in Figure 1. Similar UV-Visible absorption
maximum (kmax,UV) corresponding to maximum p-pꢃ transition was
measured at 341 nm for a-NPD and 343 nm for m-MTDAPB. However,
1-TNAPB showed kmax,UV at 331 nm. With the excitation at their
k
max,UV, PL emission peaks of these compounds are shown in
Figure 2. Maximum PL emission (kmax,PL) values were observed at
443 nm for a-NPD, 405 nm for m-MTDAPB, and 447 nm for 1-TNAPB,
respectively.
Band gap energies of these compounds were estimated from UV-
Visible absorption and PL emission spectra, and electrochemical
properties of these compounds were also listed in Table 1. Similar
band gap energies around 3.07 ꢄ 3.13 eV were measured in a-NPD
and 1-TNAPB, whereas m-MTDAPB exhibited higher band gap
energy of 3.32 eV. HOMO energy levels from ionization potential
values were measured by a RIKEN Keiki AC-2 instrument. These
compounds showed HOMO energy levels of ꢁ5.39 ꢄ ꢁ5.60 eV, which
might be matched to the work function (ꢄ ꢁ4.8 eV) of ITO anode when
they were employed as hole injecting=transporting layers in OLEDs.
Double-layered OLEDs with the configuration of ITO=N-triarylamine=
Alq3 (40 nm)=LiF (2 nm)=Al (140 nm) were fabricated, where synthe-
sized N-triarylamine derivatives were used as the hole injecting=
transporting layer, and Alq3 worked as the emitting layer. Figure 3
TABLE 1 Summary on Thermal, Photoluminescent and Electrochemical
Properties of HTL Materials
HOMO
(eV)
LUMO
(eV)
Band gap
(eV)
kmax, UV
(nm)
kmax, PL
(nm)
Tg
(ꢂC)
HTL
a-NPD
m-MTDAPB
1-TNAPB
ꢁ5.60
ꢁ5.39
ꢁ5.53
ꢁ2.47
ꢁ2.07
ꢁ2.46
3.13
3.32
3.07
341
343
331
443
405
447
100.4
107.7
149.4

Organic Light-Emitting Devices Consisting
319
FIGURE 1 UV-Visible absorption spectra of a-NPD, m-MTDAPB, and
1-TNAPB.
presents the OLED structure with energy levels of each layer. Table 2
summarizes data on the device performance. When voltages were
applied on the electrodes, devices emitted bright green light at
530 nm, indicating Alq3 layer was the exiton formation layer. CIE
FIGURE 2 Photoluminescence emission spectra of a-NPD, m-MTDAPB, and
1-TNAPB.

320
H. Y. Cho et al.
FIGURE 3 OLED structure with the configuration of ITO=N-triarylamine=
Alq3 (40 nm)=LiF (2 nm)=Al (140 nm).
coordinates of double-layered OLEDs were compared with those of
standard R, G, B, and W colors established by NTSC [8,9]. Based on
these color coordinates and Kelly’s map in CIE 1931 chromaticity dia-
gram [10], it appeared all double-layered OLEDs illuminated green
(CIE coordinates of x ¼ 0.312 ꢄ 0.345 and y ¼ 0.535 ꢄ 0.556) color.
Figure 4 presents luminescence vs. voltage curves of three
OLEDs with different thickness of the hole injecting=transporting
N-triarylamine layer. Their luminescence increased generally with
TABLE 2 Device Performance of the OLEDs with the configuration of ITO=
HTL=Alq3=LiF=Al
HTL
materials
Thickness of
HTL (nm)
Turn-on
voltage (V)
Max. luminescence
(cd=m2) at voltage
Color
coordinate (x,y)
a-NPD
30
50
20
30
50
10
30
3.0
7.0
4.0
3.0
4.0
4.5
5.0
6,710 (at 15.0 V)
17,650 (at 17.0 V)
15,970 (at 10.0 V)
13,210 (at 11.0 V)
2,880 (at 12.0 V)
248 (at 8.5 V)
0.332,0.556
0.331,0.553
0.337,0.551
0.345,0.547
0.328,0.535
0.312,0.550
0.330,0.556
m-MTDAPB
1-TNAPB
643 (at 10.5 V)

Organic Light-Emitting Devices Consisting
321
FIGURE 4 Luminescence vs. voltage curves of OLEDs with the following
configuration: (a) ITO=a-NPD=Alq3=LiF=Al, (b) ITO=m-MTDAPB=Alq3=LiF=Al,
Al, and (c) ITO=1-TNAPB=Alq3=LiF=Al.
increasing the applied voltage. It appeared in Figures 4 (a) and (b),
respectively, that OLED containing a-NPD with thickness of 50 nm
showed turn-on voltage at 7.0 V and maximum luminescence of
17,650 cd=m2 at 17.0 V, and OLED comprised of m-MTDAPB with
thickness of 20 nm presented turn-on voltage at 4.0 V and maximum
luminescence of 15,970 cd=m2 at 10.0 V. As can be seen in Figure 4
(c), on the other hand, relatively lower performance was observed in
OLED with 1-TNAPB (thickness of 30 nm), showing turn-on voltage
at 5.0 V and maximum luminescence of 643 cd=m2 at 10.5 V. In gen-
eral, the device based on m-MTDAPB as the hole injecting=transport-
ansporting layer showed better electroluminescent performance
among OLEDs fabricated in this study. This result revealed that
m-MTDAPB had suitable band gap offsets between ITO anode and
the emitting Alq3 layer within OLEDs, compared to other devices. In
addition, high energy barrier (1.03 eV) between m-MTDAPB and

322
H. Y. Cho et al.
Alq3 might also provide an additional contribution on preventing
electron transfer from Alq3 to m-MTDAPB.
CONCLUSIONS
Series of N-triarylamine-based derivatives such as a-NPD,
m-MTDAPB and 1-TNAPB were synthesized and characterized. New
synthetic procedure for 1-TNAPB was reported, providing improved
yield within relatively short reaction time. Thermal, optical and
electrochemical properties of these materials and their application to
OLEDs were studied. Specially, the hole injecting=transporting
properties of these materials were demonstrated well in the devices
in which Alq3 was used as an emitter and electron transporting layer.
It appeared that optimized band gap offsets between ITO anode and
the emitting Alq3 layer within OLEDs was one of major factors for
the use as the hole injecting=transporting layer.
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