New hole transporting materials based on hexaarylbenzene and aromatic amine moiety for organic light-emitting diodes

Article abstract of DOI:10.1166/jnn.2014.8291

Three different new hole transport compounds, namely DPAP-TB, 1-PNAP-TB and 2-PNAP-TB, were synthesized by Sonogashira coupling and Diels-Alder reaction. Synthesized materials exhibited high T_gin the range of 118 to 133 °C. These values are higher than that of NPB, which is commonly used as a hole transporting material. OLED devices were fabricated by the synthesized compounds using the solution process as a hole transporting layer. Device structure was ITO/PEDOT:PSS (40 nm)/synthesized compounds or NPB (20 nm)/Alq3 (70 nm)/LiF/Al. Luminance efficiencies and external quantum efficiencies of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB devices were 3.98, 4.62, 4.22 cd/A, and 1.35, 1.56, 1.43% at 20 mA/cm², respectively. In luminance efficiency and external quantum efficiency, 1-PNAP-TB especially had superior property to NPB. Copyright

Full text of DOI:10.1166/jnn.2014.8291

Article
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Nanoscience and Nanotechnology
Vol. 14, 6382–6385, 2014
Copyright © 2014 American Scientific Publishers
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Printed in the United States of America
New Hole Transporting Materials Based on
Hexaarylbenzene and Aromatic Amine
Moiety for Organic Light-Emitting Diodes
Seungho Kim1, Sang-Ho Lee2, Hwangyu Shin1,
Kwang-Yol Kay2ꢀ∗, and Jongwook Park1ꢀ∗
1Department of Chemistry, The Catholic University of Korea,
Bucheon, 420-743, Korea
2Department of Molecular Science and Technology, Ajou University,
Suwon, 443-749, Korea
Three different new hole transport compounds, namely DPAP-TB, 1-PNAP-TB and 2-PNAP-TB,
were synthesized by Sonogashira coupling and Diels-Alder reaction. Synthesized materials exhib-
ited high Tg in the range of 118 to 133 ꢀC. These values are higher than that of NPB, which is
commonly used as a hole transporting material. OLED devices were fabricated by the synthe-
sized compounds using the solution process as a hole transporting layer. Device structure was
Delivered by Publishing Technology to: Florida State University, College of Medicine
ITO/PEDOT:PSS (40 nm)/synthesized compounds or NPB (20 nm)/Alq3 (70 nm)/LiF/Al. Luminance
IP: 109.241.99.10 On: Sat, 31 Oct 2015 11:13:41
efficiencies and external quantum efficiencies of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB devices
Copyright: American Scientific Publishers
were 3.98, 4.62, 4.22 cd/A, and 1.35, 1.56, 1.43% at 20 mA/cm2, respectively. In luminance effi-
ciency and external quantum efficiency, 1-PNAP-TB especially had superior property to NPB.
Keywords: Hole Transport Material, Organic Light-Emitting Diode, Solution Process.
1. INTRODUCTION
phenyl}-3,4,5,6-tetraphenylbenzene-2-yl]5-(N-phenylnaphtha-
len-1-amino)pyridine(1-PNAP-TB) and 2-[1{4-(N-phenyl-
naphthalen-1-amino)phenyl-3,4-,5,6-tetraphenylbenzene-2-yl]
-5-(N-phenylnaphthalen-2-amino) pyridine (2-PNAP-TB),
were synthesized by Sonogashira coupling and Diels-Alder
reaction. Synthesized materials include aromatic amine
groups as electron donor moiety.
In particular, we analyzed dependence of the EL device
characteristics on the position at which the naphthylphenyl
amine or diphenylamine group is substituted. Then we
compared EL properties with those of NPB, a commer-
cialized HTL compound.
Electrochemical and optical properties of these materi-
als were characterized by cyclic voltammetry (CV), UV-
visible spectroscopies (UV) and photo luminescence (PL)
spectroscopies. Moreover, OLED devices were fabricated
by the synthesized compounds using the solution process
as a hole transporting layer. The device was fabricated
with hybrid configuration structure which includes solution
process of hole injection layer (HIL) and HTL as well as
vacuum deposition process of EML and cathode.
Organic light-emitting diodes (OLEDs) have been the sub-
ject of intensive investigation in the past few years because
of their potential application as emissive elements for flat
panel displays.1ꢀ2 Generally, multi-layered devices com-
prised of charge transport and emitting layers exhibit
higher electroluminescence (EL) efficiency than single-
layer devices. This is because multi-layered devices bal-
ance hole and electron carriers, and then the exciton is
formed in the middle of emitting layer.3 Among multi-
layers of OLED, charge transporting materials have played
important roles of balancing hole and electron carriers, and
hole transporting layer (HTL) and electron transporting
layer (ETL) are used.4
Three different new hole transport compounds (Sch-
eme 1), namely 2-[1-{4-(N,N-diphenylamino) phenyl}-3,
4,5,6-tetraphenylbenzene-2-yl ] 5-N,N-diphenylaminopyri-
dine (DPAP-TB), 2-[1-{4-(N-phenylnaphthalene-1-amino)
∗Authors to whom correspondence should be addressed.
6382
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 8
1533-4880/2014/14/6382/004
doi:10.1166/jnn.2014.8291

Kim et al.
New Hole Transporting Materials Based on Hexaarylbenzene and Aromatic Amine Moiety for OLEDs
2. EXPERIMENTAL DETAILS
2.1. Synthesis
NPB or synthesized materials as a hole transporting layer,
LiF as an electron injection layer, and Alq3 as an electron
transporting and emitting layer. The device was fabricated
with hybrid configuration structure which includes solution
process of HIL and HTL as well as vacuum deposition pro-
cess of EML and cathode.5 A water-dispersed PEDOT:PSS
mixture (Baytron PVP CH8000, H. C. Starck GmbH) was
spin-coated on top of indium tin oxide (ITO) in air to make
films (40 nm). The spin-coated film was heated on a hot
plate at 110 C for 5 minutes in air and 200 C for 5 min-
utes in nitrogen atmosphere. Synthesized material and NPB
films as a HTL were prepared by spin coating of a 0.6
wt% toluene solution. This HTL was spin-coated to obtain
a thickness of 20 nm. The HTL films were heated on a hot
plate under 100 ꢀC and 30 min in nitrogen atmosphere. The
emitting layer was vacuum-deposited using thermal evap-
oration at a vacuum pressure of 10−6 torr and the rate of
deposition 1 Å/s to give an emitting area of 4 mm2, and the
LiF, Al layer was continuously deposited under the same
vacuum condition.
a
b
trimethylsilylacetylene
KF, MeOH
Br
I
Br
Br
Si
Pd(PPh3)2Cl2, CuI,
rt, 24 h, 83%
1
2
(Et)3N, 48 ºC, 2 h, 95%
3
c
d
2,5-dibromopyridine
tetraphenylcyclopentadienone
Br
diphenyl ether, reflux, 48 h, 71%.
Br
N
Pd(PPh)3Cl2, CuI, (Et)3N.
48 ºC, 24 h, 84%
4
ꢀ
ꢀ
e
diphenylamine
Pd(OAc)2, tri(t-butyl)phosphine
NaOt-Bu, p-xylene,
N
N
N
Br
reflux, 12 h, 24%.
N
5
Br
N-phenyl-2-naphthylamine
Pd(OAc)2, tri(t-butyl)phosphine,
NaOt-Bu, p-xylene,
N-phenyl-1-naphthylamine,
Pd(OAc)2, tri(t-butyl)
phosphine, NaOt-Bu,
DPAP-TB
f
reflux, 12 h, 24%
g
p-xylene,\reflux,12 h, 18%
N
N
N
N
3. RESULTS AND DISCUSSION
Scheme 1 shows the chemical structures of the synthe-
sized hole transporting materials. UV-visible absorption
(UV-Vis.) of the synthesized materials are described and
summarized in Figure 1 and Table I. As shown in Table I,
N
N
1-PNAP-TB
2-PNAP-TB
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Scheme 1. Synthetic routes of new compounds.
DPAP-TB, 1-PNAP-TB, and 2-PNAP-TB in film showed
IP: 109.241.99.10 On: Sat, 31 Oct 2015 11:13:41
maximum absorption wavelengths of 309 nm, 316 nm and
Copyright: American Scientific Publishers
322 nm, respectively.
2.2. Measurements
UV-Vis. maximum values of 1-PNAP-TB and 2-PNAP-
TB had red shift compared to DPAP-TB.
The optical absorption spectra were obtained by HP 8453
UV-VIS-NIR spectrometer. Perkin Elmer luminescence
spectrometer LS50 (Xenon flash tube) was used for photo-
luminescence (PL) spectroscopy. Glass transition temper-
ature (Tgꢁ and thermal degradation temperature (Tdꢁ of
the compounds were measured by differential scanning
calorimetry (DSC) under nitrogen atmosphere using a
DSC2910 (TA instrument) and by thermo-gravimetric anal-
ysis (TGA) using SDP-TGA2960 (TA instrument). The
redox potential of the synthesized materials was measured
at the scanning rate of 50 mV/s by WBCS 3000 system
with cyclic voltammeter (CV). Acetonitrile (AN) contain-
ing 0.1 M tetrabutylammonium perchlorate was used as an
electrolyte when measuring. Indium tin oxide (ITO) was
used as a working electrode and saturated Ag/AgNO3 as a
reference electrode. Ferrocene was a reference compound
for potential calibration. The current–voltage–luminance
(I–V –L) characteristics of the fabricated EL devices were
obtained by Keithley 2400 electrometer, and light intensity
was obtained by Minolta CS-1000A.
This is because 1-PNAP-TB and 2-PNAP-TB have
longer conjugation length of naphthalene moiety than
DPAP-TB.6 All three compounds did not show any
absorption in the visible light area, and thus are suit-
able as hole transporting layer (HTL) or hole injection
layer (HIL). Based on UV-Vis. and cyclic voltammetry
(CV) results, highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) levels
2.3. Preparation of OLED Device
EL devices of ITO/PEDOT:PSS (40 nm)/synthesized mate-
rials or NPB (20 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm)
were fabricated using PEDOT:PSS as a hole injection layer,
Figure 1. UV-Visible absorption spectrum in film states.
J. Nanosci. Nanotechnol. 14, 6382–6385, 2014
6383

New Hole Transporting Materials Based on Hexaarylbenzene and Aromatic Amine Moiety for OLEDs
Kim et al.
Table I. EL performances of multilayered devices with the synthesized compounds at 20 mA/cm2 and the optical, electrical and thermal properties
of the synthesized materials.
Hole transporting
materials
ELamax (nm) C.E.b (cd/A) E.Q.Ec (%) C.I.Ed (x, y) UVemax (nm) HOMO (eV) LUMO (eV) Egf (eV) Tgg (ꢀC) Tdh (ꢀC)
DPAP-TB
1-PNAP-TB
2-PNAP-TB
NPB
533
533
533
533
3.98
4.62
4.22
4.04
1.35
1.56
1.43
1.37
0.356, 0.543
0.356, 0.543
0.356, 0.543
0.356, 0.543
309
316
322
348
5.27
5.31
5.25
5.34
1.97
2.18
2.16
2.40
3.30
3.13
3.09
2.94
118
133
125
104
394
396
400
378
Notes: aELmax: Electroluminescent spectrum maximum peak; bC.E.: Current efficiency, cE.Q.E: External quantum efficiency; dCIE: Commission internationale de l’Eclairage;
eFilm on glass; f Eg (band gap) calculated from the optical absorption edge; gꢀh: Measured by DSC and TGA at a heating rate of 10 ꢀC/min.
were calculated as shown in Table I.7 LUMO values of
DPAP-TB, 1-PNAP-TB and 2-PNAP-TB were −1.97 eV,
−2.18 eV and −2.16 eV. All three compounds showed
higher LUMO levels than NPB, which is commercialized
as HTL. Through a higher LUMO level, electron block-
ing to hole transporting layer is available when electron
is injected to emitting layer. Based on the electron block-
ing effect, luminescence in HTL layer can be prevented
and EML layer would be only allowed to emit. HOMO
values of DPAP-TB, 1-PNAP-TB and 2-PNAP-TB were
−5.27 eV, −5.31 eV and −5.25 eV. TGA and DSC were
measured to identify thermal properties of the synthesized
materials. Glass transition temperature (Tgꢁ and initial ther-
mal degradation temperature (Tdꢁ of the materials are sum-
marized in Table I. Synthesized materials exhibited high Tg
in the range of 118 to 133 ꢀC. These values are higher than
that of NPB, which is commonly used as a hole transport-
ing material.8
Non-doped OLED devices were fabricated using the syn-
thesized materials or NPB (20 nm) as a hole transport
layer. Device configuration is ITO/PEDOT:PSS (40 nm)/
synthesized materials or NPB (20 nm)/Alq3 (70 nm)/LiF
(1 nm)/Al (200 nm). EL performances of the devices are
described and summarized in Figure 2 and Table I.
Luminance efficiency and external quantum efficiency
of the devices were 3.98, 4.62, 4.22 cd/A and 1.35, 1.56,
1.43%, respectively, when DPAP-TB, 1-PNAP-TB and
2-PNAP-TB were used as a HTL in the devices. These are
comparable efficiencies compared to 4.04 cd/A and 1.37%
of NPB, a commercialized HTL material. The reason for
increased efficiencies of two compounds, 1-PNAP-TB and
Delivered by Publishing Technology to: Florida State University, College of Medicine
2-PNAP-TB seems to be caused by the increased LUMO
IP: 109.241.99.10 On: Sat, 31 Oct 2015 11:13:41
Copyright: American ScleievneltsificwPhiucbhlisphreovrside electron blocking effect between
HTL and EML.
Among three synthesized new compounds, 1-PNAP-TB
showed the highest efficiency. It can’t be explained by
clear reason, but it may be due to the improved charge
balance in this device configuration. Also, 1-PNAP-TB is
appropriate as a candidate of commercialized HTL mate-
rial in solution-processed OLED because it has high Tg
ꢀ
value of 133 C. High Tg value can support device relia-
bility and long life-time.
4. CONCLUSION
New hole transporting materials, DPAP-TB, 1-PNAP-
TB, and 2-PNAP-TB including hexaaryl benzene moiety,
were synthesized by Sonogashira coupling and Diels-Alder
reaction. UV-Vis. spectra in film state of the three com-
pounds, DPAP-TB, 1-PNAP-TB and 2-PNAP-TB, showed
maximum values of 309 nm, 316 nm and 322 nm. Tg of the
ꢀ
synthesized materials were more than 10 C higher than
that of NPB. EL devices of DPAP-TB, 1-PNAP-TB and
2-PNAP-TB were fabricated by solution process. Lumi-
nance and external quantum efficiencies of DPAP-TB,
1-PNAP-TB and 2-PNAP-TB were 3.98, 4.62, 4.22 cd/A,
and 1.35, 1.56, 1.43%, respectively. In the luminance effi-
ciency result, 1-PNAP-TB especially had superior prop-
erty to NPB. Luminance efficiency and external quantum
efficiency were increased over 1.2 times in 1-PNAP-TB
Figure 2. Device performances (a) Luminance efficiency of the devices
(b) External quantum efficiency of the devices.
6384
J. Nanosci. Nanotechnol. 14, 6382–6385, 2014

Kim et al.
New Hole Transporting Materials Based on Hexaarylbenzene and Aromatic Amine Moiety for OLEDs
(4.62 cd/A, 1.56%) more than commercialized material,
NPB (4.04 cd/A, 1.37%).
J. H. Kim, S. O. Jung, and S. K. Kwon, Macromol. Res. 17, 91
(2009).
3. (a) D. K. Olosev and S. R. F. Orrest, J. Appl. Phys. 90, 3242 (2001);
(b) Y. I. Park, J. H. Son, J. S. Kang, S. K. Kim, J. H. Lee, and J. W.
Park, Chem. Commun. 18, 2143 (2008).
4. T. Zhang, J. Wang, T. Li, M. Liu, W. Xie, S. Liu, D. Liu, C. L. Wu,
and C. T. Chen, J. Phys. Chem. C 114, 4186 (2010).
5. E. Tutis, D. Berner, and L. Zuppiroli, J. Appl. Phys. 93, 4594 (2003).
6. T. Kojima, J. Nishida, S. Tokito, H. Tada, and Y. Yamashita, Chem.
Commun. 14, 1430 (2007).
Acknowledgment: This work was supported by
the National Research Foundation of Korea (NRF)
grant funded by the Korea government (MEST)
(No. 2012001846).
References and Notes
1. C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett. 51, 913 (1987).
2. (a) S. K. Kim, Y. I. P ark, I. N. Kang, J. H. Lee, and J. W. Park,
J. Mater. Chem. 17, 4670 (2007); (b) J. S. Kim, J. Heo, P. Kang,
7. S. Janietz, D. D. C. Bradley, M. Grell, C. Giebeler, M. Inbasekaran,
and E. P. Woo, Appl. Phys. Lett. 73, 2453 (1998).
8. J. Li, D. Liu, Y. Li, C. S. Lee, H. L. Kwong, and S. T. Lee, Chem.
Mater. 17, 1208 (2005).
Received: 4 March 2013. Accepted: 4 April 2013.
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J. Nanosci. Nanotechnol. 14, 6382–6385, 2014
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