High efficiency new hole injection materials for organic light emitting diodes based on dimeric phenothiazine and phenoxazine moiety derivatives

Article abstract of DOI:10.1166/jnn.2012.5886

New hole injection materials for organic light emitting diodes (OLEDs) based on phenothiazine and phenoxazine were synthesized, and the electro-optical properties of synthesized materials were examined by through UV-visible (UV-vis), photoluminescence (PL) spectrum and cyclic voltammetry (CV). 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX showed T_g of 127 and 200 °C, which are higher than that (110 °C) of 2-TNATA, a commercial hole injection layer (HIL) material. The highest occupied molecular orbital (HOMO) level of the synthesized materials of 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX were 4.97 and 4.91 eV, indicating values well-matched between HOMO (4.8 eV) of ITO and HOMO (5.4 eV) of NPB, hole transporting layer (HTL) material. As a result of using the synthesized materials in OLED device as HIL, 1-BPNA-t-BPBPOX of 2.43 lm/W was higher than 2-TNATA of 1.98 lm/W and 1-BPNA-t-BPBP of 1.39 lm/W in power efficiency. These results indicated that 1-BPNA-t-BPBPOX shows higher excellent power efficiency which is about 18% improved over 2-TNATA a commercial HIL material. Copyright

Full text of DOI:10.1166/jnn.2012.5886

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4356–4360, 2012
High Efficiency New Hole Injection Materials for
Organic Light Emitting Diodes Based on Dimeric
Phenothiazine and Phenoxazine Moiety Derivatives
Youngil Park1, Beomjin Kim1, Changjun Lee1, Jaehyun Lee1, Ji-Hoon Lee2, and Jongwook Park1ꢀ∗
1Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, Korea
2Department of Polymer Science and Engineering and Photovoltaic Technology Institute,
Chungju National University, Chungju, 380-702, Korea
New hole injection materials for organic light emitting diodes (OLEDs) based on phenothiazine and
phenoxazine were synthesized, and the electro-optical properties of synthesized materials were
examined by through UV-visible (UV-vis), photoluminescence (PL) spectrum and cyclic voltammetry
(CV). 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX showed Tg of 127 and 200 ꢀC, which are higher than
ꢀ
that (110 C) of 2-TNATA, a commercial hole injection layer (HIL) material. The highest occupied
molecular orbital (HOMO) level of the synthesized materials of 1-BPNA-t-BPBP and 1-BPNA-t-
BPBPOX were 4.97 and 4.91 eV, indicating values well-matched between HOMO (4.8 eV) of ITO
and HOMO (5.4 eV) of NPB, hole transporting layer (HTL) material. As a result of using the synthe-
sized materials in OLED device as HIL, 1-BPNA-t-BPBPOX of 2.43 lm/W was higher than 2-TNATA
of 1.98 lm/W and 1-BPNA-t-BPBP of 1.39 lm/W in power efficiency. These results indicated that
Delivered by Ingenta to: Nanyang Technological University
1-BPNA-t-BPBPOX shows higher excellent power efficiency which is about 18% improved over
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2-TNATA a commercial HIL material.
Copyright: American Scientific Publishers
Keywords: Phenothiazine, Phenoxazine, OLED, Hole Transporting Material.
1. INTRODUCTION
the better the efficiency of the material; (3) the higher Tg,
the better it is for the endurance of device degradation
by Joule heat occurring while driving device in stabil-
ity standpoint.3–5 Recently, star-bust amine 4,4ꢁ,4ꢁꢁ-Tris
(N-(naphthalen-2-yl)-N-phenyl-amino)triphenylamine (2-
TNATAꢂꢁ4,4ꢁ,4ꢁꢁ-Tris(N-3-methylphenyl-N-phenyl-amino)
triphenylamine (m-MTDATA) and copper phthalocyanine
(CuPc) compound acting as hole injection layer were
inserted respectively between the hole transport layer and
transparent anode to improve device durability. Unfor-
tunately, CuPc absorbed slightly blue and red light, and
thus could not be used in the fabrication of full color
display. And m-MTDATA showed the disadvantage of
low glass-transition temperatures (Tgꢂ. Crystallization and
melting of amorphous organic materials caused by Joule
heat or short-circuit current due to pinholes in thin films
are considered as common causes of device degradation.
In principle, all the organic layers forming the EL devices
should have Tg as high as possible. The individual layer
that the lowest Tg is likely to limit the thermal stability of
the OLED.3ꢁ6
Organic light emitting diodes (OLEDs) are optoelectronic
devices based on the electroluminescent properties of
ꢀ-conjugated organic materials, and are useful for light-
ing applications and next-generation flat panel displays.1ꢁ2
In particular, with OLED device using small molecule,
multi-layer device can be produced through in deposition
method, thus demonstrating higher efficiency than polymer
OLED. In general, layers comprising multi-layering sys-
tem in low molecule OLED device are broadly composed
of hole injection/transport, emitting and electron injec-
tion/transporting layers. Therefore, in the development of
OLED, development of materials with characteristics opti-
mized for each layer can be one of the most important
considerations.
Good hole injection layer (HIL) materials used in
OLEDs have the following common properties: (1) highest
occupied molecular orbital (HOMO) level is between ITO
and hole transfer layer (HTL); (2) the less the absorp-
tion of red, blue, green (RGB) emitting layer (EML) is,
Therefore, in this study, new hole injection materi-
als based on dimeric phenothiazine and phenoxazine
∗Author to whom correspondence should be addressed.
4356
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 5
1533-4880/2012/12/4356/005
doi:10.1166/jnn.2012.5886

Park et al.
High Efficiency New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine
derivatives were synthesized as alternative to solve the
above problem. Phenothiazine and Phenoxazine have been
mainly developed as p-type material that can convey
hole in organic photovoltaic (OPV) as well as in organic
optoelectronic thin-film transistor (OTFT) field of opto-
electronic field.7–8 Therefore, in order to strengthen the
transfer of electron to hole transferring phenothiazine and
phenoxazine fundamentally with hole transfer character-
istics and to induce amorphous molecule condition and
proper HOMO energy level, bulky aromatic amine group
was substituted, and two new HIL materials 10,10ꢁ-Bis-
(4-tert-butyl-phenyl)-10Hꢁ10ꢁH-[3ꢁ3ꢁꢃbiphenothiazinyl-7,
7ꢁ-biphenyl-4-yl-naphthalen-1-yl-amine(1-BPNA-t-BPBP)
2. EXPERIMENTAL DETAILS
2.1. General Method
1H-NMR spectra were recorded on Bruker, Advance 300
and 500, and Fast atom bombardment (FAB) mass spec-
tra were recorded by JEOL, JMS-AX505WA and HP5890
series II. The optical absorption spectra were obtained by
HP 8453 UV-Vis-NIR spectrometer. Photo- and electro-
luminescence (EL) spectroscopy was done with the use
of Perkin Elmer luminescence spectrometer LS50 (Xenon
flash tube). The melting temperatures (Tm), glass-transition
temperatures (Tg), crystallization temperatures (Tc), and
degradation temperatures (Td) of the compounds were
measured by carrying out differential scanning calorimetry
(DSC) under a nitrogen atmosphere using a DSC2910 (TA
Instruments) and thermogravimetric analysis (TGA) using
a SDP-TGA2960 (TA Instruments). The redox potentials
of the compounds were determined with cyclic voltamme-
try (CV) using a AUTOLAB/PG-STAT128N model system
with the scanning rate of 100 mV/s. We used synthe-
sized material coated ITO as working electrode, a saturated
Ag/AgNO3 as a reference electrode and acetonitrile (AN)
with 0.1 M tetrabutylammonium perchlorate (TBAP) as
an electrolyte. Ferrocene was used for potential calibration
and
10ꢁ10ꢁ-Bis-(4-tert-butyl-phenyl)-10Hꢁ10ꢁH-ꢄ3ꢁ3ꢁꢃ
biphenoxazinyl-7ꢁ7ꢁ-biphenyl-4-yl-naphthalen-1-yl-amine
(1-BPNA-t-BPBPOX) were synthesized as shown in
Scheme 1.
The synthesized compounds based on dimeric phenoth-
iazine and phenoxazine were compared on the electro-
optical and thermal properties. Furthermore, the hole
injection layer properties of phenothiazine and phenox-
azine derivatives synthesized through electroluminescence
(EL) device experiment were compared and analyzed with
the commercial HIL material 2-TNATA as reference.
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H
N
Pd (dba) , (t-Bu) P, t-BuNa
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2
3
3
NBS
Copyright: American ScientificNPublisnh-eBursLi, Boagent
+
N
N
X
CCl4
Toluene/Reflux
X
THF/-78 ºC
Br
O
O
X
X
Br
B
+
N
X
N
X
Pd(OAc)2,Tris-(o-toly)P
N
X
N
X
NBS
AceticAcid/CHCl3
X
N
Br
X
N
O
O
Br
K2CO3, DME/H2O
Br
B
N
S
N
S
N
N
N
X
Pd2(dba)3, (t-Bu)3P, t-BuNa
Toluene/Reflux
X
N
Br
Br
HN
+
1-BPNA-t-BPBP
X = Sulfuror Oxygen
N
O
O
N
N
N
1-BPNA-t-BPBPOX
Scheme 1. Synthetic route of new hole injection materials.
J. Nanosci. Nanotechnol. 12, 4356–4360, 2012
4357

High Efficiency New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine
Park et al.
and for reversibility criteria. OLED devices were fabri-
cated as the following structure: ITO/2-TNATA or Syn-
thesized material (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF
(1 nm)/Al (200 nm), where, 2-TNATA and synthesized
material as hole injection layer, N,Nꢁ-bis(naphthalen-1-
yl)-N,Nꢁ-bis(phenyl)benzidine [NPB] as hole transport
layer, and 8-hydroxyquinoline aluminum [Alq3] as elec-
tron transporting and emission layer, lithium fluoride [LiF]
as electron injection layer, ITO as anode and Al as cath-
ode. The organic layer was vacuum-deposited using ther-
mal evaporation at a vacuum base pressure of 10−6 torr and
the rate of deposition being 1 Å/s to give an emitting area
of 4 mm2, and the Al layer was continuously deposited
under the same vacuum condition. The current–voltage
(I–V) characteristics of the fabricated OLED devices were
obtained by Keithley 2400 electrometer. Light intensity
was obtained by Minolta CS-1000A.
(d, 2 H), 6.36 (d, 2 H), 6.50 (s, 2 H), 6.73 (d, 2 H),
6.74 (s, 2 H), 6.90–6.92 (d, 4 H), 7.19 (t, 2 H), 7.26–7.34
(m, 12 H), 7.39–7.46 (m, 8 H), 7.50–7.52 (d, 4 H), 7.62–
7.64 (d, 4 H), 7.75 (d, 2 H), 7.88 (d, 2 H), 7.95 (d, 2 H),
Fab+-Mass m/e: 1215.
3. RESULTS AND DISCUSSION
New 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX which
were based on the dimeric phenothiazine and phonoxazine
moieties were synthesized through C–C and C–N coupling
reaction using palladium-catalyst as shown in Scheme 1.
The synthesized material was purified on a silica column
and recrystallized to yield a pure solid material. The chem-
ical structure of the synthetic material was confirmed by
NMR, FT-IR, and FAB-MS analysis.
As a result of TGA and DSC, 1-BPNA-t-BPBP and
ꢀ
2.2. General Synthesis of New Materials
1-BPNA-t-BPBPOX showꢀed Tg of 127 and 200 C, which
are higher than that (110 C) of 2-TNATA, a commercial
HIL material. A high Tg indicates that the morphology of
the material is not easily changed by Joule heating gener-
ated from the operation of OLED devices, and is closely
correlated with long OLED device lifetime.5 It is expected
that the device using the synthesized materials would be
longer life-time under the Joule heating due to the high Tg
compared to 2-TNATA.6
These materials were synthesized by Suzuki aryl-aryl
coupling reaction using Pd catalyst. A typical synthetic
procedure was as follows: To 7,7ꢁ-Dibromo-10,10ꢁ-bis-(4-
tert-butyl-phenyl)-10H,10ꢁH-[3,3ꢁ]biphenothiazinyl (0.5 g,
0.6 mmol) and Biphenyl-4-yl-naphthalen-1-yl-amine
(0.41 g, 1.4 mmol) and in a 250 mL round-bottomed
flask under a nitrogen atmosphere were added Pd2(dba)3
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(0.04 g, 0.04 mmol), Sodium tert-butoxide (0.40 g,
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4.1 mmol) and toluene. The temperature was increased
Copyright: American Scientific Publishers
ꢀ
the UV-vis, PL spectrum and CV of the synthesized mate-
rials in solution and film state. As for the maximum
values of UV-vis absorption spectrum of the synthesized
materials in solution state, 1-BPNA-t-BPBP and 1-BPNA-
t-BPBPOX were respectively 363 and 370 nm, and the
values of PL spectrum were 480 and 548 nm, indicat-
ing that 1-BPNA-t-BPBPOX of phenoxazine derivative
shows red-shifted result compared with 1-BPNA-t-BPBP,
phenothiazine derivative.
to 110 C. The mixed substances were stirred continu-
ously at this temperature and the reaction monitored by
TLC. When the reaction was completed, the product was
extracted with water and toluene. The residual water was
removed by MgSO4, and then dried by solvent evapora-
tion. The resulting crude mixture was passed through a
short-column of silica with THF as the eluent and then
recrystallized from THF to obtain yellow solid.
2.3. Synthesis of 10,10ꢁ-Bis-(4-tert-butyl-
phenyl)-10H,10ꢁH-[3,3ꢁ]Biphenothiazinyl-
7,7ꢁ-Biphenyl-4-yl-Naphthalen-1-yl-Amine
(1-BPNA-t-BPBP)
On the other hand, UV-vis maximum values of 1-BPNA-
t-BPBP and 1-BPNA-t-BPBPOX were 322 and 370 nm in
film state. It was found that in case of UV-visible spectrum
on the absorption edge, 1-BPNA-t-BPBPOX has virtually
no change in solution and film state, but 1-BPNA-t-BPBP
is more blue-shifted in film state than in solution state.
That could be interpreted in a way that 1-BPNA-t-BPBP
in film state maintains relatively shorter length of effective
conjugation.
Also, absorption maximum value of 1-BPNA-t-BPBP
in film state was blue shifted compared to solution state
and there was no difference in solution and film cases of
1-BPNA-t-BPBPOX. Absorption maximum value can be
variable according to the solvent polarity, thus different
solvents such as benzene, methylene chloride (MC), and
dimethylformamide (DMF) to two compounds were used
to vary the polarity. However, UV-vis maximum absorp-
tion values were not changed at different polarity. As a
The yield was 43%. 1H-NMR (500 MHz, THF-d8ꢂ: ꢅ
(ppm) 1.37 (s, 18 H), 6.05–6.07 (d, 2 H), 6.17–6.19
(d, 2 H), 6.54 (d, 2 H), 6.85 (s, 2 H), 6.86 (d, 4 H), 6.91
(d, 2 H), 7.11 (s, 2 H), 7.20 (t, 2 H), 7.30–7.34 (m, 12 H),
7.39–7.45 (m, 8 H), 7.50–7.52 (d, 4 H), 7.63 (d, 4 H), 7.75
(d, 2 H), 7.88 (d, 2 H), 7.95 (d, 2 H), Fab+-MS m/e: 1247.
2.4. Synthesis of 10,10ꢁ-Bis-(4-tert-butyl-
phenyl)-10H,10ꢁH-[3,3ꢁ]Biphenoxazinyl-
7,7ꢁ-Biphenyl-4-yl-Naphthalen-1-yl-Amine
(1-BPNA-t-BPBPOX)
The yield was 90%. 1H-NMR (500 MHz, CDCl3ꢂ: ꢅ (ppm)
ꢅ (ppm) 1.36 (s, 18 H), 5.79–5.81 (d, 2 H), 5.89–5.91
4358
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Park et al.
High Efficiency New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine
Table II. EL performances of synthesized material: ITO/2-TNATA or
Synthesized material (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al
(200 nm) at 10 mA/cm2.
Turn-on
voltagea (V)
Operating
voltage (V)
L. E.b
(cd/A)
P. E.c
(lm/W)
Compounds
1-BPNA-
t-BPBP
1-BPNA-
t-BPBPOX
2-TNATA
4.3
3.2
3.0
10.4
6.4
4.17
4.49
4.10
1.39
2.43
1.98
7.2
Notes: aTurn-on voltage measured at 2 cd/m2; bluminance efficiency; cpower
efficiency.
and 4.91 eV. Thus, it is well-matched with the required
HOMO level zone.
Based on the results above, OLED device was fabricated
by using the synthesized material as HIL layer, and the EL
characteristics were examined with the device configured
by ITO/2-TNATA or synthesized material (60 nm)/NPB
(15 nm)/Alq3 (70 nm)/LiF (1 nm)/Al (200 nm). The EL
performances of the synthesized material are summarized
in Table II.
As shown in the I–V–L graph of Figure 2, turn-on volt-
age of 1-BPNA-t-BPBPOX and 2-TNATA, about 3 V,
were similar, but that of 1-BPNA-t-BPBP was increased
to 4.3 V.
In the case of power efficiency that considered operation
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voltage, 1-BPNA-t-BPBPOX, 2-TNATA and 1-BPNA-
Fig. 1. (a) UV-visible absorption and PL spectra in benzene solution
(b) in film state: UV-visible absorption of 1-BPNA-t-BPBP (triangle),
1-BPNA-t-BPBPOX (circle).
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t-BPBP were respectively 2.43, 1.98 and 1.39 lm/W.
Copyright: American Scientific Publishers
In particular, it was shown that the power efficiency of 1-
BPNA-t-BPBPOX was about 18% higher than 2-TNATA,
a commercial HIL material. Furthermore, even at high cur-
rent density, the improved property is well maintained.
Also in luminance efficiency, 1-BPNA-t-BPBPOX showed
higher efficiency of 4.49 cd/A than 2-TNATA of 4.17 cd/A
and 1-BPNA-t-BPBP of 4.10 cd/A at the same current
density of 10 mA/cm2.
result, it is not clearly understood that absorption maxi-
mum value of 1-BPNA-t-BPBP was blue-shifted at film
state compared to solution state.
In addition, judging from the fact that the synthesized
materials both showed virtually no absorption at 450 nm or
in areas above the visible region in film state, the synthe-
sized materials may be effectively used as an HIL material
because of its transparency.
In order to explain the result, hole-only devices (ITO/2-
TNATA or synthesized materials (60 nm)/NPB (15 nm)/LiF
As shown in Table I, HOMO and LUMO were calcu-
lated through CV and optical band gap. In the case of
HOMO level, 1-BPNA-t-BPBP and 1-BPNA-t-BPBPOX
which are located between HOMO (4.8 eV) of ITO and
HOMO (5.4 eV) of NPB as the HTL material were 4.97
Table I. Optical properties of synthesized materials.
Solutiona
Film on glass
UVmax PLmax UVmax PLmax
Band
Compounds
(nm) (nm) (nm) (nm) HOMO LUMO gap
1-BPNA-
t-BPBP
1-BPNA-
t-BPBPOX
363
370
480
548
322
370
465
486
4.97
4.91
1.91
2.03
3.06
2.88
Fig. 2. I–V–L data of EL device: ITO/2-TNATA (square) or 1-BPNA-t-
BPBP (triangle) or 1-BPNA-t-BPBPOX (circle) (60 nm)/NPB (15 nm)/
Alq3 (70 nm)/LiF (1 nm)/Al (200 nm).
Note: aDichloromethane (1×10−5 M).
J. Nanosci. Nanotechnol. 12, 4356–4360, 2012
4359

High Efficiency New Hole Injection Materials for OLEDs Based on Dimeric Phenothiazine
Park et al.
(5.4 eV) of NPB, HTL material well-matched. In addition,
judging from the fact that the synthesized materials both
barely showed any absorption in the range of over 450 nm,
the synthesized materials could be effectively used as an
HIL material. As a result of using the synthesized materials
in OLED device as HIL, the power efficiency of 1-BPNA-t-
BPBPOX (2.43 lm/W) was higher than those of 2-TNATA
(1.98 lm/W) and 1-BPNA-t-BPBP (1.39 lm/W) respec-
tively. These results indicated that 1-BPNA-t-BPBPOX
shows improved power efficiency about 18% compared
with 2-TNATA a commercial HIL material.
Acknowledgment: This research was supported by a
grant (Catholic University) from the Fundamental R&D
Program for Core Technology of Materials funded by
the Ministry of Knowledge Economy (MKE), Republic
of Korea. This research was supported by Basic Sci-
ence Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (No. 20110027647).
This research was partially supported by Leading For-
eign Research Institute Recruitment Program through
the National Research Foundation of Korea funded by
the Ministry of Education, Science and Technology
(2011-0030065).
Fig. 3. I–V characteristics of hole only device: ITO/2-TNATA or syn-
thesized materials (60 nm)/NPB (15 nm)/LiF(1 nm)/Al (200 nm);
1-BPNA-t-BPBP (triangle), 1-BPNA-t-BPBPOX (circle), 2-TNATA
(square).
(1 nm)/Al (200 nm)) were fabricated. High LUMO level of
NPB to limit the electron carrier in hole-only device.9 Thus,
the device current is considered to be driven mainly by hole
carriers, which are injected from ITO to organic layer. As
shown in Figure 3, 1-BPNA-t-BPBPOX exhibited much
low turn-on voltages and left-shifted J–V curves compared
to 2-TNATA and 1-BPNA-t-BPBP. These J–V characteris-
tics of hole-only devices were well matched with the J–V–L
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References and Notes
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results of normal OLED devices (1-BPNA-t-BPBPOX > 2-
Copyright: American Scientific Publishers
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it means that 1-BPNA-t-BPBPOX and 2-TNATA are supe-
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4. CONCLUSION
In this study, new hole injection materials for OLED based
on phenothiazine and phenoxazine were synthesized, and
the electro-optical properties of synthesized materials were
examined through UV-vis, PL spectrum and CV. As for
the HOMO level of the synthesized materials, 1-BPNA-
t-BPBP and 1-BPNA-t-BPBPOX were 4.97 and 4.91 eV,
indicating that HOMO (4.8 eV) of ITO and HOMO
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Received: 12 May 2010. Accepted: 29 July 2011.
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J. Nanosci. Nanotechnol. 12, 4356–4360, 2012