Highly efficient new hole injection materials for organic light emitting diodes base on phenothiazine derivatives

Article abstract of DOI:10.1166/jnn.2014.8456

New hole injection materials for OLED based on phenothiazine were synthesized, and the electro-optical properties of synthesized materials were examined by through UV-Vis. and photolumines-cence spectroscopy as well as cyclic voltammetry 1-NDD-t-BPBP and 2-NDD-t-BPBP showed T_g of 180 and 177 °C. These are higher than that (110 °C) of 2-TNATA, commercial HIL material. As for the HOMO level of the synthesized materials, 1-NDD-t-BPBP and 2-NDD-t-BPBP were 4.99 and 5.02 eV, indicating well-matched values between HOMO (4.8 eV) of ITO and HOMO (5.4 eV) of NPB, HTL material. 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. The synthesized materials were used as the HIL in OLED device, yielding luminance efficiencies of 4.51 cd/A (1-NDD-t-BPBP) and 4.44 cd/A (2-NDD-t-BPBP). These results indicated that 1-NDD-t-BPBP shows more excellent luminance efficiency which is about 11% improved over 2-TNATA a commercial HIL material. Copyright

Full text of DOI:10.1166/jnn.2014.8456

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 6404–6408, 2014
Copyright © 2014 American Scientific Publishers
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Printed in the United States of America
Highly Efficient New Hole Injection Materials for Organic
Light Emitting Diodes Base on Phenothiazine Derivatives
Beomjin Kim, Jaehyun Lee, Youngil Park, Changjun Lee, and Jong Wook Park∗
Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, Korea
New hole injection materials for OLED based on phenothiazine were synthesized, and the electro-
optical properties of synthesized materials were examined by through UV-Vis. and photolumines-
cence spectroscopy as well as cyclic voltammetry. 1-NDD-t-BPBP and 2-NDD-t-BPBP showed Tg
of 180 and 177 ꢀC. These are higher than that (110 ꢀC) of 2-TNATA, commercial HIL material.
As for the HOMO level of the synthesized materials, 1-NDD-t-BPBP and 2-NDD-t-BPBP were 4.99
and 5.02 eV, indicating well-matched values between HOMO (4.8 eV) of ITO and HOMO (5.4 eV)
of NPB, HTL material. 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. The synthesized materials were used as the HIL in OLED device, yield-
ing luminance efficiencies of 4.51 cd/A (1-NDD-t-BPBP) and 4.44 cd/A (2-NDD-t-BPBP). These
results indicated that 1-NDD-t-BPBP shows more excellent luminance efficiency which is about
11% improved over 2-TNATA a commercial HIL material.
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Keywords: Phenothiazine, OLED, Hole Transporting Material, HIL Material.
Copyright: American Scientific Publishers
1. INTRODUCTION
(2-TNATA),
4,4ꢁ,4ꢁꢁ-Tris(N-3-methylphenyl-N-phenyl-
amino)triphenylamine (m-MTDATA) or copper phthalo-
cyanine (CuPc) compound acting the role of the hole
injection layer, was inserted between the hole transport
layer and the transparent anode to improve device dura-
bility. Unfortunately, CuPc absorbs slightly blue and red
light, it can’t use fabrication of full color display. And
m-MTDATA has disadvantage of low glass-transition tem-
peratures (Tgꢂ. Crystallization or melting of amorphous
organic materials cause by Joule heat or short-circuit cur-
rent due to pinholes in thin films are considered 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.3ꢁ7
Therefore, in this study, new hole injection materials
based on dimeric phenothiazine derivatives were synthe-
sized as alternative to solve the above problem. Phenoth-
iazine have been mainly developed as p-type material or
ambipolar materials that can transport hole and/or elec-
tron in organic photovoltaic (OPV) device and organic
thin-film transistor (OTFT) field of optoelectronic field.8–11
Therefore, in order to strengthen the transfer of electron
to basic moiety of phenothiazine fundamentally with hole
transfer characteristics and to induce amorphous molecule
condition and proper HOMO energy level, bulky aromatic
Organic light emitting diodes (OLEDs) are optoelectronic
devices based on the photoluminescent properties of ꢀ-
conjugated organic materials. It is useful for lighting
applications and next-generation flat panel displays.1ꢁ2
In particular, as for OLED device using small molecule,
multi-layer device can be produced through deposition
method, so it can implement high 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, development of mate-
rial with characteristics optimized for each layer can be
one of the most important objectives of the development.
Good hole injection layer (HIL) materials used in
OLEDs have the following common properties: (1) high-
est occupied molecular orbital (HOMO) level should be
between ITO and hole transfer layer (HTL); (2) the less the
absorption of red, blue, green (RGB) emission coming out
of emitting layer (EML) is, the better; (3) the higher Tg,
the better it is for the endurance of device degradation
by Joule heat occurring while driving device in device
stability standpoint.3–6 Recently, star-bust amine 4,4ꢁ,4ꢁꢁ-
Tris (N-(naphthalen-2-yl)-N-phenyl-amino) triphenylamine
∗Author to whom correspondence should be addressed.
6404
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 8
1533-4880/2014/14/6404/005
doi:10.1166/jnn.2014.8456

Kim et al.
Highly Efficient New Hole Injection Materials for OLEDs Base on Phenothiazine Derivatives
N
N
N
S
N
S
N
S
N
N
N
N
S
N
N
N
1-NDD-t-BPBP
2-NDD-t-BPBP
Scheme 1. Chemical structure of new phenothiazine materials.
amine group was substituted. Two new HIL materials
1-NDD-t-BPBP and 2-NDD-t-BPBP were synthesized as
shown in Scheme 1.
energy level was derived by assuming that the energy
level of ferrocene/ferrocenium (Fc/Fc+) is 4.8 eV. Finally,
their LUMO energy level was derived from HOMO energy
level and the band gap. OLED devices were fabricated
with the following structure: ITO/2-TNATA or synthe-
sized material (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF
(1 nm)/Al (200 nm), where 2-TNATA and the synthe-
sized material formed the HILs, N,Nꢁ-bis(naphthalen-1-
yl)-N,Nꢁ-bis(phenyl)benzidine (NPB) formed the HTL,
8-hydroxyquinoline aluminum (Alq3) formed the EML
and electron transport layer (ETL), lithium fluoride (LiF)
formed the electron injection layer (EIL), ITO formed
The synthesized compounds based on dimeric phenoth-
iazine were compared on the electro-optical and thermal
properties through UV-visible (UV-vis), photolumines-
cence (PL), cyclic voltammetry (CV), thermal gravimet-
ric analysis (TGA) and differential scanning calorimetry
(DSC) analysis. Furthermore, the hole injection layer prop-
erties of phenothiazine derivatives synthesized through
electroluminescence (EL) device experiment were com-
pared and analyzed with the commercial HIL material
2-TNATA as reference.
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the anode, and Al formed the cathode. The organic layer
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was vacuum-deposited by thermal evaporation at a vac-
Copyright: American Scientific Publishers
uum base pressure of 10−6 Torr and a rate of deposition
of 1/s to give an emitting area of 4 mm2; the Al layer was
continuously deposited under the same vacuum conditions.
The current–voltage (I–V ) characteristics of the fabricated
OLED devices were obtained using a Keithley 2400 elec-
trometer. Light intensity and electroluminescence (EL) was
measured using a Minolta CS-1000A. The operational sta-
bility of the devices was measured with encapsulation in
glove box.
2. EXPERIMENTAL DETAILS
2.1. General Method
1H-NMR spectra were recorded using Bruker Advance 300
and 500 instruments, fast atom bombardment (FAB) mass
spectra were recorded using a JEOL, JMS-AX505WA,
HP5890 series II. The optical absorption spectra were
obtained using an HP 8453 UV-VIS-NIR spectrometer.
A Perkin-Elmer luminescence spectrometer LS50 (Xenon
flash tube) was used for photo-luminescence (PL) spec-
troscopy. The melting temperatures (Tmꢂ, glass-transition
temperatures (Tgꢂ, crystallization temperatures (Tcꢂ, and
degradation temperatures (Tdꢂ of the compounds were
measured by DSC under a nitrogen atmosphere using a
DSC2910 (TA Instruments), and TGA was performed
using an SDP-TGA2960 (TA Instruments). The redox
potentials of the compounds were determined by CV
using an AUTOLAB/PG-STAT128N model system with
a scan rate of 100 mV/s. The working electrode was
formed by depositing the synthesized materials onto ITO
substrates, Ag/AgNO3 in acetonitrile (AN) as reference
electrode, Pt wire as counter electrode, and 0.1 M tetra-
butylammonium perchlorate (TBAP) in acetonitrile as
the electrolyte. All potentials are quoted versus the fer-
rocene/ferrocenium (Fc/Fc+) couple as the internal stan-
dard. The band gap was estimated from the tailing edges
of UV-vis. spectra. From the oxidation potential, HOMO
2.2. General Syntheses of New Materials
These synthesized materials by Suzuki aryl–aryl coupling
reaction using Pd catalyst. A typical synthetic proce-
dure was as follows: To 7,7ꢁ-Dibromo-10,10ꢁ-bis-(4-tert-
butyl-phenyl)-10H,10ꢁH-[3,3ꢁ]biphenothiazinyl (0.32 g,
0.4 mmol) and N1-(naphthalen-1-yl)-N4,N4-diphenyl-
benzene-1,4-diamine or N1-(naphthalen-2-yl)-N4,N4-di-
phenylbenzene-1,4-diamine (0.34 g, 0.9 mmol) and in a
250 mL round-bottomed flask under a nitrogen atmosphere
were added Pd2(dba)3 (0.02 g, 0.02 mmol), Sodium tert-
butoxide (0.25 g, 2.6 mmol), Tri-t-butylphosphine (1 ml)
and xylene. The temperature was increased to 140 ꢀC.
Stirring was continued at this temperature and the reac-
tion was monitored by TLC. When the reaction was com-
plete, extraction of the product was performed with water
and toluene. The organic extract was dried with MgSO4
added, and then filtered the solvent removed in vacuo. The
J. Nanosci. Nanotechnol. 14, 6404–6408, 2014
6405

Highly Efficient New Hole Injection Materials for OLEDs Base on Phenothiazine Derivatives
Kim et al.
resulting crude mixture was passed through a short-column
of silica with THF as the eluent and then recrystallized
from THF to obtain as a yellow solid.
6.85(s, 2 H), 6.97(m, 14 H), 7.05(d, 8 H), 7.14(s, 2 H),
7.18(m, 10 H), 7.25(t, 2 H), 7.30(d, 2 H), 7.35(d, 6 H),
7.58(d, 2 H), 7.68(m, 8 H), HRMS: m/z 1428.5890 (M+,
calcd 1428.5886). Anal. Calcd for C100H80N6S2: C,
84.00; H, 5.64; N, 5.88; S, 4.49 Found: C, 84.13; H, 5.69;
N, 5.85; S; 4.50.
2.3. Synthesis of 10,10ꢁ-bis(4-tert-butylphenyl)-
N7,N7ꢁ-di(naphthalen-1-yl)-N7,N7ꢁ-di
(phenyl-p-diphenylamine-yl)-10H,
10ꢁH-3,3ꢁ-biphenoxazine-7,7ꢁ-Diamine
(1-NDD-t-BPBP)
3. RESULTS AND DISCUSSION
New 1-NDD-t-BPBP and 2-NDD-t-BPBP which are based
on the dimeric phenothiazine moieties were synthesized by
through C–C and C–N coupling reaction using palladium-
catalyst as shown in Scheme 2. The synthesized material
was purified on a silica column and recrystallized to yield a
pure solid material. The chemical structure of the synthetic
material was confirmed by NMR, elementary analysis and
FAB-MS analysis.
The yield was 30%. 1H-NMR (300 MHz, THF-d8):
ꢃ (ppm) 1.39(s, 18 H), 6.02(d, 2 H), 6.17(d, 2 H),
6.48(d, 2 H), 6.75(s, 2 H), 6.82(d, 4 H), 6.87(m, 8 H),
6.89(d, 2 H), 6.94(d, 8 H), 7.00(s, 2 H), 7.08(t, 8 H),
7.14(d, 6 H), 7.36(t, 2 H), 7.42(m, 4 H), 7.61(d, 4 H),
7.74(d, 2 H), 7.86(d, 2 H), 7.95(d, 2 H), HRMS:
m/z 1428.5889 (M+, calcd 1428.5886). Anal. Calcd for
C100H80N6S2: C, 84.00; H, 5.64; N, 5.88; S, 4.49 Found:
C, 83.06; H, 5.64; N, 5.82; S; 4.43.
As a result of TGA and DSC, 1-NDD-t-BPBP and
ꢀ
2-NDD-t-BPBP showed Tg of 180 and 177 C. These are
ꢀ
2.4. Synthesis of 10,10ꢁ-bis(4-tert-butylphenyl)-
N7,N7ꢁ-di(naphthalen-2-yl)-N7,N7ꢁ-di
(phenyl-p-diphenylamine-yl)-10H,10ꢁH-
3,3ꢁ-biphenoxazine-7,7ꢁ-Diamine
(2-NDD-t-BPBP)
The yield was 41%. 1H-NMR (300 MHz, CDCl3):ꢃ (ppm)
1.39(s, 18 H), 6.14(d, 2 H), 6.18(d, 2 H), 6.62(d, 2 H),
higher than that (110 C) of 2-TNATA, commercial HIL
material. It is expected that the device using the synthe-
sized materials would be longer life-time under the Joule
heating due to the high Tg compared to 2-TNATA.7
Figure 1 and Table I show the summarized results of the
UV-vis., PL spectrum and CV of the synthesized materials
in solution and film state.
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O
B
H
N
Pd (dba) , (t-Bu) P, t-BuNa
O
O
NBS
n-BuLi
2
3
3
+
CCl
4
Toluene/Reflux
N
S
N
S
THF/-78 ºC
N
S
S
Br
O
Br
B
O
+
N
N
S
N
S
Pd(OAc) , Tris-(o-toly)P
N
S
NBS
2
S
N
S
N
K CO , DME/H O
Br
S
O
AceticAcid/CHCl
2
3
2
3
B
O
Br
Br
N
N
S
S
N
N
N
HN
N
N
N
S
1-NDD-t-BPBP
+
S
or
Br
Pd (dba) , (t-Bu) P, t-BuNa
2
3
3
Br
Toluene/Reflux
N
N
N
S
HN
S
N
N
N
N
N
2-NDD-t-BPBP
Scheme 2. Synthetic route of new hole injection materials.
6406
J. Nanosci. Nanotechnol. 14, 6404–6408, 2014

Kim et al.
Highly Efficient New Hole Injection Materials for OLEDs Base on Phenothiazine Derivatives
Figure 2. Cyclic voltammetry 0.1 M TBAP in acetonitrile; 1-NDD-t-
BPBP (square), 2-NDD-t-BPBP (circle).
reduction waves as shown in Figure 2. The area corre-
sponding to oxidation in the CV curve was larger than
that for the reduction step, suggesting that these com-
pounds favored the cationic state, consistent with a p-type
material.
Based on these results, the HOMO and LUMO energy
levels were calculated from the CV and optical band gap
measurements (see Table I).12ꢁ13
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Figure 1.
From the oxidation potential, the HOMO energy level
(a) UV-Visible absorption andIPPL: 2sp1e8ct.r8a7i.n4T6H.1F7s3oluOtinon: Fri, 16 Oct 2015 07:40:51
(b) in film state: UV-Visible absorption of 1-NDD-t-BPBP (square), 2-
NDD-t-BPBP (circle).
was derived by assuming that the energy level of fer-
rocene/ferrocenium (Fc/Fc+) is 4.8 eV. The LUMO energy
level was determined from the HOMO level and the optical
band gap. The optical band gap was derived by analyz-
ing the absorption edge with a plot of (hꢅ) versus (ꢆhꢅꢂ2,
where ꢆ, h, and ꢅ are absorbance, Plank’s constant, and
the frequency of light.
The HOMO levels for the synthesized phenothiazine
materials were 4.99 eV and 5.01 eV, intermediate between
the HOMO of ITO (4.8 eV) and that of NPB (5.4 eV), the
HTL material. Thus, the HOMO levels of the new mate-
rials were well-matched with the required HOMO level
of HIL.
Copyright: American Scientific Publishers
As for the maximum values of UV-vis absorption spec-
trum of the synthesized materials in solution state, 1-NDD-
t-BPBP and 2-NDD-t-BPBP were respectively 303 and
316 nm, and the values of PL spectrum were 474 and
479 nm.
On the other side, UV-Vis. maximum values of 1-NDD-
t-BPBP and 2-NDD-t-BPBP are 306 and 316 nm in film
state. The UV-vis. spectra and the absorption edges of
these materials displayed virtually no changes on going
from the solution to the film state. Additionally, because
the synthesized materials in the film state showed almost
no absorption at wavelengths longer than 450 nm, the syn-
thesized materials are appropriate HIL materials due to
their transparency properties.
Based on the results above, OLED device was fabricated
by using the synthesized material as HIL layer, and the EL
characteristics were examined with device configuration
of ITO/synthesized material or 2-TNATA (60 nm)/NPB
CV was performed over the range −1ꢄ50 to 1.50 V. The
four compounds displayed normal reversible oxidation and
Table II. EL performances of synthesized material: ITO/Synthesized
materials or 2-TNATA (60 nm)/NPB (15 nm)/Alq3 (70 nm)/LiF
(1 nm)/Al (200 nm) at 10 mA/cm2.
Table I. Optical properties of synthesized materials.
Solutiona
Film on glass
Operating
voltage (V)
L. E.a
(cd/A)
P. E.b
(lm/W)
UVmax PLmax UVmax PLmax
HO LU Band
MO MO gap
Compounds
C.I.E (x, y)
Compound
(nm)
(nm)
(nm)
(nm)
1-NDD-t-BPBP
2-NDD-t-BPBP
2-TNATA
8.18
8.55
6.79
4.60
4.54
4.05
1.95
1.84
2.07
(0.35, 0.54)
(0.35, 0.54)
(0.34, 0.54)
1-NDD-t-BPBP
2-NDD-t-BPBP
303
316
474
479
306
316
513
465
4.99 1.82 3.17
5.01 1.82 3.19
Note: aTetrahydrofuran (THF) solution (1×10−5 M).
Notes: aLuminance efficiency; bPower efficiency.
J. Nanosci. Nanotechnol. 14, 6404–6408, 2014
6407

Highly Efficient New Hole Injection Materials for OLEDs Base on Phenothiazine Derivatives
Kim et al.
properties of synthesized materials were examined by
through UV-Vis., PL spectrum and CV.
As for the HOMO level of the synthesized materi-
als, 1-NDD-t-BPBP and 2-NDD-t-BPBP were 4.99 and
5.01 eV, indicating well-matched values between HOMO
(4.8 eV) of ITO and HOMO (5.4 eV) of NPB, HTL
material.
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 effec-
tively used as an HIL material.
As a result of using the synthesized materials in OLED
device as HIL, 1-NDD-t-BPBP of 4.60 cd/A was higher
than 2-TNATA of about 4 cd/A. These results indicated
that 1-NDD-t-BPBP shows more excellent luminance effi-
ciency which is about 12% improved over 2-TNATA a
commercial HIL material.
Moreover, it is expected that the device using the syn-
thesized materials could be longer life-time under the Joule
heating because of the high Tg compared to 2-TNATA.
Acknowledgment: This work was supported by
the National Research Foundation of Korea (NRF)
grant funded by the Korea government (MEST)
(No. 2012001846).
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Figure 3. (a) I–V –L data, (b) Luminance and power efficiency of EL
IP: 218.87.46.173 On: Fri, 16 Oct 2015 07:40:51
References and Notes
device: ITO/2-TNATA (triangle) or 1-NDD-t-BPBP (square) or 2-NDD-
Copyright: American Scientific Publishers
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4. CONCLUSION
In this study, new hole injection materials for OLED based
on phenothiazine were synthesized. The electro-optical
Received: 19 May 2011. Accepted: 1 August 2012.
6408
J. Nanosci. Nanotechnol. 14, 6404–6408, 2014