Electroluminescence characteristics of a new green-emitting phenothiazine derivative with biphenyl benzimidazole substituent

Article abstract of DOI:10.1080/15421401003608378

We report on the synthesis and electroluminescence characteristics of a new green-emitting material, 3,7-bis(1-(biphenyl-4-yl)benzimidazol-2-yl)10- methylphenothiazine (PhBBMP) containing the electron-donating phenothiazine core and the electron-accepting biphenyl benzimidazole substituent. An optimized OLED device with the structure of [ITO/NPB (40nm)/PhBBMP (30nm)/BCP (10nm)/TPBI (30nm)/LiF (1nm)/Al (100nm)] exhibited a luminous efficiency of 4.5cd/A and an external quantum efficiency of 0.9% at 100mA/cm², and the maximum luminance of 12,000cd/m² with the CIE 1931 chromaticity coordinates of (0.21, 0.54). A more balanced charge carrier injection and the charge confinement in the emitting layer cooperatively contributed to improve the performance of the device prepared with the PhBBMP emitting layer.

Full text of DOI:10.1080/15421401003608378

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Electroluminescence Characteristics of
a New Green-Emitting Phenothiazine
Derivative with Biphenyl Benzimidazole
Substituent
Sun-Hee Lee a , Man Su Kim a , Yong-Bum Cha b , Kwang-Hyun Ahn b
& Young Chul Kim a
a Department of Chemical Engineering and MRCID , Kyung Hee
University , Yongin-si, Gyeonggi-do, Korea
b Department of Applied Chemistry and MRCID , Kyung Hee
University , Yongin-si, Gyeonggi-do, Korea
Published online: 19 Apr 2010.
To cite this article: Sun-Hee Lee , Man Su Kim , Yong-Bum Cha , Kwang-Hyun Ahn & Young Chul Kim
(2010) Electroluminescence Characteristics of a New Green-Emitting Phenothiazine Derivative with
Biphenyl Benzimidazole Substituent, Molecular Crystals and Liquid Crystals, 520:1, 36/[312]-43/[319],
DOI: 10.1080/15421401003608378
To link to this article: http://dx.doi.org/10.1080/15421401003608378
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Mol. Cryst. Liq. Cryst., Vol. 520: pp. 36=[312]–43=[319], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003608378
Electroluminescence Characteristics of a New
Green-Emitting Phenothiazine Derivative with
Biphenyl Benzimidazole Substituent
SUN-HEE LEE,1 MAN SU KIM,1 YONG-BUM CHA,2
KWANG-HYUN AHN,2 AND YOUNG CHUL KIM1
1Department of Chemical Engineering and MRCID, Kyung Hee
University, Yongin-si, Gyeonggi-do, Korea
2Department of Applied Chemistry and MRCID, Kyung Hee University,
Yongin-si, Gyeonggi-do, Korea
We report on the synthesis and electroluminescence characteristics of a new
green-emitting material, 3,7-bis(1-(biphenyl-4-yl)benzimidazol-2-yl)10-methylphe-
nothiazine (PhBBMP) containing the electron-donating phenothiazine core and
the electron-accepting biphenyl benzimidazole substituent. An optimized OLED
device with the structure of [ITO=NPB (40 nm)=PhBBMP (30 nm)=BCP
(10 nm)=TPBI (30 nm)=LiF (1 nm)=Al (100 nm)] exhibited a luminous efficiency
of 4.5 cd=A and an external quantum efficiency of 0.9% at 100 mA=cm2, and the
maximum luminance of 12,000 cd=m2 with the CIE 1931 chromaticity coordinates
of (0.21, 0.54). A more balanced charge carrier injection and the charge confine-
ment in the emitting layer cooperatively contributed to improve the performance
of the device prepared with the PhBBMP emitting layer.
Keywords Biphenyl benzimidazole; green OLED; phenothiazine
Introduction
Organic light-emitting diode (OLED) [1] is attracting increasing interest due to its
potential applicability to large area displays and two-dimensional light sources,
and small full-color display products are already on the market. Accordingly,
tremendous efforts have been made over the recent two decades to improve the emis-
sion efficiency of OLEDs by developing either high efficiency materials or efficient
device structures. In order to fabricate full-color displays or white light sources, it
is also of essential importance to have blue-, green-, and red-emitting materials with
high color purity and long-term stability as well as high luminous quantum yield.
p-conjugated donor-acceptor (D-A) molecules have been successfully employed
in OLEDs as high efficiency emitting materials [2–6] because energy transfer and
charge carrier recombination can easily occur within the molecular structure to give
Address correspondence to Prof. Young Chul Kim, Department of Chemical
Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Korea. E-mail:
kimyc@khu.ac.kr
36=[312]

Green-Emitting Phenothiazine Derivative
37=[313]
a high emission efficiency. In this work, we synthesized a new green-emitting D-A
molecule with a phenothiazine core and two biphenylbenzimidazole substituents.
Phenothiazine is a well known electron donor with electron-rich sulfur and nitrogen
heteroatoms [7] and its unique non-planar geometry can make the molecule have an
amorphous nature with decreased crystallinity, resulting in an enhanced device per-
formance [8,9]. Benzimidazole group was introduced to the material as an electron
acceptor due to its high electron affinity [10,11]. We also prepared the OLED devices
with three different multilayer structures using the new material as the emitting layer,
and characterized their performance to optimize the device structure.
Experimental
Synthesis of PhBBMP
3,7-Bis(1-(biphenyl-4-yl)benzimidazol-2-yl)10-methylphenothiazine (PhBBMP)
4
was prepared according to Scheme 1. 10H-Phenothiazine 1 was alkylated to give
10-methylphenothiazine 2 in an excellent yield (99%). Vilsmeier-Haak reaction of
10-methylphenothiazine 2 by phosphorus(III) oxychloride and DMF in 1,2-dichlor-
oethane afforded 10-methylphenothiazine-3,7-dicarbaldehyde 3 (53%). 3,7-Bis(1-
(biphenyl-4-yl)benzimidazol-2-yl)10-methylphenothiazine 4 was obtained by the
oxidative coupling of N-(biphenyl-4-yl)benzene-1,2-diamine with 3 under the
treatment of sodium metabisulfite. The compound was purified by column
chromatography using hexane and ethyl acetate (78%).
10-Methylphenothiazine (2), Mp: 101ꢀC. 1H NMR (300 MHz, CDCl3):
7.15–7.20(m, 4H), 6.87–6.98(m, 4H), 3.41(s, 3H).
d
13C NMR (75 MHz, CDCl3): d 145.74, 127.37, 127.08, 123.29, 122.38, 114.01,
35.22.
10-Methylphenothiazine-3,7-dicarbaldehyde (3), Mp: 153ꢀC. 1H NMR (300 MHz,
CDCl3): d 9.83(s, 2H), 7.69(dd, J ¼ 8.40, J ¼ 1.80 Hz, 2H), 7.16(d, J ¼ 1.80 Hz,
2H), 6.92(d, J ¼ 8.40 Hz, 2H), 3.49(s, 3H).
13C NMR (75 MHz, CDCl3): d 189.85, 149.22, 131.88, 130.48, 127.78, 123.27,
114.47, 36.20.
Scheme 1. Reagent and conditions: (a) NaH, CH3I, THF, 0ꢀC; (b) POCl3, DMF,
ClCH2CH2Cl, 90ꢀC; (c) N-(biphenyl-4-yl)benzene-1,2-diamine, Na2S2O5, DMF, 150ꢀC.

38=[314]
S.-H. Lee et al.
1
3,7-Bis(1-(biphenyl-4-yl)benzimidazol-2-yl)10-methylphenothiazine (4), Mp: 300ꢀC. H
NMR (300 MHz, CDCl3): d 7.80(d, 2H), 7.56–7.66(dd, 8H), 7.18–7.41(m, 20H), 6.55(d,
2H), 3.20(s, 3H).
13C NMR (75 MHz, CDCl3): d 151.16, 145.88, 141.55, 139.56, 137.02, 135.73,
128.98, 128.73, 128.49, 127.94, 127.90, 127.62, 127.10, 123.95, 123.93, 123.35,
123.16, 123.08, 119.40, 113.72, 110.40, 35.45.
Fabrication of OLEDs
ITO-coated glass substrates with a surface resistance of 10 X=sq were patterned by
photholithography and cleaned with trichloroethylene, acetone, deionized water,
and isopropyl alcohol in an ultrasonic bath. The cleaned ITO surface was oxygen
plasma-treated to improve adhesion with organic materials. OLED devices with three
different structures were prepared by using PhBBMP as the emitting layer, as depicted
in Figure 1. In the devices, N,N0-bis(1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-
diamine (NPB) was used as a hole transport layer (HTL), 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP) as a hole blocking layer (HBL), 1,3,5-tris
(N-phenylbenzimidazol-2-yl)benzene (TPBI) and tris-(8-hydroxyquinoline) aluminum
(Alq3) as an electron transport layer (ETL), and the bilayer of lithium fluoride (LiF)
and aluminum (Al) as a cathode. All organic and metal layers were deposited by
thermal evaporation under ꢁ10ꢂ6 torr on the ITO substrate and the deposition rates
˚
were 1.0, 0.1, 3 ꢁ 4 A=s for the organic, LiF, and Al layers, respectively. Active
emitting area of the devices was defined to be 3 ꢃ 3 mm2.
Measurements
UV-vis absorption and photoluminescence (PL) spectra were obtained using an
Agilent 8453 UV-visible spectrophotometer and a PTI QuantaMasterTM spectro-
fluorometer, respectively. The current-voltage-luminance (J-V-L) characteristics of
the devices were measured with a JBS IVL-300 EL characterization system and a
Keithley 2400 sourcemeter. The highest occupied molecular orbital (HOMO) energy
level of PhBBMP was measured by cyclic voltammetry (CV) and the lowest
unoccupied molecular orbital (LUMO) energy level was obtained by adding the
optical absorption band gap to the HOMO level.
Figure 1. Structures and energy level diagrams of the OLED devices; (a) device 1, (b) device 2,
(c) device 3.

Green-Emitting Phenothiazine Derivative
39=[315]
Results and Discussion
Thermal and Optical Properties
The thermal properties of PhBBMP were examined by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) as shown in Figure 2. The
TGA thermogram of PhBBMP measured at a rate of 5ꢀC=min showed that the
material has a fairly high thermal decomposition temperature (Td) of 394ꢀC. In
the DSC thermograms, both melting and recrystallization behavior were not
observed when heated up to 300ꢀC and then cooled to room temperature at a scan
rate of 5ꢀC=min. On the second heating, however, the thermogram showed a rela-
tively high glass transition temperature (Tg) of 162ꢀC, which is far above the normal
operating temperature of OLED. It is considered that the non-planar structure of
PhBBMP with biphenyl benzimidazole group prevents close packing of the mole-
cules. Accordingly, the PhBBMP layer can remain homogeneous and amorphous
under the joule heating during the device operation.
Figure 3 presents the normalized UV-vis absorption and PL spectra of PhBBMP
in dilute toluene solution (10ꢂ5 M). When the PhBBMP solution was excited at the
two absorption peak wavelengths of 294 and 382 nm, the solution exhibited almost
the same PL spectrum centered at 502 nm but the PL intensity was much stronger
when excited at 294 nm. This implies that there is a large Stokes shift between
absorption and emission. It has been reported that the two biphenyl rings in the ben-
zimidazole substituent have different geometric structures between the ground and
the excited states because the twisted phenyl rings transform into planar structure
when excited, leading to a large Stokes shift [12–14]. A large Stokes shift sometimes
enhances the emission efficiency by preventing the self-absorption of the emitted
light.
Electroluminescence Properties of OLED Devices
The current density-voltage-luminance characteristics of the OLED devices are
shown in Figure 4. As shown in Figure 4(a), device 1 with the TPBI (30 nm) and
Figure 2. TGA and DSC thermograms of PhBBMP measured at a scan rate of 5ꢀC=min in
nitrogen.

40=[316]
S.-H. Lee et al.
Figure 3. UV-vis absorption and photoluminescence spectra of PhBBMP in dilute toluene
solution.
Figure 4. (a) Luminance and current density vs. voltage, and (b) luminous efficiency and
& &
external quantum efficiency vs. current density characteristics of device 1 ( , ), device 2
~
(4, ), and device 3 ( ,.).
ꢀ

Green-Emitting Phenothiazine Derivative
41=[317]
Figure 5. Electroluminescence spectrum of device 3.
Alq3 (10 nm) layers as ETLs required a high driving voltage of 13 V to give a lumi-
nance of 100 cd=m2, however, the driving voltage was reduced to 9 V when the TPBI
(40 nm) single layer was employed as ETL (device 2). Although device 2 has a higher
energy barrier for electron injection (1.2 eV) from cathode to ETL than that of device
1 (1.0 eV), the higher electron mobility in the TPBI layer than in the Alq3 layer could
reduced the driving voltage [15–19]. On the other hand, device 2 gave a lower lumi-
nous efficiency than device 1 as shown in Figure 4(b), which can result from a poorly
balanced charge carrier injection.
To obtain an OLED device with a higher luminous efficiency as well as a lower
driving voltage, we introduced the BCP (10 nm) layer as a hole blocking layer (HBL)
between EML (PhBBMP) and ETL (TPBI) (device 3). Device 3 demonstrated a
luminous efficiency of 4.5 cd=A at a current density of 100 mA=cm2 and a driving
voltage of 10 V. The luminous efficiency was increased by 1.7 cd=A compared with
device 2 and the driving voltage was reduced by 3 V compared with device 1. As illu-
strated in Figure 1(b), device 2 has a higher energy barrier for electron injection than
that for hole injection therefore, without an HBL, a considerable amount of holes
can transport across EML to cause current loss. It is possible to confine the injected
holes in EML by introducing an HBL such as BCP that has a deep HOMO level of
6.7 eV, as shown in Figure 1(c), and hole confinement in EML can increase the
chance of charge carrier recombination in EML to generate more excitons. Accord-
ingly, device 3 with the optimized device structure exhibited the best performance not
Table 1. Electroluminescence characteristics of the OLED devices
Turn-on
voltage
[V]
gL [cd=A]
gP [lm=W]
gext [%]
CIE
coordinates
[x, y]
at
at
at
Device
Lmax [cd=m2] 100 mA=cm2 100 mA=cm2 100 mA=cm2
1
2
3
13
9
10
8500
8800
12000
4.2
2.8
4.5
0.6
0.6
0.9
0.9
0.6
1.0
0.22, 0.55
0.22, 0.56
0.21, 0.54

42=[318]
S.-H. Lee et al.
only by the efficient electron injection through TPBI but also by a more effective
recombination of injected holes and electrons in EML.
Also, device 3 showed a green emission spectrum peaked at 502 nm (Fig. 6) with
the CIE 1931 chromaticity coordinates of (0.20, 0.55) at a current density of
100 mA=cm2. EL characteristics of devices 1–3 were summarized in Table 1.
Conclusions
We synthesized a new green-emitting material, 3,7-bis(1-(biphenyl-4-yl)-benzimida-
zol-2-yl)-10-methylphenothiazine (PhBBMP), which contains the electron-donating
phenothiazine core and the electron-accepting biphenyl benzimidazole substituent.
We also prepared the OLED devices with three different multilayer structures using
the new material as the emitting layer, and characterized their performance to opti-
mize the device structure. The optimized device with the configuration of [ITO=NPB
(40 nm)=PhBBMP (30 nm)=BCP (10 nm)=TPBI (30 nm)=LiF (1 nm)=Al (100 nm)]
exhibited a luminous efficiency of 4.5 cd=A and an external quantum efficiency of
0.9% at 100 mA=cm2, and the maximum luminance of 12000 cd=m2 with the CIE
1931 chromaticity coordinates of (0.20, 0.55). The TPBI layer acted as an efficient
ETL for the device and the BCP layer employed as an HBL successfully confined
holes and excitons in EML to enhance the emission efficiency.
Acknowledgement
This research was supported by Basic Science Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science
and Technology (Grant No. 2009-0090177), and the ‘‘GRRC’’ Project of Gyeonggi
Provincial Government, ROK.
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