1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes
We report on the photophysical properties of 1,4-bis(2,2-diphenylethenyl)benzene (PEB) in a solution and a solid state. A poor blue photoluminescence efficiency of PEB in a solution dramatically increases in the deposited film. We explain such properties in terms of molecular dynamics and degrees o...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2007
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| Cite this: | 1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes / L. Fenenko, Guoliang Mao, Akihiro Orita, Junzo Otera, P. Smertenko, G. Svechnikov, Jun-ichi Nishide, Hiroyuki Sasabe, Chihaya Adachi // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 1. — С. 77-82. — Бібліогр.: 24 назв. — англ. |
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Fenenko, L. Guoliang Mao Akihiro Orita Junzo Otera Smertenko, P. Svechnikov, G. Jun-ichi Nishide Hiroyuki Sasabe Chihaya Adachi 2017-05-26T17:28:17Z 2017-05-26T17:28:17Z 2007 1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes / L. Fenenko, Guoliang Mao, Akihiro Orita, Junzo Otera, P. Smertenko, G. Svechnikov, Jun-ichi Nishide, Hiroyuki Sasabe, Chihaya Adachi // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 1. — С. 77-82. — Бібліогр.: 24 назв. — англ. 1560-8034 PACS 73.50.-h, 78.60.Fi https://nasplib.isofts.kiev.ua/handle/123456789/117779 We report on the photophysical properties of 1,4-bis(2,2-diphenylethenyl)benzene (PEB) in a solution and a solid state. A poor blue photoluminescence efficiency of PEB in a solution dramatically increases in the deposited film. We explain such properties in terms of molecular dynamics and degrees of intramolecular freedom in various molecular environments. PEB as an electron-transport and emitting layer in organic light-emitting diodes (OLEDs) shows bright blue-green electroluminescence (EL) with the peak wavelength at λmax ~ 495 nm. The maximum external EL quantum efficiency of ηEL = 2.5 % and maximum luminance of 41600 cd/m2 in the optimized device were obtained, indicating that PEB possesses superior electron-transport ability. We are very grateful to the 21st Century COE program of “Molecular Informatics” for financial support of this study. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics 1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes Article published earlier |
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1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes Fenenko, L. Guoliang Mao Akihiro Orita Junzo Otera Smertenko, P. Svechnikov, G. Jun-ichi Nishide Hiroyuki Sasabe Chihaya Adachi |
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1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes |
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1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes |
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1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes |
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1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes |
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1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes |
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Fenenko, L. Guoliang Mao Akihiro Orita Junzo Otera Smertenko, P. Svechnikov, G. Jun-ichi Nishide Hiroyuki Sasabe Chihaya Adachi |
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Fenenko, L. Guoliang Mao Akihiro Orita Junzo Otera Smertenko, P. Svechnikov, G. Jun-ichi Nishide Hiroyuki Sasabe Chihaya Adachi |
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2007 |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Article |
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We report on the photophysical properties of 1,4-bis(2,2-diphenylethenyl)benzene
(PEB) in a solution and a solid state. A poor blue photoluminescence efficiency of PEB in a solution dramatically increases in the deposited film. We explain such properties in terms of molecular dynamics and degrees of intramolecular freedom in various molecular environments. PEB as an electron-transport and emitting layer in organic light-emitting diodes (OLEDs) shows bright blue-green electroluminescence (EL) with the peak wavelength at λmax ~ 495 nm. The maximum external EL quantum efficiency of ηEL = 2.5 % and maximum luminance of 41600 cd/m2 in the optimized device were obtained, indicating that PEB possesses superior electron-transport ability.
|
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1560-8034 |
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https://nasplib.isofts.kiev.ua/handle/123456789/117779 |
| citation_txt |
1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material for organic light emitting diodes / L. Fenenko, Guoliang Mao, Akihiro Orita, Junzo Otera, P. Smertenko, G. Svechnikov, Jun-ichi Nishide, Hiroyuki Sasabe, Chihaya Adachi // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2007. — Т. 10, № 1. — С. 77-82. — Бібліогр.: 24 назв. — англ. |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 77-82.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
77
PACS 73.50.-h, 78.60.Fi
1,4-bis(2,2-diphenylethenyl)benzene as an efficient emitting material
for organic light emitting diodes
Larysa Fenenko1,2, Guoliang Mao3, Akihiro Orita3, Junzo Otera3, Petro Smertenko2, Georgi Svechnikov2,
Jun-ichi Nishide4, Hiroyuki Sasabe4, Chihaya Adachi1
1Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
2 V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Department of Optoelectronics, 41, prospect
Nauky, 03128 Kyiv, Ukraine
3Department of Applied Chemistry, Okayama University of Science, 1-1 Ridai, Okayama 700-0005, Japan
4Chitose Institute of Science and Technology (CIST), 758-65 Bibi, Chitose, Hokkaido 066-8655, Japan
*Corresponding author: e-mail: fenenko@ yahoo.com
Abstract. We report on the photophysical properties of 1,4-bis(2,2-diphenyl-
ethenyl)benzene (PEB) in a solution and a solid state. A poor blue photoluminescence
efficiency of PEB in a solution dramatically increases in the deposited film. We explain
such properties in terms of molecular dynamics and degrees of intramolecular freedom in
various molecular environments. PEB as an electron-transport and emitting layer in
organic light-emitting diodes (OLEDs) shows bright blue-green electroluminescence
(EL) with the peak wavelength at λmax ~ 495 nm. The maximum external EL quantum
efficiency of ηEL = 2.5 % and maximum luminance of 41600 cd/m2 in the optimized
device were obtained, indicating that PEB possesses superior electron-transport ability.
Keywords: electroluminescence, electrical transport characteristics, OLED, 1,4-bis(2,2-
diphenylethenyl)benzene (PEB).
Manuscript received 22.12.06; accepted for publication 26.03.07; published online 01.06.07.
1. Introduction
Organic light emitting diodes (OLEDs) have become
one of the most attractive research areas in recent years.
Wide possibilities of various molecular structures
through organic synthesis, simple and low temperature
processability and low cost, attributed to organic
materials, show their high potential and promising
feature as a next-generation display [1-4]. Since the
organic multilayer electroluminescence (EL) diode was
reported in 1987 [5], the OLED characteristics have
been progressively improved. Various device structures,
organic materials and fabrication techniques have been
widely developed [6-8].
Among various emitting materials, one of the
promising candidates is an arylethene and stilbene based
π-conjugated system [9-14]. Some of π-conjugated
compounds such as stilbene and poly(arylenevinylene)s
(1,2-diarylethene-based π-systems) have already
revealed their high potential as the OLED materials [9,
15-18] due to their excellent fluorescence properties [9].
Many arylethene derivatives have photoluminescence
(PL) in a visible region; the PL spectrum and the PL
efficiency significantly depend on the substituents and
their position and these derivatives emit intense PL even
in a solid state [9, 19-21].
The blue-green emitting 1,4-bis(2,2-diphenyl-
ethenyl)benzene (PEB) shown in Fig. 1 is the simplest
material in this family containing stylbene units and can
be a good model compound to understand basic
photophysical and electrical properties. As it was
reported before, PEB is highly fluorescent and can be
used as a dopant in blue emitting OLEDs [16, 22-23].
OLED with PEB dispersed into a polystyrene (PS)
matrix showed pure blue emission with the external EL
quantum efficiency of ηEL = 0.25 % [22]. Further, OLED
using PEB blended with rubrene in a PS matrix
demonstrated white emission with ηEL = 0.3 % [23].
Although the PL efficiency of PEB in a solid film was
found to be high [16], the EL efficiencies of OLEDs
with the spin-coated emitting layer containing PEB were
rather low [22-23].
In this work, we report on the fundamental
photophysical properties of PEB in a solution and a solid
state. We also study the EL and carrier transport
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 77-82.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
78
characteristics of PEB using various OLED structures. A
high efficiency OLED with PEB as an electron-transport
and emitting layer is demonstrated.
2. Experimental section
PEB was synthesized according to the preparation
method described below. All reactions were carried out
under an atmosphere of nitrogen with freshly distilled
solvents, unless otherwise noted. Silica gel (Daiso gel
IR-60) was used for column chromatography. NMR
spectra were recorded on JEOL Lambda 500 instrument.
To a DMF solution (25 mL) of terephthalaldehyde
( 268 mg, 2.0 mmol), 0.663 g of dimethyl diphenyl-
methylphosphonate and 0.449 g of potassium t-butoxide
were added. The solution was stirred at room
temperature for 12 hours. After being transferred to a
separation funnel, ethyl acetate was added to mixture.
NaCl solution was used to wash the organic phase for
3 times to remove DMF. Then, the organic layer was
washed with brine and dried over MgSO4. After
evaporation, the residue was subjected to column
chromatography on silica gel (hexane / CH2Cl2 = 10:1)
to give 1,4-bis(2,2-diphenylethenyl)benzene (PEB)
(0.617 g, 71 %) as yellow solid. H NMR (500 MHz,
CDCl3) δ 6.79 (s, 4H), 6.86 (s, 2H), 7.14-7.20 (m, 4H),
7.25-7.34 (m, 16H). C NMR (125 MHz, CDCl3) δ
127.42, 127.45, 127.57, 127.86, 128.16, 128.60, 129.15,
130.35, 135.88, 140.35, 142.49, 143.46.
To investigate the molecular aggregation properties
of PEB, the degassed solutions with various PEB
concentrations of 10−6 ~ 10−2 M in dehydrated chlo-
roform (CHCl3) and toluene (CH3C6H5) were prepared
for PL measurements. Neat PEB films were deposited in
vacuum (10-4 Pa) on pre-cleaned quartz, glass and ITO
substrates, respectively. The substrates were cleaned
using a standard cleaning procedure and exposed to UV-
ozone before vacuum deposition. The thicknesses of the
deposited films were measured with Stylus Profiler
DEKTAK 6M.
The highest occupied molecular orbital (HOMO)
level of the deposited films was measured with an
ultraviolet photoelectron spectroscopy (AC-1, Riken
Keiki Co.). The absorption spectra of the degassed
solutions and deposited films were recorded using a UV-
VIS-NIR recording spectrophotometer UV-3100
(Shimadzu Co.). The PL spectra were measured using
Spectrofluorometer FP-6500-A-51 (Jasco Co.). The
absolute PL efficiency (Φf) of degassed solutions and
neat films under argon flow was measured using an
integrating sphere [24] with 325 nm excitation source
(Xe lamp) and multi-channel spectrometer (Hamamatsu
PMA-11) as the optical detector.
To understand the carrier transport and EL
properties of PEB, we prepared three types of OLED
structures (Fig. 1): Type (I): ITO( 110 nm) / α-
NPD(50 nm) / PEB(50 nm) / Mg:Ag( 100 nm) /
Ag(20 nm); Type (II): ITO(110 nm) / α-NPD(50 nm) /
PEB(20 nm) / Bphen(30 nm) / Mg:Ag(100 nm) /
Ag(20 nm); and Type (III): ITO(110nm) / α-
NPD(50 nm) / PEB(10 nm) / Alq3(50 nm) /
Mg:Ag(100 nm) / Ag(20 nm). Here, 4,4’-bis[N-(1-
naphtyl)-N-phenyl-amino]biphenyl (α-NPD) as a hole-
transport layer (HTL), aluminum (III) tris(8-
hydroxyquinoline) (Alq3) as an electron-transport layer
(ETL), and 4,7-diphenyl-1,10-phenanthroline (Bphen) as
an ETL and hole-blocking (HBL) layer were used for
OLEDs (Fig. 1). A magnesium silver (Mg:Ag) alloy
layer (10:1) capped with a silver layer was deposited on
the top of the organic layers. Current density-voltage-
luminance (DVL) characteristics were measured using a
semiconductor parameter analyzer (Agilent, HP4155C)
with an optical power meter (Newport, Model 1835-C).
3. Results and discussion
3.1. Photoluminescent properties of PEB
in a solution and a solid state
Table and Fig. 2 summarize the optical properties of PEB
in a neat film and in solutions with the PEB
concentrations of 10−6 ~ 10−2 M in chloroform and
toluene, respectively. These PEB solutions indicate strong
absorption with the maxima at λa,max ~ 355 nm
corresponding to the 0 → 0* transition. The absorption
spectrum of the 50 nm-thick PEB film showed the
identical absorption maximum at λa,max = 356 nm (Fig. 2).
ITO
HTL
X
(HBL) ETL
Mg:Ag
ITO
HTL
X
ETL
Mg:Ag
(I) (III)(II)
ITO
HTL
X
Mg:Ag
Fig. 1. PEB molecular structure with indication of possible
intermolecular rotations around the single and double bonds.
The types of OLED structures: (I) − ITO(110 nm) / α-
NPD(50 nm) / PEB(50 nm) / Mg:Ag(100 nm) / Ag(20 nm);
(II) − ITO(110 nm) / α-NPD(50 nm) / PEB(20 nm) /
Bphen(30 nm) / Mg:Ag(100 nm) / Ag(20 nm); (III) −
ITO(110 nm) / α-NPD(50 nm) / PEB(10 nm) / Alq3(50 nm) /
Mg:Ag(100 nm) / Ag(20 nm).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 77-82.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
79
Table. Photophysical properties of PEB in solution and deposited film.
Solution Neat film X mol % PEB: CBP
(doped film)
Molar
concentration (M)
In chloroform In
toluene
HOMO (eV) 5.8
LUMO (eV) 2.9
Eg (eV)
10-6
10-5
10-4
10-3
10-2
3.1
3.1
3.1
2.9
2.8
3.1
3.1
3.1
3.0
3.0
2.9
Φf
(%)
10-6
10-5
10-4
10-3
10-2
4.5+0.5
4.0+0.5
4.0+0.5
4.0+0.5
3.5+0.5
21.0+0.5
18.0+0.5
18.0+0.5
17.5+0.5
17.5+0.5
57.0+0.5
89.0+0.5*
90.5+0.5**
ηEL
(%) 0.012 (I)
2.5 (II)
1.1 (III)
1.1* (II)
1.1 ** (II)
λa, max
(nm)
354 358 356 359 *
359 **
λf, max
(nm)
10-6
10-5-10-2
456, 467
467~470
452, 468
468
471 471 *
471 **
λEL, max
(nm) 492 (I)
497 (II)
493 (III)
477 * (II)
484 ** (II)
(I), (II) and (III) denote type of the OLED structures.
* 0.23 mol. %; ** 4.70 mol. %
In a diluted chloroform or toluene solutions of
10-6 M, PEB showed blue fluorescence with vibronic
structures having two PL maxima at λf,max ~ 454 nm and
~ 467 nm corresponded to the 0* → 0 and 0* → 1
transitions, respectively, and a weak shoulder at
λf ~ 494 nm (0* → 2 transition). With an increase of the
PEB concentration, a slight red shift of the PL peaks was
observed. The PL spectra of the 10−5 ~ 10−2 M solutions
showed the PL maximum corresponding to the 0* → 1
transition and also two shoulders at 456 and 494 nm.
Further, the PL spectrum of the PEB neat film showed
more intense red shift compared with those of the
spectra in the 10-5 ~ 10-2 M solutions. The film showed
bright blue-greenish emission with a main maximum at
λf,max = 471 nm (0* → 1 transition) and two shoulders at
~ 456 nm (0* → 0 transition) and 493 nm (0* → 2
transition).
The PL efficiencies of PEB in the 10−6 ~ 10−2 M
chloroform solutions were as low as Φf = 3 ~ 5 % and
was almost independent on the concentration (Table).
On the other hand, the PL efficiencies of PEB in the
10−6 ~ 10−2 M toluene solutions showed much higher
values of Φf = 17 ~ 21 % and a slight decrease of the PL
efficiency with an increase of the concentration. On the
other hand, the PL efficiency of the deposited film was
Φf = 57 %, indicating dramatic increase in the solid state
compared with those in the solutions. This can be
explained in terms of degrees of the intramolecular
freedom of PEB in the solutions and solid state. PEB can
be well dissolved in a chloroform solution and have a
freedom in the intramolecular rotations as shown in
Fig. 1. PEB contains stilbene units which can transform
from trans to cis and vice versa under the optical
excitation. The trans-configuration of stilbene is
radiative, while cis-configuration is completely non-
emissive. As it was found before [22], the trans-trans
configuration in PEB is optically most emissive,
although it can be flipped into less emissive cis-trans or
cis-cis PEB configurations under the optical excitation.
Thus, the prevention of the intramolecular rotations and
stabilization of the trans-configuration are necessary to
obtain high PL efficiency. In case of the toluene
solution, we suppose that PEB have less freedom for the
intramolecular rotation due to the higher viscosity of
toluene (δ = 0.68 Centistokes at 20 ºC) compared with
chloroform (δ = 0.38 Centistokes at 20 ºC).
To confirm the above consideration, we prepared
the “solid solution” of a co-deposited film of 4,4’-N,N’-
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 77-82.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
80
200 300 400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
A
bs
or
ba
nc
e
(a
. u
.)
10-6 M in CHCl3
10-6 M in toluene
neat film
Wavelength (nm)
PL
(a
. u
.)
Fig. 2. Absorption and PL spectra of PEB in a neat film
and in 10−6 M chloroform and toluene solutions.
200 300 400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0 Abs, PL
- EL (I)
- EL (II)
- EL (III)
PL
a
nd
E
L
in
te
ns
ity
(a
.u
.)
A
bs
or
ba
nc
e
(a
.u
.)
Wavelength (nm)
Fig. 3. EL spectra in OLED devices (I, II and III), and PL
and absorption spectra of PEB in a neat film.
1 10
10-6
1x10-5
1x10-4
10-3
10-2
10-1
100
101
102
103
104
105
10-2 10-1 100 101 102 103
10-3
10-2
10-1
100
Current density (mA/cm2)
E
xt
er
na
l E
L
ef
fic
ie
nc
y
(%
)
20 305
OLED (I)
OLED (II)
OLED (III)
Voltage (V)
C
ur
re
nt
d
en
si
ty
(m
A
/c
m
2 )
Fig. 4. Current density-voltage (DV) characteristics of
OLEDs (I, II and III). The corresponding external EL
efficiency-current density (ηEL-J) characteristics are shown
in the insert.
10-2 10-1 100 101 102 103
10-2
10-1
100
101
102
103
104
105
OLED (I)
OLED (II)
OLED (III)
Current density (mA/cm2)
Lu
m
in
an
ce
(c
d/
m
2 )
Fig. 5. Luminance-current density characteristics of OLEDs
(I, II and III).
dicarbazole-biphenyl (CBP) doped with 1 ~ 2 wt% PEB.
The PL spectrum of 1 ~ 2 wt% PEB: CBP co-deposited
film was similar to those in the chloroform and toluene
solutions, demonstrating the same maximum at λf,max =
471 nm and two weak shoulders at 456 and 494 nm,
respectively. It indicates that the PEB molecules are well
dispersed in the CBP host matrix and no PEB molecular
aggregation occurs. The PL efficiency was significantly
high compared with these in the solutions and the neat
film, showing Φf = (89 ~ 91) + 0.5 % (Table). Thus, the
CBP host matrix well prevents the intramolecular
rotation in PEB, leading to the significant increase of the
radiative decay process.
3.2. Electroluminescent properties of PEB
All OLEDs showed blue-green EL with the maximum at
λEL, max = 492 nm corresponding to the 0* → 2 transition
(Fig. 3). OLEDs with the Type (I) structure showed
inferior device characteristics. The low external EL
quantum efficiency of ηEL = 0.012 % with maximum
luminance of ~31 cd/m2 and poor reproducibility were
observed due to the crystallization of the PEB layer
during the metal deposition (Figs 3-5). Also, relatively
high energy barrier of ~0.85 eV at the PEB/MgAg
interface may limit efficient electron injection (Fig. 4).
The OLEDs of Type (II) with Bphen as an addi-
tional electron-transport and hole-blocking layer showed
good reproducibility and pronounced OLED
characteristics with ηEL = 2.5 % and the maximum
luminance of 41600 cd/m2 (Figs. 4 and 5). The EL
spectrum showed the maximum at λEL,max = 493 nm
corresponding to the 0* → 3 transition in PEB (Fig. 3).
Here, we observed no crystallization of the PEB layer
due to the presence of the overlaid Bphen layer. The
small electron injection barrier of ∆E = 0.1 eV from
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2007. V. 10, N 1. P. 77-82.
© 2007, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
81
ITO
α-NPD
5.0
2.4
5.4
2.9
5.8
3.7
PEB
Mg:Ag
(a)
-
+
ITO
α-NPD
5.0
2.4
5.4
2.9
5.8
3.7
PEB
Mg:Ag
6.4
3.0
Bphen
(b)
-
+
ITO
5.0
2.4
5.4
2.9
5.8
α-NPD PEB
3.7
Mg:Ag
3.1
5.8
Alq3
(c)
-
+
Fig. 6. Energy diagrams in OLEDs (I (a), II (b) and III (c)).
Bphen into the PEB layer would also provide efficient
electron injection (Fig. 6b). To clarify the major carriers
in PEB, Type (III) OLED having an Alq3 ETL was
examined (Fig. 6c). The EL spectrum was similar to the
Type (II) devices, showing the greenish emission with
the maximum at 493 nm, indicating that EL is due to
PEB layer, since the Alq3 emission locates at λEL,max
~ 510 nm. Thus, we can conclude that the PEB layer
efficiently transports electrons.
4. Conclusions
The photophysical and EL properties of PEB were
investigated. The very weak blue PL of PEB in a
solution was dramatically increased in a solid state. We
explained such PEB behavior in the terms of degrees of
freedom in the intramolecular rotations between cis- and
trans-configuration. The prevention of the intramolecular
rotations leads to the dramatic increase of Φf. The
OLEDs with PEB as an emitting layer showed blue-
greenish EL with the maximum ηEL = 2.5 %. PEB was
identified as an efficient electron-transport and emitting
material in OLEDs.
Acknowledgments
We are very grateful to the 21st Century COE program
of “Molecular Informatics” for financial support of this
study.
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