A new approach way for white organic light-emitting diodes based on single emitting layer and large stokes shift

Article abstract of DOI:10.1166/jnn.2014.8432

New red dopant, DPPZ based on porphyrin moiety was synthesized. DPPZ showed UV-Vis and PL maximum values of 412 and 638 nm, indicating the large stokes shift. New blue host compound, TATa was also synthesized and used for co-mixed white emission. TATa exhibited UV-Vis. and PL maximum values of 403 nm and 463 nm in film state. Thus, when two compounds are used as co-mixed emitter in OLED device, there is no energy transfer from blue emission of TATa to DPPZ due to large stokes shift of DPPZ. Based on the PL result, it is available to realize two-colored white in PL and EL spectra. As a result of this, two-mixed compounds showed vivid their own PL emission peaks of 466 and 638 nm in film state. Also, white OLED device using two-mixed compounds system was fabricated. EL spectrum shows 481 and 646 nm peaks and two separate EL peaks. As the operation voltage is increased from 8 to 11 V, EL spectrum does not change the peak shape and maximum wavelength values. EL performance of white device showed 0.041 cd/A, 0.018 lm/W, and CIE (0.457, 0.331) at 8 V. Copyright

Full text of DOI:10.1166/jnn.2014.8432

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 5491–5494, 2014
Copyright © 2014 American Scientific Publishers
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Printed in the United States of America
A New Approach Way for White Organic
Light-Emitting Diodes Based on Single
Emitting Layer and Large Stokes Shift
Beomjin Kim, Youngil Park, Yunseop Shin, Jiwon Lee,
Hwangyu Shin, and Jongwook Park∗
Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, Korea
New red dopant, DPPZ based on porphyrin moiety was synthesized. DPPZ showed UV-Vis and PL
maximum values of 412 and 638 nm, indicating the large stokes shift. New blue host compound,
TATa was also synthesized and used for co-mixed white emission. TATa exhibited UV-Vis. and PL
maximum values of 403 nm and 463 nm in film state. Thus, when two compounds are used as
co-mixed emitter in OLED device, there is no energy transfer from blue emission of TATa to DPPZ
due to large stokes shift of DPPZ. Based on the PL result, it is available to realize two-colored
white in PL and EL spectra. As a result of this, two-mixed compounds showed vivid their own
PL emission peaks of 466 and 638 nm in film state. Also, white OLED device using two-mixed
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compounds system was fabricated. EL spectrum shows 481 and 646 nm peaks and two separate
IP: 182.71.110.82 On: Tue, 23 Feb 2016 05:27:31
EL peaks. As the operation voltage is increased from 8 to 11 V, EL spectrum does not change the
Copyright: American Scientific Publishers
peak shape and maximum wavelength values. EL performance of white device showed 0.041 cd/A,
0.018 lm/W, and CIE (0.457, 0.331) at 8 V.
Keywords: OLED, White OLED, WOLED, Large Stoke Shift, Stoke Shift, Porphyrin.
1. INTRODUCTION
and green chromophores using non-conjugated linkage to
prevent energy transfer,6 (3) broad and red-shifted spec-
trum by using locally excited and intramolecular charge
transfer (ICT) states.7
Therefore, we investigated new approach way to realize
two-colored white in single emitting layer system by using
large stokes shift of red dopant. Red dopant having a large
stokes shift of over 200 nm has basically no absorption of
blue or green region. Thus it does not affect blue emission
and prevent Förster resonance energy transfer of singlet
exciton between blue and red emitter.
In this study, porphyrin derivative which is new
red emitter and has large stoke shift was synthesized.
4-(10-(30,50-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-
diphenylaniline (TATa), which was very high efficiency
blue emitting material in previous paper, was also used
for single layered white OLED device.8
The organic light-emitting diodes (OLEDs) based on white
color are currently the subject of intense research due to
their applications of full color display as well as lighting.
A lot of meaningful approach ways for white OLED have
been reported since the first report of Kido group.1 Many
researchers are also engaged in ongoing efforts to deposit
multilayer with two or three emissive materials in OLED
device.2–4
However, multilayered emitter device has several dis-
advantages to achieve white color such as the following:
complicated fabrication process, changeable color coordi-
nates (CIE) values, and flexible emission color. To improve
this demerits, there have been several attempts using single
emitting layer in OLED device, but it is very difficult to
control the ratio of energy transfer in terms of from blue
to yellow or red color.
Several important methods for white color in single
emitter system are (1) excited state intra-molecular proton
transfer (ESIPT),5 (2) chemical combination of blue, red,
2. EXPERIMENTAL DETAILS
2.1. General Method
1H-NMR spectra were recorded on Brucker. The opti-
cal absorption spectra were obtained by HP 8453
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 7
1533-4880/2014/14/5491/004
doi:10.1166/jnn.2014.8432
5491

A New Approach Way for White OLED Based on Single Emitting Layer and Large Stokes Shift
Kim et al.
UV-VIS-NIR spectrometer. Perkin Elmer luminescence
spectrometer LS55 (Xenon flash tube) was used for photo-
luminescence spectroscopy. The redox potentials of the
compounds were determined with cyclic voltammetry
(CV) using a AUTOLAB/PG-STAT128N model system
with a scanning rate of 100 mV/s. We used synthesized
material coated ITO as working electrode, a saturated
Ag/AgNO3 as a reference electrode and Acetonitrile (AN)
with 0.1 M tetrabutylammonium tetrafluoroborate (TBAT)
as an electrolyte. Ferrocene was used for potential calibra-
tion and for reversibility criteria.
The reaction was checked with TLC, and when the reac-
tion was completed, purified by using column chromatog-
raphy with benzene and recrystallization was performed
with chloroform and methanol. As a result, pure pur-
ple compounds were obtained. The final yield was 15%.
1H NMR (300 MHz, THF-d8ꢁ: ꢂ (ppm) 10.34 (s, 2 H),
9.42 (d, 4 H), 9.11 (d, 4 H), 8.30 (m, 4 H), 7.83 (m, 6 H),
−3ꢃ11 (s, 2 H).
2.2.3. [1,5-Diphenyl-porphyrinato]Zinc(II) (DPPZ)
Diphenylporphyrin 1 g (2.162 mmol) and zinc chloride
1.1 g (8.65 mmol) were mixed in a round flask of 250 mL
which was refluxed by using dimethylformamide of 50 mL
as solvent for 4 hour. The reaction was checked with TLC,
and when the reaction was completed, washing with dis-
tilled water, methanol and isopropyl alcohol. and recrystal-
lization was performed with chloroform and hexane. As a
result, pure purple compounds were obtained. The final
EL devices were fabricated as the following struc-
ture: ITO/2-TNATA (60 nm)/NPB (15 nm)/TATa + DPPZ
(50%:50%, 30 nm)/Bphen (30 nm)/LiF (1 nm)/Al
(200 nm), where Tris(N-(naphthalen-2-yl)-N-phenyl-
amino) triphenylamine (2-TNATA) was used as hole injec-
tion layer, N,Nꢀ-bis(naphthalen-1-yl)-N,Nꢀ-bis(phenyl)
benzidine (NPB) as hole transporting layer, 4,7-Diphenyl-
1,10-phenanthroline (Bphen) as electron transporting layer,
the TATa + DPPZ as emitting layer, LiF as electron injec-
tion layer, ITO as anode and Al as cathode. The organic
layer was vacuum-deposited using thermal evaporation at
a vacuum base pressure of 10−6 torr and the rate of depo-
sition being 1 Å/s to give an emitting area of 4 mm2, and
the aluminum layer was continuously deposited under the
same vacuum condition. The current–voltage–luminance
(I–V –L) characteristics of the fabricated EL devices were
1
yield was 70%. H NMR (300 MHz, THF-d8ꢁ: ꢂ (ppm)
10.34 (s, 2 H), 9.46 (d, 4 H), 9.16 (d, 4 H), 8.28 (m, 4 H),
7.81 (m, 6 H). FAB-Mass 524 m/z.
3. RESULTS AND DISCUSSION
Scheme 1 shows the chemical structures of TATa and
DPPZ which are used for OLED emitter.
TATa showed UV-Vis and PL maximum values of
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403 nm and 445 nm in film state (see Table I and Fig. 1).8
obtained by Keithley 2400 electrometer. Light intensity
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TATa has high Tg value, above 130 ꢁC. A high Tg indicates
that the morphology of the material is not easily changed
by the high temperatures generated by the operation of
OLED device, and is closely correlated with good OLED
device lifetime.9ꢄ10 Also, as we mentioned in introduction,
large-stokes shift for red dopant is useful approach way
to realize two-color white OLEDs. In this concept, the
main requirement issue for large-stokes shift is no optical
absorption of red dopant in the wavelength range of blue
host emission. Based on this approach way, new DPPZ
was synthesized as shown in Scheme 2.
Copyright: American Scientific Publishers
was obtained by Minolta CS-1000.
2.2. Synthesis of [1,5-Diphenyl-Porphyrinato]
Zinc(II) (DPPZ)
2.2.1. Synthesis of Dipyrromethane ꢀDPMꢁ
Paraformaldehyde 1.73 g (0.0576 mol) and pyrrole 100 mL
(1.44 mol) were mixed in a round flask of 250 mL which
wasꢁ nitrogen atmosphere. Mixture stirred and heated to
60 C. After 30 minute, reduced the heating and added
trifluoroacetic acid 0.44 ml (5.76 mmol) and stirred for
4 h. The reaction was checked with TLC, and when
the reaction was completed, extraction was performed by
using water and ethyl acetate, and moisture was removed
by using anhydrous MgSO4. By distillation, extra pyr-
role was reduced, and pure white dipyrromethane solid
For the optical and electrical data, UV-Vis. spectroscopy
and cyclic voltammetry (CV) were carried out. Based
1
was obtained. The yield was 26%. H NMR (300 MHz,
CDCl3):ꢂ (ppm) 7.88 (s, 2 H), 6.66 (d, 2 H), 6.16 (d, 2 H),
6.03 (s, 2 H), 3.98 (s, 2 H).
N
N
N
N
N
Zn
2.2.2. 1,5-Diphenyl-Porphyrin (DPP)
Dipyrromethane 1 g (6.84× mmol) and benzaldehyde
0.76 ml (6.84 mmol) were mixed in a round flask of
2 L and then boron trifluoride diethyl etherate 0.48 ml
was added, which was stirred by using chloroform of
1 L as solvent. After 1 hour, 2,3-Dichloro-5,6-dicyano-
p-benzoquinone (DDQ) 2.33 g (10.26 mmol) was added
and stirred for 5 hour, and then triethylamine was added.
(a)
(b)
Scheme 1. The chemical structures of 4-(10-(30,50-diphenylbiphenyl-
4-yl)anthracen-9-yl)-N,N-diphenylaniline ((a) TATa) and [5,15-
Diphenylporphyrinato]zinc(II) ((b) DPPZ).
5492
J. Nanosci. Nanotechnol. 14, 5491–5494, 2014

Kim et al.
A New Approach Way for White OLED Based on Single Emitting Layer and Large Stokes Shift
Table I. Optical property of synthesized compounds.
O
H
H
N
H
H
O
CF3COOH
60 ºC
+
+
Solution
Film on glass
H
H
NH
HN
DPM
Compounds UVmax (nm) PLmax (nm) UVmax (nm) PLmax (nm)
TATa
DPPZ
398
412a
476
638a
403
–
463
–
Note: aTHF solution.
N
N
NH
N
N
N
N
ZnCl2
DMF/reflux
1) CHCl3/BF3OEt2/RT
2) DDQ/RT
Zn
NH
on these data, HOMO and LUMO levels of DPPZ were
5.19 eV and 2.98 eV. DPPZ solution showed UV-Vis. and
PL maximum values of 412 nm and 638 nm, respectively
(see Table I and Fig. 1). As shown in Figure 1, there
was no absorption or emission in the range of 450 to
540 nm, indicating the large stokes shift. As a result, when
two compounds are mixed as an emitter in OLED device,
each compound absorbs similar light region of 400 nm but
two PL spectra will come out separately. DPPZ compound
does not specifically absorb or emit in the range of 450
to 540 nm. Thus DPPZ does not affect the emission of
TATa and two mixed compounds system is theoretically
available for white emission. PL spectrum of the mixed
compound is shown in Figure 2. Two-mixed compounds
showed vivid their own PL emission peaks, respectively.
Maximum values were 466 and 638 nm.
DPPZ
DPP
Scheme 2. Synthetic route of DPPZ.
On the other hand, white OLED device using two-
mixed compounds system was fabricated as follow-
ing configuration: ITO/2-TNATA (60 nm)/NPB (15 nm)/
TATa + DPPZ (50%:50%, 30 nm)/Bphen (30 nm)/LiF
(1 nm)/Al (200 nm). Two compounds were co-deposited
by using tooling-factor calibration method. As shown in
Figure 3, EL spectrum of the white device was measured
according to different applied voltage.
At first, EL spectrum shows 479 and 645 nm peaks
and two separate EL peaks, respectively. As the opera-
tion voltage is increased from 8 to 11 V, EL spectrum
does not change the peak shape and maximum wave-
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length values. It means that although operation voltage
IP: 182.71.110.82 On: Tue, 23 Feb 2016 05:27:31
was changed, recombination site was not affected and two
Copyright: American Scientific Publishers
compounds normally emit. EL performance of the device
showed 0.041 cd/A, 0.018 lm/W, and CIE (0.457, 0.331) at
8 V. Color coordinate (CIE) values in EL performance are
in good agreement with EL spectra data under the differ-
ent voltage and current density. As a result of CIE value,
CIE value is very stable and reddish white color although
device efficiency is not so high because of new approach
for the first time. Further studies on the optimized CIE
value and improved EL efficiency are underway.
Figure 1. UV-Visible absorption and PL spectrum (a) TATa (in film
state) (b) DPPZ (in THF solution).
Figure 2. PL spectrum of TATa and DPPZ mixed compound in film
(TATa: DPPZ = 50%:50%).
J. Nanosci. Nanotechnol. 14, 5491–5494, 2014
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A New Approach Way for White OLED Based on Single Emitting Layer and Large Stokes Shift
Kim et al.
Although this method showed relatively low efficiency
data compared to other method such as multi-stack layer
system, it is meaningful approach way for various ways to
make white OLED device in the future.
Acknowledgment: This work was supported by the
National Research Foundation of Korea (NRF) grant fun-
ded by the Korea government (MEST) (No. 2012001846).
References and Notes
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Figure 3. EL spectrum of the white device using two-mixed com-
pounds of TATa and DPPZ as the co-emitter.
4. CONCLUSION
By using large stokes shift, two-colored white OLED
device was demonstrated. In this device, new red dopant,
DPPZ and blue host compound, TATa were used for co-
mixed single layer made from co-deposition. There was no
energy transfer from blue emission of TATa to DPPZ due
to large stokes shift of DPPZ. When white OLED device
using two-mixed compounds system was fabricated, EL
spectrum showed 479 and 645 nm peaks and two sepa-
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Delivered by Publishing Technology to: Purdue University Libraries
rate EL peaks, respectively. As the operation voltage is
increased from 8 to 11 V, EL spectrum does not change
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Copyright: American Scientific Publishers
the peak shape and maximum wavelength values. Finally,
DPPZ does not affect the emission of TATa and two mixed
compounds system using large stokes shift is available for
white emission. EL performance of white device showed
0.041 cd/A, 0.018 lm/W, and CIE (0.457, 0.331) at 8 V. So
far, there hasn’t been an attempt to use this kind of method.
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Mater. 14, 3860 (2002).
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Received: 19 May 2011. Accepted: 1 August 2011.
5494
J. Nanosci. Nanotechnol. 14, 5491–5494, 2014