Solution processable white organic light-emitting diodes using new blue host material including substituent group

Article abstract of DOI:10.1166/jnn.2016.12026

New host material of T-TATa isomer substituted t-butyl group was investigated in solution process WOLED device compared with 4-(10-(3′,5′-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,Ndiphenylaniline [TATa]. A two-color WOLED of a co-host system using solution process method was demonstrated. The device configuration was ITO/PEDOT:PSS (40 nm)/emitting layer (50 nm)/TPBi (20 nm)/LiF (1 nm)/Al. The emitting layer consisted of TATa or T-TATa isomer, NPB, DPAVBi (blue dopant), and rubrene (yellow dopant). NPB was used as not only blue host but also helping hole carrier transport. The device using T-TATa compound as a co-host exhibited a luminance efficiency of 3.39 cd/A, which is about twice higher than TATa device of 1.58 cd/A at 10 mA/cm².

Full text of DOI:10.1166/jnn.2016.12026

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Vol. 16, 2101–2104, 2016
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Solution Processable White Organic Light-Emitting
Diodes Using New Blue Host Material Including
Substituent Group
Jaehyun Lee, Hwangyu Shin, and Jongwook Park∗
Display Research Center, The Catholic University of Korea, Bucheon, 420-743, Republic of Korea
New host material of T-TATa isomer substituted t-butyl group was investigated in solution
process WOLED device compared with 4-(10-(3ꢀ,5ꢀ-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-
diphenylaniline [TATa]. A two-color WOLED of a co-host system using solution process method was
demonstrated. The device configuration was ITO/PEDOT:PSS (40 nm)/emitting layer (50 nm)/TPBi
(20 nm)/LiF (1 nm)/Al. The emitting layer consisted of TATa or T-TATa isomer, NPB, DPAVBi (blue
dopant), and rubrene (yellow dopant). NPB was used as not only blue host but also helping hole
carrier transport. The device using T-TATa compound as a co-host exhibited a luminance efficiency
of 3.39 cd/A, which is about twice higher than TATa device of 1.58 cd/A at 10 mA/cm2.
Keywords: Organic Light-Emitting Diode, Solution Process, White OLED, Blue Host.
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1. INTRODUCTION
high performance, many molecules were designed and
synthesized.
Many researchers have extensively studied to make
high performance organic light-emitting diode (OLED).1–4
Especially, The interest based on white OLED (WOLED)
has been rapidly growing in large full color display such
as OLED TV and lighting fields.5 Until now, WOLED has
been produced using vacuum evaporation method, but this
method involves disadvantages such as high cost, difficul-
ties of large scaling and adjusting doping concentration.6ꢀ7
Therefore, there are many researches on methods of pro-
ducing solution processed WOLED such as spin-coating,
nozzle printing, and ink-jet printing method for their
advantages of large scaling and relatively inexpensive
cost.8ꢀ9
A lot of approach ways for WOLED have been reported
since the first report of Kido group.10 Many researchers are
also involved in ongoing efforts to make two or three emis-
sive layers in OLED device. Also, because host-dopant
system can significantly improve efficiency and show sta-
ble color purity, it was often used in WOLED.
In this study, T-TATa as new host material with t-butyl
group was characterized and compared to TATa without
t-butyl group in WOLED. White EML is proposed by
using blending and spin-coating method through a co-
host/dopant system based on TATa or T-TATa and NPB as
co-host.
2. EXPERIMENTAL DETAILS
2.1. General Experiment
TATa, T-TATa and N ,Nꢀ-Bis(naphthalen-1-yl)-N,Nꢀ-
bis(phenyl)-benzidine [NPB] were used for the host
materials used in the emitting layer, and 4,4ꢀ-bis[4-(di-
p-tolylamino)styryl]biphenyl [DPA-VBi] and 5,6,11,12-
tetraphenylnaphthacene [rubrene] were used as each
blue and yellow as the dopant materials. TATa:NPB or
T-TATa:NPB was mixed in a mass ratio of 60:40 for
host materials and DPAVBi with ratio of 4% and rubrene
with ratio of 0.25% were mixed for dopant materials
based on the mass of co-host. Solvent was toluene.
A water-dispersed PEDOT/PSS mixture (Baytron PVP
CH8000, H. C. Starck GmbH) was spin-coated on top
of indium tin oxide (ITO) in air for 40 nm thickness.
The spin-coated films were baked on a hot plate at
Especially, a system that shows high efficiency with a
hole or electron transporting material blended with emit-
ting compounds is being actively considered as a co-
host system.11 Also, in order to develop the host for
ꢁ
ꢁ
∗Author to whom correspondence should be addressed.
110 C for 5 minutes in air and 200 C for 5 minutes
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 2
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doi:10.1166/jnn.2016.12026
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Solution Processable White Organic Light-Emitting Diodes Using New Blue Host Material Including Substituent Group
Lee et al.
In a 250 mL round-bottomed flask, compound (1) (3.4 g,
5.37 mmol), compound (2) (1.62 g, 5.91 mmol), Pd(OAc)2
(36 mg, 0.16 mmol), (o-OMePh)3P (114 mg, 0.32 mmol),
and K3PO4 (4.6 g, 21.5 mmol) were placed with anhy-
drous DME 120 ml and water 30 ml. The temperature was
raised to 100 ꢁC. After reaction is completed, it is extracted
by chloroform and water. After removing water from the
mixture, it was filtered by using MgSO4. The mixture
was columned under chloroform:Hexane (1:3) mixed sol-
vent. The product was concentrated under reduced pressure
and re-precipitated with methanol to obtain a yellow solid
material (yield: 83.4%).
T-TATa has isomer form. So NMR is not conformed,
but EA and mass can be conformed. HRMS: m/z 781.3697
(M+, calcd 781.3709). Anal. Calcd for C60H47N1:
C, 92.15; H, 6.06; N, 1.79 Found: C, 92.02; H, 6.04;
N, 1.74.
Figure 1. Chemical structures of host and dopant materials.
3. RESULTS AND DISCUSSION
in N2 glove box. The emitting layer was spin-coated
to obtain a thickness of 50 nm. The spin-coated films
of emitter were baked on a hot plate (110 C, 30 min)
in N2 glove box. Electron-transporting layers 1,3,5-
tris(N -phenylbenzimidazole-2-yl)benzene (TPBi) with a
thickness of 20 nm were deposited by vacuum evaporation
(at a pressure about 10−6 Torr). Also, LiF (1 nm) and
Al films were sequentially deposited on the electron-
Figure 1 shows the chemical structures of host and dopant
materials. TATa and T-TATa are blue emitters as host mate-
rials. In the case of T-TATa, t-butyl group was included in
anthracene core of TATa. NPB is used as a co-host mate-
rial which has HTL property and wide band gap similar
to a blue emitter. DPAVBi and rubrene were each used as
blue dopant and yellow dopant.
ꢁ
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transporting layer. I–V –L characteristics of the WOLED
devices and EL spectra were obtained using a Keith-
The optical and electrochemical properties of the syn-
thesized materials were summarized in Table I. Figure 2
exhibits UV-Visible (UV-Vis.) absorption and PL spectra
of two compounds in toluene solution and film state. UV-
Vis. absorption in solution and film state of two com-
pounds shows the typical anthracene absorption band in
range of 350∼405 nm. Major absorption peak of TATa
and T-TATa in toluene solution state exhibited at 397 nm
and 396 nm, respectively. Compared to solution state, UV-
Vis. absorption in film state was slightly red shifted as
404 nm and 402 nm, which indicated an increase in conju-
gation length in the solid state. The increase in conjugation
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ley 2400 source measure unit and a Minolta CS-1000A
spectrophotometer. The optical absorption spectra were
obtained by HP 8453 UV-VIS-NIR spectrometer. Perkin
Elmer luminescence spectrometer LS50 (Xenon flash
tube) was used for photo spectroscopy (PL).
2.2. Synthesis of TATa and T-TATa
The method of synthesis for TATa in Figure 1 is published
in Org. electron.12 Scheme 1 shows synthetic routes of
T-TATa.
Scheme 1. Synthetic routes of T-TATa.
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J. Nanosci. Nanotechnol. 16, 2101–2104, 2016

Lee et al.
Solution Processable White Organic Light-Emitting Diodes Using New Blue Host Material Including Substituent Group
Table I. Optical and electrical properties of synthesized materials.
Solutiona Filmb
UVmax
Table II. EL performances of the WOLED devices using synthesized
materials at 10 mA/cm2.
L. E.a (Cd/A)
P. E.b (lm/W)
CIE (xꢀy)
ELmax (nm)
UVmax
PLmax
(nm)
PLmax
(nm)
HOMO LUMO
(nm)
(nm)
(eV)
(eV)
TATa
T-TATa
1.58
3.39
1.28
2.68
(0.30, 0.39)
(0.28, 0.38)
466, 550
468, 550
TATa
T-TATa 382, 396
382, 397
456
456
384, 404
384, 402
466
465
−5.37
−5.28
−2.50
−2.40
Notes: aLuminance efficiency. bPower efficiency.
Notes: aToluene solution (1×10−5 M). bFilm on glass.
It may be due to the donating effect of t-butyl group in
T-TATa.
length might be due to the conformation of solid state
ꢁ-stacked compounds being more planar.13 The PL max-
imum wavelength of TATa and T-TATa were also mea-
sured at 456 nm in solution state, 466 nm and 465 nm in
film state, respectively. TATa and T-TATa including t-butyl
group were observed similar optical maximum values of
UV-Vis. absorption and PL spectra.
Optical band gap obtained from onset of UV absorption
was 2.87 eV for TATa and 2.88 eV for T-TATa. The value
of highest occupied molecular orbital (HOMO) level was
calculated with the use of cyclovoltammetry, and lowest
unoccupied molecular orbital (LUMO) level with the use
of HOMO level and optical band gap. HOMO-LUMO val-
ues of T-TATa have at −5.28 eV and −2.40 eV slightly
higher values than −5.37 eV and −2.50 eV of TATa.
The device structure was fabricated as ITO/PEDOT
(40 nm)/emitting layer: TATa or T-TATa+NPB+dopants
(50 nm)/TPBi (20 nm)/LiF (1 nm)/Al. Device perfor-
mances of WOLED using TATa or T-TATa are summa-
rized in Table II. WOLED devices were fabricated by
spin-coating method using co-host of TATa or T-TATa
and NPB as well as dopants. Figure 3 shows the lumi-
nance efficiency (cd/A) versus current density of TATa and
T-TATa. WOLED devices based on TATa or T-TATa exhib-
ited luminance efficiencies of 1.58 cd/A and 3.39 cd/A at
10 mA/cm2, respectively. Power efficiencies (lm/W) were
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Figure 2. UV-Visible absorption and PL spectra of TATa and T-TATa:
Figure 3. (a) Luminance efficiency and (b) power efficiency of
WOLED devices.
(a) solution in toluene (1.0×10−5 M), (b) spin-coated film.
J. Nanosci. Nanotechnol. 16, 2101–2104, 2016
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Solution Processable White Organic Light-Emitting Diodes Using New Blue Host Material Including Substituent Group
Lee et al.
(0.30, 0.39) and (0.28, 0.38), respectively. In conclusion,
WOLED device based on solution process including host
material, a compound that includes substitution of t-butyl
group into the anthracene core group showed higher per-
formance than another compound device without t-butyl
group.
Acknowledgment: This research was supported by a
grant from the Fundamental R&D Program for Core Tech-
nology of Materials funded by the Ministry of Trade,
Industry and Energy, Republic of Korea (Project No.
10050215).
References and Notes
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Figure 4. EL spectra of WOLED devices.
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1.28 lm/W and 2.68 lm/W. T-TATa in which t-butyl group
was added at 2-position of anthracene showed higher effi-
ciency than TATa because t-butyl group can help to prevent
ꢁ–ꢁ∗ stacking interaction between adjacent molecules in
spin-coated film.14 As shown in Figure 4, the EL maximum
values of TATa and T-TATa were exhibited at 466 nm or
468 nm of blue emission and 550 nm of orange emission,
respectively. In case of the CIE values, similar color coor-
dinates of TATa and T-TATa were shown at (0.30, 0.39)
and (0.28, 0.38).
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4. CONCLUSION
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WOLED devices were fabricated by spin-coating method
based on co-host such as TATa or T-TATa with NPB.
Also, DPAVBi and rubrene were used as blue and orange
dopants. As a result, the device used T-TATa including t-
butyl group as the co-host showed luminance efficiency of
3.39 cd/A, which is about two times as high as 1.58 cd/A
of TATa. CIE values of TATa and T-TATa device were
Received: 19 April 2015. Accepted: 27 May 2015.
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J. Nanosci. Nanotechnol. 16, 2101–2104, 2016