A new anthracene derivative containing t-butyl group for solution process organic light-emitting diodes

Article abstract of DOI:10.1166/jnn.2015.11258

4-(10-(3',5'-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline [TATa] and a new anthracene derivative of 4-(2 or 3-tert-butyl-10-(3',5'-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline [T-TATa] isomer by introduced f-butyl group were synt

Full text of DOI:10.1166/jnn.2015.11258

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Nanoscience and Nanotechnology
Vol. 15, 8285–8288, 2015
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A New Anthracene Derivative Containing t-Butyl Group
for Solution Process Organic Light-Emitting Diodes
Jaehyun Lee1, Seungho Kim1, Jee-Hwan Kim2, and Jongwook Park1ꢀ∗
1Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, Korea
2Eton College, Windsor, Berkshire, SL4 6DW, United Kingdom
4-(10-(3ꢀ,5ꢀ-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline [TATa] and a new anthracene
derivative of 4-(2 or 3-tert-butyl-10-(3ꢀ,5ꢀ-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline
[T-TATa] isomer by introduced t-butyl group were synthesized. OLED devices of TATa and T-TATa
were fabricated by solution process. Its physical properties such as optical, electrochemical, and
electroluminescent properties were also investigated. Two compounds were used as emitting layer
(EML) in OLED device: ITO/PEDOT (40 nm)/synthesized materials (60 nm)/TPBi (20 nm)/LiF
(1 nm)/Al (200 nm). The luminance efficiency of the synthesized compounds at 10 mA/cm2 were
measured 0.85 cd/A for TATa and 1.49 cd/A for T-TATa, respectively. Moreover, the power efficiency
of T-TATa is 1.08 lm/W. Its value is almost two times higher than 0.56 lm/W of TATa. As a result,
more improved efficiency was shown with the device in a compound including t-butyl group to TATa
core part, when the deivces were prepared by solution process.
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Keywords: Organic Light-Emitting Diode, Blue Emitting Material, Solution Process.
Copyright: American Scientific Publishers
1. INTRODUCTION
material is especially characterized as low efficiency and
short device lifetimes because its wide band gap. Blue light
means that it includes high energy band gap as well as a
steep energy barrier at the hole and electron injection inter-
faces inside of device compared to red and green emitter.
Therefore, it is necessary to develop blue light emitting
materials that maintain pure color and high efficiency.10–12
In this study, we propose new approach method to
prevent packing of molecules by introducing alkyl group
into anthracene. In our previous paper, 4-(10-(3ꢀ,5ꢀ-
diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline
[TATa],13 a blue fluorescent emitter was shown to have
high efficiency through the deposition process. Here, we
changed anthracene core part using tertiary butyl group
as shown in Figure 1 and characterized compounds using
solution process.
ꢀ-Conjugated compounds are useful in a variety of appli-
cations, including organic light-emitting diodes (OLEDs),1
organic thin film transistors (OTFTs)2 and organic pho-
tovoltaic cells (OPVCs).3 OLEDs based on small organic
molecules are currently the subject of intense research due
to their promise in large full color display applications
such as OLEDs TV. In OLEDs field, commercialization
of OLEDs materials and method of manufacturing device
have been the interest of many researchers. Although depo-
sition method in OLEDs has been generally used, it has
problems to involve high cost and large consumption of
material.4ꢁ5 In order to solve problems, OLED is widely
researched by using solution process such as ink-jet print-
ing, nozzle printing, and spin-coating that uses a simple
device structure and consumes less material.6–8 In partic-
ular, solution process has such advantages as larger panel
size, higher throughput and lower cost than evaporation
process, so it has been studied extensively.9
2. EXPERIMENTAL DETAILS
2.1. General Method
Because the OLED is a self-emissive display unlike
LCD with separate light sources and color filters, the light
emitting materials in the OLED determine the whole per-
formance of the device. The red, green and blue emit-
ting materials have been used in a full color OLED. Blue
The UV-Visible (UV-Vis.) absorption spectra were
obtained by using a Lambda 1050 UV/Vis/NIR spec-
trometer (PerkinElmer). A PerkinElmer luminescence
spectrometer LS50 (Xenon flash tube) was used to perform
photoluminescence (PL) spectroscopy. Fast atom bom-
bardment (FAB) mass spectra were recorded by JEOL,
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 10
1533-4880/2015/15/8285/004
doi:10.1166/jnn.2015.11258
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A New Anthracene Derivative Containing t-Butyl Group for Solution Process OLEDs
Lee et al.
(TPBi) with a thickness of 20 nm were deposited by vac-
uum evaporation. LiF (1 nm) and Al films were sequen-
tially deposited on the electron-transporting layer under
vacuum (at a pressure about 10−6 Torr). The current–
voltage (I–V ) characteristics of the fabricated OLED
devices were obtained by Keithley 2400 electrometer.
Light intensity was obtained by Minolta CS-1000A.
2.2. General Syntheses of New Materials
The method of synthesis for TATa in Figure 1 was intro-
duced elsewhere.13 T-TATa was synthesized by Suzuki
aryl-aryl coupling reaction using Pd catalyst. A typical
synthetic procedure was as follows (see Fig. 2).
Figure 1. Chemical structures of TATa and T-TATa.
JMS-AX505WA, HP5890 series II. The optical absorp-
tion spectra were obtained by HP 8453 UV-VIS-NIR spec-
trometer. Perkin Elmer luminescence spectrometer LS50
(Xenon flash tube) was used for photo- and electro-
luminescence (EL) spectroscopy. The redox potentials
of the compounds were determined with cyclic voltam-
metry (CV) using an 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 ace-
tonitrile (AN) with 0.1 M tetrabutylammonium perchlorate
(TBAP) as an electrolyte. Ferrocene was used for poten-
tial calibration and for reversibility criteria. OLED devices
were fabricated as the following structure: ITO/PEDOT
2.3. Synthesis of and 4-(2 or 3-tert-butyl-10-
(3ꢀ,5ꢀ-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-
diphenylaniline (T-TATa)
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.
3. RESULTS AND DISCUSSION
Figure 1 shows chemical structures of the synthesized
materials. In the case of T-TATa, t-butyl group was
included in anthracene core of TATa. The optical and elec-
trochemical properties of the synthesized materials were
summarized in Table I. Figure 3 shows UV-Vis. absorption
and PL spectra of two compounds in solution state and
film state. UV-Vis. absorption in solution and film state
of two compounds shows the typical anthracene absorp-
tion 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 solu-
tion state, UV-Vis. absorption in film state was slightly red
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(40 nm)/Synthesized materials (60 nm)/TPBi (20 nm)/LiF
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(1 nm)/Al (200 nm). A water-dispersed PEDOT/PSS
Copyright: American Scientific Publishers
mixture (Baytron P VP CH8000, Starck GmbH) was
spin-coated on top of indium tin oxide (ITO) in aiꢁr. The
spin-coated films were baked on a hot plate at 110 C for
5 minutes in air and 200 ꢁC for 5 minutes in Ar. The emit-
ting layer was spin-coating to obtain a thickness of 60 nm.
The solution processed films were baked at 80 ꢁC for
30 min on a hot plate in Ar. Electron-transporting layers
Figure 2. Synthetic route of T-TATa.
8286
J. Nanosci. Nanotechnol. 15, 8285–8288, 2015

Lee et al.
A New Anthracene Derivative Containing t-Butyl Group for Solution Process OLEDs
Table I. Optical and electrical properties of synthesized materials.
Solutiona Filmb
UVmax PLmax UVmax PLmax HOMO LUMO Gap
Table II. EL performances of synthesized materials: ITO/PEDOT
(40 nm)/synthesized materials (60 nm)/TPBi (20 nm)/LiF (1 nm)/Al
(200 nm) at 10 mA/cm2.
Band
L.E.a
P. E.b
E.Q.E
(%)
CIE
ELmax
(nm)
Compounds (nm) (nm) (nm) (nm) (eV) (eV) (eV)
(Cd/A)
(lm/W)
(x,y)
TATa
T-TATa
382, 397 456 384, 404 466 −5ꢂ37 −2ꢂ50 2.87
382, 396 456 384, 402 465 −5ꢂ28 −2ꢂ40 2.88
TATa
T-TATa
0.85
1.49
0.56
1.08
0.62
1.81
0.162, 0.225
0.145, 0.156
466
460
Note: aToluene solution ꢃ1×10−5M), bFilm on glass.
Note: aLuminance efficiency; bPower efficiency.
shifted as 404 nm and 402 nm, which indicated an increase
in conjugation length in the solid state. The increase in
conjugation length might be due to the conformation of
solid state ꢀ-stacked compounds being more planar.14 The
PL maximum wavelength of TATa, 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 was measured and calculated by UV-
Vis. absorption of solid state. Band gap obtained from
onset of 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 cyclovoltam-
metry, and lowest unoccupied molecular orbital (LUMO)
level with the use of HOMO level and optical band
gap. HOMO-LUMO values 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. It may be due to the donating effect of
t-butyl group in T-TATa. TGA and DSC were measured to
identify thermal properties of synthesized materials. Glass
transition temperature (Tgꢄ and initial thermal degrꢁadation
temperature (Tdꢄ of the materials ꢁexhibited 139 C and
ꢁ
ꢁ
479 C for TATa, 130 C and 453 C for T-TATa, respec-
tively. Relatively lower temperature value of T-TATa might
come from t-butyl group effect due to packing breakage.
OLED devices were fabricated by using the synthe-
sized material as EML layer, and the EL characteristics
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Figure 4. (a) Luminance efficiency (b) power efficiency of EL devices:
Figure 3. UV-Visible absorption and PL spectra (a) in toluene solution
(1×10−5 M) (b) in film state.
ITO/PEDOT (40 nm)/synthesized materials (60 nm)/TPBi (20 nm)/LiF
(1 nm)/Al (200 nm).
J. Nanosci. Nanotechnol. 15, 8285–8288, 2015
8287

A New Anthracene Derivative Containing t-Butyl Group for Solution Process OLEDs
Lee et al.
by the substitution of t-butyl group in core structure
were examined. In the UV-Vis. absorption and PL spec-
tra of film and solution states, maximum values did not
change significantly before and after t-butyl group was
included.
Solution processed OLED devices were fabricated by
using two materials as emitting layer. Luminance effiency
of T-TATa including t-butyl group was increased almost
two times than TATa. In conclusion, in OLED device using
solution process as emitting material, a compound includ-
ing substitution of t-butyl group in core group showed
higher performance than that of without substitution.
Acknowledgment: This research was supported by a
grant from the Fundamental R&D Program for Core
Technology of Materials funded by the Ministry of
Trade, Industry and Energy, Republic of Korea (Project
No. 10050215).
Figure 5. EL spectra of synthesized materials: TATa (square) and
T-TATa (circle).
were examined with device configuration of ITO/PEDOT
(40 nm)/Synthesized materials (60 nm)/TPBi (20 nm)/LiF
(1 nm)/Al (200 nm). The results were summarized in
Table II.
Solution processed OLED devices were fabricated in
spin-coating method by using the synthesized materi-
als as EML. Figure 4 shows the luminance (cd/A) and
power (lm/W) efficiencies versus current density of TATa
and T-TATa. OLED devices based on TATa or T-TATa
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4. CONCLUSION
A new anthracene derivative that includes t-butyl group
in TATa was synthesized. The electro-optical properties
Received: 17 July 2014. Accepted: 9 December 2014.
8288
J. Nanosci. Nanotechnol. 15, 8285–8288, 2015