Physical Properties in Thin Films of a Thienoimide End-capped Compound

Article abstract of DOI:10.1080/15421406.2019.1651072

We have synthesized a new compound of a benzothiadiazole central core coupled to terminal thienoimide (TI) via vinyl linker, namely, C8-TI-BT. We investigated physical properties of vacuum-deposited and spin-coated thin films of C8-TI-BT. Optical properties of C8-TI-BT were different between thin films fabricated by vacuum deposition and spin-coating method. The vacuum-deposited films become more ordered structures by heat treatment. The thin-film transistor (TFT) shows ambipolar semiconducting behavior. We further demonstrated characteristics of electroluminescence (EL) device with the spin-coated C8-TI-BT films.

Full text of DOI:10.1080/15421406.2019.1651072

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Physical Properties in Thin Films of a Thienoimide
End-capped Compound
H. Tachibana, N. Toda & R. Azumi
To cite this article: H. Tachibana, N. Toda & R. Azumi (2019) Physical Properties in Thin Films of
a Thienoimide End-capped Compound, Molecular Crystals and Liquid Crystals, 688:1, 82-88, DOI:
10.1080/15421406.2019.1651072
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
019, VOL. 688, NO. 1, 82–88
2
https://doi.org/10.1080/15421406.2019.1651072
Physical Properties in Thin Films of a Thienoimide
End-capped Compound
H. Tachibana, N. Toda, and R. Azumi
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1,
Tsukuba 305-8565, Japan.
ABSTRACT
KEYWORDS
thienoimide; TFT;
electroluminescence:
heat treatment
We have synthesized a new compound of a benzothiadiazole central
core coupled to terminal thienoimide (TI) via vinyl linker, namely, C8-
TI-BT. We investigated physical properties of vacuum-deposited and
spin-coated thin films of C8-TI-BT. Optical properties of C8-TI-BT
were different between thin films fabricated by vacuum deposition
and spin-coating method. The vacuum-deposited films become more
ordered structures by heat treatment. The thin-film transistor (TFT)
shows ambipolar semiconducting behavior. We further demonstrated
characteristics of electroluminescence (EL) device with the spin-
coated C8-TI-BT films.
1. Introduction
The design and synthesis of high-performance n-type organic semiconductors is one of
the key factors for controlling various properties in electronic devices. Recently, inser-
tion of phthalimide (PI) [1–3] and thienoimide (TI) [4–8] as a terminal acceptor unit
has emerged as a powerful tool to obtain n-type small molecules. We have reported a
photovoltaic performance of PI derivatives as n-type semiconductors [9].
In this work, we synthesized a new compound (C8-TI-BT) with terminal TI units
bridged with central benzothiadiazole (BT) unit through vinyl linker units, and investi-
gated its physical properties for vacuum deposited and spin-coated thin films of C8-TI-
BT. We investigated thermal, optical, electrical, and optoelectronic properties evaluated
by thin-film transistor (TFT) and electroluminescence (EL) devices and discuss the
effect of fabrication methods of C8-TI-BT films.
2. Experimental
2.1. Synthesis
2.1.1. Synthesis of C8-TI-V
To a solution of bromo-thieno[3,2-c]pyrrole-4,6-dione 1 (4.72 g, 13.71 mmol), potassium
vinyltrifluoroborate (2 g, 14.93 mmol), and K CO (6.2 g, 44.86 mmol) in THF/H2O
2
3
mixture (9:1; 100 ml), Pd(PPh ) (0.35 g, 0.30 mmol) was added. This mixture was
3
4
ꢀ
heated to 85 C for 24h. A crude product was purified by chromatography on silica gel
CONTACT H. Tachibana
h-tachibana@aist.go.jp
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
1
(
toluene). H NMR (400 MHz, CDCl ): d 8.37 (d, J ¼ 16.0 Hz, 2H), 7.66 (s, 2H), 7.48 (d,
3
J ¼ 16.0 Hz, 2H), 7.38 (s, 2H), 3.61 (t, 4H), 1.65 (m, 4H), 1.27-1.33 (m, 20H), 0.88
m, 6H)
(
2.1.2. Synthesis of C8-TI-BT
To a solution of (2 g, 7.48 mmol), 1.4-dibromobenzodithiazole (0.73 g, 2.48 mmol), tetra-
butylammonium bromide (1.6 g, 4.96 mmol), and CH COONa (0.61 g, 7.44 mmol) in
3
DMF (50 ml), Pd(OAc) (0.1 g, 13.41 mmol) was added under N atmosphere. This mix-
2
2
ꢀ
ture was heated at 100 C for 48h. A crude product was purified by chromatography on
1
silica gel (CHCl ). H NMR (400 MHz, CDCl ): d 8.37 (d, J ¼ 16.0 Hz, 2H), 7.66 (s, 2H),
3
3
7
0
6
.48 (d, J ¼ 16.0 Hz, 2H), 7.38 (s, 2H), 3.61 (t, 4H), 1.65 (m, 4H), 1.27-1.33 (m, 20H),
.88 (m, 6H). Anal. Calcd. For C H N O S : C, 63.84; H, 5.92; N, 7.84. Found: C,
3
8
42 4 4 3
3.87; H, 5.90; N, 7.77.
2
.2. Film preparation
Solid thin films of C8-TI-BT were prepared by spin-coating chloroform solution and
deposited by vacuum deposition onto glass plates for UV-visible absorption, AFM, and
X-ray diffraction measurements. Heat treatments were carried out by heating the films
for 10 minutes under ambient conditions. After the heat treatment, the thin films were
cooled to room temperature prior to all the measurements.
2
.3. Characterization
UV-visible absorption spectra of C8-TI-BT thin films were recorded using a JASCO V-
80 spectrophotometer. Thermogravimetric (TGA) and differential scanning calorimetry
DSC) analysis was performed by using Seiko Instruments Inc. EXSTAR6000. Electrical
7
(
measurements were performed using Keithley 4200-SCS semiconductor parametric ana-
lyzer in nitrogen atmosphere. X-ray diffraction (XRD) was acquired using Rigaku
SmartLab with a monochromated Cu-Ka radiation. AFM measurements were performed
using a SEIKO SPA300 microscope operating in a noncontact mode using a silicon tip
ꢁ
1
with a resonance frequency of 28 kHz and a spring constant of 1.9 Nm .
Electroluminescence was measured using Keithley 2400 and Topcon BM-9 as luminance
meter and source meter in air, respectively.
2
.4. Device fabrication
For TFT measurement, the C8-TI-BT films were prepared on HMDS-treated substrates
of p-type Si wafer with thermally grown SiO . The drain-source gold electrodes were
2
evaporated through a shadow mask on top of the thin films. The channel length and
the channel width were 20 lm and 2 mm, respectively.
For EL measurement, PEDOT-PSS aqueous solutions were spin-coating on ITO-
coated glass substrate and the C8-TI-BT films were prepared on the ITO/PEDOT:PSS/
glass. Al electrodes were evaporated on the C8-TI-BT films by vacuum deposition. The
2
active area of the EL device was 4 mm .

8
4
H. TACHIBANA ET AL.
Scheme 1. Synthetic route for C8-TI-BT compound.
Figure 1. TGA and DSC thermograms of C8-TI-BT.
3. Results and Discussion
3.1. Synthesis
The synthetic routes for C8-TI-BT compound is shown in Scheme 1. C8-TI-Br was pre-
pared according to methods described previously [6]. C8-TI-BT was synthesized with a
slight modification of Suzuki-Heck coupling reactions [10,11]. The Suzuki reaction of
C8-TI-Br and potassium vinyltrifluoroborate provide the vinyl-containing C8-TI and
the Heck reaction of the resulting vinyl-containing C8-TI with 1,4-dibromobenzodithia-
zole yields C8-TI-BT. The structures of the obtained compounds were confirmed by
NMR analysis.
3
.2. Physical properties
The thermal behaviour of C8-TI-BT is analyzed by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) under N atmosphere as shown in Figure 1.
2
ꢀ
ꢀ
The C8-TI-BT shows a good thermal stability with 5% weight loss at 355 C. Heating
DSC scans of C8-TI-BT are characterized by two endothermal peaks at 100 C (4.2 J/g)

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
85
Figure 2. UV-visible absorption spectra for solution (solid line), spin-coated films (dotted line), and
vacuum deposited films (dot-dashed line) of C8-TI-BT.
ꢀ
and 220 C (50.9 J/g). Analysis of the temperature behavior of C8-TI-BT exhibit that
the first transitions corresponds to crystal–crystal transformation due to alkyl chains,
while the second one corresponds to the sample isotropization.
To compare the optical properties of vacuum-deposited and spin-coated films of C8-
TI-BT, we measured UV-visible absorption spectra. Figure 2 shows UV-visible absorp-
tion spectra for both thin films. For comparison, an absorption spectrum in chloroform
solution of C8-TI-BT is also plotted by solid lines. Although the absorption edge is
almost the same for both vacuum-deposited and spin-coated films, the absorption peak
and spectral profile are different. C8-TI-BT in solution has an absorption peak at
474 nm with a shoulder at 497 nm. C8-TI-BT for vacuum deposited films has a blue-
shifted absorption peak at 435 nm with respect to that in solution. In contrast, C8-TI-
BT shows a red-shifted absorption peak at 550 nm for spin-coated films. This is prob-
ably due to the difference in the molecular orientation of C8-TI-BT between the vac-
uum-deposited and spin-coated films. We investigated the change in UV-visible
absorption spectra after heating thin films of C8-TI-BT at various temperatures for
1
0 min. The absorption peak and spectral profile of both vacuumed-deposited and spin-
ꢀ
coated films after heating at 120 C do not almost change compared with that
before heating.
3
.3. Vacuum deposition
Thin films of C8-TI-BT were deposited by vacuum deposition and used as active layer
of TFT. We investigated the effect of heat treatment on the TFT device for C8-TI-BT
thin films. Although TFT performance is not observed in the device before heating,
ꢀ
the device after heating at 120 C for 10 min shows ambipolar TFT characteristics.
Figure 3(a) and 3(b) show transfer characteristics of C8-TI-BT measured at positive and
ꢀ
negative source-drain bias after heat treatment at 120 C, respectively. The device shows
ꢁ
6
2
ꢁ1 ꢁ1
a n-type field-effect response with a mobility of 1.7 x10 cm V s . In contrast, the
hole mobility was 1 order of magnitude lower than the electron mobility.

8
6
H. TACHIBANA ET AL.
Figure 3. Transfer curves of a TFT device for vacuum deposited films of C8-TI-BT after heating
ꢀ
at 120 C.
Figure 4. AFM images for vacuum deposited films of C8-TI-BT (a) before heating and (b) after heating
ꢀ
at 120 C for 10 min.
The morphological changes before and after heating were observed by AFM. Figure 4
ꢀ
shows the AFM images of the C8-TI-BT thin film before and after heating at 120 C for
10min. In the AFM image of before heating, the thin film surface is smooth with small
domains. In contrast, the larger rounded and well-defined terraced grains are formed by
heat treatment, which would facilitate the charge carrier transport. These behaviors are also
supported by XRD measurements. Figure 5 shows the XRD patterns of C8-TI-BT films
ꢀ
before and after heating at 120 C. In the XRD patterns of the films before hating, the C8-
ꢀ
TI-BT have no peaks other than broad peak at around 6 due to glass substrate. On the other
ꢀ
hand, XRD pattern of the C8-TI-BT films after heating at 120 C exhibits higher order peri-
odicity of 3.08nm, suggesting an almost perpendicular arrangement of the molecules respect
to the substrate.
3
.4. Solution deposition
Figure 6 shows the EL spectrum of the spin-coated C8-TI-BT film. For comparison, the
photoluminescence (PL) is also plotted by dotted lines. The EL spectrum of C8-TI-BT

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
87
Figure 5. XRD patterns for vacuum deposited films of C8-TI-BT before heating (solid line) and after
ꢀ
heating at 120 C for 10 min (dotted line).
Figure 6. EL (solid line) and PL (dotted line) spectra for spin-coated films of C8-TI-BT.
Figure 7. (a) XRD patterns, AFM images, and (b) transfer curves for spin-coated films of C8-TI-BT after
ꢀ
heating at 120 C.

8
8
H. TACHIBANA ET AL.
was identical with that of PL in the films with the main EL and PL peaks at 600 nm,
independence of heat treatment. The structures in C8-TI-BT films after heating at
ꢀ
1
20 C were investigated by XRD and AFM measurements. As shown in Figure 7(a),
the films exhibit small grains with lateral sized lower than 1 lm and shows the peak at
ꢀ
6
.36 with the interlayer distance of 1.39 nm. The XRD patterns were identical with that
of C8-TI-BT films before heating. The hole and electron mobility were the same order
of magnitude as that for vacuum-deposited films (Figure 7(b)).
4
. Conclusions
A new compound of a benzothiadiazole central core coupled to terminal thienoimide
TI) via vinyl linker, namely, C8-TI-BT was synthesized. We investigated physical prop-
(
erties of vacuum-deposited and spin-coated thin films of C8-TI-BT. Optical properties
of C8-TI-BT were different between thin films fabricated by vacuum deposition and
spin-coating method. The vacuum-deposited films become more ordered structures after
ꢀ
heating at 120 C. The thin-film transistor (TFT) shows ambipolar semiconducting
behavior. We further investigated characteristics of electroluminescence (EL) device
with the spin-coated C8-TI-BT films, in which the EL spectrum shows a peak
at 600 nm.
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