Benzothiadiazole-Based Small-Molecule Semiconductors for Organic Thin-Film Transistors and Complementary-like Inverters

Article abstract of DOI:10.1002/cplu.201700070

New benzothiadiazole derivatives, 4,7-bis(5-phenylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (PT-BTD) and 4,7-bis[4-(thiophen-2-yl)phenyl]benzo[c][1,2,5]thiadiazole (TP-BTD), were synthesized and characterized as small-molecule organic semiconductors for organic thin-film transistors (OTFTs) and complementary inverters. The thermal, optical, and electrochemical properties of the new compounds were fully characterized. Vacuum-deposition and solution-shearing methods were used to fabricate thin films based on these compounds. Thin films based on PT-BTD exhibited p-channel characteristics with hole mobilities as high as 0.10 cm² V^?1 s^?1 and current on/off ratios >10⁷ for top-contact/bottom-gate OTFT devices. With an optimized blending ratio of PT-BTD and the representative n-channel semiconductor N,N′-1H,1H-perfluorobutyl dicyanoperylenediimide, bulk heterojunction ambipolar transistors were fabricated with balanced hole and electron mobilities of 0.10 and 0.07 cm² V^?1 s^?1, respectively. Furthermore, a complementary-like inverter was fabricated using ambipolar thin-film transistors, which showed a high voltage gain of 84.

Full text of DOI:10.1002/cplu.201700070

Accepted Article
Title: Benzothiadiazole-based Small Molecule Semiconductors for
Organic Thin-Film Transistors and Complementary-like Inverters
Authors: Hyekyoung Kim, M. Rajeshkumar Reddy, Hyungsug Kim,
Donghee Choi, Choongik Kim, and SungYong Seo
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To be cited as: ChemPlusChem 10.1002/cplu.201700070
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Benzothiadiazole-based Small Molecule Semiconductors for
Organic Thin-Film Transistors and Complementary-like
Inverters
Hyekyoung Kima,† M. Rajeshkumar Reddyb,†, Hyungsug Kima, Donghee Choia, Choongik
Kima,* and SungYong Seob,*
a Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 04107,
Korea
b Department of Chemistry, Pukyong National University, Busan 48513, Korea
† These authors contributed equally to this work
*E-mail: choongik@sogang.ac.kr, tel: +82-2-705-7964, fax: +82-2-711-0439
syseo@pknu.ac.kr, tel: +82-51-629-5594, fax: +82-51-629-5583
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Abstract
New
benzothiadiazole
derivatives,
(PT-BTD)
4,7-bis(5-phenylthiophen-2-
4,7-bis(4-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazole
and
yl)phenyl)benzo[c][1,2,5]thiadiazole (TP-BTD), were synthesized and characterized as small
molecular organic semiconductors for organic thin-film transistors (OTFTs) and
complementary inverters. Thermal, optical, and electrochemical properties of the new
compounds were fully characterized. For the fabrication of thin films based on new
compounds, vacuum deposition and solution-shearing method were employed. Thin films
based on PT-BTD exhibited p-channel characteristics with hole mobility as high as 0.10 cm2
V-1 s-1 and current on/off ratio > 107 for top-contact/bottom-gate OTFT devices. With an
optimized blending ratio of PT-BTD and the representative n-channel semiconductor,
PDIFCN2, bulk heterojunction (BHJ) ambipolar transistors were fabricated with balanced
hole and electron mobility of 0.10 and 0.07 cm2 V-1 s-1, respectively. Furthermore, a
complementary-like inverter using ambipolar thin-film transistors was fabricated, which
showed high voltage gain of 84.
1. INTRODUCTION
Organic semiconductors based on π-conjugated small molecules and polymers are one
of the essential components as an active layer to realize low-cost, large-area, and printed
optoelectronics.[ 1 ] Compounds with numerous chemical cores such as pentacene and
oligothiophene have been widely developed and employed for organic thin-film transistors
(OTFTs).[2] Recently, the design strategy of molecular structures with electron-accepting and
electron-donating moieties in one unit has gained considerable interests in organic
semiconducting materials with high electrical performance.[3] Through the donor-acceptor
(D-A) design strategy, the highest occupied molecular orbital (HOMO) and lowest
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unoccupied molecular orbital (LUMO) energies of organic semiconductors can be tuned to
improve charge transport of both holes and electrons through π-conjugated backbones.[4]
Despite recent acheivements, novel solution-processable compounds with high electrical
performance are still in demand for a wider range of applications in the field. In the donor-
acceptor (D-A) structure, carbazole, thiophene and its derivatives are the majority of electron-
donating moieties and electron-accepting moieties such as diketopyrrolopyrrole (DPP),
naphthalenediimide (NDI), perylene diimide (PDI), and benzothiadiazole (BT) are the
representative units reported in the literatures.[5] Among various electron acceptor materials,
benzothiadiazole-based conjugated molecules are regarded as superb acceptors due to their
excellent stability, electron affinity, and planar structure.[6] For instance, Zhang et al. reported
new solution-processable molecules based on benzothiadiazole and dithienocoronenediimide
units and showed ambipolar device characteristics with well-balanced hole and electron
mobilities of 0.018 and 0.019 cm2 V-1 s-1, respectively.[7] In addition, Chen et al. synthesized a
solution-processable benzothiadiazole based small molecules with hole mobility of 0.016 cm2
V-1 s-1, and showed their application in solar cells with power conversion efficiency (PCE) of
1.01 %.[8] Moreover, Zhang et al. synthesized a solution-processable benzothiadiazole-based
small molecule with ambipolar transport characteristics with well-balanced hole and electron
mobilities of 0.01 cm2 V-1 s-1, and showed its application in organic inverter circuits with a
gain of 25.[9] These results clearly show that it is desirable to explore new benzothiadiazole
derivatives with high electrical performance in OTFTs.
For the applications of OTFTs in a wide range to the fullest extent, complementary
metal-oxide semiconductor (CMOS) technology is desirable because it enables the operation
of integrated circuits (ICs) with high robustness, good noise margin, and low power
consumption.[10] Especially, development of high performance inverters, the building blocks
of ICs, is required.[11] Among various strategies to fabricate organic CMOS[12], ambipolar
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OTFTs with bulk heterojunction (BHJ) structure, formed by blending two organic
semiconductors with different polarities, are one of the most promising methods, since they
not only eliminates the need to pattern the p- and n-channel semiconductors in separate
regions, but they are also readily fabricated using a single solution process.[13]
To this end, we have synthesized novel donor-acceptor-donor (D-A-D) conjugated
small molecules based on benzothiadiazole derivatives (Fig. 1). Thermal, electrochemical,
and optical properties for the new compounds were characterized and molecular structure and
HOMO/LUMO energy levels were determined via density functional theory (DFT)
calculations. Furthermore, vacuum-deposition method and solution process (solution-
shearing) were employed for the formation of thin films of new compounds for top-
contact/bottom-gate OTFTs. Thin-film microstructure and morphology were characterized by
atomic force microscopy (AFM) an X-ray diffraction (XRD). Finally, our results revealed
that films of PT-BTD (4) were active as p-channel organic semiconductor with carrier
mobility as high as 0.10 cm2/Vs and current on/off ratio > 107. Additionally, we used PT-
BTD (4) in fabricating solution-processable bulk heterojunction (BHJ) ambipolar transistors
by blending it with a representative n-channel organic semiconductor, PDIFCN2.[14] Finally,
we fabricated CMOS-like inverter with a high gain of 84 from the bulk heterojunction
ambipolar transistors.
Fig. 1. Chemical structure of compounds PT-BTD (4) and TP-BTD (5).
2. RESULTS AND DISCUSSION
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2.1 Synthesis
4,7-Dibromobenzo[c][1,2,5]thiadiazole (1) was purchased from commercial sources
or was synthesized according to reference methods. [15] Both 5-phenylthiophen-2-yl-2-boronic
acid (2)[16] and 4,5,5-tetramethyl-2-(4-(thiophen-2-yl)phenyl)-1,3,2-dioxaborolane (3)[17, 18]
were prepared in two steps as shown in Scheme 1. Organic semiconductors, 4,7-bis(5-
phenylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (4; PT-BTD) and 4,7-bis(4-(thiophen-2-
yl)phenyl)benzo[c][1,2,5]thiadiazole (5; TP-BTD) were synthesized using Suzuki coupling
of compound 1 with compounds 2 and 3, respectively.[19]
Scheme 1. Synthetic route to afford compounds PT-BTD (4) and TP-BTD (5), where (i)
Pd(PPh3)4, Na2CO3, DME; (ii) n-BuLi, B(OiPr)3, THF, -78 oC, then 10% HCl; (iii) Pd(PPh3)4,
Na2CO3, THF; (iv) PdCl2(dppf), KOAc, DMF; (v) Pd(PPh3)4, Na2CO3, Toluene.
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2.2 Theoretical calculation
Theoretical calculation was performed to determine the molecular structure and
HOMO/LUMO energy distribution of benzothiadiazole derivatives by Gaussian 03W
program (B3LYP with 6-31G(d) basis sets). As shown in Fig. 2, the HOMO orbitals of all
compounds are dispersed on the main backbone moieties, while the LUMO orbitals are
localized on the benzothiadiazole moiety. The theoretical HOMO/LUMO energy were
calculated as -5.04/-2.64 eV for PT-BTD (4), and -5.41/-2.47 eV for TP-BTD (5),
respectively. Hence, theoretically calculated HOMO/LUMO energy bandgap was 2.40 eV for
PT-BTD, and 2.94 eV for TP-BTD, respectively. Overall, strong electron-accepting
character of benzothiadiazole moiety and electron-donating character of the substituents are
projected on the HOMO/LUMO energy distribution. Notably, PT-BTD is in fully planar
structure with a dihedral angle of ~0° between each unit. Within adjacent thiophene and
benzothiadiazole units, weak and strong hydrogen bonding from sulfur and nitrogen makes
possible the minimization of dihedral angles in PT-BTD.[20] The planarity of the conjugated
backbone in PT-BTD can possibly improve the intra-/inter-molecular interaction and thus
extend the π-orbital delocalization.[21] On the other hand, TP-BTD is not planar as it showed
the dihedral angle of ~ 21° between benzothiadiazole and phenyl units. This twisted structure
is translated into a much less conjugated skeleton, resulting in poor device performance than
PT-BTD (vide infra).[24, 22]
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Fig. 2. Molecular orbital surfaces of HOMO and LUMO and geometries by DFT calculation
of compounds (a) PT-BTD (4) (left) and (b) TP-BTD (5) (right).
2.3 Optical, thermal, and electrochemical properties
Figure 3 shows the optical absorption spectra of the compounds PT-BTD (λmax = 342,
488 nm) and TP-BTD (λmax = 318, 408 nm) in diluted chloroform solution. PT-BTD
exhibited red-shift (> 24 nm) compared to TP-BTD, showing that phenyl conjugation is not
as effective as thienyl conjugation.[23] Increasing the extent of conjugation leads to red-shifts
of the absorption spectra and decreased HOMO-LUMO energy gap.[24] The HOMO-LUMO
energy gaps calculated from the onset of the experimental optical absorption were 2.20 (PT-
BTD), and 2.46 eV (TP-BTD), respectively.[25]
Thermal properties of new compounds were characterized by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). For new compounds PT-BTD
o
o
and TP-BTD, weight loss (~5%) temperatures were observed above 351 C and 368 C,
respectively (Fig. S1). Furthermore, compounds PT-BTD and TP-BTD exhibited sharp
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o
o
endotherms above 238 C and 244 C, respectively. The sharp exotherms which corresponds
o
o
to the crystallization are observed at 213 C for PT-BTD and 167 C for TP-BTD in the
reverse scans (Fig. S2). This results show that two compounds in this study have high thermal
stability.
Cyclic voltammetry (CV) measurements of the new compounds were performed in o-
C6H4Cl2 solution at room temperature. As shown in Fig. S3, PT-BTD and TP-BTD exhibited
oxidation peaks around +1.20 V and +1.24 V and reduction peaks at -1.16 V, and -1.31 V,
respectively (using Fc/Fc+ as an internal standard at +0.65 V). Hence, HOMO and LUMO
energy levels were obtained by the conversion of oxidation/reduction potential using
ferrocene/ferrocenium as the standard (Table I). The HOMO-LUMO energy gaps of the
compounds PT-BTD and TP-BTD were calculated as 2.36 eV and 2.84 eV, respectively,
showing similar trends to those determined optically.
Table I
Physical and electrochemical properties of the compounds PT-BTD (4) and TP-BTD (5).
UV-Vis
peak
peak
DSC
Tm(°C)
TGA
(°C; 5%)
Ered
(V)b
LUMO Eox
HOMO
(eV)b
Egap
λmax
(eV)b
(V)b
(eV)b
(nm)a
238
244
351
368
342, 488
318, 408
-1.16
-1.31
-2.99
-2.55
1.20
1.24
-5.35
-5.39
2.36
2.84
4
5
a measured by UV-vis spectroscopy. b measured by cyclic voltammetry in o-C6H4Cl2 at 25 °C
(using ferrocene/ferrocenium as intermal standard at +0.65 V for all compounds).
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Fig. 3. Optical spectra of benzothiadiazole derivatives in chloroform solution.
2.4 Thin-film transistor and complementary inverter characterization.
Top-contact/bottom-gate organic thin-film transistors (OTFTs) were fabricated to test
the new benzothiadiazole derivatives via two different methods of solution-shearing and
vacuum deposition. For the optimization of film forming process, various conditions
including solvents (chlorobenzene) with concentration (1 mg/mL), temperature of the
substrate (70 oC), and shearing speed (4 mm/min) were optimized. Table II and SI summarize
the OTFT data and the representative transfer and output plots are shown in Figs. 4 and S4.
Overall, only thin films of PT-BTD (4) exhibited p-channel activity with carrier mobilities as
high as 0.10 cm2 V-1 s-1 and a current on/off ratio > 107. Solution-sheared thin films showed
higher device performance than those formed via vacuum deposition method. For instance,
on PS-brush substrates, solution-sheared devices exhibited carrier mobility as high as 0.10
cm2 V-1 s-1, while vacuum-deposited devices exhibited one-order of magnitude lower
electrical performance (0.010 – 0.058 cm2 V-1 s-1). For vacuum-deposited films, hydrophobic
substrates (PS-coated or OTMS-treated SiO2) afforded higher device performance (0.029 –
0.058 cm2 V-1 s-1) than bare SiO2 substrates (~0.01 cm2 V-1 s-1). All TFT devices based on thin
films of compound 4 exhibited fairly high current on/off ratio over 107 in most devices (Table
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II). Gold electrodes used in this study have work functions better suited for injection of holes
into the HOMO than of electrons into the LUMO for PT-BTD, resulting in p-channel
characteristics of PT-BTD. Furthermore, devices based on PT-BTD measured in air
exhibited slightly lower device performance than those measured under vacuum, probably
due to its insufficiently low HOMO energy level.[26]
Table II
TFT device performance parameters based on thin films of compounds PT-BTD (4) and TP-
BTD (5) employed in this study.a (TD: deposition temperature, µ: carrier mobility, Ion/Ioff:
current on/off ratio, VT; threshold voltage).
Surface
treatment
µ
VT
(V)
Compound
Method
SS
Ion/Ioff
(cm2 V−1s−1)
0.080 0.020
Max = 0.10
PS-brush
Bare
(1.7 0.5) × 106
(1.4 1.1) × 107
(6.7 4.5) × 107
(4.4 1.0) × 108
(2.0 2.0) × 105
(3.2 2.2) × 107
(2.8 2.0) × 108
-18 10
0.0090 0.0010
Max = 0.010
0.028 0.001
Max = 0.029
0.057 0.001
Max = 0.058
0.0010 0.0000
Max = 0.010
0.036 0.002
Max = 0.038
0.042 0.003
Max = 0.045
-25
-38
-46
-12
-29
-22
7
6
5
1
7
9
Vacuum deposition
PS
( TD = 20 oC)
OTMS
Bare
4
Vacuum deposition
PS
( TD = 70 oC)
OTMS
Not active
5
a
Measured under vacuum and exhibited p-channel characteristics. The average values
obtained from 3 different devices.
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Fig. 4. Representative (a) transfer and (b) output characteristics of OTFT devices measured
under vacuum fabricated with thin films of PT-BTD (4) via solution-shearing method
(channel length: 100 ꢀm, channel width:1000 ꢀm).
Furthermore, bulk heterojunction (BHJ) ambipolar transistors were fabricated by
blending PT-BTD with a representative solution processable n-channel semiconductor,
PDIFCN2. PDIFCN2 was chosen as an n-channel semiconductor due to its high electrical
performance, excellent air-stability, and solubility in common organic solvents.[ 27 ] To
optimize blending ratios of PT-BTD and PDIFCN2 on the electrical performance of the
resulting thin film transistors, we have varied the weight ratio of PT-BTD and PDIFCN2
between 2:1 and 1:2 using chlorobenzene (1 mg/mL) as a solvent. Particularly, the ambipolar
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transistors, fabricated with 1:1 blending weight ratio of PT-BTD:PDIFCN2, exhibited
balanced hole and electron mobilities of 0.10 cm2 V-1 s-1 and 0.07 cm2 V-1 s-1, respectively
(Fig. S5 and Table SII). The threshold voltages for hole and electron was -6.12 V and -11.8 V,
respectively. Given the ambipolar characteristics of PT-BTD:PDIFCN2 (1:1 weight ratio)
OTFTs, the bulk heterojuncion structural ambipolar transistors were combined into CMOS-
like inverters (Fig. S6). As shown in Fig. 5, with the input voltage (VIN) and the supply
voltage (VDD) being either positively or negatively biased, inverting functionality was clearly
observed both in positive and negative VIN and VDD. Notably, the BHJ ambipolar transistors
were measured as a complementary-like inverter with very high gain of 84 and 65 at the first
and third quadrants, respectively.
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Fig. 5. Transfer characteristics of CMOS-like inverters using BHJ ambipolar thin-film
transistors (1:1 weight ratio of PT-BTD:PDIFCN2) measured under vacuum; (a) inverter
characteristics and gain at positive VIN and VDD (100 V), (b) inverter characteristics and gain
at negative VIN and VDD (-100 V) (channel length: 100 ꢀm, channel width:1000 ꢀm, bias step
size: 0.1 V).
2.5 Thin-film morphology and microstructure
Thin-film microstructure and surface morphology of solution-sheared and vacuum
deposited films were characterized by wide-angle θ/2θ XRD and AFM. Conventional θ/2θ
XRD scans of thin films were performed to suggest microstructural order in semiconductor
thin films. As shown in Figs. 6 and S7, thin films of PT-BTD (4) formed via solution
shearing and vacuum deposition showed multiple XRD peaks up to sixth-order, suggesting
highly crystalline microstructure of the corresponding thin films. Based on thin films with
highest device performance, as fabricated via solution-shearing and vacuum deposition, the
major primary diffraction peak (001) was observed at 2θ = 4.76°, corresponding to d-spacing
of 18.6 Å (Figs. 6a and S7). The computed molecular length of PT-BTD was found to be
19.3 Å. It indicates that the molecules of PT-BTD in thin films are aligned vertically on the
substrates, tilted only slightly. Highly crystalline thin film microstructure with vertically
aligned molecular packing could be ascribed to the high device performance of OTFT based
on PT-BTD. On the other hand, solution-sheared thin films of TP-BTD (5) did not exhibit
any significant Bragg reflections, indicating poor film texture (Fig. 6b).
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Fig. 6. θ-2θ XRD scans of thin films of (a) PT-BTD (4) and (b) TP-BTD (5) on PS-brush
coated SiO2 substrates, formed via solution-shearing method.
Thin-film surface morphologies based on PT-BTD and TP-BTD were characterized
by AFM, as shown in Figs. 7, S8, and S9. As shown, different film-forming methods afforded
different film morphologies. Thin films of PT-BTD formed by solution shearing method
exhibited microscale rods with evident terraced islands, aligned along the shearing direction
with a RMS (root-mean-square) surface roughness of ~ 27.8 nm for 10.0 ꢀm × 10.0 ꢀm scan
area (Fig. 7). On the other hand, thin films of TP-BTD formed by solution shearing method
exhibited large grains without significant crystalline features along the shearing direction
with a RMS surface roughness of ~ 81.4 nm for 10.0 ꢀm × 10.0 ꢀm scan area (Fig. S8).
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Furthermore, thin films formed by vacuum deposition method exhibited ball-shaped small
grains with a surface roughness of 3.98 – 6.79 nm range for 5.0 ꢀm × 5.0 ꢀm scan area (Fig.
S9). Using solution-shearing process, strained thin films of organic semiconductors can be
formed along the shearing direction, leading to greater electron orbital overlap and high-
performance in electronic devices.[28] As shown in Figs. 7b and S8b, solution-sheared thin
films of PT-BTD exhibited strained long-length crystals parallel to the shearing direction,
while those of TP-BTD did not show uniformly oriented crystals along the shearing direction.
PT-BTD exhibited significantly good electrical performance compared to TP-BTD, most
likely due to the highly crystalline and well-aligned grains in the solution-sheared films.
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Fig. 7. (a) AFM topographic image (10 × 10 ꢀm2) of solution-sheared thin film of PT-BTD
(4) indicating major fiber alignment directions. (b) Microscopy image (100 × 100 ꢀm2) of
solution-sheared thin film. The white arrow represents the shearing direction.
3. CONCLUSION
In conclusion, benzothiadiazole-based small molecular organic semiconductors were
synthesized and characterized. Thin-film transistors were fabricated based on thin films
formed via solution-processing of the developed semiconductors. P-channel device
characteristics with mobility as high as 0.10 cm2 V-1 s-1 was achieved. Furthermore, bulk
heterojunction ambipolar transistors using the mixture of developed compound with
PDIFCN2 at an optimized blending ratio was fabricated and showed balanced hole and
electron mobilities. Complementary-like inverters fabricated from the ambipolar transistors
exhibited high voltage gain of 84.
4. EXPERIMENTAL DETAILS
4.1 General Methods
Air and/or moisture sensitive reactions were carried out under an argon atmosphere
in oven-dried glassware and with anhydrous solvents. All compounds were purchased from
commercial sources unless otherwise noted and used without further purification. 4,7-
Dibromobenzo[c][1,2,5]thiadiazole (1) was purchased from commercial sources or were
synthesized according to reference methods.[15] Solvents were freshly distilled or dried by
passing through an alumina column. General analysis method of the synthesized compounds
was already described in previous work.[16]
4.2 Synthesis
4.2.1. Synthesis of 5-phenylthiophen-2-yl-2-boronic acid (2)
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5-Phenylthiophen-2-yl-2-boronic acid was synthesized employing a published
procedure.[17] 1H NMR (400 MHz, DMSO-d6) δ 8.25 (s, 2H), 7.68-7.64 (m, 3H), 7.52 (d, 3.7
Hz, 1H), 7.41 (t, 8.1 Hz, 2H), 7.32 (t, 7.3 Hz, 1H).
4.2.2. Synthesis of 4,4,5,5-tetramethyl-2-(4-(thiophen-2-yl)phenyl)-1,3,2-dioxaborolane
(3)
4,5,5-Tetramethyl-2-(4-(thiophen-2-yl)phenyl)-1,3,2-dioxaborolane was synthesized
according to the literature with slight modification.
1-Bromo-4-iodobenzene (1.0 g, 3.5 mmol), Pd(PPh3)4 (196.3 mg, 0.17 mmol),
4,4,5,5-tetramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane (741.6 mg, 7.0 mmol), and 2M
Na2CO3 (10 mL, 20 mmol) were dissolved in tetrahydrofuran (THF, 20 mL) at room
temperature, stirred for 10 min, and then refluxed for 12 h under N2 atmosphere. After
cooling down the reaction mixture to room temperature, the mixture was washed with water
and ethyl acetate. The organic layer was collected and concentrated in vacuo. The crude
product was purified by column chromatography (hexane) to afford 2-(4-
bromophenyl)thiophene as pale yellow solid (599.1 mg; 71.0%).[17] 1H NMR (400 MHz,
CDCl3) δ 7.49-7.43(m, 4H), 7.27 (d, 4.4 Hz, 2H), 7.07-7.04 (m, 1H).
2-(4-Bromophenyl)thiophene (600 mg, 2.5 mmol), PdCl2(dppf) (100 mg, 0.13 mmol),
4,4,5,5-tetramethyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane
(637.1 mg, 2.5 mmol), and potassium acetate (738.4 mg, 9.0 mmol) were dissolved in
dimethylformamide (DMF, 20 mL) at room temperature, stirred for 10 min, and then refluxed
for 24 h under N2 atmosphere. After cooling down the reaction mixture to room temperature,
the mixture was washed with water, brine solution, and dichloromethane. The organic layer
was collected and concentrated in vacuo. The crude product was purified by column
chromatography (hexane/ethyl acetate = 10:1) to give compound 3 as yellow solid (549.9 mg;
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76.9%).[18] 1H NMR (400 MHz, CDCl3) δ 7.83 (d, 8.08Hz, 2H), 7.62 (d, 8.44 Hz, 2H), 7.39-
7.36 (m, 1H), 7.30-7.28 (m, 1H), 7.09-7.06 (m, 1H), 1.32 (s, 12H).
4.2.3. Synthesis of 4,7-bis(5-phenylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (4)
Compound 1 (100 mg, 0.34 mmol), compound 2 (166.5 mg, 0.82 mmol), Pd(PPh3)4
(39.2 mg, 0.034 mmol), 2M Na2CO3 (2 mL, 4 mmol), and aliquot (100 µL) were dissolved in
toluene (10 mL) and refluxed for 12 h under N2 atmosphere. After cooling to room
temperature, the mixture was extracted with dichloromethane, dried over anhydrous MgSO4,
and the solvent was removed under reduced pressure. Column chromatography on silica gel
(hexane/ethyl acetate = 10:1) afforded pure 4 as dark red solid (120 mg; 79.0%).[19] 1H NMR
(400 MHz, CDCl3) δ 8.11 (d, 4.04 Hz, 2H), 7.89 (s, 2H), 7.71-7.69 (m, 4H), 7.42-7.38 (m,
13
6H), 7.32-7.28 (m, 2H). C NMR (100 MHz, CDCl3) δ 153.5, 146.6, 139.6, 135.1, 129.7,
129.5, 128.6, 126.8, 126.7, 126.2, 124.9. HRMS-EI (m/z): [M + Na+] calcd. for
C28H18N2S2Na+, 475.0368; found, 475.0373.
4.2.4. Synthesis of 4,7-bis(4-(thiophen-2-yl)phenyl)benzo[c][1,2,5]thiadiazole (5)
Compound 1 (100 mg, 0.34 mmol), compound 3 (233.6 mg, 0.81 mmol), Pd(PPh3)4
(39.2 mg, 0.034 mmol), 2M Na2CO3 (2 mL, 4 mmol), and aliquot (100 µL) were dissolved in
toluene (10 mL) and refluxed for 12 h under N2 atmosphere. After cooling to room
temperature, the mixture was extracted with dichloromethane and dried over anhydrous
MgSO4. The solvent was removed under reduced pressure and the residue was purified by
column chromatography on silica gel (hexane/ethyl acetate = 10:1) to give compound 5 as
yellowish solid (42.5 mg; 50.5%).[19] 1H NMR (400 MHz, CDCl3) δ 8.01-7.98 (m, 4H), 7.82
(s, 2H), 7.79-7.77 (m, 4H), 7.4 (dd, 3.64 Hz, 1.08 Hz, 2H), 7.31 (dd, 5.12 Hz, 1.12 Hz, 2H),
13
7.11 (q, 3.32Hz, 5.12 Hz, 2H). C NMR (100 MHz, CDCl3) δ 154.2, 144.1, 136.5, 134.6,
132.9, 129.7, 128.1, 127.8, 126.2, 125.1, 123.5. HRMS-EI (m/z): [M + Na+] calcd. for
C28H18N2S2Na+, 475.0368; found, 475.0373.
18
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4.3 Theoretical calculation
Density functional theory (DFT) calculations on the present semiconductors were
performed using the B3LYP (Becke’s 3 parameters employing the Lee-Yang-Parr) functional
and the 6-31G(d) basis set as implemented in Gaussian 03W program.
4.4 Device fabrication
Top-contact/bottom-gate organic thin-film transistor and inverter devices were
fabricated using heavily n-doped (100) silicon wafers (resistivity < 0.005 ꢁꢂcm) and a 300
nm thermally grown SiO2 layer as gate electrode and gate dielectric, respectively. After
sonication for 10 min in acetone, the substrates were dried with N2 and cleaned by air plasma
for 5 min (Harrick Plasma, 18W). The general recipes were employed for the treatment of
gate dielectric layers including polystyrene (PS) polymer layer (Mn = 20 kg/mol, Sigma
Aldrich)[29], self-assembled monolayer (octadecyltrimethoxysilane; OTMS)[30] and PS-brush
layer (Mn = 10 kg/mol, Polymer Source)[31] on the Si/SiO2 substrates. Two methods including
solution-shearing (SS)[ 32 ] and vacuum deposition were employed for the formation of
semiconducting layers. For the optimization of solution-shearing process, various solvents
including toluene, chlorobenzene, and 1,2,4-trichlorobenzene with concentration (1 mg/mL)
were employed. After film formation by solution shearing, the substrates were placed in a
vacuum oven at 90 oC overnight to remove the residual solvent. Vacuum deposition (5 × 10-6
torr) was performed on various substrates (bare SiO2, PS, and OTMS) at two different
substrate temperatures (20 oC and 70 oC) with deposition rate of 0.2 Å/s. Film thicknesses
were characterized by profilometer (DEKTAK-XT, Brucker) as 80 – 95 nm (for solution-
sheared films) and 70 – 90 nm (for vacuum-deposited films). Finally, gold source and drain
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electrodes (40 nm) were evaporated through a shadow mask with various channel lengths (L;
50 – 100 ꢀm) and widths (1000 – 2000 ꢀm).
Bulk-heterojunction ambipolar transistors were fabricated on PS-brush-coated Si/SiO2
substrates via drop casting mixture of PT-BTD (4) as p-channel semiconductor and PDIFCN2
(N1100, Polyera) as a n-channel semiconductor. For the optimization of balanced hole and
electron mobilities, blending ratio of PT-BTD:PDIF-CN2 were varied. After film formation,
the substrates were placed in a vacuum oven 90 oC overnight to remove the residual solvent.
Drop-casted film thickness was 85 nm, as characterized by profilometer. Finally, gold source
and drain electrodes (40 nm) were deposited by thermal evaporation through a shadow mask
with channel length (100 ꢀm) and width (1000 ꢀm).
4.5 Characterization
Thermogravimetric analyses (TGA, TA Instrument Q50-1555) were performed on
each sample in a platinum crucible heating from 40 to 700 °C at a heating rate of 20 °C min-1,
under N2 atmosphere. Differential scanning calorimetry (DSC) analyses (TA instrument Q20-
2487) were performed from 40 to 350 °C at a heating rate of 20 °C min-1. The UV-visible
spectra were obtained (in dilute choloroform solution) using JASCO V-530 spectrometer
with quartz cuvette (200 – 800 nm). Cyclic voltammetry (CV) experiments were performed
with a conventional three-electrode configuration (platinum-wire counter electrode, glassy
carbon working electrode, and Ag/AgCl reference electrode) with supporting electrolyte of
tetrabutylammonium tetrafluoroborate in the specified dry solvent on AUT302N
Electrochemical Analyzer (Autolab). All electrochemical potentials were referenced to an
Fc+/Fc internal standard. A Keithley 4200 SCS was used for the characterization of OTFTs
and inverters under vacuum and in air. Carrier mobilities (ꢀ) were determined in the
saturation regime by the standard relationship, ꢀsat = (2IDSL)/[WCi(VG-VT)2], (IDS; source-
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drain current, L; the channel length, W; channel width, Ci; areal capacitance of the gate
dielectric (11.4 nF cm-2), VG; gate voltage, VT; threshold voltage). The reported values are the
average of 3 different devices. The surface morphology and film microstructure of thin films
were characterized by atomic force microscope (AFM, NX10, Park Systems) and X-ray
diffraction (XRD, Miniflex, Rigaku), respectively.
Acknowledgments:
This work was supported by National Research Foundation of Korea grants (NRF-
2015R1D1A3A01019356 and NRF-2016K1A3A1A31905000) funded by the Ministry of
Science, ICT, and future planning, Republic of Korea.
Keywords: organic thin-film transistors; organic semiconductor; benzothiadiazole; self-
assembled monolayer
Table of Contents Text:
New benzothiadiazole-based small-molecule organic semiconductors were synthesized and
employed as active layers in organic thin-film transistors with the highest hole mobility of 0.1
cm2 V-1 s-1. Bulk-heterojunction ambipolar transistors were fabricated by blending the
developed p-channel material with PDIFCN2. Complementary-like inverters fabricated from
the ambipolar transistors exhibited voltage gains as high as 84.
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