Solution-processable small molecules for bulk heterojunction ambipolar thin-film transistors and complementary-like inverters

Article abstract of DOI:10.1016/j.dyepig.2018.12.054

Solution-processable thioalkylated bithiophene derivatives with different lengths of side chains (SBT-n; n = 10, 12, 14, 16, 18) have been synthesized and characterized as p-channel organic semiconductors for thin-film transistors (TFTs). Based on a combi

Full text of DOI:10.1016/j.dyepig.2018.12.054

Accepted Manuscript
Solution-processable small molecules for bulk heterojunction ambipolar thin-film
transistors and complementary-like inverters
Dongil Ho, Sureshraju Vegiraju, Donghee Choi, Chang-Hui Cho, Guhyun Kwon, Po-
Chun Huang, Gene-Hsiang Lee, Taeshik Earmme, Shueh Lin Yau, Ming-Chou Chen,
Choongik Kim
PII:
S0143-7208(18)32176-4
https://doi.org/10.1016/j.dyepig.2018.12.054
DYPI 7259
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 October 2018
Revised Date: 23 December 2018
Accepted Date: 24 December 2018
Please cite this article as: Ho D, Vegiraju S, Choi D, Cho C-H, Kwon G, Huang P-C, Lee G-H,
Earmme T, Yau SL, Chen M-C, Kim C, Solution-processable small molecules for bulk heterojunction
ambipolar thin-film transistors and complementary-like inverters, Dyes and Pigments (2019), doi: https://
doi.org/10.1016/j.dyepig.2018.12.054.
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ACCEPTED MANUSCRIPT
Solution-processable small molecules for bulk heterojunction
ambipolar thin-film transistors and complementary-like inverters
a,1
b,1
a
b
a
Dongil Ho, Sureshraju Vegiraju, Donghee Choi, Chang-Hui Cho, Guhyun Kwon, Po-
b
c
d
b
,b
Chun Huang, Gene-Hsiang Lee, Taeshik Earmme, Shueh Lin Yau, Ming-Chou Chen,*
,
a
and Choongik Kim*
a
D. Ho, D. Choi, G. Kwon, Prof. C. Kim,
Department of Chemical and Biomolecular Engineering, Sogang University, Mapo-gu, Seoul,
4107 Republic of Korea
0
E-mail: choongik@sogang.ac.kr
b
Dr. S. Vegiraju,+ Prof. M.-C. Chen, C.-H. Cho, P.-C. Huang, Prof. S. L. Yau
Department of Chemistry and Research Center of New Generation Light Driven Photovoltaic
Modules,, National Central University, Taoyuan, 32001 Taiwan
E-mail: mcchen@cc.ncu.edu.tw
c
Dr. G.-H. Lee
Instrumentation Center National Taiwan University Taipei 10617, Taiwan
d
Prof. T. Earmme
Department of Chemical Engineering, Hongik University, Mapo-gu, Seoul, 04066 Republic
of Korea.
1
These authors contributed equally to this work.
Keywords: organic thin-film transistors; organic semiconductor; bithiophene; bulk
heterojunction transistors; CMOS inverter
1

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Table of Contents
2

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Abstract
Solution-processable thioalkylated bithiophene derivatives with different lengths of
side chains (SBT-n; n = 10, 12, 14, 16, 18) have been synthesized and characterized as p-
channel organic semiconductors for thin-film transistors (TFTs). Based on a combination of
the highest performing SBT derivative (SBT-14) and quinoidals (DTTRQs) as p- and n-
channel materials, solution-processed small-molecular bulk heterojunction (BHJ) ambipolar
thin-film transistors were fabricated, and the resulting devices showed air-stable and high
2
ambipolar performance with well-balanced electron and hole mobilities as high as 0.70 cm
-1
-1
2
-1
-1
V
s
and 0.21 cm V s , respectively. Furthermore, complementary-like inverters
comprising two ambipolar thin-film transistors were demonstrated, which exhibited a high
voltage gain of up to 81. Our study clearly demonstrated that side chain engineering of small
molecular organic semiconductors had a significant influence on the electrical performance
of TFTs and BHJ transistors, as well as complementary-like inverters.
1
. Introduction
Organic electronics have been developed over the last few decades, with the
advantages of large area processability at low cost for various applications, such as organic
thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), organic photovoltaics
(OPVs), and chemical sensors [1-11]. These developments were made possible through the
design and synthesis of many new π-conjugated polymeric and small molecular organic
semiconductors [12-24]. Among these, solution-processable small molecules with high
performance and ambient stability have attracted interest due to their unique advantages of
versatile molecular structure, simple synthesis, easy purification, high purity, and synthetic
3

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reproducibility [25-30]. Aromatic building blocks with good π-conjugation are used for the
molecular design of these semiconductors as they offer optimal charge transport, and alkyl
chain substitutions are used for solution processability [31-36]. Regarding the conjugated
heterocyclic aromatics, fused thiophenes have drawn interest due to their planar backbone
structure with strong π–π stacking and intermolecular S-S interactions, which enhances the
neighboring molecular orbital overlapping, and the resulting charge transport behavior [37-
5
0].
The study of compounds with ambipolar charge transport is an important part of the
development of organic semiconductors. Since both hole and electron transport is feasible for
ambipolar TFTs, they can be applied in complementary metal-oxide semiconductor (CMOS)-
like inverters for logic circuits [51-58]. Although inverters can be constructed using two
complementary transistors in a CMOS configuration, CMOS-like inverters using ambipolar
materials need only a single ambipolar semiconductor layer. Therefore, there is no need to
pattern separate semiconductors, making it a cost-effective fabrication process for the
implementation of electrical circuits.
Compared with the high hole and electron mobilities achieved for unipolar small
molecular and polymeric organic semiconductors, small molecular ambipolar organic
materials showed rather insufficient performance [59-61]. This is due to the difficulty in
tuning the balanced energy levels with the work function of the metal electrodes, as well as
the device instability in ambient conditions [62-64]. Charge carriers from the electrodes must
be capable of transporting to both the HOMO and LUMO levels of ambipolar materials. For
instance, through the combination of electron donor and acceptor building blocks in a
molecular unit, semiconducting polymers could accomplish proper energy levels with good
ambipolar characteristics [65-72]. Although there have been a few recent reports on
4

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2
-1 -1
ambipolar polymeric semiconductors with carrier mobility > 1.0 cm V s for both hole and
electron [73-76], there are few reports on ambipolar small molecular semiconductors, and
they showed relatively low mobilities. For instance, ambipolar silylethynylated N-
2
-1 -1
heteropentacenes were reported with a hole mobility up to 0.22 cm V s (in ambient air)
2
-1 -1
and electron mobility up to 1.1 cm V s (under a vacuum), respectively [59]. Fluorine-
functionalized asymmetric tetraceno[2,3-b]thiophenes were reported to exhibit maximum
2
-1 -1
hole and electron mobilities of 0.071 and 0.37 cm V s under a nitrogen environment,
respectively [60]. Balanced ambipolar mobilities of pentacene derivatives were reported with
2
-1 -1
hole and electron mobilities of up to 0.51 and 0.46 cm V s under nitrogen atmosphere,
respectively [61].Although there have been a few reports on ambipolar small molecular
semiconductors, the semiconducting films were mostly fabricated via vacuum deposition and
experienced decreased performance in ambient conditions [59-61].
Another promising method to form ambipolar semiconductor films is to blend two
organic semiconductors with different polarities, resulting in a bulk heterojunction (BHJ)
structure [77-81]. The ambipolar organic bulk heterojunction thin-film transistors can simply
be fabricated via a single solution process, which could reduce the fabrication cost of
electrical circuits. Reports on small molecular blend ambipolar organic semiconductors are
also rare, and those that have been published show relatively low mobilities. For instance,
2
-1 -1
ambipolar characteristics with hole and electron mobilities of 0.01 and 0.001 cm V s ,
respectively, were reported by blending naphthalene diimide (NDI) derivatives [82]. An
optimized blend of thieno[3,2-b;4,5-b']dithiophene (BTDT) derivative with n-channel
semiconductor, C , was reported to result in a BHJ ambipolar transistor with balanced carrier
6
0
2
−1 −1
mobilities for holes and electrons of 0.03 and 0.02 cm V s , respectively [40]. A recent
study by Zhu et al. reported ambipolar behavior for a donor (DPTTA) and acceptor
5

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(DTTCNQ) charge transfer complex with hole and electron mobilities of 0.77 and 0.24
2
−1 −1
cm V s , respectively, for single crystal field-effect transistors [83]. However, to the best
of our knowledge, there are no example of solution-processed small molecular blends
demonstrating balanced ambipolar characteristics with both hole and electron mobilities
2
−1 −1
higher than 0.1 cm V s in ambient.
To this end, a bithiophene-based solution processable p-type semiconducting SBT
series was synthesized in this study (Fig. 1). Different length of alkyl chains connected to the
bithiophene moiety was employed to deduce the relationship between the length of the side
chain and the resulting charge transport characteristics. Furthermore, organic thin-film
transistors (OTFTs) of each series were fabricated, and their electrical performance was
investigated. In particular, SBT-14 with tetradecyl thioalkyl chain substituents (-SC H )
14
29
2
-1 -1
exhibited promising p-type characteristics with hole mobility up to 0.30 cm V s , and was
blended with n-type semiconductor material DTTRQs with various alkyl substituents
(DTTRQ (C3), (C6), (C11), and (C15)) to construct bulk heterojunction (BHJ) thin-film
transistors (Fig. 2), which exhibited ambipolar characteristics. Fused thienoquinoids were
selected as n-channel materials for blends, since they have been reported to exhibit high
2
−1 −1
electrical performance with electron mobility as high as 0.90 cm V s in ambient [84].
Furthermore, DTTRQs employed in this study exhibit high solubility enough for solution
process due to the presence of alkyl chains, while solubilities of many quinoidal molecules
are too poor to fabricate devices via solution process due to the strong π–π stacking among
molecules [84]. Through the use of chain-modulated DTTRQs and optimization of the blend
ratio between SBT-14 and DTTRQs, the BHJ transistors showed ambipolar device
2
-1 -1
operations with balanced electron and hole mobilities as high as 0.70 and 0.21 cm V s ,
respectively. To the best of our knowledge, this result is one of the highest balanced
6

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ambipolar characteristics for solution-processed small molecules reported to date. Finally,
ambipolar BHJ transistors based on SBT-14 and DTTRQ (C11) were employed for the
fabrication of complementary-like inverters, and the resulting devices showed a high voltage
gain of up to 81.
SC H
S
n
2n+1
S
S
S
S
S
S
S
H2n+1C S
n
SBT- n
n = 10; 1
1
2; 2
1
1
1
4; 3
6; 4
8; 5
Fig. 1. Chemical structure of the synthesized SBT series employed in this study.
Fig. 2. The device structure of bulk heterojunction ambipolar transistors employed in this
study with molecular structures of SBT-14 and four DTTRQs.
7

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2
. Experimental
2
.1. General Methods
1
13
H and C NMR were recorded using a Bruker 500 or 300 instrument for all
materials, with reference to solvent signals. Mass spectrometric data were obtained with a
JMS-700 HRMS instrument. Differential scanning calorimetry (DSC) was carried out under
-1
nitrogen on a Mettler DSC 822 instrument (scanning rate of 10 °C min ). Thermogravimetric
analysis (TGA) was carried out using a Perkin Elmer TGA-7 thermal analysis system, using
-1
-1
dry nitrogen as a carrier gas at a flow rate of 10 mL min (heating rate of 10 °C min ), and
the reported decomposition temperatures represent the temperature observed at 5 % mass loss.
UV-Vis absorption was carried out in the indicated solvents at room temperature with a
JASCO V-530 spectrometer. Differential pulse voltammetry (DPV) experiments were
performed with a conventional three-electrode configuration (a platinum disk working
electrode, an auxiliary platinum wire electrode, and a non-aqueous Ag reference electrode,
with a supporting electrolyte of 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF ) in
6
the specified dry solvent) using a CHI621C Electrochemical Analyzer (CH Instruments). All
+
electrochemical potentials were referenced to an Fc /Fc internal standard (at 0.6 V).
Crystallographic data (excluding structure factors) for the structure(s) reported in this paper
were deposited with the Cambridge Crystallographic Data Centre as supplementary
publication with the numbers CCDC 1819944 for SBT-10 and CCDC 1819945 for DTTRQ-3.
2
.2. Synthesis
Starting materials (from Aldrich, Arco, or TCI Chemical Co.) were of reagent grade
and used without further purification unless otherwise indicated. Reaction solvents (toluene,
ether, and tetrahydrofuran) were distilled under nitrogen from sodium/benzophenoneketyl,
8

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and halogenated solvents were distilled from CaH . 3-bromothiophene (7), 3,3'-dibromo-2,2'-
2
bithiophene (8), 3,3'-bis(alkylthio)-2,2'-bithiophene (9), 5,5'-dibromo-3,3'-bis(alkylthio)-2,2'-
bithiophene (10), tributyl(dithieno[3,2-b:2',3'-d]thiophen-2-yl)stannane (11), and SBT-14
were synthesized as reported previously [91]. DTTRQ derivatives were synthesized
according to the previous report [84].
2
.3. General Procedure for Stille Cross-Coupling Reactions to Final Targets (SBT-10 – 18)
Under anhydrous conditions, Pd(PPh ) (0.05 equiv) was added to a solution of
3
4
dibrominated compounds (10a-10e; 1 equiv) and mono-stannylated dithienothiophene(11; 2.2
equiv) in dry toluene. The resulting mixture was refluxed overnight under nitrogen. After
removal of solvent, the obtained residue was purified by column chromatography on silica
gel with dichloromethane and hexanes as the eluents. It was then further purified by
recrystallization in dichloromethane.
2
.3.1. Synthesis of SBT-10
The title compound was obtained as a red solid (yield = 67%). Mp: 151 °C. H NMR
1
(
300 MHz, CDCl ): δ 7.41 (s, 2 H), 7.39 (d, J = 5.1 Hz, 2 H), 7.29 (d, J = 5.4 Hz, 2 H), 7.19
3
(s, 2 H), 2.88 (t, J = 7.2 Hz, 4 H), 1.68-1.58 (m, 4 H), 1.41-1.37 (m, 4 H), 1.32-1.21 (m, 24
1
3
H), 0.86-0.81 (m, 6 H). C NMR (125 MHz, CDCl ): δ 141.94, 141.63, 137.12, 136.77,
3
1
2
8
2
32.57, 131.75, 130.91, 129.94, 127.16, 126.41, 120.80, 117.45, 36.44, 31.91, 29.60, 29.54,
9.36, 29.23, 28.78, 22.68, 14.10. HRMS (m/z, FAB+) calcd for C H S 898.1120, and
4
4
50 10
98.1114 was found.
.3.2. Synthesis of SBT-12
1
he title compound was obtained as a red solid (yield = 48%). Mp: 136 °C. H NMR
(
500 MHz, CDCl ): δ 7.41 (s, 2 H), 7.38 (d, J = 5 Hz, 2 H), 7.29 (d, J = 5.5 Hz, 2 H), 7.19 (s,
3
9

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2
H), 2.88 (t, J = 7.5 Hz, 4 H), 1.64-1.59 (m, 4 H), 1.39-1.35 (m, 4 H), 1.32-1.21 (m, 32 H),
13
0
.87-0.84 (m, 6 H). C NMR (125 MHz, CDCl ): δ 141.95, 141.63, 137.14, 136.78, 132.60,
3
1
31.77, 130.92, 129.95, 127.19, 126.41, 120.80, 117.46, 36.44, 31.91, 29.70, 29.65, 29.60,
2
9.53, 29.36, 29.22, 28.76, 22.68, 14.04. HRMS (m/z, FAB+) calcd for C H S 954.1746,
48
58 10
and 954.1749 was found.
2
.3.3. Synthesis of SBT-14
The title compound was obtained as a red solid (yield = 74%). Mp: 141 °C. H NMR
1
(
300 MHz, CDCl ): δ 7.42 (s, 2 H), 7.39 (d, J = 5.4 Hz, 2 H), 7.29 (d, J = 5.4 Hz, 2 H), 7.19
3
(s, 2 H), 2.29 (t, J = 7.5 Hz, 4 H), 1.68-1.58 (m, 4 H), 1.45-1.35 (m, 4 H), 1.32-1.21 (m, 40
1
3
H), 0.86 (t, J = 7.2 Hz, 6 H). C NMR (125 MHz, CDCl ): δ 142.04, 141.72, 137.19, 136.90,
3
1
2
1
2
32.79, 131.92, 131.01, 130.52, 127.32, 126.37, 120.79, 117.49, 36.51, 31.93, 29.70, 29.65,
9.61, 29.60, 29.34, 29.22, 28.79, 22.67, 14.05. HRMS (m/z, FAB+) calcd for C H S
66 10
5
2
011.2372, found 1011.2476.
.3.4. Synthesis of SBT-16
1
The title compound was obtained as a red solid (yield = 43%). Mp: 129 °C. H NMR
(
500 MHz, CDCl ): δ 7.42 (s, 2 H), 7.39 (d, J = 5.0 Hz, 2 H), 7.29 (d, J = 5.0 Hz, 2 H), 7.19
3
(s, 2 H), 2.88 (t, J = 7.5 Hz, 4 H), 1.65-1.61 (m, 4 H), 1.40-1.38 (m, 4 H), 1.32-1.21 (m, 48
1
3
H), 0.88-0.85 (m, 6 H). C NMR (125 MHz, CDCl ): δ 141.95, 141.63, 137.13, 136.78,
3
1
2
1
2
32.60, 131.77, 130.92, 129.95, 127.19, 126.40, 120.79, 117.45, 36.44, 31.93, 29.70, 29.66,
9.60, 29.53, 29.36, 29.21, 28.76, 22.69, 14.11. HRMS (m/z, FAB+) calcd for C H S
74 10
5
6
066.2998, and 1066.2999 was found.
.3.5. Synthesis of SBT-18
1
The title compound was obtained as a red solid (yield = 65%). Mp: 176 °C. H NMR
(
300 MHz, CDCl ): δ 7.42 (s, 2 H), 7.39 (d, J = 5.4 Hz, 2 H), 7.29 (d, J = 5.1 Hz, 2 H), 7.20
3
1
0

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(s, 2 H), 2.88 (t, J = 7.5 Hz, 4 H), 1.68-1.58 (m, 4 H), 1.45-1.35 (m, 4 H), 1.32-1.21 (m, 56
1
3
H), 0.89-0.85 (m, 6 H). This material was not sufficiently soluble to collect C NMR. HRMS
(
m/z, FAB+) calcd for C H S 1122.3624, and 1122.3618 was found.
60 82 10
2
. 4. Device fabrication
The OTFTs and complementary-like inverters were fabricated with the top
contact/bottom gate (TC/BG) structure. The highly n-doped silicon wafers with thermally
-2
grown 300 nm oxide (areal capacitance; C =11.4 nF cm ) were cleaned via sonication in
i
acetone for 10 min and treated by air plasma for 5 min (Harrick plasma, PDC-32G, 18 W).
Organic semiconductor layers (single component or blend) were formed via solution-shearing
-
1
on PS-brush-treated substrates. The PS-brush (M = 1.7 – 28 kg mol ) treatments were
w
implemented following the general recipe [92]. The concentration of organic material
solution, solvent type, substrate temperature, blending ratio, and shearing speed were
optimized. The solution-sheared substrates were annealed in a vacuum at various temperature
for 30 min to remove the residual solvent. The thicknesses of semiconductor layers for a
single SBT layer (34 – 86 nm) and for blend organic films (31 – 60 nm) were measured using
a profilometer (DEKTAK-XT, Brucker). The Au layers (50 nm) with various channel widths
(W; 2000, 1000, and 500 µm) and lengths (L; 100 and 50 µm) were thermally evaporated to
define the source and drain electrodes.
2
.5. Device Characterization
The electrical performances of OTFTs and complementary-like inverters were
characterized in ambient conditions at room temperature with a semiconductor parameter
analyzer (Keithley 4200-SCS) equipped with a probe station. Carrier mobilities (µ) were
1
1

ACCEPTED MANUSCRIPT
determined in the saturation regime by the standard relationship, µ = (2I L)/[WC (V -
sat
DS
i
G
2
V ) ], (I ; source-drain current, L; the channel length, W; channel width, C ; areal
T
DS
i
-2
capacitance of the gate dielectric (11.4 nF cm ), V ; gate voltage, V ; threshold voltage). The
G
T
microstructure and surface morphology of the semiconductor thin-films were measured by
atomic force microscopy (AFM, NX10, Park systems) and wide-angle X-ray diffraction
(XRD, Smartlab, Rigaku).
3
. Results and discussion
3
. 1. Synthesis
The SBT core was prepared in two steps, first the selective deprotonation of 3-
bromothiophene (7) with LDA and then homocoupling in the presence of ZnCl /CuCl to
2
2
afford dibromobithiophene 8. Next, lithium alkylthiolate anion was prepared by treating the
corresponding alkanethiol with n-BuLi, and the nucleophilic substitution of 8 with lithium
alkylthiolate in the presence of CuO/KI produces 9 after facile purification, with an overall
yield >70%. The SBT core was dibrominated using n-bromosuccinimide, then end-capped
with dithienothiophene (DTT), with a Stille cross coupling reaction of 10 with
tributylstannyl-DDT (11) then used to produce the small molecules SBT-10 – 18,
respectively (Scheme 1).
1
2

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Scheme 1. Synthetic routes to the small molecules SBT-10 – 18
3
. 2. Thermal, optical, and electrochemical properties
Thermal properties of SBT derivatives were investigated using thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). TGA revealed that the SBT
compounds 1 – 5 exhibited good thermal stability, with ~5 wt % loss occurring at 319 to
3
52 °C (Fig. S1). Note that the two-connected alkyl chain breaking occurred during the
thermal decomposition for all five SBT derivatives, which can be confirmed from their TGA
weight loss percentages. DSC showed that SBT derivatives melted at above 129 °C (Fig. S2).
The data is summarized in Table 1. The UV-Vis absorption spectra of SBT derivatives in o-
dichlorobenzene are shown in Fig. 3a. The absorption spectra of all compounds in solution
state were almost identical and exhibited the maximum absorption wavelengths (λmax) at 436-
4
39 nm. The absorption maximum of SBT-18 was slightly blue-shifted (~ 10 nm) compared
to other SBTs. The absorption maxima of the UV-Vis absorption spectra for spin-coated
films were slightly red-shifted (Fig. S3) compared to those in solution state, indicating the
enhancement of molecular stacking in the solid state.
1
3

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Table 1
Optical, Electrochemical, and Thermal Properties of SBT Derivatives. ( m
temperature, Td: decomposition temperature; 5%).
T
: melting
b)
Potential [V]
T
T
λmax
HOMO
[eV]
LUMO
[eV]
m
d
Compounds
o
o
a)
c)
c)
[
C]
[ C] [nm]
Eox
Ered
SBT-10
SBT-12
SBT-14
SBT-16
151
136
140
129
134
333
319
352
337
339
437
0.87
0.88
0.87
0.86
0.88
-1.69
-1.71
-1.62
-1.69
-1.72
-5.07
-5.08
-5.07
-5.06
-5.08
-2.51
-2.49
-2.58
-2.51
-2.48
439
436
436
426
SBT-18
a)
b)
in o-C H Cl . by DPV in o-C H Cl at 25 °C, E = Oxidative potential, E = Reductive
6
4
2
6
4
2
ox
red
c)
potential. HOMO = -(4.2+E ); LUMO = -(4.2+E ).
ox
red
The electrochemical properties were investigated by differential pulse voltammetry
+
(
DPVs) in o-dichlorobenzene at 25 ºC with a 0.1 M Bu NPF solution, and Fc/Fc was used
4
6
as an internal standard calibrated at +0.60 eV. The oxidation and reduction potential curves
are shown in Fig. 3b and Fig. S4, respectively, and the data is summarized in Table 1. The
estimated DPV data show that the oxidation and reduction potentials are located at 0.86 –
0
.88 V and -1.62 – -1.72 V, respectively. According to the equation: HOMO/LUMO = -(4.2
Eox/Ered), assuming ferrocene/ferrocenium oxidation at -4.8 eV, the highest occupied
+
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of
SBT derivatives were around -5.06 – -5.08 eV and -2.48 – -2.58 eV, respectively. The
HOMO and LUMO energy levels of SBT derivatives were almost identical, irrespective of
the thioalkyl side chain on the molecular backbone.
1
4

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Fig. 3. (a) UV-Vis spectra and (b) DPV curves of SBT derivatives in o-dichlorobenzene
solution.
3
. 3. Single crystal analyses
Single crystals of SBT-10 were obtained using a slow solvent evaporation method
from solvent mixture (hexane and dichloromethane), and the crystal structures are shown in
Fig. 4 with the results of their analysis in Table S1. As seen in Fig. 4a, all four S atoms in the
SBT unit are in one plane, and the intramolecular distance of S(Thio)··S(R) is ~3.07 Å. This
value is below the sum of the van der Waals radii of two S atoms (~1.8 Å), suggesting that
there are strong S(Thio)··S(R) intramolecular interactions. The alkyl chains are positioned
opposite to each other and the two end-capped DTT fused-thiophene moieties have the
dihedral angles of 7.2° (Fig. 4b) with the SBT unit. The molecular length of SBT-10 was
2
3.8 Å, and the inter planar distance between the SBT molecules was ~ 3.48 Å (Fig. 4c). Figs.
4
d–f show the molecular packing arrangements with slipping angles of 72° and 50°.
1
5

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Fig. 4. Single crystal structures of the SBT-10 (all hydrogen atoms have been omitted for
clarity). Red and black colors indicate sulfur and carbon atoms, respectively. (a) Top view of
two stacking SBT-10 molecules. (b) Front view of two stacking SBT-10 molecules. The
interplanar distance between the SBT-10 layers is ~ 3.48 Å (the alkyl chains are omitted). (c)
Side view of two SBT-10 molecules. (d) Packing orientation of alkyl chains (e
–f) Molecular
packing arrangement of SBT-10 molecules with slipping angles of 72° and 50°.
Single crystalline propyl alkylated DTTRQ-3 was also obtained from a solvent
mixture of hexane and dichloromethane by slow evaporation of the solvent. Note that
DTTRQ-3 with shorter alkyl chains, compared to other DTTRQs employed in this study,
might give insight into the molecular arrangement of the corresponding compounds. As
shown in Fig. 5 and Table S2, the two CN groups were almost coplanar with the DTT main
core, with small torsion angles of 4.5-5.7° and the molecular length was 13 Å (Fig. 5b). The
molecule exhibited a slipped 1D stacking arrangement (Fig. 5d). The DTTRQ molecule was
connected with the neighboring molecules through a “zig-zag” arrangement in the same layer,
with short intermolecular S-N distances of 3.08 Å and 3.05 Å (Fig. 5e). The inter planar
distance between the “zig-zag” layers was 3.40 Å (Fig. 5f).
1
6

ACCEPTED MANUSCRIPT
Fig. 5. Single crystal structure of DTTRQ-3 (all hydrogen atoms have been omitted for
clarity). Red and black colors indicate sulfur and carbon atoms, respectively. (a) Top view of
two DTTRQ-3 stacking molecules, (b) Front view of two stacking DTTRQ-3, (c) Side view;
inter planar distance of 3.4 Å. (d) Slipped 1D stacking, (e) Top view of the zig-zag packing
arrangement of DTTRQ-3 molecules with the intermolecular S-N distances of 3.08 Å and
3
.05 Å, and (f) Front view of the zig-zag packing arrangement of DTTRQ-3 molecules with
interlayer distance of 3.40 Å.
3
. 4. Thin-film transistor characterization.
The thin films of SBT derivatives were characterized as active layers for TFTs and
complementary-like inverters. The OTFTs were fabricated with a top-contact/bottom-gate
TC/BG) structure. The surface of the Si/SiO (300 nm) gate dielectric was treated with a PS
(
2
[
85]
brush. The organic layers were formed by the solution shearing method to make the highly
ordered and anisotropic crystalline structure [86-88]. The gold electrodes were thermally
evaporated on the organic layer with various channel widths and lengths. The series of SBT
materials exhibited p-type characteristics, and their electrical performances are summarized
in Table 2 and Fig. S5. These results clearly show the effects of side chain engineering on
2
−1 −1
the SBT core. Although SBT-14 exhibited decent hole mobility (up to 0.30 cm V s ), other
1
7

ACCEPTED MANUSCRIPT
SBT derivatives with different lengths of alkyl substituents showed much lower device
-4
-2
2
−1 −1
performance, with a hole mobility of 10 – 10 cm V s .
Table 2
a
TFT Device Performance Parameters based on the Thin Films of SBT Derivatives. (µ:
carrier mobility, I /I : current on/off ratio, V ; threshold voltage).
on off
T
max
µ
Surface
treatment
avg
VT
(V)
Compound Method
(µ )
I /I
on off
2
−1 −1
(
cm V s )
.025
0.019±0.003)
.0010
0.0008±0.0002)
.30
0.26±0.03)
.00053
0.00045±0.00005)
.0020
0.0017±0.0003)
0
3
3
2
3
3
SBT-10
SBT-12
(19±7.7) × 10
(54±6.5) × 10
(20±4.8) × 10
(29±5.2) × 10
(56±6.3) × 10
-8±1.2
-16±3.3
-39±5.6
-3±1.7
(
0
(
0
SBT-14
SBT-16
SBT-18
SS
PS-brush
(
0
(
0
-29±4.3
(
a
Measured in ambient and exhibited p-channel characteristics. Maximum and average value among
multiple (>10) device measurements.
The BHJ OTFTs were fabricated based on blended films of SBT-14 (as p-channel
semiconductor), the highest performing SBT derivative in this study, and quinoidal DTTRQs
(as an n-channel semiconductor) in identical solvent [84]. Four different kinds of DTTRQs
with different alkyl side chains were tested to find the effects of side chains of quinoidal
DTTRQs on the BHJ transistor performance. The electrical performances of the BHJ OTFTs
are summarized in Table 3, and the representative transfer and output curves are shown in
Fig. 6 and S6. It can be seen that the BHJ OTFTs based on SBT-14 and DTTRQs showed
ambipolar characteristics that were stable in ambient conditions. The device performance of
the ambipolar transistors was optimized by varying the blend ratio of SBT-14 and DTTRQs
(Tables S3 – S5). The optimized devices based on SBT-14 and DTTRQ (C11) showed air-
stable ambipolar charge transport behaviour with balanced electron and hole mobilities as
1
8

ACCEPTED MANUSCRIPT
2
−1 −1
high as 0.70 and 0.21 cm V s , respectively (Table 3). Note that this result is, to the best of
our knowledge, one of the highest balanced ambipolar characteristics for solution-processed
small molecules reported to date. The ambipolar transistor exhibited diode-like output curves
[
40]
which is due to the presence of both types of carriers in the active channel layer.
The
highly positive threshold voltage for the n-type performance and/or the negative threshold
voltage for the p-type operation imply electron and hole trapping at the phase-separated
interface of the two types of organic materials [89]. The BHJ transistors with other DTTRQs
showed relatively lower device performance, with electron and hole mobilities of 0.021– 0.21
2
−1 −1
and 0.075 – 0.11 cm V s , respectively. For comparison, the other SBT derivatives were
also blended with the four DTTRQs, and the results are summarized in Table S6 with the
representative transfer curves in Figs. S8 – S11. Due to the poor p-channel characteristics
compared with SBT-14, the blended films yielded relatively poor ambipolar performance
(Table S6).
Table 3
Electrical Performance of the Bulk Heterojunction Ambipolar Thin Film Transistors based on
a
Blend Films of SBT-14 and Four Different DTTRQs.
P -channel
N-channel
max
avg
max
Compound
P type)
Compound
(N type)
µh
µe
V
avg
e
−1 −1
V
T
(
T
(
µ
)
I /I
on off
(µ
)
I /I
on off
h
2
−1 −1
(V)
2
(V)
(
cm V s )
(cm V s )
0
.021
0
.075
-35
(12±3.1)
×10
-34
±5.6
(5.9±2.2)
DTTRQ (C3)
0
(0.017±0.00
0
(
(
0.069±0.004) ±4.4
×10
4)
0
.11
0.08±0.02)
.21
0.19±0.02)
-57
±7.1
-29
(10±2.3)
×10
0.21
(0.18±0.03)
0.70
9.2
±1.4
25
(4.4±3.6)
DTTRQ (C6)
DTTRQ (C11)
0
6
(
(
×10
SBT-14
0
(10±3.2)
(5.2±2.9)
0
1
±3.8
×10
(0.66±0.02)
±1.5
×10
0
.055
0
.097
-7.5
(11±5.8)
×10
-7.6
±2.7
(10±4.8)
×10
DTTRQ (C15)
0
(0.052±0.00
0
0.095±0.002) ±2.1
1)
a
Measured in ambient and exhibited ambipolar characteristics. Maximum and average value among
multiple (>10) device measurements.
1
9

ACCEPTED MANUSCRIPT
Fig. 6. (a) N-channel and (b) P–channel transfer characteristics of BHJ ambipolar transistors
based on blends of SBT-14 and DTTRQ (C11). Channel widths and lengths of 1000 µm and
1
00 µm were used, respectively.
The BHJ ambipolar transistors described herein are suitable for the fabrication of
high-performance inverters, the building blocks of integrated circuits. Therefore, we have
fabricated complementary-like inverters employing two identical ambipolar transistors, and
the transfer characteristics of the inverters are shown in Fig. 7. Because of its unique
ambipolar characteristics, the inverter was capable of operating in both positive and negative
V and V . The performance of complementary-like inverters can usually be demonstrated
IN
DD
by the transfer gain, as defined as dVOUT/dV , where V
and V are the output and input
IN
IN
OUT
voltages, respectively. We calculated the transfer gains of the inverter at two different supply
voltages (V ) of -100 V and 100 V. As shown in the results, at negative V and V , the
DD
IN
DD
2
0

ACCEPTED MANUSCRIPT
inverter showed a high maximum transfer gain of 81, while at positive V and V , a gain of
IN
DD
4
0 was obtained.
Fig. 7. Voltage transfer characteristics (VTCs) of complementary-like inverters using two
identical ambipolar thin-film transistors based on SBT-14 and DTTRQ (C11) in the first (a)
and third (b) quadrants with the corresponding voltage gain. The inset shows the circuit
diagram, and the plots of gains (-dVOUT/dV ) are given in red.
IN
Thin-film microstructure and morphology
The film microstructure and surface morphology of the new solution-processed
semiconductor thin films were characterized using wide-angle θ-2θ X-ray diffraction and
atomic force microscopy to evaluate device performance. Conventional θ-2θ XRD scans of
thin films were performed to investigate the microstructural order in the semiconductor thin
films. As shown in Figs. S12a and S12c, thin films of SBT-10 and SBT-14 showed multiple
2
1

ACCEPTED MANUSCRIPT
Bragg reflections with diffraction peaks up to the fifth- and sixth-order, respectively,
suggesting the highly crystalline microstructure of the corresponding thin films. SBT-12 did
not exhibit any significant Bragg reflections, indicating poor film texture (Fig. S12b), while
SBT-16, and SBT-18 showed relatively broad peaks with smaller intensities (Figs. S12d, e).
The film of SBT-10, SBT-12, SBT-14, SBT-16, and SBT-18 exhibited (001) diffraction peaks
at 2θ = 5.58, 4.92, 4.80, 4.34, and 4.1, corresponding to d-spacing of 15.8, 17.9, 18.4, 20.7
and 21.5 angstrom, respectively. As shown in the results, SBTs with a longer side chain
length afforded larger d-spacings. The surface morphologies of SBT derivatives were
characterized by AFM (Fig. S13). Overall, solution-sheared films of SBT derivatives
exhibited microscale terrace-like morphologies along the shearing direction, with scattered
small aggregates. In particular, films of SBT-14 showed a relatively smooth surface with a
root-mean-square (RMS) surface roughness of 0.82 nm, while films of other SBT derivatives
showed an RMS roughness of 1.21 – 7.69 nm (Fig. S13). Note that high film texture and
relatively smooth film morphology with good interconnectivity of SBT-14 could afford better
electrical performance of the corresponding films, compared to other SBT derivatives (vide
supra). It can also be implied that the different lengths of the alkyl chains in the
semiconducting molecule affect the overall film morphology [90].
Wide-angle θ-2θ XRD and AFM were also used to characterize the BHJ blend films
of SBT-14 and DTTRQs. The XRD results are shown in Fig. 8 and S14 – S18. It can be seen
that all the BHJ films of SBT-14 and DTTRQ exhibited two sets of Bragg reflections up to
the sixth peaks. Two sets of reflections for all blend films correspond to the primary peak of
the single component films (SBT-14 and DTTRQs), which indicates that BHJ films are
phase-separated (Fig. 8) [78]. In particular, the primary XRD peaks for each component for
blend films of SBT-14:DTTRQ(C11) were investigated in more detail, as shown in Fig. 8B.
2
2

ACCEPTED MANUSCRIPT
It can be seen that the relative intensity of the primary peak for SBT-14 increased, as the
blending ratio of SBT-14:DTTRQ(C11) changed from 1:2 to 2:1. This microstructural trend
matches well with the corresponding device performance, where BHJ ambipolar transistors
showed more of the p-channel characteristic as the content of SBT-14 increased in the blend
film (vide supra, Table S4).
2
3

ACCEPTED MANUSCRIPT
Fig. 8. Wide-angle θ-2θ X-ray diffraction (XRD) scans for blend films of SBT-14 and (a)
DTTRQ(C6), (b) DTTRQ(C11), and (c) DTTRQ(C15) with different blend ratios,
respectively.
Similarly, the surface morphologies of the organic BHJ layers were measured by
AFM. As shown in Fig. 9, all BHJ films exhibited two distinct phases, a relatively large
2
4

ACCEPTED MANUSCRIPT
terraced region and light regions (as small dots or connected lines). Compared to Figs. 9a and
9
c (SBT-14 with DTTRQ-6 and DTTRQ-15), the light regions in Fig. 9b showed relatively
better connectivity within a two-phase discontinuous network structure, which might indicate
higher/balanced carrier mobilities of the corresponding device (vide supra). Furthermore, the
surface morphologies of SBT-14/DTTRQ-11 blend films were measured as a function of
blend ratio (Figs. 9b and S19, S20). As the SBT-14 component increased from 1:2 (Fig. 9b)
to 1:1 (Fig. S19) and 2:1 (Fig. S20), larger grain sizes were observed with a higher roughness
and worse interconnectivity than seen with the optimized blend ratio (Fig. 9b). This surface
morphology trend matches well with the observed microscopy images (Fig. S21).
2
5

ACCEPTED MANUSCRIPT
Fig. 9. AFM topographic images for BHJ thin films of SBT-14 and (a) DTTRQ(C6), (b)
DTTRQ(C11), and (c) DTTRQ(C15), respectively. The blending ratio of SBT-14 and
DTTRQ(Cn) is 1:2 (Table 3). Scale bar denotes 5 µm and the arrow shows the shearing
direction.
4
. CONCLUSION
In conclusion, solution-processable thioalkylated bithiophene derivatives with
different lengths of side chains have been synthesized and employed as p-channel organic
2
6

ACCEPTED MANUSCRIPT
semiconducting layers for thin-film transistors. The thioalkylated bithiophene derivative with
a proper alkyl chain length showed the highest p-channel characteristics. Bulk heterojunction
ambipolar transistors were fabricated based on an optimized combination of the highest
performing p-channel bithiophene derivative and quinoidal n-channel material, and exhibited
high and air-stable performance with well-balanced electron and hole mobilities as high as
2
-1 -1
0
.70 and 0.21 cm V s , respectively. Furthermore, complementary-like inverters fabricated
from the ambipolar transistors exhibited high voltage gain of 81. Our work reports one of the
highest ambipolar TFT performance based on solution-processable small molecular blends,
and demonstrates that the side chain engineering of small molecular organic semiconductors
has a significant influence on the electrical performance of TFTs and complementary-like
inverters.
Acknowledgements:
This work was supported with grants from the Basic Science Research Program of the
National Research Foundation of Korea (NRF-2017R1A2B4001955) funded by the Ministry
of Science and ICT, Republic of Korea and the support at National Central University was
received from the Ministry of Science and Technology of Taiwan (MOST).
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