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Highly soluble small-molecule organic semiconductor with
trihexylsilyloxy side chain for high-performance organic field-effect
transistors with mobility of up to 3.10 cm2 V1 s1
Bogyu Lim b, 1, Huabin Sun a, 1, Yong-Young Noh a, *
a
b
Department of Energy and Materials Engineering, Dongguk University, 30 Pildong-ro, 1-gil, Jung-gu, Seoul 04620, Republic of Korea
Future Technology Research Center, Corporate R&D, LG Chem Research Park, 188, Moonji-ro, Yuseong-gu, Daejeon 34122, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 February 2017
Received in revised form
11 March 2017
Accepted 11 March 2017
Available online 12 March 2017
A donoreacceptor type small molecule organic semiconductor with a trihexylsilyloxy bulky side chain,
coded LGC-D127, was synthesized, and its electronic, electrochemical, and electrical properties were
investigated for use as the active layer of solution-processable organic field-effect transistors. LGC-D127
consisted of a phenyleneedithiophene moiety with a bulky trihexylsilyloxy side chain as the electrondonating core, diketopyrrolopyrrole as the electron-accepting linker, and octylrhodanine as the
electron-accepting end group. In spite of bulky trihexylsilyloxy side chains, LGC-D127 film was highly
crystalline. The charge-carrier transport properties of the LGC-D127 was investigated through the
fabrication and characterization of field-effect transistor via solution process. LGC-D127 showed significantly high field-effect hole mobility of 3.16 cm2 V1 s1 after thermal annealing due to the large
crystalline nanostructure and the small grain boundaries. In particular, LGC-D127 had good solubility in
the environmentally friendly solvent such as 2-methyltetrahydrofuran due to the bulky trihexylsilyloxy
side chain, and its high hole mobility (max. 3.06 cm2 V1 s1) was sustained from the LGC-D127 solution
in 2-methyltetrahydrofuran.
© 2017 Elsevier Ltd. All rights reserved.
Keywords:
Organic field-effect transistors
Trihexylsilyloxy side chain
Small-molecule semiconductor
Donor-acceptor
Green solvent
1. Introduction
Organic field-effect transistors (OFETs) with solution processes
have been successfully used as active elements and electronic
components such as radio-frequency identification tag, sensor,
flexible display, memory device, and are considered key elements
in large-area fabrication of logic elements on plastic and flexible
substrates [1e8]. One of important requirements for most applications is a high charge-carrier field-effect mobility (mFET). To attain
high performance OFETs, intense research effort has focused on the
development of novel materials, discovery of manufacturing technology, and utilization of new device architectures [7e14]. Recent
studies indicated that OFETs with performance clearly exceeds that
of benchmark amorphous silicon-based device (mFET > 1 cm2 V1
s1) [15], which may approach the mobility requirements to
address organic light-emitting diodes. In particular, solution-
* Corresponding author.
E-mail address: [email protected] (Y.-Y. Noh).
1
Both authors contributed equally to this work.
http://dx.doi.org/10.1016/j.dyepig.2017.03.025
0143-7208/© 2017 Elsevier Ltd. All rights reserved.
processable OTFTs possess advantageous features compared with
their vacuum-deposition counterparts, such as low-cost, lowtemperature processing, and large-area fabrication processes [16].
In addition, vacuum evaporation is unsuitable for industrial
manufacture of OFET backplanes because it is difficult to control the
uniformity of the crystalline organic semiconductors (OSC) on large
size substrates.
Conjugated polymer semiconductors composed of p-electron
donor (D) and p-electron acceptor moieties (A) (called D-A type)
have shown impressive potential for achieving high mFET owing to
their strong intermolecular interactions and excellent backbone
planarity [14,17,18]. Recently, DeA conjugated polymer semiconductors with building blocks such as isoindigo, naphthalenediimide, perylenediimide, and diketopyrrolopyrrole (DPP)
have demonstrated excellent performance with charge carrier
mobilities of over 10 cm2 V1 s1 [19e25]. By contrast, the development of DeA type small-molecular OSC materials for use as the
active layer of OFETs is lagging far behind that of DeA type polymer
semiconductors, despite the several advantages over polymer
semiconductors for less batch-to-batch variation, easy purification,
and functionalization potential [26e28]. Thus, the active
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B. Lim et al. / Dyes and Pigments 142 (2017) 17e23
development of solution-processable D-A small molecules is
essential [29].
As is well known, side chain engineering is important for device
performance as well as main chain engineering [30e32]. In
particular, solubility and molecular packing are greatly affected by
side chain selection such as side chain length and branching point
position. However, a limited number of side chain designs have
been reported. Recently, OFETs with improved performance
incorporating bulky siloxane-terminated linear alkyl chains into
polymer semiconductors due to combination of smaller p-stacking
distances and larger crystalline coherence length were reported
[33,34].
More recently, Bin et al. demonstrated high efficiency polymer
solar cell using novel D-A copolymer with tripropylsilyl substituents on thiophene conjugated side-chain, which possessed a
low-lying highest occupied molecular orbital (HOMO) energy level
and high film absorption extinction coefficients due to s*-p* bond
interaction between silicon and carbon [35].
In this study, we report a new DeA type small molecule, coded
LGC-D127 (as shown in Scheme 1), which features a phenyleneedithiophene moiety with a bulky trihexylsilyloxy side chain as
the electron-donating core, DPP as the electron-accepting linker,
and octylrhodanine as the electron-accepting end group for high-
performance OFETs. This DeA type small-molecule OSC could
effectively induce backbone planarization. In addition, S/O noncovalent intramolecular interaction between sulfur and oxygen in
the phenyleneedithiophene core causes extended conjugation of
the p-orbital, which induces strong pep stacking properties (as
shown in Scheme 1) [36e39]. In particular, LGC-D127 contains a
bulky trihexylsilyloxy side chain, which promotes high solubility in
various organic solvents [40,41]. To the best of our knowledge, this
is the first report on this bulky trihexylsilyloxy side chain in organic
semiconductor materials for OFETs. Despite the bulky trihexylsilyloxy side chain in the core, surprisingly, the LGC-D127 semiconductor film is highly crystalline, accounting for its rather high
mobility (max. 3.16 cm2 V1 s1) after annealing at 140 C, which is
the highest field-effect hole mobility reported in the literature for
DPP-based solution-processable small-molecule organic semiconductors. Furthermore, the bulky trihexylsilyloxy side chain
makes LGC-D127 readily soluble in various eco-friendly solvents,
such as 2-methyltetrahydrofuran (M-THF). The LGC-D127-based
OFETs fabricated using M-THF as the solvent showed impressive
average mobilities of up to 2.64 cm2 V1 s1 (max. 3.06 cm2 V1
s1) after annealing at 140 C. These results show that the bulky
trialkylsilyloxy side chain in organic (or even polymer) semiconductors can be used to prepare organic electronics, showing
Scheme 1. Synthetic route of LGC-D127: (i) (C6H13)3SiCl, Et3N, 4-DMAP, CH2Cl2; (ii) 2-(Tributylstannyl)thiophene, Pd2dba3, PPh3, toluene; (iii) LDA, (CH3)3SnCl, THF; (iv) Pd(pph3)4,
toluene/DMF; (v) 3-octylrhodanine, piperidine, CHCl3.
B. Lim et al. / Dyes and Pigments 142 (2017) 17e23
high crystallinity, sufficient solubility, and possibly further boosting
device performance.
2. Experimental section
2.1. Materials and characterization
Materials were characterized by 1H NMR spectroscopy (Agilent
DD1, 500 MHz). Differential scanning calorimetry (DSC) was performed using a TA Instrument Q20 differential scanning calorimeter at heating rate of 10 C/min in a nitrogen atmosphere. UVevis
spectra were obtained with a Mecasys Optizen Pop spectrophotometer. Cyclic voltammetry (CV) experiments were performed
with an AutoLab analyzer. All CV measurements were carried out in
0.1 M tetrabutylammoniumtetrafluoroborate (Bu4NBF4) in acetonitrile with platinum as the counter electrode, indium tin oxide
(ITO) coated with a thin film as the working electrode, and Ag/Agþ
electrode as the reference electrode, at a scan rate of 100 mV/s. To
estimate the polymer energy levels from the vacuum energy level,
we used the ferrocene/ferrocenium (Fc/Fcþ) redox couple as a
calibration reference. The half-wave potential (E1/2) for oxidation of
the Fc/Fcþ redox couple was assumed to be 4.8 eV, below the vacuum level.
19
owing to the bulky trihexylsilyloxy side chain. Differential scanning
calorimetry (DSC) analysis of LGC-D127 revealed a sharp endothermic peak at 185 C, while an exothermic peak was detected at
148 C, as shown in Fig. 1a and summarized in Table 1. The DSC data
means that LGC-D127 with its bulky trihexylsilyloxy side chain has
crystalline properties.
3.2. Optical and electrochemical properties
Fig. 1b shows the normalized UVevis absorption spectra of LGCD127 in dilute chlorobenzene solution and as a thin solid film; the
corresponding optoelectronic properties, including the absorption
peak wavelengths, absorption edge wavelengths, and optical band
gaps, are summarized in Table 1. Both the absorption spectra from
dilute chlorobenzene solution and the thin film exhibited dualband absorption with a lmax around 430 and 760 nm, which can
be attributed to the localized pep* transition, and an intramolecular charge transfer between the electron-donating and
electron-withdrawing units, respectively. Compared with absorption in dilute solution (lmax ~ 696 nm), the absorption in the spincoated thin film from chlorobenzene was significantly red-shifted
2.2. Film characterization of film
GIWAXS measurements of the thin film microstructure were
performed at the 9A beamlines of the Pohang Accelerator Laboratory (PAL). 11.07 KeV photons with grazing angle 0.13 were
directed onto the sample to produce 2D scattering patterns. The
surface morphology measurements were performed using an
atomic force microscopy (AFM) (NX10, Park systems).
2.3. OFET fabrication and measurement
OFET devices were fabricated in a TG/BC structure. Glass substrates were employed, and a lithographed electrode (Au/Ni ¼ 13
nm/3 nm) was used as the source and drain electrodes. The glass
substrates were sequentially cleaned with acetone, DI water, and
isopropanol, and oven dried at 110 C for 1 h. After drying, the
substrates were treated with UV/Ozone for 30 min and then moved
into a N2 filled glovebox. The pristine organic semiconductor layer
was spin coated onto glass substrate from the solution (3 mg/ml in
CB) at 1500 rpm and annealed at different temperatures for 1 h.
CYTOP was spin coated on to the organic semiconductor as the
dielectric layer (1:1 diluted) at 2000 rpm and annealed at 90 C for
1 h. Al (50 nm) was used for the gate electrode and was thermally
evaporated under vacuum (~106 Torr). Electrical characterization
was measured under nitrogen using a Keithley semiconductor
parametric analyzer (Keithley 4200-SCS). Hole mobility (m) was
determined using Ids ¼ (WCi/2L) m (Vg e Vth)2 in the saturation
regime, where Ci is the capacitance measured from OFETs (Fig. S4),
Ids is the drain-source current, Vg is gate voltage, and Vth is the
threshold voltage.
3. Results and discussion
3.1. Synthesis and characterization
LGC-D127 was obtained via palladium-catalyzed coupling and
Knoevenagel condensation, and then characterized by mass spectroscopy and 1H NMR, with synthetic routes and detailed procedures provided in the Supporting Information. LGC-D127 exhibits
good solubility in common organic solvents, such as dichloromethane, chloroform, chlorobenzene, and 1,2-dichlorobenzene,
Fig. 1. (a) Differential scanning calorimetry (DSC) curve and (b) UVevis absorption
spectra of LGC-D127.
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B. Lim et al. / Dyes and Pigments 142 (2017) 17e23
Table 1
Thermal, optical, and electrochemical properties of LGC-D127.
Material
Tma ( C)
Tcb ( C)
lmax,solc (nm)
lmax,film,asd (nm)
lmax,film,anne (nm)
HOMOf (eV)
LUMO (eV)
Eg,UVg (eV)
Eg,CVh (eV)
LGC-D127
185
148
696
768
777
5.20
3.73
1.38
1.47
a
b
c
d
e
f
g
h
Melting temperature.
Crystallization temperature.
Measurements in chlorobenzene solution.
Measurements in films were spin-casted on the glass before annealing.
Measurements in films were spin-casted on the glass after annealing.
The HOMO and LUMO level were estimated from cyclic voltammetry measurement.
Optical band gap was determined from onset of the absorption in film.
Electrochemical band gap was estimated by cyclic voltammetry measurement.
with a lmax at 768 nm and a vibronic peak at 770 nm. In particular,
the vibronic feature in the absorption spectra of the LGC-D127 film
was greatly increased and slightly red-shifted (~5 nm) after thermal
annealing at 110 C, indicating a higher degree of ordering of LGCD127. From the absorption edge in the film, the optical bandgap of
LGC-D127 was estimated as 1.38 eV.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) were evaluated by electrochemical cyclic voltammetry (CV), as shown in Fig. S1. From the
oxidation and reduction of the LGC-D127, the HOMO and LUMO
levels are 5.20 and 3.73 eV, respectively, and the electrochemical bandgap of LGC-D127 was 1.47 eV, as listed in Table 1.
3.3. Organic field effect transistor performance
To investigate the OFET properties of LGC-D127, top gate/bottom
contact (TG/BC) transistors were fabricated from the LGC-D127
solution in chlorobenzene (CB). The procedures used for OFET
fabrication are provided in the Supporting Information. Fig. 2
shows the typical transfer and output curves of the OFETs based
on LGC-D127 after annealing at 140 C, with their performance
summarized in Table 2 (transfer curves of OFETs with different
annealing conditions are shown in Fig. S2). The transistor parameters, including mFET, threshold voltage (VTH), and the on/off ratio,
were calculated in the saturation regime using the standard OFET
formula (Fig. S3) [42]. LGC-D127-based devices annealed at 100 C
showed an average mobility of 1.32 cm2 V1 s1, showing a significant increase after annealing at higher temperatures (average
mobilities of 1.53 and 1.81 cm2 V1 s1 after annealing at 120 and
140 C, respectively, and the highest mobility of 3.16 cm2 V1 s1 at
140 C). Owing to their small molecular nature, the standard deviation of the mobility value was always large because of domain
size deviation [43,44]. It can be further optimized through a
polymer/small-molecule blend system in future work [45]. In
addition to high mobility, LGC-D127 also shows good stability, with
no significant performance reduction detected after 1-week exposure to air (Fig. S4).
Fig. 2. Typical transfer and output curves of organic field-effect transistors based on
LGC-D127 after annealing at 140 C (drain voltage is 40 V).
3.4. Film morphology analysis
To investigate the surface morphology of LGC-D127, atomic
force microscopy (AFM) was performed on pristine films prepared
with different annealing temperatures. Fig. 3a, b and S5 show the
topography of the LGC-D127 films after annealing at different
temperatures. The film annealed at 100 C contained rod-like grains
with obvious grain boundaries. After annealing at 140 C, however,
the domain of LGC-D127 induced a much larger crystalline nanostructure and the grain boundaries simultaneously become much
smaller, which can explain the mobility trend observed for different
annealing temperatures.
The thin film of LGC-D127 was also studied by grazing incidence
wide angle X-ray scattering (GIWAXS) analysis to investigate the
molecular ordering structures of LGC-D127 and understand the
correlation between the microstructure and transistor performance. Fig. 4a and b shows the GIWAXS images of the LGC-D127
films annealed at 100 and 140 C. The film annealed at 100 C
exhibited intense (h00) diffraction peaks along with second and
third order peaks and a weak (010) reflection along the out-ofplane (qz) direction, and a weak (010) diffraction peak along the
in-plane (qy) direction, indicating that LGC-D127 backbones are
stacking strong edge-on and weak face-on bimodal orientation to
the substrate. The first diffraction peak along the out-of-plane
B. Lim et al. / Dyes and Pigments 142 (2017) 17e23
21
Table 2
Summary of devices incorporating different solvents and annealing conditions.
Material/
solvent
Annealing
temperature ( C)
LGC-D127/CB 100
120
140
LGC-D127/M- 140
THF
a
b
Average mobility
(cm2 V1 s1)a
1.32
1.53
1.81
2.64
±
±
±
±
0.46
0.49
0.52
0.32
Highest mobility
(cm2 V1 s1)
Subthreshold swing
(V Dec1)
2.02
2.51
3.16
3.06
4.30
2.33
3.34
3.64
±
±
±
±
1.73
1.88
0.78
0.17
Threshold
Voltage (V)
On/Off Ratio
(106)
Contact Resistance
(MU)b
8.52
7.36
14.4
4.66
0.32 ± 0.13
0.12 ± 0.10
1.6 ± 1.45
0.89 ± 0.73
0.30
0.33
0.22
0.15
±
±
±
±
0.70
0.58
3.13
1.12
±
±
±
±
0.11
0.06
0.03
0.08
Error in average mobility ¼ standard deviation (s) of values. Averaging was calculated for 4e8 devices.
Contact resistance at channel width of 1000 mm obtained using the Y-function method [47].
Fig. 4. Grazing-incidence wide-angle X-ray scattering of the LGC-D127 films annealed
at (a) 100 and (b) 140 C.
Fig. 3. Atomic force microscopy images of the LGC-D127 films annealed at (a) 100 and
(b) 140 C.
corresponds to a lamellar d-spacing of 15.09 Å. The LGC-D127 film
annealed at 140 C showed slightly decreased diffraction and
reflection peaks along the qz direction, but otherwise more intense
(010) peaks along the qy direction with the other diffraction peaks.
In addition, after being annealed at higher temperature, the LGCD127 film demonstrated a series of well-defined, mixed index
peaks indicative of 3D crystal packing. These results show that the
LGC-D127 film annealed at 140 C has an increased degree of
crystalline order, which could contribute a higher mobility.
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B. Lim et al. / Dyes and Pigments 142 (2017) 17e23
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.03.025.
References
Fig. 5. Typical transfer organic field-effect transistors fabricated from solution of LGCD127 in M-THF (Vsd ¼ 40 V).
3.5. Eco-friendly process
Environmentally friendly manufacturing processes are valuable
for the commercialization of solution-processed OFETs, attracting
much interest to the synthesis of high-performance semiconducting materials that are soluble in environmentally friendly
solvents [46]. Herein, to further expand the performance of LGCD127, we used the eco-friendly M-THF as the solvent for device
fabrication. LGC-D127 was readily soluble in M-THF, as shown in
Fig. S6, and OFETs fabricated using the corresponding M-THF solution showed stable p-type operation, as presented in Fig. 5. Surprisingly, OFETs prepared from the LGC-D127 solution in M-THF
exhibited an average mobility of 2.64 cm2 V1 s1 and the highest
mobility of 3.06 cm2 V1 s1 after thermal annealing at 140 C.
These results show that the bulky trihexylsilyloxy side chain in
OSCs can be used in high-performance organic electronic applications using eco-friendly solvents.
4. Conclusions
In conclusion, we report a new DeA type small-molecule OSC
for use in high-performance OFETs. The highest mFET of 3.16 cm2 V1
s1 and excellent solubility were achieved for LGC-D127 owing to
the strong interaction between the newly introduced trihexylsilyloxy side chains. The AFM and GIWAXS results indicated that
this remarkably high mobility could be attributed to the high
crystallinity and larger crystalline nanostructure with smaller grain
boundaries achieved by thermal annealing. Additionally, LGC-D127
was readily soluble in an environmentally friendly solvent, sustaining its high mobility. Our results demonstrate that a novel side
chain engineering approach can be used to prepare new organic
semiconductors for organic electronics, showing high crystallinity
and sufficient solubility, and possibly further boosting device
performance.
Acknowledgements
This work was supported by the Center for Advanced SoftElectronics (2013M3A6A5073183) funded by the Ministry of Science, ICT & Future Planning and the LG Research Fund. The authors
thank Prof. Dong-Yu Kim and Yunseul Kim for performing GIWAXS
measurements.
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