Liquid crystals for organic thin-film transistors

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Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm(2) V(-1) s(-1)) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

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### ARTICLE

Received 11 Dec 2014 | Accepted 3 Mar 2015 | Published 10 Apr 2015 DOI: 10.1038/ncomms7828 OPEN
## Liquid crystals for organic thin-film transistors

Hiroaki Iino 1,2, Takayuki Usui 1,2 & Jun-ichi Hanna 1,2

Crystalline thin films of organic semiconductors are a good candidate for field effect

transistor (FET) materials in printed electronics. However, there are currently two main

problems, which are associated with inhomogeneity and poor thermal durability of these

films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal

phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline

material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs

and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films

reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability

of films up to 200 �C. In addition, the mobility of FETs is dramatically enhanced by about one
order of magnitude (over 10 cm 2 V � 1 s � 1 ) after thermal annealing at 120 �C in bottom-gate
bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs

may help overcome upcoming difficulties with novel technologies for printed electronics.

1 Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, J1-2, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. 2 Japan Science
and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST), 4-1-8 Hon-cho, Kawaguchi 332-0012, Japan. Correspondence
and requests for materials should be addressed to J.H. (email: [hanna@isl.titech.ac.jp).](mailto:hanna@isl.titech.ac.jp)

ecent research concerning materials for use in organic field
effect transistors (OFETs), including small molecules and
polymers, has been focused on solution-processed OFETs
# R
with applications in printed electronics. To allow for high
mobility in these OFETs, significant effort has been directed
towards the exploration of new, large p-conjugated molecules.
Unfortunately, there is currently limited knowledge with regard
to the design of molecular alignment in crystal lattices so as to
achieve the desired level of mobility in crystalline thin films. As
for solution-processability, OFET materials are often chemically
modified with long hydrocarbon chains to improve their
solubility in common organic solvents. In this manner, high
mobilities (over 1 cm 2 V � 1 s � 1 ) have been achieved in solutionprocessed OFETs fabricated with polycrystalline thin films of
small molecules, including TIPS-pentacene 1 and 2,7-dialkyl
[1]benzothieno[3,2-b][1]benzothiophene (dialkyl-BTBTs) 2 . Even
higher mobilities of over 10 cm 2 V � 1 s � 1 have been reported in
strained or single-crystal thin films 3–5 . With regard to practical
applications, however, the use of small molecules retains two
deficits. The first is poor uniformity and rough surface
morphology in solution-processed films resulting from
recrystallization of OFET materials during solvent evaporation.
This not only hinders reliable device performance and film
fabrication but also results in wide variations in device
performance. The other challenge is the poor thermal durability
of the films, due to the low melting points brought about by
chemical modification of the molecules with long hydrocarbon
chains. These melting points are often in the vicinity of 100 �C
and thus limit the thermal processes that are used for wiring and
passivation during device fabrication. In practice, it is quite
difficult to achieve both high solubility and good thermal
durability simultaneously, since both are associated with the
p–p interactions between OFET molecules. This represents a
dilemma, due to the trade-off between the two factors.

S C10H21

Ph-BTBT-10

Ph-BTBT-10 (2.25 mg)

Because of this problem, recent research has focused on
p-conjugated main chain polymers 6–8, which favor easy
fabrication of uniform films and high thermal durability. These
include a liquid crystalline polymer of poly(2,5-bis(3alkylthiophene-2-yl)thieno[3,2-b]thiophenes) (PBTTT) 7, donor–
acceptor polymers of diketopyrrolopyrrole-based copolymers 9,
and the recently proposed polymer of poly[4-(4,4-dihexadecyl4H-cyclopenta[1,2-b:5,4-b’]dithiophen-2-yl)-alt-[1,2,5] thiadiazolo

[3,4,-c]pyridine] (PCDTPT) 10 . These polymers do indeed show
promise with respect to the formation of uniform films by wet
processes and good thermal durability, but, to ensure high
mobility, the resulting films must be thermally annealed at high
temperatures in excess of 200 �C. In addition, there are often
difficulties with regard to reproducible synthesis and purification
of the polymers, control of molecular orientations towards both
the substrate surface and the electrodes and crystallinity within
the film.

To solve these problems and take full advantage of small
molecules as an alternative to the polymers described above, we
paid attention to the liquid crystal phase that often appears in
OFET materials chemically modified with long alkyl chains 11–15 .
In particular, our work focused on a highly ordered smectic E
(SmE) mesophase in rod-like liquid crystals. Polycrystalline films
of such OFET materials readily melt into droplets at their
crystallization temperature when the OFET materials show less
ordered liquid crystal phases, such as nematic, SmA 2, or SmC
phases. However, it is expected that such films will not transition
to droplets at that temperature and thus the film shape will be
retained when they show the SmE phase because of its solid-like
nature. Furthermore, the SmE phase exhibits a herringbone
structure in its molecular alignment. Since this structure is
naturally maintained when the material is crystallized from the
SmE phase, the resulting crystals exhibit two-dimensional
conduction that favours high mobility 16–18 .

10 4

10 3

10 2

0 5 10 15 20 25 30
2 _�_ / _�_ (degree)

–1

–2

–3

143.41 °C
–67.68 mJ mg –1

50 100 150 200

Temperature (°C)

2 °C min –1

98.75 °C
59.80 mJ mg –1

Cryst.

SmE

210 °C

SmE

SmA

Liq.

223 °C

SmA

Figure 1 | Basic characteristic of Ph-BTBT-10. (a) Chemical structure, (b) texture as determined by polarized optical microscopy at 150 �C, white bar
indicates a scale of 50 mm in length, (c) XRD pattern of a liquid crystalline film exhibiting a polydomain texture acquired at 160 �C in an out-of-plain
configuration, (d) DSC curves obtained at 2 �C min � 1 and single-crystal structures of the (e) ac and (f) ab planes. Cryst., crystalline; Liq., liquid.

In this paper, we design and synthesize the SmE liquid
crystalline material of 2-decyl-7-phenyl-benzothienobenzothiophene (Ph-BTBT-10, Fig. 1a) for OFETs and describe the phase
transition behaviour, the crystallographic properties and the
fabrication of its crystalline thin films by spin coating and their
application to FET devices. All the results indicate high potential
of Ph-BTBT-10 for FET applications, including solution processability for uniform polycrystalline thin films and high thermal
durability of the films up to 200 �C. In addition, mobility of the
FETs is dramatically enhanced by about one order of magnitude
after thermal annealing at 120 �C and the highest mobility was
13.9 cm 2 V � 1 s � 1 in bottom-gate and bottom-contact FETs. We
conclude the use of SmE liquid crystals in solution-processed
OFETs may help overcome upcoming difficulties with novel
technologies for printed electronics.

Result
Molecular design of Ph-BTBT-10 and its liquid crystallinity.
Based on our new strategy for the molecular design of highly

ordered smectic liquid crystals (unpublished results), Ph-BTBT10 was designed and synthesized: a large p-conjugated fused ring
system of a benzothienobenzothiophene (BTBT) moiety was
selected to achieve ready crystallization; in addition, a phenyl
group was attached to the BTBT molecule to achieve molecular
motion around the long molecular axis, thus providing ‘disorder’
in the resulting molecular system required in the SmE phase; a
long flexible hydrocarbon chain of the decyl group was also
appended to the end of the BTBT moiety to facilitate formation of
the liquid crystal phase. Furthermore, rather than the double side
chains applied in the conventional molecular design of liquid
crystals, we adopted a single side chain to generate possible
asymmetric phase transition behaviour during the cooling and
heating processes, such that we expected that the crystallization
temperature during cooling would be far below the crystallization
temperature associated with heating. As discussed in more detail
further on, it is beneficial to the production of OFETs both to
fabricate a precursor liquid crystalline film at lower temperatures
and to maintain a crystalline film at higher temperatures.

2.8 nm

0 2 4 6 8 10
Distance (μm)

120
100 Substr ate

–20

0 10 20 03 40 50

Distance (μm)

5 45

0.00 (μm) 11.25

0 2 4 6 8 10

(μm)

4.5

3.5

2.5

0 40 80 120 160 –20

Crystal phase SmE

Substrate

0 2 4(μm)6 8 10

0.00 (nm) 11.99

120
100
80
60
40
20
0

25
20 2.6 nm
15
10
5
0

0 2 4 6 8 10
Distance (μm)

Temperature (°C)

0 10 20 30 40 50

Distance (μm)

Figure 2 | Characteristics of polycrystalline thin films of Ph-BTBT-10. The polycrystalline thin film were fabricated from a 0.5 wt% diethylbenzene
solution at B110 �C, (a,d) texture as determined by optical microscopy and (b,e) AFM images. Insets show cross-sectional profiles observed by
(a,d) confocal laser scanning microcopy and (b,e) AFM. (a,b) As-coated polycrystalline film and (d,e) after thermal stress at 150 �C for 5 min. (a,d) White
bars indicate scale of 20 mm in length. (c) Four d-spacings of Ph-BTBT-10 in crystal and SmE phase in wide- and small-angle regions as a function of
temperature.

–0.1

–0.08

–0.06

–0.04

–0.02

Vg=

–100 V

–90 V

–80 V

–70 V

–60 V

–50 V

10 –2

As-coated –100 V
After annealed

–90 V _�_ =14.7 cm 2 V –1 s –1

20 40 60 80 100 120 140

0 –20–40 –60 –80 –100

Source drain voltage (V)

10 –4

10 –6

10 –8

10 –10

Gate voltage (V) Annealing temperature (°C)

Vg=

–100 V

–90 V

–80 V

–70 V

–60 V

–50 V

–0.5

–0.4

–0.3

–0.2

–0.1

Annealed at 120 °C

10 –12

As-coated

_~~�~~_ =2.1 cm 2 V –1 s –1

_V_ DS=–100 V

_L_ =100 μm

_W_ / _L_ =5
_C_ i=10.4 nF cm –2

–100–80 –60 –40 –20 0

0 –20–40 –60 –80 –100

Source drain voltage (V)

0
0 1 2(μm)3 4 5

0.00 (nm) 58.56

2 _�_ =3.4°
_d_ =26 A
As-coated

Annealed 120 °C

2 _�_ =1.7°
_d_ =52 A

20 °C min –1

55 °C
13.37 mJ mg –1
Cooling

1st heating

Heating

100 °C
39.65 mJ mg –1

144 °C
–69.03 mJ mg –1

–5

50
40
30
20
10
0

1 1.5 2 2.5 3 3.5 4 4.5 5 0 50 100 150 200

0 1 2 3 4 5 2 _�_ / _�_ (degree)

5.6 nm
5.7 nm 11.8 nm

Ph-BTBT-10 (2.25 mg)

1 1.5 2 2.5 3 3.5 4 4.5 5

–10

Distance (μm)

2 _�_ / _�_ (degree) Temperature (°C)

Figure 3 | Characteristics of top-contact FET and its change after thermal annealing. Characteristics of top-contact type FETs made using polycrystalline
thin films of Ph-BTBT-10 fabricated from 0.5 wt% p-xylene solution in the SmE phase at ca., 80 �C. Output characteristics of FETs fabricated using the
polycrystalline thin films (a) as-coated and (c) after thermal annealing at 120 �C for 5 min, (b) transfer characteristics of FETs fabricated with polycrystalline
thin films both as-coated and after thermal annealing, (d) FET mobility as a function of annealing temperature, the error bars calculated from the standard
deviations over 10 samples in each annealing temperature, (e) a topographic image of the polycrystalline thin films after annealing at 120 �C, as obtained by
AFM, (f) out-of-plane small-angle XRD patterns for crystalline films as-coated and after annealing, with a schematic illustration of the crystalline structure
and (g) DSC data for Ph-BTBT-10 during fast cooling and heating at 20 �C min � 1 .

7×10 4

6×10 4

5×10 4

4×10 4

3×10 4

2×10 4

1×10 4

7×10 4

6×10 4

5×10 4

4×10 4

3×10 4

2×10 4

1×10 4

–60 V
–70 V
–80 V
–90 V
–100 V

Annealed at 120 °C

As-coated

0 5 10 15 20 25

Depth (nm)

Annealed at 120 °C

0 5 10 15 20 25
Depth (nm)

As-coated

Annealed at 120 °C

0 50 100 150 200 250
Channel length (μm)

–60 V
–70 V
–80 V
–90 V

–100 V

As-coated

1.2 kΩ cm

_V_ ds=–4V

0 50 100 150 200 250
Channel length (μm)

5×10 3

4×10 3

3×10 3

2×10 3

1×10 3

–100 –90 –80 –70 –60

Gate voltage (V)

5×10 3

Annealed at 120 °C

4×10 3

3×10 3

2×10 3

1×10 3

2.1 kΩ cm

_V_ ds=–4V

–100 –90 –80 –70 –60

Gate voltage (V)

Figure 4 | Characteristics of the polycrystalline thin films and contact resistance. Depth profile of sulfur atom (a) before, (e) after thermal annealing
at 120 �C observed by TOF-SIMS. Depth axis was determined by sputtering rate. Polarized optical microscope images of polycrystalline thin films of
Ph-BTBT-10 (b) before and (f) after thermal annealing at 120 �C for 5 min. White bars indicate scale of 20 mm in length. Contact resistance of FETs
fabricated with polycrystalline thin film of Ph-BTBT-10 (c,d) before and (g,h) after thermal annealing at 120 �C for 5 min evaluated by transfer line method.

–0.12

–0.1

–0.08

–0.06

–0.04

–0.02

Vth=–3.0 V

_�_ =11.2 cm 2 V –1 s –1

_V_ DS=–50 V

_L_ =100 μm

_W_ / _L_ =1

_�_ =11.1 cm 2 V –1 s –1 _�_ ave=10.0 cm 2 V –1 s –1

_�_ =0.38

0.015

0.0112

0.0075

0.00375

0 10 –13

10 –3
_I_ on/off=1.9×10 7

10 –5

10 –7

10 –9

_W_ / _L_ =1

_V_ DS=–50V

_L_ =100 μm

_C_ i=10.4 nF cm –2

1st
2nd
3rd
5th

10 –3

10 –4

10 –5

10 –6

10 –7

10 –8

10 –9

10 –10

10 –11

10 –12

_V_ DS=–50 V
_L_ =100 μm

_W_ / _L_ =1

10 –12

10 –3

10 –4

10 –5

10 –6

10 –7

10 –8

10 –9

10 –10

10 –11

Vg=

–50 V

–40 V

–30 V

–20 V

–10 V

Vth=–3.0 V _L_ =100 μm 10th

_�_ ss=–0.78 V dec=11.2 cm 2 V –1 s –1–1 10 –11 _CW_ i=10.4 nF cm/ _L_ =1 –2 10 –11 15th20th

_C_ i=10.4 nF cm –2

0 –10 –20 –30 –40 –50

–50 –40 –30 –20 –10 0 10 10 –12
–50 –40 –30 –20 –10 0 10

10 –12

–50 –40 –30 –20 –10 0

Gate voltage (V)

Source drain voltage (V) Gate voltage VG (V) Gate voltage (V)

Figure 5 | Characteristics of bottom-contact FETs after thermal annealing. Characteristics of bottom-gate, bottom-contact type FETs made with
polycrystalline thin films of Ph-BTBT-10 after thermal annealing at 120 �C. (a) output characteristics and (b–d) transfer characteristics. Transfer
characteristics focused on (c) hysteresis and (d) repeated measurement properties. dec, decade.

Ph-BTBT-10 exhibits two liquid crystal phases at low and high
temperatures: the liquid crystalline phase at the low temperatures
exhibited a typical fan-like texture with disclination lines as
observed by polarized optical microscopy (Fig. 1b); furthermore,
it had three peaks at B18�, 21� and 26� in the X-ray diffraction
(XRD) pattern attributed to the rectangular structure, in addition
to a peak at a small angle of B3� attributed to the smectic layer
structure, as shown in Fig. 1c; on the other hand, the liquid
crystalline phase at the high temperatures exhibited a fan-like
texture without disclination lines as shown in Supplementary
Fig. 1 and only a peak at a small angle of B3� in the XRD pattern.
Thus, these were identified to be the SmE and SmA phases,
respectively. As expected, Ph-BTBT-10 exhibited different phase
transition behaviours, exhibiting the SmE phase at 99 �C during
the cooling process and at 143 �C during the heating process, as
shown in the differential scanning calorimetry (DSC) chart in
Fig. 1d. This allowed us to fabricate a SmE film as a precursor to a
crystalline film at temperatures far below 143 �C (that is, 100 �C
or lower).

Fabrication of polycrystalline thin films. Ph-BTBT-10 polycrystalline thin films were fabricated by spin coating a diethylbenzene solution at the SmE phase temperature of 110 �C and
cooling the SmE films to room temperature. The resulting polycrystalline films were very uniform, which are very similar to
those fabricated from less-ordered liquid crystalline films previously reported 19,20, but contained no cracks and maintained the
crystal state up to 143 �C. The different colors in the film shown
in Fig. 2a are due to interference in the samples consisting of a
Ph-BTBT-10 layer of about 50 nm and silicon dioxide of 300 nm
on Si-substrate, indicating that the film has a small variation in
thickness within a film. The absence of cracks in the
polycrystalline thin films is attributed to similar thermal
expansion coefficients of lattices within the molecular layer in
the SmE and crystal films, which was confirmed by continuous
decrease of three d-spacings corresponding to SmE structure as a
function of temperature structure, in spite of the phase transition
from SmE to crystal phases, as shown in Fig. 2c and
Supplementary Fig. 2. These films are therefore very different
from the cracked polycrystalline thin films previously fabricated
from SmA films of a dialkyl-BTBT derivative 20 . Atomic force
microscope (AFM) image of the polycrystalline thin film shows
wide terrace structures with steps of B2.8 nm, as shown in
Fig. 2b, corresponding to the molecular length along the long
molecular axis of Ph-BTBT-10 as shown in Supplementary Fig. 3.
This indicates that the Ph-BTBT-10 molecules are aligned

perpendicular to the substrate. Thanks to the solid-like SmE
phase similar to crystal phase, the resulting films not only
maintained the film shape but also molecular flatness even after
heating at 150 �C for 5 min, as shown in Fig. 2d,e.

Top contact FETs fabricated with Ph-BTBT-10. Bottom-gate
and top-contact type FETs were fabricated with polycrystalline
thin films of Ph-BTBT-10 on SiO2 (300 nm)/Si-substrates to allow
the characterization of this compound as an OFET material.
These FETs exhibited good p-channel performance, as summarized in Fig. 3a. Figure 3b demonstrates that the FET mobility
under ambient condition was 2.1 cm 2 V � 1 s � 1 . Surprisingly, the
FET transfer characteristics were significantly improved after
brief thermal annealing at 120 �C for 5 min. Following this
annealing, the drain current dramatically increased to 10 � 3 A
and the FET mobility in the saturation region was also enhanced,
reaching a value of 14.7 cm 2 V � 1 s � 1, as shown in Fig. 3b,d.

Transition from monolayer to bilayer structure. The AFM
observations revealed that thermal annealing promotes a change
in the step structure from monolayer steps of 2.8 nm to bilayer
steps of 5.7 nm, as can be seen in the surface profile presented in
Figs 2b and 3e. Furthermore, the XRD patterns exhibit a distinct
change in the peaks at 3.4� and 1.7� before and after thermal
annealing, as shown in Fig. 3f. These variations suggest that the
monolayer structure with a thickness of 26 Å before thermal
annealing was transformed into a bilayer structure with a thickness of 52 Å, as shown in the schematic illustration in Fig. 3f.
Figure 3d summarizes the improvements in FET mobility as a
function of annealing temperature, from which it is evident that
enhancements start at B70 �C and saturate in the vicinity of
120 �C. This variation coincides with an observed exotherm in the
temperature range from 70 to 120 �C before a large endotherm
associated with the transition from the crystal to the SmE phase at
144 �C during the heating process, as shown in Fig. 3g. These
results demonstrate that the structural changes occurring in the
crystal during thermal annealing may be attributed to a crystalto-crystal phase transition that generates the same crystal structure in Ph-BTBT-10 recrystallized from solutions. In fact, a
crystallographic study of a Ph-BTBT-10 single crystal formed
from a toluene solution revealed that each unit cell contains two
molecules oriented head to head at the inter layer, and that the
Ph-BTBT moiety is aligned in a herringbone structure within a
molecular layer, as shown in Fig. 1e,f. Thus, it was concluded that
the observed significant improvements in the FET mobility result
from the structural change of the Ph-BTBT-10 from a monolayer

_�_ =11.7 cm 2 /Vs

12 _�_ =1.17

10%

10 _N_ =49

100

0.1

0.01

0 50 100 150 200

0 2 4 6 8 10 12 14 16
Thermal stress (°C)
Mobility (cm 2 V –1 s –1 )

C10H21

H21C10

Cryst.

SmE

Ph-BTBT-10

SmA

Liq.

10-BTBT-10

C10H21

30
25
20
15
10
5
0

2.8 nm

0 2 4 6 8 10
Distance (μm)

0 2 4 (μm) 6 8 10

0.00 (μm) 44.11

Figure 6 | Reliability of FETs and fabrication of uniform thin films at 40 �C. FET characteristics of bottom-gate, bottom-contact type FETs made with
polycrystalline thin films of Ph-BTBT-10 after thermal annealing at 120 �C. (a) Variations of FET mobility in five films. Thermal durability properties of FET
fabricated with polycrystalline thin films of Ph-BTBT-10. (b) FET mobility as a function of stress temperature. Polycrystalline thin films of Ph-BTBT-10
fabricated from a p-xylene solvent mixture. (c) Optical microcopy texture and (d) AFM image. (c) White bar indicates a scale of 20 mm in length. Cryst.,
crystalline; Liq, liquid.

Table 1 | Variations of FET mobility in five films.

Sample No. l (cm 2 V � 1 s � 1 ) r r/l (%)

A 11.0 0.8 7
B 12.1 1.0 8
C 12.0 1.1 10
D 11.3 1.3 11
E 12.5 1.0 8

m, s and s/m indicate the average mobility, s.d. and ratios of s.d., respectively.

to a bilayer structure. This phenomenon was confirmed by the
compositional depth profile of thin films obtained from time-offlight secondary ion mass spectrometric (TOF-SIMS) studies
before and after thermal annealing as shown in Fig. 4a,d,
respectively, in which the depth profiles of sulfur are well fitted
with 2.5 and 5 nm thick sulfur-containing layers Gaussiandistributed, respectively. On the other hand, the bilayered polycrystalline thin films were retransformed when heated over
143 �C, which is the phase transition temperature from crystal to
SmE phases as shown in Supplementary Fig. 4. This conclusion is
also reinforced by the observation that neither a change in the
10 mm grain size nor a reduction in the contact resistance of
B1 kO cm at the interface with the source/drain electrodes in
FETs was observed after thermal annealing as shown in Fig. 4b,e
and c,f, respectively. With regard to the design of materials
for OFETs, a bilayer structure consisting of mono-alkylated
p-conjugated molecules, representing a quasi-extended large
p-conjugated system, evidently offers a new strategy for achieving
high mobility without sacrificing solubility. This approach is

different from the conventional strategy of extending the
p-conjugated system in existing molecules.

Bottom contact FETs fabricated with Ph-BTBT-10. Based on
the apparent promise of Ph-BTBT-10 as a solution-processed
OFET material, we fabricated OFETs on a SiO2 (300 nm)/Si
substrate in the more practical device configuration of bottomgate and bottom-contact type OFETs, and evaluated the device
performance and reliability obtained from the present film
fabrication process. To ensure good electrical contact with
Ph-BTBT-10 in the bottom-contact configuration, we modified
the Au electrodes used for the source and drain with pentafluorobenzenethiol (PFBT) and obtained a small contact resistance of 156 O cm, which is one order of magnitude smaller than
that of the top-contact type OFETs as shown in Supplementary
Fig. 5. Figure 5a,b present the typical output and transfer
characteristics of bottom-gate and bottom-contact OFETs,
respectively. The mobility obtained after thermal annealing was
estimated to be 11.2 cm 2 V � 1 s � 1 in the saturated region, while
Vth was - 3 V and the subthreshold swing was 0.78V per decade.

Reproducibility and reliability of FET devices. To evaluate
reliability of FET performance, we investigated hysteresis of FET
performance during forward and reverse operation as well as
repeated measurements of the characteristics. FET characteristics
are hardly changed during forward and reverse operation as a
function of gate voltage and repeated measurement as shown in
Fig. 5c,d, respectively. Furthermore, to evaluate reproducibility of
FET performance in both device to device and sample to sample a
total of 49 bottom-gate and bottom-contact transistors on five

substrates were fabricated and the transfer and output
characteristics of each transistor were measured. As shown
in Fig. 6a, the transistors exhibit minimal variations in FET
mobility, with an average mobility of 11.7 ± 1.2 cm 2 V � 1 s � 1
(9.5–13.9 cm 2 V � 1 s � 1 ) for the 49 devices. Table 1 summarizes
the variations in performance of FETs fabricated with five films,
and shows a film-to-film variation of B10%. We attribute this
variation to minor differences in the uniformity and surface
smoothness of the films resulting from the fabrication process. It
should be noted that, even though the present study employed
polycrystalline OFETs, the high mobilities observed here (above
10 cm 2 V � 1 s � 1 ) are comparable to the values normally obtained
from single-crystal OFETs 3,21–24 . Considering that it could
potentially be necessary to fabricate millions of FETs for
display applications, polycrystalline OFETs may have an
advantage over single-crystal OFETs. This advantage arises
because variations in FET performance in single-crystal OFETs
is determined by the quality of each single-crystal film, which in
turn may be greatly affected by the formation of twins and/or
multiple grains, as well as the crystal orientation towards the
source and drain electrodes.

The thermal durability of the present FETs was also evaluated.
Figure 6b summarizes FET mobility values as measured under
ambient conditions after thermal stress at a given temperature for
5 min, as a function of the stress temperature. The FET mobility
values exhibit little change and remain above 10 cm 2 V � 1 s � 1 up
to 140 �C, and a value of 2.6 cm 2 V � 1 s � 1 is obtained even after
thermal stress is applied at 200 �C for 5 min. In contrast, the
mobility in FETs fabricated with polycrystalline thin films of
2,7-didecylbenzothienobenzothiophene (10-BTBT-10) abruptly
decreases after thermal stress is applied above 100 �C, at which
temperature the crystal film was observed to melt into droplets
due to the phase transition to a liquid-like, less-ordered phase of
SmA and the FETs ceased to function 25,26 .

As described above, fabrication of crystalline films from
precursor SmE films is considered to be the key factor accounting
for the minor variations in FET performance found in this work.
Surprisingly, uniform crystalline thin films could be fabricated at
temperatures down to 40 �C below the temperature associated
with the SmE phase, as shown in Fig. 6c,d, when employing spin
coating from a p-xylene:chloroform solution (4:1 v/v) of
Ph-BTBT-10. The exact reason for this phenomenon is not yet
known, but it is possible that the self-organization of liquid
crystalline Ph-BTBT-10 molecules may contribute to the
formation of a uniform film at that temperature, in spite
of the competing recrystallization process during solvent
evaporation.

In conclusion, we successfully fabricate OFETs using polycrystalline thin films obtained from Ph-BTBT-10 as a SmE liquid
crystal. This material is evaluated with regard to its potential for
practical applications in solution-processed OFETs. We demonstrate that the unique nature of the SmE phase not only enable us
to fabricate uniform and molecularly flat films, giving only minor
variations in FET performance between devices and between
samples, but also improves the thermal durability of the films up
to 200 �C, such that small-molecule OFET materials may offer the
same advantages as polymeric OFET materials. The new
molecular design strategy adopted with Ph-BTBT-10 is applicable
to different types of extended p-conjugated system and should
allow tailoring of transistor materials for particular printing
technologies. The discovery of a dramatic enhancement of FET
mobility up to 13.9 cm 2 V � 1 s � 1, resulting from the phase
transition from a monolayer to a bilayer crystal structure in
mono-alkylated liquid crystalline molecules may lead to the
possibility of designing new materials for the burgeoning field of
printed electronics.

Methods
Materials. Ph-BTBT-10 was synthesized by the Suzuki coupling reaction between
2-decyl-7-iodo-[1]benzothieno[3,2-b][1]benzothiophene(I-BTBT-10) and phenyl
borate as shown in Supplementary Fig. 6. The phase transition behaviour of this
material was characterized by DSC (DSC-60, Shimadzu), polarized optical
microscopy (OPTIPHOT2-POL, Nikon and FP82HT hot stage, Mettler Toledo)
and XRD (RAD-2X, Rigaku). Single crystals of Ph-BTBT-10 were prepared by
recrystallization from toluene and analyzed by XRD (R-AXIS RAPID II/R, Rigaku)
using CrystalStructure 4.0 software (Rigaku) to determine the structure.

Fabrication of thin films. Polycrystalline thin films of Ph-BTBT-10 were fabricated by spin coating at a given temperature, for which we preheated the solution, a
glass pipet and a substrate on a thick glass substrate of 1.1 mm for a big heat
capacity in order to make sure the spin coating at that temperature before pipetting
a small amount of the solution onto the substrate and turning on the spin coater.
We carried out this procedure as quickly as we could in B3 s, while the substrate
temperature was monitored by an infrared thermometer calibrated in advance. In
the spin coating at high temperature range of over B100 �C, spin coating was
carried out either in a vacuum oven or under illumination with an infrared lamp.
The temperatures of spin coating described in this paper indicate the initial temperature of substrate before spin coating at 3,000 r.p.m. The spin-coating solution
of Ph-BTBT-10 was 1,4-diethylbenzene or p-xylene and its concentration was B0.5
or 0.75 wt%. The resulting films were evaluated by optical microscopy, confocal
laser scanning microscopy (VK-9700, Keyence), and atomic force microscopy
(AFM, SPA300 DFM in tapping mode, Seiko).

Fabrication and evaluation of FET. To fabricate a bottom-gate top-contact type
FET, gold electrodes were deposited on the polycrystalline thin films on SiO2
(B300 nm)/p þ -Si substrates. The channel length (L)/width (W) were 100/500 mm
or 100/100 mm in normal measurements, 200/1,000 mm, 150/750 mm, 100/500 mm,
70/350 mm, 50/250 mm, 30/150 mm and 20/100 mm in transfer line method. In
bottom-gate bottom-contact type FET, polycrystalline thin film was fabricated by
spin coating on the substrate with source-drain electrodes deposited by vacuum
evaporator in 3 � 10 � 4 Pa on SiO2 (B300 nm)/p þ -Si substrates. The gold source
and drain electrodes were modified with PFBT in a glove box. Gold electrodes were
soaked in 0.1 mol l � 1 PFBT with anhydride ethanol for 20 min and then clean the
substrate by ethanol and acetone.

The FET performance was characterized by two source measurement units
(8,252, ADCMT). The mobility (m) was calculated by plotting the square root of the
source-drain current (Ids) versus gate voltage (Vg) and using the equation from the
saturated region,

Ids¼ðWCi=2LÞm�Vg � Vth�2;

where Ci is capacitance of gate insulator and Vth is threshold voltage. One
substrate, B1.5 � 1.5 mm, has 10 FET devices. s is calculated s.d. in the devices.

Evaluation of thin films by TOS-SIMS. The compositional depth profile of sulfur
atoms in the polycrystalline thin film was studied by TOF-SIMS (ION-TOF
GmbH, TOF-SIMS 5-100-AD); the sample surface was probed by 60 keV Bi3 þ þ

cluster ions and sputtered by Cs þ ions at an energy of 2 keV; the mass spectra were
measured in the negative secondary ion mode. Depth axis was determined by
sputtering rate.

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Acknowledgements
This study was supported in part by JST-CREST and Grants-in-Aid for Scientific
Research for Young Scientists (Start-up 18850008, B21750188 and B24750178) from the
Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors also
wish to thank the Rigaku Co., Ltd. for single-crystal XRD measurements. In addition,
we gratefully acknowledge the Center for Advanced Materials Analysis at the Tokyo
Institute of Technology for in-plane XRD, TOF-SIMS and high resolution mass
spectrometry measurements.

Author contributions
H.I. fabricated thin films, determined the crystal structures, phase transition behaviors,
and FET. characteristics, collected all data and co-wrote this manuscript. T.U.
synthesized and purified Ph-BTBT-10. J.H. directed the overall research project, designed
the molecules and co-wrote this manuscript. Each of the authors discussed the results
and commented on the manuscript.

Additional information
Supplementary Information accompanies this paper at [http://www.nature.com/](http://www.nature.com/naturecommunications)
[naturecommunications](http://www.nature.com/naturecommunications)

Competing financial interests: The authors declare no competing financial interests.

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How to cite this article: Iino, H. et al. Liquid crystals for organic thin-film transistors.
Nat. Commun. 6:6828 doi: 10.1038/ncomms7828 (2015).

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