High performance ambipolar organic field-effect transistors based on indigo derivatives

Article abstract of DOI:10.1039/c4tc01563k

A bio-inspired organic semiconductor 5,5′-diphenylindigo shows excellent and well-balanced ambipolar transistor properties; its hole and electron mobilities are 0.56 and 0.95 cm² V^-1 s^-1, respectively. The enhanced performance is attributed to the extended π-π overlap of the phenyl groups as well as the characteristic packing pattern that is a hybrid of the herringbone and brickwork structures. The ambipolar transistor characteristics are analyzed considering its operating regions, where a large unipolar saturated region appears due to the difference of the electron and hole threshold voltages.

Full text of DOI:10.1039/c4tc01563k

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DOI: 10.1039/C4TC01563K
Journal Name
RSCPublishing
ARTICLE
High Performance Ambipolar Organic Field-Effect
Transistors Based on Indigo Derivatives
Cite this: DOI: 10.1039/x0xx00000x
a
Oratai Pitayatanakul,* Toshiki Higashino, Tomofumi Kadoya, Masaki
a
a
a
Tanaka, Hirotaka Kojima, Minoru Ashizawa, Tadashi Kawamoto,
a
a
a
a
Hidetoshi Matsumoto, Ken Ishikawa, and Takehiko Mori*
a
ab
Received 00th January 2012,
Accepted 00th January 2012
A bio-inspired organic semiconductor 5,5’-diphenylindigo shows excellent and well-balanced
DOI: 10.1039/x0xx00000x
2
–1 –1
ambipolar transistor properties; the hole and electron mobilities are 0.56 and 0.95 cm V s ,
respectively. The enhanced performance is attributed to the extended π-π overlap of the phenyl
groups as well as the characteristic packing pattern that is a hybrid of the herringbone and
brickwork structures. The ambipolar transistor characteristics are analyzed considering
operation regions, where a large unipolar saturated region appears due to the difference of the
electron and hole threshold voltages.
www.rsc.org/
Introduction
Recently, ambipolar operation has been reported in indigo
2
3
derivatives (Scheme 1). Indigo is a plant-origin dye produced
Development of ambipolar organic semiconductors capable from Indigofera tinctoria and Isatis tintora, which have been
2
4
of both hole and electron transports has essential importance in cultivated for at least four thousand years for coloring textiles.
1,2
designing electronic and optoelectronic organic devices.
Indigo shows a very high melting point (390~392°C) and an
Recently, high-performance ambipolar transistors have been extremely low solubility in common organic solvents,
1
-6
realized in donor-acceptor polymers; examples are a polymer explained by stabilization from the inter- and intramolecular
consisting of thiophene and diketopyrrolopyrole (DPP), hydrogen bonds. Intermolecular interactions of the
-skeletons
showing the hole and electron field-effect mobilities of ꢀh ꢀe
also strongly influence the charge transport in indigo thin films.
.9/0.1 cm V s , and a polymer of thienothiophene and DPP Indigo shows ambipolar transport with well-balanced mobilities
π
/
=
2
–1 –1 7
0
2
–1 –1
8
2
–1 –1
(
ꢀh
/
ꢀe = 1.36/1.56 cm V s ). Ambipolar transport has been ꢀh
/
ꢀe = 0.01/0.01 cm
V
s
and good stability against
2
3
also achieved in some small-molecule organic semiconductors degradation in air.
Tyrian purple (6,6’-dibromoindigo,
9
-11
12,13
2
such as quinoidal oligothiophenes,
fullerenes,
and
Scheme 1) shows ambipolar transport with ꢀh
/
ꢀe = 0.4/0.4 cm
Very recently, transistor properties of
However, ambipolar transport is sometimes several halogen-substituted indigos have been reported, where
1
4-18
19
–1 –1
24,25
phthalocyaniones,
pentacene.
metal
complexes,
even
V
s
as well.
1
6,20
–
3
–3
sensitive to conditions such as annealing and oxygen 6,6’-dichloroindigo shows ꢀh
and the performance drops by several orders cm V s , and 5,5’-dibromoindigo (
in comparison with the unipolar operation. Among these transport: ꢀe = 2~4.1 × 10 cm V s . These molecules are
/
ꢀe = 1.8~2.1 × 10 /7~9 × 10
9
,10,12,13
2
–1 –1
exposure,
1
) shows only n-type
–
5
2
–1 –1 26
2
–1 –1
materials, ꢀh
/
ꢀe = 3/3 cm V
s
has been reported in a of interest not only as biological and bio-inspired materials but
2
0
pentacene ambipolar transistor, but mobilities of other small- also because of the minimal molecular structure consisting of
molecule ambipolar transistors are still lower than 1 cm V s . electron-donating nitrogen atoms working as a donor and
It has been recognized that stable ambipolar operation is electron-withdrawing carbonyl groups as an acceptor.
2
–1 –1
2
7
The
realized by the appropriate choice of passivation layers; in characteristic blue colour comes from the small energy gap, and
particular tetratetracontane (TTC, C H ) is an excellent the resulting long-wavelength absorption is closely related to
4
4
90
1
4-16,21
passivation material,
though the operation still depends the ambipolar transport.
1
6,19,21
on the choice of the contact metals.
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J. Name., 2013, 00, 1-3 |
1

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4TC01563K
DOI: 10.1039Jo/Curnal Name
potential,
and
absorption edges, the energy gaps are also estimated to be 1.70
eV for and 1.70 eV for (Table 1). These values are in good
E
(Table 1). The resulting HOMO-LUMO gaps of
red
1
2 are 1.79 and 1.65 eV, respectively. From the
1
2
agreement with the reported values in indigo, which are
estimated to be 1.7 eV both from CV and from the absorption
2
3,26
edge.
The absorption of
2 is not much different from that of
Scheme 1. Structures of indigo derivatives.
1
, suggesting that the
π
-skeleton is not really extended because,
Table 1. Redox potentials and energy levels of
Compounds
1, 2, and the parent indigo.
E
E
HOMO
LUMO
[eV]
3.86
HOMO-LUMO gap
Optical gap [eV]
ox
red
[
V]
[V]
[eV]
5.65
5.56
5.60
[eV]
1.79
1.65
1.69
35
Solution
Film
1.70
1.70
1
2
0.85
0.76
0.80
–0.94
–0.89
–0.89
1.89
1.85
3.91
2
3
23
Indigo
3.91
1.9
1.7
+
Onset redox potentials vs. Fc/Fc are assumed to be 4.8 eV from the vacuum level.
In the present work, in order to improve the performance of as will be verified from the crystal structure, the phenyl rings
indigo semiconductors, we investigate indigo derivatives are not coplanar to the indigo part. It has been reported that
substituted at the 5-position instead of the 6-position in Tyrian hole transport is realized for Au electrodes when the HOMO is
purple, namely 5,5’-dibromoindigo (
) (Scheme 1). It is a conventional way in organic the LUMO level is located deeper than 3.15 eV.
semiconductors to extend the -skeleton by connecting phenyl estimated HOMO/LUMO levels of the present compounds
groups to achieve a more parallel molecular packing.
addition, substitutions at the 5,5’-positions lead to
1) and 5,5’-diphenylindigo located above 5.6 eV, and electron transport is achieved when
3
6
(
2
The
π
2
8,29
In satisfy these conditions.
2
7,30
bathochromic shift in contrast to the 6,6’-substitutions.
We Operation regions of ambipolar transistors
have found that the 5-bromo and 6-bromo indigo derivatives
have basically the same molecular packing as the parent indigo,
whereas the 5-phenyl indigo has a new type of molecular
packing that is a hybrid of the herringbone and brickwork
packings, and exhibits an improved transistor performance. In
order to analyze the ambipolar transistor characteristics,
operation regions of ambipolar transistors are investigated.
Since there is a considerable difference between the hole and
electron threshold voltages, a large unipolar saturated region
Although a unipolar saturated region does not appear in the
1
,19
ordinary formulas of ambipolar transistors,
considerable unipolar saturated region appears due to the
difference between the electron ( TH) and hole ( TH') threshold
voltages. When and TH' are nonzero, operation regions
are divided as shown in Figure 1. This figure is depicted for
TH' < 0 < TH, but and TH' may have the same sign; in this
in general a
V
V
V
V
TH
V
V
V
V
TH
case the whole figure is translated horizontally. The transistor
characteristics are obtained assuming the gradual channel
approximation (Supporting Information). When we follow the
appears. We have also fabricated an ambipolar transistor of
2
on an organic substrate, polyethylenenaphtalate (PEN), where
polychloroparaxylylene (Parylene C) is proved to be an inert
enough gate dielectric to achieve ambipolar transport.
line (i) from right to left, the linear region (L) in
DS,
V
–
V
>
TH
GS
V
W
ꢀeC 
1 V
(V −VTH )VDS −
2
DS



(
V
+
V
<
TH
V
GS)
(1)
DS
Results and discussion
I =
D
GS
L

2
Preparation and electrochemical properties
is followed by the unipolar electron-transporting saturated
were reported before the era of the region (S),
Although
coupling reaction,
coupling reaction. Compounds
commercially available bromoindole similarly to the published
1 and 2
3
1,32
we obtained
2
using the standard
1
and
2
were prepared from the
W
ꢀe
L
C
2
(
V
+
V
TH' <
V
<
V +
DS
V
TH)
(2)
I =
(VGS −VTH )
DS
GS
D
2
3
3-35
procedures,
and purified several times by temperature
gradient sublimation.
at
V
=
V
+ VTH. An ambipolar region (A) appears below
DS
GS
The electronic structures are investigated by cyclic
voltammetry (CV) and ultraviolet-visible spectroscopy (UV-
Vis). The CV shows both oxidation and reduction peaks. The
highest occupied molecular orbital (HOMO) level is estimated
V
=
V
+ VTH',
DS
GS
W
ꢀe
2
C
2
W
ꢀh
C
2
I =
(VGS −V ) +
(VDS −VGS +VTH ')
D
TH
L
2L
from the oxidation potential,
Eox, and the lowest unoccupied
(
V
<
V
<
V +
DS
V
TH')
(3)
TH
GS
molecular orbital (LUMO) level is obtained from the reduction
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4TC01563K
which is certainly continuous to Equation 2 at
V
=
V
+
V
TH'. extract
V
and VTH' from the experimental data, because the
TH
GS
DS
This is connected to the unipolar hole-transporting saturated apparent threshold voltages extracted from the transfer
region (S') below
V
<
V
TH,
characteristics are dependent on
V
and different from the
DS
GS
intrinsic threshold voltages,
V
and VTH'.
TH
W
ꢀh
C
2
(
V
TH' <
V <
GS
V
TH)
(4)
ID =
(VDS −VGS +VTH ')
Transistor characteristics
2
L
Transistors with bottom-gate top-contact configuration were
prepared by using a heavily doped -type Si wafer with 300 nm
where the first term of Equation 3 vanishes. Finally, a reversed
hole-transporting linear region (L') appears below TH',
n
V
< V
GS
SiO as the gate insulator (Figure 2). It has been recognized
2
that in organic transistors, not only the electrode but also
treatment of the semiconductor/insulator interface is important
to determine the charge carrier types. In order to achieve
ambipolar charge carrier transport, it is effective to treat the
substrate with an appropriate passivation layer. In particular,
tetratetracontane (TTC, C H ) has been proved to be useful to
W
ꢀhC 1

DS

2
DS
(
V
<
V
TH')
(5)
GS
I =
 V −(VGS −VTH ')V 
D
L
2
which is continuous to Equation 4 at
appear successively as L
V
= VTH'. Regions
GS
→
S
→
A
→
S'
→
L', in which
I
D
D
4
4
90
2
follows
V
in the central S, A, and S' regions, whereas
linear in the L and L' regions.
I
14-16,23-25
GS
realize the ambipolar transport.
TTC is a long-chain
depends on
V
GS
alkane molecule that has an advantage in the low dielectric
constant and the reduced polarization effect. At the same time,
the high surface energy increases the crystallinity of the
semiconductor layer to improve the charge transport
1
4-16
properties.
Figure 2. Structure of an indigo field-effect transistor in bottom-
gate top-contact geometry, where
respectively.
L and W are 100 and 1000 µm,
Figure 1. Operation regions of ambipolar transistors considering
nonzero threshold voltages.
Below
V
<
V
–
V
TH', an off state appears, where
I
Transfer and output transistor characteristics measured
DS
TH
D
drops zero. When we trace the horizontal line (ii) in Figure 1, under vacuum show well-balanced electron and hole transports
regions appear successively as L Off S'
L'. (Figure 3). The cusp of the transfer characteristics, which
→
S
→
→
→
Similar relations hold for hole transport, and the lower half of corresponds to the charge neutrality point in graphene
Figure 1 has a similar shape to the upper half. The off region is transistors, is not much deviated from the zero gate voltage
represented by the rhombic domain in Figure 1, and the
ambipolar region appears only above
TH'; this is shows
the reason that ambipolar characteristics are observed only at Threshold voltages listed in Table 2 are extracted from the
comparatively large
in the actual ambipolar transistors. square root plot of the transfer characteristics. The values
The off region appears in
do not strictly follow the expectation from Figure 1, but
transistor characteristics are not observed at this
= 0 V agree with the results of the output characteristics; in Figure 3b,
limit, and the actual = 0 region is always smaller than
(
V
GS = 0 V). Although
1 shows a considerable off region, 2
V
>
V
–
TH
V
a
comparatively sharp charge neutrality point.
DS
V
I
D
DS
V
TH' <
V
<
V
at
V
= 0 V, but for
DS
1
GS
TH
V
DS
I
V
–
TH
electron conductance at
TH'. The ambipolar region is sandwiched by the unipolar = –60 V, and hole conductance at
saturated regions, whose widths are again
TH'. It is increases from DS = 40 V. From Figure 1, this indicates
therefore appropriate to extract the mobility from the formula 60 V and TH' = –40 V. For , the threshold voltages when
of the unipolar saturated region (Equation 2 and 4). Even in the and DS have the same sign, directly afford TH = 73 V and
conventional unipolar transistors, we should pay attention for = –60 V. The start of the reversed region is not obvious at
not to exceed
V
GS = 0 V starts to increase around
V
DS
D
V
VGS = 0 V in Figure 3c
V
–
V
V
V
TH
=
TH
V
2
V
GS
V
V
V
'
TH
V
GS
V
V
in order to keep the transfer = 0 V in the output characteristics (Figure 3e and f), because
DS
GS
characteristics in the saturated region. In contrast,
larger than in many ambipolar transistors. It is also
noteworthy that both the electron and hole sides have the same (Figure 3) show
importance whatever is the
sign. It should be cautious to dependence. Figure 1 demonstrates that the saturated region
V
–
V
TH' is the threshold values are comparatively large. It is also because
TH' is as large as 133 V that the transfer characteristics
TH
V
V – V
TH
DS
2
V
GS dependence rather than VGS linear
V
DS
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Table 2. Summary of mobilities of 1, 2, indigo and Tyrian purple.
Compound
V
Maximum mobilities under
Average mobilities under
vacuum [cm V s ]
Threshold Voltage
[V]
DS
2
–1 –1
2
–1 –1
[
V]
vacuum [cm V s ]
Hole
0.21
0.07
0.56
0.24
0.01
0.4
Electron
Hole
0.08
0.02
0.42
0.16
-
Electron
0.03
0.27
0.49
0.73
-
Hole
–58
–31
–31
–60
-
Electron
1
2
80
80
80
80
0.04
0.35
0.60
0.95
0.01
0.4
71
97
73
29
-
–
–
[
23]
Indigo
Tyrian purple
10
12
[
24]
-
-
-
-
Figure 3. a) Transfer characteristics of thin-film transistors of
1. b) Output characteristics at
VDS < 0, and c) at VDS > 0. d) Transfer
characteristics of thin-film transistors of
2
. e) Output characteristics at DS < 0, and f) at DS > 0. All measurements are done under
V
V
vacuum.
Crystal structures
continues at most in this voltage extent starting from the
apparent threshold voltage.
In order to explore the origin of the high transistor
performance, crystal structures of and were investigated by
single-crystal X-ray structure analyses. Due to the poor
solubility in organic solvents, crystals were grown by the gas
The mobilities are estimated by using the formulas of the
unipolar saturated region (Equation 2 and 4) as listed in Table
1
2
2
. Transistor performance of 1 is very similar to those of 6,6’-
2
5
dibromoindigo, including the sweep direction dependence of
the mobilities. The maximum hole and electron mobilities are
transport method. Crystals of
1 belong to a monoclinic space
3
7-
2
–1 –1
group similarly to indigo and 6,6’-dibromoindigo (Figure 4).
0
.21 and 0.35 cm V s , respectively, at the downward
sweep. A thin-film transistor of shows maximum hole and
electron mobilities as high as 0.56 and 0.95 cm V s ,
respectively, at the downward sweep, and the on/off ratio is
V
GS
42
The molecules are located on inversion centers, and stacked
along the axis with the interplanar spacing of 3.39 Å, where
the molecular long axis is approximately parallel to the axis.
The adjacent stacks along the axis are alternately tilted in the
opposite directions with respect to the molecular long axis
2
2
–1 –1
b
a
V
GS
7
c
more than 10 . There have been recently reported a few donor-
acceptor polymers in which both electron and hole mobilities
exceed 1 cm V s , and a pentacene ambipolar transistor
o
2
–1 –1 3,4
(Figure 4b). The molecular long axis is tilted by 41 from the a
o
2
–1 –1 20 axis so that the adjacent molecules are tilted by 81 . These
V s .
whose electron and hole mobilities are 3 cm
molecules are linked by hydrogen bonds with the N-O distance
However, mobilities of other small-molecule ambipolar
2
–1 –1
of 2.9 Å (and the H-O distance of 2.1 Å) as indicated by dashed
lines in Figure 4a . This structure is not exactly isostructural to
indigo and Tyrian purple because the lattice constant along the
transistors are still lower than 1 cm V s , and the present
values are one of the highest among the small-molecular
3
,4
ambipolar materials.
stacking axis (b = 4.462 Å) is significantly smaller than b =
4
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DOI: 10.1039/C4TC
A
01
R
5
T
6
I
3
C
K
LE
5
=
.77-5.88 Å (depending on the polymorphism) for indigo and
b
Compound
2 has a structure completely different from 1
and
3
7-42
4.842 Å for Tyrian purple.
The large
b
indicates that the the conventional indigo derivatives (Figure 5). The molecules,
molecular long axis is largely tilted with respect to the stacking located on inversion centers, are arranged nearly parallel to the
o
direction.
Nonetheless, all these crystals are basically
a axis. The phenyl planes are tilted by 29 from the central
composed of similar molecular stacks, which are connected by indigo plane. The phenyl groups form a perfect herringbone
o
interstack hydrogen bonds.
structure with the dihedral angle of 72 (Figure 5c). The indigo
part is, however, approximately parallel to the ac plane; only
o
slightly tilted by 7 from the
c axis. When this structure is
regarded as a herringbone structure, the dihedral angle is as
o
small as 14 (Figure 5d). The parallel molecules do not form
stacks either along the
b axis or along the c axis. If these indigo
parts are regarded as approximately parallel to each other, this
4
3,44
structure is recognized as a brickwork structure.
from the central molecule at = 1/2, two molecules at
Viewed
y = 0 are
y
largely but equivalently displaced along the molecular short
axis (// ). Then the lattice constant is twice the "interplanar"
distance of 3.50 Å. The hydrogen bond survives between the
parallel molecules (// ), though the N-O distance of 3.1 Å (and
the H-O distance of 2.4 Å) is slightly longer than the
corresponding distance of 2.9 Å (and the H-O distance of 2.1
c
b
c
Figure 4. Crystal structure of
The transfer integral along the stacking axis (
1
1
viewed along the a)
) is estimated to be
2.4 meV for HOMO and 21.0 meV for LUMO. c) AFM image of
b and b) c axes.
b
thin film of
1 (45 nm) deposited on a 20 nm film of TTC. d) XRD
Å) in 1.
pattern of 1.
From the molecular orbital calculation, the transfer integrals
between the HOMOs as well as between the LUMOs are
estimated as shown in the captions in Figure 4 and 5. Charge
carrier transport is confined in one dimension in
two-dimensional carrier transport is expected in
brickwork structure has been observed in cyclohexyl
1
, whereas
2
.
The
4
3
naphthalene diimide which attain high field-effect mobility.
Although the present structure is different from the ideal
brickwork structure, the network of the transfer integrals is
identical to the cyclohexyl naphthalene diimide.
It is
noteworthy that the present characteristic molecular packing,
which is a hybrid of the herringbone and brickwork structure, is
induced by the steric effect of the two phenyl groups. The
incorporation of the phenyl groups elongates the molecules to
destroy the twisted stack structure. The whole molecules
realize a herringbone structure, but interestingly the central
parts realize an approximate brickwork structure.
remarkably enhanced mobility of is mainly attributed to this
brickwork packing that realizes two-dimensional transport,
rather than the extended -skeleton coming from the
The
2
π
introduction of the phenyl groups.
Thin film properties
Atomic force microscopy (AFM) images of the thin films
deposited on a TTC layer are shown in Figure 4 and 5. The
thin films of both
due to the strong intermolecular
1
and
2
consist of highly crystalline domains
interactions together with
π-π
the influence of the TTC passivation layers. It has been
reported that the crystallinity of a wide variety of organic
semiconductors increases when deposited on TTC, to achieve
1
4-16
high mobility as well as ambipolar charge transport.
As shown in Figure 4d, a thin film of shows X-ray
diffraction (XRD) peaks at 2 = 6.49° and 13.4°, corresponding
to the -spacing of 13.6 Å. This is almost equal to the length of
Figure 5. Crystal structure of
axes. Hydrogen atoms and phenyl groups are omitted in d). The
transfer integral, , is estimated to be 20.4 meV for HOMO and 23.5
meV for LUMO. e) AFM image of thin film of (75 nm) deposited
on a 20 nm film of TTC. f) XRD pattern of
2 viewed along the a) b, b) c, and c), d)
1
a
θ
t
p
d
2
2.
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the crystallographic
a
axis, implying that the
, a sharp primary
= 4.17°, corresponding to the In summary, we have developed new indigo derivatives
a axis is Conclusion
perpendicular to the substrate. For
diffraction peak is observed at 2
-spacing of 21.2 Å (Figure 5f). This is again in good showing excellent ambipolar transistor properties.
2
θ
d
In
agreement with the crystallographic
a
axis, so that the particular, the transistor performance of
2
stems from the
molecules are standing perpendicular to the substrate. From the characteristic structure that is a hybrid of the herringbone and
crystal structures, the molecular long axis of
is inclined by brickwork structures in contrast to the stacking structure of the
6° from the normal to the substrate (Figure 4b), while the parent indigo. Another important aspect is the well-balanced
1
3
molecules
2
are almost perpendicular to the substrate. It has ambipolar properties achieved by a molecule with a minimal π-
been reported in naphthalene diimide series that the framework. Similarly, DPP in donor-acceptor polymers is not a
perpendicular orientation is most advantageous for charge simple acceptor as recognized generally, but DPP has an
4
3
transport in organic transistors.
the order of
~ tyrian purple > indigo, which is in complete and the electron-withdrawing carbonyl groups. Although the
The tilt angle decreases in ambipolar property due to the electron-donating nitrogen atoms
4
4
1
> 2
agreement with the decreasing order of the mobility.
operation of an ambipolar transistor is complicated depending
on and DS, Figure 1 clearly shows the relations of the
different operation modes. It should be emphasized that the
unipolar region is large due to the large TH', and the
reversed region is equivalently useful to the ordinary side. The
present finding also suggests great possibilities of bio-inspired
materials for the development of organic semiconductors.
V
V
GS
Transistor on an organic substrate
V
– V
TH
In order to explore a way to flexible devices, we have
fabricated a transistor of
using poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)
PEDOT:PSS) as a gate electrode, and a Parylene C film as gate
2 on an organic PEN substrate by
(
dielectric (Figure 6a). A pentacene transistor with the hole
Acknowledgements
2
–1 –1
mobility of 0.3 cm V
s
has been reported using PEDOT:
4
5,46
PSS as the gate contacts.
A pentacene ambipolar transistor The authors are grateful to Tokyo Institute of Technology
in a bottom-gate top-contact geometry has been realized by Center for Advanced Materials Analysis for XRD measurement
and Prof. Kakimoto for NMR and AFM measurements. This
using Parylene C as the hydrophobic dielectric layer (ꢀh/ꢀe =
2
–1 –1
work was partly supported by a Grant-in Aid for Scientific
Research (B) (No. 23350061) from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
0
.07-0.1/0.01-0.04 cm V s ), so Parylene C is an inert
4
7
enough material to achieve an ambipolar transistor. The XRD
shows peaks at = 21.2 Å and 6.33 Å (Figure 6b), which
respectively correspond to As shown in
d
4
8
2
and Parylene C.
Notes and references
Figure 6d-f, the transistor shows well-balanced ambipolar
characteristics, where the maximum hole and electron
mobilities are 0.13 and 0.29 cm V s , respectively. The
AFM image (Figure 6c) reveals decreased grain size, which
may be related to the reduced transistor performance. The
hole/electron threshold voltages extracted from the transfer
a
Department of Organic and Polymeric Materials, Tokyo Institute of
2
–1 –1
Technology, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan.
b
ACT-C, JST, Honcho, Kawaguchi, Saitama 332-0012, Japan.
E-mail: pitayatanakul.o.aa@m.titech.ac.jp, mori.t.ae@m.titech.ac.jp
†
1
Electronic supporting information (ESI) available: CCDC 1002555-
002556 contain the supplementary crystallographic information for
Additional information of synthesis, thermal analysis, redox
characteristics are –25/120 V for
V
= 80 V and –50/90 V for
DS
1
V
= –80 V, so the intrinsic threshold voltages are
V
= 120 V
TH
DS
and
2.
and VTH' = –50 V. These threshold voltages are proportional to
properties, UV-vis spectra, gradual channel approximation, single crystal
structures, theoretical calcutions, devive fabrication, and device air
stablility.
the trap numbers at the semiconductor/dielectric interface.
Although the threshold voltages are increased in comparison
with the TTC transistors, Parylene C is a promising gate
dielectric material for an ambipolar transistor.
1
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7

Journal of Materials Chemistry C
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DOI: 10.1039/C4TC01563K
The table of contents entry
Title
High Performance Ambipolar Organic Field-Effect Transistors Based on Indigo
Derivatives
Text (one sentence, of maximum 20 words, highlighting the novelty of the work)
A bio-inspired organic semiconductor 5,5’-diphenylindigo shows excellent and well-
balanced ambipolar transistor properties; the hole and electron mobilities are 0.56 and
2
–1 –1
0
.95 cm V s , respectively.
Keywords
organic transistor, ambipolar, organic semiconductor, natural material
Authors
Oratai Pitayatanakul, Toshiki Higashino, Tomofumi Kadoya, Masaki Tanaka,
Hirotaka Kojima, Minoru Ashizawa, Tadashi Kawamoto, Hidetoshi Matsumoto, Ken
Ishikawa and Takehiko Mori
ToC figure (maximum size 8 cm x 4 cm)