Dithienylbenzobis(thiadiazole) based organic semiconductors with low LUMO levels and narrow energy gaps

Article abstract of DOI:10.1039/b925151k

Novel OFET materials showing high mobility (0.77 cm² V ^-1 s^-1) and good air-stability were developed using a benzobis(thiadiazole) (BBT) unit and the high FET performance was attributed to the low LUMO level and the film morphology.

Full text of DOI:10.1039/b925151k

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Dithienylbenzobis(thiadiazole) based organic semiconductors with low
LUMO levels and narrow energy gapsw
Takahiro Kono,a Daisuke Kumaki,b Jun-ichi Nishida,a Shizuo Tokitoab and
Yoshiro Yamashita*a
Received 1st December 2009, Accepted 18th March 2010
First published as an Advance Article on the web 13th April 2010
DOI: 10.1039/b925151k
Novel OFET materials showing high mobility (0.77 cm2 VÀ1 sÀ1
)
and physical properties of BBT derivatives, and their semi-
conducting characteristics by FET measurements.
and good air-stability were developed using a benzobis(thia-
diazole) (BBT) unit and the high FET performance was attributed
to the low LUMO level and the film morphology.
BThBBT 2 and FPTBBT 3 (Fig. 1) were obtained by the
Stille coupling reaction of 4,10-dibromobenzobis(thiadiazole)
(5) with the corresponding stannyl reagents.6,7 Trifluoromethyl
groups were introduced in 3 since they are effective to enhance
n-type FET behaviors by improving the crystallinity of films
and lowering the LUMO energy level.5 For comparison,
dithienyl derivative ThBBT 1 was synthesized according to a
reported method.5a Black block crystals of BThBBT 2 were
obtained by slow sublimation. The ORTEP structure shows
that 2 has a planar structure and the benzothiophene rings are
disordered (Fig. 2). Intermolecular SÁ Á ÁN contacts of ca.
3.21 A are observed between the columns. This is due to the
electrostatic interaction between the positively polarized S
atom and the negatively polarized N atom.5,6
Control of energy levels and band gaps is very important since
they determine the optical and electrochemical properties of
organic compounds, and the characteristics of their electronic
devices.1 Especially, semiconductors for organic field-effect
transistors (OFETs) are required to have suitable energy levels
for efficient carrier-injection from electrodes.2 Organic redox
systems of p-conjugated molecules have been used as semi-
conductors since their HOMO and LUMO levels can be tuned
by suitable design of electron-donating and -accepting
moieties. Although some superior air-stable p-channel OFETs
are known, high-performance air-stable n-channel OFETs are
still rare and their development is crucial for low-voltage and
high-gain CMOS inverters.3 For development of n-channel
OFET materials, high electron affinity is important to match
the LUMO levels with the work function of the electrodes as
well as to enhance the air stability.
The HOMO and LUMO energies are summarized in
Table 1. The redox potentials of these compounds were
measured by cyclic voltammetry (CV) at 100 1C due to the
low solubility. The LUMOs of 1–3 are lower than that of 4,
suggesting that 1–3 would be able to work as n-channel
semiconductors. This is attributed to the bisthiadiazole unit
of BBT containing a hypervalent sulfur atom leading to a
stronger electron-accepting property. The LUMO levels esti-
mated from the CV of 2 and 3 satisfy the proposed air-stable
LUMO level (4.0–4.1 eV).3e The end-absorption of the thin
film of 3 is red-shifted from that of 1 and is further red-shifted
from that of 4 (Table 1).
Recently, BBT derivatives have been used for electronic
applications.4 However, to our best knowledge, high perfor-
mance and air-stable n-channel OFET materials using the
BBT skeleton have not been reported yet. In this context, we
expect benzobis(thiadiazole) (BBT) derivatives to be viable
n-channel organic semiconductors for the following reasons.
First, BBT is a stronger electron acceptor than benzothiadiazole
(BTD)5 because of an additional thiadiazole ring attached to
BTD with a hypervalent sulfur atom. Second, intermolecular
interactions are expected by intermolecular SÁ Á ÁN contacts
between the thiadiazole rings.5,6 Third, small HOMO–LUMO
energy gaps are expected in the BBT derivatives, which would
be useful to induce ambipolar behavior. Fourth, the BBT
derivatives with thiophene units are planar molecules with
some quinoidal characteristics.6 We report here the synthesis
Besides the CVs, the ionization potentials (IP) were measured
with ultraviolet photoelectron spectroscopy (UPS) in air to
investigate the energy levels in the thin films. The HOMO and
LUMO levels are summarized in Table 1. These IP data are
different from the CV data in solution. The difference can be attri-
buted to the difference in measurement methods (UPS or CV)
a Department of Electronic Chemistry, Interdisciplinary Graduate
School of Science and Engineering, Tokyo Institute of Technology,
Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan.
E-mail: yoshiro@echem.titech.ac.jp; Fax: (+81)45-924-5489;
Tel: (+81)45-924-5571
b NHK Science and Technical Research Laboratory, Kinuta,
Setagaya-ku, Tokyo 157-851, Japan
w Electronic supplementary information (ESI) available: Synthesis,
UV-Vis spectra, UPS and CV for 1–3, and FET characteristics of a
bottom contact device, crystal structure, CIF, SEM and AFM data
for 2. See DOI: 10.1039/b925151k
Fig. 1 BBT derivatives 1–3 and FPTBTD 4.
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Fig. 3 X-Ray diffraction pattern of film 3 deposited at 130 1C.
Fig. 2 ORTEP drawing (a) and packing view (b) of 2.
as well as the difference in the state (solution or film). In the
thin film, the LUMO level of 3 is much lower than those of 1
and 2. The large difference in 3 may be attributed to the fact
that the crystallinity of film 3 is better than those of 1 and 2,
resulting in stronger intermolecular interactions in the thin
film of 3.
The thin films of 1–3 deposited on HMDS-treated substrates
were investigated by X-ray diffraction (XRD) (Fig. S4, ESIw).
The film of 3 showed five diffraction peaks. In contrast, the
films of 1 and 2 only showed very small peaks. This result
indicates that the film 3 has higher crystallinity than 1 and 2.
The primary reflection peaks were observed at 2y = 4.621
(d-spacing = 19.11 A) for 3, 2y = 8.121 (d-spacing = 10.88 A)
for 2, and 2y = 10.881 (d-spacing = 8.12 A) for 1. The film of
3 at 130 1C showed seven peaks in the XRD (Fig. 3), indicating
that the crystallinity was enhanced with an increase of substrate
temperature at 130 1C.
To investigate the morphologies of 2 and 3 films in more
detail, scanning electron microscopy (SEM) measurements
were carried out. The SEM images of 3 are shown in Fig. 4
and ESIw, revealing the formation of large grains at r.t., 80 and
130 1C.
Fig. 4 SEM images of films of FPTBBT 3 at (a) Tsub = r.t., (b), r.t.
on electrode and (c) 130 1C. (d) AFM image of Tsub 130 1C film.
onto hexamethyldisilazane (HMDS) treated Si/SiO2 sub-
strates. The FET measurements were carried out in vacuum
at room temperature. The films of 1 and 2 with BC showed
ambipolar FET characteristics when deposited at r.t. (me = 1.6 Â
10À4 and mh = 3.4 Â 10À7 cm2 VÀ1 À1
for 1 and me = 2.0 Â
s
10À5 and mh = 1.5 Â 10À5 cm2 VÀ1 sÀ1 for 2). This is due to
their narrow HOMO–LUMO gaps and the HOMO and
LUMO levels being close to the work function of gold and
chromium which were used as the source and drain electrodes
(Au: ca, 5.1 eV and Cr: 4.5 eV).
The film 3 with BC exhibited only n-channel behavior
because the HOMO level of 3 (6.25 eV from the IP) is different
from the work function of Au and Cr, resulting in a large
injection barrier of hole-carriers between the organic layer
An interesting finding is that the number of grains on the film
surface decreases and the flat region is increased at Tsub = 130 1C.
The film at 130 1C was further investigated by AFM, where a
terrace-like structure with steps was observed (Fig. 4(d)).
The semiconductor characteristics were investigated with
top-contact (TC) and bottom-contact (BC) FETs fabricated
and electrode interface. High electron mobility of me
=
0.17–0.40 cm2 VÀ1 sÀ1 was observed when deposited at r.t,
which is much higher than those of 1 and 4.5b This is attributed
Table 1 HOMO and LUMO levels of 1–3 and FPTBTD 4
Compd.
lmaxa/nm
HOMOa (IP)/eV
Egb/eV
LUMOc/eV
HOMOd (CV)/eV
LUMOd (CV)/eV
Eg (CV)/eV
1
2
3
4
806
720
810
475
5.60
5.65
6.25
1.30
1.35
1.20
2.00
4.30
4.30
5.05
5.32
5.35
5.29
5.50
3.96
4.03
4.04
3.30
1.36
1.32
1.25
2.20
e
e
—
—
a
b
100 nm thin films on quartz plates. Measured with thin film and estimated from the end-absorption. HOMO (IP) + Eg. CV measured in
c
d
e
PhCN containing 0.1 M Bu4NPF6. HOMO and LUMO were estimated from onset of CV peaks. Not observed.
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by FET measurements. The LUMO levels are lower than that
of the BTD analogue 4 owing to the electron-withdrawing
BBT skeleton with a hypervalent sulfur atom. The film of 3 is
highly ordered, as revealed from the XRD, SEM and AFM
measurements. The FET mobility of 3 was found to be very
high even when deposited at room temperature. The top-
contact FET device showed high air-stability due to the low
LUMO levels and/or the film morphology. The mobility
reached levels up to 0.77 cm2 VÀ1 sÀ1 with the bottom-contact
FET. This result shows that BBT is a promising unit for air-
stable high-performance n-channel FETs.
We gratefully acknowledge Professor Ikuyoshi Tomita
(Tokyo Institute of Technology) for measurements of SEMs
and Dr Hiroyasu Sato (RIGAKU Co. Japan) for solving the
crystal structure of 2. This work was supported by a Grant-in-
Aid for Scientific Research (No. 19350092) from the Ministry
of Education, Culture, Science, and Technology.
Notes and references
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Fig. 5 Transfer curves (a), and Vth (red) and mobility (blue) plots
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are helpful to create suitable films for carrier accumulation
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The film 3 with TC showed a mobility of 0.28 cm2 VÀ1 sÀ1
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TC devices as shown in Fig. 5, where the transfer curves
and the change of mobility and Vth in air are depicted. It is
noteworthy that Vth showed a small positive shift (10 to 19 V)
upon standing in air and the mobility was constant at
0.16 cm2 VÀ1 sÀ1 after 50 days (Table S2, ESIw). In contrast,
the film of FPTBTD 4 showed a very large Vth shift in air.8
This fact indicates that the high air-stability of film 3 is
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CV, and lower level in the thin film, as estimated from UPS
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On the other hand, the BC based device of 3 showed lower
air stability. Although the reason is still uncertain, this may be
related to the difference in crystallinity on the HMDS treated
surface from that on the Au surface because the grains on the
HMDS surface were a little larger than those on the Au
surface (Fig. 4(b)) The film 2 (BC device) did not show FET
performance in air. This is attributed to the poor crystallinity,
as revealed from XRD, as well as to the higher LUMO level in
the film.
In conclusion, we have developed oligomer-based BBT deri-
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Chem. Commun., 2010, 46, 3265–3267 | 3267