C O M M U N I C A T I O N S
Table 1. FET Characteristics of DNTT and DNSS Devices
Fabricated on Si/SiO2 Substrates with Different Surface
Treatments and under Different Substrate Temperatures (Tsub
)
b
surface-treatment
reagent
Tsub
/
µFET
/
Vth
/
1
°
Ca
cm2 V-1 s-
Ion/Ioff
V
DNTT
DNSS
HMDS
rt
60
0.73-0.83 5 × 106
-8.5 ( 2.5
1.1-1.2
5 × 106 -12.5 ( 2.5
100 1.1-1.3
rt
60
100 1.6-1.9
rt
107
107
107
107
-5.5 ( 1.5
-13 ( 2.0
-11.0 ( 2.0
-6.5 ( 2.5
-4.5 ( 0.5
-3.0 ( 1.0
-6.5 ( 1.5
-7.5 ( 2.5
-7.5 ( 2.5
-9.5 ( 2.5
OTS
1.6-1.8
2.1-2.9
Figure 3. Crystal structure of DNTT: (a) b-axis projection; (b) herringbone
packing in the layered structure.
HMDS
OTS
0.54-0.57 5 × 106
60
0.71-1.3
5 × 106
100 0.31-0.59 5 × 106
rt
60
0.99-1.9
0.97-1.0
5 × 106
107
100 0.43-0.66 5 × 106
a rt ) room temperature. b Data from more than 10 devices.
tributes to effective molecular overlap, which can lead to high
carrier mobility in the thin-film transistor setting. From the
experimental results, we conclude that the present design strategy
for air-stable, high-performance organic semiconductors is quite
valid. Further studies to optimize DNTT- and DNSS-based devices
are under way.
Figure 4. FET characteristics of DNTT-based OFET on OTS-treated
substrate (Tsub ) 60 °C): output characteristics (left) and transfer
characteristics at Vd ) -60 V (right).
Acknowledgment. We thank Dr. E. Miyazaki (Hiroshima
University) for helpful discussion on X-ray structural analysis. This
work was partially supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology, Japan, and an Industrial Technology Research
Grant Program in 2006 from the New Energy and Industrial
Technology Development Organization (NEDO), Japan.
3.0 and 2.9 eV for DNTT and DNSS, respectively. Electrochemi-
cally defined HOMO levels for DNTT and DNSS by means of
cyclic voltammetry (Figure S1) are 5.44 and 5.38 eV below the
vacuum level, respectively.9 These experimental HOMO levels as
well as HOMO-LUMO gaps are qualitatively consistent with the
results obtained from MO calculations. Judging from these data,
we expect that DNTT and DNSS are stable organic semiconductors,
similar to DPh-BTBT and DPh-BSBS.
Supporting Information Available: Experimental details for the
synthesis and characterization of DNTT and DNSS, crystallographic
information file (CIF) for DNTT, device fabrication, XRDs, DFT
calculations, AFM images of thin films, and FET characteristics of
devices fabricated on bare Si/SiO2 substrates. This material is available
Figure 3 shows the crystal structure of DNTT elucidated by
single-crystal X-ray structural analysis. Almost planar molecules
with a small standard deviation of 0.058 Å form a layer-by-layer
structure in the crystal (Figure 3a). In each layer, herringbone
packing typical of poly fused aromatic compounds was observed,
resulting in the two-dimensional molecular network (Figure 3b).
Preliminary OFET studies were carried out using “top-contact”-
type devices with W/L ) ca. 30, which were fabricated by vacuum
deposition on Si/SiO2 substrates whose surfaces were treated with
octyltrichlorosilane (OTS) or hexamethyldisilazane (HMDS). All
the devices fabricated under various conditions showed typical
p-channel FET characteristics with µFET higher than 0.3 cm2 V-1
s-1 and Ion/Ioff > 106 under ambient conditions (Figure 4 and Table
1). In particular, excellent FET characteristics with µFET higher than
2.0 cm2 V-1 s-1 and Ion/Ioff of 107 were observed in DNTT-based
devices fabricated on the OTS-treated substrate at Tsub ) 60 °C.
The present FET characteristics are almost comparable or slightly
better than those for DPh-BTBT- and DPh-BSBS-based devices.
In summary, we have established a straightforward three-step
procedure for the synthesis of highly π-extended heteroarenes,
DNTT and DNSS. Solution UV-vis spectra and electrochemical
measurements indicated that they have relatively low-lying HOMO
levels and large HOMO-LUMO energy gaps, despite the fact that
they are highly π-extended arenes consisting of six fused aromatic
rings. These physicochemical properties are reflected by the stability
in air of OFETs fabricated with DNTT and DNSS thin films as the
active layer. In addition, the highly extended π-framework con-
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