Naphtho[2,1-b:6,5-b′]difuran: A versatile motif available for solution-processed single-crystal organic field-effect transistors with high hole mobility

Article abstract of DOI:10.1021/ja2120635

We here report naphtho[2,1-b:6,5-b′]difuran derivatives as new p-type semiconductors that achieve hole mobilities of up to 3.6 cm² V ^-1 s^-1 along with high I_on/I_off ratios in solution-processed single

Full text of DOI:10.1021/ja2120635

Communication
pubs.acs.org/JACS
Naphtho[2,1-b:6,5-b′]difuran: A Versatile Motif Available for
Solution-Processed Single-Crystal Organic Field-Effect Transistors
with High Hole Mobility
Chikahiko Mitsui,†,‡ Junshi Soeda,‡ Kazumoto Miwa,‡ Hayato Tsuji,*,† Jun Takeya,*,‡
and Eiichi Nakamura*,†
†Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡ISIR, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
S
* Supporting Information
OLED device.12 To our disappointment, however, BDFs
ABSTRACT: We here report naphtho[2,1-b:6,5-b′]-
difuran derivatives as new p-type semiconductors that
achieve hole mobilities of up to 3.6 cm2 V−1 s−1 along with
high Ion/Ioff ratios in solution-processed single-crystal
organic field-effect transistors. These features originate
from the dense crystal packing and the resulting large
intermolecular π-orbital overlap as well as from the small
reorganization energy, all of which originate from the small
radius of an oxygen atom.
performed rather poorly in single-crystal OFET devices,
probably because of poor charge injection due to the low
HOMO level and poor intermolecular interactions. We report
here that naphtho[2,1-b:6,5-b′]difuran (NDF) serves as a novel
and effective motif for single-crystal OFETs made by a
solution- and vapor-growth method, which show carrier
mobilities of up to 3.6 cm2 V−1 s−1 as well as high Ion/Ioff
ratiosvalues among the highest for solution-processed
OFETs.13 Such high performance of the NDF-based OFETs
illustrates the potential of furans in organic electronics
applications, which needs to be explored further in the future.14
Among the NDF derivatives we examined, two were found to
have the optimum structure for control of the solubility and
device processability. We synthesized DPNDF and its octyl-
substituted derivative C8-DPNDF in excellent yields by a zinc-
mediated double-cyclization reaction of dialkynes 1a and 1b,
respectively (Scheme 1).11,12,15
We first fabricated bottom-gate−top-contact OFETs by the
oriented-crystal-growth method13 using C8-DPNDF and found
that the devices showed excellent performance. We employed
the octyl group to achieve high solubility of the compound
while maintaining the crystallinity. Thus, a droplet of a 0.2 wt%
solution of C8-DPNDF in chlorobenzene was placed at an edge
of a liquid-sustaining piece on a substrate, so that the crystalline
domain would grow in the direction of evaporation of the
solvent.13 As examined for four devices, this single-crystalline
film exhibited FET mobilities of 1.5−3.6 cm2 V−1 s−1 in the
saturation regime with a high Ion/Ioff ratio of 104−105 and a
threshold voltage of −100 V (Figure 1 for device A, which
showed the highest mobility).
uran1,2 is a ubiquitous organic frame found in nature and
F
synthetically readily accessible, yet it has received far less
attention than thiophenes in organic electronics research,3−5
particularly in device applications, which include one example
of an organic light-emitting diode (OLED)6 and some
examples of organic field-effect transistors (OFETs)7,8 and
organic photovoltaic (OPV) devices.9 This lack of interest
could have arisen because of some earlier examples that showed
instability under oxidative conditions.10 We conjectured that
the use of a fused furan skeleton, such as in DPBDF (Scheme
1), would exhibit not only stability but also p-type semi-
conducting properties. Indeed, amorphous films of some
benzodifuran (BDF) derivatives showed carrier mobilities of
>10−3 cm2 V−1 s−1, which is high for amorphous materials,
using the time-of-flight technique.11 We also demonstrated that
a new ambipolar BDF led us to develop a unique homojunction
Scheme 1
The molecular orientation and homogeneity of the solution-
grown C8-DPNDF film (Figure 2a) were determined by X-ray
diffraction (XRD) measurements (Figure 2b) and atomic force
microscopy (AFM) (Figure 2c,d). The XRD data revealed that
the c axis is oriented perpendicular to the substrate and the ab
plane (the carrier conduction path) parallel to the substrate
[Figure 2b; also see the Supporting Information (SI)]. The
AFM images revealed only a few steps corresponding to a single
molecular height, indicating the homogeneity of the film, which
bears a molecularly flat area extending to a micrometer area.
Received: December 25, 2011
Published: March 14, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
Figure 1. OFET characteristics of C8-DPNDF: (a) transfer and (b)
output characteristics of device A.
Figure 3. Representative characteristics of a PVT-grown single-crystal
OFET using DPNDF (device B): (a) transfer and (b) output
characteristics.
(device C) was also fabricated by vaporizing solvent on an
inclined substrate (as for device A) using a 0.2 wt % solution of
DPNDF in o-dichlorobenzene at 150 °C to form a single
crystal on the surface-treated Si/SiO2 substrate. This solution-
crystallized OFET device C showed similar characteristics as
device B and a mobility of 1.0 cm2 V−1 s−1 (for details, see the
SI).
It is intriguing that the DPNDF derivatives showed high hole
mobility, and we examined its origin in some detail. Calculated
transfer integrals (t) among HOMOs in a crystal of the NDF
molecules provided support for these characteristics (Figure 4).
Figure 2. Molecular orientation and homogeneity of the solution-
grown single crystal of C8-DPNDF. (a) Photograph of a representative
OFET device. (b) Schematic illustration of the packing diagram of C8-
DPNDF along the b axis as revealed by XRD. The parallelogram
stands for a unit cell. (c) Top view of the AFM image (inset: bird’s-eye
view). (d) Cross-sectional AFM profile along the white dotted line in
(c). The observed 2.4 nm height of a step agrees very well with the
step size shown in (b).
Figure 4. Crystallographic axes in the NDFs and the transfer integrals
calculated using the ADF program. For C8-DPNDF, the alkyl groups
have been omitted for clarity.
Interestingly, the parent compound DPNDF showed an
OFET behavior similar to that of C8-DPNDF regardless of the
fabrication process. We first examined physical vapor transport
(PVT) to prepare single crystals of DPNDF. The platelet single
crystals thus obtained were manually laminated on surface-
treated [using a fluorinated self-assembled monolayer (F-
SAM)]16 Si/SiO2 substrates with patterned Au electrodes to
construct a bottom-gate−bottom-contact architecture. The
transfer and output characteristics of the DPNDF-laminated
device B are plotted in Figure 3. This device showed a hole
mobility of 1.3 cm2 V−1 s−1 in the saturation regime with a high
Ion/Ioff ratio of 105. The hole mobility of DPNDF is 1.6 times
higher than that of a thiophene analogue having exactly the
same carbon framework.17 It is noteworthy that the threshold
voltage of the laminated device was −10 V. This significantly
large shift of the threshold voltage to the positive side can be
ascribed to the use of F-SAM.16 The solution-processed OFET
The transfer integrals for C8-DPNDF are highly anisotropic,
with very high values for one direction (t1 = 59.3 meV and t2 =
0.3 meV along the stacking and transverse directions,
respectively).18 We consider that the perpendicular positional
relationship between the molecules along the transverse
direction is responsible for this t2 value. The t1 value for the
stacking direction is slightly larger than that for 2,7-dioctyl[1]-
benzothieno[3,2-b][1]benzothiophene (C8-BTBT) (57 meV
for the stacking direction and 41 meV for the transverse
direction),17 which shows a very high carrier mobility.19 On the
basis of these data, we could expect that there should be room
to achieve a hole mobility higher than those observed herein for
the C8-DPNDF-based OFETs. We consider the mobility in the
present case to be limited by a nonideal crystal orientation in
the solution-crystallized device, as the crystal structure of the
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Journal of the American Chemical Society
Communication
solution-crystallized C8-DPNDF film indicated that the crystal
growth direction (along which the source−drain channel was
constructed) was not completely parallel to the stacking
direction (i.e., the b axis) but ca. 20−30° from the b axis.
Hence, we expect that once the channel direction is optimized
for C8-DPNDF, the mobility will be further improved.
The transfer integrals for DPNDF along the stacking and
transverse directions are t1 = 27.7 meV and t2 = 39.9 meV,
respectively, as calculated on the basis of the experimental
packing parameters in a single crystal.18 Notably, these values
are much higher than those for the thiophene counterpart17 (11
and 9 meV, respectively). XRD analysis of the PVT-grown
single-crystal OFET of DPNDF (device C) revealed that the
channel direction of our device was almost parallel to the
transverse direction, which has a large transfer integral (see the
SI), and hence, the experimental mobility data obtained for
device C should reflect the best carrier mobility of a DPNDF
single crystal.
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
tsuji@chem.s.u-tokyo.ac.jp; takeya@sanken.osaka-u.ac.jp;
nakamura@chem.s.u-tokyo.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank MEXT (KAKENHI 22000008 for E.N. and
20685005 for H.T. along with the Global COE Program for
Chemistry Innovation) for financial support. This work was
partially supported by the Strategic Promotion of Innovative
R&D, JST.
It should be noted that the DPNDF and C8-DPNDF
molecules in single crystals stack in a herringbone manner
(Figure 4) with a shortest intermolecular distance of 2.80 Å.
The thiophene counterpart also packs in a herringbone manner,
and the shortest intermolecular distance is longer (3.31 Å)
because of the larger atomic radius of a sulfur atom (1.80 Å)
than of an oxygen atom (1.52 Å). The NDF molecules are
more densely packed in a single crystal, which could be partially
responsible for the large transfer integrals of NDFs.
The reorganization energy serves as a measure to estimate
the charge carrying ability in the solid.20 The reorganization
energies for hole formation (λh) in DPNDF and C8-DPNDF
are 0.17 and 0.18 eV, respectively, as calculated for a single
molecule at the B3LYP/6-31G* level.21 These values are
smaller than those for the sulfur analogue DPNDT17 (0.19 eV)
and α-oligofurans (0.23−0.37 eV),22 which supports the high
ability for carrier transportation by DPNDF and C8-DPNDF.
The expanded π-electron conjugation of the naphthalene motif
contributes to the small reorganization energy.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, thermal data, electrochemical measure-
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