Indacenodithiophene semiconducting polymers for high-performance, air-stable transistors

Article abstract of DOI:10.1021/ja1049324

High-performance, solution-processed transistors fabricated from semiconducting polymers containing indacenodithiohene repeat units are described. The bridging functions on the backbone contribute to suppressing large-scale crystallization in thin films. However, charge carrier mobilities of up to 1 cm²/(V s) for a benzothiadiazole copolymer were reported and, coupled with both ambient stability and long-wavelength absorption, make this family of polymers particularly attractive for application in next-generation organic optoelectronics.

Full text of DOI:10.1021/ja1049324

Published on Web 08/02/2010
Indacenodithiophene Semiconducting Polymers for High-Performance,
Air-Stable Transistors
Weimin Zhang,*,†,‡ Jeremy Smith,† Scott E. Watkins,§ Roman Gysel,| Michael McGehee,|
Alberto Salleo,| James Kirkpatrick,† Shahid Ashraf,† Thomas Anthopoulos,† Martin Heeney,† and
Iain McCulloch†
Imperial College, London SW7 2AZ, U.K., Guangxi UniVersity for Nationalities, Nanning 530006, P.R. China,
CSIRO Molecular and Health Technologies, Victoria 3169, Australia, and Stanford UniVersity,
Stanford, California 94305
Received June 9, 2010; E-mail: weimin.zhang@imperial.ac.uk
of the backbone and further reduce the energetic disorder of the
Abstract: High-performance, solution-processed transistors fab-
ricated from semiconducting polymers containing indacenodithio-
hene repeat units are described. The bridging functions on the
backbone contribute to suppressing large-scale crystallization in
thin films. However, charge carrier mobilities of up to 1 cm2/(V s)
for a benzothiadiazole copolymer were reported and, coupled with
both ambient stability and long-wavelength absorption, make this
polymer. The aryl amine unit was replaced with more planar, fused
thiophenes, such as BT and TT,2,3 the latter of which can contribute
to lowering energetic disorder through stiffening the backbone and
has been shown to help promote close intermolecular π-stacking
distances, leading to molecular ordering. The BT unit has also
previously been shown to contribute to enhanced π-stacking
morphologies, leading to very high charge carrier mobility poly-
family of polymers particularly attractive for application in next-
generation organic optoelectronics.
mers,4 despite the potential torsional backbone twisting effect of
the 1,4-phenyl link. The indacenodithiophene unit, with its periph-
eral thiophenes, promotes enhanced planarity by eliminating
potential phenyl-phenyl links.
Recently we demonstrated that, by extending the persistence
Copolymer IDT-BT was synthesized as shown in Figure 1 (and
length of arylamine polymers through incorporation of indeno-
the Supporting Information) by a Suzuki cross-coupling condensa-
fluorene (IF) units1 and thus reducing conformational energetic
tion. This synthetic route provides a well-defined backbone and
disorder, it was possible to systematically improve charge carrier
combined with the linear alkyl side chains, molecular packing is
mobilities for these semiconducting polymers while retaining their
expected to be optimized. However, attempted synthesis of IDT-
air stability. However, further exploitation of this class of polymer
TT by Suzuki polymerization only afforded low molecular weight
for both transistor and solar cell applications requires that the
material, possibly due competing deborylation of the electron rich
mobility is further increased without compromising air stability and
thienothiophene monomer under our reaction conditions. Therefore,
that the absorption maximum is significantly shifted to longer
a Stille polycondensation protocol under microwave heating condi-
wavelengths. In this Communication, we present fused ladder
tions5 was employed to form the IDT-TT copolymer. Polymeriza-
indacenodithiophene (IDT) alternating copolymers with both thieno-
tions proceeded in good yield, with sufficiently high molecular
[3,2-b]thiophene (TT) and benzothiadiazole (BT), shown in Figure
weights (Table 1), as determined by gel chromatography (GPC).
1, and describe their thin-film properties and transistor character-
The copolymers are soluble in common organic solvents such as
istics.
tetrahydrofuran, chloroform, toluene, and chlorobenzene. HOMO
energy levels of both polymers were estimated by ambient
photoelectron spectroscopy (PESA) and found to be quite similar.
Density functional theory (DFT) calculations (see Supporting
Information) support this observation, showing that the HOMO has
a similar energy and bonding antibonding motif in both polymers,
with values given in Table 1. The BT unit therefore functions
primarily to stabilize the LUMO, leading to a lower bandgap.
Table 1. Properties of Copolymers
λmax of the
IP (eV)
thin film (nm)
repeat
unit
Mn/Mw
(kDa)
µhole
PESA
DFT
UV-vis
TD-DFT
(cm2/(V s)
Figure 1. Polymerization and molecular structure of indacenodithiophene
copolymers with benzothiadiazole (IDT-BT) and thieno[3,2-b]thiophene
(IDT-TT).
BT
TT
-5.4
-5.4
-4.7
-4.6
38/108
64/168
675
550
711
550
1.0 ( 0.25
0.15 ( 0.02
The strategy for further improvement in charge carrier mobility
in comparison to the IF copolymers was to enhance the planarity
Grazing incidence X-ray scattering (GIXS) experiments were
carried out on annealed thin films of both polymers on Si substrates
to explore the microstructure, and images are shown in Figure 2.
The IDT-BT polymer was observed to be semicrystalline, while
the IDT-TT thin film was amorphous. Under the experimental
† Imperial College.
‡ Guangxi University for Nationalities.
§ CSIRO Molecular and Health Technologies.
| Stanford University.
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J. AM. CHEM. SOC. 2010, 132, 11437–11439 11437

C O M M U N I C A T I O N S
deposition conditions (see Supporting Information), the IDT-BT
backbones were observed to align preferably face-on on the
substrate, and the broad π-stacking peak (at a scattering vector
qz ) 1.54 Å-1) indicates a spacing of 4.1 Å.
from the highly nonlinear output characteristics (Supporting
Information) observed at low VD. The latter is attributed to the
energy offset between the Fermi level of the gold source/drain
electrodes and the HOMO level of IDT-BT, even when treated
with the work-function-raising self-assembled monolayer, pen-
tafluorophenyl thiol, offering the possibility that mobility could
potentially be increased further by fine-tuning of the device
architecture and improved materials processing. The IDT-TT
copolymer exhibits a lower mobility, which is attributed to the
more amorphous nature of the thin-film microstructure, revealed
by X-ray diffraction. However, the on/off ratio for this copolymer
is >106 (see Supporting Information), which is significantly
higher than the 103 exhibited by the IDT-BT copolymer.
Interestingly, these polymers also exhibit excellent ambient
stability, as shown in Figure 3b, where the on and off currents
are shown to remain relatively stable for a transistor device
continually exposed to ambient conditions in the dark over a
period of more than 1000 h. The ambient electrochemical
stability observed for both polymers is attributed to their low-
lying HOMO levels.
Figure 2. Grazing incidence X-ray scattering images of the semicrystalline
IDT-BT (a) and the amorphous IDT-TT (b), showing the π-stacking distance
at 4.1 and 4.0 Å, respectively.
The diffuse ring centered at the origin and with a radius close to
the π-stacking distance indicates that a significant ratio of the thin
film is amorphous. The GIXS image of IDT-TT shows only a
diffuse ring at a scattering vector 1.4 Å-1 but no peaks that would
suggest that it has crystallized. These thin-film data are consistent
with differential scanning calorimetry results on bulk powder
samples, which show no observable thermal transitions for either
polymer.
Bottom-contact, top-gate (BC-TG) architecture field-effect tran-
sistor (FET) devices were fabricated with the polymer semiconduc-
tors spin-cast from a 10 mg/mL chlorobenzene solution at 2000
rpm followed by an annealing step at 100 °C for 5 min in nitrogen.
Details on the deposition of Cytop and Al gate electrode can be
found elsewhere.6
The IDT-BT polymer in particular has a relatively long
wavelength absorption maximum of 675 nm, promoted by both
the donor-acceptor hybridization of the molecular orbital energy
levels, with the electron-rich indacenodithiophene and electron-
poor benzothiadiazole, and the extended coplanar backbone
conjugation induced by the bridging functionality on the
indacenodithiophene. Time-dependent density functional theory
(TD-DFT) calculations (see Supporting Information) reveal that
both copolymers containing the IDT moiety have significantly
lower torsional barriers than analogous indenofluorene (IF)
copolymers. For example, the IDT-BT copolymer has a com-
putational torsion of just 7° (compared to 37° for the IF-BT)
and is predicted to have an absorption maximum (λmax) of 711
nm, which is in good agreement with experimental values (Table
1). A higher energy LUMO, due to the weaker electron-accepting
strength of the TT unit, is responsible for the calculated λmax of
550 nm in the IDT-TT copolymer, in excellent agreement with
the measured value for the thin film. Thus, with optimization of
the alkyl side chains, this IDT-BT copolymer may also have
utility in heterojunction solar cell devices.7
The lack of pronounced thin-film crystallinity exhibited by both
polymers, combined with the suboptimal orientation of the backbone
(i.e., face-on rather than edge-on with respect to the substrate) and
relatively large π-stacking distances, would not be expected to be
favorable for charge transport. Although other seemingly amorphous
polymers have also shown high mobilities,8 it is still quite
remarkable that high carrier mobilities can be observed.
High-performance, solution-processed transistors fabricated
from semiconducting polymers containing indacenodithiohene
repeat units are described. The data reflect one of the highest
hole mobilities reported to date for a polymeric semiconductor,
which, coupled with both ambient stability and long-wavelength
absorption, makes this family of polymers particularly attractive
for application in the next generation of organic optoelectronic
devices.
Figure 3. (a) Transfer characteristic of a BC-TG (inset) IDT-BT transistor
with Au/perfluorobenzothiol-coated electrodes (L ) 30 µm, W ) 1 mm)
measured at VD ) -60 V. The right axis shows the square root of ID versus
VG, from which VT is extracted. (b) IDT-TT semiconductor transistor on
and off currents upon extended exposure to ambient conditions.
The charge carrier mobility was calculated from the transfer
characteristics of the device, in saturation, using previously
reported procedures.6 Figure 3a displays a typical transfer curve
obtained from an IDT-BT FET (channel length (L)/width (W)
of 30/1000 µm) at a drain voltage (VD) of -60 V. The transistors
yield maximum hole mobilities in the range 0.8-1.2 cm2/(V s),
with a current on/off ratio between 103 and 104 and a threshold
voltage (VT) of -25 to -34 V. Despite the very high mobility
values, FET operation is heavily injection limited, as evident
Acknowledgment. This work was carried out in part under the
EC FP7 ONE-P project no. 212311and DPI grant 678.
Supporting Information Available: Synthesis of IDT monomers
and copolymers, UV-vis absorption spectra, DSC traces, transistor
characteristics, DFT molecular modeling, and 2D GIXS of both
copolymers. This material is available free of charge via the Internet
at http://pubs.acs.org.
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C O M M U N I C A T I O N S
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Pisula, W.; Mu¨llen, K. AdV. Mater. 2009, 21, 209–212.
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