Benzo[1,2-b:4,5-b']dithiophene-based copolymers applied in bottom-contact field-effect transistors

Article abstract of DOI:10.1016/j.polymer.2010.05.022

Three copolymers of benzo[1,2-b:4,5-b']dithiophene and 3,3'-bis(alkyl)-5,5'-bithiophene (dodecyl, tetradecyl and hexadecyl side chains) have been synthesized through Stille copolymerization. The polymers have number-average molecular weights over 20 kg/mol, are well-packed in the bulk and thin film, and possess an ionization potential of -5.1 eV in thin film, which offers stability versus oxidation in environmental conditions. The thin film packing of the polymer with dodecyl side chains leads to an excimeric emission upon excitation, which is not observed for longer side chain lengths. The presence of the dimers responsible for this excimer formation results in a device performance improvement as well. Field-effect transistors fabricated from these copolymers have On/Off ratios >10⁷, equal saturation and linear hole mobilities above 10^-2 cm²/Vs, almost no hysteresis and turn-on voltages around 0 V in bottom-contact devices.

Full text of DOI:10.1016/j.polymer.2010.05.022

Polymer 51 (2010) 3099e3107
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
0
Benzo[1,2-b:4,5-b ]dithiophene-based copolymers applied in bottom-contact
field-effect transistors
a
,
b
b
c
,
d
a
a
Mark A.M. Leenen , Fabio Cucinotta , Wojciech Pisula , Jürgen Steiger , Ralf Anselmann ,
Heiko Thiem *, Luisa De Cola b
Evonik Degussa GmbH, Creavis e Technologies and Innovation, Paul-Baumann-Straße 1, D-45764 Marl, Germany
Physikalisches Institut and Center for Nanotechnology, CeNTech, Universität Münster, Mendelstraße 7, D-48149 Münster, Germany
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
a,
a
b
c
d
Evonik Degussa GmbH, Process Technology & Engineering, Process Technology e New Processes, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang, Germany
a r t i c l e i n f o
a b s t r a c t
0
0
0
Article history:
Received 19 January 2010
Received in revised form
Three copolymers of benzo[1,2-b:4,5-b ]dithiophene and 3,3 -bis(alkyl)-5,5 -bithiophene (dodecyl, tet-
radecyl and hexadecyl side chains) have been synthesized through Stille copolymerization. The polymers
have number-average molecular weights over 20 kg/mol, are well-packed in the bulk and thin film, and
possess an ionization potential of ꢀ5.1 eV in thin film, which offers stability versus oxidation in envi-
ronmental conditions. The thin film packing of the polymer with dodecyl side chains leads to an exci-
meric emission upon excitation, which is not observed for longer side chain lengths. The presence of the
dimers responsible for this excimer formation results in a device performance improvement as well.
3
May 2010
Accepted 13 May 2010
Available online 25 May 2010
Keywords:
Polymer synthesis
Polymer materials
7
Field-effect transistors fabricated from these copolymers have On/Off ratios >10 , equal saturation and
ꢀ
2
2
linear hole mobilities above 10 cm /Vs, almost no hysteresis and turn-on voltages around 0 V in
bottom-contact devices.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
oxidative polymerization [13], prompted us to resynthesize it by Stille
copolymerization. The resynthesized copolymer 1a (Scheme 1)
ꢀ
2
2
In the past decade, research towards solution-processed organic
semiconductors has increased tremendously [1,2], because these
materials are envisioned to enable low-cost electronics on flexible
surfaces [3]. Although the film-forming properties of (amorphous
or semicrystalline) conjugated polymers are superior to those of
small (crystalline) molecules, the material performance (charge
carrier mobility) is generally in favor of small molecules [4e6].
Nevertheless, also polymers are currently able to obtain mobilities
showed a hole mobility of 10 cm /Vs in bottom-contact geometry,
a performance improvement of over a factor 100 (vide infra). During
these measurements, a different research group has reported polymer
1a in a top-contact device geometry as well [14].
In this work, we have synthesized two additional polymers
with longer alkyl side chain lengths, in order to understand the
role played by the side chain length elongation on the material
performance. In fact it was shown for different types of systems
that such an increased side chain length can result in an
improved performance [8,15]. Thus, we have synthesized two
2
in excess of 0.5 cm /Vs [7e10], the benchmark performance of
amorphous silicon.
0
A commonly used building block, for p-type semiconducting
new poly[2,6-bis(3-alkylthiophen-2-yl)benzo[1,2-b:4,5-b ]dithio-
0
polymers, is benzo[1,2-b:4,5-b ]dithiophene (BDT). The 4,8-substitu-
phene]s, with tetradecyl 1b and hexadecyl 1c side chains
(Scheme 1). Furthermore, by utilizing a Soxhlet extraction, we
have greatly improved the molecular weight, the On/Off ratio
and hysteresis of 1a [14]. Finally, we have optimized the OFET
performance of these copolymers in bottom-contact (BC) device
geometry. Although a top-contact (TC) device geometry gener-
ally gives higher mobility values, material parameters can be
more reliably evaluated in the BC geometry [16] and addition-
ally, the BC geometry is most relevant for industrial application
of OFETs.
tionwith alkyl side chains, has already resulted in materials employed
in organic field-effect transistors (OFETs) with hole mobilities
2
exceeding 0.1 cm /Vs [11,12]. Also poly[2,6-bis(3-dodecylthiophen-2-
0
yl) benzo[1,2-b:4,5-b ]dithiophene] (PTBT),
a polymer of BDT
substituted with 3-alkylthiophenes, has been reported [13,14]. The
low reported mobility of PTBT (8 ꢁ 10ꢀ cm /Vs), synthesized by
5
2
*
Corresponding author. Tel.: þ49 2365 49 9483; fax: þ49 2365 49 80 9483.
E-mail address: heiko.thiem@evonik.com (H. Thiem).
0
032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.05.022

3100
M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
local film morphology [22]. Both effects are detrimental to device
performance.
3. Polymer characterization
The molecular weights of the synthesized copolymers, deter-
mined by gel permeation chromatography (GPC), are summarized
in Table 1. In all CB fractions, the number-average molecular weight,
Scheme 1. Synthesized copolymers in this work.
M
n
, is over 20 kg/mol, with a maximum M
Responsible for the higher average molecular weights, compared to
the polymer reported in recent literature (M 17 kg/mol, M 42 kg/
n
of 43 kg/mol for 1a.
n
w
mol) [14], is the Soxhlet extraction that successfully extracts olig-
omers and short polymer chains first, before collecting the polymer
fraction that will be used for device purposes.
The smallest polydispersity index (PDI) was found for 1a,
amounting to 1.8 for the CB fraction. Also the low PDI of the poly-
mer batches, in this work, is a direct result of the Soxhlet extrac-
tions. The weight-average molecular weight, M
fraction is smaller than the M of the corresponding CB fraction.
Peak molecular weights, M , of 1ae1c are comparable for both the
CHCl (12e13 kg/mol) and CB (35e41 kg/mol) fractions. Polymer
degradation could be responsible for the lower M and M of the
w 3
, of each CHCl
n
p
Scheme 2. Synthesis of Monomer 1 and Monomer 2. 4a, 5a, 6a: R ¼ C12
H25. 4b, 5b, 6b:
3
R ¼ C14 29. 4c, 5c, 6c: R ¼ C16
H
H
33.
n
w
ODCB fraction of 1b, compared to the CB fraction, due to the high
ODCB reflux temperatures during the Soxhlet extraction procedure.
1
2
. Synthesis
The synthesis of both monomers is depicted in Scheme 2. BDT 2
The polymers were characterized by H NMR (solutions of the
CHCl
3
fractions in deuterated 1,1,2,2-tetrachloroethane) and IR
spectroscopy (CB fractions as powder); both are in agreement with
data reported earlier on structurally similar polymers [12,13]. In the
IR spectra, there is almost no distinction possible between 1ae1c.
has been synthesized according to modified literature procedures
0
[17]. 2,6-Bis(tributylstannyl)benzo[1,2-b:4,5-b ]dithiophene
3
1
(
Monomer 1) has been synthesized from 2 by direct lithiation of the
- and 6-positions, followed by the addition of tributylstannyl-
chloride. The colorless oil was obtained in good yields.
-Bromo-3-alkylthiophenes 5ae5c were synthesized by selec-
tive bromination of corresponding 3-alkylthiophenes 4ae4c with
The different alkyl chain lengths can however be detected in the H
2
NMR spectra by the magnitude of the integration of the signal
around 1.00e1.50 ppm (see Experimental section). For all further
characterization and device fabrication, the CB fractions of the
polymers were used (unless stated differently).
We confirm the reported large tendency to aggregate of 1a in
diluted solution [14], which takes place to the same extent for 1b
and 1c. Such aggregate formation has been reported before for
similar copolymers as well [23]. Therefore, UVeVis absorption
2
0
0
NBS in glacial acetic acid in high yields. 2,2 -Dibromo-3,3 -bis
0
(
alkyl)-5,5 -bithiophenes 6ae6c (Monomer 2) were obtained by
homocoupling of 2-bromo-3-alkylthiophenes 5ae5c at the 5-
position of the molecule. This extraordinary homocoupling, where
the bromide functional group is preserved, was undertaken in
ꢂ
spectra have been recorded in heated solutions (60 C, chloroben-
warm DMSO, with PdCl
AgNO and KF [18,19].
2
(PhCN)
2
as catalyst, in the presence of
zene). As expected, the photophysical properties of copolymers
1ae1c in heated solution are all comparable, with an optical band
3
The copolymers 1ae1c were obtained through Stille copoly-
merization (Scheme 3) in good yields, determined from the
combined chloroform, chlorobenzene (CB) and 1,2-dichloroben-
zene (ODCB) fractions after Soxhlet extraction. A rather large excess
of Monomer 2 (1.04 eq with respect to Monomer 1) has been used
to tune chain length and therewith solubility, processability and
polymerization yield. Additionally, almost each polymer chain
should have the bithiophene monomeric unit at both chain ends by
using such an excess.
gap (E ) of 2.12 eV. Representative UVeVis absorption spectra of 1b
in solution, both heated and at room temperature, are shown in
Fig. 1.
g
The UVeVis absorption spectra of compounds 1ae1c, recorded
on drop-cast thin films, are shown in Fig. 2. For 1a, 1b and 1c, the
spectral pattern consists of a broad, slightly structured band
centered around 515 nm with a shoulder at about 550 nm. The
optical band gap E of 1a is smaller than E of 1b and 1c, that have
g
g
equal photophysical properties in thin film. For the CHCl fraction
3
To remove bromo- and tributyltin end-groups, the polymers
have been end-capped with phenyl-groups, by subsequent addition
of tributylphenyltin and bromobenzene after the polymerization
was completed. Without defined end-capping, remaining end-
groups can act as charge carrier trap sites [20,21], or can disturb
of 1a, the absorption peak is hypsochromically shifted to 473 nm, as
expected because of the lower molecular weight of the polymer. As
a consequence, the optical band gap for the CHCl fraction of 1a is
3
estimated to be 2.18 eV, more than 0.1 eV larger than those
obtained for the CB fractions of 1ae1c (Table 2).
Scheme 3. Stille copolymerization.

M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
3101
Table 1
GPC data of synthesized copolymers versus polystyrene standards, determined with
RI detector.
Polymer
Fraction
M
p
(kg/mol)
M
n
(kg/mol)
M
w
(kg/mol)
PDI
1
1
1
1
1
1
1
a
a
b
b
b
c
CHCl
CB
CHCl
CB
ODCB
CHCl
CB
3
13
38
13
35
52
12
41
11
43
12
22
20
9
17
76
19
69
66
1.6
1.8
1.5
3.1
3.3
1.8
4.0
3
a
a
a
a
3
15
116
c
29
a
Determined with UVeVis detector.
The emission spectra of 1ae1c in heated chlorobenzene solu-
tions are about equal and structured, with peaks at 540 nm and
85 nm (Fig. 3). It is noteworthy that neat films of 1a (both the CB
and the CHCl fraction) display a larger red-shift and a de-struc-
turing of emission (Fig. 3a and b) compared to the spectra of 1b and
c (Fig. 3c and d), indicating a higher structural ordering. Based on
this observation, we ascribe the different photophysical properties
of 1a not to the larger molecular weight of the CB fraction
compared to 1b and 1c, but to the possible formation of an excimer
due to the tilting of adjacent polymer chains, leading to an elec-
tronic interaction between neighbouring BDTcores. Apparently, the
longer tetradecyl 1b and hexadecyl 1c side chains do not allow for
the formation of such an excimer.
X-ray diffraction(XRD)measurementsshowanorderingof1ae1c
in thin films (Fig. 4). The films were drop cast from 1,2-dichloro-
benzene (ODCB) solutions on octyltrichlorosilane-pretreated silicon
substrates. From 2-dimensional wide-angle X-ray scattering (2D-
WAXS) experiments on extruded fibers of 1a and 1b (Fig. 5), a pep
inter-chain stacking distance of 0.37 nm (in accordance with litera-
ture [14]), and a chain-to-chain spacing related to side chain length
5
3
1
Fig. 2. Absorption spectra of 1a (CHCl
and 1c (solid lines) in thin films.
3
fraction, dashed), and the CB fractions of 1a, 1b
other hand, the hexadecyl side chains of 1c seem to have crossed the
upperlimit for side chain length that still results in a strongly defined
packing of the material.
Thermogravimetric analysis (TGA) shows a thermal stability of
the copolymers up to over 400 C in N
differential scanning calorimetry (DSC) traces are featureless, and
show no melting transitions or mesophases, comparable to
a similar dithienothiophene-based copolymer [23] but contrary to
PBTTT [8]. Also in temperature-dependent polarized optical
microscopy experiments, no phase transitions have been observed.
Finally, the ionization potentials IP of the polymers in thin film
were determined by UV Photoelectron Spectroscopy in air (PESA).
The ionization potential did not show a dependence on side chain
length, and was determined to be 5.1 ꢃ 0.02 eV. This ionization
potential is excellent for hole injection from clean gold contacts, and
is indicative of stability versus oxidation in ambient conditions [24].
ꢂ
2
-atmosphere. Subsequent
(
1.87 nm for 1a and 2.1 nm for 1b) can be extracted (Fig. 6). The latter
values are in agreement with the XRD-measurements in thin film
d ¼ 1.87 nm for 1a, d ¼ 2.05 nm for 1b, d ¼ 2.18 nm for 1c), where the
(
diffraction angle also clearly decreases with increasing side chain
length (Fig. 4). Both in bulk fiberand in thin film,1b shows diffraction
peaks with a higher intensity than 1a and 1c. This could be explained
by the excimer formation in case of 1a; even a small amount of inter-
chain BDT-dimers, that result in excimeric emission upon excitation,
could not only totally quench fluorescence, but would also disturb
supramolecular ordering to the extent shown in Figs. 4 and 5. On the
4. OFET characterization
Bottom gate, bottom-contact OFETs were fabricated to test the
electrical characteristics of these copolymers. The surface of the
Table 2
Photophysical data of 1ae1c.
ꢂ
Solution (60 C)
1a (CHCl
3
)
1a (CB)
1b (CB)
1c (CB)
a
l
abs (nm)
458
70,282
541
497
48,406
543
487
33,074
545
490
28,713
542
ꢀ
1
ꢀ1 a
)
3
max (M cm
a
lem (nm)
5
80
3%
2.25
583
3%
2.12
579
3%
2.12
580
3%
2.12
a
f
em
E
g
(eV)a,b
Neat films
c
l
abs (nm)
473
670
518
670
512
562
01
1%
513
561
596
1%
2.06
c
lem (nm)
6
c
f
em
1%
2.18
1%
2.01
E
g
(eV)c,d
2.06
a
ꢂ
Values obtained from chlorobenzene solutions heated to 60 C by a heating bath.
b
Values obtained from the onset of the absorption spectra (551 nm for 1a (CHCl
3
),
5
85 nm for the CB fractions of 1ae1c).
c
3
Values obtained from drop cast films out of a 0.02 M CHCl solution.
d
Values obtained from the onset of the absorption spectra (570 nm for 1a (CHCl
3
),
Fig. 1. Absorption spectra of the CB fraction of 1b, in chlorobenzene solution at room
temperature (RT) and heated to 60 C.
ꢂ
618 nm for 1a, 602 nm for 1b and 1c).

3102
M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
Fig. 3. (a) Normalized excitation (
CHCl fraction) in thin film (dotted line). (b) Normalized excitation (
spectrum of 1a in thin film (dotted line). The smaller intensity of the first emission band around 550 nm is due to self-absorption. (c) Normalized excitation (
emission (
exc ¼ 450 nm) spectra of 1b in chlorobenzene solution (solid line) and emission spectrum of 1b in thin film (dotted line). (d) Normalized excitation (
emission (
exc ¼ 450 nm) spectra of 1c in chlorobenzene solution (solid line) and emission spectrum of 1c in thin film (dotted line).
l
em ¼ 640 nm) and emission (
l
exc ¼ 450 nm) spectra of 1a (CHCl
em ¼ 640 nm) and emission ( exc ¼ 450 nm) spectra of 1a in chlorobenzene solution (solid line) and emission
em ¼ 585 nm) and
em ¼ 585 nm) and
3
fraction) in chlorobenzene solution (solid line) and emission spectrum of 1a
(
3
l
l
l
l
l
l
thermally grown SiO
2
dielectric was pretreated with trimethyl-
better than 1c. This can be explained by the side chain length
elongation in this series: the longer the side chain, the larger the
influence of the insulating side chains will be on film formation and
on the semiconductor film morphology at interfaces. Remarkably,
1a performs slightly better than 1b, although drop cast thin films of
1b show stronger reflections in XRD than films of 1a. Apparently,
the tilted parts of the polymer chains of 1a do not only allow for
excimer formation, but they also facilitate charge transport along
the channel, possibly by adding a third dimension for fast inter-
chain charge transport. However, just like in case of 1b and 1c, the
chlorosilane (TMCS) or octyltrichlorosilane (OTS) to obtain
a hydrophobic monolayer, on which the polymers could be spin-
coated or drop cast from ODCB-solutions. After coating the semi-
conductor layer, the devices were annealed in a glovebox for 30 min
ꢂ
at 130 C to remove residual solvent and induce film ordering. OFET
performance is summarized in Table 3.
From the transfer characteristics of 1ae1c, drop cast from ODCB
on TMCS (Fig. 7, left), it is clear that 1a and 1b perform considerably
3
OFET performance (hole mobility) of the CHCl fraction of 1a is one
order of magnitude lower than the mobility of the CB fraction,
underlining the strong dominance of molecular weight on device
performance.
Drop-casting results in rather rough and thick films
(d > 500 nm), therefore spin-coating was preferentially used for
further device optimization. Additionally, 1a was coated on
substrates with OTS monolayers. Organic thin films coated on OTS
tend to be of high quality [12,25], leading to improved device
performances compared to films coated on for instance TMCS. Next
to that, spin-coating 1b and 1c on TMCS resulted in lower m than
h
for devices fabricated by drop-casting on TMCS (Table 3).
Transfer characteristics of 1a spun on OTS are shown in Fig. 7
(right). Striking is the near-absence of hysteresis [26]. Secondly,
the linear and the saturation mobility (mlin and msat, respectively)
ꢀ
2
2
are equal and amount to 1.4$10 cm /Vs, whereas in most cases
the linear mobility is usually a bit lower than the saturation
mobility. Compared to the devices reported in literature [14], the
On/Off ratio of OFETs of 1a has been improved by over a factor 100,
Fig. 4. XRD-spectra of 1ae1c in drop cast thin films.

M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
3103
ꢂ
Fig. 5. 2D-WAXS patterns of 1a (left) and 1b (right) after annealing an extruded fiber at 180 C.
and the hysteresis has been greatly decreased. Additionally, the
same hole mobility has been achieved in a bottom-contact (BC)
geometry as previously reported in a top-contact (TC) geometry
measurements in the glovebox. The turn-on voltage is unaffected.
Ongoing dedicated long-term stability experiments suggest
a strong dominance of physical, reversible effects over (electro)
chemical, irreversible effects taking place.
[14]. Because TC OFETs have a better interface between the semi-
conductor layer and the contacts, compared to BC OFETs, they
usually perform better than BC OFETs.
Responsible for the better performance of these batches of
copolymers, compared to the polymer reported in literature [14], is
likely the Soxhlet extraction. This extraction has resulted in
a higher molecular weight and narrower PDI on the one hand, and
possibly lower impurity concentrations (that could act as dopants
5
. Conclusion
Three copolymers of BDT and bis(3-alkylthiophene)s have been
synthesized by Stille coupling (dodecyl, tetradecyl and hexadecyl
side chains) and subsequently purified and fractionated by Soxhlet
extraction. The CB fractions of the polymers all had an M > 20 kg/
n
mol. No liquid crystalline phases were observed in DSC traces. To
prevent aggregation, UVeVis absorption, excitation and photo-
luminescence spectra were recorded on heated solutions of the
polymers. The emission spectrum of the dodecyl-substituted
copolymer in thin film was distinctly different from the other two
[27]) on the other hand [28]. A small additional factor could be the
polymer chain end-capping.
Ionization potentials determined by PESA indicated that 1ae1c
would be stable versus oxidation in environmental conditions.
Therefore, OFETs of 1a were also operated in air. Transfer charac-
teristics of 1a in air are shown in Fig. 8. Immediately clear is the
increase in OFF current by a factor 100, and the appearance of
hysteresis. Both effects are reversible: if the device, measured in air,
is returned into a glovebox and shortly annealed, the original
characteristics obtained in the glovebox are re-obtained.
Measurements in air confirm that there is no irreversible oxidation
of the polymers taking place in environmental conditions. On/Off
polymers, showing
emission, both for the CHCl
a
red-shifted, ‘excimer-like’ unstructured
and the CB fractions. Our hypothesis
3
for the occurrence of such an emission spectrum is the formation of
excimers by tilted neighbouring polymer chains, resulting in an
additional electronic interaction between BDT cores. The longer
tetradecyl and hexadecyl side chains do not allow for the formation
of such an excimer.
5
ratios are still >10 , and the charge carrier mobility is
ꢀ3
2
8
.5
ꢁ
10
cm /Vs, only slightly decreased compared to
All polymers form ordered thin films, as studied by XRD. The
ionization potentials, determined in thin film in air, amount to
ꢀ
5.1 eV, indicative of stability versus oxidation in environmental
conditions. The best OFET performance in this series is obtained
with the dodecyl side chains, resulting in hole mobilities amount-
ꢀ
2
2
7
ing to 1.4 ꢁ 10 cm /Vs, On/Off ratios >10 , a negligible hysteresis
and turn-on voltages around 0 V in the industrially relevant
bottom-contact geometry. In air, the OFF current and hysteresis are
increased but no irreversible device degradation is taking place
during operation.
The results obtained in this work suggest that a polymer frac-
tionation and purification by Soxhlet extraction, which is being
omitted regularly, can clearly improve material performance in thin
film devices. We argue that the lower polymerization yield by
fractionation, and Soxhlet processing time are fully compensated
by better material performance. Furthermore, it is not unthinkable
that the excimer formation observed for the polymers described in
this work, can also occur for other fused conjugated cores with
a suitable inter-chain spacing. The accompanying device perfor-
mance improvement may suggest the incorporation of excimeric
dimers as a new design tool for high-performance OFET materials.
Fig. 6. Molecular ordering in an extruded fiber of 1a, derived from 2D-WAXS experi-
ments. The vertical lamellae correspond to the polymer long-axis along the conjugated
backbone.

3104
M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
Table 3
OFET performance of 1ae1c.
Polymer, coating
method
Silane,
device
m
sat
m
lin
On/off
Vturn-on (V)
Vth (V)
2
2
(cm /Vs)
(cm /Vs)
geometry
3
2
3
3
3
3
ꢀ3
5
5
1
1
1
1
1
1
1
1
1
1
1
a, drop cast
a, drop cast
b, drop cast
b, spun
c, drop cast
c, spun
a, drop cast
a, spun
a, spun, air
a spun [14]
a spun [13]
Bare, BC
7.5 ꢁ 10ꢀ
3.5 ꢁ 10
3 ꢁ 10
>107
ꢀ14
0
ꢀ25
ꢀ21
ꢀ27
ꢀ25
ꢀ33
ꢀ36
ꢀ21
ꢀ23
ꢀ21
ꢀ20
ꢀ32
ꢀ
ꢀ2
TMCS, BC
TMCS, BC
TMCS, BC
TMCS, BC
TMCS, BC
OTS, BC
OTS, BC
OTS, BC
HMDS, TC
ODTS-12, TC
1.2 ꢁ 10
10
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ2
ꢀ3
7
9 ꢁ 10
7.5 ꢁ 10
4 ꢁ 10
1.2 ꢁ 10
1.5 ꢁ 10
6.5 ꢁ 10
1.4 ꢁ 10
7 ꢁ 10
e
>10
ꢀ10
ꢀ4
ꢀ6
ꢀ12
0
6 ꢁ 10
>107
3 ꢁ 10
6 ꢁ 10
6
2 ꢁ 10
10
ꢀ
10
2
>107
>107
ꢀ2
ꢀ3
ꢀ2
1.4 ꢁ 10
8.5 ꢁ 10
1.5 ꢁ 10
e
0
0
5
2.5 ꢁ 10
5a
a
a
a
a
10
10
ꢀ4
8 ꢁ 10ꢀ
5
4
ꢀ31
Values extracted from transfer characteristics at Vds ¼ ꢀ60 V, linear mobility extracted at Vds ¼ ꢀ4 V. BC ¼ bottom-contact, TC ¼ top-contact. W/L ¼ 500 (this work), W/L ¼ 20
[
V
14], W/L ¼ 10 [13].
th ¼ Threshold voltage.
a
Data estimated from published characteristics.
6
. Experimental
a Nicolet Nexus FTIR spectrometer with a Diamond ATR cell (1a) or
a Bruker Tensor 27 FTIR Spectrometer with a golden gate diamond
ATR unit (1b, 1c).
6.1. Materials and methods
ꢂ
GPC traces were recorded in 80 C ODCB on a PL-GPC120 system
3
-Tetradecylthiophene and 3-hexadecylthiophene were
with a PLgel 5 mm Mixed_C column and refractive index and
purchased from Carbosynth. N-Bromosuccinimide (NBS) was
purchased from Acros organics. Tert-butyllithium, glacial acetic
acid, chlorobenzene, 1,2-dichlorobenzene and Celite 545 were
purchased from Merck. All other chemicals were purchased from
Sigma Aldrich. All chemicals were used as received without further
purification. All reactions were performed under inert (Ar) atmo-
sphere and were monitored by thin layer chromatography (TLC,
silicagel 60 F254 and Aluminium Oxide 150 F254 sheets, Merck). All
column chromatography was performed with silicagel 60 (Fluka)
unless stated otherwise.
UVeVis detectors. Polystyrene standards were used.
TGA measurements were performed on a Netzsch TG 209
thermo-microbalance. DSC measurements were performed on
a Mettler Toledo DSC820 Calorimeter on open (hole in lid)
aluminium sample pans.
UVeVis absorption spectra were recorded on a Varian Cary 5000
double-beam UVeViseNIR spectrophotometer; all spectra in
solution were measured from quartz cuvettes (1 cm, Hellma).
Steady-state emission spectra in the spectral range of 300e800 nm
were recorded on a HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3
spectrometer equipped with a 450 W Xenon arc lamp, double
grating excitation and emission monochromators (2.1 nm/mm
dispersion; 1200 grooves/mm) and a Hamamatsu R928 photo-
multiplier tube or a TBX-4-X single-photon-counting detector.
Emission and excitation spectra were corrected for source intensity
(lamp and grating) and emission spectral response (detector and
grating) by standard correction curves.
All NMR spectra were measured on a Bruker Avance NMR
1
13
spectrometer. H (500 MHz) and C (125 MHz) chemical shifts in
ppm relative to TMS (0.00 ppm) or dichloromethane (5.32 ppm for
1
H and 54.00 for 13C). q ¼ quintet. Mass spectrometry was per-
formed on a Thermo Fisher LTQ FT mass spectrometer in THF,
a Thermo Fisher Trace DSQ II GC-MS, a Fisons GC8000-MD800 GC-
MS, or a Accela LTQ FT LC-MS. IR spectroscopy was performed on
Fig. 7. Transfer characteristics of OFETs of (a) 1ae1c drop cast on TMCS and (b) 1a spun on OTS, measured in a glovebox.

M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
3105
"
#
2
1
=2
vððI ÞÞ
2L
WCi
ds
m
¼
(3)
sat
vV
g
6.2. Monomer synthesis
0
Benzo[1,2-b:4,5-b ]dithiophene 2 was synthesized according to
modified literature procedures [17].
1
H NMR (d-THF):
d
8.35 (s, 2H), 7.56 (d, J 5.5 Hz, 2H), 7.38 (d, J
.5 Hz, 2H). C NMR (d-THF): 138.7, 138.2, 128.0, 123.8, 117.5. GC-
MS: found: 189.9, calculated: 189.991 g/mol (C10 ).
13
5
d
6 2
H S
0
6
.2.1. Synthesis of 2,6-bis(tributylstannyl)benzo[1,2-b:4,5-b ]
dithiophene 3
Glassware was dried before use. A solution of 1 g (5.26 mmol, 1
ꢂ
eq) 2 in 60 ml dry THF was cooled down to ꢀ80 C. After drop-
wise addition of 7.5 ml tert-butyllithium (15% in pentane,
1
2.7 mmol, 2.4 eq), the mixture was allowed to slowly warm to
Fig. 8. Transfer characteristics of 1a, spun on OTS in air.
ꢂ
ꢂ
ꢀ
10 C. The mixture was then cooled down to ꢀ50 C again, and
3
.5 ml (4.2 g, 12.7 mmol, 2.4 eq) tributylstannylchloride was
Emission quantum yields were determined with a Hama-
matsu Photonics absolute PL quantum yield measurement
system (C9920-02) equipped with a L9799-01 CW Xenon light
source (150 W), monochromator, C7473 photonic multi-channel
analyzer, integrating sphere and employing U6039-05 PLQY
measurement software (Hamamatsu Photonics, Ltd., Shizuoka,
Japan).
added dropwise. The mixture was allowed to reach room
temperature overnight.
The next morning, the mixture was quenched with 150 ml 0.1 M
NaHCO , and the product was extracted with dichloromethane
3
(4 ꢁ 60 ml). The organic phase was further washed with 0.1 M
NaHCO (2 ꢁ 60 ml), dried with Na SO , filtered and the solvent
3
2
4
was evaporated. Column chromatography (heptane/triethylamine
A Siemens D500 Kristalloflex with a graphite-monochromized
95/5, neutral aluminium oxide) yielded 3.1 g of 3 as a colorless oil
CuK
a
X-ray beam was used for X-ray diffraction experiments on
range
(76% yield).
1
thin films. The diffraction patterns were recorded in the 2
q
2 2
H NMR (CD Cl ): d 8.32 (s, 2H), 7.46 (s, 2H), 1.68 (q, 12H), 1.42
ꢂ
ꢂ
13
from 1 to 50 and are presented as a function of the scattering
angle. The 2D-WAXS measurements were performed by means of
a rotating anode (Rigaku 18 kW) X-ray beam with a pinhole colli-
mation and a 2D Siemens detector. A double graphite mono-
(sextet, 12H), 1.25 (t, J 8 Hz, 12H), 0.96 (t, J 7.25 Hz, 18H). C NMR
(CD Cl ): 140.6, 140.5, 137.7, 130.2, 113.8, 28.1, 26.4, 12.5, 9.9.
2
2
d
6.2.2. Synthesis of 2-bromo-3-tetradecylthiophene 5b
chromator for the CuK
samples were prepared by filament extrusion at elevated temper-
atures [29].
a
radiation (
l
¼ 0.154 nm) was used. The
To a solution of 1.8 g (6.42 mmol,1 eq) 3-tetradecylthiophene 4b
in 20 ml glacial acetic acid, 1.05 g NBS was added in small amounts
over time, in absence of light. After stirring at room temperature
overnight, the mixture was quenched with demineralised water
(40 ml) and the product was extracted with dichloromethane
(3 ꢁ 20 ml). The organic phase was washed with 1 M NaOH
(2 ꢁ 30 ml), once more with brine (30 ml), dried with Na SO ,
PESA measurements were performed in air on a Riken Keiki AC2
Photoelectron spectrometer with a deuterium UV lamp and grating
2
type monochromator. UV spot area is 4 mm , spectra were cor-
rected for relative light intensity at each wavelength. PESA samples
2
4
were spin-coated on OTS-pretreated SiO
solutions.
2
-substrates from ODCB-
filtered and the solvent was evaporated. Column chromatography
(heptane) yielded 1.93 g of 5b as a colorless liquid (84% yield).
1
OFET transfer and output characteristics were measured on an
2 2
H NMR (CD Cl ): d 7.21 (d, J 5.5 Hz, 1H), 6.82 (d, J 5.5 Hz, 1H),
Agilent 4156C semiconductor parameter analyzer. The hole
2.57 (t, J 7.5 Hz, 2H), 1.58 (q, 2H), 1.24e1.36 (m, 22H), 0.89 (t, J 7 Hz,
2
13
mobility
m
h
(cm /Vs) was determined from the transfer character-
3H). C NMR (CD Cl ):
d
142.8, 129.0, 125.8, 109.2, 32.6, 30.3 (6C),
30.2, 30.0 (3C), 29.8, 23.3, 14.5. GC-MS: found: 358.4, calculated:
358.133 g/mol (C18 31SBr).
2
2
istics, using the following formula [2]:
ꢀ
ꢃ
H
W
L
ꢁ
ꢂ
1
2
2
ds
Ids
¼
m
hCi
V
g
ꢀ Vth Vds ꢀ V
(1)
6.2.3. 2-Bromo-3-dodecylthiophene 5a was synthesized in the
same procedure as 5b (86% yield)
where Ids is the measured drain-source current (A), V
g
the gate
1
H NMR (CD
2
Cl
2
):
d 7.21 (d, J 5.5 Hz, 1H), 6.82 (d, J 5.5 Hz, 1H),
voltage (V), L the channel length (20,10, 5 or 2.5
width (0.01 m) and C the capacitance of the dielectric (F/cm ). In
the linear regime (Vds << V ), equation (1) can be rewritten, and
mm), W the channel
2
2.57 (t, J 8 Hz, 2H), 1.58 (q, 2H), 1.22e1.38 (m, 18H), 0.89 (t, J 7 Hz,
i
13
3
(
H). C NMR (CD
3C), 29.9, 29.8, 29.7 (2C), 29.6, 23.1, 14.2. GC-MS: found: 330,
calculated: 330.102 g/mol (C16 27SBr).
2
Cl
2
): d 142.6, 128.7, 125.6, 108.9, 32.3, 30.1, 30.0
g
when the derivative is taken, the linear mobility
mlin can be calcu-
H
lated independently of Vth (2):
vIds
L
6.2.4. 2-Bromo-3-hexadecylthiophene 5c was synthesized in the
m
¼
(2)
lin
same procedure as 5b (87% yield)
vV
g
i
WC V
ds
1
2 2
H NMR (CD Cl ): d 7.22 (d, J 6 Hz,1H), 6.83 (d, J 6 Hz,1H), 2.57 (t,
In the saturation regime (Vds z V
rewritten as well. Taking the square root and then the deriva-
tive, the saturation mobility msat can be calculated independently
of Vth (3):
g
ꢀ Vth), equation (1) can be
13
J 8 Hz, 2H), 1.58 (q, 2H), 1.22e1.37 (m, 26H), 0.89 (t, J 7 Hz, 3H).
C
NMR (CD Cl ): 142.6, 128.7, 125.6, 108.9, 32.3, 30.1 (4C), 30.0 (4C),
2
2
d
2
9.9, 29.8, 29.7 (2 peaks), 29.6, 23.1, 14.2. LC-MS: found: 387.171
þ
[M þ H ], calculated: 386.164 g/mol (C20
H35SBr).

3106
M.A.M. Leenen et al. / Polymer 51 (2010) 3099e3107
0
0
0
ꢀ1
6
.2.5. Synthesis of 2,2 -dibromo-3,3 -bis(tetradecyl)-5,5 -
36H), 0.90 (br s, 6H). IR (cm ): 3060 (aromatic CeH), 2919, 2849
(eCH e), 1650 (CeS in BDT), 1508 (aromatic C]C), 1464 (CeS in
thiophene).
bithiophene 6b [18]
To a solution of 0.95 g (2.64 mmol, 1 eq) 5b in 30 ml dry
DMSO, 22 mg (0.057 mmol, 0.02 eq) PdCl (PhCN) and 314 mg
5.4 mmol, 2 eq) KF were added subsequently. In absence of
2
2
2
0
(
6.3.2. Poly[2,6-bis(3-tetradecylthiophen-2-yl)-benzo[1,2-b:4,5-b ]
light, 914 mg (5.4 mmol, 2 eq) AgNO
mixture was stirred at 80 C for 4 h. After cooling down to room
temperature, an additional 316 mg (5.4 mmol, 2 eq) KF and
3
was added and the
dithiophene] 1b (50% yield)
ꢂ
1
2 2
H NMR (CDCl -CDCl ): d 8.20 (br s, 2H), 7.37 (br s, 2H), 7.07 (br
s, 2H), 2.87 (br s, 4H), 1.74 (br s, 4H), 1.05e1.57 (br s þ shoulder,
ꢀ
1
9
00 mg (5.3 mmol, 2 eq) AgNO
3
were added and the mixture
44H), 0.89 (br s, 6H). IR (cm ): 3060 (aromatic CeH), 2919, 2849
(eCH e), 1650 (CeS in BDT), 1509 (aromatic C]C), 1464 (CeS in
thiophene).
ꢂ
was stirred overnight at 80 C.
2
The next morning, after cooling down to room temperature
again, an additional 152 mg (2.62 mmol, 1 eq) KF and 450 mg
0
(
2.65 mmol, 1 eq) AgNO
3
were added, and the mixture was stirred
6.3.3. Poly[2,6-bis(3-hexadecylthiophen-2-yl)-benzo[1,2-b:4,5-b ]
ꢂ
at 80 C for 4 h more. When the mixture had been cooled down to
room temperature again, it was filtered over celite. The celite cake
was washed thoroughly with dichloromethane. The resulting
organic phase was washed with demineralised water (2 ꢁ 40 ml),
the water phase was extracted with 20 ml dichloromethane. The
combined organic phases were washed with demineralised water
dithiophene] 1c (69% yield)
1
2 2
H NMR (CDCl eCDCl ): d 8.21 (br s, 2H), 7.40 (br s, 2H), 7.10 (br
s, 2H), 2.88 (br s, 4H), 1.74 (br s, 4H), 1.10e1.50 (br s þ shoulder,
ꢀ1
52H), 0.89 (br s, 6H). IR (cm ): 3062 (aromatic CeH), 2919, 2849
(eCH e), 1650 (CeS in BDT), 1510 (aromatic C]C), 1464 (CeS in
2
thiophene).
once more (40 ml), dried with Na
evaporated. Column chromatography (heptane) yielded 543 mg of
b as a yellow solid (57% yield).
2 4
SO , filtered and the solvent was
6.4. OFET fabrication
6
1
H NMR (CDCl
.23e1.37 (m, 44H), 0.88 (t, J 7 Hz, 6H). C NMR (CDCl
36.2, 124.5, 107.9, 31.9, 29.7 (5C), 29.6 (3C), 29.4 (2C), 29.2, 22.7,
3
):
d
6.76 (s, 2H), 2.51 (t, J 7.75 Hz, 4H), 1.57 (q, 4H),
Silicon wafers with a 230 nm thick SiO dielectric and patterned
2
13
1
3
):
d
143.0,
gold contacts (L ¼ 20, 10, 5, 2.5 m, W ¼ 0.01 m) were purchased
m
1
from Fraunhofer IPMS in Dresden (D). The substrates were soni-
cated for 5 min in a boiling 1:1 acetone:isopropanol mixture, and
afterwards washed on a spin-coater with isopropanol and demin-
eralised water, subsequently. After heating the substrates on a hot
þ
14.1. LC-MS: found: 715.258 [M þ H ], calculated: 714.250 g/mol
(
C
H
36 60
S
2
2
Br ).
0
0
0
ꢂ
6
.2.6. 2,2 -Dibromo-3,3 -bis(dodecyl)-5,5 -bithiophene 6a was
plate at 110 C for 1 min, the substrates were subjected to a UV/
synthesized in the same procedure as 6b (73% yield)
ozone treatment for 11 min.
1
H NMR (CD
.21e1.40 (m, 36H), 0.89 (t, J 7 Hz, 6H). C NMR (CD
36.5, 125.0, 108.1, 32.3, 30.0 (3C), 29.9 (3C), 29.7 (2C), 29.5, 23.1,
2
Cl
2
):
d
6.82 (s, 2H), 2.53 (t, J 7.5 Hz, 4H), 1.59 (q, 4H),
A trimethylsilane (TMS) or octylsilane (OTS) monolayer was
grown on the dielectric surface by putting the substrates in a 5%
solution of trimethylchlorosilane (TMCS) in dry ethanol, respec-
tively a 5% solution of octyltrichlorosilane (OTCS) in dry iso-
propanol, for 3 h. After monolayer growth, the substrates were
washed with ethanol or isopropanol to remove physisorbed silane.
The polymer thin films were spin-coated (1000 rpm, 60 s) from
0.5 wt% solutions, or drop cast from 0.1 wt% solutions, of the
chlorobenzene fractions of the polymers in 1,2-dichlorobenzene.
13
1
2 2
Cl ): d 143.6,
1
þ
14.2. LC-MS: found: 659.194 [M þ H ], calculated: 658.188 g/mol
(
C
H
32 52
S
2
2
Br ).
0
0
0
6
.2.7. 2,2 -Dibromo-3,3 -bis(hexadecyl)-5,5 -bithiophene 6c was
synthesized in the same procedure as 6b (57% yield)
1
H NMR (CD
2 2
Cl ): d 6.82 (s, 2H), 2.54 (t, J 8 Hz, 4H), 1.59 (q, 4H),
13
ꢂ
1
.21e1.40 (m, 52H), 0.89 (t, J 7 Hz, 6H). C NMR (CD
2
Cl
2
):
d
143.6,
The devices were annealed in a glovebox for 30 min at 130 C before
1
2
36.5, 125.0, 108.1, 32.3, 30.1 (5C), 30.0 (3C), 29.9 (3C), 29.7, 29.5,
the measurements were done.
þ
3.1, 14.2. LC-MS: found: 771.319 [M þ H ], calculated: 770.313 g/
mol (C40
H
68
S Br
2 2
).
Acknowledgement
6
.3. General procedure for Stille copolymerization
The authors thank Ralf Bovee for high-temperature GPC
measurements, the European Commission for financing through
the Human Potential Programme (Marie-Curie RTN NANOMATCH,
Grant No. MRTN-CT-2006-035884) and the BMBF for financing
through the MaDriX project.
To a dispersion of 0.25 mmol (1 eq) 3 and 0.26 mmol (1.04 eq) 6
in 6 ml dry toluene and 4 ml dry DMF, 0.005 mmol (0.02 eq) Pd
PPh was added, and the mixture was refluxed for 3 days. After
(
3 4
)
cooling down to room temperature, 0.1 ml tributylphenyltin was
added, and the mixture was refluxed for another 6 h. The mixture
was cooled down to room temperature again to add 0.1 ml bro-
mobenzene, and was refluxed overnight.
After cooling down to room temperature again, the mixture was
poured out in methanol, and stirred for 1 h. The polymer suspen-
sion was filtered through a Soxhlet thimble, and fractionated
through Soxhlet extraction (methanol, acetone, hexane, chloro-
form, chlorobenzene, dichlorobenzene). The chloroform, CB and
ODCB fractions were taken into account to calculate the yield
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