Charge carrier mobility study of a mesogenic thienothiophene derivative in bulk and thin films

Article abstract of DOI:10.1016/j.orgel.2014.01.017

A novel mesogenic 2,5-bis-(5-octylthiophene)-thieno[3,2b]thiophene (TT) derivative has been synthesized. The fused-ring thiophene, end-capped with two octylthiophenes, exhibits ordered lamellar mesophases which were characterized by polarizing optical microscopy, differential scanning calorimetry and small-angle X-ray diffraction at various temperatures. The charge transport properties were investigated by time-of-flight technique as a function of temperature. On cooling from isotropic phase, a maximum hole mobility value of 0.07 cm² V^-1 s^-1 was measured in the highly ordered mesophase of the bulk films. Field-effect transistor experiments on both solution and vacuum deposited thin films have also been performed. The solution-processed films exhibit charge carrier mobilities several orders of magnitude lower than values extracted from bulk time-of-flight curves and from vacuum deposited thin film transistors. This work provides evidence that the melt-processing route is an efficient alternative to commonly used solution-processing for fabrication of charge transporting layers from liquid crystalline semiconductors, with performances comparable to evaporation techniques.

Full text of DOI:10.1016/j.orgel.2014.01.017

Organic Electronics 15 (2014) 943–953
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Charge carrier mobility study of a mesogenic thienothiophene
derivative in bulk and thin films
Leszek Mazur a,b, Andrea Castiglione a, Kornel Ocytko b, Farid Kameche a, Romain Macabies e,
Abdelmalek Ainsebaa e, David Kreher a, Benoît Heinrich c, Bertrand Donnio d,
Sébastien Sanaur e, Emmanuelle Lacaze f, Jean-Louis Fave f, Katarzyna Matczyszyn b
Marek Samoc b, Jeong Weon Wu g, Andre-Jean Attias a, Jean-Charles Ribierre g,
,
⇑
Fabrice Mathevet a,
⇑
a Laboratoire des Chimie des Polymères, Université Pierre et Marie Curie, 3, rue Galilée, 94200 Ivry, France
b Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
c Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 (CNRS-Université de Strasbourg), 23, rue du Loess, 67034 Strasbourg Cedex
2, France
d Complex Assemblies of Soft Matter Laboratory (COMPASS), UMI 3254 (CNRS-Rhodia/Solvay-University of Pennsylvania), CRTB, 350 George Patterson
Boulevard, Bristol, PA 19007, USA
e Département de Bioélectronique, École Nationale Supérieure des Mines, CMP-EMSE, MOC, 880, route de Mimet, 13541 Gardanne, France
f Institut des NanoSciences de Paris, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
g Department of Physics, CNRS-Ewha International Research Center, Ewha Womans University, Seoul 120-750, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel mesogenic 2,5-bis-(5-octylthiophene)-thieno[3,2b]thiophene (TT) derivative has
been synthesized. The fused-ring thiophene, end-capped with two octylthiophenes, exhib-
its ordered lamellar mesophases which were characterized by polarizing optical micros-
copy, differential scanning calorimetry and small-angle X-ray diffraction at various
temperatures. The charge transport properties were investigated by time-of-flight tech-
nique as a function of temperature. On cooling from isotropic phase, a maximum hole
mobility value of 0.07 cm2 Vꢀ1 sꢀ1 was measured in the highly ordered mesophase of the
bulk films. Field-effect transistor experiments on both solution and vacuum deposited thin
films have also been performed. The solution-processed films exhibit charge carrier mobil-
ities several orders of magnitude lower than values extracted from bulk time-of-flight
curves and from vacuum deposited thin film transistors. This work provides evidence that
the melt-processing route is an efficient alternative to commonly used solution-processing
for fabrication of charge transporting layers from liquid crystalline semiconductors, with
performances comparable to evaporation techniques.
Received 31 October 2013
Received in revised form 13 January 2014
Accepted 15 January 2014
Available online 10 February 2014
Keywords:
Thienothiophene
Ordered mesophase
Charge transport properties
Hole mobility
Time-of-flight
Organic field-effect transistor
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
much attention as futuristic materials for the development
and production of the next generation of electronic and
Since the discovery of conducting properties in
p
-conju-
optoelectronic devices [2] such as organic solar cells
[3–6] or organic field-effect transistors [7,8] (OFETs). The
fabrication of commercial OFET devices by a solution pro-
cess is generally preferred considering its potential for
large-area applicability and low cost production. However,
the charge carrier mobility of OFET semiconducting active
gated polymers [1],
p-conjugated systems have attracted
⇑
Corresponding authors. Tel.: +33 0144275032 (F. Mathevet), Tel. +81
928026920 (J.-C. Ribierre).
E-mail addresses: ribierre@opera.kyushu-u.ac.jp (J.-C. Ribierre),
fabrice.mathevet@upmc.fr (F. Mathevet).
http://dx.doi.org/10.1016/j.orgel.2014.01.017
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

944
L. Mazur et al. / Organic Electronics 15 (2014) 943–953
layers prepared by such a process is usually lower than
that of devices fabricated by vacuum deposition. From a
general point of view, that is due to the inevitable loss of
long-range scale order and to the formation of ‘‘grain
boundaries’’ defects inherent to polycrystalline structures
formed during the solvent evaporation stage.
organization behavior by polarizing optical microscopy
(POM), differential scanning calorimetry (DSC) and tem-
perature-dependent small-angle X-ray scattering (SAXS).
Second, we investigated the hole transport properties of
this material using the TOF technique in order to correlate
self-organization and mobility in melt-processed bulk
samples. Finally, we prepared thin films on SiO2/Si sub-
strates by solution and vacuum deposition techniques for
evaluation of charge transport in OFET configuration, to
compare and discuss the experimental results on TOF
and FET mobilities of the polycrystalline thin films.
Among all the
p-conjugated architectures, liquid–crys-
talline semiconductors have emerged as promising materi-
als for organic electronics and appear as appropriate
candidates to avoid these deleterious effects due to their
dynamic and tunable supramolecular organizing and self-
healing abilities [9–12]. Recent studies performed on semi-
conducting calamitic liquid crystals showing highly or-
dered smectic mesophases have validated this approach
and high charge carrier mobilities up to 10ꢀ1 cm2 Vꢀ1 sꢀ1
were extracted from time-of-flight (TOF) experiments
and OFET measurements [13–17].
2. Results and discussion
2.1. Material design and synthesis
The TT calamitic semiconductor consists of an elon-
gated aromatic core associated with electronic charge car-
rier transport and peripheral alkyl chains which control the
solubility and the self-organization of molecules into lay-
ered structure. The synthetic route is shown in Scheme 1.
The mesogenic thieno[3,2-b]thiophene derivative was syn-
thesized by Stille cross-coupling reaction between the
dibromo[3,2b]thienothiophene and the 2-stanyl-5-octyl-
thiophene, in the presence of catalytic amounts of
Pd(PPh3)3 and DMF as solvent, with a yield of 91% [29].
The catalyst was prepared in situ via reaction of the Pd
(acetate) and PPh3. The dibromo[3,2b]thienothiophene
precursor was prepared via bromination of thieno[3,2b]
thiophene with NBS at room temperature in dichlorometh-
ane [25]. The 2-stannyl-5-octylthiophene was synthesized
from the reaction of the commercially available 2-n-octyl-
thiophene with the tributyltin chloride in THF as solvent
[30]. All these compounds were purified with column
chromatography on silica gel and the final molecule TT
was also purified with several additional crystallizations
in ultra-pure solvents. The products were characterized
by 1H and 13C NMR.
Recently, a smart melt-processing route has been re-
ported to fabricate charge-transporting layers from liquid
crystalline organic semiconductors. This approach ap-
peared as an efficient alternative to commonly used vac-
uum or solution processing for the elaboration of well-
structured polycrystalline organic films [18–20]. Indeed,
this route, which takes advantage on the self-healing abil-
ities of mesophases, allows to control the polycrystalline
film morphology on cooling from isotropic state, and can
lead to significant improvement of the layer charge trans-
port properties.
Fused thiophene, in particular thieno[3,2-b]thiophene
is a very attractive and important chemical moiety for
the design of organic semiconductors with improved
air-stability and relative good transport performances
for both low-molecular weight systems [21] and poly-
mers [22]. Indeed, due to the central cross-conjugated
double bond, this planar and stable aromatic heterocycle
can form a fully conjugated pathway and preserve the
close
p
–
p
intermolecular distances necessary for high
-conju-
charge carrier mobility when incorporated in
p
gated systems. Thus, a large number of semiconducting
molecules or oligomers based on thieno[3,2-b]thiophene
unit was recently prepared and successfully studied in
evaporated thin-film transistor configuration [23–26].
Moreover, McCulloch et al. have demonstrated the possi-
bility to confer mesomorphic properties to the fused thi-
ophene moiety by synthesizing a semiconducting main-
chain liquid–crystalline thieno[3,2-b]thiophene polymer.
This polymer exhibited enhanced charge-carrier mobility
(up to 0.6 cm2 Vꢀ1 sꢀ1), when crystallized from the meso-
phase, due to the formation of large, well organized
lamellar domains [27,28]. However, no studies on the
influence of the polymorphism on the charge transport
properties of mesogenic thieno[3,2-b]thiophene based
small molecules have been reported so far.
2.2. Cyclic voltammetry, absorption and emission spectra
measurements
Absorption and emission spectra of the TT molecule in
chloroform are shown in Fig. 1a. The absorption spectrum
shows a peak with a maximum at 382 nm while the emis-
sion spectrum presents two vibronic peaks centered
around 430 and 455 nm as well as a shoulder close to
480 nm. The optical bandgap (Eg) of TT can be approxi-
mated by determining the intersection of its absorption
and emission spectra [31], which is around 418 nm. Thus,
the Eg was estimated to be 2.96 eV (see experimental
Section 4.2).
In this context, our main objectives were to design a
mesogenic thieno[3,2-b]thiophene molecular derivative
and to demonstrate that the smart melt-processing route
can be used to enhance the morphology and the charge
transport properties of organic films based on this kind of
material. For this purpose, we prepared a novel mesogenic
compound, 2,5-bis-(5-octylthiophene)-thieno[3,2b]thio-
phene (TT). First, we fully studied its thermal and self-
The cyclic voltammetry (CV), typically used to evaluate
the energy levels of organic semiconductors in solution,
were carried out to determine the highest occupied molec-
ular orbital energy (EHOMO) of TT compounds. The electro-
chemical potential was calibrated using the ferrocene/
ferrocenium (Fc/Fc+) couple as an external standard which
was measured under same condition before and after the
measurements of the TT sample. The energy level of the

L. Mazur et al. / Organic Electronics 15 (2014) 943–953
945
S
S
Br
S
S
ii
C8H17
C8H17
i
Sn(Bu)4
Br
S
S
2
1
S
S
Br
S
iii
C8H17
C8H17
Sn(Bu)3
S
+
S
C8H17
Br
S
S
TT
1
2
Scheme 1. Reaction scheme and chemical structure of, 2,5-bis-(5-octyl-thiophene)-thieno[3,2b]thiophene (TT). Conditions: (i) NBS CH2Cl2 (ii) (a) BuLi-
78 °C THF (b) Sn(Bu)3Cl-78 °C (iii) Pd(acetate), PPH3 THF DMF.
Fig. 1. (a) Normalized absorption (solid) and emission spectra (dash) of TT chloroform solutions. (b) Cyclic voltammogram of TT at 2 ꢂ 10ꢀ4 mol/l in a
solution of dichloromethane containing 5 ꢂ 10ꢀ3 mol/l of nBu4NPF6 as the electrolyte, the reference electrode being Ag/AgCl.
Fc/Fc+ couple was assumed at ꢀ4.8 eV to vacuum [32,33].
transitions obtained from DSC traces (Fig. 3a and b) are
consistent with the observations made by POM.
The half-wave potential of the Fc/Fc+ couple, denoted
E1/2,Fc/Fc+
,
was evaluated from: E1/2,Fc/Fc+ = (Epc + Epa)/2,
On slow cooling from the isotropic liquid (0.5 °C minꢀ1),
the material displayed fleetingly both the typical fan-
shaped and the schlieren domains between neat glass
slides at 186.5 °C, suggesting a metastable tilted lamellar
mesophase, presumably a smectic C mesophase (SmC)
with both smectic planes respectively upright and parallel
to the substrate (Fig. 2a) [34]. On further cooling, the SmC
phase quickly transforms into an ordered mesophase (M1)
characterized by an abrupt textural change of the fan-
shaped and the schlieren textures into a high viscous
spherulitic and mosaic texture, respectively (Fig. 2b and
c). On further cooling, the ordered mesophase textures
are maintained and do not change down to around 50 °C
(Fig. 2c–e) where they transform into a stable crystalline
form (Cr) (Fig. 2f). These observations suggest first the
presence of a monotropic SmC phase before the appear-
ance of the highly ordered mesophase organization and,
second, the value of the crystallization temperature around
50 °C.
where Epc and Epa correspond to the cathodic and anodic
potentials. As shown in Fig. 1b, the CV of TT in dichloro-
methane solution presents one reversible oxidation wave,
with a half wave oxidation potential value E1/2,ox = 0.94 V
versus Ag/AgCl, corresponding to a value of 0.45 V versus
Fc/Fc+. The HOMO energy level was then calculated to be
ꢀ5.25 eV using the equation EHOMO = ꢀ(4.8 eV + E1/2,ox
versus Fc/Fc+). No reversible reduction wave was observed
in the range of potentials available in our experiment, indi-
cating the only electron-donor character of the TT com-
pound. The LUMO energy level of TT was then calculated
by subtracting the optical bandgap (Eg) value from the
HOMO energy level, yielding a value close to ꢀ2.29 eV.
Our results are consistent with those found in the litera-
ture (EHOMO = ꢀ5.2 eV, ELUMO = ꢀ2.5 eV) [25], even if the
way employed for the determination of the HOMO and
LUMO levels was different (combination of UV photoelec-
tron spectroscopy (UPS) in solid films and optical spectros-
copy). It should be emphasized that the method used in
our study results only in approximated value of the LUMO
level [32]. It is also worth noticing that the measured
EHOMO value suggests that holes can be injected effectively
into TT films from Au electrodes.
The DSC trace on cooling confirmed also clearly these
observations by the presence of a reproducible shoulder
in the high enthalpy transition peak around 187 °C
(
D
H ꢁ 40 J/g) (Fig. 3a) and the presence of a transition peak
at 53 °C. The DSC trace shows also an additional transition
around 97 °C not detected by POM, proving the presence of
an additional mesophase denoted M2 prior to crystalliza-
tion (Fig. 3b). The DSC traces are reversible on heating, so
that the transformations below 100 °C could be considered
as a broad transition between mesophases of similar or
2.3. Polymorphism study and structural characterization
The liquid crystalline phase behavior was studied by
POM, DSC and SAXS at various temperatures. The phase

946
L. Mazur et al. / Organic Electronics 15 (2014) 943–953
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 2. POM textures of TT (a) at 186.5 °C (SmC), (b) 186.5 °C (SmC-M transition), (c) at 185 °C (M1), (d) at 130 °C (M1), (e) at 75 °C (M2) and (f) at 30 °C (Cr).
Every scale bar is worth 50 m.
l
SmC
Iso
78.5°C
(a)
(b)
48.1°C
M1
M2
182.61°C
186.5°C
M1
97.1°C
160
165
175
180
185
190
M1270
M1 44.0 J/g
Cr
Cr
53.4°C
1.5 J/g
3.9 J/g
Cr
Iso
Iso
Endo
Endo
41.7°C
68.4°C
3.2 J/g
1.7 J/g
54.0°C
Cr
M2
M1
181.9°C
37.9 J/g
M1
M2
86.9°C
185.04°C
40
80
120
160
200
40
60
80
100
120
Temperature (C°)
Temperature (C°)
Fig. 3. (a) DSC traces of TT recorded at rates of 2 °C/min. The inset in (a) shows an enlargement of the transition peak on cooling between 190 °C and 160 °C
and (b) enlargement of weak and broad signals spreading in the 100 °C to room temperature range. The DCS traces are reproducible between heating and
cooling cycles.
Fig. 4. (a) SAXS patterns of TT in the high- and low-temperature mesophases (M1 at 120 °C and M2 at 70 °C, respectively), and in crystalline phase Cr at
40 °C. (b) Variation of the thickness of lamellae versus temperature. Note that these data were obtained on cooling from the isotropic phase.
close organizations. X-ray diffraction patterns indeed look
similar in both mesophases, with an intense first order
and weak second and third order lamellar reflections in
the small-angle region, and with several sharp reflections
overlapping the diffuse scattering around 4.5 Å in the
wide-angle region (see Fig. 4a).
The lamellae trivially occur from the alternation of sub-
layers of aliphatic chains and sublayers of mesogens, the
wide-angle scattering coming from lateral distances be-
tween molten chains confirming the assignment to a
mesophase. The presence of intense crossed (hk l) reflec-
tions indicates that the respective position of the meso-
genic cores within juxtaposed sublayers remains long-
range correlated, likely due to some hindered translational
and rotational molecular movements. These lamellar
mesophase are therefore more ordered than smectic
phases [34], and could be likened to the 3-dimensional
mesophases reported for thiophene-based calamitic mate-
rials [17]. POM textures and DSC traces are indeed compa-
rable to these previous systems and the indexation trials
also point to a system with a low symmetry such as a
monoclinic lattice. Unfortunately, the registry fluctuations

L. Mazur et al. / Organic Electronics 15 (2014) 943–953
947
lead to the early collapse of the reflections at mid and large
Bragg angles, detrimental for the proper discrimination of
the different monoclinic solutions lattices.
2.4.1. Temperature-dependent time-of-flight measurements
The bulk charge transport properties of our material are
characterized by the conventional TOF technique. In our
Beyond this unsolved aspect, both ordered mesophases
M1 and M2 have very similar structures differing merely
by the tilt of the mesogens within the sublayers. The vari-
ation of the lamellar thickness (d) shows particular the
characteristic appearance of the transition between tilted
and untilted smectic phases, such as the Smectic C–Smec-
tic A transition [35,36]: above this transition, the mesogens
are untilted (or the tilt is locked), and d slightly decreases
due to the lateral expansion of the layers; cooling down be-
low the transition results in the rapid shrinkage of d as the
tilt angle gradually increases (see Fig. 4b). The tilt angle
variation then also causes lateral shifts between successive
registered sublayers, modifying parameters of the 3-
dimensional cell. Note that by X-ray diffraction, only a di-
rect transition from Isotropic phase to mesophase M1
could be observed, since the presumed monotropic SmC
mesophase exists over a too narrow range.
experiments, 8 lm thick TT films are excited by nanosec-
ond pulses at 337 nm, where the sample strongly absorbs
light. At that specific wavelength, the absorption coeffi-
cient of the TT film is measured to be around
4 ꢂ 104 cmꢀ1 (see the absorption spectrum of the TT film
in the inset of Fig. 5a), which leads to a penetration depth
of about 250 nm. As a result, the depth of penetration of
the excitation light is indeed significantly smaller than
the sample thickness, which is a necessary condition for
a proper determination of the charge carrier mobility in
our device geometry. Note that only photocurrent caused
by hole transport is observed in this work. Fig. 5 shows
typical transient photocurrents obtained on cooling in
the isotropic phase at 200 °C, in the mesophase M1 at
130 °C, in the mesophase (M2) at 75 °C and in the crystal-
line (Cr) phase at 30 °C.
The TOF signals in these three different phases (Iso, M1
and M2) present a plateau region with a well-defined
inflection point demarcating the transit time (ttr) of the
charge carrier packet. From the determination of this tran-
2.4. Charge transport properties
sit time, the hole mobility (
l) is then calculated using the
The charge transport properties of our TT material were
investigated by two methods. The charge carrier mobility
in bulk samples was measured by temperature-dependent
TOF in the mesophases, and in the isotropic and crystalline
states. In parallel, the charge carrier mobilities in thin films
prepared by either drop-casting or thermal evaporation
were evaluated at room temperature from OFET current–
voltage characteristics (either done under 10ꢀ6 mbar vac-
uum pressure or at atmospheric pressure).
standard equation
l
= d/ttrꢃE, where d is the film thickness
and E the magnitude of the applied electric field. While the
transient photocurrent curves show that the charge trans-
port properties in M1, M2 and the isotropic phases are
non-dispersive, the material in the crystalline phase is
found to exhibit dispersive transient photocurrents, with-
out any definite cusp. In that case, as illustrated in the inset
of Fig. 5d, the transit time is measured from the intersec-
(a)
(b)
(c)
(d)
Fig. 5. Transient photocurrent curves recorded on cooling in a bulk TT sample in (a) the isotropic phase at 200 °C (E = 2.50 ꢂ 104 V/cm), (b) the mesophase
M1 at 130 °C (E = 2.50 ꢂ 103 V/cm), (c) the mesophase M2 at 75 °C (E = 2.50 ꢂ 103 V/cm) and (d) the crystalline phase Cr at 30 °C (E = 6.25 ꢂ 103 V/cm). The
inset in (d) shows the data in the double logarithmic scale for the determination of the transit time ttr and the hole mobility
the absorption spectrum of a 100 nm thick TT film and its absorption coefficient vs. wavelength.)
lh. (The inset in Fig. 5a shows

948
L. Mazur et al. / Organic Electronics 15 (2014) 943–953
Cr
M2
M1
Iso
-1
-2
-3
-4
10
10
10
10
(a)
(b)
40
80
120
160
200
240
O
temperature ( C)
Fig. 6. (a) Temperature dependence of the hole mobility in the TT material determined by TOF Measurements on cooling. The temperature ranges
associated with the different phases of the TT material are indicated. (b) POM image at room temperature of typical mosaic texture preserved from
mesophase M2 (see Section 2.3), after slow cooling from isotropic phase (at rate around 1 °C/min) during the TOF measurements.
tion of the asymptotes to the plateau and the trailing edge
of the transients in the double logarithmic scale, as de-
scribed first by Scher and Montroll [37].
ues in M2 are presumably due to a closer molecular pack-
ing, which would be more favorable to promote an efficient
charge hopping. Note that the observation of a tempera-
ture independent mobility is similar to what has been pre-
viously reported in smectic phases and discotic columnar
phases of liquid crystalline organic semiconducting mole-
cules [13,17,38–42]. This is generally explained by smaller
energetic and positional disorders and by a reduction of
the hopping barrier, which are caused by the molecular
close packing in these lamellar mesophases. In the crystal-
line phase, large aligned domains of the M2 ordered meso-
phase are well preserved, as displayed in Fig. 6b. However,
although the density further increases and motions further
decrease, which should lead to an enhancement of the
transfer integral between neighboring molecular orbitals,
the charge carrier mobilities were found to slightly
decrease with temperature and to be lower than in the
two mesophases M1 and M2, yielding a value of about
0.02 cm2/V s at room temperature. Such a reduction in
The temperature dependence of the hole mobility cal-
culated from the transient photocurrent data is displayed
in Fig. 6a. The discontinuities observed in this plot are
caused by the different phase transitions. In the isotropic
phase with no molecular order, the mobilities are found
to be the lowest with a value about 4 ꢂ 10ꢀ4 cm2/V s for
an applied electric field of 6.25 ꢂ 103 V/cm and to gradu-
ally decrease with temperature. A drastic, step-like in-
crease in the hole mobility is observed at around 190 °C,
which is due to the transition from the isotropic phase to
the mesophase M1. This behavior can be easily explained
by the sudden molecular ordering taking place in the
well-ordered lamellar mesophase. The mobilities are
nearly constant within the temperature range of each
mesophase, yielding values of 0.06 and 0.07 cm2/V s in
M1 and M2, respectively. The slightly higher mobility val-
(a)
(b)
(c)
(d)
Fig. 7. Output (a) and transfer (b) characteristics of a bottom-contact OFET based on drop-casted TT film, output (c) and transfer (d) characteristics of a
bottom-contact OFET based on evaporated TT film.

L. Mazur et al. / Organic Electronics 15 (2014) 943–953
949
Table 1
Field-effect mobility, threshold voltage and on/off current ratio of TT transistors prepared with or without an OTS treatment and using either bottom-contact
(BC) or top-contact (TC) geometry under different atmosphere conditions (vacuum, air, nitrogen).
Configuration
BC
Conditions
Gate dielectric surface
Mobility (cm2 Vꢀ1 sꢀ1
)
Threshold voltage (V)
ION/IOFF ratio
Drop-casted
Ambient
Vacuum
Ambient
Vacuum
N2
OTS
OTS
OTS
OTS
SiO2
OTS
SiO2
OTS
SiO2
OTS
9.9 ꢃ 10ꢀ6
1.3 ꢃ 10ꢀ5
8.3 ꢃ 10ꢀ5
1.9 ꢃ 10ꢀ4
7.5 ꢃ 10ꢀ3
7.4 ꢃ 10ꢀ4
1.4 ꢃ 10ꢀ2
1.9 ꢃ 10ꢀ2
9.2 ꢃ 10ꢀ3
1.8 ꢃ 10ꢀ2
ꢀ48
ꢀ62
ꢀ11
ꢀ52
ꢀ11
ꢀ45
ꢀ19
ꢀ35
ꢀ5
2.2 ꢃ 103
3.7 ꢃ 103
3.5 ꢃ 103
2.0 ꢃ 104
3.3 ꢃ 107
4.6 ꢃ 106
8.7 ꢃ 103
1.5 ꢃ 106
3.0 ꢃ 103
1.6 ꢃ 107
TC
BC
TC
TC
Evaporated
Evaporated-annealed
ꢀ32
Fig. 8. (a) Optical image and (b) topography images of TT drop-casted thin film on OTS-treated bottom contact transistor. (c) Optical image and (d)
topography images of TT evaporated thin film on OTS-treated bottom contact transistor.
mobility and the observation of a thermally-activated
behavior is attributed to the presence of deep traps formed
on defects and crystalline grain boundaries. Nevertheless,
except for the particular case of crystalline phase, overall,
these temperature dependent TOF results show a clear cor-
relation between the degree of order and the charge trans-
port properties of our TT material.
tion, we fabricated and characterized bottom-gate organic
field-effect transistor (OFET) devices in both bottom con-
tact and top contact configurations. For bottom contact de-
vices, the channel length was 25
1.9 cm. For the top contact OFETs, the channel length was
100 m and the width was 1 mm. Because its work func-
lm and the width was
l
tion matches well the HOMO energy level of the TT mate-
rial for an efficient hole injection, gold is the metal chosen
for the source and drain electrodes. Fig. 7 shows typical
output and transfer characteristics measured in the evapo-
rated and solution drop-casted bottom-contact OFETs.
2.4.2. Field-effect transistor measurements
In order to evaluate the charge carrier mobilities in thin
films prepared either by drop-casting or thermal evapora-

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L. Mazur et al. / Organic Electronics 15 (2014) 943–953
The output characteristics measured in all our unipolar
before the thermal treatment. The similar mobility values
obtained in the bulk samples and the evaporated films sug-
gest that the morphology of the bulk sample is quite opti-
mized in using the smart melt-processing route. This is
well-illustrated by the optical texture of the TT film dis-
played in Fig. 6b. Overall, this study provides evidence that
the smart melt-processing route can be used to enhance
the morphology and the charge transport properties of or-
ganic films based on a mesogenic thieno[3,2-b]thiophene
derivative.
p-channel devices exhibit clear linear and saturation re-
gions for negative applied voltages. A superlinear behavior
of the source–drain current at low source drain voltage is
observed in devices because of contact resistance effects
between the source–drain electrodes and the molecular
layer. In all our devices, field-effect mobilities are calcu-
lated in the saturation regime, using the standard equa-
tions, described elsewhere [43]. Mobilities, threshold
voltage (Vth) and on/off current ratio (Ion/off) deduced from
the device characteristics are summarized in Table 1. For
both, drop-casted and evaporated devices, the hole mobil-
ities in the top-contact geometry are at least one order of
magnitude higher than in the bottom-contact configura-
tion. The use of a self-assembled monolayer of (OTS) at
the surface of the SiO2 gate dielectric layer does not lead
to significant changes in the charge carrier mobilities but
is found to strongly modify the threshold voltage of the de-
vices, as shown in Table 1. The best mobility value mea-
sured in the drop-casted films is obtained in the top-
contact device and is found to be 1.9 ꢂ 10ꢀ4 cm2/V s. Note
that the electrical performance of the drop-casted TT
OFETs was also evaluated in ambient atmosphere. As
shown in Table 1, the mobilities are, in both bottom- and
top-contact architectures, slightly lower when the mea-
surements are carried out in air. The results also show that
the threshold voltage, in both top- and bottom-contact
cases, shifts towards more positive values after exposure
in ambient atmosphere, which is explained by the deproto-
nation processes of Si(OH) on SiO2 in the presence of
humidity and oxygen [44,45]. In case of OTS-treated de-
vices (BC and TC contacts), we also measured threshold
voltage shift towards more positive voltages when exposed
at ambient air. Previous work has explained that carbonyl
groups act as charge carriers traps and can induce different
charge carrier accumulation at organic semiconductor/
dielectric interface, resulting in a shift of the threshold
voltage towards more positive values [46]. More impor-
tantly, we found that the evaporated devices exhibit hole
mobilities as high as 1.9 ꢂ 10ꢀ2 cm2/V s, which is several
orders of magnitude higher than those in drop-casted
films. It is important to notice that this value is similar to
that obtained at room temperature by the TOF technique
in the crystalline phase of the bulk sample after melting.
In addition, we found that Ion/off is much higher in the
evaporated devices than in the drop-casted devices, reach-
ing a maximum value of 3.3 ꢂ 107 in the bottom contact
configuration.
3. Conclusions
We reported on a novel mesogenic molecular derivative
based on thieno[3,2-b]thiophene core that exhibits two
three-dimensional lamellar mesophases and compared
the charge transport properties in the mesophases, the iso-
tropic liquid and the crystalline state. Charge carrier mobil-
ities in the bulk were evaluated by temperature-dependent
time-of-flight measurements. The results show a large hole
mobility of about 0.07 cm2/V s in the highly ordered mes-
ophases. The slightly lower mobility in the crystalline
phase at room temperature is attributed to charge trapping
at the crystalline grain boundaries. Organic field-effect
transistors were also fabricated to compare the field-effect
mobilities in thin films deposited either by thermal evapo-
ration or drop-casting. The performances of the drop-
casted devices were significantly lower due to the poor
morphology of the solution-processed organic layer. How-
ever, the evaporated devices showed mobilities similar to
those obtained in the crystalline phase of the melt-pro-
cessed bulk samples. This study provides evidence that
the melt-processing route for thiophene-based mesomor-
phic semiconductors is an effective approach, involving
no solvents or vacuum-deposition processes, to improve
the morphology and the charge transport properties of or-
ganic semiconducting layers.
4. Experimental section
4.1. Synthesis
All commercially available chemicals were used with-
out further purification and solvents were carefully dried
and distilled prior to use. All glassware was previously
heat-dried under vacuum with a hot air gun before using.
Reactions were performed under an inert atmosphere. Col-
umn chromatography was performed on Geduran Silica 60
(40–63 l
m, Merck). 1H NMR spectra were recorded at 200,
We attribute the higher mobility values obtained in the
crystalline phase of the bulk sample after melting and in
the evaporated devices to an improved film morphology.
The optical and AFM images displayed in Fig. 8 show
strong differences between the surface morphology of the
drop-casted and the evaporated films. The solution-pro-
cessed TT films present a small crystalline grain size and
a large density of grain boundaries, which presumably lead
to strong trapping of the charge carriers in the layer, and,
thus, to a significant decrease of the mobility. It is also
worth noting that a thermal annealing did not lead to
any significant changes in the mobility of the evaporated
film, suggesting that its film morphology is already optimal
250 or 300 MHz on Bruker spectrometers in CDCl3. Tetra-
methylsilane was used as internal standard and the posi-
tions of the peaks are reported in ppm. J values are given
in Hz. Proton decoupled 13C NMR spectra were recorded
at 50, 66 or 75 MHz on the same spectrometer.
4.2. 2,5-dibromothieno-[3,2-b] thiophenene (1)
A solution of 1.55 g (8.7 mmol) of N-bromosuccinimide
(NBS) in 1 ml of DMF was slowly added in a stirred solution
of 0.61 g (4.35 mmol) of thieno[3,2-b]thiophene in 9 ml of
DMF. The reaction mixture was stirred overnight. The

L. Mazur et al. / Organic Electronics 15 (2014) 943–953
951
hꢃc
product was extracted with dichloromethane and the or-
ganic phase was washed 3 times with water, dried with
MgSO4 and the organic solvent was removed with roto-
evaporation. The resulting crude residue was purified by
column chromatography on silica gel with hexane/dichlo-
romethane 9/1 to give a white solid (1 g, 85% yield). (m.p.
116.4–118.6 °C) dH (200 MHz, CDCl3) 7.19 (1H, s, 1-H), dC
(50 MHz, CDCl3) 128.6, 122.2, 114.4.
Optical bandgap (Eg) calculation: Eg ¼ ¼ J, where
k
1 eV = 1.6 ꢂ 10ꢀ19 J, h = Planck’s constant = 6.626 ꢂ 10ꢀ34
J/s, c = 3.0 ꢂ 108 m/s, k = intersection of absorption and
emission spectra (418 nm). Cyclic Voltammetry (CV) mea-
surements were carried out with a usual three-electrodes
assembly cell from Metrohm and an AUTOLAB/PGSTAT100
Potentiostat/Galvanostat from the same company. The
solution in dichloromethane was purged with nitrogen
prior to recording the electrochemical data, and all mea-
surements were recorded at 25 °C.
4.3. Tributyl-(5-octyl-thiophene)-stannane (2)
In controlled atmosphere, n-Butyl-Lithium 2.5 M
(42 ml, 0.105 mol) was added drop by drop at ꢀ78 °C to a
solution of 2-n-octylthiophene (20 g, 0.1 mol) in degassed
THF (70 ml). The mixture was stirred for 1 h at this temper-
ature. The tributyltin chloride was added very slowly to
the mixture and stirred for 30 min at ꢀ78 °C. After reach-
ing room temperature, the reaction mixture was stirred
for 12 h. The crude product was solubilized in dichloro-
methane, washed 3 times with a saturated solution of
ammonium chlorate, dried with MgSO4 and the organic
solvent was removed with roto-evaporation. The resulting
crude residue was purified by column chromatography on
silica gel with hexane/dichloromethane 6/4 to give an or-
ange oil (33.38 g, 100% yield). dH (200 MHz, CDCl3) 7.02
(1H, d, J 3.2, 2-Harom), 6.94 (1H, d, J 3.0, 1-Harom), 2.89
(2H, t, J 8.6, ꢀCH2-thiophene), 1.86 (2H, m, Haliph), 1.62
(2H, m, Haliph), 1.76–0.89(44H, m, Haliph); dC (50 MHz,
CDCl3) 145, 126, 123, 33.1, 30.5, 30.3, 30.0, 29.2, 27.6,
23.1, 14, 13.7.
4.6. Polymorphism study and structural Characterization
Polarization optical microscopy (POM) was carried out
using a Leica microscope equipped with a Linkam THMS
350 heating plate connected to a Linkam TMS 93 processor.
Differential scanning calorimetry (DSC) was performed
using TA instruments Q2000 under N2 flow in aluminum
hermetic pans. The XRD patterns were obtained by trans-
mission diffraction with a Guinier experimental set-ups,
based on linear focalized monochromatic Cu Ka1 beam
(k = 1.5405 Å) obtained from
a sealed-tube generator
(900 W) equipped with a bent quartz monochromator.
The sample was filled in 1 mm Lindemann capillaries. An
initial set of diffraction patterns was recorded with a
curved Inel CPS 120 counter gas-filled detector linked to
a data acquisition computer; periodicities up to 70 Å can
be measured, and the sample temperature controlled to
within 0.01 °C from 20 to 200 °C. Alternatively, patterns
were also recorded on an image plate; periodicities up to
120 Å can be measured (scanned by STORM 820 from
Molecular Dynamics with 50 mm resolution). Exposure
times were varied from 1 to 24 h.
4.4. 2,5-Bis-(5-decyl-thiophene)-thieno[3,2-b]thiophene (3)
A degassed solution of 2,5-dibromothieno-[3,2-b] thio-
phene (1 g, 3.4 mmol), Tributyl-(5-octyl-thiophene)-stann-
ane (4.9 g, 10 mmol), palladium acetate (0.0075 g,
4.7. Time-of-flight measurements
0.0033 mmol)
and
triphenylphosphine
(0.177 g,
The samples for our TOF measurements are prepared in
nitrogen atmosphere by capillary-filling the TT molecules
at isotropic phase into commercial liquid crystal cells con-
sisting of two indium tin oxide coated glass substrates
0.068 mmol) in THF was heated to 80 °C under nitrogen.
After 12 h the reaction was cooled down to room temper-
ature. The crude product was solubilized in dichlorometh-
ane, washed 3 times with water, dried with MgSO4 and the
organic solvent was removed with roto-evaporation. The
resulting crude residue was purified by column chroma-
tography in silica gel with hexane/chloroform 8/2 to give,
after 2 recrystallizations in ethyl acetate and then 1 recrys-
tallization in high purity methanol, a white solid (1.5 g,
87% yield). dH (200 MHz, CDCl3) 7.19 (H, s, 1-Harom), 7.00
(2H, d, J 3.6, 2-Harom), 6.68 (2H, d, J 3.6, 2-Harom), 2.79
(4H, t, J 7.7, ꢀCH2-thiophene), 1.68 (4H, m, Haliph), 1.31–
1.27(20H, m, Haliph) 0.88–1(3H, m, Haliph); dC (50 MHz,
CDCl3) 146.3, 139.6, 138.4, 135.2, 125.4, 123,5, 115.4,
31.89, 31.5, 30.90, 30.18, 29.46, 29.46, 29.19, 22.76, 13.94.
spaced with 8 lm spacers from Instec Company. The sam-
ples are then mounted in the TOF setup, using a home-built
polarizing microscope and a microscopic hot stage (Instec
HCS302). Multiple in situ transitions between the liquid
crystalline and isotropic states are carried out to signifi-
cantly increase the grain size and to be able to reduce
the effects of the grain boundaries on the charge transport.
Charge mobilities are measured using the conventional
photocurrent TOF technique [40], in which the samples
are excited by a nitrogen nanosecond pulse laser (LTB
Lasertechnik Berlin GmbH, model MNL 205-L) emitting at
337 nm with a repetition rate of 1.8 Hz. The photogenera-
tion of a sheet of photocarriers in our samples leads to a
transient photocurrent signal, which is amplified and then
4.5. Cyclic voltammetry measurement and UV–Vis absorption
emission spectra
recorded by
oscilloscope.
a Tektronix TDS 5034 350 MHz digital
Absorption spectrum in solution was recorded on a
Varian Cary 300 scan spectrometer using 10 mm quartz
cuvette. The steady-state fluorescence spectrum of the
same solution was recorded by a Varian Cary Eclipse spec-
trofluorimeter using an excitation wavelength of 380 nm.
4.8. Fabrication and characterization of OFETs
Bottom gate OFETs were fabricated on silicon wafers
with thermally grown 280 nm SiO2 layer, purchased from

952
L. Mazur et al. / Organic Electronics 15 (2014) 943–953
MicroChemicals GmbH. The substrates were sonicated in
acetone and isopropanol and then placed in a piranha solu-
tion to remove the residues of organic materials and to in-
duce OH groups in the vicinity of the surface. After rinsing
the substrates in deionized water, they were cleaned for
3 min in a UV cleaner. For the preparation of the OTS-trea-
ted devices, the substrates were placed in a 3 mM OTS
solution in dry hexane for 90 min. Except for our TT mole-
cules, all materials and solvents were purchased from Sig-
ma Aldrich and were used without further purification.
Both bottom-contact and top-contact OFETs were fabri-
cated in this study. For the bottom-contact devices, inter-
digitated source and drain electrodes were deposited
directly on the OTS-treated SiO2 surface. This was achieved
by the sequential thermal evaporation of a 5 nm thick
adhesion layer of Ti and a 100 nm Au layer through a sha-
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