Open Journal of Applied Sciences, 2012, 2, 283-293
doi:10.4236/ojapps.2012.24042 Published Online December 2012 (
Organic Thin Film Transistors Based on
Distyryl-Oligothiophenes: Role of AFM Images in
Analyses of Charge Transport Properties
Noriyuki Yoshimoto1, Hugues Brisset2,3, Jörg Ackermann2, Christine Videlot-Ackermann2*
1Graduate School of Engineering, Iwate University, Morioka, Japan
2Aix Marseille Université, Marseille, CNRS, CINaM UMR 7325, France
3Université de Toulon MAPIEM, EA 4323, La Carde, France
Email: *
Received October 2, 2012; revised November 1, 2012; accepted November 11, 2012
Significant advances have been made recently in the area of organic electronics and optoelectronics based on small
molecules as a result of an improved chemistry and a better technology. Together with light emitting diodes and solar
cells, transistors are among the most studied components. The development of new semiconductors induced a real
improvement in organic thin film transistor’s performances. Additionally, the synthesis of new soluble and air-stable
molecules with the ability to process the active materials at low temperatures over large areas on substrates such as
plastic or paper provide unique technologies and generate new applications. However the control of the solid state
structure has emerged as essential to realize the full intrinsic potential that organic semiconductors possess. Atomic
force microscopy (AFM) was likely to contribute to a further advancement of knowledge. The ability of the AFM to
produce three dimensional maps at the micro- and nanometer scale has greatly increased its popularity as an imaging
tool. Recently, distyryl-oligothiophenes and their derivatives appear as a new class of molecular semiconductors.
Detailed morphological studies of organic active layers based on such new semiconductors involved in organic thin film
transistors (OTFTs) have brought a large knowledge about the impact of chemical and physico-chemical aspects on
charge transport efficiency.
Keywords: Oligomer; Thin Film; Transistor; AFM
1. Introduction
After the initial invention of the AFM in 1986 by Binnig
and Quate [1] there was a great effort focused on deve-
loping AFM instrumentation. AFM is rapidly becoming a
standard microscopy technique for visualizing and mea-
suring a material’s surface structure in the physical sci-
ences. Furthermore, AFM presents a great advantage as
little or no sample preparation is required. The types of
structures that are scanned with the AFM include: sur-
faces of bulk materials, thin films, and nanostructures
that are located on a surface. There are a large number of
materials that may be imaged with an AFM, including
polymers, ceramics, metals, crystals, and minerals. The
scan ranges for imaging in the physical sciences is from a
few nanometers all the way up to tens of microns. From
1990 to 2000 applications for the AFM moved from
fundamental physics to most areas of science and tech-
nology. It is estimated that in 2006 there are approxi-
mately 10,000 AFM’s in use around the world. The AFM
is now utilized in a much wider area than others tech-
niques as scanning probe microscopy (STM), from basic
biology through to semiconductor production lines, be-
cause of the lower restrictions on the sample structures.
Organic electronics saw the day with the discovery in
the late 1970s of the first “conductive polymer” in its
doped form which is otherwise a semiconductor. Nobel
Laureates in Chemistry in 2000, A. J. Heeger, A. G.
MacDiarmid and H. Shirakawa were awarded for their
revolutionary discovery that plastic, through some modi-
fications, can be made electrically conductive. Elec-
tronics based on organic semiconductors has evolved
from constant and large efforts in the field of materials,
processing and circuit design. Together with light emit-
ting diodes and solar cells, transistors are among the
most studied components. While development of new
semiconductors based on improved chemistry and on
better technology induced a real improvement in device’s
performances, a detailed study of the involved active
*Corresponding author.
Copyright © 2012 SciRes. OJAppS
layers is necessary not only on an electrical point of view
but also from a morphological aspect. Analogous to con-
ventional inorganic semiconductors, the performance of
organic semiconductors is directly related to their mo-
lecular structure and packing, crystallinity, growth mode,
and purity. In order to achieve the best possible perfor-
mance, it is critical to understand how organic semi-
conductors grow. As a great importance is done to the
relationship between electrical performances and mor-
phology, AFM is likely to contribute to a further advan-
cement of knowledge. The AFM since its introduction
has gained wide usage in surface topography measure-
ments at the very small scale. The ability of the AFM to
produce three dimensional maps at the micro- and nano-
meter scale has greatly increased its popularity as an imag-
ing tool of thin films. Unlike polymers, molecular ma-
terials, called small organic molecules, have advantages
such as well-defined structures, easier purification and
more easily controllable properties. Such organic materi-
als can be deposited on substrates by different processes
to form thin films as active layers in organic thin film
transistors (OTFTs). Despite different materials (metal,
insulator or semiconductors) are involved in OTFTs, the
morphology of the active layer based on such organic
semiconductors on measured performances is one of the
key parameters. After a description of AFM as an imag-
ing technique to characterize such organic active layers
involved in OTFTs, we will present the most OTFT con-
figurations or architectures commonly used in the litera-
ture together with the electrical operating mode of tran-
sistor devices. The series of distyryl-oligothiophenes and
their derivatives were recently presented as a novel class
of OTFT semiconductors. In the present paper, a direct
impact of their thin film morphology on charge trans-
port efficiency is highlighted by AFM where the mole-
cular structure, the deposition process, the substrate nature
and the thin film thickness are some of direct parameters.
2. Description of AFM as an Imaging
The AFM consists of a cantilever with a sharp tip (probe)
at its end that is used to scan the surface. The cantilever
is typically silicon or silicon nitride with a tip radius of
curvature on the order of nanometers. When the tip is
brought into proximity of a sample surface, forces be-
tween the tip and the sample lead to a deflection of the
cantilever according to Hooke’s law. Depending on the
situation, forces that are measured in AFM include me-
chanical contact force as van der Waals forces, capillary
forces, chemical bonding, electrostatic forces, magnetic
forces. Typically, the deflection is measured using a laser
spot reflected from the top surface of the cantilever into
an array of photodiodes. If the tip was scanned at a con-
stant height, a risk would exist that the tip collides with
the surface, causing damage. Hence, in most cases a
feedback mechanism is employed to adjust the tip-to-
sample distance to maintain a constant force between the
tip and the sample. Traditionally, the sample is mounted
on a piezoelectric tube, which can move the sample in
the z direction for maintaining a constant force, and the x
and y directions for scanning the sample. Alternatively a
“tripod” configuration of three piezo crystals may be em-
ployed, with each responsible for scanning in the x, y and
z directions. This eliminates some of the distortion ef-
fects seen with a tube scanner. In newer designs, the tip
is mounted on a vertical piezo scanner while the sample
is being scanned in x and y using another piezo block.
The resulting map of the area z = f(x,y) represents the
topography of the sample.
The AFM can be operated in a number of modes, de-
pending on the application. In general, possible imaging
modes are divided into static (also called contact) modes
and a variety of dynamic (non-contact or “tapping”)
modes where the cantilever is vibrated.
Contact mode: Contact mode is the most basic opera-
tion mode to observe topographic images of samples.
Because the tip directly contacts with the sample sur-
face in contact mode, damage by friction force be-
tween the tip and sample becomes problems. The
static deflection of cantilever is measured by detect-
ing the position of reflected laser beams, and used as
a feedback signal for controlling the force given to
the tip. The force between the tip and the surface is
kept constant during scanning by maintaining a con-
stant deflection.
Tapping mode: Tapping mode was developed in
1990s, and overcomes the problems peculiar to con-
tact mode to obtain high-resolution topographic im-
ages of materials without any damaging. In tapping
mode, the cantilever is driven to oscillate at its reso-
nance (50 k - 500 kHz). When the tip approaches to
sample surface, the amplitude of oscillation decreases
due to repulsion interaction between the tip and the
sample. Keeping the amplitude to be constant by a
feedback loop during scan across the surface, topog-
raphic images of samples are obtained from the am-
plitude signal. Tapping mode overcomes problems
associated with friction that a plague conventional
contact method by alternately placing the tip in con-
tact with the surface to provide high resolution and
then lifting the tip off the surface to avoid dragging of
samples by friction between the tip and the surface.
Most of the extremely high resolution images mea-
sured with an AFM in ambient air are made with tap-
ping mode. The image represents the amplitude
variation of the square root of the amplitude (rms).
The resolution is somewhat worse than in the contact
Copyright © 2012 SciRes. OJAppS
Copyright © 2012 SciRes. OJAppS
mode but molecular resolution can still sometimes be
obtained. The characterization by AFM in tapping
mode is used to determine several parameters as the
growth mode, the surface layer roughness, the grain
size of polycrystalline thin film deposited on sub-
strates, the presence and the nature of defects as grain
boundaries, cracks, and dislocations.
cially suitable for transistors with SiO2 dielectric layer. In
such cases the OSC-based layer is deposited on top of the
dielectric layer, after its growth and modification of its
surface. Thus, minimal risk exists that the technological
processes may negatively influence the structure or mor-
phology of the semiconductor layer or lead to a partial
decomposition of the semiconductor. Several techni-
ques are used to deposit the OSC layer as vapour phase
under vacuum or liquid-based process as spin coating
and drop-casting. In the first case, the semiconductor is
vaporized and then condenses into a film onto a substrate.
Some parameters as the rate of evaporation, the substrate
temperature (Tsub), the thickness can be controlled. For
solution deposition, the organic semiconductor is com-
pletely dissolved in an organic solvent. The solution is
then coated onto the substrate by a liquid-based process.
As the solvent vaporizes, the solution becomes super-sa-
turated and forms organic semiconductor layers.
3. Organic Thin Film Transistors (OTFTs):
Description and Characterization
OTFTs are desired for the manufacture of low-cost elec-
tronic devices involving in modern electronics such as
smart cards, RFID tags, flexible electronic paper, and
backplane circuitry for active matrix displays [2-4].
While must of the attention of the organic transistors
community has been focused on the search for high-mo-
bility, ambient stable, and solution-processable small
molecule and polymeric semiconductor materials, it is
now admitted that substantial improvements in OTFT
performance is also obtained by focusing on the mor-
phology of the organic active layer involved in charge
transport. A field effect transistor consists of a semicon-
ducting layer based on an organic semiconductor (OSC),
a dielectric layer and three electrodes, namely gate (G),
source (S) and drain (D). Silicon dioxide (SiO2), the oxide
of silicon, together with silicon are the most widely used
dielectric and gate materials, respectively. The most of-
ten process to grow silicon dioxide on the surface of
silicon wafers is the thermal oxidation where the silicon
is exposed to oxidizing agents such as water and oxygen
at elevated temperatures. This process has good control
over the thickness and properties of the SiO2 layer. Such
resulting bilayers are commonly named Si/SiO2 sub-
strates. There exist four device configurations where the
bottom gate (BG) configurations, with either top or bot-
tom contact (TC or BC) as shown in Figure 1, are espe-
A field-effect transistor operates as a voltage-controll-
led current source. By applying the gate voltage (VG)
across the gate dielectric, a sheet of mobile charge car-
riers is induced in the semiconductor that allows a cur-
rent (the drain current ID) to flow through the semicon-
ductor when another voltage (the drain-source voltage
VDS) is applied between drain and source. The charge
transport, operating by a hopping process in OTFT de-
vices, occurs in the OSC layer near the dielectric which
is called OSC/dielectric interface on Figure 1. In elec-
trical operating mode of transistor devices, there are three
crucial parameters. These are the charge carrier mobility
(µe or µh for electron or hole mobilities), the threshold
voltage (VT) and the Ion/Ioff ratio.
They can be determined from so called output and trans-
fer characteristics of the transistor (Figure 2). Transistor
activity is observed by application of negative or positive
drain and gate voltages to operate in the accumulation
Figure 1. Bottom gate (BG) configurations of organic thin film transistors with bottom (BC) or top (TC) source-drain con-
tacts. Zoom-in of the organic active layer.
(a) (b)
Figure 2. Typical output (a) and transfer (b) characteristics of p-channel OTFTs.
mode meaning that the organic molecules involved in the
active layer behave as p- or n-type semiconductors, re-
Many of n-channel materials only oprate in vacuum or
under an inert atmosphere due to electron trapping by
ambient oxygen and moisture. Even if several new
n-channel semiconductors have exhibited high perform-
ances together with high stability in ambient conditions
[5], it is still challenging to fabricate high-performing
ambient-stable n-channel OTFTs. Like p-channel OTFTs,
electron transport in n-channel thin films is greatly go-
verned by their morphology and molecular orientation
where key parameters are thin-film growth conditions.
Most issues such as electronic performances and air sta-
bility have been greatly influenced by quality thin films
where highly oriented polycrystalline thin films exhibit
higher charge-carrier mobility than amorphous films or
randomly oriented crystalline films. It is well-known that
the ordering of organic molecules in the anchoring re-
gion near the dielectric layer (interface OSC/dielectric on
Figure 1) are strongly affected by the substrate surface
structure and the balance of molecule-molecule and mo-
lecule-substrate interactions. Therefore, the use of visua-
lization techniques becomes important to observe the
morphology of the active layer together with its different
parameters. Most of organic molecular films are electri-
cally insulating and can therefore be investigated by
AFM and related techniques using some force interac-
tions between the scanned probe and the sample surface
such as van der Waals, frictional, electrostatic, capillary
and imaging forces. Because of the very different physi-
cal nature of the two media, OSC and dielectric, the
deposition techniques as vapour phase under vacuum,
should result in highly disordered films, leading to very
poor performance. However a good organization can be
obtained by heating the substrate at different Tsub and
depositing the organic semiconductor at a lower rate as
commonly observed for organic thin films based on
small molecules as pentacene, oligothiophene derivatives
and many others. Additionally, different parameters can
also highly influence the resulting performances of active
layers as the molecular structure, the deposition process,
the substrate nature and the thin film thickness.
4. Distyryl-Oligothiophenes and Derivatives:
A Novel Class of OTFT Semiconductors
Long range interconnected crystalline grains with small
grain boundary are favorable to charge transport as ob-
served in pentacene-based OTFTs [6,7]. Oligothiophenes,
distyryl-oligothiophenes and their derivatives do not de-
rogate from the rule. A new series of structurally simple
and readily available oligothiophenes end-capped with
styryl units, named distyryl-oligothiophenes DSnTs (n =
2 - 4), represents a novel class of OTFT semiconductors
that combine good electrical performances and excep-
tional stabilities [8]. Transistor responses were obtained
only for negative bias, which is indicative that DSnT
compounds behave as p-type semiconductors in air. The
field effect mobilities calculated in the saturation regime
are shown to increase with the substrate deposition tem-
perature (Tsub) as due to the formation of better ordered
thin films at elevated Tsub. The highest value of the hole
mobility (0.1 cm2/Vs) was obtained for the longest oli-
gomer DS4T (see molecular structure on Figure 3) at
110˚C (Table 1). Additionally, DS4T-based OTFTs were
also found to possess an exceptional long-lifetime (more
than one year) and an electrical stability toward continu-
ous operation.
The morphology of vapor deposited thin films of
DS4T grown on SiO2 was investigated using AFM where
Copyright © 2012 SciRes. OJAppS
Table 1. OTFT data of DSnTs deposited at different sub-
strate temperatures (Tsub) on SiO2 layers in BG-TC con-
figuration [8].
Material Tsub (˚C) µ (cm2/Vs) Ion/Ioff VT (V)
DS2T 30 0.002 - 0.006 1.6 - 1.9 105 4 - 8
80 0.01 - 0.02 2 - 2.3 105 0 - 19
DS3T 30 0.001 - 0.008 2.2 - 2.8 104 0.1 - 3
80 0.01 - 0.02 2.2 - 3.3 104 4 - 13
DS4T 30 0.02 - 0.04 1.2 - 1.8 103 3 - 13
80 0.03 - 0.06 1.9 - 2.5 103 12 - 20
110 0.08 - 0.1 0.8 - 1.2 103 5 - 20
the formation of islands whose size increases with the
substrate deposition temperature was observed. At Tsub =
110˚C, a terrace-and-step morphology with small grain
boundary is clearly observed, an average value of 2.9 nm
being determined for the steps height and associated to
“up-right molecules” (Figure 3). The overlap of π orbi-
tals between grains containing up-right oriented mole-
cules is by consequence favorable to an efficient carrier
path charge at the grain boundary to ensure an efficient
charge transport in OTFTs.
Recently, from thin films based on a “kite” shaped
styryl end-capped benzo[2,1-b:3,4-b’]dithiophene
(KDS2T) emerges also an obvious correlation between
the grain size and OTFT performances according to the
substrate temperature (Tsub = 30˚C or 80˚C) [9]. The mo-
lecular structure of KDS2T is shown in Figure 4. While a
Figure 3. AFM image of a DS4T (molecular structure on the left side) film deposited at Tsub = 110˚C on a Si/SiO2 substrate
with a nominal thickness of 50 nm [8].
Figure 4. AFM images of KDS2T (molecular structure on the left side) films deposited on Si/SiO2 substrates at Tsub = 30˚C (a)
nd 80˚C (b) with a nominal thickness of 50 nm [9]. a
Copyright © 2012 SciRes. OJAppS
terrace-like step structure was observed for both Tsub va-
lues together with a comparable grain boundary length,
small crystal grains of 1.5 - 3 μm in size are observed in
the AFM images of thin films obtained at 30˚C (Figure
4(a)) and the grain size increases to 3 - 7 μm at Tsub =
80˚C as shown in Figure 4(b). In the present study, a di-
rect correlation is established between on increased field
effect mobility calculated in the saturation regime with the
substrate deposition temperature where ordered thin films
with larger grains are formed at higher temperatures.
Additionally to the control of the substrate temperature,
the fine control of the thin film thickness can provide
further information as previously observed for penta-
cene-based thin films [6,7]. As an example, Figure 5
shows AFM images of thin films based on an oligomer
(DFH-4T) deposited by vacuum evaporation on Si/SiO2
substrates heated to 50˚C or 80˚C to a nominal thickness
of 3, 5, 15 or 30 nm [10]. While separate islands with
dendritic shapes are observed at Tsub = 50˚C (Figures 5(a)
and (b)) following an island growth mode, AFM images
at Tsub = 80˚C show larger and interconnected grains with
a more homogenous distribution in size and height. Fig-
ures 5(c) and (d) demonstrate an almost complete cov-
ering of the substrate surface by the first DFH-4T
monolayer (ML) as well as the successive ML formation.
The growth mode switch to a layer-to-layer growth mode
for DFH-4T films grown on SiO2 substrates heated to
80˚C, revealing the strong impact of the substrate tem-
perature on the film morphology. At low substrate tem-
perature (typically 30˚C) during the deposition of mole-
cules, an electron mobility value of 1 10–5 cm2/V.s has
been obtained. As already observed for OTFTs based on
DSnTs, the field effect mobilities calculated in the satu-
ration regime are shown to increase with the substrate
deposition temperature as due to the formation of better
ordered thin films at elevated Tsub. An electron-mobility
Figure 5. AFM images of DFH-4T (molecular structure of DFH-4T on the left side) films deposited on Si/SiO2 substrates at
sub = 50˚C (a,b) and Tsub = 80˚C (c,d) with a nominal thickness of 3 nm (a), 5 nm (b), 15 nm (c) and 30 nm (d) [10]. T
Copyright © 2012 SciRes. OJAppS
up to 6 × 10–4 cm2/V.s for DFH-4T films deposited at
80˚C on a dielectric layer based on polymethylmethacry-
late (PMMA) was measured in air. The difference of one
order in the mobility of DFH-4T between Tsub = 30˚C
and Tsub = 80˚C can be directly correlated to the grain
size determined in the morphology study, where DFH-4T
showed larger grains on heated substrates due to its
layer-to-layer growth mode. Furthermore, while most of
the results reported on transistors incorporating DFH-4T
thin films prepared by vacuum deposition on silicon di-
oxide (SiO2), hexamethyldisilazane (HMDS), poly(sty-
rene) (PS) and poly (vinyl alcohol) (PVA) dielectric lay-
ers [11] where measured in vacuum, an independent
work has demonstrated relatively air-stable n-channel
DFH-4T based transistors by using PMMA as the organic
insulating layer [10]. Despite lower electron carrier mo-
bility, such the strategy to modify the insulator surface pro-
perties by PMMA to eliminate electron trapping sites could
give rise to operational transistors in ambient conditions.
AFM study on organic thin films can also highlighted
the influence of molecular structure on growth mode.
While a layer-by-layer growth together with an island
one, characteristic of the Stranski-Krastanov mechanism,
was observed for DS3T based thin films on heated sub-
strates, the formation of the islands in diPhAc-3T thin
films is achieved by the Volmer-Weber mechanism,
characteristic of a 3-dimensional (3D) growth (Figure 6)
[12,13]. The growth of thin films is here essentially con-
trolled by two types of interaction, the interaction be-
tween molecules and substrate and the intermolecular
interaction which can be modulated by heating the sub-
strate during the solid state formation of organic thin
films. These results underline the importance of molecu-
lar structure on growth mechanism and resulting thin
film morphology for electronic applications such as
charge transport in OTFTs. Together with high perform-
ing OTFTs elaborated at low Tsub i.e. 25˚C - 50˚C, di-
PhAc-3T based thin films benefit from a compact solid
state to block the introduction of air containing oxidizing
contaminants conferring high air stability to OTFTs over
the time [13].
Furthermore, the efficient cohesion of diPhAc-3T
vacuum evaporated thin films induced by a 3D growth
offers an exceptional high physical resistance to a laser
pulse. Active layers based on diPhAc-3T for BG-TC
transistors have been printed using the laser induced
forward transfer (LIFT) technique [14]. diPhAc-3T was
vacuum evaporated on a quartz substrate prior a transfer
by laser on an acceptor substrate (typically a Si/SiO2 sub-
strate) to form an organic active layer for charge trans-
port. Resulting printed diPhAc-3T pixels on receiver
substrates have well defined morphological properties as
shown by optical microscopy and AFM (Figure 7). Elec-
trical characterizations demonstrated that transistors are
fully operative with hole mobilities up to 0.04 cm2/V.s,
threshold voltage VT near 0 V and Ion/Ioff ratio up to 2.8 ×
105. The high intermolecular interaction involved in such
growth mechanism makes thin films weakly sensitive to
the mechanical damages induced by the laser. These re-
sults underline the importance of molecular structure on
growth mechanism and resulting thin-film cohesion for
the realization of laser printed OTFTs.
In the oligothiophene series, the n-type semiconduct-
ing counterparts have been obtained by end-substitution
of the oligomer backbone with electron withdrawing
groups (EWGs) as perfluoroarenes. The synthesis and the
solid-state properties of two new perfluoroarene-term-
inated oligothiophene derivatives DFSnTs (n = 2, 4) were
reported recently [15,16]. The molecular structure of
DFS2T and DFS4T are shown on Figure 8. Actually,
these compounds are the perfluorinated analogues of
-distyryl-oligothiophenes, DSnTs (n = 2 - 4), which
exhibited p-type semiconducting properties and high
stability in OTFTs [8]. With DFSnTs, both n-type carrier
Figure 6. AFM images of DS3T and diPhAc-3T-based thin films with a nominal thickness of 50 nm deposited on Si/SiO2
heated substrates (Tsub = 80˚C for DS3T in (a) and Tsub = 50˚C for diPhAc-3T in (b)) [12,13]. Molecular structures of DS3T
and diPhAc-3T on both side of corresponding AFM images.
Copyright © 2012 SciRes. OJAppS
Figure 7. AFM image (a) and cross-sections (b), (c) of the surface of a diPhAc-3T-based pixel printed by LIFT at 0.26 J/cm2
Figure 8. AFM pictures of DFS2T and DFS4T thin films deposited with a nominal thickness of 50 nm on Si/SiO2 substrates
heated to Tsub = 80 ˚C [16]. Molecular structures of DFS2T and DFS4T on both side of corresponding AFM images.
dominance and remarkable attributes characterizing the
DSnTs have been expected to be maintained. Surpri-
singly, both compounds, DFS2T and DFS4T, do not
show n-type transport but were p-type materials for
OTFT measurements realized in air and in vacuum. By
using a controlled low evaporation rate together with a
fine control of the substrate temperature [16], highly in-
terconnected µm-long rodlike crystallites were obtained
at Tsub = 80˚C (Figure 8). Such observation by AFM has
removed the doubt as a lack of electron transport in or-
ganic-based thin films is due to a poor morphology as
suggested by Tang and Bao [17]. Analyses of these ma-
terials reveal a direct impact of the molecular structure
where the presence of the two double bonds may affect
the final transport properties by confining the electrons in
some segments of the molecular backbone bonds thus
thwarting the π-conjugation [15].
As efficient EWGs, some substitutions by either alkyl
groups (CF3) on aryl naphthalenetetracarboxylic diimide
(NTCDI) enable to lower LUMO level and help to im-
prove electron charge injection. Resulting mobilities and
air stability on n-channel OTFTs have been considerably
increased [18]. In order to expand the series of fluori-
nated distyryl-bithiophenes, two new perfluoroalkyl end-
substituted analogues of distyryl-bithiophene were syn-
thesized by introducing CF3 groups on arene end-units
depending on their number and position [19]. The mo-
lecular structure of both oligomers, CF3-DS2T and
diCF3-DS2T, is shown on Figure 9. A comparative study
with the perfluoroarene-containing distyryl-bithiophene
analogue (DFS2T) underlines the influence of perfluori-
nation by either alkyl groups (CF3) or by fluorine atoms
(F) on arene end-units of DS2T -conjugated core.
While DFS2T implemented as active layer into OTFTs
behaves as a p-type organic semiconductor, CF3-DS2T
leads to n-channel OTFTs measured in vacuum where
any obvious electron density confinement in the molecu-
lar backbone occurred [19]. With a fine microstructure
observed by AFM and XRD of CF3-DS2T-based thin
films deposited on HMDS-treated Si/SiO2 substrates
Copyright © 2012 SciRes. OJAppS
heated to 80˚C (Figure 9), electron injection can occur
from gold electrodes to LUMO level generating an elec-
tron transport. A mobility up to 9.8 10–4 cm2/V.s to-
gether with an Ion/Ioff ratio of ~107 were measured un-
der high vacuum. The absence of a fine solid-state or-
dering in diCF3-DS2T thin films at either 30˚C or 80˚C
revealed by XRD leads to a total absence of charge
transport in such films. Analyses of these materials re-
veal a direct relationship between molecular structure,
solid state microstructure and electrical properties. The
results of this study indicate that the molecular geometry
and intermolecular interactions in the crystalline state
govern the electrical properties of OTFTs by directly
influencing key factors as interface states and morphol-
ogy of organic thin films. The main result coming out of
this comparative study is the influence of the high elec-
tronic effect of CF3 groups together with a fine molecu-
lar arrangement in CF3-DS2T-based thin films to control
the charge carrier type, i.e. electron, transported in such
organic active layers.
In others cases, unfavorable macroscopic morphology
observed by AFM can explain the lack of charge trans-
port in OTFTs. Solid-state properties of two new dis-
tyryl-bithiophene derivatives with cyano groups at dif-
ferent positions on the ethylene linkage show drastic dif-
ferent behavior from each other [20]. While the surface
morphology of one of them as 50 nm thick vacuum de-
posited layer on Si/SiO2 substrates heated at 80˚C shows
numerous needle like grains going out of the plane
spaced by in-plane grains, a quasi-amorphous morpho-
logy is observed for the second one in the same condi-
tions. All thin films of the first one are field-effect tran-
sistor active in air revealing a hole charge transport. On
the contrary, thin films based on the second one show no
response under either positive or negative gate voltage
meaning that this compound shows neither an n- nor a
p-channel activity in air but behaves as an insulator. The
comparative analysis of the electrical responses obtained
under stringently identical conditions reveals the great
importance of the position of cyano groups on ethylene
linkage [20].
From the soluble properties of some oligomers, dif-
ferent techniques using OSC-based solutions can be used
in addition to vapour phase under vacuum. Based on a
small molecule, the α,ω-hexyl-distyryl-bithiophene (DH-
DS2T), a series of OTFT devices were realized by vapor
phase, spin-coating, drop casting and inkjet printing for a
comparative analysis of their electrical response/behavior
obtained under identical measurement conditions [21].
For vacuum-evaporated DH-DS2T thin films, AFM pic-
tures show a polycrystalline morphology where mono-
layer terraces are clearly observed with an average value
of 3.2 nm as determined for the steps height and corre-
sponding to the molecular length (3.4 - 3.5 nm) (Figure
10(a)). Along with these grains, a large number of nee-
dles like grains pointing out of the substrates are ob-
served. Spin-coated thin films exhibit a thin layer ho-
mogeneously dispersed on the surface together with
higher grains (Figure 10(b)). Upon changing to drop-
casted thin films, large domains can be observed with
terrace steps of 3.2 nm (Figure 10(c)). Different pa-
rameters linked directly to the processes (solvent, con-
centration, deposition method, surface, post-treatment…)
are identified as key factors controlling film quality/
crystallinity and device performances. While all OSC-
layers give rise to active p-channel OTFTs corresponding
to a hole transport, such systematic study reveals the
factors that limit efficient charge transport at the macro-
scopic scale of the channel length in OTFT devices.
5. Conclusion
While initial progress was mostly attributable to syn-
thetic efforts in the form of the creation of new molecular
Figure 9. AFM images of CF3-DS2T and diCF3-DS2T-based thin films with a nominal thickness of 50 nm deposited on
HMDS-treated Si/SiO2 substrates heated to Tsub = 80˚C. Molecular structures of CF3-DS2T and diCF3-DS2T on both side of
corresponding AFM images [19].
Copyright © 2012 SciRes. OJAppS
Figure 10. AFM images of DH-DS2T-based thin films deposited on Si/SiO2 substrates by vapor (a), spin-coating (b) and
drop-casting (c) in air at Tsub = room temperature [21]. Molecular structure of DH-DS2T.
species, and enhanced regularity and purity, more re-
cently it has also been the advancement of physico-
chemical aspects, notably materials processing, and the
inducement of increased order and control of the solid
state structure. Here we have shown that a fundamental
understanding of the latter issues is critical to realize the
full intrinsic potential that organic semiconductors pos-
sess. A complete approach including molecular struc-
ture, electrical performances and solid state morphology
as well as device design is required to provide unique
technologies and generate new applications.
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