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Topography-guided spreading and drying
of 6,13-bis(triisopropylsilylethynyl)-pentacene solution
on a polymer insulator for the field-effect mobility enhancement
Chang-Min Keum,
1
Jin-Hyuk Bae,
2
Min-Hoi Kim,
1
Hea-Lim Park,
1
Marcia M. Payne,
3
John E. Anthony,
3
and Sin-Doo Lee
1,a)
1
School of Electrical Engineering, Seoul National University, Kwanak P.O. Box 34, Seoul 151-600,
South Korea
2
School of Electronics Engineering, Kyungpook National University, 1370 Sankyuk-dong, Bukgu,
Daegu 702-701, South Korea
3
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, USA
(Received 9 April 2013; accepted 5 May 2013; published online 17 May 2013)
We report on the enhancement of the field-effect mobility of solution-processed 6,13-
bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) by unidirectional topography (UT) of an
inkjet-printed polymer insulator. The UT leads to anisotropic spreading and drying of the
TIPS-pentacene droplet and enables to spontaneously develop the ordered structures during the
solvent evaporation. The mobility of the UT-dictated TIPS-pentacene film (0.202 6 0.012 cm
2
/Vs)
is found to increase by more than a factor of two compared to that of the isotropic case
(0.090 6 0.032 cm
2
/Vs). The structural arrangement of the TIPS-penta cene m olecules in relation
to the mobility enhancement is described within an anisotropic wetting formalism. Our
UT-based approach to the mobility enhancement is easily applicable to different classes of soluble
organic field-effect transistors by adjusting the geometrical parameters such as the height, the
width, and the periodicity of the UT of an inkjet-printed insulator.
V
C
2013 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4807461]
Solution-processed organic field-effect transistors
(OFETs) have been extens ively investigated for flexible,
large-area, and low-cost electronic applicati ons
1–3
owing to
the simplicity in fabrication by simple printing and/or roll-
to-roll processing, for example, using inkjet printing.
4–6
Apart from the materials themselves being used, several
issues such as the device architecture, the energy level align-
ment at organic semiconductor (OSC)-metal-insulator inter-
faces, and the charge transport should be tailored for
developing high-performance OFETs. Of particular impor-
tance in solution-processing are the interfacial properties of
an OSC solution in contact with the gate insulator since they
play critical roles on the field-effect mobility,
7
the current
leakage,
8
and the uniformity
9
of the OSC film in a fully dried
state. In OSCs with p-orbital interactions between
co-facially stacked molecules, improved chain ordering is
known to significantly increase the mo bility.
10,11
Especially,
for soluble low-molecular OSCs such as 6,13-bis(triisopro-
pylsilylethynyl) pentacene (TIPS-pentacene), the molecular
order is induced by either dip coating,
12
solution-shearing,
13
or hollow pen writing.
14
The use of blended TIPS-pentacene
with a photo-reactive polymer
15
is another method of pro-
ducing the molecular order when the exposure of ultra-violet
light is carried out during the solvent evaporation. From the
viewpoint of the interfacial interactions, the physicochemical
modification of the gate insulator surface can be achieved
using a hydrophobic layer for the improvement of the crys-
tallinity in the soluble OSC.
6,16
In this case, the hydrophobic
layer is necessarily different from the gate insulator so that
the dielectric properties of the gate insulator are inevitably
altered. Therefore, a new approach taking into account the
fluidic and volatile nature of the OSC solution should be
explored for the mobility enhancement without sacrificing
other electrical properties of the OFET.
In this work, we introduce a concept of the unidirectional
topography (UT) of an inkjet-printed polymer insulator to pro-
duce anisotropic spreading and drying of the TIPS-pentacene
droplet and to spontaneously develop the ordered structures
over relatively large area during the solvent evaporation. The
formation of ordered structures showing optical anisotropy in
the solution-processed TIPS-pentacene film leads directly to
the enhancement of the field-effect mobility. In contrast to the
alignment of a liquid crystal, which is a highly viscous and
non-volatile fluid, on a micro-grooved surface according to
the elasticity argument,
17
the structural arrangement of the
TIPS-pentacene molecules is most likely achieved through
anisotropic wetting, spreading, and drying along the UT on
the gate insulator produced by inkjet printing. The develop-
ment of the structural order of the TIPS-pentacene molecules
is described in terms of the anisotropic wetting phenomenon
together with the directional flow during the solvent evapora-
tion along the UT.
The construction of the UT of a polymer insulator by
inkjet printing is schematically shown in Fig. 1(a). As shown
in the figure, a series of the droplets of a polymer solution
are printed in line such that the droplets are partially con-
nected. As a basic element of the UT, a ridge structure with
periodic arcs is naturally developed during the anisotropic
solvent evaporation along the direction of inkjet printing.
Note that the periodic arc boundary in the ridge is attributed
to the coffee-ring effect,
18–20
which is commonly observed
in a liquid droplet containing dispersed solids during the
a)
Electronic mail: sidlee@plaza.snu.ac.kr
0003-6951/2013/102(19)/193307/5/$30.00
V
C
2013 AIP Publishing LLC102, 193307-1
APPLIED PHYSICS LETTERS 102, 193307 (2013)
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drying process. The inset in Fig. 1(a) shows the optical
microscopic image of a single droplet pattern of poly(4-
vinylphenol) (PVP) on the uniform PVP layer, where the
coffee-ring effect is clearly seen. The geometrical factors
such as the width (w) and the height (h) of the ridge depend
primarily on the printing pitch (p) and the radius (r) of the
droplet of the polymer solution. The center-to-center dis-
tance (d) between two adjacent ridges is simply given by the
spatial interval in inkjet printing. A polymer dielectric mate-
rial of PVP mixed with methylated poly(melamine-co-form-
aldehyde) (MMF) (100 wt. % of PVP) in propylene glycol
methyl ether acetate (PGMEA) in 5 wt. % was used for pro-
ducing the gate insulator with the ridge structures by inkjet
printing. Inkjet printing was performed using a piezoelectric
printer (UJ200, Unijet Co.), equipped with a nozzle having
the orifice diameter of 50 lm (MJ-AT, MicroFab), at a fre-
quency of 1 kHz in an asymmetric bipolar type wave form.
The ridge structures of PVP were constructed on top of a uni-
form PVP layer, which was prepared on a glass substrate by
spin-coating. The substrate was maintained at 60
C during
inkjet printing. For a single PVP droplet with the volume of
40 pl in solution, the average radius (r) on the uniform PVP
layer was about 110 lm as shown in the inset of Fig. 1(a).The
optical microscopic image of the PVP ridges constructed on
the uniform PVP layer by inkjet printing was shown in
Fig. 1(b). From the geometrical profiles of the PVP ridge
measured using a surface profiler (alpha step 500, KLA-
Tencor), it was found that h ¼350 nm, w ¼30 lm, and
d ¼55 lm. These parameters depend on the experimental
conditions, such as the volume (or the radius) of the droplet,
the printing pitch, the solvent evaporation rate, and the
substrate temperature, together with the intrinsic properties of
the substrate including the surface energy and the morphol-
ogy. Note that in our case, the periodic ridges forming the UT
were built up in the PVP-on-PVP configuration to eliminate
any detrimental effect at an insulator-insulator interface.
We first examined the wetting behavior of water on two
types of PVP insulator surfaces, one of which is spatially
uniform and the other has the UT. The x-axis and the y-axis
represent the direction perpendicular to the ridge and the
direction parallel to the ridge, respectively, as shown in Fig.
1(b). The contact angle of water on a uniform PVP surface
was circularly symmetric (h
s
¼75
) as shown in Fig. 2(a).
For the UT on the uniform PVP, as seen from Figs. 2(b) and
2(c), the contact angle was h
?
¼81
along the y-axis, while
it was h
k
¼70
along the x-axis according to the geometrical
anisotropy of the UT. This is consistent with the anisotropic
wetting phenomena of liquids on a large number of textured
surfaces.
21–24
In Wenzel’s model
25–27
for anisotropic wetting
of a liquid on a textured surface, the relationship between the
apparent angle h
a
and the intrinsic contact angle h on a
FIG. 1. (a) Schematic diagram showing the construction of UT of PVP on
top of the uniform PVP insulator by inkjet printing. The radius of a single
PVP droplet and the printing pitch are denoted by r and p, respectively. The
width and the height of the ridge formed by a series of the droplets printed
in line are w and h, respectively. The center-to-center distance between two
adjacent ridges is d. The inset is the optical microscopic image of a single
PVP droplet pattern showing the coffee-ring effect. (b) The optical micro-
scopic image of PVP ridges constructed on the uniform PVP layer by inkjet
printing and the geometrical profiles of several PVP ridges. The x-axis and
y-axis are defined as the directions perpendicular and parallel to UT,
respectively.
FIG. 2. The contact angles of water on (a) the uniform PVP insulator pre-
pared by spin-coating (h
s
), (b) on PVP insulator with the UT measured along
the y-axis (h
?
), and (c) on PVP insulator with UT measured along the x-axis
(h
k
). (d) The schematic illustration of an elongated TIPS-pentacene droplet
formed on PVP insulator with UT by anisotropic wetting. The black dashed
arrows indicate the directions of preferential spreading of the droplet along
the ridges. The optical microscopic images of the TIPS-pentacene films
prepared (e) on a uniform PVP film and (f) on PVP with UT. The radial
domains on the uniform PVP and the elongated, bilateral domains along UT
of PVP are indicated by black solid arrows.
193307-2 Keum et al. Appl. Phys. Lett. 102, 193307 (2013)
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smooth surface can be expressed as cos h
a
¼ k cos h, where k
is a constant depending on the ratio of the effective surface
area in contact with the liquid to the projected surface area.
Moreover, it is well known that the geometrical anisotropy
on the textured surface breaks the symmetry of the contact
line at the liquid-solid boundary,
22
indicating that the liquid
tends to spread along the ridges but is restricted in the direc-
tion perpendicular to the ridges. This means that the liquid
droplet should be elongated along the ridge direction. In
other words, using h ¼75
, k was about 1.3 for h
a
¼70
and
about 0.6 for h
a
¼81
in a first-order approximation, where
the interfacial interactions are ignored.
22,23
Similarly, for the
TIPS-pentacene solution, an elongated droplet is formed on
the PVP insulator with the UT by anisotropic wetting as
shown schematically in Fig. 2(d). The directional flow is
then generated along the UT during the solvent evaporation,
and the structural order of the TIPS-pentacene molecules is
accordingly developed. In our study, a dropl et of 1 llof
TIPS-pentacene dissolved in anisole in 1 wt. % was placed
on the PVP insulator with the UT at room temperature and
allowed for the solvent evaporation at 60
C for 30 min. The
optical microscopic image of the TIPS-pentacene film pre-
pared on a uniform PVP film and that on the UT of the PVP
is shown in Figs. 2(e) and 2(f), respectively. It is clear that in
contrast to the random, radial domains (indicated by black
arrows) observed on the uniform PVP insulator, the elon-
gated, bilateral domains were developed along the UT of the
PVP insulator as shown in Fig. 2(f).
In analyzing the structural order of the TIPS-pentacene
molecules in terms of the optical anisotropy, the optical re-
tardation measurements were carried out using a photoelastic
modulator (PEM) (PEM90, Hinds Instruments), which has
been commonly used for characterizing liquid crystals
28,29
and pentacenes,
30
as shown in Fig. 3(a). The PEM with a
fused silica head was placed between two crossed polarizers
that were oriented at 645
with respect to the optic axis of
the PEM head. A 633 nm-laser beam was incident normal to
the TIPS-pentacene film sample. The signal fed into a lock-
in amplifier (SR830, Stanford Research Systems) from a
photodetector was monitored during rotation of the sample.
Figure 3(b) shows the optical retardation values for the
uniform PVP insulator (open circles) and those for the PVP
insulator with the UT (filled circles) as a function of the azi-
muthal rotation angle ( u). In both cases, the PVP insulators
were opt ically isotropic irrespective of the presence of the
UT. However, when the TIPS-pentacene film was crystal-
lized on the PVP insulator after the solvent evaporation, the
optical anisotropy resulting from the molecular order was
observed as shown in Figs. 3(c) and 3(d). Different color
curves represent five different samples we studied. For the
uniform PVP case, the magnitude of the optical anisotropy
and the direction of the optic axis of the TIPS-pentacene film
were quite different from sample to sample as shown in Fig.
3(c), while for the UT case, the optic axis was very well-
defined along the UT and the magnitude was fairly constant
as shown in Fig. 3(d). The average value of the retardation
maxima of the five samples was found to be 0.279 6 0.072
for the uniform PVP case and 0.366 6 0.037 for the UT case.
This indicates that the UT of the PVP insulator indeed
increases the degree of the molecular order and the crystal-
linity of the TIPS-pentacene film by the directional flow dur-
ing the solvent evaporation.
We now investigate the electrical properties of the
TIPS-pentacene OFETs with two types of PVP insulators,
one with no UT and the other with the UT, in a bottom-gate,
top-contact configuration as illustrated in Fi g. 4(a). The gate
electrode was prepared usin g 120 nm-thick-indium-tin-oxide,
and the source and drain electrodes were gold (60 nm). The
channel was produced along a direction either perpendicular
or parallel to the UT of the PVP insulator. The channel
length (L) and the channel width (W) were 50 lm and
1000 lm, respectively, defined by a metal shadow mask
FIG. 3. (a) The experimental setup for
the measurements of the optical anisot-
ropy. The optical retardation values for
(b) the uniform PVP (open circles) and
PVP with UT (filled circles), (c) TIPS-
pentacene film on the uniform PVP, and
(d) TIPS-pentacene film on PVP with
UT. Different colors represent five dif-
ferent samples.
193307-3 Keum et al. Appl. Phys. Lett. 102, 193307 (2013)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.163.8.74
On: Wed, 21 Oct 2015 19:04:43