COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Fabrication of PEDOT–OTS-patterned ITO substrates
Nicole Herzer,a Martijn M. Wienk,b Pauline Schmit,c Anne B. Spoelstra,c Chris E. Hendriks,a
Stefan D. Oosterhout,b Stephanie Hoeppener*d and Ulrich S. Schubert*ad
Received 16th May 2010, Accepted 23rd June 2010
DOI: 10.1039/c0jm01468k
The fabrication of a poly(3,4-ethylenedioxythiophene) (PEDOT)
pattern is demonstrated. As template, an n-octadecyltrichlorosilane
(OTS) monolayer self-assembled on indium tin oxide (ITO) was
structured by UV–ozone photolithography, resulting in an ITO–
OTS patterned surface. The conducting properties of the ITO were
utilized for the selective electropolymerization of 3,4-ethylenedioxythiophene (EDOT), whereby the electropolymerization was
inhibited by the insulating OTS. Differently sized PEDOT–OTS
patterns were obtained. The electronic properties of the patterns
were finally evaluated in a test OLED device.
1 Introduction
Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conductive polymer
material that is frequently used in organic electronics. It is wellknown for its excellent properties, i.e., relatively high transparency to
visible light, high conductivity, and good stability. These properties
make it a well-suited material for many applications, e.g., as electrode
material, in organic light emitting diode (OLED) devices, in solar
cells, in sensors, or in field emission displays.1–4 While thin film
preparation methods are commonly used to prepare homogeneous
PEDOT layers, other applications require the preparation of small
PEDOT features. In particular, electronic devices with substructures
on the micro- or nanometre scale are desirable for the fabrication of,
e.g., high resolution LED displays.
The fabrication of micrometre-size PEDOT features can be achieved, e.g., by printing,5 laser ablation,5,6 photolithography,5,7 or
plasma patterning.8 In this case the deposition is achieved by applying
solutions of pre-polymerized PEDOT. A disadvantage of this
approach is the limited solubility of the PEDOT itself.
This issue is frequently addressed by additives such as PSS to
obtain stable solutions with better processing properties (up to now
the highest conductivity values for PEDOT could be obtained in
cases without PSS as additive).1 In addition, the use of functionalized
conductive PEDOT derivatives is reported.9–12
a
Laboratory of Macromolecular Chemistry and Nanoscience, Center for
NanoMaterials (cNM), Eindhoven University of Technology, Den
Dolech 2, 5600 MB Eindhoven, The Netherlands
b
Laboratory of Molecular Materials and Nanosystems, Eindhoven
University of Technology, Den Dolech 2, 5600 MB Eindhoven, The
Netherlands
c
Laboratory of Polymer Technology, Department of Chemical Engineering
and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600
MB Eindhoven, The Netherlands
d
Laboratory of Organic and Macromolecular Chemistry, FriedrichSchiller-Universit€
at Jena, Humboldtstrasse 10, 07743 Jena, Germany.
E-mail: s.hoeppener@uni-jena.de; ulrich.schubert@uni-jena.de; Fax: +49
3641 948202; Tel: +49 3641 948200
6618 | J. Mater. Chem., 2010, 20, 6618–6621
A more straightforward approach to overcome the solubility
problem would be the direct polymerization of 3,4-ethylenedioxythiophene (EDOT) monomers onto a given pattern. One
possible pathway to directly prepare PEDOT films without additives
is the electropolymerization of EDOT. This method requires only
small amounts of materials as well as short polymerization times and
allows the formation of either freestanding or electrode-supported
films on the anode.1
In the method outlined, the patterning can be achieved by structuring of the anode. A possible approach to obtain micrometre-sized
structures is the patterning of self-assembled monolayers of n-octadecyltrichlorosilane (OTS). These monolayers provide stable and
transparent coatings which can be prepared easily and with reliable
quality; they allow the efficient tailoring of surface properties, such as
wettability,13 chemical addressability,14 or insulating properties.15
A large number of structuring techniques have been employed for the
patterning of self-assembled monolayers, e.g., soft lithography,
photolithography, dip-pen and electro-oxidative as well as nanoimprint lithography.16,17
Only a few examples are shown in the literature which make use of
structured self-assembled monolayers as templates for patterned
PEDOT films. Examples include microcontact printing,18–20 photolithography with polymers,21,22 and with self-assembled monolayers23
as resist. In the present paper we propose the use of a self-assembled
monolayer of OTS that can be locally degraded by means of
UV–ozone irradiation. No subsequent removal process of the
material is required, which usually involves the application of solvents
that might affect also the quality of the deposited PEDOT films.
Micrometre-sized areas on the ITO substrate where the monolayer is
removed by the UV–ozone irradiation can be generated by applying
a suitable photomask. Those areas can subsequently serve as anode
for the electropolymerization of an electrode-supported PEDOT film,
whereas the non-removed OTS monolayer protects the non-irradiated areas. The OTS serves in this case as a barrier where no PEDOT
film is formed. The anode-supported electropolymerization of EDOT
leads to the formation of stable and robust PEDOT films. Those
patterns can be used, e.g., as templates for the fabrication of micrometre-size OLED structures. The quality of the surface-grafted,
electropolymerized PEDOT structures is demonstrated in this
contribution.
2 Experimental part
Materials
n-Octadecyltrichlorosilane (Fluka), 3,4-ethylenedioxythiophene
(Aldrich), tetrabutylammonium hexafluorophosphate (Aldrich),
chloroform (Biosolve) and bicyclohexane (BCH, Fluka) were
purchased from different suppliers. BCH was distilled over sodium
before use. All other reagents were used without further purification.
This journal is ª The Royal Society of Chemistry 2010
ITO glass slides were obtained from PGO and Naranjo Substrates.
The ITO glass slides were treated on both sides for 30 minutes in
a UV–ozone chamber before use.
Preparation of patterned substrates
OTS monolayers were prepared by immersing the ITO glass slides in
a solution of OTS (5 mL) in BCH (5 mL) for 20 minutes, followed by
sonication in chloroform. Finally, the slides were dried in a stream of
air. This procedure was repeated twice. The patterning was performed by placing a TEM grid onto the OTS-modified ITO glass
slides which were then incubated for two hours in a UV–ozone
photoreactor. The TEM grids were fixed to the substrate by placing
a UV transparent quartz slide on top of them.
Electropolymerization of EDOT
A droplet of a mixture of 3,4-ethylenedioxythiophene and tetrabutylammonium hexafluorophosphate was placed on the patterned
substrate and a voltage of 2 V was applied for 30 minutes. The treated
ITO glass slide was subsequently sonicated in chloroform.
Preparation of OLED devices
OLED devices were prepared onto OTS–PEDOT-patterned ITO
substrates (Naranjo Substrates) in a glovebox. The photoactive layer
tris(8-hydroxyquinolinato)aluminium (Alq3) as well as the counter
electrode of LiF (1 nm) and aluminium (100 nm) were deposited by
vacuum evaporation at 109 bar.
Instruments
A UV–ozone photoreactor (UPV, PR-100, Upland, CA) was used to
perform the cleaning and patterning process. AFM measurements
were recorded with a Solver LS system (NT-MDT). Tapping mode
tips were obtained from mMash (NSC35). Optical images were
recorded with a Solver LS system (NT-MDT) equipped with a CCD
camera and a Hyperion 2000 FT-IR microscope from Bruker. The
optical pictures of the OLED were taken by an Axioplan Imaging 2
from Zeiss in reflection, whereby the microscope light was turned off.
XPS measurements were conducted on a VG Escalab MKII spectrometer equipped with a dual Al/Mg Ka X-ray source and a hemispherical analyzer with a five channeltron detector. Spectra were
obtained using a magnesium anode (Mg Ka ¼ 1253.6 eV) operating
at 480 W and a constant pass energy of 20 eV with a background
pressure of 2 1012 bar. Spectra were referenced to the In (3d) peak
at 451.1 eV of the ITO layer on the substrate.27
3 Results and discussions
The schematic overview of the complete preparation process of the
PEDOT–OTS patterns is depicted in Scheme 1.
Smooth ITO substrates prepared on float glass with a typical
roughness of <2 nm were utilized. On these substrates, OTS was selfassembled and the layers were inspected by contact angle goniometry,
revealing a water contact angle of >110 . This indicates the formation
of an OTS monolayer on the ITO substrate.
The OTS monolayer was used as a chemically inert and fairly
robust passivation layer. OTS was chosen due to the high stability,
This journal is ª The Royal Society of Chemistry 2010
Scheme 1 Schematic overview of the preparation of a PEDOT–OTS
pattern. (a) UV–ozone photopatterning of the OTS monolayer and
(b) electropolymerization of EDOT.
the ordered and close packing as well as the insulating properties of
the molecules.24–26
The preparation of the patterned substrate was performed by
locally applying UV–ozone irradiation on OTS through a mask
structure. As a suitable photomask, TEM grids were used that could
be fixed to the ITO substrate by placing a UV-transparent quartz
slide on top of the TEM grid. This process allowed close contact of
the entire TEM grid mask to the ITO substrate. TEM grids with
different bar structures could be used to obtain different structure
dimensions (100 mm hole size, 20 mm bar size for 200 mesh Ni grids
and 30 mm hole size, 7 mm bar size for 600 mesh Cu grids, respectively). In the UV–ozone photoreactor, ozone was produced by the
UV light; the ozone reacted with the alkyl chains of the OTS
monolayer to form carbon oxide species. In the course of the irradiation the degradation of the monolayer took place. The removal of
the OTS monolayer and the preparation of micrometre structures in
the UV–ozone photoreactor were previously reported and investigated in detail.13
When, after the irradiation process, the photomask was removed,
the surface of the slides showed a strong hydrophilic–hydrophobic
contrast which resembled the structure of the used TEM grid
photomasks: the bar structures remained hydrophobic, whereas the
open spaces were hydrophilic and represented the characteristic
hexagonal structure of the holes of the TEM grid. The patterns were
then utilized for the electropolymerization of EDOT. As a reference,
a non-structured PEDOT film was electropolymerized onto ITO
substrates. A solution of EDOT and tetrabutylammonium hexafluorophosphate (which is known to support the formation of highly
conductive PEDOT at room temperature1) was placed on top of the
ITO surface. A platinum electrode was placed in the solution and the
ITO substrate was used as anode. A voltage of 2 V was applied for
30 minutes to form an anode-supported PEDOT film. The substrate
was subsequently cleaned by sonication in chloroform. The obtained
films were characterized by XPS measurements (Fig. 1). The XPS
spectrum shows indium and tin signals from the ITO coating and the
expected sulfur as well as carbon signals from the PEDOT film.
After successful formation of the PEDOT films on the nonstructured ITO substrates, the electropolymerization of EDOT on the
structured ITO substrate was performed. In this case the patterned
ITO substrates were used as anode in the electropolymerization setup
and the polymerizations were performed utilizing the same conditions
that were tested on non-structured ITO electrodes. The electropolymerization started from the conductive ITO layer and resulted in
the formation of a 60 nm thick PEDOT film with a peak-to-peak
roughness of approximately 10 nm. AFM investigations of the
patterned PEDOT films demonstrated the selective growth of
J. Mater. Chem., 2010, 20, 6618–6621 | 6619
Fig. 1 XPS measurement on an electropolymerized PEDOT film on
ITO.
a homogeneous PEDOT layer within the irradiated hexagonal
structures (Fig. 2a–d). Due to the light blue color of the PEDOT
films, the patterns were also visible under the optical microscope and
again showed the selectivity of the electropolymerization in the
hexagons (Fig. 2e).
According to Fig. 2a, the AFM images show sharp borders, which
indicate a good selectivity of the PEDOT polymerization as well as
the good protection properties of the OTS-coated regions.
The roughness of the films, depicted as representative line profiles
(Fig. 2b and d), was not ideal and can certainly be further improved
by optimization of the polymerization conditions. The stability of the
OTS barrier structures between the individual anode structures was
rather good, as no degradation or loss of the hydrophobic properties
was observed during the electropolymerization process.
By variation of the photomask, differently sized patterns could be
obtained. In the present study TEM grids with hexagonal features
provided anode structure sizes of 30 mm and 100 mm, respectively.
In the case of 600 mesh TEM grid patterns the individual anode areas
were separated by only 5 mm OTS features. In the case of 200 mesh Ni
TEM grids, the corresponding anode structures were separated by
15 mm OTS regions. The differences in bar size between the actual
TEM grids and the produced OTS patterns are supposedly the result
of diffusion, at the edges of the TEM grids, of the active oxygen
species during the patterning process.
Patterned PEDOT–OTS structures as described above are interesting in terms of microfabrication. We have therefore tested whether
the quality of the electropolymerized films and their electronic
properties were sufficient to fabricate patterned OLED based on
these structures. The schematic setup is depicted in Scheme 2. In this
proof-of-concept study, a rather simple OLED device layout was
chosen. Further improvement of the device performance can be
expected by tuning the layer structure as well as the layer materials
used to fabricate the OLED.
Despite of its preliminary character, the device setup described was
suitable to serve as a first test for the electronic performance of the
patterned electropolymerized PEDOT–OTS films. For the device
a different ITO substrate was utilized to fit into the evaporation
system. The peak-to-peak roughness of these ITO substrates was
approximately 5 nm. A 60 nm thick layer of Alq3 was evaporated
Scheme 2 Schematic representation of the device setup.
Fig. 2 AFM investigations and optical images of the PEDOT–OTS patterns: (a) AFM height image of the 30 mm PEDOT–OTS pattern, (b) representative height profile, (c) AFM height image of the 100 mm PEDOT–OTS pattern, (d) representative height profile, and (e) optical image of the 100 mm
PEDOT–OTS pattern. Dark areas represent the areas where PEDOT films were grown on the ITO anode.
6620 | J. Mater. Chem., 2010, 20, 6618–6621
This journal is ª The Royal Society of Chemistry 2010
Acknowledgements
N.H., S.H. and U.S.S. gratefully acknowledge the financial support
of the Dutch Science Council (NWO) (VICI grant awarded to
U.S.S.) and Prof. Dieter Schubert for his helpful comments.
References
Fig. 3 Optical photograph of the OLED based on the 100 mm PEDOT–
OTS pattern.
onto the PEDOT–OTS patterns, which served as light emitting layer,
followed by a thin layer of LiF and finally a layer of 100 nm Al used
as cathode of the OLED. Contacting this device and applying a bias
voltage of 5 V led to the emission of green light from the device. For
the optical image presented in Fig. 3, the bias voltage was increased to
7 V to allow observation of the light emission under a microscope.
The image shows the light emitting from the hexagons where the
PEDOT is located. The hexagonal structure of the patterned PEDOT
films was clearly visible and the bars were not light emissive, which
underlines the good insulating properties of the OTS monolayer.
Light emission within the hexagons of Fig. 3 was not homogenous,
which may be due to the rather rough structure of the grafted
PEDOT film. It should be considered that, in principle, light could be
also emitted from ITO areas which are not coated by PEDOT; the
required voltage is, however, higher than applied here. The darker
regions in the hexagons may thus even indicate the presence of
discontinuities in the PEDOT film, which are not emissive because
the drive voltage applied was not high enough to turn on the light
without the PEDOT layer.28,29 In any case, further improvement of
the structures and optimization of the electropolymerization process
will be required.
4 Summary
We have demonstrated the functionalization of ITO-coated
substrates with self-assembled monolayers and their subsequent
patterning by photolithography. The structured surfaces were applied
as templates for the selective electropolymerization of EDOT in
micrometre structures, leading to patterned PEDOT–OTS substrates.
Films of a constant height of 60 nm and in different sizes were
prepared. The microstructures were tested regarding their application
in OLED and showed promising first results.
Patterned PEDOT devices could be of interest for the development
of micro-electronic construction elements such as transistors, displays
or microfluidics. The combination of photolithography on selfassembled monolayers and electropolymerization used here provides
an easy and fast route towards obtaining PEDOT microstructures.
This journal is ª The Royal Society of Chemistry 2010
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