MASARYK UNIVERSITY
FACULTY OF SCIENCE
DEPARTMENT OF PHYSICAL ELECTRONICS
HABILITATION THESIS
2017 Tomáš Homola
MASARYK UNIVERSITY
FACULTY OF SCIENCE
DEPARTMENT OF PHYSICAL ELECTRONICS
Plasma Processing of Surfaces and Nanostructured
Coatings for Flexible and Printed Electronics
HABILITATION THESIS
2017 RNDr. Tomáš Homola, PhD.
Bibliographic Entry
Author RNDr. Tomáš Homola, PhD.
Faculty of Science, Masaryk University
Department of Experimental Physics
Title of Thesis:
Plasma Processing of Surfaces and Nanostructured Coatings
for Flexible and Printed Electronics
Habilitation process in: Plasma Physics
Academic Year: 2017
Number of Pages: 32
Keywords: plasma treatment; flexible and printed electronics; surface
energy; roll-to-roll; mesoporous coatings
Abstract
Atmospheric pressure plasma processing of thermally sensitive surfaces has recently
become a subject of great interest in novel emerging technologies, particularly in the field of
flexible and printed electronics. This thesis consists of commentaries on the author’s published
research papers, which address atmospheric pressure plasma treatments of flexible substrates
and plasma processing of coatings on flexible substrates for flexible electronics. It is organized
into three main sections: the first provides the background of atmospheric-pressure plasma
generation, the second presents plasma pre-treatment of substrates, and the third focuses on
plasma post-treatment or plasma processing of coatings on flexible surfaces. In addition, the
thesis also briefly summarizes the problematic aspects of flexible and printed electronics and
how plasma treatments may contribute to faster, cheaper and more competitive
manufacturing comparing to the present state of the art in electronics.
Keywords: plasma treatment, flexible and printed electronics, surface energy, roll-to-roll,
mesoporous coatings
Abstrakt
Povrchová úprava teplotně citlivých materiálů atmosférickým plazmatem se stala
objektem velkého zájmu v nových technologiích, např. v oblasti flexibilní a tištené elektroniky.
Tato habilitační práce pozůstává z komentářů k mým nedávno publikovaným článkům, které se
týkají povrchové úpravy flexibilních materiálů atmosférickým plazmatem a vrstev na
flexibilních površích pro flexibilní elektroniku. Práce je členěná do troch částí: první část
poskytuje přehled zdrojů atmosférického plazmatu, druhá část prezentuje povrchovou úpravu
flexibilních substrátů a třetí část se zaměřuje na post úpravu vrstev pro flexibilní elektroniku.
Práce rovněž stručně sumarizuje hlavní výzkumné aspekty flexibilní a tištené elektroniky a jak
může právě plazma přispět k rychlejší, levnější a víc konkurence schopné výrobě v porovnání se
současnými klasickými elektronickými zařízeními.
Klíčové slova: úprava plazmatem, flexibilní a tištená elektronika, povrchová energie, roll-toroll,
mezoporézní vrstva
Preface
Atmospheric pressure plasma and plasma treatment of surfaces is an area of research
in which I worked for twelve years. First, I joined Laboratory of Surface Discharge Studies at
Comenius University in Slovakia in 2005. Afterwards, in 2008, I enrolled in the doctoral
programme in Plasma Physics. Since then, I have continued within the same laboratory, and
worked on many interesting projects. Half of my doctoral study was spent as a full-time
research Intern at the Singapore Institute of Manufacturing Technology (A*STAR, Singapore),
where participated in flexible and printed electronics research using roll-to-roll manufacturing
systems. After successful completion of my doctoral study in 2012, I joined the newlyestablished
research and development centre known as CEPLANT (R&D Centre for Low-Cost
Plasma and Nanotechnology Surface Modifications) at Masaryk University in Brno, Czech
Republic. Since then I have worked on various experimental research topics, dealing preferably
with functional thin films, surfaces and plasma treatments.
During 2012 – 2015 I accepted two long-term international internships, at
Lappeenranta University of Technology, Finland, and at Joanneum Research in Austria. At
Finland, I learned to operate various atomic layer deposition (ALD) systems which I used to
deposit TiO2, Al2O3, CeO2, ZnO coatings. Subsequently, the head of the ALD group in Finland,
Prof. David Cameron, joined CEPLANT in 2015 and since 2017 our ALD project related to optoelectronics
has received financial support from GACR. The scientific work related to ALD
resulted in nine papers published in recognized impacted journals. Furthermore, ALD has
become important in flexible electronics, because it enables the deposition of conformal thin
films with high precision of thickness onto flat, thermally sensitive or porous/mesoporous
substrates. ALD is currently employed largely in high-end applications.
Since 2016 I have been involved in very active and fruitful collaboration with the Brno
University of Technology in plasma processing of inkjet-printed coatings for flexible
electronics, particularly photovoltaics. Our first collaborative paper was published in 2016 in
ACS Applied Materials & Interfaces, IF 7.5. Since then, the team has started work on numerous
novel topics. Moreover, our network has expanded to other local and international institutes.
This habilitation thesis summarizes my main research results related to plasma
treatments of surfaces for low-cost flexible and printed electronics. It consists of seven papers
recently published in international peer-reviewed journals. The final paper included in this
thesis has been recently published in a new IOP journal, Flexible and Printed Electronics, which
further confirms the novelty and importance of this topic.
Because all my research activities were always completed jointly with others in form of
teamwork, I would like gratefully to acknowledge the support of my current and former
colleagues and collaborators:
 Prof. Mirko Černák, Prof. David C. Cameron and Dr. Richard Krumpolec of Masaryk
University,
 Dr. Petr Dzik of the Brno University of Technology,
 Dr. Linda YL Wu of the Singapore Institute of Manufacturing Technology, A*STAR,
Singapore,
 Dr. Martin Kormunda and Dr. Jindřich Matoušek of J. E. Purkyně University in Ústí nad
Labem.
I would also like to extend my thanks to my friend Dr. Tony Long (Svinošice) for sharing his
experience and helping to further my competence in written and spoken scientific English.
Of course, I must offer my heartfelt thanks to my fiancée Martina for her love, inspiration,
patience and support.
Tomáš Homola
27.11.2017 Brno
DECLARATION
I hereby declare that all the work presented in this thesis is original. Wherever
contributions made by others are involved, every effort has been made to indicate this clearly,
with due reference to the literature and appropriate acknowledgement of collaborative
research.
…………………………………………………………
In Brno, 27.11.2017 RNDr. Tomáš Homola, PhD.
Content
List of figures....................................................................................................................... i
List of abbreviations........................................................................................................... iii
List of papers commented upon ..........................................................................................iv
1. Low-temperature plasmas for surface treatments............................................................ 1
1.1. State of the art ................................................................................................................... 1
2. Plasma pre-treatment of substrates for flexible and printed electronics ........................... 6
2.1. State of the art ................................................................................................................... 6
2.2. Comments on papers: ........................................................................................................ 8
3. Plasma post-treatment of coatings for flexible and printed electronics........................... 19
3.1 State of the art .................................................................................................................. 19
3.2 Comments on papers ........................................................................................................ 20
4. Conclusions................................................................................................................... 28
References........................................................................................................................ 29
Appendix: Copies of papers commented upon................................................................... 33
i
List of figures
Figure 1. Total number of records addressing the topic of “plasma surface treatment” in the
course of the past twenty years. Web of Science......................................................................... 2
Figure 2. Commercial volume DBD (“industrial corona”) for roll-to-roll applications, by
Ahlbrandt System GmbH. Photo available from www.ceplant.cz. ............................................... 3
Figure 3. Surface plasma generated by surface DBD in ambient air for treatment of
polypropylene non-woven fabrics [15]......................................................................................... 4
Figure 4. Multiple DCSBD plasma units generating homogeneous plasma operating in an
integrated roll-to-roll manufacturing line. Photo available from www.ceplant.cz. ..................... 5
Figure 5. Total number of records addressing the topic of “atmospheric pressure plasma” in
the course of the past twenty years. Web of Science. ................................................................. 7
Figure 6. Water contact angles of PET surfaces as a function of DCSBD plasma treatment
time(1–10 s) and storage time (0–12 days). ................................................................................. 9
Figure 7. PET 1, PET2 and PET 3 rolls in the Singapore Institute of Manufacturing Technology,
A*STAR integrated clean-room laboratory: (complementary material for Ref. 3)..................... 12
Figure 8. Water contact angles of PET surfaces as a function of MyPL plasma composition
(constant argon flow 6 l.min-1
+ various oxygen amounts in sccm) (complementary material for
Ref. 3).......................................................................................................................................... 12
Figure 9. UV-Vis-NIR transmittance spectra for untreated and MyPL plasma-treated PET 1, PET
2 and PET 3: (complementary material for Ref. 3)...................................................................... 13
Figure 10. Typical defects, such as bubbles and flow-lines, in PEDOT:PSS coated on untreated
PET surface: recorded immediately after coating process, taken from Ref. 3. .......................... 14
Figure 11. Number of defects in PEDOT:PSS coated on untreated and plasma-treated PET
flexible foil, taken from Ref. 3..................................................................................................... 14
Figure 12. Interaction between ITO and DCSBD plasma followed by total extinction of
filamentary plasma in a gap smaller than 0.75 mm, taken from Ref. 5...................................... 17
Figure 13. Visualization of plasma mineralization process in ambient air, taken from Ref. 6. .. 21
Figure 14. Atomic concentration of elements in a TiO2/SiO2 mesoporous coating fabricated
using thermal sintering, UV curing and plasma treatment (complementary material to Ref. 6).
..................................................................................................................................................... 22
Figure 15. Evolution of total current measured on TiO2/SiO2 photoanodes deposited on FTO
glass together with total carbon concentration in the TiO2/SiO2 coating, taken from Ref. 6. ... 23
Figure 16. Visualization of plasma fabrication of large-area mesoporous TiO2 photoanodes by
roll-to-roll processing.................................................................................................................. 25
ii
Figure 17. a) Roll-to-roll system at Masaryk University developed by CEPLANT, b) detailed view
of TiO2 mesoporous film on PET roll treated by the roll-to-roll system. .................................... 25
Figure 18. TiO2/SiO2 hybrid nanocomposite flexible photoanodes inkjet-printed onto ITO/PET
foil, taken from Ref. 7. ................................................................................................................ 26
Figure 19. Evolution of total current measured on TiO2/SiO2 photoanodes deposited on FTO
glass and ITO together with total carbon concentration in the TiO2/SiO2 coating, taken and
merged from Ref. 6 and Ref. 7.................................................................................................... 27
iii
List of abbreviations
AFM – atomic force microscopy
CEPLANT – R&D Centre for Low-Cost Plasma and Nanotechnology Surface Modifications
DBD – dielectric barrier discharge
DCSBD – diffuse coplanar surface barrier discharge
DSSC – dye sensitized solar cell
FTO – fluorine doped tin oxide
ITO – indium tin oxide
PEDOT:PSS -- poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
PEN – poly(ethylene naphthalate)
PET – poly(ethylene terephthalate)
R2R – roll-to-roll
TCO – transparent conductive oxide
XPS – X-ray photoelectron spectroscopy
iv
List of papers commented upon
[1] T. Homola, J. Matoušek, B. Hergelová, M. Kormunda, L.Y.L. Wu, M. Černák, Activation
of poly(ethylene terephthalate) surfaces by atmospheric pressure plasma, Polym. Degrad. Stab.
97 (2012) 2249–2254. doi:10.1016/j.polymdegradstab.2012.08.001.
[2] M. Kormunda, T. Homola, J. Matousek, D. Kovacik, M. Cernak, J. Pavlik, Surface
analysis of poly(ethylene naphthalate) (PEN) films treated at atmospheric pressure using
diffuse coplanar surface barrier discharge in air and in nitrogen, Polym. Degrad. Stab. 97 (2012)
547–553. doi:10.1016/j.polymdegradstab.2012.01.014.
[3] T. Homola, L.Y.L. Wu, M. Černák, Atmospheric Plasma Surface Activation of
Poly(Ethylene Terephthalate) Film for Roll-To-Roll Application of Transparent Conductive
Coating, J. Adhes. 90 (2014) 296–309. doi:10.1080/00218464.2013.794110.
[4] T. Homola, J. Matoušek, M. Kormunda, L.Y.L. Wu, M. Černák, Plasma treatment of
glass surfaces using diffuse coplanar surface barrier discharge in ambient air, Plasma Chem.
Plasma Process. 33 (2013) 881–894. doi:10.1007/s11090-013-9467-3.
[5] T. Homola, J. Matoušek, V. Medvecká, A. Zahoranová, M. Kormunda, D. Kováčik, et al.,
Atmospheric pressure diffuse plasma in ambient air for ITO surface cleaning, Appl. Surf. Sci.
258 (2012) 7135–7139. doi:10.1016/j.apsusc.2012.03.188.
[6] T. Homola, P. Dzik, M. Veselý, J. Kelar, M. Černák, M. Weiter, Fast and lowtemperature
(70 °C) mineralization of inkjet printed mesoporous TiO2 photoanodes using
ambient air plasma, ACS Appl. Mater. Interfaces. 8 (2016) 33562–33571.
doi:10.1021/acsami.6b09556.
[7] T. Homola, M. Shekargoftar, P. Dzik, R. Krumpolec, Z. Ďurašová, M. Veselý, M. Černák,
Low-Temperature (70 °C) Ambient Air Plasma-Fabrication of Inkjet-Printed Mesoporous TiO2
Flexible Photoanodes, Flex. Print. Electron. 2 (2017) 035010. doi: 10.1088/2058-8585/aa88e6
1
1. Low-temperature plasmas for surface treatments
1.1. State of the art
Plasma is, in terms of energy, the highest state of the matter, lying above solid, liquid
and gas. During the past two centuries, plasmas have been used in numerous applications,
from low-energy plasma lighting systems to high-energy plasma fusion reactors. Possible
“natural” encounters with plasma are limited to only a few examples: perhaps the aurora
borealis, the aurora australis (northern and southern lights), lightning, and extremely rare red
sprites and ball lightning. Plasma may be categorized according to various parameters:
temperature, ratio of ions/neutrals, pressure, composition of gas, etc. One of the most crucial
of them for wide-spectrum applicability of plasma is its temperature. In natural conditions,
low-pressure plasma exists in a non-equilibrium condition, i.e. particles in plasma exist at
significantly different temperatures; such plasma is often called non-equilibrium, non-thermal
or cold plasma. The contact of cold plasma with material leads to significant changes in the
surface without affecting bulk structure. Transition from low pressure to a higher pressure is
associated with a higher number of collisions between the particles and consequently to
thermalization of the plasma. Outside the laboratory, high-pressure plasma exists in a
condition of equilibrium; a typical example is an electrical arc with temperatures reaching
several thousand degrees. The contact of such plasma with inert material leads to sudden and
profound changes; it is widely utilized in an arc-welding process.
Since the applicability of cold plasma at low-pressure is inappropriate to low-cost and
large-area surface treatments in industrial conditions, researchers have been seeking practical
ways of preventing the natural thermalization of plasma during transition from low to
atmospheric pressure. This may be achieved by i) localization of plasma, ii) fast gas flows or iii)
discharges with ultra-short lifetimes. Localization of plasma is typical of corona discharge,
which generates plasma in very small volumes, of a few mm3
. Fast gas flows may be found in
most plasma jets – although most of them, especially in ambient air, generate plasma of
temperatures higher than 150 °C. Cold plasma generation using ultra-short discharges – microdischarges
– is typical of dielectric barrier discharges.
The use of plasma become important in the 1960s in the microelectronic industry,
later in other industrial segments, mostly in the lighting that employed popular helium/neon
tubes. The number of research papers related to applications of cold plasmas is rising every
year and the state of the art of cold plasma progressively more difficult to understand. In
2
response to the massive quantity of research papers, several reviews have been published
focusing on various aspects of cold plasma science. In 2011, D. Pappas [1] presented a
comprehensive review of low-pressure plasmas together with the physical background of
breakdown voltage, while the paper further describes knowledge of atmospheric plasmas in
1990’s, compares the designs of reactors, and discusses the role of species on surface
modification. Two road-maps discussing the future potential of new applications in cold
plasma science were published in 2012 [2] and 2017 [3] by leading scientists in the field. Also
worthy of note is a review of plasma jets by Fanelli et al. published in 2017 [4].
The most recent two decades have seen a plethora of cold atmospheric plasma
sources used in a range of applications. Figure 1 shows total the number of records for the
topic “plasma surface treatment” during 1996–2016. The number of research papers is rising
because plasma surface treatments enables new fields of application, e.g. in flexible and
printed electronics, among many others.
Figure 1. Total number of records addressing the topic of “plasma surface treatment” in the
course of the past twenty years. Web of Science.
Dielectric barrier discharges
Dielectric barrier discharge (DBD) is based on the utilization of a dielectric among two
metal electrodes connected to an AC (0.05 – 500 kHz) high-voltage (typically up to 10 – 20 kV)
generator [5]. The dielectric limits the current and prevents the formation of spark and arc.
The dielectric may cover one or both electrodes, or it may be located in the space between
3
them. Typical materials for the dielectric in DBD are glass, quartz, ceramics and polymers. The
inter-electrode distance is 0.1–2 mm. The concept of DBD was introduced in 1857 by Ernst
Werner Siemens, who proposed a novel electrical discharge for ozone generation from
ambient air [6].
Dielectric barrier discharges generating volume plasma
The first application of dielectric barrier discharge for plasma treatment of material
was proposed in 1959 by J.F. McDonald, who used volume DBD to treat polymeric foils [7].
Since then, this concept have been adopted by many research groups and commercial
companies and is now widely known as the ”industrial corona”. The industrial corona should
not be mistaken for a true corona discharge, needs no dielectric barrier and is generated in a
strongly non-uniform electric field near sharp points.
The main feature of volume DBD, or industrial corona, lies in the orientation of the
micro-discharges. The micro-discharge event occurs in the volume between high-voltage
electrodes and therefore the streamer occurs perpendicular to the electrodes. The treated
surface is therefore exposed to plasma that strikes the surface energetically; in extreme
conditions, this leads to local damage arising out of overheating. The maximal power in volume
DBD is often restricted to a reasonable level. An example of commercial “industrial corona”
available the CEPLANT laboratory appears in Figure 2.
Figure 2. Commercial volume DBD (“industrial corona”) for roll-to-roll applications, by
Ahlbrandt System GmbH. Photo available from www.ceplant.cz.
Dielectric barrier discharges generating surface plasma
It is generally accepted that surface dielectric barrier discharge (surface DBD) was
observed and reported for first time in 1958 by Semyon Kirlian, a Russian scientist who was
4
investigating electrographic plates connected to high-voltage generators. The method he
developed is now widely known as a Kirlian photography [8] and can be used to visualize
various surfaces, e.g. fingerprints [8,9]. Surface DBD was further improved by Senichi Masuda,
a director in a Japanese company that was developing a system for the transport of powder by
exploiting electro-charging properties of plasma. A member of his team inadvertently used a
power level that was significantly above the standard working parameter, which led to the
occurrence of surface plasma. Work on improved surface DBD was published in 1988 by
Masuda et al. in a research paper that focused on ozone generation [10]. Afterwards, Masuda
founded a company developing ozone generators. Later, Gerhard J. Pietsch and Valentin
Gibalov contributed greatly to the further improvement of ozone generation and to the
plasma physics of surface DBDs [11,12].
In 2003, Masashi Kando and Mirko Černák (now a professor at Masaryk University and
Head of the Department of Physical Electronics) reported the first use of surface DBD to
modify non-woven polymer fabrics [13,14], and since then surface DBD has been used to
modify polymeric materials many times. Surface DBD plasma in ambient air, used for
activation of polypropylene non-woven fabrics, is shown in Figure 3 [15]. However, long-term
use (>100 hours) of surface DBD leads to erosion of the upper electrode system arising out of
bombardment by energetic particles from the plasma. This negatively affects overall lifetime
and renders its use in industrial applications problematic [16].
Figure 3. Surface plasma generated by surface DBD in ambient air for treatment of
polypropylene non-woven fabrics [15].
Limited lifetime and low levels of safety in the early systems led to a search for a better
surface plasma system, one of considerably high energy density with a longer and/or unlimited
5
lifetime. In late 1990s and early 2000s, the concept of coplanar DBD was already known from
flat plasma display panels [17], but the total power delivered to plasma was extremely low, in
terms of up to only a few watts [18]. The first high-power coplanar DBD was introduced by
Mirko Černák in 2002 [19]. He employed alumina ceramic as a thin (0.4 mm) dielectric and
parallel, strip-like molybdenum electrodes (1 mm wide, 50 μm thick, 150 mm long, 0.5 mm
strip-to-strip). A surface coplanar plasma with dimensions of approx. 20 × 8 cm2
was the first
system to provide sufficient power, in terms of several hundred watts, thus opening a window
for various novel applications.
The concept of coplanar DBD was later adapted by several groups. Novel plasma
sources based on coplanar electrode arrangement led to increased energy densities, higher
degrees of diffusivity on the plasma and new opportunities for fundamental research. The
original inventor and holder of several patents, Mirko Černák, calls the technology “diffuse
coplanar surface barrier discharge (DCSBD)”. It has been reported that coplanar DBD can
operate in a wide range of pressures [20] and generate diffuse, homogeneous plasma at
atmospheric pressure in many working gases including: ambient air, hydrogen, oxygen, argon,
helium, carbon dioxide, methane, and others. An array of coplanar DBD plasma units available
at the CEPLANT laboratory appears in Figure 4.
Figure 4. Multiple DCSBD plasma units generating homogeneous plasma operating in an
integrated roll-to-roll manufacturing line. Photo available from www.ceplant.cz.
6
2. Plasma pre-treatment of substrates for flexible and printed electronics
2.1. State of the art
Flexible electronics may be defined as a technology for assembling electronic circuits
on flexible substrates. Printing is a common deposition method for mounting electronic
devices onto flexible substrates, e.g. screen-, inkjet-, flexo- and gravure-printing [21]. Plastics
such as poly(ethylene terephthalate) PET and poly(ethylene naphthalate) PEN foils are the
materials most widely-used as flexible substrates. In addition to plastics, thin, flexible glass
(Corning and Schott) may be also used for various high-end products. Flexible and printed
electronics [22,23] have attracted increased attention in recent times because of their
potential to enable low-cost and high-throughput manufacturing of electronics on cheap
plastic substrates for various applications, including rollable and foldable displays, smart
packaging, photovoltaics, and more.
The primary advantage of flexible and printed electronics lies in their potential for lowcost
mass production using roll-to-roll technology. Roll-to-roll, often referred to as web
processing, reel-to-reel or R2R processing, may be defined as a process of creating thin films
and coatings on flexible substrates in continuous fashion starting with a roll of blank, flexible,
thin substrate placed on an unwinder. During the handling of the film, various coating and
curing steps take place as the thin film is transported to a rewinder. Roll-to-roll manufacturing
may also be combined with non-continuous manufacture, such as roll-to-sheet and sheet-tosheet
processing.
One of the most crucial difficulties (apart from any issues related to printing
resolution, web handling, etc.) is the extremely high thermal sensitivity of PET and PEN foils.
This limit is established by glass transition temperature and it varies according to density
and/or the crystalline nature of the polymer. Thermoplastics such as PET and PEN have low
glass transition temperature, less than 150 °C, and therefore cannot withstand the commonlyand
largely currently-used deposition procedures, which involve high-temperature
curing/annealing/sintering steps. Furthermore, raw plastic materials have poor surface
properties (insufficient wettability and poor printability) that often result in coatings of low
quality due to a range of defects. This has often been addressed by low-pressure plasma
activation; however, low-pressure plasma is barely compatible with atmospheric roll-to-roll
systems. Furthermore, industry demands that the entire process should take place in ambient
conditions, thus minimizing the cost of the final product and competing successfully with stateof-the
art electronics and enabling development of new products and applications.
7
Atmospheric pressure plasmas have thus become increasingly important for the processing of
surfaces and the manufacture of novel products. Applied research into atmospheric pressure
plasma is burgeoning, as documented by an ever-increasing number of published papers
dedicated to the subject (Figure 5).
Figure 5. Total number of records addressing the topic of “atmospheric pressure plasma” in
the course of the past twenty years. Web of Science.
This part of the thesis summarizes my research work on plasma pre-treatment of
substrates for flexible and printed electronics.
 The effects of ambient air plasma on common plastics for flexible electronics such as
PET and PEN are reported in Refs. 1–2. Both works were undertaken in collaboration
with J.E. Purkyně University, Ústí nad Labem, Czech Republic.
 Ref. 3 provides an introduction to roll-to-roll manufacturing and shows that plasma
pre-treatment is beneficial for subsequent coating of PET with transparent conductive
films. The entire research work was completed during my internship at the Singapore
Institute of Manufacturing Technology, A*STAR, Singapore.
 Ref. 4 discusses the use of plasma for surface treatments of glass and Ref. 5
demonstrates the activation of transparent indium-tin oxide deposited on glass.
Results presented in Refs. 4–5 were completed during my postgraduate study at
Comenius University, Slovakia.
8
2.2. Comments on papers:
[1]
T. Homola, J. Matoušek, B. Hergelová, M. Kormunda, L.Y.L. Wu, M. Černák, Activation of
poly(ethylene terephthalate) surfaces by atmospheric pressure plasma, Polym. Degrad. Stab.
97 (2012) 2249–2254. doi:10.1016/j.polymdegradstab.2012.08.001.
Number of times cited: 22 Journal IF: 3.4
This article reports on ambient air atmospheric pressure DCSBD plasma treatment of
poly(ethylene terephthalate) PET thin films. The surfaces were analyzed by water contact
angle, XPS and AFM. Water contact angle analysis of PET surfaces before and after plasma
treatment appears in Figure 6Figure 6 and shows that plasma treatment for 1 s leads to a
significant decrease of water contact angle, from 78.4° to 40.1°, corresponding to an increase
in the polar part of the surface energy and therefore better water wettability. Figure 6 also
depicts hydrophobic recovery of PET surfaces with storage time. The samples were stored
under ambient air conditions in a laboratory at a constant temperature of 22 °C and relative
humidity of 45%. It is apparent that changes to the surface induced by plasma treatment are
not stable and surface modification reverts towards its original state. Figure 6 shows the
hydrophobic recovery effect of PET surfaces treated in plasma for 1 s and 10 s. It is evident
that change in the water contact angle is at its most rapid during the beginning of the storage
time and reached saturation after approx. 24 hours. The water contact angle of PET treated for
1 s reached 59.9° after 288 hours (12 days). This is, however, lower than the water contact
angle measured on untreated samples, which was 78.1°. This hydrophobic recovery is also
referred to as the “ageing effect” and it influences the industrial use of plasma-treated
surfaces if they are not printed immediately after treatment, e.g. in roll-to-roll processing.
Since the declining water contact angle after ageing remains significantly below the untreated
value, flexible PET foils may be pre-treated in independent steps and afterwards transported
to another line or manufacturing hall for deposition of coating or bonding/laminating with
another functional material.
Activation
Detailed chemical analysis of plasma-treated PET surfaces revealed that the reason for
the decrease of water contact angle lies mainly in the incorporation of oxygen polar groups
during plasma treatment of the PET surface. Since water is a polar liquid, polar groups on the
9
PET surface lead to better wettability for water or water-based paints and/or functional
coatings. Water-based coatings are currently a pressing matter for industry because they are
cheap, easier to handle, and without toxic waste, therefore compliant with strict
environmental requirements.
Figure 6. Water contact angles of PET surfaces as a function of DCSBD plasma treatment time
(1–10 s) and storage time (0–12 days).
Cleansing and etching
XPS C1s peak analysis showed that the interaction of plasma with PET surfaces leads to
a decrease in concentration of C—C/C—H bonds. The C—C/C—H bond concentration indicates
the cleansing of the PET surface of hydrocarbons or scission in the PET chain. The cleaning and
scission of the PET chain may start simultaneously, but this usually depends on the initial
degree of PET contamination: cleansing starts first and, once major contaminants have been
removed, plasma interacts with the PET surface. Hydrocarbons (among many other
contaminants) adsorbed from ambient atmosphere lead to non-homogeneities in deposited
paints and/or functional coatings. The cleansing induced by plasma is therefore beneficial for
further surface processing of PET foils.
AFM analysis showed that an untreated PET surface has a flat surface with a roughness
of approx. 1.87 nm. Plasma treatment of PET for 1 s led to a significant increase in roughness,
10
to 6.92 nm. DCSBD plasma treatment therefore provides marked and selective etching of the
semi-crystalline polymer. This selectivity arises from the non-uniformly structured bulk, which
consists of semi-crystalline, biaxially-oriented PET polymer, i.e. it contains crystalline regions of
higher density and amorphous regions of lower density. Since the amorphous part of the
polymer is easier to etch than the crystalline, plasma treatment generates surface roughness.
Furthermore, a rougher surface at nanoscale contributes to higher adhesion because it can
provide a larger area and therefore a higher number of active surface sites for bonding.
[2]
M. Kormunda, T. Homola, J. Matousek, D. Kovacik, M. Cernak, J. Pavlik, Surface analysis of
poly(ethylene naphthalate) (PEN) films treated at atmospheric pressure using diffuse coplanar
surface barrier discharge in air and in nitrogen, Polym. Degrad. Stab. 97 (2012) 547–553.
doi:10.1016/j.polymdegradstab.2012.01.014.
Number of times cited: 21 Journal IF: 3.4
This paper reports on the ambient air atmospheric pressure DCSBD treatment of
poly(ethylene naphthalate) PEN thin foils. The surfaces were analyzed in terms of water
contact angle, XPS and AFM. Plasma treatment of PEN showed similar effects to those
reported for the plasma treatment of PET in the previous paper – Ref. 1. However, in addition
to ambient air, nitrogen was also used in the plasma treatment of PEN. Comparison between
ambient air and nitrogen environments showed that the nitrogen plasma treatment of PEN
resulted in slightly lower water contact angles. The water contact angle on an untreated PEN
surface was 82.1°, while plasma treatment for 1 s in ambient air and nitrogen led to 30.6° and
28.7° respectively. Ten seconds of treatment time resulted in water contact angles of 25.1° and
18.8° for ambient air and nitrogen respectively. The higher efficiency of nitrogen was explained
by incorporation of nitrogen into the PEN structure and, apparently, by the development of
nitrogen functional groups.
PET and PEN are the most important and widely-used flexible, low-cost substrates
for a wide range of applications in flexible and printed electronics. Both papers published in
2012 showed that DCSBD plasma treatment is a time- and energy-efficient method for
surface modification of thermally sensitive polymeric foils and could have the potential to be
used in highly improved further processing and manufacturing of flexible electronics.
11
[3]
T. Homola, L.Y.L. Wu, M. Černák, Atmospheric Plasma Surface Activation of Poly(Ethylene
Terephthalate) Film for Roll-To-Roll Application of Transparent Conductive Coating, J. Adhes. 90
(2014) 296–309. doi:10.1080/00218464.2013.794110.
Number of times cited: 3 Journal IF: 1.6
This contribution reports on the activation of large-area flexible PET foils for roll-to-roll
applications employing transparent conductive poly(3,4-ethylenedioxythiophene)
poly(styrenesulfonate) (PEDOT:PSS) coating. A commercial roll-to-roll line from Coatema
Machinery GmbH, Germany, equipped with a 1-m web-width roll-to-roll coating and
lamination system, was used, consisting of unwinder, web cleaning element, MyPL plasma
discharge unit, web alignment unit, slot die coater, thermal oven, web inspection system,
lamination unit and rewinder. The roll-to-roll operated at speeds of 1 – 30 m.min-1
.
The plasma treatments of PET films were carried out using two atmospheric plasma
generators: i) a commercially available MyPL plasma (AP Plasma, Korea [24]) which was preinstalled
within the roll-to-roll line, and ii) an experimental DCSBD system provided by
Comenius University, Slovakia. MyPL produces diffuse plasma in argon (6000 sccm) together
with small amounts of oxygen (30 sccm) that facilitates its reactivity for surface treatments.
The DCSBD plasma is described in Section 1. The basic attributes of MyPL and DCSBD plasmas
are summarized in Table 1.
Attribute MyPL DCSBD
Atmospheric pressure  
Homogeneity  
Ambient air  
Argon  
Oxygen  
Hydrogen  
Relative humidity Up to 90% Also pure H2O vapour
Effective area 10 x 1 cm2
20 x 8 cm2
Thickness of plasma 3 mm 0.3 mm
Power 75–125 W 350–600 W
Gas temperature ~ 50 °C ~ 70 °C
Table 1. Attributes of, and differences between MyPL plasma and DCSBD, taken from Ref. 3.
12
This study involved three different PET flexible foils, designated as PET 1 (Melinex S,
100 μm), PET 2 (Melinex ST506, 125 μm) and PET 3 (HK31, 125 μm). The PET films were
obtained in rolls 500-m and 1000 long, as shown in Figure 7. The interaction of MyPL and
DCSBD plasma resulted in a significant decrease of water contact angle and an increase in
wettability. The decrease of water contact angle accorded with the observations reported in
previous papers Refs. 1 – 2.
Figure 7. PET 1, PET2 and PET 3 rolls in the Singapore Institute of Manufacturing Technology,
A*STAR integrated clean-room laboratory: (complementary material for Ref. 3).
Figure 8. Water contact angles of PET surfaces as a function of MyPL plasma composition
(constant argon flow 6 l.min-1
+ various oxygen amounts in sccm) (complementary material for
Ref. 3).
Untreated PET 1 foil provided a water contact angle of 80.9°. Pure argon MyPL plasma
treatment for 1 m.min-1
led to a considerable decrease, to 43.2° ± 8.4° (Figure 8). A small
quantity (10 sccm) of oxygen admixed with the argon carrier gas had a considerable effect on
water contact angle, which then decreased to 26.4° ± 2.9°. It may be proposed that the
13
difference in plasma efficiency derived from the higher number of oxygen polar groups
induced by oxygen in the carrier gas, or a higher rate of etching of surface contaminants arising
out of electronegative oxygen. Whatever the underlying mechanism, a small concentration of
oxygen was beneficial and decreased both water contact angle and its standard error.
Interestingly, a further increase of oxygen had no significant effect on the decrease in water
contact angle.
Plasma treatment of transparent, thin polymeric materials also leads to changes in
optical properties. Figure 9 shows transmittance in the UV-Vis-NIR region for PET 1, PET 2 and
PET 3 before and after MyPL plasma treatment for 1 m.min-1
. The plasma treatment clearly led
to a slight decrease in transmittance for PET 1 and PET 2. In contrast, the transmittance
measured on PET 3 increased. The difference in behaviour and optical parameters among PET
foils was explained by their different original surface chemistry: i.e. the PET 3 was chemically
pre-treated with silica primer for better wettability, which was also confirmed by XPS.
After MyPL plasma treatment at a speed of 1 m.min-1
, the PET surface was coated in
transparent conductive PEDOT:PSS polymeric film with a thickness of 350 nm and a sheet
resistance of 100 Ω/square. Untreated, the PET flexible foil coated with PEDOS:PSS films was
visually non-homogeneous and showed various defects – lines and bubbles, as shown in Figure
10. The origin of these defects lies in the low surface energy of the PET combined with surface
contaminations that led to inferior adhesion between the PET and PEDOT:PSS.
Figure 9. UV-Vis-NIR transmittance spectra for untreated and MyPL plasma-treated PET 1, PET
2 and PET 3: (complementary material for Ref. 3).
14
Figure 10. Typical defects, such as bubbles and flow-lines, in PEDOT:PSS coated on untreated
PET surface: recorded immediately after coating process, taken from Ref. 3.
The concentration of defects in the PEDOT:PSS coating deposited on untreated PET
and MyPL-plasma treated PET was qualitatively and quantitatively evaluated by the “Dr.
Schenk web inspection system”. The data presented in Figure 11 show the number of defects
of severities ranging from minor (1) to major (5). It is evident that plasma pre-treatment of PET
prior to slot die coating of PEDOT:PSS led to a significant decrease in the total number of
defects in PEDOT:PSS, from approx. 500 to 100.
Figure 11. Number of defects in PEDOT:PSS coated on untreated and plasma-treated PET
flexible foil, taken from Ref. 3.
15
[4]
T. Homola, J. Matoušek, M. Kormunda, L.Y.L. Wu, M. Černák, Plasma treatment of glass
surfaces using diffuse coplanar surface barrier discharge in ambient air, Plasma Chem. Plasma
Process. 33 (2013) 881–894. doi:10.1007/s11090-013-9467-3.
Number of times cited: 15 Journal IF: 2.4
Since it is capable of withstanding higher temperatures than PET and PEN, glass is also
a suitable material for flexible electronics. However, the cost of thin, flexible glass is
significantly higher (AF 32 eco flexible glass from Schott AG costs 250 euro per 10 samples of 5
x 5 cm) and it is therefore largely confined to high-end applications. Glass shows excellent
transmittance and no haze, making it especially suitable for picture visualization devices such
as flexible or rollable displays. In similar fashion to PET and PEN, the surface of glass adsorbs
contaminants from the ambient environment and it must be cleaned prior to application of
functional films and coatings.
Ref. 4 pertains to a study of the DCSBD plasma treatment of soda-lime glass and the
effects of glass on the behaviour of plasma. Since the transparency of light through glass is
higher than through PET and PEN, it allows the capture of photographs of the plasma during
treatment. DCSBD plasma consists of micro-discharges that create a diffuse and homogeneous
layer of surface plasma at full power: 400 W, corresponding to 2.5 W/cm-2
. A single microdischarge
consists of a diffuse and filamentary plasma, which varies mainly in the mechanisms
of its generation. Whereas filamentary plasma generation is arises out of a streamer
mechanism, diffuse plasma is relies upon the Townsend mechanism. This contribution
reported that glass placed in close vicinity to plasma leads to suppression of streamer plasma
and therefore more diffuse plasma was observed. This was explained by the significant lack of
space, insufficient for the electron avalanche to generate the quantity of electrons required to
initiate a streamer. Furthermore, the results presented in the paper showed that diffuse and
filamentary plasma have different effects on glass surfaces (the visualization was enhanced by
hydrophobic amorphous SiO2 deposited from sol-gel). It was confirmed that diffuse plasma is
more beneficial in terms of the development of wettability on a glass surface and this
conclusion may also be transferred to other non-conductive surfaces. It should be noted that
the lower efficiency of filamentary plasma may also be related to the lower “thickness” of its
streamers because the filaments are farther from the treated surfaces. Nevertheless, this is
also beneficial for the plasma treatment of any thermally-sensitive material, because filaments
16
generally contain plasma of higher temperatures, unfavourable to the treatment of such
extremely thermally sensitive materials as ultra-thin (<100 μm) polymeric foils. The paper also
shows that the effective distance of the treated surfaces from the DCSBD ceramic is approx.
0.1–0.3 mm, which is under, or equal to, that of the thickness of the plasma.
Short plasma treatment of soda-lime glass surfaces resulted in a swift decrease of
carbon from 15 at. % to approx. 4 at. % Prolonging the plasma treatment time had no
additional effect on the concentration of carbon, probably due to ultra-fast adsorption of
carbon from ambient air during transportation of samples between plasma treater and XPS
load-lock. Plasma treatment units isolated from ambient atmosphere within dry nitrogen glove
boxes are currently being developed by our team at Masaryk University, Brno. These will
permit the team to study plasma-treated surfaces and their interaction in a controlled
atmosphere, i.e. plasma treatment in dry air or hydrogen followed by immediate storage in
inert gas.
In similar fashion to that described in the previous studies of plasma treatment of PET
and PEN (Refs. 1–2), surface modification of soda-lime glass developed hydrophilicity related
to the incorporation of oxygen polar groups combined with cleaning of the surface. In 2014,
the author presented a contribution to the Nanocon conference in Brno [25] which
demonstrated improved TiO2 coating adhesion on plasma pre-treated soda-lime glass.
[5]
T. Homola, J. Matoušek, V. Medvecká, A. Zahoranová, M. Kormunda, D. Kováčik, et al.,
Atmospheric pressure diffuse plasma in ambient air for ITO surface cleaning, Appl. Surf. Sci.
258 (2012) 7135–7139. Doi:10.1016/j.apsusc.2012.03.188.
Number of times cited: 41 Journal IF: 3.4
Indium-tin oxide (ITO) is an important semiconductor; it embodies a unique
combination properties, among them excellent light transmission in the visible spectrum, good
conductivity, and relatively high work function. In flexible electronics, ITO is widely used as a
transparent electrode (usually as an anode) in energy harvesting and light-emitting systems. Its
high work function enables its widespread use as a hole injection layer and/or electron
blocking layer in both energy-harvesting and light-emitting systems. The electrical and optical
properties of devices utilizing ITO are sensitive to the surface conditions of ITO. The work
function of ITO which, for instance, determines carrier injection efficiency into organic layers,
17
is extremely sensitive to contaminants. Even a low concentration of adsorbed carbon on an
ITO surface may lead to higher turn-on voltages and/or lowered durability and luminous
efficiency.
In Ref. 5 the effects of ambient air DCSBD plasma on ITO (ITO-coated glass) surfaces as
well as the effect of ITO on the behaviour of plasma were investigated. As is evident from the
previous paper (Ref. 4), a glass substrate placed in close vicinity to DCSBD plasma leads to
suppression of filamentary plasma and expansion of diffuse plasma. The interaction of DCSBD
plasma and ITO glass leads to the total extinction of filamentary plasma, as shown in Figure 12.
When a conductive ITO surface is exposed to an electrical field, it acts as an electrode with
floating potential. The observed extinction of the filamentary plasma in gaps smaller than 0.75
mm bears a certain resemblance to the behaviour of standard-volume DBD burning between
two dielectric-coated electrodes in dry atmospheric-pressure air at approximately the same
values of inter-electrode gap widths [26]. The method presented in this paper shows a simple
method of avoiding filamentary streamer plasma in coplanar dielectric barrier discharges.
The effect of the plasma treatment on ITO accorded with the previously presented
results in Refs. 1–4. A very short treatment time, of a few seconds, led to a rapid decrease of
carbon from 51 at.% to 11 at.%, also associated with a considerable decrease in water contact
angle from 84° to <5°. The decrease of water contact angle was explained by cleansing of
carbon and oxygen contaminants and incorporation of functional polar groups. Damage to the
surface after plasma treatment was negligible. The importance of ITO surface conditions prior
to the application of functional coating is discussed in Section 3.
Figure 12. Interaction between ITO and DCSBD plasma followed by total extinction of
filamentary plasma in a gap smaller than 0.75 mm, taken from Ref. 5.
In addition to the study in Ref. 5, our team recently published a paper entitled
Enhancement of electrical properties of flexible ITO/PET by atmospheric pressure roll-to-roll
plasma [27]. This work focuses on the plasma treatment of ITO deposited on flexible PET and
18
the effects of plasma on the electrical properties of ITO. As reported in Ref. 5, it was also
confirmed that plasma treatment led to a swift decrease of carbon followed by a decrease in
water contact angle. We examined the efficiency of plasma generated in various gas
compositions from pure nitrogen to pure oxygen, as well as various ratios between nitrogen
and oxygen and including synthetic air. The effect of plasma on ITO work function was also
investigated. It is interesting that samples stored in the controlled environment of a drynitrogen
glove-box exhibited a slower ageing effect, which confirms our hypothesis that
moisture and contaminants present in ambient atmosphere contribute to the degradation of
surfaces treated by plasma. It was also confirmed that plasma treatment decreased the sheet
resistance of the ITO surface. Most of the laboratory work presented in reference [27] was
carried out by my postgraduate student Mr. Masoud Shekargoftar, who joined our team in
2016.
19
3. Plasma post-treatment of coatings for flexible and printed electronics
3.1 State of the art
Flexible and printed photovoltaics can contribute greatly to increasing global access to
cheap energy. Dye-sensitized solar cells (DSSC) [28–31] and perovskite solar cells [32–35], in
which the photo-electrochemical system relies upon a mesoporous TiO2 layer, have emerged
as a promising low-cost photovoltaic technology and constitute a notable application field for
semiconducting photoanodes. Other suitable applications for photoanodes involve, for
example, photocatalytic treatment of water [36,37], hydrogen production [38,39], energy
storage [40] and various sensing systems.
Titanium dioxide (TiO2) photo-electroactive layers have already been deposited by a
range of techniques in both gas and liquid phases. Deposition from liquid is popular, since the
manufacturing equipment required is relatively simple and often compatible with ambient
conditions. An example may be found in TiO2 sol-gel (titanium[IV] isopropoxide, water,
hydrochloride acid and ethanol) deposition by spin-coating, dip-coating or spraying. After
deposition, the coating must be cured to facilitate solvent evaporation. This is usually
performed in an oven at 60 °C for 20 min. The coating is then visually homogeneous,
containing no defects, and consists of pure amorphous TiO2. However, amorphous TiO2 is
electrochemically inactive and the coating has to be further sintered towards crystalline
TiO2,which has photocatalytic attributes. All crystalline forms of TiO2 (anatase, rutile, brookite)
are photocatalytically active, although explanations for the variations in photocatalytical
efficiency remain largely lacking [41–43]. The temperature that is required to reach crystalline
structure in amorphous TiO2 is higher than 400 °C [44]; this is completely incompatible with all
plastic low-cost flexible substrates. Such incompatibility also applies in other deposition
techniques, such as chemical vapour deposition (CVD), atomic layer deposition (ALD) and
plasma-enhanced CVD and ALD.
This „temperature” problem may be overcome by utilizing pre-fabricated anatase
nanoparticles deposited on a surface, together with a binder from the liquid phase. However,
the viscosity and density of the binder must meet the requirements of common deposition
methods such as spraying [45], screen-printing [46], gravure printing [47], blade coating [48],
slot die coating and inkjet printing [30,49,50].
Numerous works in which TiO2 mesoporous layers have been employed in
photovoltaics have reported the use of coating formulations that need to be either i) thermally
20
sintered at temperatures higher than 150 °C [51] or ii) low-temperature sintered (UV and/or
plasma) for quite some time (more than 30 min) [52,53]. These high temperatures preclude
the use of such fabrication procedures for thermally sensitive substrates [54], such as flexible
plastics, while slow, low-temperature curing, employing UV or remote plasmas, involves
problems when fast/low-cost roll-to-roll manufacturing is envisaged. In particular, the
deposition of TiO2 photoanodes on PET/PEN foils, with all its significant potential for
applications in printed electronics, has emerged as a considerable challenge.
It is generally accepted that plasma treatment of surfaces leads to modification of
several nanometres of material (specifically, 20 nm for polymers). Since functional coatings are
usually thicker (100 nm and more), traditional plasma treatments affect only the top of the
surface whereas the remainder of the bulk remains in its original state. Such treatments would
effect only minor or negligible changes and such a procedure also fails to induce any
crystallization. These statements are based on my own negative results (unpublished) obtained
when attempting high-power-density low-temperature plasma curing of sol-gel TiO2 coatings
during my placement at the Singapore Institute of Manufacturing Technology. I have tried
various procedures (different chemical synthesis, ultra-thin multi-coatings, and extremely long
treatment times) and all of them have proven inapplicable to flexible electronics. I then began
to work on non-compact, porous and mesoporous coatings of the very high-specific surfaces
currently used in photovoltaics: mesoporous TiO2 layers are commonly used in DSSC, as well as
perovskites. A hypothesis that DCSBD surface plasma might be generated within the coating
has been confirmed by pilot results, arising out of research performed in collaboration with Dr.
Petr Dzik from the Faculty of Chemistry, Brno Technological University, Czech Republic.
Refs. 6 and Ref. 7 show the results of this collaborative work on low-temperature
plasma processing of functional mesoporous TiO2 photoanodes deposited on glass sheets and
flexible PET foils.
3.2 Comments on papers
[6]
T. Homola, P. Dzik, M. Veselý, J. Kelar, M. Černák, M. Weiter, Fast and low-temperature (70 °C)
mineralization of inkjet printed mesoporous TiO2 photoanodes using ambient air plasma, ACS
Appl. Mater. Interfaces. 8 (2016) 33562–33571. doi:10.1021/acsami.6b09556.
Number of times cited: 1 Journal IF: 7.5
21
This work reports on the plasma processing of a TiO2 mesoporous coating consisting of
pre-fabricated TiO2 anatase nanoparticles (25 nm) embedded in a methyl-silica binder. The
binder was chosen in the light of previously reported work [55] because it enables excellent
printability on a number of surfaces while preserving a mesoporous structure. Although
methyl-silica emerged as a good binding material, the methyl groups on its surface do not
support electron transport and therefore the entire coating is of low electrochemical activity.
Typically, the coating can be sintered in an oven at 400 °C or cured by UV for several hours to
allow removal of organic methyl groups, or treated in high-power-density air plasma. This
process is generally known as mineralization and an example of mineralization via air plasma is
visualized in Figure 13.
Figure 13. Visualization of plasma mineralization process in ambient air, taken from Ref. 6.
The results presented in Ref 6. show that rapid DCSBD plasma treatment leads to
mineralization of a binder of superior performance to that of binders traditionally sintered or
cured by thermal or UV processes. Since mineralization is associated with swift removal of
carbon, it may be determined by using XPS to measure the carbon. Figure 14 shows a
comparison of the concentration of elements in a coating deposited on glass after traditional
thermal sintering in an oven at 400 °C for 64 min, UV curing for 64 min, and low-temperature
plasma treatment for 64 s. All three procedures led to removal of carbon, i.e. mineralization
through oxidation of methyl groups. Thermal sintering decreased carbon from 21.6% to 8.9%
after approx. 1 hour. An hour of low-temperature UV curing was less efficient; it decreased the
atomic concentration of carbon to only 15.4%. In sharp contrast, low-temperature
mineralization by plasma treatment decreased the atomic concentration of carbon to 5.7% in
approx. 1 min. The maximum decrease, ceasing at approx. 5%, may also be explained by the
22
reasons given in Ref. 4 – transportation of the sample in ambient atmosphere may have led to
adsorption of carbon contaminants from the air.
Furthermore, thermal and UV treatments lead to an increased concentration of
sodium. This is clearly because Na+
ions migrate from the bulk of the glass onto the glass
surface and/or farther into the TiO2 coating. Sodium decreases the photocatalytic activity of
TiO2 and the migration of sodium ions is often addressed by adding a 100-nm dense SiO2
crystalline interlayer deposit between substrate and TiO2 coating [56]. Crystallization of dense
SiO2 is associated with the same temperature issues as crystalline TiO2 and is scarcely
applicable to thermally sensitive, flexible coatings. The coating treated with DCSBD plasma
showed no sodium.
Figure 14. Atomic concentration of elements in a TiO2/SiO2 mesoporous coating fabricated
using thermal sintering, UV curing and plasma treatment (complementary material to Ref. 6).
Ref. 6 shows results relating to the plasma mineralization of TiO2/SiO2 coatings as
characterized by a range of experimental techniques, including profilometry, SEM, FTIR, BET,
XRD, XPS (including C1s, Ti2p peak analysis), linear sweep voltammetry, chronoamperometric
measurements and photocatalytic tests. The results may be summarized as follows:
23
 Plasma mineralization is faster than traditional thermal and UV procedures and
because of the low temperature of plasma, only 70°C, it may be applied in the
manufacture of flexible electronics (FTIR, XPS).
 Plasma-mineralized mesoporous coatings showed increases in generated
photocurrents with respect to plasma treatment time, as shown in Figure 15.
 Plasma mineralization has no significant effect on the bulk structure of mesoporous
coating (SEM).
 Plasma mineralization has no significant effect on TiO2 nanoparticles (XRD).
Figure 15. Evolution of total current measured on TiO2/SiO2 photoanodes deposited on FTO
glass together with total carbon concentration in the TiO2/SiO2 coating, taken from Ref. 6.
[7]
T. Homola, M. Shekargoftar, P. Dzik, R. Krumpolec, Z. Ďurašová, M. Veselý, et al., LowTemperature
(70 °C) Ambient Air Plasma-Fabrication of Inkjet-Printed Mesoporous TiO2
Flexible Photoanodes, Flex. Print. Electron. 2 (2017) 035010. Doi: 10.1088/2058-8585/aa88e6.
Number of times cited: 0 Journal IF: N/A – new journal
Ref. 7 addresses a combination of plasma pre-treatment of ITO film on PET foil
(ITO/PET) and plasma post-treatment of TiO2/SiO2 mesoporous coatings deposited on ITO/PET.
While working with DCSBD plasma in recent years, my colleagues and I noticed that treatment
24
of lightweight materials, such as polymeric foils, was rendered problematic by difficulties
associated with electrostatic charges generated on the material surface. These led to
attraction of the treated material towards the plasma and consequently to contact between
the treated material and the DCSBD ceramic. Thus the ambient-air gap between the treated
material and the DCSBD ceramic has a tendency to shrink to zero, leading to plasma extinction.
As a solution to this problem, CEPLANT has developed, thanks to the contributions of Prof.
Mirko Černák, Dr. Jozef Ráheľ and Dr. Dušan Kováčík, a pilot roll-to-roll plasma treatment
apparatus with concave-curved ceramic DCSBD electrodes. Since the plastic foil can be
stretched over the roller, the distance between foil and DCSBD ceramic remains constant and
stretching forces compensate for the force from the electrostatic charges.
The team found that short plasma treatment of ITO is important prior to application of
a TiO2/SiO2 mesoporous coating. Untreated ITO exhibited poor TiO2/SiO2 wetting, resulting in
an “orange-peel texture”, an undesirable phenomenon. Plasma pre-treatment of ITO/PET for 2
s removed most of the carbon contaminants and generated electronegativity, also important
for keeping the work function as high as possible. Interestingly, XPS analysis confirmed that
oxygen contaminants were present on the ITO surface, which were also removed from its
surface by plasma. After the plasma treatment, most of the oxygen present on the surface was
bound to tin and indium in amorphous and lattice structures.
Figure 16 visualizes roll-to-roll plasma processing of a mesoporous coating on flexible
ITO/PET foil. The foil is treated by curved DCSBD plasma mounted in close vicinity to the roll
with PET foil. This set-up can be further upscaled, e.g. the speed of the line may be increased
by adding another plasma unit, and so onwards.
25
Figure 16. Visualization of plasma fabrication of large-area mesoporous TiO2 photoanodes by
roll-to-roll processing.
The apparatus for roll-to-roll treatment of flexible materials developed by CEPLANT
appears in Figure 17. The large-scale photoanodes were pre-printed at VUT (Brno University of
Technology), on a commercial Coatema printing line and afterwards the roll with the
photoanodes was treated on a roll-to-roll plasma line at Masaryk University. The plasma line
enables treatment of both sides at speeds ranging from 1 to 30 m.min-1
, fully comparable with
standard industrial processes.
Figure 17. a) Roll-to-roll system at Masaryk University developed by CEPLANT, b) detailed view
of TiO2 mesoporous film on PET roll treated by the roll-to-roll system.
26
Figure 18 shows fully-mineralized 1 cm2
flexible photoanodes on ITO/PET foil. The coating
maintains good mechanical stability on the flexible substrates. We are currently designing a
system for studying coating parameters under various bending conditions.
Figure 18. TiO2/SiO2 hybrid nanocomposite flexible photoanodes inkjet-printed onto ITO/PET
foil, taken from Ref. 7.
Figure 19 shows the effect of plasma mineralization on total atomic concentration (at.
%) in TiO2/SiO2 coating deposited on flexible ITO/PET. Figure 19 also compares total current
measured on TiO2/SiO2 deposited on FTO glass [40] on flexible ITO/PET. The lower currents
measured in flexible photoanodes were explained by the higher sheet resistance of ITO.
27
Figure 19. Evolution of total current measured on TiO2/SiO2 photoanodes deposited on FTO
glass and ITO together with total carbon concentration in the TiO2/SiO2 coating, taken and
merged from Ref. 6 and Ref. 7.
28
4. Conclusions
This habilitation thesis presents an extended summary of plasma treatment methods
for the fabrication and manufacture of low-cost flexible and printed electronics. I concentrated
my attention largely upon:
 plasma pre-treatment of flexible polymeric substrates,
 plasma processing of mesoporous TiO2 photoanodes created on flexible
ITO/PET foils.
It is clear that low-temperature diffuse coplanar surface barrier discharge (DCSBD)
plasma in ambient air is capable of efficiently removing organic contaminants from flexible
surfaces and also of mineralizing mesoporous coatings by means of removing organic moieties
from a binder while preserving its mesoporous structure. This approach may be extended in
the future to cover other organic binders and a number of other nanoparticles, among them
ZnO and WO3.
Plasma processing provides production performance superior to those techniques
currently considered standard (thermal sintering and UV curing), taking only a fraction of the
time required for them at far lower temperatures and making it particularly suitable for
incorporation into roll-to-roll fabrication units. This method could constitute an important and
major step forward in the large-scale manufacture of photonic devices, among many other
applications.
29
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Appendix: Copies of papers commented upon