Paper substrate for printed functionality

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15th Fundamental Research Symposium, Cambridge, September 2013 945
PAPER SUBSTRATE FOR
PRINTED FUNCTIONALITY
Roger Bollström and Martti Toivakka
Laboratory of Paper Coating and Converting, Centre for Functional Materials
Åbo Akademi University
ABSTRACT
Requirements for paper to be used as substrate for printed function-
ality were investigated. A recyclable, multilayer- coated paper
substrate that combines adequate barrier and printability properties
for printed electronics and sensor applications was developed. In this
multilayer structure, a thin top- coating consisting of mineral pigments
is coated on top of a dispersion- coated barrier layer. The top- coating
provides well- controlled sorption properties through controlled
thickness and porosity, thus enabling optimizing the printability of
functional materials. The optimum barrier layer structure was investi-
gated by studying the infl uence of latex type and amount in blends
with different size and shape factor kaolin pigments. Highly aligned
high shape factor kaolin improved barrier properties in general, but
was found especially useful against organic solvents, which may
degrade the latex. Dimensional stability and its infl uence on substrate
surface properties as well as on functionality of conductive tracks
were studied by exposure to high/low humidity cycles. The barrier
layer of the multilayer coated paper reduced the dimensional changes
and surface roughness increase caused by humidity and helped main-
tain the conductivity of printed tracks. As proof of concept functional
devices, hygroscopic insulator fi eld effect transistors were printed on
the multi- layer curtain coated paper using a custom- built roll- to- roll
hybrid printer.
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1 INTRODUCTION
Mass- produced paper electronics (large area organic printed electronics on paper-
based substrates, “throw- away electronics”) has the potential to introduce the use
of fl exible electronic applications in everyday life. While paper manufacturing
and printing have a long history, they were not developed with electronic applica-
tions in mind. Low- cost and recyclable paper substrates are being considered
for various novel, value- added printed applications outside the conventional
graphical arts industry [1, 2, 3, 4, 5, 6]. Electronic devices such as transistors,
capacitors, radio frequency identifi cation (RFID) antennas and batteries have
been fabricated on paper or paper- like substrates by using functional inks
containing e.g. conducting and semiconducting materials, such as silver, organic
polymers as well as carbon nanotubes [7, 8, 9, 10, 11, 12, 5]. Organic photodiodes
and photovoltaic cells, electronic paper displays, foldable thermochromic displays
and high- performance organic thin fi lm transistor arrays on paper have also been
demonstrated [13, 14, 15]. Recently also sensors for analysis of ionic concentra-
tion, analysis of modifi ed atmosphere conditions, as well as sensors for use in
diagnostics applications, have been manufactured by printing on paper [16, 17,
18, 19, 20, 21]. Suffi cient surface smoothness of substrate is necessary for many
printed electronics applications, especially for multilayer devices, where a single
peak may cause short circuits and render the device inoperable [22, 23, 24, 25].
By adjusting the substrate surface energy relative to the surface tension of the
ink, the spreading and adhesion of the functional materials can be controlled.
Penetration of ink components into the substrate can be eliminated by use of
barrier coating [26, 24, 27, 28, 29].
Poor dimensional stability, which can cause cracks and disconnects in printed
tracks, has been considered a challenge when using fi ber- based materials.
Humidity and temperature variations have a strong impact on the fi bers, the bind-
ings between them as well as the size and length of them [30, 31, 32]. Transferring
materials onto the substrate, by use of coating or printing methods, involves use
of solvents combined with harsh drying methods, all having a strong impact on
the dimensional stability of the fi ber network. From the end product point of
view, maintaining functionality in varying weather and temperature conditions is
important. One way to reduce the problems associated with the fi ber network
expansion is to apply a barrier layer on substrate surface to limit the solvent and
humidity penetration. Various types of dispersion polymers, commonly known as
latexes, either coated as pure or in combination with mineral pigments can be used
for improving barrier properties against humidity, grease or solvent penetration
[33, 34, 35].
This work focuses on understanding the requirements for paper when used
as substrate for printed functionality. This article compiles results from several
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separately published articles and summarizes important fi ndings. The interactions
between coated paper and setting of different functional inks are studied. Further-
more the requirements for a paper to withstand functional processing and storage
in harsh conditions are investigated. As proof of concept a roll- to- roll printed
transistor is demonstrated on the multilayer coated paper.
2 MATERIALS AND METHODS
The details of the multi- layer coated paper- based substrates that were used in the
current work are reported elsewhere [36, 37, 29]. The differences between the
substrates are mainly related to the thickness and the formulation of the top-
coating, which controls the printability of the functional inks. Regarding the
barrier layer formulation, the main differences are pigment volume concentration
(PVC), pigment aspect ratio, binder chemistry as well as layer thickness [38, 35].
For printing of silver electrodes, two commercial conductive inks were used;
Suntronic 5603 Ag nano particle inkjet silver ink, and Creative Materials 125–06
micron size particle fl exography silver ink. Regioregular poly(3- hexylthiophene)
(P3HT) was used as the semiconductor and poly(4- vinylphenol) (PVP) as insu-
lator. PEDOT:PSS (H.C. Starck) organic conductive polymer was used for the
gate electrode in the printed transistor. Printings were carried out either in batch
process (DMP- 2831, Dimatix- Fujifi lm Inc.) or with a custom- built roll- to- roll
hybrid printer. In roll- to- roll processing the printing speed was 10 m/min, the web
width 100 mm and eight 500 W infrared sintering units were mounted online. The
fl exographic silver printing (including source and drain electrodes for the tran-
sistor) were carried out using an ASAHI DSH® (Shore A 69º) photopolymer
printing plate with a ceramic anilox cylinder (Cheshire Engraving Cervices Ltd.,
cell angle 60º, line density 120 lines/cm, cell volume of 12 cm3/m2). The roll- to-
roll inkjet printhead fed by a custom- built ink- feed setup is a 128 nozzle and 80 pl
drop volume Xaar operated by Imaje 4400 controller and software.
3 RESULTS AND DISCUSSION
3.1 Paper structure
It is not possible to defi ne a single paper concept that could be considered a “paper
for printed electronics.” The suitability of the paper depends on the functional
materials deposited on it to fabricate the targeted device. However, there are some
general properties, which either are a prerequisite for functioning of a printed
device, or which improve the performance of it. These include surface smooth-
ness, barrier properties to maintain the functional materials on paper surface, and
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print defi nition. Considering the above, the authors have developed a multilayer-
coated, paper- based substrate concept that is suitable for printed electronics and
functionality [39, 36, 29]. In this multilayer structure, shown in Figure 1, a thin
top- coating consisting of mineral pigments is coated on top of a dispersion- coated
barrier layer. The top- coating provides well- controlled ink spreading and sorption
properties through controlled thickness and porosity, thus enabling optimizing the
printability of functional materials (section 3.3). The penetration of ink solvents
and functional materials stops at the barrier layer (section 3.2), which not only
improves the performance of the functional material but also eliminates potential
fi ber swelling and de- bonding that can occur if solvents are allowed to penetrate
into the base paper (section 3.4). Additionally, the mineral pigment coating
improves the heat stability of the paper enabling online infrared sintering with
relatively high temperatures [40]. The multi- layer coated paper under considera-
tion in the current work consists of a pre- coating and a smoothing layer under the
barrier layer. Coated fi ne paper may also be used directly as basepaper, as long a
smooth base for the barrier layer is ensured. The top coating layer is thin and
smooth (coat weight 0.5–10 g/m2, layer thickness 0.5–5 μm, root mean square
(RMS) surface roughness 55–75 nm) consisting of mineral pigments such as
kaolin, calcium carbonate, silica or blends of these. All the materials in the coating
structure are chosen in order to maintain the recyclability and sustainability of the
substrate. The substrate can be coated in steps, sequentially layer by layer, which
requires detailed understanding and tuning of the wetting properties and topo-
graphy of the barrier layer versus the surface tension of the top- coating [38]. An
alternative, cost competitive method for industrial scale production is the curtain
Figure 1. A: Cross- section scanning electron microscope (SEM) image showing the
layer structure of the paper substrate: top- coating, barrier layer, smoothing layer, pre-
coating and basepaper. Reproduced with permission. Copyright 2009, Elsevier [36].
B: Focused ion beam etched cross- section image. Reproduced with permission. Copyright
2012, Elsevier [29].
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coating, which enables simultaneous coating of all the layers in one pass [29, 37,
41, 42, 43].
3.2 Barrier properties
In functional printing and coating, conductive, semi- conductive and insulating
materials are usually dissolved in organic solvents such as dichlorobenzene or
toluene. Although these liquids are brought into a direct contact with the substrate,
the solvents evaporate quite rapidly, suggesting that short term barrier properties
might suffi ce. On the other hand, in throw- away sensor applications, for example
for medical use, acidic or basic analytes may be used and the sensoring process
might last for several minutes and long term barrier properties are needed [44, 45,
46, 47, 48, 49]. In the multilayer coating structure the barrier layer both controls
the absorption of the inks during device fabrication and ensures the end- use func-
tion. A simple example of a sorption test for a semiconductor ink, regioregular
poly(3- hexylthiophene) (P3HT) dissolved in ortho- dichlorobenzene (DCB), is
shown in Figure 2A. The amount of ink applied was the same for each sample
(5 μl), and the scanned areas were 25 × 25 mm2 except for the Mylar® A where an
Figure 2. A: A simple barrier test, application of semiconductor ink dissolved in DCB
onto different substrates and scanning of the spots from the front and backside of the
substrate. A visible spot on the backside indicates solvent and functional ink penetration.
Reproduced with permission. Copyright 2009, Elsevier [36]. B: Method for measuring
active barrier lifetime, i.e. penetration through the substrate as function of time, via change
in effective refractive index at paper- glass prism interface. Reproduced with permission.
IOP Publishing. Copyright 2012 [50].
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area of 35 × 35 mm2 was needed because of the excessive droplet spreading. To
be able to study in detail the effective barrier properties against solvents, novel
barrier measurement methods were developed. A practical method for obtaining
the active barrier lifetime is a prism method schematically presented in Figure 2B.
The penetration through the substrate is monitored as function of time as changes
in the effective refractive index at the prism- paper interface [50].
Dispersion coating has received attention as a production method for barrier
coatings, since it has been considered to be more environmentally friendly in
comparison to extrusion coating and lamination [51, 52, 53, 34, 33]. Traditional
paper- and board- based products, such as those used in packaging, require high
barrier properties against gases and liquids. If paper is to be used as a substrate in
functional applications, the required barrier properties against solvents and acids
need to be understood and developed. The current work aimed at understanding
how a dispersion coated barrier layer is optimally built up and how various
polymer dispersions and pigments function as a barrier against water vapor as
well as against an organic solvent (DCB) and an acid. High (100) (Platy kaolin)
and low (30) (Fine kaolin) shape factor pigments were blended with different
amounts of styrene- acrylate (SA), styrene- butadiene (SB) and ethylene- acrylate
(EA) latex as well as starch. For barrier dispersions, a specifi c ratio between the
pigment and the binder addition levels exists, acting as a threshold level, at which
barrier properties change signifi cantly, due to introduction of porosity into barrier
layer. For barrier coating, it is of utmost importance to have knowledge of this
critical pigment volume concentration (CPVC). The CPVC was determined by
measuring light scattering as function of drying time at different pigment addition
levels and found to be 55.7% for the platy kaolin and 62.8% for the fi ne kaolin
[53, 54, 55].
Figure 3 shows the infl uence of high (100) and low (30) shape factor kaolin
additions on barrier properties (water vapor transfer rate (WVTR), normalized to
15 μm). In the case of low shape factor kaolin, the improvement in barrier proper-
ties is only minimal compared to pure latex (SA). However, the addition of
pigments reduces the blocking problem, which occurred for the sticky surface of
the pure SA latex and caused defects in rewinding thereby also deteriorating the
barrier properties. Addition of high shape factor (100) kaolin however signifi -
cantly reduces the penetration at both PVC levels when combined with SA latex.
The standard deviation of the barrier properties measured from the coatings
containing low shape factor kaolin were signifi cantly larger compared to the prac-
tically negligible standard deviation of the barrier results obtained from the coat-
ings fi lled with high shape factor kaolin. This may be a result of nonhomogenous
or poor alignment of the particles, as shown in fi gure 4B.
Figure 5 shows the WVTR for three latexes with different chemistry, fi lled
with high shape factor (100) kaolin. As a reference material to the latexes, starch
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Figure 3. Normalized (15 μm thickness) WVTR at 23ºC and 85% relative humidity
(RH) for kaolin with a shape factor of 30 (Fine kaolin) and 100 (Platy kaolin) combined
with different amounts of SA and SB latex [35].
Figure 4. Focused ion beam images showing the alignment of the kaolin particle fi lled latex
barrier layer. A: Highly aligned high shape factor kaolin. B: Poorly aligned low shape factor
kaolin. The tortuosity of the structures is shown schematically in the inserted images [35].
was also tested as barrier polymer. The water soluble anionic starch however
dissolves resulting in very poor barrier properties against water vapor (note the
discontinuous Y- axis scale). Despite the dissolving of the starch, the thicker layer
(25 μm) clearly improves the barrier properties compared to the thinner (10 μm),
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952 Session 9: Novel Applications
which is explained by the longer pathway through the tortuous kaolin structure.
Regarding the latexes, the most obvious difference can be seen for the styrene
acrylate latex where addition of kaolin clearly improves the barrier properties at
the same layer thickness, whereas for the styrene butadiene latex the difference is
smaller. While the highest barrier properties could be obtained by using pure
ethylene acrylic latex, an addition of 44 vol- % kaolin did not signifi cantly weaken
the barrier properties against water vapor, which is an economic advantage in
commercial applications. The WVTR barrier properties for the basepaper
(including precoating and smoothing layer) were 795 g/m2/day.
In addition to barrier properties against water vapor, barrier properties against
liquids directly applied onto the surface of the substrate were measured. These
measurements were made in order to mimic a coating or printing operation, or an
analysis procedure in a printed functional application [50]. Organic solvents are
common as ink vehicles and acids are used as analytes in sensor applications, both
requiring barrier properties for varying times. Figure 6 plots the time it takes for
ortho- dichlorobenzene (DCB) to penetrate the substrates. As can be seen for all
the latexes, the addition of high shape factor kaolin clearly improves the barrier
properties. This can be related to the increased tortuosity through the particle
fi lled structure (Figure 4A). The organic solvent dissolves partially the latex but
Figure 5. Barrier properties against water vapor (WVTR) at 23ºC and 85% RH as a
function of barrier layer thickness. The values are average values of three parallel measure-
ments, with a negligible standard deviation. (B = Tendency for blocking, P = risk for
pinholes). Note the discontinuous Y- axis scale.
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the inert high shape factor mineral particles create a long pathway for the solvent
to migrate through. DCB, on the contrary to the water vapor, does not dissolve the
starch, which as a polar molecule performs well as a barrier against the nonpolar
DCB. Figure 6 only plots the short term barrier properties, but for the starch coat-
ings the barrier measurements were extended to three days by addition of DCB
to counteract the evaporation. No DCB penetrated the starch based barrier
coatings during the three days, indicating the starch is completely insoluble by
DCB. Differences in dissolving or degrading of the latexes can also be seen, the
styrene- butadiene and styrene- acrylic latexes dissolving most rapidly while
the ethylene- acrylic can withstand the organic solvent for longer.
In contrast to the barrier properties against DCB, the pure latexes and the layers
with low pigment volume concentration show the best barrier properties against
1M hydrochloric acid (Figure 7). This can be related to voids existing in the layers
fi lled to 57% by pigments, since the critical pigment volume concentration for the
platy kaolin was found to be 55.7%. The lower amount of organic material on
the surface of these layers can result in a more hydrophilic surface and more
complete wetting [35, 38]. In the case of the thin (5 μm) kaolin/ethylene acrylic
latex layer the penetration was caused by pinholes. The latexes are all inert against
the hydrochloric acid showing no degradation or dissolving tendency as was the
case for the organic solvent. The water soluble starch was dissolved immediately
Figure 6. Barrier properties against ortho- dichlorobenzene. The fi gure shows the time in
seconds for the liquid to penetrate the substrate. (B = Tendency for blocking, P = risk for
pinholes). Penetration time measured with prism method presented in Figure 2B [50].
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by the hydrochloric acid. Both the DCB and the hydrochloric acid penetrate the
basepaper (including precoating and smoothing layer) in less than 5 seconds.
The binder, whether it is latex or starch can be considered the most important
material for creating a sealed layer, but addition of mineral pigments can both
improve the barrier properties as well as ensure problem free runnability and
rewinding. Latexes, especially with low glass transition temperatures, tend to
cause blocking problems, i.e. undesired adhesion of coating layer to the back of
the adjacent paper in a roll, but addition of mineral pigment can signifi cantly
reduce the blocking problem.
3.3 Functional printability and surface properties
The ability to print narrow and well defi ned lines or structures is important,
especially when high resolution devices are produced. In addition to the high
requirements regarding line defi nition, functional printing also sets further
demands regarding the actual functionality of the printed material, which usually
is measured as electrical conductivity. Resistance is normally measured, which
can be converted to conductivity. Since exact thicknesses and thereby volume
resistivities are practically impossible to measure accurately on absorbing
Figure 7. Barrier properties against 1 M hydrochloric acid. The fi gure shows the time in
seconds for the liquid to penetrate the substrate. (B = Tendency for blocking, P = risk for
pinholes). Penetration time measured with prism method presented in Figure 2B [50].
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surfaces, surface resistivity (Ω/sq) was chosen as the main parameter for evalu-
ating conductivity. In the multilayer coating structure it is the top- coating that
determines the printability of the functional inks. Important coating layer proper-
ties affecting printability of functional inks are thickness, porosity, pore volume,
surface energy and roughness. These can be adjusted by the choice of pigment
shape, size and their distributions as well as by calendering [29, 37].
Conducting silver was printed both with fl exography (roll- to- roll) and inkjet
(batch). Two different silver inks were used, one consisting of fl aky micrometer
sized particles with a propylene glycol monomethyl ether acetate (PM acetate) as
solvent designed for fl exography and one containing nano sized particles with
ethylene glycol as solvent designed for inkjet. As can be seen in the SEM image
(Figure 8A), the nanoparticles of the silver ink penetrate into the pores of the
silica coating rendering it nonconductive, but stay on the surface of both the
kaolin and the kaolin/precipitated calcium carbonate (PCC) blend top- coatings.
The pore volume of the top- coating was measured by mercury porosimetry [56,
57, 58, 59]. Since it is not possible to measure reliably the porosity of only the
top- coating of a multilayer coated structure, pressure- fi ltrated tablets of the top-
coating formulations were measured instead. With the knowledge of the porosity
and the coat weight of the top- coating, the pore volume in the top- coating could
then be estimated. The penetration of ink particles correlates with the top- coating
dominant pore size, which for the kaolin and kaolin/PCC coatings was in the
range of 13 to 80 nm and ca 380 nm for the silica coatings. The large pore size
is a consequence of the relatively large particle size of the silica pigments used.
The fl exographic silver ink with micrometer sized fl aky particles remained on
the surface on all the top- coatings, and is thereby less sensitive to pore size
(Figure 8B). A higher porosity and total available pore volume in the top- coating
in fact allowed for faster ink vehicle uptake and thereby reduced the squeeze
Figure 8. SEM images of silver particle inks printed with inkjet (A) and fl exography
(B) (3 g/m2 top- coating). Reproduced with permission. Copyright 2012, Elsevier [29].
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(ink spreading under printing nip pressure), improving the fl exographic printa-
bility of micrometer sized particle ink [29].
Semiconductor, regioregular poly(3- hexylthiophene) (P3HT) dissolved in a
mixture (1:1:2) of xylene:chlorobenzene:ortho- dichlorobenzene, was printed with
inkjet at a solids content of 0.25 weight - %. The low solids content means a large
amount of solvent is applied which has to evaporate or absorb into the coating
structure. The solvent mixture vapor pressure was chosen to provide an evapora-
tion rate, which is slow enough to eliminate clogging of the printing nozzles but
fast enough to be printed in a roll- to- roll process. Too fast evaporation of the ink
solvent leads to viscosity increase and deposits in the inkjet nozzle, where as too
slow drying requires either slowing down of the roll- to- roll printing process or use
of additional driers. However, excessive drying can potentially render the printed
Figure 9. Focused ion beam cut and imaged cross- section of the coated paper substrate,
showing fl exography printed micrometer sized silver (LEFT) and inkjet printed nanopar-
ticle silver (RIGHT) on the multilayer coating structure. The sectioned layers in the multi-
layer structure are, from the top: top- coating (5 g/m2 fi ne kaolin), the barrier layer (10 g/m2
platy kaolin) and the basepaper coating. The platinum deposit is required for the milling
(ion beam cutting) through the coating structure.
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device inoperable, e.g. due to crack propagation or a too high temperature destroy
the semiconductive polymer. Semiconductor ink was printed at three different
amounts, giving equal theoretical uniform dry thicknesses of 20 nm, 40 nm and
80 nm. This corresponds to applied P3HT- volumes of 2, 4 and 8 nl/cm2 and to
total printed volumes of 800, 1600 and 3200 nl/cm2 respectively. These volumes
were compared to the pore volumes in the top- coatings, which were in the range
of 4 to 400 nl/cm2.
All printed amounts on all the surfaces gave a visibly purple color, with darker
color intensity for the higher amounts. Visually evaluated the most even fi lms
were achieved for the thinnest printed amounts (2 nl/cm2) whereas the larger
printed amounts (4 and 8 nl/cm2) resulted in slow and uneven drying. Especially
the substrates with low pore volume, and thereby limited absorptivity, exhibited
coffee stain effect. Surface resistivity was measured for all the printed amounts
and was correlated with the pore volume. As is shown in Figure 10, the relation-
ship between the surface resistivity and the pore volume is almost linear for
the small pore volumes (< 100 nl/cm2) and the small amount of semiconductor
(2 nl/cm2). The impact of pore volume decreases as higher amounts of ink is
applied. It is likely that for small applied ink volumes the semiconductor pene-
trates fully into the coating structure and surrounds the mineral particles, thereby
creating a connecting network. Once the pores are fi lled, with both semiconductor
and evaporating solvent, the rest of the applied amount will be on the surface and
continued evaporation leads to an irregular fi lm.
3.4 Dimensional stability
Dimensional stability was analysed by exposing substrates to humidity cycling.
As reference substrates to the multilayer curtain coated (MLCC) paper, commer-
cially available standard copy paper, double coated fi ne paper and a high quality
Figure 10. Surface resistivity as function of pore volume for 2 nl/cm2 (left), 4 nl/cm2
(middle) and 8 nl/cm2 (right) total printed (roll- to- roll) semiconductor amounts.
Reproduced with permission. Copyright 2012, Elsevier [29].
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photo paper were tested. Silver lines (20 mm by ~100 μm, non- sintered) were
inkjet printed onto the substrates at 23ºC and a relative humidity of 30%. The
substrates with the printed lines were then stored for 24 hours in 90% relative
humidity at 23ºC, then dried in a 100ºC oven for 30 minutes where after again
stored in 90% relative humidity at 23ºC for 24 hours. Finally the substrates
were stored in room conditions at 30% relative humidity at 23ºC for two days
(Figure 11). The printed lines were scanned after each stage with a high resolution
(6400 dpi) scanner and the exact line lengths were measured using image analysis
to determine the expansion/shrinkage in x/y- plane. The measurement accuracy is
± 1 pixel equaling ± 4μm or 0.02%. Exposing the substrates to the fi rst increased
humidity level led to 0.1–0.3% expansion of all the substrates except for the inkjet
paper which has a polymer fi lm coating. Subsequent drying in oven shrank all the
substrates by 0.35–0.6%, with smallest shrinkage observed for the MLCC paper
and highest for the double coated fi ne paper. The polymer coated inkjet paper did
not fully withstand the high temperature which resulted in permanent cracks in the
polymer fi lm. The second humidity level increase resulted in approximately the
same dimensional changes as the fi rst one, with the exception of the damaged
inkjet paper. Overall, the dimensional changes were largest for the fi ne paper and
copy paper. Compared to similar measurements of different nanocellulose based
sheets conducted by Torvinen et al. [22], the dimensional changes observed here
were in the same range. The multilayer coated substrate, including the strong
barrier layer both limiting penetration and strengthening the structure of the
Figure 11. Dimensional change as function of humidity and temperature cycling for the
different papers. Measurements are made after indicated time periods at each condition [37].
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substrate showed small dimensional changes. Since the MLCC paper had the
multilayer coating only on one side, this asymmetry led to curl of the paper. The
curl can potentially be reduced or eliminated by coating the backside of the paper
as well.
The changes in surface roughness of the double coated fi ne paper and MLCC
caused by exposure of paper to high humidity were measured both by atomic
force microscopy (AFM) and Parker Print- Surf (PPS) (1.0 MPa, soft backing).
PPS allowed for fast measuring as a function of “drying” (90% RH → 50%
RH @ 23ºC) after the samples had been brought into equilibrium at high
humidity. After the samples were removed from the high humidity conditions,
the PPS surface roughness increased slightly during the fi rst 15 minutes. For
MLCC the change was from 0.53 to 0.60 μm and for Fine paper from 0.87 to
1.05 μm (standard deviation ±0.03μm; values below 0.60 μm outside the ISO
standard for PPS). This is potentially due to contraction of the fi bers, which
causes shrinkage of the basepaper fi ber network. The initial surface roughness
increase was observed for both the fi ne paper and the MLCC, but after the
15 minutes no change in roughness was detected for either one. Changes in
surface roughness as a function of length scale were further studied by AFM
(Figure 12A). Since obtaining an AFM image takes approximately one hour, the
samples were stored for one hour in room conditions (25% RH and 23ºC) before
starting the measurement, in order to avoid possible roughness change during
the image acquisition. As can be seen in Figure 12A, the increase in roughness
Figure 12. A: Root mean square roughness (Sq) as a function of correlation length (Ta),
for MLCC and double coated fi ne paper before and after humidity treatment (24 h in 23ºC
and 90% RH) [37]. B: Surface resistivity for different width printed silver tracks before
and after humidity treatment.
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960 Session 9: Novel Applications
is obvious on every length scale for the fi ne paper whereas it is signifi cantly
smaller for the multilayer curtain coated paper. For the fi ne paper the changes
in surface roughness increase at longer length scales. This indicates the increase
is a result of fi ber swelling, since the roughness at longer length scales is
determined by the base paper fi bers [60, 61, 62]. Despite fi ber swelling, which
caused curl of the multilayer coated substrate, the mechanically strong barrier
layer ensured that only minimal topographical changes on the coated side
occurred.
The infl uence of humidity on conductivity was investigated by exposing printed
silver tracks, with different line widths (2–32 inkjet printed 10 pl drops, drop
spacing 20 μm), to humidity cycling. Surface resistivity was measured for the
lines as they were printed and sintered (25% relative humidity) and again after
24 hour exposure to 90% relative humidity. As shown in fi gure 12B, the con -
ductivity of the narrow printed tracks on the double coated fi ne paper, which
showed the largest dimensional and surface roughness changes when exposed
to high humidity, decreased considerably. The surface resistivity increased by
4 orders of magnitude for line widths printed with 4 and 8 drops. The impact on
conductivity of the wider lines, printed with 16 and 32 drops, was negligible.
On the multilayer coated paper, only a minimal impact on conductivity could
be observed after the humidity treatment on the thinner lines (2 and 4 drops).
No changes could be observed for the wider lines (8–32 drops). It is obvious that
poor dimensional stability and roughening caused by humidity changes in the
environment are detrimental to the functioning of narrow printed conductive
tracks. In addition to the roughness increase and expansion of the substrate, oxida-
tion of the silver particles might also play a role in slightly reducing the con -
ductivity. Similar reduction in conductivity as function of treatment in high
humidity conditions was also reported for printed tracks on different label papers
by Wood et al. [63].
3.5 Proof- of- concept device
Several electronic devices and sensors have been successfully manufactured on
the multilayer coated paper presented herein [16]. A hygroscopic insulator fi eld
effect transistor (HIFET) [64, 65] printed with a roll- to- roll hybrid printer serves
as an example here. Figure 13A shows a schematic image of the top gate bottom
contact fi eld effect transistor geometry and an optical top view image of the gate
electrode in blue, printed on top of a transparent insulator layer. Underneath is
the purple semiconductor printed on top of silver electrodes. The output and
transfer characteristics of the transistor are shown in Figure 13B and C, respec-
tively. Compared to transistors manufactured in a batch process, especially on
plastic substrates, the current throughput is rather low. Poor semiconductor
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Paper Substrate for Printed Functionality
15th Fundamental Research Symposium, Cambridge, September 2013 961
ordering and impurities of the paper surface might degrade the charge transport
[66, 67, 68].
4 CONCLUDING REMARKS
The understanding of the interactions between functional materials formulated
and applied on paper as inks, makes it possible to create a paper- based substrate
that can be used to manufacture printed electronics- based devices and sensors on
paper. The multitude of functional materials and their complex interactions make
it challenging to draw general conclusions in this topic area. The results become
partially specifi c to the device chosen and the materials needed in its manufac-
turing. Based on the results, it is clear that for inks based on dissolved or small size
functional materials, a barrier layer is essential and ensures the functionality of the
printed material in a device. The required active barrier life time depends on the
solvents or analytes used and their volatility. High aspect ratio mineral pigments,
which create tortuous pathways and physical barriers within the barrier layer limit
the penetration of solvents used in functional inks. The surface pore volume and
pore size can be optimized for a given printing process and ink through a choice
of pigment type and coating layer thickness. However, when manufacturing multi-
layer functional devices, such as transistors, which consist of several printed
layers, compromises have to be made. E.g., while a thick and porous top- coating
is preferable for printing of source and drain electrodes with a silver particle ink,
a thinner and less absorbing surface is required to form a functional semicon-
ducting layer. The possibility of printing transistors in a roll- to- roll process on
paper is demonstrated. For industrial production of the paper for printed elec-
tronics, curtain coating is a suitable coating technique allowing extremely thin
top- coatings to be applied simultaneously with a closed and sealed barrier layer.
Figure 13. A: Schematic and optical image of a roll- to- roll printed HIFET. Output
(B) and transfer (C) characteristics of the transistor on the multilayer curtain coated paper.
Reproduced with permission. Copyright 2012, Elsevier [29].
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962 Session 9: Novel Applications
5 ACKNOWLEDGEMENTS
This article is a compilation of several separately published articles. The authors
want to acknowledge Anni Määttänen, Daniel Tobjörk, Peter Dolietis, Roger
Nyqvist, Jarkko J Saarinen, Petri Ihalainen, Janet Preston, Pekka Salminen, Jouko
Peltonen and Ronald Österbacka for their contribution in development and
analyses of the multilayer coating structure. Imerys Minerals Ltd., Styron Europe
GmbH, Specialty Minerals Nordic Oy, Paramelt B.V., StoraEnso, and Basf are
acknowledged for providing materials. Academy of Finland (Grant 118650) and
Tekes (Grant 40092/09) are acknowledged for fi nancial support.
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