Conductivity of PEDOT:PSS on Spin-Coated and Drop Cast Nanofibrillar Cellulose Thin Films
Valtakari et al. Nanoscale Research Letters
Conductivity of PEDOT:PSS on Spin-Coated and Drop Cast Nanofibrillar Cellulose Thin Films
Dimitar Valtakari 0 2
Jun Liu 1
Vinay Kumar 0 2
Chunlin Xu 1
Martti Toivakka 0 2
Jarkko J. Saarinen 0 2
0 Laboratory of Paper Coating and Converting, Center for Functional Materials (FunMat), Abo Akademi University , Porthansgatan 3, 20500 Åbo/Turku , Finland
1 Laboratory of Wood and Paper Chemistry, Abo Akademi University , Porthansgatan 3, 20500 Åbo/Turku , Finland
2 Laboratory of Paper Coating and Converting, Center for Functional Materials (FunMat), Abo Akademi University , Porthansgatan 3, 20500 Åbo/Turku , Finland
Aqueous dispersion of conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was deposited on spin-coated and drop cast nanofibrillar cellulose (NFC)-glycerol (G) matrix on a glass substrate. A thin glycerol film was utilized on plasma-treated glass substrate to provide adequate adhesion for the NFC-glycerol (NFC-G) film. The effects of annealing temperature, the coating method of NFC-G, and the coating time intervals on the electrical performance of the PEDOT:PSS were characterized. PEDOT:PSS on drop cast NFC-G resulted in 3 orders of magnitude increase in the electrical conductivity compared to reference PEDOT:PSS film on a reference glass substrate, whereas the optical transmission was only slightly decreased. The results point out the importance of the interaction between the PEDOT:PSS and the NFC-G for the electrical and barrier properties for thin film electronics applications. PACS: 61.46.-w (structure of nanoscale materials); 68.37.-d (microscopy of surfaces, interfaces, and thin films); 81.05.Lg (polymers and plastics); 81.07.-b (nanoscale materials and structures: fabrication and characterization)
PEDOT; PSS; Nanofibrillar cellulose (NFC); Conductivity; Thin films
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Background
Thin, lightweight, and flexible conductive films are
useful for many applications from intelligent packaging,
solar panels, radio frequency identification (RFID) tags
to medical implants, wearable computers, and various
sensors [1]. Traditionally, such films have been produced
on plastics, glass, and other nonrenewable materials.
However, the recent environmental concerns have
resulted in the search for alternative and more sustainable
materials. These must be cost-effective, widely available,
and both ecologically and economically sustainable.
Cellulose is the most abundant biopolymer on the
Earth and thus a suitable candidate for replacing fossil
fuel-based solutions. Within the past decade,
nanostructured cellulose-based materials have raised large attention
due to their unique properties. Nanofibrillar cellulose
(NFC) has a high aspect ratio, large surface area, and high
strength [2]. NFC can be utilized in various end-use
products ranging from thin films, coatings, and composites to
aerogels and hydrogels. Recently, NFC-based conductive
films and composites for electronics applications have
been studied; see, e.g., reviews [3, 4]. NFC films have good
thermal [5–7] and chemical stability [8], tunable optical
properties [5, 6, 9], high toughness [10], and low surface
roughness [11]. Therefore, NFC films can be used as
a substrate for flexible and transparent electronic
devices for cost-effective manufacturing in a roll-to-roll
process flow.
Various strategies have been employed to induce
conductivity in NFC films. The NFC film can be coated [5,
8, 9, 12] or printed [9, 11, 13] with a conductive
material. Alternatively, the conductive material can be
intermixed with the NFC fibers that can be cast into a
conductive composite film [6, 10, 14–17]. The conductive
material can also be directly incorporated onto the NFC
fibers using, for example, in situ chemical polymerization
[18] or carbonization [19] that allows conductive films to
be produced from such conductive fibers [20]. Different
conductive materials such as silver nanowires [9, 12] or
nanoparticles [11, 13], tin-doped indium oxide [9],
carbon nanotubes (CNTs) [5, 8–10, 12, 14, 15, 17, 20,
21], graphene oxide [19], ZnSe quantum dots [6],
polypyrrole [18], polyaniline doped with
camphorsulphonic acid [16], poly(p-phenylene ethynylene) [16]
© 2015 Valtakari et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.
and
poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) [5, 9, 20, 21] have been used in
the literature. Nanostructured cellulose-based
electronics have been studied, e.g., in flexible
supercapacitors [22], flexible nanopaper transistors with high
transmission and low surface roughness [23],
photovoltaic cells based on cellulose nanocrystal substrates
[24], nonvolatile memory based on cellulose nanofiber
paper [25], subcomponents made from nanofibrillar
cellulose for “cut, stick and peel”-based flexible
electronics [26], and recently cellulose nanofibril
paperbased high-performance flexible electronics [27].
PEDOT:PSS is a solution-processable conductive
polymer that offers flexibility, high transparency, high
thermal stability, low production cost, and compatibility
with aqueous solution-based deposition techniques.
PEDOT:PSS has a lower electrical conductivity than the
other conductive polymers or metal oxides. However,
the conductivity can be improved by addition of
polyalcohols such as glycerol [28], by adding polyelectrolytes
[29], or by solvent [30] or an acid treatment [31, 32].
Highly conductive and transparent PEDOT:PSS films
have been studied, e.g., for replacing indium tin oxide
(ITO) in photovoltaics [33, 34] and touch screens [35].
Recently, a solar cell [9] and an OLED device [5] were
demonstrated using a spin-coated conductive
PEDOT:PSS on NFC substrate. Additionally, PEDOT:PSS has
been used with CNTs in the layer-by-layer coating of
wood microfibers for paper-based batteries [20] and
capacitors [21].
In this work, we report a simple fabrication method of
conductive PEDOT:PSS films on spin-coated or drop
cast NFC-glycerol (NFC-G) layer on top of an oxygen
plasma-activated glass substrate. We use glycerol as an
anchor layer between the NFC-G layer and glass
substrate since the NFC-G solution cannot be coated
directly onto a pure plasma-activated glass substrate. A
glycerol anchor layer provides good adhesion and
reproducible results that allows a systematic study of the
interaction between PEDOT:PSS and NFC-G coating.
The electrical properties are studied as a function of the
annealing temperature, the spin coating and drop casting
of NFC, and the coating time intervals during film
deposition. The effect of glycerol as a plasticizer on film
formation and the resulting conductivity is discussed as
well. PEDOT:PSS on NFC can be used to produce
humidity or amine sensors, which can find applications in
food packaging industry. These films can also be utilized
in solar cell applications due to their tunable optical
properties.
Methods
The NFC suspension was prepared from bleached birch
Kraft pulp using 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO)-mediated oxidation followed by mechanical
disintegration as reported by Liu et al. [36]. Ten grams of
the pulp fibers were dispersed in 600 mL deionized (DI)
water (Millipore 18.2 MΩ). The TEMPO (0.1 mmol/g
fiber) and NaBr (1.0 mmol/g fiber) were dissolved in
300 mL DI water and then mixed with the pulp slurry.
The concentration and the pH of the pulp were adjusted
to 0.1 % and 10.0, respectively. The oxidation was started
by adding the NaClO (10 mmol/g fiber) solution
dropwise. All NaClO solution was added for 8 h. During the
reaction, the pH of the reaction mixture was kept at 10.5
by adding 0.5 M NaOH. After 24.0-h reaction, the mixture
was precipitated in ethanol and purified by washing with
DI water and centrifugation at 3500 rpm for 10 min for
three times. The oxidized fibers were diluted to a
concentration of 0.5 % and mechanically fibrillated with a
domestic blender (OBH Nordica 6658, Denmark) for 5 min. The
carboxylate content (1.79 ± 0.11 mmol/g) of the resulting
NFC was determined by conductometric titration [36].
The chemicals were purchased from Sigma Aldrich and
used without further purification.
The NFC and glycerol (≥99.0 %, Sigma-Aldrich, St.
Louis, USA) solutions were prepared by weighing and
diluting to given strengths using DI water and small
1.5 mL microcentrifuge tubes. All NFC-G coatings were
well mixed and applied fresh on the surface immediately
after the mixture preparation.
All samples were prepared on washed, cleaned, and
dried clear microscope slide glasses cut into the size of
2.5 × 2.5 × 0.1 cm3. Furthermore, the glass slide surfaces
were oxygen plasma treated in the etch mode in a high
vacuum sample sputter coater (SCD 050, Bal-Tec AG,
Balzers, Liechtenstein; now Leica Microsystems GmbH.,
Wetzlar, Germany) for 60 s at 20 mA and 0.05 mbar
vacuum pressure for 3–15 min prior to spin coating.
The plasma treatment lowered the water contact angle
(KSV Cam 2000, Biolin Scientific Inc., Espoo, Finland)
on the glass surface from 14° to 0°.
Figure 1 shows the two different approaches for the
sample preparation with spin-coated and drop cast
NFCG mixture. The samples were spin coated (KW-4A,
Chemat Technology Inc., USA) at different speeds depending
on the material: 5.0 wt% glycerol (the anchor layer) at
1000 rpm for 60 s, NFC-G mixtures at 1000 rpm for 60 s
and homogenized standard PEDOT:PSS (Clevios PH 500,
Heraeus Holding GmbH, Germany) at 1500 rpm for 60 s.
The additional drop cast NFC-G coatings on precoated
NFC-G layer were prepared out of 200 μL NFC-G solution
of a given strength and mixture ratio covering the
substrate area completely from edge to edge and then left to
dry for 24 h at room temperature (RT) of 24 °C and
relative humidity (RH) of 50 %. All samples were left to dry
for 15 to 60 min in between the coating steps, and no
weton-wet coatings were applied at any stage of this work.
Fig. 1 The NFC-G sample preparation steps. The glass slide surface is treated by oxygen plasma (1) followed by spin coating of 5.0 wt% glycerol
anchor layer (2), NFC-G spin coating (3), and PEDOT:PSS spin coating (4). The drop cast NFC-G samples were prepared by an additional step before the
PEDOT:PSS spin coating
The spin-coated NFC-G layer serves two purposes as
shown in Fig. 1: first, the PEDOT:PSS can be directly
spin coated on top of it, and secondly, it serves as a rigid
underlayer in which the drop cast NFC-G coating
adheres preventing withdrawal from the edge during
drying. We wish to emphasize that NFC or NFC-G did
neither adhere to the plain glass slide surface nor did it
form a clean, uniform coating on the oxygen
plasmatreated glass surface. However, addition of a 5.0 wt%
glycerol coating as an anchor layer after the oxygen plasma
surface treatment step resulted in good-quality
spincoated NFC-G films. The adhesion of the glycerol
anchor layer was better on the oxygen plasma-coated
surfaces than on an untreated plain glass. The glycerol
coating has several advantages as it is colorless and
semitransparent making it suitable for transmission
measurements. It is also a plasticizer and a humectant
for the NFC films to avoid brittleness and a well-known
secondary dopant that can increase the overall electrical
conductivity of the PEDOT:PSS by 2 to 3 orders of
magnitude.
PEDOT:PSS samples were spin coated and annealed
for 20 min at a given temperature range from 60 to
130 °C (Model 200, Memmert GmbH, Schwabach,
Germany). Samples were kept aside prior to annealing
for 20 to 30 min. Samples were stored overnight in dark
and in humidity-controlled conditions at RH 50 % and
measured for electrical conductivity on the following
day. The sheet resistance was measured over two parallel
hand-painted conductive silver paint (Electrolube) stripes
with inner borders 10 mm apart. A Keithley 2100 Digital
Multimeter was used for sheet resistance measurements
at 24 °C and RH 50 %. The scanning electron microscope
(SEM, LEO 1530 VP Gemini, Carl Zeiss Microscopy
Gmbh, Oberkochen, DE) was used to image the thickness
and structure of the PEDOT:PSS-coated NFC-G samples
on glass. These were either freeze-fractured after dipping
in liquid N2 for a minimum of 5 s or alternatively
fractured under ambient conditions. The sample light
transmission was measured using a Perkin Elmer Lamba 900
spectrophotometer with an integrating sphere setup and
the UV Winlab software.
Results and Discussion
The Effect of Annealing Temperature and NFC-G Mixture
on the PEDOT:PSS Sheet Resistance
PEDOT:PSS aqueous dispersion coatings are typically
annealed to increase their conductivity. Figure 2 shows
the effect of annealing temperature on conductivity of
the spin-coated PEDOT:PSS deposited on oxygen
plasma-treated reference and glycerol-treated glass. The
used annealing temperatures vary from 60 to 130 °C that
are below the recommended drying temperature given
by the polymer manufacturer of 130 °C. The
PEDOT:PSS films become more stable above 80 °C, and the
scatter in the measurement data decreases with the
raising annealing temperature. On a pure glass substrate,
the PEDOT:PSS reaches a sheet resistance minimum at
the 80 °C annealing temperature.
Fig. 2 PEDOT:PSS sheet resistance on reference glass and glass with glycerol anchor layer
The spin-coated PEDOT:PSS on glycerol anchor layer
resulted in a significant drop in the sheet resistance
values at lower annealing temperatures from 60 to 100 °C
range. This is expected as glycerol is traditionally
considered as a secondary dopant for the PEDOT:PSS. It blends
with the PEDOT:PSS and improves the conductivity by
allowing the PEDOT and the PSS components to
restructure morphologically. The PEDOT:PSS conductivity can
also be improved by blending, e.g., some polyols into the
PEDOT:PSS that evaporate or pass through the
PEDOT:PSS layer without a trace. On the glass substrates, the
lowest sheet resistances are found in the middle and lower
end of the used annealing temperature range.
The PEDOT:PSS coatings on drop cast NFC-G with a
varying NFC concentration at 5.0 wt% glycerol in Fig. 3a
show a significant decrease in the sheet resistance by
3 orders of magnitude compared to the reference
PEDOT:PSS on glass without NFC. The drop cast NFC-G
coatings were produced with three different NFC
strengths, i.e., with three different viscosities while
keeping the glycerol plasticizer level constant (5.0 wt%) in
the NFC-G solution mixture. The 0.05 and 0.1 wt% NFC
concentrations gave almost identical results. The sheet
resistances dropped and leveled out with an increasing
annealing temperature. The gel-like blend with the
highest viscosity of 0.2 wt% NFC had the highest sheet
resistance. This can be caused by the reduced deformability of
NFC due to the high viscosity, which can result in
lowered absorption and blending at the NFC/PEDOT:PSS
interface. In the reference PEDOT:PSS, solvent
evaporation takes place from a thin layer whereas NFC and
glycerol absorb water from the aqueous PEDOT:PSS
solution resulting in a different drying mechanism that
Fig. 3 PEDOT:PSS sheet resistance on drop cast NFC-G samples with
a variable NFC concentration at a constant glycerol concentration of
5.0 wt% and b constant NFC concentration of 0.1 wt% with variable
glycerol concentration
is susceptible for cracking. Therefore, on higher NFC
concentrations, uneven shrinkage and mechanical stress
during the annealing can result in cracking and reduced
PEDOT:PSS film quality, and thus, a higher sheet
resistance as observed in Fig. 3a. However, the sheet
resistance of PEDOT:PSS on drop cast NFC-G is significantly
lower than the PEDOT:PSS reference.
Figure 3b shows the effect of glycerol concentration on
the sheet resistance of PEDOT:PSS at a constant NFC
concentration of 0.1 wt%. The NFC itself is sufficient to
reduce the reference PEDOT:PSS sheet resistance levels by a
factor of 30 from 550 kΩ/□ down to <18 kΩ/□. This may
be due to water still present in the NFC-G layer that can
allow vapor transmission through and rearrangement of
the PEDOT:PSS layer [37, 38]. The lowest sheet resistance
is observed with the 5.0 wt% glycerol concentration, which
is expected as glycerol is a secondary dopant for the
PEDOT:PSS. However, a further increase of glycerol content to
10.0 wt% does not improve conductivity as the mechanical
properties of the drop cast NFC-G layers start to fail and
disintegrate during the spin coating. The overall
PEDOT:PSS stability improves towards higher temperatures
without the sheet resistance increase as observed in Fig. 2.
We conclude that the pure drop cast NFC coating
improves the PEDOT:PSS conductivity by an order of
magnitude. However, a much larger improvement is achieved
through the combination of glycerol and water originating
from the NFC-G mixture that allows vapor transmission
and rearrangement of the PEDOT:PSS coating.
Sheet Resistance of PEDOT:PSS on Spin-Coated NFC-G
Drop casting is a slow process whereas spin coating
provides a fast track for prototyping. In our study, spin
coating of NFC-G films reduced the processing time
from 24 h down to a few minutes. In addition, the
sample to sample variation was reduced with spin coating.
This is especially important with multilayer structures as
every additional sample preparation step with successive
coating layers can be a source of error.
Figure 4 shows the PEDOT:PSS sheet resistances on a
spin-coated NFC-G. We observe that PEDOT:PSS on
0.1/5.0 wt% NFC-G behaves similarly to the reference
PEDOT:PSS spin-coated directly on a glass as shown in
Fig. 2. The large standard deviations in the 60 and 80 °C
points originate from the PEDOT:PSS instability that is
absent in the 100 and 130 °C. Similar results indicate
that the PEDOT:PSS and the NFC-G coatings act as
separate, isolate entities with no interaction between the
two layers during the spin coating or annealing.
A reduction in NFC concentration from 0.1 wt% down
to 0.08 wt% produces an NFC-G mixture with a lower
viscosity. Thus, at a constant spinning rate (1000 rpm), a
thinner film will be produced. The sheet resistance of
the PEDOT:PSS is reduced due to the PEDOT:PSS
interaction with the glycerol at 60 and 80 °C. Furthermore, a
slow initialization rate at 300 rpm will retain more liquid
on the substrate that results in a thicker film. At lower
temperatures, the sheet resistances remain at the
previous levels whereas at 100 and 130 °C, the performance is
stabilized approximately to 200 kΩ/□ that is significantly
lower compared to the PEDOT:PSS reference (550 kΩ/□).
We can conclude that the higher NFC concentration
of 0.1 wt% forms a tight and dense layer with sufficient
barrier properties to prevent the glycerol interaction
with the PEDOT:PSS. On the other hand, at a lower
NFC concentration of 0.08 wt%, the NFC-G layer is less
Fig. 4 PEDOT:PSS sheet resistance on spin coated NFC-G samples with variable NFC concentration and spin coating parameters at a constant
glycerol concentration of 5.0 wt%
dense allowing the glycerol to migrate into the
PEDOT:PSS layer during the spin coating and annealing. The
100 °C point suggests that the NFC component dries
quicker and becomes restrictive, blocking the glycerol
and water from the PEDOT:PSS. Hence, the spin-coated
NFC-G films can be customized to act as either a barrier
(0.1 wt% NFC) or as a regulator (0.08 wt% NFC) that
controls the release of glycerol and water into the
PEDOT:PSS layer.
Finally, the time dependence of the start of the
annealing from the PEDOT:PSS coating is shown in Fig. 5. It is
clearly seen that starting the annealing instantly 1 min
after the coating results in lower sheet resistance with
lower annealing temperatures than the 3-min interval.
The 1-min samples have higher moisture content, and
the samples annealed at higher temperatures may drive
the water and perhaps some glycerol rapidly out from
the PEDOT:PSS/NFC-G interface region, disallowing
PEDOT and PSS to rearrange. The sheet resistance levels
in Fig. 5 at 80 °C are approximately twice as high to
those in Fig. 4. A closer look reveals that the 3-min
samples have a similar instability issue at 60 °C as with the
PEDOT:PSS reference response shown in Fig. 2. The
overall performance improves and becomes more stable
converging to 300–400 kΩ/□ sheet resistance level
between annealing temperatures of 80 and 130 °C.
Coating Thickness, Wetting Characteristics, and Optical
Transmission
An objective with our NFC-G model system on glass
substrate was to retain the same semitransparent
character provided by unsupported, standalone NFC films. The
benefit of our model system is that the flat NFC-G films
supported on the glass are suitable for the PEDOT:PSS
spin coating. Our conductivity results in Figs. 2, 3, 4,
and 5 show that these samples are reproducible and thus
suitable both for electrical and optical characterization.
The sample thicknesses of the PEDOT:PSS-coated
NFC-G layers were determined using SEM. The samples
were either annealed at 130 °C or left as such with no
annealing. The annealing step is essential for the
conductivity, electrical stability, and smoothness of the
PEDOT:PSS layer. Annealing causes shrinkage of the
PEDOT:PSS liquid dispersion particle, vertical
segregation into a mainly PEDOT phase at the bottom, and a
PSS phase on top as well as degradation over a
prolonged exposure to heat [39].
Figure 6 shows cross sections of the PEDOT:PSS
samples with and without annealing together with the
spin-coated reference PEDOT:PSS on the oxygen
plasmaactivated glass on top (a). The annealed samples are
shown to the left in Fig. 6 with spin coating (b1) and drop
casting (c1) and the corresponding samples without
annealing to the right, spin coated (b2), and drop cast (c2).
The spin coating causes film shrinkage due to
compression and fast evaporation of water from the thin films.
Furthermore, annealing causes shrinkage due to drying.
The cross section samples for the SEM imaging are
commonly freeze fractured in liquid nitrogen. Unfortunately,
moisture absorbed from the surrounding air can cause
sample swelling with hygroscopic materials. In Fig. 6, on
the left (a1, b1, c1), all drying steps are present, whereas
on the right (b2, c2), the drying steps were reduced to a
minimum. Finally, the cross section samples to the right
were fractured at room temperature without freezing.
The spin-coated PEDOT:PSS sample without annealing
(b2) has a thickness of 127 nm. This is slightly less
compared to the 138–145 nm of the annealed and
freeze
Fig. 5 PEDOT:PSS sheet resistance on spin coated NFC-G samples with variable time interval between the spin coating and annealing
Fig. 6 Left hand column, annealed (130 °C) and freeze fractured samples: (a) PEDOT:PSS reference, thickness 47 nm; (b1) PEDOT:PSS on spin-coated
NFC-G, thickness 138–145 nm; (c1) PEDOT:PSS on drop cast NFC-G, thickness 438–458 nm. Right hand column samples without annealing and freeze
fracturing: (b2) PEDOT:PSS on spin-coated NFC-G, thickness 127 nm; (c2) PEDOT:PSS on drop cast NFC-G, thickness 274 nm
fractured counterpart (b1). This suggests that the
spincoated NFC-G has a dense texture being fairly resistant to
the moisture. This is also supported by the water contact
angle results. The spherical PEDOT:PSS water dispersion
particles can be distinguished on the surface in the sample
(b2). This suggests that the spin-coated NFC-G layer
accounts for approximately 80 nm and the PEDOT:PSS
resting on top the remaining 47 nm from the total thickness
of 127 nm.
The drop cast PEDOT:PSS samples have a significantly
thicker NFC-G layer than the spin-coated counterparts
as drop-casting produces a less dense texture with high
water release and uptake ability. The drop cast
PEDOT:PSS-coated sample shrank during annealing.
However, the sample swells after the freeze fracturing to
438–485 nm as seen in the sample (c1). Of the total
thickness of 274 nm in the sample (c2) without
annealing, approximately 230 nm belongs to the NFC
component. The water contact angle results show that the
PEDOT:PSS coating on top provides a barrier to
moisture. Therefore, it is reasonable to assume that the
swelling is mainly caused by moisture entering from the open
face of the fractured side, not from the top.
Water contact angle measurements are shown in Fig. 7.
The PEDOT:PSS-coated samples were annealed at 130 °C,
and all the samples were stored at RH 50 % and measured
24 h after sample preparation was completed. The
spincoated and drop cast pure NFC samples are highly
hydrophilic with the latter showing very strong absorption of
water between the 15 and 40-s time interval causing
swelling and deformation of the NFC layer. Some marks from
the drying water droplets were observed on the surface of
the spin-coated NFC after completed drying. This
indicates a limited water absorption into the NFC layer. We
also tested the water contact angles on glycerol film, and
the results were visually identical to the spin-coated NFC
except the strong uneven and noncircular wetting pattern
outwards from the wetting center.
PEDOT:PSS on spin coated NFC-G
PEDOT:PSS on drop cast NFC-G
water absorbed into the pure NFC film
The PEDOT:PSS films were less hydrophilic in
comparison to the uncoated NFC-G layers. The spin-coated
PEDOT:PSS on NFC-G behaved identically to the
PEDOT:PSS reference on glass. This indicates that the sessile
drop method is insensitive to the morphological changes
that can take place in the spin-coated PEDOT:PSS on
NFC-G resulting in a better conductivity. The
PEDOT:PSS on drop cast NFC-G shows approximately 15°
higher water contact angles than the reference
PEDOT:PSS. In general, the annealing step has a
smoothening effect on the PEDOT:PSS films. However, this may
be compromised by the pores and pinholes in the
NFCG layer caused by water and glycerol evaporation
through the PEDOT:PSS film. Nevertheless, the barrier
properties of the PEDOT:PSS coating are quite
remarkable as the water contact angle drops monotonously, at
the same rate as the reference, suggesting only a limited
water absorption. Visual inspection of the samples
revealed only light marks on the surface from the drying
water droplet.
The optical transmission results are in Fig. 8 for the
spin-coated PEDOT:PSS on reference glass and on
spincoated and drop cast NFC-G. The spin-coated sample
data overlaps with that of the reference with
transmission varying from approximately 94 to 84 % for the
wavelength range 350–850 nm. Both maintain the same
light blue color shade, and the components in the
NFCG layer seem not to affect the transmission. This is in
accordance by the dense texture and strong barrier
properties of the NFC-G layer observed in the SEM and
the contact angle measurements. On the other hand, the
PEDOT:PSS on drop cast NFC-G has 1 to 8 points lower
transmission across the measured spectral range of 350–
850 nm. The transmission loss is a fairly small sacrifice
in comparison to the enhanced conductivity increase of
three orders of magnitude. The transmission loss is
caused by a small color shift towards deeper blue as a
result of the PEDOT:PSS absorption into the NFC-G
layer. The PEDOT:PSS liquid dispersion consists of more
than 95 % water that is easily absorbed by the drop cast
NFC-G layer as seen in the contact angle measurement
results in Fig. 7. The same applies to the PEDOT:PSS
liquid dispersion. The submicron thickness of the
colorless and semitransparent NFC-G film has a minor
impact on the transmission drop. Transmission results
for samples annealed at temperatures from 60 to 130 °C
were rather similar, and the results stayed within one
percentage point for all spin-coated samples and within
3 percentage points for all drop cast samples.
Conclusions
The conductive polymer PEDOT:PSS is typically applied
on transparent surfaces such as glass or plastic film. Our
work is focused on PEDOT:PSS on NFC-G, which can
produce stand-alone, semi-transparent films. NFC can
provide sustainable, recyclable, and biodegradable
alternatives to glass and plastic substrates.
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PEDOT:PSS reference
PEDOT:PSS on spin coated NFC-G
PEDOT:PSS on drop cast NFC-G
Fig. 8 Optical transmission of PEDOT:PSS on reference glass, spin-coated NFC-G, and drop cast NFC-G coatings annealed at 130 °C
Pure NFC films are very brittle but can be converted
to flexible ones by blending with a suitable plasticizer.
Here, we use glycerol as a plasticizer due to its additional
multifunctional characteristics: it is colorless,
semitransparent, adhesive, water-soluble, and a well-known
secondary dopant for the PEDOT:PSS. The semi-transparent
NFC film, unlike glass and plastic, can have added
functionality and be an active substrate with tailored properties
that will interact and enhance the conductivity of the
coated PEDOT:PSS on top of the NFC-G.
NFC films are most commonly produced through drop
casting that is a time-consuming process. The dry film
does not adhere to glass or plastic substrate. Gluing the
NFC film onto a solid substrate may alter the optical
and structural film characteristics. Therefore, an
optically semi-transparent model system was developed to
study the PEDOT:PSS interaction on an NFC-G layer
immobilized on glass either through drop casting or spin
coating. These techniques enable spin coating of
PEDOT:PSS on top of the NFC-G sample combining fast
prototyping with good reproducibility. The electrical
conductivity of PEDOT:PSS improved 3 orders of
magnitude on the drop cast NFC-G samples. The high
conductivity is a combined result from the water content
and the glycerol in the NFC-G film allowing annealing
of the samples across a broader temperature range.
The spin-coated PEDOT:PSS on NFC-G samples
provided a sensitive study platform. The conductivity levels
were much lower than with drop cast NFC-G, but the
changes in conductivity levels allowed a better
understanding about the interaction between the PEDOT:PSS
and the NFC-G. Clearly, the water content has a
significant impact on the PEDOT:PSS. More
importantly, the moisture level in the NFC-G film has a
decisive impact on the PEDOT:PSS conductivity level
when the samples are annealed. Environmental stability
of the transparent, conductive PEDOT:PSS films on
NFC-G substrate is a key property for practical
applications that can either degrade the electrical functionality
or be utilized, for example, in gas or humidity sensing of
the environment. This work was carried out in
wellcontrolled conditions to study fundamental interactions
between PEDOT:PSS and NFC-G with minimized
environmental variables. We plan to return on this issue in a
future communication.
We believe that conductive PEDOT:PSS films on
NFC-G have potential for many applications in flexible
electronics and sensors in the future.
Authors’ contributions
DV, VK, MT, and JJS designed and planned the experiments. JL and CX
fabricated the nanofibrillar cellulose (NFC). DV conducted all the experiments
and performed the data analysis. DV wrote the manuscript. All authors read
and approved the final manuscript.
Acknowledgements
This work has been funded by the Academy of Finland (grants no. 250 122,
256 263 and 283 054). The Physics Laboratory staff at Abo Akademi University
are greatly acknowledged for all the help, equipment, and facilities employed in
this work. MSc Björn Törngren is acknowledged for performing the optical
transmission measurements at the Laboratory of Physical Chemistry at Abo
Akademi University. SEM samples were measured by M.Sc. Linus Silvander at
the Laboratory of Inorganic Chemistry at the Abo Akademi University, Finland.
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