conductivity of inkjet-printed pedot:pss-swcnts … · conductivity of inkjet-printed...

Conductivity of inkjet-printed PEDOT:PSS-SWCNTs on uncoated papersPeter D. Angelo, Gregory B. Cole, Rana N. Sodhi and Ramin R. Farnood

KEYWORDS: Inkjet printing, PEDOT:PSS, Carbon

nanotubes, Conductive ink, ToF-SIMS

SUMMARY: Poly(3,4-ethylenedioxythiopene): poly

(styrene-sulfonate), or PEDOT:PSS, as well as single-

walled carbon nanotubes, were incorporated into an inkjet

ink. Handsheets were prepared which contained varying

amounts of TiO2 filler, internal sizing agent, fixation agent,

and either softwood or hardwood kraft pulp. The ink was

jetted onto the handsheets to form conductive layers with

apparent conductivity as high as 0.018 S/cm on internally

alkyketene dimer-sized softwood kraft handsheets with no

other additives. Internal sizing increased conductivity at

low filler loadings by preventing PEDOT:PSS from

penetrating into the substrate, resulting in a conductive ink

film on the surface of the sample. Unsized handsheets

allowed more rapid absorption, and therefore deeper

penetration, of the PEDOT:PSS ink, which resulted in a

more diffuse conductive layer. The inclusion of a

polyethyleneimine retention aid for TiO2 filler decreased

conductivity significantly even in unfilled sheets by

interaction with PSS- counterions. A positively charged

fixation agent, poly(diallyldimethylammonium) chloride,

reduced PEDOT conductivity through the retention of non-

conductive PSS- anions.

ADDRESSES OF THE AUTHORS: Peter D. Angelo

([email protected]), Ramin R. Farnood

([email protected]), Rana N. Sodhi

([email protected]): University of Toronto,

Department of Chemical Engineering, 200 College St.,

Toronto, Canada, M5S3E5; Gregory B. Cole

([email protected]): University of Waterloo, Department

of Chemistry, 200 University Ave. W., Waterloo, Canada,

N2L3G1.

Corresponding author: Peter D. Angelo

Over the last several years, the use of poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate), also

known as PEDOT:PSS, in printed electronics has attracted

interest due to its high optical transmittance, tunable

electrical conductivity, and ease of processing by wet

methods such as spin-coating or inkjet printing (Ouyang et

al. 2005; Garnett, Ginley 2004; Tekin et al. 2008;

Yoshioka, Jabbour 2006; Sankir 2008; Ouyang et al. 2004;

Fan et al. 2008; Xia, Ouyang 2009; Hsiao et al. 2008;

Lopez et al. 2008; Kim et al. 2002; Crispin et al. 2006;

Fehse et al. 2007; Dobbelin et al. 2007; Xue, Su 2005).

PEDOT:PSS is an aqueous suspension of small, gelled

particles of water-insoluble conductive polymer (PEDOT)

with a moderate bulk conductivity of 550 S/cm (Crispin et

al. 2003), dispersed with a PSS– ion. Because it is water-

borne, it can easily form films after heat or vacuum-induced

removal of the water solvent.

The interaction of PEDOT:PSS with a paper substrate and

its effect on film conductivity has been reported by

Montibon et al. (2009, 2010), although primarily for coated

PEDOT:PSS layers rather than printed ones (Denneulin et

al. 2008; Agarwal et al. 2006, 2009). As the substrate itself

appears to cause variability in conductivity, however, either

coated or printed layers are likely to interact with it in a

similar fashion. In general, the roughness of the substrate

plays a major role in the conductivity of any films

deposited on it. Furthermore, in the case of paper, chemical

interactions such as adsorption to the cellulosic components

of paper (Montibon et al. 2009) or the inorganic coating

materials (Agarwal et al. 2006) can lead to wide variability

in conductivity. Polyelectrolytes contained in the paper

may also alter conductive layer structure and performance

(Agarwal et al. 2006, 2009). Paper, being hygroscopic, also

tends to absorb the aqueous PEDOT:PSS suspension into

its fibres. These and other physicochemical interactions

make paper-based conductive PEDOT films complex

systems with unpredictable electrical properties.

To enhance the conductivity of PEDOT:PSS films, the

addition of single-walled and multi-walled carbon

nanotubes (SWCNTs and MWCNTs) has been considered

in the literature (Kymakis et al. 2007; Moon et al. 2005;

Bhandari et al. 2009; Mustonen et al. 2007, Agarwal et al.

2006, 2009). In particular, Mustonen et al. (2007) showed

that aqueous dispersions of PEDOT:PSS-SWCNTs can be

deposited relatively uniformly by inkjet printing to form

translucent conductive films.

Inkjet printing of PEDOT:PSS is highly desirable because

the precision and flexibility offered by an inkjet printer is

ideal for fine patterning of printed electronics without the

use of labour- or time-intensive methods such as

photolithography and vapour deposition. Stable inkjet

inks are carefully formulated blends of components which

control rheology, dispersion, foaming, microbial growth,

and pH (Karsa 2005). Extensive research has been devoted

to the preparation of inkjet inks using PEDOT:PSS as a

conductive species, on a variety of printers (Garnett, Ginley

2004; Sankir 2008; Lopez et al. 2008). In our previous

work (Angelo et al. 2009; Angelo, Farnood 2010a, 2010b,

2010c), we have prepared PEDOT:PSS-SWCNT inks and

considered the optimization of conductivity, mechanical

properties, and optical properties in jetted films of this

material. Although paper has been used as a substrate in

these studies, the effect of paper furnish on the conductivity

of printed PEDOT:PSS-SWCNT films has not been

PAPER PHYSICS

486 Nordic Pulp and Paper Research Journal Vol 27 no.2/2012

addressed. It was expected that the connectivity and hence

the conductivity of the printed ink would be affected by the

paper structure and constituents, i.e. fibres, fillers, sizing

agents, and so forth.

In this work, the role of the paper’s components in the

distribution of inkjet-printed PEDOT:PSS-SWCNT inks

and conductivity of the resulting layers is investigated.

Handsheets with varying pulp type, filler content, internal

sizing, and fixation agent were prepared in the laboratory.

PEDOT:PSS-SWCNT inks were printed on the samples

using an inkjet printer and bulk resistance of printed

samples was obtained using a two-point measurement

technique. Using this approach, the interaction of paper

with inkjet-printed PEDOT:PSS-SWCNT films was

systematically investigated.

Materials & Methods Handsheet characterization

Northern bleached softwood (SW) and hardwood (HW)

kraft pulps were used in this work. 40 g of oven-dried (OD)

pulp was soaked in 500 ml of deionized water for 12 h. The

excess water was pressed out of the pulp in order to

calculate consistency, which was 23-25% for both pulp

types. Pulp was then refined in a Noram refiner for 5100

revolutions after which 24 g (dry pulp weight) of refined

pulp was added to 2 L of water, and was then mixed in a

Durant disintegrator for 15000 revolutions. The resulting

slurry was made into handsheets containing 1.5 g dry pulp

per sheet, using a non-recirculating Noram sheet former

according to TAPPI Standard Method T-205. TiO2 (≥ 99%,

Sigma-Aldrich) filler and alkylketene dimer (AKD) internal

sizing agent (Hercon 115, Hercules) were added to the pulp

slurry. Filler was added in 15 w/w% and 30 w/w%

(relative to the dry pulp weight) loadings. Internal sizing

agent was added at 0.8 w/w% on OD pulp basis. A

polyethyleneimine-based retention aid (Polymin SK, 30

w/w% active ingredient in water, BASF) was added to

sheets containing filler (0.83 g in a sheet containing 15

w/w% filler and 1.66 g in sheets containing 30 w/w% filler)

during sheet forming. An ink fixation agent,

poly(diallyldimethylammoniumchloride) or PDADMAC

(Sigma-Aldrich), was added at 2 w/w% (OD pulp basis)

during sheet forming. Handsheets were air dried in a

conditioning room at 25°C and 75% relative humidity for

24 h. All sheets were calendared at 80oC, 100 kPa and 3

nips using a Beloit Wheeler laboratory calendar, couch side

up.

Handsheets were characterized for thickness using a TMI

micrometer at 10 different points on the sheet. Average

sheet thickness over a fixed area was also used to estimate

sheet density and thereby sheet grammage. Actual retained

filler content was determined by burning a portion of each

sheet of a fixed weight in an oven at 500oC for 1 h, at which

point the fibres were completely ashed. The weight of the

filler in the sheet was determined by the difference in ash

weights between a filled and unfilled sheet. The actual

weight percent of filler retained in the sheet was estimated

by dividing ash weight by the original sample weight

(before burning). Micrographs of the unprinted handsheets

were obtained using a JEOL-7001 JSM scanning electron

microscope (SEM). Filler and surface pore distribution

were observed using the backscattered electron (BSE)

detector on the SEM to improve contrast between the filler

and fibres. Contact angle was estimated using an aqueous

solution of crystal violet dye (test ink) prepared according

to TAPPI Standard Method T431 for measuring ink

absorbency into paper. The test ink had a surface tension of

62 mN/m. 30 µL of this ink was dropped with a calibrated

micropipette onto a handsheet, and the resulting drop was

immediately photographed from the side using a Canon

Rebel XT-ME DSLR camera with a MP-E 65 mm macro

lens. Finally, absorbency of the sheets was observed by

measuring the time for complete absorption of a 30 µL

sample of the same test ink into the surface. During this

test, the samples were placed directly under a 60 W

incandescent lamp elevated 30 cm from the test specimen’s

surface. Complete absorbency was defined according to

Test Method T431 as the point at which light reflection

from the droplet on the surface was no longer visible.

PEDOT:PSS-SWCNT ink formulation and printing

Details of the formulation of a PEDOT:PSS-SWCNT ink

with suitable fluid properties for inkjet printing have been

provided in our previous work (Angelo, Farnood 2010a)

based on guidelines described elsewhere (Daniel 2007; de

Gans 2004). All reagents were supplied by Sigma-Aldrich

Canada. The basis for the ink was PEDOT:PSS suspension

(1.3 w/w% in water). SWCNTs (50-70% CNT basis, 1.2–5

nm width, 2.5 µm length) in water and sodium lauryl

sulfate (0.04 w/w% SWCNTs, 0.1 w/w% sodium lauryl

sulfate) solution were dispersed into the inks using an

ultrasonic mixer. The final optimized ink composition

was 35 w/w% DI water, 34 w/w% PEDOT:PSS suspension,

17 w/w% glycerol, 10 w/w% DMSO, 3 w/w% SWCNT

solution, 0.5 w/w% SLS, and 0.5 w/w% defoamer. This

ink met the criteria for stable drop formation and jetting

(FUJIFILM-Dimatix 2006), with a viscosity of 2.2 cP,

surface tension of 30.2 mN/m, and average particle size of

200 nm.

8 mm × 8 mm solid square patterns of PEDOT:PSS-

SWCNT ink were inkjet-printed onto each handsheet using

a FUJIFILM-Dimatix DMP2831 piezoelectric printer and

cartridge (10 pl drop volume). Jetting behaviour was

optimized by adjusting the drive voltage-time waveform for

the piezoelectric printhead, to produce stable, spherical

drops. 3 layers of ink were printed on the couch side of

each sheet, and printed samples were immediately dried at

100°C for 20 min on a hot plate.

Conductive film characterization

Time-of-flight secondary ion mass spectrometry (ToF-

SIMS) was used to map the distribution of PEDOT and PSS

ions and ion fragments relative to the paper fibres

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Nordic Pulp and Paper Research Journal Vol 27 no.2/2012 487

Table 1a. Negative ToF-SIMS ion fragments.

Species Source Ion m/z

PSS Ink (PSS) C8H7O3S- 183

PEDOT Ink (PEDOT)

C6H3O2S- C6H2O2S2- C6HO2S3-

C6O2S4-

139 138 137 136

Table 1b. Positive ToF-SIMS ion fragments.

Species Source Ion m/z

Metals Filler

Surfactant Ti4+ Na+

48 23

Cellulose Fibres C6H7O3+ C6H9O4+ C6H9O5+

127 145 161

Additives PDADMAC

Retention aid C8H16N+

C2H5N+

126 43

and filler. An ION-ToF ToF-SIMS IV apparatus was used

to perform the measurements. A Bi3 primary ion gun was

used to induce ion ejection and fragmentation. Images of

the samples’ surfaces were obtained using the high-spatial-

resolution detector. The imaged areas had a size of 100 μm

×100 μm, with a spatial resolution of 390 nm. Both

positive and negative ion spectral maps were obtained. A

full peak list is shown in Table 1; identification of the

cellulosic fragments was based on the work of Fardim &

Holmborm (2005), Sodhi et al. (2008) and Delandes et al.

(1998). It is important to note that the secondary ion

masses listed were variable by several mass units due to

protonation or deprotonation of the fragments, a common

occurrence in ToF-SIMS analysis of organic species (Lua et

al. 2005).

The thickness of the conductive layer was estimated by

imaging cross-sections of the printed samples, and

observing the dark-coloured PEDOT-SWCNT layer on the

surface and in the bulk of the sheet. Samples were fastened

between two glass slides with double-sided tape and a

cross-section was cut along the edges of the slides using a

surgical scalpel. Images were captured using a Leica DM-

LA optical microscope and attached camera. Several

images were captured at various focal depths and stacked to

generate an in-focus section using CombineZ software

(Hadley 2009). The section was then converted to a

greyscale image – the dark-coloured ink appeared as a

darker grey. The image was then colour-inverted, and the

grey levels at or above the ink colour were highlighted

using thresholding in ImageJ software, highlighting only

the inked region. The highlighted inked region was then

measured as a %-area of the entire image. Multiplying the

%-area by the image dimension parallel to the layer

thickness gave an average layer thickness for the sample.

Sheet resistance, R, was measured using a two-point

method, described by Heaney (2000). To perform this

measurement, two bus bars of carbon paint were applied

along opposite edges of the printed squares, and a silver

contact was painted on each. The two-point method with

bus bars as contacts was used due to the irregular

penetration depth of the conductive ink observed in the

sample. The bulk resistance was determined across the

printed samples using a multimeter (Innova Electronics

Corporation). Also, a mega-ohmmeter (GenRad 1863) was

used to measure the resistance of the unprinted base paper

sheets prepared in the same fashion. Ohmic behaviour of

the ink films and the base sheets was confirmed by

increasing the input potential of the megaohmmeter when

applied to the samples and noting that the resistance did not

change significantly between applied voltages of 0.1 – 100

V. Finally, any effect of retained moisture on resistance

was examined by measuring resistance across several sheets

before and after 30 minutes of dessication in a vacuum

oven at 40°C and 1 Torr. Resistance did not vary as a

result. Conductivity, , was estimated by measuring the

film thickness (h) from cross-section images, as described

above. According to the definition of conductivity in

terms of resistance and film thickness:

For homogeneous materials, is a fundamental material

property. However, because the PEDOT-SWCNT ink

penetrated into the paper structure, was an apparent

property which depended on the variations in the structure

of the conductive layer.

Results & Discussion Handsheet properties

For this study, 24 separate handsheets were prepared (Table

2). Also, control handsheets, containing retention aid but

no filler, were prepared, to assess the effect of retention aid

itself on conductivity. The handsheets containing AKD

generally had greater grammage values. At the same filler

levels, the HW handsheets retained more filler than the SW

sheets. Higher filler retention in HW sheets was likely due

to differences in the fibre morphology and hence in the

sheet structure (Gullichsen, Paulapuro 1999). BSE imaging

of the handsheets’ surfaces confirmed that the wider SW

fibres created sheets with larger surface pores and larger

clusters of TiO2 filler, whereas narrow HW fibres formed

handsheets with many smaller pores and more uniformly

distributed TiO2 (Fig 1). There were also significantly

larger clumps of filler in the sized sheets when compared to

the unsized sheets, where filler was distributed evenly

among the fibres. It is possible that the AKD internal

sizing served as an additional retention aid during sheet

forming, leading to both the higher filler amounts (Table 2)

and larger concentrations of TiO2 (Figs 1g, 1h), and greater

sheet grammage. Based on Table 2, the conductivity of the

base sheets varied in the range of about 1~4 nS/cm, but in

general was insignificant compared to the conductivity of

the printed PEDOT:PSS-SWCNT films, as described in

subsequent sections. Therefore, the paper components

themselves would not be capable of significant charge

transport and that their physicochemical interactions with

PAPER PHYSICS


the PEDOT-SWCNT ink were the determining factor in

conductivity variation in the printed sheets.

Table 2 also shows that the surface properties of the sheets

were noticeably affected by the pulp type and the presence

of additives. The unsized HW sheets generally showed

more rapid wetting and ink absorbency than the

corresponding SW samples. This suggested the presence of

a relatively fine pore network in the HW sheets capable of

rapid absorption of aqueous inks, which was absent in the

case of sheets containing large, flat SW kraft fibres. As

expected, surface wetting and absorbency were greatly

reduced in the presence of AKD sizing (Fig 2).

Absorbency was particularly low in sheets containing

sizing, which in the case of the printed PEDOT:PSS ink

lead to increased retention of conductive material at the

sheet surface. There were also moderate decreases in

contact angle, indicating improved wetting, with increases

in filler content. This was caused by the reduction in

volume of sized fibres – i.e. more of the sheet was

composed of TiO2 particles which were not sized, and

therefore more easily wetted by the ink. Table 2 also

suggests that the cationic fixation agent, PDADMAC, was

able to bind to the ink as it penetrated through the substrate,

resulting in improved wetting, but not necessarily in a faster

rate of absorbency, as ink would be bound in the upper

layers of the sheet rather than soak into the untreated fibres.

Therefore, depending on the distribution of PDADMAC in

the sheet, the ink absorbency might increase or decrease

with the addition of fixation agent.

Conductivity of printed handsheets

Printing of the liquid PEDOT:PSS-SWCNT ink onto the

porous, absorbent handsheets naturally resulted in the

penetration of the conductive species into the paper. This

had an effect on the connectivity and hence conductivity of

printed PEDOT:PSS-SWCNT patterns.

The conductivity of printed SW and HW handsheets can

be found in Fig 3. Conductivity of these samples was four

(4) orders of magnitude smaller than the bulk conductivity

of PEDOT (Crispin et al. 2003), but was comparable to that

reported in our earlier work on paper-based PEDOT-

SWCNTs. Despite slight differences in ink absorption rate

and surface structure (Table 2), there was little if any

difference between SW and HW handsheets in terms of

conductivity.

TiO2 filler and retention aid

The plot of measured conductivity versus actual retained

filler content (shown as “Ash” in Table 2) in Fig 3 shows

that with the addition of TiO2 (and therefore retention aid)

conductivity of printed HW and SW handsheets decreased

by nearly two orders of magnitude. However, this decline

was primarily due to the presence of the retention aid, as

the control sheets – containing appropriate amounts of

Polymin SK but no actual TiO2 – performed similarly to

those with both filler and retention aid. The dashed

horizontal lines in Fig 3 show the measured conductivity on

these Polymin SK-loaded control sheets, which is similar to

that in the handsheets containing both Polymin SK and

TiO2. This suggested a chemical or physical interaction

between the conductive components of the ink and the

Polymin SK which compromised conductivity. It is worth

noting that conductivity was slightly higher in sheets

containing Polymin SK and TiO2 over the control sheets

containing only Polymin SK, because of the interaction of

the Polymin SK with TiO2, which reduced the amount of

Polymin SK interacting with the ink components. The

above observations suggest that an interaction between the

Polymin SK and the PEDOT:PSS-SWCNT ink took place

resulting in compromised electrical performance.

Fig 1. SEM BSE micrographs of handsheets’ surfaces. None of the sheets shown contain fixation agent. (a) HW, 0% filler, unsized; (b) HW, 0% filler, sized; (c) HW, 30% filler, unsized; (d) HW, 30% filler, sized; (e) SW, 0% filler, unsized; (f) SW, 0% filler, sized; (g) SW, 30% filler, unsized; (h) SW, 30% filler, sized.

100 µm

(e) (f)

(g) (h)

(d) (c)

(a) (b)

(g)

(h) (g) (h)

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Table 2. Handsheet properties.

Pulp (filler w/w%)

AKD PDAD-MAC

Ash (w/w%)

Grammage (g/m2)

Conductivity (nS/cm)

Contact angle (°)

Ink absorbency (µl/min)

SW (0%)

0.049 95 3.83 81 19 0.098 104 1.94 100 0.34 0.049 89 0.94 59 33 0.15 89 1.05 108 0.33

SW (15%)

8.5 118 1.85 82 14 9.1 94 2.95 102 0.46 8.9 92 1.43 44 16 6.2 112 1.79 107 0.37

SW (30%)

13.4 101 3.41 47 18 13.5 94 1.82 74 4.1 12.7 94 2.15 30 14 14.9 86 1.51 94 1.7

HW (0%)

0.39 96 1.19 52 51 0.25 101 1.35 100 0.35 0.34 86 3.13 31 43 0.24 92 2.63 107 0.33

HW (15%)

8.2 91 3.23 50 62 11.0 81 1.78 114 0.39 9.8 78 1.07 40 30 12.2 79 1.49 102 0.36

HW (30%)

12.4 91 2.71 57 25 14.9 74 1.48 92 0.46 21.0 102 1.31 53 28 16.1 100 2.35 97 0.37

The most likely interaction is the neutralization of the

excess ionic charge provided by the PSS– counterion in the

ink, a function for which polyethyleneimide (PEI), the

active component of the retention aid, is known (Neimo

1999). Furthermore, due to this strong interaction with the

PEDOT:PSS complex, non-conductive PEI was likely

incorporated within the conductive layer, dramatically

reducing conductivity of the layer as a whole. This type of

interaction has been previously exploited by Lin et al.

(2007) for modulation of PEDOT:PSS conductivity by five

orders of magnitude at a ratio of PEDOT:PSS to PEI of 1:1.

However, in the case of printed handsheets, PEI was

dispersed throughout the sheet in a relatively low

concentration, and hence had a less drastic effect on

conductivity was observed. Furthermore, this effect was

mitigated by washout of retention aid during the handsheet

forming process, producing the plateau at 10-4

S/cm

observed in Fig 3. These results suggest that the effect of

the filler itself on conductivity was evidently negligible

compared to PEDOT interaction with PEI. ToF-SIMS

mapping of PEDOT, TiO2, and PEI (Figs 4 and 5) confirms

that the presence of isolated clusters of TiO2 did not disturb

the spatial distribution of PEDOT in the handsheets. This

observation implied that these clusters of TiO2 were also

saturated with the conductive ink, and the filler species did

not interrupt the conductive path. Hence, in the absence of

retention aid, finely divided filler particles are expected to

have little or no effect on the conductivity of paper-based

printed PEDOT-SWCNT films. However, with the

addition of a strongly positively-charged retention aid, filler

can increase conductivity by binding to the retention aid,

thereby diminishing its interaction with the PSS-

counterion. ToF-SIMS images of printed handsheets also

revealed that PEDOT and PSS were located in exactly the

same regions (Figs 5a and 5b), but were less concentrated

where PEI was localized (Fig 5c). The resulting non-

uniformity of the conductive layer would have an adverse

effect on the conductivity of filled handsheets. Comparing

Fig 5c and 5d, it appears that a very small amount of PEI is

actually interacting with the TiO2 . and the majority resides

in or on the fibres, where it can readily interact with the

PSS–. In fact, ToF-SIMS peaks for Ti

4+-PEI (m/z = 91) and

TiO2-PEI (m/z = 123) were very weak, suggesting that little

PEI was adsorbed or bonded to the surface of the filler,

leaving the rest to freely interact with PSS– in or on the

fibres. These observations are consistent with the

hypothesis that PEI-PSS– interaction was primarily

responsible for increased electrical resistance.

PDADMAC fixation agent

The addition of fixation agent, PDADMAC, decreased

conductivity of the printed handsheets, and this effect was

more pronounced at higher filler addition levels (Fig 6).

With a similar effect to the retention aid, this was likely

caused by the interaction of the cationic PDADMAC with

the PSS– counterion and the introduction of non-conductive

PDADMAC into the PEDOT film. However, the lower

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Fig 2. Contact angle of test ink on HW pulp, 30% filler: (a) no additives; (b) sized; (c) fixed; (d) fixed and sized.

Fig 4. Distribution of TiO2 in HW and SW handsheets (unsized, no fixation agent). (a) HW, 0% filler; (b) HW, 30% filler; (c) SW, 0% filler; (d) SW, 30% filler.

Fig 3. Conductivity as a function of added filler for handsheets with SW pulp (squares), and HW pulp (circles). Dashed lines (a) and (b) represent the conductivity in handsheets containing retention aid but no actual TiO2. Error bars represent standard deviation.

Fig 5. Relative distribution of PEDOT:PSS, PEI, and TiO2. Sheets contained SW fibres, 30 w/w% filler during sheet forming and no internal sizing or fixation agent. (a) PEDOT; (b) PSS; (c) PEI; (d) TiO2.

1 mm

(a) (b) (c)

(c) (d)

(a) (b)

(c) (d)

20 µm

(a) (b)

(c) (d)

20 µm

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Fig 6. Estimated conductivity of PEDOT-SWCNT printed ink on SW fibres: (a) unfurnished (b) PDADMAC fixation agent. Error bars represent standard deviation.

Fig 8. Estimated conductivity of PEDOT-SWCNT printed ink on HW fibres: (a) unfurnished; (b) with internal AKD sizing; (c) with internal AKD sizing and fixation agent. Error bars represent standard deviation.

Fig 7. Relative distribution of PEDOT:PSS and PDADMAC in HW sheets (30% filler, no sizing): (a) PEDOT; (b) PSS; (c) PDADMAC; (d) total negative ion.

concentration of PDADMAC versus PEI resulted in a

smaller decrease in estimated conductivity. In addition, the

highly-charged, cationic nature of PDADMAC may also

have caused agglomeration of the PEDOT or SWCNTs

after printing due to destabilization of these suspensions

through interaction with the PSS- and lauryl sulfate

dispersants. Such an interaction would create a non-

uniformly distributed conductive ink and hence a lower

sheet conductivity. The non-uniform distribution and the

apparent agglomeration of both PEDOT and PSS in the

ToF-SIMS maps of handsheets containing PDADMAC

supported this theory (Fig 7). With increasing filler levels,

the detrimental effect of PDADMAC also became more

pronounced as the contact angle decreased (Table 2). A

lower contact angle implied better wetting and penetration

of PEDOT:PSS into the sheet, and hence reduced

conductivity.

Internal AKD sizing

The most notable effect on conductivity aside from that of

PEI was that of internal sizing (Fig 8). In every case, the

addition of sizing agent resulted in an increase in

conductivity. As shown in Table 2, internal sizing

decreased ink spreading and absorption by increasing the

contact angle of the ink. Therefore, a more uniform ink

layer (containing fewer non-conductive fibres/filler

particles) with higher connectivity and conductivity was

obtained. Moreover, the ink absorption rate was reduced

by several orders of magnitude in the sized sheets, allowing

a longer time for the ink to rest on the surface during drying

of the PEDOT-SWCNT ink. Cross-sectional images

confirmed the presence of a relatively thin PEDOT-

SWCNT layer on the sized sheets. Fig 9 shows the cross-

sections of printed SW handsheets (30% filler) for an

0 15 30

Co

nd

uct

ivit

y (S

/cm

)

TiO2 filler added during sheet forming (w/w%)

0 15 30

Co

nd

uct

ivit

y (S

/cm

)

TiO2 filler added during sheet forming (w/w%)

(a) (b)

10-2

10-3

10-4

10-5

10-6

(a) (b)

(c) (d)

20 µm

(a) (b)

(c)

10-2

10-3

10-4

10-5

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internally AKD sized handsheet and an unsized handsheet.

In the case of the sized sheet, the PEDOT-SWCNT ink

(blue-coloured) is concentrated near the surface of the

sample while in the latter case ink is distributed throughout

the sheet thickness.

The positive effect of AKD sizing appeared to be more

pronounced for handsheets containing retention aid and/or

fixation agent. In other words, AKD sizing appeared to

largely eliminate the adverse effects of PDADMAC and

PEI that are distributed throughout the sheet, by reducing

the amount of contact of the conductive ink with these

molecules.

Because of this correlation between ink “holdout” in the

printed sheets and the performance of the conductive layers,

paper with minimal porosity and high hydrophobicity (for

aqueous inks, at least) should be considered ideal for

electrically conductive paper production. The performance

of such materials might be improved by the deposition of

larger amounts of ink, as the initial print passes would fill

the pores and become absorbed into the fibres and filler

clusters, providing a less absorbent surface for subsequent

print passes. However, this approach would of course

consume more ink and require more processing time to

achieve similar performance to appropriately sized sheets.

The significant improvement in conductivity across all the

sheets resulting from internal sizing is likely more easily

and economically achieved by sheet pretreatment.

Fig 9. Cross-sections of printed SW handsheets (30% filler): (a) internally AKD sized; (b) unsized.

Another variable that might assist in improving

conductivity, in the same vein, is sheet smoothness. A

rougher sheet, whether it be sized or not, provides greater

surface area for ink absorption and improved wetting, and

is expected to result in a considerably more irregular

conductive film and a lower conductivity. In fact, a brief

examination of printed PEDOT-SWCNT layers for

uncalendered handsheets revealed that their conductivity

was close to those of the base sheets themselves (not

shown). The universal calendering of the handsheets used

in these experiments provided a high degree of smoothness,

improving the ink holdout and conductivity of printed

samples.

ToF-SIMS mapping of the ink components on the sized

handsheets further confirmed the improved ink retention on

the sheet surface. It is evident in Fig 10 that internal sizing

allowed the PEDOT ink to coat the surface of the sheets to

a greater degree, rather than being absorbed into them,

resulting in a larger proportion of interconnected PEDOT-

SWCNT regions. However, there was still not a completely

contiguous PEDOT/PSS/ SWCNT layer on the surface of

the sized sheets, and fragments of cellulosic materials and

filler were clearly visible in the spectral maps. As is also

shown in Fig 9, even sized sheets absorbed the ink to a

certain degree. However, it is worth noting that the

intensity of the PEDOT signal in the ToF-SIMS maps of

sized sheets was significantly higher indicating a higher

concentration of conductive ink on the surface of the sized

samples.

Fig 10. ToF-SIMS images of PEDOT distribution in unfilled, unfixed sheets. % coverage by PEDOT was estimated using grey-level thresholding.

50 µm

(a)

(b)

inked region

% covered by PEDOT (a) sized HW: 45.9% (b) unsized HW: 25.6% (c) sized SW: 40.2% (d) unsized SW: 25.1%

(a) (b)

(c)

20 µm

(d)

inked region

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Once again, the addition of PDADMAC and retention aid

to the sized handsheets reduced conductivity (Fig 8).

Therefore, the AKD sizing was not able to fully prevent the

contact of the conductive ink with the PDADMAC and PEI

distributed throughout the sheet. Indeed, both PEI and

PDADMAC were detected by ToF-SIMS analysis on the

sheet surface.

Conclusions Moderately conductive paper was successfully produced by

inkjet printing of PEDOT:PSS-SWCNT ink. The flexibility

of inkjet printing, which allows for rapid patterning and is

well-adapted to the use of paper as a substrate, makes it

ideal for this application. The process represents an

alternative to the use of metallic inks or metallicized paper

fibres for conductive cellulosic materials. However, several

layers of ink or adequate level of surface treatment may be

required to achieve a desirable electrical conductivity.

Internal sizing can improve the ink holdout and therefore

result in a higher conductivity by forming a thin layer of

PEDOT:PSS on the surface of the paper. Papermaking

additives; such as retention aids and fixation agents, could

interact with PSS– and/or other ink components and lower

the connectivity and hence the conductivity of printed

PEDOT:PSS films.

However, these values of conductivity are estimated from

the dimensions of the ink layers. It should be emphasized

that the key obstacle to objectively quantify the

conductivity of printed PEDOT:PSS on paper arises from

the irregularity and unpredictability of the ink distribution.

Therefore, ideally, to optimize the performance of

conductive papers, the uniformity of the paper itself needs

to be controlled as tightly as possible. It is likely that the

best result that can be offered for a given paper type is a

range of conductivity values, which cannot be compared in

an absolute sense to the conductivity of typical PEDOT-

SWCNT structures, such as films. However, sized, unfilled

paper provides a promising alternative and novel substrate

for PEDOT-SWCNTs to be used as moderately conductive

electrode or charge-injecting materials. It should be

emphasized that this study deals with uncoated sheets;

surface treatment of paper in the form of surface sizing and

coating is an alternative opportunity that warrants its own

systematic study.

Acknowledgements

The authors would like to thank the SENTINEL Bioactive Paper Network for their financial support, and Peter Brodersen, Turn Lu, and Edgar Acosta at the University of Toronto for their technical assistance.

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