Organic Electronics 14 (2013) 3061–3069

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Reconfigurable sticker label electronics manufacturedfrom nanofibrillated cellulose-based self-adhesive organicelectronic materials

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.07.013

⇑ Corresponding author. Tel.: +46 11 202507.E-mail address: peter.andersson.ersman@acreo.se (P. Andersson

Ersman).

Jun Kawahara a,b,c, Peter Andersson Ersman a,⇑, Xin Wang a, Göran Gustafsson a,Hjalmar Granberg d, Magnus Berggren b

a Printed Electronics, Acreo Swedish ICT AB, Box 787, SE-60117 Norrköping, Swedenb Organic Electronics, Dept. of Science and Technology, Linköping University, SE-60174 Norrköping, Swedenc R&D Strategy Department, Lintec Corporation, 5-14-42 Nishiki-cho, Warabi, Saitama 3350005, Japand Innventia AB, Drottning Kristinas väg 61, SE-11428 Stockholm, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 May 2013Received in revised form 10 July 2013Accepted 12 July 2013Available online 30 July 2013

Keywords:Cellulose nanofiberElectrochromic displayElectrochemical transistorSelf-supportingSelf-adhesiveElectronic system integration

Low voltage operated electrochemical devices can be produced from electricallyconducting polymers and polyelectrolytes. Here, we report how such polymers and poly-electrolytes can be cast together with nanofibrillated cellulose (NFC) derived from wood.The resulting films, which carry ionic or electronic functionalities, are all-organic, dispos-able, light-weight, flexible, self-adhesive, elastic and self-supporting. The mechanical andself-adhesive properties of the films enable simple and flexible electronic systems byassembling the films into various kinds of components using a ‘‘cut and stick’’ method.Additionally, the self-adhesive surfaces provide a new concept that not only allows for sim-plified system integration of printed electronic components, but also allows for a uniquepossibility to detach and reconfigure one or several subcomponents by a ‘‘peel and stick’’method to create yet another device configuration. This is demonstrated by a stack oftwo films that first served as the electrolyte layer and the pixel electrode of an electrochro-mic display, which then was detached from each other and transferred to another config-uration, thus becoming the electrolyte and gate electrode of an electrochemical transistor.Further, smart pixels, consisting of the combination of one electrochromic pixel and oneelectrochemical transistor, have successfully been manufactured with the NFC-hybridizedmaterials. The concept of system reconfiguration was further explored by that a pixel elec-trode charged to its colored state could be detached and then integrated on top of a tran-sistor channel. This resulted in spontaneous discharging and associated currentmodulation of the transistor channel without applying any additional gate voltage. Ourpeel and stick approach promises for novel reconfigurable electronic devices, e.g. in sensor,label and security applications.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Since its discovery about 2000 years ago, papers are oneof the most commonly manufactured and utilized sheet

materials in our world; it represents the planar carrier thatwe utilize to record, transfer and share printed informa-tion. In fact, it is the largest surface ever manufacturedby mankind. During the digital revolution, paper has cer-tainly been challenged. However, nowadays, rather thanksto the digital revolution the interest for paper as the carrierfor information is regained. One of the prime reasons forthis revival is the birth of the technology and science field

3062 J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069

known as printed electronics. From an application point ofview paper now turns into the carrier for flexible electron-ics, systems that can be manufactured using the sameprinting tools that were originally developed for graphicsand texts. Printed electronics defined on paper labels,package board and on fine paper, is expected to extendthe function of paper and to make paper products con-nected to the digital world.

Paper is based on wood fibers comprising the mostabundant organic compound derived from biomass,namely cellulose. One such renewable cellulose material,nanofibrillated cellulose (NFC), has attracted a very largeinterest during the last decade. This material emanatesfrom the cellulose fibrils inside the wood fiber wall andthe fibrils are liberated in different ways using both chem-ical treatment and high-pressure homogenization to sepa-rate the fibrils from each other and to stabilize them inaqueous dispersion [1,2]. The attraction of NFC stems fromtheir interesting intrinsic properties such as its high spe-cific surface area, flexibility, high aspect ratio of the fibrils,good mechanical properties, and its film forming capacity.NFC dispersions converted into ‘‘nanopaper’’ films can betransparent, reach mechanical properties similar to thatof cast iron [3], or show very good oxygen barrier proper-ties. NFC has also been used as a filler to provide reinforce-ment, flexibility and transparency in nanocomposites anddisplays [4,5].

The field of printed electronics is presently attracting alot of interest since it promises for distributed intelligenceand monitoring; features that are expected to combatsome of the challenges in our society related to health care,monitoring, logistics and safety. A tremendous variety ofcomponents have been developed and manufactured onflexible substrates using different kinds of coating, printingand lamination tools [6–8]. Examples include batteries,capacitors, solar cells, displays, diodes, transistors and logiccircuits. To achieve true printed electronics applications,typically several different kinds of components need tobe integrated into a monolithic flexible system. However,integration of printed electronic subcomponents has pro-ven to be a major challenge with respect to the mergerand compatibility of different printing technologies, mate-rials and post-processing steps. In fact, in many cases spe-cific manufacturing steps are mutually exclusive sincesome steps tend to destroy or deteriorate features andcomponents manufactured earlier in the integrationscheme. This then implies a decreased production yieldthat can only be solved by typically increasing the costfor production and integration. In addition to this, the com-plexity of such production process also brings along therequirement of huge investments of non-standard manu-facturing equipment. The overall mindset when develop-ing the printed electronics platform must therefore aimat minimizing the number of materials and process steps,as well as trying to make the processes of different sub-components compatible with each other by utilizing thesame kind of materials for different functionalities [9].

One of the major advantages of using electrochemicaldevices, based on for instance an electrochemically activeconjugated polymer and an electrolyte, is that the verysame material can be used for a versatility of device

functions. The very same electrolyte component can beused both as the gate insulator in electrochemical andelectrolyte-gated field effect transistors, and also as theelectrolyte in electrochromic (EC) display devices, batteriesand capacitors. PEDOT:PSS, poly(3,4-ethylenedioxythio-phene) doped with poly(styrene sulfonate acid), can serveas the conductor, display electrode and also as the transis-tor channel in printed integrated circuits, such as displays,indicators and sensor systems. The PEDOT:PSS material iselectrically conducting and optically transparent in itspristine state and it exhibits electrochromic switch charac-teristics and control of the conductivity upon electrochem-ical switching. PEDOT is a p-conjugated electronic polymerand the PSS phase serves as its counter ion [10]. In the neu-tral state PEDOT appears deep blue [11] and exhibit semi-conducting properties. PEDOT can reversibly be switchedin between its oxidized and neutral state. The electrochro-mic effect can be utilized in transmissive displays by usingtransparent electrolytes, or in a reflective mode of displayoperation in which the counter electrode is hidden underan opaque electrolyte [12]. The latter version is typicallyused when PEDOT:PSS serves as both the counter and thepixel electrodes since such configuration maximizes thecolor switch contrast of the resulting display. However,there are different strategies in order to enhance the colorswitch contrast also for applications where transmissionmode is desirable, for example by using a bottom displayelectrode and a top display electrode that together expressa complementary EC switching characteristics with respectto each other. Polyaniline (PANI) is such an EC materialthat electrochemically switch color in a complementaryfashion with respect to PEDOT, i.e. it exhibits a faint yellow,close to transparent, color in its reduced leucoemeraldinestate, while PANI becomes dark blue, almost violet, in itsoxidized pernigraniline state. Hence, an electrochromicdisplay comprising a transparent electrolyte layer sand-wiched by one PEDOT:PSS electrode and one PANI elec-trode can switch between a close to transparent pixelstate to a dark black-blue colored state, where the lattercolor state is obtained by applying the positive andnegative voltage to the PANI and PEDOT:PSS electrode,respectively [13].

Besides reducing the number of different materials inprinted electronics to achieve a resulting robust and ra-tional platform for flexible electronics, one should alsoconsider radically new integration concepts. In packagingand graphic art industry labels are commonly adhered topaper surfaces and products to generate a final integratedsystem or to extend the functionality of a specific product.Often, these add-on stickers include a coating that providespressure-sensitive adhesion of the label to the surface ofthe carrier. In some cases those stickers can later be re-moved and then transferred to a different new carrier oritem to serve yet another application.

Previous attempts on using ‘‘peel and stick’’ or ‘‘cut andstick’’ techniques have resulted in flexible thin film solarcells [14] and free-standing dielectric layers for use in tran-sistors [15], respectively. Additionally, a novel method forintegration of memory devices onto flexible substrateshas been reported [16], as well as that nanocellulose-basedcomposite materials has been examined for utilization in

J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069 3063

sensor applications [17]. However, the approach of thepresent study aims at functionalizing the carrier substratein order to reduce the number of materials, and thus thenumber of processing steps, and also to develop a newintegration concept for a reconfigurable electronic systemby using self-adhesive electronic materials to enable a‘‘cut, stick and peel’’ technology for printed electronics,i.e. to establish an electronic equivalence to the ‘‘scrap-book’’ or ‘‘sticker book’’.

2. Materials and methods

2.1. Materials and equipment

Aqueous dispersion of anionic carboxylated NFC(0.5 wt%, pH = 7) was prepared at Innventia AB, Sweden,according to the method described by Wågberg et al.[22]. Aqueous dispersion of PEDOT:PSS ‘‘Baytron P HC’’, tol-uene solution of polyaniline ‘‘PANIPOL T’’, glycerol, andaqueous solution of poly(diallyl dimethyl ammoniumchloride) (PDADMAC, polyelectrolyte with averageM.W. < 100,000), were purchased from Heraeus, PanipolOy, and Sigma–Aldrich (the last two materials), respec-tively, and used without further treatments. PET filmcoated with a 200 nm thick layer of PEDOT:PSS ‘‘OrgaconEL-350’’ was purchased from AGFA. TiO2 powder ‘‘Kronos2300’’ was purchased from Kronos and used as opacifierin an electrolyte.

Spectrophotometer (Datacolor Mercury, aperture diam-eter of 6.5 mm for the illumination, aperture diameter of2.5 mm for the measurement, SCE (specular componentexcluded) mode and D65/10 degrees illumination) wasused to measure the color contrast based on the CIEL*a*b* color coordinates. Keithley SourceMeter 2400 andHP/Agilent 4155B Parameter Analyzer were used to applyDC voltages and evaluate current vs. voltage characteristicsof the devices.

2.2. Device preparation

P:PSS-NFC and Plyte-NFC (transparent and opaque)were prepared by mixing the following formulations de-scribed in dry wt%: P:PSS-NFC hPEDOT:PSS/glycerol/NFC = 13.3/63.5/23.2i, transparent Plyte-NFC hPDADMAC/glycerol/NFC = 94.6/4.0/1.4i, opaque Plyte-NFC hPDAD-MAC/glycerol/TiO2/NFC = 30.8/5.9/62.4/0.9i. Solutions ofthe different materials were mixed and dispersed using aDispermill blender from ATP Engineering, about 5 ml ofeach mixture was then poured into plastic petri disheswith a diameter of 50 mm and finally dried in ambientcondition for 2 days. On one of the Plyte-NFC, toluene-diluted PANI was drop cast and dried on one face (Plyte-NFC-PANI). Depending on the recipe chosen and thevolume of the mixture that is used, a wide range of thick-nesses of the resulting hybridized films can be obtained,which in turn affect the color switch contrast and theswitching time of electrochemical devices. The P:PSS-NFCfilm reported here was about 25 lm thick, while the trans-parent and the opaque Plyte-NFC both were approximately300 lm thick. Three different ECD structures were created

by simple lamination of these hybridized films. The colorswitch contrast of each ECD was characterized by the spec-trophotometer after connecting both electrodes to a DCpower supply.

3. Results and discussion

3.1. System reconfiguration of electronic devices

A semi-dried membrane of NFC mixed with appropriateplasticizers exhibits gel-like behavior when it comes toappearance and physical hardness. This property ispartially remained in the fully dried state and hence aself-supporting film can be obtained thanks to the strongscaffold functionality of NFC, which originates from hydro-gen bonds and/or ionic bonds inside the polymer matrix(not investigated further in this report), see Fig. 1. Theresulting self-supporting composite films containing ECor electrolyte materials can weakly adhere to other solidfunctional substrate surfaces by simple lamination. Thanksto its weak adhering property the film can also be detached(here called ‘‘delaminated’’) from a substrate while main-taining shape and functionality. This finally results in anextremely high degree of freedom to construct, disassem-ble and reconstruct multilayered printed electronic com-ponents. For example, in this report, NFC membraneshybridized with PEDOT:PSS (labeled as P:PSS-NFC) arestacked onto NFC membranes hybridized with polyelectro-lyte and white TiO2 pigment (denoted opaque Plyte-NFC)by hand, and a vertical symmetric electrochromic display(ECD) is established by that another layer of P:PSS-NFC islaminated on the other side of the Plyte-NFC.

Additionally, two other display architectures can also beprepared by simple lamination. A lateral symmetric ECD isconstructed by the lamination of two separated P:PSS-NFCon the same side of the Plyte-NFC, where the two P:PSS-NFC are physically isolated. A vertical non-symmetricECD is instead made from a drop-cast PANI layer on topof a Plyte-NFC (Plyte-NFC-PANI) and lamination of P:PSS-NFC on the opposite surface of the Plyte-NFC layer. Thesethree ECD structures are drawn in Fig. 2(a–c).

Upon delamination of the vertical symmetric ECD, oneof the P:PSS-NFC layers is delaminated and removed fromthe other two layers, and the remaining bilayer can insteadbe laminated onto another functional surface in order toserve as e.g. the electrolyte and the ECD pixel electrodeor the electrolyte and the gate electrode of an electrochem-ical transistor (ECT). This whole process, consisting of lam-ination, delamination and re-lamination, is here denoted‘‘system reconfiguration’’. Additionally, the NFC-basedfunctional films typically are very robust and can be cutby scissors, folded or bended, hence, they have an appear-ance similar to the sticky notes used in scrapbooks or stick-er books.

3.1.1. Reconfiguration from ECD to ECTAfter characterizing the color switch contrast of the

ECDs shown in Fig. 2(a–c), the P:PSS-NFC layer on one sideof the vertical symmetric ECD (Fig. 2(b)) was delaminatedand the remaining bilayer was cut in two pieces by using a

Fig. 1. Photographs showing the self-supporting films based on (a) NFC and PEDOT:PSS, (b) NFC and transparent polyelectrolyte, and (c) NFC and opaquepolyelectrolyte.

3064 J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069

pair of scissors. One of the cut pieces was manually lami-nated onto a PEDOT:PSS-based ECT channel prepared froman Orgacon EL-350 film patterned by a knife plotter tool.Here the laminated bilayer served as the gate electrodeand the electrolyte layer of an ECT, and the patterned PED-OT:PSS on top of the PET substrate served as the drain,source and channel material, see Fig. 2(e). The other cutpiece was put onto another Orgacon foil such that colorswitching could be observed in order to prove the reconfig-uration concept (data not recorded), see Fig. 2(f).

3.1.2. ECT characterizationThe structure of the resulting ECT is drawn in Fig. 3,

although the schematic illustration shows a complete elec-trochromic smart pixel device that will be discussed later.Instead of completing the EC smart pixel device by prepar-ing the top electrode of the ECD moiety, the ECT is obtainedby connecting a DC power supply to the drain electrodesuch that the I–V curves of the ECT could be recordedseparately.

3.1.3. Reconfiguration from ECT to EC smart pixelThe ECT characterized in Section 3.1.2 was further dis-

assembled by delamination of the gate electrode bilayer.In combination with another piece of bilayer, which ismentioned in the end of Section 3.1.1 above, the two filmswere laminated onto another similarly patterned Orgaconfilm in order to create an EC smart pixel device, seeFigs. 2(g) and 3. In this report an EC smart pixel is definedas an ECD connected in series with an ECT, hence, the ECDcoloration can be controlled by the conduction state of theaddressing ECT. Here, upon reconfiguration, one of thelaminated bilayers served as the gate electrode and theother was used as the pixel top electrode.

3.1.4. EC smart pixel characterizationThe same external circuitry was connected as in the

case of ECT characterization, except for that the pixel topelectrode, and not the drain electrode, was connected tothe equipment. The EC smart pixel characterization meth-od has been reported previously [12].

3.1.5. Reconfiguration and duplication of the EC smart pixelThe two bilayers of the EC smart pixel device described

in Section 3.1.3 serve as the gate and the ECD pixel elec-trodes, and after delamination they were laminated onto

another Orgacon film having the same pattern in order tocreate the same EC smart pixel architecture as in Section3.1.3, which was characterized according to Section 3.1.4.

3.1.6. Reconfiguration of a pre-charged hybridized NFC-basedlayer

A vertical symmetric ECD was prepared according toFig. 2(b), followed by applying a voltage across the ECD,which in turn results in electrode charging. After delami-nation of the ECD, one P:PSS-NFC layer is fully oxidizedwhile the other P:PSS-NFC layer is in its fully reduced state.The bilayer including reduced PEDOT:PSS was then recon-figured and transferred onto a PEDOT:PSS-based ECT chan-nel such that the bilayer instead became the electrolyteand the gate electrode of an ECT. However, in this casethe gate electrode is not connected to any power supply;it is only wired directly to the source electrode. So, sincethe gate electrode was electrochemically reduced in ad-vance, it is expected that this charge will be equilibratedbetween the gate and the ECT channel, hence, currentmodulation of the ECT channel should occur just uponbringing the sheets into contact with each other.

3.2. Device characteristics

3.2.1. Color switch contrast of the ECDEC pixels with three different architectures (Fig. 2(a–c))

were evaluated and the data is shown in Table 1. The mea-surement is performed by obtaining the CIE color spacevalues L*, a* and b* of the pixel electrode. Finally the colorcontrast value DE* is calculated from the following equa-tion [18,19]:

DE� ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðL�2 � L�1Þ

2 þ ða�2 � a�1Þ2 þ ðb�2 � b�1Þ

2q

Here the subscripts ‘‘1’’ and ‘‘2’’ for each color spaceparameter indicate the decolored (oxidized light blue)and colored (reduced deep blue) states of the PEDOT pixelelectrode, i.e. the on and off states, respectively. The high-est color contrast was obtained in a lateral pixel device;DE* = 35.2. This is somewhat lower as compared to re-cently reported PEDOT:PSS-based EC pixel devices [20].However, the obtained color contrast is considered to besufficient for most display applications. The color contrastresults and the appearances of the characterized displaysare shown in Table 1 and Fig. 4.

(a) (b) (c)

(d) (e)

(g) (h)

(f)

Fig. 2. Device structures and manufacturing, reconfiguration and characterization procedures are summarized in this chart. The lateral symmetric ECD, thevertical symmetric ECD and the complementary vertical non-symmetric ECD is shown in (a), (b) and (c), respectively. The electrochromic electrodes of eachECD are ionically connected by either a transparent, (a) and (c), or an opaque, (b), electrolyte layer. (d) shows an ECT achieved by delamination andreconfiguration of a pre-charged (b) electrode that is applied onto an ECT channel patterned on top of a PET foil. The ECT in (e) is created after reconfiguringthe (b) pixel electrode into an ECT gate electrode on top of a patterned ECT channel on a PET foil, while (f) proves that the ECD functionality can bemaintained after reconfiguration. To fully demonstrate the concept, the color state of an ECD can be controlled by the conduction state of an ECT in an ECsmart pixel device (g), which is obtained by delamination and reconfiguration of the (f) subcomponent onto the (e) device. Finally, this is further evidencedby simply duplicating the EC smart pixel device onto another substrate by utilizing the reconfiguration technique (h) and thereafter recording identical I–Vcharacteristics as compared to device (g).

J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069 3065

3.2.2. ECT characteristicsThe current–voltage (I–V) output curves for the ECT

device (Fig. 2(e)) are shown in Fig. 5. The ECT is operating

in depletion mode, which implies that the maximumon-current through the transistor channel is obtainedwhen the gate voltage (VG) is 0 V. This is due to that the

Fig. 3. The top view of the ECT/EC smart pixel is illustrated. The drain andsource electrodes are 20 � 20 mm2 squares and they are connected by anarrow rectangle having an area of 0.4 � 10 mm2. Approximately 3 mm ofthe length of the rectangle is covered by the electrolyte and the gateelectrode bilayer, thereby forming the active area of the ECT channel,while an area of 2 � 2 mm2 of the bilayer sheet was attached at the edgeof the drain electrode such that the bilayer serves as the coloring pixelelectrode and the drain electrode serves as the counter electrode of the ECsmart pixel device. Note that the VDS is connected between the drain andsource electrodes in the case of an ECT measurement, while the sourceand pixel electrodes are connected in the EC smart pixel measurement.

Table 1ECD color switch contrast measurements based on CIE L*a*b* colorcoordinates. The numbers 1 and 2 indicated by the subscripts beside L*,a* and b* represent the decolored (oxidized light blue) and the colored(reduced deep blue) state of the pixel top electrode, respectively.

Device L�1 a�1 b�1 L�2 a�2 b�2 DE*

(a) 58.16 �4.05 �7.55 28.83 �0.22 �26.68 35.2(b) 63.48 �4.08 �6.10 43.39 �1.49 �21.19 25.3(c) 37.15 �12.7 3.48 17.28 �3.23 �17.2 30.2

Fig. 4. Photographs showing two different display architectures that havebeen characterized: (a) lateral symmetric ECD based on two P:PSS-NFCelectrodes bridged by Plyte-NFC, and (b) complementary vertical non-symmetric ECD consisting of Plyte-NFC sandwiched by one layer of P:PSS-NFC and a drop cast layer of PANI. No photograph is available for thevertical symmetric ECD based on opaque Plyte-NFC sandwiched by P:PSS-NFC on both sides.

-0.10-0.09-0.08-0.07-0.06-0.05

-1.0 -0.5 0.0-8-7-6-5-4-3-2-10

-1.0 -0.8 -0.6 -0.4 -0.2 0.0VDS / V

I DS

/ µA

Fig. 5. I–V characteristics of an ECT utilizing Orgacon as the transistorchannel and the drain and source electrodes, Plyte-NFC as the electrolyteand P:PSS-NFC as the gate electrode. The inset graph indicates the off-current levels at elevated gate voltages.

3066 J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069

PEDOT:PSS is electrically conducting in its pristineoxidized state, hence, the channel is switched to its off-state upon applying a positive VG. Here the electric currentthrough the channel between the drain and source elec-trodes (IDS) was recorded by sweeping the drain-sourcevoltage (VDS) from 0 to �1.0 V at an incremental step of0.01 V for 6 different VG; 0 V at the first cycle and thenincreased in steps of 0.25 V until the last sweep at 1.25 V.The minimum off-current level is reached already atVG = 0.75 V, which can be observed by that the curves rep-resenting VG = 0.75, 1 and 1.25 V behave similarly, see insetof Fig. 5. The time between two adjacent data points was10 ms. An on/off-ratio of approximately 100 was observedand the off-current level was about 60–80 nA. Both thesevalues, especially the low off-current, are sufficient forthe operation of an active matrix addressed display, seenext section. Previous research projects related to PED-OT:PSS-based ECTs and ECDs have shown a number ofswitch cycles exceeding 104 and 105, respectively, there-fore we have no reason to believe that the devices reportedherein would behave differently, even though no evalua-tion of the operational lifetime was performed. However,

the ECT device did not show any issues regarding delami-nation or degradation upon storage in ambient atmosphere(22 �C and 40%RH) for 1 month. Fig. 6 shows an ECT mea-sured 19 and 31 days after device assembly. The relativelyconstant on- and off-current levels of the ECT demonstratehigh degree of stability over extended periods of time.

3.2.3. EC smart pixel: Integration of ECD and ECTThe function of the EC smart pixel, which herein is de-

fined as the circuit integration of one ECT and one ECD, isvalidated by using two previously reported measurementmethods [12]. The two methods are chosen in order toinvestigate the two-folded functionality of the ECT in thesmart pixel configuration; prevention of cross-talk along

-10

-8

-6

-4

-2

0

-1.0 -0.5 0.0

-0.04

-0.03

-0.02

-0.01

-1.0 -0.5 0.0

-10

-8

-6

-4

-2

0

-1.0 -0.5 0.0VDS / V

I DS

/ µA

VDS / V

I DS

/µA

-0.05

-0.04

-0.03

-1.0 -0.5 0.0

Fig. 6. An ECT showing identical I–V characteristics 19 (left) and 31 (right) days after device manufacturing, respectively. The inset graphs indicate the off-current levels at elevated gate voltages.

-25-20-15-10-505

10152025

0 20 40 60 80

Time /s

Cur

rent

thro

ugh

the

pixe

l /µ A

Fig. 8. The ECT shows its capability to maintain the color state of the ECD.First, the pixel is switched to its colored state by VDS, which is shown inthe first current peak at t � 3 s. Subsequently VG is set to 1 V and thetransistor channel switches off to its reduced state at t � 20 s, whichgenerates the second current peak. At t � 32 s VDS is set to 0 V, whichresults in maintained color even though a small leakage current can beobserved. When VG is set to 0 V at t � 43 s, the ECD is decolored by thedischarging current indicated by the positive current peak.

J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069 3067

the addressing lines of the ECD devices in an active-matrixdisplay and to improve the retention time of an ECD thathas been updated to its colored state. The former is evalu-ated by monitoring that the pixel does not switch from itstransparent off-state to its colored on-state upon applyingVDS and VG (the ECT is in its off-state) simultaneously. Thelatter feature can be evaluated by applying VG after switch-ing the ECD to its on-state, followed by turning off VDS,which results in color retention of the ECD introduced bythe non-conducting off-state of the ECT channel. Figs. 7and 8 show the results of the two ECT–ECD smart pixelfunctionalities.

Fig. 7 indicates that the ECD is switched on only whenVDS is turned on and when VG is 0 V (the ECT is in its on-state), as evidenced by the low leakage current flowthrough the EC smart pixel while the ECT channel is non-conducting. This is a required feature in order to preventcross-talk effects along the EC pixel addressing lines inan active matrix addressed EC display; only one ECT perEC pixel addressing line is allowed to be in its conductingstate. Fig. 8 demonstrates that the conduction state cancontrol the retention time of the EC pixel. This has a directimpact on the power consumption in large active-matrixdisplay systems. EC pixels are semi-bistable to their nature,

-30

-15

0

15

30

0 20 40 60Charging

Discharging

-20-18-16-14-12-10-8-6-4-20

0 20 40 60 80Time /s

Cur

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thro

ugh

the

pixe

l /µ

A

Fig. 7. The ECT shows that it can prevent ECD cross-talk. The first currentpeak at t � 10 s corresponds to electrochemical reduction of the channelinto its non-conducting state by applying VG. VDS was applied at t � 25 sbut no current peak due to pixel coloration can be observed, whichindicates that the off-state of the ECT is capable of keeping the ECD in itsinitial off-state. Finally, VG = 0 V is applied at t � 45 s, which brings theECT channel to its on-state that, in turn, allows for pixel coloration by theconstantly applied VDS. The inset graph shows the charging and discharg-ing behavior of an independent ECD.

thanks to their electrochemical and impedance character-istics. Further, the ability to control the color state of eachEC pixel in a larger system is advantageous; not only be-tween the on- and off-states but also to enable gray-scalelevels. Here, the negative current peaks in Figs. 7 and 8 cor-respond to electrochemical reduction, i.e. switching to thenon-conducting state and deep-blue coloration, of the PED-OT:PSS-based ECT channel or EC pixel electrode, while thepositive current peak instead represents oxidation into theconducting and transparent form of the PEDOT:PSS in theECT channel or EC pixel electrode. The area dependenceon the switching time can be observed in the graphs; therelatively narrow current peaks correspond to the redoxreactions of the ECT channel and the relatively broadercurrent peaks represent color updates in the EC pixel.The switching time of the EC pixel is typically prolongedby high strain on the ECT channel, i.e. by simultaneouslyapplying VDS and VG results in a reduction front propagat-ing within the PEDOT:PSS channel outside the electrolyteedge. This is reflected in the broadened coloration currentpeak starting at t � 45 s in Fig. 7. There are several avail-able routes how to circumvent the issue with long switch-ing times in ECTs and EC smart pixels, this is however notthe focus of the present work [21].

3068 J. Kawahara et al. / Organic Electronics 14 (2013) 3061–3069

By combining the measurements and results given inFigs. 7 and 8 we draw the conclusion that the color stateof the EC pixel can be properly controlled by the appliedgate voltage to the ECT device, and practically no leakagecurrent is observed.

3.2.4. Reconfiguration test of NFC-hybridized electrochemicaldevices

Various kinds of electronic devices have been estab-lished by combining nanofibrillated cellulose (NFC) witheither a polyelectrolyte or a conducting polymer, e.g. elec-trochromic displays, electrochemical transistors, electro-chromic smart pixels and even electrolyte capacitors;components that together as integrated systems can formlarger electronic systems, such as active matrix addresseddisplays, indicators, sensors and more. The devices ob-tained here utilize NFC in order to create ionically and elec-tronically conducting films that are self-supporting, i.e.functionalized substrates. In addition to this, it is also pos-sible to integrate, disintegrate and reconfigure the initiallycreated subcomponents by using a ‘‘cut, stick and peel’’technique similar to what is being used in a sticker book.

Reconfiguration of the gate electrode and the pixel elec-trode in the respective ECT and EC smart pixel devices,which resulted in a second set of devices, showed mostlyidentical performance as compared to the first set of de-vices shown in Figs. 5, 7 and 8 (data not shown). This dem-onstrates the robustness of the interfaces that areestablished upon reconfiguration of the functionalizedsubstrates.

The system integration concept is also demonstrated bythe measurement shown in Fig. 9. An electrochemicalcapacitor is formed by using a layer of Plyte-NFC as theintermediate layer, Orgacon as one of the electrodes anda layer of PEDOT:PSS-NFC as the other electrode. The latterelectrode is then electrochemically reduced by applying avoltage of �5 V for �10 s. The capacitor structure was thenpeeled apart by separating the bilayer from the Orgaconsheet, and the remaining bilayer was instead reconfigured

-80

-60

-40

-20

0

0 20 40 60 80 100

Time /s

I DS

/µA

Fig. 9. The graph shows the current modulation of an ECT channel. Thegate electrode was pre-charged adjacently to the ECT device, peeled offand reconfigured on top of the ECT channel. Before laminating the pre-charged gate electrode and its electrolyte layer, IDS � 55 lA can beobserved during the first few seconds of the measurement. Uponreconfiguration of the charged gate electrode, IDS decreases to �10 lA;far from the most optimized on/off-ratio in terms of transistor perfor-mance but yet it proves the concept of system integration, reconfigura-tion and energy transfer by using the peel and stick technique.

to become the electrolyte and the gate electrode of an ECTby simply sticking it onto a 0.4 � 3 mm2 PEDOT:PSS-based(Orgacon) channel. Prior to the attachment of the pre-charged gate electrode and its corresponding electrolyte,an on-current exceeding 50 lA was flowing between thesource and drain electrodes (VDS = �1 V). A spontaneousdischarging of the gate electrode occurred, immediatelyafter lamination, which then resulted in current modula-tion of the depletion mode ECT channel. Hence, the energystored in the pre-charged conducting polymer was suffi-cient to alter the conduction state of the ECT device. Theresults demonstrate that organic electronic systems canbe integrated, disintegrated and reconfigured into variousflexible autonomous electronic systems by simple meansand that information can be transferred between differentdevices.

4. Conclusions

A new concept for system integration of subcompo-nents for flexible electronics is demonstrated. The ‘‘cut,stick and peel’’ technique increases the degree of freedomto attach, detach and reconfigure electronic subcompo-nents located on a common substrate, making it a novelmethod for manufacturing of a variety of printed electronicsystems that minimizes the number of materials and pro-cessing steps. The system integration technique is obtainedby casting NFC with an electrochromic material or an elec-trolyte phase into a self-supporting film, which bringseither electronic or ionic functionality into the film. Elec-trochemical devices are then easily created by stackingthe self-adhesive films on top of each other. An electro-chromic display pixel without ordinary plastic substrate(e.g. PET film), an electrochemical transistor where theNFC hybrid layers stick to the solid PEDOT:PSS channeland an electrochromic smart pixel composed of one pixeland one transistor were all successfully achieved. The com-ponents functioned well after performing the peel andstick reconfiguration of the NFC layers, and they alsoshowed very good stability with respect to storage timein ambient atmosphere. The unique ability to integrateand reconfigure electronic systems based on self-support-ing subcomponents will pave the way for the creation ofmore advanced flexible electronic circuits.

Acknowledgements

This work was supported by the Knut and AliceWallenberg Foundation (Power Papers).

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