improving the contrast of all-printed electrochromic polymer on paper displays

improving the contrast of all-printed electrochromic polymer on paper displays

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Improving the contrast of all-printed electrochromic polymer on paper displays

Payman Tehrani,a Lars-Olov Hennerdal,c Aubrey L. Dyer,b John R. Reynoldsb and Magnus Berggren*a

Received 19th November 2008, Accepted 3rd February 2009 First published as an Advance Article on the web 16th February 2009 DOI: 10.1039/b820677e

PEDOT:PSS-based electrochromic displays have been explored for manufacture on flexible paper substrates in roll-to-roll printing presses at high volumes and low costs. Here, we report the improvement of the optical contrast of such devices by adding an extra layer of a dihexyl-substituted poly(3,4-propylenediox- ythiophene) (PProDOT-Hx2) to complement the optical absorption spectrum of PEDOT:PSS. The oxidized state of PProDOT-Hx2 is highly transparent and is an intense magenta color while in the reduced state. By adding a layer of PProDOT-Hx2 directly on top of PEDOT:PSS, we were able to improve the optical contrast by nearly a factor of two. In this report, we present optical and elec- trochemical data of PProDOT-Hx2/PEDOT:PSS-based electrochromic paper displays and compare their performance with

PEDOT:PSS-only equivalents.

Over the centuries,paper has served our societyas the main carrier for writteninformationand documentation. Associated with paper, a vast array of printing tools, coatingtechniques and other conversion techniques have been developedoffering the abilityto manufacturepaper-basedproductsathighvolumesandlowcosts.Thishas led to a vast improvementin the accessibility of information in our modern society. Overthelast 50 years,theelectronics revolutionhas made a tremendous impact on dailylife with respect to processing, transfer and displaying of information. The next natural leap would be to merge the two worlds of paper and electronics, creating combined features that cannot be achieved in either of the two formatsalone.

Several attemptshave been carriedout towardsachievingvarious kinds of e-paper technologies,in part pioneered by consumerproductscompaniessuchas Philips,andSonyInc,amongothers,andby technology providers such as E-ink. In these technologies,electrophoretic inks,1 Gyricon balls2or electro-wetting3 are used as the fundamental electronic colorant system on large area planar substrates.This technology allows for high resolution and good performance, but at a considerably higher cost when compared to ordinary printed paper. On the other hand, electrochromic (EC) displays4,5 can be fabricated usinga much simpler and robustdevice architecturewhen compared to the above-mentioned display techniques presently being considered for e-paper technologies. The developmentof organicEC materialsthat can merge with the well establishedtechnologyof paper substratespromises forhigh-volume manufacturingof paperdisplays.

Among theorganicEC systems,poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS),6,7 has received muchattention duetoitshighconductivityandchemicalstability.By electrochemicalmeans,the doping level of PEDOT:PSS filmscan be reversiblycontrolled,resultingin switchingof the bulkconductivity, as well as the color,8 of the film. The redox control of conductivity and color can be utilized in electrochemical transistors9 and EC displays,10respectively.ECswitchingof PEDOTis relativelyfastand fullyreversiblebuthighercontrastmustbedevelopedforapplications in monochromaticdisplays and electronicindicators.In its oxidized statePEDOT hasa lightblue,closeto transparent,colorwhilein its reduced state it is dark blue in color with an absorptioncentered around 650 nm as shown in Fig. 1. Careful examinationof these resultsshowsthe, as expected, low absorptivityof the oxidized form from350to850nm. Thecoloredstateisevidentuponreductionwith growth of 650 nm absorption. Interestingly, having PEDOT:PSS printeddirectly onthe papersubstrate (asopposedtousinga metallic contact electrode)inhibits full reduction of the polymerto its electricallyinsulating form.As such, the reduced form in Fig. 1 retains absorptionat 800–900nm due to polaronicchargecarriers,and the remnantconductivity allowsthematerialtobe repeatedlyswitchedas an electrochrome.

In previous studies, an anodically coloring polymer has been utilizedon the counter electrode in a verticalstructurein order to increasethe opticalcontrast.1,12 However,in this study we increase theopticalcontrastbyapplyinganadditionallayerofanECpolymer with complementary color characteristics to PEDOT in a lateral design, further creating a simplified device from a manufacturing

Fig. 1 Absorption spectra of reduced and oxidized PEDOT:PSS films.Organic Electronics, Department of Science and Technology (ITN)

Link€oping University, SE-601 74 Norrk€oping, SwedenThe George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and

Engineering, University of Florida, Gainesville, FL 32611, USAAcreo AB, Bredgatan 34, SE-601 21 Norrk€oping, Sweden

This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 1799–1802 | 1799

COMMUNICATION w.rsc.org/materials | Journal of Materials Chemistry point of view.This complementarycolor propertyrequires a highly transmissive oxidizedstate,alongwitha deeplycoloredreducedstate havinga differentcolor(shiftedlmax) relativeto PEDOT:PSS. Previously, we explored various polythiophene derivatives to enhance the contrast of PEDOT:PSS-based displays13 with the contrast values calculated using the CIE Lab color system from measurements of the transmission spectra. The previous study highlightedseveral factors of importance while choosing an EC polymerfor use in combination with PEDOT,such as the need for the materialto have a low oxidation potential, high ionic conductivity,and spectral absorptioncomplementaryto PEDOT. In this current study, we report a bi-layer display structure that includes

PProDOT-Hx2coatedontopofPEDOT:PSSfilms,wheretherepeat unitstructuresof these two componentsare givenin Fig. 2, and the optical switch contrast characteristics and its associated dynamic electricalswitchparametersare measured. Indium tin oxide (ITO) coated glass substrates were rinsed in isopropanol,acetoneanddeionizedwaterbeforeuse.PProDOT-Hx2 wasdissolvedinchloroformataconcentrationof10mg ml 1andthe solutionwasspin-coatedon thesubstrateat 1000rpm.PEDOT:PSS (400 nm thick film) coated on paper substrateswere supplied by Agfa-Gevaert(see Fig. 3a). For electrochemistry measurements,the liquid electrolyte was an aqueous solution of 0.1 M LiCF3SO3.A gelled electrolyte was utilized for the displays, consisting of 7% hydroxyethylcellulose,64% water, 21% glycerol,7% sodiumcitrate and1% LiCF3SO3. ThePEDOT:PSS-coatedpapersubstrateswerepatternedusingthe previouslydetailedsubtractivepatterningmethodby using a Nilpeter RotolabelFA3300/5 label printer14 modified with a rotary screen printer,as shown in Fig. 3b. A UV-curableelectrolyte,consisting of a commercialUV-lacquermixedwith salt(1%NaCl),was patterned through the rotary screen mesh with a conducting squeegee. The squeegee was biased as the cathode while the PEDOT:PSS film was anodically biased. A 150 V potential is applied between the PEDOT:PSSfilmandthesqueegee,andwherethePEDOT:PSSisin contact withthe electrolyte,itbecomes irreversiblyover-oxidizedand permanentlylosesitselectronicconductivity.Theelectrolyte wasthen exposed to a UV source and cured, substantially reducing its conductivity.The resulting resistance between the electrodes in the displaywas typicallygreaterthan 1 MU. This processallows for the PEDOT:PSSe lectrodest ob ep atterned at manufacturings peeds from 5t o 10m min 1.

The PProDOT-Hx2 filmswerethen spin-coatedfrom chloroform solutionsat variousconcentrationsat 1000 rpm onto the PEDOT electrodes(resultingin differentthicknessesfrom5 nmto160 nm).A 200 m thick plastic film, containingholes for the electrolyte,was then laminated onto the films to serve as a gasket for the gelled electrolyte.Theholeswerethenfilledwiththeaqueouselectrolyteand heated at 50 C for 2 min to curethe gelledelectrolyte.An adhesive tape was used to encapsulate the device for both mechanical protection andto prevent dryingoutof the electrolyte.Electricaland optical characterizations were performedfor the EC displaysusing a test pixel configuration (see Fig. 2a), which includes adjacent PEDOT:PSSelectrodespatternedviaelectrochemicalover-oxidation.

Three-electrodeelectrochemicalcellmeasurementswereperformed with platinized titanium as the counterelectrodeandAg/AgCl as the reference electrodewith a 0.1 M aqueous solutionof LiCF3SO3 as the electrolyte.Absorption spectroelectrochemicalmeasurementsof the PProDOT-Hx2 films coated on ITO were measured with a Lambda 900 UV-Vis–NIRspectrometer with varyingpotentials applied using a 283 potentiostat (EG&G Princeton Applied Research). To achieve electrochromic switching of the PEDOT:PSS/PPro-

DOT-Hx2 display,a potentialdifference of 1.5 V was appliedto the electrodes and 1.0V wasappliedfor thePEDOT:PSS-onlydisplays.

The color coordinates,of the displays, were measured with a handheld DatacolorMicroflashspectrophotometer,which yieldsthe L*, a*a nd b* coordinatesof coloredsamples.The measurementswere conductedinthegloss-trapmode,whicheliminatedspectralreflection occurringalong the plasticencapsulationfilm. The color contrastof the display cells, while switchedbetweenthe reduced and oxidized state,respectively,was calculatedfrom eqn (1).

DE* ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDL*Þ2þðDa*Þ2þðDb*Þ2 q (1)

A more in-depth description of this measurement and analysis method has been published previously.13,15 Images of the displays wereobtainedwitha commercialflatbedscanner, CanoScan 9900F fromCanon. The switching speedmeasurements wereperformed in a reflectivesetupwherethe incident light was directedat an angle of 45 to the displaysurface,and the reflectedlightwas collectedin the normaldirection. The reflected lightwas collectedin realtime using an Andor Shamrock spectrograph (SR303i) including an Andor NewtonCCDdetector(DU940N).

Spectroelectrochemicalmeasurements of PProDOT-Hx2 on ITO (Fig. 4) show switching of the absorption spectrum at positive potentials.The absorptionpeak at 550 nm disappearscompletelyat

Fig. 2 (a) The device architecture of the PEDOT:PSS/PProDOT-Hx2 paper display. The repeat unit structure of (b) PEDOT:PSS and (c)

PProDOT-Hx2.

Fig. 3 (a) PEDOT:PSS-coatings on photo paper were supplied by AGFA-Gevaert N.V. and were used in the roll-to-roll printing press. (b) The rotary screen printing unit is slightly modified to allow for electrochemical over-oxidation of the PEDOT:PSS film in the roll-to-roll printing press.

1800 | J. Mater. Chem., 2009, 19, 1799–1802 This journal is ª The Royal Society of Chemistry 2009

0.90V vs. the Ag/AgCl referenceelectrode,with initialformation of a polaronicabsorption bandin the infraredregion(ca. 900–950nm), followed by a broad bipolaronic absorption at higher applied potential.Cyclicvoltammetrydata of PProDOT-Hx2 is givenas an inset to Fig. 4 and shows that the polymer undergoesa reversible oxidation reaction with a current peak at 0.90 V. In a bi-layer structureontopofPEDOT:PSS,thePProDOT-Hx2shouldimprove the coverage of the absorption spectrum of the reduced stateof the

PEDOT:PSS,while the oxidized state of both polymers is highly transparent. The color switch contrast of the displayswas maximized by opti- mizingthethickness of the PProDOT-Hx2 filmleavingthethickness ofthecoatedPEDOT:PSSbottomfilmunchanged. Thethicknessof the PProDOT-Hx2 film cast from the 10 mg ml 1 solutionontothe PEDOT:PSS-coatedplastic was measured using a DecTak profil- ometer, while the thickness of the remaining films was calculated from theirabsorbance values relativeto the 10 mg ml 1 sample.In

Fig. 5 the opticalswitch contrast,DE*, of the displays is shown as afunctionofPProDOT-Hx2thicknessesontopofPEDOT:PSS.The optical switch contrast was found to be maximizedfor films spin- coated from the 10 mg ml 1 solution, thus, this thickness combina- tion,(tPEDOT:PSS ¼ 400 nm, tPProDOT-Hx ¼ 80 nm) was selected for further testing. The associated color switch contrastwas increased from 29 for the PEDOT:PSS-only displays to 54 for the bi-layer displayas the color changed from the blue of the PEDOT:PSS to a deep purple and almostblack. The L*, a*, b*c oordinates fort he oxidized andreduced states of theoptimizedbi-layerdisplayare(60, 5, 10) and (29, 16, 32),respectively. The bilayer of PProDOT-Hx2 and PEDOT:PSS was further evaluated as a display element in a patternedPEDOT:PSS-coated paper display as shown in Fig. 6. It can be seen that the device containing the bilayer has an improved optical switch contrast, especiallyon comparingthereducedand oxidizeddisplayelements of theprinteddisplay.Mostnotably,theaddedlayerof PProDOT-Hx2 does not decreasethe reflectivityin the paper display when in the oxidized state, while providing an increase in the absorption and darkeningof the pixelswhileswitched to the reducedstate. It is expectedthat the switchingspeed of the PEDOT:PSS/PPro-

DOT-Hx2 display will be reduced in comparison to the switching speed of the PEDOT:PSS-only displays,due to the fact that there is relativelymore material to be electrochemicallyswitched during each color switchingcycle.The dynamicsof the switchingcurrentduring cyclingof thebi-layerstructuredisplayprovidedsomeinsightintothe electrochemical switchingdynamics of the display.To monitorthe switchingoftheindividualpolymerfilmswithinthebi-layerstructure, the reflected light at wavelengths corresponding to each of the absorption peaks (550n m for PProDOT-Hx2 and 640 nm for PEDOT)wasmeasured andisshown as a functionof timeinFig.7.

As expected, the PEDOT-only displaysdo not show a noticeable differencein switchingtimes for the 550 nm and the 640 nm-wavelengths(Fig.7a).Definingtheswitchingtimeas achieving90%color saturation of a full switch at a specific applied voltage, the PEDOT:PSS-onlydisplayswitches in 2.1stotheOFF-stateand1.7s to the ON-state.The addition of PProDOT-Hx2 to the displaycells decreasestheswitchingtimeoftheoveralldisplayto6.7stotheOFF- stateand4.5s totheON-state (Fig.7b).However,theslowswitchto theOFF-stateisnotduetotheswitchingspeedofthePProDOT-Hx2

Fig. 4 In situ spectroelectrochemical measurement of PProDOT-Hx2 on ITO. The inset shows a cyclic voltammetry measurement in 0.1 M KCl aqueous electrolyte. The potentials are measured relative to Ag/AgCl reference electrode.

Fig. 5 Contrast of PEDOT displays with different thickness of PPro-

DOT-Hx2 on top. The pictures show the oxidized and reduced state of the pixel. The measurements were done on paper display with gelled elec- trolyte by applying 1.5 V to reduce and oxidize the pixels (except for the PEDOT-only display where only +1.0 V was applied to fully oxidize the PEDOT-film.).

Fig. 6 Seven-segment displays manufactured in the printing press. The added layer of PProDOT-Hx2 in the left display elements enhances the optical contrast compared to the pure PEDOT:PSS display on the right.

This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 1799–1802 | 1801

(550nm),which switches nearlyas fastas thePEDOTin Fig.7a,but is ratherattributedto a slow switchingof thePEDOT:PSS film (640 nm) underneath. We attribute this to the relatively poor ionic conductivity of the PProDOT-Hx2 in comparison to the PEDOT:PSS film and therefore it functions as a barrier to ion diffusion from the electrolyteto the PEDOT:PSS film underneath. Onemethod to overcomethisissueand increase theswitchingspeed of the device would be to improve the ionic conductivity of the

PProDOT-Hx2phaseorcreateamoreopenmorphologytoallowion diffusion to occur to the underlyingPEDOT:PSSlayer.

Additionally,for the ON-switch (see Fig. 7b), the PEDOT:PSS and on close inspection the oxidation of the PProDOT-Hx2 film occursafterthePEDOT:PSS filmhasbeenfullyoxidized.Thisisdue to the fact that the reduced PEDOT:PSS film has a low electronic conductivityandactsas a poor electrodefor switchingthe ProDOT-

Hx2 layer to its reduced state as we have illustrated in the past for conducting polymer bilayers.16–18 For the ON-switch, the

PEDOT:PSSfilm is first oxidized, increasingin conductivity, allowing the supply of electronic charges, transportedlaterally,to the

PProDOT-Hx2 film coatingon top. Another main contributor to the switching speed of aP EDOT:PSS-based displayi st he ionic conductivity/diffusion characteristicsof the electrolyteIn this study the electrolyte utilized was one that allowed for stable switching of the devices for comparative purposes and was adaptable for the manufacturing processesemployed for this specificdevicefabrication.Further work willfocuson thescreeningof additionalelectrolytesthatstillallowfor ease of fabrication and long devicelifetime while increasingelectrochemical switching of the device.

ThemanufacturingofPEDOT:PSS-basedEC displaysisrelatively simple and involves only three printing steps: Patterning PEDOT:PSS, electrolyte deposition and device encapsulation. Because of this simple manufacturingprocess, the EC displaysare promisingas low-cost displaysandindicatorsinvariousapplications. In this paper we have improved the optical contrast of the PEDOT:PSS paper display by the additionof an added layer of

PProDOT-Hx2 in a bi-layerstructure. As the synthesis of this polymercanbescaleduptomanufacturelargequantitiesandthepolymer is solution processable,it can be incorporated in the PEDOT-based display devices using the roll-to-roll manufacturing method.

We found that the switching speed of the contrast-enhanced displayswasslowerthanthePEDOT:PSS-onlydisplaysandattribute thisto the poor ionicconductivityof the PProDOT-Hx2 film in the aqueous electrolyte employed. From previousstudies,13 it has been shown that oligoethylene oxide side groups can improve the ionic conductivityofthepolymerfilm.Other routestoincreasingswitching speed are to induce a moreopen morphology in the PProDOT-Hx2 film,usetwopolymersthatareoptimizedforthesameelectrolyteand possiblyblend the twofilms; all ofwhich will be investigated in future studies.

Acknowledgements

TheUniversityof Floridaacknowledgesthe AFOSR(FA9550-06-1- 0192)forfunding.JRRappreciatestheeffortsoftheresearcherswho developed and carried out the synthesis of the PProDOT-Hx2 employed in this work. The Link€oping University acknowledge financial support from the Swedish Foundation for Strategic Research, the Swedish ResearchCouncil, VINNOVA,The Royal Academy of Sciences and Knut and AliceWallenberg Foundation.

References

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J. C. Crowley, K. A. Oraha, M. E. Howerd, M. A. Rodkin, R. Swidler and R. Sprague, J. Soc. Inf. Display, 1999, 7, 141–145. 3 R. A. Hayes and B. J. Feenstra, Nature, 2003, 425, 383–385. 4 C.-G. Granqvist, Nat. Mater., 2006, 5, 89–90. 5 G. Sonmez, H. Meng and F. Wudl, Chem. Mater., 2004, 16, 574–580. 6 B. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik and

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