Textile Based Electrochromic Cells Prepared with PEDOT: PSS and Gelled Electrolyte

sensors

Letter Textile Based Electrochromic Cells Prepared with PEDOT: PSS and Gelled Electrolyte

Carsten Graßmann 1,2,* , Maureen Mann 1, Lieva Van Langenhove 2 and Anne Schwarz-Pfeiﬀer 1

1 Research Institute for Textile and Clothing, Niederrhein University of Applied Sciences, 41065 Mönchengladbach, Germany; [email protected] (M.M.); anne.schwarz-pfeiﬀ[email protected] (A.S.-P.) 2 Center for Textile Science and Engineering, Ghent University, 9052 Ghent, Belgium ; [email protected] * Correspondence: [email protected]; Tel.: +49-2161-1866197

Received: 31 August 2020; Accepted: 3 October 2020; Published: 6 October 2020

Abstract: Electrochromic devices can act as passive displays. They change their color when a low voltage is applied. Flexible and bendable hybrid textile-ﬁlm electrochromic devices with poly-3,4-ethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) were prepared on polyethylene polyethylene terephthalate (PEPES) membranes using a spray coating technique. The electrolyte consisted of a gelatin glycerol mixture as host matrix and calcium chloride. Titanium dioxide was used as an ion storage layer and a carbon containing dispersion was used for the counter electrode on a polyester rip-stop fabric. The sheet resistance of PEDOT:PSS on PEPES was 500 Ohm/sq. A 5 5 × electrochromic matrix with individually addressable pixels was successfully designed and assembled. The switching time of the pixels was 2 s at a voltage of 2.0 V directly after assembling. The use of titanium dioxide as ion storage also increased the contrast of the dark-blue reduced electrochromic layer. Coloration was not self-sustaining. The PEDOT:PSS layer needed a constant low voltage of at least 0.5 V to sustain in the dark-blue reduced state. The switching time increased with time. After 12 months the switching time was ~4 s at a voltage of 2.8 V. The addition of glycerol into the electrolyte extended the lifetime of a non-encapsulated textile electrochromic cell, because moisture is retained in the electrolyte. Charge carriers can be transported into and out of the electrochromic layer.

Keywords: smart textile; ﬂexible device; passive display; multilayer matrix structure; gelatin electrolyte; alginate

1. Introduction Reversible color changing materials can be grouped according to the external stimulus, which induces the color change. For instance, hydrochromic materials react to the presence of water or water vapor, thermochromic materials react to the change of temperature, mechanochromic materials to mechanical stress, photochromic materials to illumination, and electrochromic materials to a voltage [1–4]. Electrochromic materials have been considered for decades. Color changing materials can not only be used as decorative elements, but they can also act as actuators. Electrochromic materials change their color reversibly depending on their current redox state. Prominent examples are electrochromic windows for buildings and rear-view mirrors in automotive applications [5]. Generally, the following performance parameters for electrochromic materials are of interest for potential applications: electrochromic contrast, coloration eﬃciency, write-erase eﬃciency, switching speed, stability, cycle life, and optical memory [6]. The electrochromic contrast or contrast ratio characterizes the transmittance of the electrochromic material in its bleached (colorless) state to the transmittance in the colored state. The ratio of the absorbance change to the charge carriers injected per

Sensors 2020, 20, 5691; doi:10.3390/s20195691 www.mdpi.com/journal/sensors Sensors 2020, 20, 5691 2 of 14 electrode area is described by the coloration efficiency. The percentage of the originally formed colored state that subsequently can be electrochemically bleached is defined as the write–erase efficiency. The switching speed, also referred to as response time in some instances, is generally described as the time an electrochromic material requires for some fraction of the color to form or become bleached. The electrochromic stability is generally associated with the electrochemical stability. The degradation of the active redox couple leads to the loss of electrochromic contrast, and therefore the performance of the electrochromic material. The cycle life describes the number of write–erase cycles that can be performed whilst the material remains stable and without a noticeable decrease in performance. Optical memory means the materials can retain the new electrochromic state, when the redox state has been switched, meaning no further charge injection needs to be provided. In electrochromic devices, a persistent but reversible color change is induced by an electrochemical oxidation or reduction process [7,8]. Materials which undergo an electrochromic color change can be inorganic or organic compounds. A widely studied and probably the most prominent material is tungsten(VI) oxide [9]. Another inorganic electrochromic material is iron(III) hexacyanoferrate(II), more commonly known as Prussian Blue [10]. The color change in electrochromic cells is possible at low voltages between 1 and 5 V [11–13]. The exact value depends on the redox potential of the electrolyte and the electrochromic material, which are used. The color change sustains in most cases only when the device is supplied with a low voltage. However materials with a memory-effect exist. These materials do not need a constant power supply to sustain the oxidized or reduced state [14]. Compared to light emitting devices like electroluminescent and (organic) light emitting devices, which must be powered continuously, electrochromic devices consume less power and are economically favored. The color impression and the contrast are independent of the observation angle and surrounding brightness. Besides inorganic thin layers, organic and metal-organic compounds can also exhibit electrochromism. These compounds are, e.g., small molecules like benzothiadiazole fluorophores, bipyridylium salts (also called viologens), and electrically conductive polymers like derivatives of polyaniline (PAni) and polythiophene (PT) [15–22]. These polymers exist in a broad variety of oxidation states, which all appear in a different color, e.g. for PAni three different colored states are known: the colorless leucoemeraldine, green emeraldine and dark blue pernigraniline [18,23]. Poly-3,4-ethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) exists in two states. The oxidized cationic state it is light blue colored and semi-transparent, and when reduced to the neutral state it is dark blue (Figure1)[24]. Compared to inorganic compounds, the presented electrochromic polymers offer some advantages. They are flexible, easily processable, and can act both as an electrode and electrochromic layer, which reduces the necessary number of layers in the cell. PEDOT:PSS has attracted much attention as a conducting polymer in various application fields due to its high conductivity, and since it is easily processable, as a water-based dispersion [25–28]. The principle built-up of an electrochromic device is shown in Figure2. Between two electrodes, of which one carries the electrochromic material, an electrolyte is sandwiched. The electrolyte layer must provide the necessary charge mobility to reduce and oxidize the electrochromic layer, thus enabling the color change. For an optimal performance, the choice of electrolyte is crucial, because it acts both as an ionic conductor between the electrodes and as a reservoir for charge carriers moving into and out of the electrochromic layer [29]. Small molecules or ionic electrolytes dissolved in any solvent are not practicable for porous substrates such as fabrics due to evaporation. Therefore, different polymer electrolyte types have been investigated in the past. Among the liquid polymer electrolytes, poly diallyldimethylammonium chloride (PDADMAC) has been investigated [30]. An ionic gel polymer electrolyte for electrochromic cells was presented by Kobayashi and co-workers using polyvinyl butyral as host polymer and tetrabutylammonium perchlorate as ionic conductor [31]. Also, natural polymers based on starch, cellulose, and chitosan were investigated as a host matrix for electrolytes not only in electrochromic cells, but also for assembling capacitors, batteries, and solar cells [32–34]. Sensors 2020, 20, x FOR PEER REVIEW 2 of 15 per electrode area is described by the coloration efficiency. The percentage of the originally formed colored state that subsequently can be electrochemically bleached is defined as the write–erase efficiency. The switching speed, also referred to as response time in some instances, is generally described as the time an electrochromic material requires for some fraction of the color to form or become bleached. The electrochromic stability is generally associated with the electrochemical stability. The degradation of the active redox couple leads to the loss of electrochromic contrast, and therefore the performance of the electrochromic material. The cycle life describes the number of write–erase cycles that can be performed whilst the material remains stable and without a noticeable decrease in performance. Optical memory means the materials can retain the new electrochromic state, when the redox state has been switched, meaning no further charge injection needs to be provided. In electrochromic devices, a persistent but reversible color change is induced by an electrochemical oxidation or reduction process [7,8]. Materials which undergo an electrochromic color change can be inorganic or organic compounds. A widely studied and probably the most prominent material is tungsten(VI) oxide [9]. Another inorganic electrochromic material is iron(III) hexacyanoferrate(II), more commonly known as Prussian Blue [10]. The color change in electrochromic cells is possible at low voltages between 1 and 5 V [11–13]. The exact value depends on the redox potential of the electrolyte and the electrochromic material, which are used. The color change sustains in most cases only when the device is supplied with a low voltage. However materials with a memory-effect exist. These materials do not need a constant power supply to sustain the oxidized or reduced state [14]. Compared to light emitting devices like electroluminescent and (organic) light emitting devices, which must be powered continuously, electrochromic devices consume less power and are economically favored. The color impression and the contrast are independent of the observation angle and surrounding brightness. SensorsB2020eside, 20s , 5691inorganic thin layers, organic and metal-organic compounds can also exhibit3 of 14 electrochromism. These compounds are, e.g., small molecules like benzothiadiazole fluorophores, bipyridylium salts (also called viologens), and electrically conductive polymers like derivatives of Anpolyaniline alternative (PAni) host and matrix polythiophene for electrolytes (PT) used [15–22 in] these. These applications polymers exist is gelatin in a broad [35]. variety Gelatin of is oxidation a mixture states,of different which animal all appear proteins, in a different mainly denatured color, e.g. collagenfor PAni derived three different from cattle colored and states pig connective are known: tissue. the Aftercolorless immersing leucoemeraldine, gelatin in water, green it emeraldi swells andne dissolves, and dark depending blue pernigraniline on its production [18,23 process]. Poly- and3,4- ethylenedioxythiopheneexact composition when heatedpolystyrene above sulfonate 50 ◦C. Mixtures (PEDOT:PSS) of glycerol exists and in two gelatin states. are The known oxidized to keep cationic water statemolecules it is light in the blue film colored due to and the semi hygroscopic-transparent effect, and of when glycerol. reduced Furthermore, to the neutral the addition state it is of dark glycerol blue (Figincreasesure 1) the[24] water. vapor barrier of the film [36].

dark blue colored Sensors 2020, 20, x FOR PEER REVIEW 3 of 15 Oxidation Reduction Figure 1. Redox scheme for PEDOT: PSS electrode- 2 e- +(for 2 e- clarification reasons the section shows five conjugated monomer units).

Compared to inorganic compounds, the presented electrochromic polymers offer some advantages. They are flexible, easily processable, and can act both as an electrode and electrochromic layer, which reduces the necessary number of layers in the cell. PEDOT:PSS has attracted much attention as a conducting polymer in various application fields due to its high conductivity, and since it is easily processable, as a water-based dispersion [25–28]. light blue colored The principle built-up of an electrochromic device is shown in Figure 2. Between two electrodes, ofFigure which 1. oneRedox carries scheme the electrochromic for PEDOT: PSS electrodematerial, (foran electrolyte clariﬁcation is reasons sandwiched. the section The shows electrolyte ﬁve layer mustconjugated provide monomer the necessary units). charge mobility to reduce and oxidize the electrochromic layer, thus enabling the color change.

Figure 2. Schematic built-up of an electrochromic device with an electrochromic electrode. Figure 2. Schematic built-up of an electrochromic device with an electrochromic electrode. Most electrochromic devices are fabricated on rigid substrates. In the last decade electrochromic devices wereFor an also optimal produced performance on flexible, substratesthe choice such of electrolyte as elastomeric is crucial substrates,, because textiles it acts and both paper as [35 an–39 ionic]. Theconductor conductive between layer can the beelectrodes combined and with as flexiblea reservoir polydimethylsiloxane, for charge carriers moving whichallows into and the out use of of the tungstenelectrochromic oxide as electrochromic layer [29]. Small material molecule despites or its ionic brittleness. electrolytes dissolved in any solvent are not practicableColored andfor colorporous changing substrates fabrics such areas fabrics sometimes due desiredto evaporation. features Therefore not only, in different the design polymer of clothing,electrolyte but alsotypes in have technical been applications,investigated wherein the past. a color Among change the can liquid be even polymer more electrolytes important, for poly displayingdiallyldimethylammonium sensed information orchloride for camouflage (PDADMAC) purposes has [been29]. Ainvestigated change in the [30 state]. An of ionic system gel canpolymer be displayedelectrolyte on anfor electrochromic electrochromic display. cells was To display presented more by than Kobayashi only binary and information co-workers (on using/off), polyvinyl a pixel matrixbutyral with as individually host polymer addressable and tetrabutylammonium pixels is necessary. perchlorate With such as aionic matrix, conductor words or[31 numbers]. Also, natural can be displayed.polymers based The availabilityon starch, cellulose of the pixels, and allowschitosan for were the flexibleinvestigated display as ofa host any matrix information, for electrolytes and a predefinitionnot only in through electrochromic printing cells or coating, but also is notfor necessary.assembling capacitors, batteries, and solar cells [32–34]. AnSeveral alternative approaches host matrix for combination for electrol ofytes electrochromic used in these devices applications and textiles is gelatin have been[35]. presented Gelatin is a in themixture past years. of different Notably, animal Moretti proteins, and co-workers mainly denatured have reduced collagen the necessary derived from layers cattle for flexible and pig electrochromicconnective tissue devices. After [40,41 immersing]. gelatin in water, it swells and dissolves, depending on its production process and exact composition when heated above 50 °C. Mixtures of glycerol and gelatin are known to keep water molecules in the film due to the hygroscopic effect of glycerol. Furthermore, the addition of glycerol increases the water vapor barrier of the film [36]. Most electrochromic devices are fabricated on rigid substrates. In the last decade electrochromic devices were also produced on flexible substrates such as elastomeric substrates, textiles and paper [35–39]. The conductive layer can be combined with flexible polydimethylsiloxane, which allows the use of tungsten oxide as electrochromic material despite its brittleness. Colored and color changing fabrics are sometimes desired features not only in the design of clothing, but also in technical applications, where a color change can be even more important for displaying sensed information or for camouflage purposes [29]. A change in the state of system can be displayed on an electrochromic display. To display more than only binary information (on/off), a pixel matrix with individually addressable pixels is necessary. With such a matrix, words or numbers can be displayed. The availability of the pixels allows for the flexible display of any information, and a predefinition through printing or coating is not necessary. Several approaches for combination of electrochromic devices and textiles have been presented in the past years. Notably, Moretti and co-workers have reduced the necessary layers for flexible electrochromic devices [40,41].

Sensors 2020, 20, x FOR PEER REVIEW 4 of 15 Sensors 2020, 20, 5691 4 of 14 In this work, textile-based electrochromic cells with two different gel-based electrolytes are presented. The color-changing material herein is PEDOT:PSS, which also acts as the front electrode. ForIn the this presented work, textile-based textile-based electrochromicelectrochromic devices cells with, a five two-layer diﬀerent assembly gel-based is necessary electrolytes (Figure are 3). presented. The color-changing material herein is PEDOT:PSS, which also acts as the front electrode. For the presented textile-based electrochromic devices, a ﬁve-layer assembly is necessary (Figure3).

Figure 3. Built-up of a 3 3 electrochromic matrix with switchable pixels. × In addition to the electrolyte pixel, an insulating material can be applied in the same layer using Figure 3. Built-up of a 3 × 3 electrochromic matrix with switchable pixels. masking technology. This guarantees that only the desired pixels are switched on when a voltage is supplied.In addition Furthermore, to the electrolyte the insulating pixel, an layer insulating can be filledmaterial with can inorganic be applied pigments, in the same providing layer using an additionalmasking andtechnology. persisting This color. guarantees that only the desired pixels are switched on when a voltage is supplied. Furthermore, the insulating layer can be filled with inorganic pigments, providing an 2. Materials and Methods additional and persisting color. 2.1. Preparation of Gel Electrolytes 2. Materials and Methods 2.1.1. Electrolyte E1 2.1. Preparation of Gel Electrolytes Electrolyte E1 was prepared by mixing 10.0 g Polydiallyldimethylammonium chloride (PDADMAC)2.1.1. Electrolyte solution E1 (20% in water, Sigma Aldrich, Steinheim, Germany) and 1.0 g titanium dioxide (Kronos 2360, Leverkusen, Germany). In a second flask 0.2 g sodium alginate (VWR, Leuven, Belgium)Electrolyte were dissolved E1 was at 60 prepared◦C in 10 by ml deionizedmixing 10.0 water. g Polydially Both solutionsldimethylammonium were mixed and stirred chloride thoroughly(PDADMAC) resulting solution in a (20 viscous% in water liquid., Sigma Aldrich, Steinheim, Germany) and 1.0 g titanium dioxide (Kronos 2360, Leverkusen, Germany). In a second flask 0.2 g sodium alginate (VWR, Leuven, 2.1.2.Belgium Electrolyte) were E2dissolved at 60 °C in 10 ml deionized water. Both solutions were mixed and stirred thoroughlyTo a prepared resulting mixture in a ofviscous 1.0 g gelatinliquid. powder (microbiology grade, Merck, Darmstadt, Germany) in 14 mL glycerol (ACS reagent grade, Sigma-Aldrich, Steinheim, Germany), 6.0 mL deionized water, 2.1.2. Electrolyte E2 0.5 g titanium dioxide in rutile modification (Kronos 2360, Leverkusen, Germany), and 0.3 g calcium chlorideTo (97%, a prepared Carl Roth, mixture Karlsruhe, of Germany) 1.0 g gelatin were powderadded. After (microbiology homogenization grade, at room Merck, temperature, Darmstadt, theGermany) mixturewas in 14 swelled mL glycerol for one hour. (ACS Then reagent the mixture grade, Sigma was slowly-Aldrich, heated Steinheim, up under Germany) stirring to, 606.0◦ C mL fordeionized 1 min. The water electrolyte, 0.5 g titanium can be stored dioxide at RT in forrutile at least modification one month (Kronos and should 2360, be Leverkusen, warmed prior Germany to use.), and 0.3 g calcium chloride (97%, Carl Roth, Karlsruhe, Germany) were added. After homogenization 2.2.at Substratesroom temperature and Equipment, the mixture was swelled for one hour. Then the mixture was slowly heated up underThe stirring textile substrates to 60 °C for were 1 min kindly. The provided electrolyte by can Schmitz-Werke, be stored at RT Emsdetten, for at least Germany. one month The and coatings should werebe warmed performed prior on to a polyesteruse. (PES) plain weave fabric (200 g/m2) and a semi-transparent polyester rip-stop fabric (120 g/m2). Further, the fabrics were pre-coated with the polyurethane dispersion Tubicoat2.2. Substrates MEA (CHT and Equipment R. Beitlich, Tübingen, Germany) to avoid bleeding blurs of the coated and sprayed electrodeThe layers textile using substrates a 40-µm were roller kindly bar. Polyethylene provided by terephthalate Schmitz-Werke, (PET) Emsdetten, film with a Germany. thickness ofThe 25coatingsµm was were provided performed by Inoviscoat on a polyester GmbH, Monheim(PES) plain am weave Rhein, fabric Germany. (200 g/m²) A transparent and a semi polyethylene-transparent polyesterpolyester membrane rip-stop (PEPES) fabric (120 was g/m²). provided Further, by Sympatex, the fabrics Unterföhring, were pre-coated Germany. with Roller the bar polyurethane coatings weredispersion performed Tubicoat with MEA a 40 µ(CHTm gap R. sizeBeitlich, bar. Tübingen, Spray coating Germany) was performedto avoid bleeding with a blurs Revell of airbrushthe coated pistol.and sprayed Electrolyte electrode E1 was layers applied using with a a 40 pipette-µm roller whereas bar. thePolyethylene electrolyte terephthalate E2 was applied (PET) with film a heated, with a double-walledthickness of 25 syringe µm was (see provided Figure4 ).by Inoviscoat GmbH, Monheim am Rhein, Germany. A transparent polyethylene polyester membrane (PEPES) was provided by Sympatex, Unterföhring, Germany. Roller bar coatings were performed with a 40 µm gap size bar. Spray coating was performed with a

Sensors 2020, 20, x FOR PEER REVIEW 5 of 15

RevellSensors 2020 airbrush, 20, 5691 pistol. Electrolyte E1 was applied with a pipette whereas the electrolyte E25 was of 14 applied with a heated, double-walled syringe (see Figure 4).

Figure 4. Double-walled syringe for the application of electrolyte E2. Figure 4. Double-walled syringe for the application of electrolyte E2. The space between the walls was filled with a supersaturated sodium acetate trihydrate solution. The space between the walls was filled with a supersaturated sodium acetate trihydrate solution. The exothermic crystallization of sodium acetate trihydrate solution provides enough heat to ensure, The exothermic crystallization of sodium acetate trihydrate solution provides enough heat to ensure, that the electrolyte remains liquid during the application [42]. Since the crystallization is a reversible that the electrolyte remains liquid during the application [42]. Since the crystallization is a reversible reaction, the applicator is reusable after reliquification of the sodium acetate trihydrate. The substrates reaction, the applicator is reusable after reliquification of the sodium acetate trihydrate. The were laminated together using a colorless Toolcraft spray adhesive from Conrad Electronic SE substrates were laminated together using a colorless Toolcraft spray adhesive from Conrad Electronic (Hirschau, Germany). SE (Hirschau, Germany). 2.3. Coating of Electrode Layers 2.3. Coating of Electrode Layers Two methods were investigated and compared for the production of the electrode layers: 1) roller bar coatingTwo methods and 2) spray were coating. investigated In the andcase com of rollerpared bar for coating, the production the dispersions of the were electrode used as layers: received 1) rollerafter stirring. bar coating For and spray 2)coating, spray coating. the dispersions In the case were of roller diluted bar with coating isopropanol, the dispersions (Carl Roth, were Karlsruhe, used as receivedGermany) after in a stirring mass ratio. For of spray 1:1. coating, the dispersions were diluted with isopropanol (Carl Roth, KarlsForruhe, the Germany) bottom electrode, in a mass stripesratio of of 1:1. Tubicoat ELH (CHT R. Beitlich, Tübingen, Germany) were coatedFor onto the thebottom textile electrode substrate., strip Thees lengthof Tubicoat of the ELH stripes (CHT was R. 100 Beitlich, mm and Tübingen, they were Germany) 25 mm width. were Acoated roller onto bar withthe textile a gap substrate size of 40. Theµm length was used, of the the stripes distance was of 100 2.5 mm cm betweenand they the were stripes 25 mm was width. fixed A roller bar with a gap size of 40 µm was used, the distance of 2.5 cm between the stripes was fixed with a removable tape. The coating was dried at 100 ◦C for 120 s and annealed afterwards at 150 ◦C for with300 s. a removable tape. The coating was dried at 100 °C for 120 s and annealed afterwards at 150 °C for 300The s. transparent top layer serves both as the electrochromic layer and counter electrode. PEDOT:PSS (CleviosThe S transparent V4, Hereaus, top Leverkusen, layer serves Germany) both as was the coated electrochromi with a rollerc layer bar (40 andµm counter gap size) electrode. onto the PEDOT:PSSsubstrate, which (Clevios was S in V4, this Hereaus, case also Leverkusen, the semi-transparent Germany) PES was rip-stop coated with fabric. a roller PEDOT:PSS bar (40 wasµm alsogap size) onto the substrate, which was in this case also the semi-transparent PES rip-stop fabric. spray coated on PET film. The coating was dried at 100 ◦C for 120 s. In the second approach, it was PEDOT:PSSspray-coated was with also a mixture spray coated of PEDOT:PSS on PET andfilm. isopropanol The coating onto was the dried PEPES at 100 membrane. °C for 120 s. In the secondTo approach realize the, it patterning,was spray-coated a stencil with prepared a mixture beforehand of PEDOT:PSS was securely and isopropanol placed on onto the the substrate. PEPES membrane.Individual stencils were prepared for the different layers to produce a 5 5-pixel ECD matrix. For the × counterTo electrode,realize the a patterning mask with, upa stencil to five prepared horizontal beforehand stripes (6 mm was width, securely 100 placed mm length, on the 6 mmsubstrate. apart), Individuala stencil with stencils similar were vertical prepared stripes, for with the thedifferent same measurements,layers to produce was a 5 prepared × 5-pixel for ECD the matrix. electrochromic For the counterelectrode. electrode A shadow, a mask mask with for theup to masking five horizont layer wasal strip preparedes (6 mm accordingly. width, 100 Inmm order length, to achieve 6 mm apart), a thin, aevenly stencil coated with substrate, similar vertical the material strip wases, sprayed with the for same 5 s from measurements, a distance of 20was cm. prepared The coating for was the electrochromic electrode. A shadow mask for the masking layer was prepared accordingly. In order dried in the oven for 60 s at a temperature of 100 ◦C. to achieve a thin, evenly coated substrate, the material was sprayed for 5 s from a distance of 20 cm. The2.4. Insulatingcoating was Layer dried for in Electrochromic the oven for Matrices60 s at a temperature of 100 °C.

2.4. InsulatingFor the insulating Layer for Electrochromic layer 9.50 g Tubicoat Matrices MEA (CHT R. Beitlich, Tübingen, Germany) were mixed with 0.5 g titanium dioxide (Kronos 2360, Leverkusen, Germany) and 10.0 g isopropanol (Carl Roth,

Sensors 2020, 20, x FOR PEER REVIEW 6 of 15 Sensors 2020, 20, 5691 6 of 14 For the insulating layer 9.50 g Tubicoat MEA (CHT R. Beitlich, Tübingen, Germany) were mixed Karlsruhe,with 0.5 g titanium Germany). dioxide The dispersion (Kronos 236 was0, sprayLeverkusen, coated Germany) on top of the and counter 10.0 g electrodeisopropanol stripes (Carl using Roth, a stencilKarlsruhe, with Germany). spaces for The the electrolyte.dispersion was For stencilsspray coated see Appendix on top ofA .the counter electrode stripes using a stencil with spaces for the electrolyte. For stencils see Appendix A. 2.5. Assembly of Electrochromic Device 2.5. Assembly of Electrochromic Device The assembly steps of electrochromic cells by spray coating is described in Figure5. Stripes of conductiveThe assembly dispersions steps were of electrochromic coated or sprayed cells on by the spray substrates coating using is described a stencil (Figure in Figure A1). 5 PEDOT:PSS. Stripes of asconductive electrochromic dispersions materials were was coated deposited or sprayed on PEPES on and the Tubicoat substrates ELH using on the a stenciltextile substrate (Figure A1). (A). ThePEDOT:PSS insulating as layerelectrochromic was applied materials on top ofwas the deposited counter electrode on PEPES via and spray Tubicoa coatingt ELH using on thethe cut textile out squaressubstrate from (A). stencilThe insulating A2 as a maskslayer was (B). applied on top of the counter electrode via spray coating using the cut out squares from stencil A2 as a masks (B).

Figure 5 5.. Assembling steps of electrochromic devices by spray coating process.process.

Afterwards t thehe electrolyte E2 was applied with the help of a heated applicator, extending thethe time window for application, which consisted of a double walled syri syringenge (C). The rest of the fabric was covered with stencil A2. After gelling of the electrolyte,electrolyte, a spray adhesive was applied on the prepared half-cell.half-cell. The cut cut-out-out squares from the stencil A2 were used to protect the electrolyte layer (D). The electrochromic electrodeelectrode waswas laminatedlaminated onon toptop ofof thethe sprayspray adhesiveadhesive (E).(E). 2.6. Functionality Tests of Assembled Electrochromic Cells 2.6. Functionality Tests of Assembled Electrochromic Cells Electrochromic cells with PDADMAC alginate gelled electrolyte (E1) were assembled with roller Electrochromic cells with PDADMAC alginate gelled electrolyte (E1) were assembled with roller bar coated top and bottom electrodes. Three cells were prepared with this electrolyte. The first cell was bar coated top and bottom electrodes. Three cells were prepared with this electrolyte. The first cell switched 25 times between “on” and “off”. The second cell was switched “on” and the power supply was switched 25 times between “on” and “off”. The second cell was switched “on” and the power unit was turned off after darkening of the PEDOT:PSS layer. The third cell was switched “on” and “off” supply unit was turned off after darkening of the PEDOT:PSS layer. The third cell was switched “on” directly after assembling and then again after two hours for the second time. and “off” directly after assembling and then again after two hours for the second time. Similar to the electrochromic cells with PDADMAC as electrolyte, the cells with the electrolyte E2 Similar to the electrochromic cells with PDADMAC as electrolyte, the cells with the electrolyte were investigated. Individual pixels of electrochromic matrices were switched on and off separately. E2 were investigated. Individual pixels of electrochromic matrices were switched on and off Over a period of one year the 5 5 electrochromic matrix was switched and off on a regular basis to separately. Over a period of one× year the 5 × 5 electrochromic matrix was switched and off on a evaluate its functionality. regular basis to evaluate its functionality. 2.7. Characterization of Electrochromic Devices 2.7. Characterization of Electrochromic Devices The prepared electrochromic devices were characterized with regard to their electrical and chromaticThe prepared properties. electrochromic A HCS-3404 power devices supply were unit characterized from Manson with Engineering regard to their was used electrical to power and chromatic properties. A HCS-3404 power supply unit from Manson Engineering was used to power the electrochromic cells. The switching time was determined by measuring the time needed to a fully the electrochromic cells. The switching time was determined by measuring the time needed to a fully colored state. Scanning electron micrographs were obtained with TM4000Plus from Hitachi and UV/Vis colored state. Scanning electron micrographs were obtained with TM4000Plus from Hitachi and spectra were recorded with a UV-2600 spectrometer from Shimadzu. Chromaticity was measured with UV/Vis spectra were recorded with a UV-2600 spectrometer from Shimadzu. Chromaticity was a Datacolor 400 chromatometer from Datacolor using D65 light with a 10-degree observer. The color measured with a Datacolor 400 chromatometer from Datacolor using D65 light with a 10-degree contrast ∆E* is given by the Euclidean distance between the color coordinates of the different redox observer. The color contrast ΔE* is given by the Euclidean distance between the color coordinates of

Sensors 2020, 20, 5691 7 of 14 statesSensors and 2020 was, 20, x calculated FOR PEER REVIEW in the CIELAB color space (see in AppendixB), which well adapts the vision7 of 15 of the human eye [43]. The respective formula is: the different redox states and was calculated in the CIELAB color space, which well adapts the vision

of the human eye [43]. The respective formulap is:2 2 2 ∆E∗ = ∆L∗ + ∆a∗ + ∆b∗ (1) ∆퐸∗ = √∆퐿∗2 + ∆푎∗2 + ∆푏∗2 (1) I/V-data were acquired using a power source, which was controlled by a LabView program. Data were acquiredI/V-data bywere two acquired point measurements using a power between source, –5 which and +was5 V. controlled by a LabView program. Data were acquired by two point measurements between –5 and +5 V. 3. Results and Discussion 3. Results and Discussion 3.1. Electrochromic Cells with Electrolyte E1 in Combination with Roller Bar Coated Electrodes 3.1. Electrochromic Cells with Electrolyte E1 in Combination With Roller Bar Coated Electrodes Directly after assembling, all cells showed the same characteristics. The switching time of the freshlyDirectly prepared after cells assembling was 4 s. Switching, all cells showed between the oxidized same characteristics. and reduced state The of switching PEDOT:PSS time occurred of the atfreshly a voltage prepared of 2.5 V.cell Thes was switching 4 s. Switching time for between the first oxidized cell remained and reduced at 4 s after state 25 of cycles.PEDOT:PSS The second occurred cell sustainedat a voltage the of reduced 2.5 V. The state switching for ten minutes, time for fromthe first the cell moment remained the oxidation at 4 s after process 25 cycles was. The observable. second Thecell switching sustained time the of reduced the third state cell for increased ten minutes, to 11 sfrom at a voltage the moment of 2.5 Vthe after oxidation two hours process indicating was aobservable. loss of functionality. The switching Textile time substrates of the third are cell permeable increased to for 11 air s at and a voltage water moisture,of 2.5 V after thus two water hours of theindicating electrolyte a loss evaporated. of functionality. The alginate Textile substrates gel was not are capable permeable to holdfor air back and enough water moisture moisture,, th andus thereforewater of the electrolyte electrochromic evaporated. cell stopped The alginate working gel within was not a fewcapable hours. to hold This back was enough supported moisture, by the factand that therefore the dried the electrochromic electrolyte was cell found stopped in small work crumbsing within distributed a few hours on.the This electrodes was supported when by the cellthewas fact disassembled.that the dried electrolyte This explains, was whyfound the in chargesmall crumbs carriers distributed could not on be transportedthe electrode ins when the dried the PDADMACcell was disassembled. alginate gel. This Nevertheless explains, why the cellthe wascharge roughly carriers characterized. could not be Eventransported though in PDADMAC the dried canPDADMA be usedC as alginate an electrolyte gel. Nevertheless itself, titanium the cell dioxide was roughly pigments characterized. were added Even to enhancethough PDADMAC the contrast can be used as an electrolyte itself, titanium dioxide pigments were added to enhance the contrast between the oxidized and the reduced state of PEDOT:PSS. Overall, the prepared electrolyte works for between the oxidized and the reduced state of PEDOT:PSS. Overall, the prepared electrolyte works electrochromic cells, though due to the lack of encapsulation, which would result in the loss of textile for electrochromic cells, though due to the lack of encapsulation, which would result in the loss of haptics, it is not suitable for textile-based electrochromic cells. textile haptics, it is not suitable for textile-based electrochromic cells. 3.2. Comparison of Electrical Resistance and Surface Morphology on Different Substrates 3.2. Comparison of Electrical Resistance and Surface Morphology on Different Substrates All electrochromic devices were fabricated method-consistently. A spray coated PEDOT:PSS electrodeAll was electrochromic always combined devices with were spray fabricated coated counter method electrode-consistently. and a A roller-bar spray coated coated PEDOT:PSS PEDOT:PSS electrodeelectrode was was combined always combinedwith a roller-bar with coatedspray coated electrode, counter respectively. electrode and a roller-bar coated PEDOT:PSS electrode was combined with a roller-bar coated electrode, respectively. In contrast to coatings on PET film, roller-bar coating on PES fabric did not result in even and In contrast to coatings on PET film, roller-bar coating on PES fabric did not result in even and smooth layers. This is due to the unevenness of the woven material on a microscopic scale (see Figures6 smooth layers. This is due to the unevenness of the woven material on a microscopic scale (see and7). Figures 6 and 7).

Figure 6. SEM crosscut micrographs of PEDOT:PSS (2) roller-bar coated PET film (1) (left) and PET Figure 6. SEM crosscut micrographs of PEDOT:PSS (2) roller-bar coated PET film (1) (left) and PET fabric (right). The coated film is flat, whereas the fabric is coated unevenly with thick (3) and thin spots fabric (right). The coated film is flat, whereas the fabric is coated unevenly with thick (3) and thin (4) on the surface. spots (4) on the surface.

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The coating dispersion accumulated in the cavities of the fabric, leading to a thicker layer than on top of the yarn intersections. Also, patterning options were quite limited with this technique. Patterning with masks or adhesive tape resulted in uneven layers and therefore in a non-uniform electrical resistance of the respective layer. Especially at the edges of the mask or tapes, the layer became thicker. The sheet resistance of roller-bar coated Tubicoat ELH on the rip-stop fabric was 50 Ωsq, whereas the coated PEDOT:PSS layer on PES rip-stop fabric was 250 Ωsq. Compared to roller-bar coating spray coating is not a direct method. The use of spray coating allowed the use of stencils with different patterns. However, the herein used commercially available conductive dispersions Clevios S V4 for the electrochromic electrode and Tubicoat ELH for the counter electrode were too viscous for spray application. For that reason, both dispersions were diluted with isopropanol, which resulted in a sprayable dispersion. Isopropanol has got a low boiling point, which allowed a fast drying of the sprayed layers. The sheet resistance of a spray coated diluted Tubicoat ELH layer on the rip-stop fabric was 130 Ωsq, whereas the spray coated PEDOT:PSS electrode on the PEPES membrane had a sheet resistance of 500 Ωsq. The switching time of electrochromic devices is known to be influenced by the speed of electron diffusion into the electrochromic layer. The diffusion rate and therefore also the switching speed is higher for thinner layers [44]. The lower conductive PEDOT:PSS layer is favored for a faster switching electrochromic device. The conductivity of the PEDOT:PSS layer is controlled by the application method and dilution of the dispersion. Nevertheless, it was not possible to spray coat the commercially available PEDOT:PSS dispersion due to its high viscosity and the viscosity of diluted PEDOT:PSS was too low for roller-bar coating. Therefore, no direct comparison was possible.

3.3. Comparision of Electrolytes The electrolytes are chemically diﬀerent. However, both electrolytes are based on natural gels. Electrolyte E1 contains alginate and electrolyte E2 is based on gelatin. Both electrolytes contain titanium dioxide as ion storage. Furthermore, titanium dioxide in the electrolyte enhances the contrast of the electrochromic layer. The ionic compound in electrolyte E1 is PDADMAC, which is a well-known water soluble electrolyte, whereas calcium chloride was chosen in electrolyte E2 as ionic conductor. Initially, textile-based electrochromic devices with both electrolytes work at low voltages (2.5 V for electrolyte E1 and 2.0 V for electrolyte E2). The devices have switching times of 4 s for electrolyte E1 and 2 s for electrolyte E2. This already indicates, that the gelatin-based electrolyte has a higher performance than the alginate based electrolyte. Electrolyte E2 shows also a clear better long-term performance in the presented electrochromic devices. The devices with this electrolyte are stable for more than one year, whereas devices with electrolyte E1 showed a reduced functionality after two hours due to drying of the electrolyte. In electrolyte E1 PDADMAC lost its functionality due to the evaporation of water. The presence of water or moisture in the electrolyte is crucial for the transportation of charge carriers into or out of the electrochromic layer. Therefore, the addition of glycerol as a hygroscopic compound in electrolyte E2 resulted in the long-term stability and functionality of the electrochromic device.

3.4. Electrochromic Cells with Electrolyte E2 A textile-based electrochromic cell was assembled with PEDOT:PSS as electrochromic layer and the gelatin-based electrolyte E2. A SEM micrograph of the crosscut electrochromic cell is shown in Figure7. The PEDOT:PSS layer cannot be resolved in the SEM micrographs due to its too low thickness. The cells are ﬂexible and bendable. Further, the textile-based devices could also be creased to some extent. The switching time directly after assembling was 2 s at a voltage of 2.0 V. Moreover, the switching time and voltage remained constant after 25 cycles. Crumpled and re-ﬂattened devices were still working, while the switching time and voltage did not change. The electrolyte was quite stable and held back moisture over an extended period of time. Even after one year, the cell could still be Sensors 2020, 20, x FOR PEER REVIEW 9 of 15

Sensors 2020, 20, 5691 9 of 14 switched on and oﬀ. After one year, the switching time increased and was found to be at around 4 s at 2.0Sensors V. A 20 520, 205,electrochromic x FOR PEER REVIEW matrix with this conﬁguration is shown in Figure8. 9 of 15 ×

Figure 7. SEM crosscut micrograph of an electrochromic device with 1 - PES fabric, 2 - polyurethane pre-coating, 3 – counter electrode, 4 – electrolyte with titanium dioxide, 5 – top substrate with PEDOT:PSS layer.

The cells are flexible and bendable. Further, the textile-based devices could also be creased to some extent. The switching time directly after assembling was 2 s at a voltage of 2.0 V. Moreover, the switching time and voltage remained constant after 25 cycles. Crumpled and re-flattened devices FigureFigure 7. 7.SEM SEM crosscutcrosscut micrographmicrograph ofof anan electrochromicelectrochromic devicedevice with 1—PES1 - PES fabric fabric,, 2 2—polyurethane - polyurethane were still working, while the switching time and voltage did not change. The electrolyte was quite pre-coating,pre-coating, 3—counter 3 – counter electrode, electrode, 4—electrolyte 4 – electrolyte withwith titanium titanium dioxidedioxide,, 5 5—top – top substrate with with stable and held back moisture over an extended period of time. Even after one year, the cell could PEDOT:PSSPEDOT:PSS layer. layer. still be switched on and off. After one year, the switching time increased and was found to The cells are flexible and bendable. Further, the textile-based devices could also be creased to some extent. The switching time directly after assembling was 2 s at a voltage of 2.0 V. Moreover, the switching time and voltage remained constant after 25 cycles. Crumpled and re-flattened devices were still working, while the switching time and voltage did not change. The electrolyte was quite stable and held back moisture over an extended period of time. Even after one year, the cell could still be switched on and off. After one year, the switching time increased and was found to be at around 4 s at 2.0 V. A 5 × 5 electrochromic matrix with this configuration is shown in Figure 8.

Figure 8. 5 × 55 electrochromic device device where where two pixels have been switched at 2.0 V as indicated on × power supply unit. As it can be seen the pixels can be switched individually.

The color contrast ∆E* was calculated according to formula 1 to be 21.25 for a pixel of the 5 5 × pixel electrochromic device. For the human eye the color changes from a very light blue shade to a

Figure 8. 5 × 5 electrochromic device where two pixels have been switched at 2.0 V as indicated on power supply unit. As it can be seen the pixels can be switched individually.

Sensors 2020, 20, x FOR PEER REVIEW 10 of 15 Sensors 2020, 20, 5691 10 of 14 The color contrast ΔE* was calculated according to formula 1 to be 21.25 for a pixel of the 5 × 5 pixel electrochromic device. For the human eye the color changes from a very light blue shade to a darkdark blue blue color color,, as one can also see in Figure8 8.. TheThe colorcolor of of a a reduced reduced PEDOT:PSS PEDOT:PSS pixel pixel sustains sustains for for a amoment, moment and, and therefore therefore two two pixels pixels in in this this figure figure are are shown shown in in the the colored colored state. state. ColorationColoration efficiency efficiency is is an an important important characteristic characteristic for for electrochromic electrochromic devices. devices. It It is is calculated calculated usingusing the the transmittance transmittance of of bleached bleached and and colored colored state. state. However, However, in in this this case case no no transmittance transmittance in in the the visiblevisible range range can can be be measu measuredred due due to to the the high high reflection reflection and and light light scattering scattering on on titanium titanium dioxide dioxide particlesparticles in in the the electrolyte electrolyte and and the the high high absorption absorption of of light light on on the the black black counter counter electrode. electrode. Therefore Therefore,, reflectancereflectance spectra of bleachedbleached andand coloredcolored states states were were measured measured in in the the visible visible light light range range between between 350 350and and 800 800 nm nm (Figure (Figure9). 9)

bleached 70 colored

reflectance R] [% 20

0 400 500 600 700 800 wave length [nm]

Figure 9. Reflectance spectra of electrochromic cell in bleached and colored states. Figure 9. Reflectance spectra of electrochromic cell in bleached and colored states. The diffuse reflectance of the bleached device in the visible range is between 50% and 60%, which can beThe explained diffuse reflectance by the high of amountthe bleached of titanium device in dioxide the visible in the range electrolyte. is between The 50 reflection% and 60%, decreases which canin the be higherexplained wavelengths, by the high which amount is explainedof titanium by dioxide the light in bluethe electrolyte. shimmer ofThe the reflection electrochromic decreases cell inand the by higher the absorption wavelengths, of light which in is the explained PEDOT:PSS by the layer light on blue the frontshimmer electrode. of the Whenelectrochromic the device cell is andswitched by the on, absorption the overall of reflectionlight in the decreases. PEDOT:PSS The maximumlayer on the at front 425 nm electrode. is now atWhen 51% reflectionthe device and is switchedthe reflection on, the decreases overall reflection steeper until decreases. a minimum The maximum reflectance at of 425 23% nm is is found now at around 51% reflection 660 nm. Thisand thespectrum reflection coincides decreases with steeper the deep un bluetil a colorminimum of PEDOT:PSS reflectance in of the 23 reduced% is found state. around 660 nm. This spectrumThe electrochromiccoincides with devicethe deep shows blue ancolor ohmic of PEDOT:PSS behavior between in the reduced 0 and + 2.8state. V, when it is measured fromThe 0 to electrochromic+5 V (see inlay device in Figure shows 10). an The ohmic total behavior resistance between of the device 0 and +2.8 was V, calculated when it tois measured be around from15 kΩ 0 .to When +5 V (see the inlay PEDOT:PSS in Figure is 10). reduced The total completely resistance to of its the insulting device state,was calculate the currentd to be drops around and remains constant above 3.1 V. Starting the measurement at 5 V, results in a slightly different behavior 15 kΩ. When the PEDOT:PSS is reduced completely to its− insulting state, the current drops and remains(Figure 10 constant). Between above –3 V3.1 and V. + Starting1 V, the the I–V-curve measurement is linear, at which −5 V, means results that in the a slightly resistance different of the behaviordevice is (Figure constant. 10). The Between current – increases3 V and +1 steeper V, the aboveI–V-curve+1 V, is but linear, again which the increase means that is linear. the resistance When the ofvoltage the device is reduced, is constant. the current The current decreases increases only slightly steeper and above is non-ohmic +1 V, but until again the the voltage increase reaches is linear.+2 V, Whenthen a the linear voltage decrease is reduced, of the current the current is observed, decreases which only becomes slightly steeperand is non at –1-ohmic V. A smalluntil hysteresisthe voltage is reobservedaches +2 atV, thethen start a linear and enddecrease point of –5 the V. current The final is currentobserved, is awhich little bitbecomes higher steeper than the at – current1 V. A small in the hysteresisbeginning is (Figure observed 10). at the start and end point –5 V. The final current is a little bit higher than the current in the beginning (Figure 10)

Sensors 2020, 20, x FOR PEER REVIEW 11 of 15 Sensors 2020, 20, 5691 11 of 14

0,3 voltage up 0,2 voltage down

0,1

0,0

-0,1 0,00025

0,00020

current[mA]

-0,2 0,00015

current [A] current 0,00010 -0,3 0,00005

0,00000 0 1 2 3 4 5 -0,4 voltage [V]

-6 -4 -2 0 2 4 6 voltage [V]

Figure 10. Voltage-current data for one cycle from –5 to +5 V and from 0 to +5 V (inlay). Figure 10. Voltage-current data for one cycle from -5 to +5 V and from 0 to +5 V (inlay). The fabricated textile ECD prototype has a 5 5-pixel matrix (Figure7). So far, it is a simple × displayThe which fabricated is patternable. textile ItECD has theprototype further has potential a 5 × to5- bepixel integrated matrix into(Figure an autonomously 7). So far, it is actuated a simple systemdisplay in thewhich future. is patternable. Color can be It switched has the further on and opotentialff at the (active) to be integrated addressable into pixels. an autonomously actuated system in the future. Color can be switched on and off at the (active) addressable pixels. 4. Summary and Conclusions 4. SummaryThe successful and Conclusions fabrication of a textile integrated ECD with individually addressable pixels for the controllable ability to change the display of patterns and coloration is demonstrated. Extension and The successful fabrication of a textile integrated ECD with individually addressable pixels for miniaturization of the pixel matrix allow a flexible display for information or camouflage purposes. the controllable ability to change the display of patterns and coloration is demonstrated. Extension A moisture-retaining agent in the electrolyte ensures that the electrolyte does not dry out and the and miniaturization of the pixel matrix allow a flexible display for information or camouflage electrochromic cell retains its functionality. The white backing of the pixel electrodes through the purposes. A moisture-retaining agent in the electrolyte ensures that the electrolyte does not dry out opaque white pigmentation of the electrolyte enhances the color contrast of the pixel electrode in the and the electrochromic cell retains its functionality. The white backing of the pixel electrodes through layer above. The electrochemical behavior of this electrochromic cell is not yet well understood and the opaque white pigmentation of the electrolyte enhances the color contrast of the pixel electrode in must be further investigated. the layer above. The electrochemical behavior of this electrochromic cell is not yet well understood Authorand must Contributions: be furtherConceptualization, investigated. C.G., M.M. and A.S.-P.; Investigation, C.G. and M.M.; Methodology, C.G., M.M. and A.S.-P.; Supervision, L.V.L. and A.S.-P.; Validation, C.G.; Writing—original draft, C.G. and M.M.; Author Contributions: Conceptualization, C.G., M.M. and A.S.-P.; Investigation, C.G. and M.M.; Methodology, Writing—review & editing, L.V.L. and A.S.-P. All authors have read and agreed to the published version of theC.G. manuscript., M.M. and A.S.-P.; Supervision, L.V.L. and A.S.-P.; Validation, C.G.; Writing – original draft, C.G. and M.M.; Writing – review & editing, L.V.L. and A.S.-P. All authors have read and agreed to the published version of the Funding: This research received no external funding. manuscript. Acknowledgments: The authors thank Andrea Ehrmann from Bielefeld University of Applied Sciences and SandraFunding: Gellner This from research Niederrhein received University no external of funding Applied. Sciences for valuable discussions. ConflictsAcknowledgments: of Interest: The The authors authors declare thank no Andrea conflict Ehrmann of interest. from Bielefeld University of Applied Sciences and Sandra Gellner from Niederrhein University of Applied Sciences for valuable discussions. Appendix A. Preparation of stencils Conflicts of Interest: The authors declare no conflict of interest. All stencils were cut out of PET film. The grey shaded areas were cut out. The cut out squares from stencilAppendix A2 were A: Preparation used to mask of the stencils electrolyte pixels when spraying the polyurethane masking layer. All stencils were cut out of PET film. The grey shaded areas were cut out. The cut out squares from stencil A2 were used to mask the electrolyte pixels when spraying the polyurethane masking layer.

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Figure A1. Stencil for the 5 × 5 ECD pixel electrodes. FigureFigure A1.A1. Stencil for the 5 × 5 ECD pixel electrode electrodes.s. ×

Figure A2. Stencil for 5 5 ECD electrolyte layer. × Figure A2. Stencil for 5 × 5 ECD electrolyte layer. Appendix B. Color Contrast Figure A2. Stencil for 5 × 5 ECD electrolyte layer. AppendixTable A1.B: ColorChromaticity contrast of EC devices in CIELAB color space (light source: D65, 10 degree observer). Appendix B: Color contrast Table B1. Chromaticity of EC devicesIn Oxidized in CIELAB State color space (light In Reduced source: D65, State 10 degree observer). Table B1. ChromaticityL* of EC devices in CIELAB 80.28 color space (light source: 65.48 D65, 10 degree observer). in Oxidized State in Reduced State a* 3.61 5.83 L* in Oxidized80.28− State in Reduced65.48 State− b* 2.18 17.27 L*a* 80.28−3.61− 65.48−5.83 − b*a* According−3.612.18 to formula 1: ∆E* =−21.25.−17.275.83 According to formula 1: ΔE* b*= 21.25 −2.18 −17.27 References According to formula 1: ΔE* = 21.25 1.ReferencesSheng, L.; Li, M.; Zhu, S.; Li, H.; Xi, G.; Li, Y.-G.; Wang, Y.; Li, Q.; Liang, S.; Zhong, K.; et al. Hydrochromic References1. molecularSheng, L.; Li, switches M.; Zhu, for S.; water-jet Li, H.; Xi, rewritable G.; Li, Y. paper.-G.; Wang,Nat. Commun.Y.; Li, Q.; 2014Liang,, 5 ,S.; 3044. Zhong, [CrossRef K.; et al.][PubMed Hydrochromic] 2. Parkin, I.P.; Manning, T.D. Intelligent Thermochromic Windows. J. Chem. Educ. 2006, 83, 393. [CrossRef] 1. Sheng,molecular L.; Li,switches M.; Zhu, for S.; water Li, H.;-jet Xi,rewritable G.; Li, Y. paper.-G.; Wang, Nat. Commun.Y.; Li, Q.; 2014Liang,, 5 ,S.; 3044, Zhong, doi:10.1038/ncomms4044. K.; et al. Hydrochromic 3. Lee, C.K.; Beiermann, B.A.; Silberstein, M.N.; Wang, J.; Moore, J.S.; Sottos, N.R.; Braun, P.V. Exploiting Force 2. molecularParkin, I.P.; switches Manning, for water T.D.-jet Intelligen rewritablet paper. Thermochromic Nat. Commun. Windows. 2014, 5, 3044,J. Chem. doi:10.1038/ncomms4044. Educ. 2006, 83, 393, Sensitive Spiropyrans as Molecular Level Probes. Macromolecules 2013, 46, 3746–3752. [CrossRef] 2. Parkin,doi:10.1021/ed083p393. I.P.; Manning, T.D. Intelligent Thermochromic Windows. J. Chem. Educ. 2006, 83, 393, doi:10.1021/ed083p393.

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