Single-substrate encapsulated cholesteric LCDs: Coatable, drapable, and foldable

Irina ShiyanovskayaAsad KhanSeth GreenGreg MagyarOleg PishnyakDuane MarhefkaJ. William Doane

Abstract — The first ever, reflective cholesteric liquid-crystal displays (ChLCDs) on single textile sub-strates made with simple coating processes have been developed. A novel approach for fabricationof ultra-thin encapsulated ChLCDs with transparent conducting polymers as bottom and top electrodeswill be reported. These displays are fabricated from the bottom-up by sequential coating of variousfunctional layers on fabric materials. Encapsulation of the cholesteric liquid-crystal droplets in a poly-mer matrix and the mechanical flexibility of the conducting polymers allow for the creation of durableand highly conformable textile displays. The development and status of this next-generation displaytechnology for both monochrome and multicolor cholesteric displays will be discussed.

Keywords — Cholesteric liquid crystals, encapsulated cholesteric LCs, emulsion, single substratedisplay, fabric substrate, flexible display, drapable display.

1 Introduction

Due to their excellent electro-optical properties, reflectivecholesteric liquid-crystals displays (ChLCDs) hold greatpotential and promise for emerging flexible portable appli-cations. The development of the cholesteric glass displayshas been on-going for several years with dramatic improve-ments in brightness, color, driving speed, and power.1,2

Reflective cholesteric displays have been made with conven-tional LCD manufacturing methods for over a decade. Thisinvolves two ITO-coated rigid glass substrates with LC mate-rial being filled in between. These displays are now startingto appear in more applications and even consumer-elec-tronic devices.

Cholesteric liquid-crystal materials naturally providehigh brightness and require low power due to the inherentbistability of the cholesteric textures. The pure reflectivenature of the cholesteric materials does not require the useof filters, polarizers, and backlighting. All these features enablethe fabrication of light-weight low-power ChLCDs on flex-ible substrates if special care is taken to prevent liquid-crys-tal flow under the pressure caused by display deformationwhich is unavoidable for flexible applications. The elegantsolution is to confine small liquid-crystal droplets in a poly-mer matrix. In addition to the elimination of the materialflow under pressure, encapsulation of the LC provides goodfilm-forming properties and high durability of the encapsu-lated LC layer.

There are two well-known methods to encapsulateLCs: phase separation and emulsification. The phase-sepa-ration method produces polymer-dispersed LCs by coatingwith an isotropic LC/polymer mixture and by curing it etherwith heat, radiation, or solvent evaporation to create LCdroplets.3–4 The emulsification method in contrast allowsone to first create LC droplets in an aqueous solution andthen to cast the emulsion with the addition of a film-formingpolymer binder to form a thin layer.5

The motivation of our work was to go beyond a flex-ible-display concept by developing highly conformablecholesteric displays which can be draped and folded in threedimensions. We focus on the emulsification technique toencapsulate ChLCs because it provides excellent paint-likeLC dispersions compatible with existing coating and print-ing techniques. Moreover, this method allows us to developa sequential coating approach when multiple layers can becoated on the top of each other.

In our approach to create fully coatable displays, wechoose to use conducting polymers to replace conventionalITO electrodes. The conducting polymers combine the excel-lent electrical and optical properties of metal or semicon-ductors with unique mechanical flexibility and ease ofprocessing of polymers.6

We employed a single-substrate approach, which allowsfor the fabrication of highly conformable displays by replac-ing the conventional sandwiched substrate approach forLCDs. Our choice of the single substrate was woven andnon-woven textile materials, which enable high display con-formability and provide opportunities for a variety of novelapplications. Moreover, the choice of such challenging sub-strates was motivated to develop the technology whichwould be applicable for emerging “smart” and interactivetextile applications. Smart textiles will incorporate displaysand sensors to offer wearable information and communica-tion systems in various military, biomedical, security, safety,and fashion applications. It should be emphasized that ourtechnology is fully applicable to conventional substratessuch as plastic and glass and to other unconventional sub-strates, including paper, ultra-thin plastic, walls, and clothing.

Two outstanding contributions in the development ofnew single-substrate LCDs have recently been made. Theinitial research was performed by Philips, based on thephase separation method with two UV light exposures andglass substrate with ITO interdigitated electrodes for in-plane

Extended version of a paper presented at the 2005 SID International Symposium held May 24–27, 2005, in Boston, Massachusetts.

The authors are with Kent Displays, Inc., 343 Portage Blvd., Kent, OH 44240, U.S.A.; telephone 330/673-8784, fax -4408, e-mail: ishiyanovskaya@kentdisplays.com

© Copyright 2006 Society for Information Display 1071-0922/06/1402-0181$1.00

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switching (IPS) of twisted-nematic LC (TNLC).7,8 The sec-ond contribution was performed by Kodak with top-downdisplay fabrication on the ITO/plastic substrate using emul-sified ChLCs as the electro-optical layer and non-transpar-ent conductive inks as the second electrode.9

We took the single-substrate display approach to thenext step by incorporating several new important features.The novelty of our work can be summarized as follows:

� the use of transparent conducting polymers as bot-tom and top electrodes;

� bottom-up fabrication on single opaque textile sub-strate;

� color-display fabrication using sequential coating ofmultiple cholesteric layers with different colors;

� the use of various unconventional single substrates,such as textile, paper, or ultra-thin plastic.

In this paper, we report on a new technology that willenable the fabrication of low-cost large-area reflective ChLCDson a single textile substrate using conventional coating andprinting techniques. The paper will discuss the fabricationsteps for monochrome and multi-color coated ChLCDs.

2 Encapsulation of ChLCsTo fully encapsulate the ChLC and make it into a coatablematerial, we employed an emulsification process usingeither the conventional shear mixing method or a novelmembrane injection method. Emulsions are heterogeneoussystems of immiscible liquids that form dispersed andcontinuous phases. Shear mixing is a simple method thatinvolves mixing together two immiscible liquids (in our caseliquid crystal and water) and the formation of the disperseddroplets by shear flow. The shear-flow energy should behigh enough to overpower the interfacial tension betweentwo liquids. Typically, droplets tend to coalesce after emul-sification to decrease their interfacial energy. Adding surfac-tants with a particular hydrophile-lipophile balance resultsin surfactant adsorption at the interface and prevents coales-cence. The resulting heterogeneous system is kinetically sta-ble and has a shelf life of several years. The second method,membrane emulsification, involves pressing LC materialthrough a porous glass membrane into an aqueous solutionwith a surfactant. The distinctive feature of this method isthat it produces uniformly sized droplets controlled by themembrane pore size.10

We emulsified the ChLC in an aqueous solution con-taining the surfactant and polyurethane binder material toform LC droplets with an average diameter of about 10 µm.Figure 1 shows a polarizing optical microscope image of thedroplets of the ChLC with selective reflection in the yellowrange of the spectrum. Typically, the ratio between the LCand the polymer binder was about 5:1. Paint-like emulsionis then coated to form a thin film on various substrates usinga blade technique. Encapsulated ChLC combines all theintrinsic properties of the bulk material, including bistabil-ity and optical characteristics, with properties common to

polymers such as film-forming capability, compatibility withcoating and printing techniques, and mechanical flexibility.Confinement of LC material in the polymer matrix changesthe conventional method of handling LC materials.

Because the LC is confined in the droplets, propertiessuch as droplet shape, droplet-size distribution, fill factor,and liquid-crystal/polymer interface, become dominant pa-rameters in the electro-optical behavior of the display layer.Emulsion-based encapsulation brings three major advan-tages in comparison with conventional bulk cholesterics.First, the LC confined in the droplet does not flow and as aresult the encapsulated ChLC film becomes pressure insensi-tive. Typically, pressure sensitivity is one of the main prob-lems with cholesteric displays and requires specialprotective measures. Second, the thin film formed by theencapsulated ChLCs is rugged enough to allow for sequen-tial coating of the transparent conductive electrodes. Third,a LC/water emulsion is coatable using conventional meth-ods, including a web-coating process, and can form thinfilms on various substrates, including substrates which canbe flexed and draped in three dimensions. These three fea-tures of the encapsulated ChLC offer an opportunity for asingle-substrate approach in the fabrication of ChLCDs.

3 Coated monochrome Ch-LCDs

3.1 Fabrication processThe schematic representation of the entire display system isshown in Fig. 2. The displays are built from the bottom upon the black-rayon-fabric substrate.

The fabrication includes several coating and printingprocesses to sequentially deposit several functional layers onthe top of one other. This layer-by-layer approach employs thedirect-coating techniques compatible with continuous roll-to-roll processing. Web compatibility and ease of proc-essability offers great potential for low-cost manufacturing.

The first step is planarization of the fabric substrate.The surface smoothness of the fabric substrates is critical forthe integrity of the conductive electrodes and the encapsu-lated LC layer. The surface of the fabric substrate can beundesirably rough and can cause pinholes in LC layers result-

FIGURE 1 — Polarizing optical microscope image of the ChLC dropletsin the thin-film coating. Emulsion was made using the shear-mixingmethod which provides a rather broad droplet-size distribution.

182 Shiyanovskaya et al. / Single-substrate encapsulated cholesteric LCDs

ing in electrical shorts. The smoothed fabric surface ontowhich the first conductive electrode may be deposited requiresa planarization procedure. A planarization layer of eitherpolyurethane or acrylic aqueous borne dispersions with adry thickness on the order of 15 µm is coated using a doc-tor-blade technique. Moreover, smoothing out the surfacehelps to maintain a constant LC-layer thickness. Figure 3shows the SEM images of the fabric surface before and afterthe planarization step.

For electrodes, we use highly transparent conductingpolymers (CP) with an excellent flexibility and low process-ing temperatures essential for the textile substrates. Con-ducting polymers based on secondary doped π-conjugatedpolymer PEDOT-PSS, poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate), developed by AGFA are used as topand bottom electrodes. These CP materials exhibited excel-lent adhesion, low electrical resistivity, and high opticaltransmission. The first conducting polymer (CP1) is screen-printed on the planarized fabric substrate to create a pat-terned electrode. The emulsion is then blade coated on thesubstrate with a conducting electrode, and the water fromthe emulsion is allowed to evaporate. The thickness of theencapsulated LC layer is in the range of 10–12 µm.

The top conducting polymer (CP2) electrode is depos-ited over the encapsulated layer using air-brushing methodwith a stencil for patterning and is cured at room tempera-ture. Figure 4 shows the dependence of the transmission (at5860 nm) on the surface resistance of the top conductingpolymer air-brushed on the plastic substrate in the visiblerange of the spectrum. The conducting polymer exhibitshigh transmission, high conductivity, and flexibility, whichmake this material a promising alternative for conventionalITO electrodes.

To increase the durability of the display, a protectiveclear layer is deposited on the top conducting polymer elec-trode. The total thickness of the coatings on the fabric sur-face is on the order of 30–35 µm. Flexible printed circuitsare bonded to the flexible display using conductive tape overthe conductive polymer electrodes on the fabric substrate.Figure 5 illustrates all the steps in the fabrication processand is a schematic representation of the final display assem-bly.

3.2 Display performanceA distinctive feature of the ChLCDs is their reflectivity andbistability. The color of cholesterics originates from the pureselective reflection sensitive to the wavelength and polariza-tion of the incident light. The helical molecular structure ofChLCs exhibits two stable textures controllable by the appliedelectric field. The first texture, planar, is reflective for theincident light with one of the circular polarizations. The sec-ond texture, focal conic, is transparent and produces a darkstate due to the absorptive black background. Intrinsic bis-tability of these two states, planar and focal conic, allows forretaining information without any applied electric field. Thebistability of ChLCs has the advantage of multiplexing withpassive driving. To passively drive the displays row by row,we employed the conventional driving scheme describedelsewhere.11 The driving voltage is slightly higher than thatfor bulk non-encapsulated ChLCDs and is in the range of70–90 Vrms. The display can be addressed with rather fastswitching speeds of 5–20 msec/row.

We have achieved excellent optical performance forChLCs encapsulated in different polymer binders. The cur-rent displays with a polyurethane latex binder exhibit reflectivityup to 34% at the maximum wavelength of the reflected lightand have a high contrast ratio of up to 20:1. Figure 6 showsphotographs of the passive-matrix encapsulated ChLCDson fabric substrates with 16 × 13 pixels.

FIGURE 2 — Scheme of the functional layers in the encapsulated ChLCDcoated on the single textile substrate. CP stands for conductive polymers.The fabrication is described in the text.

FIGURE 3 — SEM images of the surface of (a) pre-treated rayon fabricand (b) the same fabric surface after planarization.

FIGURE 4 — Transmission vs. surface resistance for the CP layer on theplastic substrate.

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Encapsulated ChLCs exhibit outstanding viewingproperties. Incorporation of ChLCs in the droplets dis-persed in a polymer matrix eliminates the angular depend-ence of the reflected light. The curved shape of the dropletsand parallel alignment of LC molecules at the dropletboundaries result in selective reflection with a wide viewingangle.

The encapsulated ChLCDs are robust and can befolded, flexed, and draped during image addressing withoutany image distortion. Figure 7 illustrates fully functionalChLCDs on fabric substrates encountering bending, folding,rolling, and draping deformations. Since the displays do nothave the conventional second substrate, there is no cell-gap

deformation. The thin top conducting-polymer electrodewith a thickness of approximately 1 µm is deposited directlyon the encapsulated LC layer. Conducting polymer followsthe profile of this layer and provides the natural cell-gapcontrol when the display is curved due to the intrinsic mechani-cal flexibility of the polymer. Testing in an environmentalchamber shows that displays can sustain 95% RH at 50°C formore than 200 hours without degradation.

4 Coated multi-color ChLCDsThe unique optical properties of ChLCs allow for the fabri-cation of full-color displays by stacking several display ele-ments.12,13 The helical molecular structure of the ChLCthat assembles either in right-handed or left-handed helicesresults in a maximum reflectance of 50% of the incidentlight with one of circular polarization. Wavelength selectivitypermits the Bragg reflection at a certain wavelength, deter-mined by the helical pitch of the ChLC material. The inci-dent light with all other wavelengths is fully transmittedthrough the LC layer. The transmitted light is available forfurther reflection if cholesteric-display elements with dif-ferent pitch lengths are stacked on top of each other. Thisenables the high brightness of the reflected light due to thefact that the full area of the display reflects each color. Thisfeature of cholesterics of additive color mixing allows for thefabrication of full-color displays with triple stacking of pri-

FIGURE 5 — Fabrication process of the encapsulated ChLCD on thesingle textile substrate.

FIGURE 6 — ChLCDs fabricated on the textile substrate, 16 × 13 passivematrix.

FIGURE 7 — Fully functional deformed ChLCDs coated on the textilesubstrates, segmented and 16 × 13 passive matrix.

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mary colors. We have adapted this feature to develop themulti-color display based on the sequential coating of twoindividually addressed cholesteric layers. Each pixel con-sists of two individually addressed layers with yellow andblue cholesterics. Depending on switching state of the lay-ers, the display is able to generate white, black, blue, andyellow colors.

The costs of a stacked display and drive electronics canbe minimized through the use of a shared-electrode driveapproach. With this approach, rather than building and sub-sequently stacking two independent displays, the blue layerand yellow layer are constructed together and separated bya shared electrode which is used for driving both of the lay-ers. Thus, one set of drive electronics is eliminated becausethere are only three ledges to drive rather than four. The twolayers are generally driven in succession using the multi-plexing properties of the LC. In this case, the layer beingupdated experiences the full drive voltages while the otherlayer receives non-select voltages that do not alter the dis-played image. Simultaneous drive of both LC layers is pos-sible if the drive properties of both layers are well matched.

The detailed description of the fabrication processand optical behavior of the double-stack Ch-LCDs on thesingle substrate will be presented in a future publication.The development of a full-color display is a focus of ourcurrent efforts.

5 ConclusionsWe report on the novel technology for fully coatable Ch-LCDs on a single textile substrate. We demonstrated thefeasibility of fabrication of monochrome and multi-colordisplays using sequential coating of functional layers withsimple existing coating and printing techniques. The emul-sification approach enables the coating of paint-like Ch-LCemulsion to form thin encapsulated display layers virtuallyon any substrate. Transparent flexible conducting polymersare used to serve as top and bottom electrodes. The rugged-ness of the encapsulated Ch-LC layer, flexibility of the con-ducting polymer electrodes, and the use of a single substrateprovide highly conformable displays, which can be folded,flexed, and draped during image addressing. Coated dis-plays exhibit excellent optical performance with a reflectiv-ity of up to 34% and a contrast ratio of 20:1. This technologywas successfully demonstrated on a variety of unconven-tional substrates, including paper, ultra-thin plastic, and tex-tile materials. Our technology provides a totally new way forfabrication of the LCDs compatible with roll-to-roll proc-essing.

AcknowlegmentsThe authors would like to acknowledge the entire team atKent Displays, especially Forrest Nicholson, Nick Miller,Todd Ernst, Scott Scheibe, and Steve Oliveri for constantsupport in the development of this technology.

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Irina Shiyanovskaya is principal scientist at KentDisplays, Inc. She received her B.S. and M.S degreesin physics and material science from Kiev StateUniversity, Ukraine. In 1996, she received herPh.D. degree in physics and mathematics fromthe Institute of Physics, Ukrainian Academy ofSciences. In 2003, after working as a research scien-tist at Case Western Reserve University, she joinedKent Displays to carry out research and develop-ment of materials for the next generation of coat-able cholesteric LCDs.

Asad Khan received his B.A. degree in physicsfrom the College of Wooster in 1993, his M.S. degreein physics from Kent State University in 1995, andhis Ph.D. in chemical physics from the LiquidCrystal Institute at Kent State University in 2003.He has been a member of the technical staff atKent Displays, Inc., since 1995. He currentlyleads R&D efforts in cholesteric materials andflexible displays.

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Seth Green received his B.S. degree in physicsfrom Hiram College in 1996. He joined Kent Dis-plays, Inc., as a display engineer in 2003, wherehe works on flexible cholesteric display process-ing.

Greg Magyar received his B.A. degree in chemis-try and M.A. degree in secondary education fromKent State University, Kent, Ohio, in 1984 and1987, respectively. He was a Research Specialistat the Liquid Crystal Institute at Kent State Univer-sity from 1989 to 2002, where he worked onpolymer-dispersed liquid crystals (PDLC), bistablecholesteric liquid crystals, and other display appli-cations. He has been a Display Technician at KentDisplays, Inc., since 2004, where he is currently

working on emulsion-based encapsulations of cholesteric liquid crystalsin flexible bistable displays.

Oleg Pishnyak graduated from the Engineering-Technical Department of Chernivtsi State Univer-sity, Ukraine, in 1997. Currently he is a graduatestudent at the Liquid Crystal Institute, Departmentof Chemical Physics, Kent State University, OH.His research area includes displays, phase sepa-ration in LC/polymer systems, and electro-opticapplications of liquid crystals.

Duane Marhefka received his Ph.D. degree fromthe Department of Electrical Engineering at TheOhio State University, Columbus, Ohio, in 2000.His current interest is drive waveforms forcholesteric LCDs. He is a member of the Societyfor Information Display and the IEEE.

J. William Doane is co-founder of Kent Displays,Inc., and serves on its Board of Directors. Heserves part time as Chief Technical Officer, fol-lowing his retirement from Kent State University,where he was Director of the Liquid Crystal Insti-tute and Professor of Physics.

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