Progress in Advanced Properties of Electrowetting Displays

micromachines

Review Progress in Advanced Properties of Electrowetting Displays

Yi Lu 1,2 , Biao Tang 1,2, Guisong Yang 1,2,* , Yuanyuan Guo 3,4 , Linwei Liu 1,2 and Alex Henzen 1,2,4,*

1 Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China; [email protected] (Y.L.); [email protected] (B.T.); [email protected] (L.L.) 2 National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, China 3 Academy of Shenzhen Guohua Optoelectronics, Shenzhen 518110, China; [email protected] 4 Shenzhen Guohua Optoelectronics Tech. Co. Ltd., Shenzhen 518110, China * Correspondence: [email protected] (G.Y.); [email protected] (A.H.)

Abstract: Electrowetting display (EWD) has promising prospects in the electronic paper industry due to it having superior characteristics, such as the ability to provide a comfortable reading experience and quick response. However, in real applications, there are also problems related to dielectric deterioration, excess power consumption, optical instability and narrow color gamut etc. This paper reviewed the existing challenges and recent progress made in terms of improving the optical performance and reliability of EWD. First, the principle of electrowetting applied in small and conﬁned conﬁgurations is introduced and the cause of the failure of the dielectric layer is analyzed. Then, the function of the pixel structures is described to avoid display defects. Next, electric signal modulations are compared in terms of achieving good image quality and optical stability. Lastly, the methods are presented for color panel realization. It was concluded that multi-layer dielectrics, three-dimensional pixel structures, proper electric frequency-and-amplitude modulation and an RGB

Citation: Lu, Y.; Tang, B.; Yang, G.; color panel are expected to resolve the current limitations and contribute to designing advanced Guo, Y.; Liu, L.; Henzen, A. Progress reﬂective displays. in Advanced Properties of Electrowetting Displays. Keywords: electrowetting display; technical developments; dielectric breakdown; bi-stable; opti- Micromachines 2021, 12, 206. cal stability https://doi.org/10.3390/mi12020206

Academic Editor: Athanasios G. Papathanasiou 1. Introduction Received: 23 December 2020 Electrowetting is the phenomenon of modulating surface wettability via an external Accepted: 15 February 2021 Published: 18 February 2021 electric ﬁeld, which has drawn great attention in lab-on-a-chip microﬂuidics [1]. Electrowet- ting display (EWD) uses electrowetting to control the state of oil inks and display desired

Publisher’s Note: MDPI stays neutral images, which has been at the center of attention since 2003, when Hayes and Feenstra with regard to jurisdictional claims in reported on the possibilities contained in the system [2]. The road since then has been long published maps and institutional affil- and arduous. A plethora of challenges have required solving one by one. Research was iations. later focused on the prediction of the working principle, dielectric material preparation, pixel structure design, electric signal modulation and color panel realization in order to achieve market applications and mass production. In display theory, the basic principle of electrowetting has been demonstrated to apply in confined pixels. The surface wettability change between the hydrophobic (oleophilic) Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. state and hydrophilic (oleophobic) state causes a switch between the “on” and “off” status in This article is an open access article each pixel unit. Tang et al. [3] illuminated the mechanism of oil film opening and predicted distributed under the terms and the threshold voltage based on the electro-capillary instability of the oil–water interface in conditions of the Creative Commons the pixel. Zhou et al. [4] formulated a theoretical model of dynamic electrowetting display Attribution (CC BY) license (https:// to predict the opening rate of oil film. creativecommons.org/licenses/by/ Researchers have also dedicated efforts to the development of robust dielectric materi- 4.0/). als, as the deterioration of the dielectric layer will directly lead to the failure of the display

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device and greatly reduce the service time. The current preparation of dielectric layers is a lengthy and error-prone process which includes sample cleaning, film coating, surface treatment and photolithographic patterning [5]. Chen et al. [6] reported the advantages of screen printing techniques for preparing uniform and compact dielectric materials as compared with the conventional spin coating process. Guo et al. [7] selectively injected a fluoropolymer to form hydrophobic patterns based on a combined approach of inkjet printing and phase change filling. In this way, changes to the material properties during the surface treatment of reactive ion etching were avoided. These technical developments enabled the fast preparation of uniform and compatible multi-layer dielectrics. A well-designed shape and structure of pixel guide the flows of oil inks in the confined system which resolve display defects. Early studies mainly focused on the planar surfaces by varying the shape and size of the pixel to restrict the fluid flow in the display unit [8]. Various shapes of the pixel, including quadrilateral, rectangle, triangle and trapezoid shapes were designed and the response rates and brightness of the images produced were compared. The quadrilateral pixel has a better response rate and brightness, but the improvement is still limited. Additionally, the planar surface still relies on the electric signal, while displaying images in case of backflows [9]. For a limited pixel size and spacing, overflows of the oil inks [10] to the adjacent pixels may take place. Three-dimensional structures reserve excess oil inks and prevent oil backflows and overflows, while the bi- stable features only require electric energy during image conversion, thus, enabling them to be used for a longer time [11]. In addition, “symmetry breaking structure” [12] on the original planar surface realizes the breaking of the oil film under a low voltage and ensures the oil ink flows in a uniform direction. For application of the electric signal, a proper driving waveform can not only achieve a good optical performance, but also improves the stability of the gray level [13]. The display performance refers to the image quality (e.g., reflectivity, brightness, resolution). Optical stability means that the generated picture will not fluctuate in a certain period of time. The optical performance and stability of EWD are mainly dependent on the electro-fluidic behaviors of oil inks in the pixel. Generally, a higher opening rate and larger aperture ratio are achieved by a larger driving voltage. However, a large voltage may cause oil splitting [14,15]. Conversely, lower voltages can ensure more stable oil ink flows and prevent the breakdown of the dielectric layer. Therefore, the selection of electric voltage requires a tradeoff between performance and stability. A proper electric driving scheme can achieve both expectations in the real operation. By using frequency-and-amplitude modulation and converting a constant voltage to several segments, Yi et al. [16] achieved both high levels of brightness and optical stability and also improved the response speed of oil inks. Current challenges for electrowetting displays are mainly related to dielectric deterioration, display defects, energy consumption in mono-stable states and color panel realization. In the following sections, the electrowetting principles in the confined config- uration will be introduced. The failure mode of the dielectric layer will be analyzed in more details. Suitable pixel structures and driving schemes will be discussed to achieve high performance, optical stability and energy saving displays. Lastly, the latest status and highlights of R&D will be elaborated on in this article, providing the latest insights in the behavior of oil films in pixels, and the creation of colorful images.

2. Principles: Electrowetting in Confined Systems Electrowetting produces its active influence by changing surface wettability using an external electric field. This effect can be used to control the contact angle of the electrolyte on a solid surface, which was first described in Young–Lippmann’s equation [17].

CV2 cos θ = cos θ + (1) 0 2σ Micromachines 2021, 12, x FOR PEER REVIEW 3 of 13

performance, optical stability and energy saving displays. Lastly, the latest status and highlights of R&D will be elaborated on in this article, providing the latest insights in the behavior of oil films in pixels, and the creation of colorful images.

2. Principles: Electrowetting in Confined Systems Electrowetting produces its active influence by changing surface wettability using an external electric field. This effect can be used to control the contact angle of the electrolyte on a solid surface, which was first described in Young–Lippmann’s equation [17].

𝐶𝑉 Micromachines 2021, 12, 206 co𝑠𝜃 cos𝜃 3 of(1) 13 2𝜎

where 𝜃 is the initial contact angle, 𝜃 is the contact angle after applying electrowetting. 𝐶 (= ) is the capacitance. 𝜀 is the vacuum permittivity. 𝜀 and 𝑑 are the relative per- where θ0 is the initial contact angle, θ is the contact angle after applying electrowetting. mittivityε0ε rand thickness of the dielectric layer, respectively. 𝜎 is the interfacial tension be- C (= d ) is the capacitance. ε0 is the vacuum permittivity. εr and d are the relative tweenpermittivity the liquid and and thickness vapor and of the V is dielectric the applied layer, voltage. respectively. For a largeσ is capacitance the interfacial material, tension thebetween contact the angle liquid can and be vaporgreatly and reducedV is the via applied a relatively voltage. small For voltage, a large capacitance which results material, in a surfacethe contact wettability angle canchange be greatly from hydrophobic reduced via (o a relativelyleophilic) smallto hydrophilic voltage, which(oleophobic). results in a surface wettability change from hydrophobic (oleophilic) to hydrophilic (oleophobic). In electrowetting displays, two types of fluid exist in the confined unit; electrically In electrowetting displays, two types of fluid exist in the confined unit; electrically insulating oil inks and electrically conductive liquid electrolytes (e.g., DI water) in the insulating oil inks and electrically conductive liquid electrolytes (e.g., DI water) in the sur- surrounding environment. Surface wettability plays a crucial role in the process of mod- rounding environment. Surface wettability plays a crucial role in the process of modulating ulating the state of oil inks in order to change the aperture ratio of the pixel. A typical unit the state of oil inks in order to change the aperture ratio of the pixel. A typical unit of the of the pixel is schematically shown in Figure 1. In Figure 2, the oil ink is initially filled in pixel is schematically shown in Figure1. In Figure2, the oil ink is initially filled in and and completely covers each pixel unit. Once the electric signal is applied, the oil film re- completely covers each pixel unit. Once the electric signal is applied, the oil film recoils and coilsan aqueous and an aqueous phase occupies phase occupies the open the section open of section the pixel. of the The pixel. electro-capillary The electro-capillary instability instabilitywas considered was considered in the initial in the opening initial opening of oil films of oil in films electro-fluidic in electro-fluidic displays displays [3]. It [3]. was Itderived was derived from from the pressure the pressure balance balance on the on oil-and-water the oil-and-water interface, interface, including including the capillary the ca- pillarypressure pressure to maintain to maintain a flat interfacea flat interface and the and electric the electric stress stress to undulate to undulate the interface. the interface.p(r, t ) 𝑝𝑟, 𝑡 represents the total force pressure considering a reference pressure of 𝑝 . represents the total force pressure considering a reference pressure of p0.

11 p𝑝(x𝑥,, y, 𝑦,t) 𝑡 =𝜎𝛻 (−σ2 + C𝐶"ℎ𝑉00 (h)V2)𝜁𝑥,ζ(x, y, t 𝑦,) + p 𝑡 𝑝 (2)(2) 22 0

Micromachines 2021, 12, x FOR PEER REVIEW 4 of 13 FigureFigure 1. 1. SchematicSchematic plot plot of of electrowetting electrowetting display. display. (Images (Images are are reproduced reproduced from from reference reference [6] [6 with] with thethe permission permission of of Wiley-VCH Wiley-VCH Verlag). Verlag).

FigureFigure 2.2. Experimental demonstrationdemonstration toto showshow “on”“on” and and “off” “off” state state of of electrowetting electrowetting display display (EWD) display(EWD) display units under units electrowetting. under electrowetting.

AA positive value value of of this this pressure pressure promotes promotes film ﬁlm growth. growth. 𝜁𝑥,ζ(x, 𝑦,y, 𝑡 t is) theis the real-time real-time in- interfacialterfacial location. location. 𝐶"ℎC00 (h ) isis the the second second derivative derivative of of the the capacitance capacitance of the oil ﬁlmfilm withwith anan undisturbed thickness of h. In Figure 3, it is theoretically shown that the applied electric signal leads to the generation of a cascade of voltage dependent wave modes. A large voltage or a small oil film thickness will make the interface unstable, and consequently lead to the rupture of the oil film. In the course of validating the theory, it was experimentally observed that a number of undulation modes appear prior to film rupture, and the rupture location does not correspond to the maximum electric field strength in the case of the standard convex water/oil interface used in working devices. These findings provided an improved understanding of the dynamics of the oil rupture process inside the pixel after applying a voltage.

Figure 3. The experimentally observed wave modes and the theoretically expected sequences are compared in solid squares (the aspect ratio is defined as the ratio of width to the height of a pixel). (Images are reproduced from reference [3] with the permission of Springer Nature).

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Figure 2. Experimental demonstration to show “on” and “off” state of electrowetting display Micromachines 2021, 12, 206 (EWD) display units under electrowetting. 4 of 13

A positive value of this pressure promotes film growth. 𝜁𝑥, 𝑦, 𝑡 is the real-time interfacial location. 𝐶"ℎ is the second derivative of the capacitance of the oil film with an undisturbedundisturbed thicknessthickness ofofh. h.In In FigureFigure3 ,3, it it is is theoretically theoretically shown shown that that the the applied applied electric electric signalsignal leadsleads toto thethe generation generation of of a a cascade cascade of of voltage voltage dependent dependent wave wave modes. modes. AA large large voltagevoltage or or a a small small oil oil film film thickness thickness will will make make the interfacethe interface unstable, unstable, and consequentlyand consequently lead tolead the to rupture the rupture of the of oil the film. oil film. In the In course the course of validating of validating the theory, the theory, it was it experimentallywas experimen- observedtally observed that a numberthat a number of undulation of undulation modes appearmodes priorappear to prior film rupture, to film rupture, and the ruptureand the locationrupture doeslocation not does correspond not correspond to the maximum to the maxi electricmum electric field strength field strength in the in case the ofcase the of standardthe standard convex convex water/oil water/oil interface interface used used in working in working devices. devices. These These findings findings provided provided an improvedan improved understanding understanding of the of dynamicsthe dynamics of the of oil the rupture oil rupture process process inside inside the pixel the afterpixel applyingafter applying a voltage. a voltage.

FigureFigure 3. 3.The The experimentallyexperimentally observedobserved wave modes an andd the theoretically theoretically expected expected sequences sequences are are comparedcompared in in solid solid squaressquares (the(the aspectaspect ratioratio isis deﬁneddefined asas thethe ratioratio ofof widthwidth toto thethe heightheight ofof aa pixel).pixel). (Images(Images are are reproduced reproduced from from reference reference [3 [3]] with with the the permission permission of of Springer Springer Nature). Nature).

(initiation stage), oil-dewetting and a slower droplet rearrangement stage (Figure4). For “off” switching, fast oil wetting and reforming of the surface to the flat (dark) state (Figure5) occur. A dynamic model derived from the energy balance perspective has been employed to describe the electro-fluidic response and corresponding optical performance inside an electro-fluidic based display (EFD) pixel. The variability of oil ink motion between on and off-switching and the optical response delay during the on-switching process have been well described and addressed. According to the theoretical approach of static and dynamic conditions for oil ink in confined pixels, this provided a straightforward approach to describe the complex electro- fluidic switching dynamics under electrowetting modulation, which may guide the further optimization of EFD device design and driving schemes. Micromachines 2021, 12, x FOR PEER REVIEW 5 of 13

Furthermore, the dynamic behaviors of oil inks in the dewetting and rewetting pro- cesses, corresponding to the “on” and “off” switching of the pixel unit, were analyzed in an electro-fluidic pixel [4]. For “on” switching, the oil motion leads to oil film rupture (initiation stage), oil-dewetting and a slower droplet rearrangement stage (Figure 4). For “off” switching, fast oil wetting and reforming of the surface to the flat (dark) state (Figure 5) occur. A dynamic model derived from the energy balance perspective has been employed to describe the electro-fluidic response and corresponding optical performance inside an electro-fluidic based display (EFD) pixel. The variability of oil ink motion between Micromachines 2021, 12, 206 side an electro-fluidic based display (EFD) pixel. The variability of oil ink motion5 of 13 between on and off-switching and the optical response delay during the on-switching process have been well described and addressed.

FigureFigure 4. 4.Stages Stages in in the the on on switching switching of an of EWDan EWD pixel. pixel. (Images (Images are reproduced are reproduced from reference from reference [4] [4] withwith the the permission permission of MDPI). of MDPI).

Figure 5. Stages in the off-switching process. (Images are reproduced from reference [4] with the FigureFigure 5. 5.Stages Stages in in the the off-switching off-switching process. process. (Images (Images are reproduced are reproduced from referencefrom reference [4] with [4] the with the permission of Elsevier). permissionpermission of of Elsevier). Elsevier).

3. Materials:According Multilayer to the Dielectrictheoretical approach of static and dynamic conditions for oil ink in confinedDong etpixels, al. [18 this] fabricated provided a Parylene a straightforward C and AF 1600 approach double layer to describe structure the to complex prevent electro- dielectricfluidic switching breakdown. dynamics The advantages under ofelectrowe a doubletting layered modulation, dielectric material which may in terms guide of the fur- stability and reliability over a single layer dielectric material were evaluated by measuring ther optimization of EFD device design and driving schemes. the leaking electric charge. Zhou et al. further characterized the service time by comparing three3. Materials: different dielectric Multilayer materials Dielectric (Cytop (AGC), Teﬂon (Chemours) and Hyﬂon (Solvay)) through3. Materials: thermal Multilayer aging testing Dielectric [19]. The size and shape of the defects were measured underDong a scanning et al. electron[18] fabricated microscope, a Parylene as shown C an ind Figure AF 16006, and double the failure layer modes structure were to prevent analyzeddielectric in breakdown. the development The ofadvantages manufacturing of a techniques.double layered The pinhole-freedielectric material and multi- in terms of layerstability structure and reliability can effectively over avoid a single dielectric layer breakdown,dielectric material therefore, were this evaluated largely extends by measuring thethe reliability leaking electric of the device. charge. Zhou et al. further characterized the service time by comparing three different dielectric materials (Cytop (AGC), Teflon (Chemours) and Hyflon (Solvay)) through thermal aging testing [19]. The size and shape of the defects were measured under a scanning electron microscope, as shown in Figure 6, and the failure modes were analyzed in the development of manufacturing techniques. The pinhole-free and multi-

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Micromachines 2021, 12, 206 6 of 13 layerlayer structure structure can can effectively effectively avoid avoid dielectric dielectric breakdown, breakdown, therefore, therefore, this this largely largely extends extends thethe reliability reliability of of the the device. device.

FigureFigure 6.6. LeakageLeakage currentcurrent measurementsmeasurements withwith stepwisestepwise appliedapplied voltagevoltage atat differentdifferent thermalthermal agingaging testingtesting conditionsconditions forfor Figure 6. Leakage current measurements with stepwise applied voltage at different thermal aging testing conditions for ((aa)) bilayerbilayer AFXAFX (Teflon(Teflon®®AF AF series)/ParyleneC. series)/ParyleneC.( b(b)) Single Single AFX AFX (AFX: (AFX: 1359 1359 nm) nm) device. device. TheThe correspondingcorresponding microscopymicroscopy (a) bilayer AFX (Teflon® AF series)/ParyleneC. (b) Single AFX (AFX: 1359 nm) device. The corresponding microscopy picture of the damage is given. (Images are reproduced from reference [18] with the permission of the Royal Society ). picturepicture of of the the damage damage is is given. given. (Images (Images are are reproduced reproduced from from reference reference [18 [18]] with with the the permission permission ofthe of the Royal Royal Society). Society ). 4. Pixel Architecture: Three-Dimensional Structures 4.4. PixelPixel Architecture:Architecture: Three-DimensionalThree-Dimensional StructuresStructures 4.1. Symmetry Breaking Structure 4.1.4.1. SymmetrySymmetry BreakingBreaking Structure Structure A "symmetry breaking structure" in the pixel facilitates directional openings to AA “symmetry "symmetry breaking breaking structure” structure" in the in pixelthe facilitatespixel facilitates directional directional openings openings to achieve to achieve image/pixel symmetry consistency. The design of a symmetry breaking structure image/pixelachieve image/pixel symmetry symmetry consistency. consistency. The design The design of a symmetry of a symmetry breaking breaking structure structure also also increases the local electric field so that the oil film opens near to or above the sym- increasesalso increases the local the electriclocal electric field sofield that so the that oil the film oil opens film opens near to near or aboveto or above the symmetry the sym- breakingmetrymetry breakingbreaking structure. structure.structure. The effects TheThe of effects theeffects first ofof effect, ththee firstfirst breaking effect,effect, the breakingbreaking symmetry thethe of symmetrysymmetry the square of pixel,of thethe thussquaresquare determine pixel, pixel, thus thus the determine locationdetermine of the the location oillocation film ruptureof of the the oil oil and film film effectively rupture rupture and guideand effectively effectively the ink movement guide guide the the toinkink a movement specificmovement place to to a insidea specific specific the place pixel,place inside inside as can the the be pixel, seenpixel, in as as Figure can can be be7 [seen 12seen]. in in Figure Figure 7 7 [12]. [12].

Figure 7. Symmetry breaking structure (EPS) in different pixel locations results in various oil FigureFigure 7. 7.Symmetry Symmetry breaking breaking structure structure (EPS) (EPS) in in different different pixel pixel locations locations results results in variousin various oil openingoil opening shapes. (Images are reproduced from reference [12] with the permission of Elsevier). shapes.opening (Images shapes. are (Images reproduced are reproduced from reference from [refe12]rence with the[12] permission with the permission of Elsevier). of Elsevier). 4.2. Oil Reserve Structure 4.2.4.2. Oil Oil Reserve Reserve Structure Structure For the conventional planar display unit, the size of the pixel is determined by the oil ﬁlm thickness. Figure8 shows a set of curves for the relationship between aperture ratio, pixel size and oil ﬁlm thickness [20].

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For the conventional planar display unit, the size of the pixel is determined by the oil Micromachines 2021, 12, 206 film thickness. Figure 8 shows a set of curves for the relationship between7 ofaperture 13 ratio, pixel size and oil film thickness [20].

Figure 8. 8.Relationship Relationship between between aperture aperture ratio, pixel ratio, size pixel and oilsize film and thickness oil film (Images thickness are reproduced (Images are repro- ducedfrom reference from reference [20]). [20]). Taking the 5 micron (um) thick oil film as an example, when the pixel size is less than 50Taking um, the the opening 5 micron rate (um) will thick drop tooil 70%. film Ifas the an pixel example, size is when less than the 40pixel um, size the is less than 50opening um, the rate opening will drop rate to 50%. will Indrop order to to 70%. achieve If the smaller pixel pixels size is and less a higherthan 40 opening um, the opening raterate, will a feasible drop methodto 50%. is In to order make theto achieve oil film thinner.smaller However,pixels and the a oil higher film cannot opening be rate, a fea- siblevery thinmethod since ais thin to oilmake film the will oil reduce film the thinner. illuminating However, contrast. the The oil design film ofcannot the “oil be very thin sincereserve a structure”thin oil film realizes will the reduce storage the of the illuminating oil inks and iscontrast. free of the The limitations design of of the the oil “oil reserve film thickness. Consequently, this effectively avoids the occurrence of the oil flow over the structure”wall (overflows). realizes As athe result, storage this effectively of the oil prevents inks and the opticalis free defects.of the limitations In addition, theof the oil film thickness.oil grooves Consequently, potentially limit oilthis movement effectively which avoids makes the bi-stable occurrence electrowetting of the oil displayflow over the wall (overflows).possible [11]. InAs 2009, a result, Heikenfeld this eteffectively al. [21] developed prevents a three-dimensional the optical defects. electrowetting In addition, the oil groovesdisplay, as potentially shown in Figure limit9 oil. The movement honey-comb which like structuremakes bi-stable enables oilelectrowetting spreading and display pos- siblestoring. [11]. When In 2009, there Heikenfeld is no electric et signal, al. [21] the de colorveloped ink collects a three-dimensional in the storage tank. electrowetting The display,transparent as shown oil layer in Figure is laid on9. The the surface honey-comb of the hydrophobic like structure insulation enables layer, oil spreading the light and stor- reflects through the oil layer and displays as white. When the voltage is applied, the ing.ink is When distributed there to is the no surfaceelectric of signal, the hydrophobic the color insulationink collects layer in duethe tostorage the effect tank. of The trans- parentelectrowetting. oil layer In is the laid meantime, on the surface the oil phaseof the originallyhydrophobic laid on insulation the display layer, surface the islight reflects throughpushed to the the oil conduit, layer theand light displays reflects as and white. displays When the the color voltage of the is ink. applied, The structure the ink is distrib- utedis good to forthe the surface storage of of the the hydrophobic inks and increases insulati the openingon layer rate due of to the the pixels, effect therefore of electrowetting. Inimproving the meantime, the reflectivity. the oil phase originally laid on the display surface is pushed to the con- In addition, the “oil reserve structure” was also applied on a conventional square duit, the light reflects and displays the color of the ink. The structure is good for the stor- display panel [22], as shown in Figure 10. This structure enables good control over the agemovement of the ofinks the and ink throughincreases the the asymmetric opening distribution rate of the of pixels, the electric therefore field, withoutimproving the reflectivity.affecting the visual effect. However, further improvements are required to resolve the issue related to the groove structure producing a local pinning force which has a certain impact on the reversibility of the ink movement.

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(a) (b)

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(a) (b) (a) (c) (b)

Figure 9. Three-dimensional honey-comb pixel structure and display principle for oil spreading and storing (a) honey- comb oil reservoir (b) oil-spreading for pixel opening (c) oil-storing for pixel closing (Images are reproduced from reference [21] with the permission of Springer Nature).

In addition, the “oil reserve structure” was also applied on a conventional square display panel [22], as shown in Figure 10. This structure enables good control over the movement of the ink through the (asymmetricc) distribution of the electric field, without Figure 9. Three-dimensionalaffecting honey-comb the visual pixel effect. structure However, and( displayc )further principle improvements for oil spreading are required and storing to resolve (a) honey-comb the is- Figure 9. Three-dimensionalsue related honey-comb to the groove pixel structure structure and producing display principle a local for pinning oil spreading force which and storing has a ( acertain) honey- oilcomb reservoir oil reservoir (b) oil-spreading (b) oil-spreading for pixel for opening pixel opening (c) oil-storing (c) oil-storing for pixel for closing pixel closing (Images (Images are reproduced are reproduced from reference from refer- [21] Figure 9. Three-dimensionalimpact onhoney-comb the reversibility pixel structure of the inkand movement. display principle for oil spreading and storing (a) honey- withence the[21] permission with the permissi of Springeron of Nature).Springer Nature). comb oil reservoir (b) oil-spreading for pixel opening (c) oil-storing for pixel closing (Images are reproduced from reference [21] with the permission of Springer Nature). In addition, the “oil reserve structure” was also applied on a conventional square displayIn addition,panel [22], the as “oil shown reserve in Figure structure” 10. This was structure also applied enables on agood conventional control over square the movementdisplay panel of the[22], ink as throughshown in the Figure asymmetric 10. This di structurestribution enables of the goodelectric control field, overwithout the affectingmovement the of visual the ink effect. through However, the asymmetric further improvements distribution are of therequired electric to resolvefield, without the is- sueaffecting related the to visual the groove effect. However,structure producing further improvements a local pinning are requiredforce which to resolve has a certain the is- impactsue related on the to reversibilitythe groove structure of the ink producing movement. a local pinning force which has a certain impact on the reversibility of the ink movement.

(a) (b)

Figure 10. Illustration of the bi-stable “groove” EWD pixel structure (a) On-state “planner” pixel Figure 10. Illustration of the bi-stable “groove” EWD pixel structure (a) On-state “planner” pixel structurestructure (b) On-state (b) On-state bi-stable bi-stable “groove” “groove” pixel structur pixel structure.e. (Images (Images are reproduced are reproduced from reference from reference [22] [22] withwith the the permission permission of ofAmerican American Chemical Chemical Society). Society). 5. Electrical Driving: Optimization of Opening Behavior and Greyscale Control 5. Electrical Driving: Optimization of Opening Behavior and Greyscale Control A good optical performance of an electrowetting display is realized by controlling the A good optical performance of an electrowetting display is realized by controlling movement of ink under(a) optimized driving signals. When the initial(b) voltage is very low, the movement of ink under optimized driving signals. When the initial voltage is very the ink area is basically invariable. However, the voltage should not be too high otherwise low, the ink area is basically(a) invariable. However, the voltage should (notb) be too high oth- itFigure will 10. cause Illustration the oil ﬁlmof the to bi-stable break into“groove” a number EWD pixel of small structure oil droplets, (a) On-state which “planner” reduces pixel the erwisestructure it will cause (b) On-state the oil bi-stablefilm to break “groove” into pixel a number structur ofe. small (Images oil aredroplets, reproduced which from reduces reference displayFigure 10. area. Illustration In addition, of the bi-stable it can easily“groove” penetrate EWD pixel the structure hydrophobic (a) On-state insulation “planner” layer pixel and the display[22] with area. the In permission addition, of it Americancan easily Chemical penetrate Society). the hydrophobic insulation layer and damagestructure the(b) On-state pixel unit. bi-stable “groove” pixel structure. (Images are reproduced from reference damage the pixel unit. [22] withIn order the permission to display of different American gray Chemical scales inSociety). an electrowetting display, a voltage sequence 5. Electrical Driving: Optimization of Opening Behavior and Greyscale Control Inmust order be to applied display in different order to controlgray scales the grayin an scale. electrowetting This voltage display, sequence a voltage is related se- to the quencedriving5. must ElectricalA begood waveform. applied Driving: optical in The orderperformance Optimization driving to control waveforms of ofthean Opening electrowegray commonly scale. Behaviortting This useddisplay voltage and in Greyscale anis sequence realized electrowetting isControl by related controlling display to the arethedriving pulsemovement waveform. width of modulation ink The under driving (PWM)optimized wavefo [23] driving andrms amplitudecommonly signals. modulation Whenused in the an initial (AM)electrowetting voltage [16]. However, is very bothlow, the of these ink area methods is basically have drawbacks.invariable. However, The high framethe voltage rate of should conventional not be too PWM high limits otherwise it will cause the oil film to break into a number of small oil droplets, which reduces the display area. In addition, it can easily penetrate the hydrophobic insulation layer and damage the pixel unit.

In order to display different gray scales in an electrowetting display, a voltage sequence must be applied in order to control the gray scale. This voltage sequence is related to the driving waveform. The driving waveforms commonly used in an electrowetting

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Micromachines 2021, 12, 206 display are pulse width modulation (PWM) [23] and amplitude modulation (AM) [16]9 of 13. However, both of these methods have drawbacks. The high frame rate of conventional PWM limits the gray level and causes large visual oscillations. The disadvantage of AM method is mainly attributed to the hysteresis effect. the gray level and causes large visual oscillations. The disadvantage of AM method is mainlyA more attributed functional to the driving hysteresis waveform effect. is based on the modified PWM square waves, as shownA more in Figure functional 11 [24] driving . The waveformfaster rising is rate based results on the in modified a shorterPWM ink rearrangement square waves, timeas shown after oil in film Figure rupture 11[ 24 [25].]. The When faster a risingsharp ratedriving results waveform in a shorter is applied, ink rearrangement the oil film breakstime after into oiloil filmdrops rupture which [move25]. When to different a sharp corners; driving a waveform process called is applied, oil splitting the oil [15]. film Slowbreaks waveforms, into oil drops such whichas sinusoidal move to waveforms different corners; (as shown a process in Figure called 11b), oil on splitting the other [15]. hand,Slow can waveforms, effectively such reduce as sinusoidal the oil splitting waveforms and slowly (as shown open the in Figure pixel. However,11b), on the the other ris- inghand, time can is long, effectively which reduce limits thetheoil response splitting rate. and Therefore, slowly open the the rising pixel. voltage However, waveform— the rising astime shown is long, in Figure which 11a—that limits the is response employed rate. to start Therefore, from the the threshold rising voltage voltage waveform—as and rise to theshown pixel in opening Figure 11voltagea—that in isthe employed form of a to si startne wave, from not the only threshold improves voltage the andopening rise torate, the butpixel also opening reduces voltage the oil insplitting the form effect. of a sine wave, not only improves the opening rate, but also reduces the oil splitting effect.

(a) (b)

Figure 11. Modified pulse width modulation (PWM) driving waveforms (a) sinusoidal driving waveform after optimization Figure 11. Modified pulse width modulation (PWM) driving waveforms (a) sinusoidal driving waveform after optimization(b) sinusoidal(b) sinusoidal driving driving waveform waveform (c) ramp (c) ramp driving driving waveform waveform (d) square(d) square wave wave driving driving waveform. waveform. (Images (Images are reproducedare repro- ducedfrom referencefrom reference [24] with [24] the with permission the permission of Springer of Springer Japan). Japan). When a voltage is applied, the oil film opens and the oil ink is pushed towards the When a voltage is applied, the oil film opens and the oil ink is pushed towards the corner of the pixel. After a certain period of time, some ions from the liquid electrolyte enter corner of the pixel. After a certain period of time, some ions from the liquid electrolyte the insulation layer and are trapped, thus causing ink “backflows” [26]. The experimental enter the insulation layer and are trapped, thus causing ink “backflows” [26] . The exper- results showed that when an alternating current voltage is applied, the captured charge imental results showed that when an alternating current voltage is applied, the captured can be repeatedly released and recaptured [23]. Therefore, the charge capture behavior charge can be repeatedly released and recaptured [23]. Therefore, the charge capture be- can be controlled by applying the alternating current signal so as to limit the ink backflow havioreffect andcan be improve controlled the displayby applying stability. the alternating As shown incurrent Figure signal 12, an so initial as to limit voltage the ofink 15 backflowV is applied effect to and both improve upper the and display lower plates,stability. the As voltage shown differencein Figure 12, is 0an and initial the voltage oil film ofremains 15 V is inapplied place. to When both theupper bottom and voltagelower plates, changes the tovoltage−15 V difference the pixels is slowly 0 and openthe oil to filmshow remains the state in atplace. T2, and When the the ink bottom shrinks voltage to the corner changes of the to − pixel.15 V Atthe T3,pixels due slowly to the chargeopen tocapture, show the the state oil droplets at T2, and gradually the inkflow shrinks back. to Ifthe the corner reflectivity of the is pixel. reduced At T3, to onedue tenth to the of chargethe maximum capture, value,the oil the droplets voltage gradually between flow the plates back. is If set the to reflectivity 15 V. In this is way,reduced the trappedto one charges are neutralized. Once the signal is set to −15 V, the reflectivity will rise again. A periodic reset will ensure a stable and high reflectivity.

Micromachines 2021, 12, x FOR PEER REVIEW 10 of 13 Micromachines 2021, 12, x FOR PEER REVIEW 10 of 13

tenth of the maximum value, the voltage between the plates is set to 15 V. In this way, the trappedtenth of chargesthe maximum are neutralized. value, the Oncevoltage the between signal is th sete plates to −15 is V,set the to 15re ﬂV.ectivity In this willway, rise the Micromachines 2021, 12, 206 again.trapped A periodiccharges arereset neutralized. will ensure Once a stable the and signal high is reflectivity.set to −15 V, the reﬂectivity will10 ofrise 13 again. A periodic reset will ensure a stable and high reflectivity.

Figure 12. The voltage drive waveform of periodic reset and the change of reflectivity. (Images are reproducedFigure 12. The from voltage reference drive [26] waveform with the of permission periodic reset of Wiley-Blackwell). and the change of reflectivity. reflectivity. (Images are reproducedreproduced fromfrom referencereference [[26]26] with with the the permission permission of of Wiley-Blackwell). Wiley-Blackwell). To provide a brief discussion of the amplitude-and-frequency modulation of the electric signal,To provide the chosen a a brief brief voltage discussion discussion and frequencyof ofthe the amplit amplitude-and-frequency shouldude-and-frequency be able to prevent modulation the modulation display of thedefects of elec- the (splitting,electrictric signal, signal, overflow the chosen the chosenand voltage backflow) voltage and infrequency and order frequency to shimproveould should be the able display be to able prevent performance to prevent the display the and display defects opti- caldefects(splitting, stability. (splitting, overflow Therefore, overflow and the backflow) andinitial backflow) voltage in order inpotential to order improve to is improve set the to display athe value display performance just performancebelow the and rising opti- and thresholdopticalcal stability. stability. and Therefore, the Therefore, applied the thevoltage initial initial voltageis voltageslowly potential potentialincreased is is setin set theto to a forma value value of just justa sinusoidal below below the wave. risingrising Thisthresholdthreshold will both and improve the appliedapplied the opening voltage israte slowly and re increasedduce ink insplitting the form and of oil a sinusoidalsinusoidaloverflows to wave. the adjacentThisThis willwill pixels. bothboth improveimprove The frequency thethe openingopening of the raterate periodic andand reducere squareduce inkink waves splittingsplitting is also andand fitted oiloil overflowsoverflows to prevent toto the thethe phenomenonadjacent pixels. of inkThe backflow frequency so ofthat the the periodic pixel can square maintain waves a constant is also fittedfitted optical toto reflectivity. preventprevent thethe phenomenonphenomenon ofof inkink backflowbackflow soso that the pixel can maintain a constant optical reflectivity.reflectivity. 6. Demonstration Displays and Color Reproduction 6. Demonstration Displays and Color Reproduction 6.1.6. Demonstration Color Mixing Displays and Color Reproduction 6.1. Color Mixing 6.1. ColorThe color Mixing system for this display is based on a cyan, magenta, yellow subtractive The color system for this display is based on a cyan, magenta, yellow subtractive color Thesystem. color This system means for that this three display layers is basedof color on have a cyan, to be magenta, assembled yellow and subtractiveseparately color system. This means that three layers of color have to be assembled and separately driven.color system. Therefore, This displaysmeans that have three to be layers alig nedof color accurately, have to substrat be assemblede thickness and separatelyhas to be driven. Therefore, displays have to be aligned accurately, substrate thickness has to be minimizeddriven. Therefore, to limit parallaxdisplays (Figurehave to 13) be andalig lightned accurately, losses have substrat to be minimizede thickness in hasorder to tobe minimized to limit parallax (Figure 13) and light losses have to be minimized in order to achieve brilliant colors. achieveminimized brilliant to limit colors. parallax (Figure 13) and light losses have to be minimized in order to achieve brilliant colors.

FigureFigure 13. 13. ParametersParameters influencing inﬂuencing parallax. parallax. (Images (Images are are reproduced reproduced from from reference reference [27] [27]). ). Figure 13. Parameters influencing parallax. (Images are reproduced from reference [27] ). 6.2.6.2. System System 6.2. SystemTheThe target target for for the the display display demonstration demonstration is is to—as to—as much much as as possible—use possible—use “standard” “standard” drivingdrivingThe equipment. equipment. target for the In In orderdisplay order to to demonstrationachieve achieve this this,, the the is to—asimage image sourcemuch source as of of possible—use the the demonstrated “standard” panel consistsdriving equipment. of a standard In order Android to achieve system, this providing, the image standard source of RGB the outputdemonstrated to drive panel the display panels. The adaptation is surprisingly easy, once it is realized that cyan is the complement of red, magenta is the complement of green, and yellow is the complement of blue. Therefore, by driving the cyan panel with the red signal, the signal generates exactly the right response (less “cyan” means more “red”). Micromachines 2021, 12, x FOR PEER REVIEW 11 of 13

consists of a standard Android system, providing standard RGB output to drive the display panels.

The adaptation is surprisingly easy, once it is realized that cyan is the complement of red, magenta is the complement of green, and yellow is the complement of blue. There- Micromachines 2021, 12, 206 11 of 13 fore, by driving the cyan panel with the red signal, the signal generates exactly the right response (less “cyan” means more “red”).

Of course, some adaptation may be necessary for grayscale spacing, but this is rela- Of course, some adaptation may be necessary for grayscale spacing, but this is rel- tively simple, and can be done by adjusting the greyscale “gamma” value. Apart from atively simple, and can be done by adjusting the greyscale “gamma” value. Apart from this,this, thethe systemsystem hashas toto accommodateaccommodate forfor thethe peculiaritiespeculiarities ofof thethe electrowettingelectrowetting system,system, makingmaking sure sure the the charge charge rate rate of of the the pixel pixel does does not not exceed exceed a a certain certain limit limit value, value, as as described described inin previousprevious sections.sections. Furthermore,Furthermore, thethe systemsystem makesmakes useuse ofof thethe ﬁeldfield responseresponse andand lowlow leakageleakage currentcurrent propertiesproperties ofof thethe display,display, making making it it possible possible to to refresh refresh a a still still image image with with longlong intervals, intervals, thereby thereby minimizing minimizing the the power. power.

7.7. ConclusionsConclusions TheThe electrowetting electrowetting display display theory, theory, technological technological development development of dielectric of dielectric layers, layers, pixel structurepixel structure designs, designs, electric electric signal modulationsignal modula andtion the and current the progresscurrent progress made in made terms ofin colorterms displayof color are display reviewed. are reviewed. The physical The mechanismphysical mechanism of electrowetting of electrowetting was first introduced was first intro- and theduced failure and mode the failure of the dielectricmode of layerthe dielectric was analyzed. layer was Then, analyzed. the roles Then, of the the pixel roles structure of the andpixel driving structure mechanism and driving in solvingmechanism display in solving defects display were discussed. defects were The discussed. “asymmetric The breaking“asymmetric structure” breaking is helpful structure” to control is helpful the opening to control direction the opening of the direction oil film, which of the ensuresoil film, thewhich consistency ensures the of the consistency pixel opening. of the The pixel use open of theing. “oil The reserve use of structure”the “oil reserve is good structure” for ink storageis good andfor ink control storage of ink and movement, control of ink which movement, greatly preventswhich greatly display prevents drawbacks, display such draw- as backflowbacks, such and as overflow. backflow Theand bi-stableoverflow. structural The bi-stable features structural also reduce features energy also reduce consumption energy andconsumption extend the and service extend time. the The service driving time. scheme The driving is adopted scheme by a two-stepis adopted method. by a two-step Firstly, amethod. threshold Firstly, voltage a threshold is set in the voltage preparation is set in stage, the preparation and then it stage, slowly and rises then to theit slowly threshold rises forto the opening threshold the oil for film opening in a gradual the oil film waveform. in a gradual Secondly, waveform. after maintainingSecondly, after this maintain- voltage differenceing this voltage for a period difference of time, for periodic a period alternating of time, periodic signal regulation alternating is carriedsignal regulation out in time. is Thiscarried waveform out in time. effectively This waveform prevents theeffectively oil film splittingprevents intothe oil several film splitting smaller oilinto droplets several andsmaller also oil avoids droplets the and oil backflowsalso avoids caused the oil backflows by the capture caused of by charges the capture in the of dielectrics. charges in Therefore,the dielectrics. future Therefore, research canfuture explore research the fabricationcan explore of the multi-layer fabrication dielectrics, of multi-layer the design dielec- oftrics, hierarchical the design pixel of structureshierarchical and pixel the optimizationstructures and of the driving optimization schemes, of which driving are expectedschemes, towhich resolve are theexpected current to limitations resolve the in current EWD andlimitations improve in displayEWD and performance. improve display Lastly, per- a fullformance. color reflective Lastly, a video full color speed reflective electrowetting video speed display electrowetting panel (Figure display 14) is proposedpanel (Figure for future14) is proposed applications. for future applications.

FigureFigure 14. 14.Prototype Prototype 5.8”5.8” full full color color reﬂective reflective video video speed speed electrowetting electrowetting display. display.

Author Contributions: Y.L., B.T., G.Y. and Y.G., wrote the initial draft of the manuscript. A.H. and Y.L., revised the paper. L.L., contributed to the experiments which were designed by A.H. All authors have read and agreed to the published version of the manuscript. Micromachines 2021, 12, 206 12 of 13

Funding: This work was supported by National Key R&D Program of China (No.2016YFB0401502), Science and Technology Program of Guangzhou (No. 2019050001), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), Program for Guangdong Innovative and Enterpreneurial Teams (No. 2019BT02C241), Science and technology project of Guangdong Province (No. 2018A050501013), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No. 2017B030301007), Guangzhou Key Laboratory of Electronic Paper Displays Materials and Devices, Science and Technology Program of Guangzhou (No.201904020007), MOE International Laboratory for Optical Information Technologies and the 111 Project. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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