Electrowetting optical element

Abstract

The present invention relates to an electrowetting optical element comprising: a first electrode layer stack comprising a substrate and a first electrode layer; a second electrode layer stack including a substrate and a second electrode layer; an accommodation space formed between the first and second electrode layer stacks; one or more cell walls extending between the first and second electrode stacks for defining a side of the accommodation space forming a cell; the accommodating space comprises a polar liquid and a non-polar liquid, and the polar liquid and the non-polar liquid are not dissolved with each other; wherein the first and second electrode layers are configured to apply a voltage between the electrodes to reconfigure the polar liquid relative to the non-polar liquid; wherein the first electrode layer stack further comprises an insulating layer disposed between the first electrode layer and the receiving space, forming an interface with the receiving space, and wherein the insulating layer comprises a first and a second group of polymers, the first group of polymers comprises functional hydrophilic chemical groups configured to form covalent bonds with the first electrode layer stack, and the second group comprises polymers without functional hydrophilic chemical groups, and wherein the first and second groups of polymers are configured as an entangled polymer network in the insulating layer.

Description

Technical Field

[0001] The present disclosure generally relates to electrowetting optical elements and methods of fabricating electrode layer stacks in such electrowetting optical elements. More specifically, the present disclosure relates to an insulating layer that is included in an electrode layer stack in such an electrowetting optical element, where the insulating layer improves the operational reliability of the optical element over a wide temperature range.

[0002] The present disclosure also relates to methods of fabricating such electrode layer stacks in electrowetting optical elements and in displays that include such elements. Background Art

[0003] Electrowetting is based on the change that, by varying the intensity of an electric field applied across an insulating layer, the effective wetting preference of the hydrophobic surface of the insulating layer for a non-polar liquid relative to a polar liquid is changed. The insulating layer, the polar liquid, and the non-polar liquid thus become part of a capacitor assembly that also includes electrodes across which a voltage can be applied to establish an electric field across the insulating layer.

[0004] An electrowetting optical element, which may also be referred to as an electrowetting element in the present disclosure, according to the state of the art, from bottom to top, i.e., when viewed in the reverse observation path, may include a first electrode layer stack and a second electrode layer stack, respectively. The first electrode layer stack includes at least a substrate, a first electrode layer, and an electrically insulating layer on top of the first electrode layer. The second electrode layer stack includes at least an upper substrate and a second electrode layer.

[0005] An accommodation space is formed between the first electrode layer stack and the second electrode layer stack, in which a polar liquid and a non-polar liquid are accommodated between a plurality of cell walls that extend between the first electrode layer stack and the second electrode layer stack. The polar liquid and the non-polar liquid are immiscible with each other. Separate cells are formed between these cell walls and the first electrode layer stack and the second electrode layer stack. In an optical device in which an electrowetting optical element is arranged, at least one cell, but preferably a plurality of cells together, can define a separate pixel. A pixel is considered the smallest addressable element in a display and thus, in the present disclosure, a pixel element includes at least one electrowetting cell. A pixel can accommodate one, but can also be any number of cells, such as an even number of cells or an odd number of cells.

[0006] It is stated that the definition of the accommodation space should be interpreted in a functional and non-restrictive manner. This means that polar or non-polar liquids can flow freely or partially from one part of the accommodation space to another part of the accommodation space. To allow or facilitate such migration, the cell walls disposed between the first electrode layer stack and the second electrode layer stack do not completely encapsulate polar and non-polar liquids in isolated spaces, but allow these liquids to migrate completely or restrictedly from one cell to adjacent cells, and thus from one part of the accommodation space to an adjacent part of the accommodation space. Therefore, the definition of the accommodation space is used as a known definition, but is not limited to examples that prevent liquid migration between adjacent accommodation spaces.

[0007] For example, the above-described electrowetting element is known from US 9,274,331B2, which has the same applicant as the present disclosure. The electrowetting element disclosed therein is configured to energize the first electrode layer and the second electrode layer to enable reconfiguration of polar liquids relative to non-polar liquids. The element has a first electrode layer stack and a second electrode layer stack and cell walls with a typical structure, where the cell walls extend between the first electrode layer stack and the second electrode layer stack, and where the cell walls are fixedly mounted on the second electrode layer stack but not fixedly mounted on the first electrode layer stack. This non-fixed mounting can also mean that the end face of the cell wall faces the first electrode layer stack in such a way that the end face contacts the first electrode layer stack in a loose manner.

[0008] Thus, as described, the insulating layer in the first electrode layer stack is electrically insulating to substantially prevent electrical conduction and / or short-circuiting between the first electrode layer and the polar liquid in the accommodation space. The insulating layer can also form a hydrophobic interface with the accommodation space including polar and non-polar liquids.

[0009] It has been found that known electrowetting displays including such electrowetting optical elements exhibit aging phenomena during display operation at elevated temperatures.

[0010] At room temperature, the electrowetting display can operate normally, but when the temperature rises above room temperature or drops significantly, it is observed that the electrowetting performance of the display deteriorates, which is particularly relevant for outdoor applications. Summary of the Invention

[0011] The object of the present disclosure is to eliminate the above problems and drawbacks existing in the prior art, and more specifically to provide an electrowetting optical element that has improved and more reliable electrowetting effects at temperatures significantly different from room temperature.

[0012] The object of the present invention is also to provide a method for manufacturing or processing an electrowetting optical element that has improved and more reliable electrowetting effects at temperatures significantly different from room temperature.

[0013] According to a first aspect of the present disclosure, the above object is achieved by an electro-wetting optical element, which includes:

[0014] A first electrode layer stack including a substrate and a first electrode layer;

[0015] A second electrode layer stack including an upper substrate and a second electrode layer;

[0016] A receiving space formed between the first electrode layer stack and the second electrode layer stack;

[0017] One or more unit walls extending between the first electrode stack and the second electrode stack for defining sides of the receiving space forming the unit;

[0018] The receiving space includes a polar liquid and a non-polar liquid, and the polar liquid and the non-polar liquid are immiscible with each other;

[0019] Wherein, the first electrode layer and the second electrode layer are configured to apply a voltage between the electrodes to reconfigure the polar liquid relative to the non-polar liquid;

[0020] Wherein, the first electrode layer stack further includes an insulating layer disposed between the first electrode layer and the receiving space, forming an interface with the receiving space, wherein the insulating layer includes a first set of polymers and a second set of polymers, the first set of polymers includes functional hydrophilic chemical groups configured to form covalent bonds with the first electrode layer stack, and the second set includes polymers without functional hydrophilic chemical groups, and wherein the first set of polymers and the second set of polymers are configured as a tangled polymer network in the insulating layer.

[0021] The working principle of the electrowetting element is as follows. In the non-powered state or disabled power mode of the first electrode and the second electrode, that is, when no voltage is applied between the first electrode and the second electrode, when the non-polar liquid forms a boundary layer between the polar liquid and the hydrophobic surface of the insulating layer, the system is in the state of the lowest energy. This is because the non-polar liquid preferentially wets the hydrophobic surface, which effectively repels the contact of the polar liquid with the hydrophobic surface. If the non-polar liquid is an optically absorbing liquid in at least part of the visible light wavelength region, the optical absorption of the non-polar liquid forms an obstacle to the incident light penetrating the system, thereby generating an electrowetting element with a reduced optical transmittance in at least part of the visible light wavelength region. When a voltage is applied between the first electrode and the second electrode, an electric field is established between the second electrode (short-circuited with the conductive polar liquid) and the first electrode across the total thickness of the insulating layer and the non-polar liquid, and the lowest energy state of the system becomes the case where the poorly conductive or insulating non-polar liquid is at least partially pushed aside by the force of the applied electric field by the conductive polar liquid. In fact, applying a voltage between the electrodes reduces the preferential wetting of the hydrophobic surface by the non-polar liquid. If the applied voltage is large enough, the hydrophobic surface is preferentially wetted by the polar liquid, thereby displacing the non-polar liquid from the hydrophobic surface to a large extent. Inside the electrowetting unit, the shape of the displaced non-polar liquid thus changes from a lenticular liquid film to a contracted droplet. In this case, if the polar liquid is a liquid that is substantially optically non-absorbing in the visible light wavelength region, the degree of absorption of the incident light penetrating the system by the non-polar liquid is reduced, thereby improving the optical transmittance of the electrowetting element to the incident light.

[0022] When the electrodes are switched from the energized state back to the non-energized state, by eliminating the voltage applied between the electrodes, the electric field on the insulating layer fails, and the system changes back to the lowest energy state of the system before the electrodes are energized, where the hydrophobic layer is preferentially wetted by the non-polar liquid into the shape of a non-polar liquid film, thereby displacing the polar liquid from the hydrophobic surface of the insulating layer.

[0023] It has been found that for the electrowetting element to maintain the structural integrity and high hydrophobicity of the insulating layer during electrowetting operation within a wide temperature range from -30 °C to 70 °C, the physical and chemical properties of the insulating layer play an important role.

[0024] The insulating layers of the prior art mainly include inorganic materials, and defects and pores are prone to appear in the inorganic part of the insulating layer. When a potential difference is applied across the insulating layer, it is very easy to generate a conduction current and / or short circuit during the electrowetting operation.

[0025] During electrowetting operation, especially during operation at elevated (outdoor) temperatures, prior art hydrophobic hydrocarbon or fluorocarbon monolayers disposed on top of an insulating organic or inorganic layer will rapidly undergo chemical degradation. Chemical degradation of the interface between the insulating layer and the polar liquid tends to make the interface more hydrophilic, which impairs the electrowetting performance under the application of a potential difference across the insulating layer: Therefore, the interface must be able to maintain a high degree of hydrophobicity over a wide temperature range during electrowetting operation.

[0026] Insulating layers used in known devices may consist of only a single layer thickness of fluorocarbon material, such as Teflon AF (DuPont), Cytop-S (AGC Chemicals), or Fluoropel (Cytonics). Such insulating layers are known to have problems with adhesion to electrode materials and are also known to exhibit pores and defects.

[0027] It has been found that insulating layers composed of layers of these fluorocarbon materials typically lack functional chemical groups capable of bonding to electrode materials, e.g., unable to form covalent bonds with hydroxyl groups on the surface of the electrode (ITO) and / or the glass substrate surface, and thus adhesion problems with electrode materials may occur and delamination may occur.

[0028] On the other hand, an insulating polymer material known to be able to bond to the electrode and / or glass surface is Cytop-M (AGC Chemicals). Cytop-M has functional aminosilyl groups attached to both ends of the polymer chain for chemical bonding with hydroxyl groups on the surface of the electrode (ITO) and / or the glass surface. However, it has been found that especially at higher temperatures, when the insulating layer of Cytop-M material is exposed to a polar liquid, it gradually loses its hydrophobicity. It has been determined that the decrease in the degree of hydrophobicity is due to the transfer of non-bonded hydrophilic aminosilane end groups on the polymeric fluorocarbon chain to the interface between the insulating layer and the polar liquid. In addition, the fluorocarbon layer composed of Cytop-M exhibits many defects when used as an insulating layer, resulting in rapid electrical breakdown.

[0029] However, an insulating layer including a chemically crosslinked hydrocarbon material covalently bonded to an ITO electrode material or another inorganic insulating material disposed on the ITO electrode material can also be achieved using parylene. Parylene can initially achieve the required electrowetting characteristics in a polar liquid environment, but the hydrophobicity of the parylene / polar liquid interface gradually decreases, which may be due to hydrolysis of the parylene material, thereby generating oxygen-containing hydrophilic chemical groups at the interface. Polar liquids typically include polar fluids such as water, ethylene glycol, and / or glycerol or mixtures thereof. Non-polar liquids typically consist of alkane fluids such as decane and dodecane.

[0030] An electrowetting optical element according to the present disclosure includes a first electrode layer stack and a parallel second electrode layer stack, which are spaced apart from each other by a certain distance. The distance between the first electrode layer stack and the second electrode layer stack is bridged by a plurality of cell walls, which form different spaces between the two stacks. These spaces can be referred to as cells and can be embodied as completely sealed accommodation spaces in which immiscible polar and non-polar liquids are enclosed. The space can also be an accommodation space in which these liquids are only loosely enclosed, such that to a certain extent, one of these liquids, such as the polar liquid, is allowed to migrate into an adjacent space. For example, the accommodation space can be constructed in such a way that the cell walls are fixedly attached to one of the electrode layer stacks and are only placed in loose mechanical contact with the opposite electrode layer stack, and this loose mechanical contact allows a small amount of non-polar liquid to migrate into an adjacent cell or accommodation space. The present disclosure is not limited to any of the above configurations, but can be implemented in any configuration of an electrowetting element that at least has a substrate and an insulating layer in its first electrode layer stack.

[0031] The first electrode layer stack includes an insulating layer. The insulating layer is disposed on top of the stack and interfaces with the accommodation space, thereby interfacing with the polar and non-polar liquids included in the space.

[0032] According to the inventors' insights, an insulating layer including two polymers with different properties can combine the advantages of two aspects. On the one hand, it can be strongly bonded to the first electrode layer stack (such as the ITO material of the electrode layer and / or the glass substrate), and on the other hand, it can maintain a high degree of hydrophobicity at the interface with the polar and non-polar liquids.

[0033] The insulating layer includes a first set of polymers and a second set of polymers, where the first set of polymers includes functional hydrophilic chemical groups capable of bonding to the first electrode layer stack because it is configured to form covalent bonds with the layer stack, and where the second set of polymers includes polymers without functional hydrophilic chemical groups. When the polymers of the second set of polymers migrate across the insulating layer to the interface with the polar liquid, due to their lack of hydrophilic chemical groups, they can maintain the hydrophobicity at the interface and prevent hydrolysis.

[0034] Since the first set of polymers and the second set of polymers form an entangled polymer network, a strong mechanical bond is formed between the two sets of polymers. The first set has already established a strong chemical (covalent) bond with the first electrode layer stack and has a polymer chain length long enough to allow the polymers from the second set of polymers to entangle with the first set of polymers, thereby achieving an entangled polymer network.

[0035] Entanglement can be defined as entanglement between molecular chains. Therefore, there may be a certain degree of entanglement between the chains of polymers in the same group, especially for polymers with longer polymer chain lengths. However, according to the present disclosure, the first group of polymers and the second group of polymers are bonded through entanglement between molecular chains.

[0036] The entanglement can be achieved or promoted by pendant loops present in the chains of the polymers from the first group of polymers. The loops are formed between two functional groups that are evenly covalently bonded to the first electrode layer stack on the chain. The loops between the bonded functional groups provide space for the chains of the second group of polymers to pass through during deposition, thereby achieving a certain degree of entanglement. The size of the loops depends on multiple aspects, such as the number of functional hydrophilic chemical groups per unit chain length, the polymer chain length, and the free volume of the polymer cross - structure (matrix) formed by the voids between the entangled polymer chains. Therefore, the effective free volume or average size of the loops can be determined by the average distance between two adjacent covalent bonds by which a single polymer chain from the first group of polymers is attached to the first electrode layer stack and the free volume that may exist between multiple polymer chains from the first group of polymers that have multiple covalent bonds with the first electrode layer stack. Thus, the entanglement can be interpreted as the entanglement of the polymers of the second group of polymers through the loops of the polymer chains of the first group of polymers on the surface of the first electrode layer stack or through the free volume existing between multiple polymer chains of the first group of polymers.

[0037] The deposition method, degree, or height of the first group of polymers is such that the second group of polymers has sufficient loops or free volume during and after its deposition. Preferably, the polymers from the first group of polymers are deposited in the form of a polymer monolayer, characterized in that all the deposited polymer chains are covalently bonded to the first electrode layer stack. Subsequently, the polymers from the second group of polymers are deposited in the form of multiple layers, or at least in such a volume or quantity, so as to achieve a deposition thickness that greatly exceeds the thickness of the deposited first group of polymers and to cover the deposited polymers from the first group with the deposited polymers from the second group. Preferably, the volume of the second group of polymers arranged on the deposited first group of polymers reaches a thickness that is at least substantially consistent with the extended chain length of the deposited polymers from the first group, but preferably exceeds that length. This has the effect that even if any one of the deposited polymer chains of the first group extends from the covalent bond site attached to one end of the polymer chain towards the interface with the accommodation space, the opposite end of the polymer chain cannot reach the surface of the insulating layer and thus cannot reach the interface with the polar liquid due to its limited chain length. Therefore, the deposited second group of polymers covers the deposited first group of polymers, making it substantially covered and buried.

[0038] Thus, the insulating layer can be considered to include a stacked fluorocarbon layer having two sub-layers, where the first layer includes a first polymeric fluorocarbon having a functional chemical group configured to form a covalent bond with a hydroxyl group included in or attached to the support layer or the electrode layer stack, and where the second layer includes a second polymeric fluorocarbon located above the first layer and preferably includes only a fluorocarbon moiety.

[0039] Preferably, the insulating layer can adhere or bond to a support layer included in the first electrode layer stack, and the support layer can be formed of any one or more of the following materials:

[0040] - Electrode material on the substrate surface,

[0041] - Inorganic non-metallic material,

[0042] - Organic material, preferably a chemically cross-linked organic material, where, except for the electrode material itself, the material forming the support layer is bonded to the electrode material on the substrate surface in an ionic or covalent manner. In the case where the electrode material on the substrate surface exists in the form of a patterned electrode material on glass, the support layer is also bonded to the glass in an ionic or covalent manner.

[0043] In one example, the first set of polymers and the second set of polymers have polymer chains including at least 100 monomers, preferably at least 250 monomers, more preferably at least 500 monomers.

[0044] In one example, the first set of polymers and the second set of polymers have polymer chains with a length of at least 100 nm, preferably at least 200 nm, more preferably at least 250 nm.

[0045] Some (intrinsic) properties of the polymers may affect, determine, or promote the degree of entanglement between the two deposited sets of polymers. These properties may be related to the chain length of the polymers. Longer polymer chains may allow more free space or loops, thus promoting chain entanglement. The number of covalent bonds of each polymer, and thus defined by the number of functional chemical groups of each polymer chain, may also determine the degree to which the two sets of polymers tend to entangle. Therefore, during the deposition of the second set of polymers, the degree of entanglement can be controlled or determined by selecting one or more of the polymer chain length, the number of functional chemical groups on each polymer chain, and thus also the free volume in the deposited composite polymer layer and the size of the pendant loops of the deposited first set of polymers.

[0046] In one example, the deposited polymers from the first set of polymers exist in the form of a single-layer polymer, characterized in that all the polymers in the single layer are covalently bonded to the first electrode layer stack.

[0047] In one example, the deposited polymers from the second group of polymers exist in the form of a multilayer polymer.

[0048] Preferably, the layer thickness of the deposited polymers from the first group of polymers is thinner than the layer thickness of the deposited polymers from the second group of polymers. More specifically, the first group of polymers can be deposited substantially as a single layer, while the second group of polymers can be deposited as a thicker multilayer. At the interface between the single layer and the multilayer, the deposited first and second groups of polymers exist in the form of an entangled cross structure.

[0049] In one example, the deposited entangled polymer network includes a single layer of the first group of polymers having a covalent bond with a functional chemical group on the first electrode layer stack, and a multilayer of the second group of polymers entangled with the first group of polymers through pendant polymer chains of the first group of polymers, with both ends of the rings attached to the functional chemical groups on the first electrode layer stack.

[0050] In one example, the second group of polymers includes polymers composed of hydrophobic chemical moieties.

[0051] In one example, the first group of polymers includes polymers having a single functional chemical end group configured to form a covalent bond with a hydroxyl group included in and attached to the first electrode layer stack.

[0052] In one example, the first group of polymers includes two monofunctional end groups configured to form a covalent bond with a hydroxyl group included in and attached to the first electrode layer stack.

[0053] The first group of polymers includes at least a single functional chemical group on each polymer chain, but preferably includes polyfunctional chemical groups. Polymers having multiple such functional chemical groups are configured to form covalent bonds with the hydroxyl groups on the first electrode layer stack, which has the effect of promoting stronger bonding between the deposited first group of polymers and the first electrode layer stack compared to polymers having only one such functional chemical group.

[0054] In one example, the first electrode layer stack further includes a support layer disposed between the first electrode layer and the insulating layer, and includes one or more materials selected from the group consisting of inorganic non-metallic materials and organic materials having a high resistivity, more preferably chemically crosslinked organic materials having a high resistivity.

[0055] The insulating layer can also be (covalently) bonded to the stack through the support layer, which is preferably formed of an organic or inorganic non-metallic material.

[0056] In one example, the support layer includes poly(p-xylylene), and preferably poly(p-xylylene) covalently bonded to the hydroxyl groups included in and attached to the first electrode layer, and the covalent bonding is preferably achieved through a silane adhesion promoter.

[0057] In one example, the poly(p-xylylene) is poly(p-xylylene)-C, more preferably poly(p-xylylene)-N, and most preferably poly(p-xylylene)-HT.

[0058] Poly(p-xylylene) can initially achieve the desired electro-wetting behavior in a polar liquid environment, but the hydrophobicity of the poly(p-xylylene) / polar liquid interface gradually decreases, which may be due to hydrolysis of the poly(p-xylylene) material, thereby generating oxygen-containing hydrophilic chemical groups at the interface, thus reducing the hydrophobicity of the interface exposed to polar and non-polar liquids.

[0059] It is known that compared with a single inorganic layer, the support layer in the form of an inorganic multi-layer stack has significantly fewer defects, but the deposition process cost is too high to be practical. In addition, the inorganic multi-layer stack needs to be top-coated with a fluorocarbon material in such a way that the fluorocarbon material is not affected by the fluorocarbon problems that will sooner or later affect the electro-wetting performance.

[0060] In one example, one or both of the first group of polymers and the second group of polymers include a fluoropolymer, and the fluoropolymer has at least one or more of the properties of optical transparency, electrical insulation, high hydrophobicity, and high chemical corrosion resistance.

[0061] In one example, the first group of polymers consists of Cytop-M.

[0062] In one example, the second group of polymers consists only of a fluorocarbon moiety.

[0063] In one example, the second group of polymers consists of Cytop-S.

[0064] In another aspect, the present disclosure provides a method for processing an electrode layer stack for manufacturing an electro-wetting optical element, the method comprising the following steps:

[0065] - Providing an electrode layer stack including a substrate and an electrode layer;

[0066] - Providing an insulating layer on the electrode layer stack for forming an interface with the accommodation space in the electro-wetting optical element; wherein the insulating layer is formed by the following consecutive steps:

[0067] - Deposit a first set of polymers on the electrode layer stack, wherein the first set of polymers includes at least one functional hydrophilic chemical group configured to form a covalent bond with a hydroxyl group included in and attached to the electrode layer stack;

[0068] - Remove those polymers of the first set that are not covalently bonded to the electrode layer stack;

[0069] - Deposit a second set of polymers on the electrode layer stack, wherein the second set of polymers includes polymers that do not have a functional hydrophilic chemical group;

[0070] Entangle the first set of polymers and the second set of polymers in the insulating layer to form a polymer network.

[0071] In one example, the first set of polymers and the second set of polymers are deposited from corresponding polymer solutions by means of a wet chemical dip coating method.

[0072] In one example, by immersing the electrode layer stack in a fluorocarbon solvent of a polymer configured to dissolve the first set of polymers, those polymers of the first set that are not covalently bonded to the hydroxyl groups included in and attached to the electrode layer stack are dissolved and removed, thereby removing a portion of the first set of polymers.

[0073] In one example, between the step of depositing the first set of polymers and the step of removing a portion of the first set of polymers, the method further includes the following steps:

[0074] - Anneal the electrode layer stack including the first set of polymers, and wherein the annealing step is preferably carried out at an elevated temperature > 100 °C.

[0075] In one example, before the step of setting the insulating layer, the method further includes the following steps:

[0076] - Chemically activate the first electrode layer to enable covalent bonding with the first set of polymers.

[0077] In one example, before the step of setting the insulating layer, the method further includes the following consecutive steps:

[0078] - Set a support layer on the first electrode layer, the support layer including one or more materials selected from the group consisting of inorganic non-metallic materials and organic materials having a high resistivity, more preferably chemically crosslinked organic materials having a high resistivity;

[0079] - Chemically activate the surface of the support layer so as to be able to covalently bond with the first set of polymers in the insulating layer.

[0080] In one example, the chemical activation is configured by providing hydroxyl groups for it.

[0081] In one example, the support layer is an organic parylene layer that covalently bonds to hydroxyl groups on the ITO material of the substrate electrode during parylene deposition with the aid of a silane A-174 adhesion promoter. Chemical activation is used to hydrophilize the substrate electrode material, enabling a higher density of silane A-174 molecules to covalently bond to the electrode material, thereby improving the adhesion of the parylene layer to the substrate electrode. After the parylene layer is deposited, the density of hydroxyl groups on the surface of the parylene layer is controlled by means of a reasonably selected UV-O 3 or oxygen-plasma treatment, thereby activating the parylene layer itself. When the first perfluorinated polymer material is deposited on the activated parylene layer (e.g., by dip coating), a first annealing is carried out at a temperature T > 100 °C, preferably in an oxygen-free environment, and these hydroxyl groups can form covalent bonds with reactive silyl groups attached to the backbone of the first perfluorinated polymer material. Subsequently, all perfluorinated polymer materials that are not covalently bonded to the parylene are removed by immersing the substrate in a fluorocarbon solvent that can dissolve the first perfluorinated polymer material. After removing the solvent by evaporation and annealing the first fluorocarbon sublayer at a temperature T > 100 °C, only the covalently bonded fluorocarbon material remains on the parylene layer, effectively forming a first fluorocarbon sublayer that includes a monolayer of perfluorinated polymer lying flat on the parylene surface. Subsequently, a second fluorocarbon sublayer is cast on the first fluorocarbon sublayer, and the second fluorocarbon material preferably uses the same fluorocarbon material as the first fluorocarbon material but does not have reactive silyl groups or other hydrophilic chemical groups. During the deposition of the second fluorocarbon material from the fluorocarbon solvent, the first perfluorinated polymer material is again surrounded by solvent molecules and partially detached from the support surface, thereby forming pendant loops between two covalent bond sites on its backbone. This allows the second perfluorinated polymer material to entangle with the first perfluorinated polymer material during deposition in the presence of the fluorocarbon solvent, and the entanglement is frozen when the solvent is removed during a second annealing of the entangled polymer network at a temperature T > 100 °C (the second annealing is preferably carried out in an inert oxygen-free environment). The second fluorocarbon sublayer is substantially thicker than the first fluorocarbon sublayer to prevent any unreacted functional groups on the first perfluorinated polymer material from being exposed at the interface between the composite fluorocarbon layer and the polar liquid in the electrowetting display. As a result, the composite fluorocarbon layer is firmly secured to the support layer while maintaining a hydrophobic interface with the polar liquid. The support layer is firmly secured to the substrate electrode material.

[0082] In one example, a method for processing an electrode layer stack to manufacture an electrowetting optical element includes the following steps:

[0083] - Provide an electrode layer stack including a substrate and an electrode layer;

[0084] - Provide cell walls on the electrode layer stack and extending from the electrode layer stack in a manner defining a three-dimensional non-planar topographical surface for at least partially defining a receiving space encapsulating a polar liquid and a non-polar liquid;

[0085] - Provide an insulating layer on the electrode layer and above the cell walls on the electrode layer stack for forming an interface with the receiving space;

[0086] Wherein, the insulating layer is formed through the following sequential steps:

[0087] - Deposit a first set of polymers on the electrode layer stack, wherein the first set of polymers includes at least one functional hydrophilic chemical group configured to form a covalent bond with a hydroxyl group included in and attached to the electrode layer stack;

[0088] - Remove those polymers of the first set that are not covalently bonded to the electrode layer stack;

[0089] - Deposit a second set of polymers on the electrode layer stack, wherein the second set of polymers includes polymers without functional hydrophilic chemical groups,

[0090] Entangle the first set of polymers and the second set of polymers into a polymer network in the insulating layer.

[0091] An insulating interface layer formed according to the present disclosure by deposition of two sets of polymers (e.g., Cytop-M and Cyto-S) can be formed on a flat planar two-dimensional surface of the electrode layer, but according to the above example, it can also be formed on a non-flat, non-planar three-dimensional surface. When cell walls are provided on the electrode layer, the planar surface of the electrode layer becomes a relief surface with raised cell walls and recessed cells defining a receiving space encapsulating a polar liquid and a non-polar liquid.

[0092] An insulating interface layer formed according to the present disclosure by deposition of two sets of polymers (e.g., Cytop-M and Cytop-S) can alternatively be disposed on top of a support layer formed of one or more of the following materials selected from the group consisting of inorganic non-metallic materials and organic materials, preferably chemically crosslinked organic materials, wherein when cell walls are provided on the electrode layer, the support layer is bonded to the electrode layer material and to the cell wall material in an ionic or covalent manner.

[0093] In another aspect of the present disclosure, there is provided an electrowetting optical display including one or more electrowetting optical elements according to any one of the foregoing aspects or examples thereof.

[0094] Each example described in relation to the first aspect of the present invention is also applicable to the second aspect or other aspects of the present invention. Correspondingly, all the advantages and other examples of the first aspect are also applicable to the second aspect or other aspects and their examples or the examples of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0095] The present invention will be further described with reference to the accompanying drawings which illustrate embodiments of the present invention, and in which:

[0096] Figure 1 An electrode layer stack of an electrowetting element according to the prior art is shown illustratively;

[0097] Figure 2 An electrode layer stack of an electrowetting element according to the present disclosure is shown illustratively;

[0098] Figures 3a to 3c Multiple steps of processing an electrode layer stack of an electrowetting element according to the present disclosure are shown;

[0099] Figure 4 , Figure 5 and Figure 6 Multiple embodiments of an electrode layer stack of an electrowetting element according to the present disclosure are shown illustratively. DETAILED DESCRIPTION

[0100] Figure 1 Details of an electrowetting optical element (or further also referred to as an electrowetting element) according to the prior art are shown. An electrowetting display (EWD) includes an electrowetting optical element which includes a transparent substrate, a transparent upper substrate aligned parallel to the substrate, and a liquid layer sandwiched between the two plates. Both plates are provided with electrodes facing the liquid layer, and the liquid layer includes a transparent polar liquid layer and a colored non-polar liquid immiscible with the polar liquid. The substrate is also provided with an insulating layer disposed on the substrate electrode.

[0101] As Figure 1 shown, the insulating layer 102 separates the polar liquid in electrical short-circuit contact with the upper substrate electrode from the substrate electrode layer 103 or the support layer 103 on top of the substrate electrode layer. When the substrate electrode and the upper substrate electrode are set to different potentials, a potential difference across the insulating layer 102 is generated. The material and thickness of the insulating layer should be selected to minimize the probability of problems such as electrical breakdown and / or the occurrence of leakage current across the insulating layer, such as at the maximum applied potential drop. These problems are typically caused by defects in the insulating layer.

[0102] The non-polar liquid 106 is a colored oil that resides at the interface between the insulating layer 102 and the polar liquid layer. In this device, it is desirable that when the potential drop across the insulating layer is zero, the oil completely spreads over the interface between the insulating layer 102 and the polar liquid layer (i.e., completely wets with a substantially zero contact angle). This is achieved by ensuring that the surface of the insulating layer exposed to the polar liquid is hydrophobic.

[0103] When a potential difference across the insulating layer is established by applying different potentials to the substrate electrode and the upper substrate electrode, the oil layer on the insulating layer breaks into droplets that form a non-zero contact angle (i.e., partial wetting) on the insulating layer in the polar liquid environment, and the contact angle increases as the potential difference increases. This is the so-called electrowetting effect, a key aspect of EWD operation, which is well documented in the literature. The partial oil wetting of the oil on the insulating layer makes the EWD partially transparent, and complete oil wetting makes the EWD opaque to an extent determined by the color of the oil and the thickness of the oil layer.

[0104] Electrowetting devices in the prior art may include an insulating layer that is either entirely composed of a hydrocarbon material, entirely composed of a fluorocarbon material, or composed of at least two dissimilar materials. In the latter case, the insulating layer is disclosed as including an organic or inorganic support layer in contact with the substrate electrode, and a hydrophobic organic top layer in contact with the oil and the polar liquid. The hydrophobic top layer can be a silane-type hydrocarbon monolayer or a fluorocarbon monolayer, or a thicker hydrocarbon or fluorocarbon layer deposited by means of a coating process.

[0105] Figure 1 Show details of such prior art electrowetting devices, particularly the manner in which the insulating layer 102 is constructed. Figure 1 The insulating layer 102 is composed of fluorocarbon polymers 104a-d having chemical functional groups (end groups) (shown as open dots) capable of forming covalent bonds with hydroxyl groups on the layer (such as a glass substrate layer or an ITO electrode layer 103 or a support layer 103 disposed on the ITO electrode layer) below the insulating layer 102. As Figure 1 shown, some of the polymers 104a, 104b have formed such covalent bonds with the layer 103, while others 104c, 104d have not.

[0106] When the insulating layer 102 includes only a single fluorocarbon material, such as Cytop-M (AGC Chemicals), which has functional silyl groups for chemical bonding with ITO or glass, it has been observed to gradually lose its hydrophobicity when exposed to polar liquids, especially at higher temperatures. This is in Figure 1As shown, some of the polymers 104c, 104d in the insulating layer 102 migrate to the surface and reduce the degree of hydrophobicity on the surface of the fluorinated polymer in contact with the polar liquid as hydrophilic non-reactive functional silane groups. In addition, when used as an insulating layer, many defects occur in the fluorocarbon layer, resulting in rapid electrical breakdown. Therefore, its structural integrity and hydrophobicity cannot be maintained, leading to a decrease in the electro-wetting effect.

[0107] Figure 2 An example of an electro-wetting element 200 according to one aspect of the present disclosure is shown. Figure 2 The insulating layer of the element 200 shown includes a transparent layer of a substrate electrode material, which can be chemically activated to provide a high surface density of hydroxyl groups thereon, thereby producing a high degree of surface hydrophilicity. On top of the electrode layer, a chemically activated support layer 203 can be provided. Its activation causes the hydroxyl groups on its surface to face away from the substrate electrode. An insulating layer 202 is provided on top of the support layer 203. The insulating layer 202 includes a composite fluorocarbon layer on the activated support layer 203 having a mixture of two sets of polymers. The first set of polymers is formed from a fluorinated polymer material that is capable of forming a covalent bond with the hydroxyl groups on the support layer via a single functional chemical end group attached to each polymer chain. It is illustrated by polymers 204a, 204b. The second set of polymers formed on the first set includes a second polymer material containing only fluorocarbon moieties 205a, 205b and thus cannot form a covalent bond with the hydroxyl groups on the support layer 203.

[0108] The first set and the second set are capable of mixing with each other to form an entangled polymer network 204, 205, thereby forming a firm adhesive contact between the fluorocarbon sub-layers, wherein the first set of fluorocarbon materials present in the form of a polymer monolayer is provided with functional chemical groups capable of forming covalent chemical bonds with the hydroxyl groups on the surface of the support layer, and the second set of fluorocarbon materials present in the form of a polymer multi-layer does not have functional chemical groups or other hydrophilic chemical groups.

[0109] The insulating layer can be formed directly on top of the electrode layer, such as an ITO electrode, but can also be formed on top of a support layer formed from one or more materials from the group including inorganic non-metallic materials and organic materials, preferably chemically cross-linked organic materials.

[0110] Except for the electrode material itself, the material forming the support layer is bonded to the electrode material on the substrate surface in an ionic or covalent manner. In the case where the electrode on the substrate surface exists in the form of a patterned electrode on glass, the support layer is also bonded to the glass in an ionic or covalent manner.

[0111] In Figures 3a to 3cSteps of processing an electrode layer stack are shown, and in particular steps of forming an insulating layer on the electrode layer stack for an electrowetting optical element according to the present disclosure.

[0112] In a first step, an electrode layer stack including a substrate and an electrode layer is provided. An insulating layer is provided on top thereof for forming an interface with an accommodation space that has not been formed during this processing step but initially contains air 307.

[0113] The insulating layer is formed by first depositing a first set of polymers, for example by dip coating. Hydroxyl groups on a support layer 303, for example made of parylene, can form covalent bonds 304a, 304b with the first set of polymers. For example, when a first fluoropolymer carbon material is deposited on the activated parylene layer, a first annealing is performed at a temperature T > 100 °C, preferably in an oxygen-free environment, and reactive silyl groups attach to the main chain of the first fluoropolymer carbon material.

[0114] Subsequently, as Figure 3b shown, all fluorocarbon materials that are not covalently bonded to the parylene are removed. This can be achieved by dipping the substrate 320 into a fluorocarbon solvent capable of dissolving the first fluoropolymer carbon material. Then, only the covalently bonded fluorocarbon materials 304a, 304b remain on the parylene layer 303. After removing the solvent by a first annealing at a temperature T > 100 °C, a first set of polymers or fluorocarbon sublayers are effectively formed, which include a monolayer of fluoropolymer carbon lying flat on the parylene surface.

[0115] Subsequently, a second set of fluorocarbon polymers 305a, 305b for forming a second fluorocarbon sublayer are cast on the first set of fluorocarbon polymers 304a, 304b. The second set preferably uses the same fluorocarbon material as the first fluorocarbon material but is not provided with reactive silyl groups or other hydrophilic groups, as shown by the absence of dots on the second set of polymers 305a, 305b.

[0116] During the deposition of the second set of fluorocarbon materials from the fluorocarbon solvent, the first fluoropolymer carbon material itself is surrounded by solvent molecules and partially detached from the support surface, thereby forming pendant loops between adjacent covalent bond sites on its main chain. In the presence of the fluorocarbon solvent, this allows the second fluoropolymer carbon material to easily become entangled with the first fluoropolymer carbon material. When the solvent is subsequently removed during a second annealing at T > 100 °C (the second annealing is preferably performed in an inert oxygen-free environment), the entanglement becomes stable or frozen, thereby producing an electrowetting optical element, or more specifically, the first electrode layer stack of the electrowetting optical element includes an insulating layer with improved properties, as Figure 3cAs shown, there are strongly covalently bonded reactive silane groups at the bottom of the stack, while there are no reactive silane groups or other hydrophilic groups at the top of the stack, where the insulating layer maintains its structural integrity through the entanglement of the two sets of polymers.

[0117] Figure 4 , Figure 5 and Figure 6 Several embodiments of a first electrode layer stack of an electrowetting element according to the present disclosure are shown.

[0118] exist Figure 4 In the embodiment of the present invention, on top of a chemically activated hydrophilic ITO electrode 420 on a glass substrate 410, an insulating layer 440, 450 is disposed on top of a parylene layer 430. The insulating layer 440, 450 comprises a composite fluorocarbon layer comprising a first sublayer 440 of a first polymeric fluorocarbon material and a second sublayer 450 of a second polymeric fluorocarbon material.

[0119] exist Figure 5 , insulating layers 540, 550 are disposed on top of a chemically activated hydrophilic ITO electrode 520 of a glass substrate 510. The insulating layers 540, 550 include a composite fluorocarbon layer including a first sublayer 540 of a first polymeric fluorocarbon material and a second sublayer 550 of a second polymeric fluorocarbon material.

[0120] Figure 6 An example with an insulating support layer 635 is shown. The support layer 635 is arranged on top of a chemically activated hydrophilic ITO electrode 620 on a glass substrate 610. Insulating layers 640, 650 are arranged on the chemically activated support layer 635, and the insulating layers 640, 650 include a first sublayer 640 of a first polymeric fluorocarbon material and a second sublayer 650 of a second polymeric fluorocarbon material, Figure 4 and Figure 5 The other examples shown in are similar. The support layer 635 itself can also be a composite support layer including an insulating inorganic sublayer and an insulating organic sublayer.

[0121] As will be appreciated by those skilled in the art, the present invention may be implemented in ways other than those specifically described herein. Obvious changes and specific design choices to the disclosed embodiments will be apparent to readers of this art. The scope of the present invention is limited only by the appended claims.

Claims

1. An electrowetting optical element, comprising: a first electrode layer stack including a substrate and a first electrode layer; a second electrode layer stack including an upper substrate and a second electrode layer; a receiving space formed between the first electrode layer stack and the second electrode layer stack; one or more unit walls extending between the first electrode stack and the second electrode stack for defining sides of the receiving space forming the unit; the receiving space including a polar liquid and a non-polar liquid, the polar liquid and the non-polar liquid being immiscible with each other; wherein the first electrode layer and the second electrode layer are configured to apply a voltage between the electrodes to reconfigure the polar liquid relative to the non-polar liquid; wherein the first electrode layer stack further includes an insulating layer disposed between the first electrode layer and the receiving space, forming an interface with the receiving space, wherein the insulating layer includes a first set of polymers and a second set of polymers, the first set of polymers including functional hydrophilic chemical groups configured to form a covalent bond with the first electrode layer stack, and the second set including polymers without functional hydrophilic chemical groups, and wherein the first set of polymers and the second set of polymers are configured in the insulating layer as an entangled polymer network.

2. The electrowetting optical element according to claim 1, wherein the first set of polymers and the second set of polymers have polymer chains including at least 100 monomers, preferably at least 250 monomers, more preferably at least 500 monomers.

3. The electrowetting optical element according to claim 1 or 2, wherein the first set of polymers and the second set of polymers have polymer chains with a length of at least 100 nm, preferably at least 200 nm, more preferably at least 250 nm.

4. The electrowetting optical element according to any one of the preceding claims, wherein the first set of polymers is a polymer monolayer, wherein at least substantially all, and preferably all, of the polymers in the polymer monolayer are covalently bonded to the first electrode layer stack.

5. The electrowetting optical element according to any one of the preceding claims, wherein the second set of polymers is a multi-layer polymer.

6. The electrowetting optical element according to any one of the preceding claims, wherein the entangled polymer network includes a monolayer of the first set of polymers covalently attached to the first electrode layer stack, and multi-layers of the second set of polymers entangled with the first set of polymers via pendant polymer chain loops formed between two functional chemical groups on the chains of the polymers from the first set of polymers, wherein both of the two functional chemical groups are covalently bonded to the first electrode layer stack.

7. The electrowetting optical element according to any one of the preceding claims, wherein the second set of polymers includes polymers composed of hydrophobic chemical moieties.

8. The electrowetting optical element according to any one of the preceding claims, wherein the first set of polymers comprises polymers having a single functional chemical end group, the functional chemical end group being configured to form a covalent bond with a hydroxyl group included in and attached to the first electrode layer stack.

9. The electrowetting optical element according to any one of the preceding claims, wherein the first set of polymers comprises two functional end groups, the functional end groups being configured to form a covalent bond with a hydroxyl group included in and attached to the first electrode layer stack.

10. The electrowetting optical element according to any one of the preceding claims, wherein the first electrode layer stack further comprises a support layer disposed between the first electrode layer and the insulating layer, the support layer comprising one or more materials selected from the group consisting of inorganic non-metallic materials and organic materials having a high resistivity, more preferably chemically crosslinked organic materials having a high resistivity.

11. The electrowetting optical element according to claim 10, wherein the support layer comprises poly(p-xylene), and preferably, poly(p-xylene) is disposed between the first electrode layer and the poly(p-xylene) layer, the poly(p-xylene) being covalently bonded to a hydroxyl group included in and attached to the first electrode layer or to a hydroxyl group included in and attached to the inorganic non-metallic layer, the covalent bonding preferably being achieved through a silane adhesion promoter.

12. The electrowetting optical element according to claim 11, wherein the poly(p-xylene) is poly(p-xylene)-C, more preferably poly(p-xylene)-N, and most preferably poly(p-xylene)-HT.

13. The electrowetting optical element according to any one of the preceding claims, wherein one or both of the first set of polymers and the second set of polymers comprise a fluoropolymer having at least one or more of the properties of optical transparency, electrical insulation, high hydrophobicity, and high chemical resistance.

14. The electrowetting optical element according to any one of the preceding claims, wherein the first set of polymers consists of Cytop-M.

15. The electrowetting optical element according to any one of the preceding claims, wherein the second set of polymers consists only of a fluorocarbon moiety.

16. The electrowetting optical element according to any one of the preceding claims, wherein the second set of polymers consists of Cytop-S.

17. A method for fabricating an electrode layer stack for an electrowetting optical element, the method comprising the steps of: - providing an electrode layer stack including a substrate and an electrode layer; - providing an insulating layer on the electrode layer stack for forming an interface with an accommodation space within the electrowetting optical element; wherein, the insulating layer is formed by the following consecutive steps: - depositing a first set of polymers on the electrode layer stack, wherein the first set of polymers comprises at least one functional hydrophilic chemical group, the functional hydrophilic chemical group being configured to form a covalent bond with a hydroxyl group included in and attached to the electrode layer stack; - Remove those polymers in the first group that are not covalently bonded to the electrode layer stack; - Deposit a second group of polymers on the electrode layer stack, where the second group of polymers includes polymers having no functional hydrophilic chemical groups, Entangle the first group of polymers and the second group of polymers in the insulating layer to form a polymer network.

18. The method for processing and manufacturing an electrode layer stack for an electrowetting optical element according to claim 17, wherein, The first group of polymers and the second group of polymers are deposited from their respective polymer solutions by means of wet chemical dip coating.

19. The method for processing and manufacturing an electrode layer stack for an electrowetting optical element according to claim 17 or 18, wherein, Before depositing the second group of polymers, by immersing the electrode layer stack in a fluorocarbon solvent of the polymer configured to dissolve the first group of polymers, those polymers in the first group that are not covalently bonded to the hydroxyl groups included in and attached to the electrode layer stack are removed, thereby removing a part of the deposited first group of polymers.

20. The method for processing and manufacturing an electrode layer stack for an electrowetting optical element according to any one of claims 17 to 19, wherein, Between the step of depositing the first group of polymers and the step of removing a part of the deposited first group of polymers, the method further includes the following steps: - Anneal the electrode layer stack including the deposited first group of polymers, and the step of annealing is preferably carried out at an elevated temperature >100 °C.

21. The method for processing and manufacturing an electrode layer stack for an electrowetting optical element according to any one of claims 17 to 20, wherein, Before the step of setting the insulating layer, the method further includes the following steps: - Chemically activate the first electrode layer to achieve the covalent bonding with the first group of polymers.

22. The method for processing and manufacturing an electrode layer stack for an electrowetting optical element according to any one of claims 17 to 21, wherein before the step of setting the insulating layer, the method further includes the following consecutive steps: - Set a support layer on the first electrode layer, the support layer includes one or more materials selected from the group consisting of inorganic non-metallic materials and organic materials having high resistivity, more preferably chemically crosslinked organic materials having high resistivity; - Chemically activate the surface of the support layer so as to be able to carry out the covalent bonding with the first group of polymers in the insulating layer subsequently.

23. The method for processing and manufacturing an electrode layer stack for an electrowetting optical element according to claim 21 or 22, wherein the chemical activation is configured by providing it with hydroxyl groups.

24. A method for processing and manufacturing an electrode layer stack for an electrowetting optical element, the method comprises the following steps: - Set an electrode layer stack including a substrate and an electrode layer; - A cell wall is provided on the electrode layer stack and extends from the electrode layer stack in a manner that defines a three-dimensional non-planar topographical surface, for at least partially defining a receiving space for encapsulating a polar liquid and a non-polar liquid; - An insulating layer is provided on the electrode layer and the cell wall and on the electrode layer stack, for forming an interface with the receiving space; Wherein, the insulating layer is formed by the following consecutive steps: - Depositing a first set of polymers on the electrode layer stack, wherein the first set of polymers includes at least one functional hydrophilic chemical group, and the functional hydrophilic chemical group is configured to form a covalent bond with a hydroxyl group included in and attached to the electrode layer stack; - Removing those polymers of the first set of polymers that are not covalently bonded to the electrode layer stack; - Depositing a second set of polymers on the electrode layer stack, wherein the second set of polymers includes polymers that do not have functional hydrophilic chemical groups, Entangling the first set of polymers and the second set of polymers into a polymer network in the insulating layer.

25. An electrowetting optical display includes one or more electrowetting optical elements according to any one of the preceding claims 1 to 16.

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