WO2010136925A1

WO2010136925A1 - Electrophoretic device

Info

Publication number: WO2010136925A1
Application number: PCT/IB2010/052102
Authority: WO
Inventors: Kars-Michiel Hubert Lenssen; Manfred Mueller; Martinus Hermanus W.M. Van Delden
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-05-29
Filing date: 2010-05-12
Publication date: 2010-12-02
Also published as: TW201106079A

Abstract

The invention relates to an electrophoretic device comprising one or more cells. Each of the one or more cells of the device comprises a bottom substrate having a bottom substrate area (110), a top substrate (120) having a top substrate area, and a spacer structure (130) spacing apart the bottom and top substrates to form a space containing a suspension of particles (140) in a medium, the particles being movable through application of an electric field. The bottom substrate comprises a bottom electrode having an effective bottom electrode area, and the top substrate comprises one or more independently- addressable top electrodes (121, 122) having an effective top electrode area. The bottom and top electrodes are arranged to establish the electric field, and to control a distribution of the particles from a distributed state to a collected state. The effective top electrode area is in the form of a pattern. The ratio of the effective bottom electrode area to the effective top electrode area is larger than one, and the ratio of the top substrate area to the effective top electrode area is larger than or equal to 5. The electrophoretic device of the invention has built-in grey scales, an improved manuf acturability through a relatively simple electrode arrangement, and allows the formation of a transparent state.

Description

ELECTROPHORETIC DEVICE

FIELD OF THE INVENTION

The invention relates to an electrophoretic device comprising one or more cells, each cell comprising a bottom substrate having a bottom substrate area, a top substrate having a top substrate area, and a spacer structure spacing apart the bottom and top substrates to form a space containing a suspension of particles in a medium, the particles being movable through application of an electric field, wherein the bottom substrate comprises a bottom electrode structure having an effective bottom electrode area, and the top substrate comprises a top electrode structure having an effective top electrode area in the form of a pattern, the effective bottom electrode area being larger than the effective top electrode area, the bottom and top electrodes being arranged to establish the electric field, and to control a distribution of the particles from a distributed state to a collected state.

BACKGROUND OF THE INVENTION

Electrophoretic devices have been known for years, particularly in the form of display devices. However, electrophoretic devices may also be used to electronically change the visual appearance of a surface covered thereby.

The operation of electrophoretic devices is based on the principle of electrophoresis, which denotes the motion of particles relative to a medium under the influence of an externally applied electric field. The medium may be a (transparent or colored) fluid, and the particles may be electrically charged particles, or uncharged dielectric particles. When dielectric particles are used, and when the electric field comprises an AC component, one speaks of dielectrophoresis, which, in the context of the invention, is considered to be a special kind of electrophoresis. Known electrophoretic devices typically comprise a bottom substrate, a top substrate, and a spacer structure spacing apart the bottom and top substrates such that a space between the bottom and top substrates is subdivided into one or more cells containing a suspension of particles, the particles being movable through application of an electric field that is supplied by an electrode arrangement. The electrode arrangement comprises at least two electrodes, each having an effective electrode area. The effective electrode area indicates the area of the electrode that is available for the creation of an electric field. For example, the effective electrode area can be the electrode area that is in direct contact with the particle suspension. The electrode arrangement is for controlling a distribution of the particles from a distributed state to a collected state. In the distributed state, the particles are positioned within the cell such that the optical appearance of the cell is predominantly determined by the optical properties of the particles. The distributed state can be accomplished by dispersing the particles throughout the cell, or by having the particles form a (multi) layer on one of the substrate surfaces. In the collected state, the particles are positioned within the cell such that the optical appearance of the cell is predominantly determined by the optical properties of the substrates, the electrode arrangement and/or the medium (possibly containing a different type of particles still in a distributed state, see also below). The collected state can be accomplished by compacting the particles in a volume that has, in a direction parallel to the normal of the substrates, a projected area that is much smaller than the substrate area. In other words, the electrode arrangement is for moving the particles within the cell to make them more or less visible to a viewer, thereby providing the cell with an electrically controllable visual appearance. In case the medium contains more than one type of particles (for example, a dual particle suspension), a collected state may refer to one of these types of particles. In other words, in a collected state one type of particles may be compacted as described above, while any other type of particles stays in a distributed state.

Electrophoretic devices can be classified according to the main direction of particle movement resulting from the particular orientation of the electric field lines within the cells. In a first type of electrophoretic devices, known as out-of-plane (or top-down, or vertically) switching electrophoretic devices, the electrode arrangement comprises electrodes on both bottom and top substrates, arranged to provide an electric field for moving the particles in a direction substantially perpendicular to the plane of the bottom and top substrates (or, in other words, substantially parallel to the normal of the bottom and top substrates). A basic out-of-plane switching electrophoretic device is disclosed in US patent 3612758.

In a second type of electrophoretic devices, known as in-plane (or laterally, or horizontally) switching electrophoretic devices, the electrode arrangement comprises multiple electrodes on a single substrate, arranged to provide an electric field for moving the particles in a direction substantially parallel to the plane of that substrate. A basic in-plane switching electrophoretic device is disclosed in Japanese patent application S49-24695. A drawback of out-of-plane switching electrophoretic devices is that they cannot provide a transparent state, which is important for bright and full color applications (because it allows stacking), and which is required for applications having electrically switchable light transmissive properties, such as smart windows.

In-plane switching electrophoretic devices can provide a transparent state, but at the cost of a relatively complex electrode arrangement, rendering their manufacture more difficult. Furthermore, in-plane switching electrophoretic devices may have a relatively long switching time as the particles usually have to move a greater distance than in out-of-plane switching electrophoretic devices.

A further electrophoretic device that can provide a transparent state is disclosed in US patent 4648956. This electrophoretic device has an array of unit cells that comprise two parallel substrates separated by a distance in which an electrophoretic suspension with pigment particles is provided. The unit cells of the device have an electrode covering the surface of one substrate, and strip electrodes covering the surface of the other substrate. With respect to the strip electrodes, the ratio of the width to the interspacing distance should be as small as possible to maximize transmission of light when a pixel of the device is in a condition wherein the pigment particles are collected on the strip electrodes. In operation, the electrophoretic device disclosed in US patent 4648956 will exhibit particle motion in directions having both out-of-plane and in-plane components. Consequently, the device is neither a purely out-of-plane switching electrophoretic device, nor a purely in-plane switching electrophoretic device. Rather, the device may be called a hybrid electrophoretic device that, in operation, exhibits particle movement characteristics that can be found in both out-of-plane and in-plane switching electrophoretic devices.

The unit cells of the hybrid electrophoretic device disclosed in US patent 4648956 may only be switched between states that either block a relatively large portion of light (the "off state), or a relatively small portion of light (the "on" state) from passing through the cell. In other words, this device cannot provide intermediate states, such as grey scales, without the use of elaborate driving schemes.

SUMMARY OF THE INVENTION

It is an object of the invention to solve the above-mentioned drawback of the known hybrid electrophoretic device. The object is realized by an electrophoretic device according to the opening paragraph, characterized in that the top electrode structure comprises two or more independently addressable top electrodes, wherein the ratio of the top substrate area to the effective top electrode area is larger than or equal to 5, the bottom and top electrode structures being further arranged to control the distribution of particles to one of a plurality of collected states.

In the electrophoretic device of the invention, a non-uniform (or inhomogeneous) electric field is generated between a relatively large effective electrode area on a first substrate, and relatively small effective electrode areas in the form of a pattern on a second substrate, opposite the first substrate. In operation, the electric field will induce particle motion in directions having both out-of-plane and in-plane components, so that the device is a hybrid electrophoretic that, in operation, exhibits particle movement characteristics that can be found in both out-of-plane and in-plane switching electrophoretic devices. The combination of the bottom and top electrode structures allows the formation of a distributed state as well as a plurality of different collected states, thereby enabling the cell to be switched between a dark state, a transparent state, and several grey states.

The two or more independently addressable top electrodes comprised in the top electrode structure enable the formation of grey scales. In known electrophoretic devices grey scales are produced by using elaborate and time-consuming driving schemes that increase the manufacturing costs, and that are sensitive to changes in the properties of a particle suspension, that may occur, for example, due to temperature changes and/or aging. The electrophoretic device according to the invention provides a way to realize so-called built-in grey scales, wherein the term "built-in" is used to denote that the grey scales are predominantly dependent on the application of electric potentials to the independently addressable bottom and top electrodes, so that there is no need for elaborate and/or time- consuming driving schemes. Additionally, built-in grey scales are relatively insensitive to small variations in particle suspension characteristics. In an embodiment of the electrophoretic device of the invention, the effective top electrode area has a line, a comb, or a ladder pattern, which is a particularly convenient way of providing the top electrodes according to the invention.

In an embodiment of the electrophoretic device of the invention, the bottom electrode area is substantially equal to the bottom substrate area. This essentially means that the bottom substrates of the one or more cells are completely covered by a common bottom electrode. In other words, the bottom electrode of each cell is part of a continuous bottom electrode that is common to all of the one or more cells. This is a particularly convenient way of providing a bottom electrode according to the invention. In an embodiment of the electrophoretic device of the invention, the spacer structure has an irregular shape and/or the effective top electrode area has a non-periodic pattern, in order to avoid optical artifacts, such as Moire effects.

In an embodiment of the electrophoretic device of the invention, the medium and the bottom electrode are optically transparent, enabling the device to be switched into a transparent state.

In an embodiment of the electrophoretic device of the invention, the particles are dielectric particles, and the electric field comprises an AC component. The AC component has a magnitude that is sufficient to move the dielectric particles. This embodiment allows for the use of dielectric particles, which generally have improved suspension stability and improved particle selectivity.

The electrophoretic device of the invention can be used in a broad range of applications, such as electronically switchable windows, wall papers, and foils for incorporation into consumer products, but also in (low-resolution) displays, for example for use in digital signage.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. Ia (top view) and Ib (cross-sectional view) show a cell of an electrophoretic device according to the invention, in a distributed state;

Figs. 2a (top view) and 2b (cross-sectional view) show the same cell as shown in Fig. 1, in a collected state;

Fig. 3 shows the top view of the same cell as shown in Figs. 1 and 2, when driven to various built-in states;

Fig. 4 shows cross-sectional views corresponding to the top views as shown in Fig. 3; Fig. 5 shows microscope images of part of an electrophoretic device according to the invention, when driven to various built-in states.

DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 1 shows a cell 100 of an electrophoretic device according to the invention, in a top view (Fig. Ia), and in a cross-sectional view (Fig. Ib). An electrophoretic device according to the invention may contain one or more cells, such as the cell 100. For example, the electrophoretic device may contain a plurality of cells arranged in a matrix of rows and columns.

The cell 100 has a bottom substrate 110, a top substrate 120, and a spacer structure 130 spacing apart the bottom substrate 110 and the top substrate 120.

The spacer structure 130 has a rectangular shape with a width (wl) of 100 micrometer, defining a regular grid pattern of spacer walls in the electrophoretic device. The spacer structure 130 may be embossed in one of the top and bottom substrates, or it may be made of a resist material, such as SU-8 photoresist. The spacer structure 130 confines a suspension of particles 140 in compartments, in order to avoid inhomogenization over time. Instead of having a rectangular shape, the spacer structure may have any other shape, preferably an irregular shape, in order to avoid the occurrence of optical artifacts, such as Moire effects.

On a side facing the top substrate 120, the bottom substrate 110 is provided with a bottom electrode structure comprising a bottom electrode 111. Save from the parts covered by the spacer structure 130, the bottom electrode 111 is fully exposed to the suspension of particles 140. In other words, the bottom electrode 111 has an effective bottom electrode area that is substantially equal to the total bottom electrode area.

The bottom electrode 111 is manufactured from electrically conductive and optically transparent indium tin oxide (ITO). Instead of ITO, any material combining electrical conductivity with optical transparency may be used to form the bottom electrode 111. In fact, if there is no need for a transparent state of the cell 100, any material may be used to manufacture the bottom electrode 111, as long as it is electrically conductive.

On a side facing the bottom substrate 110, the top substrate 120 is provided with a top electrode structure comprising a first top electrode 121, and a second top electrode 122, both having an effective top electrode area in the form of a periodic pattern of uniformly spaced lines having a width (w2) of 4 micrometer, spaced apart by a distance (d) of 40 micrometer. For the purpose of the invention, the patterns of the effective areas of the two top electrodes may also be different in shape and/or size. For example, they can have different line widths, have a non-uniform or asymmetric spacing between them, be in the form of a grid pattern, or any non-periodic pattern so that the occurrence optical artifacts, such as Moire effects, can be avoided. The top electrodes 121 and 122 are both fully exposed to the suspension of particles 140. In other words, the top electrode structure has an effective top electrode area that is substantially equal to the total area of the top electrode structure. Alternatively, an electrode structure may be provided with a patterned effective electrode area by covering the electrode structure with a patterned insulative layer.

In Fig. 1, the bottom electrode 111, the top electrode 121, and the top electrode 122, are each shown as being formed from a single electrically conductive material. Alternatively, these electrodes may be formed from more than material. For example, the electrodes may comprise an electrically conductive layer covered by an electro- and/or dispersion-protective layer, wherein the presence of the protective layer does not influence the effective electrode area.

The cell 100 may further comprise other layers (not shown in Fig. 1). For example, at a side of the bottom electrode 111 facing away from the top substrate 120, the cell 100 may comprise a reflective layer. The effective area of the top electrode structure is 16 % of the total area of the top substrate 120 (the top electrodes 121 and 122 each cover 8 % of the total area of the top substrate 120).

The bottom electrode 111 is a continuous electrode covering the full area of the side of the bottom substrate 110 on which it is provided. The ratio of the effective bottom electrode area to the effective top electrode area is 6.25.

For the purpose of the invention, the bottom electrode may also have an effective bottom electrode area in the form of a pattern, as long the effective bottom electrode area is larger than the effective top electrode area. Preferably, the ratio of the effective bottom electrode are to the effective top electrode area is equal to or larger than 5, and more preferably equal to or larger than 10. When the bottom electrode has an effective bottom electrode area in the form of a pattern, the pattern is preferably a non-periodic pattern, so that the occurrence of optical artifacts, such as Moire effects, can be avoided.

For the purpose of the invention, the ratio of the top substrate area to the effective top electrode area is larger than or equal to 5, preferably larger than or equal to 10, and more preferably larger than or equal to 20.

When a potential difference is applied between the bottom electrode structure and the top electrode structure, an electric field will be created that is non-uniform (or inhomogeneous) over most of the cell 100, due to the difference in effective electrode areas. When subjected to the non-uniform electric field, the particles 140 will move in directions that have two mutually orthogonal components, viz. a component in the plane of the substrates 110 and 120, and a component parallel to the normal of the substrates 110 and 120.

In other words, in operation the particles 140 will exhibit both out-of-plane and in-plane movement. The particles 140 may either be uncharged polarisable dielectric particles or electrically charged particles, preferably exhibiting threshold behaviour. In case of the particles 140 being uncharged polarisable dielectric particles, the electric field that is established between the bottom electrode structure and the top electrode structure is an electric field comprising an AC component. By changing the frequency of the AC component the particles 140 can be moved towards regions with a high density of electric field lines (i.e. close to the top electrode structure) or towards regions with a low density of electric field lines (i.e. close to the bottom electrode structure).

The particles 140 may be black particles, or may have any other color.

Preferably, the particles 140 are non-backscattering to allow a CMY(K) color scheme. The particles 140 are suspended in a medium, which may be any kind of fluid, such as a liquid or a gas (for example, air).

Depending on the electric potentials applied to the bottom and top electrode structures, respectively, the cell 100 may be in a distributed state, wherein the particles 140 are concentrated adjacent to the effective area of the bottom electrode 111, or in a collected state, wherein the particles 140 are concentrated adjacent to the effective area of the top electrode structure.

In the distributed state, which may also be called a dark state or an opaque state, the particles 140 are distributed over the volume of the cell 100 so that the optical appearance of the cell 100 is dominated by the optical properties of the particles 140. In the distributed state the particles 140 do not have to remain kept on the effective area of the bottom electrode 111, but may also be released to spread freely throughout the volume of the cell 100, so that no power is consumed anymore, as is shown in Fig. 1.

For the cell 100, a collected state is shown in Fig. 4b. In this state, which may also be called a light state or a transparent state, the optical appearance of the cell 100 is dominated by the optical properties of the bottom and top substrates 110 and 120, respectively, of the bottom electrode 111, and of the medium wherein the particles 140 are suspended.

The distributed state of the cell 100, as shown in Figs. 1, is an extreme optical state of the cell 100 as it represents the maximum opaque state of the cell 100. Another extreme optical state of the cell 100 may be obtained when the transparency of the cell 100 is maximized. This will obviously be a collected state of the cell 100. In between the extreme optical states of the cell 100, a plurality of intermediate states may be obtained, in the form of collected states in which the opacity/transparency of the cell 100 is not maximized. Such intermediate collected states are grey states of the cell 100.

A method of operating the cell 100 will be described below, wherein the particles 140 are negatively charged, and the bottom electrode 111 is grounded. However, the generalization to positively charged particles and/or a non-grounded bottom electrode will be obvious to the skilled person. The cell 100 is operated by applying voltages of 1.5 V or higher, but preferably in a range of 3 V to 7 V.

In a distributed state of the cell 100, the first and second top electrodes 121 and 122, respectively, are grounded or no voltage is applied to them. The distributed state is an extreme optical state in that the opacity (transparency) of the cell 100 is maximized (minimized). In a first collected state of the cell 100, a negative potential is applied to the first top electrode 121, while the second top electrode 122 is floating or grounded. This state may also be obtained when a negative potential is applied to the second top electrode 122, while the first top electrode 121 is floating or grounded. However, when the effective area of the first top electrode 121 is different (in size and or shape) from that of the second top electrode 122, reversing the potentials as described before results in the formation of two different collected states.

In a second collected state of the cell 100, a negative potential is applied to both the first top electrode 121 and the second top electrode 122.

In a third collected state of the cell 100, a positive potential is applied to both the first top electrode 121 and the second top electrode 122.

In a fourth collected state of the cell 100, a positive potential is applied to the first top electrode 121, while the second top electrode 122 is floating or grounded. This state may also be obtained when a positive potential is applied to the second top electrode 122, while the first top electrode 121 is floating or grounded. However, when the first top electrode 121 has an effective area that is different (in size and or shape) from that of the second top electrode 122, reversing the potentials as described before results in the formation of two different collected states. The optical characteristics of the above-mentioned states of the cell 100 have been measured (see Table 1).

Table 1 :

Characteristics of states for the cell 100, depending on potentials applied to the bottom electrode 111, the first top electrode 121, and the second top electrode 122.

The states listed in Table 1 are illustrated in Figs. 3 and 4, wherein for each of the states of the cell 100, Fig. 3 shows the top view, and Fig. 4 a cross-sectional view. In Figs. 3 and 4, (a) illustrates the distributed state, (b) the first collected state, (c) the second collected state, (d) the third collected state, and (e) the fourth collected state.

The states listed in Table 1 and illustrated in Figs. 3 and 4 are built-in states, as they are predominantly dependent on the application of electric potentials to the independently addressable bottom electrode 111, first top electrode 121, and second top electrode 122, respectively

From Table 1 , it is clear that the distributed state (lowest transmission) and the fourth collected state (highest transmission) are two extreme optical states, while the first, second, and third collected states are intermediate (grey) states. The grey scales are built-in grey scales, as they are predominantly dependent on the application of electric potentials to the independently addressable bottom electrode 111 and top electrodes 121 and 122, respectively.

With respect to the two patterned effective areas of the top electrodes 121 and 122, the line widths, the distances between the lines, and/or the shape may be optimized to achieve a desired distribution of built-in grey scales. Fig. 5 shows microscope images of an electrophoretic device according to the invention. The device of Fig. 5 comprises hexagonally shaped cells with an electrode arrangement (bottom and top electrode structures) that is similar to that of the cell 100 shown in Figs. 1-4. In Fig. 4, (a) and (e) represent the dark and light states of the device, respectively (comparable to the distributed state and the fourth collected state of cell 100), while (b), (c), and (d) represent three intermediate (grey) states of the device, (comparable to the first, second, and third collected states of cell 100).

In the embodiments described above, the top electrode structure comprises two independently addressable top electrodes, resulting in the ability to switch the cell in one of five different built-in states. The top electrode structure may comprise more than two independently addressable top electrode, to further increase the number of built-in states. For example, when the top electrode structure comprises 3, 4, 5, etc. independently addressable top electrodes will yield 9, 17, 33, etc. built-in states, respectively.

The built-in grey scales may be combined with a basic driving scheme to enhance the driving speed of the electrophoretic devices, or to generate intermediate grey scales by timed and/or dynamic driving. For example, one could first apply a voltage to all top electrodes to move the particles as fast as possible in a direction substantially perpendicular to the plane of the substrates (out-of-plane particle movement) to quickly obtain a first image. Subsequently, after for instance 10 to 400 ms one could switch to the originally desired voltage pattern to get the full grey scales, or the full transparency, by moving the particles in a direction substantially parallel to the plane of the substrates (in- plane particle movement).

The invention as described hereinabove may be used in electrophoretic devices comprising particles of different kinds and/or with different properties, for example a dual-particle suspension, reflective particles, particles active in the visible and/or IR/UV regime, or any combination of such particles.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An electrophoretic device comprising one or more cells, each cell comprising: a bottom substrate having a bottom substrate area, a top substrate having a top substrate area, and a spacer structure spacing apart the bottom and top substrates to form a space containing a suspension of particles in a medium, the particles being movable through application of an electric field, wherein the bottom substrate comprises a bottom electrode structure having an effective bottom electrode area, and the top substrate comprises a top electrode structure having an effective top electrode area in the form of a pattern, the effective bottom electrode area being larger than the effective top electrode area, the bottom and top electrode structures being arranged to establish the electric field, and to control a distribution of the particles from a distributed state to a collected state, characterized in that the top electrode structure comprises two or more independently addressable top electrodes, wherein the ratio of the top substrate area to the effective top electrode area is larger than or equal to 5, the bottom and top electrode structures being further arranged to control the distribution of particles to one of a plurality of collected states.

2. The electrophoretic device of claim 1, wherein the effective top electrode area has a line, a comb, or a ladder pattern.

3. The electrophoretic device of claim 1, wherein the bottom electrode area is substantially equal to the bottom substrate area.

4. The electrophoretic device of claim 1, wherein the spacer structure has an irregular shape and/or the effective top electrode area has a non-periodic pattern.

5. The electrophoretic device of claim 1, wherein the medium and the bottom electrode are optically transparent.

6. The electrophoretic device of claim 1, wherein the particles are dielectric particles, and wherein the electric field comprises an AC component.