Variable light transmission device and method of operation thereof

Abstract

The invention discloses a variable light transmission device and a method of operating the same. The method of operation of the variable light transmissive device comprises the steps of: (a) providing a variable light transmissive device, the device comprising a microcell layer having a plurality of microcells, each microcell comprising a raised structure and a channel, and comprising an electrophoretic medium; (b) applying a first electric field on the micro-unit layer to enable the micro-unit layer to be in an open optical state; and (c) applying a second electric field on the microcell layer to place it in a closed optical state having a lower percentage of light transmission than the open optical state.

Description

[0001] Related Applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 436,127, filed Dec. 30, 2022, the entire content of which, as well as all other patents and patent applications disclosed herein, are incorporated herein by reference. Background of the Invention

[0003] The present invention relates to a variable light transmission device and a method of operating the same. Specifically, the present invention relates to a microcell electro-optical device that includes an electrophoretic medium that includes charged pigment particles and a charge control agent. The charge control agent is present in the electrophoretic medium in a weight percentage based on the weight of the charge control agent in the electrophoretic medium. The electrophoretic medium is capable of switching between optical states using an electric field. The present invention also relates to a driving method for switching between optical states. Variable light transmission devices regulate the amount of light and other electromagnetic radiation passing therethrough. They can be used in mirrors, windows, skylights, and the like. For example, the present invention can be applied to windows that can regulate infrared radiation to control the temperature inside buildings and vehicles. Examples of electrophoretic media that can be incorporated into various embodiments of the present invention include, for example, those described in U.S. Patent Nos. 7,116,466, 7,327,511, 8,576,476, 10,319,314, 10,809,590, 10,067,398, 10,067,398, and 11,143,930, and U.S. Patent Application Publication Nos. 2014 / 0055841, 2017 / 0351155, 2017 / 0235206, 2011 / 0199671, 2020 / 0355979, 2020 / 0272017, 2021 / 0096439, and U.S. Patent Application Serial No. 17 / 935,386, filed Sep. 27, 2022, the entire content of which is incorporated herein by reference.

[0004] Particle-based electrophoretic displays, in which a plurality of charged pigment particles move through a suspension fluid under the influence of an electric field, have been the subject of intensive research and development for many years. Such displays have characteristics such as good brightness and contrast, wide viewing angles, bistable states, and low power consumption when compared to liquid crystal displays.

[0005] As used herein, the terms "bistable" and "bistability" in their ordinary sense in the art refer to a display including a display element having a first display state and a second display state that differ in at least one optical property, and such that after driving any given element with an addressing pulse of finite duration to assume its first or second display state, after termination of the addressing pulse, the state will persist for at least several times (e.g., at least four times) the shortest duration of the addressing pulse required to change the state of the display element. Published U.S. Patent Application Serial No. 2002 / 0180687 shows that some particle-based grayscale electrophoretic displays are stable not only in their extreme black and extreme white states, but also in their intermediate gray states, as are some other types of electro-optic displays. Displays of this type are more appropriately referred to as "multistable" rather than "bistable", but for convenience, the term "bistable" may be used herein to encompass both bistable and multistable displays.

[0006] As noted above, an electrophoretic medium requires the presence of a suspending fluid. In most prior art electrophoretic media, this suspending fluid is a liquid, but a gaseous suspending fluid can be used to produce an electrophoretic medium. When the medium is used in a direction that permits particle sedimentation (e.g., in a sign where the medium is in a vertical plane), this gas-based electrophoretic medium appears to be susceptible to the same type of particle sedimentation problems as liquid-based electrophoretic media. In fact, particle sedimentation appears to be a more severe problem in gas-based electrophoretic media than in liquid-based electrophoretic media because the gaseous suspending fluid has a lower viscosity than liquid electrophoretic media, which allows the charged pigment particles to sediment more quickly.

[0007] Numerous patents and applications assigned to the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC and their related companies or filed in their names describe various techniques for encapsulating and microcell electrophoresis and other electro-optic media. Encapsulated electrophoretic media include numerous small vesicles, each of which itself includes an internal phase that includes electrophoretically mobile particles in a liquid medium, and a vesicle wall that surrounds the internal phase. Typically, the vesicles themselves are held in a polymeric binder to form a continuous layer between two electrodes. In a microcell electrophoretic display, the charged pigment particles and the liquid are not encapsulated in microcapsules, but are retained in a plurality of cavities formed within a carrier medium (usually a polymeric film). The techniques described in these patents and applications include:

[0008] (a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814.

[0009] (b) Vesicles, binders, and encapsulation processes; see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719.

[0010] (c) Microcell structures, wall materials, and methods of forming microcells; see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906.

[0011] (d) Methods for filling and sealing microcells; see, e.g., U.S. Patent Nos. 7,144,942 and 7,715,088.

[0012] (e) Thin films and subassemblies containing electro-optic materials; see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564.

[0013] (f) Backplanes, adhesive layers, and other auxiliary layers and methods used in displays; see, e.g., U.S. Patent Nos. 7,116,318 and 7,535,624.

[0014] (g) Color formation and color adjustment; see, e.g., U.S. Patent Nos. 7,075,502 and 7,839,564.

[0015] (h) Methods for driving displays; see, e.g., U.S. Patent Nos. 7,012,600 and 7,453,445.

[0016] (i) Applications of displays; see, e.g., U.S. Patent Nos. 7,312,784 and 8,009,348.

[0017] (j) Non-electrophoretic displays, as described in U.S. Patent No. 6,241,921 and U.S. Patent Application Publication No. 2015 / 0277160; and applications of encapsulation and microcell technologies other than displays; see, e.g., U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710.

[0018] Many of the above patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced with a continuous phase, resulting in so-called polymer dispersed electrophoretic displays, where the electrophoretic medium includes a plurality of discrete droplets of nonpolar liquid and a continuous phase of polymer material, and the discrete droplets of electrophoretic medium within such polymer dispersed electrophoretic displays can be considered capsules or microcapsules even though each individual droplet does not have an associated independent capsule membrane; see, e.g., 2002 / 0131147 above. Thus, for the purposes of this application, such polymer dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

[0019] One related type of electrophoretic display is the so-called "microcell electrophoretic display". In a microcell electrophoretic display, charged pigment particles and a suspending liquid are not encapsulated in microcapsules, but instead are retained in a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02 / 01281 and Published U.S. Application Serial No. 2002 / 0075556, both of which are assigned to Sipix Imaging, Inc.

[0020] Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be set to operate in a so-called "shutter mode", in which one display state is substantially opaque and another display state is light transmissive. See, for example, U.S. Patent Nos. 6,130,774 and 6,172,798, as well as U.S. Patent Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on changes in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optic displays may also be capable of operating in a shutter mode.

[0021] Encapsulated electrophoretic displays or microcell electrophoretic displays generally do not suffer from the aggregation and sedimentation failure modes of conventional electrophoretic devices, and offer further advantages such as the ability to print or coat the display on a variety of flexible and rigid substrates. The use of the term "print" is intended to include all forms of printing and coating, including but not limited to: pre-metered coating such as block die coating, slot coating or extrusion coating, slide coating or cascade coating, curtain coating; roll coating such as knife-over-roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition; and other similar techniques. Thus, the resulting display can be flexible. In addition, since the display medium can be printed (using a variety of methods), the display itself can be manufactured inexpensively.

[0022] A potentially important market for electrophoretic media is windows with variable light transmittance. As the energy performance of buildings and vehicles becomes increasingly important, electrophoretic media can be used in windows to electronically control the proportion of incident radiation transmitted through the window by changing the optical state of the electrophoretic medium. Effective implementation of this "variable transmittance" ("VT") technology in buildings is expected to bring the following benefits: (1) reducing unnecessary heat effects during hot weather, reducing the energy required for cooling, the size of air conditioning equipment, and peak electricity demand; (2) increasing the use of natural daylight, thus reducing lighting energy consumption and peak electricity demand; and (3) enhancing the comfort of occupants by improving thermal and visual comfort. Since the ratio of the glass surface to the enclosed volume in automobiles is significantly greater than that in ordinary buildings, automobiles are expected to benefit more. Specifically, effective implementation of variable transmittance technology in automobiles is expected to bring not only the above benefits but also the following benefits: (1) improving driving safety; (2) reducing glare; (3) enhancing mirror performance (by using electro-optic coatings on mirrors); and (4) improving the ability to use head-up displays. Other potential applications of variable transmittance technology include privacy glass and anti-glare glass in electronic devices.

[0023] There are already some examples of devices in the prior art that include an electrophoretic medium sandwiched between electrode layers, capable of achieving a closed optical state (opaque state) and an open optical state (transparent state), and switching between these states by applying an electric field to the electrophoretic medium. However, traditional electrophoretic devices with traditional structures and waveforms require a long switching time, making these devices less than ideal. The inventors of the present invention unexpectedly found that a specific device with a micro-unit layer and a specific waveform can achieve efficient switching between the open and closed optical states. Summary of the Invention

[0024] On the one hand, the present invention provides a method for operating a variable light transmittance device, comprising the following steps: (a) providing a variable light transmittance device; (b) applying a first electric field to switch the variable light transmittance device to an open optical state; and (c) applying a second electric field to switch the variable light transmittance device to a closed optical state.

[0025] The variable light transmission device includes a first light-transmissive electrode layer, a second light-transmissive electrode layer, and a micro-unit layer. The micro-unit layer includes a plurality of micro-units and a sealing layer. The micro-unit layer is disposed between the first light-transmissive electrode layer and the second light-transmissive electrode layer. Each micro-unit includes an electrophoretic medium, which includes charged pigment particles in a fluid and a charge control agent. The content of the charge control agent in the electrophoretic medium is expressed as a weight percentage of the charge control agent in the weight of the electrophoretic medium. Each of the plurality of micro-units has a micro-unit opening. The sealing layer spans the micro-unit openings of the plurality of micro-units. Each of the plurality of micro-units includes a micro-unit bottom layer, a raised structure, a micro-unit wall, and a channel. The micro-unit bottom layer has a micro-unit bottom inner surface, which includes an exposed micro-unit bottom inner surface and an unexposed micro-unit bottom inner surface. The raised structure has a raised base, a raised surface, a raised apex, and a raised height. The raised apex is a point or a set of points on the raised structure, and the distance from this point or this set of points to the micro-unit opening is shorter than the distance from all other points on the raised structure to the micro-unit opening. The raised height is the distance between the raised base and the raised apex. The raised surface is the surface of the raised structure that does not include the raised apex in contact with the electrophoretic medium. The micro-unit wall has a micro-unit inner wall surface and a micro-unit wall upper surface. The micro-unit inner wall surface is the surface of the micro-unit wall of the micro-unit in contact with the electrophoretic medium. The micro-unit wall upper surface is the surface of the micro-unit wall of the micro-unit in contact with the sealing layer. The channel has a channel height, and the channel height is 50% of the raised height. The unexposed micro-unit bottom inner surface is in contact with the raised base. The channel is the volume between the exposed micro-unit bottom inner surface, the raised surface, and the micro-unit inner wall surface. The weight percentage of the charge control agent in the electrophoretic medium of the variable light transmission device can be 1% to 8% of the weight of the electrophoretic medium.

[0026] The method of operating the variable light transmission device includes the following steps: applying a first electric field between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a first waveform to cause the charged pigment particles to move towards the channel, thereby switching the variable light transmission device to an open optical state. The charged pigment particles in the open optical state are located in the channel.

[0027] The method of operating the variable light transmission device includes the following steps: applying a second electric field between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a second waveform to cause the charged pigment particles to move towards the first light-transmissive electrode layer at a certain speed, and the speed has a lateral component, thereby resulting in a closed optical state. The second waveform can be DC unbalanced. The second waveform can include at least one positive voltage and at least one negative voltage, and the second waveform has a net positive impulse or a net negative impulse. The closed optical state has a lower light transmission percentage than the open optical state.

[0028] In one embodiment, the second waveform of the method of operating a variable light transmission device may include an alternating current (AC) waveform having a frequency and a duty cycle ranging from 5% to 45%. The duty cycle of the AC waveform may be higher than 50%, higher than 55%, higher than 60%, or higher than 65%. The duty cycle of the AC waveform may be 55% to 95%, 58% to 90%, 60% to 88%, 65% to 85%, or 70% to 80%. The duty cycle of the AC waveform may be lower than 50%, lower than 45%, lower than 40%, or lower than 35%. The duty cycle of the AC waveform may be 5% to 45%, 8% to 40%, 10% to 38%, 10% to 40%, 15% to 35%, or 20% to 30%. The AC waveform may be a square wave, a sine wave, a triangular wave, or a sawtooth wave. The ratio of the frequency of the AC waveform in hertz to the content of the charge control agent in the electrophoretic medium expressed as a weight percentage of the charge control agent in the electrophoretic medium may be from 400 to 2000 Hz.

[0029] The AC waveform may be a square wave having two or more cycles. In this case, the positive and negative voltages of the AC waveform have the same amplitude; the amplitude of the AC waveform may be 10 V to 200 V; the frequency of the AC waveform may be 0.1 Hz to 6000 Hz or 100 Hz to 3000 Hz; the amplitude of the AC waveform may be 10 V to 200 V or 20 V to 180 V, and the frequency of the AC waveform may be 0.1 Hz to 6000 Hz or 100 Hz to 3000 Hz. The ratio of the frequency of the AC waveform in hertz to the content of the charge control agent in the electrophoretic medium expressed as a weight percentage of the charge control agent in the electrophoretic medium may be from 400 to 2000 Hz.

[0030] In another embodiment, the second waveform of the method of operating a variable light transmission device may include a waveform formed by superimposing a DC voltage component and an AC waveform having a frequency and an amplitude. The frequency of the AC waveform may be 0.1 Hz to 6000 Hz, 100 Hz to 3000 Hz, or 400 Hz to 2000 Hz. The amplitude of the AC waveform may be 10 V to 200 V or 20 V to 180 V. The DC voltage component has an amplitude of 0.1 V to 500 V. The ratio of the frequency of the AC waveform in hertz to the content of the charge control agent in the electrophoretic medium expressed as a weight percentage of the charge control agent in the electrophoretic medium may be from 400 to 2000 Hz. The second waveform may include an AC waveform having a DC offset. The AC waveform may be selected from the group consisting of a square wave, a sine wave, a triangular wave, and a sawtooth wave.

[0031] The convex structure can be a geometric body selected from the group consisting of: (a) a cone; (b) a cone on a cylinder, the cylinder having a base, and the base of the cylinder being the convex base of the convex structure; (c) a tetrahedron; (d) a tetrahedron on a triangular prism, the triangular prism having a triangular base, and the triangular base being the convex base of the convex structure; (e) a triangular prism having a square base, and the square base being the convex base of the convex structure; (f) a quadrangular pyramid having a square base, and the square base being the convex base of the convex structure; (h) a quadrangular pyramid on a cube, the cube having a base, and the base of the cube being the convex base of the convex structure; (i) a quadrangular pyramid on a right parallelepiped, the right parallelepiped having a right parallelogram base, and the right parallelogram base being the convex base of the convex structure; (j) a pentagonal pyramid having a pentagonal base; and the pentagonal base being the convex base of the convex structure; (k) a pentagonal pyramid on a pentagonal prism, the pentagonal prism having a pentagonal base, and the pentagonal base being the convex base of the convex structure; (l) a hexagonal pyramid having a hexagonal base, and the pentagonal base being the convex base of the convex structure; (m) a hexagonal pyramid on a hexagonal prism, the hexagonal prism having a hexagonal base, and the hexagonal base being the convex base of the convex structure. The convex structure can be a cone, and the slope of the cone can be from 5 degrees to 10 degrees. The convex structure can be a cone on a cylinder. The cylinder can have a base, and the base of the cylinder is the convex base of the convex structure; and the slope of the cone can be 10 degrees or less. The convex structure can be a geometric body of a pyramid having a base with n sides, and the base with n sides is the convex base of the convex structure, where n is an integer from 7 to 12; (m) a pyramid having a base with n sides, the pyramid being located on a prism having a base with n sides, and the base of the prism having n sides is the convex base of the convex structure, where n is from 7 to 12.

[0032] The electrophoresis medium can include first-type charged pigment particles and second-type charged pigment particles. The first-type charged pigment particles can reflect light. And the second-type charged pigment particles can absorb light. The first-type charged pigment particles can be white. The second-type charged pigment particles can be black. The first-type charged pigment particles can have the same polarity as the second-type charged pigment particles. The first-type charged pigment particles can have the opposite polarity to the second-type charged pigment particles.

[0033] The upper surface of the micro-unit wall can have a light-blocking layer, and the light-blocking layer is located between the upper surface of the micro-unit and the sealing layer. The light-blocking layer can include a light-absorbing pigment. The light-blocking layer can include a black pigment.

[0034] On the other hand, the present invention provides a variable light transmission device. The variable light transmission device includes a first light-transmissive electrode layer, a second light-transmissive electrode layer, and a microcell layer. The microcell layer includes a plurality of microcells and a sealing layer. The microcell layer is disposed between the first light-transmissive electrode layer and the second light-transmissive electrode layer. Each microcell includes an electrophoretic medium, which includes charged pigment particles in a fluid and a charge control agent. Each of the plurality of microcells has a microcell opening. The sealing layer spans the microcell openings of the plurality of microcells. Each of the plurality of microcells includes a microcell bottom layer, a raised structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface, which includes an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. The raised structure has a raised base, a raised surface, a raised apex, and a raised height. The raised apex is a point or a set of points on the raised structure, and the distance from this point or this set of points to the microcell opening is shorter than the distance from all other points on the raised structure to the microcell opening. The raised height is the distance between the raised base and the raised apex. The raised surface is the surface of the raised structure that does not include the raised apex in contact with the electrophoretic medium. The microcell wall has a microcell inner wall surface and a microcell wall upper surface. The microcell inner wall surface is the surface of the microcell wall of the microcell in contact with the electrophoretic medium. The microcell wall upper surface is the surface of the microcell wall of the microcell in contact with the sealing layer. The channel has a channel height, and the channel height is 50% of the raised height. The unexposed microcell bottom inner surface is in contact with the raised base. The channel is the volume between the exposed microcell bottom inner surface, the raised surface, and the microcell inner wall surface.

[0035] A first electric field is applied between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a first waveform to cause the charged pigment particles to move toward the channel, thereby switching the variable light transmission device to an open optical state. The charged pigment particles in the open optical state are located in the channel.

[0036] A second electric field is applied between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a second waveform to cause the charged pigment particles to move toward the first light-transmissive electrode layer at a certain speed, and the speed has a lateral component, thereby resulting in a closed optical state. The second waveform can be DC unbalanced. The second waveform can include at least one positive voltage and at least one negative voltage, and the second waveform has a net positive impulse or a net negative impulse. The closed optical state has a lower light transmission percentage than the open optical state.

[0037] The second waveform can include an alternating current waveform, which has a frequency, and the alternating current waveform has a duty cycle ranging from 5% to 45%. The alternating current waveform can be a square wave, a sine wave, a triangular wave, or a sawtooth wave.

[0038] The alternating current waveform can be a square wave having two or more cycles, wherein the positive and negative voltages of the alternating current waveform have the same amplitude. The amplitude of the square wave can be from 10V to 200V, and the frequency can be from 0.1Hz to 6000Hz or from 100Hz to 3000Hz. The amplitude of the square wave can be from 10V to 200V or from 20V to 180V, and the frequency can be from 0.1Hz to 6000Hz or from 100Hz to 3000Hz. The ratio of the frequency of the alternating current waveform expressed in Hertz to the content of the charge control agent in the electrophoresis medium expressed as a weight percentage of the charge control agent in the electrophoresis medium by weight can be from 400 to 2000Hz.

[0039] Alternatively, the second waveform can include a waveform formed by superimposing a direct current voltage component and an alternating current waveform, the alternating current waveform having an amplitude and a frequency, wherein the direct current voltage component has an amplitude. The frequency of the alternating current waveform can be from 0.1Hz to 6000Hz, from 100Hz to 3000Hz or from 400Hz to 2000Hz, and the amplitude of the alternating current waveform can be from 10V to 200V or from 20V to 180V. The amplitude of the direct current voltage component can be from 0.1V to 500V. The ratio of the frequency of the alternating current waveform expressed in Hertz to the content of the charge control agent in the electrophoresis medium expressed as a weight percentage of the charge control agent in the electrophoresis medium by weight can be from 400 to 2000Hz. The second waveform can include an alternating current waveform having a direct current offset. The alternating current waveform can be selected from the group consisting of a square wave, a sine wave, a triangular wave, and a sawtooth wave. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Shows a cylindrical particle in a liquid under the influence of an electric field and the forces generated on the particle.

[0041] Figure 2A 、 2B 2C and 2D show side views of examples of a part of the variable light transmission device of the present invention.

[0042] Figure 3 Shows a side view of a micro unit in an open optical state and a side view of a micro unit in a closed optical state.

[0043] Figure 4 is an example of the first embodiment of the present invention; this example is a direct current unbalanced waveform that can be applied to a variable light transmittance device to achieve a closed state; this waveform includes an alternating current waveform with a duty cycle higher than 50%.

[0044] Figure 5 is an example of the second embodiment of the present invention, which is a direct current unbalanced waveform that can be applied to a variable light transmittance device to achieve a closed state, and this waveform is a superposition of a direct current voltage component and an alternating current waveform.

[0045] Figure 6 Shows the force exerted by charged pigment particles on the conical protrusion surface of the variable light transmission device of the present invention.

[0046] Figure 7 Shows a part of a variable light transmission device having an electrophoretic medium, the electrophoretic medium including first-type charged pigment particles and second-type charged pigment particles, and the first-type charged pigment particles and the second-type charged pigment particles having the same polarity.

[0047] Figure 8 Shows a part of a variable light transmission device having an electrophoretic medium, the electrophoretic medium including first-type charged pigment particles and second-type charged pigment particles, and the first-type charged pigment particles and the second-type charged pigment particles having opposite polarities.

[0048] Figure 9 Provides a graph showing the variation of light reflection, transmission, and absorption with layer thickness for a layer including light-absorbing pigment particles.

[0049] Figure 10 Provides a graph showing the variation of light reflection, transmission, and absorption with layer thickness for a layer including light-reflecting pigment particles.

[0050] Figure 11 Shows a graph of the effects of the variation of reflection, transmission, and absorption with layer thickness for a closed optical state layer including a combination of light-reflecting pigment particles and light-absorbing pigment particles.

[0051] Figure 12 Shows a part of a variable light transmission device, the device including a light-blocking layer on the inner surface of the bottom of the exposed microcell.

[0052] Figure 13 Shows a part of a variable light transmission device, the device including a light-blocking layer on the upper surface of the microcell wall.

[0053] Figure 14 Shows a plan view of the microcell of the variable transmission device used in the example.

[0054] Figure 15 Shows a cross-sectional view of the microcell of the variable transmission device used in the example.

[0055] Figure 16 Provides micrographs of the open and closed optical states of the variable light transmission device of Example 1, these optical states being generated by various waveforms.

[0056] Figure 17 Provides micrographs of the open and closed optical states of the variable light transmission device of Example 2, the electrophoretic medium of the device including different concentrations of a charge control agent.

[0057] Figure 18 Micrograph of the microcell array of the variable light transmittance device of Example 5; the light-blocking composition includes black pigment particles that are distributed throughout the microcell (closed optical state).

[0058] Figure 19 Is a micrograph of the microcell array of the variable light transmittance device of Example 5, where the black pigment particles of the light-blocking composition are driven into the channels of the microcells (closed optical state).

[0059] Figure 20 Is a micrograph of the closed optical state of the microcell array of the variable light transmittance device of Example 6; the white charged pigment particles of the light-blocking composition are distributed throughout the microcell (closed optical state).

[0060] Figure 21 Is a micrograph of the open optical state of the microcell array of the variable light transmittance device of Example 6; the white charged pigment particles of the light-blocking composition are driven into the channels (open optical state).

[0061] Figure 22 Is a micrograph of the open optical state of the microcell array of the variable light transmittance device of Example 7, the electrophoretic medium of which includes white and black pigment particles. Detailed Description

[0062] The distance from a point to a plane is the shortest perpendicular distance from the point to the plane. The shortest distance from a point to a plane is the length of the parallel perpendicular line from the given point to the normal vector of the given plane.

[0063] The distance between two planes in three-dimensional space is the shortest distance between the planes, that is, the shortest distance from any point on one plane to any point on the other plane.

[0064] The slope of a cone is defined as an angle having: (a) a vertex (A) that lies on the circumference of the base of the cone, (b) a first arm that is the line connecting point A (the vertex) and the center of the base of the cone, and (c) a second arm that is the line connecting point A (the vertex) and the apex of the cone.

[0065] The term "charged pigment particle" can refer to a charged pigment particle whose surface does not have any polymeric material. The term "charged pigment particle" can also refer to a pigment particle whose surface has a polymeric material.

[0066] The "inner wall surface of the microcell" is the surface of the microcell wall that contacts the electrophoretic medium of the microcell.

[0067] The "upper surface of the micro-unit wall" is the surface of the micro-unit wall that contacts the sealing layer of the micro-unit. When there is a light-blocking layer on the upper surface of the micro-unit wall, the light-blocking layer is located between the upper surface of the micro-unit wall and the sealing layer.

[0068] The "DC balanced waveform" or "DC balanced driving waveform" applied to a pixel refers to a driving waveform in which the integral of the driving voltage applied to the pixel over the entire waveform application period is substantially zero. DC balance can be achieved by balancing each stage of the waveform, that is, by selecting a first positive voltage such that the integral of the subsequent negative voltage is zero or substantially zero. If the waveform is not DC balanced, it is called a "DC unbalanced waveform" or "DC unbalanced driving waveform". The driving waveform applied to a pixel can have a part of a DC unbalanced waveform and at least one additional pulse of reverse impulse to ensure that the overall waveform applied to the pixel is DC balanced. This additional pulse can be applied before the DC unbalanced part (pre-pulse) of the waveform. Typical examples of DC unbalanced waveforms include: (a) a square or sinusoidal AC waveform with a duty cycle less than (or greater than) 50%, and (b) a square or sinusoidal AC waveform with a DC offset.

[0069] The term "impulse" refers to the integral of voltage over time. That is, for a waveform pulse with a voltage V applied for a time t, its impulse is V×t. If the polarity of the voltage V is positive, the impulse is positive; if the polarity of the voltage V is negative, the impulse is negative.

[0070] The "net positive impulse" of a waveform means that during the application of the waveform, negatively charged pigment particles will be attracted to and move towards the first light-transmitting electrode layer.

[0071] In the micro-unit of the variable light transmission device of the present invention, the term "lateral component of velocity" related to the movement of charged pigment particles refers to the velocity in the horizontal direction. In this definition, we assume that the velocity of the charged particle is a vector obtained by adding the horizontal direction velocity (Vh) and the vertical direction velocity (Vv), and in the case where the charged pigment particle moves within the electrophoretic micro-unit, the vertical direction is the direction from the first light-transmitting electrode layer to the second light-transmitting electrode layer, or from the second light-transmitting electrode layer to the first light-transmitting electrode layer. In the same system, the horizontal direction in which the charged pigment particle moves within the electrophoretic micro-unit is the direction from one side of the micro-unit wall to the other side of the micro-unit wall, and this direction is parallel to the first light-transmitting electrode layer. Therefore, the statement "the velocity of the charged pigment particle has a lateral component" means that the magnitude of the velocity in the horizontal direction is greater than zero.

[0072] The induced-charge electroosmosis (ICEO) phenomenon can be used to laterally move polarizable particles present in an electrophoresis medium, such as pigment particles. That is, the polarizable particles can be moved parallel to the electrode layers sandwiching the electrophoresis medium. Under the action of an electric field, the particles may be subject to forces caused by particle polarization (or by polarization of a conductive coating adsorbed on the particle surface, or by polarization of the double electric layer around the particle). This force may cause a perturbation in the flow of mobile charges (such as ions or charged micelles) in the electrophoresis medium, as Figure 1 shown, where the cylindrical particle 101 is surrounded by the liquid of the electrophoresis medium under the action of the applied electric field. This figure is reproduced from the following article: Bazant and Squires, J. Fluid Mech., 2004, 509, 217-252.

[0073] Perfectly symmetric spherical particles are not subject to any net force, but less symmetric particles are subject to forces with a component perpendicular to the direction of the applied field. The cooperative flow generated by a group of particles each subject to such a force can lead to a "vortex" in the electrophoresis medium including multiple particles. According to the theory proposed by Bazant and Squires in the article, the maximum velocity u of such a vortex for a specific particle is approximately given by expression (1).

[0074] Expression (1)

[0075] In expression (1), E is the field strength, ε is the dielectric constant of the solvent, η is the viscosity of the electrophoresis liquid, is the frequency of the applied sinusoidal alternating current, and τ is the time scale for the solvent charge carriers to move around the charge to form a screening charge layer. This time scale τ is given by formula (2).

[0076] Formula (2)

[0077] In formula (2), λ D is the Debye screening length, R is the particle radius, and D is the diffusion constant of the charge carriers in the fluid.

[0078] According to expression (1), as the frequency increases, the value of increases, and the maximum velocity of the induced-charge flow decreases. In addition, for the value of is much greater than 1, the maximum rotational velocity is proportional to the square of the ratio The induced-charge flow occurs in the same direction regardless of the polarity of the applied electric field, and thus can be driven by an alternating field.

[0079] When the electrophoretic medium is included in microcells (which is preferred in electrophoretic displays), the geometry of the induced flow is affected by the shape of the particular microcells used. For example, in the simplest case of two parallel electrodes, it has been shown that, using an appropriate electric field strength and AC frequency, the flow can adopt a scroll-like structure with a periodic spacing corresponding to the gap width between the electrodes.

[0080] The inventors of the present invention have utilized a complex microcell structure formed by an imprinting method to fabricate a switchable device. In one example, the imprinted structure includes a conical protrusion at the bottom of each microcell. Figure 2A 、 2B Figures 2A, 2B, and 2C show examples of a variable light transmission device according to the present invention, wherein the protrusion structure of the variable light transmission device is a cone on a cylinder. As Figure 2A 、 2B shown in Figures 2A, 2B, and 2C, the cone of the protrusion structure can direct the flow of electrophoretic particles into the channels. If the electric field applied to the electrophoretic medium has a suitable polarity related to the polarity of the charged pigment particles, the charged pigment particles will move towards the channels. For example, if the charged pigment particles are positively charged and the voltage applied via the light-transmissive electrode causes the second light-transmissive electrode to be negatively polarized, the charged pigment particles will move towards the channels. The same movement will occur if the charged pigment particles are negatively charged and the voltage applied via the light-transmissive electrode causes the second light-transmissive electrode to be positively polarized. Figure 2A 、 2B Figures 2A, 2B, and 2C show a cross-section (not drawn to scale) of a portion of the variable light transmission device, which shows only one microcell out of the multiple microcells of the device. Figures 2A, 2B, and 2C are identical in the device structure shown, but different parts of the device are labeled in each figure.

[0081] Figure 2A 、 2B The portion of the variable light transmission device 200 shown in Figures 2A, 2B, and 2C includes a microcell layer that includes a plurality of microcells and a sealing layer. Although Figure 2A 、 2B only one microcell is shown in Figures 2A, 2B, and 2C, the entire variable light transmission device including a plurality of microcells can be envisioned. The variable light transmission device can include a first transparent substrate 201, a first light-transmissive electrode layer 202, a microcell layer 203 that includes a plurality of microcells 204 and a sealing layer 206, a second light-transmissive electrode layer 207, and a second transparent substrate 208. Each of the plurality of microcells 204 includes an electrophoretic medium 209 that includes charged pigment particles and a charge control agent in a fluid. Figure 2A 、 2BThe components of the electrophoretic medium are not shown in FIGS. 2B and 2C. Each of the plurality of micro-units 204 has a micro-unit opening 205, and a sealing layer 206 extends across the micro-unit openings 205 of the plurality of micro-units 204. Each of the plurality of micro-units 204 includes a micro-unit bottom layer 210, a raised structure 217, a micro-unit wall 212, and a channel 215. The micro-unit bottom layer 210 has a micro-unit bottom inner surface 211, which includes an exposed micro-unit bottom inner surface 211a and an unexposed micro-unit bottom inner surface 211b. The unexposed micro-unit bottom inner surface 211b contacts the raised base 218. Figure 2A The exposed micro-unit bottom inner surface 211a is highlighted with a thicker line in FIG.

[0082] In this example, the raised structure 217 is a cone on a cylinder. The raised structure 217 has a raised base 218, a raised surface 221, a raised vertex 219, and a raised height 220. The raised vertex 219 is a point or a set of points on the raised structure 217 that are closer to the micro-unit opening 205 than all other points of the raised structure 217. In Figure 2A , 2B and the example of the variable light transmission device shown in FIGS. 2B and 2C, the raised vertex 219 is the vertex of the cone of the raised structure. The raised height 220 is the distance between the raised base 218 and the raised vertex 219. If the raised structure 217 has a raised vertex 219 that includes more than one point, such as a plane, the raised height 220 is the distance between the plane of the raised structure 217 and the raised base 218. A micro-unit layer including a plurality of micro-units 204 having raised structures 217 can be manufactured by using a pre-patterned stamper to imprint a thermoplastic or thermosetting precursor layer and then demolding. The precursor layer can be hardened by radiation, cooling, solvent evaporation, or other means during or after the imprinting step.

[0083] The micro-unit wall 212 has a micro-unit inner wall surface 213 and a micro-unit wall upper surface 214. The micro-unit inner wall surface 213 contacts the electrophoretic medium 209. The micro-unit wall upper surface 214 is the surface of the micro-unit wall 212 of the micro-unit that contacts the sealing layer 206. The micro-unit inner wall surface 213 is highlighted with a thicker line in FIG. Figure 2B

[0084] The channel 215 is the volume between the exposed micro-unit bottom inner surface 211a, the micro-unit inner wall surface 213, and the raised surface 221. The channel 215 is the volume position where most of the charged particles are located in the device in the open optical state. The channel 215 has a channel height 216 that is 50% of the raised height 220. Thus, the channel height, as well as the exposed micro-unit bottom inner surface 211a, the micro-unit inner wall surface 213, and the raised surface 221, further define the channel. InFigure 2C In it, the raised surface 221 is highlighted with a thicker line.

[0085] Figure 2D An example of a variable light transmission device according to the present invention is shown, where the raised structure of the variable light transmission device is a cone on a cylinder. Figure 2D The variable light transmission device shown is similar to Figure 2A , 2B , and the device shown in 2C, but shows a larger portion of the device including four micro-units. The variable light transmission device 200 includes a first transparent substrate 201, a first light-transmissive electrode layer 202, a micro-unit layer 203 including a plurality of micro-units 204 and a sealing layer 206, a second light-transmissive electrode layer 207, and a second transparent substrate 208. Each of the plurality of micro-units includes an electrophoretic medium that includes charged pigment particles 222 and a charge control agent in a fluid. Each of the plurality of micro-units 204 has a micro-unit opening, and the sealing layer 206 extends across the micro-unit openings of the plurality of micro-units. Each of the plurality of micro-units includes a micro-unit bottom layer 210, a raised structure 217, a micro-unit wall 212, and a channel 215. Figure 2D The variable light transmission device shown is in the closed optical state.

[0086] When a first electric field is applied between the first light-transmissive electrode layer 202 and the second light-transmissive electrode layer 207 via a first waveform, if the polarity of the charged pigment particles 222 is opposite to the voltage polarity of the second light-transmissive electrode layer, the charged pigment particles 222 will move towards the channel. If the polarity of the charged pigment particles 222 is opposite to the voltage polarity of the second light-transmissive electrode layer, the charged pigment particles 222 will be attracted to the second light-transmissive electrode, and the variable light transmission device will switch to the open optical state, which has a higher light transmission percentage than the closed optical state. The open optical state is as shown in Figure 3 a, where the charged pigment particles 222 are represented by black solid circles. In this example, the electrophoretic medium includes one type of charged pigment particles 222.

[0087] A second electric field is applied between the first light-transmissive electrode layer 202 and the second light-transmissive electrode layer 207 via a second waveform, causing the charged pigment particles 222 to move towards the first light-transmissive electrode layer 202 at a certain speed. This results in the closed optical state, as shown in Figure 3As shown in b. This velocity has a lateral component. If there is no velocity with a lateral component, the closed optical state will not occur because the charged pigment particles 222 will move from the channels in the open state towards the first light-transmissive electrode layer 202, but these charged pigment particles 222 will occupy the regions near the periphery of the microcells near the sealing layer 206. That is to say, the charged pigment particles 222 will not cover the entire surface of the first light-transmissive electrode layer 202. Therefore, the closed optical state cannot be effectively formed because the light transmittance of the closed optical state is relatively high.

[0088] The above shows that the transition from the closed optical state to the open optical state will be easier to achieve because when the charged pigment particles impact the raised surface of the raised structure during their movement towards the second light-transmissive electrode layer, the slope of the raised structure (e.g., Figure 3 the cones in FIGS. 3a and 3b) will impart a lateral component to the velocity of the charged pigment particles.

[0089] By making the conductivities of the electrophoretic medium and the cones significantly different from each other, it is possible to form an electric field within the variable light-transmission device. For example, if the conductivity of the cones is much lower than that of the electrophoretic medium, the field lines will tend to guide the charged pigment particles into the channels. However, even in such a case, it may still be necessary to provide a greater horizontal force component to redisperse the charged pigment particles from the channels. In addition, at the current state of the art, when the resistivity of the cone material and the electrophoretic medium are approximately equal (about 10 10 Ω*cm), it is easier to assemble and operate the device. In this case, the electric field lines will pass through the cell approximately perpendicularly. Therefore, it is preferable to use a waveform that causes the charged pigment particles to have lateral movement.

[0090] The method of operating a variable light-transmission device includes the following steps: applying a first electric field between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a first waveform to cause the charged pigment particles to move towards the channels, thereby switching the variable light-transmission device to the open optical state, and the charged pigment particles in the open optical state are located within the channels. The method of operating a variable light-transmission device further includes the following steps: applying a second electric field between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a second waveform to cause the charged pigment particles to move towards the first light-transmissive electrode layer at a certain velocity that has a lateral component and results in the closed optical state. The second waveform includes a series of at least two positive and negative pulses having a net positive impulse or a net negative impulse, wherein the closed optical state has a lower light transmittance percentage than the open optical state.

[0091] The second waveform can be DC unbalanced. The second waveform can include at least one positive voltage and at least one negative voltage, and the second waveform has a net positive impulse or a net negative impulse. The choice of the net positive impulse or the net negative impulse depends on the polarity of the charged pigment particles. Specifically, if the charged pigment particles are negatively charged, a net positive impulse is required to move the charged pigment particles from the particles in the channel to the first light-transmissive electrode layer. In other words, this movement requires that the final result of applying the voltage is that the positive voltage of the first light-transmissive electrode layer attracts the negatively charged particles relative to the second light-transmissive electrode layer. Conversely, if the charged pigment particles are positively charged, a net negative impulse is required to move the charged pigment particles from the channel near the second light-transmissive electrode layer 207 to the first light-transmissive electrode layer.

[0092] The second electric field applied between the two light-transmissive electrode layers via the second waveform achieves the off optical state.

[0093] In the first embodiment, the second waveform includes an alternating current waveform having a duty cycle different from 50%. An example of the second waveform of the first embodiment is as Figure 4 shown.

[0094] The alternating current waveform can have a positive or negative DC bias. The DC bias can be achieved by controlling the duty cycle of the waveform. The duty cycle of the positive DC bias waveform is higher than 50%. The duty cycle of the positive DC bias waveform can be higher than 55%, higher than 60%, or higher than 65%. The duty cycle of the positive DC bias waveform can be from 55% to 95%, from 58% to 90%, from 60% to 88%, from 65% to 85%, or from 70% to 80%. Similarly, the duty cycle of the negative DC bias waveform is lower than 50%. The duty cycle of the negative DC bias waveform can be lower than 45%, lower than 40%, or lower than 35%. The duty cycle of the negative DC bias waveform can be from 5% to 45%, from 8% to 40%, from 10% to 38%, from 15% to 35%, or from 20% to 30%.

[0095] Figure 4 The waveform shown in the example includes an alternating current square wave having two or more cycles. Each cycle can include a first pulse with an amplitude of V1 applied to a time period t1 and a second pulse with an amplitude of V2 applied to a time period t2, where V1 is positive, V2 is negative, and where t1 is greater than t2. When the amplitude of V1 is equal to the amplitude of V2 (|V1| = |V2|), the DC bias is achieved by the difference in time periods. In Figure 4 the example case, there is a positive DC bias because the application time period (t1) of the positive voltage V1 is longer than the application time period (t2) of the negative voltage V2. The positive DC bias means that if the charged pigment particles of the variable light transmission device are negatively charged, the charged pigment particles will move towards the first light-transmissive electrode layer of the device. The duty cycle of the waveform can be calculated by formula (3).

[0096] Duty cycle = 100 × (V1 · t1) / [(V1 · t2) + ((V2 · t2)] Formula (3)

[0097] In Figure 4 In the waveform example, the amplitude of V1 can be equal to the amplitude of V2 (|V1| = |V2|), but generally, the amplitudes of V1 and V2 can be different from each other.

[0098] Figure 4 The example of the driving waveform shown is DC unbalanced. However, Figure 4 One or more additional pulses of reverse impulse can be included in the waveform shown, which can ensure that the overall waveform applied to the pixel is DC balanced. The additional pulse (or additional pulses) can be applied before the DC unbalanced waveform (pre-pulse). In addition, Figure 4 The waveform example shown is a square wave AC waveform. Other example AC waveforms that can be used include sine waves, triangular waves, and sawtooth waves.

[0099] The AC waveform in the first embodiment can have an amplitude ranging from 10V to 200V and a frequency ranging from 0.1 to 6000Hz. The AC waveform can have an amplitude ranging from 15V to 180V, from 20V to 160V, from 25V to 150V, or from 30V to 140V. The AC waveform can have a frequency ranging from 0.5Hz to 5000Hz, from 1Hz to 4000Hz, from 5Hz to 3000Hz, from 10Hz to 2000Hz, from 15Hz to 1000Hz, from 20Hz to 800Hz, or from 25Hz to 600Hz. The ratio of the frequency of the AC waveform to the weight percentage content of the charge control agent in the electrophoretic medium can range from 400Hz to 2000Hz.

[0100] In the second embodiment, the second waveform can include a waveform formed by superimposing a DC voltage component and an AC waveform. An example of the second waveform of the second embodiment is as Figure 5 shown.

[0101] Figure 5 The waveform in has a net negative impulse due to the DC offset (Vd). Although the application time period (t3) of the positive pulse is equal to the application time period (t4) of the negative pulse, a DC bias is generated due to the difference in pulse amplitudes. Specifically, the amplitude V3 of the positive pulse is less than the amplitude V4 of the negative pulse. This is caused by the DC voltage component Vd of the waveform. That is, Figure 5 The waveform shown has a DC offset.

[0102] Figure 5 The example of the driving waveform shown is DC unbalanced. However, Figure 5One or more additional pulses of reverse impulse may be included in the waveform shown, which can ensure that the overall waveform applied to the pixel is DC balanced. The additional pulse (or pulses) may be applied before the waveform with DC imbalance (pre-pulse). In addition, Figure 5 The waveform example shown is a square wave AC waveform. Other example AC waveforms that can be used include sine waves, triangular waves, and sawtooth waves.

[0103] The AC waveform may have an amplitude ranging from 10V to 200V and a frequency ranging from 0.1 to 6000Hz. The AC waveform may have an amplitude ranging from 15V to 180V, from 20V to 160V, from 25V to 150V, or from 30V to 140V. The AC waveform may have a frequency ranging from 0.5Hz to 5000Hz, from 1Hz to 4000Hz, from 5Hz to 3000Hz, from 10Hz to 2000Hz, from 15Hz to 1000Hz, from 20Hz to 800Hz, or from 25Hz to 600Hz. The ratio of the frequency of the AC waveform to the weight percentage content of the charge control agent in the electrophoretic medium may range from 400Hz to 2000Hz.

[0104] In the case where the ICEO-induced movement of the charged pigment particles is relatively low, even when using a DC balanced AC waveform driving device, the raised structure of the micro unit contributes to the effective operation of the variable transmission device. In an example where the raised structure is a cone, any charged pigment particles located on the surface of the cone will be subject to a net force that will cause them to move towards the apex of the cone. As Figure 6 shown, Figure 6 shows charged pigment particles 222 in contact with the raised structure 617 (cone) in an electric field 602. In this case, the ICEO flow direction is indicated by the curved arrow, and the "uphill" side of the cone is more constrained than the "downhill" side. This exerts a force on the particles, as shown by the dashed horizontal arrow. There will be a reaction force perpendicular to the cone, forcing the particles to move towards the apex of the cone. By appropriately selecting the AC field and frequency, the particles can be moved out of the channel region and up the side of the cone in this way.

[0105] The electrophoretic medium of the variable light transmission device of the present invention includes charged pigment particles, a charge control agent, and a fluid.

[0106] The electrophoretic medium may include two types of charged pigment particles: the first type of charged pigment particles and the second type of charged pigment particles. The first type of charged pigment particles can reflect light, while the second type of charged pigment particles can absorb light. Typical examples of light-reflecting pigment particles are titanium dioxide, which is white in color. Typical examples of light-absorbing pigment particles include organic and inorganic pigment particles having black, blue, cyan, magenta, red, green, yellow, and other colors. The first type of charged pigment particles can have the same polarity as the second type of charged pigment particles. The first type of charged pigment particles can have the opposite polarity to the second type of charged pigment particles.

[0107] In an electrophoretic medium having the first type of charged pigment particles and the second type of charged pigment particles (which have the same charge polarity), the Zeta potential of the first type of charged pigment particles can be lower than the Zeta potential of the second type of charged pigment particles. Additionally, the average particle size of the first type of charged pigment particles can be larger than the average particle size of the second type of charged pigment particles, where the particle size is determined by the average diameter of the pigment particles. Figure 7 a shows the open optical state of an example of a variable light transmission device. In this example, after applying an appropriate electric field between the first light-transmitting electrode layer 202 and the second light-transmitting electrode layer 207, both types of charged pigment particles 222a (the first type) and 222b (the second type) will move into the channels of the microcells to form the open state. However, since the first type of charged pigment particles 222a has a lower charge (and a larger size), the second type of charged pigment particles 222b (light-absorbing) will be located below the first type of charged pigment particles 222a (light-reflecting). In other words, the second type of charged pigment particles 222b will be closer to the inner surface of the exposed bottom of the microcell (the bottom of the channel) than the first type of charged pigment particles 222a. Figure 7 b shows the closed optical state of this example of the variable light transmission device. After applying an appropriate electric field between the first light-transmitting electrode layer 202 and the second light-transmitting electrode layer 207, both charged pigment particles 222a and 222b will move towards the first light-transmitting electrode layer, thus achieving the closed optical state. However, the second type of charged pigment particles 222b (light-absorbing) will be closer to the sealing layer 206 than the first type of charged pigment particles 222a (light-reflecting) because the first type of charged pigment particles 222a has a lower charge (and a larger size).

[0108] In another example, the variable light transmission device has an electrophoretic medium including the first type of charged pigment particles (light-reflecting) and the second type of charged pigment particles (light-absorbing), where the first type of charged pigment particles and the second type of charged pigment particles have opposite charge polarities. Figure 8Figures 8a and 8b respectively show two possible open optical states of the example. After applying an appropriate electric field between the first light-transmissive electrode layer 202 and the second light-transmissive electrode layer 207, depending on the polarity of the applied electric field, the first charged pigment particles 223a (light-reflecting) or the second charged pigment particles 223b (light-absorbing) will move towards the channels (open optical state). A closed optical state can be formed by applying an appropriate electric field (second electric field) between the first light-transmissive electrode layer 202 and the second light-transmissive electrode layer 207, which causes one type of charged pigment particles to move towards the first light-transmissive electrode layer 202 at a certain speed that has a lateral component. Depending on the applied electric field, the first type of charged pigment particles 223a or the second type of charged pigment particles 223b will spread over the microcell regions (microcell openings) near the sealing layer 206.

[0109] The number of the second type of charged pigment particles that can be black will be selected to be sufficient to cover the white pigment in the channels when viewed from below (open optical state), but not too high so as not to cause excessive light absorption in the closed state. "Viewed from below" means that the observer is located on the side of the variable light transmission device near the second light-transmissive electrode layer 207, opposite to the side near the first light-transmissive electrode layer 202.

[0110] It is desirable that the open optical state of the variable transmission device has a high transmittance and a low haze. In addition, in some applications, such as building windows or vehicle skylights, it is desirable to manage the heating of the building or the vehicle. When the variable transmission device includes an electrophoretic medium (including light-absorbing charged pigment particles), heat management is difficult. In the closed optical state of such a device, the light incident on the device may be absorbed by the device, resulting in its heating up. The incident light can include wavelengths within the solar spectrum, namely ultraviolet rays, visible light, and infrared rays. Another problem may be that the closed optical state is not completely opaque. That is, part of the incident light will penetrate the building or the vehicle, warming the interior of the building or the vehicle.

[0111] Figure 9 Graphs showing the variation of light reflection, transmission, and absorption with layer thickness for a layer including a black pigment (light-absorbing) are provided. That is, for each layer thickness, these graphs provide the ratios of the amounts of light reflected, transmitted, and absorbed to the incident light.

[0112] Figure 10 Graphs showing the variation of light reflection, transmission, and absorption with layer thickness for a layer including a white pigment (light-reflecting) are provided. That is, for each layer thickness, these graphs provide the ratios of the amounts of light reflected, transmitted, and absorbed to the incident light.

[0113] Figure 9 and 10It is shown that white pigments reflect significantly more light than black pigments and do not absorb incident light. In fact, white pigments provide opacity through light reflection / scattering. As the layer thickness increases, more incident light is reflected and less incident light is transmitted. However, it has been observed that a variable light transmission device including a medium with white pigment particles (reflective) can exhibit significant haze in the open optical state.

[0114] The inventors of the present invention have found that a variable light transmission device including an electrophoretic medium provides significant benefits such as reduced haze. The electrophoretic medium has charged particles (first type) including a reflective pigment and charged particles (second type) including an absorbing pigment. The reflective pigment can be titanium dioxide and the absorbing pigment can be an inorganic black pigment such as iron oxide black.

[0115] Figure 11 Shows the effect of the layer thickness on the reflectance, transmittance, and absorptance of a closed optical state layer of a composition including a white pigment (light reflective) and a black pigment (light absorbing). The weight ratio of the black pigment to the white pigment is 0.1. That is, for each layer thickness, these graphs provide the ratio of the amount of reflected, transmitted, and absorbed light to the incident light.

[0116] As the layer thickness increases, the rate of decrease in transmittance remains almost constant, but the upper limit of the reflectance of the white pigment is 30%. The absorptance increases with the increase in layer thickness, but is only about half of that of the Figure 9 pure black pigment layer shown. Placing the white pigment layer in front of the black pigment layer instead of mixing the two can achieve a reduction in absorptance and its unnecessary heat effect.

[0117] In an example of a variable transmission device having an electrophoretic medium, the electrophoretic medium includes a first type of charged pigment particles (white or reflective, having a negative charge polarity) and a second type of charged pigment particles (black or absorptive, having a positive charge polarity). The relative positions of the two types of charged pigment particles in the open and closed optical states can be controlled by an applied electric field. For each type of charged pigment particle, the polarizability and size of the particles determine the frequency required to achieve optimal movement. The maximum ICEO speed of these two types of pigment particles with opposite charges can be achieved by using an electric field including different frequency alternating current waveforms. In Example 7 below related to this scenario, the characteristic alternating current waveform frequency of the white pigment is much higher than that of the black pigment. Therefore, a relatively low alternating current frequency can be used to switch the black pigment into the channel, and an appropriate direct current offset is superimposed to move the black pigment in the microcell channel, as shown in the figure. Since the black pigment is positively charged, applying an alternating current voltage and a positive offset voltage on the first light-transmissive electrode layer while grounding the second light-transmissive electrode layer will cause the black pigment to move into the channel. The alternating current frequency is relatively low (10 Hz); at this frequency, both the white and black pigments have strong ICEO-induced lateral movement. Once the black pigment is in the channel, the alternating current frequency is increased to a higher value. At high frequencies, the ICEO-induced movement of the black pigment is attenuated, but the movement of the white pigment is maintained. Therefore, the white pigment can be switched into the channel by applying a negative direct current offset to the alternating current waveform. Both white and black particles are present in the channel (as shown in Figure 7 as shown in a). Although under certain conditions, the electric field in the second step can drive some of the black pigment particles out of the channel without causing the black pigment to move laterally. Therefore, the black particles that can be removed from the channel will experience vertical movement, thus providing an open optical state similar to that shown in Figure 8 as shown in a).

[0118] Figure 12 Another method for reducing the haze of a variable light transmission device, which is caused by the light scattering effect of charged pigment particles, is shown. Figure 12 A side view of an innovative variable light transmission device is shown, which includes a first light-transmissive electrode layer 202, a second light-transmissive electrode layer, and an electrophoretic medium including charged pigment particles 222 that have light reflectivity. The variable light transmission device further includes a first light-blocking layer 1202 located on the exposed bottom inner surface of the microcell. The first light-blocking layer 1202 may include light-absorbing black pigment particles. In the open optical state of the device, the charged pigment particles of the electrophoretic medium are included in the channels of the microcells ( Figure 12a). When observing the device from the bottom, the first light-blocking layer 1202 can block the haze. Observing from the bottom means that the observer observes the device from the side close to the second light-transmitting electrode layer 207. However, when observing the device from the top, the haze is still obvious. Observing from the top means that the observer observes the device from the side close to the first light-transmitting electrode layer 202. Figure 12 b shows the closed optical state of the device. Figure 12 The variable light-transmitting device shown can be used in applications where the observer is typically located on one side of the variable light-transmitting device, such as, for example, a skylight.

[0119] Figure 12 The manufacturing method of the variable light-transmitting device shown includes the following steps: (a) providing an assembly including a third electrode layer, a second light-transmitting electrode layer, and a layer including a plurality of micro-units, the layer including a plurality of micro-units being disposed between the third electrode layer and the second light-transmitting electrode layer, each of the plurality of micro-units including a light-blocking composition, the light-blocking composition including (i) light-absorbing charged pigment particles, (ii) a polymer, oligomer or monomer, and (iii) an optional solvent, each of the plurality of micro-units having a micro-unit opening, the third electrode layer spanning the micro-unit openings of the plurality of micro-units, each of the plurality of micro-units including a micro-unit bottom layer, a raised structure, a micro-unit wall and a channel, the micro-unit bottom layer having a micro-unit bottom inner surface, the micro-unit bottom inner surface including an exposed micro-unit bottom inner surface and an unexposed micro-unit bottom inner surface; (b) applying an electric field between the third electrode layer and the second light-transmitting electrode layer via a waveform that causes the light-absorbing charged pigment particles of the light-blocking composition to move into the channel, so that the light-absorbing charged pigment particles are in a state within the channel; (c) curing the light-blocking composition to form a light-blocking layer on the exposed micro-unit bottom inner surface; (d) removing the third electrode layer; (e) filling each of the plurality of micro-units with an electrophoretic medium including charged pigment particles, a charge control agent and a fluid; (f) sealing each of the plurality of micro-units with a sealing layer; and (g) attaching the first light-transmitting electrode layer to the sealing layer.

[0120] If the light-blocking composition includes a solvent, the solvent evaporates during the curing of the light-blocking composition. The curing of the light-blocking composition can be achieved by ultraviolet irradiation, heat treatment or solvent evaporation.

[0121] Figure 12Another manufacturing method of the device includes the following steps: (a) providing a component that sequentially includes a second light-transmissive electrode layer, a second light-transmissive electrode layer, and a layer including a plurality of micro-units disposed on the second light-transmissive electrode layer, each of the plurality of micro-units having a micro-unit opening, each of the plurality of micro-units including a micro-unit bottom layer, a raised structure, a micro-unit wall, and a channel, the micro-unit bottom layer having a micro-unit bottom inner surface, the micro-unit bottom inner surface including an exposed micro-unit bottom inner surface and an unexposed micro-unit bottom inner surface, the raised structure having a raised base, a raised surface, a raised apex, and a raised height, the raised apex being a point or a set of points of the raised structure that is closer to the micro-unit opening than all other points of the raised structure, the raised height being the distance between the raised base and the raised apex, the raised surface being the surface of the raised structure that does not include the raised apex and the raised base, the micro-unit wall having a micro-unit inner wall surface and a micro-unit upper wall surface, the channel having a channel height, the channel height being 50% of the raised height, the unexposed micro-unit bottom inner surface being in contact with the raised base, and the channel being the volume between the exposed micro-unit bottom inner surface, the raised surface, and the micro-unit inner wall surface; (b) dispersing a light-blocking composition onto the exposed micro-unit bottom inner surface of each micro-unit, the light-blocking composition including (i) light-absorbing pigment particles, (ii) a polymer, an oligomer, or a monomer, and (iii) an optional solvent; (c) curing the light-blocking composition to form a light-blocking layer on the exposed micro-unit bottom inner surface; (d) filling each of the plurality of micro-units with an electrophoretic medium including charged pigment particles, a charge control agent, and a fluid; (e) sealing each of the plurality of micro-units with a sealing layer; and (f) attaching a first light-transmissive electrode layer to the sealing layer.

[0122] If the light-blocking composition includes a solvent, the solvent evaporates during the curing of the light-blocking composition. The curing of the light-blocking composition can be achieved by ultraviolet irradiation, heat treatment, or solvent evaporation.

[0123] Examples 5 and 6 describe experiments of forming a blocking layer using a light-blocking composition. In Example 5, the light-blocking composition includes black charged pigment particles, while in Example 6, the light-blocking composition includes white charged pigment particles.

[0124] By using Figure 13 the device shown, a variable light transmission device with improved off-optical state performance can also be achieved. Figure 13The variable light transmission device shown includes a first light-transmissive electrode layer (202), a second light-transmissive electrode layer (207), and a microcell layer (203). The microcell layer is disposed between the first light-transmissive electrode layer (202) and the second light-transmissive electrode layer (207). The microcell layer includes a plurality of microcells and a sealing layer. Each of the plurality of microcells includes an electrophoretic medium that includes charged pigment particles in a fluid and a charge control agent. Each of the plurality of microcells has a microcell opening, and the sealing layer spans the microcell openings of the plurality of microcells. Each of the plurality of microcells includes a microcell bottom layer, a raised structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface that includes an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. The raised structure has a raised base, a raised surface, a raised apex, and a raised height. The raised apex is a point or a set of points in the raised structure that is closer to the microcell opening than all other points in the raised structure, and the raised height is the distance between the raised base and the raised apex. The raised surface is the surface of the raised structure that does not include the raised apex that contacts the electrophoretic medium. The microcell wall has a microcell inner wall surface, a microcell wall upper surface, and a second light-blocking layer 1311. The microcell inner wall surface is the surface of the microcell wall of the microcell that contacts the electrophoretic medium. The second light-blocking layer 1311 is located between the microcell wall upper surface and the sealing layer. The channel has a channel height that is 50% of the raised height. The unexposed microcell bottom inner surface contacts the raised base. The channel is the volume between the exposed microcell bottom inner surface, the raised surface, and the microcell inner wall surface. Figure 13 The variable light transmission device in Figure 13 may further include a first light-blocking layer 1202 that is disposed on the exposed microcell bottom inner surface layer. As described above, the first light-blocking layer 1202 can reduce the haze when viewing the device from below. The second light-blocking layer 1311 promotes the improvement of the off state by increasing the opacity of the device, which may be caused by a partially light-transmissive wall material. The second light-blocking layer 1311 can be black, white, or any other color. The second light-blocking layer 1311 can be conductive, thus contributing to the switching of the device. Figure 13 The variable light transmission device shown may further include an auxiliary layer 1312 that is disposed between the sealing layer and the second light-blocking layer 1311. The auxiliary layer 1312 may include an adhesive material. The auxiliary layer 1312 may include light-reflective pigments to further improve the opacity in the off optical state. The auxiliary layer 1312 may further include a encapsulated electrophoretic layer that includes an electrophoretic medium of charged pigment particles. Applying an electric field on the encapsulated electrophoretic layer of the auxiliary layer 1312 can switch the color (or image) of the auxiliary layer 1312, which can also affect Figure 13 the appearance of the variable light transmission device shown.

[0125] Charge control agents are generally oligomeric or polymeric materials that are soluble in the fluid of the electrophoretic medium. Charge control agents are surfactant-type molecules having one or more polar functional groups (heads) and non-polar moieties (tails). The electrophoretic medium may include a charge control agent in a concentration of 0.1% to 10% by weight of the weight of the electrophoretic medium. The electrophoretic medium may include a charge control agent in a concentration of 0.5% to 9% by weight, 0.7% to 8% by weight, 1% to 7% by weight, or 1% to 6% by weight of the weight of the electrophoretic medium.

[0126] The fluid of the electrophoretic medium may include aliphatic hydrocarbons, cycloaliphatic hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons, polydimethylsiloxane, or mixtures thereof.

[0127] The electrophoretic medium may also include a flocculant, also known as a dissipative agent. The dissipative agent creates an osmotic pressure difference between pigment particles and between pigment particles and dissipative agent molecules. Thus, the bistability of the optical state (on and off) of the device is enhanced. The dissipative agent is typically a polymeric material such as polyisobutene and polydimethylsiloxane.

[0128] Example

[0129] Example 1 : A device was prepared by laminating a poly(ethylene terephthalate) (PET) sheet coated with indium tin oxide (ITO) transparent conductor with an imprinted microcell array on a second sheet containing PET / ITO and an electrophoretic medium. The structure of the device corresponds to the illustration in Figures 2A to 2D but without the addition of the sealing layer 212. The structure of the imprinted microcell array is as shown in Figure 14 , which is a plan view of the microcell in the device. Figure 15 shows a corresponding cross-sectional view of a microcell in the device. Table 1 shows the external dimensions of the microcell.

[0130] Table 1: Device Structure

[0131] Element Distance (micrometers) Cavity pitch: center to center of micro-units 500 Height of cylinder (base) below the cone protrusion 15.2 Maximum height of particle filling level (above the base) 15.2 Minimum wall width at the first substrate 15 Draft angle of the wall and cone base 8 Wall height 50 Wall notch / groove width at the first substrate 10 Draft angle of the wall notch / groove 26.6 Depth of the wall notch / groove 5 Gap between the protrusion vertex and the wall top 9 Cone slope (degrees) 7.3

[0132] The electrophoretic medium includes a white pigment, a hydrocarbon solvent, a charge control agent (CCA), and a dissipative agent. In this example, the preparation of the electrophoretic medium sample was to mix 10 wt% of the white pigment and 5 wt% of the charge control agent (cationic charge control agent of CCA111 from Example 1 of US2020 / 0355978) in Isopar E solvent. The device was switched using a 50V square wave AC waveform with a duty cycle of 50%. As shown in Figure 16As shown, by increasing the alternating current frequency from 0 Hz to 5000 Hz, three different pigment movement patterns were observed. The white pigment particles were prepared with titanium dioxide pigment as the core and polymer as the shell, as described in Example 1 of US Patent No. 8,582,196.

[0133] At a low frequency of 10 Hz and after multiple switches, the white pigment moves towards the edge of the micro-unit (near the periphery) ( Figure 16 as shown in (a) in Figure 16 ), which may be caused by a slight downward push along the slope of the cone. Once in the channel, the pigment switches up and down in the vertical direction. At a low frequency of 10 Hz, the movement is mainly normal electrophoresis. By increasing the frequency to 100 Hz, the pigment tends to laterally diffuse into the area above the cone in the imprinted micro-unit structure ( Figure 16 as shown in (b) in Figure 16 ). At a frequency of 1000 Hz, the white pigment completely diffuses into the circle of the imprinted micro-unit structure, as shown in

[0134] Example 2 : In Example 2, the effect of the charge control agent (CCA) concentration on the movement of white pigment in the imprinted micro-unit device was studied.

[0135] As mentioned above, it is expected that increasing the concentration of the charge control agent in the electrophoresis medium will reduce the Debye length associated with the surface of the charged pigment particles, thus increasing the frequency required for a specific ICEO flow. To verify this hypothesis, three variable light transmission devices with similar electrophoresis media but using different charge control agent concentrations were prepared. The charge control agent used in this example is the cationic polymer disclosed in Example 1-CCA111 of US2020 / 0355978. Three different electrophoresis media were prepared with charge control agent concentrations of 0.1 wt%, 1 wt%, and 5 wt% of the weight of the electrophoresis medium, respectively. Each electrophoresis medium also included 10 wt% of white pigment and Isopar E solvent. The white pigment particles were prepared with a titanium dioxide pigment core that included a polymer coating, as described in Example 1 of US Patent No. 8,582,196.

[0136] The switching performance of the three samples was evaluated in an imprinted micro-unit device (as shown in Figure 14 and 15 ). The waveform was an alternating current with a square wave of + / -50 V and a 50% duty cycle. As shown in Figure 17As shown, as the concentration of the charge control agent increases, the frequency required to achieve the off-optical state also increases. That is, for the electrophoretic medium containing 0.1 wt% CCA, a frequency of 50 Hz is required to achieve the off-optical state; for the electrophoretic medium containing 1 wt% CCA, a frequency of 100 Hz is required to achieve the off-optical state; for the electrophoretic medium containing 5 wt% CCA, a frequency of 1000 Hz is required to achieve the off-optical state. In these three experiments, the ratios of frequency to CCA concentration are 500 Hz, 1000 Hz, and 1000 Hz, respectively.

[0137] The electrophoretic medium of the device includes 1 wt% CCA, and can be switched from the on-optical state to the off-optical state using (a) a simple square wave alternating current of + / -100 V, 0.5 Hz, or (b) a square wave alternating current of + / -50 V, 50 Hz frequency superimposed on a -50 V direct current voltage. The on-optical state is achieved by a + / -50 V square wave alternating current with a duty cycle of 5%, while the off-optical state requires a + / -50 V square wave alternating current with a duty cycle of 95%. The switching time in each case is approximately 1 second.

[0138] Example 3 : The effect of the charge of the white pigment on the switching performance of the variable light transmission device was studied.

[0139] A variable light transmission device was prepared, and its electrophoretic medium includes 10 wt% positively charged white pigment particles in Isopar E. The white pigment was functionalized with 1.6% wt of silane Z6030 and grafted with lauryl polymethacrylate (PLMA). Titration with CCA (the cationic polymer disclosed in Example 1-CCA111 of US Patent No. 2020 / 0355978) was used to make the Zeta potential of the treated pigment +35 mV. The electrophoretic medium also includes 1 wt% of CCA (the cationic polymer disclosed in Example 1-CCA111 of US Patent No. 2020 / 0355978). The waveform used to switch the device from the on-optical state to the off-optical state is direct current superimposed on alternating current, that is, a square wave waveform, an alternating current of + / -50 V with a frequency of 500 Hz. The waveform for switching the device from the off-optical state to the on-optical state is an alternating current of + / -50 V offset by a direct current of +50 V. Therefore, the behavior of the device in Example 3 is very similar to that of the device in Example 2 using an electrophoretic medium containing 1 wt% CCA, except that the polarity of the direct current offset required to achieve the on-optical state is opposite.

[0140] Example 4 : In this example, the solvent of the electrophoretic medium matches the polymer forming the imprinted micro-units.

[0141] "Haze" refers to the percentage of diffusely transmitted light in the total transmitted light. Diffusely transmitted light refers to the light that is scattered during the transmission process. In order to manufacture a variable light transmission device with low haze, it is necessary to match the refractive index of the electrophoretic medium liquid solvent with the refractive index of the polymer material used to manufacture the imprinted micro-units, as described in U.S. Patent No. 7,327,511.

[0142] Typically, the solvents used in the electrophoretic medium have a low dielectric constant (preferably less than 10, ideally less than 3), low viscosity, low vapor pressure, and a relatively high refractive index. Examples of solvents include, but are not limited to, aliphatic hydrocarbons such as heptane, octane, and petroleum fractions such as Isopar® (ExxonMobil) or Isane® (Total), terpenes such as limonene (e.g., 1-limonene), and aromatic hydrocarbons such as toluene. A particularly preferred solvent is limonene because it has both a low dielectric constant (2.3) and a relatively high refractive index (1.47). The refractive index of the electrophoretic medium can be changed by adding a refractive index matching agent. For example, an electrophoretic medium suitable for a variable light transmission device as described in the aforementioned U.S. Patent No. 7,679,814, wherein the non-polar liquid of the electrophoretic medium comprises a mixture of partially hydrogenated aromatic hydrocarbons and terpenes, and the preferred mixture is d-limonene and partially hydrogenated terphenyl, which is commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009.

[0143] To reduce haze, it is preferable that the refractive index of the encapsulated electrophoretic medium closely matches the refractive index of the encapsulating material. In most instances, it is beneficial to use an electrophoretic medium having a refractive index between 1.51 and 1.57 at 550 nm, preferably having a refractive index of approximately 1.54 at 550 nm.

[0144] Example 4A : A variable light transmission device including micro-units was prepared, the electrophoretic medium of which included 5 wt% white pigment and 1 wt% CCA (a cationic polymer disclosed in Example 1 - CCA111 of US2020 / 035597811), and the solvents were Cargille® 5040 and Isopar E. The waveform used was an alternating square wave with a frequency of 10 Hz and an amplitude of + / -50 V. To achieve the closed optical state, a duty cycle of 5% was used. To achieve the open optical state, a duty cycle of 95% was first used, and then the duty cycle was changed to 50%.

[0145] Example 4B:Another variable light transmission device including micro-units was prepared using an electrophoretic medium similar to the composition of Example 4A. The device also includes polyisobutene (PIB) as a dissipative agent. The dissipative agent is used to improve the bistability of the device, that is, to ensure that when no electric field is applied, the device remains in the open optical state and the closed optical state. The electrophoretic medium includes 10 wt% white pigment, 1 wt% CCA (a cationic polymer disclosed in Example 1-CCA111 of US2020 / 035597811), 0.5 wt% Cargille® 5040 polyisobutene, and Isopar E solvent. The waveform used is an alternating square wave current of + / -50V and a frequency of 10Hz. To achieve the closed optical state, a pattern of alternating 5% duty cycle and 50% duty cycle was used. To achieve the open optical state, we used a 95% duty cycle followed by a 50% duty cycle. The time required for complete switching is about 20 seconds. This time is much longer than the time required for switching in a non-refractive index matching solvent without a dissipative agent (Example 4A).

[0146] Example 5 : A light-blocking layer including black particles.

[0147] A variable transmission device was prepared by laminating a PET sheet coated with an ITO transparent conductor with an imprinted micro-unit array layer on another PET / ITO sheet. The imprinted micro-unit array includes an electrophoretic medium. The structure of the imprinted micro-unit array is as Figures 2A to 2D shown, but the micro-units in this example do not include a sealing layer.

[0148] The light-blocking composition for the light-blocking layer includes black pigment, solvent, charge control agent, and dissipative agent. In this example, the light-blocking composition was prepared by mixing 10 wt% black pigment, 1 wt% CCA (a cationic charge control agent from Example 1-CCA111 of US2020 / 035597811), and 0.5 wt% polyisobutene in a solvent mixture of partially hydrogenated terphenyl, which is commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55Commerce Rd, Cedar Grove N.J.07009, and also includes limonene, Isopar M, and Isopar E. The black pigment particles have a core including black iron oxide (Pigment Black 11) and a shell of polymer. As Figure 18As shown, during device fabrication, black pigment particles are dispersed throughout the electrophoretic medium. A square wave of 0.5 Hz, 50 V, and a +50 V offset (i.e., switching between +100 V and 0 V) with a 50% duty cycle is applied to the first light-transmissive electrode layer while the second light-transmissive electrode layer is held at 0 V. The electric field between the two electrodes induces electrophoresis and drives the positively charged black pigment to the bottom surface of the unexposed microcells (inside the channels), as Figure 19 shown. After the voltage is released, the black pigment remains on the bottom surface of the unexposed microcells. Then the PET / ITO first electrode is peeled off the device to allow for thin film evaporation.

[0149] Example 6 : The preparation of the variable light transmission device is the same as that of the variable light transmission device in Example 5, except that the light-blocking composition is formulated by mixing 10 wt% of white pigment and 5 wt% of CCA (cationic charge control agent from Example 1 - CCA111 of US2020 / 0355978111) in Isopar E solvent. The device easily switches between the off optical state (0 V offset, Figure 20 ), and the on optical state (-50 V offset, Figure 21 ) under a square wave of 50 Hz / 50 V.

[0150] Example 7 : Black and white pigments with opposite charges are switched into the channels of the imprinted microcells.

[0151] A variable transmission device is prepared by laminating a sheet of PET-coated and ITO transparent conductor together onto an array of imprinted microcells including the electrophoretic medium on another sheet of PET / ITO. The structure of the microcells corresponds to the illustration in Figures 2A to 2D , except that the device does not include a sealing layer. The structure of the array of imprinted microcells is as shown in Figure 14 and 15 .

[0152] First, an electrophoretic composition including a black pigment is prepared and switched to an optically on state (the black pigment is included in the channels). Then, the first light-transmissive electrode layer is removed and the solvent is evaporated. After the solvent is evaporated, an electrophoretic medium including a white pigment, a solvent, and a charge control agent (CCA) is prepared. In this example, the preparation of the electrophoretic medium is to mix 10 wt% white pigment and 1 wt% CCA111 in a solvent mixture of partially hydrogenated terphenyl, which is commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009, and also includes limonene, Isopar M, and Isopar E. The white pigment particles are prepared using a titanium dioxide pigment core that includes a polymer coating as described in Example 1 of U.S. Patent No. 8,582,196.

[0153] The electrophoretic medium is assembled into a microcell device, where the black pigment is switched into the channels and the solvent is evaporated. Thus, the device includes white and black pigment particles of opposite charge. Both pigments can be switched electrophoretically. To switch both pigments into the channels, first, the black pigment is switched into the channels using a positive DC offset on an AC voltage of relatively low frequency, and then the white pigment is switched into the channels using a negative DC offset on an AC voltage of relatively high frequency.

[0154] In the first step, a waveform of 10 Hz, 50 V square wave, 50% duty cycle, and +2 V offset is applied to the first light-transmissive electrode layer 202 while the second light-transmissive electrode layer is held at 0 V. The electric field between the electrodes induces electrophoresis by superimposing the induced charge electroosmosis and drives the positively charged black pigment into the channels. In the second step, a waveform of 500 Hz / 50 V square wave, 50% duty cycle, and alternating +2 V and -2 V offsets is applied to the first electrode while the second electrode is held at 0 V. At the higher frequency (500 Hz, while it is 10 Hz in the first step), the lateral movement of the black pigment is less than that of the white pigment. That is, during the phase when the waveform has a negative DC offset, the white pigment is driven into the channels. As Figure 22 shown, the result is that both the white and black pigments are switched into the channels of the microcell.

[0155] Partial structures in the figure: 200 variable transmission device; 201 first transparent substrate; 202 first light-transmitting electrode layer; 203 microcell layer; 204 multiple microcells; 205 microcell opening; 206 sealing layer; 207 second light-transmitting electrode layer; 208 second transparent substrate; 209 electrophoretic medium; 210 microcell bottom layer; 211 inner surface of the microcell bottom; 211a exposed inner surface of the microcell bottom; 211b unexposed inner surface of the microcell bottom; 212 microcell wall; 213 inner wall surface of the microcell; 214 upper surface of the microcell wall; 125 channel; 216 channel height; 217 raised structure; 218 raised base; 219 raised vertex; 220 raised height; 221 raised surface; 222 charged pigment particles; 222a first type of charged pigment particles; 222b second type of charged pigment particles; 223a first type of charged pigment particles having a charge polarity opposite to that of the second type of charged pigment particles 223b; 223b second type of charged pigment particles having a charge polarity opposite to that of the first type of charged pigment particles 223a; 1202 first light-blocking layer; 1311 second light-blocking layer; 1312 auxiliary layer.

Claims

1. A method of operating a variable light transmission device, comprising the following steps: Providing a variable light transmission device (200), the variable light transmission device (200) comprising: A first light-transmissive electrode layer (202); A second light-transmissive electrode layer (207); and A microcell layer (203), the microcell layer (203) being disposed between the first light-transmissive electrode layer (202) and the second light-transmissive electrode layer (207), the microcell layer (203) comprising a plurality of microcells (204) and a sealing layer (206), Each of the plurality of microcells (204) includes an electrophoretic medium (209), the electrophoretic medium (209) comprising charged pigment particles and a charge control agent in a non-polar liquid, Each of the plurality of microcells (204) has a microcell opening (205), and the sealing layer (206) spans the microcell openings (205) of the plurality of microcells (204), Each of the plurality of microcells (204) includes a microcell bottom layer (210), a raised structure (217), a microcell wall (212), and a channel (215), The microcell bottom layer (210) has a microcell bottom inner surface (211), the microcell bottom inner surface (211) comprising an exposed microcell bottom inner surface (211a) and an unexposed microcell bottom inner surface (211b), The raised structure (217) has a raised base (218), a raised surface (221), a raised vertex (219), and a raised height (220), the raised vertex (219) being a point or a set of points of the raised structure (217), the point or the set of points being closer to the microcell opening (205) than all other points of the raised structure (217) are to the microcell opening (205), the raised height (220) being the distance between the raised base (218) and the raised vertex (219), and the raised surface (221) being the surface of the raised structure (217) that does not include the raised vertex in contact with the electrophoretic medium (209), The microcell wall (212) has a microcell inner wall surface (213) and a microcell wall upper surface (214), the microcell inner wall surface (213) being the surface of the microcell wall (212) of the microcell in contact with the electrophoretic medium (209), and the microcell wall upper surface (214) being the surface of the microcell wall (212) of the microcell in contact with the sealing layer (206), The channel (215) has a channel height (216), the channel height (216) being 50% of the raised height (220), The unexposed microcell bottom inner surface (211b) is in contact with the raised base (218), The channel (215) is the volume between the exposed microcell bottom inner surface (211a), the raised surface (221), and the microcell inner wall surface (213), Apply a first electric field between the first transparent electrode layer (202) and the second transparent electrode layer (207) via a first waveform, so that the charged pigment particles move towards the channel (215), thereby switching the variable light transmission device (200) to an open optical state, and the charged pigment particles in the open optical state are located in the channel (215). Apply a second electric field between the first transparent electrode layer (202) and the second transparent electrode layer (207) via a second waveform, so that the charged pigment particles (222) move towards the first transparent electrode layer (202) at a certain speed, the speed has a lateral component, and this results in a closed optical state. The second waveform includes at least one positive voltage and at least one negative voltage, and the second waveform has a net positive impulse or a net negative impulse, wherein the closed optical state has a lower light transmission percentage than the open optical state.

2. The method for operating a variable light transmission device according to claim 1, wherein the content of the charge control agent in the electrophoretic medium is 1% by weight to 8% by weight of the weight of the electrophoretic medium.

3. The method for operating a variable light transmission device according to claim 1 or 2, wherein the second waveform includes an alternating current waveform, the alternating current has a frequency, and the alternating current waveform has a duty cycle from 5% to 45%.

4. The method for operating a variable light transmission device according to claim 3, wherein the alternating current waveform is a square wave, a sine wave, a triangular wave or a sawtooth wave.

5. The method for operating a variable light transmission device according to claim 3 or 4, wherein the alternating current waveform is a square wave having two or more cycles, wherein the positive voltage and the negative voltage of the alternating current waveform have the same amplitude, the amplitude is from 10V to 200V, and wherein the frequency of the alternating current waveform is from 0.1Hz to 6000Hz.

6. The method for operating a variable light transmission device according to claim 5, wherein the frequency of the alternating current waveform is from 100Hz to 3000Hz, and the amplitude of the alternating current waveform is from 20V to 180V.

7. The method for operating a variable light transmission device according to any one of claims 3 to 6, wherein the ratio of the frequency of the alternating current waveform expressed in hertz to the content of the charge control agent in the electrophoretic medium expressed as a weight percentage of the charge control agent in the electrophoretic medium is from 400 to 2000Hz.

8. The method for operating a variable light transmission device according to claim 1 or 2, wherein the second waveform includes a waveform formed by superimposing a direct current voltage component and an alternating current waveform, the alternating current waveform has a frequency and an amplitude, the frequency is from 0.1Hz to 6000Hz, the amplitude is from 10V to 200V, the direct current voltage component has an amplitude, and the amplitude of the direct current voltage component is from 0.1V to 500V.

9. The method for operating a variable light transmission device according to claim 8, wherein the alternating current waveform is a square wave, a sine wave, a triangular wave or a sawtooth wave.

10. The method of operating a variable light transmission device according to claim 8 or 9, wherein the ratio of the frequency of the alternating current waveform expressed in Hertz to the content of the charge control agent in the electrophoretic medium expressed as a weight percentage of the charge control agent in the electrophoretic medium is from 400 to 2000 Hz.

11. The method of operating a variable light transmission device according to any one of claims 1 to 10, wherein the raised structure is a geometric body selected from the group consisting of: (a) a cone, (b) a cone on a cylinder, the cylinder having a base, the base of the cylinder being the raised base of the raised structure, (c) a tetrahedron, (d) a tetrahedron on a triangular prism, the triangular prism having a triangular base, the triangular base being the raised base of the raised structure, (e) a triangular prism having a square base, the square base being the raised base of the raised structure, (f) a square pyramid having a square base, the square base being the raised base of the raised structure, (h) a square pyramid on a cube, the cube having a base, the cube base being the raised base of the raised structure, (i) a square pyramid on a rectangular parallelepiped, the rectangular parallelepiped having a rectangular parallelogram base, the rectangular parallelogram being the raised base of the raised structure, (j) a pentagonal pyramid, the pentagonal pyramid having a pentagonal base, the pentagonal base being the raised base of the raised structure, (k) a pentagonal pyramid on a pentagonal prism, the pentagonal prism having a pentagonal base, the pentagonal base being the raised base of the raised structure, (l) a hexagonal pyramid, the hexagonal pyramid having a hexagonal base, the hexagonal base being the raised base of the raised structure, (m) a hexagonal pyramid on a hexagonal prism, the hexagonal prism having a hexagonal base, the hexagonal base being the raised base of the raised structure.

12. The method of operating a variable light transmission device according to claim 11, wherein the raised structure is a cone, and the slope of the cone is from 5 degrees to 10 degrees.

13. The method of operating a variable light transmission device according to claim 11, wherein the raised structure is a cone on a cylinder, the cylinder having a base, the base of the cylinder being the raised base of the raised structure, and wherein the slope of the cone is 10 degrees or less.

14. The method of operating a variable light transmission device according to claim 1, wherein the raised structure is a geometric body of a pyramid having a base with n sides, the base with n sides being the raised base of the raised structure, wherein n is an integer from 7 to 12, (m) a pyramid having a base with n sides, the pyramid being located on a prism having a base with n sides, the base of the prism having n sides being the raised base of the raised structure, wherein n is from 7 to 12.

15. A method of operating a variable light transmission device according to any one of claims 1 to 14, wherein the electrophoretic medium comprises first type charged pigment particles and second type charged pigment particles, the first type charged pigment particles being light-reflective and the second type charged pigment particles being light-absorbing.

16. A method of operating a variable light transmission device according to claim 15, wherein the first type charged pigment particles are white.

17. A method of operating a variable light transmission device according to claim 15, wherein the second type charged pigment particles are black.

18. A method of operating a variable light transmission device according to claim 15, wherein the first type charged pigment particles and the second type charged pigment particles have the same polarity.

19. A method of operating a variable light transmission device according to claim 15, wherein the first type charged pigment particles and the second type charged pigment particles have opposite polarities.

20. A variable light transmission device (200) comprising: a first light-transmissive electrode layer (202); a second light-transmissive electrode layer (207); and a micro-unit layer (203) disposed between the first light-transmissive electrode layer (202) and the second light-transmissive electrode layer (207), the micro-unit layer (203) comprising a plurality of micro-units (204) and a sealing layer (206), each of the plurality of micro-units (204) comprising an electrophoretic medium (209) comprising charged pigment particles in a non-polar liquid and a charge control agent, each of the plurality of micro-units (204) having a micro-unit opening (205), the sealing layer (206) spanning the micro-unit openings of the plurality of micro-units (204), each of the plurality of micro-units (204) comprising a micro-unit bottom layer (210), a raised structure (217), a micro-unit wall (212) and a channel (215), the micro-unit bottom layer (210) having a micro-unit bottom inner surface (211), the micro-unit bottom inner surface (211) comprising an exposed micro-unit bottom inner surface (211a) and an unexposed micro-unit bottom inner surface (211b), the raised structure (217) having a raised base (218), a raised surface (221), a raised apex (219) and a raised height (220), the raised apex (219) being a point or a set of points of the raised structure (217) that are closer to the micro-unit opening (205) than all other points of the raised structure (217), the raised height (220) being the distance between the raised base (218) and the raised apex (219), the raised surface (221) being the surface of the raised structure (217) that does not include the raised apex (219) in contact with the electrophoretic medium (209), The microcell wall (212) has a microcell inner wall surface (213) and a microcell wall upper surface (214). The microcell inner wall surface (213) is the surface of the microcell wall (212) of the microcell in contact with the electrophoretic medium (209). The microcell wall upper surface (214) is the surface of the microcell wall (212) of the microcell in contact with the sealing layer (206). The channel (215) has a channel height (216), and the channel height (216) is 50% of the protrusion height (220). The unexposed microcell bottom inner surface (211b) is in contact with the protrusion base (218). The channel (215) is the volume between the exposed microcell bottom inner surface (211a), the protrusion surface (221), and the microcell inner wall surface (213). Wherein, a first electric field is applied between the first light-transmissive electrode layer (202) and the second light-transmissive electrode layer (207) via a first waveform to cause the charged pigment particles to move towards the channel (215), thereby causing the variable light transmission device (200) to switch to an open optical state. The charged pigment particles in the open optical state are located within the channel (215), and Wherein, a second electric field is applied between the first light-transmissive electrode layer (202) and the second light-transmissive electrode layer (207) via a second waveform to cause the charged pigment particles (222) to move towards the first light-transmissive electrode layer (202) at a certain speed. The speed has a lateral component and causes a closed optical state. The second waveform includes at least one positive voltage and at least one negative voltage. The second waveform has a net positive impulse or a net negative impulse, wherein the closed optical state has a lower light transmission percentage than the open optical state.

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