Variable light transmission device comprising electrophoretic medium having combination of light reflecting and light absorbing pigment particles

Abstract

The invention discloses a variable light transmission device. The device comprises two light transmission electrode layers and a micro-cell layer with a plurality of micro-cells. Each microcell includes a first type of charged pigment particles, a second type of charged pigment particles, a charge control agent, and a non-polar liquid. After the electric field is applied, the amount of light passing through the device can be adjusted.

Description

[0001] Related Applications

[0002] This application claims the priority of U.S. Provisional Patent Application No. 63 / 436,124, filed on December 30, 2022, the entire content of which, as well as all other patents and patent applications disclosed herein, are incorporated herein by reference. Background Art

[0003] The present invention relates to a variable light transmission device. Specifically, the present invention relates to a microcell electro-optical device including an electrophoretic medium that includes first type charged pigment particles, second type charged pigment particles, and a charge control agent. The electrophoretic medium is capable of switching between optical states using an electric field. The first type charged pigment particles include light-reflecting pigments, while the second type charged pigment particles include light-absorbing pigments. The present invention also relates to a driving method for switching between optical states. Variable light transmission devices can regulate the amount of light and other electromagnetic radiation passing through them. They can be used in mirrors, windows, skylights, and the like. For example, the present invention can be applied to windows capable of regulating 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 on September 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 action 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, that 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 allows the particles to settle (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 settlement problems as liquid-based electrophoretic media. In fact, particle settlement seems to be a more serious 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 settle 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 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, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814.

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

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

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

[0012] (e) Thin films and subassemblies containing electro-optic materials; see, for example, 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, for example, U.S. Patent Nos. 7,116,318 and 7,535,624.

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

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

[0016] (i) Applications of displays; see, for example, 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, for example, 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, thereby producing so-called polymer dispersed electrophoretic displays, where the electrophoretic medium includes a plurality of discrete droplets of a non-polar liquid and a continuous phase of a polymeric material, and the discrete droplets of the electrophoretic medium within such polymer dispersed electrophoretic displays can be regarded as capsules or microcapsules, even though each individual droplet does not have an associated independent capsule membrane; see, for example, 2002 / 0131147 above. Thus, for the purposes of this application, such polymer dispersed electrophoretic media are regarded as 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 are retained within 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 the 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 doctor blade 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; electrostatographic printing processes; thermographic 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 as window coatings to electronically control the proportion of incident radiation transmitted through the window by changing the optical state of the electrophoretic media. 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, thereby reducing the energy required for cooling, the size of air conditioning units, and peak electricity demand; (2) increasing the use of natural daylight, thereby reducing lighting energy consumption and peak electricity demand; and (3) enhancing occupant comfort by improving thermal and visual comfort. Since the ratio of the glass surface area to the enclosed volume in automobiles is significantly greater than that in ordinary buildings, automobiles are expected to benefit more. Specifically, effective implementation of VT 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 an electro-optic coating on the mirror); and (4) enhancing the ability to use a head-up display. Other potential applications of VT 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 and can achieve a closed optical state (opaque state) and an open optical state (transparent state), and switch 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 microcell layer and a specific waveform can achieve efficient switching between the open and closed optical states. Summary of the Invention

[0024] One aspect of the present invention provides a variable light transmission device, which includes two light-transmitting electrode layers and a microcell layer including a plurality of microcells. Each microcell includes a first type of charged pigment particles, a second type of charged pigment particles, a charge control agent, and a non-polar liquid. After applying an electric field, the amount of light passing through the device can be adjusted.

[0025] The variable light transmission device of the present invention 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 first-type charged pigment particles, second-type charged pigment particles, a charge control agent, and a non-polar liquid. 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. The molecular structure of the charge control agent can include a quaternary ammonium functional group and a non-polar tail. The non-polar liquid of the electrophoretic medium can include materials selected from the group consisting of aliphatic hydrocarbons, cycloaliphatic hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons, polydimethylsiloxane, or mixtures thereof.

[0026] The first-type charged pigment particles can reflect light, while 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 charge polarity as the second-type charged pigment particles. The first-type charged pigment particles and the second-type charged pigment particles can be positively charged. If both the first-type charged pigment particles and the second-type charged pigment particles are positively charged, the Zeta potential of the first-type charged pigment particles can be lower than the Zeta potential of the second-type charged pigment particles.

[0027] The first type of charged pigment particles and the second type of charged pigment particles may be negatively charged. If both the first type of charged pigment particles and the second type of charged pigment particles are negatively charged, the Zeta potential of the first type of charged pigment particles may be higher than the Zeta potential of the second type of charged pigment particles. A higher Zeta potential of the first type of charged pigment particles means that the Zeta potential of the first type of charged pigment particles is less than the Zeta potential of the second type of charged pigment particles. That is to say, for example, if the Zeta potential of the second type of charged pigment particles is -15 eV, the Zeta potential of the first type of charged pigment particles may be -10 eV.

[0028] The charge polarity of the first type of charged pigment particles may be opposite to that of the second type of charged pigment particles. The first type of charged pigment particles may be negatively charged while the second type of charged pigment particles may be positively charged. Alternatively, the first type of charged pigment particles may be positively charged while the second type of charged pigment particles may be negatively charged. The average particle size of the first type of charged pigment particles may be larger than the average particle size of the second type of charged pigment particles. The average particle size corresponds to the average diameter of the largest dimension of the charged pigment particles.

[0029] 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; (g) a quadrangular pyramid on a square cuboid, the cuboid having a square base, and the square base being the convex base of the convex structure; (h) a quadrangular pyramid on a right parallelepiped, the right parallelepiped having a right parallelogram base, and the right parallelogram being the convex base of the convex structure; (i) a pentagonal pyramid having a pentagonal base, and the pentagonal base being the convex base of the convex structure; (j) 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; (k) a hexagonal pyramid having a hexagonal base, and the hexagonal base being the convex base of the convex structure; (l) 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; (m) a polyhedron pyramid having a polygonal base, and the polygonal base being the convex base of the convex structure; (n) a polyhedron pyramid on a polyhedron prism, the polyhedron prism having a polygonal base, and the polygonal 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 20 degrees, or 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; the slope of the cone can be 10 degrees or less, and the slope of the cone can be from 5 degrees to 20 degrees. The convex structure can be the following geometric body: a pyramid having a base with n sides, and the base with n sides being 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 on a prism having a base with n sides, and the base of the prism having n sides being the convex base of the convex structure, where n is from 7 to 12.

[0030] By applying a first electric field between the first light-transmissive electrode layer and the second light-transmissive electrode layer via a first waveform, the variable light transmission device can be switched to the open optical state.

[0031] Applying a first waveform between the first light-transmissive electrode layer and the second light-transmissive electrode layer can cause the first charged pigment particles to move towards the channel, thereby switching the variable light transmission device to the open optical state. The second type of charged pigment particles in the open optical state can be located within the channel.

[0032] Applying the second waveform can cause the first type of charged pigment particles to move towards the first light-transmissive electrode layer at a certain speed, which has a lateral component, thereby resulting in a closed optical state. 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 transmittance percentage than the open optical state. The second waveform can include an alternating current (AC) waveform that has a duty cycle ranging from 5% to 45%. Alternatively, the second waveform can include a DC offset waveform formed by superimposing a DC voltage component and an AC waveform. The second waveform can be DC unbalanced.

[0033] In one example, the second waveform can include an AC waveform that has a frequency and a duty cycle ranging from 5% to 45%. The duty cycle of the AC waveform can be higher than 50%, higher than 55%, higher than 60%, or higher than 65%. The duty cycle of the AC waveform can be 55% to 95%, 58% to 90%, 60% to 88%, 65% to 85%, or 70% to 80%. The duty cycle of the AC waveform can be lower than 50%, lower than 45%, lower than 40%, or lower than 35%. The duty cycle of the AC waveform can be 5% to 45%, 8% to 40%, 10% to 38%, 10% to 40%, 15% to 35%, or 20% to 30%. The AC waveform can 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 weight of the electrophoretic medium can be from 400 to 2000 Hz.

[0034] The AC waveform can be a square wave with 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 can be 10 V to 200 V; the frequency of the AC waveform can be 0.1 Hz to 6000 Hz or 100 Hz to 3000 Hz; the amplitude of the AC waveform can be 10 V to 200 V or 20 V to 180 V, and the frequency of the AC waveform can 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 weight of the electrophoretic medium can be from 400 to 2000 Hz.

[0035] In another example, the second waveform may include a waveform formed by superimposing a DC voltage component and an AC waveform, and the AC waveform has a frequency and an amplitude. The frequency of the AC waveform may be from 0.1 Hz to 6000 Hz, from 100 Hz to 3000 Hz, or from 400 Hz to 2000 Hz. The amplitude of the AC waveform may be from 10 V to 200 V or from 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 expressed in hertz to the content of the charge control agent in the electrophoretic medium expressed as the 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 with 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.

[0036] The electrophoretic medium may include a first type of charged pigment particles and a second type of charged pigment particles. The first type of charged pigment particles may reflect light. And the second type of charged pigment particles may absorb light. The first type of charged pigment particles may be white. The second type of charged pigment particles may be black. The first type of charged pigment particles may have the same polarity as the second type of charged pigment particles. The first type of charged pigment particles may have an opposite polarity to the second type of charged pigment particles.

[0037] The upper surface of the microcell wall may have a light-blocking layer located between the upper surface of the microcell and the sealing layer. The light-blocking layer may include a light-absorbing pigment. The light-blocking layer may include a black pigment.

[0038] 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 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 and a charge control agent, and a non-polar liquid. 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.

[0039] 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 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.

[0040] 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 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.

[0041] The second waveform can include an AC waveform, the AC waveform has a frequency, and the AC waveform has a duty cycle ranging from 5% to 45%. The AC waveform can be a square wave, a sine wave, a triangular wave, or a sawtooth wave.

[0042] 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 the weight percentage of the charge control agent in the electrophoresis medium can be from 400 to 2000Hz.

[0043] 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 the weight percentage of the charge control agent in the electrophoresis medium 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

[0044] Figure 1 Shows a cylindrical particle in a liquid under the influence of an electric field and the forces generated on the particle.

[0045] Figure 2A 、 2B Figures 2C and 2D show side views of examples of a part of the variable light transmission device of the present invention.

[0046] 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. The electrophoresis medium includes one type of charged pigment particles.

[0047] 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 the variable light transmittance device to achieve a closed state; this waveform includes an alternating current waveform with a duty cycle higher than 50%.

[0048] 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 the 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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. The electrophoretic medium includes one type of charged pigment particles.

[0056] 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. The electrophoretic medium includes one type of charged pigment particles.

[0057] Figure 14 Shows a plan view of the microcell of the variable transmission device used in the embodiment.

[0058] Figure 15 Shows a cross-sectional view of the microcell of the variable transmission device used in the embodiment.

[0059] 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.

[0060] Figure 17Micrographs of the open and closed optical states of the variable light transmission device of Example 2, the electrophoretic medium of which includes charge control agents at different concentrations.

[0061] Figure 18 Micrograph of the microcell array of the variable light transmission device of Example 5; the light-blocking composition includes black pigment particles, which are distributed throughout the microcell (closed optical state).

[0062] Figure 19 Micrograph of the microcell array of the variable light transmission device of Example 5, in which the black pigment particles of the light-blocking composition are driven into the channels of the microcells (closed optical state).

[0063] Figure 20 Micrograph of the closed optical state of the microcell array of the variable light transmission device of Example 6; white charged pigment particles of the light-blocking composition are distributed throughout the microcell (closed optical state).

[0064] Figure 21 Micrograph of the open optical state of the microcell array of the variable light transmission device of Example 6; white charged pigment particles of the light-blocking composition are driven into the channels (open optical state).

[0065] Figure 22 Micrograph of the open optical state of the microcell array of the variable light transmission device of Example 7, the electrophoretic medium of which includes white and black pigment particles. Detailed Description

[0066] 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 perpendicular line parallel to the normal vector from the given point to the given plane.

[0067] 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.

[0068] The slope of a cone is defined as an angle having: (a) a vertex (A), which is located on the circumference of the base of the cone, (b) a first arm, which is the line connecting point A (the vertex) and the center of the base of the cone, and (c) a second arm, which is the line connecting point A (the vertex) and the apex of the cone.

[0069] 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.

[0070] The "inner wall surface of the microcell" is the surface of the microcell wall that contacts the electrophoretic medium of the microcell.

[0071] 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.

[0072] The "DC balance waveform" or "DC balance drive waveform" applied to a pixel refers to a drive waveform in which the integral of the drive 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 drive waveform". The drive waveform applied to a pixel can have a portion 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 portion (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.

[0073] 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.

[0074] 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.

[0075] In the micro-units 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 sum of 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.

[0076] 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 the polarization of a conductive coating adsorbed on the particle surface, or by the 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.

[0077] 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 particular particle is approximately given by expression (1).

[0078] Expression (1)

[0079] 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).

[0080] Formula (2)

[0081] 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.

[0082] 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 can thus be driven by an alternating field.

[0083] When the electrophoresis medium is included in micro-units (which is preferred in electrophoretic displays), the geometry of the induced flow is affected by the shape of the particular micro-units used. For example, in the simplest case of two parallel electrodes, it has been shown that with appropriate electric field strengths and AC frequencies, the flow can adopt a scroll-like structure with a periodic spacing corresponding to the gap width between the electrodes.

[0084] The inventors of the present invention utilize a complex micro-unit 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 micro-unit. Figure 2A 、 2B Figures 2A, 2B, and 2C show an example 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 electrophoresis 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. 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, the same movement will occur. 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 micro-unit out of the multiple micro-units of the device. The three figures 2A, 2B, and 2C are the same in terms of the device structure shown, but different parts of the device are labeled in each figure.

[0085] Figure 2A 、 2B shown in Figures 2A, 2B, and 2C, the variable light transmission device 200 portion includes a micro-unit layer that includes a plurality of micro-units and a sealing layer. Although Figure 2A 、 2B only one micro-unit is shown in Figures 2A, 2B, and 2C, the entire variable light transmission device including a plurality of micro-units can be envisioned. The variable light transmission device can include a first light-transmissive substrate 201, a first light-transmissive electrode layer 202, a micro-unit layer 203 that includes a plurality of micro-units 204 and a sealing layer 206, a second light-transmissive electrode layer 207, and a second light-transmissive substrate 208. Each of the plurality of micro-units 204 includes an electrophoresis medium 209 that includes charged pigment particles, a charge control agent, and a non-polar liquid. Figure 2A 、 2BThe components of the electrophoretic medium (charged pigment particles, charge control agent, and non-polar liquid) are not shown in FIGS. 2A and 2C. Each of the plurality of micro-units 204 has a micro-unit opening 205, and a sealing layer 206 spans 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.

[0086] 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 the example of the variable light transmission device shown in FIGS. 2A 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. The micro-unit layer including a plurality of micro-units 204 having the raised structure 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.

[0087] 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

[0088] Channel 215 is the volume between the inner surface 211a of the exposed microcell bottom, the inner wall surface 213 of the microcell, and the raised surface 221. Channel 215 is the volume location where most charged particles are located in the device in the open optical state. The channel height 216 of Channel 215 is 50% of the raised height 220. Thus, the channel height, along with the inner surface 211a of the exposed microcell bottom, the inner wall surface 213 of the microcell, and the raised surface 221, further defines the channel. In Figure 2C the raised surface 221 is highlighted with a thicker line.

[0089] 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 the devices shown in Figure 2A and 2B and 2C, but shows a larger portion of the device including four microcells. The variable light transmission device 200 includes a first light-transmissive substrate 201, a first light-transmissive electrode layer 202, a microcell layer 203 including a plurality of microcells 204 and a sealing layer 206, a second light-transmissive electrode layer 207, and a second light-transmissive substrate 208. Each of the plurality of microcells includes an electrophoretic medium that includes charged pigment particles 222, a charge control agent, and a non-polar liquid. Each of the plurality of microcells 204 has a microcell opening, and the sealing layer 206 spans the microcell openings of the plurality of microcells. Each of the plurality of microcells includes a microcell bottom layer 210, a raised structure 217, a microcell wall 212, and a channel 215. Figure 2D The variable light transmission device shown is in the closed optical state.

[0090] 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 solid black circles. In this example, the electrophoretic medium includes one type of charged pigment particles 222.

[0091] Applying a second electric field between the first light-transmissive electrode layer 202 and the second light-transmissive electrode layer 207 via a second waveform causes 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 Fig. b. This velocity has a lateral component. If there is no velocity with a lateral component, the optical-off state will not occur because the charged pigment particles 222 will move from the channels in the on 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, the charged pigment particles 222 will not cover the entire surface of the first light-transmissive electrode layer 202. Therefore, the optical-off state cannot be effectively formed because the light transmittance of the optical-off state is relatively high.

[0092] The above shows that the transition from the optical-off state to the optical-on state will be easier to achieve because when the charged pigment particles strike 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. a and 3b) will impart a lateral component to the velocity of the charged pigment particles.

[0093] 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 direct 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 into the entire microcell volume. Additionally, 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 be oriented to pass through the microcells approximately vertically. Therefore, it is preferable to use a waveform that causes the charged pigment particles to have a lateral movement.

[0094] The variable light-transmission device can be switched to the optical-on state by 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 first type of charged pigment particles to move towards the channels, such that the variable light-transmission device is switched to the optical-on state and the first type of charged pigment particles in the optical-on state are located within the channels. The variable light-transmission device can be switched to the optical-off state by 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 first type of 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 optical-off 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 optical-off state has a lower light transmittance than the optical-on state.

[0095] 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 to be moved to a position near the sealing layer in the electrophoretic medium. Specifically, if the off state involves the movement of negatively charged first-type charged pigment particles, a net positive impulse is required to move these particles from the channel towards 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 off state involves the movement of positively charged first-type charged pigment particles, a net negative impulse is required to move these charged pigment particles from the channel near the second light-transmissive electrode layer 207 towards the first light-transmissive electrode layer.

[0096] The second electric field applied between the two light-transmissive electrode layers via the second waveform achieves the off optical state.

[0097] 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.

[0098] 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%.

[0099] 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 through the difference in time periods. In Figure 4In the case of the example, there is a positive DC bias because the application period (t1) of the positive voltage V1 is longer than the application 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-transmitting electrode layer of the device. The duty ratio of the waveform can be calculated by formula (3).

[0100] Duty ratio = 100 × (V1 · t1) / [(V1 · t2) + ((V2 · t2)] Formula (3)

[0101] In Figure 4 the waveform example of, the amplitude of V1 can be equal to the amplitude of V2 (|V1| = |V2|), but generally speaking, the amplitudes of V1 and V2 can be different from each other.

[0102] 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.

[0103] 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 electrophoresis medium can range from 400Hz to 2000Hz.

[0104] 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.

[0105] Figure 5The waveform in has a net negative impulse due to the DC offset (Vd). Although the application period (t3) of the positive pulse is equal to the application 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.

[0106] Figure 5 The example drive waveform shown is DC unbalanced. However, Figure 5 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 pulses) can be applied before the DC unbalanced waveform (pre-pulse). Additionally, Figure 5 The example waveform shown is a square wave AC waveform. Other example AC waveforms that can be used include sine waves, triangular waves, and sawtooth waves.

[0107] The AC waveform can have an amplitude from 10V to 200V and a frequency from 0.1 to 6000Hz. The AC waveform can have an amplitude from 15V to 180V, from 20V to 160V, from 25V to 150V, or from 30V to 140V. The AC waveform can have a frequency 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 be from 400Hz to 2000Hz.

[0108] 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 the charged pigment particles 222 in contact with the raised structure 617 (cone) in the 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 area and up the side of the cone in this way.

[0109] The electrophoretic medium of the variable light transmission device of the present invention includes first-type charged pigment particles, second-type charged pigment particles, a charge control agent, and a nonpolar liquid.

[0110] The first-type charged pigment particles can reflect light, while the second-type 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 of black, blue, cyan, magenta, red, green, yellow, and other colors. 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.

[0111] In the electrophoretic medium having first-type charged pigment particles and second-type charged pigment particles (which have the same charge polarity), the Zeta potential of the first-type charged pigment particles can be lower than that of the second-type charged pigment particles. In addition, the average particle size of the first-type charged pigment particles can be larger than that of the second-type charged pigment particles, and 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 the 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 (first type) and 222b (second type) will move into the channels of the micro-units to form the open state. However, since the first-type charged pigment particles 222a have a lower charge (and larger size), the second-type charged pigment particles 222b (light-absorbing) will be located below the first-type charged pigment particles 222a (light-reflecting). In other words, the second-type charged pigment particles 222b will be closer to the inner surface of the exposed bottom of the micro-unit (the bottom of the channel) than the first-type 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 charged pigment particles 222b (light-absorbing) will be closer to the sealing layer 206 than the first-type charged pigment particles 222a (light-reflecting) because the first-type charged pigment particles 222a have a lower charge (and larger size).

[0112] In another example, the variable light transmission device has an electrophoretic medium including first-type charged pigment particles (light-reflecting) and second-type charged pigment particles (light-absorbing), wherein the first-type charged pigment particles and the second-type charged pigment particles have opposite charge polarities. Figure 8Figs. 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 region (microcell opening) near the sealing layer 206.

[0113] 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.

[0114] It is desired 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 desired 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. The incident light can include wavelengths within the solar spectrum, i.e., ultraviolet light, visible light, and infrared light. 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.

[0115] 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 reflected, transmitted, and absorbed light to the incident light.

[0116] 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 reflected, transmitted, and absorbed light to the incident light.

[0117] Figure 9 and 10It shows 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.

[0118] 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 reflective pigments and charged particles (second type) including absorbing pigments. The reflective pigment can be titanium dioxide and the absorbing pigment can be an inorganic black pigment such as iron oxide black.

[0119] Figure 11 The effects of the reflection, transmission, and absorption rates of a closed optical state layer of a composition including white pigment (light reflecting) and black pigment (light absorbing) on the layer thickness are shown. In the example shown, the weight ratio of black pigment to white pigment is 0.1. That is, for each layer thickness, these figures provide the ratios of the amount of light reflected, transmitted, and absorbed to the incident light.

[0120] As the layer thickness increases, the rate of decrease in transmission remains almost constant, but the upper limit of the reflectivity of the white pigment is 30%. The absorption rate increases with the increase in layer thickness, but compared with Figure 9 the pure black pigment layer shown, the absorption rate is only about half of it. Placing the white pigment layer in front of the black pigment layer instead of mixing the two can achieve a reduction in the absorption rate and its unnecessary thermal effects.

[0121] 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 velocity 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 can be 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 the high frequency, 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).

[0122] Figure 12 Another method for reducing the haze of a variable light transmission device is shown, which is caused by the light scattering effect of charged pigment particles. 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. In this example, the electrophoretic medium includes one type of charged pigment particles. The variable light transmission device also includes a first light-blocking layer 1202 located on the inner surface of the exposed bottom 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 channel of the microcell ( Figure 12a). When observing the device from the bottom, the first light-blocking layer 1202 can block haze. Observing from the bottom means that the observer observes the device from the side close to the second light-transmissive 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-transmissive electrode layer 202. Figure 12 b shows the off-optical state of the device. Figure 12 The variable light-transmission device shown can be used in applications where the observer is usually located on one side of the variable light-transmission device, such as, for example, a skylight.

[0123] By using Figure 13 the device shown, a variable light-transmission device with improved off-optical state performance can also be achieved. Figure 13 The variable light-transmission device shown includes a first light-transmissive electrode layer (202), a second light-transmissive electrode layer (207), and a micro-unit layer (203). The micro-unit layer is disposed between the first light-transmissive electrode layer (202) and the second light-transmissive electrode layer (207). The micro-unit layer includes a plurality of micro-units and a sealing layer. Each micro-unit of the plurality of micro-units includes an electrophoretic medium, which includes charged pigment particles, a charge control agent, and a non-polar liquid. In this example, the electrophoretic medium includes one type of charged pigment particles. Each micro-unit of the plurality of micro-units has a micro-unit opening, and the sealing layer extends across the micro-unit openings of the plurality of micro-units. Each micro-unit 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 in the raised structure where the distance to the micro-unit opening is less 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 in contact with the electrophoretic medium. The micro-unit wall has a micro-unit inner wall surface, a micro-unit wall upper surface, and a second light-blocking layer 1311. 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 second light-blocking layer 1311 is located between the micro-unit wall upper surface and the sealing layer. The channel has a channel height, which 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. Figure 13The variable light transmission device in [[ ]] may further include a first light-blocking layer 1202, which is disposed on the inner surface layer of the bottom of the exposed microcell. As described above, the first light-blocking layer 1202 can reduce the haze when observing the device from below. The second light-blocking layer 1311 promotes the improvement of the closed state by increasing the opacity of the device, and the opacity can be caused by the 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, which 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-reflecting pigments to further improve the opacity in the closed optical state. The auxiliary layer 1312 may further include an encapsulated electrophoresis layer, which includes an electrophoresis medium with charged pigment particles. Applying an electric field on the encapsulated electrophoresis 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.

[0124] The charge control agent is generally an oligomeric or polymeric material that is soluble in the nonpolar liquid of the electrophoresis medium. The charge control agent is a surfactant-type molecule having one or more polar functional groups (heads) and a nonpolar part (tail). The electrophoresis medium may include a charge control agent at a concentration of 0.1% by weight to 10% by weight of the weight of the electrophoresis medium. The electrophoresis medium may include a charge control agent at a concentration of 0.5% by weight to 9% by weight, 0.7% by weight to 8% by weight, 1% by weight to 7% by weight, or 1% by weight to 6% by weight of the weight of the electrophoresis medium.

[0125] The nonpolar liquid of the electrophoresis medium may include aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons, polydimethylsiloxane, or a mixture thereof.

[0126] The electrophoresis medium may further 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. Therefore, the bistability of the optical state (open and closed) of the device is enhanced. The dissipative agent is generally a polymeric material, such as polyisobutene and polydimethylsiloxane.

[0127] Example

[0128] 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 electrophoresis medium. The structure of the device is the same as that of Figures 2A to 2Dcorresponds to the illustration in, but without the addition of the sealing layer 212. The structure of the imprinted microcell array is as Figure 14 shown, which is a plan view of the microcell in the device. Figure 15 A corresponding cross-sectional view of a microcell in the device is shown. Table 1 shows the external dimensions of the microcell.

[0129] Table 1: Device Structure

[0130] Component Distance (micrometers) Cavity spacing: center to center of micro-units 500 Height of the 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 the 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

[0131] 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 is to mix 10 wt% of the white pigment and 5 wt% of the charge control agent (a cationic charge control agent from Example 1 - CCA111 of US2020 / 0355978) in Isopar E solvent. The device is switched using a 50V square wave AC waveform with a duty cycle of 50%. As Figure 16 shown, by increasing the AC frequency from 0 Hz to 5000 Hz, three different pigment movement patterns were observed. The white pigment particles were prepared with a titanium dioxide pigment core and a polymer shell, as described in Example 1 of US Patent No. 8,582,196.

[0132] At a low frequency of 10 Hz and after multiple switches, the white pigment moves towards the edge of the microcell (near the periphery) ( Figure 16 in (a)), 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 microcell structure ( Figure 16 in (b)). At a frequency of 1000 Hz, the white pigment completely diffuses into the circle of the imprinted microcell structure, as Figure 16 shown in (c). At a frequency of 1000 Hz, it is believed that the behavior of the pigment is mainly dominated by the lateral movement of induced charges, which may be the result of ICEO. When the frequency is 5000 Hz, the white pigment particles tend to the center of the area near the cone of the imprinted microcell structure, as Figure 16 shown in (d).

[0133] Example 2 : In Example 2, the effect of the charge control agent (CCA) concentration on the movement of the white pigment in the imprinted microcell device was studied.

[0134] As described above, it is expected that increasing the concentration of the charge control agent in the electrophoretic medium will reduce the Debye length associated with the surface of the charged pigment particles, thereby increasing the frequency required for a specific ICEO current. To verify this hypothesis, three variable light transmission devices with similar electrophoretic media but 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 electrophoretic media were prepared with charge control agent concentrations of 0.1 wt%, 1 wt%, and 5 wt% of the weight of the electrophoretic medium, respectively. Each electrophoretic medium also included 10 wt% of white pigment and Isopar E solvent. The white pigment particles were prepared using a titanium dioxide pigment core that included a polymer coating as described in Example 1 of US Patent No. 8,582,196.

[0135] The switching performance of the three samples was evaluated in an imprinted microcell device (such as Figure 14 and 15 shown). The waveform was an alternating current with a square wave of + / -50V and a 50% duty cycle. As Figure 17 shown, as the concentration of the charge control agent increased, the frequency required to reach the off optical state also increased. That is, for the electrophoretic medium containing 0.1 wt% CCA, a frequency of 50 Hz was required to achieve the off optical state; for the electrophoretic medium containing 1 wt% CCA, a frequency of 100 Hz was required to achieve the off optical state; for the electrophoretic medium containing 5 wt% CCA, a frequency of 1000 Hz was required to achieve the off optical state. In these three experiments, the ratios of frequency to CCA concentration were 500 Hz, 1000 Hz, and 1000 Hz, respectively.

[0136] 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 alternating current square wave of + / -100V and 0.5 Hz, or (b) an alternating current square wave of + / -50V and 50 Hz frequency superimposed on a -50V direct current voltage. The on optical state is achieved by an alternating current square wave of + / -50V with a 5% duty cycle, while the off optical state requires an alternating current square wave of + / -50V with a 95% duty cycle. The switching time in each case is approximately 1 second.

[0137] Example 3 : The effect of the charge of the white pigment on the switching performance of the variable light transmission device was studied.

[0138] A variable light transmission device was prepared, and its electrophoretic medium includes 10 wt% of positively charged white pigment particles in Isopar E. The white pigment was functionalized with 1.6% wt of silane Z6030 and grafted with lauryl methacrylate (PLMA). Titration with CCA (cationic polymer disclosed in Example 1 - CCA111 of US Patent No. 2020 / 0355978) was performed to give the treated pigment a Zeta potential of +35 mV. The electrophoretic medium also includes 1 wt% of CCA (cationic polymer disclosed in Example 1 - CCA111 of US Patent No. 2020 / 0355978). The waveform for switching the device from the open optical state to the closed optical state is direct current superimposed on alternating current, i.e., a square wave waveform, with an alternating current of + / -50 V at a frequency of 500 Hz. The waveform for switching the device from the closed optical state to the open optical state is an alternating current of + / -50 V with a direct current offset of +50 V. Thus, the behavior of the device in Example 3 is very similar to that of the device in Example 2 using an electrophoretic medium including 1 wt% CCA, except that the polarity of the direct current offset required to achieve the open optical state is opposite.

[0139] Example 4 : In this example, the solvent of the electrophoretic medium matches the polymer forming the embossed micro-units.

[0140] "Haze" refers to the percentage of diffusely transmitted light in the total transmitted light. Diffusely transmitted light is the light that is scattered during transmission. To manufacture a variable light transmission device with low haze, it is necessary to match the refractive index of the liquid solvent of the electrophoretic medium with the refractive index of the polymer material used to manufacture the imprinted micro-units, as described in US Patent No. 7,327,511.

[0141] Typically, the solvents used in electrophoretic media 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 altered 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 nonpolar liquid of the electrophoretic medium comprises a mixture of partially hydrogenated aromatic hydrocarbons and terpenes, and a 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.

[0142] To reduce haze, it is desirable for the refractive index of the encapsulant of the electrophoretic medium to closely match 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.

[0143] Example 4A : A variable light transmission device including microcells was prepared, wherein the electrophoretic medium comprised 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 off optical state, a duty cycle of 5% was used. To achieve the on optical state, a duty cycle of 95% was first used and then the duty cycle was changed to 50%.

[0144] 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 included polyisobutene (PIB) as a dissipative agent. The dissipative agent was used to improve the bistability of the device, that is, to ensure that when no electric field was applied, the device remained in the open optical state and the closed optical state. The electrophoretic medium included 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 was 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, a 95% duty cycle was used, followed by a 50% duty cycle. The time required for complete switching was approximately 20 seconds. This time was much longer than the switching time required for the non-refractive index matching solvent without the dissipative agent (Example 4A).

[0145] Example 5 : A light-blocking layer including black particles.

[0146] 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 included an electrophoretic medium. The structure of the imprinted micro-unit array was as shown in Figures 2A to 2D shown, but the micro-units in this example did not include a sealing layer.

[0147] The light-blocking composition for the light-blocking layer included 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 was commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55Commerce Rd, Cedar Grove N.J. 07009, and also included limonene, Isopar M, and Isopar E. The black pigment particles had a core including black iron oxide (Pigment Black 11) and a shell of polymer. As shown in Figure 18As shown, during the device fabrication process, 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 transparent electrode layer, while the second transparent 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.

[0148] 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 can easily switch 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.

[0149] Example 7 : Black and white pigments with opposite charges are switched into the channels of the imprinted microcells.

[0150] A variable transmission device is prepared by laminating a sheet of PET-coated and ITO transparent conductor 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 shown.

[0151] 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 electrophoretic medium is prepared by mixing 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.

[0152] 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.

[0153] 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 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 was 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 of the waveform having 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.

[0154] Partial structures in the figure: 200 variable transmission device; 201 first light-transmitting substrate; 202 first light-transmitting electrode layer; 203 micro-unit layer; 204 multiple micro-units; 205 micro-unit opening; 206 sealing layer; 207 second light-transmitting electrode layer; 208 second light-transmitting substrate; 209 electrophoretic medium; 210 micro-unit bottom layer; 211 inner surface of the micro-unit bottom; 211a exposed inner surface of the micro-unit bottom; 211b unexposed inner surface of the micro-unit bottom; 212 micro-unit wall; 213 inner wall surface of the micro-unit; 214 upper surface of the micro-unit wall; 125 channel; 216 channel height; 217 raised structure; 218 raised base; 219 raised apex; 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 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) includes an electrophoretic medium (209), the electrophoretic medium (209) comprising first-type charged pigment particles, second-type charged pigment particles, a charge control agent, and a non-polar liquid, Each of the plurality of micro-units (204) has a micro-unit opening (205), and the sealing layer (206) spans 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), 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) has 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) are to the micro-unit opening (205), the raised height (220) being the distance between the raised base (218) and the raised apex (219), and the raised surface (221) being the surface of the raised structure (217) that does not include the raised apex that contacts the electrophoretic medium (209), 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) being the surface of the micro-unit wall (212) of the micro-unit that contacts the electrophoretic medium (209), and the micro-unit wall upper surface (214) being the surface of the micro-unit wall (212) of the micro-unit that contacts 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 micro-unit bottom inner surface (211b) contacts the raised base (218), The channel (215) is the volume between the exposed micro-unit bottom inner surface (211a), the raised surface (221), and the micro-unit inner wall surface (213).

2. The variable light transmission device according to claim 1, wherein the first type of charged pigment particles reflect light and the second type of charged pigment particles absorb light.

3. The variable light transmission device according to claim 1 or 2, wherein the first type of charged pigment particles are white.

4. The variable light transmission device according to any one of claims 1 to 3, wherein the second type of charged pigment particles are black.

5. The variable light transmission device according to any one of claims 1 to 4, wherein the first type of charged pigment particles and the second type of charged pigment particles have the same charge polarity.

6. The variable light transmission device according to any one of claims 1 to 5, wherein the first type of charged pigment particles and the second type of charged pigment particles are positively charged, and wherein the Zeta potential of the first type of charged pigment particles is lower than the Zeta potential of the second type of charged pigment particles.

7. The variable light transmission device according to any one of claims 1 to 6, wherein the first type of charged pigment particles and the second type of charged pigment particles are negatively charged, and wherein the Zeta potential of the first type of charged pigment particles is higher than the Zeta potential of the second type of charged pigment particles.

8. The variable light transmission device according to any one of claims 1-4, wherein the first type of charged pigment particles and the second type of charged pigment particles have opposite charge polarities.

9. The variable light transmission device according to claim 8, wherein the first type of charged pigment particles are negatively charged and the second type of charged pigment particles are positively charged.

10. The variable light transmission device according to any one of claims 1 to 9, wherein the average particle size of the first type of charged pigment particles is larger than the average particle size of the second type of charged pigment particles.

11. The variable light transmission device according to any one of claims 1 to 10, 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.

12. The variable light transmission device according to any one of claims 1 to 11, wherein the molecular structure of the charge control agent includes a quaternary ammonium functional group and a non-polar tail.

13. The variable light transmission device according to claim 1, wherein the non-polar liquid of the electrophoretic medium includes a material selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons, polydimethylsiloxane, or mixtures thereof.

14. The variable light transmission device according to any one of claims 1 to 13, wherein the convex 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, and the base of the cylinder being the convex base of the convex structure. (c) Tetrahedron; (d) Tetrahedron on a triangular prism having a triangular base which is the raised base of the raised structure; (e) Triangular prism having a square base which is the raised base of the raised structure; (f) Square pyramid having a square base which is the raised base of the raised structure; (g) Square pyramid located on a square cuboid having a square base which is the raised base of the raised structure; (h) A quadrangular pyramid on a regular parallelepiped, the regular parallelepiped having a regular parallelogram base, the regular parallelogram being the raised base of the raised base structure, (i) A pentagonal pyramid having a pentagonal base which is the raised base of the raised structure, (j) A pentagonal pyramid on a pentagonal prism, the pentagonal prism having a pentagonal base which is the raised base of the raised structure, (k) A hexagonal pyramid having a hexagonal base which is the raised base of the raised structure, (l) A hexagonal pyramid on a hexagonal prism, the hexagonal prism having a hexagonal base which is the raised base of the raised structure, (n) A polyhedron pyramid having a polygonal base which is the raised base of the raised structure, (o) A polyhedron pyramid on a polyhedron prism, the polyhedron prism having a polygonal base which is the raised base of the raised structure.

15. The variable light transmission device according to claim 1, wherein the raised structure is a cone on a cylinder, the cylinder having a bottom which is the raised bottom of the raised structure, and wherein the slope of the cone is from 5 degrees to 20 degrees.

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

17. The variable light transmission device according to any one of claims 1 to 16, wherein a first electric field is applied between the first light-transmitting electrode layer and the second light-transmitting electrode layer via a first waveform to switch the variable light transmission device to an open optical state.

18. The variable light transmission device according to any one of claims 1 to 17, wherein a second electric field is applied between the first light-transmitting electrode layer and the second light-transmitting electrode layer via a second waveform, causing the first type of charged pigment particles to move towards the first light-transmitting electrode layer at a certain speed, the speed having a lateral component, thereby resulting in a closed optical state, the second waveform including at least one positive voltage and at least one negative voltage, the second waveform having 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.

19. The variable light transmission device according to claim 18, wherein the second waveform includes an alternating current waveform having a duty cycle from 5% to 45%.

20. The variable light transmission device according to claim 18, wherein the second waveform includes a direct current offset waveform formed by superimposing a direct current voltage component and the alternating current waveform.

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