Variable optical transmission device and method for manufacturing same

Abstract

A variable light transmission device and a method for fabricating the same are disclosed. The variable light transmission device includes two light-transmitting electrodes and a microcell layer having a plurality of microcells, each of which includes a protrusion structure and a channel and contains an electrophoretic medium. The opacity of the device is controlled by an applied electric field. In one embodiment, each microcell of the plurality of microcells includes a first light-blocking layer, the first light-blocking layer contacting the exposed microcell bottom inner surface and the electrophoretic medium.

Description

[Technical Field]

[0001] (Related Applications) This application claims priority to U.S. Provisional Patent Application No. 63 / 436,119, filed December 30, 2022, which is incorporated by reference in its entirety along with any other patents and patent applications disclosed herein.

[0002] The present invention relates to variable light transmission devices and methods for manufacturing the same. Specifically, the present invention relates to microcellular electro-optical devices comprising an electrophoretic medium, the electrophoretic medium comprising charged pigment particles and a charge control agent. The electrophoretic medium is switchable between optical states using an electric field. The present invention also relates to methods for manufacturing the devices. Variable light transmission devices can modulate the amount of light and other electromagnetic radiation passing through them. Variable light transmission devices can be used on mirrors, windows, sunroofs, and similar items. For example, the present invention can be applied on windows that can modulate infrared radiation to control the temperature inside buildings and vehicles. Specifically, the present invention relates to variable light transmission devices that use a particle-based electrophoretic medium to control light modulation. Examples of electrophoretic media that may be incorporated into various embodiments of the present invention are described, for example, 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 / 0055842, 2017 / 0055843, and 2017 / 0055844. 0351155, 2017 / 0235206, 2011 / 0199671, 2020 / 0355979, 2020 / 0272017, 2021 / 0096439, and U.S. patent application Ser. No. 17 / 935,386, filed Sep. 27, 2022 (the contents of which are incorporated herein by reference in their entirety). [Background technology]

[0003] Particle-based electrophoretic displays, in which a plurality of charged pigment particles migrate through a suspending fluid under the influence of an electric field, have been the subject of intensive research and development over the past few years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays.

[0004] The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to a display having display elements with first and second display states that differ in at least one optical property, where any given element is driven with a finite-duration address pulse to adopt either its first or second display state, and where that state persists after the address pulse has terminated for at least several times, e.g., at least four times, the minimum duration of the address pulse required to change the state of the display element. Published U.S. Patent Application No. 2002 / 0180687 shows that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true for several other types of electro-optic displays. Displays of this type are properly referred to as "multistable" rather than bistable, although for convenience the term "bistable" may be used herein to include both bistable and multistable displays.

[0005] As described above, electrophoretic media require the presence of a suspending fluid. In most prior art electrophoretic media, the suspending fluid is liquid, but electrophoretic media can be produced using gaseous suspending fluids. Such gas-based electrophoretic media are believed to be susceptible to the same types of problems resulting from particle settling as liquid-based electrophoretic media when the media is used in an orientation that allows for such settling, for example, in signs where the media is placed on a vertical surface. In fact, particle settling is believed to be a more severe problem in gas-based electrophoretic media than in liquid-based ones, because the lower viscosity of gaseous suspending fluids compared to liquid ones allows for more rapid settling of charged pigment particles.

[0006] Numerous patents and applications assigned to or in the name of Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC, and related companies describe various techniques used in encapsulation and microcell electrophoresis and other electro-optic media. Encapsulated electrophoretic media comprise a multitude of small capsules, each of which itself comprises an internal phase containing electrophoretically movable particles in a liquid medium and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymer binder to form a coherent layer positioned between two electrodes. In microcell electrophoretic displays, charged pigment particles and liquid are not encapsulated within microcapsules, but instead are held within multiple cavities formed within a carrier medium, typically a polymer film. Techniques described in these patents and applications include the following:

[0007] (a) electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814);

[0008] (b) capsules, binders, and encapsulation processes (see, e.g., U.S. Patent Nos. 6,922,276 and 7,411,719);

[0009] (c) microcell structures, wall materials, and methods of forming the microcells (see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906);

[0010] (d) methods of filling and sealing microcells (see, e.g., U.S. Patent Nos. 7,144,942 and 7,715,088);

[0011] (e) films and subassemblies containing electro-optical materials (see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564);

[0012] (f) backplanes, adhesive layers, and other auxiliary layers and methods used in displays (see, e.g., U.S. Patent Nos. 7,116,318 and 7,535,624);

[0013] (g) color formation and color control (see, e.g., U.S. Patent Nos. 7,075,502 and 7,839,564);

[0014] (h) methods of driving displays (see, e.g., U.S. Patent Nos. 7,012,600 and 7,453,445);

[0015] (i) display applications (see, e.g., U.S. Patent Nos. 7,312,784 and 8,009,348);

[0016] (j) Non-electrophoretic displays (see, e.g., U.S. Pat. No. 6,241,921 and U.S. Patent Application Publication No. 2015 / 0277160) and non-display applications of encapsulation and microcell technology (see, e.g., U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710).

[0017] Many of the aforementioned patents and applications recognize that the walls surrounding the separate microcapsules in an encapsulated electrophoretic medium may be replaced with a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of separate droplets of a non-polar liquid and a continuous phase of a polymer material, and that the separate droplets of electrophoretic medium in such a polymer-dispersed electrophoretic display may be considered capsules or microcapsules even though no separate capsule membrane is associated with each individual droplet. See, for example, the aforementioned 2002 / 0131147. Therefore, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0018] A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, the charged pigment particles and suspension liquid are not encapsulated in microcapsules, but instead are held within a plurality of cavities formed in a carrier medium, typically a polymer film. See, for example, International Application Publication No. WO 02 / 01281 and Published U.S. Application No. 2002 / 0075556 (both of which are assigned to SiPix Imaging, Inc.).

[0019] Although electrophoretic media are often opaque (e.g., because 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 fabricated to operate in a so-called "shield mode," where one display state is substantially opaque and one is light-transmitting. See, for example, U.S. Patent Nos. 6,130,774 and 6,172,798, and 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 variations 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 shield mode.

[0020] Encapsulated or microcell electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices and offer additional advantages, such as the ability to print or coat displays on a wide variety of flexible and rigid substrates. The use of the term "printing" is intended to include, but is not limited to, all forms of printing and coating, including pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, and curtain coating; roll coatings such as knife-over-roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silkscreen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition; and other similar techniques. Thus, the resulting display can be flexible. Furthermore, because the display medium can be printed (using a variety of methods), the display itself can be inexpensively fabricated.

[0021] One potentially important market for electrophoretic media is windows with variable light transmission. As energy performance in buildings and vehicles becomes increasingly important, electrophoretic media could be used as coatings on windows, allowing a percentage of incident radiation to be transmitted through an electronically controlled window by varying the optical state of the electrophoretic media. Effective implementation of such "variable transmittance" ("VT") technology in buildings is expected to provide increased occupant comfort by: (1) reducing unwanted heating effects during high outdoor temperatures, thereby reducing the amount of energy required for cooling, the size of air conditioning equipment, and peak electrical demand; (2) increasing the use of natural daylight (thus reducing the energy used for lighting and peak electrical demand); and (3) increasing both thermal and visual comfort. Further benefits would be expected to accrue in automobiles, where the ratio of glossy surface to enclosed volume significantly exceeds that in a typical building. Specifically, effective implementation of VT technology in automobiles is expected to provide not only the aforementioned benefits, but also (1) increased driving safety, (2) reduced glare, (3) enhanced mirror performance (by using electro-optical coatings on the mirrors), and (4) increased ability to use head-up displays. Other potential applications involving VT technology include privacy glass and glare protection in electronic devices.

[0022] The technical field provides examples of devices comprising an electrophoretic medium sandwiched by electrode layers that achieve a closed optical state (opaque state) and an open optical state (transparent state) and are switchable between these states by application of an electric field across the electrophoretic medium. However, the open optical state of variable transmittance electrophoretic devices using conventional structures is blurred, making the device less desirable. The present inventors have surprisingly found that devices having a plurality of microcells including a protrusion structure and a light-blocking layer exhibit improved performance. [Prior art documents] [Patent documents]

[0023] [Patent Document 1] U.S. Patent Application No. 2002 / 0180687 [Patent Document 2] U.S. Patent No. 7,002,728 [Patent Document 3] U.S. Patent No. 6,130,774 Summary of the Invention [Means for solving the problem]

[0024] In one aspect, the present invention provides a variable light transmission device comprising two light-transmitting electrodes and a microcell layer having a plurality of microcells, each of which comprises a protrusion structure, a channel, and a light-blocking layer, and each of which contains an electrophoretic medium with charged pigment particles. The opacity of the device is controlled by an applied electric field.

[0025] The variable light transmission device of the present invention comprises a first light-transmitting electrode layer, a second light-transmitting electrode layer, and a microcell layer. The microcell layer comprises a plurality of microcells and a sealing layer. The microcell layer is disposed between the first light-transmitting electrode layer and the second light-transmitting electrode layer. Each microcell contains an electrophoretic medium, the electrophoretic medium comprising charged pigment particles and a charge control agent in a non-polar liquid. Each microcell of the plurality of microcells has a microcell opening. The sealing layer spans the microcell openings of the plurality of microcells. Each microcell of the plurality of microcells comprises a microcell bottom layer, a protrusion structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface, the microcell bottom inner surface comprising an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. Each microcell of the plurality of microcells comprises a first light-blocking layer, the first light-blocking layer contacting the exposed microcell bottom inner surface and the electrophoretic medium. The protrusion structure has a protrusion bottom surface, a protrusion surface, a protrusion apex, and a protrusion height. The protrusion apex is a point or set of points on the protrusion structure that is a shorter distance from the microcell opening than any other point on the protrusion structure. The protrusion height is the distance between the protrusion bottom surface and the protrusion apex. The protrusion surface is the surface of the protrusion structure that does not include the protrusion apex that contacts the electrophoresis medium. The microcell wall has a microcell inner wall surface and a microcell wall upper surface. The microcell inner wall surface is the surface of the microcell wall of the microcell that contacts the electrophoresis medium. The microcell wall upper surface is the surface of the microcell wall of the microcell that contacts the sealing layer. The channel has a channel height, and the channel height is 50% of the protrusion height. The unexposed microcell bottom inner surface contacts the protrusion bottom surface. The channel is the volume between the first light-blocking layer, the protrusion surface, and the inner wall surface of the microcell. The charge control agent in the electrophoretic medium of the variable light transmission device can be 1 weight percent to 8 weight percent by weight of the electrophoretic medium.

[0026] Application of a first electric field between the first and second light-transmitting electrode layers via the first waveform can cause the charged pigment particles to migrate toward the channel, resulting in switching of the variable light transmission device to an open optical state, where the charged pigment particles are located inside the channel.

[0027] Application of a second electric field between the first and second light-transmitting electrode layers via a second waveform can cause the charged pigment particles to move toward the first light-transmitting electrode layer at a velocity having a lateral component that leads to 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 or net negative impulse. The closed optical state has a lower percent light transmission than the open optical state.

[0028] In one embodiment, the second waveform may comprise an AC waveform, the AC waveform having a frequency and the AC waveform having a duty cycle of 5% to 45%. The AC waveform may have a duty cycle greater than 50%, greater than 55%, greater than 60%, or greater than 65%. The AC waveform may have a duty cycle of 55% to 95%, 58% to 90%, 60% to 88%, 65% to 85%, or 70% to 80%. The AC waveform may have a duty cycle less than 50%, less than 45%, less than 40%, or less than 35%. The AC waveform may have a duty cycle of 5% to 45%, 8% to 40%, 10% to 38%, 10% to 40%, 15% to 35%, or 20% to 30%. The AC waveform may be a square waveform, a sinusoidal waveform, a triangular waveform, or a sawtooth waveform. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge control agent in the electrophoretic medium, expressed as a weight percent of the charge control agent by weight of the electrophoretic medium, can be 400 to 2,000 Hz. The AC waveform can be a square waveform having two or more cycles. In this case, the positive and negative voltages of the AC waveform have the same amplitude, the amplitude of the AC waveform can be 10 V to 200 V, the frequency of the AC waveform can be 0.1 Hz to 6,000 Hz or 100 Hz to 3,000 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 6,000 Hz or 100 Hz to 3,000 Hz. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge control agent in the electrophoretic medium, expressed as a weight percent of the charge control agent by weight of the electrophoretic medium, can be 400 to 2,000 Hz.

[0029] In another embodiment, the second waveform may comprise a waveform formed by superimposing a DC voltage component and an AC waveform, the AC waveform having a frequency and amplitude. The frequency of the AC waveform may be 0.1 Hz to 6,000 Hz, 100 Hz to 3,000 Hz, or 400 Hz to 2,000 Hz. The amplitude of the AC waveform may be 10 V to 200 V or 20 V to 180 V. The DC voltage component has an amplitude of 0.1 V to 500 V. The ratio of the frequency of the AC waveform, expressed in Hz, to the content of the charge control agent in the electrophoretic medium, expressed as a weight percent of the charge control agent by weight of the electrophoretic medium, may be 400 to 2,000 Hz. The second waveform may comprise an AC waveform with a DC offset. The AC waveform may be selected from the group consisting of a square waveform, a sinusoidal waveform, a triangular waveform, and a sawtooth waveform.

[0030] The protruding structure is (a) a cone, (b) a cone on a cylinder, where the cylinder has a base and the base of the cylinder is the protruding base of the protruding structure, (c) a tetrahedron, (d) a tetrahedron on a triangular prism, where the triangular prism has a triangular base and the triangular base is the protruding base of the protruding structure, (e) a triangular prism, where the triangular prism has a square base and the square base is the protruding base of the protruding structure, (f) a square pyramid with a square base and the square base is the protruding base of the protruding structure, (h) a square pyramid on a cube, where the cube has a base and the cube base is the protruding base of the protruding structure, and (i) a square pyramid on a rectangular parallelepiped. (j) a square pyramid on a rectangular parallelepiped, where the rectangular parallelepiped has a right-angled parallelogram base, and the right-angled parallelogram is the protruding base of the protruding structure; (k) a pentagonal pyramid on a pentagonal prism, where the pentagonal prism has a pentagonal base, and the pentagonal base is the protruding base of the protruding structure; (l) a hexagonal pyramid on a hexagonal prism, where the hexagonal pyramid has a hexagonal base, and the pentagonal base is the protruding base of the protruding structure; and (m) a hexagonal pyramid on a hexagonal prism, where the hexagonal prism has a hexagonal base, and the hexagonal base is the protruding base of the protruding structure. The protruding structure may be a cone, and the inclination of the cone may be 5 to 10 degrees. The protruding structure may be a cone on a cylinder. The cylinder may have a base, and the base of the cylinder is the protruding base of the protruding structure, and the inclination of the cone may be 10 degrees or less. The protruding structure may be a geometric solid of a pyramid having a base with n sides, and the base with n sides is the protruding base of the protruding structure, and n is an integer from 7 to 12; (m) a pyramid having a base with n sides on a prism has a base with n sides, and the base of the prism having n sides is the protruding base of the protruding structure, and n is an integer from 7 to 12.

[0031] The electrophoretic medium may comprise a first type of charged pigment particles and a second type of charged pigment particles. The first type of charged pigment particles may be light reflective. The second type of charged pigment particles may be light absorbing. 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.

[0032] The first light-blocking layer may comprise light-absorbing pigment particles and a polymer. The light-absorbing pigment particles may be charged. The light-absorbing pigment particles may be black. The first light-blocking layer may comprise light-reflecting pigment particles and a polymer. The light-reflecting pigment particles may be charged. The light-reflecting pigment particles may be white.

[0033] The first light-blocking layer can be formed by curing a first light-blocking composition, which can comprise a light-absorbing pigment, a polymer, an oligomer, or a monomer, and optionally a solvent. Curing of the first light-blocking composition can be achieved thermally, by UV irradiation, by solvent evaporation, or a combination thereof.

[0034] The variable light transmission device of the present invention may include a second light-blocking layer. The second light-blocking layer may be disposed between the upper surface of the microcell wall and the sealing layer. The second light-blocking layer may be conductive. The second light-blocking layer may include light-absorbing pigment particles and a polymer. The second light-blocking layer may include light-reflecting pigment particles and a polymer.

[0035] The second light-blocking layer can be formed by coating the upper surface of the microcell walls with a dispersion composition and curing the coated dispersion composition thermally, by UV irradiation, by solvent evaporation, or a combination thereof. The dispersion composition can comprise a light-absorbing pigment, a polymer, an oligomer, or a monomer, and, optionally, a solvent.

[0036] The variable light transmission device of the present invention may further include an auxiliary layer disposed between the second light-blocking layer and the sealing layer. The auxiliary layer may be an adhesive layer. The auxiliary layer may include light-reflecting pigment particles. The auxiliary layer may include an encapsulated electrophoretic layer comprising an electrophoretic medium including charged pigment particles, and upon application of an electric field across the encapsulated electrophoretic layer of the auxiliary layer, color or image switching of the auxiliary layer may be achieved.

[0037] In another aspect, the present invention provides a method for manufacturing a variable light transmission device, the method comprising the steps of: (a) providing an assembly comprising a third electrode layer, a second light-transmitting electrode layer, and a layer comprising a plurality of microcells, the layer comprising a plurality of microcells disposed between the third electrode layer and the second light-transmitting electrode layer, each microcell of the plurality of microcells comprising a light-blocking composition comprising (i) light-absorbing charged pigment particles, (ii) a polymer, oligomer, or monomer, and (iii) optionally a solvent, each microcell of the plurality of microcells having a microcell opening, the third electrode layer spanning the microcell openings of the plurality of microcells, each microcell of the plurality of microcells comprising a microcell bottom layer, a protrusion structure, a microcell wall, and a channel, the microcell bottom layer having a microcell bottom inner surface, (b) applying an electric field between the third electrode layer and the second light-transmitting electrode layer via a waveform that causes migration of light-absorbing, charged pigment particles of the light-blocking composition toward the channel, resulting in a state in which the light-absorbing, charged pigment particles are located inside the channel, (c) curing the light-blocking composition to form a light-blocking layer on the exposed microcell bottom inner surface, (d) removing the third electrode layer, (e) filling each microcell of the plurality of microcells with an electrophoretic medium comprising charged pigment particles, a charge control agent, and a non-polar liquid, (f) sealing each microcell of the plurality of microcells with a sealing layer, and (g) attaching the first light-transmitting electrode layer onto the sealing layer. If the light-blocking composition comprises a solvent, the solvent is evaporated during curing of the first dispersion composition. Curing of the light blocking composition can be achieved by UV radiation, thermally, or by solvent evaporation.

[0038] In yet another aspect, the present invention provides a method for manufacturing a variable optical transmission device. the protrusion structure has a protrusion bottom surface, a protrusion surface, a protrusion apex, and a protrusion height, the protrusion apex being a point or set of points on the protrusion structure, the point or set of points having a shorter distance from the microcell opening than any other point on the protrusion structure; the protrusion height being the distance between the protrusion bottom surface and the protrusion apex; the protrusion surface being a surface of the protrusion structure excluding the protrusion apex and the protrusion bottom surface; the microcell wall being a surface of the protrusion structure excluding the protrusion apex and the protrusion bottom surface; (b) dispensing a light-blocking composition onto the exposed microcell bottom inner surface of each microcell, the light-blocking composition comprising: (i) light-absorbing pigment particles; (ii) a polymer, oligomer, or monomer; and (iii) optionally, a solvent; (c) curing the light-blocking composition to form a light-blocking layer on the exposed microcell bottom inner surface; (d) filling each microcell of the plurality of microcells with an electrophoretic medium comprising charged pigment particles, a charge control agent, and a non-polar liquid; (e) sealing each microcell of the plurality of microcells with a sealing layer; and (f) attaching a first light-transmitting electrode layer onto the sealing layer. If the light-blocking composition comprises a solvent, the solvent is evaporated during curing of the second dispersion composition. Curing of the light-blocking composition can be accomplished by UV irradiation, thermally, or by solvent evaporation. [Brief explanation of the drawings]

[0039] [Figure 1] FIG. 1 is an illustration of a cylindrical particle in a liquid under the influence of an applied electric field and the resulting force on the particle.

[0040] [Figure 2-1] 2A, 2B, 2C and 2D show side views of some example variable optical transmission devices of the present invention. [Figure 2-2] 2A, 2B, 2C and 2D show side views of some example variable optical transmission devices of the present invention.

[0041] [Figure 3] FIG. 3 illustrates a side view of a microcell in an open optical state and a side view of a microcell in a closed optical state.

[0042] [Figure 4] FIG. 4 is an example of a first embodiment of the present invention, which is a DC unbalanced waveform that may be applied to a variable optical transmission device to achieve a closed state, the waveform including an AC waveform having a duty cycle greater than 50%.

[0043] [Figure 5] FIG. 5 is an example of a second embodiment of the present invention, which is a DC unbalanced waveform that may be applied to a variable optical transmission device to achieve a closed state, where the waveform is a superposition of a DC voltage component and an AC waveform.

[0044] [Figure 6] FIG. 6 illustrates the force exerted by charged pigment particles on the surface of a conical protrusion of a variable light transmission device of the present invention.

[0045] [Figure 7]FIG. 7 illustrates a portion of a variable light transmission device having an electrophoretic medium comprising charged pigment particles of a first type and charged pigment particles of a second type, the charged pigment particles of the first type having the same polarity as the charged pigment particles of the second type.

[0046] [Figure 8] FIG. 8 illustrates a portion of a variable light transmission device having an electrophoretic medium comprising a first type of charged pigment particles and a second type of charged pigment particles, the first type of charged pigment particles having an opposite polarity to the second type of charged pigment particles.

[0047] [Figure 9] FIG. 9 provides a graph of light reflection, transmission, and absorptance versus layer thickness for a layer comprising light absorbing pigment particles.

[0048] [Figure 10] FIG. 10 provides a graph of light reflection, transmission, and absorption versus layer thickness for a layer comprising light-reflecting pigment particles.

[0049] [Figure 11] FIG. 11 shows a graph providing the effect of reflection, transmission, and absorptance on layer thickness for a closed optical state layer comprising a combination of light reflecting and light absorbing pigment particles.

[0050] [Figure 12] FIG. 12 illustrates a portion of a variable light transmission device with a light blocking layer on the exposed bottom interior surface of the microcell.

[0051] [Figure 13] FIG. 13 illustrates a portion of a variable light transmission device with a light blocking layer on the upper surface of the microcell walls.

[0052] [Figure 14] FIG. 14 shows a plan view of the microcell of the variable transmission device used in the examples.

[0053] [Figure 15] FIG. 15 shows a cross-sectional view of the microcell of the variable transmission device used in the examples.

[0054] [Figure 16] FIG. 16 provides micrographs of the open and closed optical states of the variable optical transmission device of Example 1, the optical states resulting from the various waveforms.

[0055] [Figure 17] FIG. 17 provides photomicrographs of the open and closed optical states of the variable light transmission device of Example 2, where the electrophoretic medium of the device comprises different concentrations of charge control agent.

[0056] [Figure 18] FIG. 18 is a photomicrograph of the microcell array of the variable light transmission device of Example 5, in which the light-blocking composition comprises black pigment particles that are dispersed throughout the microcells (closed optical state).

[0057] [Figure 19] FIG. 19 is a photomicrograph of the microcell array of the variable light transmission device from Example 5, with the black pigment particles of the light-blocking composition driven into the channels of the microcells (closed optical state).

[0058] [Figure 20] FIG. 20 is a photomicrograph of the closed optical state of the microcell array of the variable light transmission device of Example 6, in which the white charged pigment particles of the light-blocking composition are dispersed throughout the microcells (closed optical state).

[0059] [Figure 21] FIG. 21 is a photomicrograph of the open optical state of the microcell array of the variable light transmission device of Example 6, in which the white charged pigment particles of the light-blocking composition are driven into the channels (open optical state).

[0060] [Figure 22] FIG. 22 is a photomicrograph of the open optical state of the microcell array of the variable optical transmission device of Example 7, in which the electrophoretic medium of the device comprises white and black pigment particles. DETAILED DESCRIPTION OF THE INVENTION

[0061] The distance of a point from 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 a perpendicular line that is parallel to the normal vector from the given point to the given plane.

[0062] The distance between two planes in three-dimensional space is the shortest distance between the planes: it is the shortest distance between any point on one plane and any point on the other plane.

[0063] The slope of a cone is defined as the angle between (a) the vertex (A) on the circumference of the base of the cone, (b) a first line segment that is the line connecting point A (vertex) to the center of the base of the cone, and (c) a second line segment that is the line connecting point A (vertex) to the apex of the cone.

[0064] The term "charged pigment particles" can refer to charged pigment particles that do not have any polymeric material on the surface of the pigment particle. The term "charged pigment particles" can also refer to pigment particles that have polymeric material on the surface of the pigment particle.

[0065] The "inner wall surface of the microcell" is the surface of the microcell wall that contacts the electrophoretic medium of the microcell.

[0066] The "microcell wall upper surface" is the surface of the microcell wall that contacts the sealing layer of the microcell. When a light-blocking layer is on the microcell wall upper surface, the light-blocking layer is disposed between the microcell wall upper surface and the sealing layer.

[0067] The term "DC balanced waveform" or "DC balanced drive waveform" refers to a drive waveform applied to a pixel in which the drive voltage applied to the pixel, integrated over the entire application period of the waveform, is substantially zero. DC balance can be achieved by balancing each stage of the waveform; i.e., the first positive voltage would be selected so that the integration over the subsequent negative voltage results in zero or substantially zero. If a waveform is not DC balanced, it is referred to as a "DC imbalanced waveform" or "DC imbalanced drive waveform." The drive waveform applied to a pixel can have a portion that is DC imbalanced and at least one additional pulse with an opposing impulse to ensure that the overall waveform applied to the pixel is DC balanced. This additional pulse can be applied before the DC imbalanced portion of the waveform (a leading pulse). Typical examples of DC imbalanced waveforms include (a) square or sinusoidal AC waveforms with a duty cycle of less than (or greater than) 50% and (b) square or sinusoidal AC waveforms with a DC offset.

[0068] The term "impulse" is the integral of voltage with respect to time. That is, for a waveform pulse with voltage V applied for time t, impulse is V × t. Impulse can be positive if the polarity of voltage V is positive, or negative if the polarity of voltage V is negative.

[0069] The term "net positive impulse" of the waveform means that the negatively charged pigment particles will be attracted to and move towards the first light-transmitting electrode layer during application of the waveform.

[0070] The term "lateral component of velocity" in relation to the movement of charged pigment particles within the microcell of the variable light transmission device of the present invention refers to the velocity in the horizontal direction. For this definition, we assume that the velocity of a charged particle is the vector resulting from the vector addition of its velocity in the horizontal direction (Vh) and its velocity in the vertical direction (Vv), with the vertical direction in the case of the movement of a charged pigment particle inside the electrophoretic microcell being 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 of the movement of a charged pigment particle inside the electrophoretic microcell is the direction from one side of the microcell wall to the other side of the microcell wall, which direction is parallel to the first light-transmitting electrode layer. Therefore, the statement "the velocity of a charged pigment particle has a lateral component" means that the magnitude of the velocity in the horizontal direction is greater than zero.

[0071] The phenomenon of induced charge electroosmosis (ICEO) can be used to induce the lateral movement of polarizable particles, such as pigment particles, within an electrophoretic medium. That is, the polarizable particles can move parallel to the electrode layers that sandwich the electrophoretic medium. In the presence of an electric field, the particles can experience a force caused by the polarization of the particle (or by the polarization of a conductive coating adsorbed on the particle surface or the electric double layer surrounding the particle). This force can cause a disturbance in the flow of mobile charges, such as ions or charged micelles, in the electrophoretic medium, as shown in Figure 1 for a cylindrical particle 101 surrounded by the liquid of the electrophoretic medium in an applied electric field. This figure is reproduced from the paper by Bazant and Squires (J. Fluid Mech., 2004, Vol. 509, pp. 217-252).

[0072] A perfectly symmetric spherical particle will experience no net force, while a less symmetric particle will experience a force with a component perpendicular to the applied field direction. The cooperative flow generated by a group of particles, each experiencing such a force, can lead to a "vortex" in the electrophoretic medium containing multiple particles. The maximum velocity of this vortex, u, for a particular particle will be approximately given by equation (1), according to theory proposed in a paper by Bazant and Squires:

number

number

[0073] In equation (2), λ D is the device cleaning length, R is the particle radius, and D is the diffusion constant of the charge carriers in the fluid.

[0074] According to equation (1), as the frequency is increased, ω 2 τ 2 As the value of increases, the maximum velocity of induced charge flow decreases. Furthermore, for values ​​of ω significantly greater than 1, 2 τ 2 For values ​​of ω, the maximum vortex velocity is proportional to the square of the ratio E / ω. Induced charge flow occurs in the same direction regardless of the polarity of the applied electric field and can therefore be driven by an AC field.

[0075] When the electrophoretic medium is contained in microcells, as is preferred in electrophoretic displays, the geometry of the induced flow is influenced by the shape of the particular microcell 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 roll structure with periodic spacing corresponding to the width of the gap between the electrodes.

[0076] The present inventors used complex microcell structures formed by an embossing method to create switchable devices. In one example, the embossed structure includes conical protrusions on the bottom of each microcell. Figures 2A, 2B, and 2C illustrate an example of a variable light transmission device according to the present invention, in which the protrusion structure is a cone on a cylinder. The cone of the protrusion structure can direct the electrophoretic flow of particles into the channel, as shown in Figures 2A, 2B, and 2C. Charged pigment particles will move toward the channel if the electric field applied across the electrophoretic medium has the appropriate polarity relative to the polarity of the charged pigment particles. For example, charged pigment particles will move toward the channel if the charged pigment particles are positively charged and a voltage applied through a light-transmitting electrode results in a negative polarity on the second light-transmitting electrode. The same movement would occur if the charged pigment particles were negatively charged and a voltage applied across one light-transmitting electrode resulted in a positive polarity on the second light-transmitting electrode. Figures 2A, 2B, and 2C illustrate a cross section of a portion of a variable light transmission device (not drawn to scale) showing only one microcell of the device. All three Figures 2A, 2B, and 2C are identical in terms of the device structure depicted, although different parts of the device are identified in each of the figures.

[0077] A portion of the variable light transmission device 200 in Figures 2A, 2B, and 2C may include a microcell layer including multiple microcells and a sealing layer. While only one microcell is depicted in Figures 2A, 2B, and 2C, an entire variable light transmission device including multiple microcells may be envisioned. The variable light transmission device may include a first transparent substrate 201, a first light-transmitting electrode layer 202, a microcell layer 203 including multiple microcells 204 and a sealing layer 206, a second light-transmitting electrode layer 207, and a second transparent substrate 208. Each microcell of the multiple microcells 204 includes an electrophoretic medium 209 including charged pigment particles and a charge control agent in a non-polar fluid. The components of the electrophoretic medium (charged pigment particles, charge control agent, non-polar fluid) are not shown in Figures 2A, 2B, and 2C. Each microcell of the plurality of microcells 204 has a microcell opening 205, and a sealing layer 206 spans the microcell openings 205 of the plurality of microcells 204. Each microcell of the plurality of microcells 204 includes a microcell bottom layer 210, a protrusion structure 217, a microcell wall 212, and a channel 215. The microcell bottom layer 210 includes a microcell inner bottom surface 211, which includes an exposed microcell inner bottom surface 211a and an unexposed microcell inner bottom surface 211b. The unexposed microcell inner bottom surface 211b is in contact with the protrusion bottom surface 218.

[0078] Each microcell (204) of the plurality of microcells includes a first light-blocking layer (232) that contacts the exposed microcell bottom inner surface (211a) and the electrophoretic medium (209).

[0079] In this example, the protrusion structure 217 is a cone on a cylinder. The protrusion structure 217 has a protrusion base 218, a protrusion surface 221, a protrusion apex 219, and a protrusion height 220. The protrusion apex 219 is a point or set of points on the protrusion structure 217 that is a shorter distance from the microcell opening 205 than any other point on the protrusion structure 217. In the example variable light transmission device of FIGS. 2A, 2B, and 2C, the protrusion apex 219 is the apex of the cone of the protrusion structure. The protrusion height 220 is the distance between the protrusion base 218 and the protrusion apex 219. If the protrusion structure 217 has a protrusion apex 219 with more than one point, such as a planar surface, the protrusion height 220 is the distance between the planar surface and the protrusion base 218 of the protrusion structure 217. A microcell layer comprising a plurality of microcells 204 with protruding structures 217 can be fabricated by embossing a thermoplastic or thermosetting precursor layer using a pre-patterned male mold and then releasing the mold. The precursor layer can be hardened by radiation, cooling, solvent evaporation, or other means during or after the embossing step.

[0080] The microcell wall 212 has an inner microcell wall surface 213 and an upper microcell wall surface 214. The inner microcell wall surface 213 contacts the electrophoretic medium 209. The upper microcell wall surface 214 is the surface of the microcell wall 212 of the microcell that contacts the sealing layer 206.

[0081] Channel 215 is the volume between first light-blocking layer 232, microcell inner wall surface 213, and protrusion surface 221. Channel 215 is the volume location where the majority of electrically charged particles reside in the open optical state of the device. Channel 215 has a channel height 216 that is 50% of protrusion height 220. Thus, the channel height, together with exposed microcell bottom inner surface 211a, microcell inner wall surface 213, and protrusion surface 221, further define the channel.

[0082] The variable light transmission device of the present invention may comprise a second light blocking layer 233 .

[0083] A second light-blocking layer 233 may be disposed between the microcell wall upper surface 214 and the sealing layer 206. An auxiliary layer 234 may be disposed between the second light-blocking layer 233 and the sealing layer 206, as shown in FIG. 2B.

[0084] FIG. 2D illustrates an example of a variable light transmission device according to the present invention, in which the protruding structure of the variable light transmission device is a cone on a cylinder. The variable light transmission device of FIG. 2D is similar to that illustrated by FIGS. 2A, 2B, and 2C, but shows a larger portion of a device including four microcells. The variable light transmission device 200 includes a first transparent substrate 201, a first light-transmitting electrode layer 202, a microcell layer 203 including a plurality of microcells 204 and a sealing layer 206, a second light-transmitting electrode layer 207, and a second transparent substrate 208. Each microcell of the plurality of microcells includes an electrophoretic medium including charged pigment particles 222 and a charge control agent in a nonpolar fluid. Each microcell of the plurality of microcells 204 has a microcell opening, and a sealing layer 206 spans the microcell openings of the plurality of microcells. Each microcell of the plurality of microcells comprises a microcell bottom layer 210, a protruding structure 217, a microcell wall 212, and a channel 215. The variable light transmission device is illustrated in Figure 2D in a closed optical state.

[0085] When a first electric field is applied between the first light-transmitting electrode layer 202 and the second light-transmitting electrode layer 207 via a first waveform, migration of charged pigment particles 222 toward the channel is induced when the polarity of the charged pigment particles 222 and the voltage polarity of the second light-transmitting electrode layer are opposite to each other. When the polarity of the charged pigment particles 222 and the voltage polarity of the second light-transmitting electrode layer are opposite to each other, the charged pigment particles 222 are attracted by the second light-transmitting electrode layer, and the variable light transmission device switches to an open optical state, which has a higher percent light transmission than the closed optical state. The open optical state is illustrated in FIG. 3a, where the charged pigment particles 222 are represented by solid circles. In this example, the electrophoretic medium comprises one type of charged pigment particles 222.

[0086] Application of a second electric field between the first light-transmitting electrode layer 202 and the second light-transmitting electrode layer 207 via a second waveform causes the charged pigment particles 222 to move with a velocity toward the first light-transmitting electrode layer 202. This leads to the closed optical state, which is shown in FIG. 3b. The velocity has a lateral component. Without the lateral component of the velocity, the closed optical state would not occur. This is because, although the charged pigment particles 222 will migrate from the open channels towards the first light-transmitting electrode layer 202, these charged pigment particles 222 will occupy an area near the periphery of the microcells in the vicinity of the sealing layer 206. That is, the charged pigment particles 222 will not be dispersed over the entire surface of the first light-transmitting electrode layer 202. Therefore, a closed optical state will not actually be formed because the closed optical state will have a relatively high optical transmission.

[0087] The above shows that it is somewhat easier to achieve the transition from the closed optical state to the open optical state, since the inclination of the protrusion structure (e.g., the cone in Figures 3a and 3b) will impart a lateral component to the velocity of the charged pigment particles when they hit the protrusion surface of the protrusion structure during their movement towards the second light-transmitting electrode layer.

[0088] It is possible to shape the electric field within a variable light transmission device by making the electrical conductivities of the electrophoretic medium and the cone substantially different from one another. For example, if the cone is much less conductive than the electrophoretic medium, the field lines will serve to direct the charged pigment particles into the channel. However, even in such cases, it may still be necessary to provide a larger, substantial horizontal force component to redisperse the charged pigment particles from the channel. In addition, the current state of the art requires that the resistances of the cone material and the electrophoretic medium are approximately equal, each about 10 10 It is easier to assemble and operate the device when the electrical field is approximately Ω*cm, in which case the electric field lines will be nearly vertical through the unit. It would therefore be preferable to use a waveform that imparts lateral motion to the charged pigment particles.

[0089] A method of operating a variable light transmission device includes applying a first electric field between a first light-transmitting electrode layer and a second light-transmitting electrode layer via a first waveform, causing the charged pigment particles to move toward the channel, resulting in switching of the variable light transmission device to an open optical state, where the charged pigment particles in the open optical state are located inside the channel. The method of operating a variable light transmission device also includes applying a second electric field between the first light-transmitting electrode layer and the second light-transmitting electrode layer via a second waveform, causing the charged pigment particles to move toward the first light-transmitting electrode layer with a velocity, where the velocity has a lateral component, leading to a closed optical state, where the second waveform comprises a series of at least two positive and negative pulses having a net positive or net negative impulse, where the closed optical state has a lower percent light transmission than the open optical state.

[0090] The second waveform may be DC unbalanced. The second waveform may comprise at least one positive voltage and at least one negative voltage, with the second waveform having a net positive or net negative impulse. The choice of net positive or net negative impulse depends on the polarity of the charged pigment particles. Specifically, if the charged pigment particles are negatively charged, a net positive impulse is required to move the particles from the channel toward the first light-transmitting electrode layer. In other words, this movement requires that the net result of the applied voltages is the attraction of the negatively charged particles by the positive voltage of the first light-transmitting electrode layer relative to the second light-transmitting electrode layer. In contrast, if the charged pigment particles are positively charged, a net negative impulse is required to move the charged pigment particles from the channel near the second light-transmitting electrode layer 207 toward the first light-transmitting electrode layer.

[0091] A second electric field applied between the two light-transmitting electrode layers via a second waveform achieves the closed optical state.

[0092] In a first embodiment, the second waveform comprises an AC waveform having a duty cycle different from 50%. An example of the second waveform of the first embodiment is illustrated in FIG.

[0093] The AC 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 for a positive DC biased waveform is greater than 50%. The duty cycle for a positive DC biased waveform can be greater than 55%, greater than 60%, or greater than 65%. The duty cycle for a positive DC biased waveform can be 55%-95%, 58%-90%, 60%-88%, 65%-85%, or 70%-80%. Similarly, the duty cycle for a negative DC biased waveform is less than 50%. The duty cycle for a negative DC biased waveform can be less than 45%, less than 40%, or less than 35%. The duty cycle for a negative DC biased waveform can be 5%-45%, 8%-40%, 10%-38%, 15%-35%, or 20%-30%.

[0094] The waveform illustrated in the example of FIG. 4 comprises an AC square waveform having two or more cycles. Each cycle may comprise a first pulse of amplitude V1 applied for a period t1 and a second pulse of amplitude V2 applied for a period t2, where V1 is positive and V2 is negative, and t1 is longer than t2. When the amplitude of V1 is equal to the amplitude of V2 (|V1| = |V2|), a DC bias is achieved by the difference in the periods. In the example of FIG. 4, a positive DC bias exists because the positive voltage V1 is applied for a longer period (t1) than the negative voltage V2 (t2). A 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 toward the device's first light-transmitting electrode layer. The duty cycle of the waveform can be calculated by equation (3): Duty cycle = 100 × (V1·t1) / [(V1·t2)+((V2·t2)] Equation (3)

[0095] In the example waveform of FIG. 4, the amplitude of V1 can be equal to the amplitude V2 (|V1|=|V2|), although in general, the amplitudes V1 and V2 can differ from each other.

[0096] The example drive waveform of Figure 4 is DC unbalanced. However, one or more additional pulses may be included in the waveform of Figure 4 of opposite impulse to ensure that the overall waveform applied on the pixel is DC balanced. This additional pulse (or multiple additional pulses) may be applied before the DC unbalanced waveform (preceding pulse). The example waveform of Figure 4 is also a square AC waveform. Other examples of AC waveforms that may be used include sinusoidal waveforms, triangular waveforms, and sawtooth waveforms.

[0097] The AC waveform of the first embodiment may have an amplitude of 10 V to 200 V and a frequency of 0.1 to 6,000 Hz. The AC waveform may have an amplitude of 15 V to 180 V, 20 V to 160 V, 25 V to 150 V, or 30 V to 140 V. The AC waveform may have a frequency of 0.5 Hz to 5,000 Hz, 1 Hz to 4,000 Hz, 5 Hz to 3,000 Hz, 10 Hz to 2,000 Hz, 15 Hz to 1,000 Hz, 20 Hz to 800 Hz, or 25 to 600 Hz. The ratio of the frequency of the AC waveform to the weight percent content of the charge control agent by weight of the electrophoretic medium may be 400 Hz to 2,000 Hz.

[0098] In a second embodiment, the second waveform may comprise a waveform formed by the superposition of a DC voltage component and an AC waveform. An example of the second waveform of the second embodiment is illustrated in FIG.

[0099] The waveform in Figure 5 has a net negative impulse due to the DC offset (Vd). The period of application of the positive pulse (t3) is equal to the period of application of the negative pulse (t4), but the DC bias is achieved by the difference in the amplitudes of the pulses. Specifically, the amplitude V3 of the positive pulse is smaller than the amplitude V4 of the negative pulse. This is caused by the DC voltage component Vd of the waveform. That is, the waveform illustrated in Figure 5 has a DC offset.

[0100] The example drive waveform of Figure 5 is DC unbalanced. However, one or more additional pulses can be included in the waveform of Figure 5 of opposite impulse, which can ensure that the overall waveform applied on the pixel is DC balanced. This additional pulse (or multiple additional pulses) can be applied before the DC unbalanced waveform (preceding pulse). The example waveform of Figure 5 is also a square AC waveform. Other examples of AC waveforms that can be used include sinusoidal waveforms, triangular waveforms, and sawtooth waveforms.

[0101] The AC waveform may have an amplitude of 10V to 200V and a frequency of 0.1 to 6,000 Hz. The AC waveform may have an amplitude of 15V to 180V, 20V to 160V, 25V to 150V, or 30V to 140V. The AC waveform may have a frequency of 0.5Hz to 5,000Hz, 1Hz to 4,000Hz, 5Hz to 3,000Hz, 10Hz to 2,000Hz, 15Hz to 1,000Hz, 20Hz to 800Hz, or 25Hz to 600Hz. The ratio of the frequency of the AC waveform to the weight percent content of the charge control agent by weight of the electrophoretic medium may be 400Hz to 2,000Hz.

[0102] If the ICEO-induced motion of the charged pigment particles is relatively low, the protruding structure of the microcell contributes to the effective operation of the variable transmission device even when the device is driven using a DC-balanced AC waveform. In an example where the protruding structure is a cone, any charged pigment particles located on the surface of the cone will experience a net force that will move them toward the apex of the cone, as shown in Figure 6. Figure 6 shows a charged pigment particle 222 in contact with a protruding structure 617 (a cone) within an electric field 602. In this case, the ICEO flow, illustrated by the curved arrows, is more constrained on the "uphill" side of the cone than on the "downhill" side. This imparts a force to the particle, indicated by the dotted horizontal arrow. There is an opposing force perpendicular to the cone, which will urge the particle toward the apex of the cone. By using an appropriate choice of AC field and frequency, particles can thus be moved out of the channel region and up the side of the cone.

[0103] The electrophoretic medium of the variable light transmission device of the present invention comprises charged pigment particles, a charge control agent, and a non-polar fluid.

[0104] The electrophoretic medium may comprise two types of charged pigment particles: a first type of charged pigment particles and a second type of charged pigment particles. The first type of charged pigment particles may be light-reflective, and the second type of charged pigment particles may be light-absorbing. A typical example of light-reflecting pigment particles is titanium dioxide, which has a white color. Typical examples of light-absorbing pigment particles include organic and inorganic pigment particles having black, blue, cyan, magenta, red, green, yellow, and other colors. The first type of charged pigment particles may have the same polarity as the second type of charged pigment particles. The first type of charged pigment particles may have the opposite polarity to the second type of charged pigment particles.

[0105] In an electrophoretic medium having a first type of charged pigment particles and a second type of charged pigment particles with the same charge polarity, the zeta potential of the first type of charged pigment particles may be lower than that of the second type of charged pigment particles. Furthermore, 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, as determined by the average diameter of the pigment particles. The open optical state of this example of a variable light transmission device is illustrated in FIG. 7a. In this example, upon application of 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 migrate into the channel of the microcell, forming the open state. However, the second type of charged pigment particles 222b (light-absorbing) will be located below the first type of charged pigment particles 222a (light-reflective) because the first type of charged pigment particles 222a have a lower charge (and larger size). In other words, the second type of charged pigment particles 222b will be located closer to the first light-blocking layer (the bottom of the channel) than the first type of charged pigment particles 222a. The closed optical state of this example of a variable light transmission device is illustrated in FIG. 7b. Upon application of 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 and 222b will migrate toward the first light-transmitting electrode layer, achieving the closed optical state. However, the second type of charged pigment particles 222b (light absorbing) will be positioned closer to the sealing layer 206 than the first type of charged pigment particles 222a (light reflective) because the first type of charged pigment particles 222a have a lower charge (and larger size).

[0106] In another example, a variable light transmission device has an electrophoretic medium including a first type of charged pigment particles (light-reflective) and a second type of charged pigment particles (light-absorbing), where the first type of charged pigment particles and the second type of charged pigment particles have opposite charge polarities. Two possible open optical states of this example are illustrated in Figures 8a and 8b, respectively. Upon application of an appropriate electric field between the first light-transmitting electrode layer 202 and the second light-transmitting electrode layer 207, either the first charged pigment particles 223a (light-reflective) or the second charged pigment particles 223b (light-absorbing), depending on the polarity of the applied electric field, will migrate toward the channel (open optical state). The closed optical state can be formed by application of a suitable electric field (second electric field) between the first light-transmitting electrode layer 202 and the second light-transmitting electrode layer 207 that causes one type of charged pigment particles to move towards the first light-transmitting electrode layer 202 with a velocity, the velocity having a lateral component. Depending on the electric field applied, either the first type of charged pigment particles 223 a or the second type of charged pigment particles 223 b will disperse in the microcell area (microcell opening) near the sealing layer 206.

[0107] The amount of the second type of charged pigment particles, which may have a black color, will be chosen to be sufficient to hide the white pigment in the channel (open optical state) when viewed from below, but not so high as to lead to excessive light absorption in the closed state. By "viewed from below" we mean that the observer is positioned on the side of the variable light transmission device closer to the second light transmissive electrode layer 207, as opposed to the side closer to the first light transmissive electrode layer 202.

[0108] The open optical state of a variable transmission device desirably has high transmittance and low haze. Furthermore, in certain applications, such as building windows or vehicle sunroofs, it is desirable to manage heat buildup in the building or vehicle. Thermal management is difficult when the variable transmission device includes an electrophoretic medium containing light-absorbing, charged pigment particles. In the closed optical state of such a device, light incident on the device is absorbed by the device, which can cause the device to heat up. The incident light can include wavelengths within the solar thermal spectrum, i.e., ultraviolet, visible, and infrared. Another problem can be that the closed optical state is not completely opaque; that is, some of the incident light penetrates into the building or vehicle, warming the interior of the building or vehicle.

[0109] 9 provides a graph of light reflection, transmission, and absorption versus layer thickness for a layer comprising a black pigment (light absorbing). That is, for each layer thickness, the graph provides the amount of light reflected, transmitted, and absorbed as a percentage of the incident light.

[0110] 10 provides a graph of light reflection, transmission, and absorption versus layer thickness for a layer comprising a white pigment (light-reflective). That is, for each layer thickness, the graph provides the amount of light reflected, transmitted, and absorbed as a percentage of the incident light.

[0111] 9 and 10 show that white pigments reflect light significantly more than black pigments, but do not absorb incident light. In fact, white pigments provide opacity by light reflection / scattering. As the layer thickness increases, more incident light is reflected and little is transmitted. However, it is observed that variable light transmission devices comprising a medium with white pigment particles (reflective) can exhibit significant haze in the open optical state.

[0112] The present inventors have discovered that a variable light transmission device comprising an electrophoretic medium having charged particles comprising a reflective pigment (first type) and charged particles comprising an absorptive pigment (second type) provides significant benefits, such as reduced haze. The reflective pigment may be titanium dioxide and the absorptive pigment may be an inorganic black, such as iron oxide black.

[0113] Figure 11 shows the effect of reflection, transmission, and absorptance on layer thickness for a closed optical state layer comprising a combination of white pigment (light-reflecting) and black pigment (absorbing). The weight ratio of black pigment to white pigment is 0.1. That is, for each layer thickness, the graph provides the amount of light reflected, transmitted, and absorbed as a percentage of the incident light.

[0114] The rate of decrease in transmittance with increasing layer thickness remains nearly constant, but the reflection from the white pigment is capped at 30%. The absorption rate increases with layer thickness, but is only about half that rate compared to the pure black pigment layer shown in Figure 9. Absorption with little of the undesirable thermal effect could be achieved by layering the white pigment in front of the black pigment instead of mixing the two.

[0115] In the example of a variable transmission device having an electrophoretic medium containing a first type of charged pigment particles (white or reflective and having a negative charge polarity) and a second type of charged pigment particles (black or absorptive and having a positive 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 particle's polarizability and size determine the frequency required for optimal motion. The maximum ICEO velocity for these two types of oppositely charged pigment particles can be achieved by using an electric field with AC waveforms of different frequencies. In Example 7 below, which relates to this scenario, the characteristic AC waveform frequency is much higher for the white pigment than for the black pigment. Therefore, the black pigment can be switched into a channel using a relatively low AC frequency with an appropriate superimposed DC offset to move the black pigment in the channel of a microcell, as illustrated in the figure. Because the black pigment is positively charged, applying a positive offset voltage to the AC voltage on the first light-transmitting electrode layer while the second light-transmitting electrode layer remains grounded will cause the black pigment to migrate into the channel. The AC frequency is relatively low (10 Hz), and at this frequency, both the white and black pigments have strong ICEO-induced lateral motion. Once the black pigment is located in the channel, the AC frequency is increased to a higher value. At high frequencies, the ICEO-induced motion of the black pigment is reduced, while that of the white pigment is maintained. Therefore, the white pigment can be switched into the channel by applying a negative DC offset to the AC waveform. Both white and black particles are present in the channel, as shown in Figure 7a. Under certain conditions, the electric field of the second step may drive some of the black pigment particles out of the channel, but it will not cause the black pigment to migrate laterally. Therefore, the black particles that can migrate out of the channel will undergo vertical motion similar to that shown in Figure 8a, providing an open optical state.

[0116] Another method for reducing haze in a variable light transmission device caused by the light scattering effect of charged pigment particles is shown in FIG. 12. FIG. 12 illustrates a side view of a variable light transmission device of the present invention comprising a first light-transmitting electrode layer 202, a second light-transmitting electrode layer, and an electrophoretic medium containing charged pigment particles 222, where the charged pigment particles are light-reflective. The variable light transmission device also comprises a first light-blocking layer 232 on the exposed bottom inner surface of the microcell. The first light-blocking layer 232 may comprise black pigment particles that are light-absorbing. In the open optical state of the device, the charged pigment particles of the electrophoretic medium are contained in the channels of the microcells (FIG. 12a). The first light-blocking layer 232 blocks haze when the device is viewed from the bottom. By "viewed from the bottom," we mean that the observer views the device from the side adjacent to the second light-transmitting electrode layer 207. However, the haze is still evident when the device is viewed from the top. By "viewed from the top" we mean that the viewer views the device from the side near the first light-transmitting electrode layer 202. Figure 12b illustrates the closed optical state of the device. The variable light transmission device shown in Figure 12 can be used for applications where the viewer is generally positioned to one side of the variable light transmission device, such as, for example, a sunroof.

[0117] A method of manufacturing the variable light transmission device shown in FIG. 12 includes the steps of: (a) providing an assembly comprising a third electrode layer, a second light-transmitting electrode layer, and a layer comprising a plurality of microcells, the layer comprising a plurality of microcells disposed between the third electrode layer and the second light-transmitting electrode layer, each microcell of the plurality of microcells comprising a light-blocking composition, the light-blocking composition comprising (i) light-absorbing charged pigment particles, (ii) a polymer, oligomer, or monomer, and (iii) optionally a solvent, each microcell of the plurality of microcells having a microcell opening, the third electrode layer spanning the microcell openings of the plurality of microcells, each microcell of the plurality of microcells comprising a microcell bottom layer, a protrusion structure, a microcell wall, and a channel, the microcell bottom layer having a microcell bottom inner surface. wherein the microcell bottom inner surfaces comprise an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface; (b) applying an electric field between the third electrode layer and the second light-transmitting electrode layer via a waveform that causes migration of light-absorbing, charged pigment particles of the light-blocking composition toward the channel, resulting in a state where the light-absorbing, charged pigment particles are located inside the channel; (c) curing the light-blocking composition to form a light-blocking layer on the exposed microcell bottom inner surface; (d) removing the third electrode layer; (e) filling each microcell of the plurality of microcells with an electrophoretic medium comprising charged pigment particles, a charge control agent, and a non-polar fluid; (f) sealing each microcell of the plurality of microcells with a sealing layer; and (g) attaching the first light-transmitting electrode layer onto the sealing layer. If the light-blocking composition comprises a solvent, the solvent is evaporated during curing of the light-blocking composition. Curing of the light blocking composition can be achieved by UV radiation, thermally, or by solvent evaporation.

[0118] Another method of fabricating the device of FIG. 12 includes the steps of: (a) providing an assembly comprising, in order, a second light-transmitting electrode layer; a second light-transmitting electrode layer; and a layer comprising a plurality of microcells disposed on the second light-transmitting electrode layer, wherein each microcell of the plurality of microcells has a microcell opening, and each microcell of the plurality of microcells comprises a microcell bottom layer, a protrusion structure, a microcell wall, and a channel, and wherein the microcell bottom layer comprises a microcell bottom inner surface; the microcell bottom inner surface comprises an exposed microcell bottom inner surface and a non-exposed microcell bottom inner surface; the protrusion structure has a protrusion bottom surface, a protrusion surface, a protrusion apex, and a protrusion height, the protrusion apex being a point or set of points on the protrusion structure, the point or set of points having a shorter distance from the microcell opening than any other point on the protrusion structure; the protrusion height being the distance between the protrusion bottom surface and the protrusion apex; the protrusion surface being a surface of the protrusion structure excluding the protrusion apex and the protrusion bottom surface; the channel has an inner microcell wall surface and an upper microcell wall surface, the channel has a channel height, the channel height is 50% of the protrusion height, the unexposed inner microcell bottom surface is in contact with the bottom protrusion surface, and the channel is a volume between the exposed inner microcell bottom surface, the protrusion surface, and the inner microcell wall surface; and (b) dispensing a light-blocking composition onto the exposed inner microcell bottom surface of each microcell, the light-blocking composition comprising: (i) light-absorbing pigment particles; and (ii) a polymer, oligomer, or polymeric material. (iii) optionally, a solvent; (c) curing the light-blocking composition to form a light-blocking layer on the exposed bottom inner surface of the microcells; (d) filling each microcell of the plurality of microcells with an electrophoretic medium comprising charged pigment particles, a charge control agent, and a fluid; (e) sealing each microcell of the plurality of microcells with a sealing layer; and (f) attaching a first light-transmitting electrode layer onto the sealing layer. If the light-blocking composition comprises a solvent, the solvent is evaporated during curing of the second dispersion composition. Curing of the light-blocking composition can be achieved by UV irradiation, thermally, or by solvent evaporation.

[0119] Examples 5 and 6 describe experiments involving the formation of a barrier layer using a barrier composition. In Example 5, the barrier composition comprises black charged pigment particles, while in Example 6, the barrier composition comprises white charged pigment particles.

[0120] A variable light transmission device with improved performance in the closed optical state can also be achieved by using the device illustrated in FIG. 13. The variable light transmission device illustrated in FIG. 13 includes a first light-transmitting electrode layer (202), a second light-transmitting electrode layer (207), and a microcell layer (203). The microcell layer is disposed between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207). The microcell layer includes a plurality of microcells and a sealing layer. Each microcell of the plurality of microcells includes an electrophoretic medium, the electrophoretic medium comprising charged pigment particles and a charge control agent in a non-polar fluid. Each microcell of the plurality of microcells has a microcell opening, and the sealing layer spans the microcell openings of the plurality of microcells. Each microcell of the plurality of microcells includes a microcell bottom layer, a protrusion structure, a microcell wall, and a channel. The microcell bottom layer has a microcell bottom inner surface, which includes an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface. The variable light transmission device of FIG. 13 further includes a first light-blocking layer 232 disposed on the exposed microcell bottom inner surface layer. The protrusion structure has a protrusion bottom surface, a protrusion surface, a protrusion apex, and a protrusion height. The protrusion apex is a point or set of points on the protrusion structure that is closer to the microcell opening than any other point on the protrusion structure, and the protrusion height is the distance between the protrusion bottom surface and the protrusion apex. The protrusion surface is the surface of the protrusion structure that does not include the protrusion apex, which contacts the electrophoretic medium. The microcell wall has a microcell inner wall surface, a microcell wall upper surface, and a second light-blocking layer 233. The microcell inner wall surface is the surface of the microcell wall of the microcell that contacts the electrophoretic medium. The second light-blocking layer 233 is disposed between the upper surface of the microcell wall and the sealing layer. The channel has a channel height, which is 50% of the protrusion height. The unexposed microcell bottom inner surface contacts the protrusion bottom surface. The channel is the volume between the first light-blocking layer 232, the protrusion surface, and the microcell inner wall surface.

[0121] As explained above, the first light-blocking layer 232 reduces haze when the device is viewed from below. The second light-blocking layer 233 contributes to an improved closed state by increasing the opacity of the device, which can be caused by partially light-transmitting wall materials. The second light-blocking layer 233 can be black, white, or any other color. The second light-blocking layer 233 can be conductive, which can facilitate switching of the device. The second light-blocking layer (233) can be formed by coating a pigment dispersion on the upper surface of the microcell wall and curing the coating via UV irradiation, thermally, or solvent evaporation. The pigment dispersion can comprise pigment particles, polymers, oligomers, or monomers, and optionally, a solvent. The variable light transmission device of FIG. 13 can further comprise an auxiliary layer 234, which is disposed between the sealing layer and the second light-blocking layer 233. The auxiliary layer 234 can comprise an adhesive material. The auxiliary layer 234 can comprise a light-reflecting pigment to further improve the opacity of the closed optical state. Auxiliary layer 234 may also comprise an encapsulated electrophoretic layer comprising an electrophoretic medium including electrically charged pigment particles. Application of an electric field across the encapsulated electrophoretic layer of auxiliary layer 234 may switch the color (or image) of auxiliary layer 234, which may also affect the appearance of the variable light transmission device of FIG. 13.

[0122] Charge control agents are typically oligomeric or polymeric materials that are soluble in the non-polar fluid of the electrophoretic medium. Charge control agents are surfactant-type molecules with one or more polar functional groups (heads) and a non-polar portion (tail). The electrophoretic medium may comprise the charge control agent at a concentration of 0.1 to 10 weight percent by weight of the electrophoretic medium. The electrophoretic medium may comprise the charge control agent at a concentration of 0.5 to 9 weight percent, 0.7 to 8 weight percent, 1 to 7 weight percent, or 1 to 6 weight percent by weight of the electrophoretic medium.

[0123] The non-polar fluid of the electrophoretic medium may comprise an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, an aromatic hydrocarbon, a halogenated aliphatic hydrocarbon, a polydimethylsiloxane, or mixtures thereof.

[0124] The electrophoretic medium may also contain a flocculating agent, also called a depleting agent. The depleting agent induces an osmotic pressure difference between the pigment particles and between the pigment particles and the depleting agent molecules. As a result, the bistability of the optical state (open / closed) of the device is improved. The depleting agent is typically a polymer material such as polyisobutylene and polydimethylsiloxane.

[0125] (Example)

[0126] Example 1: A device was prepared by laminating together a sheet of polyethylene terephthalate (PET) coated with an indium tin oxide (ITO) transparent conductor to an embossed microcell array on a second sheet of PET / ITO containing the electrophoretic medium. The structure of the device corresponded to the illustration in Figures 2A-2D, except that a sealing layer 212 was not incorporated. The structure of the embossed microcell array is illustrated in Figure 14, which is a plan view of a microcell of the device. Figure 15 shows the corresponding cross-section of one microcell of the device. Table 1 shows the dimensions of the microcell.

[0127] [Table 1]

[0128] The electrophoretic medium comprised a white pigment, a hydrocarbon solvent, a charge control agent (CCA), and a depletion agent. In the example, an electrophoretic medium sample was prepared by mixing 10 wt. % white pigment and 5 wt. % charge control agent (the cationic charge control agent from Example 1—CCA111 in US 2020 / 0355978) in Isopar E solvent. The device was switched with a 50 V square-wave AC waveform using a 50% duty cycle. Three different pigment motion styles were observed by increasing the AC frequency from 0 Hz to 5,000 Hz, as shown in Figure 16. White pigment particles were prepared using a titanium dioxide pigment core and a polymer shell, as described in Example 1 of U.S. Patent No. 8,582,196.

[0129] At a low frequency of 10 Hz, after multiple switching, the white pigment migrated toward the edge of the microcell (near the periphery), likely by slight sliding along the cone's slope (Figure 16(a)). Once positioned within the channel, the pigment switched vertically up and down. At this low frequency of 10 Hz, the motion was generally dominated by electrophoresis. By increasing the frequency to 100 Hz, the pigment tended to disperse laterally into the region above the cone within the embossed microcell structure (Figure 16(b)). At a frequency of 1,000 Hz, the white pigment was completely dispersed within the circular region of the embossed microcell structure, as shown in Figure 16(c). At a frequency of 1,000 Hz, the pigment behavior was likely dominated by induced charge lateral motion, likely a result of ICEO. At a frequency of 5,000 Hz, the white pigment particles tended toward the center of the region near the cone of the embossed microcell structure, as shown in Figure 16(d).

[0130] Example 2: In Example 2, the effect of charge control agent (CCA) concentration on the movement of white pigment in an embossed microcell device was studied.

[0131] As described above, it is expected that increasing the concentration of charge control agent in the electrophoretic medium will decrease the Debye length associated with the charged pigment particle surface and therefore increase the frequency required for a particular ICEO flow. To test this hypothesis, three variable light transmission devices were prepared with similar electrophoretic media but using different charge control agent concentrations. The charge control agent used in this example is the cationic polymer (CCA111) disclosed in Example 1 of US 2020 / 0355978. Three different electrophoretic media were prepared with charge control agent concentrations of 0.1 wt%, 1 wt%, and 5 wt% by weight of the electrophoretic medium. Each electrophoretic medium also contained 10 wt% white pigment and Isopar E solvent. The white pigment particles were prepared using titanium dioxide pigment cores, which were coated with a polymer coating as described in Example 1 of U.S. Patent No. 8,582,196.

[0132] The switching performance of the three samples was evaluated in embossed microcell devices (represented by Figures 14 and 15). The waveform was a square wave ±50V AC with a 50% duty cycle. As shown in Figure 17, increasing the concentration of the charge control agent increased the frequency required to reach the closed optical state. That is, a frequency of 50 Hz was required to achieve the closed optical state for an electrophoretic medium containing 0.1 wt% CCA, a frequency of 100 Hz was required to achieve the closed optical state for an electrophoretic medium containing 1 wt% CCA, and a frequency of 1,000 Hz was required to achieve the closed optical state for an electrophoretic medium containing 5 wt% CCA. The ratios of frequency to CCA concentration for the three experiments were 500 Hz, 1,000 Hz, and 1,000 Hz, respectively.

[0133] A device with an electrophoretic medium comprising 1 wt. % CCA could be switched from the open optical state to the closed optical state using (a) a simple square wave AC of + / -100 V at 0.5 Hz, or (b) a square wave AC of + / -50 V with a 50 Hz frequency with a superimposed DC voltage of -50 V. The open optical state was reached by a square wave AC of + / -50 V with a 5% duty cycle, whereas the closed optical state required a square wave AC of + / -50 V with a 95% duty cycle. Switching times were approximately 1 second in both cases.

[0134] Example 3: The effect of the charge of the white pigment on the switching performance of the variable light transmission device was studied.

[0135] A variable optical transmission device was prepared having an electrophoretic medium comprising 10 wt. % positively charged white pigment particles in Isopar E. The white pigment was functionalized with 1.6 wt. % silane Z6030 and grafted with polylauryl methacrylate (PLMA). The zeta potential of the treated pigment was +35 mV as determined by titration with CCA (cationic polymer disclosed in Example 1—CCA111 in US2020 / 0355978 CCA111). The electrophoretic medium also contained 1 wt. % CCA (cationic polymer disclosed in Example 1—CCA111 in US2020 / 035597811). The waveform used to switch the device from an open optical state to a closed optical state was DC-superimposed AC, i.e., a square wave waveform with a 500 Hz frequency, + / - 50 V AC. The waveform for switching the device from the closed to the open optical state was + / - 50V AC with a +50V DC offset. The behavior of the device of Example 3 was therefore very similar to that of the device with the electrophoretic medium comprising 1 wt% CCA of Example 2, except that the DC offset required to achieve the open optical state was of the opposite polarity.

[0136] Example 4: In this example, the solvent of the electrophoretic medium matched the polymer that formed the embossed microcells.

[0137] "Haze" refers to the percentage of diffusely transmitted light compared to the total transmitted light. Diffusely transmitted light is light that is scattered as it is transmitted. To create a variable light transmission device with low haze, it was necessary to match the refractive index of the electrophoretic medium liquid solvent with the polymer material used to create the embossed microcells, as described in U.S. Pat. No. 7,327,511.

[0138] Typically, solvents used in electrophoretic media have a low dielectric constant (preferably less than 10, desirably less than 3), low viscosity, low water 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 distillates 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 combines a low dielectric constant (2.3) with a relatively high refractive index (1.47). The refractive index of the electrophoretic medium can be modified using the addition of refractive index matching chemicals. For example, the aforementioned U.S. Pat. No. 7,679,814 describes an electrophoretic medium suitable for use in a variable light transmission device in which the non-polar liquid of the electrophoretic medium comprises a mixture of partially hydrogenated aromatic hydrocarbons and terpenes, a preferred mixture being d-limonene and partially hydrogenated terphenyl, commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove NJ 07009.

[0139] To reduce haze, it is preferable to closely match the refractive index of the encapsulated electrophoretic medium to that of the encapsulating material. In most cases, it is beneficial to use an electrophoretic medium having a refractive index of 1.51 to 1.57 at 550 nm, preferably about 1.54 at 550 nm.

[0140] Example 4A: A variable optical transmission device comprising a microcell was prepared having an electrophoretic medium comprising 5 wt. % white pigment and 1 wt. % CCA (cationic polymer disclosed in Example 1—CCA111 in US 2020 / 035597811) in Cargille® 5040 and Isopar E solvents. The waveform used was a square wave AC with a 10 Hz frequency and an amplitude of + / −50 V. A 5% duty cycle was used to reach the closed optical state. A 95% duty cycle was initially employed to reach the open optical state, and then the duty cycle was changed to 50%.

[0141] Example 4B: Another variable optical transmission device comprising a microcell was prepared using an electrophoretic medium having a similar composition to Example 4A, further comprising polyisobutylene (PIB) as a depletion agent. The depletion agent is used to improve the bistability of the device, i.e., to ensure that the open and closed optical states are maintained when no electric field is applied across the device. The electrophoretic medium comprises 10 wt.% white pigment, 1 wt.% CCA (the cationic polymer disclosed in Example 1—CCA111 in US 2020 / 035597811), and 0.5 wt.% polyisobutylene in Cargille® 5040 and Isopar E solvents. The waveform used was a + / −50V square wave AC with a 10 Hz frequency. To reach the closed optical state, a 5% duty cycle alternating with a 50% duty cycle was used. To reach the open optical state, a 95% duty cycle was employed, followed by a 50% duty cycle. The time required for complete switching was approximately 20 seconds. This time is significantly longer than the time required to switch a non-index-matched solvent without any depletion agent (Example 4A).

[0142] Example 5: Light-blocking layer with black particles.

[0143] A variable transmission device was prepared by laminating an embossed microcell array containing an electrophoretic medium on a second sheet of PET / ITO, together with a sheet of PET coated with an ITO transparent conductor. The structure of the embossed microcell array is illustrated in Figures 2A-2D, except that the microcells in this example do not have a sealing layer.

[0144] The light-blocking composition used for the light-blocking layer comprised a black pigment, a solvent, a charge control agent, and a depletion agent. In this example, the light-blocking composition was prepared by mixing 10 wt. % black pigment, 1 wt. % CCA (the cationic charge control agent from Example 1—CCA111 in US 2020 / 0355978111 ), and 0.5 wt. % polyisobutylene in a solvent mixture of partially hydrogenated terphenyl, limonene, Isopar M, and Isopar E, commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove, NJ 07009. The black pigment particles had a core comprising black iron oxide (pigment black 11) and a polymer shell. As shown in FIG. 18, the black pigment particles were dispersed throughout the electrophoretic medium when the device was fabricated. A 0.5 Hz waveform, i.e., a 50 V square waveform with a +50 V offset (i.e., switching between +100 V and 0 V) ​​and a 50% duty cycle, was applied to the first light-transmitting electrode layer, while the second light-transmitting electrode layer was held at 0 V. The electric field between the two electrodes induced electrophoresis, driving the positive black pigment to the unexposed bottom surface of the microcell (inside the channel), as demonstrated in Figure 19. The black pigment remained on the unexposed bottom surface of the microcell after the voltage was released. The PET / ITO first electrode was then peeled off from the device, allowing the film to evaporate.

[0145] Example 6: A variable light transmission device was prepared as the variable light transmission device of Example 5, except that the light-blocking composition was formulated by mixing 10 wt% white pigment and 5 wt% CCA (cationic charge control agent from Example 1 - CCA111 in US2020 / 0355978111) in Isopar E solvent. The device was easily switched between a closed optical state (0V offset, FIG. 20) and an open optical state (-50V offset, FIG. 21) under a 50Hz / 50V square wave.

[0146] Example 7: Switching oppositely charged black and white pigments into channels in embossed microcells.

[0147] Variable transmission devices were prepared by laminating a sheet of PET-coated ITO transparent conductor together with an embossed microcell array containing an electrophoretic medium on a second sheet of PET / ITO. The microcell structure corresponded to the illustrations in Figures 2A-2D, except that the device did not include a sealing layer. The structure of the embossed microcell array is illustrated in Figures 14 and 15.

[0148] First, an electrophoretic composition comprising a black pigment was prepared and switched to an open optical state (black pigment contained in the channel). The first light-transmitting electrode layer was then removed, and the solvent was evaporated. After solvent evaporation, an electrophoretic medium comprising a white pigment, a solvent, and a charge control agent (CCA) was prepared. In this example, the electrophoretic medium was prepared by mixing 10 wt. % white pigment and 1 wt. % CCA111 in a solvent mixture of partially hydrogenated terphenyl, limonene, Isopar M, and Isopar E, commercially available as Cargille® 5040 from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove, NJ 07009. The white pigment particles were prepared using titanium dioxide pigment cores, which were provided with a polymer coating as described in Example 1 of U.S. Pat. No. 8,582,196.

[0149] The electrophoretic medium was assembled into a microcell device, in which the black pigment was switched into the channel and the solvent was evaporated. Thus, the device contained both white and black pigment particles of opposite charge. Both pigments could be electrophoretically switched. To switch both pigments into the channel, first the black pigment was switched into the channel using a relatively low frequency and a positive DC offset on the AC voltage, and then the white pigment was switched into the channel using a relatively high frequency and a negative DC offset on the AC voltage.

[0150] In the first step, a 10 Hz waveform, i.e., a 50 V square waveform with a 50% duty cycle and a +2 V offset, was applied to the first light-transmitting electrode layer 202, while the second light-transmitting electrode layer was held at 0 V. The electric field between the electrodes induced electrophoresis using superimposed induced charge electroosmotic flow, driving the positive black pigment into the channel. In the second step, a 500 Hz / 50 V square waveform with a 50% duty cycle and alternating +2 V and -2 V offset was applied to the first electrode, while the second electrode was held at 0 V. At higher frequencies (500 Hz compared to 10 Hz in the first step), less lateral movement of the white pigment than the black pigment was observed. That is, the white pigment was driven into the channel during the phase of the waveform with the negative DC offset. As shown in Figure 22, both white and black pigments were switched into the channel of the microcell.

[0151] Parts of the structure in the drawing: 200 variable transmission device; 201 first transparent substrate; 202 first light-transmitting electrode layer; 203 microcell layer; 204 multiple microcells; 205 microcell opening; 206 sealing layer; 207 second light-transmitting electrode layer; 208 second transparent substrate; 209 electrophoretic medium; 210 microcell bottom layer; 211 microcell bottom inner surface; 211a exposed microcell bottom inner surface; 211b unexposed microcell bottom inner surface; 212 microcell wall; 213 microcell inner wall surface; 214 microcell wall upper surface; 125 channel; 21 6 channel height; 217 protrusion structure; 218 protrusion bottom surface; 219 protrusion top surface; 220 protrusion height; 221 protrusion surface; 222 charged pigment particles; 222a first type charged pigment particles; 222b second type charged pigment particles; 223a first type charged pigment particles having opposite charge polarity to the second type charged pigment particles 223b; 223b second type charged pigment particles having opposite charge polarity to the first type charged pigment particles 223a; 232 first light blocking layer; 233 second light blocking layer; 234 auxiliary layer.

Claims

1. A variable light transmission device, comprising: a first light-transmitting electrode layer (202); a second light-transmitting electrode layer (207); Microcell layer (203) Equipped with the microcell layer (203) is disposed between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207), and the microcell layer (203) comprises a plurality of microcells (204) and a sealing layer (206); Each microcell of the plurality of microcells (204) contains an electrophoretic medium (209), the electrophoretic medium (209) comprising charged pigment particles and a charge control agent in a non-polar liquid; Each microcell of the plurality of microcells (204) has a microcell opening (205), and the sealing layer (206) spans the microcell openings (205) of the plurality of microcells (204); each microcell of the plurality of microcells (204) comprises a microcell bottom layer (210), a protruding structure (217), a microcell wall (212), and a channel (215); the microcell bottom layer (210) has a microcell bottom inner surface (211), the microcell bottom inner surface (211) comprising an exposed microcell bottom inner surface (211a) and an unexposed microcell bottom inner surface (211b); each microcell of the plurality of microcells (204) comprises a first light-blocking layer (232), the first light-blocking layer (232) contacting the exposed microcell bottom inner surface (211 a) and the electrophoretic medium (209); the protrusion structure (217) has a protrusion bottom surface (218), a protrusion surface (221), a protrusion apex (219), and a protrusion height (220), the protrusion apex (219) being a point or a set of points on the protrusion structure (217), the point or the set of points having a shorter distance from the microcell opening (205) than any other point on the protrusion structure (217), the protrusion height (220) being the distance between the protrusion bottom surface (218) and the protrusion apex (219), and the protrusion surface (221) being a surface of the protrusion structure (217) that is in contact with the electrophoresis medium (209) and does not include the protrusion apex; The microcell wall (212) has a microcell inner wall surface (213) and a microcell wall upper surface (214), the microcell inner wall surface (213) being the surface of the microcell wall (212) of the microcell that contacts the electrophoretic medium (209), and the microcell wall upper surface (214) being the surface of the microcell wall (212) of the microcell that contacts the sealing layer (206); the channel (215) has a channel height (216), the channel height (216) being 50% of the protrusion height (220); the non-exposed microcell bottom inner surface (211b) contacts the protrusion bottom surface (218); A variable light transmission device, wherein the channel (215) is a volume between the first light-blocking layer (232), the protrusion surface (221), and the microcell inner wall surface (213).

2. The protrusion structure is (a) a cone, (b) a cone on a cylinder, the cylinder having a base, the base of the cylinder being the protrusion base of the protrusion structure, (c) a tetrahedron, (d) a tetrahedron on a triangular prism, the triangular prism having a triangular base, the triangular base being the protrusion base of the protrusion structure, (e) a triangular prism, the triangular prism having a square base (f) a triangular prism having a square base, the square base being the protruding base of the protruding structure; (h) a square pyramid on a cube, the cube having a base, the cube having a base, the cube bottom being the protruding base of the protruding structure; (i) a square pyramid on a rectangular parallelepiped, the rectangular parallelepiped having a square base; (j) a pentagonal pyramid on a rectangular parallelepiped, wherein the pentagonal pyramid has a right-angled parallelogram base, and the right-angled parallelogram is the protruding base of the protruding structure; (k) a pentagonal pyramid on a pentagonal prism, wherein the pentagonal prism has a pentagonal base, and the pentagonal base is the protruding base of the protruding structure; 2. The variable optical transmission device of claim 1, wherein the geometric solid is selected from the group consisting of: (1) a pentagonal pyramid on a pyramid; (2) a hexagonal pyramid, wherein the hexagonal pyramid has a hexagonal base, the pentagonal base being the protrusion base of the protrusion structure; and (3) a hexagonal pyramid on a hexagonal prism, wherein the hexagonal prism has a hexagonal base, the pentagonal base being the protrusion base of the protrusion structure.

3. The variable light transmission device of claim 1 or claim 2, wherein the first light blocking layer comprises light absorbing pigment particles and a polymer.

4. The variable light transmission device according to any one of claims 1 to 3, wherein the light-absorbing pigment particles are electrically charged.

5. 5. The variable light transmission device according to claim 3 or claim 4, wherein the light absorbing pigment particles are black.

6. 6. The variable light transmission device of claim 1, wherein the first light-blocking layer is formed by curing a first light-blocking composition, the first light-blocking composition comprising a light-absorbing pigment, a polymer, an oligomer, or a monomer, and optionally a solvent.

7. 7. The variable light transmission device of claim 6, wherein the curing of the first light blocking composition is achieved thermally, by UV irradiation, by solvent evaporation, or by a combination thereof.

8. A variable light transmission device according to any one of claims 1 to 7, further comprising a second light-blocking layer, the second light-blocking layer being disposed between the upper surface of the microcell wall and the sealing layer.

9. The variable light transmission device of claim 8 , wherein the second light blocking layer comprises light absorbing pigment particles and a polymer.

10. The variable light transmission device of claim 8 , wherein the second light blocking layer is conductive.

11. The variable light transmission device of any one of claims 8 to 10, wherein the second light-blocking layer is formed by coating the upper surface of the microcell walls with a dispersion composition and curing the coated dispersion composition thermally, by UV irradiation, by solvent evaporation, or by a combination thereof, the dispersion composition comprising a light-absorbing pigment, a polymer, an oligomer, or a monomer, and optionally a solvent.

12. The variable light transmission device of any one of claims 8 to 11, further comprising an auxiliary layer (234), the auxiliary layer being disposed between the second light blocking layer (233) and the sealing layer (206).

13. The variable optical transmission device according to claim 12 , wherein the auxiliary layer is an adhesive layer.

14. The variable light transmission device of claim 12 , wherein the auxiliary layer comprises light-reflecting pigment particles.

15. 13. The variable light transmission device of claim 12, wherein the auxiliary layer comprises an encapsulated electrophoretic layer comprising an electrophoretic medium including charged pigment particles, and wherein color or image switching of the auxiliary layer is achieved upon application of an electric field across the encapsulated electrophoretic layer of the auxiliary layer.

16. Application of a first electric field between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207) via a first waveform causes the charged pigment particles to move into the channel (215), resulting in switching of the variable light transmission device (200) to an open optical state, and the charged pigment particles in the open optical state are located inside the channel (215), and application of a second electric field between the first light-transmitting electrode layer (202) and the second light-transmitting electrode layer (207) via a second waveform causes the charged pigment particles to move into the channel (215), resulting in switching of the variable light transmission device (200) to an open optical state.

17. The variable light transmission device of claim 1, wherein the applied voltage causes the charged pigment particles (222) to move towards the first light-transmitting electrode layer (202) with a velocity, the velocity having a lateral component leading to a closed optical state, the second waveform comprising at least one positive voltage and at least one negative voltage, the second waveform having a net positive or net negative impulse, and the closed optical state having a lower percentage of light transmission than the open optical state.

17. 17. The variable optical transmission device of claim 16, wherein the second waveform comprises an AC waveform, the AC having a frequency, and the AC waveform having a duty cycle between 5% and 45%.

18. 17. The variable optical transmission device of claim 16, wherein the second waveform comprises a waveform formed by the superposition of a DC voltage component and an AC waveform.

19. 1. A method for manufacturing a variable optical transmission device, the method comprising: providing an assembly comprising a third electrode layer, a second light-transmitting electrode layer, and a layer comprising a plurality of microcells, the layer comprising a plurality of microcells disposed between the third electrode layer and the second light-transmitting electrode layer, each microcell of the plurality of microcells comprising a light-blocking composition comprising (i) light-absorbing charged pigment particles, (ii) a polymer, oligomer, or monomer, and (iii) optionally a solvent, each microcell of the plurality of microcells having a microcell opening, the third electrode layer spanning the microcell openings of the plurality of microcells, each microcell of the plurality of microcells comprising a microcell bottom layer, a protrusion structure, a microcell wall, and a channel, the microcell bottom layer having a microcell inner bottom surface, the microcell inner bottom surface comprising an exposed microcell inner bottom surface and an unexposed microcell inner bottom surface; applying an electric field between the third electrode layer and the second light-transmitting electrode layer via a waveform, the waveform causing the light-absorbing charged pigment particles of the light-blocking composition to migrate toward the channel, resulting in the light-absorbing charged pigment particles being located inside the channel; curing the light-blocking composition to form a light-blocking layer on the exposed inner bottom surface of the microcell; removing the third electrode layer; filling each microcell of the plurality of microcells with an electrophoretic medium comprising charged pigment particles, a charge control agent, and a non-polar liquid; sealing each microcell of the plurality of microcells with a sealing layer; attaching a first light-transmitting electrode layer onto the sealing layer; A method comprising:

20. 1. A method for manufacturing a variable optical transmission device, the method comprising: providing an assembly comprising, in order, a second light-transmitting electrode layer, a second light-transmitting electrode layer, and a layer comprising a plurality of microcells, the layer comprising the plurality of microcells being disposed on the second light-transmitting electrode layer, each microcell of the plurality of microcells having a microcell opening, each microcell of the plurality of microcells comprising a microcell bottom layer, a protrusion structure, a microcell wall, and a channel, the microcell bottom layer having a microcell bottom inner surface, the microcell bottom inner surface comprising an exposed microcell bottom inner surface and an unexposed microcell bottom inner surface, the protrusion structure having a protrusion bottom surface, a protrusion surface, a protrusion top end, and a protrusion height, an edge is a point or set of points on the protrusion structure, the point or set of points having a shorter distance from the microcell opening than any other point on the protrusion structure; the protrusion height is the distance between the protrusion bottom surface and the protrusion apex; the protrusion surface is the surface of the protrusion structure excluding the protrusion apex and the protrusion bottom surface; the microcell wall has a microcell inner wall surface and a microcell wall upper surface; the channel has a channel height, the channel height being 50% of the protrusion height; the unexposed microcell bottom inner surface is in contact with the protrusion bottom surface; and the channel is the volume between the exposed microcell bottom inner surface, the protrusion surface, and the microcell inner wall surface. dispensing a light-blocking composition onto the exposed inner bottom microcell surface of each microcell, the light-blocking composition comprising: (i) light-absorbing pigment particles; (ii) a polymer, oligomer, or monomer; and (iii) optionally, a solvent; curing the light-blocking composition to form a light-blocking layer on the exposed inner bottom surface of the microcell; filling each microcell of the plurality of microcells with an electrophoretic medium comprising charged pigment particles, a charge control agent, and a non-polar liquid; sealing each microcell of the plurality of microcells with a sealing layer; attaching a first light-transmitting electrode layer onto the sealing layer; A method comprising:

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