Method for driving electrophoretic display device

Abstract

To provide a favorable method for driving an electrophoretic display device.SOLUTION: An electrophoretic medium comprises fluid and first (B), second (Y), third (R) and fourth (W) particles. The first (B) and third (R) particles bear charges of one polarity, and the second (Y) and fourth (W) particles bear charges of the opposite polarity. The first particles (B) have a greater potential than the third particles (R), and the second particles (Y) have a greater potential than the fourth particles (W). One of the particles is white, one of the non-white particles is partially light-transmissive, and the remaining two non-white particles are light-reflective. To display the color of a mixture of the first (B) and second (Y) particles at a viewing surface, the medium is driven to display the second particles (Y) at the viewing surface, then a first driving voltage is applied for a first period to drive the second (Y) and fourth (W) particles towards the viewing surface, then a second driving voltage having a polarity opposite to and a magnitude lower than that of the first voltage is applied for a second period less than the first period, and finally the applications of the two driving voltages are repeated.SELECTED DRAWING: Figure 3G

Description

[Background technology]

[0001] (Reference to Related Application)

[0002] This application is related to U.S. Patent Nos. 9,170,468, 9,361,836, 9,513,527, 9,640,119, 9,812,073, 10,147,366, 10,234,742, 10,431,168, 10,509,293, 10,586,499, and 10,782,586, and U.S. Application Publication No. 2021 / 0382369 A1.

[0003] The entire contents of the aforementioned patents and co-pending patent applications, as well as all other US patents and published and co-pending applications mentioned below, are hereby incorporated by reference.

[0004] BACKGROUND OF THEINVENTION

[0005] The aforementioned patents and published applications describe electrophoretic media, methods of driving such media, and electrophoretic display devices incorporating such media. The electrophoretic medium comprises a fluid and first, second, third, and fourth types of particles dispersed in the fluid, and such media may hereafter be referred to as a "four-particle electrophoretic medium." The four types of particles have different optical properties (typically color) from one another. The first type of particles carries a high positive charge, the second type of particles carries a high negative charge, the third type of particles carries a low positive charge, and the fourth type of particles carries a low negative charge (the charge strength may be measured in terms of zeta potential). In an electrophoretic display device, the electrophoretic medium is disposed between a front electrode and a rear electrode, and the display is typically viewed from the front electrode (viewing) side. In a typical multi-pixel display, the front electrode is continuous and spans multiple pixels, typically the entire display, while a separate back electrode is provided for each pixel to allow the displayed color to be controlled on a pixel-by-pixel basis.

[0006] The optical properties of the first and second types of particles can (in principle) be displayed on the viewing side by applying a high electric field of appropriate polarity across the electrophoretic medium for a period sufficient to allow either the first or second type of particles to be placed adjacent to the front electrode. To display the optical properties of the third type of particles, the second type of particles are first driven to the viewing surface by applying a high electric field of appropriate polarity, and then a low electric field of opposite polarity is applied to cause the third type of particles to be placed adjacent to the viewing surface while the first, second, and fourth types of particles are spaced from this surface (note that the second part of this sequence involves a change from the optical properties of high negative particles (second type of particles) to the optical properties of low positive particles (third type of particles)). Similarly, to display the optical properties of a fourth type of particle, the first type of particle is first driven to the viewing surface by applying a high electric field of appropriate polarity, and then a low electric field of opposite polarity is applied to cause the fourth type of particle to be positioned adjacent to the viewing surface while the first, second, and third types of particles are spaced from this surface (again, note that the second part of this sequence involves a change from the optical properties of highly positive particles (first type of particle) to the optical properties of low negative particles (fourth type of particle)). In practice, to achieve optimal separation of the various types of particles, the waveforms (sequences of drive pulses) may be significantly more complex than the simple outline above would suggest, and may include any one or more of the following: (a) repetition of one or two basic drive pulses already described; (b) periods of zero voltage between drive pulses; (c) the use of oscillating pulses (rapidly alternating positive and negative pulses) intended to homogenously mix the various types of particles; and (d) direct current (DC) balanced pulses intended to bring the overall impulse of the waveform to zero or close to zero (it is known that repeated application of DC unbalanced waveforms on an electrophoretic display can ultimately cause damage to the display which can reduce the quality of the displayed image and ultimately cause the display to fail completely). For all of the above waveform features, see, for example, the aforementioned U.S. Patent No. 9,640,119.

[0007] Although in most cases it is not explicitly stated, the four-particle electrophoretic media in the aforementioned patents use light-scattering ("reflective") particles rather than light-transmitting particles, so that the color (or other optical property) seen on the viewing side is determined only by the color of the particles directly adjacent to the front electrode, and the relative positions of the other particles are not relevant. Thus, although such electrophoretic media display only four independent optical states, they may also display a "particle mixture" state (typically grayish) in which various types of particles are randomly mixed, and other mixed states in which two types of particles are placed adjacent to the viewing side; for example, an orange color may be produced by mixing red and yellow particles adjacent to the viewing surface.

[0008] This limitation of four-particle electrophoretic media to four independent optical states is a serious practical disadvantage since in many applications, e.g., electronic signage such as electronic shelf labels, it is desirable to be able to display black, white, and three primary colors, e.g., red, green, and blue, or blue, red, and yellow. While good black and white states are important for text, the three primary colors allow full color display by dithering. Heretofore, four-particle electrophoretic media typically had either good black and white with two "highlight" colors (usually red and yellow), or white and the three primary colors, relying on a mixture of the three primary colors to produce a (often poor) "process" black.

[0009] It is known to overcome the aforementioned disadvantages of four-particle electrophoretic media by incorporating a fifth and optionally a sixth type of particles into the electrophoretic medium, see, for example, U.S. Patent Nos. 9,541,814 and 9,922,603. However, increasing the number of types of particles in the electrophoretic medium makes it more difficult to select suitable particles due to the increased need for strict control of the charge on the various particles, the increased possibility of interactions between the various particles (which may result in increased color contamination), and the extended waveforms, and the five- and six-particle electrophoretic media described in U.S. Patent Nos. 9,541,814 and 9,922,603 ​​require at least one three-step waveform, first to display the color of the highly charged particles of one polarity, then the color of the low charged particles of the opposite polarity, and finally the color of the intermediate charged particles of one polarity.

[0010] The aforementioned US 2021 / 0382369 A1 describes a four-particle electrophoretic medium generally similar to that described in the aforementioned patent, but in which one of the particles is white, one of the non-white particles is partially light-transmitting, and the other two non-white particles are light-reflective. Preferably, one pair of particles of the same polarity comprises blue and red particles, one of which is light-transmitting and the other is light-reflective, and the visible spectrum of the red and blue particles is selected such that a mixture of the two types of particles adjacent to the front electrode produces a good process black. One embodiment of this four-particle system, illustrated in Figures 3A-3F, has light-transmitting blue particles, enabling the display of black, white, red, blue, and yellow, and orange can also be displayed. However, in order to display a good full-color image, practice requires that such a black / white / blue / red / yellow system also be capable of displaying a green state, since dithering such a system does not provide a sufficiently saturated green color. The system of Figures 3A-3F of US 2021 / 0382369 A1 (Patent Document 3) includes both blue and yellow particles, and therefore should be capable of displaying green by mixing blue and yellow particles adjacent to the viewing surface of the display, or by placing blue particles adjacent to the viewing surface with yellow particles directly below them, but the application does not describe any method of producing such a green state.

[0011] (US 2021 / 0382369 A1 also describes a second embodiment of a four-particle system having light-transmitting red particles and reflective blue particles, illustrated in Figures 6A-6F, which is capable of displaying the color green. However, for practical reasons such as the availability of suitable pigments, it may be preferable to use a system with light-transmitting blue particles rather than red particles.)

[0012] Thus, a need exists for a method of driving a display such as that shown in Figures 3A-3F above to produce the color green, and the present invention provides such a method. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] U.S. Pat. No. 9,640,119 [Patent Document 2] US Patent Application Publication No. 2021 / 0382369 [Patent Document 3] US Patent Application Publication No. 2021 / 0382369 Summary of the Invention [Means for solving the problem]

[0014] (Summary of the invention)

[0015] The present invention provides a method of driving an electrophoretic display comprising a layer of electrophoretic medium having a viewing surface on one side and a second surface on an opposite side, the electrophoretic display further comprising voltage control means for applying an electric field through the layer of electrophoretic medium, the electrophoretic medium comprising a fluid and first, second, third and fourth types of particles dispersed in the fluid, the first, second, third and fourth types of particles having first, second, third and fourth colours respectively different from one another, the first and third types of particles having a charge of one polarity and the second and fourth types of particles having a charge of the opposite polarity, the first type of particles having a greater zeta potential or electrophoretic mobility than the third type of particles and the second type of particles having a greater zeta potential or electrophoretic mobility than the fourth type of particles, one of the types of particles is white, one of the types of non-white particles is partially light transmissive and the remaining two types of non-white particles are light reflective. The driving method of the present invention includes the steps of: (i) driving the electrophoretic medium to display a second color at the viewing surface; (ii) after step (i), applying a first drive voltage for a first period of time, the first drive voltage having a polarity that drives the second and fourth particles toward the viewing surface; (iii) after step (ii), applying a second drive voltage for a second time period, the second drive voltage having an opposite polarity and a smaller magnitude than the first drive voltage, the second time period being shorter than the first time period; and (iv) repeating steps (ii) and (iii), thereby displaying a color of a mixture of the first and second types of particles on the viewing surface. Includes.

[0016] In the present driving method, a period of zero voltage may be inserted between each step (ii) and the subsequent step (iii) and / or between each step (iii) and the subsequent step (ii).

[0017] Step (i) of the driving method of the present invention comprises: a) applying a third drive voltage for a third time period, the third drive voltage having the same polarity and substantially the same magnitude as the first drive voltage, but the third time period being shorter than the first time period; b) after step a), applying a fourth drive voltage for a fourth time period, the fourth drive voltage having the same polarity and a smaller magnitude than the second drive voltage, the fourth time period being longer than the third time period; c) repeating steps a) and b); This can be brought about by:

[0018] This preferred step (i) may be performed starting from a mixed state in which all four types of particles are randomly distributed. A period of zero voltage may be inserted between each step a) and the subsequent step b). A period of zero voltage may also be inserted between the last repetition of step b) and the first occurrence of step (ii).

[0019] The driving methods of the present invention may be preceded by one or more periods of an oscillating waveform and / or one or more periods of a DC balanced waveform (i.e., periods during which a non-zero voltage is applied to the display to reduce or eliminate the overall impulse of the entire waveform applied).

[0020] In any of the driving methods of the present invention, when the sequence of driving pulses is repeated, the repetition may be at least four times.

[0021] In some embodiments of the present invention, the white type particles are third or fourth type particles, i.e., one of the low charge type particles. If the white particles are one of the low charge type particles, the partially light-transmitting type particles can be highly charged type particles of opposite polarity to the white particles. In this case, it is advantageous that the light-reflecting type particles, which carry the same charge as the partially light-transmitting type particles, have optical properties such that the mixture of the two types of particles absorbs substantially all visible radiation, i.e., provides process black color.

[0022] In the electrophoretic medium of the present invention, the fourth particles may be white, the second particles may be yellow in color, the first particles may be blue and light-transmitting, while the second particles, in turn, may advantageously be red.

[0023] The four-particle electrophoretic medium of the present invention can thus display six colors (not counting the complete mixture state in which all four types of particles are randomly mixed). The colors of the white particles and the two light-reflective types of particles can be displayed simply by bringing each type of particle adjacent to the viewing surface. The three other colors are displayed by forming a binary mixture of partially light-transmitting type particles (typically blue) with each of the other three types of particles adjacent to the viewing surface. The mixture of light-transmitting and white particles causes light entering through the viewing surface to be scattered by the white particles and partially passed through the light-transmitting particles, eventually re-emerging from the viewing surface with the color of the light-transmitting type particles, typically blue (see discussion below with reference to FIG. 3E for practical details of this color formation). The fifth color displayed is process black, which is displayed by bringing light-transmitting particles adjacent to the viewing surface with reflective particles of the same polarity charge directly behind (i.e., immediately on the opposite side of the light-transmitting particles from the viewing surface) so that light entering through the viewing surface passes through the light-transmitting type particles and is then essentially totally absorbed by the reflective type particles directly behind the light-transmitting particles. Obviously, for this process black to be satisfactory, the combined absorption by the two types of particles needs to be spread across the entire visible spectrum, which is why the two types of particles are preferably red and blue, since it is relatively easy to arrange red and blue particles together to absorb substantially all visible light. An example of the absorption spectrum in which red and blue pigments are capable of producing a good process black is given below. The sixth color is produced by bringing a mixture of light-transmitting particles and other light-reflective particles adjacent to the viewing surface, typically a mixture of blue and yellow, producing a green state, as already explained. Some media can display a seventh state by bringing a mixture of two non-white light reflective types of particles adjacent to the viewing surface; when these two types of particles are yellow and red, it produces the color orange.

[0024] As already indicated, in the electrophoretic medium used in the driving method of the present invention, one type of particle is white, another type is partially light-transmitting, while the remaining two or three types of particles are light-reflective (i.e., light-scattering). In practice, of course, there is no such thing as a completely light-scattering particle or a completely non-light-scattering light-transmitting particle, and the minimum light scattering degree of light scattering particles and the maximum allowable light scattering degree acceptable in light-transmitting particles may vary somewhat depending on factors such as the exact pigment used, its refractive index and size, its color, the thickness of the particle layer (which itself depends on the thickness of the electrophoretic medium layer and the loading of each type of particle in the medium), and the user's ability or ability to tolerate some deviation from the ideal desired color. The scattering and absorption properties of pigments can be assessed by measuring the diffuse reflectance of a sample of the pigment dispersed in a suitable matrix or liquid against a white and dark background. Results from such measurements can be interpreted according to several models well known in the art, such as the one-dimensional Kubelka-Munk process.

[0025] The light transmittance of a pigment is most conveniently measured by its contrast ratio, which is defined (for the purposes of this application) as the ratio of the luminous reflectance (Rb) of a sample against a background of black material of a defined reflectance to the reflectance (Rw) of the same sample against a background of white material of a defined reflectance: CR=Rb / Rw The contrast ratio (CR) is an indicator of opacity and will of course vary with the thickness of the layer of pigment present in the electrophoretic medium and the type of pigment used. In general, full opacity is obtained at CR=0.98. The hiding power of a paint is understood to be its ability to eliminate the contrast between black and white substrates to the extent that the reflectance obtained over the black substrate is 98% of that obtained over the white substrate. The light-transmitting pigment layer used in the present electrophoretic medium should have a contrast ratio of about 0.5 or less, preferably 0.3 or less. The blue pigment used in the experiments described below has a contrast ratio of about 0.2. The reflective pigment should have a contrast ratio of about 0.6 or more, preferably about 0.7 or more.

[0026] The electrophoretic medium used in the present invention may be encapsulated or non-encapsulated. If encapsulated, the electrophoretic medium may be contained within a plurality of microcells, as described in U.S. Pat. No. 6,930,818, the contents of which are incorporated herein by reference in their entirety. The display cells, regardless of their shape or size, may be other types of microcontainers, such as microcapsules, microchannels, or equivalents. Alternatively, the electrophoretic medium may be encapsulated within a capsule, or in the form of a so-called polymer-dispersed electrophoretic medium, which comprises a plurality of separate droplets of electrophoretic fluid and a persistent phase of polymeric material. The separate droplets of electrophoretic fluid in such a polymer-dispersed electrophoretic display may be considered capsules or microcapsules, even if no separate capsule membrane is associated with each individual droplet. See, for example, U.S. Pat. No. 6,866,760. The present specification also provides, for example, the following: (Item 1) 1. A method of driving an electrophoretic display comprising a layer of electrophoretic medium, the electrophoretic display having a viewing surface (13) on one side thereof and a second surface (14) on an opposite side thereof, the electrophoretic display further comprising voltage control means for applying an electric field through the layer of electrophoretic medium, the electrophoretic medium comprising a fluid and particles of a first (B), second (Y), third (R) and fourth (W) type dispersed in the fluid, the first (B), second (Y), third (R) and fourth (W) type particles having first, second, third and fourth colours respectively which are distinct from one another; the first (B) and third (R) types of particles have charges of one polarity and the second (Y) and fourth (W) types of particles have charges of the opposite polarity; the first (B) type of particles has a higher zeta potential or electrophoretic mobility than the third (R) type of particles and the second (Y) type of particles has a higher zeta potential or electrophoretic mobility than the fourth (W) type of particles; one of the particle types (W) is white, one of the non-white particle types (B) is partially light-transmitting and the remaining two types of non-white particles (Y, R) are light-reflective; The driving method includes: (i) driving the electrophoretic medium to display the second color (Y) at the viewing surface (13); (ii) after step (i), applying a first drive voltage for a first period of time, the first drive voltage having a polarity that drives the second (Y) and fourth (W) particles towards the viewing surface (13); (iii) after step (ii), applying a second drive voltage for a second time period, the second drive voltage having an opposite polarity and a smaller magnitude than the first drive voltage, the second time period being shorter than the first time period; and (iv) repeating steps (ii) and (iii), whereby a mixture of the colors of the first (B) and second (Y) types of particles is displayed on the viewing surface; The method is characterized by: (Item 2) 2. The driving method according to item 1, wherein a period of zero voltage is inserted between each step (ii) and the subsequent step (iii) and / or between each step (iii) and the subsequent step (ii). (Item 3) Step (i) is a) applying a third drive voltage for a third time period, the third drive voltage having the same polarity and substantially the same magnitude as the first drive voltage, but the third time period being shorter than the first time period; b) after step a), applying a fourth drive voltage for a fourth time period, the fourth drive voltage having the same polarity and a smaller magnitude than the second drive voltage, the fourth time period being longer than the third time period; and c) repeating steps a) and b); and The driving method according to item 1, (Item 4) 4. The method of claim 3, wherein step (i) is performed starting from a mixed state (M) in which all four types of particles (B, Y, R, W) are randomly dispersed. (Item 5) 4. The driving method according to item 3, wherein a period of zero voltage is inserted between each step a) and the subsequent step b). (Item 6) 4. The driving method according to item 3, wherein a period of zero voltage is inserted between the last repetition of step b) and the first occurrence of step (ii). (Item 7) The driving method is preceded by one or more periods of an oscillating waveform and / or one or more periods of a DC balanced waveform. (Item 8) 2. The driving method according to item 1, wherein steps (ii) and (iii) are repeated at least four times. (Item 9) 4. The driving method according to item 3, wherein steps a) and b) are repeated at least four times. (Item 10) 2. The driving method according to item 1, wherein the white particles (W) are either the third or fourth type particles. (Item 11) Item 11. The driving method according to item 10, wherein the partially light-transmitting type particles (B) are highly charged type particles of opposite polarity to the white particles (W). (Item 12) Item 12. The method of claim 11, wherein the light-reflective type particles (R) carrying the same charge as the partially light-transmitting type particles (B) have optical properties such that a mixture of the two types of particles (R, B) absorbs substantially all visible radiation. (Item 13) The driving method according to item 1, wherein the fourth particles (W) are white, the second particles (Y) are yellow, the first particles (B) are blue and light-transmitting, and the third particles are red. (Item 14) 2. The driving method according to item 1, wherein the light transmitting pigment layer has a contrast ratio of about 0.5 or less. (Item 15) 2. The driving method according to item 1, wherein the light-transmitting pigment layer has a contrast ratio of about 0.3 or less. [Brief description of the drawings]

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] [Figure 1] Figure 1 of the accompanying drawings is a schematic cross-section through a four-particle display device which can be driven by the method of the present invention.

[0029] [Diagram 2] FIG. 2 shows the absorption spectrum of preferred pigment particles for use in the display device of FIG.

[0030] [Figure 3A] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo. [Figure 3B] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo. [Figure 3C] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo. [Figure 3D] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo. [Figure 3E] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo. [Figure 3F] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo. [Figure 3G] 3A-3G are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 can undergo.

[0031] [Figure 4] FIG. 4 illustrates DC balanced and oscillatory waveforms that may be incorporated into the drive method of the present invention.

[0032] [Figure 5A]5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5B] 5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5C] 5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5D] 5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5E] 5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5F] 5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5G] 5A-5G show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively.

[0033] [Figure 6] FIG. 6 is a graph showing the variation in green color achieved in the transition of FIG. 3G as a function of one of the drive voltage pulses applied using the waveform of FIG. 5G. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] (Detailed Description)

[0035] As shown above, the present invention provides a method of driving a four-particle electrophoretic medium to display at least six distinct optical states. The electrophoretic medium comprises a fluid and first, second, third, and fourth types of particles dispersed in the fluid, all four types of particles having different colors. The first and third types of particles carry a charge of one polarity, and the second and fourth types of particles carry a charge of the opposite polarity. The first type of particles has a higher zeta potential or electrophoretic mobility than the third type of particles, and the second type of particles has a higher zeta potential or electrophoretic mobility than the fourth type of particles (so that for two pairs of oppositely charged particles, one pair carries a stronger charge than the other pair. Thus, the four types of particles may also be referred to as high positive particles, high negative particles, low positive particles, and low negative particles). One type of particle is white. One of the non-white particle types is partially light transmissive, while the remaining two types of non-white particle are light reflective.

[0036] As an example shown in Figure 1, blue particles (B) and yellow particles (Y) are a first pair of oppositely charged particles, in which the blue particles are highly positive and the yellow particles are highly negative. Red particles (R) and white particles (W) are a second pair of oppositely charged particles, in which the red particles are low positive and the white particles are low negative. Of course, it should be understood that the aforementioned charges could be reversed in polarity and the display could continue to function in the same manner, except that the polarity of the driving waveforms described below would need to be reversed.

[0037] The white particles may be formed from inorganic pigments such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4, etc.

[0038] The non-white and non-black particles are independent of color, such as red, green, blue, magenta, cyan, or yellow. Pigments for color particles may include, but are not limited to, CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155, or PY20. These are commonly used organic pigments described in the Color Index Handbook "New Pigment Application Technology" (CMC Publishing Co, Ltd, 1986) and "Printing Ink Technology" (CMC Publishing Co, Ltd, 1984). Specific examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine red L3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD, Sun Chemical Phthalocyanine Blue, Phthalocyanine Green, Dialilide Yellow, or Dialilide AAOT Yellow. A preferred partially light-transmitting blue pigment for use in the display of Figure 1 is Kremer 45030, "Ultramarine Blue, greenish extra", a sodium aluminum sulfosilicate pigment, CI Pigment Blue 29: 77007, available from Kremer Pigmente GmbH & Co. KG, Hauptstr. 41-47, DE-88317 (Aichstetten, Germany). This light-transmitting blue pigment may be usefully used in combination with the Hostaperm Red D3G 70 pigment described above.

[0039] As illustrated in Figure 2, this blue pigment has a peak transmittance at about 450 nm and substantial transmittance visible over the range of 400 to about 530 nm. The Hostaperm Red D3G 70 pigment, on the other hand, is essentially non-reflective below about 555 nm. Thus, when the two pigments are arranged as shown in Figure 3A, with the light-transmitting blue pigment adjacent to the viewing surface and the reflective red pigment immediately opposite the blue pigment from the viewing surface, all visible radiation that enters through the viewing surface and passes through the blue pigment will be absorbed by the red pigment and the viewing surface will appear black.

[0040] The non-white particles can also be inorganic pigments such as red, green, blue, and yellow. Examples can include, but are not limited to, CI Pigment Blue 28, CI Pigment Green 50, and CI Pigment Yellow 227.

[0041] In addition to color, the four types of particles may also have other different optical properties such as optical transmittance, reflectance, luminescence, or, in the case of displays intended for machine reading, pseudocolor in the sense of changes in reflectance of electromagnetic wavelengths outside the visible range.

[0042] A display layer utilizing the display fluid of the present invention has two surfaces, a first surface (13) on the viewing side and a second surface (14) on the opposite side from the first surface (13), as shown in Figure 1. The display fluid is sandwiched between the two surfaces. On the first surface (13) side there is a common electrode (11), which is a transparent electrode layer (e.g. ITO) that extends across the entire top of the display layer. On the second surface (14) side there is an electrode layer (12) that comprises a plurality of pixel electrodes (12a). However, the present invention is not limited to any particular electrode configuration.

[0043] Pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. It should be noted that although active matrix drive with thin film transistor (TFT) backplanes is described with respect to a layer of pixel electrodes, the scope of the invention encompasses other types of electrode addressing so long as the electrodes perform the desired function.

[0044] Each space between two vertical dotted lines in Figure 1 represents a pixel. As shown, each pixel has a corresponding pixel electrode (12a). An electric field is generated for the pixel by the potential difference between the voltage applied to the common electrode and the voltage applied to the corresponding pixel electrode (note that in the various waveforms illustrated in the accompanying drawings, the potential difference plotted is that applied to the pixel electrode 12a, and the common electrode is assumed to be held at ground voltage, as is typically the case. The color displayed by a pixel depends on the particles adjacent to the common electrode 11, so when a positive potential difference is shown in the drawings, the common electrode is negative with respect to the pixel electrode and positively charged particles are attracted to the common electrode).

[0045] The solvent in which the four types of particles are dispersed is clear and colorless, preferably has low viscosity for high particle mobility, and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15. Examples of suitable dielectric solvents include hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene, or alkylnaphthalenes, halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene, dichlorononane, or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70, or FC-5060 from 3M Company (St. Paul MN), low molecular weight halogen containing polymers such as poly(perfluoropropylene oxide) from TCI America (Portland, Oregon), Halocarbon from Halocarbon Product Corp. (River Edge, NJ). perfluoropolyalkyl ethers such as Galden Oils from Ausimont or the Krytox Oils and Greases K-Fluid Series from DuPont (Delaware); polydimethylsiloxane-based silicone oils from Dow-corning (DC-200).

[0046] In one embodiment, the charge carried by a "low charge" particle may be less than about 50%, preferably about 5% to about 30%, of the charge carried by a "high charge" particle. In another embodiment, a "low charge" particle may be less than about 75%, or about 15% to about 55% of the charge carried by a "high charge" particle. In a further embodiment, the comparison of charge levels as indicated applies to two types of particles having the same charge polarity.

[0047] Charge strength may be measured in terms of zeta potential. In one embodiment, zeta potential is determined by a Colloidal Dynamics AcoustoSizer IIM with CSPU-100 signal processing unit, ESA EN#Attn flow-through cell (K:127). Instrument constants such as density of the solvent used in the sample, dielectric constant of the solvent, speed of sound in the solvent, viscosity of the solvent, all of which are inputted prior to testing at the test temperature (25°C). The pigment sample is dispersed in a solvent (usually a hydrocarbon fluid with less than 12 carbon atoms) and diluted to be 5-10% by weight. The sample also contains a charge control agent (Solsperse 19K, available from Lubrizol Corporation, a subsidiary of Berkshire Hathaway, "Solsperse" is a registered trademark) at a 1:10 weight ratio of charge control agent to particle. The mass of the diluted sample is determined and then the sample is loaded into the flow-through cell for determination of zeta potential.

[0048] The amplitudes of "high positive" and "high negative" particles may be the same or different. Similarly, the amplitudes of "low positive" and "low negative" particles may be the same or different. However, the zeta potential of a "high positive" or positive particle with a higher charge strength or charge magnitude is greater than the zeta potential of a "low positive" or positive particle with a lower charge strength or charge magnitude, and the same logic applies with respect to high negative and low negative particles. In the same medium under the same field, a more highly charged particle will have a greater electrophoretic mobility, i.e., a more highly charged particle will traverse the same distance in less time than a less highly charged particle.

[0049] It should also be noted that in the same fluid, two pairs of high and low charged particles may have different levels of charge difference: for example, in one pair, the low positively charged particle may have a charge strength that is 30% of the charge strength of the high positively charged particle, and in another pair, the low negatively charged particle may have a charge strength that is 50% of the charge strength of the high negatively charged particle.

[0050] The following examples illustrate display devices utilizing such display fluids.

[0051] (Example) An example of this is demonstrated in Figures 3A-3G. The highly positive light-transmitting particles are blue (B), the highly negative particles are yellow (Y), the low positive particles are red (R), and the low negative particles are white (W). The transition shown in Figure 3A is caused by starting from a fully mixed state (denoted as "(M)") and applying a vibration pulse as described below. When alternating pulses of high positive potential difference (e.g., +15V) and no potential difference (0V) are applied to the pixel electrode 22a for a sufficiently long period of time, the blue (B) and red (R) particles are driven toward the common electrode (21) or viewing side, and the yellow and white particles are driven toward the pixel electrode 22a side. The red (R) and white (W) particles carry a weaker charge and therefore move slower than the highly charged blue and yellow particles. As a result, the blue particles are placed directly adjacent to the common electrode with the red particles directly below them (as illustrated in Figure 3A). For the reasons already discussed above, this causes the pixel to appear black (indicated as "(K)" in Figure 3A), and the white and yellow particles are masked by the reflective red particles and do not affect the displayed color.

[0052] Similarly, the transition shown in FIG. 3B starts from the fully mixed state (M) and is caused by applying a shaking pulse as described below. When alternating pulses of high negative potential difference (e.g., −15V) and no voltage (0V) are applied to the pixel electrode 22a for a sufficiently long period of time, the blue (B) and red (R) particles are driven toward the pixel electrode 22a side, and the yellow and white particles are driven toward the common electrode side. The red (R) and white (W) particles move slower than the highly charged blue and yellow particles because they carry a weaker charge. As a result, the reflective yellow particles are placed directly adjacent to the common electrode, thus making the pixel appear yellow (indicated by “(Y)” in FIG. 3B). All of the white, red, and blue particles are masked by the reflective yellow particles and do not affect the displayed color. In principle, yellow could be produced by alternating pulses of -15V and 0V, but in practice more complex waveforms are preferred, as will be explained below with reference to FIG. 5B.

[0053] The transition shown in FIG. 3C starts from a completely mixed state (M). When alternating pulses of high negative potential difference (e.g., −15V) and low positive potential difference (e.g., +8V) are applied to the pixel electrode 22a for a sufficiently long period, with a low positive pulse that is much longer than the high negative pulse, the red (R) particles are driven toward the common electrode 21 side and the white particles (W) are driven toward the pixel electrode 22a side. The effect of the oscillating electric field is to cause the highly charged blue and yellow particles to repeatedly pass each other in the center of the thickness of the electrophoretic layer, and the strong electrical attraction between the highly charged positive and negative particles significantly slows down the movement of these particles, which tends to keep them in the center of the thickness of the electrophoretic layer. However, the electric field generated by the low positive pulse is sufficient to separate the low charged white and red particles, thereby allowing the low positive red particles (R) to move to the edge of the common electrode 21 side and the low negative white particles to move toward the pixel electrode 22a side. As a result, the reflective red particles are placed directly adjacent to the common electrode, thus causing the pixel to appear red (denoted with an "(R)" in FIG. 3C), and the white, yellow, and blue particles are all masked by the reflective red particles and do not affect the displayed color. Importantly, this system allows the weaker charged particles to be separated from the more strongly charged particles of the opposite polarity.

[0054] The transition shown in FIG. 3D starts from a completely mixed state (M). Alternating pulses of high positive potential difference (e.g., +15V) and low negative potential difference (e.g., −8V) are applied to the pixel electrode 22a for a long enough period of time, with the low negative pulse being much longer than the high positive pulse, so that the red (R) particles are driven toward the pixel electrode 22a side and the white particles (W) are driven toward the common electrode 21 side. As in the transition shown in FIG. 3C, the effect of the oscillating electric field is to cause the highly charged blue and yellow particles to remain together in the center of the thickness of the electrophoretic layer. However, the electric field generated by the low negative pulse is sufficient to separate the low charged white and red particles, thereby allowing the low positive red particles (R) to move to the edge of the pixel electrode 22a side and the low negative white particles to move toward the common electrode 21 side. As a result, the white particles are placed directly adjacent to the common electrode, thus making the pixel appear white (designated "(W)" in FIG. 3D), and the red, yellow, and blue particles are all masked by the white particles and do not affect the displayed color. In principle, white could be produced by alternating pulses of +15V and -8V, but in practice more complex waveforms are preferred, as will be explained below with reference to FIG. 5D.

[0055] The transition shown in Figure 3E starts from the white state (W) shown in Figure 3D. In this state, a positive potential difference pulse is applied to the device whose total impulse is not sufficient to drive the device to the black state (K) shown in Figure 3A. The positive pulse causes the highly charged blue particles to move towards the common electrode 21 side and the white particles to move towards the pixel electrode 22a side. However, because the highly charged blue particles move more quickly than the less charged white particles, a mixture of blue and white particles is visible through the viewing surface such that the pixel appears blue.

[0056] At first, it would appear from Figure 3E that the reflection from the white pigment located immediately adjacent to the front electrode would substantially reduce the saturation of the blue seen at the viewing surface. However, it should be understood that Figure 3E (as well as Figures 3A-3D and 3F-3G) are all highly schematic. In practice, the pigment particles are not spherical (the crystalline pigments used tend to fracture preferentially along certain crystal planes; for example, rutile titania, commonly used as a white pigment in electrophoretic media, is tetragonal and forms regular square prisms), the particles vary significantly in size, the "reflection" from the white particles is essentially Lambertian light scattering rather than specular, and there are several more layers of particles than are illustrated in Figure 3E (the exact number of layers will, of course, depend on the particle loading in the electrophoretic medium, the thickness of this medium, and the size of the individual particles, but in practice at least 5-10 layers are typically present). The overall effect of all the aforementioned factors is that only a very small percentage of visible light that enters the electrophoretic medium through the viewing surface is reflected directly back through the viewing surface by the white particles, and in practice a fully saturated blue color can be achieved.

[0057] It should also be understood that while Figure 3A shows blue and red particles in completely separate layers, Figure 3E shows a complete mixture of blue and white particles, these represent two extremes, and that in practice there may be a continuous gradation between completely separate layers and complete mixture. Provided that the requisite color is obtained, the present invention is not limited to any theoretical commentary regarding the exact location of the particles and their degree of mixing with other particles.

[0058] The transition shown in Figure 3F starts from the red state (R) shown in Figure 3C. A negative potential difference pulse is applied to the device in this state, whose overall impulse is not sufficient to drive the device to the yellow state (Y) shown in Figure 3B. The negative pulse causes the highly negative yellow particles to move towards the common electrode (21) side, while the low positive red particles move much more slowly towards the pixel electrode (22a) side. The result is that a mixture of red and yellow particles is visible through the common electrode 21, and the pixel appears orange.

[0059] The transition shown in FIG. 3G also starts from the red state (R) shown in FIG. 3C. A high negative potential difference pulse is applied to the device in this state, which drives the device from the red state shown in FIG. 3C toward the yellow state (as shown), with the yellow and white particles moving upwards and the blue and red particles moving downwards. The high negative pulse is followed by an intermediate positive potential difference pulse that reverses the aforementioned movement of the particles. The high negative pulse and the intermediate positive pulse are then repeated. Since the blue and yellow particles are collected in the red state where the transition begins, the impulses of the high negative pulse and the intermediate positive pulse are not balanced, but the tendency of the highly charged yellow and blue particles to collect when a potential difference smaller than the high potential difference is applied causes the population to be substantially maintained, whereby the overall effect of the alternating negative and positive pulses causes the blue / yellow population to move toward the common electrode (21) while the red particles move in the opposite direction, until the red particles pass through the blue / yellow population. As shown in FIG. 3G, the end result is that the blue / yellow population is placed adjacent to the common electrode (21), resulting in a green display, and the red and white particles are masked by the blue / yellow population (the positions of the red and white particles in FIG. 3G are largely arbitrary, but because both types of particles are masked, their exact positions have no effect on the visible color of the pixel).

[0060] To ensure both color brightness and color purity, DC balance and / or oscillatory waveforms may be used prior to any of the transitions discussed above. The oscillatory waveform consists of a pair of opposing drive pulses repeated over many cycles. For example, the oscillatory waveform may consist of a +15V pulse for 20 ms and a -15V pulse for 20 ms, with such a pair of pulses repeated 50 times. The total time of such an oscillatory waveform would be 2,000 ms. In practice, there may be at least 10 repetitions in the oscillatory pulse (i.e., 10 pairs of positive and negative pulses). The oscillatory waveform may be applied regardless of the optical state (black, white, red, or yellow) before the drive voltage is applied. After the oscillatory waveform is applied, the optical state will not be pure white, pure black, pure yellow, or pure red. Instead, the color state will be derived from a mixture of four types of pigment particles.

[0061] Each of the driving pulses in the oscillatory waveform is applied for a time not exceeding 50% (or not exceeding 30%, 10%, or 5%) of the driving time required from a complete transition from one highly charged particle color to another highly charged particle color (in this example, from blue to yellow, or vice versa). For example, if it takes 300 milliseconds to drive a display device from a fully black state to a fully yellow state (or vice versa), the oscillatory waveform may consist of positive and negative pulses, each applied for 150 milliseconds or less. In practice, shorter pulses are preferred. The oscillatory waveform as described may be used in the driving method of the present invention. In all of the figures throughout this application, the oscillatory waveform is shortened (i.e., the number of pulses is less than the actual number).

[0062] A DC balanced waveform is designed to reduce the overall impulse (i.e., integral of voltage over time) of the overall waveform to a small value, zero if possible. For example, as discussed in U.S. Patent Nos. 6,531,997 and 6,504,524, problems can be encountered and the useful life of the display can be reduced if the method used to drive the display does not result in a zero or near-zero net time-averaged applied electric field across the electro-optic medium. Waveforms that result in a zero net time-averaged applied electric field across the electro-optic medium are conveniently referred to as "direct current balanced" or "DC balanced" waveforms.

[0063] Figure 4 illustrates a combined DC balanced / oscillating waveform comprising a DC balanced section 42 followed by an oscillating section 44. Although Figure 4 illustrates the DC balanced section 42 as having a high positive potential difference, it should be understood that the DC balanced section may have a high or low, positive or negative potential difference, or zero potential difference, depending on the remaining impulses of the applied waveform.

[0064] Furthermore, although FIG. 4 illustrates a single DC balance section followed by a single vibration section, a combined DC balance / vibration waveform may include multiple DC balance sections and multiple vibration sections alternating with each other, beginning and ending with either a DC balance section or a vibration section. The use of multiple DC balance sections may be advantageous in that it may be possible to achieve a closer approach to a zero overall waveform impulse than with a single DC balance section by (for example) setting one or more DC balance sections to a high voltage and one or more to zero. Multiple DC balance sections may vary from each other in both duration and applied potential difference. Similarly, multiple vibration sections may differ from each other in duration, magnitude of potential difference, and frequency.

[0065] 5A illustrates the waveforms used to effect the transition of FIG. 3A to produce a black optical state. After a DC balance section and oscillation section S of duration t1 at a high negative voltage VH2, (i) a period of zero voltage of duration t2, (ii) a period of high positive drive voltage VH1 of duration t3, (iii) a period of zero voltage of duration t4 substantially greater than t3, and (iv) several repetitions of (ii) and (iii), typically 4-8 repetitions, are applied to the pixel electrode to achieve a mixed state M (the durations of both t1 and oscillation section S are significantly reduced in FIG. 5A, and multiple DC balance and oscillation sections could, of course, be used).

[0066] FIG. 5B illustrates a waveform used to effect the transition of FIG. 3B to produce a yellow optical state (i.e., the color of the second particle). As already indicated, in principle, yellow can be produced by applying alternating pulses of high negative potential difference (e.g., −15V) and no voltage (0V) to the pixel electrode 22a for a sufficiently long period of time. However, to ensure a pure yellow color, a much more complex waveform as shown in FIG. 5B is preferred. After a DC balance section of duration t1 and an oscillation section S that are essentially the same as those already described with reference to FIG. 5A, the waveform of FIG. 5B comprises a period of zero voltage of duration t5 followed by (i) a short period of duration t6 of high negative potential difference VH2, (ii) a period of zero voltage of duration t7, and (iii) a period of low positive potential difference VL1 for a period t8 longer than t6. Typically, the magnitude of VL1 is about half that of VH2, t7 is comparable in length to t6, and t8 is about 10 times as large as t6. For example, each of t6 and t7 may be 50 milliseconds, while t8 may be 500 milliseconds. Steps (i), (ii), and (iii) are then repeated several times, as indicated by "[Xm]" in FIG. 5B, and typically these steps may be repeated 4-6 times. Following these repetitions, (iv) a high negative potential difference VH2 is applied for a period t9 longer than t6, and then (v) a low positive potential difference VL3, lower than VL1 and typically about one-third of VH1, is applied for a period t10 shorter than t8. Steps (iv) and (v) are then repeated, as indicated by "[Xn]" in FIG. 5B, and typically these steps may be repeated 2-3 times. The final portion of the waveform in Figure 5B comprises an application of a high negative voltage difference VH2 for a duration t11 longer than t9, a period of zero voltage of duration t12, and a second application of a high negative voltage difference VH2 for duration t11. As will be readily apparent, the number of applications of VH2 in this portion of the waveform and the duration t11 can be adjusted experimentally.

[0067] FIG. 5C illustrates a waveform used to effect the transition of FIG. 3C to produce a red optical state (i.e., the color of the second particle). The waveform shown in FIG. 5C closely resembles the first portion of the waveform shown in FIG. 5B, and after a DC balance section of duration t1 and an oscillation section S that are essentially the same as those already described with reference to FIG. 5A, the waveform of FIG. 5C comprises a period of zero voltage of duration t13 followed by (i) a short period of duration t14 of a high negative potential difference VH2, (ii) a period of zero voltage of duration t15, and a period of low positive potential difference VL1 for a duration t16 longer than t14. Typically, the magnitude of VL1 is about half that of VH2, t15 is comparable in length to t14, and t16 is about 10 times as large as t14. For example, each of t6 and t7 may be 50 milliseconds, while t8 may be 500 milliseconds. Steps (i), (ii), and (iii) are then repeated several times as shown in FIG. 5C, typically these steps may be repeated 6-10 times. The waveform is terminated by a transition from the final application of VL1 to 0V to ensure a good red color. As will be readily apparent, the number of applications of VH2 and VL1 in this waveform, and the durations t14 and t16, can be adjusted experimentally.

[0068] Figure 5D illustrates the waveform used to effect the transition of Figure 3D to produce a white optical state (i.e., the color of the fourth particle). Not surprisingly, the first part of the waveform shown in Figure 5D closely resembles the "red" waveform shown in Figure 5C, but with a change in polarity; after a DC balance section of duration t1' (the DC balance section is high positive in this case) and an oscillation section S that are essentially the same as those already described with reference to Figure 5A, the waveform of Figure 5D comprises (i) a short period of duration t17 of a high positive potential difference VH1 (note that in this case there is no period of zero voltage between the application of the oscillation section S and the high driving potential difference), (ii) a period of zero voltage of duration t18, and a period of low negative potential difference VL2 for a period t19 longer than t17. Typically, the magnitude of VL2 is about half that of VH1, t18 is comparable in length to t17, and t19 is about 10 times as large as t17. For example, each of t6 and t7 may be 50 milliseconds, while t8 may be 500 milliseconds. Steps (i), (ii), and (iii) are then repeated several times, as shown in FIG. 5D, and typically these steps may be repeated 6 to 10 times. However, to ensure a pure white color, it has been found that (iv) the repetition of steps (i), (ii), and (iii) with a period of zero potential difference of duration t20, followed by (v) application of a low negative potential difference VL2 for a period t21, and a repetition of steps (iv) and (v) is advantageous. Typically, steps (iv) and (v) are repeated 6 to 10 times, with t20 being comparable to t18, and t21 being shorter than t19. As will be readily apparent, the number of VH1 and VL2 applications in this waveform and the durations t17, t18, t19, t20, and t21 can be adjusted experimentally.

[0069] FIG. 5E illustrates the waveform used to effect the transition of FIG. 3E to produce a blue optical state (i.e., the color of the first particle). Not surprisingly, the first portion of the waveform shown in FIG. 5E is the same as the "white" waveform shown in FIG. 5D. However, after the repetition of steps (iv) and (v) discussed in the previous section, the waveform of FIG. 5E continues with (vi) application of a high positive potential difference VH1 for a period t22 shorter than t17, (vii) application of a zero potential difference for a period t23 shorter than t18, (viii) application of a low negative potential difference VL4 having a magnitude smaller than VL2 for a period t24 shorter than t19 or t21, and repetition of steps (vi)-(viii), but terminating after the repetition of step (vi) and not continuing with step (viii) (i.e., terminating with a final positive drive pulse, as described above with reference to FIG. 3E). Typically, the magnitude of VL4 is about 75 percent of VL2, and typically steps (vi)-(viii) may be repeated 10 to 20 times. As will be readily apparent, the number of applications of VH1 and VL4 in this waveform, and the durations t22, t23, and t24, can be adjusted empirically.

[0070] Figure 5F shows the waveform used to effect the transition shown in Figure 3F to produce the orange optical state. The waveform shown in Figure 5F is the same as the "red" waveform shown in Figure 5C, except for the addition of a final short low negative potential difference pulse SP, the impulse of which is insufficient to drive the electrophoretic medium from the red optical state (R) to the yellow optical state (Y) (see Figure 3B). The magnitude and duration of the pulse SP can be widely varied, and the optical combination of magnitude and duration can be determined experimentally.

[0071] Finally, Figure 5G shows the waveform used to generate the green optical state shown in Figure 3G. The first portion of the waveform shown in Figure 5G, up to period t8, is the same as the "red" waveform shown in Figure 5C. However, this "red" waveform is followed by a period t17 of zero voltage, typically lasting about 250-450 milliseconds. This period t17 of zero voltage is followed by several repetitions of the following: (i) a high negative potential difference pulse of duration T18 typically lasting about 250-450 milliseconds; (ii) a short period of zero voltage T19, typically less than about 50 milliseconds; (iii) an intermediate positive voltage V of duration t typically lasting about 250-450 milliseconds L5 (In this case, V L1 <V L5 <V H1 ) short-term t20, and (iv) A second short period of zero voltage T21, typically lasting less than about 50 milliseconds. (The final iteration of step (iv) may, of course, be omitted).

[0072] A four-particle electrophoretic medium as shown in Figure 1 was formulated using the aforementioned Kremer 45030 as a partially light-transmitting blue pigment, rutile titania white pigment, light-reflecting 1254 DPP Red 254 (available from DCL Corporation), and Novoperm Yellow HR 70-EDS (available from Clariant Corporation, Holden MA) in Isopar E with the addition of a charge control agent pigment. Even using the non-optimized waveform, the following five colors were produced: [Table 1]

[0073] This four-particle electrophoretic medium has also been found to produce a green color using the waveform shown in FIG. 5G with t17=t18=350 ms and t19=t20=t21=40 ms. The green color produced is V L5 It was found that the values ​​of V L5 The induced green L as a function of * , a * , and b * From Figure 6, V L5 Accompanying a * The fluctuation of b * Therefore, VL5 It will be seen that by careful selection of , colors ranging from yellow-green to cyan-green can be obtained.

[0074] The electrophoretic media shown in Figures 1, 3A-3G, and 5A-5G are capable of displaying the seven colors shown in Figures 3A-3G, respectively, with additional colors being generated by area modulation (dithering).

[0075] From the foregoing, it will be seen that the present invention can provide a four-particle electrophoretic medium capable of producing at least six useful colors using only four different types of particles.

[0076] Electrophoretic media and devices of the present invention may utilize any of the particles, fluids, encapsulation materials, and electrophoretic device designs described in the prior art, for example, as described below: (a) electrophoretic particles, fluids, and fluid additives (see U.S. Pat. Nos. 7,002,728 and 7,679,814); (b) capsules, binders, and encapsulation processes (see U.S. Patent Nos. 6,922,276 and 7,411,719); (c) microcell structures, wall materials, and methods of forming the microcells (see U.S. Patents Nos. 7,072,095 and 9,279,906); (d) methods of filling and sealing microcells (see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088); (e) Films and subassemblies including electro-optical materials (e.g., U.S. Pat. Nos. 6,825,829, 6,982,178, 7,112,114, 7,158,282, 7,236,292, 7,443,571, 7,513,813, 7,561,324, 7,636,191, 7,649,666, 7,728,811, 7,729,039, 7,791,782, 7,826,129, 7,839,564, 7,843,621, 7,843,624, 8,034,209, 8,068,272, 8,07 No. 7,381, No. 8,177,942, No. 8,390,301, No. 8,482,835, No. 8,786,929, No. 8,830,553 , No. 8,854,721, No. 9,075,280, No. 9,238,340, No. 9,470,950, No. 9,554,495, No. 9,563 ,099, 9,733,540, 9,778,536, 9,835,925, 10,444,591, and 10,466,564, and U.S. Patent Application Publication Nos. 2007 / 0237962, 2009 / 0168067, and 2011 / 0164301); (f) backplanes, adhesive layers, and other auxiliary layers and methods used in displays (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624); (g) Color formation and color control (e.g., U.S. Patent Nos. 6,017,584, 6,545,797, 6,664,944, 6,788,452, 6,864,875, 6,914,714, 6,972,893, 7,038,656, 7,038,670, 7,046,228, 7,052,571, 7,075,502, 7,167,155, 7,385,751, 7,492,505, 7,667,684, 7,684,108, 7,791,789, 7,800,813, 7, No. 821,702, No. 7,839,564, No. 7,910,175, No. 7,952,790, No. 7,956,841, No. 7,98 No. 2,941, No. 8,040,594, No. 8,054,526, No. 8,098,418, No. 8,159,636, No. 8,213, No. 076, No. 8,363,299, No. 8,422,116, No. 8,441,714, No. 8,441,716, No. 8,466,85 No. 2, No. 8,503,063, No. 8,576,470, No. 8,576,475, No. 8,593,721, No. 8,605,354, No. 8,649,084, No. 8,670,174, No. 8,704,756, No. 8,717,664, No. 8,786,935, No. 8 ,797,634, No.8,810,899, No.8,830,559, No.8,873,129, No.8,902,153, No.8,9 No. 02,491, No. 8,917,439, No. 8,964,282, No. 9,013,783, No. 9,116,412, No. 9,146 ,439, No. 9,164,207, No. 9,170,467, No. 9,170,468, No. 9,182,646, No. 9,195,11 No. 1, No. 9,199,441, No. 9,268,191, No. 9,285,649, No. 9,293,511, No. 9,341,916 , No. 9,360,733, No. 9,361,836, No. 9,383,623, No. 9,423,666, No. 9,436,056, No. No. 9,459,510, No. 9,513,527, No. 9,541,814, No. 9,552,780, No. 9,640,119, No. 9,6 No. 46,547, No. 9,671,668, No. 9,697,778, No. 9,726,959, No. 9,740,076, No. 9,759,No. 981, No. 9,761,181, No. 9,778,538, No. 9,779,670, No. 9,779,671, No. 9,812,073, No. 9,829,764 , No. 9,921,451, No. 9,922,603, No. 9,989,829, No. 10,032,419, No. 10,036,929, No. 10,036,931, No. No. 10,332,435, No. 10,339,876, No. 10,353,266, No. 10,366,647, No. 10,372,010, No. 10,380,931 , No. 10,380,955, No. 10,431,168, No. 10,444,592, No. 10,467,984, No. 10,475,399, No. 10,509,293 Nos. 2008 / 0043318, 2008 / 0048970, 2009 / 0225398, 2010 / 0156780, 2011 / 0043543, 2012 / 0326957, 2013 / 0242378, 2013 / 0278995, 2014 / 027911, and 2015 / 027911. 4 / 0055840, No. 2014 / 0078576, No. 2015 / 0103394, No. 2015 / 0118390, No. 2015 / 0124345, No. 2015 / 0268531, No. 2015 / 0301246, No. 2016 / 0026062, No. 2016 / 0048054, and No. 2016 / 0116818); (h) methods of driving displays (see, e.g., U.S. Pat. Nos. 7,012,600 and 7,453,445); (i) Display applications (see, e.g., U.S. Pat. Nos. 7,312,784 and 8,009,348).

[0077] An electrophoretic display typically comprises a layer of electrophoretic material and at least two other layers disposed on opposite sides of the electrophoretic material, one of these two layers being an electrode layer. In most such displays, both of the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongated row electrodes and the other into elongated column electrodes extending at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode, and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display.

[0078] The manufacture of a three-layer electrophoretic display usually involves at least one lamination operation. For example, some of the aforementioned patents and applications describe a process for manufacturing an encapsulated electrophoretic display, in which an encapsulated electrophoretic medium comprising capsules in a binder is coated onto a flexible substrate, which comprises an indium tin oxide (ITO) or similar conductive coating on a plastic film (which serves as one electrode of the final display), and the capsule / binder coating is dried to form a coherent layer of electrophoretic medium that is firmly attached to the substrate. Separately, a backplane is prepared that includes an array of pixel electrodes and an appropriate arrangement of conductors for connecting the pixel electrodes to a driving circuit. To form the final display, the substrate with the capsule / binder layer thereon is laminated to the backplane using a lamination adhesive. In one preferred form of such a process, the backplane itself is flexible, and the backplane is prepared by printing pixel electrodes and conductors onto a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive.

[0079] As discussed in the aforementioned U.S. Patent No. 6,982,178 (see paragraph 3, line 63 to paragraph 5, line 46), many of the components used in electrophoretic displays and the methods used to manufacture such displays are derived from the technology used in liquid crystal displays (LCDs). For example, electrophoretic displays utilize an active matrix backplane comprising an array of transistors or diodes and a corresponding array of pixel electrodes on a transparent substrate, and a "continuous" front electrode (in the sense of an electrode that spans multiple pixels and typically the entire display), and these components can be essentially the same as LCDs. However, the methods used to assemble LCDs cannot be used with encapsulated electrophoretic displays. LCDs are typically assembled by forming a backplane and a front electrode on separate glass substrates, then gluing and fixing these components together, leaving a small opening between them, placing the resulting assembly under a vacuum, and submerging the assembly in a bath of liquid crystals, so that liquid crystals flow through the openings between the backplane and the front electrode. Finally, with the liquid crystals in place, the openings are sealed to provide the final display.

[0080] This LCD assembly process cannot be easily transferred to encapsulated electrophoretic displays. Because electrophoretic materials are typically solid (i.e., have a solid outer surface), the electrophoretic material must be provided between the backplane and the front electrode before these two entities are secured to one another. Furthermore, in contrast to liquid crystalline materials that are simply placed between the front electrode and the backplane without being attached to either, the solid electro-optic medium usually needs to be secured to both; in most cases, the solid electro-optic medium is formed on the front electrode, because this is generally easier than forming the medium on the backplane that contains the circuitry, and the front electrode / electro-optic medium combination is then laminated to the backplane, typically by covering the entire surface of the electro-optic medium with an adhesive and laminating under heat, pressure, and possibly vacuum. Thus, while most prior art methods for final lamination of solid-state electrophoretic displays are essentially batch processes in which the electro-optic medium, lamination adhesive, and backplane are (typically) directly combined prior to final assembly, it would be desirable to provide a method that is better suited for mass production.

[0081] The aforementioned U.S. Patent No. 6,982,178 describes a method of fabricating solid electro-optic displays (including encapsulated electrophoretic displays) that are well adapted for mass production. Essentially, this patent describes a so-called "front plane laminate" ("FPL") that comprises, in order, a light-transmitting conductive layer, a layer of solid electro-optic medium in electrical contact with the conductive layer, an adhesive layer, and a release sheet. Typically, the light-transmitting conductive layer will be carried on a light-transmitting substrate that is preferably flexible, in the sense that the substrate can be manually rolled around a 10 inch (254 mm) diameter drum (for example) without permanent deformation. The term "optically transparent" is used in this patent and specification to mean that the layer so designated transmits sufficient light to enable an observer looking through the layer to observe a change in the display state of the electro-optic medium, usually as viewed through the conductive layer and adjacent substrate (if present); if the electro-optic medium exhibits a change in reflectance at non-visible wavelengths, the term "optically transparent" should of course be interpreted to refer to the transmission of the relevant non-visible wavelengths. The substrate is typically a polymeric film and will usually have a thickness within the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). The conductive layer may conveniently be a thin metal or metal oxide layer, for example, aluminum or ITO, or may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are commercially available, for example as "aluminized Mylar" from EI du Pont de Nemours & Company (Wilmington Del.) ("Mylar" is a registered trademark), and such commercially available materials can be used with good results in front plane laminates.

[0082] Assembly of an electro-optic display using such a front plane laminate can be effected by removing the release sheet from the front plane laminate and contacting the backplane and adhesive layer under conditions effective to adhere the adhesive layer to the backplane, thereby affixing the adhesive layer, the layer of electro-optic medium, and the conductive layer to the backplane. This process is well suited to mass production, since the front plane laminate can be produced in large quantities, typically using roll / roll coating techniques, and then cut into pieces of any size required for use with a particular backplane.

[0083] No. 7,561,324 describes a so-called "dual release sheet" that is essentially a simplified version of the front plane laminate of the aforementioned U.S. Pat. No. 6,982,178. One form of the dual release sheet comprises a layer of solid electrophoretic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the dual release sheet comprises a layer of solid electrophoretic medium sandwiched between two release sheets. Both forms of dual release film are generally intended for use in a process similar to the process for assembling electro-optic displays from the front plane laminates already described, but with two separate laminations, typically in a first lamination the dual release sheet is laminated to a front electrode to form a front subassembly, and then in a second lamination the front subassembly is laminated to a backplane to form the final display, although the order of these two laminations can be reversed if desired.

[0084] No. 7,839,564 describes a so-called "inverted front plane laminate," which is a variation of the front plane laminate described in the aforementioned U.S. Pat. No. 6,982,178. This inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light-transmissive conductive layer, an adhesive layer, a layer of solid electrophoretic medium, and a release sheet. This inverted front plane laminate is used to form an electro-optic display having a layer of laminating adhesive between the electrophoretic layer and the front electrode or front substrate, and a second, typically thin layer of adhesive may or may not be present between the electrophoretic layer and the backplane. Such electro-optic displays can combine good low temperature performance with good resolution.

Claims

[Claim 1] The invention described in this specification.

Images