Background
The term "electro-optic" as used herein in the context of materials or displays is its conventional meaning in the imaging arts, and refers to a material having first and second display states differing in at least one optical property, the material being transitioned from the first display state to the second display state by application of an electric field to the material. Although this optical property is typically a color that is perceptible to the human eye, other optical properties are possible, such as optical transmission, reflectance, luminescence, or, in the case of displays for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume either its first or second display state by an addressing pulse of finite duration, that state will last for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. Shown in the aforementioned U.S. patent application publication No. 2002/0180687: some particle-based electrophoretic displays capable of displaying gray levels are stable not only in their extreme black and white states, but also in their intermediate gray states, as are some other types of electro-optic displays. This type of display is properly referred to as "multi-stable" rather than bi-stable, but the term "bi-stable" as used herein covers both bi-and multi-stable displays for convenience.
Various types of electro-optic displays are known. One type of electro-optic display is the rotating bichromal member type (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is more accurate since the rotating member is not spherical in some of the above patents), such as disclosed in U.S. patent nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467 and 6,147,791. Such displays use a large number of small bodies (typically spherical or cylindrical) having two or more portions with different optical properties and an internal dipole. These bodies are suspended in liquid-filled vacuoles in a matrix, which vacuoles are filled with liquid so that the bodies can rotate freely. An electric field is applied to the display, thereby rotating the bodies to various positions and changing the location of those bodies as seen through the viewing surface, thereby changing the appearance of the display. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, such as in the form of a color-changing film (nanochromic film), which includes an electrode formed at least in part of a semiconducting metal oxide and a plurality of reversibly color-changeable dye molecules attached to the electrode. See, e.g., O' Regan, B. et al, Nature, 1991, 353, 737; wood, d., Information Display, 18(3), 24 (3.2002), and see Bach, u.et al, adv.mater.2002, 14(11), 845. Color shifting films of this type are also described, for example, in U.S. patent nos. 6,301,038, 6,870,657, and 6,950,220. This type of media is also typically bistable.
Another type of electro-optic display that has been extensively studied and developed over the years is a particle-based electrophoretic display in which a plurality of charged particles are passed through a suspending fluid under the influence of an electric field. Electrophoretic displays contribute to good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, long-term image quality issues of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in inadequate service life for these displays.
As indicated above, the presence of a fluid in an electrophoretic medium is desirable. In most prior art electrophoretic media this fluid is referred to as a liquid, but the electrophoretic medium may be made of a gaseous fluid; see, for example, "movement of electronic toner in electronic Paper-like displays" by Kitamura, T.et al, "toner displays using electrostatically charged insulating particles" by IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y.et al, "toner displays using electrostatically charged insulating particles" by IDW Japan, 2001, Paper AMD 4-4. See also U.S. patent publication No. 2005/0001810; european patent applications 1,462,847, 1,482,354, 1,484,635, 1,500,971, 1,501,194, 1,536,271, 1,542,067, 1,577,702, 1,577,703, 1,598,694; and international applications WO 2004/090626, WO2004/079442 and WO 2004/001498. Such gas-based electrophoretic media are susceptible to the same types of problems associated with particle settling as liquid-based electrophoretic media when the media is used in an orientation that allows such settling, for example for signage, in which the media is positioned on a vertical flat panel. In fact, the problem of particle settling is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media, because the lower viscosity of gaseous suspending fluids compared to liquid fluids causes the electrophoretic particles to settle more quickly.
A number of patents and applications, assigned to the institute of technology and technology (MIT) and the eink corporation, or both, have recently been published which describe encapsulated electrophoretic media. Such encapsulated media comprise a plurality of capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form an adhesive layer between two electrodes. For example, U.S. Pat. Nos. 5,930,026, 5,961,804, 6,017,584, 6,067,185, 6,118,426, 6,120,588, 6,120,839, 6,124,851, 6,172,798, 6,124,851, 6,262,833, 6,124,851, 36, 2003/0222315, 2004/0014265, 2004/0075634, 2004/0094422, 2004/0105036, 2004/0112750, 2004/0119681, 2004/0136048, 2004/0155857, 2004/0180476, 2004/0190114, 2004/0196215, 2004/0226820, 2004/00239614, 2004/0257635, 2004/0263947, 2005/0000813, 2005/0007336, 2005/0012980, 2005/0017944, 2005/0018273, 2005/0024353, 2005/0062714, 2005/0067656, 2005/0078099, 2005/0099672, 2005/0122284, 2005/0122306, 2005/0122563, 2005/0122565, 2005/0134554, 2005/0146774, 2005/0151709, 2005/0152018, 2005/0152022, 2005/0156340, 2005/0168799, 2005/0179642, 2005/0190137, 2005/0212747, 2005/0213191, 2005/0219184, 2005/0253777, 2005/0270261, 2005/0280626, 2006/0007527, 2006/0024437, 2006/0038772, 2006/0139308, 2006/0139310, 2006/0139311, 2006/0176267, 2006/0181492, 2006/0181504, 2006/0194619, 2006/0197736, 2006/0197737, 2006/0197738, 2006/0198014, 2006/0202949 and 2006/0209388, and international application publication nos. WO 00/38000, WO 00/36560, WO 00/67110 and WO 01/07961, and european patent nos. 1,099,207B 1 and 1,145,072B 1.
Many of the above patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium can be replaced with a continuous phase, thus producing a so-called dispersed polymer (polymer-dispersed) electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and the discrete droplets of the electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even if no discrete capsule membrane is associated with each individual droplet; see, for example, the aforementioned U.S. patent No.6,866,760. Thus, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.
Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely on changes in electric field strength, can also operate in a similar mode; see U.S. patent No.4,418,346.
Encapsulated electrophoretic displays typically do not suffer from the aggregation and settling failure modes of conventional electrophoretic display devices and have additional advantages such as the ability to coat or print the display on a variety of flexible and rigid substrates. (use of the word "printing" is intended to include, but is not limited to, various forms of printing and coating such as pre-metered coating such as patch die coating, slot or die coating, slide or cascade coating, curtain coating, roll coating such as knife over roll coating, forward and reverse roll coating, gravure coating, dip-draw coating, spray coating, meniscus (meniscus) coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, and other similar techniques). Thus, the manufactured display may be flexible. In addition, since the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.
A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and suspending fluid are not encapsulated in microcapsules but are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, International application publication No. WO02/01281 and published U.S. application No.2002/0075556 (both assigned to Sipix Imaging, Inc.).
Other types of electro-optic media may also be used in the displays of the present invention.
Typically, an electro-optic display comprises a layer of electro-optic material and at least two further layers, one of which is an electrode layer, disposed on opposite sides of the electro-optic material. In most such displays both layers are electrode layers, and one or both of the electrode layers is patterned to define the pixels of the display. For example, where one electrode layer is patterned into elongate row electrodes and the other electrode layer is patterned into elongate column electrodes extending at right angles to the row electrodes, the pixels are defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer is in the form of a single continuous electrode, while the other electrode layer is patterned as a matrix of pixel electrodes, each defining one pixel of the display. In another type of electro-optic display intended to employ a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent to the electro-optic layer includes an electrode, the layer on the opposite side of the electro-optic layer generally acting as a protective layer to prevent the movable electrode from damaging the electro-optic layer.
The manufacture of a three-layer electro-optic display typically involves at least one lamination process. For example, in the aforementioned multiple MIT and E Ink patents and applications, a process is described for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in an adhesive is coated on a flexible substrate comprising Indium Tin Oxide (ITO), or similar conductive coating coated on a plastic film, which serves as one electrode of the final display, and the capsule/adhesive coating is dried to form an adhesive layer of the electrophoretic medium that adheres strongly to the substrate. A backplane (backplane) is separately prepared, the backplane comprising an array of pixel electrodes and appropriately arranged conductors for connecting the pixel electrodes to drive circuitry. To form the final display, the substrate with the bladder/adhesive layer is laminated to the backplane using a laminating adhesive (by replacing the backplane with a simple protective layer such as a plastic film, a stylus or similar movable electrode is allowed to slide over it, so that a very similar process can be used to prepare an electrophoretic display that can be used with a stylus or similar movable electrode). In a preferred form of the process, the backplane is itself flexible and it can be prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. One obvious lamination technique for mass production of displays by this process is roll lamination with a lamination adhesive. Similar manufacturing techniques may also be used for other types of electro-optic displays. For example, a microcell electrophoretic medium or a rotating bichromal element medium may be laminated to the backplane in substantially the same manner as the encapsulated electrophoretic medium.
As described in the aforementioned U.S. patent No.6,982,178, many of the components used in solid state electro-optic displays, and the methods for making such displays, are derived from the technology used in Liquid Crystal Displays (LCDs), which are obviously also electro-optic displays, although they employ liquid rather than solid media. For example, a solid-state electro-optic display may employ an active matrix backplane comprising an array of transistors or diodes, and a corresponding array of pixel electrodes, substantially as in an LCD, and a "continuous" front electrode (in the sense of an electrode that extends over a plurality of pixels, typically the entire display) on a transparent substrate. However, the methods used to assemble LCDs cannot be used with solid state electro-optic displays. Generally, assembling an LCD requires that the liquid crystal flow through the aperture between the back plate and the front electrode by making the back plate and the front electrode on separate glass substrates, then adhesively securing the components together with a small void between them, and immersing the resulting device under vacuum into a bath of liquid crystal. Finally, the apertures are sealed as the liquid crystal flows into place, thereby obtaining the final display.
This LCD assembly process cannot be easily transferred to a solid state electro-optic display. Since the electro-optic material is solid, it must be positioned between the backplane and the front electrode before the two entities are fixed to each other. Furthermore, the liquid crystal material may simply be placed between the front electrode and the backplane without being attached to either of these, as is the case with liquid crystal materials, where solid electro-optic media typically need to be secured to both the front electrode and the backplane; the solid electro-optic medium is in most cases formed on the front electrode because this is usually easier than forming the medium on a backplane containing the circuitry, and the front electrode/electro-optic medium combination is then typically laminated to the backplane by covering the entire surface of the electro-optic medium with an adhesive and laminating under heat, pressure and possibly vacuum.
As mentioned in the aforementioned U.S. patent No.6,312,304, there is a problem with the manufacture of solid state electro-optic displays in that the optical components (electro-optic medium) and the electronic components (in the backplane) have different performance criteria. For example, it is desirable for optical components to optimize reflectivity, contrast, and response time, while for electronic components to optimize conductivity, voltage-current relationship, and capacitance, or to have the capability of memory, logic, or other advanced (higher-order) electronics. Thus, the process for manufacturing optical components is not ideal for manufacturing electronic components, and vice versa. For example, a process for manufacturing an electronic component may include processing at an elevated temperature, which may be in the range of about 300 ℃ to about 600 ℃. However, subjecting many optical components to such high temperatures can cause damage to the optical components by chemically degrading the electro-optic medium or causing mechanical damage.
This patent describes a method of manufacturing an electro-optic display comprising providing a modulation layer (modulating layer) comprising a first substrate and an electro-optic material disposed adjacent to the first substrate, the modulation layer being capable of changing a visible state in accordance with an applied electric field; providing a pixel layer comprising a second substrate, a plurality of pixel electrodes disposed on a front surface of the second substrate, and a plurality of contact pads disposed on a back surface of the second substrate, wherein each pixel electrode is connected to a contact pad by a via extending through the second substrate; providing a circuit layer comprising a third substrate and at least one circuit element; and laminating the modulation layer, pixel layer, and circuit layer to form an electro-optic display.
Electro-optic displays are generally expensive, for example, the cost of a color LCD found in a laptop computer is generally a significant portion of the total cost of the computer. As electro-optic displays are widely used in devices such as cell phones and Personal Digital Assistants (PDAs) that are much cheaper than portable computers, there is a pressing need to reduce the cost of such displays. As described above, this ability to form layers of some solid electro-optic media on flexible substrates by printing techniques makes it possible to reduce the cost of the electro-optic components of displays by mass production of displays using techniques such as roll-to-roll coating, which uses commercial equipment for the production of coated paper, polymer films, and similar media. However, such devices are expensive and the area of electro-optic medium currently on the market is not sufficient to allow the use of dedicated devices, and so this typically requires the transfer of the coated medium from a commercial coating apparatus to an apparatus for the final assembly of an electro-optic display without damage to the relatively fragile layer of electro-optic medium.
Moreover, most of the existing methods for final lamination of electro-optic displays are essentially batch methods in which the electro-optic medium, lamination adhesive and backplane are only combined together just prior to final assembly, and thus there is a need for methods that are better suited for high volume production.
The aforementioned U.S. patent No.6,982,178 describes a method of assembling solid state electro-optic displays, including particle-based electrophoretic displays, which is well suited for mass production. This patent essentially describes a so-called "front plane laminate" (FPL) comprising, in order, a light-transmissive electrically conductive layer, a layer of solid electro-optic medium in electrical contact with the electrically conductive layer, an adhesive layer, and a release sheet. Typically, the light-transmissive electrically conductive layer is mounted on a light-transmissive substrate, which is preferably flexible in the sense that the substrate can be manually wound into, for example, a 10 inch (254 mm) diameter cylinder without permanent deformation. The term "light transmissive" as used herein and in this patent means that the layer in question is capable of passing sufficient light to enable a viewer to observe through the layer, typically viewed through the electrically conductive layer and adjacent substrate (if any), a change in the display state of the electro-optic medium. The substrate is typically a polymer film and typically has a thickness in the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). Suitably, the electrically conductive layer is a thin metal or metal oxide layer, such as aluminium or ITO, or a conductive polymer. Polyethylene terephthalate (PET) films coated with aluminum or ITO are commercially available, for example, "aluminized Mylar" (Mylar "is a registered trademark) from dupont DE Nemours & Company, Wilmington DE, Wilmington, a Company of Wilmington, delaware, such commercial materials can have good results for front plane lamination.
The combination of electro-optic displays with such front plane lamination can be achieved by: the release layer is removed from the front plane lamination and the adhesive layer is contacted with the backplane under conditions effective to promote adhesion of the adhesive layer to the backplane, thereby securing the adhesive layer, the layer of electro-optic medium, and the electrically conductive layer to the backplane. The process is well suited for mass production, as front plane lamination is typically mass produced using roll-to-roll coating techniques and then cut into pieces of any size required for use with a particular backplane.
The aforementioned U.S. patent No.6,982,178 also describes a method for testing electro-optic media in a front plane laminate prior to incorporating the front plane laminate into a display. In this test method a release layer is provided with an electrically conductive layer and a voltage sufficient to change the optical state of the electro-optic medium is applied between the electrically conductive layer and the electrically conductive layer on the opposite side of the electro-optic medium. Observation of the electro-optic medium then reveals any imperfections in the medium and thus avoids lamination of imperfect electro-optic medium into the display, which would otherwise ultimately cost discarding the entire display rather than just the imperfect front plane lamination.
The aforementioned U.S. patent No.6,982,178 also describes a second method for testing electro-optic media in front plane lamination by placing an electrostatic charge on a release layer to form an image on the electro-optic medium. The image is then viewed in the same manner as before to detect any imperfections in the electro-optic medium.
The foregoing 2004/0155857 describes a so-called "dual release film" which is essentially a simplified version of the front plane lamination of the foregoing U.S. Pat. No.6,982,178. One form of dual release sheet comprises a layer of solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of dual release sheet comprises a layer of solid electro-optic medium sandwiched between two release layers. Both forms of the dual release film are used in a process substantially similar to that already described for assembling an electro-optic display from a front plane lamination, but include two separate laminations, typically a first lamination in which the dual release layer is laminated to the front electrode to form a front sub-assembly (frontsub-assembly) and then a second lamination in which the front sub-assembly is laminated to the backplane to form the final display.
Electro-optic displays fabricated using the aforementioned front plane lamination or dual release film have a layer of lamination adhesive between the electro-optic layer itself and the backplane, and the presence of this lamination adhesive layer affects the electro-optic characteristics of the display. In particular, the electrical conductivity of the laminate adhesive layer can affect the low temperature performance and resolution of the display. The low temperature performance of the display (which has been empirically found) can be improved by increasing the electrical conductivity of the lamination adhesive layer, for example, by doping the layer with tetrabutylammonium hexafluorophosphate or other material as described in U.S. Pat. No.7,012,735 and publication No. 2005/0122565. However, increasing the conductivity of the laminated adhesive layer in this manner tends to increase pixel shading (a phenomenon in which the area of the electro-optic layer that transitions to an optical state in response to a voltage change at the pixel electrode is greater than the area of the pixel electrode itself), and this shading tends to reduce the resolution of the display. Therefore, this type of display obviously essentially requires a compromise between low temperature performance and display resolution, which in practice usually sacrifices low temperature performance.
This low temperature performance and display resolution variation with the conductivity of the laminating adhesive can be understood as a stacked resistor module whereby the electro-optic layer and the laminating adhesive layer are modularized into two resistors connected in series between the display electrodes. As the electrical conductivity of the lamination adhesive layer increases, a greater voltage is applied between the electrodes to fall on the electro-optic layer. When the electrical conductivity of the lamination adhesive layer is about 10 times greater than the electrical conductivity of the electro-optic layer, substantially the full value of the applied voltage is used to switch the electro-optic layer, so that further increases in the electrical conductivity of the lamination adhesive do not improve the electro-optic performance. However, since the lamination adhesive cannot sustain the lateral difference in potential, the electrical conductivity of the lamination adhesive cannot be made too high, with the result that a loss of resolution will result, at least part of the spatial information in the backplane being destroyed due to short-circuiting of the lamination adhesive between adjacent electrodes.
For all known useful lamination adhesives, the temperature dependence of the electrical conductivity of the lamination adhesive is greater than that of the electro-optical layer, at least when the electro-optical medium is an encapsulated electrophoretic medium. The conductivity of both layers decreases with temperature, but the conductivity of the laminating adhesive decreases more rapidly. If the laminating adhesive is formulated so that it can only provide good electro-optical properties at room temperature, the electrical conductivity of the laminating adhesive will quickly become weaker than the electro-optical layer as the temperature is lowered. In these cases the applied voltage is divided so that only a very low potential drop is applied across the electro-optic layer, and instead a large portion of the potential drop is applied across the lamination adhesive layer, which therefore does not contribute to the switching of the electro-optic layer.
There is therefore a need for an electro-optic display having improved low temperature performance without affecting the resolution of the display, and the present invention seeks to provide such an electro-optic display and components, and methods for making them.
Preferred forms of the invention can also facilitate the manufacture of colour electro-optic displays. Most types of electro-optic media have only a limited number of optical states, such as a dark (black) state, a bright (white) state, and in some cases one or more intermediate gray states. Therefore, in order to construct full-color displays using these media, it is common practice to place the electro-optic medium adjacent to a color filter array (color filter array) having, for example, a plurality of red, green and blue regions, and to provide the electro-optic medium with drive means that allow individual control of the medium adjacent to each red, green or blue region. The use of certain color filter arrays for electrophoretic displays is described in the aforementioned U.S. patent No.864,875. The foregoing 2003/0011560 describes ways to improve the optical performance of electrophoretic displays by assembling optical biasing elements in any of the various components of the display.
Detailed Description
As already explained, the present invention alleviates, or even eliminates, the problem of compromise between resolution and low temperature performance currently encountered in electro-optic displays by reversing the order of the electro-optic layer and the laminated adhesive layer in the stack forming the final display, so that the high resolution parts of the display, such as the backplane, in particular the Thin Film Transistor (TFT) backplane, are either in direct contact with the electro-optic layer, or are separated only by a secondary adhesive layer having only a small thickness. With a display of this construction, the display resolution is substantially independent of the electrical conductivity of the laminating adhesive, so that a laminating adhesive with sufficient electrical conductivity to obtain good low temperature performance can be used without affecting the display resolution.
The primary adhesive layer present between the electro-optic layer and the front electrode or between the electro-optic layer and the front protective layer in the displays of the present invention may provide a suitable color filter array.
Figure 1 is a highly schematic drawing of a cross-section through a first electro-optic display according to the invention. The display, generally designated 100, comprises a backplane 102, said backplane 102 comprising a plurality of pixel electrodes, and which may be of any conventional type, such as a TFT active matrix backplane or a direct drive backplane in which each pixel electrode is provided with a separate voltage supply line so that a controller (not shown) can independently control the voltage of each pixel. Any of the types of electro-optic layers 104 described above are in direct contact with the backplane 102; optionally, a thin (typically less than 10 μm, or less than half the thickness of the primary lamination adhesive described below) auxiliary layer of lamination adhesive (not shown) may be provided between the backplane 102 and the electro-optic layer 104. On the other side of the electro-optic layer 104 opposite the backplane 102 is disposed a primary laminate adhesive layer 106, the primary laminate adhesive layer 106 being selected to provide good low temperature performance, for example, it may be a highly doped polyurethane adhesive. The last two layers of the display 100 are a front light-transmissive electrically conductive electrode layer 108 and a light-transmissive protective layer 110, as described in the aforementioned U.S. Pat. Nos. 6,982,178 and 7,110,164, and U.S. patent application publication No.2004/0155857, the layers 108 and 110 may conveniently be provided by commercially available polymeric films coated with a very thin conductive layer, such as Indium Tin Oxide (ITO) or aluminum-coated polyethylene terephthalate (PET) films.
As explained above, only one of the layers 108 and 110 need be present in the display or inverted front plane laminate of the present invention. At least in theory, the protective layer 110 may be omitted if the electrode layer 108 has sufficient mechanical robustness to remain intact in normal operation; in practice, however, the light-transmissive electrode is typically thin and requires some form of protective layer. In order to be able to provide a maximized voltage drop over the electro-optical layer 104 and thus a fastest switching speed, it is obvious that a protective layer should be arranged on the other side of the electrode layer opposite the electro-optical layer. In some types of displays, such as those using a stylus or an external print head, the electrode layer 108 may be omitted.
Fig. 2 illustrates a second display (generally indicated at 200) of the present invention which is substantially similar to that of fig. 1, but which is intended to display full color images. The display 200 includes a backplane 202 which may still be of any conventional type. The backplane 202 is illustrated as including three pixel electrodes 203R, 203G, and 203B. An electro-optic layer 104, equivalent to that shown in FIG. 1, is in direct contact with the backplane 202. A main laminating adhesive layer is arranged on the other side of the electro-optical layer 104 opposite to the back plate 202, which is colored to form red, green and blue stripes 206R, 206G and 206B, respectively, which are aligned with the corresponding pixel electrodes 203R, 203G and 203B, respectively. As is well known to those skilled in the art of displays, the alignment of these differently colored stripes with the pixel electrodes is necessary for accurate color reproduction, thereby ensuring that colors are written independently of each other on the display. Thus, such a laminated adhesive layer may also act as a color filter array. The last two layers of the display, equivalent to those shown in figure 1, are a front light-transmissive electrically conductive electrode layer 108 and a light-transmissive protective layer 110.
FIG. 3 is a schematic cross-sectional view through a third display (generally designated 300) of the present invention, which is substantially similar to the display 200 shown in FIG. 2, but in which a thin secondary adhesive layer 312 is disposed between the backplane 202 and the electro-optic layer 104. The secondary adhesive layer 312 is not colored; because most electro-optic media are opaque and the display 300 is viewed through the protective layer 110, the secondary adhesive layer 312 is not visible to a viewer and thus the secondary adhesive layer 312 typically cannot be functionalized as a color filter array. However, if the electro-optic layer 104 is intended to operate in a shutter mode and the backplane 202 is made light transmissive so that the display 300 is viewable in transmission, the secondary adhesive layer may act as a color filter array.
Displays having the structure shown in fig. 1 to 3 and having a similar structure can also be manufactured in a large number of ways. One method for manufacturing such displays is to coat an electro-optic layer directly onto a backplane, the electro-optic layer being of a type that allows coating on the backplane (e.g., an encapsulated electrophoretic layer). In most cases, this method is not preferred because it requires the separate preparation of a large number of separate, expensive components, i.e., separate backplanes; this is also difficult to do, since it is easier and more economical to achieve coating on a roll-to-roll substrate or at least on a large flat substrate that can be coated using bar coating or hopper coating (coating) than on a large number of small, separate backing plates.
In most cases it is convenient to coat the electro-optical layer on a release sheet, i.e. a removable sheet covered with a release layer. The resulting electro-optic medium/release plate assembly is laminated to a laminating adhesive layer, which may be coated on either the second release plate or a conductive transparent electrode or a transparent protective layer, such as the aforementioned PET/ITO film. If the layer of laminating adhesive is an electrode or protective layer, the resulting structure is an inverted front plane laminate of the present invention, so named because the final structure is substantially identical to the front plane laminate described in the aforementioned U.S. Pat. No.6,982,178, except that the order of the electro-optic layer and laminating adhesive layer is inverted.
This inverted Front Plane Lamination (FPL) can be used to form the final display by removing the release sheet adjacent to the electro-optic layer and laminating the remaining layers to the backplane. If the backplane is sufficiently smooth and the lamination conditions used are closely respected, good, void-free lamination can be achieved and the resulting display can exhibit good low temperature performance and high resolution. If a problem with void formation (i.e., areas where the electro-optic medium fails to adhere to the backplane) is found, the release sheet adjacent the electro-optic layer can be removed from the inverted FPL and the remaining layers laminated to a thin layer of a thin laminating adhesive that has previously been coated on a separate release sheet to form a modified inverted FPL comprising a layer of the secondary laminating adhesive. After the release sheet covering the secondary adhesive layer is removed, the modified inverted FPL is laminated to the backplane with improved adhesion to the backplane in the same manner as previously described. Since the surface of the electro-optical layer exposed by removing the release sheet is very smooth (since the electro-optical layer is coated on a smooth support), a very thin (in some cases only 1 μm or less) layer of the auxiliary laminating adhesive is sufficient in most cases. Such a small thickness of the adhesive layer is not sufficient to affect the electro-optical performance and the display resolution. In fact, any thickness of the secondary laminate adhesive layer that is less than the primary laminate adhesive layer will result in some improvement in performance. The electrical conductivity of the secondary laminate adhesive layer can also be varied if desired. The thicker the secondary layer, the weaker its electrical conductivity, but if it is very thin (about 1 to 10 μm), the secondary layer generally has better electrical conductivity than the primary laminate adhesive layer without affecting the performance enhancement provided by the present invention.
For some applications, such a completely symmetrical structure of a layer of lamination adhesive having the same thickness on each side of the electro-optical layer is beneficial. The structure should have effectively the same symmetrical electrical response, which is expected to reduce or eliminate some type of electro-optic artifacts (artifacts). As described in 2004/0155857 above, this display structure can be made using a symmetrical dual release film.
Optionally, the electro-optical layer/release sheet subassembly described previously is laminated to one of the laminate adhesive layers coated on the second release sheet, resulting in a structure (in effect, a modified dual release film) comprising, in order, the first release sheet, the electro-optical layer, the laminate adhesive layer, and the second release sheet. Alternatively, it has been shown possible to remove either release plate from the modified double release film. In fact, this improved dual release film is equivalent to a free standing electro-optic layer, and the device can be constructed in a variety of ways as described in the aforementioned U.S. patent No.7,110,164.
The invention is particularly applicable to flexible color displays. Although considerable progress has been made in recent years in the manufacture of flexible backplanes including flexible Thin Film Transistor (TFT) backplanes, substantial difficulties remain in: the method includes the steps of fabricating a flexible Color Filter Array (CFA), aligning pixel electrodes with CFA elements in a display assembly, and maintaining this alignment when the display is bent during use.
More specifically, one important challenging aspect in manufacturing flexible color displays is the manufacturing of the CFA itself. In general, the use of flexible transparent substrates requires low processing temperatures, which is a problem when manufacturing CFAs with conventional photoresists. Due to the lack of dimensional stability during processing, achieving alignment and positioning over large areas is difficult, and in addition to these problems, there are substrate non-uniformity issues.
The use of a pigmented laminate adhesive layer as a CFA according to the invention has many advantages. When dyes are used to provide color in the CFA, the dyes may be incorporated into the laminating adhesive polymer, which may be a water-based polymer latex or a solvent-based polymer used as a laminating adhesive. A dye having a suitable solubility must be selected for use in the laminating adhesive to be used. The laminating adhesive may also be colored with a pigment. Water or oil soluble pigments should be selected as appropriate for the laminating adhesive to be used. The pigment has the advantage that it does not have a major influence on the dielectric or conductive properties of the laminating adhesive and it does not flow in the laminating adhesive, but, on the contrary, some dyes do flow in the laminating adhesive.
It is desirable that the pigmented lamination adhesive layer be thin (typically 10-50 μm) to reduce the voltage drop across the lamination adhesive layer and thus reduce the drive voltage required for the display. In such a thin laminated adhesive layer, it is difficult to obtain sufficient extinction with the dye because the dye has limited solubility in water or a solvent. Therefore, in many cases, finely divided pigments (particle size typically 5-50nm) are preferably used in order to still allow a thin laminate adhesive layer to have a high matting effect while maintaining light transmission.
As described above, in the manufacturing process of the color display according to the present invention, the laminating adhesive may be applied directly to the exposed surface of the electro-optic layer previously coated on the support, or the laminating adhesive may be applied to a separate substrate (which may be a release sheet or an electrode) and the resulting assembly laminated to the electro-optic layer. This provides a low cost method of manufacturing an inverted front plane laminate that includes the entire CFA. The use of such an FPL with an overall CFA can broaden the range of materials available for the front substrate, which can now employ essentially any light-transmissive conductive or protective layer (made of glass, plastic or other material) and need not have the ability to form a CFA by painting. The use of an inverted FPL with an entire CFA also allows for a wider range of backplane materials to be used. Furthermore, since the CFA is built in a laminate adhesive layer, it is flexible in itself, and closely laminating the CFA to the electro-optic layer reduces misalignment issues that may be encountered when the display is bent during use.
The single-color form (single-color form) of the displays of the present invention can be formed using unpatterned pigmented lamination adhesives and the lamination and combinations shown above.
Various methods may be used to apply, coat and/or color the lamination adhesive layer to form the CFA therein; the method selected varies depending on the size of the individual coloring elements of the CFA. For example, the lamination adhesive may be deposited by a screen printing process; a pigmented polymer latex is formed having suitable rheological and wetting properties to enable printing by screen printing type processes. Alternatively, the laminating adhesive may be deposited by offset printing, and a pigmented polymer latex with appropriate rheological and wetting characteristics may be formed for offset printing. Since offset printing is typically performed on webs (webs), such as when making newspapers, offset printing should be able to form sheets with the entire CFA of laminating adhesive at low cost in preparation for lamination to an electro-optic layer. Microcontact printing may also be used; colored polymer latexes having suitable rheological and wetting properties are formed for microcontact printing of fluids.
The lamination adhesive layer with the entire CFA can also be made by an inkjet or bubble jet (bubble jet) printing process of a colored lamination adhesive. Most inkjet or bubble jet printers use droplets of a colored fluid (typically about 10 μm in diameter) to eject the aqueous colored fluid onto the substrate to be printed. As described in many of the aforementioned E Ink and MIT patents and applications, polyurethane latexes are commonly used as lamination adhesives in electro-optic displays, and the particle diameter in such latexes is typically on the order of 100nm, and is therefore very small relative to the Ink-jet droplets. Thus, such emulsions are fully compatible with inkjet and bubble inkjet printing. Pigment particles on the order of 10nm in diameter, if properly suspended, can also be readily carried by inkjet and bubble droplets, while dyes and solvated polymers can be readily carried in such droplets. Finally it should be noted that inkjet and bubble inkjet printing are low temperature processes and therefore pose a lower risk on dimensional stability problems when patterning is achieved on plastic substrates.
The laminated adhesive layer with the entire CFA can also be formed by ink-jet or bubble-jet printing a dye onto a layer of pre-made laminated adhesive. Depending on the type of dye used, it is possible to print and/or diffuse the dye into the laminate adhesive layer. The impact of the inkjet fluid on the electrical properties of the laminating adhesive as well as its laminating characteristics should be taken into account.
The lamination adhesive with the entire CFA can also be made by a resist process. If the laminating adhesive is curable (crosslinkable) in localized areas, a distinction can be made in solubility from conventional solvents, allowing patterning to be performed. For example, it is known to cure polyurethane-polyacrylate latexes using ultraviolet or visible radiation. This material may be cured using a laser, or through a photomask or other process to form a non-dissolving rubbery material. The unexposed areas of the polymer can then be rinsed away and the operation repeated to form the different colored elements of the CFA in sequence. Depending on the nature of the lamination adhesive, the smoothness of the electro-optic layer, and the degree of curing, the CFA may be patterned on the release sheet to form the CFA as a separate subassembly prior to lamination to the electro-optic layer. Alternatively, the patterning can be done directly on the electro-optic layer if the required processing conditions are compatible with the electro-optic layer. Thermal curing may be used instead of radiation curing if it is possible to generate a sufficiently high temperature in a local area without damaging other components of the display.
The pattern need not be limited to a CFA sub-pixel array. The laminating adhesive may be patterned as a color overlay (e.g., by screen printing) and laminated directly to the optical layer.
Additives other than coloring materials can be effectively incorporated into the lamination adhesive layer used in the display of the present invention. For example, a uv absorbing compound (e.g., tianlero (Tinuvin) -registered trademark) may be incorporated into the lamination adhesive to protect the electro-optic layer from uv exposure. Such uv absorbers incorporated into the laminating adhesive may eliminate the need for a uv filter layer applied to the front protective layer of the display. Similarly, a diffusion layer can be formed by incorporating light scattering or diffractive materials (e.g., glass beads) into the laminating adhesive, thereby forming a display with a matte appearance.
Regardless of how precise the method used to form the lamination adhesive with the entire CFA is, it is necessary to align the coloring elements of the CFA with the pixel electrodes of the backplane. These can be achieved by placing aligned marks on one side of the CFA during the printing process, or by using an optical registration mechanism to form such marks afterwards.
The invention has the advantages that: a color filter array is formed in a lamination adhesive layer disposed between an electro-optic layer and a viewing surface of the display, and is positioned adjacent to the electro-optic layer to minimize parallax problems. A color filter array may also be provided in a flexible polymer layer already present in the display. Alignment of the colour filter array with the backplane electrode is easily achieved and in the case of a flexible display it is also easy to maintain alignment when the display is flexed in use. The choice of materials for the front protective layer and similar layers is broadened because the layers need not have the ability to incorporate or support a color filter array. Depending on performance and cost requirements, there are a number of methods for patterning color filter arrays at different resolutions. Some of these methods can be performed on a continuous web of material and form a front plane laminate with the entire color filter array that is inexpensive. Additional additives such as uv absorbers may be incorporated into the laminate adhesive layer, reducing the requirements for other layers of the display. For some applications requiring a single color, the use of a colored laminating adhesive is an inexpensive method of providing a large number of different colors.
The incorporation of uv absorbers (optical filters) in the displays of the invention and similar electro-optic displays, especially thin flexible electro-optic displays, is a considerable further consideration because many types of electro-optic media are sensitive to uv radiation. There are three basic methods of providing the desired uv absorbing layer. In the first method, an ultraviolet absorbing dye is incorporated into a polymer layer forming a protective layer (front substrate) of a display. In the second method, the uv-absorbing material is coated as a separate layer on one (or possibly both) surfaces of the front substrate. Such uv absorbing coatings are well known in the display industry and it is therefore within the level of skill of the art to apply such coatings to polymer films commonly used as front substrates for displays of the present invention. Since the surface of the front substrate facing the electro-optic layer will typically carry ITO or similar electrodes, it is preferred to coat the other (usually exposed) surface of the front substrate with an ultraviolet absorber. In the third method, an ultraviolet absorber is contained in the adhesive layer. The incorporation of the absorber in the front adhesive layer has already been described. However, in many cases, the front substrate used in the display of the present invention may be a complex multi-layer structure that needs to be combined by at least one lamination operation using a lamination adhesive, and it is also more convenient to incorporate an ultraviolet absorber in the lamination adhesive used to combine such a multi-layer front substrate.
It will be apparent to those skilled in the art of electro-optic displays that the displays of the present invention, whether monochrome or color, may incorporate any of the optional features of the prior art electro-optic displays described in the aforementioned U.S. Pat. Nos. 6,982,178 and 7,110,164 and application publication No. 2004/0155857. For example, the display of the present invention and the inverted front plane lamination may incorporate any of the various conductive vias, edge seals, protective layers, and other optional features described in these published applications.
It has been found that the electro-optic performance of displays made from prior art FPLs and from the inverted FPL of the invention is similar despite the fact that the latter display has an additional layer (lamination adhesive layer) between the electro-optic layer and the viewing surface of the display. Although the following examples are given by way of illustration only, to illustrate the different aspects of the invention and the improvements in performance that can be achieved in the displays of the invention.
Example 1: fabrication of experimental displays
Slurries containing electrophoretic cells in a polymeric binder were prepared substantially as described in U.S. patent publication No.2002/0180687, paragraphs [0067] to [0074], except that Dow Corning Q2-521, "super wetting agent" was added as a coating aid. The slurry bar was then coated onto the aluminum coated surface of an aluminized PET release sheet. In one procedure, a 20 μm lamination adhesive layer of the type described in U.S. Pat. No.7,012,735, doped with 20000ppm tetrabutylammonium hexafluorophosphate, has been pre-coated on the same aluminized surface; in a second procedure, a layer of the same laminating adhesive, also 20 μm, was laminated to the ITO-covered surface of the PET/ITO release sheet and the resulting subassembly was laminated to the electrophoretic layer/release sheet assembly, with the adhesive layer laminated to the electrophoretic layer.
The laminating adhesives used are known to achieve relatively good low temperature performance, but give displays with poor room temperature resolution.
When two structures manufactured as described above are used to produce different types of displays, it can be obtained by the following three different processes:
1. the release layer covering the electrophoretic layer is removed and the remaining layer (PET/ITO/laminating adhesive/electrophoretic layer) is laminated to a second layer with the same laminating adhesive on another release sheet, such that the electrophoretic layer is arranged between two similar laminating adhesive layers, so that a substantially symmetrical structure is obtained. The other release sheet was then removed from the second laminate layer and the remaining layers were laminated to a 2 inch (51 mm) square carbon black coated backsheet to obtain a functional experimental single pixel electrophoretic display, designated "FPL 1".
2. With some care, it was found that the release layer could be peeled off from either side of the dual release structure. If the release sheet covering the electrophoretic layer is removed, the remaining layer will (unexpectedly) be laminated directly to the ITO on the polymer film to obtain a non-inverted FPL structure (denoted "FPLnorm") equivalent to the prior art.
3. Alternatively, after removing the release sheet covering the electrophoretic layer, the remaining layer is laminated to a second layer of the same laminating adhesive on another release sheet, in order to obtain a second substantially symmetrical structure, which differs from the symmetrical structure produced in process step 1 only in which side of the electrophoretic layer is smooth, which will undoubtedly be adjacent to the coating support. The release layer on either side of the resulting structure can be peeled off and the remaining layers laminated to the ITO-covered surface of the ITO/PET film to obtain either of two related front plane laminates, denoted "FPL 1 '" and "FPL 1", where "FPL 1'" is structurally equivalent to the aforementioned FPL 1 and "FPL 1 '" has an electrophoretic layer inverted with respect to FPL 1 and FPL 1'.
Each of FPLnorm, FPL 1', and FPL 1 "was laminated to the carbon backplane in the foregoing manner, so as to obtain an experimental single-pixel electrophoretic display. (it should be noted that the surface of the carbon black backsheet should be rough enough to prevent the preparation of the inverted structure of the present invention without an adhesive layer laminated between the electrophoretic layer and the backsheet). Similar experimental displays were also made using an ITO-on-glass backplane for resolution testing as described in example 3 below, however in this display a true inverted structure was obtained since the ITO-on-glass backplane was sufficiently smooth that lamination of the electrophoretic layer could be achieved without any intermediate lamination adhesive. All displays were incubated for 5 days at 30% relative humidity before being subjected to the electro-optic test described in example 2 below.
The following table summarizes the structures made on a carbon black backsheet, where "BP" represents the backsheet, "LA" represents the lamination adhesive layer, and "ELP" represents the electrophoretic (including capsule) layer.
| Numbering | Structure of the product | Remarks for note |
| Control substance | BP/LA/ELP/ITO | Coating the slurry on ITO to obtain FPL |
| FPLnorm | BP/LA/ELP/ITO | Made of an electrophoretic layer on a release sheet as described above |
| FPL 1 | BP/LA/(smooth) ELP/LA/ITO; first ITO lamination | Symmetrical structure with the smooth surface of the electrophoretic layer facing the back plate |
| FPL 1′ | BP/LA (smooth)/ELP/ITO; | symmetrical structure with the electrophoretic layer facing smoothly |
| First release sheet lamination | To the back plate, but differs from FPL 1 in lamination order |
| FPL 1＂ | BP/LA/ELP (smooth)/LA/ITO; first release sheet lamination | The symmetric structure was observed from the rough electrophoretic layer side and was made by laminating to the adhesive layer on the release sheet, removing the "rough" side of the release sheet, and laminating to "ITO/PET". |
Example 2: electro-optical testing
The experimental display prepared in example 1 above was driven between extreme black and white optical states using a 500 millisecond drive pulse at 15V and the reflectance of the two extreme optical states was measured. The tests were carried out at room temperature (20 ℃) and 0 ℃. FIG. 4 of the drawings shows the dynamic range (measured in L)*Difference between extreme black and white states in units (where L*The general commission internationale for illumination (CIE) definition was used:
L*＝116(R/R0)1/3-16，
wherein R is reflectance and R0Is a standard reflectance value). In each test, the left column shows the results obtained at 20 ℃ and the right column shows the results obtained at 0 ℃.
As can be seen from fig. 4, both the control and the "inverted" structures of the invention (the two sets of columns on the left hand side of fig. 4, the inverted layer being inverted in the sense that the electrophoretic layer is inverted) show very similar test results at 20 ℃ and 0 ℃. The symmetrical structure also showed similar performance at 20 c, but poor performance at lower temperatures due to the second lamination adhesive layer.
For all experimental displays, kickback (kick-back) or self-erasing (self-erasing) (an activity where the optical state leaves from an extreme optical state with the end of the drive pulse) and dwell time dependence (the change in the extreme optical state related to the time a pixel dwells in the opposite optical state before transitioning to one) are similar.
Example 3: resolution ratio
The resolution of the control display and the display of the present invention can be evaluated by microscopic examination of the display formed on the glass backplane; figure 5 shows the observed differences. As indicated in fig. 5, each display used for inspection comprised two pixels separated by a 100 μm gap, which was free of the ITO layer and therefore was not switchable. On the left hand side of fig. 5 is shown a control display (conventional FPL structure), and on the right hand side of fig. 5 is shown a display according to the invention. Each side of fig. 5 is a combination of micrographs of three separate associated displays. The upper part of fig. 5 illustrates the display when the right pixel is switched to its white extreme optical state, while the lower part of fig. 5 illustrates the display when the right pixel is switched to its black extreme optical state.
As can be seen from fig. 5, in the control display the shading (shading) results in switching across the entire width of the inter-pixel spacing, that is to say the shading is at least 100 μm. On the other hand, in the inverted FPL display of the present invention, the vignetting is less than one cell width (less than 20 μm) and the inter-pixel spacing is clearly visible in both the upper and lower portions of FIG. 5.
These results illustrate a substantial advantage in resolution that can be obtained with the inverted FPL structure of the present invention while maintaining the low temperature electro-optic response of the display. In this way, the present invention has both good greenhouse resolution and good low temperature performance of the same display, thereby avoiding the trade-off between these two performance parameters in prior art displays.