Patent Publication Number: US-2011052801-A1

Title: Green color filter element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. 11/393,767 filed Mar. 30, 2006, entitled “Efficient white Light OLED Display With Filters”, by Hatwar et al., the disclosure of which is incorporated herein by reference. This application is part of a series of four applications filed concurrently under attorney dockets 93114, 93116, 93153, and 93154. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to color filters for electronic displays. 
     BACKGROUND OF THE INVENTION 
     In recent years, it has become necessary that image display devices have high-resolution and high picture quality, and it is desirable for such image display devices to have low power consumption and be thin, lightweight, and visible from wide angles. With such requirements, display devices (displays) have been developed where thin-film active elements (thin-film transistors, also referred to as TFTs) are formed on a glass substrate, with display elements (for example, organic light-emitting diode layers to produce light, or liquid-crystal layers to block light from a backlight) then being formed on top. 
     A problem with displays combining white-emitting devices with color filters is that the combination of emitter and color filters must provide a good color gamut for the reproduction of a wide range of colors. Color filters used in this way must have good spectroscopic characteristics, with sufficient transmittance with the predetermined visible light region and no unnecessary transmittance in other regions of the visible spectrum. 
     Much work has been done to identify good color filters and color filter combinations for liquid crystal displays (LCD), e.g. “Liquid Crystal Displays”, Ernst Leudner ed., John Wiley &amp; Sons (2001), pp. 28-296; “High Performance Pigments”, Hugh M. Smith, John Wiley &amp; Sons, pp. 264-265; Kudo et al.,  Jpn. J. Appl. Phys.,  37 (1998), pp. 3594-3603; Kudo et al.,  J. Photopolymer Sci. Tech.  9 (1996), pp. 109-120; Sugiura,  J. of the SID,  1(3) (1993), pp. 341-346; FU et al., SPIE, Vol. 3560, pp. 116-121; Ueda et al., U.S. Pat. No. 6,770,405; and Machiguchi et al. U.S. Pat. Nos. 6,713,227 and 6,733,934. 
     Despite such improvements, display color reproduction has remained full of compromises. For example, the standards for color television gamut, as described by Fink in “Color Television Standards”, McGraw-Hill, New York (1955), and in Recommendation ITU-R BT.709-5, “Parameter values for the HDTV standards for production and international programme exchange”, have seldom been met. The former NTSC reference describes a good red primary as having 1931 CIE x,y chromaticity coordinates of x=0.67 and y=0.33, while a good green primary has coordinates of x=0.21 and y=0.71. The latter HDTV reference defines a good blue primary as the original PAL/SECAM blue having coordinates of x=0.15 and y=0.06. Commercially available televisions fall short of these standards and have a compromised color gamut. Takizawa, in US 2004/0105265, teaches a red filter that can achieve an x value as high as 0.65 and a y value as high as 0.33, which falls short of the NTSC reference red primary in x. Yamashita, in U.S. Pat. No. 6,856,364, teaches a red filter that can achieve an x value as high as 0.665 and a y in the range from 0.31 to 0.35. While this is an improvement over Takizawa, a red primary that meets or exceeds the x value of the NTSC primary would result in a purer red color. Yamashita further teaches a blue filter wherein the x value can range from 0.13 to 0.15 and the y value can only be as low as 0.08, and a green filter wherein the x value can range from 0.22 to 0.34 with a y value ranging from 0.56 to 0.65. Both of these fall short of the respective desired primary x,y values, which if achieved would result in purer blue and green colors, respectively. 
     Additionally, liquid crystal displays commonly available often use a backlight such as a cold-cathode fluorescent light (CCFL). It is a characteristic of CCFL sources commonly available that, while it provides white light consisting of a variety of wavelengths of the visible spectrum, the light is often more intense in a few narrow bands of the spectrum. These bands are generally centered in the red, the green, and the blue regions of the spectrum. The color filters needed with such light sources do not need to be especially narrow to provide a good color gamut. For example, a red filter can allow a transmission “tail” into parts of the green region of the spectrum, so long as the tail region does not include the major green emission peak, and still provide good color with such a light source. 
     Organic light-emitting diodes (OLEDs) provide another light source for displays. Unlike LCDs, which have a single full-display light source, OLED displays only produce light at the pixels that are required to be bright at a given time. Therefore, it is possible for OLED devices to provide displays that have reduced power requirements under normal usage. There has been much interest in broadband-emitting OLED devices in color displays. Each pixel of such a display is coupled with a color filter element as part of a color filter array (CFA) to achieve a pixilated multicolor display. The broadband-emitting structure is common to all pixels, and the final color as perceived by the viewer is dictated by that pixel&#39;s corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the emitting structure. An example of a white CFA top-emitting device is shown in U.S. Pat. No. 6,392,340. Kido et al., in  Science,  267, 1332 (1995) and in  Applied Physics Letters,  64, 815 (1994), Littman et al. in U.S. Pat. No. 5,405,709, and Deshpande et al., in  Applied Physics Letters,  75, 888 (1999), report white light-producing OLED devices. Other examples of white light producing OLED devices have been reported in U.S. Pat. No. 5,683,823 and JP 07,142,169. 
     One property of broadband OLED displays is that, while they can vary somewhat in emission intensity at different wavelengths, they generally do not have the intense peaks characteristic of CCFL sources. Therefore, common color filters that provide adequate color gamut when coupled with a CCFL display may not provide good results with OLED displays. The example above of a red color filter with a “tail” into a portion of the green region of the spectrum can provide an adequate red emission for a CCFL source, but be totally unsuitable for use with an OLED device. 
     Therefore, it is a problem to be solved to produce color filters that can be coupled with broadband OLED devices to provide displays with improved color rendition. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a green color filter to provide improved color rendition, particularly with a broadband-emitting OLED device; a display containing the filter; and a method of making the filter. Improved color rendition includes improved color gamut and related properties such as improved 1931 CIE x,y chromaticity coordinates and improved spectral curve shape. 
     This object is achieved by a green color filter having a green filter layer comprising:
         a. a bridged aluminum phthalocyanine pigment; and   b. a second pigment having its maximum absorption at a wavelength from 400 to 500 nm.       

     This object is also achieved by a green color filter having a green filter layer comprising:
         a. a first pigment having its maximum absorption at a wavelength from 600 to 700 nm wherein at least 90 volume percent of the first pigment particles have a particle size less than 300 nm;   b. a second pigment having its maximum absorption at a wavelength from 400 to 500 nm wherein at least 90 volume percent of the second pigment particles have a particle size less than 300 nm; and   c. the green filter layer has a transmittance of 60% or more at a wavelength of 520 nm, of no more than 10% at a wavelength of 590 nm, and of no more than 10% at a wavelength of 480 nm.       

     ADVANTAGES 
     It is an advantage of this invention that it can produce color displays with an improved combination of color and efficiency relative to existing displays. This invention can provide better color gamut and thus better rendition of colors. While existing color filters could provide better color gamut by using thicker filters, this invention can provide the improved color gamut with reduced efficiency loss. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  to  1   d  show example pixel configurations that can be used for an electronic display using this invention; 
         FIG. 2   a  shows a cross-sectional view of one embodiment of an electronic display that can be used with this invention; and 
         FIG. 2   b  shows another embodiment of an electronic display that can be used with this invention. 
     
    
    
     Since device feature dimensions such as layer thicknesses are frequently in sub-micrometer ranges, the drawings are scaled for ease of visualization rather than dimensional accuracy. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The term “electronic display” refers to a display wherein electronic entities control the intensity of the different areas of the display. Such electronic entities can include e.g. an off-panel driver and a series of horizontal and vertical electrodes in a passive-matrix display, or an array of thin-film transistors (TFTs) in an active-matrix display. Such displays can include liquid crystal displays (LCDs) and organic light-emitting-diode (OLED) displays. The term “OLED display”, “OLED device” or “organic light-emitting display” is used in its art-recognized meaning of a display device comprising organic light-emitting diodes as pixels. The term “multicolor” is employed to describe a display panel that is capable of emitting light of a different hue in different areas. In particular, it is employed to describe a display panel that is capable of displaying images of different colors. These areas are not necessarily contiguous. The term “full color” is commonly employed to describe multicolor display panels that are capable of emitting in at least the red, green, and blue regions of the visible spectrum and displaying images in any combination of hues. The complete set of colors that can be generated by a given display is commonly called the color gamut of the display. 
     The red, green, and blue colors constitute the three primary colors from which all other colors can be generated by appropriate mixing. However, the use of additional colors to extend the color gamut or within the color gamut of the device is possible. The term “hue” refers to the intensity profile of light emission within the visible spectrum, with different hues exhibiting visually discernible differences in color. The term “pixel” is employed in its art-recognized usage to designate an area of a display panel that can be stimulated to emit light independently of other areas. It is recognized that in full-color systems, several pixels of different colors will be used together to generate a broad range of colors, and a viewer can term such a group a single pixel. For the purposes of this discussion, such a group will be considered several different colored pixels. 
     The terms “maximum absorption” and “maximum transmittance” as used herein refer to the maximum light absorption and maximum light transmission, respectively, of color filters and color filter layers within the visible portion of the spectrum, i.e. from 400 nm to 700 nm. Red color filters are color filters that have a maximum transmittance substantially in the range of 600 nm to 700 nm. Green color filters are color filters that have a maximum transmittance substantially in the range of 500 nm to 600 nm. Blue color filters are color filters that have a maximum transmittance substantially in the range of 400 nm to 500 nm. 
       FIG. 1  illustrates example pixel configurations that can be used for an electronic display using this invention.  FIG. 1   a  shows a stripe pattern configuration of a device with group of pixels  20   a.  Group of pixels  20   a  includes red, green, and blue color-gamut-defining pixels  21   a,    21   b,  and  21   c.    FIG. 1   a  is a common example of an RGB display.  FIG. 1   b  shows a configuration of a device with group of pixels  20   b  including red, green, and blue color-gamut-defining pixels  21   a,    21   b,  and  21   c  as well as extra pixel  21   d,  which can be a within-gamut pixel (e.g. white) or can be another color-gamut-defining pixel. One common arrangement utilizing  FIG. 1   b  is an RGBW display, wherein portions of the display, e.g. within-gamut pixel  21   d,  would not have a color filter.  FIG. 1   c  shows another pattern configuration of a device with group of pixels  20   c.    FIG. 1   d  shows another pattern configuration of a device with group of pixels  20   d.  Other patterns can also be applied to the present invention, including patterns with more than 4 pixels. While in the above-mentioned examples, the pixels are shown to be arranged in a certain order, the pixels can be arranged in other embodiments having different orders, and other embodiments can have pixels with differing sizes and shapes. 
     There are numerous configurations of color filters and displays with which this invention can be practiced. Turning now to  FIG. 2   a,  there is shown a cross-sectional view of one embodiment of a bottom-emitting electronic display  10  that can be used with this invention. Electronic display  10  is an OLED device well known in the art. An organic electroluminescent (EL) element  70 , comprising hole-injecting layer  35 , hole-transporting layer  40 , light-emitting layers  45  and  50 , electron-transporting layer  55 , and electron-injecting layer  60 , is provided over an OLED substrate  80 . Current is provided by cathode  90  and anodes  30   a,    30   b,  and  30   c.  The display includes at least three separate filters, e.g. 
     red color filter  25   a,  green color filter  25   b,  and blue color filter  25   c,  each of which is a separate emitting unit with its own anode  30   a,    30   b,  and  30   c,  respectively. 
     Color filters are often provided on a substrate. In  FIG. 2   a , the substrate is also the device substrate  20 . Turning now to  FIG. 2   b , there is shown another embodiment of an electronic display with color filters. Electronic display  15  is a top-emitting device. Color filters  25   a,    25   b,  and  25   c  have been provided on a separate color filter substrate  85  that is placed over the electronic display after the electronic and emissive layers are provided. It will be understood that other arrangements of color filters commonly known in the art can be used with this invention. Further, other embodiments of electronic displays can be used, e.g. tandem OLED devices, liquid crystal displays, etc. 
     Color Filter Pigment Preparation 
     The milling that has been used in the art for color filter pigments commonly produces material with a wide range of particle sizes up to 500 nm. It has been found that by milling the pigment particles to a narrow particle size range, where the particle size is predominantly less than 100 nm, yields improved color filter properties. A method for producing particles of this type has been taught by Santilli et al. in U.S. Pat. No. 5,738,716, and by Czekai et al. in U.S. Pat. No. 5,500,331, the contents of which are incorporated herein by reference. This method will be referred to herein as micromedia milling. 
     The process of preparing color filters from pigments commonly involves three steps: (a) a dispersing or milling step to break up the pigment to a dispersion of the primary particle; (b) a dilution and/or mixing step in which the dispersed pigment concentrate is diluted with a carrier and other addenda, which can include other pigment dispersions, to a coating-strength pigment dispersion; and (c) coating a color filter layer from the coating-strength pigment dispersion onto a substrate. Step (a) can be further detailed as: (a1) providing a pigment mixture containing a pigment and a carrier for the pigment, and optionally a dispersant; (a2) mixing the pigment mixture with milling media; (a3) introducing the mixture into a high-speed mill; (a4) milling the mixture to obtain a pigment dispersion wherein the pigment particles have the desired size; and (a5) separating the dispersion from the milling media. 
     In the milling step, the pigment is usually suspended in a carrier (typically the same carrier as that in the coating-strength slurry) along with rigid, inert milling media. Mechanical energy is supplied to this pigment dispersion, and the collisions between the milling media and the pigment cause the pigment to deaggregate into its primary particles. A dispersant or stabilizer, or both, is commonly added to the pigment dispersion to facilitate the deaggregation of the raw pigment, to maintain colloidal particle stability, and to retard particle reaggregation and settling. 
     There are many different types of materials which can be used as milling media, such as glasses, ceramics, metals, and plastics. In a useful embodiment, the grinding media can comprise particles, preferably substantially spherical in shape, e.g., beads, consisting essentially of a polymeric resin. Desirably the beads have sizes in the range of 10 to 100 microns, as described by Czekai et al. 
     In general, polymeric resins suitable for use as milling media are chemically and physically inert, substantially free of metals, solvent, and monomers, and of sufficient hardness and friability to enable them to avoid being chipped or crushed during milling. Suitable polymeric resins include crosslinked polystyrenes, such as polystyrene crosslinked with divinylbenzene, styrene copolymers, polyacrylates such as poly(methyl methylacrylate), polycarbonates, polyacetals, such as Derlin™, vinyl chloride polymers and copolymers, polyurethanes, polyamides, poly(tetrafluoroethylenes), e.g., Teflon™, and other fluoropolymers, high density polyethylenes, polypropylenes, cellulose ethers and esters such as cellulose acetate, poly(hydroxyethyl methacrylate), poly(hydroxyethyl acrylate), silicone containing polymers such as polysiloxanes and the like. The polymer can be biodegradable. Exemplary biodegradable polymers include polylactides, polyglycolids, copolymers of lactides and glycolide, polyanhydrides, poly(imino carbonates), poly(N-acylhydroxyproline) esters, poly(N-palmitoyl hydroxyprolino) esters, ethylene-vinyl acetate copolymers, poly(orthoesters), poly(caprolactones), and poly(phosphazenes). The polymeric resin can have a density from 0.9 to 3.0 g/cm 3 . Higher density resins are especially useful inasmuch as it is believed that these provide more efficient particle size reduction. Especially useful are crosslinked or uncrosslinked polymeric media based on styrene. 
     Milling can take place in any suitable grinding mill. Suitable mills include an airjet mill, a roller mill, a ball mill, an attritor mill, a vibratory mill, a planetary mill, a sand mill, and a bead mill. A high speed mill is particularly useful. By high speed mill we mean milling devices capable of accelerating milling media to velocities greater than about 5 meters per second. The mill can contain a rotating shaft with one or more impellers. In such a mill the velocity imparted to the media is approximately equal to the peripheral velocity of the impeller, which is the product of the impeller revolutions per minute, n, and the impeller diameter. Sufficient milling media velocity is achieved, for example, in Cowles-type saw tooth impeller having a diameter of 40 mm when operated at 9,000 rpm. Useful proportions of the milling media, the pigment, the liquid dispersion medium and dispersant can vary within wide limits and depends, for example, upon the particular material selected and the size and density of the milling media etc. The process can be carried out in a continuous or batch mode. 
     In batch milling, a slurry of &lt;100 μm milling media, liquid, pigment, and dispersant is prepared using simple mixing. This slurry can be milled in conventional high energy batch milling processes such as high speed attritor mills, vibratory mills, ball mills, etc. This slurry is milled for a predetermined length of time to allow comminution of the active material to a minimum particle size. After milling is complete, the dispersion of active material is separated from the milling media by a simple sieving or filtration with a barrier to the milling media but not the milled pigment, e.g. a filter with a pore size of 5 μm. 
     In continuous media recirculation milling, a slurry of &lt;100 μm milling media, liquid, pigment, and dispersant can be continuously recirculated from a holding vessel through a conventional media mill which has a media separator screen adjusted to &gt;100 μm to allow free passage of the media throughout the circuit. After milling is complete, the dispersion of active material is separated from the milling media by simple sieving or filtration. 
     With either of the above modes the useful amounts and ratios of the ingredients of the mill grind will vary widely depending upon the specific materials. The contents of the milling mixture comprise the mill grind and the milling media. The mill grind comprises pigment, dispersant and a liquid carrier such as water. For aqueous filter slurries, the pigment is usually present in the mill grind at 1 to 50 weight %, excluding the milling media. The weight ratio of pigment to dispersant is 20:1 to 1:2. The high speed mill is a high agitation device, such as those manufactured by Morehouse-Cowles, Hockmeyer et al. 
     The dispersant is another important ingredient in the mill grind. Useful dispersants include sulfates (e.g. sodium dodecyl sulfate), sulfonates (e.g. N-methyl-N-oleoyl taurate), acrylic and styrene-acrylic copolymers such as those disclosed in U.S. Pat. Nos. 5,085,698 and 5,172,133 (e.g. Joncryl 678), and sulfonated polyesters and styrenics such as those disclosed in U.S. Pat. No. 4,597,794. Other patents referred to above in connection with pigment availability also disclose a wide variety of useful dispersants. The dispersants used in the examples are potassium N-methyl-N-oleoyl taurate (KOMT) and Joncryl 678. 
     The milling time can vary widely and depends upon the pigment, mechanical means and residence conditions selected, the initial and desired final particle size, etc. For aqueous mill grinds using the useful pigments, dispersants, and milling media described above, milling times will typically range from 1 to 100 hours. The milled pigment concentrate is conveniently separated from the milling media by filtration. 
     The carrier for the pigment can be an aqueous carrier medium or a non-aqueous solvent. Useful solvents have been disclosed by Czekai et al., and also in U.S. Pat. No. 5,145,684, U.S. Pat. No. 5,679,138, and EP 498,492, the disclosures of which are incorporated herein by reference. The aqueous carrier medium is water, an aqueous salt solution, or an aqueous solvent mixture comprising water and at least one water-miscible co-solvent. Selection of a suitable mixture depends on requirements of the specific application, such as desired surface tension and viscosity, the selected pigment, drying time of the color filter layer, and the type of material onto which the pigment dispersion will be coated. Representative examples of water-miscible co-solvents that can be selected include (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) ketones or ketoalcohols such as acetone, methyl ethyl ketone, and diacetone alcohol; (3) ethers, such as tetrahydrofuran and dioxane; (4) esters, such as ethyl acetate, ethyl lactate, ethylene carbonate, and propylene carbonate; (5) polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetraethylene glycol, polyethylene glycol, glycerol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, and thioglycol; (6) lower alkyl mono-or di-ethers derived from alkylene glycols, such as ethylene glycol mono-methyl (or -ethyl) ether, diethylene glycol mono-methyl (or -ethyl) ether, propylene glycol mono-methyl (or -ethyl) ether, triethylene glycol mono-methyl (or -ethyl) ether, and diethylene glycol di-methyl (or -ethyl) ether; (7) nitrogen containing cyclic compounds, such as pyrrolidone, N-methyl-2-pyrrolidone, pyrrolidone, and 1,3-dimethyl-2-imidazolidinone; and (8) sulfur-containing compounds such as dimethyl sulfoxide and tetramethylene sulfone. 
     Useful non-aqueous solvents include hydrocarbons, alcohols, polyols, ethers, and esters. Solvents known to be useful for this process include toluene, hexane, ethanol, butanol, glycol, and PGMEA. 
     This treatment results in pigment particles wherein at least 90 weight percent of the particles have a particle size less than 300 nm. Often, 100% of the particles have a particle size less than 300 nm, and conveniently less than 200 nm. It is suitable that 100% of the particles have a particle size less than 100 nm; however, this is not possible in all cases, and it is useful that at least 90 volume percent of the pigment particles have a particle size less than 100 nm, and desirably less than 50 nm. In some cases, 90 volume percent of the pigment particles can have a particle size less than 30 nm. Usefully, no more than 10 volume percent of the pigment particles have a particles size less than 5 nm. 
     Coating-Strength Dispersion Preparation 
     In general it is desirable to make the pigment dispersion in the form of a concentrated mill grind, which is subsequently diluted to the appropriate concentration and further processed if necessary for use in coating. This technique permits preparation of a greater quantity of pigment slurry from the equipment. If the mill grind was made in a solvent, it can be diluted with water and/or optionally other solvents to the appropriate concentration. If it was made in water, it can be diluted with either additional water or water-miscible solvents to the desired concentration. If the color filter requires a mixture of pigments, it is useful at this point to mix pigment dispersions that have been milled separately. By dilution and/or mixing, the pigment dispersion is adjusted to the desired viscosity, color, hue, saturation density, and area coverage for the particular application. 
     In the case of organic pigments, the coating dispersion can contain up to approximately 30% pigment by weight, but will generally be in the range of approximately 0.1 to 20%, and conveniently approximately 5 to 15%, by weight of the total dispersion composition for most color filter coating applications. If an inorganic pigment is selected, the dispersion will tend to contain higher weight percentages of pigment than with comparable dispersions employing organic pigments, and can be as high as approximately 75% in come cases, since inorganic pigments generally have higher specific gravities than organic pigments. 
     The amount of aqueous carrier medium is in the range of approximately 70 to 98 weight %, and conveniently approximately 80 to 95 weight %, based on the total weight of the dispersion. A mixture of water and a polyhydric alcohol, such as diethylene glycol, is useful as the aqueous carrier medium. In the case of a mixture of water and diethylene glycol, the carrier medium usually contains from about 30% water/70% diethylene glycol to about 95% water/5% diethylene glycol. Useful ratios are approximately 60% water/40% diethylene glycol to about 95% water/5% diethylene glycol. Percentages are based on the total weight of the carrier medium. 
     It can be desirable to add additional dispersant to the mixture. Useful dispersants have been described above. 
     The ability to coat a given surface can be affected by the surface tension of the coating-strength dispersion. Control of surface tensions is accomplished by additions of small amounts of surfactants. The level of surfactants to be used can be determined through simple trial and error experiments. Anionic, nonionic, and cationic surfactants can be selected from those disclosed in U.S. Pat. Nos. 5,324,349; 4,156,616 and 5,279,654 as well as many other surfactants. Commercial surfactants include the Surfynols® from Air Products; the Zonyls® from DuPont and the Fluorads® from 3M. A useful surfactant for these dispersions is Surfactant 10G from Dixie Chemical. 
     Coating of Pigments 
     To form color filters, pigments are often coated onto a substrate. For example, a color filter layer including the pigments can be coated onto any of a variety of rigid and non-rigid transparent or semi-transparent materials, such as glass or plastic. The substrate can be a substrate used solely for forming a color filter, which can be attached to a display device. In another embodiment, the substrate can have other uses as well. For example, a color filter layer or array of color filter layers can be coated onto the bottom of a bottom-emitting display device substrate. In yet another useful embodiment, the pigments can be coated over the top of an emitting layer that forms part of a display device. The display device can be an electronic display, such as an LCD display or an OLED display. 
     Any of a variety of well-known coating and printing techniques can be used to prepare a color filter from the coating-strength pigment dispersion. These techniques can include, but are not limited to, extrusion-type hopper (X-hopper) coating, spin coating, spray coating, ultrasonic spray coating, knife coating, and gravure coating. The dispersion can be aqueous or non-aqueous. The coated dispersion is then typically allowed to dry to form a solid or semi-solid coating. Alternatively, the slurry can include e.g. gelling materials or crosslinking monomers to produce a solid or semi-solid coating. The coating-strength pigment dispersion can include one or more photoresist compounds well-known in the art as useful for patterning color filters, e.g. in an array of colored pixels for an electronic display. In such a case, processing of the coated dispersion can include patterned exposure and post-exposure processing to form a patterned color filter. 
     The final color filter layers desirably comprise at least 10% color pigment, conveniently at least 25% color pigment, and usefully at least 50% color pigment by weight. 
     Green Filter Pigments 
     A useful green color filter according to this invention has good light transmittance in the green region of the spectrum (500 to 600 nm) and good light absorption in the red and blue regions of the spectrum. One useful embodiment of this green color filter has a first pigment having good transmittance in the green region and a maximum absorption at a wavelength within the range of from 600 to 700 nm, and a second pigment having good transmittance in the green region and a maximum absorption at a wavelength within the range from 400 to 500 nm. 
     One useful class of pigments having good transmittance in the green region and a maximum absorption at a wavelength within the range of from 600 to 700 nm is the metallophthalocyanines. Although commercially available metallophthalocyanine pigments such as pigment blue 15 (copper phthalocyanine) are well-known for their excellent lightfastness, they tend to be more blue than green in hue, and thus are less than optimal for use in a green color filter. Hydroxyaluminum phthalocyanine exhibits a greener hue than copper phthalocyanine, but suffers from relatively poor lightfastness. One class of pigments which display both excellent hue for the first pigment requirements and lightfastness are the so-called bridged aluminum phthalocyanines as described by Regan in U.S. Pat. No. 4,311,775, the contents of which are incorporated herein by reference. These pigments are siloxane-bridged aluminum phthalocyanines and phosphonate-bridged aluminum phthalocyanines, which are generically represented by the following formulas respectively: 
       PcAl—O—[SiR 2 —O] n —AlPc  (1)
 
       PcAl—O—[POR] n —AlPc  (2)
 
     where Pc represents a substituted or unsubstituted phthalocyanine ring, R is an alkyl group, an aryl group, or an aralkyl group, and n is an integer from 0 to 4. For a more complete description of these pigments, see U.S. Pat. No. 4,311,775. A useful siloxane-bridged aluminum phthalocyanine is bis(phthalocyanylalumino)tetraphenyldisiloxane (Structure 3 below, Pc is unsubstituted, R is phenyl, and n is 2). Mixtures of bis(phthalocyanylalumino)tetraphenyldisiloxane with either copper phthalocyanine, hydroxyaluminum phthalocyanine, or both can also be used provided that bis(phthalocyanylalumino)tetraphenyldisiloxane comprises at least 80 weight percent of the mixture. 
     
       
         
         
             
             
         
       
     
     While bridged aluminum phthalocyanine compounds are more green than copper phthalocyanine compounds, they still have significant transmittance in the blue region. To be effective in a green filter, they are desirably combined with a second pigment having a maximum absorption at a wavelength within the range from 400 to 500 nm. One class of pigments that can be used is that which is commercially known as the monoazo yellow pigment class, or more simply monoazo pigments. Useful yellow pigments include Pigment Yellow 138, Pigment Yellow 139, Pigment Yellow 180, Pigment Yellow 74, Pigment Yellow 185, Pigment Yellow 154 and mixtures thereof are preferred. Especially preferred is Pigment Yellow 74. Pigment numbers are as designated in the Color Index. 
     When pigment particles are prepared as described herein, useful ratios of the first pigment to the second pigment have been found to be in the range of 40:60 to 75:25 by weight. A color filter layer prepared from such dispersions can have good transmittance in the green region of the visible spectrum while having good absorption in other regions of the spectrum. The green filter layer so prepared can have a maximum transmittance of 60% or more at a wavelength of 520 nm, but of no more than 10% at a wavelength of 590 nm and usefully no more than 10% at a wavelength of 580 nm, and of no more than 10% at a wavelength of 480 nm. The width at half-height of such a filter layer can be 80 nm or less. The width at half-height is defined as the width of the transmittance peak at one-half of the maximum transmittance. Such a filter layer has chromaticity coordinates (x,y) in the 1931 CIE XYZ colorimetric system, when calculated using CIE Standard Illuminant D65 or Standard Illuminant C, that satisfy the expressions 0.19≦x≦0.24 and 0.68≦y≦0.72. As will be seen, this is a very pure green color. 
     Dispersion Preparation 
     Preparation of Bridged Aluminum Phthalocyanine Dispersion. 
     A dispersion of bridged aluminum phthalocyanine was prepared by adding 1600 g bridged aluminum phthalocyanine pigment, 960 g potassium N-oleyl-N-methyltaurate dispersant, 5440 g high purity water and 8000 g 50-micron milling media composed of crosslinked polystyrene divinyl benzene to a 37L stainless steel vessel jacketed with 25C water. The mixture was stirred with a 152 mm diameter Hockmeyer® Poly high shear disperser blade at an average rate of 2546 rpm for 16 hours. After milling, the dispersion was separated from the milling media with a 5 micron filter and further diluted with high purity water to a concentration of 11.65 wt % pigment. 
     Preparation of Pigment Yellow 74 Dispersion. 
     A mixture of Pigment Yellow 74 was prepared by adding 150 g Pigment Yellow 74 Birchwood Yellow (Dominion Colour) pigment, 129.3 g of a 29.1 wt % aqueous solution of Joncryl® 678 dispersant neutralized 95% with potassium hydroxide, 720.7 g high purity water to a 5L stainless steel vessel jacketed with chilled water. The mixture was premixed with a 50 mm diameter rotor-stator blade at an average rate of 1400 rpm for 3 hours. After the premix step, 1200 g 50-micron milling media composed of crosslinked polystyrene divinyl benzene were added and the mixture was stirred with a 70 mm diameter high-shear Cowles disperser blade at an average rate of 1400 rpm for 115 hours. After milling, the dispersion was separated from the milling media by filtration through a 5 micron fiberglass filter and further diluted with high purity water to a concentration of 10.42 wt % pigment. 
     Table 1 shows the relative amounts of pigment, dispersant, and water in the dispersions prepared above that were used to make color filters as described below. Table 1 also shows particle size distribution of the pigments in the dispersions, as measured by dynamic light scattering using a Microtrac® UPA150 particle analyzer. Examination of the dispersion by transmitted light microscopy at 1110× magnification showed all particles to be well dispersed. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Bridged 
                 Pigment 
               
               
                   
                 Pigment 
                 Al Pc 
                 Yellow 74 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 dispersant 
                 KOMT 
                 Joncryl 678- 
               
               
                   
                   
                   
                 KOH 
               
               
                   
                 wt % pigment 
                 11.66 
                 10.42 
               
               
                   
                 wt % dispersant 
                 7.00 
                 2.61 
               
               
                   
                 wt % water 
                 81.34 
                 86.98 
               
               
                   
                 100% by volume 
                 0.1445 
                 0.0608 
               
               
                   
                 less than (microns): 
                   
                   
               
               
                   
                 90% by volume less 
                 0.0435 
                 0.0113 
               
               
                   
                 than (microns): 
                   
                   
               
               
                   
                 50% by volume less 
                 0.0136 
                 0.0089 
               
               
                   
                 than (microns): 
                   
                   
               
               
                   
                 10% by volume less 
                 0.0104 
                 0.0074 
               
               
                   
                 than (microns): 
               
               
                   
                   
               
            
           
         
       
     
     Filter Preparation 
     Inventive Green Filter (G j ) 
     54.24 g of the above bridged aluminum phthalocyanine dispersion was mixed with 30.19 g of the above Pigment Yellow 74 dispersion, 19.07 g Joncryl 678, and 10 drops of a 10% Surfactant 10G solution. The resulting slurry was then coated onto a polyester sheet using an x-hopper syringe coater at a rate of 1.2 cm 3 /ft 2 . This provided a coating with an average thickness when dry of 2.2 microns. 
     First Comparative Filters 
     Comparative Green Filter 1 (G c1 ) was obtained from a commercially available LCD television. 
     The visible transmittance spectra of the above filters were measured with a Perkin-Elmer Lambda  12  spectrometer with an integrating filter. The results are shown in the table below. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Inventive  
                 Comparative  
               
               
                 Color Filter: 
                 Green Filter 
                 Green Filter 1 
               
               
                   
               
             
            
               
                 Peak Transmittance 
                 518 nm 
                 517 nm 
               
               
                 % T at Peak Transmittance 
                 61% 
                  84% 
               
               
                 Bandwidth at ½ Peak T 
                  61 nm 
                 102 nm 
               
               
                 % T at 550 nm 
                 40% 
                  76% 
               
               
                 % T at 573 nm 
                 10% 
                  57% 
               
               
                 % T at 579 nm 
                  5% 
                  49% 
               
               
                 % T at 587 nm 
                  2% 
                  36% 
               
               
                 Red Transmittance 
                 &lt;1% 593 to 700 nm 
                 &lt;1% 634 to 674 nm,  
               
               
                   
                   
                 up to 5% at 700 nm 
               
               
                 % T at 500 nm 
                 40% 
                  75% 
               
               
                 % T at 490 nm 
                 10% 
                  65% 
               
               
                 % T at 486 nm 
                  4% 
                  57% 
               
               
                 Blue Transmittance 
                 &lt;1% 480 to 400 nm 
                 &lt;1% 451 to 400 nm 
               
               
                   
               
            
           
         
       
     
     The bandwidth at half peak transmittance of the inventive green filter is significantly narrower than that of the comparative green filter. Also the hypsochromic and bathochromic tails for the inventive green filter are much lower in the red and blue spectral regions than those of the comparative green filter. These spectral transmittance characteristics mean that the inventive green filter, although lower in peak transmittance, is a much purer green than the comparative green filter. 
     The color purity of the filters can be further demonstrated with excitation purity, which is a common CIE metric used to measure the purity of a color point plotted on the 1931 CIE chromaticity diagram. The spectral characteristics of selected illuminants can be cascaded with the spectral transmittance of the color filters, and with the 1931 CIE color matching functions, as described in “Colorimetry”, CIE Publication 15:2004 3rd Edition, published by the CIE Central Bureau in Vienna, Austria. The result of this cascade is a set of chromaticity coordinates that pertains to a given illuminant on the 1931 CIE chromaticity diagram. The excitation purity is the length of the line segment joining the illuminant point with the color point relative to the length of the line segment joining the illuminant point, the color point, and the spectrum locus point. A color with an excitation purity of 1.0 lies on the spectrum locus and represents the purest spectral color possible. One can further calculate a difference from the predetermined standard by the formula: 
       Delta CIE x,y=SQRT[( x   1   −x   NTSC ) 2 +( y   1   −y   NTSC ) 2 ] 
     When the green filters are cascaded with CIE Standard Illuminant C, and the 1931 CIE color matching functions, the following table shows the resulting chromaticity coordinates. The resulting CIE chromaticity coordinates for the inventive green filter are lower in x and significantly higher in y than those for the comparative green filter, making the inventive green filter less yellow and a much more pure green color. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Green Filter 
               
            
           
           
               
               
               
               
            
               
                   
                 Inventive 
                 Comparative 
                 NTSC Green 
               
               
                 Filter or Primary: 
                 Green 
                 Green 
                 Primary 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1931 CIE x,y  
                 0.2049, 0.6958 
                 0.2738, 0.5845 
                 0.21, 0.71 
               
               
                 (Std. III. C) 
                   
                   
                   
               
               
                 Excitation  
                 0.8057 
                 0.6448 
                 0.8462 
               
               
                 Purity (C) 
                   
                   
                   
               
               
                 Delta CIE x,y  
                 0.0151 
                 0.1408 
                    0 
               
               
                 from NTSC 
                   
                   
                   
               
               
                 Primary (C) 
                   
                   
                   
               
               
                 1931 CIE x,y  
                 0.2005, 0.7021 
                 0.2704, 0.5931 
                 0.21, 0.71 
               
               
                 (Std. III. D65) 
                   
                   
                   
               
               
                 Excitation Purity  
                 0.8049 
                 0.6494 
                 0.8426 
               
               
                 (D65) 
                   
                   
                   
               
               
                 Delta CIE x,y  
                 0.0124 
                 0.1316 
                    0 
               
               
                 from NTSC 
                   
                   
                   
               
               
                 Primary (D65) 
               
               
                   
               
            
           
         
       
     
     The inventive green has a much higher excitation purity than the comparative green filter, and it is very close to the excitation purity for the NTSC green primary. The NTSC green primary is the purest standard green primary made to date. When the NTSC standards were set in 1953, the green primary was based on zinc silicate. The luminance of the green zinc silicate NTSC primary was unacceptably low. As the TV industry moved away from the green zinc silicate primary, new green primaries were found that would deliver twice the luminance of the zinc silicate at a cost of less pure green chromaticities. The TV industry has not been able to get back to the pure NTSC green primary chromaticities since 1953 because of the luminance problem. This inventive green allows the purest green primary set by the NTSC standard in 1953 to come back with luminance higher than the zinc silicate green primary of 1953. The inventive green chromaticity lies close to the NTSC green primary chromaticity. The x,y delta for the comparative green filter is much larger than that for inventive green filter, making the comparative green filter a much less pure green color. The inventive green filter is a much purer green than either the comparative green filter or the green filter taught by Yamashita in U.S. Pat. No. 6,856,364. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     The patents and other publications referred to herein are incorporated herein by reference. 
     Parts List 
       10  electronic display 
       15  electronic display 
       20   a  group of pixels 
       20   b  group of pixels 
       20   c  group of pixels 
       20   d  group of pixels 
       21   a  pixel 
       21   b  pixel 
       21   c  pixel 
       21   d  pixel 
       25   a  red color filter 
       25   b  green color filter 
       25   c  blue color filter 
       30   a  anode 
       30   b  anode 
       30   c  anode 
       35  hole-injecting layer 
       40  hole-transporting layer 
       45  light-emitting layer 
       50  light-emitting layer 
       55  electron-transporting layer 
       60  electron-injecting layer 
       70  organic EL element 
       80  OLED substrate 
       85  filter substrate 
       90  cathode