Patent Publication Number: US-2005118923-A1

Title: Method of making an electroluminescent device including a color filter

Description:
This application claims the benefit of U.S. Provisional Application No. 60/523,187, filed Nov. 18, 2003, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      Generally, the present disclosure relates to electroluminescent devices. In particular, the present disclosure relates to methods of forming electroluminescent devices that include an electroluminescent element and at least one color filter.  
     BACKGROUND  
      Light emitting devices, such as organic or inorganic electroluminescent devices, are useful in a variety of display, lighting, and other applications. Generally, these light emitting devices include one or more device layers, including at least one light emitting layer, disposed between two electrodes (an anode and a cathode). A voltage drop or current is provided between the two electrodes, thereby causing a light emitting material, which can be organic or inorganic, in the light emitting layer to luminesce. Typically, one or both of the electrodes is transparent so that light can be transmitted through the electrode to a viewer or other light receiver.  
      An electroluminescent device may be constructed such that it is either a top emitting device or a bottom emitting device. In a top emitting electroluminescent device, the light emitting layer or layers are positioned between the substrate and a viewer. In a bottom emitting electroluminescent device, a transparent or semitransparent substrate is positioned between the light emitting layer or layers and the viewer.  
      In a typical color electroluminescent display, one or more electroluminescent devices can be formed on a single substrate and arranged in groups or arrays. Several approaches exist for producing a color electroluminescent display. For example, one approach includes an array having red, green, and blue electroluminescent device subpixels placed next to each other. Another approach, for example, utilizes a white pixelated display in conjunction with red, green, and blue color filters.  
     SUMMARY  
      The present disclosure provides methods of making electroluminescent devices that include color filters in optical association with an electroluminescent element. In particular, the present disclosure provides techniques that include selective thermal transfer (e.g., Laser Induced Thermal Imaging (LITI)) of color filters for use with electroluminescent devices.  
      Patterning of red-, green-, and blue-emitting primary organic light emitting diode (OLED) materials for full color devices has proven to be difficult. Many techniques have been described for such patterning, including laser thermal patterning, ink jet patterning, shadow mask patterning, and photolithographic patterning.  
      Alternative techniques of providing a full color display without patterning the emitting materials include the use of color filters as described herein. However, the use of these alternative techniques with the traditional bottom emitting electroluminescent device construction is limited by physical and optical factors. For practical reasons, the color filters must be patterned either on a separate piece of glass or on the substrate. In this case, the effect of the distance between the light emitting layer and the color filter leads to parallax problems. In other words, Lambertian emission from the electroluminescent device allows the light to reach the corresponding color filter as well as a number of adjacent color filters. As a result, the color saturation level of the electroluminescent display is reduced.  
      On the other hand, top emitting electroluminescent devices may allow for more complex pixel control circuitry as well as more flexibility in the choice of semiconductor and substrate. In a typical top emitting device, the electroluminescent device layers can be deposited onto a substrate, followed by the formation of a thin, transparent metal electrode, and a protective layer.  
      In some embodiments, the present disclosure provides selective thermal transfer (e.g., LITI) techniques for forming top emitting electroluminescent devices that include color filters that are formed on the top electrode of an electroluminescent element or on a protective layer formed over the electroluminescent element. Providing color filters directly on the top electrode or on a protective layer may help to eliminate alignment difficulties. The present disclosure also provides selective thermal transfer (e.g., LITI) techniques for forming bottom emitting electroluminescent devices that include color filters formed on a substrate surface opposite an electroluminescent element.  
      Further, selective thermal transfer patterning (e.g., LITI patterning, which is a dry, digital method), may be more compatible with the materials used for organic electroluminescent devices. Because it is a dry technique, selective thermal transfer may also allow for patterning of multiple layers on a single substrate without concern for the relative solubility of each layer.  
      In addition, selective thermal transfer patterning of color filters may provide a technique that is more easily reversible. For example, if the pattern of the color filters does not pass quality control inspection, the filters can be washed off and formed again without undue harm to the electroluminescent element.  
      In one aspect, the present disclosure provides a method of making an electroluminescent device. The method includes forming an electroluminescent element on a substrate. The method further includes selectively thermally transferring a plurality of color filters to the electroluminescent element.  
      In another aspect, the present disclosure provides a method of making an electroluminescent device. The method includes forming an electroluminescent element on a first major surface of a substrate. The method further includes selectively thermally transferring a plurality of color filters to a second major surface of the substrate.  
      In another aspect, the present disclosure provides a method of making an electroluminescent device. The method includes forming an electroluminescent element on a substrate. The method further includes forming a protective layer over at least a portion of the electroluminescent element; and selectively thermally transferring a plurality of color filters to the protective layer.  
      In another aspect, the present disclosure provides a method of making an electroluminescent color display including at least one electroluminescent device. The method includes forming the at least one electroluminescent device on a substrate. Forming the at least one electroluminescent device includes forming an electroluminescent element on the substrate; and selectively thermally transferring a plurality of color filters to the electroluminescent element.  
      As used herein, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably.  
      The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description that follow more particularly exemplify illustrative embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of one embodiment of a top emitting electroluminescent device that includes color filters formed on an electroluminescent element.  
       FIG. 2  is a schematic diagram of another embodiment of a top emitting electroluminescent device that includes color filters formed on a protective layer.  
       FIG. 3  is a schematic diagram of another embodiment of a top emitting electroluminescent device that includes color filters formed on an electroluminescent element in apertures of a black matrix.  
       FIG. 4  is a schematic diagram of an embodiment of a bottom emitting electroluminescent device that includes color filters formed on a substrate. 
    
    
     DETAILED DESCRIPTION  
      In the following detailed description of illustrative embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown, by way of illustration, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
      The present disclosure is believed to be applicable to methods of making electroluminescent devices. Electroluminescent devices can include organic or inorganic light emitters or combinations of both types of light emitters. An organic electroluminescent (OEL) display or device refers to an electroluminescent display or device that includes at least one organic emissive material, whether that emissive material is a small molecule (SM) emitter (e.g., nonpolymeric emitter), a SM doped polymer, a SM blended polymer, a light emitting polymer (LEP), a doped LEP, a blended LEP, or another organic emissive material whether provided alone or in combination with any other organic or inorganic materials that are functional or non-functional in the OEL display or devices. Inorganic light emissive materials include phosphors, semiconductor nanocrystals, etc.  
      Generally, electroluminescent devices have one or more device layers, including at least one light emitting layer, disposed between two electrodes (an anode and a cathode). A voltage drop or current is provided between the two electrodes, thereby causing the light emitter to luminesce.  
      Electroluminescent devices can also include thin film electroluminescent displays or devices. A thin film electroluminescent device includes an emissive material sandwiched between transparent dielectric layers and a matrix of row and column electrodes. Such thin film electroluminescent displays may include those described, e.g., in U.S. Pat. No. 4,897,319 (Sun) and U.S. Pat. No. 5,652,600 (Khormaei et al.).  
       FIG. 1  is a schematic diagram of one embodiment of an electroluminescent device  10 . The electroluminescent device  10  includes a substrate  12 , an electroluminescent element  20  formed on a major surface  14  of the substrate  12 , and color filters  30   a ,  30   b , and  30   c  (hereinafter referred to collectively as color filters  30 ) formed on the electroluminescent element  20 . The electroluminescent element  20  includes a first electrode  22 , a second electrode  26 , and one or more device layers  24  positioned between the first electrode  22  and the second electrode  26 .  
      The substrate  12  of electroluminescent device  10  can be any substrate suitable for electroluminescent device or display applications. For example, the substrate  12  can be made of glass, clear plastic, or other suitable material(s) that are substantially transparent to visible light. The substrate  12  can also be opaque to visible light, for example stainless steel, crystalline silicon, poly-silicon, or the like. In some instances, the first electrode  22  of electroluminescent element  20  can be the substrate  12 . Because materials used in at least some electroluminescent devices can be particularly susceptible to damage due to exposure to oxygen or water, a suitable substrate can be selected to provide an adequate environmental barrier, or is supplied with one or more layers, coatings, or laminates that provide an adequate environmental barrier.  
      The substrate  12  can also include any number of devices or components suitable in electroluminescent devices and displays, such as transistor arrays and other electronic devices; color filters, polarizers, wave plates, diffusers, and other optical devices; insulators, barrier ribs, black matrix, mask work, and other such components; and the like. The substrate  12  may also include a plurality of independently addressable active devices as is described, e.g., in European Patent Application No. 1,220,191 (Kwon).  
      The electroluminescent device  10  also includes an electroluminescent element  20  formed on major surface  14  of the substrate  12 . Although  FIG. 1  illustrates electroluminescent element  20  as being formed on and in contact with major surface  14  of substrate  12 , one or more layers or devices may be included between the electroluminescent element  20  and the major surface  14  of substrate  12 . The electroluminescent element  20  includes a first electrode  22 , a second electrode  26 , and one or more device layers  24  positioned between the first electrode  22  and the second electrode  26 . The first electrode  22  can be the anode and the second electrode  26  can be the cathode, or the first electrode  22  can be the cathode and the second electrode  26  can be the anode.  
      The first electrode  22  and the second electrode  26  are typically formed using electrically conducting materials such as metals, alloys, metallic compounds, metal oxides, conductive ceramics, conductive dispersions, and conductive polymers. Examples of suitable materials include, for example, gold, platinum, palladium, aluminum, calcium, titanium, titanium nitride, indium tin oxide (ITO), fluorine tin oxide (FTO), and polyaniline. The first and second electrodes  22  and  26  can be single layers of conducting materials or they can include multiple layers. For example, either one or both of the first electrode  22  and the second electrode  26  can include a layer of aluminum and a layer of gold, a layer of calcium and a layer of aluminum, a layer of aluminum and a layer of lithium fluoride, or a metal layer and a conductive organic layer.  
      Formed between the first electrode  22  and the second electrode  26  are the one or more device layers  24 . The one or more device layers  24  include a light emitting layer. Optionally, the one or more device layers  24  can include one or more additional layers such as, for example, a hole transport layer or layers, an electron transport layer or layers, a hole injection layer or layers, an electron injection layer or layers, a hole blocking layer or layers, an electron blocking layer or layers, a buffer layer or layers, or any combination thereof.  
      The light emitting layer includes light emitting material. Any suitable light emitting material may be used in the light emitting layer. A variety of light emitting materials, including LEP and SM light emitters, can be used. The light emitters include, for example, fluorescent and phosphorescent materials. Examples of classes of suitable LEP materials include poly(phenylenevinylene)s (PPVs), poly-para-phenylenes (PPPs), polyfluorenes (PFs), other LEP materials now known or later developed, and co-polymers or blends thereof. Suitable LEPs can also be molecularly doped, dispersed with fluorescent dyes or other materials, blended with active or non-active materials, dispersed with active or non-active materials, and the like. Examples of suitable LEP materials are described in Kraft, et al., Angew. Chem. Int. Ed., 37, 402-428 (1998); U.S. Pat. No. 5,621,131 (Kreuder et al.); U.S. Pat. No. 5,708,130 (Woo et al.); U.S. Pat. No. 5,728,801 (Wu et al.); U.S. Pat. No. 5,840,217 (Lupo et al.); U.S. Pat. No. 5,869,350 (Heeger et al.); U.S. Pat. No. 5,900,327 (Pei et al.); U.S. Pat. No. 5,929,194 (Woo et al.); U.S. Pat. No. 6,132,641 (Rietz et al.); and U.S. Pat. No. 6,169,163 (Woo et al.); and PCT Patent Application Publication No. 99/40655 (Kreuder et al.).  
      SM materials are generally non-polymeric organic or organometallic molecular materials that can be used in OEL displays and devices as emitter materials, charge transport materials, as dopants in emitter layers (e.g., to control the emitted color) or charge transport layers, and the like. Commonly used SM materials include metal chelate compounds, such as tris(8-hydroxyquinoline) aluminum (AlQ), and N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD). Other SM materials are disclosed in, for example, C. H. Chen, et al.,  Macromol. Symp.,  125:1 (1997); Japanese Laid Open Patent Application 2000-195673 (Fujii); U.S. Pat. No. 6,030,715 (Thompson et al.); U.S. Pat. No. 6,150,043 (Thompson et al.); and U.S. Pat. No. 6,242,115 (Thomson et al.); and, PCT Patent Applications Publication Nos. WO 00/18851 (Shipley et al.) (divalent lanthanide metal complexes); WO 00/70655 (Forrest et al.) (cyclometallated iridium compounds and others); and WO 98/55561 (Christou).  
      The one or more device layers  24  may also include a hole transport layer. The hole transport layer facilitates the injection of holes from an anode into the electroluminescent element  20  and their migration towards a recombination zone. The hole transport layer can further act as a barrier for the passage of electrons to the anode. Any suitable material or materials may be used for the hole transport layer, e.g., those materials described in Nalwa et al.,  Handbook of Luminescence, Display Materials and Devices , Stevens Ranch, Calif., American Scientific Publishers, 2003, p. 132-195; Chen et al.,  Recent Developments in Molecular Organic Electroluminescent Materials, Macromol. Symp.,  1:125 (1997); and Shinar, Joseph, ed.,  Organic Light - Emitting Devices , Berlin, Springer Verlag, 2003, p. 43-69.  
      The one or more device layers  24  may also include an electron transport layer. The electron transport layer facilitates the injection of electrons and their migration towards the recombination zone. The electron transport layer can further act as a barrier for the passage of holes to a cathode if desired. Any suitable material or materials may be used for the electron transport layer, e.g., those materials described in Nalwa et al.,  Handbook of Luminescence, Display Materials and Devices , Stevens Ranch, Calif., American Scientific Publishers, 2003, p. 132-195; Chen et al.,  Recent Developments in Molecular Organic Electroluminescent Materials, Macromol. Symp.,  1:125 (1997); and Shinar, Joseph, ed.,  Organic Light - Emitting Devices , Berlin, Springer Verlag, 2003, p. 43-69.  
      It may be preferred that the electroluminescent element  20  be capable of emitting white light. Those skilled in the art will understand that materials for the light emitting layer of the electroluminescent element  20  may be selected such that the electroluminescent element  20  is capable of emitting white light, such as those described in European Patent Application No. 1,187,235 (Hatwar).  
      The one or more device layers  24  can be formed between the first electrode  22  and the second electrode  26  by a variety of techniques, e.g., coating (e.g., spin coating), printing (e.g., screen printing or ink jet printing), physical or chemical vapor deposition, photolithography, and thermal transfer methods (e.g., methods described in U.S. Pat. No. 6,114,088 (Wolk et al.)). The one or more device layers  24  can be formed sequentially, or two or more of the layers can be disposed simultaneously. After formation of the one or more device layers  24  or simultaneously with deposition of the device layers  24 , the second electrode  26  is formed or otherwise disposed on the one or more device layers  24 . Alternatively, the electroluminescent element  20  may be formed using LITI techniques that include a multilayer donor sheet as described, e.g., in U.S. Pat. No. 6,114,088 (Wolk et al.).  
      Electroluminescent element  20  may also include a protective layer or layers (not shown) formed over the electroluminescent element  20  as is further described herein.  
      The electroluminescent device  10  also includes color filters  30  formed on the electroluminescent element  20 . One, two, or more color filters  30  may be formed on the electroluminescent element  20  such that at least a portion of light emitted from the electroluminescent element  20  is incident upon one or more color filters  30 . In other words, the color filters  30  are in optical association with the electroluminescent element  20 . The color filters  30  attenuate particular wavelengths or frequencies while passing others with relatively no change in wavelength. For example, color filter  30   a  may pass red light, color filter  30   b  may pass green light, and color filter  30   c  may pass blue light. As used herein, the term “red light” refers to light having a spectrum predominantly in an upper portion of the visible spectrum. As further used herein, the term “green light” refers to light having a spectrum predominantly in a middle portion of the visible spectrum. And “blue light” refers to light having a spectrum predominantly in a lower portion of the visible spectrum.  
      Color filters  30  may include any suitable material or materials. For example, color filters  30  may include any suitable colorant or colorants, e.g., color dyes, color pigments, or any other materials provided that they can selectively attenuate particular wavelengths or frequencies of radiation. These materials may be dispersed in a curable binder, e.g., a monomeric, oligomeric, or polymeric binder.  
      Color filters  30  may be formed on electroluminescent element  20  using any suitable technique, e.g., coating (e.g., spin coating), printing (e.g., screen printing or ink jet printing), physical or chemical vapor deposition, photolithography, and thermal transfer methods (e.g., methods described in U.S. Pat. No. 6,114,088 (Wolk et al.)). It may be preferred that color filters  30  are formed on electroluminescent element  20  using LITI techniques as further described herein.  
      In processes of the present disclosure, emissive materials, including light emitting polymers (LEPs) or other materials, color conversion elements, and color filters, can be selectively transferred from the transfer layer of a donor sheet to a receptor substrate by placing the transfer layer of the donor element adjacent to the receptor (e.g., the electroluminescent element  20 ) and selectively heating the donor element. See, for example, copending U.S. application Ser. No. ______ (Attorney Docket No. 59012US007, entitled “A METHOD OF MAKING AN ELECTROLUMINESCENT DEVICE INCLUDING A COLOR CONVERSION ELEMENT” and filed the same day herewith) for examples of selective transfer of color conversion elements. Illustratively, the donor element can be selectively heated by irradiating the donor element with imaging radiation that can be absorbed by light-to-heat converter (LTHC) material disposed in the donor, often in a separate LTHC layer, and converted into heat. Alternatively, LTHC can occur in any one or more of the layers in either the donor element and/or the receptor substrate. In these cases, the donor can be exposed to imaging radiation through the donor substrate, through the receptor, or both. The radiation can include one or more wavelengths, including visible light, infrared radiation, or ultraviolet radiation, for example from a laser, lamp, or other such radiation source. Other selective heating techniques can also be used, such as using a thermal print head or using a thermal hot stamp (e.g., a patterned thermal hot stamp such as a heated silicone stamp that has a relief pattern that can be used to selectively heat a donor). Material from the thermal transfer layer can be selectively transferred to a receptor in this manner to imagewise form patterns of the transferred material on the receptor. In many instances, thermal transfer using light from, for example, a lamp or laser, to patternwise expose the donor can be advantageous because of the accuracy and precision that can often be achieved. The size and shape of the transferred pattern (e.g., a line, circle, square, or other shape) can be controlled by, for example, selecting the size of the light beam, the exposure pattern of the light beam, the duration of directed beam contact with the donor sheet, or the materials of the donor sheet. The transferred pattern can also be controlled by irradiating the donor element through a mask.  
      As mentioned, a thermal print head or other heating element (patterned or otherwise) can also be used to selectively heat the donor element directly, thereby pattern-wise transferring portions of the transfer layer. In such cases, the light-to-heat converter material in the donor sheet or receptor is optional. Thermal print heads or other heating elements may be particularly suited for making lower resolution patterns of material or for patterning elements whose placement need not be precisely controlled.  
      Transfer layers can also be transferred in their entirety from donor sheets. For example, a transfer layer can be formed on a donor substrate that, in essence, acts as a temporary liner that can be released after the transfer layer is contacted to a receptor substrate, typically with the application of heat or pressure. Such a method, referred to as lamination transfer, can be used to transfer the entire transfer layer, or a large portion thereof, to the receptor.  
      The mode of thermal transfer can vary depending on the type of selective heating employed, the type of irradiation if used to expose the donor, the type of materials and properties of the optional LTHC layer, the type of materials in the transfer layer, the overall construction of the donor, the type of receptor substrate, and the like. Without wishing to be bound by any theory, transfer generally occurs via one or more mechanisms, one or more of which may be emphasized or de-emphasized during selective transfer depending on imaging conditions, donor constructions, and so forth. One mechanism of thermal transfer includes thermal melt-stick transfer whereby heating at the interface between the thermal transfer layer and the rest of the donor element results in adherence to the receptor more strongly than to the donor so that when the donor element is removed, the selected portions of the transfer layer remain on the receptor. Another mechanism of thermal transfer includes ablative transfer whereby localized heating can be used to ablate portions of the transfer layer off of the donor element, thereby directing ablated material toward the receptor. Yet another mechanism of thermal transfer includes sublimation whereby material dispersed in the transfer layer can be sublimated by heat generated in the donor element. A portion of the sublimated material can condense on the receptor. The present invention contemplates transfer modes that include one or more of these and other mechanisms whereby selective heating of a donor sheet can be used to cause the transfer of materials from a transfer layer to receptor surface.  
      A variety of radiation-emitting sources can be used to heat donor sheets. For analog techniques (e.g., exposure through a mask), high-powered light sources (e.g., xenon flash lamps and lasers) are useful. For digital imaging techniques, infrared, visible, and ultraviolet lasers are particularly useful. Suitable lasers include, for example, high power (≧100 mW) single mode laser diodes, fiber-coupled laser diodes, and diode-pumped solid state lasers (e.g., Nd:YAG and Nd:YLF). Laser exposure dwell times can vary widely from, for example, a few hundredths of microseconds to tens of microseconds or more, and laser fluences can be in the range from, for example, about 0.01 to about 5 J/cm 2  or more. Other radiation sources and irradiation conditions can be suitable based on, among other things, the donor element construction, the transfer layer material, the mode of thermal mass transfer, and other such factors.  
      When high spot placement accuracy is desired (e.g., when patterning elements for high information content displays and other such applications) over large substrate areas, a laser can be particularly useful as the radiation source. Laser sources are also compatible with both large rigid substrates (e.g., 1 m×1 m×1.1 mm glass) and continuous or sheeted film substrates (e.g., 100 μm thick polyimide sheets).  
      During imaging, the donor sheet can be brought into intimate contact with a receptor (as might typically be the case for thermal melt-stick transfer mechanisms) or the donor sheet can be spaced some distance from the receptor (as can be the case for ablative transfer mechanisms or material sublimation transfer mechanisms). In at least some instances, pressure or vacuum can be used to hold the donor sheet in intimate contact with the receptor. In some instances, a mask can be placed between the donor sheet and the receptor. Such a mask can be removable or can remain on the receptor after transfer. If a light-to-heat converter material is present in the donor, radiation source can then be used to heat the LTHC layer (or other layer(s) containing radiation absorber) in an imagewise fashion (e.g., digitally or by analog exposure through a mask) to perform imagewise transfer or patterning of the transfer layer from the donor sheet to the receptor.  
      Typically, selected portions of the transfer layer are transferred to the receptor without transferring significant portions of the other layers of the donor sheet, such as an optional interlayer or LTHC layer as is further described herein. The presence of the optional interlayer may eliminate or reduce the transfer of material from an LTHC layer or other proximate layers (for example, other interlayers) to the receptor or reduce distortion in the transferred portion of the transfer layer. Preferably, under imaging conditions, the adhesion of the optional interlayer to the LTHC layer is greater than the adhesion of the interlayer to the transfer layer. The interlayer can be transmissive, reflective, or absorptive to imaging radiation, and can be used to attenuate or otherwise control the level of imaging radiation transmitted through the donor or to manage temperatures in the donor, for example to reduce thermal or radiation-based damage to the transfer layer during imaging. Multiple interlayers can be present.  
      Large donor sheets can be used, including donor sheets that have length and width dimensions of a meter or more. In operation, a laser can be rastered or otherwise moved across the large donor sheet, the laser being selectively operated to illuminate portions of the donor sheet according to a desired pattern. Alternatively, the laser may be stationary and the donor sheet or receptor substrate moved beneath the laser.  
      In some instances, it may be necessary, desirable, or convenient to sequentially use two or more different donor sheets to form electronic devices on a receptor. For example, multiple layer devices can be formed by transferring separate layers or separate stacks of layers from different donor sheets. Multilayer stacks can also be transferred as a single transfer unit from a single donor element as is described, e.g., in U.S. Pat. No. 6,114,088 (Wolk et al.). For example, a hole transport layer and a LEP layer can be co-transferred from a single donor. As another example, a semiconductive polymer and an emissive layer can be co-transferred from a single donor. Multiple donor sheets can also be used to form separate components in the same layer on the receptor. For example, electroluminescent elements (e.g., electroluminescent element  20 ) can be patterned by selective thermal transfer of electrically active organic materials (oriented or not), followed by selective thermal transfer patterning of one or more pixel or sub-pixel elements such as color filters (e.g., color filters  30 ), emissive layers, charge transport layers, electrode layers, and the like.  
      Materials from separate donor sheets can be transferred adjacent to other materials on a receptor to form adjacent devices, portions of adjacent devices, or different portions of the same device. Alternatively, materials from separate donor sheets can be transferred directly on top of, or in partial overlying registration with, other layers or materials previously patterned onto the receptor by thermal transfer or some other method (e.g., photolithography, deposition through a shadow mask, etc.). A variety of other combinations of two or more donor sheets can be used to form a device, each donor sheet used to form one or more portions of the device. It will be understood that other portions of these devices, or other devices on the receptor, may be formed in whole or in part by any suitable process including photolithographic processes, ink jet processes, and various other printing or mask-based processes, whether conventionally used or newly developed.  
      The donor substrate can be a polymer film. One suitable type of polymer film is a polyester film, for example, polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) films. However, other films with sufficient optical properties, including high transmission of light at a particular wavelength, or sufficient mechanical and thermal stability properties, depending on the particular application, can be used. The donor substrate, in at least some instances, is flat so that uniform coatings can be formed thereon. The donor substrate is also typically selected from materials that remain stable despite heating of one or more layers of the donor. However, as described herein, the inclusion of an underlayer between the substrate and an LTHC layer can be used to insulate the substrate from heat generated in the LTHC layer during imaging. The typical thickness of the donor substrate ranges from 0.025 to 0.15 mm, preferably 0.05 to 0.1 mm, although thicker or thinner donor substrates may be used.  
      The materials used to form the donor substrate and an optional adjacent underlayer can be selected to improve adhesion between the donor substrate and the underlayer, to control heat transport between the substrate and the underlayer, to control imaging radiation transport to the LTHC layer, to reduce imaging defects and the like. An optional priming layer can be used to increase uniformity during the coating of subsequent layers onto the substrate and also increase the bonding strength between the donor substrate and adjacent layers.  
      An optional underlayer may be coated or otherwise disposed between a donor substrate and the LTHC layer, for example to control heat flow between the substrate and the LTHC layer during imaging or to provide mechanical stability to the donor element for storage, handling, donor processing, or imaging. Examples of suitable underlayers and techniques of providing underlayers are disclosed in U.S. Pat. No. 6,284,425 (Staral et al.).  
      The underlayer can include materials that impart desired mechanical or thermal properties to the donor element. For example, the underlayer can include materials that exhibit a low specific heat×density or low thermal conductivity relative to the donor substrate. Such an underlayer may be used to increase heat flow to the transfer layer, for example to improve the imaging sensitivity of the donor.  
      The underlayer may also include materials for their mechanical properties or for adhesion between the substrate and the LTHC. Using an underlayer that improves adhesion between the substrate and the LTHC layer may result in less distortion in the transferred image. As an example, in some cases an underlayer can be used that reduces or eliminates delamination or separation of the LTHC layer, for example, that might otherwise occur during imaging of the donor media. This can reduce the amount of physical distortion exhibited by transferred portions of the transfer layer. In other cases, however it may be desirable to employ underlayers that promote at least some degree of separation between or among layers during imaging, for example to produce an air gap between layers during imaging that can provide a thermal insulating function. Separation during imaging may also provide a channel for the release of gases that may be generated by heating of the LTHC layer during imaging. Providing such a channel may lead to fewer imaging defects.  
      The underlayer may be substantially transparent at the imaging wavelength, or may also be at least partially absorptive or reflective of imaging radiation. Attenuation or reflection of imaging radiation by the underlayer may be used to control heat generation during imaging.  
      An LTHC layer can be included in donor sheets of the present disclosure to couple irradiation energy into the donor sheet. The LTHC layer preferably includes a radiation absorber that absorbs incident radiation (e.g., laser light) and converts at least a portion of the incident radiation into heat to enable transfer of the transfer layer from the donor sheet to the receptor.  
      Generally, the radiation absorber(s) in the LTHC layer absorb light in the infrared, visible, or ultraviolet regions of the electromagnetic spectrum and convert the absorbed radiation into heat. The radiation absorber(s) are typically highly absorptive of the selected imaging radiation, providing an LTHC layer with an optical density at the wavelength of the imaging radiation in the range of about 0.2 to 3 or higher. Optical density of a layer is the absolute value of the logarithm (base 10) of the ratio of the intensity of light transmitted through the layer to the intensity of light incident on the layer.  
      Radiation absorber material can be uniformly disposed throughout the LTHC layer or can be non-homogeneously distributed. For example, as described in U.S. Pat. No. 6,228,555 (Hoffend, Jr., et al.), non-homogeneous LTHC layers can be used to control temperature profiles in donor elements. This can give rise to donor sheets that have improved transfer properties (e.g., better fidelity between the intended transfer patterns and actual transfer patterns).  
      Suitable radiation absorbing materials can include, for example, dyes (e.g., visible dyes, ultraviolet dyes, infrared dyes, fluorescent dyes, and radiation-polarizing dyes), pigments, metals, metal compounds, metal films, black body absorbers, and other suitable absorbing materials. Examples of suitable radiation absorbers includes carbon black, metal oxides, and metal sulfides. One example of a suitable LTHC layer can include a pigment, such as carbon black, and a binder, such as an organic polymer. Another suitable LTHC layer includes metal or metal/metal oxide formed as a thin film, for example, black aluminum (i.e., a partially oxidized aluminum having a black visual appearance). Metallic and metal compound films may be formed by techniques, such as, for example, sputtering and evaporative deposition. Particulate coatings may be formed using a binder and any suitable dry or wet coating techniques. LTHC layers can also be formed by combining two or more LTHC layers containing similar or dissimilar materials. For example, an LTHC layer can be formed by vapor depositing a thin layer of black aluminum over a coating that contains carbon black disposed in a binder.  
      Dyes suitable for use as radiation absorbers in a LTHC layer may be present in particulate form, dissolved in a binder material, or at least partially dispersed in a binder material. When dispersed particulate radiation absorbers are used, the particle size can be, at least in some instances, about 10 μm or less, and may be about 1 μm or less. Suitable dyes include those dyes that absorb in the IR region of the spectrum. A specific dye may be chosen based on factors such as, solubility in, and compatibility with, a specific binder or coating solvent, as well as the wavelength range of absorption.  
      Pigmentary materials may also be used in the LTHC layer as radiation absorbers. Examples of suitable pigments include carbon black and graphite, as well as phthalocyanines, nickel dithiolenes, and other pigments described in U.S. Pat. No. 5,166,024 (Bugner et al.) and U.S. Pat. No. 5,351,617 (Williams et al.). Additionally, black azo pigments based on copper or chromium complexes of, for example, pyrazolone yellow, dianisidine red, and nickel azo yellow can be useful. Inorganic pigments can also be used, including, for example, oxides and sulfides of metals such as aluminum, bismuth, tin, indium, zinc, titanium, chromium, molybdenum, tungsten, cobalt, iridium, nickel, palladium, platinum, copper, silver, gold, zirconium, iron, lead, and tellurium. Metal borides, carbides, nitrides, carbonitrides, bronze-structured oxides, and oxides structurally related to the bronze family (e.g., WO 2.9 ) may also be used.  
      Metal radiation absorbers may be used, either in the form of particles, as described for instance in U.S. Pat. No. 4,252,671 (Smith), or as films, as disclosed in U.S. Pat. No. 5,256,506 (Ellis et al.). Suitable metals include, for example, aluminum, bismuth, tin, indium, tellurium and zinc.  
      Suitable binders for use in the LTHC layer include film-forming polymers, such as, for example, phenylic resins (e.g., novolak and resole resins), polyvinyl butyral resins, polyvinyl acetates, polyvinyl acetals, polyvinylidene chlorides, polyacrylates, cellulosic ethers and esters, nitrocelluloses, and polycarbonates. Suitable binders may include monomers, oligomers, or polymers that have been, or can be, polymerized or crosslinked. Additives such as photoinitiators may also be included to facilitate crosslinking of the LTHC binder. In some embodiments, the binder is primarily formed using a coating of crosslinkable monomers or oligomers with optional polymer.  
      The inclusion of a thermoplastic resin (e.g., polymer) may improve, in at least some instances, the performance (e.g., transfer properties or coatability) of the LTHC layer. It is thought that a thermoplastic resin may improve the adhesion of the LTHC layer to the donor substrate. In one embodiment, the binder includes 25 to 50 wt. % (excluding the solvent when calculating weight percent) thermoplastic resin, and, preferably, 30 to 45 wt. % thermoplastic resin, although lower amounts of thermoplastic resin may be used (e.g., 1 to 15 wt. %). The thermoplastic resin is typically chosen to be compatible (i.e., form a one-phase combination) with the other materials of the binder. In at least some embodiments, a thermoplastic resin that has a solubility parameter in the range of 9 to 13 (cal/cm 3 ) 1/2 , preferably, 9.5 to 12 (cal/cm 3 ) 1/2 , is chosen for the binder. Examples of suitable thermoplastic resins include polyacrylics, styrene-acrylic polymers and resins, and polyvinyl butyral.  
      Conventional coating aids, such as surfactants and dispersing agents, may be added to facilitate the coating process. The LTHC layer may be coated onto the donor substrate using a variety of coating methods known in the art. A polymeric or organic LTHC layer can be coated, in at least some instances, to a thickness of 0.05 μm to 20 μm, preferably, 0.5 μm to 10 μm, and, more preferably, 1 μm to 7 μm. An inorganic LTHC layer can be coated, in at least some instances, to a thickness in the range of 0.0005 to 10 μm, and preferably, 0.001 to 1 μm.  
      At least one optional interlayer may be disposed between the LTHC layer and transfer layer. The interlayer can be used, for example, to minimize damage and contamination of the transferred portion of the transfer layer and may also reduce distortion or mechanical damage of in the transferred portion of the transfer layer. The interlayer may also influence the adhesion of the transfer layer to the rest of the donor sheet. Typically, the interlayer has high thermal resistance. Preferably, the interlayer does not distort or chemically decompose under the imaging conditions, particularly to an extent that renders the transferred image non-functional. The interlayer typically remains in contact with the LTHC layer during the transfer process and is not substantially transferred with the transfer layer.  
      Suitable interlayers include, for example, polymer films, metal layers (e.g., vapor deposited metal layers), inorganic layers (e.g., sol-gel deposited layers and vapor deposited layers of inorganic oxides (e.g., silica, titania, and other metal oxides)), and organic/inorganic composite layers. Organic materials suitable as interlayer materials include both thermoset and thermoplastic materials. Suitable thermoset materials include resins that may be crosslinked by heat, radiation, or chemical treatment including, but not limited to, crosslinked or crosslinkable polyacrylates, polymethacrylates, polyesters, epoxies, and polyurethanes. The thermoset materials may be coated onto the LTHC layer as, for example, thermoplastic precursors and subsequently crosslinked to form a crosslinked interlayer.  
      Suitable thermoplastic materials include, for example, polyacrylates, polymethacrylates, polystyrenes, polyurethanes, polysulfones, polyesters, and polyimides. These thermoplastic organic materials may be applied via conventional coating techniques (for example, solvent coating, spray coating, or extrusion coating). Typically, the glass transition temperature (T g ) of thermoplastic materials suitable for use in the interlayer is 25° C. or greater, preferably 50° C. or greater. In some embodiments, the interlayer includes a thermoplastic material that has a T g  greater than any temperature attained in the transfer layer during imaging. The interlayer may be either transmissive, absorbing, reflective, or some combination thereof, at the imaging radiation wavelength.  
      Inorganic materials suitable as interlayer materials include, for example, metals, metal oxides, metal sulfides, and inorganic carbon coatings, including those materials that are highly transmissive or reflective at the imaging light wavelength. These materials may be applied to the light-to-heat-conversion layer via conventional techniques (e.g., vacuum sputtering, vacuum evaporation, or plasma jet deposition).  
      The interlayer may provide a number of benefits. The interlayer may be a barrier against the transfer of material from the light-to-heat conversion layer. The interlayer may also act as a barrier that can prevent any material or contamination exchange to or from layers proximate thereto. It may also modulate the temperature attained in the transfer layer so that thermally unstable materials can be transferred. For example, the interlayer can act as a thermal diffuser to control the temperature at the interface between the interlayer and the transfer layer relative to the temperature attained in the LTHC layer. This may improve the quality (i.e., surface roughness, edge roughness, etc.) of the transferred layer. The presence of an interlayer may also result in improved plastic memory in the transferred material.  
      The interlayer may contain additives, including, for example, photoinitiators, surfactants, pigments, plasticizers, and coating aids. The thickness of the interlayer may depend on factors such as, for example, the material of the interlayer, the material and properties of the LTHC layer, the material and properties of the transfer layer, the wavelength of the imaging radiation, and the duration of exposure of the donor sheet to imaging radiation. For polymer interlayers, the thickness of the interlayer typically is in the range of 0.05 μm to 10 μm. For inorganic interlayers (e.g., metal or metal compound interlayers), the thickness of the interlayer typically is in the range of 0.005 μm to 10 μm. Multiple interlayers can also be used; for example, an organic-based interlayer can be covered by an inorganic-based interlayer to provide additional protection to the transfer layer during the thermal transfer process.  
      A thermal transfer layer is included in the donor sheet. The transfer layer can include any suitable material or materials, disposed in one or more layers, alone or in combination with other materials. The transfer layer is capable of being selectively transferred as a unit or in portions by any suitable transfer mechanism when the donor element is exposed to direct heating or to imaging radiation that can be absorbed by light-to-heat converter material and converted into heat. In some embodiments, the thermal transfer layer can include light-to-heat conversion material.  
      The thermal transfer layer can be used to form, for example, color filters, electronic circuitry, resistors, capacitors, diodes, rectifiers, electroluminescent lamps, memory elements, field effect transistors, bipolar transistors, unijunction transistors, MOS transistors, metal-insulator-semiconductor transistors, charge coupled devices, insulator-metal-insulator stacks, organic conductor-metal-organic conductor stacks, integrated circuits, photodetectors, lasers, lenses, waveguides, gratings, holographic elements, filters (e.g., add-drop filters, gain-flattening filters, cut-off filters, and the like), mirrors, splitters, couplers, combines, modulators, sensors (e.g., evanescent sensors, phase modulation sensors, interferometric sensors, and the like), optical cavities, piezoelectric devices, ferroelectric devices, thin film batteries, or combinations thereof; for example, the combination of field effect transistors and organic electroluminescent lamps as an active matrix array for an optical display. Other items may be formed by transferring a multicomponent transfer unit and/or a single layer.  
      The transfer layer can be selectively thermally transferred from the donor element to a proximately located receptor substrate. There can be, if desired, more than one transfer layer so that a multilayer construction is transferred using a single donor sheet. The receptor substrate may be any item suitable for a particular application including, but not limited to, glass, transparent films, reflective films, metals, semiconductors, and plastics. For example, receptor substrates may be any type of substrate or display element suitable for display applications, e.g., emissive displays, reflective displays, transflective displays, micromechanical displays, and the like. Receptor substrates suitable for use in displays such as liquid crystal displays or emissive displays include rigid or flexible substrates that are substantially transmissive to visible light. Examples of suitable rigid receptors include glass and rigid plastic that are coated or patterned with indium tin oxide or are circuitized with low temperature poly-silicon (LTPS) or other transistor structures, including organic transistors.  
      Suitable flexible substrates include substantially clear and transmissive polymer films, reflective films, transflective films, polarizing films, multilayer optical films, metallic films, metallic sheets, metallic foils, and the like. Flexible substrates can also be coated or patterned with electrode materials or transistors, for example transistor arrays formed directly on the flexible substrate or transferred to the flexible substrate after being formed on a temporary carrier substrate. Suitable polymer substrates include polyester base (e.g., polyethylene terephthalate, polyethylene naphthalate), polycarbonate resins, polyolefin resins, polyvinyl resins (e.g., polyvinyl chloride, polyvinylidene chloride, polyvinyl acetals, etc.), cellulose ester bases (e.g., cellulose triacetate, cellulose acetate), and other conventional polymeric films used as supports. For making organic electroluminescent devices on plastic substrates, it is often desirable to include a barrier film or coating on one or both surfaces of the plastic substrate to protect the organic light emitting devices and their electrodes from exposure to undesired levels of water, oxygen, and the like.  
      Receptor substrates can be pre-patterned with any one or more of electrodes, transistors, capacitors, insulator ribs, spacers, color filters, black matrix, hole transport layers, electron transport layers, and other elements useful for electronic displays or other devices.  
      To form color filters (e.g., color filters  30  of  FIG. 1 ), any suitable color material may be included in one or more transfer layers of the donor sheet. The colors of the transfer layer may be selected as needed by the user from amongst the many available colors normally or specially used in color filters, such as cyan, yellow, magenta, red, blue, green, white and other colors and tones of the spectrum as contemplated. The dyes can be transmissive of preselected specific wavelengths when transferred to the receptor substrate. For many applications, highly transmissive dyes, e.g., dyes having an optical density of less than 0.5 optical density units within a narrow wavelength distribution of 10 nanometers or less when those dyes are present on the receptor substrate, may be preferred. Dyes with even lower absorption characteristics within those narrow wavelength bands may be even more preferred.  
      A typical color filter transfer layer can include at least one organic or inorganic colorant or pigment and optionally an organic polymer or binder. The transfer layer materials may be optionally crosslinked before or after laser transfer in order to improve performance of the imaged color filter. Crosslinking of the color filter material may be effected by radiation, heat, and/or chemical curative. The color filter transfer layer may also contain a variety of additives including but not limited to dyes, plasticizers, UV-stabilizers, film forming additives, photoinitiators for photo-crosslinked or photo-crosslinkable color filter transfer layers, and adhesives. When a dye is used as an additive, it is generally preferable that the dye absorbs light of the same frequency as the imaging light source. The optional adhesive layer may also contain a dye that absorbs light of the same frequency as the imaging laser or light source.  
      For color filter transfer layers including pigments, any suitable pigment may be used, but preferred pigments may include those listed as having good color permanency and transparency in the NPIRI Raw Materials Data Handbook, Volume 4 (Pigments). Either non-aqueous or aqueous dispersions of pigment in binder may be used. In the non-aqueous case, solvent based pigment dispersions may be used along with an appropriate solvent based binder (i.e. Elvacite™ acrylic resins available from DuPont). However, it may be preferred to use an aqueous dispersion of pigment in binder. In this case, the most preferred pigments may be in the form of binderless aqueous dispersions (i.e. Aquis II™ supplied by Heucotech) and the most preferred binders may be those specifically designed for pigment wetting (i.e. Neocryl BT™ acrylic resins from Zeneca Resins). The use of appropriate binders may promote the formation of sharp, well defined lines during transfer. When the color filter transfer layer is induced by a high powered light source (i.e., xenon flash lamp), it may be necessary to include as binder an energetic or gas producing polymer such as disclosed in U.S. Pat. No. 5,308,737 (Bills et al.) and U.S. Pat. No. 5,278,023 (Bills et al.).  
      The pigment/binder ratio is typically 1:1 but may range from 0.25:1 to 4:1. A Mayer bar may be used to coat the colorant layer. Typically, a #4 bar is used to coat the dispersion which contains approximately 10 wt. % solids to give a dry coating thickness of about 1 micron. Other combinations of dispersion % solids and Mayer bar number are used to achieve different coating thicknesses. In general, a dry coating thickness of 0.1 to 10 microns may be desired.  
      A method of making an electroluminescent device will now be described in reference to the electroluminescent device  10  of  FIG. 1 . The electroluminescent element  20  of device  10  is formed on the major surface  14  of the substrate  12  using any suitable technique, e.g., LITI patterning as described herein. Color filters  30  are selectively thermally transferred to the electroluminescent element  20  as is also described herein. The color filters  30  may be transferred to the electroluminescent element  20  such that the color filters  30  are on the second electrode  26 . Alternatively, the color filters  30  may be transferred to a protective layer (not shown) that is formed over at least a portion of the electroluminescent element  20  as is further described herein. In some embodiments, a black matrix may be formed on the electroluminescent element  20  and the color filters  30  then transferred to apertures in the black matrix as is further described herein.  
       FIG. 2  is a schematic diagram of another embodiment of an electroluminescent device  100 . The electroluminescent device  100  is similar in many respects to electroluminescent device  10  of  FIG. 1 . In the embodiment shown in  FIG. 2 , electroluminescent device  100  includes a substrate  112 , an electroluminescent element  120  formed on a major surface  114  of substrate  112 , and color filters  130   a ,  130   b , and  130   c  (hereinafter referred to collectively as color filters  130 ) formed on a protective layer  140 . The electroluminescent element  120  includes a first electrode  122 , a second electrode  126 , and one or more device layers  124  positioned between the first electrode  122  and the second electrode  126 . All of the design considerations and possibilities described herein with respect to the substrate  12 , the electroluminescent element  20 , and the color filters  30  of the embodiment illustrated in  FIG. 1  apply equally to the substrate  112 , the electroluminescent element  120 , and the color filters  130  of the embodiment illustrated in  FIG. 2 .  
      Electroluminescent device  100  also includes a protective layer  140  formed over at least a portion of the electroluminescent element  120 . The protective layer  140  may be formed on and in contact with the electroluminescent element  120 . Alternatively, an optional layer or layers may be included between the electroluminescent element  120  and the protective layer  140 .  
      The protective layer  140  may be any suitable type of layer or layers that protect the electroluminescent element  120 , e.g., barrier layers, encapsulant layers, etc. The protective layer  140  may be formed using any suitable material or materials, e.g., as described in U.S. Patent Application Publication No. 2004/0195967 (Padiyath et al.) and U.S. Pat. No. 6,522,067 (Graff et al.).  
      The color filters  130  are transferred to a major surface  142  of the protective layer  140 . As described herein in regard to color filters  30  of electroluminescent device  10  of  FIG. 1 , the color filters  130  of electroluminescent device  100  may be formed using any suitable technique, e.g., coating (e.g., spin coating), printing (e.g., screen printing or ink jet printing), physical or chemical vapor deposition, photolithography, and thermal transfer methods (e.g., methods described in U.S. Pat. No. 6,114,088 (Wolk et al.)). It may be preferred that the color filters  130  are transferred to the protective layer  140  using LITI techniques as described herein.  
      Other elements may be formed on the electroluminescent element or protective layer, e.g., black matrix, etc. For example,  FIG. 3  is a schematic diagram of another embodiment of an electroluminescent device  200 . Electroluminescent device  200  is similar in many respects to electroluminescent device  10  of  FIG. 1  and electroluminescent device  100  of  FIG. 2 . Electroluminescent device  200  includes a substrate  212 , an electroluminescent element  220  formed on a major surface  214  of the substrate  212 , and color filters  230   a ,  230   b , and  230   c  (hereinafter referred to collectively as color filters  230 ) formed on the electroluminescent element  220 . Electroluminescent element  220  includes a first electrode  222 , a second electrode  226 , and one or more device layers  224  positioned between the first electrode  222  and the second electrode  226 . All of the design considerations and possibilities described herein with respect to substrate  12 , electroluminescent element  20 , and color filters  30  of the embodiment illustrated in  FIG. 1  apply equally to the substrate  212 , the electroluminescent element  220 , and the color filters  230  of the embodiment illustrated in  FIG. 3 .  
      Electroluminescent device  200  further includes an optional black matrix  260  formed on the electroluminescent element  220 . Black matrix  260  includes a plurality of apertures  262   a ,  262   b , and  262   c  (hereinafter referred to collectively as apertures  262 ). Although the embodiment illustrated in  FIG. 3  includes only three apertures  262 , the black matrix  260  can include any suitable number of apertures. Each aperture  262  may take any suitable shape, e.g., oval, rectangular, polygonal, etc.  
      In general, black matrix coatings are used in many display applications to absorb ambient light, improve contrast, and protect TFTs. The black matrix  260  (typically including absorbing or non-reflecting metals, metal oxides, metal sulfides, dyes or pigments) is formed around individual pixels, color conversion elements, or color filters of the display. In many displays, the black matrix  260  is a 0.1 to 0.2 μm coating of black chromium oxide on a display substrate. Resin black matrix (a pigment in a resin matrix) is an alternative to black chromium oxide. The resin black matrix can be coated onto the display substrate or electroluminescent device and then patterned using photolithography. To achieve a high optical density in a thin resin black matrix coating, it is typically necessary to use relatively high pigment loadings, which can be difficult to pattern using photolithography. Alternatively, the black matrix  260  can be transferred from a donor sheet to the device using a thermal transfer method, such as described in U.S. Pat. No. 6,461,775 (Pokorny et al.).  
      In some embodiments, the color filters  230  may be transferred to the electroluminescent element  220  such that each color filter  230  is transferred to an aperture  262  of optional black matrix  260  using any suitable technique as described herein. For example, color filter  230   a  can be transferred to aperture  262   a  of black matrix  260 .  
      In some embodiments, one or more of the substrate  212 , the one or more device layers  224 , and the color filters  230  may be configured to provide polarized light as is further described, e.g., in U.S. Pat. No. 6,485,884 (Wolk et al.) and U.S. Pat. No. 5,693,446 (Staral et al.).  
      As described herein, electroluminescent devices may be either top emitting (e.g., electroluminescent device  10  of  FIG. 1 ) or bottom emitting. One such embodiment of a bottom emitting device is illustrated in  FIG. 4 , which is a schematic diagram of another embodiment of an electroluminescent device  300 . Electroluminescent device  300  is similar in many respects to electroluminescent device  10  of  FIG. 1 . Electroluminescent device  300  includes a substrate  312  and an electroluminescent element  320  formed on a first major surface  314  of the substrate  312 . The electroluminescent element  320  includes a first electrode  322 , a second electrode  326 , and one or more device layers  324  positioned between the first electrode  322  and the second electrode  326 .  
      One difference between electroluminescent device  300  and electroluminescent device  10  of  FIG. 1  is that device  300  is a bottom emitting electroluminescent device. In this embodiment, color filters  330   a ,  330   b , and  330   c  (hereinafter referred to collectively as color filters  330 ) are formed on a second major surface  316  of the substrate  312  such that the color filters  330  are in optical association with the electroluminescent element  320 . In other words, at least a portion of light emitted by electroluminescent element  320  passes through substrate  312  and is incident on at least one color filter  330 . Although only three color filters  330  are illustrated, electroluminescent device  300  may include any suitable number of color filters, e.g., red and green; red, green, blue, etc. Further, electroluminescent device  300  may include a black matrix formed on the second major surface  316  of substrate  312  as is further described herein.  
      All of the design considerations and possibilities described herein with respect to the substrate  12 , the electroluminescent element  20 , and the color filters  30  of  FIG. 1  apply equally to similar elements of the embodiment illustrated in  FIG. 4 .  
      All documents and publications cited herein are expressly incorporated herein by reference in their entirety into this disclosure. Illustrative embodiments of this invention are discussed and reference has been made to possible variations within the scope of this invention. These and other variations and modifications in the invention will be apparent to those skilled in the art without departing from the scope of the invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. Accordingly, the invention is to be limited only by the claims provided below.