Abstract:
This invention addresses how an end user can create an electroluminescent device ( 1, 31 ) using basic components obtained by the end user as retail items. The layers or subassemblies ( 15, 17 ) are created first. Then the layers are assembled to form a completed device customized as selected by the end user. The subassemblies may be created layer-by-layer by thermal inkjet. Elements used typically will be made by manufacturers and sold commercially separately. This encompasses the printing of conductive patterns for electroluminescence on paper. In one aspect a display has a main body that may be permanent and useful indefinitely, while a part carrying the conductive pattern defining the display is readily removed and is replaced by another such part on which a new conductive pattern is printed. For tight contact with the main body, the display provides releasable pressure. Air pockets are minimized with a thin layer of highly viscous dielectric liquid.

Description:
TECHNICAL FIELD 
   This invention relates to luminous displays, specifically electroluminescent displays, that may be readily varied in the patterns or text displayed. 
   BACKGROUND OF THE INVENTION 
   In electroluminscence (EL), light is produced by applying an electric field upon an emitter material. Applying this field upon the emitter supplies the material with energized electrons that can then decay to produce photons. This results in devices that can be used for illumination. The structure of these devices can be relatively simple containing two electrodes, an emitter layer and some passivation layers. There may also be other dielectric or transport layers as deemed necessary in particular applications. Currently, these layers are assembled by a manufacturer and the completed EL device is purchased by an end user. 
   Electroluminescent displays are known which produce bright, attractive light pictures and messages at low cost in power. Since such devices are currently completed by a manufacturer, the end user necessarily has a limited selection. A need exists for such displays that can be customized or modified readily by the user at low cost. 
   U.S. Patent Publication No. 2002/0090495 A1 describes multilayer sheet forming a complete electroluminescent device that is to receive inkjet printing on an outer surface. The inkjet printing forms a mask and the electrolumination serves a back lighting for the mask printer. This requires an inkjet printer that can print on such a multilayer device and requires the multilayer device to be resistant to damage by the printing operation. 
   Accordingly, a need exists for customization of electroluminescent devices by printing on easily manipulated and sturdy substrates, including inexpensive substrates such as paper. 
   DISCLOSURE OF THE INVENTION 
   This invention addresses how an end user can create an EL device structure using basic components obtained by the end user as retail items. The layers or subassemblies are created first. Then the layers are matched, mated and assembled to form a completed EL device customized as selected by the end user. The subassemblies may be created in a layer-by-layer procedure using application by thermal inkjet. The subassemblies typically will be made by manufacturers and sold commercially as subassemblies. Layers may be added to either one or possibly two original substrates. They can be formed into a functional EL device using joining techniques. 
   This invention encompasses the printing of conductive patterns for electroluminescence on substrates, which may be paper. The optical display is formed at least in part by the pattern of conductor printed. This paper or other substrate with a printed, conductive pattern is then combined in a laminate including other layers of elements of an electroluminescent display. 
   In one aspect, this invention provides an electroluminescent display in which the main body of the display may be permanent and useful indefinitely, while a part carrying the conductive pattern defining the display is readily removed and is replaced by another such part on which a new conductive pattern is printed. The printing is by conductive ink, preferably by inkjet printing. The removable part is in sheet form suitable to receive printing. The conductive pattern printed may be composed by standard techniques on a computer and delivered from the computer to the printer for printing by standard techniques. To have the conductive pattern on the sheet tightly contact the main body, the display provides releasable pressure. 
   In forming the final display, it is important to minimize air pockets. This invention provides for an inert, highly viscous liquid applied at the boundary between a removable conductive substrate and the dielectric layer of the electroluminescent device. The liquid may be a high dielectric. This liquid also facilitates separation of the conductive layer or sheet for substitution with a different conductive layer. Where the EL device is permanent and formed under substantial pressure using adhesive, the dielectric liquid is not necessary. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The details of this invention will be described in connection with the accompanying drawings in which 
       FIG. 1  illustrates a basic electroluminescent device consistent with this invention; 
       FIG. 2  illustrates two subassemblies of such a device and their combination; 
       FIG. 3  illustrates a basic electroluminescent device having an outer sealing layer. 
       FIG. 4  is a business flow diagram illustrating application of this invention; 
       FIG. 5  illustrates a basic electroluminescent device intended to be held together by external pressure; 
       FIG. 6  illustrates a mechanical assembly in accordance with this invention; 
       FIG. 7  illustrates a different mechanical assembly in accordance with this invention; and 
       FIG. 8  illustrates a illustrates a variation of the assembly of  FIG. 7   
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Electroluminescent devices produce light by applying an electric field upon an emitter material. Applying this field upon the emitter supplies the material with energized electrons that decay to produce photons. This results in devices that can be used for illumination or for display of pictures or text. The structure of these devices can be relatively simple, containing two electrodes, an emitter layer and a dielectric layer or layers to provide internal barriers. 
   An electroluminescent device  1  of this invention is represented illustratively in  FIG. 1 . The front, light emitting side of device  1  is top layer  3 , which also may be a support layer, such as glass. 
   The next layer from the top is layer  5 , which is typically a continuous, transparent conductive layer. Various very thin metal and alloys are transparent conductors suitable for this purpose, specifically indium tin oxide. 
   The next layer from the top is layer  7 , the emitter layer, which is phosphorescent material suspended in a resin matrix. Representative phosphorescent materials are AnS:Cu,Al (zinc sulfide doped with copper and aluminum) and AnS:Cu, Mn (zinc sulfide doped with copper and manganese). 
   The next layer from the top is layer  9 , a dielectric layer that serves to physically protect emitter layer  7  while passing electrical drive signals through its dielectric characteristics. The dielectric layer  9  may comprise ceramic particles such as barium titanate suspended in a binder matrix. 
   The next element from the top is an insulating layer  11  having supporting electrically conductive elements  13  as an electrode in a pattern next to dielectric layer  9 . 
   The drawing shows conductive elements  13  in close contact with dielectric layer  9 , as is best for good operation to produce illumination in a pattern as defined by conductive elements  13 . The structure of insulating layer  11  with conductive elements  13  can be formed by printing conductive elements  13  on a substrate such as ordinary or porous paper or a synthetic polymer such as MYLAR polyester films or other polymeric films. The paper or film is applied as insulating layer  11  and elements  13  are the electrode of the completed EL device. 
   In the drawings, elements directly corresponding to elements as described for earlier figures are given the same reference numeral. 
   In one embodiment of the invention, a device structure such as in  FIG. 1  is constructed using a basic stack comprising of a transparent substrate such as polyester having a transparent conductor and then depositing the subsequent layers by different methods including inkjet printing. These subsequent layers include but are not limited to EL emitter layers, dielectric layers, transport layers, conductive layers and passivation layers. 
   A second possible embodiment is the first embodiment altered to allow one or more of the stated inkjet printed layers to be formed in a separate coating process prior to inkjet printing. In this embodiment, a precoated sheet used in the printing process may already contain a substrate, transparent conductor and an emitter layer. Following this embodiment, only a dielectric and conductor layer would need to be inkjet printed to form a device. 
   By extrapolation of this second embodiment, the precoated sheet could also contain the dielectric layer and/or other layers upon the emitter layer. This would leave only the conductor to be printed by an inkjet process. The choice of which layers would be included in the original substrate (provided by a manufacturer) would be governed by materials cost, sourcing, as well as the desired functionality of the resulting device. 
   Another possible structure begins by using a foil-based film as a substrate. Foil based films are common in packaging, and inexpensive to purchase. The initiation of a structure using the foil film presents a back plane electrode for the building of an El device. This film would frequently be purchased with passivation and/or dielectric layers already intact. A device structure could then be built upon this film by inkjet printing techniques. Adding an emitter layer and a transparent electrode would produce a simple device structure from this foil based substrate. 
   Construction and Assembly of the Stack 
   In another embodiment of the invention, two different stacks are created or obtained separately and then joined by various methods. The two stacks are initiated through two separate substrates: a polyester and a paper substrate, or any other desired substrates and the appropriate layers deposited as mentioned in the previous embodiments. These two substrates with desired layer structure would then be joined by one of several joining techniques to form the functional device structure. Joining techniques could include pressure sensitive adhesives, curable adhesives, heat sealant, or other mechanical joining methods. Important to this process is the formation of an intimate contact between these joined layers. Presence of air gaps could lead to poor device performance. 
   Adhesive Interlayer 
   In a preferred demonstration of this embodiment, as illustrated in  FIG. 2 , one subassembly  15  is built up having a transparent polyester substrate  3 , a transparent indium tin oxide conductor  5 , and a phosphor based emitter layer  7 . A second subassembly  17  is built consisting of a paper substrate  19  having a patterned conductive electrode  13 . An adhesive layer  21  is applied to one of the subassemblies, and the layers are then joined together using this adhesive joining technique. Careful attention is paid to removing all air pockets from the joint in order to achieve optimal performance. The structure of such a device is seen in  FIG. 2  with the adhesive layer  21  on subassembly  15 . 
   The adhesive layer used to join on subassembly  15  or, alternatively, on subassembly  17  can be either thermoplastic or thermosetting adhesives in the form of liquids, pastes or films. These can be dispensed using many different techniques including but not limited to screen printing, stenciling, spraying, laminating, wire rod or roll coating, spin coating, inkjet, etc. The adhesives can be dispensed or placed on the surfaces of either subassembly  15  or subassembly  17  or both and then mated together for joining. Then, depending on the adhesives selected, the mated structures could be exposed to various temperatures and/or pressure or other environments for intimate contact and setting or curing. 
   Some of the important characteristics of the adhesives for joining such structures while promoting the intensity of illumination of the EL display include but are not limited to adhesion to the different surfaces, no air entrapment in the critical areas, chemistry and structure of the adhesive, thickness, compliancy, rheology, mechanical, thermal, environmental and electrical properties. 
   Table I lists the different types of adhesives and processes used for joining subassembly  15  and subassembly  17  and the intensity of illumination achieved. The adhesives and laminating films mentioned in Table I are laminated onto either subassembly  15  or subassembly  17 , the liner films (if present) are peeled away and then the two structures are mated together and laminated. 
   Lamination can be done using a hand roller, a cold or hot roll laminator (electric, oil or steam heated) or a variable temperature press. The temperature and speed of the lamination is varied to suit the adhesive being used. When using a roll laminator, nip clearance, stiffness/compliancy of the rolls, speed/residence time, temperature and pressure, are selected based on the characteristics of the adhesive and the components in the structure. Elimination of trapped air is critical. 
   For standardization, a roll laminator (Heat Seal H 300) or an electric iron was used for making the EL devices. After the adhesive was introduced and the two subassemblies  15  and  17  were mated, the composite stack was placed between polytetrafluoroethylene (PTFE) films and then laminated either with or without heat depending on the adhesives. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE I 
             
             
                 
             
             
                 
                 
                 
                 
               Dielectric 
                 
             
             
                 
                 
               Thickness 
                 
               constant 
               Intensity LUX 
             
             
               Adhesives 
               Polymer 
               (inch) 
               Process 
               @ 1 KHz 
               (lumens/sq. meter) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               3M 9449S 
               acrylic - PSA 
               0.0003 
               cold roll 
               — 
               115 
             
             
                 
                 
                 
               lamination 
             
             
               3M 9461P 
               acrylic - PSA 
               0.001 
               cold roll 
               — 
               53 
             
             
                 
                 
                 
               lamination 
             
             
               3M 845 
               polyolefin 
               0.0025 
               hot roll 
               2.3 
               1.2 
             
             
                 
               bonding film 
             
             
               3M 583 
               phenolic based 
               0.002 
               hot roll 
               — 
               144.7 
             
             
                 
               laminating film 
             
             
               3M 588 
               phenolic based 
               0.006 
               hot roll 
               — 
               3.4 
             
             
                 
               laminating film 
             
             
               Rogers 
               phenolic 
               0.001 
               hot roll 
               4.0 
               226 
             
             
               1000B100 
               based 
             
             
                 
               laminating film 
             
             
               Allied 
               Two part 
                 
               Mix 2 parts, 
               — 
               290 
             
             
               Epoxyset 
               epoxy liquid 
                 
               coat and cure 
             
             
               #145-20005 
             
             
               Tesa 8410 
               phenolic based 
               0.0025 
               hot roll 
               — 
               68.9 
             
             
               tape 
               laminating film 
             
             
                 
             
           
        
       
     
   
   The intensity of illumination reported in Table I was measured using an International Light meter Model IL 1400A against the transparent surface. The intensity is affected by the thickness of the adhesive and the dielectric properties of the layers involved, i.e., a higher dielectric constant of the adhesive and elimination of air gaps increases the intensity of illumination. A thin layer of a high dielectric polymeric adhesive, such as an epoxy or a phenolic layer provides higher intensity of illumination. 
   Printing the adhesive by different techniques as screen or inkjet can provide a thin layer and also enable selectivity such as printing directly onto the pattern or around the pattern. 
   Intensity of illumination can also be optimized by tuning the frequency for each type of polar adhesive 
   Structure with External Sealant Layer (A Preferred Embodiment) 
   Whichever method of assembly is selected, customer appreciation is realized when the display provides a higher intensity of illumination. In order to have a higher intensity level, the capacitance contribution from the adhesive should be minimal. Thus, having no adhesive and elimination of air between subassembly  15  and subassembly  17  is often critical. 
   This is achieved through mating subassembly  15  and subassembly  17  with no adhesive in-between and then sealing the subassembly  17  on the back of subassembly  15  with an adhesive or sealant. The adhesives can be either in the form of liquids, pastes, solids, films, and the like. The adhesive/sealant can either be a thermoplastic or a thermoset—curable through radiation energy of various forms/wavelengths and or moisture. 
     FIG. 3  represents an EL device in which subassembly  17  is sealed against subassembly  15 . Some of the adhesives that work well for this concept include but are not limited to adhesive films such as 3M 845, 3M 560EG, 3M AHS148 and others mentioned in Table I. The most favorable adhesive was 3M 845 with a thickness of 0.002 to 0.004 inches. The liner/carrier layers of these adhesives were left intact through and after the lamination. Other bonding films/sealants with appropriate rheology, mechanical, chemical, thermal and adhesion properties in conjunction with process conditions could work well for this application. 
   In order to build an EL device with this method, the width of subassembly  17  needs to be smaller than the corresponding dimensions of subassembly  15  such that sufficient boundary area for adhesion is provided. The two structures are mated and then the adhesive/sealant film is placed on the outside of subassembly  17 . This adhesive film extends over and beyond the dimensions of subassembly  17  but not beyond subassembly  15 . The stack is then placed in between the PTFE sheets and laminated in a roll laminator (Heat Seal Model H300) under heat. The thermoplastic sealant layer  23  conforms around subassembly  17  and adheres directly to subassembly  15 . In this method, the conductor pattern intimately adheres to the dielectric layer on subassembly  15 . 
   Using an International Light meter Model IL 1400A, and similar settings as above, the reading was 706 Lux. (lumens/sq. meter). The intensity of the EL display using this method is improved beyond what is achieved by the previous method. 
   Other films coated with pressure sensitive adhesives (AEROSET 1880 acrylic adhesive from Ashland Chemicals, Inc.,) or thermosetting adhesives can also be used with hot roll lamination. 
   Alternatively, the adhesion of the conductive pattern on subassembly  17  to the dielectric on subassembly  15  can be achieved through the co-solvents present in the conductive ink formulation. The co-solvent(s) which can plasticize the binder in the dielectric layer on subassembly  15  can improve the adhesion between the conductor and the dielectric through pressure and/or heat. 
   Another approach would be to apply an adhesion promoter such as a silane coupling agent between the two subassemblies prior to lamination. 
   The business advantages and efficiencies encompassed by this invention are explained further with reference to  FIG. 4 .  FIG. 4  is a business-flow diagram, which starts with commercial manufacturers  25   a ,  25   b  and  25   c , which make separate items for commercial sale. Manufacturer  25   a  produces an electroluminescent stack subassembly, illustrated illustratively as subassembly  15 . Manufacturer  25   b  produces a porous paper well suited for inkjet printing of a conductive ink, illustrated illustratively as paper  19 . Manufacturer  25   c  produces an inkjet bottle or printhead  27  containing a silver ink. As envisioned by this invention, all of these items are readily available to the public by normal sale at one or more retails stores  29 . 
   The retail customer is the end user. The retail customer prints the desired image on paper  19  conveniently at that person&#39;s home or business using the printhead  27  in an existing inkjet printer  29 . The two subassemblies  15  and paper  19  with conductive pattern are then joined as described in the foregoing. The resulting device is conveniently and readily customized as desired by the end used (the retail customer). 
   Dismountable EL Display (Another Preferred Embodiment) 
   Another method of assembly uses mechanical means. This method allows a user to dismount the assembly and reuse subassembly  15 , which provides cost effectiveness. The user can purchase subassembly  15  as a supply item. Then the pattern of choice is created by the customer through the printing of conductive ink on subassembly  17 . The assembly is done through mechanical means. 
   In order for the electroluminescent device to provide high illumination intensity, one of the important factors is to eliminate the air gap that can exist between the dielectric layer  9  and the conductors  13 . Thus, the planarity and smoothness of both conductors  13  and dielectric layer  9  are necessary to provide high illumination. Any non-planarity of the two layers will trap air and reduce the intensity when illuminated. Thus, when device  1  is held together mechanically it is important that enough pressure is acting on the two layers to provide intimate contact. However, this is very difficult to achieve. 
   In the embodiment of  FIG. 5 , the complete EL device  31  corresponds to the EL device  1  except that it will have a liquid interface layer and is to be held together mechanically, as described in the following. 
   The material of dielectric layer  9  of device  31  as normally supplied commercially has a surface roughness of Ra 1.144-1.319 micrometers. When pre-processed in a hot roll laminator, the surface roughness is reduced to about 50%, to Ra of 0.485-0.727 micrometers. Such smoothing in a hot roll laminator is performed. However, some minute air gaps still would exist when conductors  13  and conductive layer  9  are pressed directly together. 
   To overcome this, an ultra-thin layer of highly viscous fluid is applied between subassembly  17  and dielectric layer  9  as an interface layer. This fluid helps eliminate the small air gaps and increases the intensity of illumination. This fluid also aids as a release coating and prevents the conductive pattern from transferring to the dielectric layer  9 . In the embodiment of  FIG. 5 , during the assembly process, a thin layer of glycerol (not shown in  FIG. 5 ) is coated preferably on the dielectric layer  9  prior to mating with the electrode conductors  13  on sheet  19 . Glycols are a direct alternative to glycerol for this purpose, as well as glycerin, glycerol mixtures. 
   As shown in  FIG. 6 , in an embodiment this laminate is then placed between two parallel but rigid plates  33   a  and  33   b  separated by a spacer  34  and having a pedestal or base  36 . Device  31  is shown with a film  38  of glycerol (actually very thin) between conductor  13  and dielectric layer  9 . A resilient material  40  is held on plate  33   a  to provide pressure to hold conductors  13  firmly against dielectric layer  9 . Plates  33   a  and  33   b  alternatively can be held in a frame (not shown), depending on the design. The space between the plates  33   a  and  33   b  is held constant and uniform. If the plates  33   a  and  33   b  cover the top lamination  3  of device  31 , then the plate  33   a  or  33   b  covering lamination  3  must be transparent so that a viewer can observe the illuminated display when an electrical potential is applied to the electrodes. 
   Another type of holder or frame is shown in  FIG. 7 . The device  31  has a plate  50  on the surface of top lamination  3 . Bottom sheet  19  (shown in  FIG. 5 ), with conductors  13 , optionally within a sealant layer  23  as in the  FIG. 3  embodiment, is contacted by a resilient material  40 . Resilient material  40  is supported on a rigid bottom plate  52 . One or both of plates  50  and  52  can be made from thick glass or a clear, transparent, but rigid plastic. This assembly is supported on its sides by brackets  54   a  and  54   b  having ledges  54   aa  and  54   bb  blocking upper plate  50  and lower plate  52  and by intermediate spacers  56   a  and  56   b  supported on brackets  54   a  and  54   b  respectively blocking device  1  from lateral movement. 
   In conjunction with the stiffness of the plates  50  and  52 , resilient material  40  provides the pressure for sufficient contact between conductors  13 , the interface liquid  38 , and dielectric layer  9 . Other shapes and designs of holders with suitable spacers and clamps or fasteners can also be used to provide the pressure uniformly. The holders can be made from either metal, plastic or composites, or glass. 
   Resilient material  40  provides sufficient spring force and uniformity. The resilient material  40  can be a spring or set of springs. The spring inserts can be metal, polymeric or composites, foam or solid, for example. In the assembly used for these descriptions, preferably the resilient material  40  is made of closely spaced, laterally unconnected resilient elements. The actual embodiment is the hook needles portion of a commercially available loop and hook fastener sold under VELCRO brand. Only one sheet is used, so it is used for the resilient properties of the small, curved elements of that material. 
     FIG. 8  depicts an assembly in which no sealant film  23  is used. The remainder of the embodiment is like the  FIG. 7  embodiment. These devices can be dismounted and reassembled a number of times with no damage to the elements of device  31 . A new sheet  19  with a customized conductive pattern  13  can be readily prepared by inkjet printer. When the assembly is reassembled to the conditions as shown in  FIGS. 6-8 , the display has changed at low cost. 
   As illustrative of other releasable holding means, adhesive may be used or a vacuum may be generated across the front of sheet  19 . If adhesive is used, it must stay with the sheet  19  as it is pulled from the device  31  or it must remain in device  31  in good form to hold the next sheet  19 . If the adhesive stays with sheet  19 , then the new sheet  19  would have adhesive applied for the same purpose. 
   In one design sheet  19  is a microporous synthetic paper, which readily accepts conductive ink from an inkjet printer. The sheet can be printed in a standard manner using silver ink or other conductive ink. The pattern can be generated in a personal computer or the like using standard capabilities and the printer is then driven from the computer as is also standard and very common. 
   Table II. below lists the effect of the glycerol upon the intensity of the display. Measurements were taken with an International light meter. The electroluminescent display is connected to a single ended ringing choke converter that is powered from a 9 volt battery and the voltage applied to the display was approximately +/−90 volts, AC. Other fluids with high permittivity (i.e., high dielectric constant) with suitable high viscosities and release properties can be used. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE II 
             
             
                 
             
             
                 
                 
                 
               Luminescence 
             
             
               Substrate I 
               Dielectric fluid 
               Assembly 
               (LUX) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               No pre-treatment 
               None 
               clamps/glass plates 
               72 
             
             
               No pre-treatment 
               Glycerol 
               clamps/glass plates 
               150 
             
             
               Pretreatment 
               None 
               clamps/glass plates 
               190 
             
             
               Pretreatment 
               Fluid - PEG 
               clamps/glass plates 
               170 
             
             
                 
               400 
             
             
               Pretreatment 
               Glycerol 
               clamps/glass plates 
               207 
             
             
                 
             
           
        
       
     
   
   Accordingly, the device as a whole has an indefinite life, while the sheet  11  can be replaced with various patterns, which may be locally made, all at low cost.