Abstract:
An array of light emitting diodes coupled between a substrate and a transparent electrode include a pair of equipotential bus bars supplying electrical current simultaneously to at least two light emitting diodes, each located in its own area of transparent conductive material. In accordance with another aspect of the invention, a linear array of light emitting diodes has an electrode that includes a conductive island for each light emitting diode and a bus bar interconnecting and surrounding the conductive islands.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application relates to application Ser. No. 11/193,305, filed Jul. 29, 2005, entitled Acicular ITO for LED Array, and assigned to the assignee of this invention. The contents of the filed application are incorporated by reference into this disclosure. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to light sources or displays utilizing an array of light emitting diodes (LEDs) and, in particular, to improving the uniformity of light emission from two or more LEDs by improving the conductive path that supplies electrical current to the LEDs. 
       BACKGROUND OF THE INVENTION 
       [0003]    For back lighting, display, and other applications, one wants as uniform a light source as possible, and therein lies a problem. LEDs have numerous advantages over incandescent lamps but, like incandescent lamps, are point sources of light. Various forms of light guides or light channels have been used to diffuse the light. An alternative is to provide a plurality of sources in an array. Depending upon the spacing of the LEDs, an object appears uniformly lit at some minimum distance from the array. In some applications, more often than not decorative, the point sources are acceptable but cost is a primary consideration. 
         [0004]    Arrays of LEDs have long been known in the art. For example, U.S. Pat. No. 4,047,075 (Schoberl) discloses an array of LEDs made by simply stacking a plurality of packaged LEDs in a small volume. Packaged LEDs occupy considerably more volume than the semiconductor die or chip within the package. U.S. Pat. No. 4,335,501 (Wickenden et al.) discloses an array of LED dice on a single semiconductor substrate. U.S. Pat. No. 4,728,999 (Dannatt et al.) discloses a plurality of bus bars for powering subsets of diodes in an array. U.S. Pat. No. 6,595,671 (Lefebvre et al.) discloses an extra bus bar for providing a higher or lower voltage than another bus bar. 
         [0005]    An LED is a non-linear device. Like most diodes, an LED does not conduct until a forward bias exceeds a threshold, e.g. 0.6 volts, and then conduction must be limited by some sort of ballast, typically a series resistance. Current is typically 10-60 ma. and brightness is roughly proportional to current. The color of the emitted light may also change with changes in current. As with any device, LEDs produce heat. Unfortunately, LEDs typically have a negative temperature coefficient of resistance, which means that current increases with temperature. Thus, controlling current is important for several reasons. 
         [0006]    Two LEDs of the same type number do not necessarily have the same electrical characteristics. If a single resistor is used as ballast for two parallel LEDs, then the failure of one LED can result in the second LED being overdriven (too much current) and, consequently, failing soon thereafter. Larger arrays, with LEDs in series and in parallel have the same problem, only compounded by a greater number of LEDs. Although it is separately well known in the art that current through an LED must be carefully controlled, many of the patents on arrays of LEDs have a paucity of disclosure on how to drive the array, except to limit or to “condition” current in some undisclosed manner. It may be that uniform brightness is not a concern or is too difficult to achieve in the disclosed configurations. 
         [0007]    A material referred to as acicular ITO (indium tin oxide) is known in the art as a transparent conductor; see U.S. Pat. No. 5,580,496 (Yukinobu et al.) and the divisional patents based thereon (U.S. Pat. Nos. 5,820,843, 5,833,941, 5,849,221). Acicular ITO has a fibrous structure composed of 2-5 μm thick by 15-25 μm long ITO needles. The needles are suspended in an organic resin, e.g. polyester. Acicular ITO is different in kind from other forms of the material. A cured, screen printed layer of acicular ITO is approximately five times more conductive than conventional layers containing ITO powder but is about two thirds less conductive than sputtered ITO, which is more difficult to pattern than screen printable materials. 
         [0008]    Even assuming that there is uniform conductivity in the transparent electrode, the placement of die on an electrode can greatly affect luminance. The current spreads through the electrode, flowing through an area to the die. Placement can affect the effective area. Because transparent conductors generally have lower conductivity than opaque conductors, a bus bar is often used to provide a low resistance path along one or more sides of an array. Unfortunately, a bus bar exaggerates the problem of die placement because a slight misplacement can, for example, increase the distance to the nearest bus bar by ten percent or more. The increase in distance causes a disproportionate decrease in current and brightness. U.S. Pat. No. 7,052,924 (Daniels et al.) discloses a conductive grid overlying an ITO coated substrate and an LED located in each grid section. While suited for some applications, an ITO coated substrate is expensive and does not allow for adjusting the current to each LED. 
         [0009]    In view of the foregoing, it is therefore an object of the invention to reduce the effect of die placement on uniformity of luminosity. 
         [0010]    Another object of the invention is to provide an improved transparent, conductive electrode for diode arrays. 
         [0011]    A further object of the invention is to provide an array of LEDs that has a more uniform ballast resistance associated with each LED. 
         [0012]    Another object of the invention is to provide an array of LEDs that are substantially uniformly bright when lit, either simultaneously or in subsets of the entire array. 
         [0013]    A further object of the invention is to provide an array of LEDs in which the failure of one LED has substantially no effect the brightness of other LEDs in the array. 
         [0014]    Another object of the invention is to provide an array of LEDs that can be manufactured at lower cost than in the prior art. 
       SUMMARY OF THE INVENTION 
       [0015]    The foregoing objects are achieved in this invention wherein an array of light emitting diodes coupled between a substrate and a transparent electrode include a pair of equipotential bus bars supplying electrical current simultaneously to at least two light emitting diodes, each located in its own area of transparent conductive material. In accordance with another aspect of the invention, a linear array of light emitting diodes has an electrode that includes a conductive island for each light emitting diode and a bus bar interconnecting and surrounding the conductive islands. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  is a cross-section of a pair of LEDs in an array constructed in accordance with the prior art; 
           [0018]      FIG. 2  is a plan view of LEDs in an array constructed in accordance with the prior art; 
           [0019]      FIG. 3  is a plan view of LEDs in an array constructed in accordance with a preferred embodiment of the invention; 
           [0020]      FIG. 4  is a plan view of LEDs in an array constructed in accordance with an alternative embodiment of the invention; 
           [0021]      FIG. 5  is a plan view of a linear LED array constructed in accordance with the prior art; 
           [0022]      FIG. 6  is a plan view of a linear LED array constructed in accordance with another aspect of the invention; 
           [0023]      FIG. 7  is a cross-section of a linear LED array constructed in accordance with the invention; 
           [0024]      FIG. 8  is a cross-section of an array constructed in accordance with the invention; and 
           [0025]      FIG. 9  is a cross-section of an array constructed in accordance with the invention and illustrates an alternative sequence for depositing layers. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 1  is a cross-section of a pair of LEDs in an array constructed in accordance with the prior art. Array  10  includes transparent substrate  11  of polyester or polycarbonate material. A transparent, front electrode (not shown in  FIG. 1 ) of indium tin oxide having a thickness of 1000 Å or overlies substrate  11  to provide electrical contact to the die. Bus bar  12  overlies the transparent conductor, increasing the conductivity of the area. Bus bar  12  is typically screen printed from a silver bearing ink, although other conductive particles can be used instead, e.g. carbon. Die  14  has an area of a first conductivity, generally p-type, in contact with the transparent conductor near bus bar  12 . The region between LEDs is filled with a suitable dielectric, such as epoxy. Bus bar  16  and insulating layer  19  overlie the die as shown. Bus bar  16  provides electrical contact to the n-region of die  14 . The construction of the array can be symmetric about the dice, i.e. layer  19  can be the same material, if not the same thickness, as layer  11  and bus bar  16  can be the same material as bus bar  12 , e.g., silver particles that are screen printed in suitable matrix, such as fluoropolymer, polyester, vinyl, epoxy. 
         [0027]      FIG. 2  is a plan view of LEDs in an array constructed in accordance with the prior art, as though one were looking up into the array in  FIG. 1 . In a plan view, the substrate and the rear electrode are not seen. LED  14  is in contact with transparent conductive layer  21 , which, in turn, is in contact with bus bar  16 . Bus bar  16  and bus bar  23  are connected together by feed  23  and coupled to lead  24 , which provides external connection to the array. 
         [0028]    As illustrated, somewhat exaggeratedly, in  FIG. 2 , LED  31  is off-center under transparent conductor  32  and LED  33  is off-center under transparent conductor  34 . Because LED  31  is further from bus bar  16  than the other LEDs, it will be somewhat dimmer, even for allowing for differences in light emission among the LEDs in the array. LED  33  is closer to bus bar  22  than the other LEDs and will be brighter because the current path has a lower resistance. 
         [0029]    Bus bars improve brightness, by reducing resistance, but make die placement more critical. A slight misplacement can be a significant fraction of the length of the current path to a bus. One could make the contact areas larger and move the bus bars further away from the LEDs but this is counterproductive. Many transparent conductors other than sputtered ITO are available but are expensive. 
         [0030]    In accordance with one aspect of the invention, illustrated in  FIG. 3 , a pair of bus bars is coupled to the same electrode of at least two LEDs. Bus bars  43  and  44  have substantially the same voltage thereon, supplied by feed  45 . The bus bars can extend in the same direction, as illustrated in  FIG. 3 , or extend in different directions, as illustrated in  FIG. 4 . Conductive areas, such as areas  51 ,  52 , and  53  are preferably screen printed from a conductive ink, such as Baytron® conductive polymer, Orgacon™ conductive polymer (PEDOT—polyethylene dioxythiophene), or particles of indium oxide, ITO, acicular ITO, gallium doped zinc oxide, or aluminum doped zinc oxide in a suitable polymer. Bus bars  43  and  44  can be printed over the edges of the areas or the areas can be printed between the bus bars, using the bus bars for definition. 
         [0031]    If an LED is located closer to one bus bar than the other, the changes in current paths are compensating. For example, LED  55  is located closer to bus bar  43  than to bus bar  44 . The current from LED  55  to bus bar  43  increases while the current from LED  55  to bus bar  44  decreases. The sum of the currents is substantially constant. Thus, a misplaced LED is as bright as its neighbors, other factors being equal. 
         [0032]    When other factors are not equal, for example, an LED needing a ballasting resistor having a resistance slightly different from other LEDs, the electrode can be trimmed as disclosed in the above-identified co-pending application. This could occur, for example, if LEDs having different color were used in a single array. Different colors can be produced instead by including cascading material, such as phosphor or dye, in the transparent conductive layer. Different colors can also be produced by including cascading material in a layer printed over the transparent conductive layer. 
         [0033]    The conductive areas can be bounded by closed curves or polygons. Preferably, the line of contact between a bus bar and the conductive area exceeds the diameter of the LED. The line of contact can be curved, jagged, or straight. In  FIG. 3  the conductive areas are substantially square. In  FIG. 4 , the conductive areas have curved boundaries. Also in  FIG. 4 , two feeds are used, with separate leads,  61  and  62 . The bus bars are interdigitated; that is, they alternate and extend in opposite directions. Leads  61  and  62  are connected to the same power source, although one lead can be left floating for dimming. 
         [0034]    The rear electrode, not shown, provides the second electrical connection to the LEDs. The rear electrode can be any conductive layer, transparent or opaque, and can be reflective. A metal layer, such as copper or aluminum, can be used or a layer can be screen printed from conductive ink, such as an ink containing particles of carbon, silver, tin oxide, or indium tin oxide. The rear electrode can be a conductive area on a printed circuit board or on a “flex circuit.” 
         [0035]    In  FIG. 3 , LEDs between a pair of equipotential bus bars can share a common transparent electrode, such as electrode  57 . This can simplify the screen printing without unduly increasing the print area. Because of the dual bus bars, one can locate LEDs as desired under a transparent electrode. For example, the LEDs in contact with transparent conductor  59  can be placed asymmetrically and can emit different colors, e.g. red, green, and blue. 
         [0036]      FIG. 5  is a plan view of a linear array constructed in accordance with the prior art.  FIG. 6  is a plan view of a linear array constructed in accordance with another aspect of the invention.  FIG. 7  is a cross-section of an array constructed as illustrated in  FIG. 6 . 
         [0037]    In  FIG. 5 , bus bar  71  overlies transparent conductive layer  72 , which overlies LEDs  75  and  76 . The area covered by transparent conductive layer  72  is significant. By comparison, in  FIG. 6  LEDs  85  and  86  underlie transparent conductive islands  87  and  88 . Bus bar  81  interconnects and surrounds the islands to provide reduce resistivity. Constructing the linear array of  FIG. 6  in accordance with the invention is significantly less expensive than constructing the array illustrated in  FIG. 5  because far less material is used for the transparent conductive areas. 
         [0038]      FIG. 8  and  FIG. 9  represent cross-sections that can be taken along line A-A in  FIG. 3 ,  FIG. 4 , or  FIG. 6  and illustrate processes for making an LED array in accordance with the invention. In  FIG. 8 , bus bars  102  and  103  are deposited on substrate  91 , e.g. by screen printing. A region of cascading material, represented by area  106  is also deposited on substrate  91 . Either the bus bars or the cascading material can be deposited first. Transparent, conductive layer  92  is then deposited and at least touches the bus bars. Preferably, layer  92  overlaps the bus bars, as illustrated. LED  101  is positioned on layer  92  and surrounded with insulator  93 . Rear electrode  94  and insulating or protective layer  95  are then applied. 
         [0039]    The array is thus built. “top down” rather than “bottom up,” which can lead to some confusion because the device is usually described as though the array were constructed “bottom up” or back to front. For example, cascading material layer  106  is “on” transparent conductive layer  92 . Similarly, the transparent conductor is “on” or “overlies” LED  101 . Those of ordinary skill in the art understand that the device is built the other way around and there is no confusion. 
         [0040]      FIG. 9  illustrates an alternative method of assembly in which bus bars  102  and  103  are applied after transparent conductive layer  92 . In this process, region  106  must be deposited first because LED  101  must contact layer  92 . It is not necessary that bus bars  102  and  103  be deposited prior to placing LED  101  but it is preferred because the bus bars provide a reference for locating the LED. Insulating layer  93 , rear electrode  94  and insulating layer  95  are then applied as in the process illustrated in  FIG. 8 . 
         [0041]    The invention thus provides a reliable, consistent connection to LEDs using a reduced area of expensive, transparent, conductive material for one electrode. The invention also reduces the effect of die placement on uniformity of luminosity, enabling one to obtain an array of LEDs that are substantially uniformly bright when lit, either simultaneously or in subsets. The failure of one LED has substantially no effect the brightness of other LEDs in the array because of individual ballasting and dual connections. For LEDs in contact with individual transparent electrodes, LEDs of different current ratings can be accommodated by adjusting the geometry of the transparent conductor, by using different material for some of the transparent conductors, or by combinations thereof. 
         [0042]    Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, although illustrated with 2×3 arrays, dual bus bars can be used for linear arrays (1×n) as well.