Abstract:
An array having a plurality of column electrodes and a plurality of rows of individually addressable OLED pixels, each row including a commonly shared electrode including wherein at least one OLED pixel in each row has a current limiting component and an organic electroluminescent diode and such at least one OLED pixel is connected between said commonly shared electrode and one of the plurality of column electrodes for conducting current therebetween, and wherein the at least one organic electroluminescent diode is connected in series with the current limiting component.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to fault-tolerant OLED apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     Organic light-emitting diode (OLED) technology holds significant promise as a display technology that is well suited to a broad range of applications. Self-emitting, OLED displays are advantaged over other display technologies, providing high luminance, good quality color, and relatively wide viewing angle. OLED display components are thin and lightweight, making them particularly adaptable for use with handheld components, such as cameras, cell phones, personal digital assistants (PDAs) and laptop computing devices.  
         [0003]     The basic bottom-emitting OLED pixel  10  is constructed as shown in  FIG. 1 . An organic layer  12 , typically fabricated as a stack of multiple thin organic layers, is sandwiched between a cathode  14  and a transparent anode  16 , built onto a glass substrate  18 . Organic layer  12  includes an electroluminescent layer (EL) that provides illumination when appropriate voltage is applied between anode  16  and cathode  14 . Pixel  10  is formed in the overlap area between cathode  14  and anode  16 . An OLED display is formed from an ordered spatial arrangement of individually addressable OLED pixels  10  arranged as an array, in successive rows and columns.  
         [0004]     There are two basic types of OLED arrays, passive matrix and active matrix. Active matrix OLED displays integrate current control circuitry within the display itself, with separate control circuitry dedicated for each individual pixel element on the substrate for producing high-resolution color graphics at a high refresh rate. Passive matrix OLED displays, on the other hand, have current control circuitry that is only external to the display itself. Thus, passive matrix OLED displays are of simpler construction than are active matrix displays, and permit simpler, lower cost fabrication techniques.  
         [0005]     The basic arrangement of a passive matrix OLED array  20  is shown in the simplified schematic of  FIG. 2   a . In array  20 , each individually addressable OLED pixel  10  has an electroluminescent diode  11  connected between an anode line  26  (column) and a cathode line  24  (row). Each anode line  26  has a current source  22  that is switched ON to anode line  26  in order to illuminate pixel  10  in each column, according to image data. Cathode line  24  is commonly shared by each electroluminescent diode  11  in a row. A switch  30  for each cathode line  24  switches to ground to enable illumination of pixels  10  in each successive row, one row at a time, using a scanned row sequence. An electroluminescent diode  11  illuminates when its current source  22  is switched ON and its corresponding row switch  30  switches to ground. Otherwise, cathode lines  24  are typically switched to an intermediate voltage Vi. Electroluminescent diodes  11 , whose cathode potential is at V i , do not illuminate. Having its cathode line  24  at intermediate voltage Vi turns pixels  10  off for any row that is not being scanned, but maintains a potential on the row. This reduces the amount of power necessary to charge the parasitic capacitance of each row as it is addressed. Using this straightforward arrangement, passive matrix OLED array  20  can be constructed to have several thousand pixels  10 , organized in a matrix of rows and columns. Control logic in a display apparatus (not shown) provides control of current source  22  for each column and of switch  30  for each row, making each OLED pixel  10  individually addressable, using control and timing techniques well known in the display component arts.  
         [0006]     It must be emphasized that the above description and schematic of  FIG. 2   a  provide a simplified explanation of the control mechanisms and composition of passive matrix OLED array  20 . More detailed information on prior art passive matrix OLED arrays and array driver solutions can be found, for example, in U.S. Pat. No. 5,844,368 to Okuda et al. and in U.S. Pat. No. 6,594,606 Everitt.  
         [0007]     While smaller OLED displays of several inches in diagonal have been successfully built, fabrication defects still present obstacles to the development of large area OLED displays of the passive matrix type. Defects can be due to dust or contamination during fabrication, asperities due to electrode surfaces, pinholes, and nonuniformities in organic layer thickness, for example.  
         [0008]     Of particular concern for display operation is the defect caused by a shorted electroluminescent diode  11 . Referring back to  FIG. 2   a , it can be observed that a shorted electroluminescent diode  11  for a pixel  10  effectively connects current source  22  directly to ground when the corresponding row is scanned. When other rows are scanned, a shorted electroluminescent diode  11  effectively sets intermediate voltage Vi onto anode line  26 . Because of this, the complete column of pixels  10  is blacked out during display operation. Whereas some number of dead pixels  10  can be tolerated in a viewed image, defects affecting an entire line, in general, are not acceptable. Thus, in practice, there is zero tolerance for shorted pixel defects over the entire area of OLED array  20 .  
         [0009]      FIGS. 2   b ,  2   c ,  2   d , and  2   e  show how various configurations of OLED array  20  behave in response to a shorted diode condition.  FIGS. 2   b ,  2   c , and  2   d  show OLED array  20  where switches  28  are either open or closed (to ground), without connection to intermediate voltage Vi. Referring first to  FIG. 2   b , there is shown a small section of OLED array  20  having electroluminescent diodes  11   a ,  11   b ,  11   c , and  11   d  at individually addressable pixels  10   a ,  10   b ,  10   c , and  10   d , respectively, in an arrangement of rows  44   a  and  44   b  and columns  42   a  and  42   b . In the example of  FIG. 2   b , electroluminescent diode  11   d  is shorted, as indicated by a short  46 . During row scanning, row  44   a  is enabled, while adjacent row  44   b  is disabled, as shown at respective switches  28 . Current source  22  for a column  42   a  is ON to illuminate pixel  10   a  (by providing current through electroluminescent diode  11   a ) at the intersection of column  42   a  and row  44   a . However, short  46  is at the position of pixel  10   d  for the next row  44   b  at a column  42   b . Short  46  thus provides an unwanted current path to column  42   b , through electroluminescent diode  11   c . Depending on the amount of current flowing through short  46 , electroluminescent diode  11   c  can illuminate, thereby being permanently ON for scanning all rows  44  in array  20 . Even dim constant illumination of electroluminescent diode  11   c  would be undesirable. As  FIG. 2   c  shows, when both current sources  22  are ON, pixel  10   c  would have the desired state. As  FIG. 2   d  shows, when row  44   b  is scanned, and electroluminescent diode  11   c  is ON, short  46  would be effectively bypassed.  
         [0010]     Referring to  FIG. 2   e , there is shown an OLED array  20  arrangement in which switch  28  is at intermediate voltage Vi until a row is scanned. With short  46  in the position shown, when switch  28  for row  44   b  connects to idle voltage Vi and when row  44   a  is scanned, or any other row except row  44   b  is scanned, column  42   b  is held at Vi. Because of this, column  42   b  is effectively disabled. It is instructive to observe that current source  22  is designed to provide current to only a single electroluminescent diode  11  at a time; meanwhile, intermediate voltage Vi is provided to a full row  44   a ,  44   b . It would be unpractical to size current source  22  in each column  42   a ,  42   b  to compensate for the condition caused by short  46 .  
         [0011]     The likelihood of a fabrication defect increases dramatically as the display area increases. Assuming that the overall defect density for array  20  exhibits a Poisson distribution characteristic, then the probability that array  20  has zero defects is the yield Y and can be expressed in the equation (1): 
 
 Y=e   −DA   (1) 
 
 where D is the shorting defect density per area and, for a shorted diode  11 , A is the full area of array  20 . 
 
         [0012]     The exponential scaling impact of defect density D and area A is particularly significant. For example, for a reasonable defect density D of 0.01 per cm and an area A of 0.5 square meter, the yield Y is as follows: 
 
 Y= 2×10 −22 . 
 
 In other words, chances for a good display yield with a very large passive matrix OLED display are effectively nil. Only a dramatic reduction of factors D and A in the exponent of equation (1) can permit a reasonable yield for OLED arrays. 
 
         [0013]     Active matrix OLED design provides one solution to this defect-related performance problem. In an active matrix OLED array, each individual OLED pixel  10  can be independently addressed, using an arrangement of thin-film transistors (TFTs) and storage capacitors. Active matrix display circuitry is disclosed in U.S. Pat. No. 6,392,617 to Gleason and U.S. Pat. No. 6,433,485 to Tai et al., for example. With an active matrix configuration, a shorted diode defect at any one OLED pixel  10  does not impact other OLED pixels  10 . However, as is noted above, active matrix OLED array design is considerably more complex, requiring a number of additional support components for each OLED pixel  10 .  
         [0014]     U.S. Pat. No. 6,605,903 to Swallow discloses, as an alternative passive matrix approach, an array having sections that can be selectively activated or deactivated to compensate for OLED pixel  10  defects. In the OLED array of U.S. Pat. No. 6,605,903, each column has two separate sections, either of which can be activated or deactivated in the event of a shorted diode. While this approach can mitigate defect problems, the array requires a considerable number of additional components, many of which would not be used. Moreover, defects occurring after manufacture, and testing would still have a negative effect on display performance.  
         [0015]     Although clearly not directed to an OLED array used for addressable image display, one solution proposed for large-scale OLED cells or modules used in room lighting and signage applications, outlined in U.S. Patent Application Publication 2002/0190661 A1 to Duggal et al., is of some interest. U.S. Patent Application Publication 2002/0190661 A1 discloses a serial connection of multiple, large area OLED modules directly to an AC power source. Each OLED cell or module is a single diode, having an emissive surface that is at least a few square centimeters in area. OLED cells are connected in series fashion, with the anode of one OLED cell connected to the cathode of the previous one, for example. Advantageously for the lighting and signage lettering uses described in U.S. Patent Application Publication 2002/0190661 A1, this solution permits OLED devices to be used with alternating current at line voltage (nominally at 120 VAC, 60 Hz), so that a separate DC power supply is not required. Series-connected OLED cells are arranged to illuminate during each half cycle of AC current. In a paper entitled “Fault-tolerant, scalable organic light-emitting device architecture” in  Applied Physics Letters , Vol. 82, Number 16, 21 April 2003, this type of series connection for large area OLED cells for illumination applications is also disclosed and further discussed with reference to the impact of faults on other OLED devices in the series. Not surprisingly, a shorted OLED cell diode in the series causes a corresponding increase in brightness among other OLED cells in the same series. However, the straightforward series connection described does have advantages over more complex fault response mechanisms.  
         [0016]     Thus, while there have been a few solutions proposed for limiting or minimizing the impact of a faulted OLED on other nearby OLEDs, none of these solutions is particularly well suited for use with a passive matrix OLED array used in imaging display applications, where each OLED electroluminescent diode  11  serves as one individually addressable pixel  10  for forming an image. The active matrix designs disclosed in U.S. Pat. Nos. 6,392,617 and 6,433,485 add considerable complexity, as does the dual-column solution disclosed in U.S. Pat. No. 6,605,903. The solution proposed in U.S. Patent Application Publication 2002/0190661 A1 applies for discrete, modular OLED lighting devices that are used as banks of large-scale illuminators, rather than for OLED arrays where each individually addressable OLED electroluminescent diode  11  serves as one pixel  10  for forming an image. In considering any practical solution, it can be appreciated that there are benefits in maintaining the low cost and relative simplicity of the passive matrix OLED array design, as is shown in the schematic diagram of  FIG. 2 .  
         [0017]     Thus, it can be seen that there is a need for an OLED array apparatus providing multiple individually addressable pixels and a method that provides a degree of tolerance to short conditions without adding substantial fabrication complexity or requiring complex support circuitry.  
       SUMMARY OF THE INVENTION  
       [0018]     It is an object of the present invention to provide an OLED array having improved fault tolerance. The present invention provides an array having a plurality of column electrodes and a plurality of rows of individually addressable OLED pixels, each row including a commonly shared electrode, comprising: 
        a) wherein at least one OLED pixel in each row has a current limiting component and an organic electroluminescent diode and such at least one OLED pixel is connected between said commonly shared electrode and one of the plurality of column electrodes for conducting current therebetween; and     b) wherein the at least one organic electroluminescent diode is connected in series with the current limiting component.        
 
         [0021]     It is a feature of the present invention that it provides a pixel having an electroluminescent diode in series with a current limiting component.  
         [0022]     It is an advantage of the present invention that it employs passive components in series with the electroluminescent diode, rather than requiring the use of active components, such as switching transistors, for each OLED pixel or row of pixels. The solution of the present invention provides a low-cost addition to OLED array fabrication that reduces the likelihood of a dark OLED line defect in a display.  
         [0023]     It is a further advantage of the present invention that it permits increased yields in OLED array manufacture by limiting one cause of line dropout due to shorting of a single pixel.  
         [0024]     It is a further advantage of the present invention that it provides a manufacturing method for isolating shorted pixels to minimize their impact on OLED array performance.  
         [0025]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a cutaway side view showing the basic components of a prior art OLED pixel;  
         [0027]      FIG. 2   a  is a schematic diagram showing the basic arrangement of a prior art passive matrix OLED array;  
         [0028]      FIGS. 2   b ,  2   c ,  2   d , and  2   e  are schematic diagrams showing a segment of a prior art passive matrix OLED array having a short, under various current source and row scanning conditions;  
         [0029]      FIG. 3   a  is a schematic diagram showing an OLED pixel arrangement in a first embodiment of the present invention;  
         [0030]      FIG. 3   b  is a plan view representation of OLED components for the first embodiment shown in  FIG. 3   a;    
         [0031]      FIG. 3   c  is a cutaway side view showing OLED pixel components for the first embodiment;  
         [0032]      FIG. 4   a  is a schematic diagram showing an OLED pixel arrangement in a second embodiment of the present invention;  
         [0033]      FIG. 4   b  is a plan view representation of OLED components for the second embodiment shown in  FIG. 4   a;    
         [0034]      FIG. 5   a  is a schematic diagram showing an OLED pixel arrangement in a third embodiment of the present invention;  
         [0035]      FIG. 5   b  is a plan view representation of OLED components for the third embodiment shown in  FIG. 5   a;    
         [0036]      FIG. 6   a  is a schematic diagram showing an OLED pixel arrangement in a fourth embodiment of the present invention;  
         [0037]      FIG. 6   b  is a plan view representation of OLED components for the fourth embodiment shown in  FIG. 6   a;    
         [0038]      FIG. 7  is a schematic diagram showing an arrangement of OLED cells in an embodiment of an area lighting apparatus, using the techniques of the present invention; and  
         [0039]      FIG. 8  is a schematic diagram showing an alternate arrangement of OLED cells in an embodiment of an area lighting apparatus using a DC source, using the techniques of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described can take various forms well known to those skilled in the art.  
         [0041]     The present invention addresses the problem of protection from a shorted diode by introducing a current limiting element in series with the electroluminescent diode  11  within OLED pixel  10 . Various different types of current limiting elements can be used, singly or in combination, as is described for each of the embodiments outlined below.  
         [0042]     Referring to  FIG. 3   a , there is shown a first embodiment of the present invention, wherein a plurality of electroluminescent diodes  11  are connected in series within each single OLED pixel  10 . In this top-emitting configuration, the cathode is transparent. With this arrangement, shorting of one electroluminescent diode  11  simply adds to the current load of the other electroluminescent diode(s)  11  connected in series within OLED pixel  10 . Electroluminescent diodes  11  can be themselves the current limiting elements, in which case, each diode  11  within OLED pixel  10  provides some portion of the overall emitted light. Alternately, one or more of the series-connected diodes could be a diode that does not emit light, but simply acts as a current limiting element.  
         [0043]     Referring to  FIG. 3   b , there is shown a plan view of component positioning within OLED pixel  10  for this first embodiment. Electroluminescent diodes  11  are formed from suitably doped OLED materials and are arranged in series, such that the cathode of one electroluminescent diode  11  connects to the anode of the next electroluminescent diode  11 . As is shown in the side view of  FIG. 3   c , a jumper  36  makes this connection between cathode  14  and anode  16 . At each electroluminescent diode  11  position, an insulator  34  is provided to isolate anode line  26  from cathode line  24 .  
         [0044]     This first display embodiment admits a number of alternative arrangements. For example, the number of series electroluminescent diodes  11  can be varied based on factors such as driver characteristics. The greater the number of electroluminescent diodes  11  connected in series within OLED pixel  10 , the greater is the relative insensitivity to a short condition. However, at the same time, the voltage required to drive pixel  10  increases with an increased number of series-connected electroluminescent diodes  11 .  
         [0045]     Referring to  FIG. 4   a , there is shown, in schematic form, a second embodiment in which OLED pixel  10  employs a series resistor  38  for current limiting. The resistance value of series resistor  38  would be selected with a suitable value for limiting current if electroluminescent diode  11  is shorted. Referring to  FIG. 4   b , there is shown a plan view of component positioning within OLED pixel  10  for this second embodiment. Series resistor  38  connects from the cathode of electroluminescent diode  11  in the embodiment shown; however, series connection of resistor  38  at the anode would be equivalent for providing current limiting protection.  
         [0046]     This second display embodiment also admits a number of alternative arrangements, including combination with other embodiments. For example, series resistor  38  could be used in OLED pixel  10  that contains two or more electroluminescent diodes  11 .  
         [0047]     Referring to  FIG. 5   a , there is shown, in schematic form, a third embodiment in which OLED pixel  10  employs a fuse  40  for current limiting. An overcurrent condition caused by shorting of electroluminescent diode  11  would blow fuse  40 , effectively causing an open circuit for this OLED pixel  10 . A single dark pixel would result; however, other pixels from other OLED pixel  10  would not be affected by this failure. Referring to  FIG. 5   b , there is shown a plan view of possible component positioning within OLED pixel  10  for this third display embodiment, with fuse  40  connected between the cathode of electroluminescent diode  11  and cathode line  24 . It can be appreciated that fuse  40  can alternately be connected between the anode of electroluminescent diode  11  and anode line  26  or between the cathode of electroluminescent diode  11  and cathode line  24 . Alternative variations on the embodiment of  FIGS. 6   a  and  6   b  include series combination of fuse  40  with multiple electroluminescent diodes  11  or with other current limiting elements in series.  
         [0048]     Fuse  40  can be fabricated using any of a number of materials and techniques. Materials useful for forming fuses are generally alloys that have a relatively low melting point. In particular, binary, ternary, quaternary, and quinternary alloys selected from the elements Bi, In, Pb, Sn, and Cd are preferred. By way of example, but not of limitation, fuse  40  materials could include any of the following: 
        a) quinternary eutectic alloy Bi(44.7%) Pb(22.6%) In(19.1%) Sn(8.3%)Cd(5.3%) which has a melting point of 47° C.;     b) quaternary Wood&#39;s Metal (Bi(50.0%) Pb(25.0%) Sn(12.5%) Cd(12.5%)) having a melting point of 70° C.;     c) ternary eutectic Bi(52.5%) Pb(32.0%) Sn(15.5%) with a melting point of 95° C.; or     d) binary eutectic solder (Sn(63%) Pb(37%)) that melts at 183° C.        
 
         [0053]     It can be appreciated that many other eutectic and non-eutectic alloys of these and other elements are useful for forming fuse  40  according to the present invention.  
         [0054]     Fuse  40  has an added advantage during fabrication of OLED array  20 . Where electroluminescent diode  11  at any pixel  10  is shorted, it would be advantageous to selectively open the circuit connection, effectively isolating and disabling pixel  10  at that location. Applying a high reverse-bias voltage to array  20  would direct a high level of current only through shorted pixels  10 . By a applying a reverse-bias voltage of sufficient value, only those fuses  40  at pixels  10  having shorted electroluminescent diodes  11  would be blown. This would enable high yields. With respect to yield equation (1) given earlier, area A is greatly reduced, effectively to the dimensions of a single pixel  10  area.  
         [0055]     Referring to  FIG. 6   a , there is shown a fourth embodiment of the present invention, in which, for any single OLED pixel  10 , multiple electroluminescent diodes  11  are disposed electrically in parallel between anode line  26  and cathode line  24 , each electroluminescent diode  11  having a separate series fuse  40 . With this arrangement, shorting of a single electroluminescent diode  11  blows its corresponding fuse  40 , opening this part of the circuit, but permitting continued flow of current through other electroluminescent diodes  11  connected in parallel. Unlike the arrangement of the third display embodiment, however, separate material need not be used to fabricate fuse  40 . Instead, the current-carrying capacity of OLED materials themselves, or that of nearby cathode or anode support structures, effectively provides a fusing element with this embodiment. An overcurrent condition melts or bums away conductive material that forms electroluminescent diode  11 , opening the circuit at that point.  
         [0056]     Referring to  FIG. 6   b , there is shown a plan view of component positioning within OLED pixel  10  for this fourth display embodiment. Five OLED pixels  10  are shown. Each parallel electroluminescent diode  11  within OLED pixel  10  is formed by the light-emitting organic material that lies between a thin strip  48  of cathode  14  material and the underlying anode  16  material. This patterning of cathode  14  into parallel thin strips  48 , spaced apart in the overlap region between cathode  14  and anode  16  where electroluminescent diodes  11  form, enables multiple electroluminescent diodes  11  to be fabricated as a set of parallel sub-pixel elements for pixel  10 . Electrode material that is not in the diode-forming area is conductive and has a limited current-carrying capacity. In particular, OLED material in a fuse area  41 , not directly in the overlap area between cathode and anode lines  24  and  26 , as indicated in  FIG. 6   b , is likely to be damaged and produce an open electrical condition when subjected to excessive current. However, because cathode  14  is segmented into thin strips  48 , it is also possible that an overcurrent condition would cause melting anywhere along the length of thin strip  48  wherein electroluminescent diode  11  is shorted. In this way, all of thin strip  48  would serve as a fusing mechanism. Note that anode  16  could be similarly formed using sections of strips to effect fuse behavior. For thermal considerations, thin strips  48  work best when formed on the electrode that is not in direct contact with the substrate. Otherwise, the substrate could provide unwanted dissipation of the heat that could otherwise cause the fuse to open.  
         [0057]     Addition of a separate fusing component for providing fuse  40  to implement the embodiment of  FIG. 6   a  is one alternative. However, the overall arrangement of  FIG. 6   b  offers advantages for manufacturability, since it does not require that a separate type of fusing material be introduced into OLED array  20  fabrication processes.  
         [0058]     A hybrid arrangement is also possible, using some combination of localized overheating of OLED material, overheating of anode  16  or cathode  14  segments, or use of a fuse material, as was described with respect to the third display embodiment. Alternative arrangements of this fourth embodiment also include replacing one or more fuses  40  with a corresponding series resistor  38 . As another alternative, one or more parallel circuits could use an arrangement with multiple electroluminescent diodes  11  connected in series within each circuit.  
         [0059]     Fault tolerance solutions provided for individually addressable OLED pixels  10  in an array  20  can also have application to large-scale OLED cells used for area lighting, where the OLED cells are not individually addressed, but are energized at the same time. In particular, the solutions offered by the third and fourth display embodiments in the schematic diagrams of  FIGS. 5   a  and  6   a  could have particular usefulness with large-area OLEDs used for room or other area lighting devices. As was noted in the background section hereinabove, series connection of large-scale OLED cells or modules has been proposed for the purpose of permitting an apparatus including multiple OLED cells to connect directly to AC line current for lighting applications. As is noted in U.S. Patent Application Publication 2002/0190661 A1, series connection of these devices also provides a measure of fault tolerance in the event of a short to any individual OLED cell.  
         [0060]     Referring to  FIG. 7 , there is shown an area lighting apparatus  50  having an array of large-scale OLED cells  52 . Each OLED cell  52  is formed by connecting a single, large-scale OLED diode  58  in series with a fuse  56 . A bank  54  of OLED cells  52  is then formed by connecting a plurality of OLED cells  52  in parallel. Bank  54  could provide a usable area lighting apparatus  50  or module; however, a more practical application would connect successive banks  54  of OLED cells  52  in series, as is shown in  FIG. 7 . Using this fault-tolerant arrangement, a short at any large scale OLED diode  58  in OLED cell  52  blows its corresponding fuse  56 , but permits neighboring OLED cells  52  in the same bank  54  of OLED cells  52  to function.  
         [0061]     The arrangement of  FIG. 7  would be suitable for application of AC power, particularly when using series-connected banks  54 . However, with the arrangement of  FIG. 7 , OLED cells  52  would illuminate only during alternate half-cycles. By reversing the orientation of some OLED cells  52 , alternate groups of  30  OLED cells  52  would be illuminated with each half-cycle of AC current.  
         [0062]     Referring to  FIG. 8 , there is shown an arrangement of banks  54  of OLED cells  52  for an embodiment of area lighting apparatus  50  using a DC power source. Here, banks  54  of OLED cells  52  are formed as in the embodiment of  FIG. 7 , but are connected in parallel across the DC source. This lighting arrangement can be especially well suited to automotive applications and other environments using battery current or other DC sources.  
         [0063]     Unlike active matrix OLED arrays that employ transistor switches and their necessary support components, the overall approach of the display embodiments of  FIGS. 3   a ,  3   b ,  3   c ,  4   a ,  4   b ,  5   a ,  5   b ,  6   a , and  6   b  uses simple current limiting components, such as additional electroluminescent diodes  11 , passive resistors  38 , or fuses  40 , arranged in series with electroluminescent diode  11 .  
         [0064]     Using the display embodiments of  FIGS. 3   a ,  3   b ,  3   c ,  4   a ,  4   b ,  5   a ,  5   b ,  6   a , and  6   b , manufacturing yields would be increased without significant cost impact. In terms of equation (1) in the Background of the Invention, the effective area A of a fault is reduced to a pixel  10  area using these methods, rather than the area A of the complete display.  
         [0065]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, the various current limiting solutions of the embodiments described hereinabove could be combined with each other to achieve a favorable component arrangement. Designation of row and column electrodes and device polarity could be reversed in practice for implementing some of the embodiments disclosed herein. In the spirit of the invention, series-connected current limiting components could be connected to either anode or cathode terminals of electroluminescent diodes  11 , depending on ease of fabrication. The present invention could be applied to both top- and bottom-emitting OLED architectures.  
       Parts List  
       [0000]    
       
           10  Oled pixel  
           10   a ,  10   b ,  10   c ,  10   d  pixel  
           11  electroluminescent diode  
           11   a ,  11   b ,  11   c ,  11   d  electroluminescent diode  
           12  organic layer  
           14  cathode  
           16  anode  
           18  substrate  
           20  passive matrix oled array  
           22  current source  
           24  cathode line  
           26  anode line  
           28  switch  
           30  switch  
           34  insulator  
           36  jumper  
           38  resistor  
           40  fuse  
           41  fuse area  
           42   a ,  42   b  column  
           44   a ,  44   b  row  
           46  short  
           48  strip  
           50  area lighting apparatus  
           52  oled cell  
           54  bank  
           56  fuse  
           58  large-scale oled diode