Patent Publication Number: US-7714514-B1

Title: Large area organic electroluminescent display using bimorph MEMS devices

Description:
RELATED APPLICATIONS 
     This is a continuation in part under 35 U.S.C. 120 of copending U.S. patent application Ser. No. 11/065,157 filed Feb. 25, 2005, which is a continuation in part under 35 U.S.C. 120 of U.S. patent application Ser. No. 10/277,500, filed Oct. 22, 2002 now U.S. Pat. No. 6,861,810, which also claims priority under U.S.C. 119(e) of Provisional Application 60/335,216 filed Oct. 23, 2001. Priority of the present application is also claimed under U.S.C. 119(e) of Provisional Application 60/797,692 filed May 5, 2006. 
    
    
     INTRODUCTION 
     The present invention relates generally to an organic electroluminescent (EL) device and more particularly to a driving method and apparatus for such a device. This active matrix drive method works for all known types of light-emitting diodes (LED) whose emission electroluminescent layer comprises a film of organic compounds. Examples of different types are small-molecule OLED (SM-OLED), polymer light-emitting diodes (PLED), and phosphorescent OLED (PHOLED). 
     BACKGROUND 
     There are two types of organic electroluminescent (EL) display devices, passive (simple matrix) organic EL and active matrix organic EL. Organic EL materials are divided into low molecular weight organic EL materials and high molecular weight (polymer) organic EL materials. Low molecular weight organic EL materials are mainly applied by evaporation to the substrate. High molecular weight or polymer organic EL materials are applied by ink jet printing or similar techniques. 
     The active matrix drive method works for all known types of light-emitting diodes (LED) whose emission electroluminescent layer comprises a film of organic compounds. Examples of different types are small-molecule OLED (SMOLED), polymer light-emitting diodes (PLED) and phosphorescent OLED (PHOLED). 
     Small molecule organic EL substances are disclosed in U.S. Pat. Nos. 4,720,432 (VanSlyke et al.), 4,769,292 (Tang et al.), 5,151,629 (VanSlyke), 5,409,783 (Tang et al.), 5,645,948 (Shi et al.), 5,683,823 (Shi et al.), 5,755,999 (Shi et al.), 5,908,581 (Chen at al), 5,935,720 (Chen et al.), 6,020,078 (Chen et al.), and 6,069,442 (Hung et al.), 6,348,359 (VanSlyke), and 6,720,090 (Young et al.), all incorporated herein by reference. 
     Large molecule or polymeric OLED substances are disclosed in U.S. Pat. Nos. 5,247,190 (Friend et al.), 5,399,502 (Friend et al.), 5,540,999 (Yamamoto et al.), 5,900,327 (Pei et al.), 5,804,836 (Heegar et al.), 5,807,627 (Friend et al.), 6,361,885 (Chou), and 6,670,645 (Grushin et al.), all incorporated herein by reference. The polymer light-emitting devices may be called PLED. 
     Organic luminescent substances also include OLEDs doped with phosphorescent compounds as disclosed in U.S. Pat. No. 6,303,238 (Thompson et al.), incorporated herein by reference. Organic photoluminescent substances are also disclosed in U.S. Patent Application Publication Nos. 2002/0101151 (Choi et al.), 2002/0063525 (Choi et al.), 2003/0003225 (Choi et al.) and 2003/0052595 (Yi et al.); U.S. Pat. Nos. 6,610,554 (Yi et al.) and 6,692,326 (Choi et al.); and International Publications WO 02/104077 and WO 03/046649, all incorporated herein by reference. 
     Active matrix organic LED devices are difficult to drive using simple two-terminal schemes because of their lack of memory. The rise and decay time of an organic LED device is very fast and it does not have intrinsic memory. To overcome this problem, thin-film-transistor (TFT) circuits have been developed to drive organic LED devices. Such circuits include four or more TFTs, a storage capacitor, and an organic LED pad arranged on a substrate. The storage capacitor enables the excitation power to an addressed organic LED element to stay on once it is selected. 
     While successfully overcoming the above mentioned problem, new problems in manufacturing are created. The storage capacitor process and deposition are very complicated and difficult to achieve in a fabrication process. The TFTs fabrication requires several mask steps whose difficulty and cost increase dynamically as the display size increases. If the substrate is plastic an expensive laser annealing process is used in fabrication of the TFT. 
     RELATED PRIOR ART 
     Electronic Circuitry 
     The following prior art relates to electronic circuitry for addressing an organic LED display and is incorporated herein by reference: U.S. Pat. Nos. 4,134,132 (Magos et al.), 5,828,181 (Okuda), 6,084,579 (Hirano), 6,356,029 (Hunter), 6,542,138 (Shannon et al.), 6,861,810 (Rutherford), and U.S. Patent Application Publication Nos. 2001/0001050 (Miyashita et al.), 2001/0054711 (Numano), 2002/0047839 (Kasai), 2002/0167474 (Everitt), 2003/0011960 (Koning et al.), and 2004/0084986 (Arbogast et al.). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a capacitor current memory Bimorph MEMS device positioned for address period. 
         FIG. 2  is a circuit diagram of a capacitor current memory Bimorph MEMS device positioned for light emission. 
         FIG. 3  is a circuit diagram of a sub-pixel for a Bimorph MEMS device in its OFF position. 
         FIG. 4  is a diagram of electrode waveforms. 
         FIG. 5  is a circuit diagram of a sub-pixel for an IPMC MEMS. 
         FIG. 6  is a diagram of IPMC MEMS electrode waveforms. 
         FIG. 7  is a sub-pixel one plastic sheet structure top edge view. 
         FIG. 8  is a sub-pixel two plastic sheet structure side edge view. 
     
    
    
     SUMMARY OF INVENTION 
     In accordance with this invention, there is provided a low power active matrix organic LED electroluminescent display device with a capacitor current memory circuit that enables increased component tolerances, and reduces problems caused by dimensional instability of plastic substrates. A Bimorph MEMS device connects a capacitor to the device&#39;s column electrode during the addressing period and to the organic LED electroluminescent device during light emission period. 
     DETAILED EMBODIMENTS 
     The capacitor current memory drive method and apparatus of this invention makes possible the low cost manufacture of very large area flexible substrate active matrix organic LED electroluminescent displays including HDTV. In accordance with this invention, display sizes up to 60 inches or more in diagonal can be manufactured. The three basic components in each of the sub-pixels that comprise this display are an organic LED electroluminescent (EL) device, a Bimorph MEMS device, and a memory capacitor. No TFTs are used or required in this active matrix drive method. Thus, some of the disadvantages associated with a TFT active matrix drive, such as more components in each pixel or sub-pixel, close manufacturing tolerances, and higher process temperatures are eliminated. All process temperatures for this display are low temperature and there are no destructive temperatures affecting manufacture on plastic substrates. Manufacturing non-uniformity has a much lesser effect on display brightness uniformity when using this structure compared to that of TFT circuits. The light output from the display is controlled by external ICs supplying a precise amount of charge to the memory capacitors. Because all of this charge is dissipated as forward current through the organic LED electroluminescent device, all pixels have the same light output even though there may be variations in the display&#39;s components. Variation in organic LED electroluminescent layer thickness across the display results in sub-pixels having different device threshold voltages. Adjustment to variation and/or changes in organic LED electroluminescent diode threshold voltage due to aging is automatic. Tolerances effecting a variation in sub-pixel size are automatically compensated by small changes in current density through the organic LED electroluminescent device. Therefore, because the display brightness uniformity is controlled by the charge stored in its memory capacitors, it is not necessary that manufacturing tolerances be as closely controlled as they are for the TFT drive method. Also, wide component tolerances helps in the manufacture of very large displays. 
     Low power results from row and column electrodes made of highly conductive opaque metal such as copper that minimizes the resistive losses in these electrodes. Energy stored in the Bimorph MEMS device during its operation is recovered in the memory capacitor. All charge added to the memory capacitor during the addressing period is used to produce light output. A ramp voltage applied to the memory capacitor controls a small forward current through the organic electroluminescent device. This results in an additional advantage in energy savings over that of the TFT drive method. In the TFT drive method forward current through the organic LED that produces the light output is dissipated to ground. In this drive method forward current through the organic LED is recovered in the V offset  power supply. The result of this energy recovery is a power savings resulting in even higher luminescent efficiency than can obtain by the TFT drive method. This display is a front emission type and has a very large aperture ratio (&gt;95%). Typically displays having very large aperture ratio can operate at a better luminous efficiency because of their lower current density. 
     This new drive method eliminates obstacles to the manufacture of small as well as very large area (60 inch or greater) HDTV organic LED electroluminescent displays on plastic substrates. It has the capability for manufacturing very large area low power organic LED electroluminescent displays with reduced cost and increased performance that cannot be matched by other active matrix drive methods. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Capacitor Current Memory Drive 
       FIG. 1  shows the electrical equivalent circuit for driving a cathode front structure organic LED electroluminescent display. The front emissions display with capacitor current memory drive consists of Row Electrodes  42  connected to the memory capacitors C 1  and driven by row ICs  8  with switches SW 3  and SW 4  and ramp circuit  6  with switches SW 1  and SW 2 . Also shown are Positive Voltage Electrode  44  and Column Electrode  47 . The constant current output from the Column Data IC  7  loads a specific amount of charge into the memory capacitors C 1 . The Column Data IC  7  includes a constant current source  7 - 1  and switch SW 7 . There is one common front electrode  61  (V OFFSET ) covering the entire front surface area of the display. To prevent excessive reverse bias on the organic LED after completion of the Ramp Voltage  45  (illustrated in  FIG. 4 ) portion of the waveform, a circuit  9  with switches SW 5  and SW 6  generates the Return to Center OFF Pulse  48  (illustrated in  FIG. 4 ). This pulses the Bimorph MEMS device&#39;s  3  bottom electrode a and returns the device to its center OFF position (illustrated in  FIG. 3 ). Also shown is top electrode c, which is grounded. The center electrode b is connected to the positive (+) lead of capacitor C 1 . Electrodes  42 ,  47 ,  61 , and  44  interconnect the display sub-pixels components. Each basic sub-pixel  1  has an organic LED electroluminescent device  2 , a Bimorph MEMS device  3  and a simple capacitor C 1  used as a memory device. Dependent upon the orientation of the sub-pixel components, this front emission display can have either the organic LED electroluminescent device cathode or anode to its front as disclosed in U.S. Pat. No. 6,861,810 (Rutherford), cited above and incorporated herein by reference. 
     As shown in  FIG. 1  the Bimorph MEMS device  3  is positioned for the address period. A mounting block  16  supports the bimorph MEMS device  3 . Pad  3 - 2  makes contact to pad  3 - 3  during the address period  43  (illustrated in  FIG. 4 ) and to pad  3 - 1  during the light emission period during ramp voltage  45  (illustrated in  FIG. 4 ). The arrowed line  10  is the current path from the Column Data IC Constant Current Source  7 - 1  to the Column Electrode  47 , through the Bimorph MEMS device  3  center electrode b, then to the memory capacitor C 1 . The charge stored in C 1  is for one video frame. The amount of charge determines the gray scale level for this sub-pixel. As illustrated in  FIG. 3 , after the address period  43  (illustrated in  FIG. 4 ), the Bimorph MEMS device  3  is repositioned to its center OFF position. 
     Shown in  FIG. 2  is an arrowed line  11  that follows the current path of memory capacitor C 1  discharge. The discharge is from C 1 , through Bimorph MEMS device  3  center electrode b, which is now positioned pad  3 - 2  to pad  3 - 1  connecting to the organic LED device  2 . The current path  11  continues through the organic LED device  2  to the V OFFSET    61 . A ramp voltage on the Row Electrode  42  controls the rate of capacitor C 1 &#39;s discharge through the organic LED device  2 . Also, a voltage equal to the Ramp Voltage  45 , shown in  FIG. 4  plus the voltage from the charge stored in C 1 , during the addressing period  43 , is applied to the Bimorph MEMS device  3  center electrode b. As the voltage increases a level is reached that determines when the Bimorph MEMS device  3  repositions from its center OFF position to the organic LED  2  position as shown in  FIG. 2 . This process controls the amount of reverse bias voltage that is applied to the organic LED  2  and can be from none to some desired amount. The effect upon the organic LED  2  is a small constant current until the end of the ramp voltage  45 , at which time the charge stored in the memory capacitor C 1  from the Column Data IC  7  is dissipated. Therefore, the total light emission during this frame time is directly proportional to the amount of charge stored in the memory capacitor C 1  during the Address Period  43  in  FIG. 4 . 
     Bimorph MEMS Device 
     Recent advances in electron-irradiated copolymer poly (vinylidene fluoride-trifluoroethyline) exhibit large energy density and recoverable strains up to 4%. See U.S. Patent Application Publication 2004/0084986 (Arbogast et al.), cited above and incorporated herein by reference. Another novel electromechanical device is ionic polymer metal composite materials. Bimorph actuation of systems utilizing high-density electrostrictive materials presents a compound actuator system. As illustrated in  FIG. 3  the Bimorph MEMS device  3  is mounted on mounting block  16 . The first  14  and second  15  electrostrictive layers are positioned between the a, b, c electrodes of the Bimorph MEMS device  3 . The electrodes a, b, c may be any suitable electrical conductor (gold, copper or aluminum). The electrodes a, b, c may be sheet conductors, or may be conductors that are sputtered or chemically vapor-deposited on the electrostrictive layers  14 ,  15 . As illustrated in  FIG. 3  when the voltage across both electrostrictive layers  14 ,  15  is equal, the Bimorph MEMS device  3  is in the center OFF position. As illustrated in  FIG. 1  when the voltage across electrodes a and b is greater than the voltage across electrodes b and c, then the electrostrictive layer  14  between electrodes a and b constricts and positions the Bimorph MEMS device  3  connecting the memory capacitor C 1  to the Column Data Electrode  47 . As illustrated in  FIG. 2  when the voltage across electrodes b and c is greater than the voltage across electrodes a and b, the electrostrictive layer  15  between electrodes b and c constricts and positions the Bimorph MEMS device  3  to connect memory capacitor C 1  to the organic LED device  2 . 
     Voltage Waveforms 
     The voltage waveforms of  FIG. 4  are for the Row Electrode  42 , Positive Voltage Electrode  44  and Column Electrode  47 . The waveforms have a period of time in which the Bimorph MEMS device  3  ( FIG. 3 ) is in the center OFF position, an Addressing Period  43  in which the Bimorph MEMS device  3  ( FIG. 3 ) is positioned to contact the memory capacitor C 1  ( FIG. 1 ) and light output period during the Ramp Voltage  45  in which the Bimorph MEMS device  3  ( FIG. 3 ) repositions from the center OFF position to a position making contact to the organic LED device  2  ( FIG. 2 ). During the Addressing Period  43 , the SW 4  ( FIG. 1 ) is ON and grounds the Row Electrode  42 , the result is that the voltage across electrodes a and b is greater than the voltage across electrodes b and c and the Bimorph MEMS device  3  ( FIG. 3 ) is positioned to connect the Column Electrode  47  to the memory capacitor C 1  ( FIG. 1 ). This enables charge to be stored in memory capacitor C 1  ( FIG. 1 ). The amount of charge added to the memory capacitor C 1  ( FIG. 1 ) will result in the desired light output. Charging is stopped by the pulse width modulation (not shown) of Column Data IC  7  ( FIG. 1 ) turning SW  7  OFF. At the end of the Address Period  43  the Row Electrode  42  Waveform steps voltage to V 1  such that the voltage across a and b is equal to the voltage across b and c which returns the Bimorph MEMS device  3  ( FIG. 3 ) to the center OFF position. As the Ramp Voltage  45  increases the voltage applied to the Bimorph MEMS device  3  ( FIG. 3 ), center electrode b increases. This center electrode voltage is the sum of the Ramp Voltage  45  plus the voltage level in the memory capacitor C 1  ( FIG. 3 ) due to the amount of charge stored in C 1  ( FIG. 3 ). When the voltage across electrode b and c reaches a certain level greater than the voltage across electrodes a and b, the Bimorph MEMS device  3  ( FIG. 3 ) repositions from the center OFF position to the organic LED position as discussed for  FIG. 2 . At this time the increasing Ramp Voltage  45  forces the discharge of capacitor C 1  through the organic LED  2  ( FIG. 2 ). 
     The small forward current level through the organic LED  2  ( FIG. 2 ) is controlled by an equilibrium condition created when the rate of reduction of the C 1  voltage due to discharge, is equal to the rate of voltage rise of the Ramp Voltage  45 . At the end of the ramp period, the discharge of C 1  stops when its voltage has been reduced to a level that is equal to the organic LED  2  ( FIG. 2 ) threshold voltage. Therefore, the spatial charge voltage remaining on the memory capacitor C 1  is indicative of the organic LED  2  ( FIG. 2 ) threshold voltage. The result is each sub-pixel has its own memory for its organic LED  2  ( FIG. 2 ) threshold voltage. In subsequent frames, charge is added to the memory capacitor C 1 , which is used to produce the desired light output. At the end of the Ramp Voltage  45  a Return to Center OFF Pulse  48  helps return the Bimorph MEMS device  3  ( FIG. 3 ) to its center OFF position before the Ramp Voltage  45  steps down from V 2  to V 1 . This prevents a reverse bias voltage on the organic LED during this step down in voltage. 
     Ionic Polymer Metal Composites (IPMC) MEMS Device 
     An IPMC has two parallel electrodes and electrolyte between the electrodes. For this invention an electro-active solid-state actuator comprising a solid polymer electrolyte film having first and second main surfaces facing each other, and first and second conductive polymer layers infiltrated into the first and second main surfaces of the solid polymer electrolyte film. 
       FIG. 5  illustrates a sub-pixel  50  using the IPMC MEMS device  53 . When the voltage is applied from memory capacitor C 1  and the bias electrode  54 , the First Conductive Polymer Layer  58  and Second Conductive Polymer Layer  59  are caused to be polar. Further, the cationic component of the Solid Electrolyte Film  52  is shifted to Conductive Polymer Layer  58  or  59  as a cathode. Thereby, expansion occurs at the electrolyte portion adjacent to the Conductive Polymer Layer  58  or  59 , while constriction occurs at the electrolyte portion adjacent to the other Conductive Polymer Layer. Thus, curvature toward the other Conductive Polymer Layer takes place. Also shown is  FIG. 5  is Pad  55 - 1  for connection to organic LED  2  ( FIG. 5 ), Pad  55 - 2  for connection to First Conductive Polymer Layer  58  and Pad  55 - 3  for connection to Column Electrode  47 . Dielectric  56  isolates  55 - 2  from Second Conductive Polymer Layer  59 . 
       FIG. 6  illustrates the voltage waveforms controlling the IPMC MEMS device  53  ( FIG. 5 ). When both the Bias Electrode waveform  64  and Row Electrode waveform  62  are at voltage V 1 , the IPMC MEMS  53  ( FIG. 5 ) device is in its center OFF position. During the Address Period  63  the Row Electrode  42  grounds memory capacitor C 1  ( FIG. 5 ) and the Bias Electrode waveform  64  is at V 1 . In this case the memory capacitor C 1 +voltage to First Conductive Polymer Layer  58  ( FIG. 5 ) results in expansion of the solid electrolyte film at this electrode and constriction at  59 . Thus, curvature to the Column Electrode  47  takes place to make the connection between the Column Electrode  47  ( FIG. 5 ) and Memory Capacitor C 1 . After the Address Period  63 , the voltage applied to the First Conductive Polymer Layer  58  is the charge stored in the Memory Capacitor C 1  ( FIG. 5 ) plus the voltage level due to increasing Ramp Voltage  65  ( FIG. 6 ). When the voltage level is reached such that the constriction of the Solid Electrolyte Film  52  ( FIG. 5 ) at the Memory Capacitor C 1  ( FIG. 5 ) and the expansion at the Bias Electrode  54  ( FIG. 5 ) results in enough curvature to make contact to the Pad  55 - 1  and organic LED  2  ( FIG. 5 ). The Memory Capacitor C 1  then discharges through the organic LED  2  ( FIG. 5 ) while the current level is controlled by the rate of Voltage Ramp  65  increase. At the end of the Ramp Voltage  65  a Return to Center OFF Pulse  66  helps return the IPMC MEMS device to its center OFF position before the Ramp Voltage goes down from V 2  to V 1 . This prevents a reverse bias of the organic LED  2  ( FIG. 5 ). 
     Single Substrate Structure 
     The structure for the organic LED electroluminescent display can have all its components located on its topside or on both sides of a single substrate  76  such as a plastic sheet. Illustrated in  FIG. 7  is one sub-pixel whose components are on both sides of a single substrate  76 . On the topside are the protective cover  70 , buffer layer  71 , translucent cathode electrode  72 , organic luminescent layers  73 , and optical trap consisting of a thin metal layer  74 - 1 , a transparent conductive layer  74 - 2 , organic LED anode electrode  74 - 3 . A plated through hole  76 - 1  makes connection between anode electrode  74 - 3  to bottom side of plastic substrate  76  pad  76 - 2 . On the bottom side are the Bimorph MEMS device  3  ( FIG. 7 ) and memory capacitor C 1 . This display is a top emission type and in the figure the organic LED cathode is to the front. However, by changing the orientation of components and waveforms the organic LED anode electrode can be to the front. 
     In the display&#39;s plastic substrate  76 , each sub-pixel has a plated through hole  76 - 1 , which makes the electrical connection between the top and bottom surfaces. Metal on the topside is formed into the shape of the organic LED anode electrodes  74 - 3 . A metal pad  76 - 2  on the bottom side is for contact to the Bimorph MEMS device  3  ( FIG. 7 ) pad  76 - 3 . 
     The topside processes begin with a destructive-interference contrast-enhancement stack. This stack has three layers consisting of the anode electrode  74 - 3 , transparent layer  74 - 2  and thin metal layer  74 - 1  that are formed into the shape of the sub-pixel. These layers also serve as the anode electrode for the organic light-emitting device. Next partitioning walls  75  (banks) are formed to fill the spaces between the sub-pixel electrodes. The organic luminescent layers  73  are formed in vertical strips of red, green, and blue color. The translucent cathode layer  72  covers the entire front surface area of the display. A buffer layer  71  covers the cathode layer  72 . Finally the cover layer  70  seals the display. To be noted the anode electrode  74 - 3  is flat, therefore the organic luminescent layers have a flat surface to be formed upon. As stated earlier the display can also be configured as a top emission type with the anode to the front. These different structure types for this display apparatus are discussed in previous papers. 
     Next testing of the organic electroluminescent layers selects reliable devices and the display process is continued. For those rejected displays, processing is discontinued, which results in a cost saving. 
     The bottom side processing continues with a spacer layer  70   s  with an open area for the Bimorph MEMS device  3  ( FIG. 7 ). The first part of the open area is filled with a temporary material. Then a metal pad  76 - 3  and c electrode are formed. A ground electrode is formed on the spacer layer. A second electrostrictive layer  15  is added to the c electrode. A spacer layer (not shown) equal to the thickness of the electrostrictive layer is added. A b electrode is formed on the second electrostrictive material  15  and also, extends to a plated through hole  78 - 1 . A first electrostrictive layer  14  is formed on the b electrode. A spacer layer (not shown) equal to the first electrostrictive layer  14  is added. The a electrode and pad  76 - 4  is formed on the first electrostrictive material  14 . The pad  76 - 4  is used to make contact to the column electrode  47 . The temporary material (not shown) under the c electrode is removed by under etching. A positive voltage electrode  44  is formed on the spacer layer and makes connection to the a electrode. An insulating film  77  fills the area between the spacer layer  70   s  and the metal for the Memory Capacitor C 1  second electrode  78  which is deposited and formed into the shape of a sub-pixel. A space  77 - 1  between second electrodes  78  isolates each C 1 . A memory capacitor dielectric layer  79  covers the entire surface. See U.S. Patent Application Publication 2003/0011960 (Koning et al.), incorporated herein by reference. Also see Y. Bai, Z.-Y. Cheng, V. Bharti, H. S. Xu, and Z. M. Zhang, “High-dielectric-constant ceramic-powder polymer composites”, Appl. Phys. Lett., Vol. 76, No. 25, 2000, cited above and incorporated herein by reference. Metal is deposited on the dielectric surface and formed into horizontal row electrodes  42  (memory capacitor first electrode). The memory capacitor second electrode  78 , memory capacitor dielectric  79  and row electrode  42  make memory capacitor C 1 . This completes the components on the bottom side. 
     Dual Substrate Structure 
     The dual substrate structure is such that manufacturing can be performed on three separate parts of the display simultaneously. Shown in  FIG. 8  is the structure for a side view of a pixel. 
     The Memory Capacitor C 1  is on the substrate  80   s . Row electrodes  42  are formed on the substrate  80   s . These electrodes also serve as the memory capacitors C 1  first electrode. A dielectric layer  89  is formed covering the entire display area of the substrate. A metal for the memory capacitors second electrode  88  is deposited upon the dielectric layer and then formed into its sub-pixel shape. The processing of the Row Electrode  42 , Memory Capacitor dielectric  89  and Memory capacitor second electrode  88  completes Memory Capacitor C 1 . 
     The first plastic substrate  87  such as a plastic sheet has interconnects for the memory capacitors second electrode  88 . It also has the MEMS device and its mounting. A conductive through hole  87 - 1  for each sub-pixel makes the electrical connection from the memory capacitor C 1  second electrode  88  to the topside of the plastic sheet  87 . Vertical column data electrodes  47  are formed on this topside. A nonconductive material  85 - 2  is formed in horizontal strips across the first plastic substrate  87 . An opening is maintained at the through hole location  87 - 1 . A ground electrode is formed on top of the horizontal stripe  85 - 2 . Also, during this process metal  87 - 2  from the through hole  87 - 1  to the top of the horizontal stripe makes an electrical conductor for the MEMS device center electrode b. The elements of the MEMS device are formed on a transfer substrate. The transfer substrate is moved to the first plastic sheet substrate  87  and the MEMS device is mounted upon the ground electrode. Conductive adhesive makes the electrical contact from ground electrodes to electrode a and metal  87 - 2  from through hole  87 - 1  to center electrode b of the MEMS device. The transfer substrate (not shown) is removed through a liftoff process, using radiation energy, such as from laser or other appropriate device. The positive voltage electrode  44  is formed across the display connecting to the c electrode of the MEMS device. A nonconductive spacer layer  85 - 1  is formed filling the space above the positive voltage electrode  44  and the horizontal strip-mounting layer  85 - 2 . This completes the processing of the first plastic substrate  87 . 
     The organic LED device is on the second plastic substrate  86 . There is a conductive through hole  86 - 1  for each sub-pixel. On the bottom side, at the conductive through hole, is a pad  86 - 2  area used for contact to pad  86 - 3 , which is connected to the cantilever Bimorph MEMS device center b electrode. The topside of the second plastic substrate begins with the anode electrode  84  for the organic light-emitting device connected to through hole  86 - 1 . Next partitioning walls (banks not shown) are formed to fill the spaces between the sub-pixels electrodes. Organic luminescent layers  83  are formed in vertical strips of red, green and blue color. A translucent cathode layer  82  covers the entire front surface display area. A buffer layer  81  covers the cathode layer. Finally the protective cover layer  80  is added to the front of the display. 
     Mating the part with the MEMS device to the part with the memory capacitor and then the part with the organic LED device to the part with the MEMS device completes the assembly of the display. 
     Display Life 
     This front emission type display has a very large aperture ratio. The resultant lower current density will increase life due to less heating of the organic LED element and less metal migration. 
     The organic LED electroluminescent layers reversed bias may be controlled to some desired amount. It is known that application of reverse bias to an organic LED electroluminescent element is an effective means to increase the life of the display. See U.S. Patent Application Publication 2002/0047839 (Kasai), cited above and incorporated herein by reference. 
     The spatial charge voltage on the memory capacitor is indicative of the threshold voltage of the organic LED device. It has been recognized that as the organic LED display elements degrade over time, their impedance increases and the potential difference between anode and cathode increases. The value of the change in potential difference provides a reasonable indication of the state of the element in terms of its light emission/drive current characteristic. See U.S. Pat. No. 6,356,029 (Hunter), cited above and incorporated herein by reference. Thus by using feedback of this voltage to adjust the amount of charge stored in each memory capacitor C 1 , compensation for temperature and aging degradation is possible. 
     Low Power 
     The row and column electrodes are made of highly conductive opaque metal such as copper. A resistive loss in these electrodes is minimal. Much of the energy used to drive the organic LED electroluminescent display is restored to the power supply and recovered in the memory capacitor. For example the energy of the forward current through the organic LED during the light emission is recovered in the V offset  power supply. Charge stored in the MEMS device is recovered in C 1 . These energy recovery methods result in significant reduction of power consumed by this display. This display is a front emission type and has a very large aperture ratio. Displays having very large aperture ratio can operate at a better luminous efficiency because of their lower current density. 
     Wide Component Tolerance 
     Manufacturing non-uniformity has a lesser effect on display brightness uniformity, when using this structure compared to that of TFT circuits. The light output from the display is controlled by external ICs supplying a precise amount of charge to the memory capacitor C 1 . Because all of this charge is dissipated as forward current through the organic LED device, all pixels have the same light output even though there are variations in the display&#39;s components. For example the memory capacitor value can vary because of non-uniform dielectric thickness and/or electrode area. Variation in organic LED layer thickness results in sub-pixels with different organic LED device threshold voltages across the display. The spatial charge voltage remaining in the memory capacitor helps compensates for these different threshold voltages. Tolerances affecting sub-pixel size are automatically compensated by small changes in current density through the organic LED. Therefore, because the display brightness uniformity is controlled by the charge stored in its memory capacitors, it is not as important that manufacturing tolerances be as closely controlled. 
     Display Brightness Uniformity 
     The capacitor current memory structure uses external column data IC&#39;s for driving the column electrode. The currents from these crystalline silicon drivers will always be more accurate and stable than that of the TFT pixel circuits. Light output is directly proportional to the amount of charge stored in the memory capacitors C 1 . All sub-pixel memory capacitors C 1  have the same charge added to them regardless of circuit variations. A more accurate charge stored in the memory capacitor results in better display brightness uniformity. 
     Adjustment to variation and/or changes in organic light-emitting diode threshold voltage is automatic. This is because after producing light by discharging the memory capacitor C 1  through the organic light-emitting diode a spatial voltage equal to the threshold voltage of the organic LED remains on the memory capacitor C 1 . In subsequence display cycles charge is added to the memory capacitor C 1  proportional to the desired display brightness. 
     Memory Capacitor Capability of Storing One Complete Frame of Data 
     The memory capacitor is able to store enough charge during an address period to light the display for a complete frame period. Therefore, sub-field addressing and the resultant visual artifacts are eliminated. 
     Deep Color 
     This large display can be addressed at very high speed. This results in a 120 Hz frame rate capability. An extended color gamut and 16-bit depth per color is then possible. 
     SUMMARY 
     The new drive method eliminates obstacles to the manufacture of very large area organic LED displays on plastic substrates. It has the capability for manufacturing low power very large area organic LED displays with reduced cost and increased performance that cannot be matched by other active matrix drive methods. 
     The foregoing description of various preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims to be interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.