Abstract:
An organic light-emitting diode display which can display independent images on both sides is described. This display can be driven with passive matrix or active matrix schemes. The invention combines a unique stacked organic diode structure and special driving schemes involving time-sequential reversed fields.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority based upon U.S. Provisional Application Ser. No. 60/731,391, filed Oct. 31, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates an organic light-emitting diode (OLED) display, and more particularly to a double sided emission OLED display, where the images can be independently controlled.  
       BACKGROUND OF THE INVENTION  
       [0003]     OLEDs are thinner, lighter, consume less power, and have faster response times than liquid crystal displays (LCD). Current designs of double-sided displays involve physically putting two displays back-to-back in an assembly. The present invention allows such double-sided displays to be made using a single display.  
         [0004]     The double-side-emitting OLED can provide independent pictures on both sides at the same time, or it can provide images on either side. There are many situations requiring such double-sided displays in order to provide more information as well as saving space and being lighter. Examples of such applications are flip-style mobile phones, personal digital assistants, digital still cameras, and video recorders.  
         [0005]     U.S. Pat. No. 6,762,436 B1 (Huang) presents a method for manufacturing a double-sided display structure for an organic light-emitting diode (OLED), involving plating an organic protection layer on the organic layer to protect various organic layers from being damaged by electron bombardment when the OLED element is subject to ITO sputtering during manufacture. The method invovles plating an electron injecting layer and a thin metal film of a mating energy level on the organic protection layer; and plating a transparent conductive film on the electron injecting layer and the thin metal film to increase conductivity and protect the thin metal film from corrosion.  
         [0006]     U.S. Published Patent Application No. 2004/0075628 A1 (Chien) provides a double-sided display device employing a transparent cathode that enables an OLED display device to two illuminate concurrently. The device provides a conventional single side display OLED such that only one driving module is needed to output signals to display the same picture on the positive (anode) side and the negative (cathode) side concurrently.  
         [0007]     U.S. Pat. No. 6,909,233 B2 (Cok) discusses an OLED device that includes a pixel having a plurality of individually addressable light emitting elements including a light emitting element for emitting white light, and one or more light emitting elements for emitting colored light; wherein at least one of the light emitting elements being stacked on top of another of the light emitting elements; and wherein the white light emitting elements are more efficient than at least one of the colored light emitting elements.  
         [0008]     U.S. Pat. No. 6,882,383 B1 (Su) discusses a full-color organic light emitting diode (OLED) display which comprises a substrate, a white light emitting OLED, a first passivation layer, stacked layers of a color-converting layer and a color filter, and a second passivation layer. The white light emitting OLED comprises an anode, a cathode, and at least one white light emitting organic material layer disposed between the anode and the cathode. The first passivation layer covers the surface and sidewalls of the white light-emitting OLED. The stacked layers of the first passivation layer are separated at intervals. The second passivation layer covers the surface and sidewalls of the stacked layers.  
         [0009]     U.S. Pat. No. 6,844,957 B2 (Matsumoto) describes a structure and fabrication technology for a reflective, ambient light, low cost display having a plurality of cells laid out side by side and stacked in as many as three levels on top of each other. Each stack of three cells is driven by an array of TFT&#39;s positioned on the bottom layer. Each cell comprises a light transmitting front window, three levels of individual cells RGB (Red, Green, and Blue) stacked on top of each other, each level having its own individual electrode, each electrode being connected by vertical conducting via holes running through each transparent dielectric spacer and being connected to an individual TFT. The bottom panel has a reflective surface so as to provide maximum reflectivity of the ambient light.  
       SUMMARY OF THE INVENTION  
       [0010]     The disadvantages of prior devices can be overcome, and the advantages of the invention can be realized by providing in one embodiment of the invention a display constructed of one or a plurality of double-side-emission organic light emitting diodes comprising: a substrate; a stack of two organic light emitting diodes in tandem; each organic light emitting diode having an anode, a cathode and a number of organic layers between the anode and the cathode; the anode of the first organic light emitting diode being in contact with the substrate, and the cathode of the first organic light emitting diode being in contact with the anode of the second organic light emitting diode; the anode of the first organic light emitting diode and the cathode of the second organic light emitting diode being transparent or semitransparent; the cathode of the first organic light emitting diode and the anode of the second organic light emitting diode being opaque to light; the anode of the first organic light emitting diode and the cathode of the second organic light emitting diode being connected to each other and are connected to the driving circuit by an electrode; and the cathode of the first organic light emitting diode and the anode of the second organic light emitting diode being in contact with each other and are connected to the driving circuit.  
         [0011]     In another embodiment the invention provides a display constructed of one or a plurality of double-side-emission organic light emitting diodes comprising: a substrate; a stack of two organic light emitting diodes in tandem; each organic light emitting diode having an anode, a cathode and a number of organic layers between the anode and the cathode; the anode of the first organic light emitting diode being in contact with the substrate, and the cathode of the first organic light emitting diode being in contact with the anode of the second organic light emitting diode; the anode of the first organic light emitting diode and the cathode of the second organic light emitting diode being transparent or semitransparent; the cathode of the first organic light emitting diode and the anode of the second organic light emitting diode being opaque to light; the anode of the first organic light emitting diode and the cathode of the second organic light emitting diode being connected to each other and are connected to the driving circuit by an electrode; and the cathode of the first organic light emitting diode and the anode of the second organic light emitting diode being in contact with each other and are connected to the driving circuit.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Further features and advantages of the present invention will become apparent upon review of the following detailed description of the preferred embodiments with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  is a diagram showing implementation of the parallel diodes using a stacked structure of one embodiment of the invention;  
         [0014]      FIG. 2  is a schematic diagram of the parallel diodes for each pixel in the double-side emission OLED of the present invention;  
         [0015]      FIG. 3A  shows the embodiment of  FIG. 2  having parallel diodes using stacked structure with both sides emitting green light;  
         [0016]      FIG. 3B  shows experimental efficiency results from the SOLED shown in  FIG. 3A ;  
         [0017]      FIG. 3C  shows experimental results on emission spectra from the SOLED shown in  FIG. 3A ;  
         [0018]      FIG. 4  is an internal circuit diagram of a passive matrix driven display using double side emission OLED&#39;s as pixels;  
         [0019]      FIG. 5  is a plane view and cross-sectional view of the double-side-emission passive matrix OLED display;  
         [0020]      FIG. 6  is a circuit schematic of one stacked OLED pixel in an active matrix driving scheme;  
         [0021]      FIG. 7  provides schematics of (a) relative values of common supply voltage and middle electrode voltage, and (b) the resulting display mode;  
         [0022]      FIG. 8  illustrates the driving TFT in a double sided emission AM OLED display;  
         [0023]      FIG. 9  is a cross-sectional view of a pixel in double sided emission AM OLED display; and  
         [0024]      FIG. 10  is a plane view of fabrication process of a pixel in double sided emission AM OLED display.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     The present invention allows an organic light-emitting diode to emit light from both sides with independent driving voltages. This is possible with the combination of two innovations: (1) the OLED architecture is such that there are two OLEDs arranged with their anodes and cathodes connected as shown in  FIG. 1 . We shall call this the parallel stacked OLED (PS-OLED), and (2) the driving scheme is such that the electronic signals from two different images are interlaced in time together to driving the matrix display, with the two signals being opposite in sign. Thus, positive signals will drive one OLED and negative signals will drive the other OLED.  
         [0026]     In order to implement the PS-OLED, we fabricated a stacked OLED where the middle electrode can function as an anode for one OLED, as well as a cathode for the other OLED. Also, the middle electrode has to be connected to the driving circuit and the outer most anode and cathode have to be connected together and connected to the driving circuit as shown in  FIG. 2 . The middle electrode of the stacked OLED has to be opaque as well. In one embodiment of the present invention, we employ opaque Ag or Au layer (with thickness over 70 nm) as the middle black reflective electrode and treat the surface of silver or gold with a CF 4 -Plasma to enhance the hole carrier injection from it when acting as the anode of top emission OLED to fabricate a double-sided emission OLED.  
         [0027]     The middle black reflective electrode which is Ag or Au (with thickness over 70 nm) can function well as the cathode of bottom emission OLED when combined with efficient electron injection layer, e.g., LiF(1 nm)/Al(3 nm) and at the same time well as the anode of top emission OLED due to the CF 4 -Plasma treatment on it. Although the bottom and top emission OLED share the common electrode Ag or Au, the electroluminescent performance of them can be independently controlled and optimized.  
         [0028]     This invention teaches how to drive this stacked OLED as a pixel in double-sided emission active matrix OLED display using passive matrix as well as active matrix. For passive matrix driving, the scanning signal and the data signal have to have a positive as well as a negative field, corresponding to the timing of the two OLEDs.  
         [0029]     For active matrix driving, a-Si TFT or p-Si TFT can be used as pixel driving elements. The a-Si TFT or p-Si TFT used to drive this OLED pixel has alternative positive and negative biasing on its source and drain, i.e., one duration for V d &lt;V s  and another duration for V s &lt;V d  depending on either front or rear panel is selected to display images. This is schematically shown in  FIG. 7 . This alternative biasing on source and drain of the a-Si TFT or p-Si TFT actually has another benefit in prolonging the lifetime of the TFT. It has been proven to be helpful for enhancing stability of driving TFT during operation.  
         [0030]      FIG. 2  shows an exemplary embodiment of a stacked OLED (SOLED)  200  as a pixel in PM/AM OLED display utilizing the present invention. The SOLED structure  200  of  FIG. 2  includes two OLEDs, one is bottom emission OLED  202 , the other is top emission OLED  204 . The SOLED structure  200  is fabricated on a substrate  206 , which is composed of a substantially transparent material such as glass in this embodiment. Above substrate  206  is the bottom electrode layer  208 , which is ITO in this case acting as the anode of bottom emission OLED  2002 . The bottom emission OLED  202  comprises organic emissive unit  210 , which normally includes the hole transporting layer (HTL), the emission layer (EML) and electron transporting layer (ETL) in sequence. A further electron injection layer  212  is deposited on top of organic emissive unit  210 . An opaque metal layer  214 , silver or gold, is deposited on the electron injection layer  212  to act as the cathode of bottom emission OLED  202 .  
         [0031]     After deposition of opaque metal layer  214 , silver or gold, the partially fabricated device was transferred to CF 4 -plasma treatment chamber for  10  seconds under the constant pressure of  20  Pascal. After that, the treated device was returned to the deposition vacuum chamber for fabricating the top emission OLED  204 . The top emission OLED  204  uses silver or gold  214  (treated by CF 4 -plasma) as the anode and includes of another organic emissive unit  216  and EIL  218  deposited sequentially on top of it. Finally, a semitransparent metal layer  220  was formed, which functions as the cathode of top emission OLED  204 . The opaque silver or gold function as the cathode of  214 , as well as anode of  204  and is the middle electrode of the total SOLED. The semitransparent metal layer  220  is the top electrode of the total SOLED and will be in direct connection with sputtering ITO when fabricating the AM/PM OLED display.  
         [0032]     When applying appropriate voltage across the bottom and top emission OLED through the external driving circuit, the front side and rear side light emission can be achieved selectively.  
         [0033]      FIG. 3A  shows a schematic cross-section of a SOLED with green bottom and green top light emission  300 , wherein the detailed materials used are also included. All functional layers marked are matching with corresponding ones in  200 . The materials used to form the layers will be abbreviated as follows: 
    ITO: indium-tin-oxide;     CF 4 : carbon tetrafluoride; used for working gas in the plasma treatment chamber with 20 Pa;     m-MTDATA:4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine;     NPB: 4,4′-N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine;     Alq 3 : tris(8-hydroxyquinoline)aluminium(III);     C545T: 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H, 11H-benzo[1]pyrano[6,7,8-ij]quinolizin-11-one;     BCP: 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline;     LiF/Al: lithium fluoride/aluminum; used for electron injection layer; and     Ag or Au: silver or gold; used for opaque reflective electrode or semitransparent top cathode.      
         [0043]      FIG. 3B  shows the experimental emission efficiency results obtained using this SOLED.  FIG. 3C  shows the emission spectra from the top and bottom OLEDs. These results demonstrate that the double side organic light emitting diode works. What is then needed will simply be the driving scheme necessary for driving an array of such SOLED.  
         [0044]      FIG. 4  shows the internal circuit diagram of PM OLED display. Such display comprises a plurality of scan lines Com 1 , Com 2 , Com 3 , Com 4 , Com 5 ; a plurality of data lines Seg 1 , Seg 2 , Seg 3 , Seg 4 , Seg 5 ; and a pixel array; wherein each pixel is a stacked OLED in which the top emission OLED and bottom emission OLED are connected as shown in  FIG. 1 . Within one pixel, the middle electrode of SOLED has to be connected to one scan line and the outer most anode and cathode have to be connected together and connected to the corresponding data line. Positive and negative voltages interlaced together may be used to drive this PM OLED display. Selection and de-selection of a particular scan line is respectively achieved by putting a definite voltage on or putting in a high impedance state the port of the scan circuit to which the line is connected. More importantly, the present invention can be well combined with a-Si or p-Si TFT to form AM OLED display with improved stability.  
         [0045]      FIG. 5  shows a plane view and cross-sectional view of an example of a double sided emission passive matrix driven OLED display. On the transparent substrate  501 , the ITO layers  502  are patterned to form rows of anodes in the first direction. The organic layers  503  of the bottom emitting OLED are coated next. The middle metal layers  504  are then deposited through a shadow mask to form columns of anode-cathodes electrodes. These middle electrode lines  504  are perpendicular to the anode  501 . The organic layers  505  of the top emitting OLED are then deposited on this middle electrode  504 . It is then followed by the semitransparent metals cathodes  506  in the same direction as the anode  501  through the same shadow mask. The problems of electrical shorting can be avoided with carefully designed layer dimension as well as shadow mask when fabricating.  
         [0046]      FIG. 6  shows the circuit diagram of one SOLED pixel in the AM OLED display. This circuit comprises one data line  601 ; one scan line  602 ; common power supply line  603  (for driving TFT); one addressing TFT  604 ; one capacitor  605  for voltage signal storage; one driving TFT  606 ; bottom emission OLED  607  and top emission OLED  608 , wherein the anode-cathode layer (opaque silver or gold) is shared as the common electrode  609 . The anode  607  (ITO) and Cathode  608  (semitransparent metal) are also connected together. Source or drain of addressing TFT  604  is connected with  601 , drain or source of  604  is connected with one port  605 , and gate  604  is connected with scanline  602 .  
         [0047]     Assuming that all TFTs are N-type (a-Si or p-Si), when the voltage of scanline  602  was high, drain or source  604  was switched on, voltage signal on the data line  601  was written in  605 . When the voltage of scanline  602  was back to low, the voltage signal was stored in capacitor  605 . One port of capacitor  605  is connected with common power supply line  603 ; the other is connected with gate  606 . The voltage signal stored in capacitor  605  will decide whether driving TFT  606  are switched on or off. The anode of  607  and the cathode of  608  are connected with the source/drain  606 . The voltage difference between common electrode  609  and common power supply line  603  will decide the working status of OLED  607  and  608 .  
         [0048]      FIG. 7  shows the specific working status of the AM OLED display including the present invention. For this double sided emission display, the front and rear side emission can be the same or different. The direct voltage  701  supplied to  603 , common power supply line which we assume is five volts here. The square voltage pulse  702  supplied to common electrode  609 , in which for the sake of discussion, low voltage is zero volts and high voltage is ten volts. If displaying images are standard video images, the frame period is 16 ms, and duration of low and high voltage is set equally to be 8 ms. When square voltage pulse electrode  702  is high, it will generate positive bias voltage on top emission OLED  608  and reversed for bottom emission OLED  607 . Therefore, in this 8 ms time duration, images  703  will display on the top side. When the square voltage pulse  702  is low, it will generate positive bias voltage on OLED  607  while reversed for cathode  608 , images  704  will display on bottom side. In the course of displaying images, the relative voltage polarity between direct voltage  701  and square voltage pulse  702  are alternatively positive and negative. Images on both sides can be the same or different, or one of them is non-selected to be dark. These features are fully satisfied with the requirements of folding-type mobile phone.  
         [0049]      FIG. 8  shows the specific status of the driving TFT  606 . We define, in  606 , the port connecting with common power supply line  603  as the drain, denoted as D, the port connecting with bottom emission OLED  607  and top emission OLED  608  as the source, denoted as S. In the Nth frame period, common electrode  609  is low for the first half of the frame; electrical potential of D (V D ) is larger than that of S (V S ) and current flows from D to S. Common electrode  609  is high for the second half of the frame; V D  is smaller than V S  and current flows from S to D.  
         [0050]      FIG. 9  shows cross-section view of a pixel in present double sided AM OLED display. This fabrication process comprises depositing a buffer layer of LTO/SiNx sequentially  902  on a glass substrate  901 ; then depositing ITO electrode  904  covering the drain electrode  903  of driving TFT; then depositing LTO insulator layer  905  to avoid edge short; then depositing organic emissive units  906  of bottom emission OLED; then depositing middle metal (silver or gold)  907  acting as both the cathode of bottom emission OLED and anode of top emission OLED; then depositing organic emissive units  908  of top emission OLED and then semitransparent metal cathode  909  of top emission OLED connecting to  904  via holes. The size of organic and metal layers can be precisely controlled by micro region masking technology.  
         [0051]      FIG. 10  shows the plane view of a pixel structure in present double-side-emission AM OLED display. This structure comprises driving matrix  1001  consisting of addressing TFT, driving TFT and storage capacitor and corresponding metal electrode; bottom electrode  1002  acting as the anode of bottom emission OLED; ITO electrode  1003  connecting to top semitransparent metal cathode of top emission OLED; organic emissive units  1004  of bottom emission OLED; middle electrode (silver or gold)  1005  acting as both the cathode of bottom emission OLED and anode of top emission OLED; organic emissive units  1006  of top emission OLED and semitransparent metal cathode  1007  of top emission OLED connecting to  1003 .  
         [0052]     The present invention can be applicable to small size display market such as personal digital assistant (PDA), digital still camera (DSC) and especially the folding-type cell phone due to its capable of displaying two independent images one each side.  
         [0053]     While the invention has been described in connection with preferred embodiments, it should be apparent to those of ordinary skill in the art that various alterations and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended that the invention be defined by the following claims.