Abstract:
An electroluminescent display device includes a display panel having scan lines, data lines, and pixel circuits. The pixel circuit includes an electroluminescent element having a first electrode layer, a first insulation film, and an emitting layer for displaying images. A driving circuit is coupled to the electroluminescent element. The first electrode layer is superimposed on a power source line, a scan line, or both, with a second insulation film therebetween.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korea Patent Application No. 10-2003-0083589 filed on Nov. 24, 2003 and No. 10-2004-0000594 filed on Jan. 06, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a display device. More specifically, the present invention relates to an organic electroluminescent (EL) display device with an improved aperture ratio. 
     (b) Description of the Related Art 
     The organic EL display device, which is a display device for electrically exciting a fluorescent organic compound to emit light, has organic light-emitting cells that are voltage—or current-driven to display an image. These organic, light-emitting cells have a structure composed of an anode layer, an organic thin film, and a cathode layer. To balance the electrons and holes in order to enhance luminescent efficiency, the organic thin film has a multi-layer structure that includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The multi-layer structure of the organic thin film can also include an electron injecting layer (EIL) and a hole injecting layer (HIL). 
     As shown in  FIG. 1 , an organic EL display device includes an organic EL display panel (referred to as “display panel” hereinafter)  100 , a data driver  200 , and a scan driver  300 . 
     The display panel  100  includes a plurality of data lines D 1  to Dm arranged in the column direction, a plurality of scan lines S 1  to Sn arranged in the row direction, and a plurality of pixel circuits. 
     Each of the pixel circuits includes a driving transistor  20  for controlling the current flowing to an organic EL element  40 , a switching transistor  10  for applying a voltage at the data line D 1  to a gate of the driving transistor  20  in response to a select signal provided by the scan line S 1 , and a capacitor  30  coupled between the gate and the source of the driving transistor. The source of the driving transistor  20  is coupled to a power source line  50  for transmitting a power source voltage V DD . 
     The data driver  200  supplies data voltages to the data lines D 1  to Dm, and the scan driver  300  sequentially applies select signals for selecting pixel circuits to the scan lines S 1  to Sn. 
       FIG. 2  shows a plan view of a pixel circuit coupled to the scan line S 1  and the data line D 1  in the organic EL display device shown in  FIG. 1 , and  FIG. 3  shows a cross-sectional view of the part of A-A′ of  FIG. 2 . 
     As shown in  FIGS. 2 and 3 , a gate electrode  16  of the switching transistor  10  is formed on the same electrode layer as that of the scan line S 1 , and a source region  13  of the switching transistor  10  is coupled to the data line D 1  by a contact hole. Drain region  14  of the switching transistor  10  is coupled to a gate electrode of the driving transistor  20  through a contact hole. The drain region of the driving transistor  20  is also coupled to the power source line  50  through a contact hole, and a source region is coupled to the pixel electrode layer  42  of the organic EL element  40  by a contact. 
     Transparent insulation film  12  is formed on a substrate film  11 . A first insulation film  15  is formed on the polycrystalline silicon layer, and a gate electrode  16  is formed to cross the polycrystalline silicon layer on the first insulation film  15 . 
     Part of the polycrystalline silicon layer beneath the gate electrode  16  is not doped, and two parts thereof are doped with n-type dopant. The regions doped with the dopant form a source region  13  and a drain region  14  respectively, and the undoped region forms a channel region. 
     A source electrode  18  is formed on the source region  13 , and the source region  13  is coupled to the data line D 1  through the source electrode  18 . A drain electrode  19  is formed on a drain region  14 , and the drain electrode  19  is coupled to a gate electrode of the second transistor  20 . 
     The organic EL element  40  comprises an organic EML  41  and a pixel electrode layer  42 , such as indium tin oxide (ITO). The organic EL element  40  is positionally separated from the power source line  50 . A cathode electrode  21  is formed on the organic EML  41   
     The organic EML  41  is formed at a pixel region defined by an insulation film which forms an aperture on the pixel electrode layer  42 . That is, since the organic EML  41  is formed within the pixel electrode layer  42 , the region for forming the organic EML  41  is limited by the pixel electrode layer  42 . Therefore, the narrow region of the generated organic EML  41  degrades the aperture ratio of the pixel circuit. It is therefore desirable to improve the aperture ratio of an organic EL display device. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, an electroluminescent (EL) display device includes a display panel including a plurality of scan lines, a plurality of data lines, and a plurality of pixel circuits. The pixel circuit includes an EL element with a first electrode layer, a first insulation film, and an emitting layer (EML). The circuit further includes a driving circuit coupled to the EL element. The first electrode layer of the EL element is superimposed on a power source line, with a second insulation film therebetween. Within the context of this disclosure, “superimposed” indicates that the element is covering, overlapping, or aligned in a vertical direction with another element, with or without intervening elements therebetween. 
     In an alternate embodiment, the first electrode layer of the EL element is superimposed on the scan line with the second insulation film therebetween. 
     In another embodiment, a method is provided for manufacturing an EL display device that includes an EL element, a first insulation film, and a driving circuit, as described above. The method includes forming a power source line for supplying power to the driving circuit, covering the power source line with a second insulation film, forming a first electrode layer of the EL element on the second insulation film, and superimposing part of the first electrode layer on the power source line. The embodiment further includes forming a third insulation film with an aperture on a part of the first electrode layer that is spaced horizontally from the power source line, forming an emitting layer of the EL element on the aperture, and forming a second electrode layer on the emitting layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional voltage programming organic EL display device. 
         FIG. 2  shows a plan view of a pixel circuit of the organic EL display device shown in  FIG. 1 . 
         FIG. 3  shows a cross-sectional view of the part of A-A′ of the pixel circuit of  FIG. 2 . 
         FIG. 4  shows a simplified plan view of a pixel circuit according to an exemplary embodiment of the present invention. 
         FIG. 5  shows a detailed plan view of a pixel circuit according to another exemplary embodiment of the present invention. 
         FIG. 6  shows a cross-sectional view of the part of B-B′ of the pixel circuit of  FIG. 5 . 
         FIG. 7  shows an alternate embodiment of the cross-sectional view of the part of B-B′ of the pixel circuit of  FIG. 5  with a wider organic EML. 
         FIG. 8  shows an application of a pixel circuit according to an exemplary embodiment of the present invention to a front-type light emitting display device. 
         FIG. 9  shows a pixel circuit according to another exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout this description, thicknesses are magnified in the drawings to clearly depict the plurality of layers and regions. Similar parts or units have the same reference numerals throughout the specification. In the context of this disclosure, when a layer, a film, a region, or a substrate is described as being “on” another part, “on” should be understood to include either direct contact or coupling through at least one intervening material. 
     The exemplary embodiments described are applied to a rear-type light emitting display device. However, it is within the scope of the invention to apply the embodiments to front-type light emitting display devices as well. 
       FIG. 4  shows a simplified plan view of a pixel circuit according to an exemplary embodiment of the present invention. For ease of description, a single pixel circuit driven by a scan line S 1 , a data line D 1 , and a power source line  250  is described. 
     As shown in  FIG. 4 , the pixel circuit comprises an organic EL element  240  for displaying images in correspondence to an amount of the applied current, and a driving circuit  280  for driving the organic EL element  240 . 
     The organic EL element  240  comprises an organic EML, a first electrode layer for forming an anode (ITO), and a second electrode layer (not illustrated) for forming a cathode. 
     The driving circuit  280  can be formed by using a voltage programming or current programming driving circuit, and it controls the current flowing to the organic EL element  240  according to images signals applied to the data line to thereby represent desired images when a select signal is applied from the scan line. 
     The first electrode layer forming the anode is formed to be superimposed on the power source line  250 . Since a constant power source voltage is applied to the power source line  250 , a minor variation of data applied to the first electrode layer substantially causes no influence to the power source line  250 . 
     Therefore, when the first electrode layer is formed to be superimposed on the power source line  250 , the organic EML is formed more widely, and the aperture ratio of the organic EL display device is improved. 
       FIG. 5  shows a detailed plan view of a pixel circuit according to an exemplary embodiment of the present invention, and  FIG. 6  shows a cross-sectional view of the part of B-B′ of the pixel circuit of  FIG. 5 . 
     As shown in  FIGS. 5 and 6 , a driving circuit  180  comprises a driving transistor  120  for controlling the current flowing to the organic EL element  140  according to the voltage applied to a gate, a switching transistor  110  for transmitting image signals applied to the data line D 1  to the driving transistor  120  in response to a select signal, and a capacitor  130 . 
     The gate electrode  116  of the switching transistor  110  is formed on the same electrode layer as that of the scan line S 1 , and a source region  113  of the switching transistor  110  is coupled to the data line D 1  through a contact hole. A drain region  114  of the switching transistor  110  is coupled to a gate electrode  116  of the driving transistor  120  through a contact hole. 
     Drain region of the driving transistor  120  is coupled to the power source line  150  by a contact hole, and a source region of the driving transistor  120  is coupled to the electrode layer  142  of the organic EL element  140  by a contact hole. 
     In this embodiment, an insulation film is formed between the electrode layer  142  of the organic EL element  140  and the power source line  150 , and part of the electrode layer  142  is formed to be superimposed on the power source line  150  with the insulation film therebetween. 
     The capacitor  130  is formed by the power source line  150  and the gate electrode of the driving transistor  120 . 
     As a result, when the switching transistor  110  is turned on by the select signal, the data voltage is transmitted to the gate of the driving transistor  120 , and a predetermined current is applied to the electrode layer  142 . Holes injected from the electrode layer  142  are transferred to the EML via the HTL of the organic EML  141 , and electrons are injected to the EML via the ETL of the organic EML  141  from a cathode electrode layer (not illustrated). The electrons and the holes are recombined in the EML to generate excitrons, and phosphorous molecules of the EML emit light as the excitrons are modified to the ground state from the excitation state. In this instance, the emitted light is output through the transparent electrode layer  142 , the insulation film, and the substrate to thus form images. 
     As shown, the organic EL display device is a rear-type light emitting display device in which a polycrystalline silicon layer is formed on a transparent insulation film  112 . The transparent insulation film  112  is formed on a substrate film  111 . A first insulation film  115  made of SiO2 or SiNx is formed on the polycrystalline silicon layer. 
     A gate electrode  116  made of Al and Cr is formed to cross the polycrystalline silicon layer on the first insulation film  115 . 
     Part of the polycrystalline silicon layer beneath the gate electrode  116  is not doped, and two parts thereof are doped with n-type dopant. The regions doped with the dopant form a source region  113  and a drain region  114  respectively, and the undoped region forms a channel region. 
     A source electrode  118  is formed on the source region  113 , and the source region  113  is coupled to the data line D 1  through the source electrode  118 . A drain electrode  119  is formed on a drain region  114 , and the drain electrode  119  is coupled to a gate electrode of the second transistor  120 . 
     The power source line  150  is formed on the first insulation film  115 , and is covered by a second insulation film  117 . The electrode layer  142  of the organic EL element  140  is formed on the second insulation film  117  between the transistor  110  and the power source line  150 . The electrode layer  142  is extended to the top of the power source line  150 , and a third insulation film  125  with an aperture is formed on the electrode layer  142 . In this instance, the third insulation film  125  is formed to cover part of an edge of the electrode layer  142 . 
     In the case that the organic EL display device is a rear-type light emitting display device, the aperture of the third insulation film  125  is formed on a part where the electrode layer  142  is not superimposed on the power source line  150 , and an organic EML  141  is formed on the aperture of the third insulation film  125 . A deposited cathode electrode  121  is formed on the organic EML  141 , and the cathode electrode  121  is formed as a metallic layer. 
       FIG. 7  shows a cross-sectional view of the part of B-B′ of the pixel circuit in an alternate embodiment showing the organic EML  141 ′ formed the most widely. 
     As shown, the organic EML  141 ′ is formed nearest the power source line  150  within a range that the organic EML  141 ′ is not bent. 
     When the thickness of the second insulation film  117  is defined as ‘b’ and the thickness of the electrode layer  142  is defined as ‘a,’ the aperture of the third insulation film  125  is separately formed from the power source line  150  by a distance equal to the summation of the thickness ‘a’ and ‘b.’ The aperture ratio of the organic EL display device can thereby become maximized. 
     The aperture ratio is increased because the organic EML  141 ′ is formed more widely by superimposing the electrode layer  142  on the power source line  150  with the second insulation film  117  therebetween. 
       FIG. 8  applies an alternate embodiment of the pixel circuit to a front-type light emitting display device. 
     As shown, the case of applying the concept of the present invention to the front-type light emitting display device is different from the rear-type light emitting display device shown in  FIG. 6  in that a substantially flattened film  122  is formed on the second insulation film  117 . 
     The flattened film  122  is formed with an organic film. Also, the electrode layer  142 ′ is formed with a metallic layer for reflecting light, and the electrode layer  121 ′ is formed with a transparent electrode layer. The electrode layer  142 ′ is formed to be superimposed on the power source line  150  with the second insulation film  117  therebetween. 
       FIG. 9  shows a pixel circuit according to another exemplary embodiment of the present invention. 
     As shown, the pixel circuit is different from the pixel circuit of  FIG. 5  in that the electrode layer  142 ″ is superimposed on the power source line  150  and the scan line S 2 . 
     Since a constant voltage signal is applied to the scan line S 2  during a select time of the pixel circuit, minor voltage variation caused by the electrode layer  142 ″ substantially generates no influence to the select signal applied to the scan line S 2 . 
     The light emitting region is maximized and the aperture ratio of the organic EL display device is increased by superimposing the electrode layer  142 ″ on the scan line S 2  to which the constant voltage is applied. 
     The third insulation film with the aperture is formed on the electrode layer  142 ″, and the organic EML  141  is formed on the aperture. In this embodiment, the aperture of the third insulation film is horizontally separated from the power source line  150  at least by a distance equal to the summation of the thickness of the second insulation film and the electrode layer  142 ″. 
       FIG. 9  shows that the electrode layer  142 ″ is superimposed on the next-row scan line S 2 . However, it is also within the scope of the invention for the electrode layer  142  to be superimposed on the current scan line S 1 . The electrode layer  142  can alternatively be superimposed on the next-row scan line S 2  without being superimposed on the power source line  150 . 
     Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concept taught herein, which may appear to those skilled in the art, will still fall within the spirit and scope of the present invention, as defined in the appended claims. 
     Specifically, the above-described driving circuit has been described in terms of a voltage programming circuit including a driving transistor and a switching transistor. Without being restricted to this circuit type, the driving circuit can be formed with the current programming method as well as various voltage programming methods. 
     The power source line  150  has been described to be a separately formed element, but it can alternatively be formed as a gate electrode of a transistor or a source/drain electrode. In these alternate embodiments, the electrode layer of the organic EL element is superimposed on the gate electrode or the source/drain electrode with an insulation film therebetween. 
     The above-described driving transistor and the switching transistor have been described to have N channel transistors, but one skilled in the art will realize that the switching transistor may be formed in any suitable manner including a first electrode, a second electrode, and a third electrode, with the voltage applied to the first electrode and the second electrode controlling the current flowing to the third electrode from the second electrode.