Patent Publication Number: US-2011062861-A1

Title: Organic electroluminescent display panel

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/438,357, filed May 23, 2006, which claims the benefit of Taiwan Patent Application Serial No. 094127642, filed Aug. 12, 2005, the subject matter of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an organic electroluminescent display panel with an anti-reflective layer. 
     (2) Description of the Related Art 
     Contrast ratio (CR) is an important index for the consumers to pick out and buy the display. It is defined as the brightness ratio of the lightest state to the darkest state at the same point on the display. High contrast ratio means that relatively high brightness and bright colors. Accordingly, a higher contrast ratio means that better image quality of the display. Under the circumstance, black is blacker, and white is whiter, the display can appear more colors. 
     The organic electroluminescent display panel is self-luminous, so it has the minimum brightness when no light emitted from it. According to the definition, if the contrast ratio is measured in a darkroom, it tends to infinity because the minimum brightness tends to zero. It makes the contrast ratio lose its significance. Therefore, the contrast ratio of the organic electroluminescent display panel should be measured in a lightroom. The way is to provide an environmental light for the organic electroluminescent display panel to measure the brightness of the light reflected by the organic electroluminescent display panel which does not emit light. Industry standards currently allow for 500 lux maintained for the lightroom. 
     One way of obtaining the better contrast ratio is to reduce the environmental brightness. Usually, the organic electroluminescent display panel uses the black matrix to reduce the reflection in order to increase the contrast ratio. 
     Refer to  FIG. 1A , the conventional organic electroluminescent display panel  10  includes an organic light emitting diode  11  and its driving transistor  12 , which are placed in an active area  13 . The organic light emitting diode  11  has a bottom electrode  111 , a top electrode  112  and an organic emissive layer  113  sandwiched between the two electrode  111 ,  112 . The driving transistor  12  includes a source metal  121 , a gate metal  122 , a drain metal  123  and a channel  124 . The channel  124  is isolated from the gate metal  122  by an inner layer dielectric  125 . The channel  124  has a source contacting area  1241  and a drain contacting area  1242  to respectively contact with the source metal  121  and the drain metal  123 . The drain metal  123  is electrically connected the bottom electrode  111  of the organic light emitting diode  11 . 
     A part of the active region  13  is covered by a black matrix pattern to form a black matrix region  131 , the other part is not covered to form an opening region  132 . As shown, a patterning black matrix  15  is formed on the substrate  14  in the black matrix region  131 . The driving transistor  12  is disposed on the black matrix  15 . Between the driving transistor  12  and the black matrix  15  has a buffer layer  17 , such as silicon oxide. The opening region  132  is below the organic light emitting diode  11  to allow the light to exit, and is covered by a color filter layer  18  to define the light color. The edge of the color filter layer  18  has a black photoresist  19  corresponding to the black matrix  15 . 
     It is note that, there is no anti-reflective structure formed between the substrate  14  and the organic light emitting diode  11  in the opening region  132 , so that the reflectivity of the whole display panel is lager than 20% not to rise the contrast ratio efficiently. 
       FIG. 1B  is a diagram showing the relation between the reflectivity and the area ratio of the black matrix region to the active region. The ordinate is the reflectivity of the organic electroluminescent display panel in the light of 550 nm. The abscissa is the area percent of the black matrix region  131  to the active region  13 .  FIG. 1B  shows that the reflectivity decreases in proportion to the area percent of the black matrix region  131  to the active region  13 . For example, when the black matrix region  131  covers the 20% of the active region  13 , the reflectivity is about 60%. If the black matrix region  131  covers the active region  13  to reach to 68%, the reflectivity reduces to 25%. 
     Still refer to  FIG. 1A , if the area ratio of the black matrix region  131  to the active region  13  is constant, an external anti-reflective film  16  is adhered to the outside of a light-emitting surface of the organic electroluminescent display panel  10  to reduce the reflectivity. However, the thickness of the organic electroluminescent display panel  10  is increased and the light transmission of the opening region  132  is reduced, so the brightness is reduced. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an organic electroluminescent display panel and its fabricating method, which can form an anti-reflective layer between a substrate and a organic light emitting diode in the organic electroluminescent display panel to increase contrast ratio. 
     According to the present invention, the fabricating method of the organic electroluminescent display panel includes the steps of: forming an anti-reflective layer on the substrate; forming a light-shielding layer on the anti-reflective layer; defining the light-shielding layer as a first region and a second region; removing at least part of the light-shielding layer from the first region so as to expose the anti-reflective layer; forming a transistor on the light-shielding layer in the second region; and forming an organic light emitting diode on the anti-reflective layer in the first region. 
     The organic electroluminescent display panel fabricated by the above-mentioned method is described as follows. The substrate has an upper surface and a lower surface opposite to the upper surface. The anti-reflective layer is disposed above the upper surface of the substrate. The light-shielding layer is disposed directly above the anti-reflective layer and has an aperture to expose at least part of the anti-reflective layer. The transistor is disposed on the light-shielding layer. The organic light emitting diode is connected to the transistor and disposed directly above the at least part of the anti-reflective layer which is exposed via the aperture. According to the present invention, it is unnecessary to adhere an anti-reflective film outside the organic electroluminescent display panel, but can form the anti-reflective layer inside the organic electroluminescent display panel by a simplified process. Thus, the organic electroluminescent display panel increases contrast ratio, but reduces the thickness without the influence of the brightness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be specified with reference to its preferred embodiment illustrated in the drawings, in which 
         FIG. 1A  is an organic electroluminescent display panel according to the related art; 
         FIG. 1B  shows that the relation between the reflectivity and the area ratio of the black matrix region to the active region; 
         FIG. 2  is an organic electroluminescent display panel according to the present invention; 
         FIGS. 3A-3E  show the fabricating process of the organic electroluminescent display panel according to the present invention; 
         FIGS. 4A-4C  show the fabricating process of the anti-reflective layer with a plurality of sub-layers; and 
         FIG. 5  is a diagram which shows the intensity of the reflective light changes with the wavelength in range of visible light. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  is a sectional view of an organic electroluminescent display panel of the present invention. Each pixel of the organic electroluminescent display panel  20  includes a thin film transistor  22  and an organic light emitting diode  24 , which are disposed on a substrate  26 . Between the thin film transistor  22  and the substrate  26  have an anti-reflective layer  28  and a light-shielding layer  29 . But there lacks the light-shielding layer  29  between the organic light emitting diode  24  and the substrate  26 . The anti-reflective layer  28  is possible to form directly on the surface of the substrate  26 . The light-shielding layer  29  covers on the anti-reflective layer  28 , and has an aperture  292  to expose at least part of the anti-reflective layer  28 . The organic light emitting diode  24  is disposed on the anti-reflective layer  28  exposed by the aperture  292 . The thin film transistor  22  is disposed on the light-shielding layer  29  to drive the organic light emitting diode  24 . 
     The anti-reflective layer  28  includes at least one metal compound layer, for example, the metal oxide, metal nitride, metal sulfide or the combinations of these compounds. The metal compound includes the metallic elements, such as Cr, Mo, Cu, Zn, In, Ti, Al or Ag etc. The light-shielding layer  29  can be a metal layer made of Cr, Mo, Cu, Zn, In, Ti, Al or Ag etc., or a black photoresist. Preferably, the anti-reflective layer  28  and the light-shielding layer  29  are made of the metals belonging to the same group in the periodic table of chemical elements. Usually, a flat layer  27  is disposed on the anti-reflective layer  28  and the light-shielding layer  29  to provide a flat base for the thin film transistor  22  or the organic light emitting layer  24 . Note that the flat layer  27  is not required essentially according to the present invention. 
     The thin film transistor  22  has a semiconductor layer  221 , a gate insulating layer  222 , a gate electrode  223 , a source electrode  224  and a drain electrode  225 . The semiconductor layer  221  can be made of amorphous silicon or ploy-silicon. There are two contacting areas  2212 ,  2214  for the source electrode  224  and the drain electrode  225  to be heavily doped with p-type or n-type dopant, which is selected according to the types of the thin film transistor. The gate insulating layer  222 , which is usually a oxide layer, is sandwiched between the semiconductor layer  221  and the gate electrode  223 . An inner layer dielectric (ILD)  25  is sandwiched between the gate electrode  223 , the source electrode  224  and the drain electrode  225 . A passivation layer  23  is made of an insulating materials, such as silicon oxide or silicon nitride, and is disposed on the source electrode  224  and the drain electrode  225 . The passivation layer  23  has a through hole to make the drain electrode  225  contact with one transparent electrode  242  so as to drive the organic light emitting diode  24 . 
     Besides the transparent electrode  242 , the organic light emitting diode  24  includes an organic emissive layer  244  and a reflective electrode  246 . In this embodiment, a spacer layer  21  is formed on the passivation layer  23  and at least part of the transparent electrode  242  to separate one organic light emitting diode  24  from others. The spacer layer  21  also has an aperture (not numbered) corresponding to the aperture  292  of the anti-reflective layer  29 . There is disposed the organic emissive layer  244 , the reflective electrode  246 , an electron injecting/transporting layer (not shown) or a hole injecting/transporting layer (not shown) in the aperture of the spacer layer  21 . 
       FIGS. 3A-3E  show the fabricating method of the organic electroluminescent display panel. First, the anti-reflective layer  28  is formed on the substrate  26 . Next the light-shielding layer  29  is formed on the anti-reflective layer  28  and is defined as a first region  294  and a second region  296 . Subsequently, The light-shielding layer  29  is removed from the first region  294  to expose the anti-reflective layer  28 . In the second region  296 , the thin film transistor  22  is fabricated on the light-shielding layer  29 . And then, in the first region  294 , the organic light emitting diode  24  is formed on the anti-reflective layer  28 . The result structure is the organic electroluminescent display panel  20  shown as  FIG. 2 . 
     As shown in  FIG. 3A , the anti-reflective layer  28  can be entirely formed on the substrate  26 . As shown in  FIG. 3B , a metallic material or a black photoresist can be deposited on the anti-reflective layer  28  to form the light-shielding layer  29 .  FIGS. 3C-3D  show a patterning process of the light-shielding layer  29 . A photomask (not shown) is provided for the light-shielding layer  29  to define it as the first region  294  and the second region  296 . The anti-reflective layer  28  is exposed by etching the light-shielding layer  29  in the first region  294 . It is worth observing that, if the etching velocity of the light-shielding layer  29  is larger than that of the anti-reflective layer  28 , then the preferable etching efficiency and result are obtained. As shown in  FIG. 3E , the gate insulating layer  222 , the inner layer dielectric  25  and the passivation layer  23  are allowed to cover their lower structure entirely. 
     The way of etching control includes selecting a suitable etchant to control the etching velocity, or to control the etching time. Since the light-shielding layer  29  is a metal, and the anti-reflective layer  28  is a metal oxide, their etching is controlled by the suitable etchant which has the strong power to corrode metals, but the weak power to corrode metal oxides. In respect of other metals or metal oxides, different etchants can be chosen. When the light-shielding layer  29  and the anti-reflective layer  28  both include metals belonging to the same group in the periodic table of the chemical elements, the suitable etchants may be found more easily. Accordingly, the etching of the light-shielding layer  29  is controlled to be faster than that of the anti-reflective layer  28 . In order to prevent the anti-reflective layer  28  from damage, the etching time can be controlled as follows. When the light-shielding layer  29  is removed completely from the first region  294 , the etching can be stopped. By the way, the suitable etchants can help to reduce the etching time. 
     Refer to  FIGS. 4A-4C , the steps of forming anti-reflective layer  28  further include forming a metal oxide layer  281  on the substrate  26 , and then forming a metal nitride layer  282  on the metal oxide layer  281 . Preferably, the thickness of the metal oxide layer  281  is larger than that of the metal nitride layer  282 . As shown in  FIG. 4B , in the first region  294 , the light-shielding layer  29  is etched until exposing the metal nitride layer  282 . In the organic electroluminescent display panel  40  shown in  FIG. 4C , the ant-reflective layer  28  includes a plurality of metal compound layers, which include respectively different non-metallic elements combining with the same metallic element, or different metallic elements combining with the same non-metallic elements. The metallic elements include Cr, Mo, Cu, Zn, In, Ti, Al or Ag etc. The non-metallic elements include N, O or S etc. 
     In above-mentioned embodiments, the compounds, which are respectively required in the first region  294  and the second region  296 , are formed by the same one process. These metal compound layers are disposed in the first region  294 , and exposed by controlling the etching velocity of the black matrix. The result structure is the anti-reflective layer  28 , which is able be reduce the reflectivity but increase the contrast ratio of the whole display panel. Compare with the related art, the present invention uses etching velocity control and only one photomast to form the anti-reflective layer  28  with one or a plurality of sub-layers in the first region  294 . The anti-reflective layer  28  has a thickness that is odd multiples of ¼ wavelength in the range of visible light, so as to reduce the reflection of the environmental light but to increase the contrast ratio efficiently. 
     Continued from the preceding paragraph, the anti-reflective layer  28  includes a plurality of sub-layers, such as metal oxide layer and the metal nitride layer. Their total thickness is less than about 3000 nm to avoid affecting the light transmission. Regarding the organic electroluminescent display panel  20  or  40 , they prefer odd multiples of ¼ wavelength of visible light in a distance, which does not include the thickness of the reflective electrode  246  and the transparent substrate  26 , between the reflective electrode  246  and the transparent substrate  26 . 
       FIG. 5  illustrates that the intensity of the reflective light changes with the wavelength of the visible light. The abscissa is light wavelength (nm), and the ordinate is the intensity of the reflective light (a.u.). The curve  51  illustrates when a common organic electroluminescent display panel reflects the visible light, the intensity of the reflective light is changed substantially from 0.3 a.u. to 0.9 a.u. The curve  52  illustrates when the organic electroluminescent display panel of the present invention reflects the visible light, the intensity of the reflective light is changed substantially from 0 a.u. to 0.4 a.u. The light intensity 1.0 a.u. represents the total internal reflection occurs. 
     While the preferred embodiments of the present invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the present invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the present invention.