Patent Publication Number: US-2007102714-A1

Title: Display device and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority from and the benefit of Korean Patent Application No. 10-2005-0107536, filed on Nov. 10, 2005, which is hereby incorporated by reference for all purposes as if fully set forth herein.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a display device and a manufacturing method thereof, and more particularly, to a display device having a electrode layer with different thicknesses depending on a wavelength of emitted light, and a manufacturing method thereof.  
      2. Description of the Background  
      Recently, an organic light emitting diode (OLED) has become more popular. The OLED may be a passive matrix OLED or an active matrix OLED according to its driving method. The passive matrix OLED has a simple production process, but requires an increase in power consumption because of an increase in its size and resolution. Thus, the passive matrix OLED is preferred in small display devices. On the other hand, the active matrix OLED has a wide screen and high resolution requiring a complex production process.  
      In the active matrix OLED, a thin film transistor is provided in every pixel region to control the emission of light from an organic light emitting layer in each pixel region. A pixel electrode is provided in each of the pixel regions, and each pixel electrode is insulated electrically from an adjacent pixel electrode so that each pixel electrode is independently driven. A wall, which is higher than the pixel electrode, is formed on every pixel region to prevent a short-circuit between adjacent pixel electrodes and to partition adjacent pixel regions. A hole injecting layer and the organic light emitting layer are sequentially formed on each pixel electrode that is formed between the walls. A common electrode is formed on the organic light emitting layer.  
      The OLED may be a bottom emission OLED or a top emission OLED according to an emission direction of light generated by the organic light emitting layer.  
      In the bottom emission OLED, light generated by the organic light emitting layer is emitted toward the thin film transistor. The bottom emission OLED has a stable process, but experiences a decrease in an aperture ratio due to the thin film transistor and wires.  
      In the top emission OLED, light generated by the organic light emitting layer is emitted to the outside through the common electrode. Thus, the top emission OLED may have a higher aperture ratio without a decrease in the aperture ratio due to the thin film transistor; however, a transparent common electrode is not easily formed.  
      The transparent electrode layer may be an anode or cathode in both the bottom emission and top emission OLEDs. According to its thickness, the transparent electrode layer may have a varying transmittance corresponding to a wavelength of the emitted light. Generally, the thickness of the transparent electrode layer is uniform regardless of the wavelength of light.  
     SUMMARY OF THE INVENTION  
      This invention provides a display device which includes a electrode layer having different thicknesses to improve light transmittance.  
      This invention also provides a manufacturing method for a display device which includes a electrode layer having different thickness to improve light transmittance.  
      Additional aspects and/or advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the present invention.  
      The foregoing and/or other aspects of the present invention can be achieved by providing a display device including a substrate, a light emitting layer arranged on a substrate, a first electrode arranged on the light emitting layer, and a second electrode facing the first electrode, where the light emitting layer is arranged between the second electrode and the first electrode. A thickness of the first electrode varies depending on the wavelength of a light emitted from the light emitting layer.  
      The foregoing and/or other aspects of the present invention can also be achieved by providing a method of manufacturing a display device including the steps of forming a light emitting layer on a substrate, where the light emitting layer comprises a red light emitting layer, a blue light emitting layer, and a green light emitting layer; forming a first electrode layer on the light emitting layer; forming a second electrode layer on the first electrode layer at a position corresponding to the red light emitting layer and the green light emitting layer, where the second electrode layer is thicker than the first electrode layer; and forming a third electrode layer on the second electrode layer at a position corresponding to the red light emitting layer, where the third electrode layer is thicker than the second electrode layer.  
      The foregoing and/or other aspects of the present invention can also be achieved by providing a method of manufacturing a display device including the steps of forming a light emitting layer on a substrate, where the light emitting layer comprises a red light emitting layer, a blue light emitting layer, and a green light emitting layer; forming a first electrode layer on the light emitting layer; forming a second electrode layer on the first electrode layer at a position corresponding to the green light emitting layer and the blue light emitting layer, where the second electrode layer is thinner than the first electrode layer; and forming a third electrode layer on the second electrode layer at a position corresponding to the blue light emitting layer, where the third electrode layer is thinner than the second electrode layer.  
      The foregoing and/or other aspects of the present invention can also be achieved by providing a method of manufacturing a display device including the steps of forming a light emitting layer on a substrate; applying a transparent electrode material on the light emitting layer; applying a photosensitive material on the transparent electrode material; and forming a transparent electrode layer having different thicknesses by patterning the photosensitive material using a mask, the mask comprising non-uniform light transmittance regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.  
       FIG. 1  illustrates a sectional view of a display device according to a first exemplary embodiment of the present invention.  
       FIG. 2  illustrates a graph of light transmittance according to the thickness of a transparent electrode layer according to the first exemplary embodiment of the present invention.  
       FIG. 3A ,  FIG. 3B ,  FIG. 3C ,  FIG. 3D ,  FIG. 3E ,  FIG. 3F ,  FIG. 3G , and  FIG. 3H  illustrate a manufacturing method of the display device according to the first exemplary embodiment of the present invention.  
       FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D , and  FIG. 4E  illustrate a manufacturing method of a display device according to a second exemplary embodiment of the present invention.  
       FIG. 5A ,  FIG. 5B , and  FIG. 5C  illustrate a manufacturing method of a display device according to a third exemplary embodiment of the present invention.  
       FIG. 6A ,  FIG. 6B , and  FIG. 6C  illustrate a manufacturing method of a display device according to a fourth exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
      The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative size of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements.  
      It will be understood that when an element such as a layer, film, region or substrate is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may also be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present.  
       FIG. 1  illustrates a sectional view of a display device according to a first exemplary embodiment of the present invention, which illustrates a driving transistor Tdr connected to a second electrode  181 .  
      As shown in  FIG. 1 , a gate electrode  121  is arranged on a substrate  110  which may include an insulating material such as glass, quartz, ceramic or plastic. A gate insulating film  131  which may be formed from silicon nitride (SiNx) is arranged on the substrate  110  and the gate electrode  121 .  
      A semiconductor layer  132  made of amorphous silicon and an ohmic contact layer  133  comprising a n+ hydrogenated amorphous silicon highly doped with an n-type dopant are sequentially arranged on the gate insulating film  131  corresponding to the gate electrode  121 . The ohmic contact layer  133  is separated into two parts, each part of the ohmic contact layer  133  being arranged on either side of the gate electrode  121  with the gate electrode  121  being the center.  
      A source electrode  141  and a drain electrode  142  are arranged on the ohmic contact layer  133  and the gate insulating film  131 . The source electrode  141  and the drain electrode  142  are arranged on either side of the gate electrode  121  with the gate electrode  121  being the center.  
      A passivation layer  151  is arranged on the source electrode  141 , the drain electrode  142 , and a portion of the semiconductor layer  132  exposed between the source electrode  141  and the drain electrode  142 . The passivation layer  151  may be formed from silicon nitride (SiNx). A portion of the passivation layer  151  corresponding to the drain electrode  142  is removed.  
      An organic layer  171  is arranged on the passivation layer  151  which covers the thin film transistor Tdr. An upper portion of the organic layer  171  is generally flat, and a portion of the organic layer  171  corresponding to the location of the drain electrode  142  may be partially removed. The organic layer  171  may be formed from any one of benzocyclobutene (BCB), olefin, acrylic resin, polyimide, Teflon, cytop and perflourocyclobutane (FCB).  
      A second electrode  181  is arranged on the organic layer  171 . The second electrode  181  may be an anode for supplying holes an organic layer  220  and refer to a pixel electrode. Generally, the second electrode  181  may be formed from an opaque material such as aluminum, silver, nickel or chrome. The second electrode  181  is connected to the drain electrode  142  through a contact hole  153 . The second electrode  181  includes a metal, which has a high work function for efficiently supplying holes to the organic layer  220 . Alternatively, the second electrode  181  may be formed from a transparent conductive material similar to a first electrode  231 . In this case, light may be emitted to opposite sides of the substrate  110  instead of being solely emitted toward the first electrode  231 .  
      A wall  211  surrounding the second electrode  181  is arranged on a portion of the second electrode  181  and the organic layer  171 . The wall  211  partitions the second electrode  181  to define a pixel region. The wall  211  prevents the source electrode  141  and the drain electrode  142  of the thin film transistor Tdr from short-circuiting with the first electrode  231 . The wall  211  may be made from a photosensitive material, which has heat-resistant and solvent-resistant properties, such as acrylic resin and polyimide resin, or an inorganic material such as silicon oxide (SiO 2 ) and titanium oxide (TiO 2 ). The wall  211  may be formed with a dual-layered structure having organic and inorganic layers.  
      The organic layer  220  is arranged on a portion of the second electrode  181  which is not covered by the wall  211 . The organic layer  220  includes a hole injecting layer  221  and a light emitting layer  222  ( 222 R,  222 G and  222 B).  
      The hole injecting layer  221  may be formed from a hole injecting material, such as poly  3 , 4 -ethylenedioxythiophene (PEDOT) and polystyrene sulfonic acid (PSS). The hole injecting layer  221  is formed by mixing the hole injecting material with water and then processing the mixture by an inkjet method in an aquatic suspension state.  
      The light emitting layer  222  includes a red light emitting layer  222 R, a green light emitting layer  222 G, and a blue light emitting layer  222 B. The light emitting layers  222 R,  222 G and  222 B emit light in different colors onto the neighboring second electrode  181 . The emitted light have different wavelength bands according to the respective colors. The light emitting layer  222  may also be formed by an inkjet method.  
      The first electrode  231  is arranged on the wall  211  and the light emitting layer  222 . The first electrode  231  may be a cathode or a common electrode for supplying electrons to the light emitting layer  222 . A hole supplied by the second electrode  181  and an electron transmitted from the first electrode  231  are combined in the light emitting layer  222  to become an exciton, thereby generating light during a non-activation process of the exciton. The first electrode  231  may be formed from a transparent conductive material, such as indium tin oxide (ITO) or indium zinc oxide (IZO). Light emitted from the light emitting layer  222  is transmitted to the outside of the display device  1  through the first electrode  231 .  
      The first electrode  231  varies in thickness depending on the colors of the emitted light, i.e., dependent on the light emitted from the light emitting layers  222 R,  222 G and  222 B. The thickness d R  of the first electrode  231  arranged on the red light emitting layer  222 R is greater than the thickness d G  of the first electrode  231  arranged on the green light emitting layer  222 G. The thickness d B  of the first electrode  231  arranged on the blue light emitting layer  222 B is less than the thickness d G  of the first electrode  231  arranged on the green light emitting layer  222 G. As described above, every material has a different wavelength band having its highest light transmittance according to the thickness of the first electrode  231  formed from that material. The highest transmittance of red light is available when the first electrode  231 , formed of ITO or IZO, has a thickness of about 1000 Å to about 2000 Å. The highest transmittance of green light is available when the first electrode  231 , formed of ITO or IZO, has a thickness of about 1300 Å to about 1500 Å. The highest transmittance of blue light is available when the first electrode  231 , formed of ITO or IZO, has a thickness of about 900 Å to about 1200 Å. Preferably, the thickness d R  of the first electrode  231  arranged on the red light emitting layer  222 R is about 1700 Å to about 2000 Å. Also, the thickness d G  of the first electrode  231  arranged on the green light emitting layer  222 G is about 1300 Å to about 1500 Å, and the thickness d B  of the first electrode  231  arranged on the blue light emitting layer  222 B is about 900 Å to about 1200 Å. Light transmittance characteristics may be improved by providing the first electrode  231  with the thickness having the highest transmittance depending on the wavelength of light.  
      The display device  1  according to the first exemplary embodiment of the present invention is a top emission OLED, which emits light toward the first electrode  231 . However, in a bottom emission OLED, which emits light toward the substrate  110 , an anode may vary in thickness depending on the wavelength of light.  
       FIG. 2  illustrates a graph of light transmittance with respect to the thickness of the first electrode  231 . Referring to  FIG. 2 , the thickness of the first electrode  231  according to the first exemplary embodiment of the present invention will be described.  
       FIG. 2  illustrates a plurality of curves representing light transmittance versus a wavelength corresponding to the first electrode  231  with a predetermined thickness.  
      At the highest light transmittance (app 98%) for a red-colored light, the first electrode  231  has a thickness of about 1700 Å to about 2000 Å, more particularly about 1800 Å, and the red-colored light has a wavelength band of about 650 to about 700 nm. At the highest light transmittance (app 98%) for a green-colored light, the first electrode  231  has a thickness of about 1300 Å to about 1500 Å, more particularly about 1400 Å, and the green-colored light has a wavelength band of about 530 to about 500 nm. At the highest light transmittance (app 95%) for a blue-colored light, the first electrode  231  has a thickness of about 900 Å to about 1200 Å, more particularly about 1000 Å, and the blue-colored light having a wavelength band of about 530 to about 550 nm.  
      The first electrode  231  in  FIG. 1  includes at least a single layer of ITO. Alternatively, the first electrode  231  may further include other alloy layers. In this case, a single light transmittance corresponding to a single thickness may be calculated by multiplying the light transmittance of the respective first electrodes. Then, a thickness of the first electrode  231  corresponding to the highest light transmittance per wavelength band may be calculated from the transmittance curves with respect to the thicknesses for the respective first electrodes using  FIG. 2 .  
      For example, when the first electrode  231  includes a double layer having an alloy metal layer formed of magnesium and silver, and a layer formed of ITO, the transmittance of the metal layer with a thickness of about 2000 Å is about 60% and the transmittance of the metal layer with a thickness of 1000 Å is 70% in the red wavelength band, while the transmittance of the ITO layer with a thickness of 1000 Å is 85% in the red wavelength band. Thus, the thickness of the metal layer having the highest transmittance can be determined per wavelength using  FIG. 2 .  
       FIG. 3A ,  FIG. 3B ,  FIG. 3C ,  FIG. 3D ,  FIG. 3E ,  FIG. 3F ,  FIG. 3G , and  FIG. 3H  illustrate a method of manufacturing the display device  1  according to the first exemplary embodiment of the present invention. For convenience of description, a transistor is not shown Additionally, a first electrode material and a first electrode having the transparent electrode material are given an identical reference number  231 , where the reference numbers  231   a,    231   b,  and  231   c  designate a first electrode layer, a second electrode layer and a third electrode layer of the first electrode according to the forming sequence.  
      As shown in  FIG. 3A , the second electrode  181  is arranged on the substrate  110 . The second electrode  181  is electrically connected to a driving transistor (not shown) through the contact hole. The second electrode  181  may be formed by depositing a material through sputtering and then patterning.  
      The wall  211  is arranged on the substrate  110  to partition the second electrode  181  from an adjacent second electrode  181 . The red light emitting layer  222 R, the green light emitting layer  222 G, and the blue light emitting layer  222 B are sequentially arranged on a respective second electrode  181 .  
      A first electrode layer  231   a  is arranged over the light emitting layers  222 R,  222 G,  222 B and the wall  211 . The thickness d B  of the first electrode layer  231   a  corresponds to the first electrode  231  which is arranged on the blue light emitting layer  222 B.  
      As shown in  FIG. 3B , a first photosensitive material  10  is arranged on the first electrode layer  231   a.  The foregoing process includes a pre-baking process for removing moisture from the first electrode layer  231   a  on which the first photosensitive material  10  is to be applied to improve adhesion between the first photosensitive material  10  and the first electrode layer  231   a;  a spin coating process for uniformly coating the first photosensitive material  10  onto the first electrode layer  231   a  by using centrifugal force; and a soft-baking process for vaporizing a solvent remaining on the first photosensitive material  10  to cure the first photosensitive material  10 .  
      As shown in  FIG. 3C , the first photosensitive material  10  on the red light emitting layer  222 R and the green light emitting layer  222 G is removed using a first mask  20 , thereby exposing the first electrode layer  231   a.  The first photosensitive material  10  is removed through well-known exposure, etching, and development processes.  
      As shown in  FIG. 3D , a second transparent layer  231   b  is deposited on the exposed first electrode layer  231   a,  and the first photosensitive material  10 .  
      As shown in  FIG. 3E , a second photosensitive material  11  is deposited on the second electrode layer  231   b,  and then the second photosensitive material  11  remaining on the blue light emitting layer  222 B is removed by a second mask  21 . The second electrode layer  231   b  is exposed above the blue light emitting layer  222 B where the second photosensitive material  11  has been removed.  
      As shown in  FIG. 3F , the second electrode layer  231   b  on the blue light emitting layer  222 B is removed by an etching liquid.  
      After removing the second photosensitive material  11  remaining on the red light emitting layer  222 R and the green light emitting layer  222 G, the first electrode  231 , which includes the first electrode layer  231   a  and the second electrode layer  231   b,  is formed on the red light emitting layer  222 R and the green light emitting layer  222 G, as shown in  FIG. 3   g.  The thickness d G  of the first electrode  231  on the green light emitting layer  222 G corresponds to the sum of the thicknesses of the first electrode layer  231   a  and the second electrode layer  231   b.  Thus, the thickness d G  of the first electrode  231  on the green light emitting layer  222 G is greater than the thickness d B  of the first electrode layer  231   a  on the blue light emitting layer  222 B.  
      By repeating the process illustrated in  FIGS. 3B  to  3 G, a third electrode layer  231   c  is formed, where the first electrode  231 , which includes the first electrode layer  231   a,  the second electrode layer  231   b,  and the third electrode layer  231   c  is arranged on the red light emitting layer  222 R, as shown in  FIG. 3H . A third photosensitive material  12  is arranged on the first electrode  231  of the green light emitting layer  222 G and the blue light emitting layer  222 B, and the first photosensitive material  10 .  
       FIG. 4A ,  FIG. 4B ,  FIG. 4C ,  FIG. 4D , and  FIG. 4E  illustrate a method of manufacturing a display device according to a second exemplary embodiment of the present invention. In the display device  1  according to the first exemplary embodiment, the thinnest electrode layer  231   a  and the thicker electrode layers  231   b  and  231   c  are formed through photolithography by using the masks  20  and  21 , and the photosensitive materials  10  and  11 . In the display device according to the second exemplary embodiment of the present invention, the thickest electrode layer  241   a  may be formed first, followed by formation of the thinner electrode layers.  
      As shown in  FIG. 4A , the first electrode  241   a  is arranged on a light emitting layer  222 , a wall  211 , and a second electrode  181 .  
      As shown in  FIG. 4B , a first photosensitive material  13  is arranged on the transparent electrode layer  241   a.  The first photosensitive material  13  remaining on a green light emitting layer  222 G and a blue light emitting layer  222 B is removed using a third mask  22 , which covers the first photosensitive material  13  at a position corresponding the red light emitting layer  222 R.  
      As shown in  FIG. 4C , the first electrode  241   a  on the green light emitting layer  222 G and the blue light emitting layer  222 B is etched so that a thickness d G  of the first electrode remains on the green light emitting layer  222 G using an etching liquid. The etching time is determined by the degree of reaction of the first electrode  241   a  with the etching liquid.  
      As shown in  FIG. 4D , a second photosensitive material  14  is arranged on the first electrode  241   a.  The second photosensitive material  14  arranged on the blue light emitting layer  222 B is removed using a fourth mask  23 , which covers the second photosensitive material  14  at a position corresponding to the blue light emitting layer  222 B.  
      As shown in  FIG. 4E , the first electrode  241   a  on the blue light emitting layer  222 B is etched so that a thickness d B  of the first electrode remains on the blue light emitting layer  222 B using the etching liquid.  
      With the foregoing process, the first electrodes are formed with various thicknesses by using the photolithography described in the first exemplary embodiment of the present invention.  
      Hereinafter, a method of manufacturing a display device according to a third exemplary embodiment of the present invention will be described with reference to  FIG. 5A ,  FIG. 5B , and  FIG. 5C .  
      The display device according to the present invention is manufactured through photolithography by using a mask. Using a single mask having different light transmittance, a first electrode with different thicknesses may be formed through etching, exposure, and development.  
      As shown in  FIG. 5A , a fifth mask  24  includes a transmission region  24   b  and a transmission region  24   c,  which are not uniform. More specifically, the fifth mask  24  includes a block region  24   a,  which completely blocks light; a transmission region  24   c,  which allows light to be completely transmitted; and a semi-transmission region  24   b,  which allows light to be partially transmitted. A plurality of slit patterns is arranged in the semi-transmission region  24   b,  thereby controlling the amount of transmitted light.  
      A photosensitive material  15  is deposited on a light emitting layer  222 , a wall  211 , and a second electrode  181 , and then the photosensitive material  15  is exposed with light through the fifth mask  24 .  
      Next, the photosensitive material  15  is formed through etching and development, as shown in  FIG. 5B . The photosensitive material  15  deposited on a red light emitting layer  222 R maintains its initial thickness, while the photosensitive material  15  deposited on a blue light emitting layer  222 B is completely removed. The photosensitive material  15  deposited on a green light emitting layer  222 G is developed at an intermediate level to form a different thickness.  
      Next, a first electrode  251  is etched according to the thickness of the photosensitive material  15  using an etching liquid, thereby forming the first electrode  251  as shown in  FIG. 5   c.    
      The different thicknesses of the first electrode  251  may be formed using the single mask  24 , allowing the first electrode  251  to be partially exposed through the slit patterns of the semi-transmission region  24   b.    
      A mask  24  according to another exemplary embodiment of the present invention may include a semi-transparent layer instead of physical patterns, such as the slit patterns of the semi-transmission region  24   b.  Light transmittance may be adjusted through the adjustment of the semi-transparent degree of the semi-transparent layer, thereby offering a similar effect as with the slit patterns.  
      A method of manufacturing a display device according to a fourth exemplary embodiment of the present invention will be described with reference to  FIG. 6A ,  FIG. 6B , and  FIG. 6C .  
      First, as shown in  FIG. 6A , a first electrode layer  261   a,  which has a thickness d B  of the first electrode arranged on a blue light emitting layer  222 B is formed.  
      Then, as shown in  FIG. 6B , a second electrode layer  261   b  is arranged using a first shadow mask  25 , which exposes the first electrode layer  261  a at a position corresponding to a red light emitting layer  222 R and a green light emitting layer  222 G. The second electrode layer  261   b  is formed by a sputtering method, which uses plasma discharge and includes use of indium tin oxide (ITO) or indium zinc oxide (IZO)  30 . Then, the first electrode, including the first electrode layer  261   a  and the second electrode layer  261   b,  with the desired thickness d G  is formed on the green light emitting layer  222 G.  
      As shown in  FIG. 6   c,  a third electrode layer  261   c  is formed through the sputtering method by using a second shadow mask  26 , which exposes the first electrode layer  261   a  and the second electrode layer  261   b  at a position corresponding to the red light emitting layer  222 R alone.  
      The display device can be manufactured without difficulty by sputtering ITO or IZO  30  through the shadow masks  25  and  26 .  
      According to another exemplary embodiment, electrode layers may be independently formed using a shadow mask which is open at locations corresponding to the respective light emitting layers, instead of being formed with the gradually-enlarged thickness.  
      The present invention provides a display device which has a electrode layer with different thicknesses to improve light transmittance, and various manufacturing methods thereof.  
      It will be apparent to those skill in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.