Abstract:
The invention is directed to an improved organic electroluminescent device. In one embodiment, the OLED includes a thin film transistor formed in a non-emission region on an insulating substrate that also includes source and drain electrodes. The OLED further includes a lower electrode formed in an emission region on the insulating substrate and connected to one electrode of the source/drain electrodes through a contact hole. The OLED yet further includes an organic emission layer formed in the emission region on the lower electrode, and an upper electrode formed on the organic emission layer, wherein the lower electrode has a surface with its corners rounded off. The lower electrode acts as a pixel electrode. Having its surface with corners rounded off prevents short-induced defects caused by outgassing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Korean Patent Application No. 2003-86154, filed Nov. 29, 2003, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic electroluminescent display device and method for manufacturing the same. More particularly, the invention is directed to an organic electroluminescent display device and method for fabricating the same, in which corners of a lower (pixel) electrode are rounded off, thereby preventing contamination caused by outgassing and short-induced defects. 
     2. Description of the Related Art 
     With the development of active matrix organic light emitting devices (AMOLED) into flat panel display devices using organic electroluminescence devices (OELDs) and with application of AMOLEDs to mobile phones, great reduction in thickness, size and fabricating cost have been achieved. 
       FIG. 1A  shows a planar structure of a conventional AMOLED having two transistors and a capacitor.  FIG. 1B  is a cross-sectional view taken along the line I-I of  FIG. 1A . 
     Referring to  FIG. 1B , the conventional AMOLED includes an emission region  110  and a non emission region  130 . In the emission region  110 , a lower electrode  131  (e.g., a pixel electrode), an organic emission layer  132  and an upper electrode  133  are formed. In the non-emission region  130  two thin film transistors (TFTS) and a capacitor are formed. 
     A buffer layer  140  is formed on a transparent insulating substrate  100  such as a glass substrate, and an amorphous silicon layer is deposited on the buffer layer to form a semiconductor layer  111 . The semiconductor layer  111  is formed by performing a crystallization process after patterning the deposited amorphous silicon layer. Then, a gate insulating layer  150  is formed on the entire surface of the substrate. Thereafter, a metallic material for a gate electrode is deposited and patterned on the gate insulating layer  150  to form a gate  112  on the semiconductor layer  111 . Additionally, a capacitor lower electrode  122  is simultaneously formed at this time. Upon formation of the gate  112  and the capacitor lower electrode  122 , a gate line  102  of  FIG. 1A  is also formed. 
     Thereafter, source/drain regions  113  and  114  are formed by ion implantation of, for example, P or N type impurities, into the semiconductor layer  111 . 
     Next, an interlayer insulating layer  160  is formed on the entire surface of the substrate. Then the interlayer insulating layer  160  and the gate insulating layer  150  are etched to expose portions of the source/drain regions  113  and  114 , thereby forming contact holes  161  and  162  for source/drain electrodes. 
     Then, after a metallic material for the source/drain electrodes is deposited on the interlayer insulating layer  160 , the source/drain electrodes  115  and  116  are formed to contact with the source/drain regions  113  and  114  through the contact holes  161  and  162 . Then a capacitor upper electrode  126  is formed which extends from any one electrode, for example, the source electrode  115 , of the source/drain electrodes  115  and  116 ; at the same time, a data line  104  and a power line  106  of  FIG. 1A  are also formed. 
     Thereafter, a passivation layer  170  is formed on the interlayer insulating layer  160 . The passivation layer  170  is etched as to expose a portion of the other electrode, for example, the drain electrode  116 , of the source/drain electrodes  115  and  116 , thus forming a contact hole  171  for a pixel electrode. 
     Then, a transparent conductive layer is deposited on the passivation layer  170  and is patterned to form the lower electrode  131  contacting with the drain electrode  116  through the contact hole  171  for pixel electrode in the emission region  130 . 
     After an insulating layer  180  is formed on the passivation layer  170 , an opening  181  is formed to expose the lower electrode  131 . An organic emission layer  132  is formed on a planarization layer  180  including the opening  181 , and an upper electrode  133  is formed thereon. 
     In the conventional OLED, although the insulating layer  180  is similar to a pixel definition layer (PDL) defining the emission region, the PDL is commonly formed of an organic layer. However, a problem arises in that the organic emission layer  132  becomes contaminated due to outgassing. To address this problem, an organic light emitting display device may be formed without such pixel definition layer. However, another problem often experienced in conventional OLEDs formed without pixel definition layers is short-induced defects that generate a dark spot in their corresponding pixels. 
     For example,  FIG. 2A  shows an embodiment of the planar structure of an organic light emitting display device without a pixel definition layer, and  FIG. 2B  is a cross-sectional view taken along the line II-II of  FIG. 2A . 
     A method for manufacturing the organic light emitting display device without such pixel definition layer is now explained with reference to  FIGS. 2A and 2B . 
     Referring to  FIGS. 2A and 2B , a thin film transistor and a capacitor are formed on a non emission region  220  in the same manner and structure as illustrated in  FIGS. 1A  and  1 B. Then, a lower electrode  261  is formed by depositing a transparent conductive layer in the emission region on the entire surface of a substrate so as to contact source/drain electrodes formed in the non-emission region through a contact hole  255   b  for pixel electrode and patterning the same. 
     Now, steps for forming the lower electrode  261  are illustrated through  FIGS. 2C to 2E .  FIG. 2C  is a perspective view showing the steps for forming the lower electrode  261 ,  FIG. 2D  is a plan view of the lower electrode  261 , and  FIG. 2E  is a cross-sectional view taken along the line III-III of  FIG. 2D . 
     Referring to  FIGS. 2C to 2E , corners of upper and lower portions of the lower electrode  261  are formed in an angle shape. As shown in  FIG. 2E , a length L 2  of the lower portion of the lower electrode  261  is formed longer than a length L 1  of the upper portion of the lower electrode  261 . That is, the lower electrode  261  has a side slanted at a tapered angle. 
     An organic emission layer  262  is formed on the lower electrode  261 . Then an upper electrode  263  made a metallic material is formed on the organic emission layer. 
     Upon the formation of the organic emission layer  262  on the lower electrode  261 , a step is formed. However, in use, a short-induced defect may be generated due to an open edge phenomenon. Thus, in the case where the upper and lower portions of the transparent conductive layer are angled, the organic emission layer  262  often deteriorates at its corner portion to expose the lower electrode  261 . 
     To address this problem, embodiments of the invention provide a lower pixel electrode having rounded corners. 
     Illustratively,  FIG. 2F  shows an enlarged view of an emission region upon the generation of a short-induced defect. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved organic electroluminescent display device that has no pixel definition layer. Embodiments of the present invention also provide a method for fabricating the same, which prevents outgassing and short-induced defects by forming a lower (e.g. pixel) electrode having corners that are rounded off. 
     In one embodiment, an organic electroluminescent display device includes a thin film transistor formed in a non-emission region on an insulating substrate, which includes source/drain electrodes. The OELD further includes a lower electrode formed in an emission region on the insulating substrate and connected to one electrode of the source/drain electrodes through a contact hole. It also includes an organic emission layer formed in the emission region on the lower electrode, and an upper electrode formed on the organic emission layer, wherein the lower electrode has a surface with its corners rounded off. 
     In one embodiment, the lower electrode has an upper surface having a center of radius of curvature is at its corners so that the radius of curvature is zero. The lower surface of is the lower electrode has a center of radius of curvature at a point on the same axis as an axis perpendicular to a plane on which the center of radius of curvature of the upper surface is positioned so that the radius of curvature is larger than zero. In this manner, curvature is formed at all four corners of the upper and lower surfaces, respectively. In one embodiment, the upper electrode is a cathode electrode and the lower electrode is an anode electrode. 
     Another aspect of the present invention provides a method for manufacturing an organic electroluminescent display device. A thin film transistor having source/drain electrodes is formed in a non-emission region on an insulating substrate. A lower electrode is formed in an emission region on the insulating substrate and connected to one electrode of the source/drain electrodes through a contact hole. An organic emission layer is formed in the emission region on the lower electrode. An upper electrode is formed on the organic emission layer, wherein the corners of the lower electrode are rounded off. 
     The lower electrode has an upper surface whose surface has a rectangular shape in which the center of the radius of curvature is at its corners so that the radius of curvature is zero. The lower electrode also has a lower surface in which the center of the radius of curvature is at a point on the same axis as the center of the radius of curvature of the upper surface so that the radius of curvature of the lower surface is larger than zero, wherein curvature is formed at all four corners of the upper and lower surfaces, respectively. 
     The lower electrode has an upper surface whose surface is formed rounded and having the center of the radius of curvature at any point thereon, and a lower surface in which the center of the radius of curvature is at a point on the same axis as the center of the radius of curvature of the upper surface so that the radius of curvature of the lower surface is larger than that of the upper surface, wherein curvature is formed at all four corners of the upper and lower surfaces, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a planar structure of a conventional AMOLED; 
         FIG. 1B  is a cross-sectional view taken along the line I-I of  FIG. 1A ; 
         FIG. 2A  shows a planar structure of another conventional organic electroluminescent display device; 
         FIG. 2B  is a cross-sectional view taken along the line II-II of  FIG. 2A ; 
         FIG. 2C  is a perspective view showing steps for forming the lower electrode of  FIG. 2A ; 
         FIG. 2D  is a plan view of the lower electrode of  FIG. 2C ; 
         FIG. 2E  is a cross-sectional view taken along the line III-III of  FIG. 2D ; 
         FIG. 2F  is an enlarged photograph of the emission region upon generation of the short-induced defect; 
         FIG. 3A  shows a planar structure of an organic electroluminescent display device according to a first embodiment of the present invention; 
         FIG. 3B  is a cross-sectional view taken along the line IV-IV of  FIG. 3A ; 
         FIG. 3C  is a perspective view showing steps for forming the lower electrode of  FIG. 3A ; 
         FIG. 3D  is a plan view of the lower electrode of  FIG. 3C ; 
         FIG. 3E  is a cross-sectional view taken along the line V-V of  FIG. 3D ; 
         FIG. 4A  shows a planar structure of an organic electroluminescent display device according to a second embodiment of the present invention; 
         FIG. 4B  is a perspective view showing steps for forming the lower electrode of  FIG. 4A ; 
         FIG. 4C  is a plan view of the lower electrode of  FIG. 4A ; and 
         FIG. 4D  is a cross-sectional view taken along the line VII-VII of  FIG. 4C . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the specification. 
       FIG. 3A  shows a planar structure of an active matrix organic electroluminescent display device (AMOLED) according to a first embodiment of the present invention. The AMOLED of  FIG. 3A  illustrates a device consisting of two transistors and a capacitor. 
       FIG. 3B  shows a cross-sectional view taken along the line IV-IV of  FIG. 3A . 
     Referring to  FIG. 3B , in an AMOLED formed according to one embodiment of the present invention, a buffer layer  340  is formed on a transparent insulating substrate  300 , such as a glass substrate. The substrate includes a non-emission region  310  and an emission region  330 . Then, an amorphous silicon layer is deposited on the buffer layer  340  to form a semiconductor layer  311 . The semiconductor layer  311  is formed by performing a crystallization process after patterning the amorphous silicon layer. Then, a gate insulating layer  350  is formed on the entire surface of the substrate, and a metallic material for a gate electrode is deposited and patterned on the gate insulating layer  350  to form a gate  312  over the semiconductor layer  311 . A capacitor lower electrode  322  is simultaneously formed at this time. Now, upon the formation of the gate  312  and the capacitor lower electrode  322 , a gate line  302  of  FIG. 3A  is also formed. 
     Thereafter, source/drain regions  313  and  314  are formed by ion implantation of, for example, P or N type impurities, into the semiconductor layer  311 . 
     Next, an interlayer insulating layer  360  is formed on the entire surface of the substrate, and the interlayer insulating layer  360  and the gate insulating layer  350  are etched to expose a portion of the source/drain regions  313  and  314 . Thereby, contact holes  361  and  362  for source/drain electrodes are formed. 
     Then, after a metallic material for the source/drain electrodes is deposited on the interlayer insulating layer  360 , the source/drain electrodes  315  and  316  are formed to contact with the source/drain regions  313  and  314  through the contact holes  361  and  362 . Here, a capacitor upper electrode  326  is formed which extends from any one electrode, for example, the source electrode  315 , of the source/drain electrodes  315  and  316 , and at the same time, a data line  304  and a power line  306  of  FIG. 3A  are also formed. 
     Thereafter, a passivation layer  370  is formed on the interlayer insulating layer  360 . The passivation layer  370  is etched to expose a portion of the other electrode, for example, the drain electrode  316 , of the source/drain electrodes  315  and  316 , thus forming a contact hole  371  for a pixel electrode. 
     Then, a transparent conductive layer is deposited on the passivation layer  370  in an emission region  330 , and is patterned to form a lower electrode  331  contacting with the drain electrode  316  through the contact hole  371  for the pixel electrode. 
     Now, steps for forming the lower electrode  331  of  FIG. 3A  are illustrated through  FIGS. 3C to 3E . 
       FIG. 3C  is a perspective view showing a whole shape of the lower electrode  331 ,  FIG. 3D  is a plan view of the lower electrode  331 , and  FIG. 3E  is a cross-sectional view taken along the line V-V of  FIG. 3D . 
     As shown in  FIGS. 3C to 3E , it can be understood that the upper surface  331   a  of the lower electrode  331  has a rectangular shape, and a lower surface  331   b  of the lower electrode  331  has rounded corners, so that the lower electrode  331  is tapered. 
     This can be expressed as following equation.
 
e 31 =0,e 32   32 &gt;0
 
     Here, e 31  is the radius of curvature of the upper surface  331   a , and e 32  is the radius of curvature of the lower surface  331   b . The lower electrode  331  has the upper surface  331   a  in which the center of the radius of curvature e 31  is at its corners so that the radius of curvature is zero. Meanwhile, the center of the radius of curvature e 32  of the lower surface  331   b  is located at a point on the same axis as an axis perpendicular to a plane on which the center of the radius of curvature of the upper surface  331   a  is positioned so that the radius of curvature e 32  is larger than zero. Thus, upper surface  331   a  has four angled corners and the lower surface  331   b  has four rounded corners. Accordingly, since the four corners of the lower electrode  331  are not angled, an open edge defect due to cut-off of the organic emission layer  332  can be prevented even after the organic emission layer  332  is deposited. 
     Next, an organic emission layer  332  is formed on the passivation layer  370  including the lower electrode  331 . Thereafter, an upper electrode  333  made of a metallic material is formed thereon, thereby creating the active matrix organic electroluminescent display device. 
     In the embodiment just described, the lower electrode acts as an anode electrode and the upper electrode acts as a cathode electrode. 
       FIG. 4A  shows a planar structure of an active matrix organic electroluminescent display device (AMOLED) according to a second embodiment of the present invention. The AMOLED of  FIG. 4A  illustrates a device having two transistors and a capacitor. 
     A cross-sectional view taken along the line VI-VI of  FIG. 4B  illustrates structure similar to that shown in  FIG. 3B . Additionally, a method of manufacturing the second embodiment of the present invention will be easily understood by a person skilled in the art with reference to the above description given with regard to the first embodiment. Thus,  FIG. 4A  need not be described in detail. 
     However, the steps of an exemplary method for forming the lower electrode  431  of  FIG. 4A  are explained with reference to  FIGS. 4B to 4D . 
       FIG. 4B  is a perspective view showing a whole shape of the lower electrode  431 ,  FIG. 4C  is a plan view of the lower electrode  431 , and  FIG. 4D  is a cross-sectional view taken along the line VII-VII of  FIG. 4C . 
     Upon the formation of the lower electrode  431 , shown in  FIG. 4C , four corners of an inner portion of an upper surface  431   a  are rounded on a plane, four corners of an outer portion of a lower surface  431   b  are rounded. As shown in  FIG. 4D , the lower electrode is formed to be tapered so that the lower surface  431   b  is formed wider than upper surface  431   a.    
     This can be expressed as following equation.
 
e 42 &gt;0,e 41 &gt;0
 
     Here, e 41  is the radius of curvature of the upper surface  431   a  of the lower electrode  431 , and e 42  is the radius of curvature of the lower surface  431   b  of the lower electrode  431 . 
     The lower electrode  431  has the upper surface  431   a  whose four corners each have curvature and the center of the curvature is positioned on the upper surface  431   a . The center of the radius of curvature e 42  of the lower surface  431   b  is at a point on the same axis as an axis perpendicular to a plane including the center of the radius of curvature e 41  of the upper surface  431   a , wherein the radius of curvature e 42  of the lower surface  431   b  is larger than that of the upper surface  431   a , and the four corners of the lower surface  431   b  are also rounded. Thus, four corners of the upper surface  431   a  and the lower surface  431   b  as well of the lower electrode are rounded, so that an open edge defect due to cut-off of the organic emission layer (see  332  of  FIG. 3B ) is further prevented even after the organic emission layer is deposited. 
     The upper electrode (see  333  of  FIG. 3B ) is formed on the organic emission layer ( 332  of  FIG. 3B ) to supply a common power source. 
     In this embodiment, the lower electrode  431  is an anode electrode and the upper electrode formed on the organic emission layer is a cathode electrode. 
     Although several embodiments of the present invention have been described above, curvature of the lower electrode may also be provided at a side portion so that a shape of the surface of the lower electrode forms an oval or circular shape. 
     According to the embodiment of the present invention as described above, in manufacturing an AMOLED having no pixel definition layer, the lower electrode is formed of a transparent metallic material and has the corners of its upper and lower surfaces rounded off, to prevent outgassing and short-induced defects. Such a configuration improves luminosity and increases the life of the organic electroluminescent display device. 
     Although preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.