Abstract:
An organic light emitting diode (OLED) display is provided. The OLED display includes: a substrate; a first electrode on the substrate; a first pixel defining layer exposing at least a portion of the first electrode; a medium layer on the first pixel defining layer and the first electrode, the medium layer including a first region and a second region; a second pixel defining layer overlapping the first pixel defining layer with the first region therebetween; a light emission layer overlapping the first electrode with the first region therebetween; and a second electrode covering the second pixel defining layer and the light emission layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2012-0099777, filed on Sep. 10, 2012 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Aspects of embodiments of the present invention relate to an organic light emitting diode (OLED) display and a method of fabricating the same. 
     2. Description of the Related Art 
     An OLED display is a self-emissive display device that displays images by exciting an emissive organic material to emit light. The OLED display includes an anode (i.e., a hole injection electrode), a cathode (i.e., an electron injection electrode), and an organic light emission layer interposed therebetween. When the holes and the electrons are injected into the light emission layer, they recombine to form excitons, which emit light when they change from an excited state to a ground state. 
     The organic light emission layer is disposed at each pixel of the OLED display. The organic light emission layer in each pixel is spatially separated by a pixel defining layer. The pixel defining layer may be formed to have a larger thickness than the organic light emission layer. A surface of the pixel defining layer may protrude more than a surface of the organic light emission layer. 
     The thickness of the pixel defining layer is associated with pixel protection. For example, when an encapsulating substrate is pressed toward pixels in a structure having the encapsulating substrate provided on the organic light emission layer, if the thickness of the pixel defining layer is too small, the pixels may be pressed (for example directly pressed), causing dark spots. In addition, if the thickness of the pixel defining layer is small, it may cause unwanted capacitance between lines under the pixel defining layer and electrodes over the pixel defining layer. 
     SUMMARY 
     Aspects of embodiments of the present invention relate to an OLED display and a method of fabricating the OLED display. Further aspects relate to an OLED display including a pixel defining layer and an organic light emission layer, and a method of fabricating the OLED display. Embodiments of the present invention provide for an OLED display that can reduce or prevent dark spots from being caused to pixels due to pressing of a pixel defining layer by forming the pixel defining layer to have a sufficiently large thickness, and can reduce capacitance between lines over and under a pixel defining layer. Further embodiments of the present invention provide for a method of fabricating an OLED display having a pixel defining layer with a sufficiently large thickness. These and other aspects of the present invention will be described in or be apparent to one of ordinary skill in the art from the following description of exemplary embodiments. 
     In an exemplary embodiment according to the present invention, an organic light emitting diode (OLED) display is provided. The OLED display includes: a substrate; a first electrode on the substrate; a first pixel defining layer exposing at least a portion of the first electrode; a medium layer on the first pixel defining layer and the first electrode, the medium layer including a first region and a second region; a second pixel defining layer overlapping the first pixel defining layer with the first region therebetween; a light emission layer overlapping the first electrode with the first region therebetween; and a second electrode covering the second pixel defining layer and the light emission layer. 
     The second region may be formed by surface modification of the first region, or the first region may be formed by surface modification of the second region. 
     The second region may have lower wettability than the first region. 
     The second region may overlap the first pixel defining layer while not overlapping the second pixel defining layer. 
     The first pixel defining layer may have a first thickness of 1 μm or less. A sum of the first thickness and a thickness of the second pixel defining layer may be greater than 1 μm. 
     The medium layer may be a common layer for pixels of the OLED display. 
     According to another exemplary embodiment of the present invention, an organic light emitting diode (OLED) display is provided. The OLED display includes: a substrate supporting a plurality of pixels; a first electrode in each of the pixels; a medium layer on the substrate and the first electrode, the medium layer including a first region and a second region; a pixel defining layer on the first region and positioned at boundary portions of the pixels; a light emission layer on the first region and overlapping the first electrode; and a second electrode covering the pixel defining layer and the light emission layer. 
     The second region may be formed by surface modification of the first region, or the first region may be formed by surface modification of the second region. 
     The second region may have lower wettability than the first region. 
     The second region may not overlap the pixel defining layer. 
     The medium layer may be a common layer of the pixels. 
     According to yet another exemplary embodiment of the present invention, a method of fabricating an organic light emitting diode (OLED) display is provided. The method includes: forming a first electrode on a substrate; forming a first pixel defining layer on the substrate while exposing at least a portion of the first electrode; forming a medium layer on the first pixel defining layer and the first electrode, the medium layer including a first region and a second region; forming a second pixel defining layer overlapping the first pixel defining layer with the first region therebetween; forming a light emission layer overlapping the first electrode with the first region therebetween; and forming a second electrode covering the second pixel defining layer and the light emission layer. 
     The forming of the medium layer may include forming the second region by selective surface modification of the first region, or forming the first region by selective surface modification of the second region. 
     The selective surface modification may include selective UV radiation. 
     The forming of the second pixel defining layer may include nozzle printing or inkjet printing. 
     The first pixel defining layer may have a first thickness of 1 μm or less. A sum of the first thickness and a thickness of the second pixel defining layer may be greater than 1 μm. 
     According to still yet another exemplary embodiment of the present invention, a method of fabricating an organic light emitting diode (OWED) display is provided. The method includes: forming a first electrode on a substrate; forming a medium layer on the substrate and the first electrode, the medium layer including a first region and a second region; forming a pixel defining layer on the first region at boundary portions of the pixels; forming a light emission layer on the first region and overlapping the first electrode; and forming a second electrode covering the pixel defining layer and the light emission layer. 
     The forming of the medium layer may include forming the second region by selective surface modification of the first region, or forming the first region by selective surface modification of the second region. 
     The selective surface modification may include selective UV radiation. 
     The forming of the pixel defining layer may include nozzle printing or inkjet printing. 
     Embodiments of the present invention provide for an OLED display having a pixel defining layer that can be formed to have a sufficiently large thickness to protect the pixels and reduce unwanted capacitance while maintaining film uniformity of a first medium layer. Accordingly, the pixels are not pressed (for example, directly pressed) when an encapsulating substrate is pressed toward the pixels, thereby suppressing dark spots from being generated in the pixels. In addition, as the overall thickness of the pixel defining layer is increased, unwanted capacitance can be reduced or prevented from being generated between electrodes over and under the pixel defining layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and aspects of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic view of an OLED display according to embodiments of the present invention; 
         FIG. 2  is a layout view of an OLED display according to an embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of the OLED display shown in  FIG. 2 ; 
         FIGS. 4 to 9  are cross-sectional views illustrating process steps of a method of fabricating the OLED display shown in  FIGS. 2 and 3 ; 
         FIG. 10  is a layout view of an OLED display according to another embodiment of the present invention; 
         FIG. 11  is a cross-sectional view of the OLED display shown in  FIG. 10 ; and 
         FIGS. 12 to 15  are cross-sectional views illustrating process steps of a method of fabricating the OLED display shown in  FIGS. 10 and 11 . 
     
    
    
     DETAILED DESCRIPTION 
     Aspects and features of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description of exemplary embodiments and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided to more fully convey concepts of the present invention to those skilled in the art. The scope of the present invention is defined by the appended claims, and their equivalents. 
     When an element or layer is referred to as being “on” another element or layer, it can be directly on the other element or layer, or intervening elements or layers may be present. Like numbers refer to like elements throughout. Although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the corresponding embodiment. Embodiments of the present invention will now be described with reference to the accompanying drawings. 
       FIG. 1  is a schematic view of an OLED display  10  according to embodiments of the present invention. Referring to  FIG. 1 , the OLED display  10  includes a display region  11  and a non-display region  12 . The display region  11  may be positioned, for example, at a central portion of the OLED display  10 . The display region  11  includes a plurality of pixels PX. Each of the pixels PX emits light having a particular color wavelength. In an exemplary embodiment, the pixels PX include red, green, and blue pixels. The non-display region  12  may be positioned, for example, around the display region  11 . The non-display region  12  may include a driver that supplies an electrical signal, such as a data signal or a scanning signal, to the display region  11 . 
       FIG. 2  is a layout view of an OLED display  100  according to an embodiment of the present invention. Referring to  FIG. 2 , each pixel includes a light emission layer  150 . In an exemplary embodiment, the light emission layer  150  of each pixel emits light of any one of red (R), green (G), or blue (B) colors. The light emission layers  150  of different pixels are divided by a pixel defining layer. The pixel defining layer includes a first pixel defining layer  130  and a second pixel defining layer  160 . As shown in  FIG. 2 , the first pixel defining layer  130  is adjacent to the light emission layer  150 . The second pixel defining layer  160  may be spaced a set distance (such as a predetermined distance) apart from the light emission layer  150 , but aspects of the present invention are not limited thereto. A configuration of a pixel PX of the display region  11  (see  FIG. 1 ) as embodied in the OLED display  100  of  FIG. 2  will now be described in more detail. 
       FIG. 3  is a cross-sectional view of the OLED display  100  shown in  FIG. 2 . 
     Referring to  FIGS. 2 and 3 , a first electrode  120  for each pixel is formed on a substrate  110 . The first electrodes  120  of different pixels are physically and electrically separated from each other. The substrate  110  may include, for example, an insulating substrate. The insulating substrate may be made, for example, of a transparent glass material having transparent SiO 2  as a main component. In some embodiments, the insulating substrate may be made of an opaque material or a plastic material. Further, the insulating substrate may be, for example, a flexible substrate. The substrate  110  may further include other structures formed on the insulating substrate. Examples of the other structures include wirings, electrodes, insulation films, or the like. When the OLED display  100  is an active OLED display, the substrate  110  may include a plurality of thin film transistors formed on an insulating substrate. Drain electrodes of some of the thin film transistors may be electrically connected to the first electrodes  120 . 
     The first pixel defining layer  130  is formed on the substrate  110  and possibly portions of the first electrodes  120 . The first pixel defining layer  130  is positioned on boundary portions of pixels to define the respective pixels. In addition, the first pixel defining layer  130  may define an opening where the light emission layer  150  is positioned. Some or all of the first electrode  120  is exposed by the opening of the first pixel defining layer  130 . In the exemplary embodiment of  FIG. 3 , lateral portions of the first electrode  120  extend toward and are partially overlapped by the first pixel defining layer  130 . That is, in an overlapping area of the first pixel defining layer  130  and the first electrode  120 , the first pixel defining layer  130  is positioned above the first electrode  120  with respect to the substrate  110 . 
     The first electrode  120  may be an anode or cathode electrode of the OLED display  100 . When the first electrode  120  is an anode electrode, a second electrode  180  is a cathode electrode. For ease of description, the embodiment of  FIG. 3  will be described with the first electrode  120  being an anode electrode. However, in other embodiments, the first electrode  120  may be a cathode electrode and the second electrode  180  may be an anode electrode. The first electrode  120  may be made, for example, of a conductive material having a high work function. When the OLED display  100  is a rear emission display, the first electrode  120  may be made, for example, of ITO, IZO, ZnO, or In 2 O 3 , or may be formed of stacked films of these materials. When the OLED display  100  is a front emission display, the first electrode  120  may further include a reflection film made of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, or Ca. 
     The first pixel defining layer  130  may be made of an insulating material. For example, the first pixel defining layer  130  may include at least one organic material selected from the group consisting of benzocyclobutene (BCB), polyimide (PI), polyamaide (PA), acryl resin, and phenol resin. In other embodiments, the first pixel defining layer  130  may include an inorganic material, such as silicon nitride. 
     As shown in the exemplary embodiment of  FIG. 3 , the first medium layer  140  is formed on the first pixel defining layer  130  and the portion of the first electrode  120  exposed by the opening of the first pixel defining layer  130 . The first medium layer  140  helps with injection or transport of electrons or holes between the first electrode  120  and the light emission layer  150 . When the first electrode  120  is an anode electrode (as described in this embodiment), the first medium layer  140  is associated with hole injection or transport. For example, the first medium layer  140  may be formed as a single layer of a hole injection layer or a hole transport layer, or as a stacked layer of a hole injection layer and a hole transport layer. The hole injection layer may include, for example, a phthalocyanine compound, such as copper phthalocyanine, or a Starburst type amine, such as TCTA, m-MTDATA, or m-MTDAPB. The hole transport layer may include, for example, N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′-diamine (TPD), or N,N′-di(naphthalen-1-yl)-N,N′-diphenyl benzidine (α-NPD). 
     In other embodiments, the first medium layer  140  extends, for example, to a top surface of the first pixel defining layer  130 . More generally, in other embodiments, the first medium layer  140  is separated by each pixel. However, as shown in the exemplary embodiment of  FIG. 3 , the first medium layer  140  is integrally formed over the entire surface of the OLED display  100 . In such an embodiment, the first medium layer  140  is a common layer and not related with pixel distinction. 
     In embodiments where the first medium layer  140  is a common layer (such as  FIG. 3 ), it may be desired for the first medium layer  140  to have a uniform thickness throughout the OLED display  100 . However, the film thickness uniformity of the first medium layer  140  is related to the thickness of the first pixel defining layer  130 , as measured by a distance from the bottom surface of the first pixel defining layer  130  to the top surface of the first pixel defining layer  130 . If the first pixel defining layer  130  is too thick (for example, greater than 1 μm), it may be difficult to secure film thickness uniformity of the entire first medium layer  140 . Therefore, the thickness of the first pixel defining layer  130  may be adjusted to 1 μm or less, but aspects of the present invention are not limited thereto. 
     In the exemplary embodiment of  FIG. 3 , the first medium layer  140  includes a first region  140   a  and a second region  140   b . The first region  140   a  and the second region  140   b  differ from each other in terms of wettability (or wetting, that is, the ability to maintain contact with liquid). For example, the first region  140   a  may have relatively high wettability (for example, attracts or adheres to liquid) and the second region  140   b  may have relatively low wettability (for example, repels liquid). The first region  140   a  and the second region  140   b  may acquire this different wettability through selective surface treatment. For example, a material film having the same property as the first region  140   a  (that is, high wettability) is coated over the entire OLED display  100  and a portion of the material film is subjected to selective surface treatment to reduce the wettability, thereby forming the second region  140   b . Examples of the selective surface treatment include UV radiation using an optical mask. 
     For example, in the exemplary embodiment of  FIG. 3 , the first region  140   a  is positioned on a top surface of the first electrode  120  and on a top surface of the first pixel defining layer  130 , while the second region  140   b  is positioned on a lateral surface or a tilted surface of the first pixel defining layer  130 . Thus, the first region  140   a  includes two portions, namely a first portion on the first electrode  120  (i.e., the first region  140   a  on the first electrode  120 ) and a second portion on the first pixel defining layer  130  (i.e., the first region  140   a  on the first pixel defining layer  130 ). In some embodiments, the second region  140   b  may partially extend more to the top surface of the first pixel defining layer  130 . However, as will be described later, in order to secure a portion for forming the second pixel defining layer  160 , the second region  140   b  may not extend to entirely cover the top surface of the first pixel defining layer  130 . 
     The light emission layer  150  is positioned on the first medium layer  140 . As shown in  FIG. 3 , the light emission layer  150  overlaps the first electrode  120  in the opening of the first pixel defining layer  130 . In addition, because of the wettability differences in the first region  140   a  and the second region  140   b , the light emission layer  150  directly contacts the first region  140   a  on the first electrode  120 , but may not directly contact the second region  140   b . Therefore, the first region  140   a  and the second region  140   b  may define the region where the light emission layer  150  is formed in the opening of the first pixel defining layer  130 . 
     When the first region  140   a  has relatively high wettability and the second region  140   b  has relatively low wettability, a difference in the wettability may selectively define a location where a coating liquid for forming the light emission layer  150  is applied. For example, when the light emission layer  150  is formed by nozzle printing, it may be selectively formed only on the first region  140   a  on the first electrode  120  while not being formed on the second region  140   b  adjacent to this portion of the first region  140   a . For instance, with nozzle printing, the spray for forming the light emission layer  150  may be somewhat repelled by the second region  140   b  and somewhat attracted by the first region  140   a  on the first electrode  120 , thus forming the light emission layer  150  directly on this portion of the first region  140   a  and away from the second region  140   b.    
     As shown in the exemplary embodiment of  FIG. 3 , a top surface of the light emission layer  150  is lower than the top surface of the first pixel defining layer  130 . That is to say, the top surface of the first pixel defining layer  130  may protrude with respect to the light emission layer  150 . Among other things, this helps create the lateral surfaces of the first medium layer (corresponding to the second region  140   b ) and helps form the light emission layer  150  on the first region  140   a  on the first electrode  120 . It also helps protect the light emission layer  150  from being pressed. 
     The light emission layer  150  may include, for example, a polymeric organic material, a small-molecule organic material, or a combination thereof. In some embodiments, the light emission layer  150  may include a host material and a dopant material. Examples of the host material include tris(8-hydroxy-quinolinato)aluminum (Alq3), 9,10-di(naphth-2-yl)anthracene (AND), 3-Tert-butyl-9,10-di(naphth-2-yl)anthracene (TBADN), 4,4′-Bis(2,2-diphenyl-ethen-1-yl)biphenyl (DPVBi), 4,4′-Bis[2,2-di(4-methylphenyl)-ethen-1-yl]biphenyl(p-DMDPVBi), Tert(9,9-diaryffluorene)(TDAF), 2-(9,9′-spirobifluorene-2-yl)-9,9′-spirobifluorene (BSDF), 2,7-bis(9,9′-spirobifluorene-2-yl)-9,9′-spirobifluorene (TSDF), bis(9,9-diarylfluorene)s (BDAF), 4,4′-Bis[2-(4-tert-butyl-phen-4-yl)-ethen-1-yl]biphenyl (p-TDPVBi), 1,3-bis(carbazole-9-yl)benzene (mCP), 1,3,5-tris(carbazole-9-yl)benzene (tCP), 4,4′,4″-tris(carbazole-9-yl)triphenylamine (TcTa), 4,4′-bis(carbazole-9-yl)biphenyl (CBP), 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl (CBDP), 4,4′-bis(carbazole-9-yl)-9,9-dimethyl-fluorene (DMFL-CBP), 4,4′-bis(carbazole-9-yl)-9,9-bisbis(9-phenyl-9H-carbazole)fluorene (FL-4CBP), 4,4′-bis(carbazole-9-yl)-9,9-di-tolyl-fluorene (DPFL-CBP), and 9,9-bis(9-phenyl-9H-carbazole)fluorene (FL-2CBP). Examples of the dopant material include 4,4′-bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi), 9,10-di(naph-2-tyl)anthracene (ADN), and 3-tert-butyl-9,10-di(naph-2-tyl)anthracene (TBADN). 
     As shown in the exemplary embodiment of  FIG. 3 , the second pixel defining layer  160  is positioned on the first region  140   a  on the first pixel defining layer  130 . The second pixel defining layer  160  thus overlaps the first pixel defining layer  130 . In addition, the second pixel defining layer  160  is not positioned on the second region  140   b . When the first region  140   a  has relatively high wettability and the second region  140   b  has relatively low wettability, a difference in the wettability may selectively define a location where a coating liquid for forming the second pixel defining layer  160  is applied. For example, when the second pixel defining layer  160  is formed by nozzle printing, it may be selectively formed only on the first region  140   a  on the first pixel defining layer  130  while not being formed on the second region  140   b  adjacent to this portion of the first region  140   a.    
     In addition, while the second pixel defining layer  160  is related to achieving a particular pixel defining layer thickness, its contribution is less significant, as will be described later. Accordingly, the second pixel defining layer  160  may be less sensitive to patterning accuracy or to film thickness uniformity. Therefore, even if the second pixel defining layer  160  is formed by inkjet printing, display quality may be less affected by the second pixel defining layer  160 . As described above, even if the second pixel defining layer  160  is formed by inkjet printing, an ink jet location can be easily and selectively defined by the wettability difference between the first region  140   a  and the second region  140   b , like when the second pixel defining layer  160  is formed by nozzle printing. 
     The second pixel defining layer  160  may include at least one organic material selected from the group consisting of benzocyclobutene (BCB), polyimide (PI), polyamaide (PA), acryl resin, and phenol resin. In some embodiments, the second pixel defining layer  160  is made of the same material as the first pixel defining layer  130 . In other embodiments, however, the second pixel defining layer  160  is made of a different material from the first pixel defining layer  130 . 
     Since the second pixel defining layer  160  is formed on the first pixel defining layer  130 , a total thickness of the overall pixel defining layer including the first pixel defining layer  130  and the second pixel defining layer  160  may increase from embodiments where only a single pixel defining layer is used. As described above, the thickness of the first pixel defining layer  130  may be limited (for example, no greater than 1 μm) due to the negative effects on the film thickness uniformity of the first medium layer  140 . Since the second pixel defining layer  160  is formed on the first pixel defining layer  130 , however, a sufficiently large thickness of the pixel defining layer can be achieved without impairing the film thickness uniformity of the first medium layer  140 . For example, a sum of thicknesses of the first pixel defining layer  130  and the second pixel defining layer  160  may be greater than 1 μm. 
     As described above, if the thickness of the overall pixel defining layer is increased, pixels may no longer be pressed (for example, directly pressed) when an encapsulating substrate is pressed toward the pixels, thereby suppressing dark spots from being caused. 
     As shown in  FIG. 3 , the second pixel defining layer  160  is not formed in the opening defined by the first pixel defining layer  130 . Accordingly, when the second pixel defining layer  160  is formed after the light emission layer  150 , the light emission layer  150  is exposed without being covered by the second pixel defining layer  160 . However, in the described method of fabricating the OLED display  100  below, the second pixel defining layer  160  is formed before the light emission layer  150 . A second medium layer  170  is formed on the exposed light emission layer  150 . The second medium layer  170  helps injection or transport of electrons or holes between the second electrode  180  and the light emission layer  150 . When the second electrode  180  is a cathode electrode, the second medium layer  170  is associated with electron injection or transport. 
     For example, the second medium layer  170  may be formed as a single layer of an electron transport layer or an electron injection layer, or as a stacked layer of an electron transport layer and an electron injection layer. The electron transport layer may include, for example, Alq3. The electron injection layer may include, for example, LiF, NaCl, CsF, Li2O, BaO, Liq, or the like. 
     As shown in the exemplary embodiment of  FIG. 3 , the second medium layer  170  extends to a lateral surface of the first pixel defining layer  130 , a lateral surface of the second pixel defining layer  160 , and a top surface of the second pixel defining layer  160 . In other embodiments, the second medium layer  170  may be separated by each pixel. As shown in  FIG. 3 , the second medium layer  170  is integrally formed over the entire surface of the OLED display  100 . Thus, the second medium layer  170  is a common layer and not related with pixel distinction. In some embodiments, the second medium layer  170  may not be provided. 
     The second electrode  180  is formed on the second pixel defining layer  160  (for example, on the second medium layer  170 , as shown in  FIG. 3 ). When the second electrode  180  is used as the cathode electrode, it may be made of a conductive material having a low work function. For example, the second electrode  180  may be made of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, or Ca. 
     In  FIG. 3 , an encapsulating substrate  190  is positioned on the second electrode  180 . The encapsulating substrate  190  may be an insulating substrate. A spacer may be disposed between the second electrode  180  on the second pixel defining layer  160  and the encapsulating substrate  190 . In other embodiments of the present invention, the encapsulating substrate  190  may not be provided. In this case, an encapsulating film made of an insulating material may cover and protect the entire structure. 
     Hereinafter, a method of fabricating the aforementioned OLED display will be described.  FIGS. 4 to 9  are cross-sectional views illustrating process steps of a method of fabricating the OLED display shown in  FIGS. 2 and 3 . 
     Referring first to  FIG. 4 , the first electrode  120  is formed on the substrate  110 . The first electrode  120  may be formed by depositing a conductive material and then patterning the same by a photolithography process. In other embodiments, the first electrode  120  may be formed by printing. 
     Referring to  FIG. 5 , the first pixel defining layer  130  having an opening is formed on the substrate  110  having the first electrode  120 . The first pixel defining layer  130  may be formed by photolithography or printing, such as nozzle printing or inkjet printing. In order to secure film thickness uniformity of the first medium layer  140  to be deposited later, the first pixel defining layer  130  may be formed to a thickness of about 1 μm or less. 
     Referring to  FIG. 6 , a coating film  140   p  for forming a first medium layer is formed on the entire surface of the structure shown in  FIG. 5 . The first medium layer forming coating film  140   p  may be formed by slit coating. When the first medium layer forming coating film  140   p  is a stacked film of two or more materials, the respective materials are sequentially coated. 
     Referring to  FIG. 7 , the first medium layer forming coating film  140   p  is selectively subjected to surface treatment. The first medium layer forming coating film  140   p  will be described as having the same property as the first region  140   a  (e.g., relatively high wettability) and then a portion of the first medium layer forming coating film  140   p  is modified to have the same property as the second region  140   b  (e.g., relatively low wettability) with UV radiation. 
     An optical mask  200  including a light-transmitting portion  210  and a light-shielding portion  220  is placed on the structure shown in  FIG. 6  and UV rays are then radiated. Here, the first medium layer forming coating film  140   p  disposed on the first electrode  120  and the first medium layer forming coating film  140   p  disposed on the top surface of the first pixel defining layer  130  correspond to the light-shielding portion  220  while the lateral or tilted surface of the first pixel defining layer  130  corresponds to the light-transmitting portion  210 . The UV rays transmitted through the light-transmitting portion  210  of the optical mask  200  then modify a surface of the first medium layer forming coating film  140   p . Thus, a region of the first medium layer forming coating film  140   p  corresponding to the light-shielding portion  220  remains as the first region  140   a , while a region of the first medium layer forming coating film  140   p  corresponding to the light-transmitting portion  210  is modified to the second region  140   b . Accordingly, the first medium layer  140  including the first region  140   a  and the second region  140   b  is completed. 
     In some other embodiments of the present invention, the first medium layer forming coating film  140   p  has the same property as the second region  140   b  (e.g., relatively low wettability) and is then modified to have the same property as the first region  140   a  (e.g., relatively high wettability) with UV radiation. In this case, the arrangement of the light-shielding portion  220  and the light-transmitting portion  210  is reversed in the optical mask  200 . 
     Referring to  FIG. 8 , the second pixel defining layer  160  is formed on the first region  140   a  on the first pixel defining layer  130 . The second pixel defining layer  160  may be printed, for example, using nozzle printing or inkjet printing. During printing, a coating liquid for forming the second pixel defining layer  160  may be directed toward the second region  140   b  adjacent to the first region  140   a  on the first pixel defining layer  130 . However, since the second region  140   b  has low wettability, the coating liquid may move toward the first region  140   a  on the first pixel defining layer  130 . Therefore, a desired pattern of the second pixel defining layer  160  can be accurately implemented. 
     The second pixel defining layer  160  may also be formed by inkjet printing. Since inkjet printing is advantageous in forming various patterns, it may be selected for forming the second pixel defining layer  160  in place of nozzle printing. During inkjet printing, it is possible to reduce or prevent an unwanted pattern from being formed in the second region  140   b  due to a wettability difference between the first region  140   a  and the second region  140   b.    
     Referring to  FIG. 9 , the light emission layer  150  is formed by printing an organic light emission material on the first region  140   a  on the first electrode  120 . The printing may be, for example, nozzle printing. During printing, a coating liquid for forming the light emission layer  150  may be partially directed toward the second region  140   b  adjacent to the first region  140   a . However, since the second region  140   b  has low wettability, this portion of the coating liquid may move toward the first region  140   a  on the first electrode  120 . In addition, since the first pixel defining layer  130  and the second pixel defining layer  160  may be used as printing barriers, patterning accuracy of the light emission layer  150  can be improved. 
     Referring back to  FIG. 3 , the second medium layer  170  is formed by depositing a material for forming the second medium layer  170  using an open mask. The second electrode  180  is formed by depositing a conductive film on the second medium layer  170 . The encapsulating substrate  190  is disposed on the second electrode  180  (for example, with a spacer on the second electrode  180  on the second pixel defining layer  160  to separate the encapsulating substrate  190  from the second electrode  180 ), thereby completing the OLED display  100  shown in  FIG. 3 . 
     Hereinafter, another embodiment of the present invention will be described. In the following embodiment, the same components as those of the previous embodiment will not be described or may only briefly be described. 
       FIG. 10  is a layout view of an OLED display  101  according to another embodiment of the present invention.  FIG. 11  is a cross-sectional view of the OLED display  101  shown in  FIG. 10 . 
     Referring to  FIGS. 10 and 11 , the OLED display  101  is different from the OLED display  100  shown in  FIGS. 2 and 3  in that a pixel defining layer  131  is formed of a single layer without being separated into two layers and is formed on a first medium layer  141 . The first medium layer  141  includes a first region  141   b  and a second region  141   a , with the first region  141   b  corresponding to the pixel defining layer  131  and a light emission layer  150  having corresponding portions on the substrate  110  and the first electrode  120 . 
     In more detail, the first medium layer  141  is formed on (for example, directly formed on) a substrate  110  having a first electrode  120  without a pixel defining layer interposed between the first medium layer  141  and the substrate  110 . Like in the embodiment shown in  FIG. 3 , the first medium layer  141  includes the first region  141   b  and the second region  141   a . However, unlike in the embodiment shown in  FIG. 3 , the first region  141   b  is positioned on the first electrode  120  (in a region where a bottom surface of the light emission layer  150  is positioned) and on the substrate  110  (in a region where a bottom surface of the pixel defining layer  131  is positioned) Further, the second region  141   a  is positioned in a space on the first electrode  120  and between the light emission layer  150  and the pixel defining layer  131 . In the embodiment shown in  FIG. 3 , the first medium layer  140  does not overlap the second pixel defining layer  160  but does overlap the first pixel defining layer  130 . However, in the present embodiment of  FIG. 11 , the first medium layer  141  does not overlap the pixel defining layer  131 . 
     The light emission layer  150  is positioned on the first region  141   b  on the first electrode  120 , and the pixel defining layer  131  is positioned on the first region  141   b  on the substrate  110 . In the illustrated embodiment of  FIG. 11 , the second region  141   a  is positioned in part on the first electrode  120 , but aspects of the present invention are not limited thereto. A second medium layer  171  and a second electrode  181  are sequentially formed on the light emission layer  150  and the pixel defining layer  131 . 
     In the present embodiment of  FIG. 11 , since the first medium layer  141  is not formed on the pixel defining layer  131  but is instead formed on (for example, directly formed on) the substrate  110  having the first electrode  120 , a thickness of the pixel defining layer  131  does not affect film thickness uniformity of the first medium layer  141 . Rather, since the pixel defining layer  131  is not provided between the first medium layer  141  and the substrate  110 , planarity of the underlying structure is improved, thereby improving film thickness uniformity of the first medium layer  141 . 
     Further, a thickness of the pixel defining layer  131  may be adjusted to have a sufficiently large thickness without having to adjust the thickness to 1 μm or less. For example, the thickness of the pixel defining layer  131  may be greater than 1 μm. Therefore, when an encapsulating substrate is pressed toward pixels, the pixels may avoid being pressed (for example, directly pressed), thereby suppressing dark spots from being caused to the pixels. 
     Hereinafter, a method of fabricating the OLED display  101  will be described.  FIGS. 12 to 15  are cross-sectional views illustrating process steps of a method of fabricating the OLED display  101  shown in  FIGS. 10 and 11 . 
     In the present embodiment, process steps up to the step of forming the first electrode  120  on the substrate  110  are the same as those of the embodiment shown in  FIG. 4 . Next, referring to  FIG. 12 , a coating film  141   p  for forming a first medium layer  141  is formed. The first medium layer forming coating film  141   p  may be formed by slit coating. When the first medium layer forming coating film  141   p  is a stacked film of two or more materials, the respective materials are sequentially coated. 
     Next, referring to  FIG. 13 , the first medium layer forming coating film  141   p  is selectively subjected to surface treatment. The first medium layer forming coating film  141   p  will be described as having the same property as the first region  141   b  (e.g., relatively high wettability) and is then modified to have the same property as the second region  141   a  (e.g., relatively low wettability) with UV radiation. 
     An optical mask  201  including a light-transmitting portion  211  and a light-shielding portion  221  is placed on the structure shown in  FIG. 12  and UV rays are then radiated. Here, the light-shielding portion  221  is placed to correspond to locations of a first electrode  120  and a pixel defining layer  131 , while the light-transmitting portion  211  is placed to correspond to spaces between the first electrode  120  and the pixel defining layer  131 . The UV rays transmitted through the light-transmitting portion  211  of the optical mask  201  then modify a surface of the first medium layer forming coating film  141   p . Thus, a region of the first medium layer forming coating film  141   p  corresponding to the light-shielding portion  221  remains as the first region  141   b , while a region of the first medium layer forming coating film  141   p  corresponding to the light-transmitting portion  211  is modified to the second region  141   a . Accordingly, the first medium layer  141  including the first region  141   b  and the second region  141   a  is completed. 
     In some other embodiments of the present invention, the first medium layer forming coating film  141   p  has the same property as the second region  141   a  (e.g., relatively low wettability) and is then modified to have the same property as the first region  141   b  (e.g., relatively high wettability) with UV radiation. In this case, the arrangement of the light-shielding portion  221  and the light-transmitting portion  211  is reversed in the optical mask  201 . 
     Referring to  FIG. 14 , the pixel defining layer  131  is formed on the first region  141   b  on the substrate  110 . The pixel defining layer  131  may be printed, for example, using nozzle printing or inkjet printing. 
     Referring to  FIG. 15 , the light emission layer  150  is formed by printing an organic light emission material on the first region  141   b  on the first electrode  120 . Here, the pixel defining layer  131  may be used as a printing barrier. Next, referring to  FIG. 11 , a second medium layer  171  and a second electrode  181  are formed and an encapsulating substrate  190  is then positioned, thereby completing the OLED display  101  shown in  FIG. 11 . 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims, and their equivalents. The above embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims and their equivalents, rather than the foregoing description, to indicate the scope of the present invention.