Patent Publication Number: US-2015069357-A1

Title: Display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Korean Patent Application No. 10-2013-0107336, filed on Sep. 6, 2013, in the Korean Intellectual Property Office, and entitled: “Display Device,” which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     Embodiments are directed to a display device. 
     2. Description of the Related Art 
     Display devices may include a liquid crystal display, an electrophoretic display panel, an organic light emitting display, an electroluminescent display, a FED (Field Emission Display), an SED (Surface-conduction Electron-emitter Display), a plasma display, or a CRT (Cathode Ray Tube) display. 
     SUMMARY 
     A display device according to exemplary embodiments may include a first electrode, an organic light emitting layer on the first electrode, a second electrode on the organic light emitting layer, and an electron transport layer between the organic light emitting layer and the second electrode, and including an electron transport material and an electron injection material. One side portion of the electron transport layer that is adjacent to the organic light emitting layer may include a greater amount of the electron injection material than the electron transport material. 
     An other side portion of the electron transport layer that faces the one side portion of the electron transport layer may include a greater amount of the electron transport material than the electron injection material. 
     The electron transport material may include at least one of a pyrene series material, a triazine series material, and an anthracene series material, and the electron injection material may include at least one of LiF, LiQ, and NaQ. 
     The display device may also include an electron injection layer between the electron transport layer and the second electrode. The electron injection layer may include the electron injection material. 
     The electron transport layer may include a first electron transport layer that is adjacent to the organic light emitting layer and a second electron transport layer on the first electron transport layer. A volume ratio between the electron transport material and the electron injection material in the first electron transport layer may range from about 1:9 to about 5:5, and a volume ratio between the electron transport material and the electron injection material in the second electron transport layer may range from about 5:5 to about 9:1. 
     The electron transport layer may include an intermediate layer between the first electron transport layer and the second electron transport layer, and a ratio of the electron injection material to the electron transport material in the intermediate layer may increase in a direction towards the organic light emitting layer. 
     The electron transport layer may include a first electron transport layer that is adjacent to the organic light emitting layer, a second electron transport layer on the first electron transport layer, and a third transport layer on the second electron transport layer. A volume ratio between the electron transport material and the electron injection material in the first electron transport layer may range from about 1:9 to about 3:7, a volume ratio between the electron transport material and the electron injection material in the second electron transport layer may range from about 3:7 to about 7:3, and a volume ratio between the electron transport material and the electron injection material in the third electron transport layer may range from about 7:3 to 9:1. 
     A ratio of the electron injection material to the electron transport material in the electron transport layer may increase in a direction towards the organic light emitting layer. 
     The organic light emitting layer may include a first organic light emitting layer and a second organic light emitting layer on the first organic light emitting layer, the electron transport layer may include a first electron transport layer on the first organic light emitting layer and a second electron transport layer on the second organic light emitting layer, and one side portion of the first electron transport layer that is adjacent to the first organic light emitting layer and one side portion of the second electron transport layer that is adjacent to the second organic light emitting layer may each include a greater amount of the electron injection material than the electron transport material. 
     The display device according to exemplary embodiments may also include a charge generation layer between the first electron transport layer and the second organic light emitting layer. 
     The electron transport layer may be formed by vacuum-depositing the electron transport material and the electron injection material. 
     A display device according to exemplary embodiments may include a first electrode, an organic light emitting layer on the first electrode, a second electrode on the organic light emitting layer, and a hole transport layer between the first electrode and the organic light emitting layer, and including a hole transport material and a hole injection material. One side portion of the hole transport layer that is adjacent to the organic light emitting layer may include a greater amount of the hole injection material than the hole transport material. 
     An other side portion of the hole transport layer that faces the one side portion of the hole transport layer may include a greater amount of the hole transport material than the hole injection material. 
     The display device according to exemplary embodiments may also include a hole injection layer between the first electrode and the hole transport layer. The hole injection layer may also include the hole injection material. 
     A ratio of the hole injection material to the hole transport material in the hole transport layer may increase in a direction towards the organic light emitting layer. 
     A display device according to exemplary embodiments may include a substrate including a first region and a second region, a hole transport layer on the substrate, and an electron transport layer on the hole transport layer. The electron transport layer may include an electron transport material and an electron injection material, and a portion of the electron transport layer that is adjacent to the hole transport layer may include a greater amount of the electron injection material than the electron transport material. The hole transport layer and the electron transport layer may cover an entire surface of the substrate, and the hole transport layer and the electron transport layer may be in direct contact with each other in the second region and may not be in direct contact with each other in the first region. 
     A portion of the electron transport layer which faces the portion that is adjacent to the hole transport layer may include a greater amount of the electron transport material than the electron injection material. 
     A display device according to exemplary embodiments may include an organic light emitting layer in the first region between the hole transport layer and the electron transport layer. 
     A display device according to exemplary embodiments may include a pixel-defining film in the second region between the substrate and the hole transport layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of ordinary skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates a cross-sectional view of a display device according to an embodiment; 
         FIG. 2  illustrates an enlarged cross-sectional view of an upper portion (A portion) of a first region of the display device of  FIG. 1 ; 
         FIG. 3  illustrates an enlarged cross-sectional view of an upper portion (B portion) of a second region of the display device of  FIG. 1 ; 
         FIG. 4  illustrates an enlarged cross-sectional view of an upper portion of a first region of a display device according to another embodiment; 
         FIG. 5  illustrates an enlarged cross-sectional view of an upper portion of a second region of the display device of  FIG. 4 ; 
         FIG. 6  illustrates an enlarged cross-sectional view of an upper portion of a first region of a display device according to still another embodiment; 
         FIG. 7  illustrates an enlarged cross-sectional view of an upper portion of a second region of the display device of  FIG. 6 ; 
         FIG. 8  illustrates an enlarged cross-sectional view of an upper portion of a first region of a display device according to still another embodiment; 
         FIG. 9  illustrates an enlarged cross-sectional view of an upper portion of a second region of the display device of  FIG. 8 ; 
         FIG. 10  illustrates an enlarged cross-sectional view of an upper portion of a first region of a display device according to still another embodiment; 
         FIG. 11  illustrates an enlarged cross-sectional view of an upper portion of a second region of the display device of  FIG. 10 ; and 
         FIG. 12  illustrates an enlarged cross-sectional view of an upper portion of a first region of a display device according to still another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may 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 exemplary implementations to those skilled in the art. 
     In the drawing figures, the dimensions of layers and regions may be exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout. 
     In some embodiments, well-known structures and devices are not shown in order not to obscure the description of the exemplary embodiments with unnecessary detail. 
     It will be understood that when an element or layer is referred to as being “on,” or “connected to” another element or layer, it can be directly on or connected to the other element or layer or intervening elements or layers may 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. It will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “below,” “beneath,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
     Embodiments described herein will be described referring to plan views and/or cross-sectional views by way of ideal schematic views of exemplary embodiments. Accordingly, the exemplary views may be modified depending on manufacturing technologies and/or tolerances. Therefore, the exemplary embodiments are not limited to those shown in the views, but include modifications in configuration formed on the basis of manufacturing processes. Therefore, regions exemplified in figures have schematic properties and shapes of regions shown in figures exemplify specific shapes of regions of elements and not limit aspects of exemplary embodiments. 
     A “display device” that is described in the description may be one of various display devices. In an exemplary embodiment, the display device described in the description may be any one of a liquid crystal display, an electrophoretic display panel, an organic light emitting display, an electroluminescent display, a FED (Field Emission Display), an SED (Surface-conduction Electron-emitter Display), a plasma display, and a CRT (Cathode Ray Tube) display, but is not limited thereto. Hereinafter, an organic light emitting display is described as an example of the display device according to exemplary embodiments. However, the display device according to exemplary embodiments is not limited thereto, and various kinds of display devices may be used. 
     Hereinafter, embodiments will be described with reference to the accompanying drawings. 
       FIG. 1  illustrates a cross-sectional view of a display device according to an embodiment.  FIG. 2  illustrates an enlarged cross-sectional view of an upper portion (A portion) of a first region I of the display device of  FIG. 1 , and  FIG. 3  illustrates an enlarged cross-sectional view of an upper portion (B portion) of a second region II of the display device of  FIG. 1 . Referring to  FIGS. 1 to 3 , a display device according to an embodiment may include a substrate  100 , a first electrode  120 , a pixel-defining film  140 , a hole injection layer  160 , a hole transport layer  180 , an organic light emitting layer  200 , an electron transport layer  220 , an electron injection layer  240 , a second electrode  260 , and a cover layer  280 . 
     The substrate  100  may include an insulating substrate. The insulating substrate may be formed of a transparent glass material having SiO 2  as a main component. In some embodiments, the insulating substrate may be made of an opaque material. 
     The substrate  100  may include a flexible substrate that can change shape, such as rolling, folding, and bending. The flexible substrate may be made of a plastic material having superior heat resistance and durability, such as polyarylate, polyetherimide, polyethersulfone, or polyimide. However, embodiments are not limited thereto, and various flexible materials may be used. 
     Although not illustrated in the drawing, the substrate  100  may further include other structures formed on the insulating substrate. Examples of other structures may be a wiring, an electrode, an insulating film, and the like. In the case where the display device according to this embodiment is an active organic light emitting display, the substrate  100  may include a plurality of thin film transistors that are formed on the insulating substrate. The thin film transistor may include a gate electrode, a source electrode, a drain electrode, and a semiconductor layer that is a channel region. The semiconductor layer may be formed of amorphous silicon, fine crystalline silicon, polycrystalline silicon, or monocrystalline silicon. In alternative embodiments, the semiconductor layer may be made of oxide semiconductor. The drain electrode of at least a part of the plurality of thin film transistors may be electrically connected to the first electrode  120 . 
     The substrate  100  may include a plurality of regions. Such a plurality of regions may include a first region I and a second region II. In an exemplary embodiment, the first region I may be a light emitting region that emits light in the display device, and the second region II may be a non-light emitting region that does not emit light in the display device. In another exemplary embodiment, the first region I may be a region in which the first electrode  120  is positioned, and the second region II may be a region in which the pixel-defining film  140  is positioned. In still another exemplary embodiment, the first region I may be a region in which the organic light emitting layer  200  is positioned, and the second region II may be a region in which the organic light emitting layer  200  is not positioned. In still another exemplary embodiment, the first region I may be a region in which the hole transport layer  180  and the electron transport layer  220  do not mutually come in direct contact with each other, and the second region II may be a region in which the hole transport layer  180  and the electron transport layer  220  mutually come in direct contact with each other. Although not illustrated in the drawing, on a plan view, the second region II may have a lattice shape, and the first region I may be a region surrounded by the second region II. 
     The first electrode  120  may be positioned on the substrate  100 . In an exemplary embodiment, the first electrode  120  may be positioned on the first region I of the substrate  100 . The first electrode  120  may be formed to come in direct contact with the substrate  100 , or a material, such as an insulating film, and may be interposed between the first electrode  120  and the substrate  100 . 
     The first electrode  120  may be an anode electrode or a cathode electrode. If the first electrode  120  is an anode electrode, the second electrode  260  is a cathode electrode. Hereinafter, embodiments will be described with a first electrode  120  as an anode electrode. However, the first electrode  120  may also be a cathode electrode, and the second electrode  260  may be an anode electrode. 
     The first electrode  120  that is used as an anode electrode may be made of a conductive material having high work function. In the case where the organic light emitting display is a bottom emission display device, the first electrode  120  may be formed of ITO, IZO, ZnO, In 2 O 3 , or a laminated film thereof. In the case where the organic light emitting display is a top emission display device, the first electrode may further include a reflective film that is formed of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, Ca, or a combination thereof. The first electrode  120  may include various modifications, such as a two or more layer structure using two or more of the above-described materials. 
     The pixel-defining film  140  may be positioned on the substrate  100 . In an exemplary embodiment, the pixel-defining film  140  may be positioned on the second region II of the substrate  100 . Further, the pixel-defining film  140  may be formed to make direct contact with the substrate  100 , or a material, such as an insulating film, and may be interposed between the pixel-defining film  140  and the substrate  100 . Further, the pixel-defining film  140  may include an opening for exposing a region in which a pixel is to be formed. The opening may be positioned on the first region I. Further, the pixel-defining film  140  may be thicker than the first electrode  120 . 
     The pixel-defining film  140  may include at least one organic material selected from the group including benzo cyclo butene (BCB), polyimide (PI), poly amaide (PA), acrylic resin, and phenol resin, or may include an inorganic material, such as silicon nitride. 
     The hole injection layer  160  may be positioned on the first electrode  120  and the pixel-defining film  140 . That is, the hole injection layer  160  may be separated by pixels, or may be formed to cover the whole surface of the substrate  100  as illustrated in  FIG. 1 . The hole injection layer  160  may be a common layer that is commonly formed on the first region I and the second region II. In some embodiments, the hole injection layer  160  may be omitted. 
     The hole injection layer  160  may include a hole injection material. The hole injection material may be selected from known hole injection materials. For example, the hole injection material may include TCTA or m-MTDATA, which is a phthalocyanine compound, such as copper phthalocyanine, or starburst amine derivatives, Pani/DBSA (Polyaniline/Dodecylbenzenesulfonic acid) or PEDOT/PSS (Poly(3,4-ethylenedioxythiophene)/Poly(4-styrenesulfonate)), which is a conductive polymer, Pani/CSA (Polyaniline Camphor sulfonic acid), or PANI/PSS (Polyaniline)/Poly(4-styrent-sulfonate), but the hole injection material is not limited thereto. 
     The hole transport layer  180  may be positioned on the hole injection layer  160 . That is, the hole transport layer  180  may be separated by pixels, or as illustrated in  FIG. 1 , may be formed to cover the whole surface of the substrate  100 . That is, the hole transport layer  180  may be a common layer that is commonly formed in the first region I and the second region II. 
     The hole transport layer  180  may include a hole transport material. The hole transport layer  180  may be selected from known hole transport materials. For example, the hole transport material may include 1,3,5-tri(carbazolyl)benzene, 4,4′-bis(carbazolyl)biphenyl, polyvinylcarbazol, m-bis(carbazolyl)phenyl, 4,4′-bis(carbazolyl)-2,2′-dimethylbiphenyl, 4,4′,4″-tri(N-carbazolyl)triphenylamine,
     1,3,5-tri(2-carbazolylphenyl)benzene, 1,3,5-tris(2-carbazolyl-5-methoxyphenyl)benzene, bis(4-carbazolylphenyl)silane, N,N′-bis(3-methyphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′ diamine (TPD), N,N′-di(naphthalene-1-i1)-N,N′-dephenylbenzidine (NPD), N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB), poly(9,9-dioctylfluorene-co-N-(4-buthylphenyl)diphenylamine) (TFB), or poly(9,9-dioctylfluorene-co-bis-(4-buthylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB), but is not limited thereto.   

     The hole injection layer  160  or the hole transport layer  180  may be formed using various methods, such as a vacuum deposition method, a spin coating method, a cast method, and an LB method. In the case where the hole injection layer  160  or the hole transport layer  180  is formed by a vacuum deposition method, the deposition conditions may differ depending on the compounds used as materials of the hole injection layer  160  or the hole transport layer  180 , the structure and the thermal properties of the hole injection layer  160  or the hole transport layer  180  that is targeted. For example, a deposition temperature of 100 to 500° C., a vacuum of 10 −8  to 10 −3  torr, and a deposition speed of 0.01 to 100 Å/sec, may be selected. 
     The organic light emitting layer  200  may be positioned on the hole transport layer  180 . In an exemplary embodiment, the organic light emitting layer  200  may be formed on the hole transport layer  180 , which is positioned on the first region I of the substrate  100 . Further, the organic light emitting layer  200  may not be formed on the hole transport layer  180  that is positioned on the second region II of the substrate  100 . Further, the organic light emitting layer  200  may completely overlap the first electrode  120 . Further, an edge of the organic light emitting layer  200  may be positioned on an edge of the pixel-defining film  140 . 
     The organic light emitting layer  200  may emit light of a specific color. Specifically, holes and electrons, which are respectively generated by the first electrode  120  and the second electrode  260  in the organic light emitting layer  200 , may be combined to form excitons, and the organic light emitting layer  200  may emit light having a color that corresponds to the energy level that is changed when the excitons shift from an excited state to a ground state. 
     The organic light emitting layer  200  may include a red organic light emitting layer that emits red light, a green organic light emitting layer that emits green light, and a blue organic light emitting layer that emits blue light. Further, the organic light emitting layer  200  may include a white organic light emitting layer that emits white light. 
     The red organic layer may be made of a high-molecular or low-molecular organic material, of which the inherent light emitting color is red, or a high-molecular/low-molecular mixed material. In some embodiments, the red organic light emitting layer may include a red host material and a red dopant material. The red host material may be one or more selected from the group including bis(2-(2-hydroxyphenyl)benzothiazolato) zinc (Zn(BTZ)2) and bis-(2-methyl-8-quinolinolate)-4-(phenylphenolate)aluminum, but is not limited thereto. Further, the red dopant material may include Pt0EP, Ir(piq) 3 , Btp 2 Ir(acac), and DCJTB, but is not limited thereto. 
     The green organic light emitting layer may be made of a high-molecular or low-molecular organic material, of which the inherent light emitting color is green, or a high-molecular/low-molecular mixed material. In some embodiments, the green organic light emitting layer may include a green host material and a green dopant material. The green host material may be one or more selected from the group including anthracene derivatives and compounds in the carbazole series, but is not limited thereto. As the anthracene derivatives, 9,10-(2-dinaphtyl)anthracene (ADN), or the like, may be used, and as the compounds in the carbazole series, 4,4′-(carbazole-9-i1)biphenyl (CBP), or the like, may be used. Further, the green dopant material may include Ir(ppy) 3  (ppy=phenylpyridine), Ir(ppy) 2 (acac), Ir(mpyp) 3 , or C545T, but is not limited thereto. 
     The blue organic light emitting layer may be made of a high-molecular or low-molecular organic material, of which the inherent light emitting color is blue, or a high-molecular/low-molecular mixed material. In some embodiments, the blue organic light emitting layer may include a blue host material and a blue dopant material. Here, the blue host material may be one or more selected from the group including anthracene derivatives and compounds in the carbazole series, but is not limited thereto. Here, as the anthracene derivatives, 9,10-(2-dinaphtyl)anthracene (ADN), and the like, may be used, and as the compounds in the carbazole series, 4,4′-(carbazole-9-i1)biphenyl (CBP), and the like, may be used. Further, the blue dopant material may include F 2 Irpic, (F 2  ppy) 2 Ir(tmd), Ir(dfppz) 3 , or ter-fluorene, but is not limited thereto. 
     The electron transport layer  220  may be positioned on the organic light emitting layer  200  and the hole transport layer  180 . That is, the electron transport layer  220  may be separated by pixels, or may be formed to cover the whole surface of the substrate  100  as illustrated in  FIG. 1 . That is, the electron transport layer  220  may be a common layer that is commonly formed on the first region I and the second region II. 
     The electron transport layer  220  may include an electron transport material. The electron transport material may be selected from known electron transport materials. For example, the electron transport material may include at least one of a pyrene series material, a triazine series material, and an anthracene series material, but is not limited thereto. As another example, the electron transport material may include quinoline derivatives, and in particular, tris(8-quinolinolate)aluminum (Alq3), TAZ, or Balq, but is not limited thereto. 
     The electron transport layer  220  may include not only the above-described electron transport material but also an electron injection material. The electron injection material may be selected from known electron injection materials. For example, the electron injection material may include at least one of LiF, LiQ, and NaQ, but is not limited thereto. As another example, the electron injection material may include NaCl, CsF, Li 2 O, or BaO, but is not limited thereto. 
     One side portion of the electron transport layer  220  that is adjacent to the organic light emitting layer  200  may include a greater amount of the electron injection material than the electron transport material. Further, the other side portion of the electron transport layer  220  that faces the one side portion of the electron transport layer  220  may include a greater amount of the electron transport material than the electron injection material. 
     The electron transport layer  220  may include a first electron transport layer  220   a  that is adjacent to the organic light emitting layer  200  and a second electron transport layer  220   b  that is positioned on the first electron transport layer  220   a . In an exemplary embodiment, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  220   a  may be about 1:9 to about 5:5, and the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  220   b  may be about 5:5 to about 9:1. In another exemplary embodiment, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  220   a  may be about 3:7 to about 4:6, and the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  220   b  may be about 7:3 to about 6:4. In still another exemplary embodiment, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  220   a  may be about 4:6, and the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  220   b  may be about 6:4. That is, although the overall volume ratio between the electron transport material and the electron injection material in the electron transport layer  220  may be about 5:5, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  220   a  and the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  220   b  may not be 5:5. 
     The electron transport layer  220  may be formed by vacuum-depositing the electron transport material and the electron injection material. In an exemplary embodiment, the electron transport layer  220  may be formed by adjusting a speed per unit time, at which the electron transport material is put into a deposition chamber and a speed per unit time, at which the electron injection material is put into the deposition chamber. For example, the first electron transport layer  220   a , in which the volume ratio of the electron transport material to the electron injection material is about 4:6, may be formed by putting the electron transport material into the deposition chamber so that the electron transport material is deposited with a thickness of about 4 Å per second, and putting the electron injection material into the deposition chamber so that the electron injection material is deposited with a thickness of about 6 Å per second. Further, the second electron transport layer  220   b , in which the volume ratio of the electron transport material to the electron injection material is about 6:4, may be formed by putting the electron transport material into the deposition chamber so that the electron transport material is deposited with a thickness of about 6 Å per second, and putting the electron injection material into the deposition chamber so that the electron injection material is deposited with a thickness of about 4 Å per second. 
     When the electron transport layer  220  is formed through vacuum deposition, the first electron transport layer  220   a  and the second electron transport layer  220   b  may be separately formed in separate vacuum deposition processes. In an exemplary embodiment, the first electron transport layer  220   a  may be formed in the first deposition chamber, and the second electron transport layer  220   b  may be formed in the second deposition chamber that is different from the first deposition chamber. In another exemplary embodiment, the first electron transport layer  220   a  and the second electron transport layer  220   b  may be formed in the same deposition chamber, and in this case, a process of initializing the deposition chamber may be provided between a process of forming the first electron transport layer  220   a  and a process of forming the second electron transport layer  220   b . However, embodiments are not limited thereto, and the first electron transport layer  220   a  and the second electron transport layer  220   b  may be consecutively formed in the same chamber. 
     Referring to  FIGS. 1 and 2 , the electronic transport layer  220  may be positioned on the organic light emitting layer  200  provided on the first region I. Specifically, the electron transport layer  220  may come in direct contact with the organic light emitting layer  200  on the first region I, and may not come in direction contact with the hole transport layer  180 . 
     Referring to  FIGS. 1 and 3 , the electronic transport layer  220  may be positioned on the hole transport layer  180  provided on the second region II. Specifically, the electron transport layer  220  may come in direct contact with the hole transport layer  180 . In an exemplary embodiment, the first electron transport layer  220   a  may come in direct contact with hole transport layer  180  on the second region II. That is, a portion of the electron transport layer  220  that is adjacent to the hole transport layer  180  may include a greater amount of the electron injection material than the electron transport material. 
     The electron injection layer  240  may be positioned on the electron transport layer  220 . That is, the electron injection layer  240  may be separated by pixels, or may be formed to cover the whole surface of the substrate  100  as illustrated in  FIG. 1 . The electron injection layer  240  may be a common layer that is commonly formed on the first region I and the second region II. In some embodiments, the electron injection layer  240  may be omitted. 
     The electron injection layer  240  may include an electron injection material. The electron injection material may be selected from known electron injection materials. For example, the electron injection material may include at least one of LiF, LiQ, and NaQ, but is not limited thereto. As another example, the electron injection material may include NaCl, CsF, Li 2 O, or BaO, but is not limited thereto. The electron injection layer  240  may include a material that is different from the electron injection material that is included in the electron transport layer  220 . 
     The electron transport layer  220  or the electron injection layer  240  may be formed by various methods, such as a vacuum deposition method and a spin coating method. In the case where the electron transport layer  220  or the electron injection layer  240  are formed with the vacuum deposition method or the spin coating method, the deposition conditions and the coating conditions may differ depending on the compounds used, and in general, may be selected to be substantially the same as the conditions for forming the hole injection layer  160 . 
     The second electrode  260  may be positioned on the electron injection layer  240 . In the case where the second electrode  260  is a cathode electrode, it may be made of a conductive material having a low work function. The second electrode  260  may be formed of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, Li, Ca, or a combination thereof. 
     The cover layer  280  may be positioned on the second electrode  260 . The cover layer  280  may protect laminated films below the cover layer  280 . The cover layer  280  may be made of an insulating material. A spacer (not illustrated) may be arranged between the second electrode  260  and the cover layer  280 . In some embodiments, the cover layer  280  may be omitted. In this case, an encapsulation film that is made of an insulating material may cover the whole structure to protect the structure. 
     As described above, according to the display device of exemplary embodiments, when one side portion of the electron transport layer  220  that is adjacent to the organic light emitting layer  200  includes a greater amount of the electron injection material than the electron transport material, and the other side portion of the electron transport layer  220  that is adjacent to the second electrode  260  includes a greater amount of the electron transport material than the electron injection material, both the light emitting efficiency and the lifespan requirements of the display device may be satisfied. Specifically, when one side portion of the electron transport layer  220  that is adjacent to the organic light emitting layer  200  includes a greater amount of the electron injection material than the electron transport material, the stability of the display device can be improved, and when the other side portion of the electron transport layer  220  that is adjacent to the second electrode  260  includes a greater amount of the electron transport material than the electron injection material, the electron transport property can be improved. Accordingly, both the light emitting efficiency and the lifespan requirements of the display device can be satisfied. In other words, when one side portion of the electron transport layer  220  that is adjacent to the organic light emitting layer  200  includes a greater amount of the electron injection material than the electron transport material, and the other side portion of the electron transport layer  220  that is adjacent to the second electrode  260  includes a greater amount of the electron transport material than the electron injection material, the light emitting efficiency and the lifespan of the display device may be improved. 
     Further, according to exemplary embodiments of the display device, electrons or holes may be prevented from moving to a non-light emitting region that does not emit light, and thus the light emitting efficiency and the lifespan of the display device can be increased. If electrons or holes move to the second region II that does not emit light, the quantity of electrons or holes that move to the first region I that emits light may be relatively decreased, and thus the light emitting efficiency and the lifespan of the display device may be decreased. Accordingly, when the one side portion of the electron transport layer  220 , which is adjacent to the hole transport layer  180  on the second region II that does not emit light includes a greater amount of the electron injection material than the electron transport material, the stability of the display device may be improved. Thus, the electrons or holes may be prevented from moving to the second region II. 
       FIG. 4  illustrates an enlarged cross-sectional view of an upper portion of a first region I of a display device according to another embodiment, and  FIG. 5  illustrates an enlarged cross-sectional view of an upper portion of a second region II of the display device of  FIG. 4 . For convenience, the same reference numerals are used for substantially the same elements as the elements illustrated in  FIGS. 1 to 3 , and duplicate explanations thereof will be omitted. 
     Referring to  FIGS. 4 and 5 , an electron transport layer  221  may include a first electron transport layer  221   a  that is adjacent to the organic light emitting layer  200 , a second electronic transport layer  221   b  positioned on the first electron transport layer  221   a , and a third electron transport layer  221   c  positioned on the second electron transport layer  221   b . In an exemplary embodiment, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  221   a  may be about 1:9 to about 3:7, the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  221   b  may be about 3:7 to about 7:3, and the volume ratio of the electron transport material to the electron injection material in the third electron transport layer  221   c  may be about 7:3 to about 9:1. In another exemplary embodiment, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  221   a  may be about 2:8 to about 4:6, the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  221   b  may be about 4:6 to about 6:4, and the volume ratio of the electron transport material to the electron injection material in the third electron transport layer  221   c  may be about 6:4 to about 8:2. That is, although the overall volume ratio between the electron transport material and the electron injection material in the electron transport layer  221  may be about 5:5, the volume ratio of the electron transport material to the electron injection material in the first electron transport layer  221   a , the volume ratio of the electron transport material to the electron injection material in the second electron transport layer  221   b , and the volume ratio of the electron transport material to the electron injection material in the third electron transport layer  221   c  may not be 5:5. 
       FIG. 6  illustrates an enlarged cross-sectional view of an upper portion of a first region I of a display device according to still another embodiment, and  FIG. 7  illustrates an enlarged cross-sectional view of an upper portion of a second region II of the display device of  FIG. 6 . For convenience, the same reference numerals are used for substantially the same elements as the elements illustrated in  FIGS. 1 to 3 , and a duplicate explanation thereof is omitted. 
     Referring to  FIGS. 6 and 7 , the ratio of the electron injection material to the electron transport material in the electron transport layer  222  may increase in a direction towards the organic light emitting layer  200 . In an exemplary embodiment, the ratio of the electron injection material to the electron transport material in the electron transport layer  222  may linearly increase in a direction toward the organic light emitting layer  200 . In another exemplary embodiment, the ratio of the electron injection material to the electron transport material in the electron transport layer  222  may increase in steps in a direction toward the organic light emitting layer  200 . 
       FIG. 8  illustrates an enlarged cross-sectional view of an upper portion of a first region I of a display device according to still another embodiment, and  FIG. 9  illustrates an enlarged cross-sectional view of an upper portion of a second region II of the display device of  FIG. 8 . For convenience, the same reference numerals are used for substantially the same elements as the elements illustrated in  FIGS. 1 to 3 , and a duplicate explanation thereof is omitted. 
     Referring to  FIGS. 8 and 9 , an electron transport layer  223  may include an intermediate layer  223   c  that is interposed between a first electron transport layer  223   a  and a second electron transport layer  223   b . The intermediate layer  223   c  may be intentionally formed or may be unintentionally formed when the first electron transport layer  223   a  and the second electron transport layer  223   b  are formed in order. In an exemplary embodiment, a ratio of the electron injection material to the electron transport material in the intermediate layer  223   c  may become higher in a direction towards the organic light emitting layer  200 . In an exemplary embodiment, a ratio of the electron injection material to the electron transport material in the entire intermediate layer  223   c  may be specifically fixed. 
       FIG. 10  illustrates an enlarged cross-sectional view of an upper portion of a first region I of a display device according to still another embodiment, and  FIG. 11  illustrates an enlarged cross-sectional view of an upper portion of a second region II of the display device of  FIG. 10 . For convenience, the same reference numerals are used for substantially the same elements as the elements illustrated in  FIGS. 1 to 3 , and a duplicate explanation thereof is omitted. 
     Referring to  FIGS. 10 and 11 , a hole transport layer  184  may include a hole transport material and a hole injection material. That is, the hole transport layer  184  may include not only the hole transport material but also the hole injection material. Further, one side portion of the hole transport layer  184  that is adjacent to the organic light emitting layer  200  may include a greater amount of the hole injection material than the hole transport material, and the other side portion of the hole transport layer  184  that faces the one side portion of the hole transport layer  184  may include a greater amount of the hole transport material than the hole injection material. 
     The hole transport layer  184  may include a first hole transport layer  184   a  that is adjacent to the organic light emitting layer  200  and a second hole transport layer  184   b  that is adjacent to the first electrode  120 . In an exemplary embodiment, the volume ratio between the hole transport material and the hole injection material in the first hole transport layer  184   a  may be about 1:9 to about 5:5, and the volume ratio between the hole transport material and the hole injection material in the second hole transport layer  184   b  may be about 5:5 to about 9:1. In another exemplary embodiment, the volume ratio between the hole transport material and the hole injection material in the first hole transport layer  184   a  may be about 3:7 to about 4:6, and the volume ratio between the hole transport material and the hole injection material in the second hole transport layer  184   b  may be about 7:3 to about 6:4. In still another exemplary embodiment, the volume ratio between the hole transport material and the hole injection material in the first hole transport layer  184   a  may be about 4:6, and the volume ratio between the hole transport material and the hole injection material in the second hole transport layer  184   b  may be about 6:4. That is, although the overall volume ratio between the hole transport material and the hole injection material in the hole transport layer  184  may be about 5:5, the volume ratio between the hole transport material and the hole injection material in the first hole transport layer  184   a  and the volume ratio between the hole transport material and the hole injection material in the second hole transport layer  184   b  may not be 5:5. 
     The ratio of the hole injection material to the hole transport material in the hole transport layer  184  may increase in a direction of the organic light emitting layer  200 . In an exemplary embodiment, the ratio of the hole injection material to the hole transport material in the hole transport layer  184  may linearly increase in a direction of the organic light emitting layer  200 . In another exemplary embodiment, the ratio of the hole injection material to the hole transport material in the hole transport layer  184  may increase in steps in a direction of the organic light emitting layer  200 . 
     The electron transport layer  224  may include only the electron transport material, but is not limited thereto. The electron transport layers  220 ,  221 ,  222 , and  223  according to the above-described embodiments may be employed. 
       FIG. 12  illustrates an enlarged cross-sectional view of an upper portion of a first region I of a display device according to still another embodiment. For convenience, the same reference numerals are used for substantially the same elements as the elements illustrated in  FIGS. 1 to 3 , and a duplicate explanation thereof is omitted. 
     Referring to  FIG. 12 , a display device according to still another embodiment may be a white organic light emitting display. At least two organic light emitting layers may be laminated on the light emitting region, and light having different wavelengths that is emitted from the at least two organic light emitting layers may be mixed to emit white light. 
     Specifically, the display device according to still another embodiment may be formed by laminating in order a substrate  100 , a first electrode  120 , a first hole injection layer  165   a , a first hole transport layer  185   a , a first organic light emitting layer  205   a , a first electron transport layer  225   a , a first electron injection layer  245   a , a charge generation layer  300 , a second hole injection layer  165   b , a second hole transport layer  185   b , a second organic light emitting layer  205   b , a second electron transport layer  225   b , a second electron injection layer  245   b , a second electrode  260 , and a cover layer  280 . A first stack S1 between the first electrode  120  and the charge generation layer  300  and a second stack S2 between the charge generation layer  300  and the second electrode  260  may interact with each other to emit white light. In other words, the combination of light emitted from first stack S1 and light emitted from second stack S2 may be white. 
     One side portion of the first electron transport layer  225   a  that is adjacent to the first organic light emitting layer  205   a  and one side portion of the second electron transport layer  225   b  that is adjacent to the second organic light emitting layer  205   b  may include a greater amount of the electron injection material than the electron transport material. Specifically, the first electron transport layer  225   a  may be divided into a lower layer  225   a - 1  and an upper layer  225   a - 2 , and the lower layer  225   a - 1  that may include a greater amount of the electron injection material than the electron transport material may be adjacent to the first organic light emitting layer  205   a , and the upper layer  225   a - 2  that may include a greater amount of the electron transport material than the electron injection material may be adjacent to the charge generation layer  300 . Further, the second electron transport layer  225   b  may be divided into a lower layer  225   b - 1  and an upper layer  225   b - 2 . The lower layer  225   b - 1  that may include a greater amount of the electron injection material than the electron transport material may be adjacent to the second organic light emitting layer  205   b , and the upper layer  225   b - 2  that may include a greater amount of the electron transport material than the electron injection material may be adjacent to the second electrode  260 . 
     Hereinafter, the light emitting efficiency and the lifespan of the display device according to an embodiment will be described. The following Examples and Comparative Examples are provided in order to highlight characteristics of one or more embodiments, but it will be understood that the Examples and Comparative Examples are not to be construed as limiting the scope of the embodiments, nor are the Comparative Examples to be construed as being outside the scope of the embodiments. Further, it will be understood that the embodiments are not limited to the particular details described in the Examples and Comparative 
     EXAMPLES 
     Example 1 
     First electrode  120  was formed with a thickness of 1000 Å by depositing ITO on a substrate  100  having SiO 2  as a main component by a sputtering method. 
     A hole injection layer  160  was formed with a thickness of 100 Å by depositing m-MTDATA on the first electrode  120 . 
     A hole transport layer  180  was formed with a thickness of 1200 Å by depositing NPB on the hole injection layer  160 . 
     A blue organic light emitting layer, which included 4,4′-(carbazole-9-i1)biphenyl (CBP) as a blue host material and F 2 Irpic as a blue dopant material, was deposited on the hole transport layer  180 . Then the blue organic light emitting layer was formed with a thickness of 100 Å. 
     An electron transport layer  220 , which included a first electron transport layer  220   a , in which the volume ratio between 9,10-(2-dinaphtyl)anthracene (ADN) and LiF was 4:6, and a second electron transport layer  220   b , in which the volume ratio between 9,10-(2-dinaphtyl)anthracene (ADN) and LiF was 6:4, was formed by vacuum-depositing 9,10-(2-dinaphtyl)anthracene (ADN) and LiF on the blue organic light emitting layer. Then the first electron transport layer  220   a  and the second electron transport layer  220   b  were respectively formed with a thickness of 150 Å. 
     An electron injection layer  240  was formed with a thickness of 13 Å by depositing LiF on the electron transport layer  220 . 
     A second electrode  260  was formed with a thickness of 100 Å by depositing MgAg on the electron injection layer  240 . 
     A cover layer  280  was formed with a thickness of 600 Å by depositing SiO 2  on the second electrode  260 . 
     Comparative Example 1 
     In the same manner as the Example 1, the electron transport layer was formed with a thickness of 300 Å by depositing only 9,10-(2-dinaphtyl)anthracene (ADN) on the blue organic light emitting layer. 
     Comparative Example 2 
     In the same manner as Example 1, the electron transport layer, in which the volume ratio between 9,10-(2-dinaphtyl)anthracene (ADN) and LiF was 4:6, was formed with a thickness of 300 Å by vacuum-depositing 9,10-(2-dinaphtyl)anthracene (ADN) and LiF on the blue organic light emitting layer. 
     Comparative Example 3 
     In the same manner as Example 1, the electron transport layer, in which the volume ratio between 9,10-(2-dinaphtyl)anthracene (ADN) and LiF was 4:6, was formed with a thickness of 300 Å by vacuum-depositing 9,10-(2-dinaphtyl)anthracene (ADN) and LiF on the blue organic light emitting layer. 
     Comparative Example 4 
     In the same manner as Example 1, the electron transport layer that includes a first electron transport layer, in which the volume ratio between 9,10-(2-dinaphtyl)anthracene (ADN) and LiF was 6:4, and a second electron transport layer, in which the volume ratio between 9,10-(2-dinaphtyl)anthracene (ADN) and LiF was 4:6, was formed by vacuum-depositing 9,10-(2-dinaphtyl)anthracene (ADN) and LiF on the blue organic light emitting layer. Then the first electron transport layer and the second electron transport layer were respectively formed with a thickness of 150 Å. 
     The light emitting efficiencies and the lifespans of display devices according to the above-described Example and Comparative Examples are shown in Table 1 below: 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Drive 
                   
                   
                   
                 97% Y Drop 
               
               
                   
                 Voltage 
                 Efficiency  
                   
                 Converted  
                 Lifespan 
               
               
                   
                 [Volts] 
                 [Cd/A] 
                 CIE_Y 
                 Efficiency 
                 (time) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative 
                 3.8 
                 3.5 
                 0.041 
                 86.5 
                 32 
               
               
                 Example 1 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 5.5 
                 4.1 
                 0.042 
                 98 
                 400 
               
               
                 Example 2 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 4.0 
                 5.7 
                 0.043 
                 132.3 
                 87 
               
               
                 Example 3 
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 6.0 
                 4.1 
                 0.042 
                 98.2 
                 270 
               
               
                 Example 4 
                   
                   
                   
                   
                   
               
               
                 Example 1 
                 5.1 
                 4.2 
                 0.041 
                 101.6 
                 500 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, above, the display device according to Example 1 satisfied both the light emitting efficiency and the lifespan requirements as compared with the display devices of the Comparative Examples. In other words, Example 1 surprisingly and unexpectedly exhibited a combination of both good light emitting efficiency and good lifespan as compared with Comparative Examples 1-4. 
     By way of summation and review, as one example of a display device, an organic light emitting display may include an anode electrode, a cathode electrode, and organic films interposed between the anode electrode and the cathode electrode. The organic films may include at least an organic light emitting layer (EML), and may further include a hole injection layer (HIL), a hole transport layer (HTL), an electron transport layer (ETL), and an electron injection layer (EIL). Such an organic light emitting display may generate excitons through reception of holes and electrons from the anode electrode and the cathode electrode, respectively, and emit light of various colors through changing of the energy level of the excitons. 
     The electron transport layer included in the organic light emitting display may also be included in other display devices. For example, the electron transport layer may be included in an electroluminescent display. 
     The electron transport layer may be made only of a generally known electron transport material. However, if the electron transport layer is made only of a known electron transport material, the light emitting efficiency and the lifespan of the display device may be reduced. 
     In order to increase the light emitting efficiency and the lifespan of a display device, an electron transport layer, which includes not only an electron transport material but also an electron injection material, may be used. The electron injection material may be a material that is generally used in electron injection layers. That is, if the electron transport layer, in which the electron transport material and the electron injection material are mixed with a predetermined ratio is used, the light emitting efficiency and the lifespan of the display apparatus may be increased. 
     However, when using the electron transport layer in which the electron transport material and the electron injection material are mixed with a predetermined ratio, it may be difficult to satisfy both the light emitting efficiency and the lifespan requirements of the display device. That is, if the amount of electron injection material is decreased in the electron transport layer, the light emitting efficiency may be greatly improved, but the lifespan improvement effect may be low. Further, if the amount of electron injection material is increased in the electron transport layer, the light emitting efficiency may be low, but the lifespan improvement effect may be high. As described above, the light emitting efficiency and the lifespan of the display device may be in conflict with each other depending on the relative amount of the electron injection material to the electron transport material in the electron transport layer. 
     Accordingly, present embodiments provide a display device that can satisfy both the light emitting efficiency and the lifespan requirements of the display device by making relative amounts of an electron injection material and an electron transport material differ from each other depending on their positions in the electron transport layer. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.