Patent Publication Number: US-9853238-B2

Title: Flexible display device including plurality of protrusions

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2015-0029105, filed on Mar. 2, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     Field 
     The described technology generally relates to a flexible display device. 
     Description of the Related Technology 
     Currently, research and development is being conducted on providing various types of display devices to be installed in electronic devices of various forms, for example, flexible display devices. 
     Organic light-emitting diode (OLED) display do not require an additional light source and thus can be driven with a low voltage and manufactured to be light and thin. Also, the OLED displays have favorable characteristics such as a wide viewing angle, a high contrast, and a fast response speed and thus have drawn attention as a next-generation display. However, OLEDs are can degrade due to external moisture, oxygen, etc. Thus, the OLED displays can include a barrier layer to block external moisture or oxygen. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One inventive aspect relates to a flexible display device in which cracks or delamination do not occur in a barrier layer even when the flexible display device is repeatedly bent or stretched. 
     Another aspect is a flexible display device that includes: a substrate; a display unit formed over the substrate; an encapsulation substrate formed opposite the substrate; a filler applied between the substrate and the encapsulation substrate to cover the display unit; and a barrier layer formed over the encapsulation substrate. The encapsulation substrate includes a base layer; and a plurality of protrusions formed over a first surface of the base layer to be spaced apart from each other. The barrier layer is formed over the first surface to cover the plurality of protrusions and a portion of the base layer exposed between the plurality of protrusions. The first surface faces the display unit. 
     Each of the plurality of protrusions can have a columnar shape extending from the first surface in a direction perpendicular to the first surface. 
     Each of the plurality of protrusions can include a first end portion in contact with the first surface; and a second end portion formed opposite the first end portion. A horizontal sectional area of each of the plurality of protrusions can decrease in a direction from the first end portion to the second end portion. 
     The second end portion can have an outwardly curved shape. 
     The encapsulation substrate can be two-dimensionally stretchable, and the plurality of protrusions and the base layer can be integrally formed of a same material. 
     When the encapsulation substrate is stretched, an elongation percentage of the encapsulation substrate at the first end portion can be greater than an elongation percentage of the encapsulation substrate between the plurality of protrusions. 
     The barrier layer can include at least one organic layer; and at least one inorganic layer. 
     The flexible display device can further include a sealer applied between the substrate and the encapsulation substrate along outer sides of the display unit. 
     The flexible display device can further include a getter between the outer sides of the display unit and the sealer. 
     The display unit can include an organic light-emitting device; and a thin-film transistor electrically connected to the organic light-emitting device. 
     Another aspect is a flexible display device that includes: a substrate; a display unit formed over the substrate; an encapsulation substrate formed opposite the substrate and including a base layer and a plurality of protrusions formed over a first surface of the base layer; a filler applied between the substrate and the encapsulation substrate to cover the display unit; and a barrier layer formed over the first surface to cover the plurality of protrusions and a portion of the base layer exposed between the plurality of protrusions. The substrate, the filler, and the encapsulation substrate are two-dimensionally stretchable. Each of the plurality of protrusions includes a first end portion in contact with the first surface, and a second end portion formed opposite the first end portion. When the encapsulation substrate is stretched, an elongation percentage of the encapsulation substrate at the first end portion is greater than an elongation percentage of the encapsulation substrate between the plurality of protrusions. 
     The first surface can face the display unit. 
     The plurality of protrusions can be spaced apart from each other and formed as a same material as the base layer to be integrally formed with the base layer. 
     Each of the plurality of protrusions can have a columnar shape extending from the first surface in a direction perpendicular to the first surface. 
     A horizontal sectional area of each of the plurality of protrusions can decrease in a direction from the first end portion to the second end portion. 
     The barrier layer can include at least one organic layer; and at least one inorganic layer. 
     Spaces between the plurality of protrusions can be filed with the filler. 
     The flexible display device can further include a sealer applied between the substrate and the encapsulation substrate along outer surfaces of the display unit. 
     The flexible display device can further include a getter between the outer surfaces of the display unit and the sealer. 
     The display unit can include an organic light-emitting device; and a thin-film transistor electrically connected to the organic light-emitting device. 
     Another aspect is a flexible display device, comprising: a substrate; a display unit formed over the substrate; a filler formed over the substrate and the display unit; an encapsulation substrate formed over the encapsulation substrate; and a barrier layer formed over the encapsulation substrate. The encapsulation substrate comprises: a base layer; and a plurality of protrusions formed over a first surface of the base layer and spaced apart from each other, wherein the barrier layer is formed over i) the first surface so as to cover the plurality of protrusions and ii) a portion of the base layer exposed between the plurality of protrusions, and wherein the first surface faces the display unit. 
     In the above flexible display device, each of the plurality of protrusions has a columnar shape extending in a direction crossing the first surface. 
     In the above flexible display device, each of the plurality of protrusions comprises: a first end portion contacting the first surface; and a second end portion formed below the first end portion, wherein a horizontal sectional area of each of the plurality of protrusions at the first end portion is greater than a horizontal sectional area of each of the plurality of protrusions at the second end portion. 
     In the above flexible display device, the second end portion is curved outwardly. 
     In the above flexible display device, the encapsulation substrate is configured to be stretched in two dimensions, wherein the plurality of protrusions and the base layer are integrally formed of the same material. 
     In the above flexible display device, an elongation percentage of the encapsulation substrate at the first end portion is greater than an elongation percentage of the encapsulation substrate between the plurality of protrusions when the encapsulation substrate is stretched. 
     In the above flexible display device, the barrier layer comprises: at least one organic layer; and at least one inorganic layer. 
     The above flexible display device further comprises a sealer formed between the substrate and the encapsulation substrate, wherein the sealer is formed to the left of the leftmost side of the display unit and to the right of the rightmost side of the display unit. 
     The above flexible display device further comprises a getter formed between the display unit and the sealer. 
     In the above flexible display device, the display unit comprises: an organic light-emitting diode; and a thin-film transistor electrically connected to the organic light-emitting diode. 
     Another aspect is a flexible display device, comprising: a substrate; a display unit formed over the substrate; an encapsulation substrate formed over the substrate and the display unit and including a base layer and a plurality of protrusions formed over a first surface of the base layer; a filler interposed between the substrate and the encapsulation substrate so as to cover the display unit; and a barrier layer formed over i) the first surface so as to cover the protrusions and ii) a portion of the base layer is exposed to the barrier layer between the plurality of protrusions, wherein the substrate, the filler, and the encapsulation substrate are configured to be stretched in two dimensions. Each of the plurality of protrusions comprises: a first end portion contacting the first surface; and a second end portion formed below the first end portion, wherein an elongation percentage of the encapsulation substrate at the first end portion is greater than an elongation percentage of the encapsulation substrate between the plurality of protrusions when the encapsulation substrate is stretched. 
     In the above flexible display device, the first surface of the base layer faces the display unit. 
     In the above flexible display device, the plurality of protrusions are spaced apart from each other, formed of the same material as the base layer, and integrally formed with the base layer. 
     In the above flexible display device, each of the plurality of protrusions has a columnar shape extending in a direction crossing the first surface. 
     In the above flexible display device, a horizontal sectional area of each of the plurality of protrusions at the first end portion is greater than a horizontal sectional area of each of the plurality of protrusions at the second end portion. 
     In the above flexible display device, the barrier layer comprises: at least one organic layer; and at least one inorganic layer. 
     In the above flexible display device, the filler is formed between the protrusions. 
     The above flexible display device further comprises a sealer formed between the substrate and the encapsulation substrate, wherein the sealer is formed to the left of the leftmost side of the display unit and to the right of the right most side of the display unit. 
     The above flexible display device further comprises a getter formed between the display unit and the sealer. 
     In the above flexible display device, the display unit comprises: an organic light-emitting diode; and a thin-film transistor electrically connected to the organic light-emitting diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a flexible display device according to an exemplary embodiment. 
         FIG. 2  is a schematic cross-sectional view of an example of a pixel unit included in a display unit of the flexible display device of  FIG. 1 . 
         FIG. 3  is a schematic enlarged cross-sectional view of a region A of  FIG. 1 . 
         FIG. 4  is a schematic plan view of an encapsulation substrate of the flexible display device of  FIG. 1  according to an exemplary embodiment. 
         FIG. 5  is a schematic cross-sectional view of the encapsulation substrate when the flexible display device of  FIG. 1  is stretched. 
         FIG. 6  is a schematic plan view of an encapsulation substrate of the flexible display device of  FIG. 1  according to another exemplary embodiment. 
         FIG. 7  is a schematic plan view of an encapsulation substrate of the flexible display device of  FIG. 1  according to another exemplary embodiment. 
         FIG. 8  is a schematic cross-sectional view of a modified example of the flexible display device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present exemplary embodiments can have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the exemplary embodiments are merely described below, by referring to the figures, to explain aspects of the present description. It would be obvious to those of ordinary skill in the art that exemplary embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the described technology. In the following description, well-known functions or constructions are not described in detail if it is determined that they would obscure the inventive concept due to unnecessary detail. 
     It will be understood that although the terms “first”, “second”, etc. can be used herein to describe various components, these components should not be limited by these terms. These terms are only used to distinguish one component from another. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the described technology. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. In the drawings, elements can be exaggerated, omitted, or schematically illustrated for convenience of explanation. In other words, since sizes and thicknesses of components in the drawings are illustrated for convenience of explanation, the following embodiments are not limited thereto. 
     It will be understood that when a layer, region, or component is referred to as being “formed on” or “formed under,” another layer, region, or component, it can be directly or indirectly formed on or under the other layer, region, or component. That is, for example, intervening layers, regions, or components can be present. 
     Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings. In the drawings, components that are substantially the same or that correspond to each other will be denoted by the same reference numeral and will not be redundantly described here. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. In this disclosure, the tem). “substantially” includes the meanings of completely, almost completely or to any significant degree under some applications and in accordance with those skilled in the art. The term “connected” can include an electrical connection. 
       FIG. 1  is a schematic cross-sectional view of a flexible display device  10  according to an exemplary embodiment.  FIG. 2  is a schematic cross-sectional view of an example of a pixel unit (or pixel) P included in a display unit  200  of the flexible display device  10  of  FIG. 1 .  FIG. 3  is a schematic enlarged cross-sectional view of a region A of  FIG. 1 .  FIG. 4  is a schematic plan view of an encapsulation substrate  300  of the flexible display device  10  of  FIG. 1  according to an exemplary embodiment. 
     Referring to  FIGS. 1 to 4 , the flexible display device  10  according to an exemplary embodiment includes a substrate  100 , the display unit  200  formed on the substrate  100 , an encapsulation substrate  300  formed opposite the substrate  100 , a filler  400  for covering the display unit  200  between the substrate  100  and the encapsulation substrate  300 , and a barrier layer  330  formed on the encapsulation substrate  300 . 
     The substrate  100  can be formed of a material having a high elongation percentage. For example, the substrate  100  is formed of a material having a Poisson&#39;s ratio of about 0.4 or more. The Poisson&#39;s ratio is a shrinkage ratio of a material in one direction when the material is stretched in another direction to increase the length thereof. That the material of the substrate  100  has the Poisson&#39;s ratio of about 0.4 or more means that the substrate  100  has a property of being easily stretched. 
     In the present embodiment, the substrate  100  is formed of at least one of silicone-based polymer, polyurethane, polyurethane acrylate, acrylate polymer, and acrylate terpolymer. The silicone-based polymer can include, for example, polydimethylsiloxane (PDMS), hexamethyldisiloxane (HMDSO), etc. Thus, the substrate  100  can have a flexible property and be two-dimensionally stretchable. 
     The substrate  100  can include a plurality of islands  110  arranged in a plane lattice form, and a base unit (not shown) that is formed between the islands  110  to be lower than the islands  110  in terms of height. Thus, when the substrate  100  is stretched, the length of the base unit can increase and the islands  110  which are thick can change less in shape compared to the base unit. 
     The display unit  200  can include a plurality of pixel units P. For example, the pixel units P formed on the islands  110 . Thus, even if the substrate  100  is two-dimensionally stretched or is bent, the pixel units P formed on the islands  110  that are changed less in shape can be prevented from being damaged. 
     The pixel units P can each include R, G, and B sub-pixels. The R, G, and B sub-pixels can each include a thin-film transistor (TFT) M 1  and an organic light-emitting diode (OLED) on the substrate  100 . 
     An insulating layer  120  such as a barrier layer and/or a buffer layer can be formed on an upper surface of the substrate  100 , e.g., at least upper surfaces of the islands  110 , to prevent impurity ions from diffusing via the substrate  100 , prevent moisture or air from permeating the flexible display device  10 , and provide a flat surface. 
     An active layer  207  of the TFT M 1  can be formed on the insulating layer  120  by using a semiconductor material, and a gate insulating film  203  can be formed to cover the active layer  207 . The active layer  207  can be formed of an inorganic semiconductor such as amorphous silicon or polysilicon, or an organic semiconductor. 
     A gate electrode  208  is formed on the gate insulating film  203 , and an interlayer insulating film  204  is formed to cover the gate electrode  208 . A source electrode  209   a  and a drain electrode  209   b  are formed on the interlayer insulating film  204 . A passivation film  205  and a pixel defining film  206  are sequentially formed to cover the source electrode  209   a  and the drain electrode  209   b.    
     The gate electrode  208 , the source electrode  209   a , and the drain electrode  209   b  can be formed of a metal such as Al, Mo, Au, Ag, Pt/Pd, Cu, etc. but is not limited thereto. The gate electrode  208 , the source electrode  209   a , and the drain electrode  209   b  can be formed by applying a resin paste including one or more of these metals in a powder form or formed using a conductive polymer. 
     The gate insulating film  203 , the interlayer insulating film  204 , the passivation film  205 , and the pixel defining film  206  can be insulators, have a single-layer structure or a multi-layer structure, and be formed of an organic material, an inorganic material, or a combination thereof. 
     The OLED displays image information by emitting red, green, blue, or white light according to the flow of current. The OLED can include a pixel electrode  210  connected to one of the source electrode  209   a  and the drain electrode  209   b  of the TFT M 1 , an opposite electrode  212  formed to cover all pixels, and an organic emission film  211  formed between the pixel electrode  210  and the opposite electrode  212  to emit light. 
     The pixel units P can be directly formed on the islands  110 , or formed on a carrier substrate (not shown) and transferred onto the islands  110 . 
     The filler  400  is formed to fill a space between the substrate  100  and the encapsulation substrate  300 , and can prevent or delay permeation of external moisture and oxygen into the flexible display device  10 . Also, the filler  400  can protect the flexible display device  10  to not be damaged by shock, etc. and secure the mechanical stability of the flexible display device  10 . The filler  400  can be formed of a stretchable material. For example, the filler  400  is formed of elastomeric polyurethane, elastomeric silicone, etc. 
     The encapsulation substrate  300  can be formed of at least one of silicone-based polymer, polyurethane, polyurethane acrylate, acrylate polymer, and acrylate terpolymer. The silicon-based polymer can include, for example, at least one of polydimethylsiloxane (PDMS) and hexamethyldisiloxane (HMDSO). Thus, the encapsulation substrate  300  can have a flexible property and be two-dimensionally stretched. 
     The encapsulation substrate  300  can include a base layer  310  and a plurality of protrusions  320  formed on a first surface of the base layer  310 . The first surface can face the display unit  200 . 
     The base layer  310  and the protrusions  320  can be integrally formed of the same material. For example, the base layer  310  and the protrusions  320  are substantially simultaneously or concurrently formed by molding, or the protrusions  320  can be formed on the first surface of the base layer  310  by photolithography. 
     The protrusions  320  can be spaced apart from each other, and have a columnar shape extending from the first surface in a direction substantially perpendicular to the first surface. Thus, the protrusions  320  can each include a first end portion U 1  that is in contact with the first surface, and a second end portion U 2  opposite the first end portion U 1 . 
     The protrusions  320  can be spaced a predetermined distance from each other to form a predetermined pattern. For example, the protrusions  320  have a pattern in which triangles, quadrangles, honeycomb structures, or the like are repeatedly arranged. Otherwise, the protrusions  320  can have a random pattern. A shape of a horizontal sectional area of each of the protrusions  320  is not limited. That is, the horizontal sectional areas of the protrusions  320  can have a round shape as illustrated in  FIG. 4  or have other various shapes such as a polygonal shape. 
     The barrier layer  330  covers the protrusions  320  and a portion of the base layer  310  exposed between the protrusions  320 . That is, the barrier layer  330  is formed on surfaces of the protrusions  320  and the portion of the base layer  310  exposed between the protrusions  320 . Thus, external moisture and oxygen can be effectively prevented from permeating the display unit  200  using the encapsulation substrate  300 . 
     The barrier layer  330  can include at least one organic layer  332  and at least one inorganic layer  334 . The at least one organic layer  332  and the at least one inorganic layer  334  can be alternately stacked. When the at least one organic layer  332  and the at least one inorganic layer  334  are alternately stacked, one of the at least one organic layer  332  can be first formed in consideration of an adhesive strength between the barrier layer  330  and the encapsulation substrate  300 . 
     The at least one organic layer  332  can be formed of at least one of acryl-based resin, methacryl-based resin, polyisoprene, vinyl-based resin, epoxy-based resin, urethane-based resin, cellulose-based resin, and perylene-based resin. 
     The at least one inorganic layer  334  can be formed of at least one of silicon nitride, aluminum nitride, zirconium nitride, titanium nitride, hafnium nitride, tantalum nitride, silicon oxide, aluminum oxide, titanium oxide, tin oxide, cerium oxide, and silicon oxynitride (SiON). 
     The protrusions  320  on which the barrier layer  330  are formed can be formed to face downward, i.e., to face the display unit  200 . The spaces between the protrusions  320  on which the barrier layer  330  is formed can be filled with the filler  400 . 
       FIG. 5  is a schematic cross-sectional view of the encapsulation substrate  300  when the flexible display device  10  of  FIG. 1  is stretched. As illustrated in  FIG. 5 , when the encapsulation substrate  300  is stretched, an elongation percentage of the encapsulation substrate  300  at the first end portion U 1  can be different from that of the encapsulation substrate  300  at the second end portion U 2 . For example, when the encapsulation substrate  300  is stretched, the area of the first end portion U 1  to which a force is applied increases but the area of the second end portion U 2  on which the barrier layer  330  having a hard property is formed does not increase. Thus, if vertical sections of the protrusions  320  have, for example, a substantially rectangular shape, the vertical sections of the protrusions  320  can change into a substantially trapezoidal shape, the upper side of which is longer than the lower side thereof when the encapsulation substrate  300  is stretched. 
     The at least one inorganic layer  334  included in the barrier layer  330  has a hard property. Thus, when the shapes of the protrusions  320  change, cracks or the like can occur in the at least one inorganic layer  334 . 
     A strain applied to the protrusions  320  when the first end portions U 1  are stretched 100% without changing the shape of the second end portions U 2  of the protrusions  320  can be calculated by Equation 1 below. 
     
       
         
           
             
               
                 
                   
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                             1 
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                                   W 
                                   
                                     2 
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                                     L 
                                   
                                 
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                               2 
                             
                           
                         
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                     × 
                     100 
                   
                 
               
               
                 
                   Equation 
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                   1 
                 
               
             
           
         
       
     
     According to Equation 1, if a ratio between the width W and length L of the protrusions  320  is 1:16 or more, the strain applied to the protrusions  320  is 0.05% or less when the first end portions U 1  of the protrusions  320  are stretched 100%. In this case, the at least one inorganic layer  334  of the barrier layer  330  formed on side surfaces of the protrusions  320  is not damaged. That is, if the ratio between the width W and length L of the protrusions  320  is 1:16 or more, the at least one inorganic layer  334  of the barrier layer  330  can be stably maintained even when the encapsulation substrate  300  is stretched 100%. 
     Similarly, when the first end portions U 1  of the protrusions  320  are stretched about 50% without changing the shapes of the second end portions U 2  of the protrusions  320 , the ratio between the width W and length L of the protrusions  320  can be set to be about 1:8 or more so that the at least one inorganic layer  334  of the barrier layer  330  is not damaged. When the first end portions U 1  of the protrusions  320  are stretched about 30% without changing the shapes of the second end portions U 2  of the protrusions  320 , the ratio between the width W and length L of the protrusions  320  can be set to be about 1:5 or more so that the at least one inorganic layer  334  of the barrier layer  330  is not damaged. Accordingly, the length L and width W of the protrusions  320  can be appropriately adjusted according to the elongation percentage of the flexible display device  10 . 
     When the flexible display device  10  is stretched, the filler  400  is also stretched. In this case, an elongation percentage of the filler  400  in a first region P 1  between the first end portions U 1  can be different from that of the filler  400  in a second region P 2  between the second end portions U 2 . That is, in some embodiments, when the filler  400  is stretched, the area of the second region P 2  to which a force is applied can increase and the area of the first region P 1  that is in contact with the barrier layer  330  having a hard property does not increase. Thus, when the encapsulation substrate  300  is stretched, the elongation percentage of the filler  400  at the first end portions U 1  can be greater than that of the filler  400  between adjacent first end portions U 1 . 
     Also, stress applied to the barrier layer  330  when the flexible display device  10  is stretched can be substantially evenly dispersed due to the first end portions U 1  and the second regions P 2 . Thus, even if the flexible display device  10  is repeatedly bent or stretched, cracks or delamination do not occur in the barrier layer  330 . 
       FIG. 6  is a schematic plan view of an encapsulation substrate  300 B of the flexible display device  10  of  FIG. 1  according to another exemplary embodiment.  FIG. 7  is a schematic plan view of an encapsulation substrate  300 C of the flexible display device  10  of  FIG. 1  according to another exemplary embodiment. 
     Referring to  FIG. 6 , the encapsulation substrate  300 B includes a base layer  310  and a plurality of protrusions  320  formed on a first surface of the base layer  310 . The base layer  310  and the protrusions  320  can be integrally formed of the same material. The base layer  310  and the protrusions  320  can each be formed of at least one of silicone-based polymer, polyurethane, polyurethane acrylate, acrylate polymer, and acrylate terpolymer. The silicon-based polymer can be formed of, for example, polydimethylsiloxane (PDMS), hexamethyldisiloxane (HMDSO), etc. 
     The protrusions  320  can be spaced apart from each other, and have a columnar shape extending from the first surface in a direction substantially perpendicular to the first surface. The protrusions  320  can each include a first end portion U 1 ′ that is in contact with the first surface, and a second end portion U 2 ′ formed opposite the first end portion U 1 ′. A barrier layer  330  can be formed on surfaces of the protrusions  320 . A filler  400  can be filled between the protrusions  320  on which the barrier layer  330  is formed. 
     Horizontal sectional areas of the protrusions  320  can decrease in a direction from the first end portions U 1 ′ to the second end portions U 2 ′. Thus, the protrusions  320  can be easily manufactured by molding, the barrier layer  330  can be substantially uniformly formed on the surfaces of the protrusions  320 , and a filler (not shown) can be easily filled between the protrusions  320  on which the barrier layer  330  is formed. 
     Since the horizontal sectional areas of the protrusions  320  decrease in the direction from the first end portions U 1 ′ to the second end portions U 2 ′, the areas of spaces between the first end portions U 1 ′ can decrease. Thus, the barrier layer  330  can be more effectively prevented from being damaged when the flexible display device  10  is repeatedly bent or stretched. 
     The encapsulation substrate  300 C of  FIG. 7  can include a base layer  310  and a plurality of protrusions  320  formed on a first surface of the base layer  310 . A barrier layer  330  can be formed on surfaces of the protrusions  320  and a portion of the base layer  310  exposed between the protrusions  320 . A filler  400  can be filled between the protrusions  320  on which the barrier layer  330  is formed. 
     The encapsulation substrate  300 C of  FIG. 7  includes curved surfaces between first end portions U 1 ″ and at second end portions U 2 ″, compared to the encapsulation substrate  300 B of  FIG. 6 . That is, the second end portions U 2 ″ can have an outwardly curved shape, and spaces between the first end portions U 1 ″ can be inwardly curved toward the base layer  310 . Thus, a stress that is likely to be intensively applied to an angular portion can be more effectively dispersed. 
       FIG. 8  is a schematic cross-sectional view of a flexible display device  20  which is a modified example of the flexible display device  10  of  FIG. 1 . 
     The flexible display device  20  of  FIG. 8  can include a substrate  100 , a display unit  200  on the substrate  100 , an encapsulation substrate  300  formed opposite the substrate  100 , a filler  400  for covering the display unit  200  between the substrate  100  and the encapsulation substrate  300 , and a sealer  500  applied along outer surfaces of the display unit  200  to fill a space between the substrate  100  and the encapsulation substrate  300 . 
     The substrate  100 , the display unit  200 , the encapsulation substrate  300 , and the filler  400  are as described above with reference to  FIGS. 1 to 5  and are thus not described again here. 
     The sealer  500  can be applied on the outer surfaces of the display unit  200  to combine the substrate  100  and the encapsulation substrate  300 , and prevent external moisture and oxygen from permeating the flexible display device  20 . The sealer  500  can be formed of, for example, UV curing resin or thermosetting resin such as elastomeric silicon. 
     The flexible display device  20  of  FIG. 8  can further include a getter  600 . The getter  600  can be applied between the outer surfaces of the display unit  200  and inner sides of the sealer  500 . The getter  600  can be formed of one of alkali metal oxide, alkaline-earth metal oxide, metal halide, lithium sulfate, metal sulfate, metal chlorate, silica gel, and phosphorous pentoxide that easily react with moisture and oxygen so as to prevent the lifetime of an OLED or the like from decreasing due to moisture and oxygen. 
     As described above, according to the one or more of the above exemplary embodiments, even if a flexible display device is repeatedly bent or stretched, cracks or delamination can be prevented from occurring in a barrier layer. 
     It should be understood that exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments. 
     While the inventive technologies have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details can be made therein without departing from the spirit and scope as defined by the following claims.