Patent Publication Number: US-2018034001-A1

Title: Window for a display device and a flexible display device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2016-0095703 filed on Jul. 27, 2016 in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     Exemplary embodiments of the present invention relate to a display device, and more particularly to a window for a flexible display device and a flexible display device including the same. 
     DISCUSSION OF RELATED ART 
     Flexible display devices that are bendable or foldable during use or manufacturing have been increasingly applied and used. 
     A display device may include a transparent window. The window covers a display surface on which an image is displayed. The window protects the display device from the occurrence of scratches or the like on the display device. 
     The window may include a glass material. However, in a flexible display device, the window including the glass material may be relatively easily broken when the flexible display device is bent or folded. When the glass material of the window is broken, a user may become injured. The window may include a plastic material having flexible properties. However, the window including the plastic material may have a relatively low surface hardness. Therefore, scratches on a surface of the window of the flexible display device may occur relatively easily. 
     SUMMARY 
     Exemplary embodiments of the present invention provide a window for a display device with an increased strength, and more particularly a flexible display device including the same. 
     One or more exemplary embodiments of the present invention provide a window for a display device. The window includes a glass layer. The window also includes a functional coating layer. The functional coating layer is disposed on the glass layer. The functional coating layer has an elastic modulus less than an elastic modulus of the glass layer. A thickness of the functional coating layer is in a range from about 1 micrometer (um) to about 10 um. 
     According to an exemplary embodiment of the present invention, a thickness of the glass layer may be less than or substantially equal to about 100 um. 
     According to an exemplary embodiment of the present invention, the glass layer may include a chemical enhancing layer. 
     According to an exemplary embodiment of the present invention, the functional coating layer may include an urethane-based resin, an epoxy-based resin, a polyester-based resin, a polyether-based resin, an acrylate-based resin, an acrylonitrile butadiene styrene (ABS) resin, or a rubber. 
     According to an exemplary embodiment of the present invention, the functional coating layer may include polyurethane, a combination of polyurethane and a rubber, or a combination of polyurethane and an acrylic monomer. 
     According to an exemplary embodiment of the present invention, the elastic modulus of the functional coating layer may be in a range from about 1.52 GPa to about 5 GPa. 
     According to an exemplary embodiment of the present invention, the functional coating layer may be combined with the glass layer. 
     According to an exemplary embodiment of the present invention, the functional coating layer may be disposed on an entire surface of the glass layer. 
     According to an exemplary embodiment of the present invention, a light transmittance of the functional coating layer may be greater than or substantially equal to about 88%. 
     According to an exemplary embodiment of the present invention, the window may have an impact resistance as indicated by a drop height of at least about 6 cm as determined by a pen drop measurement using a pen of about 5.7 g. 
     According to an exemplary embodiment of the present invention, the window may have a radius of curvature less than or substantially equal to about 4.5 mm. 
     One or more exemplary embodiments of the present invention provide a display device. The display device includes a flexible display panel. The display device also includes a window. The window is disposed on the display panel. The window includes a glass layer. The window also includes a functional coating layer. The functional coating layer is disposed between the glass layer and the display panel. A thickness of the functional coating layer may be in a range from about 1 um to about 10 um. 
     According to an exemplary embodiment of the present invention, an elastic modulus of the functional coating layer may be less than an elastic modulus of the glass layer. 
     According to an exemplary embodiment of the present invention, a thickness of the glass layer may be less than or substantially equal to about 100 um. 
     According to an exemplary embodiment of the present invention, the glass layer may include a chemical enhancing layer. 
     According to an exemplary embodiment of the present invention, the functional coating layer may be combined with the glass layer. 
     According to an exemplary embodiment of the present invention, the functional coating layer may be disposed on an entire surface of the glass layer. 
     According to an exemplary embodiment of the present invention, the flexible display device may be bent or folded in order that portions of a surface of the display panel face each other. 
     According to an exemplary embodiment of the present invention, the display panel may include a flexible substrate; at least one transistor disposed on the substrate; an insulation layer covering the transistor; an organic light-emitting element disposed on the insulation layer and electrically connected to the transistor; and an encapsulation member disposed on the substrate. The organic light-emitting element may emit light from an organic light-emitting layer disposed between opposing electrodes. 
     According to an exemplary embodiment of the present invention, the display device may further include a touch sensing member. The display device may also include an optical film. The touch sensing member and the optical film may be disposed between the display panel and the window. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating a flexible display device according to an exemplary embodiment of the present invention; 
         FIG. 2  is a cross-sectional view illustrating a stacked structure of a flexible display device of  FIG. 1  according to an exemplary embodiment of the present invention; 
         FIG. 3  is a cross-sectional view illustrating a flexible display device of  FIG. 2  according to an exemplary embodiment of the present invention; 
         FIG. 4  is a cross-sectional view illustrating an unfolded state of a window of  FIG. 3  according to an exemplary embodiment of the present invention; and 
         FIG. 5  is a cross-sectional view illustrating a folded state of a window of  FIG. 3  according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. In this regard, the exemplary embodiments may have different forms and should not be construed as being limited to the exemplary embodiments of the present invention described herein. 
     Like reference numerals may refer to like elements throughout the specification and drawings. 
     Sizes of elements in the drawings may be exaggerated for clarity of description. 
     It will be understood that when a component, such as a layer, a film, a region, or a plate, is referred to as being “on” another component, the component can be directly on the other component or intervening components may be present. 
     Hereinafter, a window for a display device and a flexible display device including the same in accordance with exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a perspective view illustrating a flexible display device according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a flexible display device  10  may have flexible properties. The flexible display device  10  may also be bendable or foldable. Since an area of the flexible display device  10  may be reduced due to folding the flexible display device  10 , the flexible display device  10  may be stored more easily when folded. A user may use the flexible display device  10  by unfolding the flexible display device  10 . 
     As illustrated in  FIG. 1 , the flexible display device  10  may include a first surface  10   a . An image may be displayed on the first surface  10   a . The flexible display device  10  may also include a second surface  10   b . The second surface  10   b  may face the first surface  10   a . According to an exemplary embodiment of the present invention, when the flexible display device  10  is bent or folded, portions of the second surface  10   b  may face each other. As illustrated in  FIG. 1 , the flexible display device  10  may be bent or folded once. However, exemplary embodiments of the present invention are not limited thereto. For example, the flexible display device  10  may be bent or folded at least two times. A folding direction or a folding form of the flexible display device  10  may be variously modified and is not limited to the illustration in  FIG. 1 . 
       FIG. 2  is a cross-sectional view illustrating a stacked structure of a flexible display device of  FIG. 1  according to an exemplary embodiment of the present invention.  FIG. 3  is a cross-sectional view illustrating a flexible display device of  FIG. 2  according to an exemplary embodiment of the present invention. 
     Referring to  FIGS. 2 and 3 , the flexible display device  10  may include a display panel  100 , a touch sensing member  200 , an optical film  300 , and a window  400 . The display panel  100 , the touch sensing member  200 , the optical film  300 , and the window  400  may be stacked in a direction from the second surface  10   b  to the first surface  10   a.    
     The display panel  100  may display an image. The display panel  100  may be flexible. The display panel  100  may include a plurality of pixels. Since each pixel may emit light, the display panel  100  may realize a predetermined image. For example, the display panel  100  may display an image to the first surface  10   a  of the flexible display device  10 . 
     The display panel  100  may be an organic light-emitting display panel. However, exemplary embodiments of the present invention are not limited thereto. For example, the display panel  100  may be a liquid crystal display panel or a plasma display panel. 
     Referring to  FIG. 3 , the display panel  100  may include a flexible substrate  110 , at least one transistor  120 , an insulation layer  130 , an organic light-emitting element  140 , and an encapsulation member  150 . The transistor  120  may be disposed on the substrate  110 . The insulation layer  130  may cover the transistor  120 . The organic light-emitting element  140  may be disposed on the insulation layer  130 . The encapsulation member  150  may be disposed on the substrate  110 . The encapsulation member  150  may encapsulate the organic light-emitting element  140 . The organic light-emitting element  140  may be electrically connected to the transistor  120 . The organic light-emitting element  140  may emit light from an organic light-emitting layer. The organic light-emitting layer may be disposed between opposing electrodes. 
     The substrate  110  may include a flexible material. The flexible material may be bendable or foldable. For example, the substrate  110  may include a plastic such as polyimide (PI), polyethylene naphtahlate (PEN), polyethylene terephthalate (PET), polyether ether ketone (PEEK), polyethersulfone (PES), polymethyl methacrylate (PMMA), polycarbonate (PC), and/or polypropylene (PP). Alternatively, the substrate  110  may include a thin plate glass or a thin metal film. 
     A buffer layer  161  may be formed on the substrate  110 . The buffer layer  161  may planarize a top surface of the substrate  110 . The buffer layer  161  may decrease or prevent the penetration of impurities into the substrate  110 . The buffer layer  161  may have a single layer structure. Alternatively, the buffer layer  161  may have a multi-layered structure. The buffer layer  161  may include a layer including an inorganic material such as silicon oxide and/or silicon nitride. The buffer layer  161  may be formed by various deposition methods. 
     A pixel circuit unit may be disposed on the buffer layer  161 . The pixel circuit unit may include at least one transistor  120 . The pixel circuit unit may also include at least one capacitor.  FIG. 3  illustrates a top-gate type transistor. The top-gate type transistor may include an active pattern  121 , a gate electrode  123 , a source electrode  125 , and a drain electrode  126 . The active pattern  121 , the gate electrode  123 , the source electrode  125 , and the drain electrode  126  may be arranged over the substrate  110 . However, exemplary embodiments of the present invention are not limited thereto. For example, various types of transistors may be used, such as a bottom-gate type transistor. 
     The active pattern  121  may be formed on the buffer layer  161 . The active pattern  121  may include a semiconductor material. For example, the active pattern  121  may include amorphous silicon (a-Si) or poly-crystalline silicon (poly-Si). The active pattern  121  may include a source region  121   a , a drain region  121   c , and a channel region  102   b . The source region  121   a  and the drain region  121   c  may be respectively connected to the source electrode  125  and the drain electrode  126 . The channel region  121   b  may be disposed between the source region  121   a  and the drain region  121   c.    
     A gate insulation layer  122  may be formed on the active pattern  121 . The gate insulation layer  122  may have a single layer structure. Alternatively, the gate insulation layer  122  may have a multi-layered structure. The gate insulation layer  122  may include a layer including an inorganic material such as silicon oxide and/or silicon nitride. The gate insulation layer  122  may insulate the gate electrode  123  and the active pattern  121  from each other. 
     The gate electrode  123  may be formed on the gate insulation layer  122 . The gate electrode  123  may substantially overlap the channel region  121   b  of the active pattern  121 . The gate electrode  123  may be connected to a gate line. The gate line may apply ON/OFF signals to the transistor  120 . The gate electrode  123  may have a single layer structure. Alternatively, the gate electrode  123  may have a multi-layered structure. The gate electrode  123  may include a layer including a conductive material such as molybdenum (Mo), aluminum (Al), copper (Cu), titanium (Ti), or any combination thereof. 
     An insulation interlayer  124  may be formed on the gate electrode  123 . The insulation interlayer  124  may have a single layer structure. Alternatively, the insulation interlayer  124  may have a multi-layered structure. The insulation interlayer  124  may include a layer including an inorganic material such as silicon oxide and/or silicon nitride. The insulation interlayer  124  may insulate the gate electrode  123  from the source electrode  125  and the drain electrode  126 . 
     The source electrode  125  and the drain electrode  126  may be formed on the insulation interlayer  124 . The source electrode  125  may be connected to the source region  121   a  of the active pattern. The drain electrode  126  may be connected to the drain region  121   c  of the active pattern  121 . The source electrode  125  and the drain electrode  126  may be respectively connected to the source region  121   a  and the drain region  121   c  of the active layer  121  through a contact hole. The contact hole may be formed in the gate insulation layer  122  and the insulation interlayer  124 . The source electrode  125  and the drain electrode  126  may have a single layer structure. Alternatively, the source electrode  125  and the drain electrode  126  may have a multi-layered structure. The source electrode  125  and the drain electrode  126  may each include a layer including a conductive material selected from molybdenum (Mo), aluminum (Al), copper (Cu), titanium (Ti), or any combination thereof. 
     The insulation layer  130  may cover the transistor  120 . The insulation layer  130  may reduce or prevent a step height caused by the transistor  120 . The insulation layer  130  may planarize an upper surface of the substrate  110 . Thus, the insulation layer  130  may reduce or prevent the occurrence of defects in the organic light-emitting element  140  due to an unevenness below the insulation layer  130 . The insulation layer  130  may have a single layer structure. Alternatively, the insulation layer  130  may have a multi-layered structure. The insulation layer  130  may include a layer including an inorganic material, an organic material, or any combinations thereof. 
     The transistor  120  may be electrically connected to the organic light-emitting element  140 . The organic light-emitting element  140  may emit light. The organic-light emitting element  140  might not emit light. The organic light-emitting element  140  may emit light according to a turn-on state or a turn-off state of the transistor  120 . 
     The organic light-emitting element  140  may be formed on the insulation layer  130 . The organic light-emitting element  140  may include a pixel electrode  141 , an opposing electrode  142 , and an intermediate layer  143 . The opposing electrode  142  may be disposed opposite to the pixel electrode  141 . The intermediate layer  143  may be disposed between the pixel electrode  141  and the opposing electrode  142 . According to an emission direction of the organic light-emitting element  140 , a display device may be a bottom emission type, a top emission type or a dual emission type. In a bottom emission type display device, the pixel electrode  141  may be a light-transmitting electrode. The opposing electrode  142  may be a reflective electrode. In a top emission type display device, the pixel electrode  141  may be a reflective electrode. The opposing electrode  142  may be a transflective electrode. In a dual emission type display device, the pixel electrode  141  and the opposing electrode  142  may each be light-transmitting electrodes.  FIG. 3  illustrates the flexible display device  10  as a top emission type display device. However, the exemplary embodiments of the present invention are not limited thereto. For example, the flexible display device  10  may be a bottom emission type display device or a dual emission type display device. 
     The pixel electrode  141  may be patterned. The pixel electrode  141  may be patterned in the form of a discrete island respectively corresponding to each pixel. The pixel electrode  141  may be connected to the transistor  120 . The pixel electrode  141  may be connected to the transistor  120  through a via hole. The via hole may be formed in the insulation layer  130 . 
     The pixel electrode  141  may include a transparent electrode layer. The pixel electrode  141  may also include a reflective electrode layer. The reflective electrode layer may reflect light in a direction from the pixel electrode  141  to the opposing electrode  142 . When the pixel electrode  141  is an anode, the transparent electrode layer may include a transparent conductive oxide with a relatively high work function, such as indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium oxide (In 2 O 3 ), indium gallium oxide (IGO), aluminum zinc oxide (AZO), or any combination thereof. The reflective electrode layer may include a relatively high reflective metal, such as silver (Ag). 
     A pixel defining layer  162  may be formed on the insulation layer  130 . The pixel defining layer  162  may be formed by a method, such as a spin coating. The pixel defining layer  162  may be formed by using an organic insulating material such as polyimide, polyamide, an acryl resin, benzocyclobutane or a phenol resin. The pixel defining layer  162  may cover an edge portion of the pixel electrode  141 . The pixel defining layer  162  may include an opening. The opening may expose at least a center portion of the pixel electrode  141 . The opening may correspond to a light-emitting region of the pixel. The intermediate layer  143  may be formed in the opening. 
     The intermediate layer  143  may include the organic light-emitting layer. The organic light-emitting layer may be configured to emit red, green or blue light. The organic light-emitting layer may include a low molecular weight organic material or a polymer organic material. When the organic light-emitting layer includes the low molecular weight organic material, a hole transport layer (HTL) and a hole injection layer (HIL) may be stacked in a direction from the organic light-emitting layer to the pixel electrode  141 . Additionally, an electron transport layer (ETL) and an electron injection layer (EIL) may be stacked in a direction from the organic light-emitting layer to the opposing electrode  142 . 
     The opposing electrode  142  may cover an entire surface of the pixel defining layer  162 . The opposing electrode  142  may include a metal. When the opposing electrode  142  is a cathode, the opposing electrode  142  may include a material with a relatively low work function, such as lithium (Li), calcium (Ca), LiF/Ca, LiF/Al, aluminum (Al), magnesium (Mg), or silver (Ag). The metal included in the opposing electrode  142  may be formed as a thin film. The thin film may transmit light therethrough. 
     A capping layer  163  may be formed on the opposing electrode  142 . The capping layer  163  may maintain a work function of the opposing electrode  142 . The capping layer  162  may reduce or prevent damage of the organic material included in the intermediate layer  143  when the encapsulation member  150  is formed. The encapsulation member  150  may be formed by a sputtering process or a plasma enhanced chemical vapor deposition (PECVD) process. 
     The encapsulation member  150  may be formed over an entire surface of the substrate  110 . The encapsulation member  150  may protect the organic light-emitting element  140  from external moisture or oxygen. The encapsulation member  150  may include one or more inorganic layers  151  and  153 . The encapsulation member  150  may also include one or more organic layers  152 . For example, as illustrated in  FIG. 3 , the encapsulation member  150  may include a first inorganic layer  151 , a second inorganic layer  153 , and an organic layer  152 . The organic layer  152  and the second inorganic layer  153  may be stacked on the first inorganic layer  151 . The organic layer  152  and the second inorganic layer  153  stacked on the first inorganic layer  151  may form the encapsulation member  150 . The first inorganic layer  151  and the second inorganic layer  153  may each include silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, titanium oxide, tantalum oxide, hafnium oxide, zinc oxide, or any combination thereof. The organic layer  152  may include polyethylene terephthalate, polyimide, polycarbonate, epoxy, polyethylene, polyacrylate, or any combination thereof. 
     According to an exemplary embodiment of the present invention, since display panel  100  may include the flexible substrate  110  and the encapsulation member  150  having flexibility, the display panel  100  may be bent, folded, or unfolded. 
     As illustrated in  FIGS. 2 and 3 , a touch sensing member  200  may be disposed on the display panel  100 . The touch sensing member  200  may include an electrostatic capacitive type sensing member, a resistive type sensing member, an electro-magnetic type sensing member, or an infrared type sensing member. The touch sensing member  200  may be electrically connected to the display panel  100 . 
     As illustrated in  FIG. 2 , an adhesive member  500  may be disposed between the display panel  100  and the touch sensing member  200 . The adhesive member  500  may attach the display panel  100  and the touch sensing member  200  to each other. For example, the adhesive member  500  may include an optically clear adhesive (OCA) or pressure sensitive adhesive (PSA); however, exemplary embodiments of the present invention are not limited thereto. 
     The optical film  300  may be disposed on the touch sensing member  200 . The optical film  300  may include a circular polarization film or a linear polarization film; however, exemplary embodiments of the present invention are not limited thereto. The optical film  300  may reduce or prevent a reflection of external light. Therefore, the optical film  300  may increase a user&#39;s ability to observe an image. 
       FIGS. 2 and 3  illustrate the touch sensing member  200  and the optical film  300  disposed over the display panel  100 . However, exemplary embodiments of the present invention are not limited thereto. For example, the touch sensing member  200  and the optical film  300  may be embedded in the display panel  100 . Thus, the touch sensing member  200  and the optical film  300  may be disposed between the substrate  110  and the encapsulation member  150  of the display panel  100 . 
     The window  400  may be disposed on the optical film  300 . The window  400  may protect the display panel  100 , the touch sensing member  200 , and the optical film  300 . A user may observe an image displayed by the display panel  100  through the window  400 . 
       FIG. 4  is a cross-sectional view illustrating an unfolded state of a window of  FIG. 3  according to an exemplary embodiment of the present invention.  FIG. 5  is a cross-sectional view illustrating a folded state of a window of  FIG. 3  according to an exemplary embodiment of the present invention. 
     Referring to  FIGS. 4 and 5 , the window  400  may include a glass layer  410 . The glass layer  410  may be bendable or foldable. The window  400  may also include a functional coating layer  420 . The functional coating layer  420  may be disposed on the glass layer  410 . 
     The glass layer  410  may include a glass material. The glass material may have a relatively high strength, surface flatness, and transparency. 
     According to an exemplary embodiment of the present invention, the glass layer  410  may include a chemical enhancing layer. The chemical enhancing layer may be formed on an outer surface of the glass layer  410  by, for example, performing a chemically enhancing process. For example, a compressive stress may be formed in the chemical enhancing layer. Additionally, a tensile stress may be formed in a portion of the glass layer  410  disposed inside the chemical enhancing layer. The strength of the glass layer  410  may be increased by forming the chemical enhancing layer in the glass layer  410 . 
     Various methods may be used to form the glass layer  410 . According to an exemplary embodiment of the present invention, after preparing a mother glass substrate having a thickness of about 100 micrometer (um) or less, the mother glass substrate may be formed into a glass layer having a predetermined shape through cutting, polishing, and firing. The glass layer may then be chemically enhanced. According to an exemplary embodiment of the present invention, after preparing a relatively thick mother glass substrate, a slimming process may be performed to provide a slimmed mother glass substrate. A shape manufacturing and chemically enhancing process may be performed on the slimmed mother glass substrate. The slimming process may be performed using mechanical methods and/or chemical methods. 
     The chemically enhancing process may be performed on the shape manufactured glass substrate by firing the glass substrate for about 15 hours to about 18 hours in a temperature from about 400° C. to about 450° C. after exposing an outer surface of the glass substrate to a KNO 3  solution. Sodium (Na) on a surface of the glass substrate may be replaced by potassium (K). Thus, the strength of the surface of the glass substrate may be increased. Since sodium (Na) on the surface of the glass substrate is replaced by potassium (K), the chemical enhancing layer may be formed on the surface of the glass layer  410 . 
     As illustrated in  FIG. 5 , the window  400  may be bent or folded according to a bending or folding direction of the display panel  100  illustrated in  FIG. 1 . Layers included in the window  400  may have a relatively small bending stiffness. Therefore, the window  400  may be easily bent or folded. The bending stiffness of a single layer may be calculated by Equation 1: 
       BS∝E×TH 3 [Equation 1]
 
     In Equation 1, BS may indicate a bending stiffness of the single layer; E may indicate an elastic modulus of the single layer; and TH may indicate a thickness of the single layer. 
     The bending stiffness of the glass layer  410  may be proportional to the cube of the thickness of the glass layer  410 . Therefore, the thickness of the glass layer  410  may be relatively small so that the glass layer  410  may have a relatively small bending stiffness. 
     According to an exemplary embodiment of the present invention, the thickness of the glass layer  410  may be less than about 100 um. Therefore, the window  400  including the glass layer  410  may be bent or folded with a relatively small radius of curvature. 
     When the window  400  is deformed or impacted, the window  400  may be damaged. When the window  400  including the glass layer  410  is deformed or impacted, a tensile stress may be applied on the glass layer  410 . The tensile stress may break the glass layer  410 . Fine glass fragments formed by a broken glass layer  410  may be scattered. When the glass layer  410  includes the chemical enhancing layer, the tensile stress on the glass layer  410  may be determined by Equation 2: 
     
       
         
           
             
               
                 
                   CT 
                   = 
                   
                     
                       CS 
                       × 
                       DOL 
                     
                     
                       T 
                       - 
                       
                         2 
                          
                         
                             
                         
                          
                         DOL 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 2, CT may indicate a tensile stress on the glass layer  410 ; CS may indicate a compressive stress applied on the surface of the glass layer  410 ; DOL may indicate a thickness of the chemical enhancing layer; and T may indicate a thickness of the glass layer  410 . 
     As shown in Equation 2, the tensile stress CT on the glass layer  410  may increase as the thickness of the glass layer  410  decreases. Additionally, the glass layer  410  may have a relatively large tensile stress CT. The tensile stress CT of the glass layer  410  may be about three times greater than that of a general glass layer when substantially the same compressive stress are applied to a surface of the glass layer  410 . When a relatively large tensile stress CT is applied to the glass layer  410 , the glass layer  410  may break. Therefore, fine glass fragments formed when the glass layer  410  broke may be scattered. A user may then be exposed to and injured by the scattered glass fragments. 
     The window  400  may include the glass layer  410 . The glass layer  410  may be chemically enhanced. The glass layer  410  may also have a relatively small thickness. Therefore, the bending stiffness of the glass layer  410  may be a relatively small and the window  400  may be bent or folded. However, the glass layer  410  may be broken or scattered by an impact on the window  400 . Accordingly, a countermeasure may be needed to reduce or prevent the breaking or scattering of the glass layer  410 . 
     The functional coating layer  420  may be disposed on the glass layer  410 . The functional coating layer  420  may increase the strength of the window  400 . The functional coating layer  420  may reduce or prevent the scatter of the glass layer  410 . 
     According to an exemplary embodiment of the present invention, the functional coating layer  420  may be disposed on a surface of the glass layer  410  facing the display panel  100 . For example, the functional coating layer  420  may be disposed between the glass layer  410  and the display panel  100 . Therefore, the glass layer  410  may be disposed on the first surface  10   a  of the flexible display device  10 . 
     When a portion of the glass layer  410  is impacted, the functional coating layer  420  may offset a tensile stress formed on the glass layer  410  by the impact. Accordingly, the functional coating layer  420  may reduce or prevent the glass layer  410  from breaking. The functional coating layer  420  may also absorb an impact energy formed when the glass layer  410  is broken. Therefore, the functional coating layer  420  may reduce or prevent fine glass fragments from being scattered. The functional coating layer  420  may include an elastic material. The elastic material may absorb the impact energy. The functional coating layer  420  may include a flexible material. The flexible material may be bendable or foldable. Since the functional coating layer  420  may be in direct contact with the glass layer  410 , an increased adhesion between the functional coating layer  420  and the glass layer  410  may be needed. 
     According to an exemplary embodiment of the present invention, the functional coating layer  420  may include a urethane-based resin, an epoxy-based resin, a polyester-based resin, a polyether-based resin, an acrylate-based resin, an ABS resin, and/or rubber. For example, the functional coating layer  420  may include polyurethane (PU), a combination of polyurethane and rubber, or a combination of polyurethane and acrylic monomer. 
     According to an exemplary embodiment of the present invention, the functional coating layer  420  may be combined with the glass layer  410 . The functional coating layer  420  may be formed on the glass layer  410  by using, for example, a coating method. For example, the functional coating layer  420  may be formed on the glass layer  410  by using a slip coating method, a bar coating method, or a spin coating method. According to an exemplary embodiment of the present invention, the functional coating layer  420  may be formed on an entire surface of the glass layer  410 . 
     A thickness of the functional coating layer  420  may be in a range from about 1 um and about 10 um, for example, from about 3 um to about 10 um. When the thickness of the functional coating layer  420  is less than about 1 um, the functional coating layer  420  might not absorb the impact energy when the glass layer  410  is impacted. When the thickness of the functional coating layer  420  is greater than about 10 um, the bending stiffness of the functional coating layer  420  may increase as described in Equation 1. Therefore a deformation of the glass layer  410  by the impact may increase and the tensile stress on the glass layer  410  may increase. 
     The functional coating layer  420  may have an elastic modulus. The elastic modulus may be less than the elastic modulus of the glass layer  410 . The functional coating layer  420  may include an elastic material. The elastic material may absorb the impact energy occurring from the glass layer  410  being impacted. Since the bending stiffness of the functional coating layer  420  may be proportional to the elastic modulus of the functional coating layer  420  as described in Equation 1, the functional coating layer  420  may have an elastic modulus less than that of the glass layer  410 . Therefore, the functional coating layer  420  might not alter the flexible properties of the window  400 . 
     According to an exemplary embodiment of the present invention, the elastic modulus of the functional coating layer  420  may be in a range from about 1.52 gigapascal (GPa) to about 5 GPa, for example, from about 2 GPa to about 4 GPa. For example, the elastic modulus of the glass layer  410  directly combined with the functional coating layer  420 , may be about 69.3 GPa. When the elastic modulus of the functional coating layer  420  is less than about 1.52 GPa, the degree of the deformation of the glass layer  410  by impact may increase. Therefore, a tensile stress on the glass layer  410  may increase. When the elastic modulus of the functional coating layer  420  is greater than about 5 GPa, the functional coating layer  420  might not absorb the impact energy occurred when the fine glass fragments are scattered. 
     According to an exemplary embodiment of the present invention, a light transmittance of the functional coating layer  420  may be greater than or equal to about 88%. The light transmittance of the functional coating layer  420  may be greater than or equal to about 90%. Light emitted from the pixels of the display panel  100  may be visible to a user through the window  400 . The light emitted from the pixels may transmit the functional coating layer  420  disposed on the entire surface of the glass layer  410 . The functional coating layer  420  may have enough light transmittance to reduce or prevent a luminance of light emitted from the display panel  100 . 
     When the window  400  is impacted, the functional coating layer  420  may reduce or prevent breakage of the glass layer  410 . Therefore, an impact resistance of the window  400  may be increased. For example, the window  400  may have an impact resistance as indicated by a drop height of at least 6 centimeter (cm) as determined by a pen drop measurement using a 5.7 gram pen. The window  400  may be not broken when the 5.7 gram pen is dropped at a height less than or equal to about 6 cm from the window  400 . 
     As illustrated in  FIG. 5 , the window  400  may be folded so that portions of the functional coating layer  420  may face each other. According to an exemplary embodiment of the present invention, the window  400  may have a radius curvature RI less than or equal to about 4.5 millimeter (mm). The functional coating layer  420  might not detach from the glass layer  410  in the radius curvature RI less than or equal to about 4.5 mm. Furthermore, the functional coating layer  420  may maintain adhesion with the glass layer  410 . 
     Exemplary embodiments of the present invention will be explained in detail below with reference to experimental results. Exemplary embodiments of the present invention described below are for description purposes and exemplary embodiments of the present invention are not limited thereto. 
     Tables 1 to 3 illustrate experimental results observing scatter preventing effect, impact resistance, and curvature reliability of windows according to exemplary embodiments of the present invention and comparative examples. The scatter preventing effect represents whether fine glass fragments are scattered or not when a window is broken. The impact resistance represents the drop height in order to break a window when a 5.7 gram pen is dropped. The curvature reliability represents whether a window is detached or not when the window is bended in about 4.5 mm radius curvature. 
     Table 1 illustrates experimental results observing scatter preventing effect, impact resistance, and curvature reliability of the window with changing the material composition and the thickness of the functional coating layer. A polyethylene terephthalate (PET) layer having about 50 um thickness and a pressure sensitive adhesive (PSA) layer having about 50 um thickness are stacked on a metal plate, and the functional coating layer and the glass layer are stacked thereon for the experiments. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Material 
                   
                 Scatter 
                 Impact 
                   
                   
               
               
                   
                 (elastic 
                 Thickness 
                 preventing 
                 resistance 
                 Curvature 
                 Window 
               
               
                   
                 modulus) 
                 (um) 
                 effect 
                 (cm) 
                 reliability 
                 availability 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 First 
                 Polyurethane 
                 3 
                 ◯ 
                 10 
                 ◯ 
                 ◯ 
               
               
                 embodiment 
                 (3.61 GPa) 
               
               
                 Second 
                   
                 5 
                   
                 7 
               
               
                 embodiment 
               
               
                 Third 
                   
                 10 
                   
                 7 
               
               
                 embodiment 
               
               
                 First 
                   
                 20 
                   
                 6 
                 X 
                 X 
               
               
                 comparative 
               
               
                 example 
               
               
                 Second 
                   
                 25 
                   
                 5 
               
               
                 comparative 
               
               
                 example 
               
               
                 Fourth 
                 Polyurethane + 
                 3 
                   
                 11 
                 ◯ 
                 ◯ 
               
               
                 embodiment 
                 Rubber 
               
               
                 Fifth 
                 (2.78 GPa) 
                 5 
                   
                 9 
               
               
                 embodiment 
               
               
                 Sixth 
                   
                 10 
                   
                 8 
               
               
                 embodiment 
               
               
                 Third 
                   
                 20 
                   
                 5 
                   
                 X 
               
               
                 comparative 
               
               
                 example 
               
               
                 Fourth 
                   
                 25 
                   
                 4 
               
               
                 comparative 
               
               
                 example 
               
               
                 Seventh 
                 Polyurethane + 
                 1 
                   
                 6 
                   
                 ◯ 
               
               
                 embodiment 
                 Acrylic 
               
               
                 Eighth 
                 monomer 
                 3 
                   
                 10 
               
               
                 embodiment 
                 (4.25 GPa) 
               
               
                 Ninth 
                   
                 10 
                   
                 11 
               
               
                 embodiment 
               
               
                 Fifth 
                   
                 20 
                   
                 7 
                 X 
                 X 
               
               
                 comparative 
               
               
                 example 
               
               
                 Sixth 
                   
                 25 
                   
                 5 
               
               
                 comparative 
               
               
                 example 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, the windows have a scatter preventing effect in the exemplary embodiments of the present invention and comparative examples. The windows according to exemplary embodiments of the present invention have impact resistances greater than or equal to about 6 cm and curvature reliability. However, the windows according to comparative examples have impact resistances less than or equal to about 5 cm or do not have curvature reliability. Therefore, the windows according to exemplary embodiments of the present invention may be suitable to be included as windows in a flexible display device. However, the windows according to comparative examples are not suitable to be included as windows in a flexible display device. 
     Table 2 illustrates experimental results observing scatter preventing effect, impact resistance, and curvature reliability of the window without the functional coating layer. A polyethylene terephthalate (PET) layer having about 50 um thickness and a pressure sensitive adhesive (PSA) layer having about 50 um thickness are stacked on a metal plate, and the glass layer  410  are stacked thereon for the experiments. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Material 
                   
                 Scatter 
                 Impact 
                   
                   
               
               
                   
                 (elastic 
                 Thickness 
                 preventing 
                 resistance 
                 Curvature 
                 Window 
               
               
                   
                 modulus) 
                 (um) 
                 effect 
                 (cm) 
                 reliability 
                 availability 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Seventh 
                 X 
                 X 
                 X 
                 5 
                 ◯ 
                 X 
               
               
                 comparative 
               
               
                 example 
               
               
                   
               
            
           
         
       
     
     Referring to Table 2, the window according to seventh comparative example has curvature reliability, however, does not have scatter preventing effect and has impact resistance less than or equal to about 5 cm. Therefore, the window according to seventh comparative example is not suitable to be included as a window in a flexible display device. 
     Table 3 illustrates experimental results observing scatter preventing effect, impact resistance and curvature reliability of the window with a generally used optically cleared adhesive (OCA) film instead of the functional coating layer. A polyethylene terephthalate (PET) layer having about 50 um thickness and a pressure sensitive adhesive (PSA) layer having about 50 um thickness are stacked on a metal plate. The optically cleared adhesive (OCA) including a pressure sensitive adhesive (PSA) layer having about 30 um thickness and a polyethylene terephthalate (PET) layer having about 50 um thickness, and the glass layer  410  are stacked thereon for the experiments. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Material 
                   
                 Scatter 
                 Impact 
                   
                   
               
               
                   
                 (elastic 
                 Thickness 
                 preventing 
                 resistance 
                 Curvature 
                 Window 
               
               
                   
                 modulus) 
                 (um) 
                 effect 
                 (cm) 
                 reliability 
                 availability 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Eighth 
                 PSA/PET 
                 30/50 
                 X 
                 3 
                 X 
                 X 
               
               
                 comparative 
               
               
                 example 
               
               
                   
               
            
           
         
       
     
     Referring to Table 3, the window according to the eighth comparative example has scatter preventing effect. However, the window according to the eighth comparative example has an impact resistance equal to about 3 cm and does not have curvature reliability. Therefore, the window according to eighth comparative example is not suitable to be included as a window for a flexible display device. 
     The windows for a display device and a flexible display device according to exemplary embodiment of the present invention may be included in various display devices. For example, the windows and the flexible display devices may be applied to personal computers, notebook computers, mobile phones, smart phones, tablet computers, personal media players (PMP), personal digital assistance (PDA), or MP3 players; however, exemplary embodiments of the present invention are not limited thereto. 
     Although windows for a display device and a flexible display device including the same in accordance with exemplary embodiments of the present invention have been described with reference to the accompanying drawings, exemplary embodiments of the present invention are not limited thereto. Those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments of the present invention without materially departing from the scope of the present inventive concept.