Patent Publication Number: US-2022236769-A1

Title: Foldable Display Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/688,931 filed on Nov. 19, 2019, which claims priority to Republic of Korea Patent Application No. 10-2018-0157274, filed on Dec. 7, 2018, each of which is incorporated herein by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to a foldable display device. 
     Description of the Related Art 
     An image display device that implements various information on a screen is a core technology in an information communication era and is developing to be thinner, lighter, more portable, and have higher performance. Therefore, an organic light emitting display device that displays images through an organic light emitting diode is getting the spotlight. 
     The organic light emitting diode is a self-emitting device using a thin light emitting layer between two electrodes so that it is advantageous to be manufactured to be thin. The organic light emitting display device has a structure in which a driving circuit and an organic light emitting diode are formed on a substrate and displays images using light emitted from the organic light emitting diode. 
     Since the organic light emitting display device may be implemented without having a separate light source, it is easy to be applicable to a flexible display device. 
     When the organic light emitting display device is implemented as a flexible display device, studies are being conducted to develop the organic light emitting display device as a foldable display device in which several parts of the display device can be bent. 
     SUMMARY 
     In various embodiments, when a cover window of a foldable display device is formed of a glass, an optical element can protect a user from scattering of fragments due to external impact or stress. Further, an optical element having various function without increasing an overall thickness of the foldable display device can be advantageous and provide a solution of the above-mentioned problem. 
     Embodiments of the present disclosure include a foldable display device including an optical element having various functions without increasing the overall thickness. 
     An object to be achieved by the present disclosure is to provide a foldable display device capable of minimizing or reducing a stress and an impact due to the folding. 
     A replaceable adhesive layer having a high adhesiveness may be disposed between the optical element and the cover window. 
     An object to be achieved by the present disclosure is to provide a foldable display device including an adhesive layer having a high adhesiveness with the optical element and a releasing property. 
     Objects of the present disclosure are not limited to the above-mentioned objects, and other objects, which are not mentioned above, can be clearly understood by those skilled in the art from the following descriptions. 
     In order to solve the above-described problems, according to an aspect of the present disclosure, a foldable display device, includes: a panel assembly divided into a folding area and a non-folding area; and an optical element on the panel assembly, where the optical element includes zigzag grooves repeatedly patterned on an upper surface of the optical element. 
     In order to solve the above-described problems, according to an aspect of the present disclosure, a foldable display device includes: a panel assembly divided into a folding area and a non-folding area; a back plate supporting the panel assembly below the panel assembly; an optical element on the panel assembly; and an adhesive layer between the panel assembly and the optical element, where the optical element includes zigzag grooves repeatedly patterned on an upper surface of the optical element. 
     Other detailed matters of the exemplary embodiments are included in the detailed description and the drawings. 
     According to the present disclosure, the stress and the impact due to the folding may be minimized or reduced in a folding area to reduce the damage of the display panel. 
     According to the present disclosure, as another function, the display device is flexibly bendable and a scratch resistance, impact resistance, and optical transparency may be improved or ensured. 
     The effects according to the present disclosure are not limited to the contents exemplified above, and more various effects are included in the present specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  is an exploded perspective view of a foldable display device according to an embodiment of the present disclosure. 
         FIGS. 2A and 2B  are cross-sectional views of a panel assembly of a foldable display device according to the embodiment of the present disclosure illustrated in  FIG. 1 . 
         FIGS. 3A and 3B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
         FIG. 4  is a cross-sectional view illustrating a foldable display device according to an embodiment of the present disclosure. 
         FIG. 5  is a cross-sectional view illustrating another foldable display device according to an embodiment of the present disclosure. 
         FIG. 6  is a cross-sectional view illustrating another foldable display device according to an embodiment of the present disclosure. 
         FIG. 7  is a graph of dynamic shear storage modulus of an adhesive layer in accordance with an embodiment of the present disclosure. 
         FIG. 8  is a table for comparing characteristics in accordance with a tensile elastic modulus and a thickness of an optical element according to various embodiments of the present disclosure. 
         FIG. 9  is a table for comparing characteristics in accordance with a thickness of an adhesive layer according to various embodiments of the present disclosure. 
         FIGS. 10A and 10B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
         FIGS. 11A and 11B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
         FIGS. 12A and 12B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
         FIGS. 13A and 13B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Advantages and characteristics of the present disclosure and a method of achieving the advantages and characteristics will be clear by referring to exemplary embodiments described below in detail together with the accompanying drawings. However, the present disclosure is not limited to the exemplary embodiments disclosed herein but may be implemented in various forms. The exemplary embodiments are provided by way of example so that those skilled in the art can fully understand the disclosures of the present disclosure and the scope of the present disclosure. Therefore, the present disclosure will be defined by the scope of the appended claims. 
     The shapes, sizes, ratios, angles, numbers, and the like illustrated in the accompanying drawings for describing the exemplary embodiments of the present disclosure are merely examples, and the present disclosure is not limited thereto. Like reference numerals generally denote like elements throughout the specification. Further, in the following description of the present disclosure, a detailed explanation of known related technologies may be omitted to avoid unnecessarily obscuring the subject matter of the present disclosure. The terms such as “including,” “having,” and “consist of” used herein are generally intended to allow other components to be added unless the terms are used with the term “only.” Any references to singular may include plural unless expressly stated otherwise. 
     Components are interpreted to include an ordinary error range even if not expressly stated. 
     When the position relation between two parts is described using the terms such as “on,” “above,” “below,” and “next,” one or more parts may be positioned between the two parts unless the terms are used with the term “immediately” or “directly.” 
     When an element or layer is disposed “on” another element or layer, another layer or another element may be interposed directly on the other element or therebetween. 
     Although the terms “first,” “second,” and the like are used for describing various components, these components are not confined by these terms. These terms are merely used for distinguishing one component from the other components. Therefore, a first component to be mentioned below may be a second component in a technical concept of the present disclosure. 
     Like reference numerals generally denote like elements throughout the specification. 
     A size and a thickness of each component illustrated in the drawing are illustrated for convenience of description, and the present disclosure is not limited to the size and the thickness of the component illustrated. The figures may not necessarily be drawn to scale. 
     The features of various embodiments of the present disclosure can be partially or entirely adhered to or combined with each other and can be interlocked and operated in technically various ways, and the embodiments can be carried out independently of or in association with each other. 
     Hereafter, various embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is an exploded perspective view of a foldable display device according to an embodiment of the present disclosure. 
       FIGS. 2A and 2B  are cross-sectional views of a panel assembly of a foldable display device according to the embodiment of the present disclosure illustrated in  FIG. 1 . 
     In this case,  FIG. 2B  illustrates a cross-sectional structure of a panel assembly  110  and  FIG. 2A  illustrates that an optical element  120  is additionally provided above the panel assembly  110  illustrated in  FIG. 2B . The optical element  120  may be provided above the panel assembly  110  with an adhesive layer  115  therebetween. 
     Referring to  FIGS. 1, 2A, and 2B , a foldable display device  100  according to an embodiment of the present disclosure may include a panel assembly  110 , a set frame  170 , a hinge unit  150 , an adjustable supporting unit  160 , and an optical element  120 . 
     The panel assembly  110  may include a display panel  111  to display images. 
     In the display panel  111 , a display element for displaying images and a circuit, wiring line, components, and the like for driving the display element may be disposed. 
     A predetermined optical element  120  may be additionally attached above the panel assembly  110  with the adhesive layer  115  therebetween. 
     The panel assembly  110  may be divided into a folding area FA and a non-folding area NFA, which may include one or more non-folding areas separated by a folding area as shown in  FIG. 2A . 
     The folding area FA is an area where the panel assembly  110  can be folded. When the foldable display device  100  is unfolded, the folding area FA may be maintained to be flat, and when the foldable display device  100  is folded, the folding area FA may be folded or bended. 
     The non-folding area NFA is an area where the panel assembly  110  is maintained to be a flat (e.g., unbent) state in both the unfolded state and the folded state of the panel assembly  110 . The non-folding areas NFA can be connected to opposite sides of the folding area FA and may rotate (e.g., about a folding axis) as the folding area FA is folded. The non-folding area NFA may be on a same plane as the folding area FA in the unfolded state and the non-folding areas NFA may be opposite to each other in the folded state. In the folded state, a first non-folding area NFA may overlap or cover a second non-folding area NFA. 
     The set frame  170  is disposed below the panel assembly  110  to support at least a part of the panel assembly  110 . Specifically, the non-folding area NFA of the panel assembly  110  may be supported on an inner surface of the set frame  170 . 
     Among the surfaces forming the inner surfaces of the set frame  170 , a lower surface of the set frame  170  in contact with a lower surface of the panel assembly  110  may be formed by a first lower surface  171 , an inclined surface  172 , a seating surface  173 , and a second lower surface  174 . 
     Specifically, the second lower surface  174  supports the non-folding area NFA of the panel assembly  110  and the inner surface may be formed to be thinnest on the second lower surface  174  among the lower surfaces of the set frame  170  (e.g., the first lower surface  171 , inclined surface  172 , seating surface  173 , and second lower surface  174 ). An adhesive material is applied between the second lower surface  174  and the non-folding area NFA of the panel assembly  110  to fix the panel assembly  110  to the set frame  170 . 
     The seating surface  173  is a surface on which an end portion of the adjustable supporting unit  160  is seated in the unfolded state of the foldable display device  100 , and the adjustable supporting unit  160  may be hung on the seating surface  173 . In this case, the seating surface  173  has a step lower than the second lower surface  174  so that an end portion of the adjustable supporting unit  160  does not move to the second lower surface  174 . In this case, a thickness of the step between the seating surface  173  and the second lower surface  174  may be the same as the thickness of the adjustable supporting unit  160 . 
     The inclined surface  172  connects the seating surface  173  to the first lower surface  171  and may form an inclination relative to the first lower surface  171  and the second lower surface  174 . 
     Among the lower surfaces of the set frame  170 , the inner surface is formed to be deepest on the first lower surface  171 . Among the lower surfaces of the set frame  170 , the first lower surface  171  is the closest to the hinge unit  150 . When the foldable display device  100  is folded, the first lower surface  171  may be in contact with the adjustable supporting unit  160 . 
     First hinge coupling units  175  that extend to the hinge unit  150  are provided at both ends of one end portion of the set frame  170  at a side of the hinge unit  150 . The first hinge coupling unit  175  may be hinged to a first hinge shaft  151  of the hinge unit  150  and the set frame  170  may rotate about the first hinge shaft  151 , but is not limited thereto. 
     A recessed portion may be defined in the set frame  170  by the first lower surface  171 , the inclined surface  172 , an outer surface of the hinge unit  150 , and the first hinge coupling unit  175 . The recessed portion is an area that is recessed from the inner surface of the set frame  170  adjacent to the hinge unit  150 . That is, the recessed portion is an empty space formed by the inner surface of the set frame  170 , which is adjacent to the hinge unit  150  and retreated from the hinge unit  150 . The recessed portion is an empty space between the first lower surface  171 , the inclined surface  172 , and the hinge unit  150  and in the unfolded state, the adjustable supporting unit  160  may be disposed above the recessed portion, but is not limited thereto. 
     A guide groove  176  may be disposed on an inner surface of the set frame  170 . That is, the guide groove  176  is disposed on a guide surface at a side of the first hinge coupling unit  175  on the inner surface of the set frame  170 . On the guide surface, the guide groove  176  may be disposed to be adjacent to a boundary of the guide surface, the seating surface  173 , the inclined surface  172 , and the first lower surface  171 , but is not limited thereto. 
     Even though not illustrated in the drawing, the guide surface is one of the side surfaces of the inner surface of the set frame  170  and is a side connected from both ends of the end portion of the set frame  170  at the side of the hinge unit  150  to the first hinge coupling unit  175 . Therefore, the guide surface may be disposed to be perpendicular to the first lower surface  171 . 
     The guide groove  176  may coincide with a rotation trajectory of the end portion of the adjustable supporting unit  160  so that the adjustable supporting unit  160  does not deviate during the rotation. Specifically, for example, a protruding portion  161  of the adjustable supporting unit  160  may be fitted to the guide groove  176  and the adjustable supporting unit  160  may be slidably guided along the guide groove  176  when the foldable display device  100  is switched between the folded state and the unfolded state. Therefore, in the unfolded state, one end of the guide groove  176  is adjacent to the seating surface  173  on which the end portion of the adjustable supporting unit  160  is disposed, and in the folded state, the other end of the guide groove  176  may be adjacent to the first lower surface  171  on which the end portion of the adjustable supporting unit  160  is disposed. 
     The adjustable supporting unit  160  is disposed below the panel assembly  110  to support the panel assembly  110  together with the set frame  170 . Specifically, for example, the adjustable supporting unit  160  may support the folding area FA of the panel assembly  110 . In the unfolded state, one pair of adjustable supporting units  160  and one pair of set frames  170  form one plane (e.g., the pair of adjustable supporting units  160  are on a same plane as the pair of set frames  170 ). In the folded state, the adjustable supporting unit  160  may retreat toward the inner surface of the set frame  170 . That is, in the folded state, the adjustable supporting unit  160  rotates toward the lower surface of the set frame  170  to be in contact with the first lower surface  171  of the set frame  170 . However, in contrast to the second lower surface  174  of the set frame  170 , a separate adhesive material is not applied on the adjustable supporting unit  160  and the adjustable supporting unit  160  is not fixed to the panel assembly  110 . 
     The adjustable supporting unit  160  may include a second hinge coupling unit (not illustrated) that extends from the lower surface of the adjustable supporting unit  160  to the hinge unit  150  at an arbitrary intermediate point between both end portions of the adjustable supporting unit  160 . The second hinge coupling unit may provide additional mechanical support to the foldable display device  100 . 
     The second hinge coupling unit may be rotatably coupled to a second hinge shaft (not illustrated) of the hinge unit  150 . Therefore, the adjustable supporting unit  160  may rotate about the second hinge shaft on the second hinge shaft, and a rotation trajectory of both end portions of the adjustable supporting unit  160  may be determined with respect to the second hinge shaft. 
     The adjustable supporting unit  160  may include protruding portions  161  disposed at both ends of the end portion of the adjustable supporting unit  160  at a side of the set frame  170 . The protruding portion  161  extends toward the guide surface of the set frame  170  to be fitted to the guide groove  176  of the guide surface. Therefore, when the adjustable supporting unit  160  rotates about the second hinge shaft, the protruding portion  161  may slide along the guide groove  176 . 
     The hinge unit  150  is disposed below the panel assembly  110  to be hinged to one pair of set frames  170  and one pair of adjustable supporting units  160 . Therefore, one pair of set frames  170  and one pair of adjustable supporting units  160  may rotate about the hinge unit  150 . The hinge unit  150  is disposed below the folding area FA of the panel assembly  110  and includes one pair of first hinge shafts  151  and one pair of second hinge shafts, but is not limited thereto. The folding area FA may have a width corresponding (e.g., equal) to a width of the hinge unit  150 . 
     The first hinge shafts  151  protrude from outer surfaces of both ends of the hinge unit  150  to be coupled to the first hinge coupling unit  175  of the set frame  170 . In this case, one pair of the first hinge shafts  151  is coupled to one pair of set frames  170 , respectively. For example, one of the first hinge shafts  151  is coupled to one pair of first hinge coupling units  175  in the set frame  170  disposed on one side of the hinge unit  150  and the other first hinge shaft  151  is coupled to one pair of first hinge coupling units  175  in the set frame  170  disposed on the other side of the hinge unit  150 . Therefore, the foldable display device  100  according to the embodiment shown in  FIG. 1  may be implemented by a biaxial hinge structure, but it is not limited thereto. Other embodiments may include a different number or arrangement of hinges to facilitate folding or bending of the foldable display device  100 . 
     The second hinge shaft protrudes from inner surfaces at both ends of the hinge unit  150  to be coupled to the second hinge coupling unit of the adjustable supporting unit  160 . In this case, the second hinge shafts are coupled to one pair of adjustable supporting units  160 . For example, one second hinge shaft is coupled to the second hinge coupling unit of the adjustable supporting unit  160  disposed at one side of the hinge unit  150  and the other second hinge shaft is coupled to the second hinge coupling unit of the adjustable supporting unit  160  disposed at the other side of the hinge unit  150 . 
     The foldable display device  100  according to an embodiment of the present disclosure is a display device in which a display element, a circuit, and the like are formed on a flexible substrate. Therefore, even though the foldable display device is bendable or foldable, images can be displayed in a folded state, an unfolded state, or while bending between the folded state and the unfolded state. The foldable display device  100  may be switched between an unfolded state in which the foldable display device  100  is unfolded to be flat and a folded state in which the upper surfaces of the foldable display device  100  are folded to be in contact with each other. 
     Hereinafter, the panel assembly  110  will be described in more detail. 
     Referring to  FIG. 2B , the panel assembly  110  according to an embodiment of the present disclosure may include a display panel  111 , a back plate  113 , a frame  114 , a cover window  112 , and a plurality of adhesive layers  115   a ,  115   b ,  115   c , and  115   d.    
     As described above, the display panel  111  may be divided into an active area and a non-active area as well as a folding area FA and a non-folding area NFA. The active area and/or the non-active area may overlap with at least a part of the folding area FA and/or the non-folding area NFA. 
     The active area is an area where images are displayed and a plurality of pixels may be disposed. 
     In the active area, a display element for displaying images and a circuit unit for driving the display element may be disposed. For example, when the foldable display device  100  is an organic light emitting display device, the display element may include an organic light emitting diode. 
     Hereinafter, for the convenience of description, it is assumed that the display device according to various exemplary embodiments of the present disclosure is a foldable display device  100  including an organic light emitting diode, but is not limited thereto. 
     The circuit units may include various thin film transistors, capacitors, wiring lines, and the like for driving the organic light emitting diode. For example, the circuit units may include various components such as a driving thin film transistor, a switching thin film transistor, a storage capacitor, a gate line, and a data line, but are not limited thereto. 
     The non-active area is an area where images are not displayed but a circuit, a wiring line, a component, and the like for driving the display element of the active area are disposed. 
     In the non-active area, various ICs such as a gate driver IC and a data driver IC and driving circuits may be disposed. For example, various ICs and the driving circuit may be mounted in the non-active area of the display panel  111  by a gate in panel (GIP) or connected to the display panel  111  by a tape carrier package (TCP) or a chip on film (COF) manner. 
     The folding area FA is an area where the panel assembly  110  is folded and may overlap with or include at least a part of the active area and at least a part of the non-active area. 
     Further, in the non-folding area NFA, the panel assembly  111  maintains a flat state. 
     The non-folding area NFA may extend to opposite sides of the folding area FA. That is, one pair of non-folding areas NFA may be defined with the folding area FA therebetween. Further, when the panel assembly  110  is folded, one pair of non-folding areas NFA may be disposed to be opposite to each other, e.g., overlapping or covering at least a portion of each other. 
     The back plate  113  may be disposed below the display panel  111 . 
     When a substrate included in the display panel  111  is formed of a plastic material such as polyimide (PI), the manufacturing process of the foldable display device  100  is performed in a situation that the support substrate formed of glass is disposed below the substrate. Further, after forming components such as a polarizing plate or a touch panel on the display panel  111 , the support substrate may be released. However, since a component for supporting the substrate is necessary even after releasing the support substrate, a back plate  113  formed of a metal foil for supporting the substrate may be disposed below the substrate. The back plate  113  may protect the display panel  111  from moisture or other contaminants, heat, impact or other mechanical damage, and the like from the outside. 
     The frame  114  may be disposed below the back plate  113 . The frame  114  may protect the back plate  113  and the display panel  111  above the frame  114 . 
     In some embodiments, a plurality of frames  114  may be provided. 
     The frame  114  may be disposed in the non-folding area NFA excluding the folding area FA. Since the frame  114  is not disposed in the folding area FA, the frame  114  is not folded and is not applied with the stress due to the folding. 
     Though not illustrated in  FIG. 2B , a polarizing plate may be disposed above the display panel  111 . 
     The polarizing plate selectively transmits light to reduce the reflection of external light incident onto the display panel  111 . Specifically, the display panel  111  may include various metal materials applied to the thin film transistor, the wiring line, the organic light emitting diode, and the like. Therefore, the external light incident onto the display panel  111  may be reflected from the metal material so that the visibility of the foldable display device  100  may be reduced due to the reflection of the external light. Therefore, when the polarizing plate is disposed, the polarizing plate suppresses the reflection of the external light to increase the outdoor visibility of the foldable display device  100 . However, the polarizing plate may be omitted in some embodiments depending on an implementation of the foldable display device  100 . 
     Further, even though not illustrated in  FIG. 2B , a touch panel may be further disposed between the polarizing plate and the display panel  111 . The touch panel is a device that senses a screen touch on the foldable display device  100  or a touch input of the user such as a gesture and may be a resistive type, a capacitive type, an optical type, or an electromagnetic type. 
     The cover window  112  may be disposed above the polarizing plate. 
     The cover window  112  may protect the polarizing plate and the display panel  111  below the cover window  112  from the impact or other mechanical damage, the moisture or other contaminants, heat, and the like from the outside. The cover window  112  may be formed of a glass. 
     The adhesive layers  115   a ,  115   b ,  115   c , and  115   d  are disposed between the above-described components to be used to bond a component with another component. The adhesive layers  115   a ,  115   b ,  115   c , and  115   d  may be disposed between the cover window  112  and the display panel  111 , between the display panel  111  and the back plate  113 , between the back plate  113  and the frame  114 , and between the frame  114  and another layer, respectively. For example, the adhesive layers  115   a ,  115   b ,  115   c , and  115   d  may be formed of an optical clear adhesive (OCA) or a pressure sensitive adhesive (PSA), but is not limited thereto. 
     The foldable display device  100  according to an embodiment of the present disclosure may be in-folded and/or out-folded. When the foldable display device  100  is in-folded, the foldable display device  100  may be folded such that upper surfaces of the frame  114  are opposite to each other at both sides. In other words, the foldable display device  100  may be folded such that upper surfaces of the optical element  120  are opposite to each other at both sides. 
     When the foldable display device  100  is out-folded, the foldable display device  100  may be folded such that lower surfaces of the frame  114  are opposite to each other at both sides. 
     When the foldable display device  100  configured as described above is an organic light emitting display device, the display panel  111  may be an organic light emitting display panel. 
     Even though not illustrated, the organic light emitting display panel may include an organic light emitting diode disposed on the substrate, a capping layer disposed on the organic light emitting diode, a protective layer disposed on the capping layer, and an encapsulating layer disposed on the protective layer. 
     The organic light emitting diode may be formed on one surface of the substrate for each unit pixel. The organic light emitting diode may include a first electrode supplied with a current from the driving transistor, a second electrode corresponding to the first electrode, and a light emitting layer disposed between the first electrode and the second electrode. 
     The first electrode may be provided as a transparent electrode or a reflective electrode. 
     When the first electrode is configured as a transparent electrode, the first electrode may be formed of ITO, IZO, ZnO, or In 2 O 3 . When the first electrode is configured as a reflective electrode, the first electrode may include a reflective film formed of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr, or a compound thereof and a transparent film formed of ITO, IZO, ZnO, or In 2 O 3  thereon. 
     The light emitting layer may include a red light emitting layer, a green light emitting layer, and a blue light emitting layer. In another embodiment, the light emitting layer may include a white light emitting layer and separately include color filters of red, green, and blue. 
     In addition to the light emitting layer, at least one of a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer may be further provided between the first electrode and the second electrode. 
     The light emitting layer, the hole injection layer, the hole transport layer, the electron transport layer, and the electron injection layer are also referred to as organic layers. The organic layer may be formed of a low molecular organic material or a high molecular organic material. 
     The second electrode may be formed of a material generally used in the art. The second electrode may also be provided as a transparent electrode or a reflective electrode. 
     When the second electrode is provided as a transparent electrode, the second electrode may include a film formed of Li, Ca, LiF/Ca, LiF/Al, Al, Mg, or a compound thereof and a film formed of a material for forming a transparent electrode such as ITO, IZO, ZnO, or In 2 O 3  thereon. When the second electrode is provided as a reflective electrode, the second electrode may be provided by depositing Li, Ca, LiF/Ca, LiF/Al, Al, Mg, or a compound thereto. 
     The capping layer may be further disposed between the second electrode and the protective layer. 
     The protective layer disposed on the capping layer may perform a function of protecting the encapsulating layer so that the side surface of the encapsulating layer is not peeled off or the uniformity is not affected during the manufacturing process of the encapsulating layer. 
     Further, the encapsulating layer may be disposed to protect the organic light emitting diode. The encapsulating layer may serve to block the permeation of oxygen or moisture into the organic light emitting diode. The encapsulating layer disposed on the protective layer may include metal oxide. 
     The encapsulating layer may be formed using metal oxide, for example, a material selected from calcium oxide, alumina, silica, titania, indium oxide, tin oxide, and silicon oxide, but the material is not necessarily limited to the above examples. 
     A predetermined optical element  120  is attached above the panel assembly  110  configured as described above, with the adhesive layer  115  therebetween. When the cover window  112  of a foldable display device  100  is formed of a glass, an optical element  120  can protect a user from scattering of fragments due to external impact or stress. Further, it is advantageous to have a replaceable adhesive layer  115  having high adhesiveness between the optical element  120  and the cover window  112 . Embodiments of a foldable display device  100  including an optical element  120  having various functions and an adhesive layer  115  having a high adhesiveness and a releasing property without increasing an overall thickness is advantageous and provides a solution of the above-described problem. 
       FIGS. 3A and 3B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
     Referring to  FIGS. 3A and 3B , the optical element  120  of an embodiment of the present disclosure may be a protective film having a low elastic modulus to protect the panel assembly  110  and may be disposed on an uppermost or outermost portion of the panel assembly  110 . In this case, the adhesive layer  115  has a high adhesiveness to suppress delamination from an anti-finger (AF) processed surface. Therefore, according to an embodiment of the present disclosure, the foldability may be facilitated or ensured. 
     For the optical element  120 , it is advantageous for a polymer film to have an impact resistance, a scratch resistance, a foldability, and a transparency that is not accompanied with a significant increase of modulus due to the surface treatment such as hard coating (HC) or anti-finger process (AF). 
     The tensile elastic modulus of the polymer film to be used may be approximately 0.8 to 4.5 GPa. If the tensile elastic modulus is lower than 0.8 GPa, the rigidity of the film itself is insufficient, so that the impact absorbing ability against the external impact is greatly deteriorated. If the tensile elastic modulus is greater than 4.5 GPa, the foldability is significantly lowered. A thickness of the polymer film may be approximately 70 to 125 μm. If the thickness is 70 μm or smaller, the rigidity of the film itself is insufficient, so that the impact absorbing ability against the external impact is greatly deteriorated. If the thickness is 125 μm or greater, the foldability is significantly lowered. 
     The thickness and the hardness of the HC are appropriately 2 to 3 μm and 2H, respectively. 
     Further, the adhesive layer  115  should decouple the stress between the optical element  120  and the cover window  112  and have high adhesiveness and high transparency. 
     In order to compensate a reduced surface energy due to a surface treatment of the HC or AF, a high adhesiveness of 200 to 500 gf/in is required. When the adhesiveness is 500 gf/in or lower, the peeling-off is generated due to the folding and when the adhesiveness is 500 gf/in or greater, the encapsulating layer EL is peeled off. 
     In the optical element  120  according to the embodiment shown in  FIGS. 3A-B , zigzag grooves  125  are repeatedly patterned on a surface of the upper portion  120   a  with a predetermined depth. The zigzag grooves  125  may include straight segments connected at corners (e.g., as shown in  FIG. 3B ) or curves (e.g., having a sinusoidal shape). At least a first part of the grooves may be in a first direction and at least a second part of the grooves may be in a second direction different than the first direction. A pattern of the grooves  125  may be repeated a constant period or interval (e.g., distance between grooves), or at a variable period or interval, for example, gradually increasing or decreasing. The depth of each groove may be a portion of a thickness of the optical element  120 . Therefore, the optical element  120  may include a lower portion  120   b  serving as a base and an upper portion  120   a  in which the grooves  125  are patterned above the lower portion  120   b.    
     The groove  125  may be patterned in a direction (a vertical direction in  FIG. 3B ) perpendicular to or intersecting a folding direction (a horizontal direction in  FIG. 3B ) of the panel assembly  110 . The groove  125  may be patterned in the same direction as a folding axis of the panel assembly  110 . 
     The groove  125  may have a shape selected from a triangular wave shape, a sine wave shape, and a pulsed wave shape, among other types of shapes. 
     The groove  125  may include a plurality of first grooves  125   a  patterned in the folding area FA and a plurality of second grooves  125   b  patterned in the non-folding area NFA, but is not limited thereto. In some embodiments, the groove  125  may be patterned in a zigzag form in the folding area FA and not in the non-folding area NFA. 
     The plurality of first grooves  125   a  may be more densely patterned than the plurality of second grooves  125   b . That is, according to an embodiment of the present disclosure, a pattern interval between the plurality of first grooves  125   a  in the folding area FA is smaller than a pattern interval between the plurality of second grooves  125   b  in the non-folding area NFA. Therefore, the stress due to the folding in the folding area FA may be minimized or reduced. Since a pattern interval between the plurality of second grooves  125   b  in the non-folding area NFA is larger than a pattern interval between the plurality of first grooves  125   a  in the folding area NFA, the rigidity in the non-folding direction may be maintained. 
     The groove  125  may be patterned with a thickness that is 10% to 20% of the entire thickness of the optical element  120 . 
     That is, a thickness of the upper portion  120   a  of the optical element  120  with the patterned groove  125  is smaller than a thickness of the lower portion  120   b  of the optical element  120  in which the groove  125  is not patterned. Further, the upper portion has a thickness that is 10% to 20% of the entire thickness of the optical element  120  so that the stress due to the folding may be minimized or reduced and the damage due to physical impact may be minimized or reduced. 
     The plurality of first grooves  125   a  according to an embodiment of the present disclosure may be patterned on the upper surface of the optical element  120  with a constant interval, but is not limited thereto. 
     In order to impart flexibility to the folding direction, the zigzag shaped groove  125  may have a predetermined pitch angle (α, β) (e.g., where 0&lt;α&lt;180 degrees and 0&lt;β&lt;180 degrees). However, for smaller values of α and β, the grooves may be more densely patterned. 
     In some embodiments, the groove  125  may be patterned with a round wavy shape (having curves), rather than a zigzag pattern (having straight segments connected at corners). 
     The left and the right sides of the groove  125  may be symmetrical with respect to the center of the folding area FA, but are not limited thereto and may be asymmetrical. 
     As described above, the optical element  120  of an embodiment of the present disclosure may be a protective film protecting the panel assembly  110  and an additional layer may be further provided thereon. 
       FIG. 4  is a cross-sectional view illustrating a foldable display device according to an embodiment of the present disclosure. 
     The foldable display device  100 ′ illustrated in  FIG. 4  may substantially have the same configuration as the foldable display device  100  according to the embodiment shown in  FIGS. 1 and 2A -B, except that a hard coating HC, anti-finger AF, or UV blocking functional layer  130  is added on the optical element  120  of the protective film. Therefore, a description of the same configuration will be omitted for purposes of clarity. 
     Referring to  FIG. 4 , according to the present disclosure, a hard coating HC, anti-finger AF, or UV blocking functional layer  130  may be added on the optical element  120  of the protective film. A hard coating HC functional layer  130  may be added for an impact resistance, and an anti-finger AF functional layer  130  may be added for a wear resistance. 
     In this case, an adhesive layer  115  having a low storage elastic modulus is introduced to compensate for an increased folding stress. 
     As illustrated in  FIG. 4 , the functional layer  130  is disposed at the uppermost portion of the panel assembly  110 , rather than the optical element  120 , so that zigzag grooves are not patterned on the upper surface of the optical element  120 , but the aforementioned zigzag grooves may be patterned on the upper surface of the functional layer  130 . The shape and the characteristic of the grooves are as described above. 
       FIG. 5  is a cross-sectional view illustrating another foldable display device according to an embodiment of the present disclosure. 
     The foldable display device  100 ″ illustrated in  FIG. 5  may substantially have the same configuration as the foldable display device  100  according to the embodiment shown in  FIGS. 1 and 2A -B, except that a hard coating HC, anti-finger AF, or UV blocking functional layer  130  is added on an optical element  125  of a mura-free protective film. Therefore, a description of the same configuration will be omitted for purposes of clarity. 
     Referring to  FIG. 5 , according to the present disclosure, a hard coating HC, anti-finger AF, or UV blocking functional layer  130  may be added on the optical element  125  of the mura-free protective film. A hard coating HC functional layer  130  may be added for an impact resistance, and an anti-finger AF functional layer  130  may be added for a wear resistance. 
     In this case, the adhesive layer  115  having a high creep and recovery rate is introduced to compensate for a folding stress. 
     As illustrated in  FIG. 5 , the functional layer  130  is disposed at the uppermost portion of the panel assembly  110 , rather than the optical element  125 , so that zigzag grooves are not patterned on the upper surface of the optical element  125 , but the aforementioned zigzag grooves may be patterned on the upper surface of the functional layer  130 . The shape and the characteristic of the grooves are as described above. 
       FIG. 6  is a cross-sectional view illustrating another a foldable display device according to an embodiment of the present disclosure. 
     The foldable display device  100 ″ illustrated in  FIG. 6  may substantially have the same configuration as the foldable display device  100  according to the embodiment shown in  FIGS. 1 and 2A -B, except that a low refractive primer  140  and a hard coating HC, anti-finger AF, or UV blocking functional layer  130  are added on an optical element  125  of a protective film. Therefore, a description of the same configuration will be omitted for purposes of clarity. 
     Referring to  FIG. 6 , according to the present disclosure, a low refractive primer  140  for improving an optical characteristic and a hard coating HC, anti-finger AF, or UV blocking functional layer  130  may be added on the optical element  125  of the protective film. The primer  140  may be added to improve the optical characteristic of rainbow mura-free. 
     Further, a hard coating HC functional layer  130  may be added for an impact resistance, and an anti-finger AF functional layer  130  may be added for a wear resistance. 
     In this case, the adhesive layer  115  having a high creep and recovery rate is introduced to compensate for a folding stress. Further, material or manufacturing costs of the display device may be reduced as compared with the mura-free protective film. 
     As illustrated in  FIG. 6 , the functional layer  130  is disposed at the uppermost portion of the panel assembly  110 , rather than the optical element  125 , so that zigzag grooves are not patterned on the upper surface of the optical element  125 , but the aforementioned zigzag grooves may be patterned on the upper surface of the functional layer  130 . The shape and the characteristic of the grooves are as described above. 
     The adhesive layer  115  according to an embodiment of the present disclosure has a high adhesiveness to suppress the separation from the anti-finger AF processed surface. The adhesive layer  115  may include a silicon optical clear adhesive (OCA) or an acryl OCA having a storage modulus of approximately 10 4  to 10 5  Pa. In this case, the folding separation is not generated, a stress relaxation is 90% or greater, and a creep and recovery rate is 90% or greater. 
     For the purpose of relaxation of the stress of the entire panel assembly  110  due to the folding, the adhesive layer  115  according to an embodiment of the present disclosure has a low modulus and a high adhesiveness. In contrast to the OCA of the related art, in an optical temperature range of −30 to 100 degrees Celsius, the foldable OCA should be gently maintained without causing a sharp gradient of the modulus. This is because the sharp modulus increase should be minimized or reduced at a low temperature, and process issues caused due to the reduction of the modulus should be minimized or reduced at a room temperature and a high temperature. 
       FIG. 7  is a graph of dynamic shear storage modulus of an adhesive layer in accordance with an embodiment of the present disclosure. The graph includes curves showing how a dynamic shear storage modulus of a certain material varies based on temperature of the material. 
     In  FIG. 7 , a first comparative example 1 illustrates a curve of an acryl OCA of the related art and a second comparative example 2 illustrates a curve of a silicon OCA of the related art. Further, a dotted line rectangular box includes a curve of a shear storage modulus of the foldable OCA (e.g., a silicon OCA having a low modulus relative to the first and second comparative examples) according to an embodiment. 
     Referring to  FIG. 7 , in order to relieve a stress of the entire panel assembly due to the folding, the adhesive layer according to an embodiment may have a low modulus and a high adhesiveness. 
     In some embodiments, a flexible storage modulus (G′) at which the tension and shear deformation behavior by the folding is allowed is approximately 10 5  Pa. 
     It is advantageous for a level of G′ for maintaining an adhesiveness to be maintained at approximately 1.4×10 5  Pa at −20 degrees, approximately 9×10 4  Pa at 25 degrees, and approximately 9.3×10 4  Pa at 60 degrees. 
     Further, the deformation and recovery behavior of the OCA for the tensile deformation and the shear deformation of the cover window and the protective film due to the repeated folding is important. 
     In an embodiment, when a stress of 10 KPa is applied for 10 minutes, at a room temperature, a creep amount may be 100% or greater, and a recovery rate may be 92% or greater. When the creep amount is less than 90%, the folding stress is transmitted to the lower structure to cause crack or encapsulating layer EL separation. 
       FIG. 8  is a table for comparing characteristics in accordance with a tensile elastic modulus and a thickness of an optical element according to various embodiments of the present disclosure. 
       FIG. 8  compares results of an optical transparency, a foldability, a hard coating HC crack, and an impact resistance when tensile elastic moduli of the protective film as the optical element are 0.5, 0.8, 4.5, and 6.5 GPa and thicknesses of the protective film are 75, 100, and 125 μm. In this case, the thickness of the OCA serving as an adhesive layer is 25 μm. 
     Referring to  FIG. 8 , it is understood that when a tensile elastic modulus of a polymer film used as a protective film is 0.8 to 4.5 GPa, the optical transparency, the foldability, the hard coating HC crack resistance, and the impact resistance are excellent (corresponding to columns 1 to 6 of the table shown in  FIG. 8 ). 
     It is understood that if the tensile elastic modulus is 0.8 GPa or lower, for example, if the tensile elastic modulus is 0.5 GPa, the rigidity of the film itself is insufficient regardless of the film thickness so that an impact absorbing ability of a steel ball or a pen metal tip against the external impact may be significantly deteriorated. For example, the distances in the ball-drop and pen-drop tests decrease from a range of 5-17 cm (corresponding to columns 1-6 of the table shown in  FIG. 8 ) to a range of 1-2 cm (corresponding to columns 7 to 9 of the table shown in  FIG. 8 ). 
     It is understood that if the tensile elastic modulus is 4.5 GPa or greater, for example, if the tensile elastic modulus is 6.5 GPa, the foldability is significantly lowered regardless of the film thickness. For example, the number of folds at room temperature decreases from 50K (corresponding to columns 1-6 of the table shown in  FIG. 8 ) to 5-10K (corresponding to columns 10 to 12 of the table shown in  FIG. 8 ). 
     Further, the appropriate film thickness is 70 to 125 μm and when the film thickness is 70 μm or smaller and 125 μm or larger, the result is substantially the same as the above-described result of the tensile elastic modulus. That is, the impact absorbing ability and foldability may deteriorate for a film having thickness that is less than 70 μm or greater than 125 μm. 
     As described above, in examples corresponding to columns 1 to 6 of the table shown in  FIG. 8 , the hard coating HC crack, the high temperature dynamic foldability, and the high temperature and humidity static foldability are improved as compared with the examples corresponding to columns 7 to 12. 
       FIG. 9  is a table for comparing characteristics in accordance with a thickness of an adhesive layer according to various embodiments of the present disclosure. 
       FIG. 9  compares a result of the foldability, the indentation, the hard coating HC crack, and the impact resistance when the adhesiveness of the OCA serving as an adhesive layer is 20 to 30 gf/in and a thickness is 10, 15, 20, 25, 30, 35, 40, 45, and 50 μm. In this case, the tensile elastic modulus and the thickness of the polymer film as a protective film are 4.5 GPa and 125 μm, respectively. 
     It is advantageous to decouple the stress between the cover window and the protective film and have high adhesiveness and high transparency for the adhesive layer. 
     As described above, in order to compensate for a reduced surface energy due to a surface treatment for hard coating HC or anti-finger AF, a high adhesiveness of 200 to 500 gf/in is required. If the adhesiveness is 200 gf/in or lower, the peeling-off is generated due to the folding and if the adhesiveness is 500 gf/in or greater, the encapsulating layer EL is prone to be peeled off. 
     It is understood that when the thickness of the OCA is 25 to 40 μm, the foldability, the indentation, the hard coating HC crack, and the impact resistance are excellent (corresponding to columns 1 to 4 of the table shown in  FIG. 9 ). 
     If the thickness of the OCA is 25 μm or smaller, for example, if the thickness of the OCA is 10, 15, and 20 μm, a foldability and an impact absorbing ability of a steel ball or a pen metal tip against the external impact may be significantly weakened. For example, the distances in the pen-drop tests decrease from a range of 13-18 cm (corresponding to columns 1-4 of the table shown in  FIG. 9 ) to a range of 10-11 cm (corresponding to columns 5 and 6 of the table shown in  FIG. 9 ). 
     It is understood that if the thickness of the OCA is 40 μm or larger, for example, if the thickness of the OCA is 45, or 50 μm, an indentation failure occurs (corresponding to columns 8 and 9 of the table shown in  FIG. 9 ). However, an indentation failure does not occur when the thickness of the OCA is 10 to 40 μm (corresponding to columns 1 through 7 of the table shown in  FIG. 9 ). 
     In some embodiments, the pattern of the groove may not be formed in the non-folding area. In this case, the rigidity gradient of the folding area and the non-folding area is maintained and the rigidity lowering phenomenon due to the groove may be mitigated or compensated. This will be described in detail below. 
       FIGS. 10A and 10B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
     Referring to  FIGS. 10A and 10B , a foldable display device  200  according to an embodiment of the present disclosure may include a panel assembly divided into a folding area FA and a non-folding area NFA and an optical element  220  above the panel assembly. 
     The optical element  220  of the embodiment shown in  FIG. 10A-B  may be a protective film having a low elastic modulus to protect the panel assembly and may be disposed on an uppermost portion of the panel assembly. In this case, similar to the above-described embodiments of the present disclosure, the adhesive layer has a high adhesiveness to suppress the separation from the anti-finger AF processed surface. Therefore, the foldability of the foldable display device  200  may be facilitated or ensured. 
     As the optical element  220 , it is advantageous for the polymer film to have an impact resistance, a scratch resistance, a foldability, and a transparency. Additionally, the polymer film should not result in a significant increase of modulus due to the surface treatment such as hard coating (HC) or anti-finger (AF) process. 
     The tensile elastic modulus of the polymer film to be used may be approximately 0.8 to 4.5 GPa. If the tensile elastic modulus is 0.8 GPa or lower, the rigidity of the film itself is insufficient, so that the impact absorbing ability against external impact is greatly deteriorated, and if the tensile elastic modulus is 4.5 GPa or greater, the foldability is significantly lowered. An appropriate thickness of the film may be approximately 70 to 125 μm. If the thickness is 70 μm or smaller, the rigidity of the film itself is insufficient, so that the impact absorbing ability against external impact is greatly deteriorated, and if the thickness is 125 μm or larger, the foldability is significantly lowered. 
     The thickness and the hardness of a hard coating HC layer may be appropriately 2 to 3 μm and 2H, respectively. 
     Further, the adhesive layer should decouple the stress between the optical element  220  and the cover window and have high adhesiveness and high transparency. 
     In order to compensate for a reduced surface energy due to a surface treatment of the hard coating HC or anti-finger AF, a high adhesiveness of 200 to 500 gf/in may be required. When the adhesiveness is 500 gf/in or lower, the peeling-off is generated due to the folding and when the adhesiveness is 500 gf/in or greater, the encapsulating layer EL is prone to be peeled off. 
     In the optical element  220  according to the embodiment shown in  FIGS. 10A-B , zigzag grooves  225  are repeatedly patterned on a surface of the upper portion  220   a  with a predetermined depth. Therefore, the optical element  220  may include a lower portion  220   b  serving as a base and an upper portion  220   a  in which the groove  225  is patterned above the lower portion  220   b.    
     The groove  225  may be patterned in a direction (a vertical direction in  FIG. 10B ) perpendicular to or intersecting a folding direction (a horizontal direction in  FIG. 10B ) of the panel assembly. The groove  225  may be patterned in the same direction as a folding axis of the panel assembly. 
     The groove  225  may have a shape selected from a triangular wave shape, a sine wave shape, and a pulsed wave shape, among other types of shapes. 
     According to the embodiment shown in  FIGS. 10A-10B , a plurality of grooves may be patterned only in the folding area FA. That is, the grooves are not patterned in the non-folding area NFA. 
     The plurality of grooves  225  may be patterned with a thickness that is 10% to 20% of the entire thickness of the optical element  220 . That is, a thickness of the upper portion  220   a  of the optical element  220  with the plurality of patterned grooves  225  is smaller than a thickness of the lower portion  220   b  of the optical element  220  in which the plurality of grooves  225  is not patterned. Further, the upper portion has a thickness that is 10% to 20% of the entire thickness of the optical element  220  so that the stress due to the folding may be minimized or reduced and the damage due to the impact may be minimized or reduced. 
     Further, the plurality of grooves  225  may be patterned on the upper surface of the optical element  220  only in the folding area FA with a constant interval, but is not limited thereto. 
     In some embodiments, the plurality of grooves  225  may be patterned not with a zigzag pattern (having straight segments connected at corners), but with a round wavy shape (having curves). 
     The left and the right sides of the plurality of grooves  225  may be symmetrical with respect to the center of the folding area FA, but are not limited thereto and may be asymmetrical. 
     Substantially similar to the above-described embodiments of the present disclosure, the optical element  220  may be a protective film protecting the panel assembly and an additional layer may be further provided thereon. 
     The rigid gradient of the folding area FA and the non-folding area NFA is maintained and the rigidity lowering phenomenon due to the groove  225  may be compensated. Further, excessive deformation of the hinge unit due to increase of resistance for the torsional stress may be suppressed. 
       FIGS. 11A and 11B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
     Referring to  FIGS. 11A and 11B , a foldable display device  300  according to an embodiment of the present disclosure may include a panel assembly divided into a folding area FA and a non-folding area NFA and an optical element  320  above the panel assembly. 
     The optical element  320  of the embodiment shown in  FIGS. 11A-11B  may be a protective film having a low elastic modulus to protect the panel assembly and may be disposed on an uppermost portion of the panel assembly. However, the present disclosure is not limited thereto and the optical element  320  may be configured by a protective film protecting the panel assembly and an additional layer may be provided thereon. 
     In this case, except for the configuration of the optical element  320 , the substantially same configuration as the foldable display devices  100  and  200  described above may be configured. Therefore, description of the same configuration will be omitted for purposes of clarity. 
     In the optical element  320  according to the embodiment shown in  FIGS. 11A-11B , zigzag grooves  325  are repeatedly patterned on a surface of the upper portion  320   a  with a predetermined depth. Therefore, the optical element  320  may include a lower portion  320   b  serving as a base and an upper portion  320   a  in which the groove  325  is patterned above the lower portion  320   b.    
     The groove  325  may be patterned in a direction (a vertical direction in  FIG. 11B ) perpendicular to or intersecting a folding direction (a horizontal direction in  FIG. 11B ) of the panel assembly. The groove  325  may be patterned in the same direction as a folding axis of the panel assembly. 
     The groove  325  may have a shape selected from a triangular wave shape, a sine wave shape, and a pulsed wave shape, among other types of shapes. 
     The groove  325  may include a plurality of first grooves  325   a  patterned in the folding area FA and a plurality of second grooves  325   b  patterned in the non-folding area NFA, but as described above, it is not limited thereto. 
     The plurality of first grooves  325   a  and the plurality of second grooves  325   b  may be more densely patterned in the folding area FA as compared with the non-folding area NFA. That is, according to an embodiment of the present disclosure, a pattern interval between the plurality of first grooves  325   a  in the folding area FA is smaller than a pattern interval between the plurality of second grooves  325   b  in the non-folding area NFA. 
     Therefore, the stress due to the folding in the folding area FA may be minimized or reduced. Since a pattern interval between the plurality of second grooves  325   b  in the non-folding area NFA is larger than a pattern interval between the plurality of first grooves  325   a  in the folding area NFA, the rigidity in the non-folding direction may be maintained. 
     In an embodiment, the intervals of the plurality of first grooves  325   a  and the plurality of second grooves  325  are gradually widened from the center of the folding area FA to the edge of the non-folding area NFA. 
     The plurality of grooves  325  may be patterned with a thickness that is 10% to 20% of the entire thickness of the optical element  320 . That is, a thickness of the upper portion  320   a  of the optical element  320  with the plurality of patterned grooves  325  is smaller than a thickness of the lower portion  320   b  of the optical element  320  in which the plurality of grooves  325  is not patterned. Further, the upper portion has a thickness that is 10% to 20% of the entire thickness of the optical element  320  so that the stress due to the folding may be minimized or reduced and the damage due to the impact may be minimized or reduced. 
     In some embodiments, the plurality of grooves  325  may be patterned not with a zigzag pattern (having straight segments connected at corners), but with a round wavy shape (having curves). 
     The left and the right sides of the plurality of grooves  325  may be symmetrical with respect to the center of the folding area FA, but are not limited thereto and may be asymmetrical. 
       FIGS. 12A and 12B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
     Referring to  FIGS. 12A and 12B , a foldable display device  400  according to an embodiment of the present disclosure may include a panel assembly divided into a folding area FA and a non-folding area NFA and an optical element  420  above the panel assembly. 
     The optical element  420  of the embodiment shown in  FIGS. 12A-12B  may be a protective film having a low elastic modulus to protect the panel assembly and may be disposed on an uppermost portion of the panel assembly. However, the present disclosure is not limited thereto and the optical element  420  may be configured by a protective film protecting the panel assembly and an additional layer may be provided thereon. 
     In this case, except for the configuration of the optical element  420 , the substantially same configuration as the foldable display devices  100 ,  200 , and  300  described above may be configured. Therefore, a description of the same configuration will be omitted for purposes of clarity. 
     In the optical element  420  according to the embodiment shown in  FIGS. 12A-B , zigzag grooves  425  are repeatedly patterned on a surface of the upper portion  420   a  with a predetermined depth. Therefore, the optical element  420  may include a lower portion  420   b  serving as a base and an upper portion  420   a  in which the groove  425  is patterned above the lower portion  420   b.    
     The groove  425  may be patterned in a direction (a vertical direction in  FIG. 12B ) perpendicular to or intersecting a folding direction (a horizontal direction in  FIG. 12B ) of the panel assembly. The groove  425  may be patterned in the same direction as a folding axis of the panel assembly. 
     The groove  425  may have a shape selected from a triangular wave shape, a sine wave shape, and a pulsed wave shape, among other types of shapes. 
     The groove  425  may include a plurality of first grooves  425   a  patterned in the folding area FA and a plurality of second grooves  425   b  patterned in the non-folding area NFA, but as described above, it is not limited thereto. 
     The plurality of first grooves  425   a  may be more densely patterned than the plurality of second grooves  425   b . That is, according to an embodiment of the present disclosure, a pattern interval between the plurality of first grooves  425   a  in the folding area FA is smaller than a pattern interval between the plurality of second grooves  425   b  in the non-folding area NFA. Therefore, the stress due to the folding in the folding area FA may be minimized or reduced. Since a pattern interval between the plurality of second grooves  425   b  in the non-folding area NFA is larger than a pattern interval between the plurality of first grooves  425   a  in the folding area NFA, the rigidity in the non-folding direction may be maintained. 
     In an embodiment, the plurality of first grooves  425   a  may be patterned to have a larger width than that of the plurality of second grooves  425   b.    
     The plurality of grooves  425  may be patterned with a thickness that is 10% to 20% of the entire thickness of the optical element  420 . Therefore, the stress due to the folding may be minimized or reduced and the damage due to the impact may be minimized or reduced. 
     In some embodiments, the plurality of grooves  425  may be patterned not with a zigzag pattern (having straight segments connected at corners), but with a round wavy shape (having curves). 
     The left and the right sides of the plurality of grooves  425  may be symmetrical with respect to the center of the folding area FA, but are not limited thereto and may be asymmetrical. 
       FIGS. 13A and 13B  are a cross-sectional view and a plan view, respectively, illustrating an optical element in a foldable display device according to an embodiment of the present disclosure. 
     Referring to  FIGS. 13A and 13B , a foldable display device  500  according to an embodiment of the present disclosure may include a panel assembly divided into a folding area FA and a non-folding area NFA and an optical element  520  above the panel assembly. 
     The optical element  520  of the embodiment shown in  FIGS. 13A-13B  may be a protective film having a low elastic modulus to protect the panel assembly and may be disposed on an uppermost portion of the panel assembly. However, the present disclosure is not limited thereto and the optical element  520  may be configured by a protective film protecting the panel assembly and an additional layer may be provided thereon. 
     In this case, except for the configuration of the optical element  520 , the substantially same configuration as the foldable display devices  100 ,  200 ,  300 , and  400  described above may be configured. Therefore, a description of the same configuration will be omitted for purposes of clarity. 
     In the optical element  520  according to the embodiment shown in  FIGS. 13A-13B , zigzag grooves  525  are repeatedly patterned on a surface of the upper portion  520   a  with a predetermined depth. Therefore, the optical element  520  may include a lower portion  520   b  serving as a base and an upper portion  520   a  in which the groove  525  is patterned above the lower portion  520   b.    
     The groove  525  may be patterned in a same direction as, or parallel to, a folding direction (a horizontal direction in  FIG. 13B ) of the panel assembly. The groove  525  may be patterned in a direction perpendicular to or intersecting a folding axis of the panel assembly. 
     The groove  525  may have a shape selected from a triangular wave shape, a sine wave shape, and a pulsed wave shape, among other types of shapes. 
     The plurality of grooves  525  patterned to be in a same direction as, or parallel to, the folding direction may include a first groove  525   a  patterned in the folding area FA and a second groove  525   b  patterned in the non-folding area NFA, respectively. 
     In this case, for each of the plurality of grooves  525 , the zigzag pattern is more densely patterned in the folding area FA than in the non-folding area NFA. That is, according an embodiment of the present disclosure, a zigzag pattern of the first groove  525   a  in the folding area FA is more densely patterned than a zigzag pattern of the second groove  525   b  in the non-folding area NFA. Therefore, the stress due to the folding in the folding area FA may be minimized or reduced. Since a zigzag pattern of the second groove  525   b  in the non-folding area NFA is looser than a zigzag pattern of the first groove  525   a  in the folding area FA, the rigidity in the non-folding direction may be maintained. 
     The plurality of grooves  525  may be patterned with a thickness that is 10% to 20% of the entire thickness of the optical element  520 . Therefore, the stress due to the folding may be minimized or reduced and the damage due to the impact may be minimized or reduced. 
     The plurality of grooves  525  may be patterned not with a zigzag pattern, but with a round wavy shape. 
     The left and the right sides of the plurality of grooves  525  may be symmetrical with respect to the center of the folding area FA, but are not limited thereto and may be asymmetrical. 
     The exemplary embodiments of the present disclosure can also be described as follows: 
     According to an aspect of the present disclosure, there is provided a foldable display device. The foldable display device includes a panel assembly divided into a folding area and a non-folding area and an optical element on the panel assembly, wherein the optical element includes grooves with a zigzag pattern repeatedly patterned on an upper surface of the optical pattern. 
     The foldable display device may include an adhesive layer between the panel assembly and the optical element. 
     The grooves may be patterned in a direction perpendicular to or intersecting a folding direction of the panel assembly. 
     The grooves may have a shape selected from a triangular wave shape, a sine wave shape, and a pulsed wave shape. 
     The grooves may include a plurality of first grooves patterned in the folding area and a plurality of second grooves patterned in the non-folding area. 
     The plurality of first grooves may be more densely patterned than the plurality of second grooves. 
     The grooves may be patterned to each have a depth of 10% to 20% of a thickness of the optical element. 
     The plurality of first grooves may be patterned on the upper surface of the optical element with a constant interval between grooves of the plurality of first grooves. 
     The grooves may be patterned only in the folding area. 
     The grooves may be more densely patterned in the folding area than in the non-folding area. 
     An interval between the grooves may be gradually widened from a center of the folding area to an edge of the non-folding area. 
     A first width of each groove of the plurality of first grooves may be greater than a second width of each groove of the plurality of second grooves. 
     The grooves may be patterned in a same direction as a folding direction of the panel assembly. 
     The zigzag pattern of the groove may be denser in the folding area than in the non-folding area. 
     The grooves may be patterned with the zigzag pattern only in the folding area. 
     According to another aspect of the present disclosure, there is provided a foldable display device. The foldable display device includes a panel assembly divided into a folding area and a non-folding area, a back plate supporting the panel assembly below the panel assembly, an optical element above the panel assembly and an adhesive layer between the panel assembly and the optical element, wherein the optical element includes grooves with a zigzag pattern repeatedly patterned on an upper surface of the optical element. 
     The grooves may include a plurality of first grooves patterned in the folding area and a plurality of second grooves patterned in the non-folding area, and the plurality of first grooves may be more densely patterned than the plurality of second grooves. 
     The grooves may be patterned only in the folding area. 
     An interval between the grooves may be gradually widened from a center of the folding area to an edge of the non-folding area. 
     In an embodiment, a foldable display device comprises: a panel assembly including: a folding area capable of being folded to a folded state of the foldable display device, and a display panel emitting light toward an upper surface of the panel assembly; and an optical element on the upper surface of the panel assembly, the optical element including grooves patterned on an upper surface of the optical element to facilitate folding of the folding area to the folded state, wherein at least a first part of the grooves is in a first direction and at least a second part of the grooves is in a second direction different than the first direction. 
     The panel assembly may further include a first non-folding area and a second non-folding area, wherein the folding area is between the first non-folding area and the second non-folding area. 
     The optical element may overlap at least a part of each of the folding area, the first non-folding area, and the second non-folding area. 
     An interval between the grooves may be gradually widened from a center of the folding area to an edge of the first non-folding area or the second non-folding area. 
     The grooves may include a plurality of first grooves patterned in the folding area and a plurality of second grooves patterned in the first non-folding area and the second non-folding area, wherein the plurality of first grooves is more densely patterned than is the plurality of second grooves. 
     The grooves may be patterned in a same direction as a folding axis of the panel assembly. 
     Although the exemplary embodiments of the present disclosure have been described in detail with reference to the accompanying drawings, the present disclosure is not limited thereto and may be embodied in many different forms without departing from the technical concept of the present disclosure. Therefore, the exemplary embodiments of the present disclosure are provided for illustrative purposes only but not intended to limit the technical concept of the present disclosure. The scope of the technical concept of the present disclosure is not limited thereto. Therefore, it should be understood that the above-described exemplary embodiments are illustrative in all aspects and do not limit the present disclosure. The protective scope of the present disclosure should be construed based on the following claims, and all the technical concepts in the equivalent scope thereof should be construed as falling within the scope of the present disclosure.