Patent Publication Number: US-2022216436-A1

Title: Foldable display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2021-0000537 under 35 U.S.C. § 119, filed on Jan. 4, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     Embodiments described herein relate to a display device. Specifically, embodiments described herein relate to a foldable display device capable of folding and unfolding. 
     2. Description of the Related Art 
     A display device may provide user with information by displaying an image. Unlike a flat panel display device, in a flexible display device, a part of the flexible display device may be bent. Also, the flexible display device can display an image even in a bent portion. The flexible display device is easy to carry and can improve user convenience. 
     Recently, among the flexible display devices, a foldable display device has attracted attention. The foldable display device may be repeatedly folded or unfolded. When the foldable display device is repeatedly folded or unfolded, repulsive force may be generated in the foldable display device. Accordingly, a crease may occur in the foldable display device, and when crease is recognized by user, a display performance of the foldable display device may be degraded. 
     SUMMARY 
     Embodiments provide a foldable display device capable of reducing repulsive force during folding and unfolding. 
     A foldable display device according to embodiments may include a flexible display module and a first plate disposed under the flexible display module. The flexible display module may include a foldable area and non-foldable area disposed in at least two sides of the foldable area. The foldable area may include a first area, a second area spaced apart from the first area and disposed in at least two sides of the first area, and a second area disposed between the first area and the second area. The first plate may include a plurality of grooves overlapping the second area and penetrating the first plate. The first plate overlapping the third area may not include grooves. 
     In an embodiment, in case that the flexible display module is folded, the flexible display module may have a first curvature radius in the first area, and may have a second curvature radius in the second area. The second curvature radius may be greater than the first curvature radius. 
     In an embodiment, in case that the flexible display module is folded, a curvature center of the flexible display module in the second area may be disposed on a non-display surface of the flexible display module, and a curvature center of the flexible display module in the first area may be disposed on a display surface of the flexible display module. 
     In an embodiment, the foldable display device may further include a protection layer disposed under the first plate and overlapping the second area. 
     In an embodiment, the foldable display device may further include a second plate disposed under the first plate and overlapping each of the non-foldable area and the third area. 
     In an embodiment, a rigidity of the second plate may be greater than a rigidity of the first plate. 
     A foldable display device according to embodiments may include a flexible display module and a first plate disposed under the flexible display module. The flexible display module may include a foldable area and non-foldable area disposed in at least two sides of the foldable area. The foldable area may include a first area, a second area spaced apart from the first area and disposed in at least two sides of the first area, and a second area disposed between the first area and the second area. The first plate may include a plurality of grooves overlapping the second area and not penetrating the first plate. The first plate overlapping the third area may not include grooves. 
     In an embodiment, in case that the foldable display device is folded, the flexible display module may have a first curvature radius in the first area and may have a second curvature radius in the second area. The second curvature radius may be greater than the first curvature radius. 
     In an embodiment, a thickness of each of the plurality of grooves not penetrating the first plate compared to a thickness of the first plate may be about 40% or greater and about 60% or less. 
     In an embodiment, the foldable display device may further include a second plate disposed under the first plate and overlapping each of the non-foldable area and the third area. 
     In an embodiment, a rigidity of the second plate may be greater than a rigidity of the first plate. 
     A foldable display device according to embodiments may include a flexible display module and a first plate disposed under the flexible display module. The flexible display module may include a foldable area and non-foldable area disposed in at least two sides of the foldable area. The foldable area may include a first area, a second area spaced apart from the first area and disposed in at least two sides of the first area, and a second area disposed between the first area and the second area. A thickness of the first plate overlapping the second area may be less than a thickness of the first plate overlapping the non-foldable area. The first plate overlapping the third area may not include grooves. 
     In an embodiment, in case that the flexible display module is folded, the flexible display module may have a first curvature radius in the first area, and may have a second curvature radius in the second area. The second curvature radius may be greater than the first curvature radius. 
     In an embodiment, the thickness of the first plate overlapping the second area compared to the thickness of the first plate overlapping the non-foldable area may be about 50% or greater. 
     In an embodiment, the first plate may further include a plurality of grooves overlapping the second are and penetrating the first plate. 
     In an embodiment, the foldable display device may further include a protection layer disposed under the first plate and overlapping the second area. 
     In an embodiment, the first plate may further include a plurality of grooves overlapping the second area and not penetrating the first plate. 
     In an embodiment, a thickness of each of the plurality of grooves not penetrating the first plate compared to the thickness of the first plate overlapping the second area may be about 40% or greater and about 60% or less. 
     In an embodiment, a thickness of each of the plurality of grooves not penetrating the first plate compared to the thickness of the first plate overlapping the second area may be about 40% or greater and about 60% or less. 
     In an embodiment, the foldable display device may further include a second plate disposed under the first plate and overlapping each of the non-foldable area and the third area. 
     In an embodiment, a rigidity of the second plate may be greater than a rigidity of the first plate. 
     The foldable display device according to embodiments may include a first plate including a plurality of grooves. Accordingly, repulsive force of the first plate may be relatively reduced, and crease generated when the foldable display device is folded or unfolded may not occur. 
     In the foldable display device according to embodiments, a thickness of a first plate overlapping a second area may be smaller than a thickness of the first plate overlapping a non-foldable area. Accordingly, repulsive force of the first plate may be relatively reduced, and crease generated when the foldable display device is folded or unfolded may not occur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate illustrative embodiments of the disclosure, and together with the description serve to explain the disclosure. 
         FIG. 1  is a perspective view schematically illustrating an unfolded state of a foldable display device according to an embodiment of the disclosure. 
         FIG. 2  is a perspective view schematically illustrating a folded state of the foldable display device of  FIG. 1 . 
         FIG. 3  is a cross-sectional view schematically illustrating a flexible display module included in the foldable display device of  FIG. 1 . 
         FIG. 4  is a plan view schematically illustrating a display panel included in the flexible display module of  FIG. 3 . 
         FIG. 5  is a schematic cross-sectional view taken along line III-III′ of  FIG. 4 . 
         FIG. 6  is a cross-sectional view schematically illustrating a foldable display device according to another embodiment. 
         FIG. 7  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 6 . 
         FIGS. 8, 9, and 10  are cross-sectional views schematically illustrating other embodiments of the foldable display device of  FIG. 6 . 
         FIG. 11  is a cross-sectional view schematically illustrating a foldable display device according to still another embodiment of the disclosure. 
         FIG. 12  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 11 . 
         FIGS. 13, and 14  are cross-sectional views schematically illustrating other embodiments of the foldable display device of  FIG. 11 . 
         FIG. 15  is a cross-sectional view schematically illustrating a foldable display device according to further still another embodiment of the disclosure. 
         FIG. 16  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 15 . 
         FIGS. 17, 18, 19, 20, and 21  are cross-sectional views schematically illustrating other embodiments of the foldable display device of  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the disclosure will be explained in detail with reference to the accompanying drawings. 
     The phrase “at least one of” is intended to include the meaning of “at least one selected from the group of” for the purpose of its meaning and interpretation. For example, “at least one of A and B” may be understood to mean “A, B, or A and B.” The terms “about” or “approximately” as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system). For example, “about” may mean within one or more standard deviations, or within ±30%, 20%, 10%, 5% of the stated value. 
     Unless otherwise defined or implied herein, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this disclosure pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the disclosure, and should not be interpreted in an ideal or excessively formal sense unless clearly so defined herein. 
       FIG. 1  is a perspective view schematically illustrating an unfolded state of a foldable display device according to an embodiment of the disclosure.  FIG. 2  is a perspective view schematically illustrating a folded state of the foldable display device of  FIG. 1 . 
     Referring to FIG. 1  and  FIG. 2 , a foldable display device  1000  according to an embodiment of the disclosure may include a flexible display module  100  and a first plate  200 . 
     Although not shown in  FIG. 1  and  FIG. 2 , the foldable display device may further include a case covering the flexible display module  100  and the first plate  200 , hinge, etc. 
     The flexible display module  100  may include a display surface  101  and a non-display surface  102  opposed to the display surface  101 . The flexible display module  100  may display an image in direction of the display surface  101 . The non-display surface  102  may not display an image. 
     The flexible display module  100  may include a flexible substrate made of flexible plastic instead of a rigid substrate made of rigid glass. Accordingly, the flexible display module  100  may be freely folded or unfolded within a predetermined range. 
     The first plate  200  may be disposed under the flexible display module  100 . A rigidity of the first plate  200  may be greater than the flexible display module  100 , so the first plate  200  may support the flexible display module  100 . For example, the first plate  200  may prevent deformation of the flexible display module  100  due to touch of users. The first plate  200  may include nobinite, stainless steel, invar, or an alloy thereof. A thickness of the first plate  200  may be about 130 μm or greater and about 170 μm or less. The thickness of the first plate  200  may be about 150 μm. 
     The flexible display module  100  may include a foldable area FA, and a non-foldable area NFA positioned in at least two sides or both sides of the foldable area FA. The foldable area FA may include a first area FA 1 , a second area FA 2  positioned apart from the first area FA 1  and positioned in both sides of the first area FA 1 , and a third area PA positioned between the first area FA 1  and the second area FA 2 . 
     In case that the flexible display module  100  is folded, the flexible display module  100  may have a curved surface in the first area FA 1 . For example, in case that the flexible display module  100  is folded, the flexible display module  100  may have a first curvature radius R 1  in the first area FA 1 . Also, in case that the flexible display module  100  is folded, a curvature center of the flexible display module  100  in the first area FA 1  may be a first curvature center C 1 . The first curvature center C 1  may be disposed on the display surface  101  of the flexible display module  100 . In other words, the first curvature center C 1  may be adjacent to the display surface  101  of the flexible display module  100 , and a shortest distance between the first curvature center C 1  and the display surface  101  of the flexible display module  100  may be less than a shortest distance between the first curvature center C 1  and the non-display surface  102  of the flexible display module  100 . 
     In case that the flexible display module  100  is folded, the flexible display module  100  may have a curved surface in the second area FA 2 . For example, in case that the flexible display module  100  is folded, the flexible display module  100  may have a second curvature radius R 2  in the second area FA 2 . The second curvature radius R 2  may be greater than the first curvature radius R 1 , or the second curvature radius R 2  may be substantially equal to the first curvature radius R 1 . In case that the flexible display module  100  is folded, a curvature center of the flexible display module  100  in the second area FA 2  may be a second curvature center C 2 . The second curvature center C 2  may be disposed on the non-display surface  102  of the flexible display module  100 . In other words, the second curvature center C 2  may be adjacent to the non-display surface  102  of the flexible display module  100 , and a shortest distance between the second curvature center C 2  and the non-display surface  102  of the flexible display module  100  may be greater than a shortest distance between the second curvature center C 2  and the display surface  101  of the flexible display module  100 . 
     In case that the flexible display module  100  is folded, the flexible display module  100  may have a substantially flat surface in the non-foldable area NFA. 
     In case that the flexible display module  100  is folded, the flexible display module  100  may have a substantially flat surface in the second area FA 2 . However, the disclosure is not limited thereto, and in case that the flexible display module  100  is folded, the flexible display module  100  may have a curved surface in the third area PA. 
     In an embodiment, the foldable display device  1000  may include a support element attached under the first plate  200  overlapping the third area PA. A rigidity of the support element may be relatively large. The support element may support the first plate  200  overlapping the third area PA. 
     In an embodiment, the support element may be further attached under the first plate  200  overlapping the non-foldable area NFA. The support element may be connected to the case, the hinge, etc. The support element may support the first plate  200  overlapping the non-foldable area NFA. 
     In case that the foldable display device  1000  is folded and then unfolded, repulsive force may be generated in the first plate  200  overlapping the foldable area FA. For example, in case that the foldable display device  1000  is folded and then unfolded, repulsive force may be generated in the first plate  200  overlapping each of the second area FA 2  and the first area FA 1 . 
     In the disclosure, the second curvature radius R 2  may be greater than the first curvature radius, so repulsive force of the first plate  200  overlapping the first area FA 1  may be greater than repulsive force of the first plate  200  overlapping the second area FA 2 . 
     In case that repulsive force is generated in the first plate  200 , a crease may occur in the foldable display device  1000 . For example, in case that the foldable display device is folded and then unfolded, the crease may occur in each of the first area FA 1  and the second area FA 2 . 
     In the disclosure, in case that the flexible display module  100  is folded, the first curvature center C 1  may be disposed on the display surface  101  of the flexible display module  100 , and the second curvature center C 2  may be disposed on the non-display surface  102  of the flexible display module  100 . Accordingly, a depth of the crease in the second area FA 2  may be greater than a depth of the crease in the first area FA 1 . 
     In order to reduce repulsive force of the first plate  200 , a groove (for example, a groove  210  of  FIG. 6 ) may be formed in the first plate  200 . In case that the groove is formed in the first plate  200 , a flexible characteristic of the first plate  200  may increase, so repulsive force may decrease. 
     In order to reduce repulsive force of the first plate  200 , a thickness of the first plate  200  overlapping the foldable area FA may be less than a thickness of the first plate  200  overlapping the non-foldable area NFA. In this case, a thickness of the first plate  200  overlapping the third area PA may be substantially equal to a thickness of the first plate  200  overlapping the non-foldable area NFA. As the thickness of the first plate  200  decreases, the flexible characteristic of the first plate  200  may increase, so repulsive force of the first plate  200  may decrease. As the thickness of the first plate  200  increases, the rigidity of the first plate  200  may increase. 
     In order to reduce a generation of the crease in the first plate  200 , the support element may be attached under the first plate  200  overlapping the third area PA. The rigidity of the support element may be relatively large. Accordingly, the support element may suppress occurrence of the crease due to deformation of the first plate  200  overlapping the third area PA. Since the support element may be attached under the first plate  200  overlapping the third area PA, the groove may not be formed in the first plate  200  overlapping the third area PA. 
       FIG. 3  is a cross-sectional view schematically illustrating a flexible display module included in the foldable display device of  FIG. 1 . 
     Referring to  FIG. 3 , a flexible display module  100  may include a window WD, a polarizing layer  400 , a sensing layer  500 , and a display panel  600 . As described above, since the flexible display module  100  may be folded or unfolded, each of the window WD, the polarizing layer  400 , the sensing layer  500 , and the display panel  600  may have flexible characteristics. 
     The window WD may protect the polarizing layer  400 , the sensing layer  500 , and the display panel  600  from external impact. In an embodiment, the window WD may include a glass, or a polymer resin such as a polycarbonate, polymethyl methacrylate, polyacrylate, polyethersulfone, polyethylene terephthalate, polyethylene naphthalate. 
     The polarizing layer  400  may be disposed under the window WD. The polarizing layer  400  may reduce reflection of external light from the display device  1000 . For example, in case that external light passes through the polarizing layer  400  and is reflected from a lower portion of the polarizing layer  400  (for example, the display panel  600 ) and then passes through the polarizing layer  400  again, the polarizing layer  400  may change a phase of the external light. Accordingly, since a phase of a reflected light is different from a phase of the incident light entering the polarizing layer  400 , extinction interference may occur, and reflection of external light may be reduced, thereby improving visibility of the foldable display device  1000 . 
     The sensing layer  500  may be disposed under the polarizing layer  400 . The sensing layer  500  may detect an external input, such as in case that an external object contacts or approaches the foldable display device  1000 . For example, the sensing layer  500  may sense the external input in a capacitive manner. Optionally, the sensing layer  500  may be disposed on the polarizing layer  400 . 
     The display panel  600  may be disposed under the sensing layer  500 . The display panel  600  may include pixels, and may generate an image in which light emitted from each of the pixels is combined. The display panel  600  may include a first surface and a second surface opposite to each other. The display panel  600  may display an image on the first surface, and the second surface of the display panel  600  may correspond to the non-display surface  102  of the flexible display module  100 . 
       FIG. 4  is a plan view schematically illustrating a display panel included in the flexible display module of  FIG. 3 .  FIG. 5  is a schematic cross-sectional view taken along line of  FIG. 4 . 
     Referring to  FIG. 4  and  FIG. 5 , the display panel  600  may include pixels including a switching transistor T 1 , a driving transistor T 2 , a capacitor CAP, and an emission element (or light-emitting element or light-emitting diode) EE. In this case, the pixel means a minimum unit for displaying an image. 
       FIG. 4  and  FIG. 5  illustrate that two transistors and one capacitor are disposed in one pixel, but the disclosure is not limited thereto. For example, a pixel includes three or more transistors and/or two or more capacitors. 
     The display panel  600  may include a substrate  610 , a gate line  631  disposed on the substrate  610 , a data line  641  intersecting the gate line  631  and insulated from the gate line  631 , and common line  642 . In general, a pixel may be defined as a boundary with the gate line  631 , the data line  641 , and the common line  642 , but the pixel is not limited to the above definition. The pixel may be defined by a black matrix or a pixel defining layer. 
     The substrate  610  may include flexible material such as plastic. For example, the substrate  610  may be formed of polyethersulfone, polycarbonate, polyimide, polyethylene terephthalate, polyethylene naphthalate, polyacrylate, fiber reinforced plastic, etc. 
     A thickness of the substrate  610  may be about 5 μm or greater and about 200 μm or less. In case that the thickness of the substrate  610  is less than about  5  the emission element EE may not be stably supported by the substrate  610 . In case that the thickness of the substrate  610  is greater than about  200  flexibility of the substrate  610  may deteriorate. 
     A buffer layer  611  may be disposed on the substrate  610 . The buffer layer  611  may prevent penetration of impurities, and an upper surface of the buffer layer may be substantially flat. The buffer layer  611  may be formed of silicon nitride, silicon oxide, silicon oxynitride, etc. The buffer layer  611  is not necessarily required, and may be omitted depending on type and process conditions of the substrate  610 . 
     A switching semiconductor layer  621  and a driving semiconductor layer  622  may be disposed on the buffer layer  611 . The switching semiconductor layer  621  and the driving semiconductor layer  622  may be formed of any one of polycrystalline silicon, amorphous silicon, and oxide semiconductors such as indium gallium zinc oxide (IGZO) and indium zinc tin oxide (IZTO). For example, in case that the driving semiconductor layer  622  is formed of polycrystalline silicon, the driving semiconductor layer  622  may include a channel region that is not doped with impurities, and a source region and a drain region formed by doping impurities in both sides of the channel region. In this case, the impurities may be a p type dopant such as boron, and B 2 H 6  may be mainly used. The impurities may vary depending on the type of transistor. In the disclosure, a PMOS transistor using a p type dopant is used as the driving transistor T 2 , but the driving transistor T 2  is not limited thereto, and the driving transistor T 2  may be a NMOS transistor or a CMOS transistor. 
     A gate insulating layer  612  may be disposed on the switching semiconductor layer  621  and the driving semiconductor layer  622 . The gate insulating layer  612  may be formed of (or include) tetraethoxysilane, silicon nitride, silicon oxide, etc. 
     A gate pattern including gate electrodes  632  and  635  may be disposed on the gate insulating layer  612 . The gate pattern may further include the gate line  631 , and a first capacitor electrode  638 . The gate electrodes  632  and  635  may be disposed to overlap at least a portion (for example, a channel region) of the semiconductor layers  621  and  622 . The gate electrodes  632  and  635  may block doping of impurities in the channel region when the source and the drain region of the semiconductor layers  621  and  622  are doped during formation of the semiconductor layers  621  and  622 . 
     The gate electrodes  632  and  635  and the first capacitor electrode  638  may be disposed on the same layer and may be formed of substantially same metal(s). For example, the gate electrodes  632  and  635  and the first capacitor electrode  638  may be formed of molybdenum, chromium, tungsten, etc. 
     An interlayer insulating layer  613  covering (or overlapping) the gate electrodes  632  and  635  may be disposed on the gate insulating layer  612 . The interlayer insulating layer  613  may be formed of tetraethoxysilane, silicon nitride, silicon oxide, etc. However, a material of the interlayer insulating layer  613  is not limited thereto. 
     A data pattern including source electrodes  643  and  646  and drain electrodes  644  and  647  may be disposed on the interlayer insulating layer  613 . The data pattern may further include the data line  641 , the common line  642 , and a second capacitor electrode  648 . The source electrodes  643  and  646  and the drain electrodes  644  and  647  may be electrically connected to the source and drain regions of the semiconductor layers  621  and  622  through contact holes formed in the gate insulating layer  612  and the interlayer insulating layer  613  respectively. 
     The switching transistor T 1  may include the switching semiconductor layer  621 , the switching gate electrode  632 , the switching source electrode  643 , and the switching drain electrode  644 , and the driving transistor T 2  may include the driving semiconductor layer  622 , the driving gate electrode  635 , the driving source electrode  646 , and the driving drain electrode  647 . The capacitor CAP may include a first capacitor electrode  638  and a second capacitor electrode  648 . In this case, the interlayer insulating layer  613  may be disposed between the first capacitor electrode  638  and the second capacitor electrode  648 . 
     The switching transistor T 1  may be used as a switching element for selecting a pixel to emit light. The switching transistor T 1  may be electrically connected to the gate line  631 . The switching source electrode  643  may be electrically connected to the data line  641 . The switching drain electrode  644  may be apart from the switching source electrode  643  and may be electrically connected to the first capacitor electrode  638 . 
     The driving transistor T 2  may apply driving power to a pixel electrode  650  to emit light for an emission layer  660  of the emission element EE in a selected pixel. The driving gate electrode  635  may be electrically connected to the first capacitor electrode  638 . Each of the driving source electrode  646  and the second capacitor electrode  648  may be electrically connected to the common line  642 . The driving drain electrode  647  may be electrically connected to the pixel electrode  650  of the emission element EE through a contact hole. 
     The switching transistor T 1  may be operated by a gate voltage applied to the gate line  631  and may transfer a data voltage applied to the data line  641  to the driving transistor T 2 . A voltage corresponding to the difference between a common voltage applied from the common line  642  to the driving transistor T 2  and the data voltage transmitted from the switching transistor T 1  may be stored in the capacitor CAP, and a current corresponding to the voltage stored in the capacitor CAP may be transmitted to the emission element EE through the driving transistor T 2 , and the emission element EE may emit light. 
     A planarization layer  614  covering or overlapping the data pattern including the data line  641 , the common line  642 , the source electrodes  643  and  646 , the drain electrodes  644  and  647 , and the second capacitor electrode  648  may be disposed on the interlayer insulating layer  613 . 
     The planarization layer  614  may remove a step (or height difference) in order to increase a luminous efficiency of the emission element EE. The planarization layer  614  may be formed of polyacrylate resin, epoxy resin, phenolic resin, polyamide resin, polyimide resin, unsaturated polyester resin, polyphenylene resin, polyphenylene sulfide resin, benzocyclobutene, etc. 
     The pixel electrode  650  of the emission element EE may be disposed on the planarization layer  614 . The pixel electrode  650  may be electrically connected to the drain electrode  647  through a contact hole formed in the planarization layer  614 . 
     A pixel defining layer  615  that defines a pixel area by exposing at least a portion of the pixel electrode  650  may be disposed on the planarization layer  614 . The pixel electrode  650  may be disposed to correspond to a pixel area of the pixel defining layer  615 . The pixel defining layer  615  may be formed of polyacrylate resin, polyimide resin, etc. 
     The emission layer  660  may be disposed on the pixel electrode  650  in the pixel area, and a common electrode  670  may be disposed on the pixel defining layer  615  and the emission layer  660 . The emission layer  660  may be formed of a low molecular weight organic material or a high molecular weight organic material. At least one of a hole injection layer (HIL) and a hole transport layer (HTL) may be further disposed between the pixel electrode  650  and the emission layer  660 , and at least one of an electron transport layer (ETL) and an electron injection layer (EIL) may be further disposed between the emission layer  660  and the common electrode  670 . 
     Each of the pixel electrode  650  and the common electrode  670  may be formed of any one of a transparent electrode, a transflective electrode, and a reflective electrode. 
     An encapsulation layer TFE may be disposed on the common electrode  670 . The encapsulation layer TFE may include one or more inorganic layers  681 ,  683 , and  685  and one or more organic layers  682  and  684 . In addition, the encapsulation layer TFE may have a structure in which the inorganic layers  681 ,  683 , and  685  and the organic layers  682  and  684  are alternately stacked. In this case, the inorganic layer  681  may be disposed at a lower surface of the encapsulation layer TFE. In other words, the inorganic layer  681  may be disposed closest to the emission element EE. 
       FIG. 5  illustrates that the encapsulation layer TFE includes three inorganic layers  681 ,  683 , and  685  and two organic layers  682  and  684 , but the disclosure is not limited thereto. 
     A thickness of the encapsulation layer TFE may be about 10 μm or less. Accordingly, a thickness of the display panel  600  may be very thin. As the encapsulation layer TFE  15  disposed on the emission element EE, a flexible characteristic of the display panel  600  may be maximized. 
       FIG. 6  is a cross-sectional view schematically illustrating a foldable display device according to another embodiment.  FIG. 7  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 6 . 
     Referring to  FIG. 6  and  FIG. 7 , a first plate  200  may be disposed under a flexible display module  100 . The flexible display module  100  may be substantially identical or similar to the flexible display module  100  described with reference to  FIGS. 1 to 5 . The first plate  200  may be different from the first plate  200  described with reference to  FIGS. 1 to 5  at least in a groove  210 . For example,  FIG. 6  may be a schematic cross-sectional view taken along line I-I′ of  FIG. 1 , and  FIG. 7  may be a schematic cross-sectional view taken along line II-II′ of  FIG. 2 . 
     The first plate  200  may include grooves  210  overlapping the second area FA 2  and penetrating the first plate  200 . The grooves  210  may relatively reduce repulsive force of the first plate  200  overlapping the second area FA 2 . Accordingly, in case that the foldable display device  1000  is folded and then unfolded, the crease may not occur in the second area FA 2  or a depth of the crease may be relatively small. 
       FIG. 8  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 6 . A description overlapping with that of the foldable display device of  FIG. 6  will be omitted. 
     Referring to  FIG. 8 , the foldable display device  1000  may further include a protection layer  230  disposed under the first plate  200  and overlapping the second area FA 2 . A part of the protection layer  230  may also overlap a part of the third area PA adjacent to the second area FA 2 . Another part of the protection layer  230  may also overlap a part of the non-foldable area NFA adjacent to the second area FA 2 . The protection layer  230  may prevent external impurities from penetrating into the lower surface of the flexible display module  100  through the groove  210  penetrating the first plate  200 . The protection layer  230  may include an elastomer having a relatively large elastic force or a relatively large restoring force. For example, the protective layer  230  may include an elastic material such as silicon, urethane, thermoplastic polyurethane, nylon, etc. 
       FIG. 9  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 6 .  FIG. 10  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 9 . A description overlapping with that of the foldable display device of  FIG. 6  will be omitted. 
     Referring to  FIG. 9  and  FIG. 10 , a second plate  300  may be disposed under the first plate  200 . The second plate  300  may overlap each of the non-foldable area NFA and the third area PA. The second plate  300  may not be folded or bent. The second plate  300  may support the first plate  200  overlapping the non-foldable area NFA and the third area PA. A rigidity of the second plate  300  may be greater than a rigidity of the first plate  200 . 
       FIG. 11  is a cross-sectional view schematically illustrating a foldable display device according to still another embodiment of the disclosure.  FIG. 12  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 11 . 
     Referring to  FIG. 11  and  FIG. 12 , a first plate  200  may be disposed under the flexible display module  100 . The flexible display module  100  may be substantially identical or similar to the flexible display module  100  described with reference to  FIGS. 1 to 5 . The first plate  200  may be different from the first plate  200  described with reference to  FIGS. 1 to 5  at least in a groove  220 . For example,  FIG. 11  may be a schematic cross-sectional view taken along line I-I′ of  FIG. 1 , and  FIG. 12  may be a schematic cross-sectional view taken along line II-II′ of  FIG. 2 . 
     The first plate  200  may include grooves  220  overlapping the second area FA 2  and not penetrating the first plate  200 . The grooves  220  may relatively reduce repulsive force of the first plate  200  overlapping the second area FA 2 . Accordingly, in case that the foldable display device  1000  is folded and then unfolded, the crease may not occur in the second area FA 2  or a depth of the crease may be relatively small. A depth D 220  of the grooves  220  not penetrating the first plate  200  compared to a thickness D 200  of the first plate  200  may be about 40% or greater and about 60% or less. 
       FIG. 13  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 11 .  FIG. 14  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 13 . A description overlapping with that of the foldable display device of  FIG. 11  will be omitted. 
     Referring to  FIG. 13  and  FIG. 14 , a second plate  300  may be disposed under the first plate  200 . The second plate  300  may overlap the non-foldable area NFA and the third area PA. The second plate  300  may not be folded or bent. The second plate  300  may support the first plate  200  overlapping the non-foldable area NFA and the third area PA. A rigidity of the second plate  300  may be greater than a rigidity of the first plate  200 . 
       FIG. 15  is a cross-sectional view schematically illustrating a foldable display device according to further still another embodiment of the disclosure.  FIG. 16  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 15 . 
     Referring to  FIG. 15  and  FIG. 16 , a first plate  200  may be disposed under a flexible display module  100 . The flexible display module  100  may be substantially identical or similar to the flexible display module  100  described with reference to  FIGS. 1 to 5 . The first plate  200  may be substantially different from the first plate  200  described with reference to  FIGS. 1 to 5  at least in a thickness D 2  of the first plate  200  overlapping the second area FA 2 . For example,  FIG. 15  may be a schematic cross-sectional view taken along line I-I′ of  FIG. 1 , and  FIG. 16  may be a schematic cross-sectional view taken along line II-II′ of  FIG. 2 . 
     The thickness D 2  of the first plate  200  overlapping the second area FA 2  may be less than a thickness D 200  of the first plate  200  overlapping the non-foldable area NFA. Accordingly, repulsive force of the first plate  200  overlapping the second area FA 2  may be relatively reduced. So, in case that the foldable display device  1000  is folded and then unfolded, the crease may not occur in the second area FA 2  or a depth of the crease may be relatively small. 
     In an embodiment, the thickness D 2  of the first plate  200  overlapping the second area FA 2  compared to the thickness D 200  of the first plate  200  overlapping the non-foldable area NFA may be about 50% or greater. In case that the thickness D 2  of the first plate  200  overlapping the second area FA 2  compared to the thickness D 200  of the first plate  200  overlapping the non-foldable area NFA is less than about 50%, the flexible display module  100  may not be stably supported by the first plate  200  overlapping the second area FA 2 . 
       FIG. 17  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 15 . A description overlapping with that of the foldable display device of  FIG. 15  will be omitted. 
     Referring to  FIG. 17 , the thickness D 2  of the first plate  200  overlapping the second area FA 2  may be less than the thickness D 200  of the first plate  200  overlapping the non-foldable area NFA. The first plate  200  may include grooves  210  overlapping the second area FA 2  and penetrating the first plate  200 . 
     The grooves  210  overlapping the second area FA 2  may relatively reduce repulsive force of the first plate  200  overlapping the second area FA 2 . Accordingly, in case that the foldable display device  1000  is folded and then unfolded, the crease may not occur in the second area FA 2  or a depth of the crease may be relatively small. 
       FIG. 18  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 17 . A description overlapping that of the foldable display device of  FIG. 17  will be omitted. 
     Referring to  FIG. 18 , the foldable display device  1000  may further include a protection layer  230  disposed under the first plate  200  and overlapping the second area FA 2 . A part of the protection layer  230  may also overlap a part of the third are PA adjacent to the second area FA 2 . Another part of the protection layer  230  may also overlap a part of the non-foldable area NFA adjacent to the second area FA 2 . The protection layer  230  may prevent external impurities from penetrating into a lower surface of the flexible display module  100  through the groove  210  penetrating the first plate  200 . The protective layer  230  may include an elastomer having a relatively large elastic force or a relatively large restoring force. For example, the protective layer  230  may include an elastic material such as silicon, urethane, thermoplastic polyurethane, nylon, etc. 
       FIG. 19  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 15 . A description overlapping with that of the foldable display device of  FIG. 15  will be omitted. 
     Referring to  FIG. 19 , the thickness D 2  of the first plate  200  overlapping the second area FA 2  may be less than the thickness D 200  of the first plate  200  overlapping the non-folded area NFA. The first plate  200  may include grooves  220  overlapping the second area FA 2  and not penetrating the first plate  200 . A depth D 220  of the plurality of grooves  220  not penetrating the first plate  200  compared to the thickness D 200  of the first plate  200  may be about 40% or greater and about 60% or less. 
     The grooves  220  overlapping the second area FA 2  may relatively reduce repulsive force of the first plate  200  overlapping the second area FA 2 . Accordingly, in case that the foldable display device  1000  is folded and then unfolded, the crease may not occur in the second area FA 2  or a depth of the crease may be relatively small. 
       FIG. 20  is a cross-sectional view schematically illustrating other embodiment of the foldable display device of  FIG. 15 .  FIG. 21  is a cross-sectional view schematically illustrating a folded state of the foldable display device of  FIG. 20 . A description overlapping with that of the foldable display device of  FIG. 15  will be omitted. 
     Referring to  FIG. 20  and  FIG. 21 , a second plate  300  may be disposed under the first plate  200 . The second plate  300  may overlap the non-foldable area NFA and the third area PA. The second plate  300  may not be folded or bent. The second plate  300  may support the first plate  200  overlapping the non-foldable area NFA and the third area PA. A rigidity of the second plate  300  may be greater than a rigidity of the first plate  200 . 
     The disclosure may be applicable to numerous display devices such as smartphones, smart pads, PMP, PDA, MP3 players, etc. 
     It should be understood that embodiments described herein should be considered in a descriptive sense only not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope.