Patent Publication Number: US-2022216289-A1

Title: Flexible display device and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2021-00016416, filed in the Korean Intellectual Property Office on Jan. 6, 2021, the entire contents of which are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present disclosure relates to a display device and, more particularly, to a flexible display device and a manufacturing method of the flexible display device. 
     DISCUSSION OF THE RELATED ART 
     A display device such as an organic light emitting diode (OLED) display device and the like includes a display panel, and the display panel is manufactured by including a plurality of layers and elements on a substrate. Conventionally, glass was used as the substrate of the display panel. However, the glass substrate is rigid and thus it is difficult to twist or deform the display device to achieve a display device of a desired shape. Recently, a flexible display device that uses a flexible substrate, such as light and deformable plastic, has been researched and developed. 
     Depending on the usage or shape, the flexible display device may be classified into a bendable display device, a foldable display device, a rollable display device, and the like. Such a flexible display device may be twisted or folded owing to the use of a flexible substrate such as plastic. 
     SUMMARY 
     A flexible display device includes a first flexible substrate including a signal line disposed on a first side of the first flexible substrate; a second flexible substrate disposed on a second side of the first flexible substrate that is located opposite to the first side of the first flexible substrate; an opening that penetrates both the first flexible substrate and the second flexible substrate; a driver that is attached to the second flexible substrate; and a connection portion that electrically connects the driver to the signal line through the opening. 
     The flexible display device may further include an adhesive layer that attaches the second side of the first flexible substrate to a first side of the second flexible substrate. 
     The flexible opening may be continuous through the first flexible substrate, the adhesive layer, and the second flexible substrate. 
     The flexible display device may further include a metal mask that is disposed on a second side of the second flexible substrate, and includes a metal mask opening that corresponds to the opening penetrating both the first flexible substrate and the second flexible substrate. 
     The opening penetrating both the first flexible substrate and the second flexible substrate may expose a rear side of the signal line, and the connection portion may be connected to the rear side of the signal line. 
     The connection portion may contact a second side of the metal mask. 
     A plurality of pixels may be formed on the first side of the first flexible substrate, and the connection portion may be connected to the signal line in a non-display area where the plurality of pixels is not located. 
     A plurality of pixels may be formed on the first side of the first flexible substrate, and the connection portion may overlap a display area where the plurality of pixels is disposed. 
     The signal line may include a signal line opening, and the signal line opening, may overlap the opening that penetrates both the first flexible substrate and the second flexible substrate. 
     The flexible display device may further include an additional connection portion that is connected to the connection portion through the signal line opening, and the additional connection portion may contact the first side of the signal line. 
     The first flexible substrate may include two polyimide layers, the second flexible substrate may include two inorganic insulating layers, and the adhesive layer may bond the polyimide layer of the first flexible substrate to the polyimide layer of the second flexible substrate. 
     The flexible display device may further include: a plurality of pixels that is formed on the first side of the first flexible substrate; an encapsulation layer that may cover the plurality of pixels; a touch detector disposed on the encapsulation layer; and a window that may be disposed on the touch detector. 
     Two or more of the flexible display devices may be attached to each other. 
     A flexible display device includes: a first flexible substrate including pixels formed on a first side of first flexible substrate; a second flexible substrate disposed on a second side of the first flexible substrate that is located opposite to the first side of the first flexible substrate; an adhesive layer bonding the second side of the first flexible substrate and a first side of the second flexible substrate; and an optical element that is attached to a second side of the second flexible substrate. 
     The optical element may overlap a display area where the pixels are formed. 
     A light transmission area may overlap the optical element. 
     A method of manufacturing a flexible display device includes: forming, a signal line and pixels on a flexible substrate; folding the flexible substrate with the signal line and pixels formed thereon; bonding the folded flexible substrate using an adhesive layer to maintain the folded state; and cutting a folded portion of the flexible substrate to divide it into a first flexible substrate where the signal line and the pixels are located, and a second flexible substrate located on a rear side of the first flexible substrate. 
     The forming of the signal line and the pixels on the flexible substrate may further include forming a metal mask that includes an opening, and the method may further include: forming an opening that overlaps the signal line on the folded and attached flexible substrate using the metal mask as a mask; and forming a connection portion that is connected to the signal line through the opening of the flexible substrate. 
     The method may further include attaching a driver to the flexible substrate after the forming of the signal line and the pixels on the flexible substrate and before the folding of the flexible substrate. 
     The method may further include attaching an optical element onto the flexible substrate after the forming of the signal line and the pixels on the flexible substrate and before the folding of the flexible substrate. 
     A method for manufacturing a display device includes disposing an array of pixels on a top surface of a flexible substrate, disposing a signal line on the top surface of the flexible substrate, the signal line being connected to the array of pixels, applying an adhesive to a bottom surface of the flexible substrate, folding the flexible substrate such that a first portion of the bottom surface contacts a second portion of the bottom surface and the first portion is bound to the second portion by the adhesive, etching an opening though the folded flexible substrate, and disposing a connector within the opening to electrically connect the signal line to a driver through the folded flexible substrate. 
     The folded flexible substrate may be cut to divide the folded flexible substrate into a first flexible substrate and a second flexible substrate that are bound to each other by the adhesive. 
     A metal mask may be used to etch the opening though the folded flexible substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present disclosure and many of the attendant aspects thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure; 
         FIG. 2  is an enlarged top plan view of a part of the flexible display device according to an embodiment of the present disclosure; 
         FIG. 3  to  FIG. 7  illustrate a method of manufacturing the flexible display device of  FIG. 1 ; 
         FIG. 8  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure; 
         FIG. 9  is a top plan view of a partially enlarged portion of the flexible display device shown in  FIG. 8 ; 
         FIG. 10  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure; 
         FIG. 11  is a top plan view of an enlarge portion of the flexible display device shown in  FIG. 10 ; 
         FIG. 12  to  FIG. 15  illustrate a method for manufacturing a flexible display device according to an embodiment of the present disclosure; 
         FIG. 16  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure; 
         FIG. 17  to  FIG. 19  illustrate a manufacturing method of the flexible display device of  FIG. 1 . 
         FIG. 20  is a cross-sectional view of a flexible display device according to an embodiment of the present: disclosure; and 
         FIG. 21  is a schematic view of a large-scale display device including the flexible display device according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
     In describing the present invention, like reference numerals may designate like elements throughout the specification. 
     The present invention is not necessarily limited to the relative size and thickness of the various elements illustrated in the drawings. In the drawings, the thickness of layers, films, panels, regions, etc., may be exaggerated for clarity. 
     It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Further, throughout the specification, the word “on” a target element will be understood to mean positioned above or below the target element, and will not necessarily be understood to mean positioned “at an upper side” based on a direction of gravitational pull. 
     In addition, unless explicitly described to the contrary, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
     Further, throughout the specification, the phrase “on a plane” or “in a plan view” means viewing a target portion from the top, and the phrase “on a cross-section” or “in a cross-sectional view” means viewing a cross-section formed by vertically cutting a target portion from the side. 
     Hereinafter, a display device according to the present embodiment will be described with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure, and  FIG. 2  is an enlarged top plan view of a part of the flexible display device according to the embodiment of the present disclosure. 
     A flexible display device  10 , according to an embodiment of the present disclosure, includes a portion formed on a first flexible substrate  110 - 1 , a portion formed on a second flexible substrate  110 - 2 , an adhesive layer  150  that bonds the first flexible substrate  110 - 1  to the second flexible substrate  110 - 2 , a driver  300  attached to one end of the second flexible substrate  110 - 2 , and a connection portion  160  formed through both the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2 . 
     First, the portion formed on the first flexible substrate  110 - 1  will be described. 
     The portion formed on the first flexible substrate  110 - 1  includes a signal line  120 , red pixels R, green pixels G, and blue pixels B connected to the signal line  120 , and an encapsulation layer  200  that covers the signal line  120  and the pixels R, G, and B may be included on the portion. 
     An area where the respective pixels R, G, and B are located is referred to as a display area, and an area where the pixels R, G, and B are not located is referred to as a non-display area. Since the encapsulation layer  200  is a constituent element that protects the pixels R, G, and B, a space in which the encapsulation layer  200  is formed cannot be reduced, and thus, to reduce a width of the non-display area, the signal line  120  is extended to the outside of the encapsulation layer  200  to reduce the size of the part connected to the driver  300 . According to an embodiment of the present disclosure, an opening  116  is continuously formed on the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150  to electrically connect the signal line  120  and the driver  300  such that a connection portion  160  is electrically connected to a lower portion of the signal line  120 . Thus, a fan-out area located in the non-display area is removed, and an area where the driver  300  is attached is located at a rear side of the first flexible substrate  110 - 1  while overlapping the display area, and accordingly, the size of the non-display area is reduced. 
     According to an embodiment of the present disclosure, an additional constituent element may be further included in an upper portion of the encapsulation layer  200  of the first flexible substrate  110 - 1 . This will be described later with reference to  FIG. 20 . 
     The portion formed on the second flexible substrate  110 - 2  includes a metal mask  170  disposed at a rear side of the second flexible substrate  110 - 2 . The metal mask  170  is formed of an electrically conductive metal, and includes an opening OP. The opening OP of the metal mask  170  may be located in an area that corresponds to an opening  116 , which is formed while penetrating the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150 . Therefore, the metal mask  170  may be used as a mask for forming the opening  116 . In addition, the opening OP of the metal mask  170  and the opening  116  formed in the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150  may overlap each other in a plan view. 
     The metal mask  170  is electrically connected to the connection portion  160  formed in the opening  116  and the pad  310  of the driver  300  to serve to transmit a signal output from the driver  300  to the connection portion  160 . 
     The adhesive layer  150  is disposed between the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2 . The adhesive layer  150  attaches the first flexible substrate  110 - 1  to the second flexible substrate  110 - 2 . The adhesive layer  150  may include an organic material. The adhesive layer  150  is formed of an organic material such that the adhesive layer  150  can be removed by the same process when the opening  116  that penetrates the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  are formed. 
     The opening  116  is formed while the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  are attached by the adhesive layer such that the opening  116  can be continuously formed in the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150 . When the opening  116  is formed without attaching of the adhesive layer  150 , the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  move with respect to each other and thus misalignment may occur. In particular, in  FIG. 1 , one signal line  120 , one opening  116 , and one connection portion  160  are illustrated, but it is to be understood that the opening  116  and the connection portion  160  need to be formed for each signal line  120 , and thus there may be a problem in that the signal is not applied to signal line  120  or is applied to another signal line  120  in a case of misalignment of the opening  116 . Thus, the opening  116  is formed after attaching the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  using the adhesive layer  150 . 
     The driver  300  is attached to one end of the second flexible substrate  110 - 2 . The driver  300  may include a driver IC, and the driver IC ma be formed on a printed circuit board or a flexible printed circuit board. The driver  300  further includes the pad  310 , which is a portion of the driver  300  that outputs a signal to the signal line  120 . The pad  310  is electrically connected with the metal mask  170  and transmits an output signal to the metal mask  170 , and the output signal is transmitted to the signal line  120  through the metal mask  170  and the connection portion  160 . 
     According to embodiments of the present disclosure, the driver IC may be directly attached to a bottom surface of the second flexible substrate  110 - 2 . In this case, the metal mask  170  may electrically connect the connection portion  160  and an output terminal of the driver IC. In addition, a printed circuit board or a flexible printed circuit board may be additionally attached to the other end of the second flexible substrate  110 - 2  where the driver IC is not positioned. 
     The connection portion  160  is disposed in the opening  116  that is formed while penetrating the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2 , and the connection portion  160  is formed while filling the opening  116  with an electrically conductive material (e.g., a metal and the like). 
     A connection structure of the signal line  120  and the connection portion  160  will be described in more detail with reference to  FIG. 2 . In  FIG. 2 , the display device  10  is viewed from above the first flexible substrate  110 - 1 , and a portion where one signal line  120  and the connection portion  160  are connected is enlarged. 
     As shown in  FIG. 2 , pixels R, G, and B and an encapsulation layer  200  that covers the pixels R, G, and B are formed on the first flexible substrate  110 - 1 . The signal line  120  extends to the non-display area from the pixels R, G, and B, and may extend to a portion not covered by the encapsulation layer  200  in the non-display area. The connection portion  160  and the opening  116  overlap the non-display area on a plane. The opening  116  that penetrates the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  is formed in the rear surface of the signal line  120 , and the signal line  120  has a structure in which the connection portion  160  contacts the rear surface of the signal line  120  through the opening  116 . 
     Thus, the driver  300  is hidden by the rear surface of the flexible substrates  110 - 1  and  110 - 2  such that a size of the non-display area is reduced. In addition, since the fan-out region in which the signal line  120  is formed to receive a signal from the driver  300  is not formed (e.g., omitted), to the size of the non-display area is reduced. According to embodiments of the present disclosure, since the signal line  120  is not electrically connected from the side but electrically connected through the rear surface, the size of the on-display area is reduced. 
     In  FIG. 2 , a thickness of the signal line  120  is enlarged, but the signal line  120  may have a thickness of about several micrometers to several tens of micrometers, and depending on the technology development, it can be formed with a thin wire. 
     In addition, as shown in  FIG. 1 , although the connection portion  160  is formed in the non-display area, the width of the non-display area may be several micrometers to several tests of micrometers, which is not visible to a user. 
     A method for manufacturing the display device  10 , according to an embodiment will be sequentially described with reference to  FIG. 3  to  FIG. 7 . 
       FIG. 3  to  FIG. 7  show a method of manufacturing the flexible display device of  FIG. 1 . 
       FIG. 3  illustrates forming the signal line  120  and the pixels R, G, and B on the flexible substrate  110 , and forming the metal mask  170  including the opening OP on the flexible substrate  110 . 
     The flexible substrate  110  is formed on the glass substrate  400 , and then the signal line  120  and the pixels R, G, and B are formed on the flexible substrate  110 . In this case, the metal mask  170  including the opening OP can be formed together. The metal mask  170  and the signal line  120  may be formed through the same process using the same mask with the same material. According to embodiments of the present disclosure, the metal mask  170  may be formed through the same process using the same mask with the same material of a metal layer included in pixels R, G, and B. 
     After that, the encapsulation layer  200  covering the pixels R, G, and B is formed. The encapsulation layer  200  protects the pixels R, G, and B from moisture or oxygen, and particularly, when the pixels R, G, and B include an organic emission layer, the encapsulation layer  200  is formed to block moisture or oxygen to thereby prevent deterioration of the organic emission layer because the organic emission layer is susceptible to moisture or oxygen. The encapsulation layer  200  includes an organic layer and an inorganic layer, and the organic layer and the inorganic layer may be alternately formed, and according to embodiments of the present disclosure, a triple layer of an inorganic layer, an organic layer, and an inorganic layer may be defined. 
     The metal mask  170  formed on the flexible substrate  110  includes an opening OP, and one opening OP is formed for each signal line  120  and thus a plurality of opening OPs are formed. Each opening OP may have a circular cross-section, and may have various other shapes. 
     As shown in  FIG. 3 , the flexible substrate  110  is placed on the glass substrate  400 , and then the signal lines  120 , the metal mask  170 , and the like are formed. Accordingly, the use of the glass substrate  400  may prevent the flexible substrate  110  from being deformed due to heat applied during laminating and etching processes. 
     As shown in  FIG. 4 , an attachment process of the driver  300  and a process P 2  for folding the flexible substrate  110  are performed on a display device  10 - 1 , which has been formed as shown in  FIG. 3 . Before forming the flexible substrate  110 , a process for folding after detaching the glass substrate  400  is carried out. Thereafter, the glass substrate  400  may be detached from the flexible substrate  110  through a lift-off method, for example, using a laser. 
     In  FIG. 4 , the process P 1  for attaching the driver  300  on the flexible substrate  110  and the process P 2  for forming the flexible substrate  110  are illustrated. 
     Referring to  FIG. 4 , the driver  300  is attached to electrically connect the metal mask  170  and the pad  310  of the driver  300 . Here, according to embodiments of the present disclosure, the driver  300  may be formed during a process for forming the signal line  120 , the pixels R, G, and B, and the encapsulation layer  200 . 
     The driver  300  of the display device  10 - 2  undergoes a process P 2  for folding after detaching the glass substrate  400 . 
     In this case, as shown in  FIG. 5 , the folded flexible substrate  110  and a rear surface of the flexible substrate  110  are bonded to each other by using an adhesive layer  150 . According to embodiments of the present disclosure, the adhesive layer  150  may be formed on the rear surface of the flexible substrate  110  before the process P 2  is performed. Thus, when the folding process P 2  is carried out, the rear surfaces of the flexible substrate  110  can be bonded to each other by the adhesive layer  150 . Meanwhile, according to embodiments of the present disclosure, the adhesive layer  150  may be formed on the rear surface of the substrate  110  after performing the folding process P 2 , and accordingly, the rear surfaces of the flexible substrate  110  can be attached to each other by the adhesive layer  150  in a separate process from the folding process P 2 . 
     After that, as shown in  FIG. 5 , the opening  116  is formed in the flexible substrate  110  and the adhesive layer  150 . 
     In  FIG. 5 , the signal line  120  and the pixels R, G, and B are located in an upper part of the flexible substrate  110 . A remainder of the flexible substrate  110  is disposed in a lower part of the flexible substrate  110  and this lower part overlaps the upper part. The signal line  420  is attached to the upper part of the folded flexible substrate  110  using the adhesive layer  150 , and the opening  116  overlaps the signal line  120  on a plane in the folded state. The metal mask  170  includes an opening OP that corresponds to the opening  116  of the flexible substrate  110 . 
     Here, the opening  116  may be formed through a laser etching method such as laser drilling or dry etching using the metal mask  170  as a mask. For example, the opening  116  is formed while the flexible substrate  110  exposed through the opening OP portion of the metal mask  170  is removed. In particular, since the rear surfaces of the flexible substrate  110  are bonded to each other by the adhesive layer  150 , the openings  116  formed on the flexible substrate  110  on both sides can be maintained without shifting. In addition, as the flexible substrate  110  is removed by the opening  116 , the bottom surface of signal line  120  is exposed. In addition, the adhesive layer  150  can be removed by the same process during laser etching or dry etching to form the opening  116  penetrating through the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  by forming the adhesive layer  150  of an organic material. 
     After that, as shown in  FIG. 6 , a connection portion  160  is formed in a display device  10 - 3  where the opening  116  is formed (e.g., the connection portion  160  is formed within the opening  116 ). 
     In  FIG. 6 , a step for forming the connection portion  160  connected with the signal line  120  through the openings  116  of the flexible substrate  110  is illustrated. 
     The connection portion  160  is formed in the opening OP of the metal mask  170  and the opening  116  of the flexible substrate  110  and the adhesive layer  150 . The connection portion  160  may be formed by filling a space with a metallic material (e.g., a metal). For example, the connection portion  160  may be formed by an inkjet printing method or a screen printing method. The connection portion  160  is electrically connected to the rear surface of the signal line  120  exposed through the opening  116  and the metal mask  170 . Since the metal mask  170  is electrically connected to the driver  300 , the driver  300  is electrically connected to the signal line  120 . 
     After that, as shown in a display device  10 - 4  where the connection portion  160  is formed, the flexible substrate  110  is cut along a cutting line CL. 
       FIG. 7  illustrates separating the flexible substrate  110  into the first flexible substrate  110 - 1 , where the signal line  120  and the pixels R, G, and B are located, and the second flexible substrate  110 - 2 , disposed in the rear surface of the flexible substrate  110 - 1 , by cutting the folded portion of the flexible substrate  110  along the cutting line CL. 
     The cutting of the flexible substrate  110  may he performed, for example, using a laser. 
     In a display device  10 - 5  that has undergone the cutting process, the flexible substrate  110  is separated into the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  such that the structure shown in  FIG. 1  is completed. 
     In the above-described arrangement, the rear surface of the signal line  120  is exposed through the opening  116  formed in the flexible substrates  110 - 1  and  110 - 2  and the adhesive layer  150  such that the connection portion  160  is directly connected with the rear surface of the signal line  120 . 
     However, according to embodiments of the present disclosure, an opening may be additionally formed in the signal line  120 , and an additional connection portion may be formed and electrically connected to the signal line  120 . 
     This will be described with reference to  FIG. 8  and  FIG. 9 . 
       FIG. 8  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure, and  FIG. 9  is a top plan view of a partially enlarged portion of the flexible display device according to the arrangement illustrated in  FIG. 8 . 
     In the arrangement illustrated in  FIG. 8  and  FIG. 9 , the signal line  120  includes an opening  121  such that a rear surface of the signal line  120  is not exposed by an opening  116  formed in flexible substrates  110 - 1  and  110 - 2 , and the signal line  120  and the connection portion  160  are to electrically connected with each other through the additional connection portion  260 . 
     Referring to  FIG. 9 , the opening  116  formed in the flexible substrates  110 - 1  and  110 - 2  may be smaller than the opening  121  formed in the signal line  120 . Thus, it is difficult for the connection portion  160  to be directly connected with the signal line  120 . Thus, the connection portion  160  and the signal line  120  are electrically connected by using the additional connection portion  260 . The additional connection portion  260  may have a structure that contacts the top surface of the signal line  120 . 
     In the arrangement illustrated in  FIG. 8  and  FIG. 9 , the signal line  120  is electrically connected not through a side surface but through a rear surface, and thus a size of the non-display area is reduced. 
     In the above, an arrangement in which the signal line  120  extends to the non-display area, and the signal line  120  and the connection portion  160  are connected to each other through the rear surface in the non-display area, has been described. 
     However, according to embodiments of the present disclosure, the connection portion  160  may be connected through the rear surface of the signal line  120  in the display area. This will be described with reference to  FIG. 10  and  FIG. 11 . 
       FIG. 10  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure, and  FIG. 11  is a top plan view of an enlarged portion of the flexible display device shown in  FIG. 10 . 
     In the arrangement illustrated in  FIG. 10  and  FIG. 11 , a rear side of a signal line  120  and a connection portion  160  are connected with each other in a display area, and the connection portion  160  and an opening  116  overlap the display area on a plane. For example, an opening  116  formed in a first flexible substrate  110 - 1  and a second flexible substrate  110 - 2 , and an adhesive layer  150  is formed on a rear side of the display area. The connection portion  160  is formed through the opening  116  and thus is connected to the rear side of the signal line  120 . 
     The connection portion  160  is also disposed in an opening OP of a metal mask  170 , and electrically connects the metal mask  170  and the signal line  120  while being electrically connected with the metal mask  170 . 
     The metal mask  170  is connected to a driver  300  through a pad  310  and thus an output signal of the driver  300  is transmitted to the signal line  120 . 
     In the arrangement illustrated in  FIG. 10  and  FIG. 11 , the size of a non-display area can be further reduced as compared to the arrangement illustrated in  FIG. 1  to  FIG. 9 . For example, in the arrangement illustrated in  FIG. 1  to  FIG. 9 , the signal line  120  extends to the non-display area and thus is connected to the connection portion  160  in the non-display area such that a size of the non-display area is made larger by as much as the space in which the connection portion  160  is formed. However, in the arrangement illustrated in  FIG. 10  and  FIG. 11 , the signal line  120  and the connection portion  160  overlap in the display area and thus the signal line  120  does not need to extend to the non-display area, thereby further reducing the non-display area. 
     Hereinafter, a method for manufacturing the display device according to an arrangement illustrated in  FIG. 1  will be described. 
     In  FIG. 3  to  FIG. 7 , the flexible substrate  110  is folded and then cut into the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2 , but according to example embodiments of the present disclosure, the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  may be separately manufactured from the beginning as opposed to being cut from a single structure and then attached to one another, and then the opening  116  may be formed. 
     This arrangement will be described with reference to  FIG. 12  to  FIG. 15 . 
       FIG. 12  to  FIG. 15  illustrate a method for manufacturing a flexible display device according to an embodiment of the present disclosure. 
     First, referring to  FIG. 12 , a first flexible substrate  110 - 1  is formed on a first glass substrate  400 - 1 , and then a signal line  120 , pixels R, G, and B, and an encapsulation layer  200  are formed on the flexible substrate  110 - 1  such that a first portion  10 - 11  of a display device is formed. 
     Separately, a second flexible substrate  110 - 2  is formed on a second glass substrate  400 - 2  and then a metal mask  170  that includes an opening OP is formed on the second flexible substrate  110 - 2 , and a driver  300  is attached to electrically connect a pad  310  to the metal mask  170  such that a second portion  10 - 12  of the display device is formed. 
     After that, as shown in  FIG. 13 , the first glass substrate  400 - 1  and the second glass substrate  400 - 2  are detached from the first portion  10 - 11  and the second portion  10 - 12  of the display device, respectively. After that, a rear side of the first flexible substrate  110 - 1  and a rear side of the second flexible substrate  110 - 2  are bonded to each other by an adhesive layer  150  such that a display device  10 - 13  is formed. 
     After that, as shown in  FIG. 14 , an opening  116  that penetrates the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150  is formed. The opening  116  may have the same shape as the opening OP of the metal mask  170  by using the metal mask  170  as a mask when etching the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150 . 
     After that, as shown in  FIG. 15 , a connection portion  160 , that is connected to a rear side of the signal line  120  through the opening  116 , is formed on a display device  10 - 14  having the opening  116  formed through the process of  FIG. 14 . The connection portion  160  electrically connects the signal line  120  and the metal mask  170  by contacting a side surface and/or a bottom surface of the metal mask  170 . Accordingly, a display device  10  that is the same as that of  FIG. 1  is firmed. 
     Therefore, as shown in the display device  10  of  FIG. 1 , the driver  300  is hidden by the rear sides of the flexible substrates  110 - 1  and  110 - 2  such that a size of the non-display area is reduced. In addition, the fan-out region formed by the signal line  120  to receive a signal from the driver  300  is not formed, thereby reducing the non-display area. According to the present arrangement, since the signal line  120  is not electrically connected from the side but electrically connected through the rear side, the size of the non-display area is reduced. 
     Meanwhile, the manufacturing method of  FIG. 12  to  FIG. 15  can be applied to a structure in which the opening  121  is formed in the signal line  120  and is connected through the additional connection portion  260  as shown in  FIG. 8  and  FIG. 9 , and may also be applied to a structure in which the signal line  120  and the connection portion  160  are connected in the display area as shown in  FIG. 10  and  FIG. 11 . 
     Hereinabove, an arrangement in which the driver  300  and the signal line  120  are electrically connected by using the opening  116  that penetrates the first flexible substrate  110 - 1 , the second flexible substrate  110 - 2 , and the adhesive layer  150 , and the connection portion  160 , has been described. 
     However, the method in which the flexible substrate is folded and then cut to reduce the non-display area as shown in  FIG. 3  to  FIG. 7  may also be used in manufacturing of a display device where an optical element  500  is formed on a rear side of a display area. 
     This arrangement will be described with reference to  FIG. 16  to  FIG. 19 . 
     First, a cross-sectional structure of a display device where an optical element  500  is disposed on a rear side of a display area will be described with reference to  FIG. 16 . 
       FIG. 16  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure. 
     A flexible display device  10 , according to an example embodiment of the present disclosure, includes a portion formed on a flexible display device  10 , a portion formed on a second flexible substrate  110 - 2 , an adhesive layer  50  that bonds the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2 , and an optical element  500  attached to the adhesive layer  150  and the second flexible substrate  110 - 2 . 
     The portion formed on the first flexible subs  110 - 1  is substantially the same as the portion shown in  FIG. 1 . For example, the portion formed on the first flexible substrate  110 - 1  includes the first flexible substrate  110 - 1 , and may include a signal line  120 , pixels R, G, and B that are connected to the signal line  120 , and an encapsulation layer  200  that covers the respective pixels R, G, and B, which are formed on the first flexible substrate  110 - 1 . 
     The portion formed on the second flexible substrate  110 - 2  includes the second flexible substrate  110 - 2  and the optical element  500  disposed in the rear surface of the second flexible substrate  110 - 2 . Various optical elements such as an optical sensor, a bio sensor, a camera, a flash, and the like may be used as the attachable optical element  500 . The optical sensor may he, for example, an infrared sensor, and the bio sensor may be, for example, a fingerprint recognition sensor. 
     An adhesive layer  150  is disposed between the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2 . The adhesive layer  150  bonds the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  to each other. 
     The optical element  500  is disposed on the rear side of the display area while the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  are attached to each other such that the optical element  500  overlaps the display area on a plane. Some of the display area may include a light transmission area LTA such that light can be incident on the optical element  500 . According to example embodiments of the present disclosure, the pixels R, G, and B located in the light transmission area LTA have lower density per unit area compared to other portions of the display area, thereby increasing light transmittance. 
     Hereinafter, a method of manufacturing the arrangement shown in  FIG. 16  will be described with reference to  FIG. 17  to  FIG. 19 . 
       FIG. 17  to  FIG. 19  illustrate a manufacturing, method of the flexible display device of  FIG. 16 . 
     In  FIG. 17 , forming the signal line  120  and the pixels R, G, and B on the flexible substrate  110  and attaching the optical element  500  onto the flexible substrate  110  are illustrated. 
     As shown in  FIG. 17 , the flexible substrate  110  is formed on a class substrate  400 , the signal line  120  and the pixels R, G, and B are formed on the flexible substrate  110 , and then the encapsulation layer  200  covering the signal line  120  and the pixels R, G, and B is formed. 
     In addition, the optical element  500  is additionally attached to the non-display area of the flexible substrate  110  such that a display device  10 - 21  is completed. 
     After that, as shown m  FIG. 18 , a process P 2  for folding a display device  10 - 22  from which the glass substrate  400  is detached is carried out. 
     In  FIG. 18 , the process P 2  for folding the flexible substrate  110  is illustrated. 
     The adhesive layer  150  is formed on the rear side of the flexible substrate  110  before or after the folding process P 2  of  FIG. 18  such that the rear sides of the flexible substrate  110  are bonded to each other as shown in  FIG. 19 . For example, according to example embodiments of the present disclosure, the folding process P 2  is performed after forming the adhesive layer  150  in the rear side of the flexible substrate  110  such that the rear sides of the flexible substrate  110  are attached to each other in the folding process P 2 . Meanwhile, depending on example embodiments of the present disclosure, the adhesive layer  150  is formed on the rear side of the flexible substrate  110  after the folding process P 2  such that the rear sides of the flexible substrate  110  may be attached to each other by the adhesive layer  150  in a process that is separate from the folding process P 2 . When the rear sides of the flexible substrate  110  are attached to each other, they may be attached while matching a position her the optical element  500  is to be aligned. 
     In  FIG. 18 , the folded flexible substrate  110  is attached by using the adhesive layer  150  such that a portion of the flexible substrate  110 , in which the signal line  120  and the pixels R, G, and B are located, is disposed in an upper portion, and a part of the rest of the flexible substrate  110  is located in a lower portion that overlaps the signal line  120 . 
     After that, the flexible substrate  110  is cut along a cutting line CL of  FIG. 19 . 
     In  FIG. 19 , separating a folded portion of the flexible substrate  110  into the first flexible substrate  110 - 1  where the signal line  120  and the pixels R, G, and B are disposed and the second flexible substrate  110 - 2  located in the rear side of the first flexible substrate  110 - 2  by cutting along the cutting lite CL is illustrated. 
     The flexible substrate  110  is divided into the first flexible substrate  110 - 1  and the second flexible substrate  110 - 2  through the cutting process such that a structure shown in  FIG. 16  is completed. 
     Depending on example embodiments of the present disclosure, a structure in which the connection portion  160  disposed on the rear side of the signal line  120  is used for connection as shown in  FIG. 1  to  FIG. 15 , and the structure in which the optical element  500  is formed on the rear side of the second flexible substrate  110 - 2  as shown in  FIG. 16  to  FIG. 19 , can be formed together. For example, one side of the flexible substrate  110  is folded and thus connected with the connection portion  160  through the rear side of the signal line  120 , while folding the other side of the flexible substrate  110  to locate the optical element  500  in a lower portion of the display area. 
     Hereinabove, arrangements in which the portion formed on the first flexible substrate  110 - 1  includes the first flexible substrate  110 - 1 , and includes the signal line  120  formed on the first flexible substrate  110 - 1 , the pixels R, G, and B connected to the signal line  120 , and the encapsulation layer  200  covering the signal line  120  and the respective pixels R, G, and B, has been described. 
     Hereinafter, a structure that can be additionally formed on the encapsulation layer  200  will be described in detail with reference to  FIG. 20 . 
       FIG. 20  is a cross-sectional view of a flexible display device according to an embodiment of the present disclosure. 
     Referring to  FIG. 20 , a display device  10  is provided above the first flexible substrate  110 - 2 . A touch detector  600  and a window  700  are disposed on the encapsulation layer  200 . 
     After forming an inorganic insulation layer on the encapsulation layer  200 , a touch detection electrode is formed such that the touch detector  600  can be formed. 
     In addition, the window  700  that protects the touch detector  600  and the pixels R, G, and B may be further included. 
     In  FIG. 20 , a configuration of the flexible substrates  110 - 1  and  110 - 2  is illustrated in more detail. 
     For example, each of the flexible substrates  110 - 1  and  110 - 2  is formed of four layers, and each includes two polyimide (PI) layers  110 - 11 ,  110 - 13 ,  110 - 21 , and  110 - 23  and two inorganic insulating layers  110 - 12 ,  110 - 14 ,  110 - 22 , and  110 - 24 . Layers attached by the adhesive layer  150  include a first polyimide layer  110 - 11  of the first flexible substrate  110 - 1  and a first polyimide layer  110 - 21  of the second flexible substrate  110 - 2 . 
     In  FIG. 20 , each of the polyimide layers  110 - 11 ,  110 - 13 ,  110 - 21 , and  110 - 23  are thicker than each of the inorganic insulating layers  110 - 12 ,  110 - 14 ,  110 - 22 , and  110 - 24 , but the may have almost the same thickness depending on various different arrangements. In addition, the thickness of the polyimide layers  110 - 11 ,  110 - 13 ,  110 - 21 , and  110 - 23  may be about 10 μm or more and about 20 μm or less, and the thickness may be about 15 μm or more and about 16 μm or less in the present arrangement. 
     However, depending on the various arrangements, each of the flexible substrates  110 - 1  and  110 - 2  may be formed of one polyimide layer and one inorganic insulating layer. 
     Hereinabove, the display device  10  in which a signal is received through the rear side of the signal line  120  and thus the size of the non-display area is reduced may be formed as a large-scale display device by attaching a plurality of display devices  10 . This will be described with reference to  FIG. 21 . 
       FIG. 21  is a schematic view of a large-scale display device including the flexible display device according to example embodiments of the present disclosure. 
     As shown in  FIG. 21 , a large-scale display device  1  is formed by attaching a plurality of display devices  10 . In the arrangement shown in  FIG. 21 , four display devices  10  are attached. However, depending on the various arrangements, the large-scale display device  1  can be formed by attaching two or more display devices  10 . 
     In the display device  10 , according to the arrangement shown in  FIG. 1  to  FIG. 20 , the signal is received through the rear side of the signal line  120  and thus the size of the non-display area is reduced or removed entirely, and even though two display devices  10  are attached, a gap Gp between two display areas is narrow such that a user cannot recognize the gap Gp. In the present arrangement, as shown in  FIG. 1 , even though the connection portion  160  is formed on the non-display area, the width of each of the non-display is as narrow as several μm to several tens of μm such that the user cannot recognize the gap Gp between the two display areas even though the gap Gp is doubled. Therefore, when the large-scale display device is formed while attaching a display panel, a problem that a black colored stripe is viewed due to the non-display area in a portion where the display panel is attached can be solved. 
     While this disclosure has been described in connection with what is presently considered to be practical embodiments, it is to be understood that the invention is not necessarily limited to the disclosed embodiments. On the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the present disclosure.