Patent Publication Number: US-2019170924-A1

Title: Backlight unit having a light guide plate with a patterned capping layer and display device including the same

Description:
This application claims priority to Korean Patent Application No. 10-2017-0164930, filed on Dec. 4, 2017 in the Korean Intellectual Property Office, under 35 U.S.C. § 119, the disclosure of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a backlight unit and, more specifically, to a backlight unit having a light guide plate with a patterned capping layer and a display device including the same. 
     DISCUSSION OF THE RELATED ART 
     A variety of display devices such as liquid-crystal display (LCD) devices and organic light-emitting diode display (OLED) devices are currently being developed to satisfy a demand for multimedia devices. 
     For example, a liquid-crystal display device may include a liquid-crystal display panel with field generating electrodes such as pixel electrodes and a common electrode, and a liquid-crystal layer in which an electric field is formed by the field generating electrodes. A backlight unit may provide light to the liquid-crystal display panel. The liquid-crystal display device displays images by re-aligning liquid crystals in the liquid-crystal layer by using the electric field generating electrodes to thereby control the amount of light passing through the liquid-crystal layer for each pixel. 
     As display devices find a variety of applications, demands on curved display devices are increasing. Curved display devices may have a curved screen to provide viewers with a more immersive viewing experience. 
     SUMMARY 
     A backlight unit includes a light guide plate and a wavelength conversion layer disposed on a surface of the light guide plate. The wavelength conversion layer is configured to convert a color of incident light. The wavelength conversion layer includes an emboss pattern thereon. The emboss pattern includes a plurality of peak portions and a plurality of valley portions. The plurality of peak portions includes a first peak portion, a second peak portion proximate to the first peak portion in a first direction, and a third peak portion proximate to the first peak portion in a second direction. The plurality of valley portions includes a first valley portion disposed between the second peak portion and the third peak portion. 
     A display device includes a light guide plate. A wavelength conversion layer is disposed on a surface of the light guide plate and is configured to convert a color of incident light. The wavelength conversion layer includes an emboss pattern having a plurality of peak portions and a plurality of valley portions. A display panel is disposed on the wavelength conversion layer. The plurality of peak portions includes a first peak portion, a second peak portion proximate to the first peak portion in a first direction, and a third peak portion proximate to the first peak portion in a second direction. The plurality of valley portions includes a first valley portion disposed between the second peak portion and the third peak portion. 
    
    
     
       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 an exploded, perspective view illustrating a display device according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view taken along line II-II′ in  FIG. 1 ; 
         FIG. 3  is an enlarged, perspective view of the backlight unit of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view taken along line IV-IV′ of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view taken along line V-V′ of  FIG. 3 ; 
         FIG. 6  is a cross-sectional view taken along line VI-VI′ of  FIG. 3 ; 
         FIG. 7  is a cross-sectional view of a display device according to an exemplary embodiment of the present disclosure; 
         FIG. 8  is an exploded perspective view of a display device according to an exemplary embodiment of the present disclosure; 
         FIG. 9  is a cross-sectional view taken along line IX-IX′ of  FIG. 8 ; 
         FIG. 10  is an enlarged, perspective view of the backlight unit of  FIG. 8 ; 
         FIG. 11  is a cross-sectional view taken along line XI-XI′ of  FIG. 10 ; 
         FIG. 12  is a cross-sectional view taken along line XII-XII′ of  FIG. 10 ; and 
         FIG. 13  is a cross-sectional view taken along line XIII-XIII′ of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In describing exemplary embodiments of the present disclosure illustrated in the drawings, specific terminology is employed for sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner . 
     It will be understood that when an element or layer is referred to as being “on,” “connected to” or “coupled to” another element or layer, the element or layer can be directly on, connected or coupled to another element or layer or intervening elements or layers. As used herein, connected may refer to elements being physically, electrically and/or fluidly connected to each other. 
     Like numbers may refer to like elements throughout the specification and the drawings. 
     It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections may be otherwise enumerated. These terms are used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the invention. 
     Spatially relative terms, such as “below,” “lower,” “under,” “above,” “upper” and the like, may be used herein for ease of description to describe the relationship of one element or feature to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. 
       FIG. 1  is an exploded, perspective view illustrating a display device according to an exemplary embodiment of the present disclosure.  FIG. 2  is a cross-sectional view taken along line II-II′ in  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , a display device  1  according to an exemplary embodiment of the present disclosure includes a display panel  10  and a backlight unit  21  for providing light to the display panel  10 . 
     The display panel  10  may be a panel-type member including elements used by the display device  1  to display an image. A plurality of pixels may be defined in the display panel  10 . The plurality of pixels may be arranged as a matrix of rows and columns. As used herein, a “pixel” refers to a smallest independent unit of image display. Each single pixel may display a predetermined one of a set of primary colors. For example, a single pixel may be a minimum unit that can represent a color independently of another pixel. 
     The display panel  10  may have a generally rectangular shape when viewed from the top with a pair of longer sides and a pair of shorter sides. For example, the longer sides of the display panel  10  may generally extend in the first direction X, and the shorter sides thereof may generally extend in the second direction Y. The drawings, the corners of the display panel DP may be right angles or may be chamfered or rounded. 
     In an exemplary embodiment of the present disclosure, the display panel  10  may be a liquid-crystal display panel including a bottom plate  10   a,  a top plate  10   b,  and a liquid-crystal layer interposed therebetween. However, to be understood that the display panel  10  may have other arrangements. The display panel  10  may be any other display panel requiring a backlight unit for image display. In some exemplary embodiments of the present invention, the display panel  10  may be at least partially bent in the first direction X, and the display device  1  may be a curved display device. According to an exemplary embodiment of the present invention, the display panel  10  may be bent in the first direction X and/or the second direction Y. As used herein, a phrase “an element is bent in a direction or along a direction” means that the slope of a surface of the element varies along the direction so that the surface forms a curved surface. For example, when the element bent in a particular direction is cut along the particular direction, the cross section becomes a curved surface. 
     The backlight unit  21  may be disposed such that it at least partially overlaps with the display panel  10  in a third direction Z and the backlight  21  may be configured to emit light having a particular wavelength in a direction toward the display panel  10 . For example, the backlight unit  21  may emit white light including red light, green light, and blue light. When the display device  1  is a curved display device, the backlight unit  21  may be, but need not be, disposed above the convex surface of the display panel  10 . 
     In an exemplary embodiment of the present disclosure, the backlight unit  21  may include a light guide plate  101 , a light source unit  200  disposed on the side of the light guide plate  101  where light is incident, and a wavelength conversion layer  301  disposed on the side of the light guide plate  101  where light exits. 
     The light guide plate  101  may guide the light provided from the light source unit  200  so that the light exits toward the display panel  10 . For example, one side surface of the light guide plate  101  that faces the light source unit  200  defines a light-incidence face, and the top surface of the light guide plate  101  facing the display panel  10  defines a light-exiting face. 
     The light guide plate may include a material having a high light transmittance so as to be at least partially transparent. For example, it may include a glass material, a quartz material, or a polymer material such as polyethylene terephthalate, polymethyl methacrylate and/or polycarbonate. 
     The light guide plate  101  may be at least partly bent in the first direction X so that the top surface of the light guide plate  101  may form a concave surface. For example, in the cross section taken along the bending direction of the light guide plate  101  (e.g., the first direction X), the top surface of the light guide plate  101  may form a part of an arc, or a part of an elliptical arc. The radius of curvature R of the light guide plate  101  bent in the first direction X may be, but toned not be, within a range of approximately 1,500 mm to 5,000 mm. According to an exemplary embodiment of the present invention, the light guide plate  101  may be bent in both the first direction X and the second direction Y. 
     To facilitate the exit of the light traveling within the light guide plate  101  with total reflection, a negative or positive optical pattern may be formed on the convex back surface (shown as the lower surface in  FIG. 2 ) of the light guide plate  101 . Alternatively, a pattern for facilitating the exit of the light may be further disposed on the back surface of the light guide plate  101 . 
     The light source unit  200  may be disposed above the light-incidence face of the light guide plate  101 . According to an exemplary embodiment of the present disclosure where the light guide plate  101  is at least partially bent in the first direction X, the light source unit  200  may be disposed on a side of the light guide plate  101  in the second direction Y perpendicular to the first direction X, and the back light unit  21  may be an edge-lit backlight unit. For example, one of the side surfaces of the light guide plate  101  in the second direction Y may be the light incidence-face. The side surface of the light guide plate  101  on one side in the second direction Y and the side surface on the other side in the second direction Y may be substantially parallel. For example, the side surface of the light guide plate  101  on one side in the first direction X and the side surface of the light guide plate  101  on the other side in the first direction X might not be parallel to each other. 
     The light source unit  200  may include light sources  210  that emit light, and a light source circuit board  230 . 
     The light sources  210  may be light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), laser diodes (LDs), or the like. For example, each of the light sources  210  may include a light-emitting diode chip configured to generate and emit light. The light source  210  may emit blue light having a peak wavelength in the range of approximately 430 nm to 480 nm or may emit light in the ultraviolet wavelength band. The light sources  210  may be disposed on the mounting surface of the light source circuit board  230  and may be spaced apart from one another along the first direction X. 
     The light source circuit board  230  may supply various signals and power for driving the light sources  210  and may further provide a space for mounting the light sources  210 . For example, the light source circuit board  230  may be a printed circuit board (PCB). The light sources  210  may be mounted on one of the side surfaces of the light source circuit board  230 . The side surface of the light source circuit board  230  on which the light sources  210  are mounted defines the mounting surface. The mounting surface of the light source circuit board  230  may face the light incidence-face of the light guide plate  101 . 
     The light source circuit board  230  may be extended generally in the first direction X and may have a shape conforming to the light-incidence face of the light guide plate  101 . For example, the light source circuit board  230  may be at least partially bent in the first direction X. For example, the top surface of the light source circuit board  230  may be at least partially bent in the first direction X, so that the top surface of the light source circuit board  230  may form a concave surface. 
     The wavelength conversion layer  301  may be disposed on the light guide plate  101 . According to an exemplary embodiment of the present disclosure, the color conversion layer  301  may include a base resin  301   a,  and wavelength shifters  301   b  and  301   c  dispersed or dissolved within the base resin  301   a.  The color conversion layer  301  may further include scattering particles (scatterers)  301   d  dispersed within the base resin  301   a.  The wavelength conversion layer  301  may have a shape conforming to the light guide plate  101 . For example, the wavelength conversion layer  301  may be at least partially bent in the first direction X. 
     The wavelength conversion layer  301  may convert the color of incident light so that the color of the transmitted light is at least partially different from that of the incident light. For example, the light, after passing through the wavelength conversion layer  301 , may be converted into light of a certain wavelength band, such that the color of the light provided from the backlight unit  21  toward the display panel  10  can be controlled. 
     The base resin  301   a  may form the shape of the wavelength conversion layer  301 . In addition, the base resin  301   a  may work as a dispersion base for the wavelength shifters  301   b  and  301   c  and the scatterers  301   d.  The base resin  301   a  may include various materials that may have high light transmittance and exhibits excellent dispersion characteristics for the wavelength shifters  301   b  and  301   c  and the scatters  301   d.  For example, the base resin  301   a  may be made of an organic material such as an epoxy resin, an acrylic resin, a cardo resin, and/or an imide resin. 
     The wavelength shifters  301   b  and  301   c  may convert or shift the peak wavelength of the incident light to another peak wavelength. The wavelength shifters  301   b  and  301   c  may have a particulate form (e.g. they may each be comprised substantially of individual particles). Examples of the wavelength shifters  301   b  and  301   c  may include quantum dots, quantum rods, and/or phosphors. For example, a quantum dot is a structure that can emit light of a particular color as an electron transition from conduction band to valence band. The quantum dot material may have a core-shell structure. The core may be a semiconductor nanocrystalline material. Examples of the core of the quantum dots may include, silicon (Si) nanocrystals, II-VI group compound nanocrystals, and III-V group compound nanocrystals, etc. but other materials may also be used. For example, the wavelength shifters  301   b  and  301   c  may each include a core made of cadmium selenide (CdSe), cadmium telluride (CdTe), cadmium sulfide (CdS) or indium phosphide (InP), and an outer shell made of zinc sulfide (ZnS). 
     In an exemplary embodiment of the present disclosure, the wavelength shifters  301   b  and  301   c  may include a first wavelength shifter  301   b  that emits red light having a single peak wavelength in a range of approximately 600 nm to 650 nm, and a second wavelength shifter  301   c  that emits green light having a single peak wavelength in a range of approximately 510 nm to 570 nm. The exiting light converted by the first wavelength shifter  301   b  and the second wavelength shifter  301   c  may have a narrow wavelength band around the peak wavelength, so that color purity and clarity can be increased. In some exemplary embodiments of the present disclosure, the wavelength shifters  301   b  and  301   c  may include only the first wavelength shifter  301   b  and the second wavelength shifter  301   c.    
     According to an exemplary embodiment of the present disclosure in which the light sources  210  provide light in the blue wavelength band, the blue light guided through the light guide plate  101  may be incident on the wavelength conversion layer  301  through the light-exiting face (for example, the upper face) of the light guide plate  101 . At least some of the blue light incident on the wavelength conversion layer  301  may be converted into red light by the first wavelength shifter  301   b,  at least some of the blue light may be converted into green light by the second wavelength shifter  301   c,  and at least some of the blue light may transmit through the base resin  301   a  and remain blue. In this manner, the blue light provided from the light sources  210  may transmit through the wavelength conversion layer  301  and then may be converted into white light that comprises light of the red wavelength band, the green wavelength band and the blue wavelength band. After having passed through the wavelength conversion layer  301 , the white light may be provided toward the display panel  10 . 
     The scatterers  301   d  may have a refractive index different from that of the base resin  301   a  and may form an optical interface with the base resin  301   a.  For example, the scatterers  301   d  may include light scattering particles. The material of the scatters  301   d  is not particularly limited as long as they can scatter at least a part of the transmitted light to modulate the light path. For example, the scatterers  301   d  may be metal oxide particles or organic particles. Examples of suitable metal oxides may include titanium oxide (TiO 2 ), zirconium oxide (ZrO 2 ), aluminum oxide (Al 2 O 3 ), indium oxide (In 2 O 3 ), zinc oxide (ZnO), tin oxide (SnO 2 ) and the like. The scatters  301   d  can scatter light in various directions regardless of the incidence angle without substantially changing the wavelength of the light passing through the wavelength conversion layer  301 . By doing so, the length of the path in which the light passes through the wavelength conversion layer  301  can be increased, and the color conversion efficiency by the wavelength shifters  301   b  and  301   c  can be increased. 
     The wavelength conversion layer  301  may have an emboss pattern  301   p  such as a repeating set of raised mounds and/or recessed depressions. The wavelength conversion layer  301  and the emboss pattern  301   p  will be described in detail below. 
     According to some exemplary embodiments of the present disclosure, the backlight unit  21  may further include a low-refractive layer  400  and a capping layer  501 . 
     The low-refractive layer  400  may be disposed between the light guide plate  101  and the wavelength conversion layer  301 . For example, the low-refractive layer  400  may be in contact with the light guide plate  101  and the wavelength conversion layer  301 . The top surface of the low-refractive layer  400  in contact with the wavelength conversion layer  301  and the bottom surface of the wavelength conversion layer  301  in contact with the low-refractive index layer  400  may be substantially flat. The thickness of the low-refractive layer  400  can be generally uniform. The thickness of the low-refractive layer  400  may be, but is not limited to, approximately 1.0 μm or less, approximately 0.5 μm or less, or approximately 0.1 μm or less. 
     The low-refractive layer  400  may have a refractive index smaller than that of the base resin  301   a  of the light guide plate  101  and that of the wavelength conversion layer  301 . For example, the refractive index of the low-refractive layer  400  may be approximately 1.0 to 1.4, or approximately 1.2 to 1.3. The difference between the refractive index of the light guide plate  101  and the refractive index of the low-refractive index layer  400  may be approximately 0.2 or more. By disposing the low-refractive layer  400  having a relatively low refractive index directly on the light guide plate  101 , it is possible to facilitate the total reflection between the light guide plate  101  and the low-refractive layer  400 , and it is possible to increase the guiding efficiency of the light traveling in the light guide plate  101 . 
     The material of the low-refractive layer  400  may have a refractive index lower than that of the light guide plate  101  and that of the wavelength conversion layer  301 . For example, the low-refractive layer  400  may include an inorganic layer including an inorganic material. Examples of suitable inorganic materials include silicon nitride, silicon oxide, silicon oxynitride and the like. 
     The capping layer  501  may be disposed on the wavelength conversion layer  301 . The capping layer  501  can block impurities such as moisture or air from permeating into the wavelength conversion layer  301 , which would otherwise damage the wavelength shifters  301   b  and  301   c.  The capping layer  501  may be an inorganic layer including silicon nitride, silicon oxide, or silicon oxynitride. 
     The capping layer  501  may be disposed directly on the wavelength conversion layer  301 . The capping layer  501  may at least partially cover the side surfaces of the wavelength conversion layer  301  and may be partially in contact with the low-refractive layer  400  to encapsulate the wavelength conversion layer  301 . When the top surface of the wavelength conversion layer  301  has the emboss pattern  301   p,  the capping layer  501  may have a shape conforming to the emboss pattern  301   p.  The thickness of the capping layer  501  may be generally uniform. The thickness of the capping layer  501  may be within the range of approximately 1.0 μm or less, approximately 0.5 μm or less, or approximately 0.1 μm or less. However, the capping layer  501  may alternatively have a thickness greater than 1.0 μm. 
     Hereinafter, referring to  FIGS. 3 to 6 , the wavelength conversion layer  301  according to an exemplary embodiment of the present disclosure will be described in detail. 
       FIG. 3  is an enlarged, perspective view of the backlight unit of  FIG. 1 .  FIG. 4  is a cross-sectional view taken along line IV-IV′ of  FIG. 3 , showing a first peak portion  311 , a second peak portion  321  and a third peak portion  331  of  FIG. 3 .  FIG. 5  is a cross-sectional view taken along line V-V′ of  FIG. 3 , showing the second peak portion  321  and the third peak portion  331  of  FIG. 3 . Specifically,  FIG. 5  is a cross-sectional view of an example of the emboss pattern  301   p  cut along the second direction Y.  FIG. 6  is a cross-sectional view taken along line VI-VI′ of  FIG. 3 , showing the first peak portion  311  and the fourth peak portion  341  of  FIG. 3 . Specifically,  FIG. 6  is a cross-sectional view of the example of the emboss pattern  301   p  cut along the first direction X. 
     Referring to  FIGS. 1 to 6 , the top surface of the wavelength conversion layer  301  may have the emboss pattern  301   p.  For example, the top surface of the wavelength conversion layer  301  may have the emboss pattern  301   p  with a plurality of peak portions  311 ,  321 ,  331  and  341  and a plurality of valley portions  351  defined therein. As used herein, a “peak portion” refers to the height part around a given area, and a “valley portion” refers to the lowest part around a given area. For example, each of the plurality of peak portions  311 ,  321 ,  331  and  341  may be a part where the thickness of the wavelength conversion layer  301  in the third direction Z is greatest around them (e.g. a local maximum). In addition, each of the plurality of valley portions  351  may be a part where the thickness of the wavelength conversion layer  301  in the third direction Z is smallest around them (e.g. a local minimum). 
     The peak portions  311 ,  321 ,  331  and  341  may each have a convex rounded surface. The optical interface formed by the top surface having the emboss pattern  301   p  of the wavelength conversion layer  301  can be reduced as each of the peak portions of the wavelength conversion layer  301  has a rounded surface. Accordingly, it is possible to suppress the optical path modulation characteristics, for example, condensing or dispersion characteristics, which are generated in the vicinity of each of the peak portions of the wavelength conversion layer  301 . Furthermore, by using the emboss pattern  301   p  of the wavelength conversion layer  301  having rounded surfaces, it is possible to increase resistance to compressive stress or tensile stress generated by bending. 
     Each of the plurality of valley portions  351  may have a rounded concave surface. The optical interface formed by the top surface having the emboss pattern  301   p  of the wavelength conversion layer  301  can be reduced as each of the valley portions of the wavelength conversion layer  301  has a rounded surface. Accordingly, it is possible to suppress the optical path modulation characteristics, which are generated in the vicinity of each of the valley portions of the wavelength conversion layer  301 . Furthermore, by using the emboss pattern  301   p  of the wavelength conversion layer  301  having rounded surfaces, it is possible to increase resistance to compressive stress or tensile stress generated by bending. 
     According to an exemplary embodiment of the present disclosure, the plurality of peak portions  311 ,  321 ,  331  and  341  may be arranged in a matrix and spaced apart from one another in both a first oblique direction OD 1  and a second oblique direction OD 2 . In addition, the plurality of valley portions  351  may be arranged in a matrix and spaced apart from one another in both the first oblique direction OD 1  and the second oblique direction OD 2 . 
     For example, the plurality of peak portions  311 ,  321 ,  331  and  341  may include a first peak portion  311  and a second peak portion  321  adjacent to the first peak portion  311  in the first oblique direction OD 1 . The height of the first peak portion  311  may be substantially equal to the height of the second peak portion  321 . The plurality of peak portions  311 ,  321 ,  331  and  341  may further include a third peak portion  331  adjacent to the first peak portion  311  in the second oblique direction OD 2 . The first oblique direction OD 1  may be perpendicular to the second oblique direction OD 2 , however, the first and second oblique direction OD 1  and OD 2  may alternatively meet at other angles. 
     A valley portion  351  may be disposed between the second peak portion  321  and the third peak portion  331 . For example, in a cross section of the emboss pattern  301   p  cut along the second direction Y including the second peak portion  321  and the third peak portion  331 , the valley portion  351  may be disposed between the second peak portion  321  and the third peak portion  331 . 
     In some exemplary embodiments of the present disclosure, the horizontal spacing distance d 1  between the second peak portion  321  and the third peak portion  331  may be equal to or greater than three times the vertical shortest distance d 2  between one of the peak portions (e.g., the second peak portion  321 ) and the valley portion  351 . For example, a pitch of the emboss pattern  301   p  in the horizontal direction may be equal to or greater than three times the height difference of the emboss pattern  301   p  in the height direction. In addition, the vertical shortest distance d 2  between one of the peak portions (e.g., the second peak portion  321 ) and the valley portion  351  may range from approximately 1.0 μm to approximately 10.0 μm. 
     As the horizontal spacing distance d 1  between the second peak portion  321  and the third peak portion  331  of the emboss pattern  301   p  is equal to or greater than three times the vertical shortest distance d 2  between the second peak portion  321  and the valley portion  351 , the angle of the sloped surface of the emboss pattern  301   p  can be sufficiently low. Accordingly, the optical path modulation characteristic generated by the emboss pattern  301   p  of the wavelength conversion layer  301  can be suppressed. 
     In some exemplary embodiments of the present disclosure, the plurality of peak portions  311 ,  321 ,  331  and  341  may further include a fourth peak portion  341  that is adjacent to the second peak portion  321  in the second oblique direction OD 2  and adjacent to the third peak portion  331  in the first oblique direction OD 1 . In addition, a valley portion  351  may be disposed between the first peak portion  311  and the fourth peak portion  341 . For example, in a cross section of the emboss pattern  301   p  cut along the second direction Y including the first peak portion  311  and the fourth peak portion  341 , the valley portion  351  may be disposed between the first peak portion  311  and the fourth peak portion  341 . The valley portion  351  disposed between the first peak portion  311  and the fourth peak portion  341  may be the same point as the valley portion  351  disposed between the second peak portion  321  and the third peak portion  331 . In some exemplary embodiments of the present disclosure, the horizontal spacing distance between the first peak portion  311  and the fourth peak portion  341  may be substantially equal to the horizontal spacing distance d 1  between the second peak portion  321  and the third peak portion  331 . For example, when viewed from the top, the first peak portion  311 , the second peak portion  321 , the third peak portion  331  and the fourth peak portion  341  may be disposed at the corners of a quadrangle, respectively, and the valley portion  351  may be disposed around the center of the quadrangle. 
     For example, a compressive stress or a tensile stress may be applied to the wavelength conversion layer  301  and/or the capping layer  501  if the display device  1  is twisted due to an external impact, when a curved display device is produced, or if there is a difference in thermal compression or thermal expansion characteristics between the low-refractive layer  400  and the wavelength conversion layer  301 . 
     In light of the above, according to exemplary embodiments of the present disclosure, the wavelength conversion layer  301  has the emboss pattern  301   p,  so that it can increase the stress resistance of the wavelength conversion layer  301  and/or the capping layer  501 . For example, when compressive stress is applied to the top surface of the wavelength conversion layer  301  and the capping layer  501 , the top surface of the wavelength conversion layer  301  can have a space for compression by virtue of the emboss pattern  301   p,  and thus it is possible to prevent the capping layer  501  from being separated from the wavelength conversion layer  501  or to suppress the occurrence of cracks in the capping layer  501 . In addition, when compressive stress is applied to the top surface of the wavelength conversion layer  301  and the capping layer  501 , the top surface of the wavelength conversion layer  301  can have a tensile margin by virtue of the emboss pattern  301   p,  and thus it is possible to suppress the occurrence of cracks in the wavelength conversion layer  301  and the capping layer  501 . 
     In addition, as the emboss pattern  301   p  includes the plurality of peak portions  311 ,  321 ,  331  and  341  and the plurality of valley portions  351  spaced apart from one another in the first oblique direction OD 1  and the second oblique direction OD 2  to form a curved surface, the resistance to the bending stress in the bending direction (e.g., the first direction X) of the light guide plate  101  may be increased and also the resistance to the compressive stress or tensile stress in the second direction Y may be increased. 
     In addition, one of the valley portions  351  is formed between the first peak portion  311  and the second peak portion  321  adjacent to each other in the bending direction (e.g., the first direction X) of the light guide plate  101 , so that the highest height difference of the emboss pattern  301   p  may be formed in the bending direction of the light guide plate  101 . By doing so, it is possible to increase the resistance of the wavelength conversion layer  301  to the bending stress in the bending direction. For example, by arranging the plurality of peak portions  311 ,  321 ,  331  and  341  and the valley portions  351  such that they intersect with the bending direction of the light guide plate  101 , a structure more robust to bending can be implemented. 
     Hereinafter, other exemplary embodiments of the present disclosure will be described. To the extent that a description of certain elements is omitted, it may be assumed that these elements are at least similar to corresponding elements that have already been described. 
       FIG. 7  is a cross-sectional view of a display device according to an exemplary embodiment of the present disclosure. 
     Referring to  FIG. 7 , the display device  2 , according to an exemplary embodiment of the present disclosure, is different from the display device  1  shown in  FIG. 2  in that the display device  2  includes a display panel  10  and a backlight unit  22 , and a light guide plate  102  of the backlight unit  22  has partly different radii of curvature. 
     The light guide plate  102  may be at least partly bent in the first direction X so that the top surface of the light guide plate  102  may form a concave surface. According to an exemplary embodiment of the present invention, in a cross section cut along the bending direction of the light guide plate  102  (e.g., the first direction X), a top surface of a first area A 1  disposed at the center of the light guide plate  102  may be bent at a first radius of curvature R 1  to form a part of an arc, or a part of an elliptical arc. In addition, a top surface of a second area A 2  disposed on a side of the first area A 1  may have a second radius of curvature that is larger than the first radius of curvature R 1  (e.g., indefinite radius of curvature). For example, according to an exemplary embodiment of the present disclosure, the center portion of the display device  2  including the light guide plate  102  may be bent at a predetermined curvature while the edges thereof may be flat with substantially no curvature. As an alternative to the arrangement shown in  FIG. 7 , the center portion of the display device  2  including the light guide plate  102  may be bent at a predetermined radius of curvature while the edges thereof may be bent at a smaller extent of curvature than that of the center portion (e.g., bent with a larger radius of curvature than that of the center portion). 
     The wavelength conversion layer  302  may be disposed on the light guide plate  102 . The top surface of the wavelength conversion layer  302  may have an emboss pattern  302   p.  For example, the top surface of the wavelength conversion layer  302  may have the emboss pattern  302   p  with a plurality of peak portions  352  and  362 , and a plurality of valley portions defined therein. 
     According to an exemplary embodiment of the present disclosure, the size of the emboss pattern  302   p  in the first area A 1  of the light guide plate  102  may be larger than the size of the emboss pattern  302   p  in the second area A 2  of the light guide plate  102 . For example, the emboss pattern  302   p  may include a fifth peak portion  352  disposed in the first area A 1  and a sixth peak portion  362  disposed in the second area A 2 . The thickness T 5  of the wavelength converting layer  302  at the fifth peak portion  352  may be larger than the thickness T 6  of the wavelength converting layer  302  at the sixth peak portion  362 . 
     As described above, when the display device  2  is implemented as a curved display device, for example, compressive stress or tensile stress may be applied to the wavelength conversion layer  302  and the capping layer  502 . In this case, the fifth peak portion  352  in the first area A 1  bent at a relatively large curvature may be formed to have a sufficient size in the height direction, to thereby increase the resistance to compressive stress or tensile stress. On the other hand, the sixth peak portion  362  in the second area A 2 , which is substantially flat or bent with a smaller extent of curvature (larger radius of curvature), is formed to have a relatively small size in the height direction, so that the optical path modulation characteristic by the wavelength conversion layer  302  can be further suppressed. 
       FIG. 8  is an exploded perspective view of a display device according to an exemplary embodiment of the present disclosure.  FIG. 9  is a cross-sectional view taken along line IX-IX′ of  FIG. 8 .  FIG. 10  is an enlarged, perspective view of the backlight unit of  FIG. 8 .  FIG. 11  is a cross-sectional view taken along line XI-XI′ of  FIG. 10 , showing the first peak portion  313  and the second peak portion  323  of  FIG. 10 . Specifically,  FIG. 11  is a cross-sectional view of the emboss pattern  303   p  cut along the first direction X.  FIG. 12  is a cross-sectional view taken along line XII-XII′ of  FIG. 10 , showing the first peak portion  313  and the third peak portion  333  of  FIG. 10 . Specifically,  FIG. 12  is a cross-sectional view of the emboss pattern  303   p  cut along the second direction Y.  FIG. 13  is a cross-sectional view taken along line XIII-XIII′ of  FIG. 10 , showing the second peak portion  323  and the third peak portion  333  of  FIG. 10 . 
     Referring to  FIGS. 8 to 13 , a display device  3 , according to an exemplary embodiment of the present disclosure, is different from the display device  1  shown in  FIG. 2  in that the display device  3  includes a display panel  10  and a backlight unit  23 , and an emboss pattern  303   p  of a wavelength conversion layer  303  of the backlight unit  23  is a linear pattern. 
     The top surface of the wavelength conversion layer  303  has an emboss pattern  303   p  in which a plurality of peak portions  313 ,  323  and  333  and a plurality of valley portions  353  and  363  are defined. The emboss pattern  303   p  may include a linear first emboss pattern  313   p  forming the first peak portion  313 , a linear second emboss pattern  323   p  forming the second peak portion  323 , and a linear third emboss pattern  333   p  forming the third peak portion  333 . 
     The first emboss pattern  313   p,  the second emboss pattern  323   p  and the third emboss pattern  333   p  may each be extended generally in the second direction Y and may be spaced apart from one another in the first direction X. The first emboss pattern  313   p,  the second emboss pattern  323   p  and the third emboss pattern  333   p  may form the first peak portion  313 , the second peak portion  323  and the third peak portion  333   p,  respectively, which protrude most in the height direction. Each of the plurality of peak portions  313 ,  323  and  333  may have a convex rounded surface. 
     Each of the first emboss pattern  313   p,  the second emboss pattern  323   p  and the third emboss pattern  333   p  may have a curved shape in the form of a wave propagating in the first direction X. The first emboss pattern  313   p  and the second emboss pattern  323   p  may partially overlap with each other in the second direction Y, and the first emboss pattern  313   p  and the third emboss pattern  333   p  may partially overlap with each other in the second direction Y. 
     As the first emboss pattern  313   p,  the second emboss pattern  323   p  and the third emboss pattern  333   p,  each of which has a curve shape in the form of a wave propagating in the bending direction (for example, the first direction X) of the light guide plate  103 , partially overlap with one another in the extending direction (for example, the second direction Y), the resistance to compressive stress or tensile stress caused by bending may be improved. 
     A plurality of valley portions  353  and  363  may be formed between the first emboss pattern  313   p  and the second emboss pattern  323   p  and between the first emboss pattern  313   p  and the third emboss pattern  333   p.  Each of the plurality of valley portions  353  and  363  may have a rounded concave surface. 
     For example, in a cross-sectional view showing the second peak portion  323  of the second emboss pattern  323   p  and the third peak portion  333  of the third emboss pattern  333   p,  the first emboss pattern  313   p,  the first valley portion  353  and the second valley portion  363  may be disposed between the second peak portion  323  of the second emboss pattern  323   p  and the third peak portion  333  of the third emboss pattern  333   p.  The first valley portion  353  may be disposed between the first emboss pattern  313   p  and the second peak portion  323 , and the second valley portion  363  may be disposed between the first emboss pattern  313   p  and the third peak portion  333 . 
     Exemplary embodiments described herein are illustrative, and many variations can be introduced without departing from the spirit of the disclosure or from the scope of the appended claims. For example, elements and/or features of different exemplary embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.