Patent Publication Number: US-10770685-B2

Title: Display device and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 15/455,037, filed on Mar. 9, 2017, and claims priority from and the benefit of Korean Patent Application No. 10-2016-0114434, filed on Sep. 6, 2016, which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND 
     Field 
     Exemplary embodiments relate to a display device and a manufacturing method thereof. 
     Discussion of the Background 
     There are various display devices such as a liquid crystal display (LCD), an organic light-emitting display, and the like. 
     The organic light-emitting display, unlike the LCD, does not need a backlight unit, and thus, the thickness of the organic light-emitting display can be minimized. Accordingly, studies have been conducted on a flexible, stretchable, foldable, bendable, or rollable organic light-emitting display. 
     However, as the thickness of a display device decreases, the impact resistance of the display device may weaken, and for this reason, a protective film may be formed below a display element unit of the display device. However, UV irradiation or thermal treatment that may be performed during the formation of the protective film may cause damage to elements of the display device. 
     If the amount of UV light or heat applied to form the protective film is minimized to prevent damage to the elements of the display device, the thickness of the protective film may not be able to be properly increased. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the inventive concept, and, therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     Exemplary embodiments provide a display device including a protective layer, which is for protecting the display device against physical impact and preventing moisture or foreign materials from infiltrating into the display device, and a manufacturing method of the display device. 
     Exemplary embodiments also provide a display device including a protective layer, which is formed by depositing an organic material at low temperature without irradiation so as to prevent elements in the display device from being damaged by light or heat, and a manufacturing method of the display device. 
     Additional aspects will be set forth in the detailed description which follows, and, in part, will be apparent from the disclosure, or may be learned by practice of the inventive concept. 
     Exemplary embodiments provide a display device including: a flexible substrate; a display element unit disposed on a first surface of the flexible substrate and including a thin-film transistor (TFT) and an organic light-emitting element coupled to the TFT; and a protective layer including an organic material and disposed directly on a second surface of the flexible substrate, the second surface being opposite to the first surface. 
     Exemplary embodiments provide a display device including: a display element unit including a TFT and an organic light-emitting element coupled to the TFT; and a protective layer disposed on a first surface of the display element unit and comprising at least one material of Formula 1: 
     
       
         
         
             
             
         
       
     
     where n is a natural number equal to or greater than 2, m is an integer number between 0 and 4, X is hydrogen or halogen, and R is halogen or alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, cycloalkynyl, aryl, or a halide thereof having 1 to 8 carbon atoms. 
     Exemplary embodiments provides a manufacturing method of a display device, comprising: forming a flexible substrate on a sacrificial substrate; forming a display element unit, including a TFT and an organic light-emitting element, on a first surface of the flexible substrate; separating the sacrificial substrate from the flexible substrate; and forming a protective layer by depositing an organic material on a second surface of the flexible substrate, the second surface being opposite to the first surface. 
     According to exemplary embodiments, a display device can be protected against physical impact, and can be prevented from being damaged by moisture or foreign materials infiltrating thereto. 
     In addition, since the protective layer may be formed by depositing an organic material at low temperature without irradiation, elements in the display device can be prevented from being damaged by light or heat. 
     The foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the inventive concept, and, together with the description, serve to explain principles of the inventive concept. 
         FIG. 1  is a cross-sectional view illustrating a stack structure of a display device according to an exemplary embodiment. 
         FIG. 2  is an enlarged cross-sectional view of a part A of the display device of  FIG. 1 . 
         FIG. 3  is a cross-sectional view illustrating an arbitrary multilayer stack bent by external stress. 
         FIG. 4  is a cross-sectional view illustrating neutral planes of display devices. 
         FIG. 5  is a cross-sectional view illustrating a stack structure of a display device according to another exemplary embodiment. 
         FIG. 6 ,  FIG. 7 , and  FIG. 8  are cross-sectional views illustrating stack structures of display devices according to other exemplary embodiments. 
         FIG. 9 ,  FIG. 10 , and  FIG. 11  are cross-sectional views illustrating stack structures of display devices according to still other exemplary embodiments. 
         FIG. 12 ,  FIG. 13 ,  FIG. 14 ,  FIG. 15 , and  FIG. 16  cross-sectional views illustrating a manufacturing method of a display device, according to an exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of various exemplary embodiments. It is apparent, however, that various exemplary embodiments may be practiced without these specific details or with one or more equivalent arrangements. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring various exemplary embodiments. 
     In the accompanying figures, the size and relative sizes of layers, films, panels, regions, etc., may be exaggerated for clarity and descriptive purposes. Also, like reference numerals denote like elements. 
     When an element or layer is referred to as being “on,” “connected to,” or “coupled to” another element or layer, it may be directly on, connected to, or coupled to the other element or layer or intervening elements or layers may be present. When, however, an element or layer is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. For the purposes of this disclosure, “at least one of X, Y, and Z” and “at least one selected from the group consisting of X, Y, and Z” may be construed as X only, Y only, Z only, or any combination of two or more of X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and ZZ. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are used to distinguish one element, component, region, layer, and/or section from another element, component, region, layer, and/or section. Thus, a first element, component, region, layer, and/or section discussed below could be termed a second element, component, region, layer, and/or section without departing from the teachings of the present disclosure. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for descriptive purposes, and, thereby, to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the drawings. Spatially relative terms are intended to encompass different orientations of an apparatus in use, operation, and/or manufacture in addition to the orientation depicted in the drawings. For example, if the apparatus in the drawings is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. Furthermore, the apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations), and, as such, the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, the singular forms, “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Various exemplary embodiments are described herein with reference to sectional illustrations that are schematic illustrations of idealized exemplary embodiments and/or intermediate structures. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments disclosed herein should not be construed as limited to the particular illustrated shapes of regions, but are to include deviations in shapes that result from, for instance, manufacturing. Accordingly, the regions illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to be limiting. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure is a part. Terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense, unless expressly so defined herein. 
     Display devices according to exemplary embodiments may be self-emissive display devices such as organic light-emitting displays or plasma display devices, or may be light-receiving display devices such as liquid crystal display (LCDs) or electrophoretic displays (EPDs). Display devices according to exemplary embodiments will hereinafter be described with respect to a flexible organic light-emitting display device as an example, but the exemplary embodiments are not limited thereto. 
       FIG. 1  is a cross-sectional view illustrating a stack structure of a display device  1  according to an exemplary embodiment. 
     Referring to  FIG. 1 , a display device  1  includes a protective layer  10 , a flexible substrate  20 , which is disposed on the protective layer  10 , and a display element unit  100 , which is disposed on the flexible substrate  20 . 
     The display device  1  may include a plurality of pixels, which are arranged in any suitable formation, such as a matrix formation over a plane. The display element unit  100  of the display device  1  may include a driving element, which drives each of the pixels. The driving elements include signal transmission elements such as a gate line, a data line, and a thin-film transistor (TFT) and a light-emitting element such as an organic light-emitting layer. 
     At least one of a first surface (e.g. a top surface) and a second surface (e.g. a bottom surface) of the display element unit  100  may be display surfaces of the display device  1 . 
     The flexible substrate  20 , which is disposed on the second surface of the display element unit  100 , not only supports the display element unit  100 , but also provides flexibility to the display device  1  such that the display device  1  may be bendable, foldable, or rollable. The flexible substrate  20  may include a first surface contacting the display element unit  100  and a second surface opposite to the first surface. 
     The protective layer  10 , which is disposed on the second surface of the flexible substrate  20 , not only prevents moisture or foreign materials from infiltrating into the flexible substrate  20  or the display element unit  100 , but also protects the display element unit  100  against physical impact. The protective layer  10  may be disposed directly on the second surface of the flexible substrate  20  without any adhesive layer interposed therebetween. 
     In exemplary embodiments, the display device  1  may further include a protective film  220 , which is disposed on the first surface of the display element unit  100 . The protective film  220 , like the protective layer  10 , may protect the display element unit  100  against moisture or foreign materials. The protective film  220 , unlike the protective layer  10 , may be disposed on the display element unit  100  with an adhesive layer  210  interposed therebetween. 
     A cross-sectional structure of the display device  1  will hereinafter be described in further detail with reference to  FIG. 2 . 
       FIG. 2  is an enlarged cross-sectional view of a part A of the display device  1  of  FIG. 1 . 
     Referring to  FIG. 2 , the protective layer  10  is disposed at a lowermost part of the display device  1 . The protective layer  10  protects the display device  1  against foreign materials or physical impact and may lower a neutral plane of the display device  1 . 
     The combined thickness of the protective layer  10  and the flexible substrate  20  may be set to be 20 μm or thicker. In a case where the combined thickness of the protective layer  10  and the flexible substrate  20  is 20 μm or thicker, the neutral plane may be lowered such that most tensile force may act on the protective layer  10  and the flexible substrate  20 , and thus, the display element unit  100  may be protected against the tensile force. For example, the neutral plane may be extended along at least one of the protective layer  10  and the flexible substrate  20 . The lowering of the neutral plane may depend on the combined thickness of the protective layer  10  and the flexible substrate  20 . Thus, even if the thickness of the flexible substrate  20  is less than 20 μm, the insufficient thickness of the flexible substrate  20  may be compensated for by the protective layer  10 , and thus, the neutral plane may be effectively lowered. As a result, flexible substrates of various thicknesses may be selected for the display device  1 . The lowering of the neural plane will be described later in further detail. 
     The protective layer  10  may include an organic material. The protective layer  10  may be formed by depositing the organic material. That is, the protective layer  10  may include an organic deposition layer. Since the protective layer  10  may be formed by depositing the organic material on the second surface of the flexible substrate  20 , the protective layer  10  may be disposed to directly contact the second surface of the flexible substrate  20  without any adhesive layer interposed therebetween. By forming the protective layer  10  directly on the second surface of the flexible substrate  20 , the thickness of the display device  1  may be reduced, and the neutral plane may be effectively lowered. 
     The organic material of the protective layer  10  may be a material that can be deposited at a low temperature of, for example, 90° C. or lower. 
     For example, the organic material of the protective layer  10  may include at least one of parylene (i.e., a poly(p-xylylene) polymer) and a derivative thereof, which may be represented by Formula 1: 
     
       
         
         
             
             
         
       
     
     where n is a natural number equal to or greater than 2, m is an integer number between 0 and 4, X is hydrogen or halogen, and R is halogen or alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, cycloalkynyl, aryl, or a halide thereof having 1 to 8 carbon atoms. 
     The material represented by Formula 1 may include at least one of materials represented by Formulas 2 through 5: 
     
       
         
         
             
             
         
       
     
     where n is a natural number equal to or greater than 2. 
     The protective layer  10  may have a Young&#39;s modulus of 20 GPa or lower. In this case, the protective layer  10  may protect the display device  1  without reducing the overall flexibility of the display device  1 . 
     The Young&#39;s modulus of the protective layer  10  may be lower than, or similar to, the Young&#39;s modulus of the flexible substrate  20 . In this case, the protective layer  10  may protect the display device  1  without adversely affecting the flexibility or the curvature of the display device  1 . 
     The protective layer  10  may have a water vapor transmission rate (WVTR) of 10 −3  g·mm/m 2 /day or less. In this case, moisture in the outside air may be effectively prevented from infiltrating into the display element unit  100  of the display device  1 . Thus, the provision of any additional waterproof member in the display device  1  may be unnecessary. 
     The protective layer  10  may have a coefficient of thermal expansion (CTE) of 15×10 −6 /K or lower. In this case, the protective layer  10  may be prevented from being peeled off from the display device  1  by repeating thermal expansion and contraction. 
     The bottom of the protective layer  10  may be hard-coated by UV irradiation or thermal treatment. As mentioned above, if the protective layer  10  has a CTE of 15×10 −6 /K or lower, the protective layer  10  may be prevented from being peeled off from the display device  1  during a hard coating process using thermal treatment. 
     The flexible substrate  20  is disposed on the protective layer  10 . The flexible substrate  20  may be flexible enough to allow the display device  1  to maintain its display performance even in a bent state. The flexible substrate  20  may be formed to have a thin thickness and may include a material such as flexible glass having elasticity. 
     For example, the flexible substrate  20  may include polyimide (PI), but the exemplary embodiments are not limited thereto, the flexible substrate  20  may include flexible glass. 
     The display element unit  100  is disposed on the flexible substrate  20 . The display element unit  100  may include a buffer layer  110 , an active layer  121 , a gate insulating layer  140 , a gate electrode  151 , an interlayer dielectric layer  160 , a source electrode  172 , a drain electrode  173 , a passivation layer  180 , an organic light-emitting element E, and an encapsulation layer  194 . 
     The buffer layer  110  is disposed at a lowermost part of the display element unit  100 . The buffer layer  110  may be disposed on the flexible substrate  20 . The buffer layer  110  may include silicon nitride (SiN x ), silicon oxide (SiO x ), or silicon oxynitride (SiO x N y ) and may be formed as a single layer or a multilayer. The buffer layer  110  prevents the infiltration of impurities, moisture, or the outside air that may degrade semiconductor characteristics, and may planarize the surface of the flexible substrate  20 . 
     The active layer  121  is disposed on the buffer layer  110 . The active layer  121  may include a semiconductor and may be formed of polysilicon. 
     The active layer  121  may include a channel region  123  and source and drain regions  122  and  124 , which are disposed on both sides of the channel region  123 . The channel region  123  may be an intrinsic semiconductor such as polysilicon not doped with impurities, and the source and drain regions  122  and  124  may be impurity semiconductors such as polysilicon doped with conductive impurities. 
     The gate insulating layer  140  is disposed on the active layer  121 . The gate insulating layer  140  may include an insulating layer having silicon nitride, silicon oxide, or silicon oxynitride and may be formed as a single layer or a multilayer. 
     The gate electrode  151  is disposed on the gate insulating layer  140  and overlaps the channel region  123  of the active layer  121 . The gate electrode  151  may be connected to a gate line (not illustrated) and a gate pad (not illustrated). The gate electrode  151  may include aluminum (Al), molybdenum (Mo), copper (Cu), or an alloy thereof and may have a multilayer structure. 
     The interlayer dielectric layer  160  is disposed on the gate electrode  151 . The interlayer dielectric layer  160  may include an insulating layer having silicon nitride, silicon oxide, or silicon oxynitride and may be formed as a single layer or a multilayer. 
     The source and drain electrodes  172  and  173  are disposed on the interlayer dielectric layer  160 . The source electrode  172  may be connected to the source region  122  of the active layer  121  via a source hole  161  formed in the gate insulating layer  140  and the interlayer dielectric layer  160 , and the drain electrode  173  may be connected to the drain region  124  of the active layer  121  via a drain hole  162  formed in the gate insulating layer  140  and the interlayer dielectric layer  160 . The source electrode  172  may be connected to a data line (not illustrated) and a data pad (not illustrated). 
     Each of the source and drain electrodes  172  and  173  may include Al, Mo, chromium (Cr), tantalum (Ta), titanium (Ti), any other refractive metal, or an alloy thereof and may have a multilayer structure. 
     The active layer  121 , the gate electrode  151 , and the source and drain electrodes  172  and  173  of the display device  1  may form a TFT T. The gate electrode  151 , which is the control terminal of the TFT T, may be connected to the gate line, the source electrode  172 , which is the input terminal of the TFT T, may be connected to the data line, and the drain electrode  173 , which is the output terminal of the TFT T, may be electrically connected to an anode electrode  191  via a contact hole  181 . 
     The passivation layer  180  is disposed on the source and drain electrodes  172  and  173 . The passivation layer  180  may include silicon nitride, silicon oxide, silicon oxynitride, or an acrylic organic compound having a low dielectric constant, benzocyclobutane (BCB), or perfluorocyclobutane (PFCB). 
     The passivation layer  180  may protect the source and drain electrodes  172  and  173  and may also planarize the top surfaces of the source and drain electrodes  172  and  173 . The contact hole  181  may be formed to penetrate the passivation layer  180  and thus to expose the drain electrode  173  therethrough. 
     The organic light-emitting element E is disposed on the passivation layer  180 . The organic light-emitting element E includes the anode electrode  191 , a pixel-defining layer  190 , an organic light-emitting layer  192 , and a cathode electrode  193 . 
     The anode electrode  191  is disposed at a lowermost part of the organic light-emitting element E. The anode electrode  191  may be electrically connected to the drain electrode  173  via the contact hole  181 , which is formed in the passivation layer  180 , and may function as a pixel electrode of the organic light-emitting element E. 
     The anode electrode  191  may include a material layer having a high work function such as a layer of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), or indium oxide (In 2 O 3 ). The anode electrode  191  may consist of a stack of the aforementioned material layer and a reflective metal layer formed of lithium (Li), calcium (Ca), lithium fluoride (LiF)/Al, Al, silver (Ag), magnesium (Mg), or gold (Au). 
     The pixel-defining layer  190  is disposed on the anode electrode  191 . The pixel-defining layer  190  may include a polyacrylate resin or a PI resin. The pixel-defining layer  190  may define each pixel of the organic light-emitting element E and may include an opening  195 , which exposes the anode electrode  191 . 
     The organic light-emitting layer  192  is disposed on a part of the anode electrode  191  exposed through the opening  195 . The organic light-emitting layer  192  may be formed as a multilayer including at least one of a hole injection layer (HIL), a hole transport layer (HTL), an electron transport layer (ETL), and an electron injection layer (EIL). 
     The cathode electrode  193  is disposed on the pixel-defining layer  190  and the organic light-emitting layer  192 . The cathode electrode  193  may include Li, Ca, LiF/Ca, LiF/Al, Al, Mg, Ag, platinum (Pt), palladium (Pd), nickel (Ni), Au-neodymium (Nd), iridium (Ir), Cr, barium fluoride (BaF), Ba, or a compound or mixture thereof (e.g. a mixture of Ag and Mg). The cathode electrode  193  may function as a common electrode of the organic light-emitting element E. 
     The encapsulation layer  194  is disposed on the cathode electrode  193 . The encapsulation layer  194  may prevent moisture or air from infiltrating into, and oxidizing, the organic light-emitting element E and may planarize the top surface of the organic light-emitting element E. 
     In exemplary embodiments, the display element unit  100  may further include a touch screen (not illustrated), which is attached to, or embedded in, the display element unit  100 . 
     The adhesive layer  210  and the protective film  220  are disposed on the display element unit  100 . The adhesive layer  210  may be disposed directly on the encapsulation layer  194 , which is located at an uppermost part of the display element unit  100 . 
     The adhesive layer  210  attaches and fixes the protective film  220  onto the display element unit  100 . For example, the adhesive layer  210  may include a pressure sensitive adhesive (PSA), but the exemplary embodiments are not limited thereto. 
     The protective film  220 , like the protective layer  10 , protects the display device  1  against foreign materials and physical impact. 
     The protective film  220  may be elastic and flexible enough to allow the display device  1  to maintain its flexible characteristics. For example, the protective film  220  may be formed as a single layer or a multilayer including a material such as tempered glass, polyurethane (PU), PI, polyethylene terephthalate (PET), polycarbonate (PC), or polymethylmethacrylate (PMMA), and the protective film  220  may further include at least one of a hard coating layer, an anti-finger (AF) coating layer, an anti-reflection (AR) coating layer, an anti-glare (AG) coating layer, and a polarizer. However, the exemplary embodiments are not limited to this example. 
     The lowering of the neutral plane by the protective layer  10  will hereinafter be described with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a cross-sectional view illustrating an arbitrary multilayer stack bent by external stress, and  FIG. 4  is a cross-sectional view illustrating neutral planes of display devices  1 ′ and  1 . 
     Referring to  FIG. 3 , in a case in which a multilayer stack is bent in one direction by external force, a first layer TL, which is a part of the multilayer stack that stretches, receives tensile stress, and a second layer CL, which is a part of the multilayer stack that contracts, receives compressive stress, and a neutral plane NP where the tensile stress and the compressive stress offset each other and thus become zero may be formed between the first and second layers TL and CL. The location of the neutral plane NP may vary depending on the rigidities of the first and second layers TL and CL and the overall thickness of the multilayer stack. 
     A flexible display device is generally bent in the same manner as illustrated in  FIG. 3 , and elements for realizing an image viewing function are generally robust against compressive stress, but may be susceptible to tensile strength. 
     Referring to  FIG. 4 , by additionally providing a protective layer  10 , which has a given thickness (20 μm or thicker) and a given Young&#39;s modulus (e.g. 20 Gpa or lower), at the bottom of a display device  1 ′, the location of the neutral plane NP may be lowered close to the protective layer  10  and the flexible substrate  20 . In  FIG. 4 , the neutral plane NP of the display device  1 ′ extends along the display element unit  100 , but the neutral plane NP of the display device  1 , which includes the protective layer  10  disposed on the bottom of the flexible substrate  20 , extends along the flexible substrate  20 . As a result, most tensile stress acts only on the protective layer  10  and the flexible substrate  20 , which are sufficiently elastic and flexible, and thus, elements above the neutral plane NP may only receive compressive stress and may be protected against tensile stress. 
     Other exemplary embodiments will hereinafter be described. 
       FIG. 5  is a cross-sectional view illustrating a stack structure of a display device according to another exemplary embodiment. 
     A display device  1 _ 1  of  FIG. 5  is the same as the display device  1  of  FIGS. 1 and 2  except that a protective layer  10 _ 1  thereof includes layers having different densities, and thus will hereinafter be described, focusing mainly on the difference from the display device  1  of  FIGS. 1 and 2 . 
     Referring to  FIG. 5 , the protective layer  10 _ 1  may have a structure in which high-density layers  11  and low-density layers  12  having a lower density than the high-density layers  11  are alternately stacked. 
     Since the density of the protective layer  10 _ 1  may vary from one layer to another layer, the Young&#39;s modulus of the protective layer  10 _ 1  may also vary from one layer to another layer. For example, the high-density layers  11  may have a high Young&#39;s modulus, and the low-density layers  12  may have a lower Young&#39;s modulus than the high-density layers  11 . 
     More than one high-density layer  11  and more than one low-density layer  12  may be alternately stacked in the protective layer  10 _ 1 , and the lowermost and uppermost layers of the protective layer  10 _ 1  may both be high-density layers  11 , may be a high-density layer  11  and a low-density layer  12 , respectively, may be a low-density layer  12  and a high-density layer  11 , respectively, and may both be low-density layers  12 . 
     In the protective layer  10 _ 1 , a low-density layer  12  inserted between adjacent high-density layers  11  may be provided to increase the overall thickness of the protective layer  10 _ 1  and maintain the overall elasticity or flexibility of the protective layer  10 _ 1 , and may thus further lower the neutral plane NP. The high-density layers  11  may strengthen the functions of the protective layer  10 _ 1 , particularly, the function of preventing infiltration of moisture, air, or foreign materials. 
       FIGS. 6 through 8  are cross-sectional views illustrating stack structures of display devices according to other exemplary embodiments. 
     Display devices  1 _ 2 ,  1 _ 3 , and  1 _ 4  of  FIGS. 6, 7, and 8  are the same as the display device  1  of  FIGS. 1 and 2  except that the density of protective layers  10 _ 2 ,  10 _ 3 , and  10 _ 4  thereof gradually increase or decrease along a vertical direction, and thus will hereinafter be described, focusing mainly on the differences from the display device  1  of  FIGS. 1 and 2 . 
     Referring to  FIG. 6 , the density of the protective layer  10 _ 2  may gradually increase from the bottom to the top of the protective layer  10 _ 2 . By controlling the density of the protective layer  10 _ 2 , as illustrated in  FIG. 6 , the thickness of the protective layer  10 _ 2  may be increased while maintaining the overall elasticity or flexibility of the protective layer  10 _ 2 , and as a result, a neutral plane may be effectively lowered. 
     Since the density of the protective layer  10 _ 2  may vary from one part to another part of the protective layer  10 _ 2 , the Young&#39;s modulus of the protective layer  10 _ 2  may also vary from one part to another part of the protective layer  10 _ 2 . For example, the Young&#39;s modulus of the protective layer  10 _ 2  may gradually increase from the bottom to the top of the protective layer  10 _ 2 . 
     Referring to  FIG. 7 , the density of the protective layer  10 _ 3  may gradually increase from the top to the bottom of the protective layer  10 _ 3 . Referring to  FIG. 8 , the density of the protective layer  10 _ 4  may gradually increase from the top or bottom to the middle of the protective layer  10 _ 4 , or vice versa. As mentioned above, by controlling the density of the protective layer  10 _ 3  or  10 _ 4 , as illustrated in  FIG. 7 or 8 , a neutral plane may be effectively lowered. 
     As mentioned above, the Young&#39;s modulus of the protective layer  10 _ 3  or  10 _ 4  may also vary depending on the density of the protective layer  10 _ 3  or  10 _ 4 . 
       FIGS. 9 through 11  are cross-sectional views illustrating stack structures of display devices according to still other exemplary embodiments. 
     A display device  1 _ 5  of  FIG. 9  is the same as the display device  1  of  FIGS. 1 and 2  except that it does not include any flexible substrate  20 , and thus will hereinafter be described, focusing mainly on the difference with the display device  1  of  FIGS. 1 and 2 . 
     Referring to  FIG. 9 , in the display device  1 _ 5 , a protective layer  10  may be disposed directly on a second surface of a display element unit  100  without any flexible substrate  20  interposed between the protective layer  10  and the display element unit  100 . 
     Since the protective layer  10  may have a given Young&#39;s modulus, WVTR, and CTE, the protective layer  10  not only protects the display device  1 _ 5 , but also serves as a base substrate of a flexible display device, replacing the flexible substrate  20 . In this case, the protective layer  10  may have a thickness of 20 μm or thicker. 
     A display device  1 _ 6  of  FIG. 10  is the same as the display device  1 _ 5  of  FIG. 9  except that a protective layer  10 _ 1  thereof includes layers having different densities. 
     A display device  1 _ 7  of  FIG. 11  is the same as the display device  1 _ 6  of  FIG. 10  except that a protective film  21  is further provided on a second surface of a protective layer  10 _ 1  and is attached on the protective layer  10 _ 1  by an adhesive layer  2 . 
     The adhesive layer  2  and the protective film  21  at the second surface of the protective layer  10 _ 1  may include the same materials or the same layers as an adhesive layer  210  and a protective film  220  on a first surface of a display element unit  100 . 
       FIGS. 12 through 16  are cross-sectional views illustrating a manufacturing method of a display device, according to an exemplary embodiment. 
     Referring to  FIG. 12 , a flexible substrate  20  is formed on a sacrificial substrate  13 , which is formed of a material with high rigidity such as glass. The sacrificial substrate  13  may serve as a supporting substrate during the formation of a TFT or an organic light-emitting element and may be separated and removed after the formation of such element. To facilitate the separation of the sacrificial substrate  13 , a sacrificial layer (not illustrated) may be formed between the sacrificial substrate  13  and the flexible substrate  20 . 
     During the fabrication of a display device having no flexible substrate  20 , such as the display device  1 _ 5  of  FIG. 9 , the formation of the flexible substrate  20  may not be performed, in which case, a buffer layer  110  may be formed directly on the sacrificial substrate  13 . 
     Referring to  FIG. 13 , a display element unit  100  including a TFT T and an organic light-emitting element E is formed on a first surface of the flexible substrate  20 . The first surface of the flexible substrate  20  is opposite to a second surface of the flexible substrate  20  which is closer to the sacrificial substrate  13 . 
     Referring to  FIG. 14 , the sacrificial substrate  13  is separated from the flexible substrate  20 . For example, the sacrificial substrate  13  may be detached from the flexible substrate  20  by applying light such as laser light or physical force to the sacrificial substrate  13 , but the exemplary embodiments are not limited thereto. In a case in which the sacrificial layer is formed between the sacrificial substrate  13  and the flexible substrate  20 , the sacrificial substrate  13  may be separated from the flexible substrate  20  by applying laser light to the sacrificial layer. 
     Referring to  FIG. 15 , a protective layer  10  is formed by depositing an organic material on the second surface of the flexible substrate  20 . The deposition of the organic material may be performed at a low temperature of, for example, 90° C. or lower. If the deposition of the organic material is performed at low temperature, various elements or layers already formed in the display element unit  100  may be prevented from being damaged during the deposition of the organic material. Also, the degree of freedom in selecting materials for various elements or layers of the display element unit  100  may be increased. 
     A material that can be deposited at a low temperature of about 90° C. may be selected as the organic material. For example, the organic material may include at least one of parylene (i.e., a poly(p-xylylene) polymer) and a derivative thereof, which may be represented by Formula 1: 
     
       
         
         
             
             
         
       
     
     where n is a natural number equal to or greater than 2, m is an integer number between 0 and 4, X is hydrogen or halogen, and R is halogen or alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, cycloalkynyl, aryl, or a halide thereof having 1 to 8 carbon atoms. 
     The material represented by Formula 1 include at least one of materials represented by Formulas 2 through 5: 
     
       
         
         
             
             
         
       
     
     where n is a natural number equal to or greater than 2. 
     In a case in which the protective layer  10  is formed using parylene-N, which is the material of Formula 2, a parylene-N monomer layer may be formed by introducing and polymerizing a parylene-N monomer, starting from locations on the flexible substrate  20  where temperature is low. Parylene-N may be a low-temperature deposition material that can be deposited at a temperature of as low as about −40° C. 
     In a case in which the protective layer  10  is formed using parylene-F, which is the material of Formula 5, a parylene-F layer may be formed by introducing and polymerizing a parylene-F dimer, starting from the locations on the flexible substrate  20  where temperature is low. Parylene-F may be a low-temperature deposition material that can be deposited at room temperature. 
     As mentioned above, since the protective layer  10  may be formed by depositing the organic material at low temperature without the irradiation of light, the use of an additional adhesive is unnecessary, and the elements in the display device  1  may be prevented from being damaged by light or heat. 
     By changing the conditions for the deposition of the organic material over time, the concentration of the organic material, i.e., the density of the protective layer  10 , may be controlled in the same manner as that illustrated in any one of  FIGS. 5 through 8 . A structure in which the density of the protective layer  10  varies from one part to another part of the protective layer  10  has already been described above, and thus, a detailed description thereof will be omitted. As mentioned above, since the density of the protective layer  10  may vary from one part to another part, the Young&#39;s modulus of the protective layer  10  may also vary from one part to another part. 
     Referring to  FIG. 16 , a protective film  220  is attached on the display element unit  100 . The protective film  220  may be attached and fixed on the display element unit  100  via the adhesive layer  210 , which includes an adhesive. 
       FIGS. 12 through 16  illustrate an example in which the attachment of the protective film  220  is performed after the separation of the sacrificial substrate  13  and the formation of the protective layer  10 , but the exemplary embodiments are not limited thereto. For example, the separation of the sacrificial substrate  13  and the formation of the protective layer  10  may be performed after the attachment of the protective film  220  on the display element unit  100 . For another example, the attachment of the protective film  220  on the display element unit  100  may be performed after the separation of the sacrificial substrate  13  before the formation of the protective layer  10 . 
     Experimental examples showing that the impact resistance of a display device can be strengthened by lowering a neutral plane with the use of a protective layer will hereinafter be described. 
     Manufacturing Example 
     A display device in which an urethane cushion layer having a thickness of 150 μm, a first PSA layer having a thickness of 30 μm, a PI layer having a thickness of 38 μm, a second PSA layer having a thickness of 25 μm, a parylene-F layer having a thickness of 10 μm, a display element unit including a TFT and an organic light-emitting element, a third PSA layer having a thickness of 35 μm, a PI layer having a thickness of 50 μm, and a PU layer having a thickness of 200 μm were sequentially stacked was fabricated. The parylene-F layer was formed according to the method described above with reference to  FIGS. 12 through 16 . 
     Comparative Example 
     A display device according to a comparative example was fabricated in the same manner as the display device according to the manufacturing example except that the parylene-F layer was not provided and the display element unit was directly stacked on the second PSA layer. 
     Experimental Example 
     The display device according to the manufacturing example and the display device according to the comparative example were evaluated for their impact resistances by dropping each of a 5.8 g Bic Ball a 5.5 g steel pen thereover. 
     More specifically, the dropping heights of the Bic Ball and the steel pen that produced contrast points for the first time in each of the display device according to the manufacturing example and the display device according to the comparative example were measured, and the results are as shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Manufacturing Example 
                 Comparative Example 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 5.8 g Bic Ball 
                 8 cm 
                 6 cm 
               
               
                 5.5 g Steel Pen 
                 7 cm 
                 4 cm 
               
               
                   
               
            
           
         
       
     
     In response to the Bic Ball or the steel pen colliding with each of the display device according to the manufacturing example and the display device according to the comparative example, the bottom surfaces of the display device according to the manufacturing example and the display device according to the comparative example both stretched, and the top surfaces of the display device according to the manufacturing example and the display device according to the comparative example both contracted. However, as is apparent from Table 1, display elements in the display device according to the manufacturing example that are above a neutral plane were less affected by tensile stress because a parylene layer was further provided to lower the neutral plane. 
     Although certain exemplary embodiments and implementations have been described herein, other embodiments and modifications will be apparent from this description. Accordingly, the inventive concept is not limited to such embodiments, but rather to the broader scope of the presented claims and various obvious modifications and equivalent arrangements.