Patent Publication Number: US-10330842-B2

Title: Polarizer and display device comprising the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 14/958,795, entitled POLARIZER AND DISPLAY DEVICE COMPRISING THE SAME, and filed on Dec. 3, 2015, which claims priority to and the benefit of Korean Patent Application No. 10-2015-0033329, filed on Mar. 10, 2015, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     Field 
     The described technology generally relates to a polarizer and a display device including the same. 
     Description of the Related Technology 
     Organic light-emitting diode (OLED) displays generate images without the use of a backlight, unlike a liquid crystal display (LCD), and thus, they can have thinner profile and be lightweight. Further, OLED displays have favorable characteristics, such as low power consumption, high luminance, and rapid response rate, and thus, they have drawn attention as a next generation display device. 
     OLED displays generally include various types of electrodes and wirings which reflect light from external sources such as ambient light. Due to such reflection, an OLED display has difficulty in representing the deepest black in a relatively bright environment and exhibits a relatively lower contrast ratio. 
     Typically, in order to prevent reflection of external light, a polarizer film is formed over the pixels. 
     It is to be understood that this background of the technology section is intended to provide useful background for understanding the technology and as such disclosed herein, the technology background section can include ideas, concepts or recognitions that were not part of what was known or appreciated by those skilled in the pertinent art prior to a corresponding effective filing date of subject matter disclosed herein. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One inventive aspect relates to a polarizer that can effectively prevent reflection of external light and a display device including the same. 
     Another aspect is a polarizer that includes: a linear polarizer; and a phase retardation layer disposed on the linear polarizer and including a quarter-wave plate and a half-wave plate, wherein the half-wave plate includes a refractive index anisotropic layer, the refractive index anisotropic layer has a refractive index Nx in a direction of an x axis, a refractive index Ny in a direction of a y axis, and a refractive index Nz in a direction of a z axis, and Nx=Nz&gt;Ny. 
     The refractive index anisotropic layer can include a liquid crystal layer. 
     The liquid crystal layer can include a discotic compound. 
     The discotic compound can have a disc surface, and the disc surface can have an inclination angle in a range of about 60 degrees to about 90 degrees with respect to a surface of the half-wave plate. 
     Nx and Nz can be in a range of about 1.6 to about 1.8. 
     Ny can be in a range of about 1.5 to about 1.7. 
     A difference between Nx and Ny can be in a range of about 0.05 to about 0.2. 
     The refractive index anisotropic layer can have a thickness in a range of about 1 micrometer (μm) to about 5 μm. 
     The refractive index anisotropic layer can have a thickness of “a” μm, and when Nx−Ny=“b”, 0.1&lt;a×b&lt;0.5. 
     The half-wave plate can include an alignment layer, and the refractive index anisotropic layer can be disposed on the alignment layer. 
     The quarter-wave plate can include a cyclic olefin polymer (COP). 
     The quarter-wave plate can be disposed on the linear polarizer, and the half-wave plate can be disposed on the quarter-wave plate. 
     The half-wave plate can be disposed on the linear polarizer, and the quarter-wave plate can be disposed on the half-wave plate. 
     Another aspect is a display device that includes: a display panel; and a polarizer disposed on the display panel, wherein the polarizer includes: a linear polarizer; and a phase retardation layer disposed on the linear polarizer and including a quarter-wave plate and a half-wave plate, the half-wave plate includes a refractive index anisotropic layer, the refractive index anisotropic layer has a refractive index Nx in a direction of an x axis, a refractive index Ny in a direction of a y axis, and a refractive index Nz in a direction of a z axis, and Nx=Nz&gt;Ny. 
     The refractive index anisotropic layer can include a liquid crystal layer. 
     The liquid crystal layer can include a discotic compound. 
     The discotic compound can have a disc surface, and the disc surface can have an inclination angle in a range of about 60 degrees to about 90 degrees with respect to a surface of the half-wave plate. 
     Nx and Nz can be in a range of about 1.6 to about 1.8. 
     Ny can be in a range of about 1.5 to about 1.7. 
     A difference between Nx and Ny can be in a range of about 0.05 to about 0.2. 
     The refractive index anisotropic layer can have a thickness in a range of about 1 μm to about 5 μm. 
     The refractive index anisotropic layer can have a thickness of “a” μm, and when Nx−Ny=“b”, 0.1&lt;a×b&lt;0.5. 
     The half-wave plate can further include an alignment layer, and the refractive index anisotropic layer is disposed on the alignment layer. 
     The quarter-wave plate can include a COP. 
     The quarter-wave plate can be disposed on the linear polarizer, and the half-wave plate can be disposed on the quarter-wave plate. 
     The half-wave plate can be disposed on the linear polarizer, and the quarter-wave plate can be disposed on the half-wave plate. 
     The display panel can include: a first substrate; a first electrode disposed on the first substrate; an organic light emitting layer disposed on the first substrate; and a second electrode disposed on the organic light emitting layer. 
     The display device can further include: a second substrate disposed on the second electrode, wherein the polarizer is disposed on the second substrate. 
     The display device can further include: a thin film encapsulation layer disposed on the second electrode, wherein the polarizer is disposed on the thin film encapsulation layer. 
     Another aspect is a polarizer, comprising: a linear polarizer; and a phase retardation layer on the linear polarizer and comprising a quarter-wave plate and a half-wave plate, wherein the half-wave plate comprises a refractive index anisotropic layer, wherein the refractive index anisotropic layer has a refractive index Nx defined in a direction of an x axis, a refractive index Ny defined in a direction of a y axis, and a refractive index Nz defined in a direction of a z axis, and wherein Nx=Nz&gt;Ny. 
     In the above polarizer, the refractive index anisotropic layer comprises a liquid crystal layer. In the above polarizer, the liquid crystal layer comprises a discotic compound. In the above polarizer, the discotic compound has a disc surface, and wherein the disc surface has an inclination angle in the range of about 60 degrees to about 90 degrees with respect to a surface of the half-wave plate. In the above polarizer, each of Nx and Nz is in the range of about 1.6 to about 1.8. In the above polarizer, Ny is in the range of about 1.5 to about 1.7. In the above polarizer, the difference between Nx and Ny is in the range of about 0.05 to about 0.2. In the above polarizer, the refractive index anisotropic layer has a thickness in the range of about 1 micrometer (μm) to about 5 μm. 
     In the above polarizer, the refractive index anisotropic layer has a thickness of “a” μm, and wherein “b”=Nx−Ny, where 0.1&lt;a×b&lt;0.5. In the above polarizer, the half-wave plate comprises an alignment layer, and wherein the refractive index anisotropic layer is on the alignment layer. In the above polarizer, the quarter-wave plate comprises a cyclic olefin polymer (COP). In the above polarizer, the quarter-wave plate is on the linear polarizer, and wherein the half-wave plate is on the quarter-wave plate. In the above polarizer, the half-wave plate is on the linear polarizer, and wherein the quarter-wave plate is on the half-wave plate. 
     Another aspect is a display device, comprising: a display panel; and a polarizer on the display panel, wherein the polarizer comprises: a linear polarizer; and a phase retardation layer on the linear polarizer and comprising a quarter-wave plate and a half-wave plate, wherein the half-wave plate comprises a refractive index anisotropic layer, wherein the refractive index anisotropic layer has a refractive index Nx defined in a direction of an x axis, a refractive index Ny defined in a direction of a y axis, and a refractive index Nz defined in a direction of a z axis, and wherein Nx=Nz&gt;Ny. 
     In the above display device, the refractive index anisotropic layer comprises a liquid crystal layer. In the above display device, the liquid crystal layer comprises a discotic compound. In the above display device, the discotic compound has a disc surface, and wherein the disc surface has an inclination angle in the range of about 60 degrees to about 90 degrees with respect to a surface of the half-wave plate. In the above display device, each of Nx and Nz is in the range of about 1.6 to about 1.8. In the above display device, Ny is in the range of about 1.5 to about 1.7. In the above display device, the difference between Nx and Ny is in a range of about 0.05 to about 0.2. In the above display device, refractive index anisotropic layer has a thickness in the range of about 1 micrometer (μm) to about 5 μm. In the above display device, the refractive index anisotropic layer has a thickness of “a” μm, and wherein Nx−Ny=“b”, 0.1&lt;a×b&lt;0.5. 
     In the above display device, the half-wave plate further comprises an alignment layer, and wherein the refractive index anisotropic layer is on the alignment layer. In the above display device, the quarter-wave plate comprises a cyclic olefin polymer (COP). In the above display device, the quarter-wave plate is on the linear polarizer, and wherein the half-wave plate is on the quarter-wave plate. In the above display device, the half-wave plate is on the linear polarizer, and wherein the quarter-wave plate is on the half-wave plate. In the above display device, the display panel comprises: a first substrate; a first electrode on the first substrate; an organic light emitting layer on the first substrate; and a second electrode on the organic light emitting layer. The above display device further comprises a second substrate on the second electrode, wherein the polarizer is on the second substrate. The above display device further comprises a thin film encapsulation layer on the second electrode, wherein the polarizer is on the thin film encapsulation layer. 
     Another aspect is a polarizer, comprising: a linear polarizer; and a quarter-wave plate on the linear polarizer; and a half-wave plate on the quarter-wave plate and comprising a refractive index anisotropic layer, wherein the refractive index anisotropic layer has a refractive index Nx defined in a direction of an x axis, a refractive index Ny defined in a direction of a y axis, and a refractive index Nz defined in a direction of a z axis, and wherein Nx=Nz&gt;Ny. 
     In the above display device, each of the half-wave and quarter-wave plates is thinner than the linear polarizer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a polarizer according to a first exemplary embodiment. 
         FIG. 2  is an exploded perspective view of  FIG. 1 . 
         FIG. 3  is a structural view illustrating a half-wave plate. 
         FIG. 4  is an exploded perspective view illustrating a polarizer according to a second exemplary embodiment. 
         FIG. 5  is a cross-sectional view illustrating a polarizer according to a third exemplary embodiment. 
         FIG. 6  is a plan view illustrating an organic light-emitting diode (OLED) display device according to a fourth exemplary embodiment. 
         FIG. 7  is a cross-sectional view taken along line I-I′ of  FIG. 6 . 
         FIG. 8  is a cross-sectional view illustrating an OLED display device according to a fifth exemplary embodiment. 
         FIGS. 9A and 9B  are graphs illustrating results of experiments on reflectivity of a polarizer. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they can be embodied in different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey exemplary implementations to those skilled in the art. 
     All terminologies used herein are merely used to describe exemplary embodiments and can be modified according to the relevant art and the intention of an applicant. Therefore, the terms used herein should be interpreted as having a meaning that is consistent with their meanings in the context of the present disclosure, and is not intended to limit the exemplary embodiments. 
     In the drawings, certain elements or shapes can be simplified or exaggerated to better illustrate the described technology, and other elements present in an actual product can also be omitted. Thus, the drawings are intended to facilitate the understanding of the described technology. Like reference numerals refer to like elements throughout the specification. 
     When a layer or element is referred to as being “on” another layer or element, the layer or element can be directly on the other layer or element, or one or more intervening layers or elements can be interposed therebetween. In this disclosure, the term “substantially” includes the meanings of completely, almost completely or to any significant degree under some applications and in accordance with those skilled in the art. The term “connected” can include an electrical connection. 
     Hereinafter, a first exemplary embodiment will be described with reference to  FIGS. 1 through 3 . 
       FIG. 1  is a cross-sectional view illustrating a polarizer  101  according to the first exemplary embodiment.  FIG. 2  is an exploded perspective view of  FIG. 1 . 
     The polarizer  101  according to the first exemplary embodiment can include a linear polarizer  110  and a phase retardation layer  140  formed on the linear polarizer  110 . The polarizer  101  can serve to prevent reflection of external light. 
     The linear polarizer  110  can serve to linearly polarize an external light incident on the polarizer  101 . 
     The linear polarizer  110  can use a film obtained by a dichroic dye being adsorbed onto and aligned in a polyvinyl alcohol (PVA) resin. Examples of the PVA resin can include a homopolymer of vinyl acetate or a copolymer of a monomer different from vinyl acetate. 
     The polarizer  101  can be prepared through a scheme including, for example, a process of uniaxially stretching a PVA resin film, a process of dyeing the PVA resin film with a dichroic dye such that the dichroic dye is adsorbed onto the PVA resin film, a process of treating the PVA resin film, onto which the dichroic dye is adsorbed, using a boron aqueous solution, and a washing process. However, the scheme of manufacturing the linear polarizer  110  according to the first exemplary embodiment is not limited thereto. 
     The dichroic dye can use, for example, iodine and another dichroic dye used in the pertinent art. In the case of using iodine as the dichroic dye, the linear polarizer  110  can be prepared through a process of dyeing the PVA resin film by immersing the PVA resin film in an aqueous solution containing iodine and potassium iodine. 
     Iodine can be arranged in a direction parallel to a direction in which the PVA resin film is stretched. A transmission axis A 1  of the linear polarizer  110  can be determined based on the arrangement of iodine, and the transmission axis A 1  of the linear polarizer  110  can be substantially parallel to the direction of the arrangement of iodine. 
     A thickness of the linear polarizer  110  can vary based on a product to which the linear polarizer  110  is to be applied. For example, the linear polarizer  110  has a thickness in the range of about 5 micrometers (μm) to about 50 μm. 
     The phase retardation layer  140  can be formed on a surface of the linear polarizer  110 . 
     The phase retardation layer  140  can retard a phase of light. The phase retardation layer  140  can convert a linearly polarized light to a circularly polarized light, or can convert a circularly polarized light to a linearly polarized light. For example, an external light incident on the polarizer  101  is linearly polarized by the linear polarizer  110 , or circularly polarized by the phase retardation layer  140 . The circularly polarized external light can be reflected within a device to which the polarizer  101  is attached, for example, a display device, to thereby be converted to a reflected light. During the reflection process, a phase and a polarization axis of the light can vary. In some embodiments, the reflected light with the varied phase does not pass through the polarizer  101 , and thus, reflection of the external light can be prevented by the polarizer  101 . 
     The phase retardation layer  140  can use a phase difference plate having a film shape. For example, a single layer of the phase difference plate is used, or a plurality of layers of the phase difference plate are stacked to be used. 
     The phase retardation layer  140  according to the first exemplary embodiment can include a quarter-wave plate (QWP)  120  and a half-wave plate (HWP)  130 . 
     For example, the phase retardation layer  140  can include the QWP  120  formed on the linear polarizer  110  and the HWP  130  formed on the QWP  120 . However, the first exemplary embodiment is not limited thereto, and the HWP  130  can be formed on the linear polarizer  110  and the QWP  120  can be formed on the HWP  130 . 
     The QWP  120  can be prepared by stretching a film. For example, the QWP  120  is be prepared by stretching a film formed of polycarbonate (PC), polyvinyl alcohol (PVA), polystyrene (PS), poly(methyl methacrylate) (PMMA), polypropylene (PP), polyolefin, polyarylate (PAR), or polyamide (PA). 
     The QWP  120  can include a film formed of a cyclic olefin polymer (COP). The COP film can be prepared using, for example, a norbornene-based COP. The COP film can have high light transmittance, high thermal resistance, high strength, low moisture absorption, and high cost-efficiency. A phase difference plate formed of such a COP film can represent a relatively stable phase retardation value, irrespective of a wavelength of incident light. 
     In addition, the QWP  120  can be prepared using a photocurable liquid crystal compound. For example, an alignment layer is formed on a base film, and liquid crystal compounds is arranged on the alignment layer to be patterned thereon, such that the QWP  120  can be prepared. In this instance, based on the alignment layer and the arrangement of the liquid crystal compounds, the type of the phase difference plate to be prepared, that is, whether the quarter-wave plate or the half-wave plate, can be determined. 
     The type or the manufacturing scheme of the QWP  120  is not limited to the above description. 
     The QWP  120  can have a transmission axis A 2 . The transmission axis A 2  of the QWP  120  can be determined by a stretching axis of a film or a direction in which the liquid crystal compounds are aligned. 
     The QWP  120  according to the first exemplary embodiment can be prepared by stretching the COP film, and the transmission axis A 2  of the QWP  120  can be substantially parallel to a stretching axis of the COP film. 
     The QWP  120  according to the first exemplary embodiment can be designed to be optimized for preventing reflection of an external light incident frontwards, that is, an external light vertically incident on a surface of the polarizer  101 . Accordingly, in a case of a polarizer using only the QWP  120  as the phase retardation plate  140 , reflection of an external light incident on the polarizer at a predetermined incident angle is not effectively prevented by the polarizer. In other words, in the case of using only the QWP  120  as the phase retardation layer  140 , a path of an external light can vary based on an incident angle of the external light such that a phase retardation effect can change, and the effect of preventing reflection of the external light can also change based on the incident angle. 
     In this case, when the incident angle of the external light increases, reflectivity of the polarizer can increase, and when a viewing angle of a user increases, an amount of reflected light to be visible to the user can increase. 
     In order to prevent the increase in reflectivity of the polarizer in response to the increase in the incident angle of the external light, the HWP  130  can be used along with the QWP  120 . 
     The HWP  130  according to the first exemplary embodiment can have a refractive index anisotropic layer. 
     The refractive index anisotropic layer can have a refractive index Nx in a direction of an x axis, a refractive index Ny in a direction of a y axis, and a refractive index Nz in a direction of a z axis, and a relationship thereamong in which “Nx=Nz&gt;Ny” can be satisfied. A phase difference plate satisfying the relationship in which “Nx=Nz&gt;Ny” can be referred to as a negative A-plate. 
       FIG. 3  is a structural view illustrating the HWP  130 . The HWP  130  of  FIG. 3  can include a liquid crystal layer  132  as the refractive index anisotropic layer. 
     In detail, the HWP  130  according to the first exemplary embodiment can include a support member  131  and the liquid crystal layer  132  formed on the support member  131 . The liquid crystal layer  132  can have a refractive index anisotropy. 
     The support member  131  can serve to support the liquid crystal layer  132 . The support member  131  can be formed of a transparent plastic film and can include, for example, one or more of the following materials: tri-acetyl cellulose (TAC), polyethylene terephthalate (PET), polyimide (PI), and polycarbonate (PC). 
     In addition, an alignment layer can be used as the support member  131 . A description pertaining to the alignment layer will be provided further below. 
     The liquid crystal layer  132  can include a discotic compound  133 . For example, the liquid crystal layer  132  can include a binder compound, and the discotic compounds  133  can be arranged through being dispersed in the binder compound. The liquid crystal layer  132  including the discotic compound  133  can also be referred to as a discotic liquid crystal layer. 
     The discotic compound  133  of  FIG. 3  can have a predetermined inclination angle θ 1  with respect to a surface of the HWP  130 . In detail, the discotic compound  133  of  FIG. 3  can have a disc surface DS, and the disc surface DS can have an inclination angle θ 1  in the range of about 60 degrees to about 90 degrees with respect to a surface RS of the support member  131 . 
     In general, the inclination angle θ 1  of the discotic compound  133  can vary based on the type of the discotic compound  133 , the alignment scheme of the discotic compound  133 , the type of material used in the alignment of the discotic compound  133 , and the type of alignment layer. 
     The liquid crystal layer  132  can be prepared by aligning the discotic compound  133  using the alignment layer and fixing the discotic compound  133  in the aligned state. 
     A discotic compound known to those skilled in the art can be used to form the liquid crystal layer  132 , and a polymerization scheme known to those skilled in the art can be used to fix the discotic compound  133 . 
     For example, to fix the discotic compound  133  by polymerization, a compound having a polymerizable group is used. Examples of the compound having the polymerizable group include a monomer or an oligomer. In this instance, a linking group can be introduced between the discotic compound  133  and the polymerizable group. For example, the liquid crystal layer  132  can include a compound expressed by Formula 1.
 
D L−P   n   [Formula 1]
 
     In Formula 1, “D” denotes the discotic compound  133 , “L” denotes the linking group, and “P” denotes the polymerizable group. “n” denotes an integer in the range of 4 to 12. 
     The liquid crystal layer  132  can be prepared by coating, drying, and heating a composition for forming a liquid crystal layer including the discotic compound  133  up to a temperature at which a discotic nematic phase is formed, on the alignment layer and then polymerizing and cooling the composition for forming the liquid crystal layer. The temperature for forming the discotic nematic phase can be in the range of about 70 degrees Celsius (° C.) to about 300° C., which is lower than a thermal deformation temperature of the alignment layer, and more particularly, in the range of about 70° C. to about 170° C. 
     The composition for forming the liquid crystal layer can include the discotic compound  133  in an amount of about 1 percentage by weight (wt %) to about 70 wt % with respect to a total weight of the composition for forming the liquid crystal layer. 
     In addition, the composition for forming the liquid crystal layer can further include a polymerizable monomer, a polymerization initiator, and an additive, in addition to the discotic compound  133 . 
     The polymerizable monomer can use a monomer having a vinyl group, a vinyloxy group, an acryl group, and a methacryl group. The composition for forming the liquid crystal layer can include the polymerizable monomer in an amount of about 1 wt % to about 50 wt % with respect to the total weight of the composition for forming the liquid crystal layer. 
     The additive can use a polymer, a plasticizer, and a surfactant. 
     The polymer can use a polymer which is highly compatible with the discotic compound  133 . An example of such a polymer includes a cellulose ester polymer. Examples of the cellulose ester polymer include cellulose acetate, cellulose acetate propionate, hydroxypropyl cellulose, cellulose acetate butyrate, and the like. The composition for forming the liquid crystal layer can include the polymer in an amount of about 1 wt % to about 30 wt % with respect to the total weight of the composition for forming the liquid crystal layer. 
     The plasticizer and the surfactant can use any plasticizer and surfactant commonly used in manufacturing a polymerizable composition. 
     The polymerization initiator can use a photopolymerization initiator and a thermal polymerization initiator. 
     The polymerization scheme of the composition for forming the liquid crystal layer can include a thermal polymerization scheme using a thermal polymerization initiator and a photopolymerization scheme using a photopolymerization initiator. In particular, in order to maintain the aligned state of the discotic compound  133  and polymerize the composition for forming the liquid crystal layer, the photopolymerization scheme can be used. 
     For the photopolymerization, the composition for forming the liquid crystal layer can include a photopolymerization initiator. Examples of the photopolymerization initiator include one or more of the following materials: an α-carbonyl compound, acyloin ether, an α-hydrocarbon-substituted aromatic acyloin compound, a polynuclear quinone compound, a combination of triarylimidazole dimer and p-aminophenylketone, an acridine compound, a phenazine compound, and an oxadiazole compound. The composition for forming the liquid crystal layer can include the photopolymerization initiator in an amount of about 0.01 wt % to about 20 wt % with respect to the total weight of the composition for forming the liquid crystal layer. 
     For the photopolymerization of the composition for forming the liquid crystal layer, light can be irradiated on the composition for forming the liquid crystal layer. Such light to be irradiated thereon can use ultraviolet (UV) light having energy in the range of about 20 millijoules per square centimeter (mJ/cm 2 ) to about 5000 mJ/cm 2 . The light irradiation can be conducted during the heating process in order to accelerate the photopolymerization reaction. 
     The composition for forming the liquid crystal layer can be cured to thereby form the liquid crystal layer  132  and to fix the discotic compound  133 . 
     As such, as the discotic compounds  133  are arranged at an inclination angle θ 1  in the range of about 60 degrees to about 90 degrees, the liquid crystal layer  132  can have the refractive index Nx in the direction of the x axis and the refractive index Nz in the direction of the z axis in the range of about 1.6 to about 1.8, and the refractive index Ny in the direction of the y axis in the range of about 1.5 to about 1.7. In addition, a difference between Nx and Ny can have a value in the range of about 0.05 to about 0.2. 
     However, the first exemplary embodiment is not limited thereto, and the refractive index Nx in the direction of the x axis, the refractive index Ny in the direction of the y axis, and the refractive index Nz in the direction of the z axis can vary based on the materials forming the liquid crystal layer  132  and the arrangement of the materials. 
     A thickness of the liquid crystal layer  132  can be adjusted to allow phase retardation, which is required for circular polarization, to occur. The thickness of the liquid crystal layer  132  can vary based on a refractive index of the liquid crystal layer  132 . The liquid crystal layer  132  according to the first exemplary embodiment can have a thickness in the range of about 1 μm to about 5 μm. 
     In addition, in order to compensate for a phase retardation variation based on an incident angle of an external light, a thickness and a refractive index of the liquid crystal layer  132  can be adjusted to satisfy the following relationship. For example, when the liquid crystal layer  132  has a thickness of “a” μm and a difference between the refractive index Nx and the refractive index Ny is given by an equation in which “Nx−Ny=b”, the thickness and the refractive index of the liquid crystal layer  132  is adjusted so as to satisfy the relationship in which “0.1≤a×b≤0.5”. 
     In a case in which a value calculated by “a×b” is in the range of about 0.1 to about 0.5, a phase retardation variation in the range of about 100 nanometers (nm) to about 500 nm can be compensated. For example, even in the case in which a laterally incident external light has the phase retardation variation in the range of about 100 nm to about 500 nm, as compared to an external light incident frontwards, the laterally incident external light can be circularly polarized by the polarizer  101 . 
     More particularly, the value calculated by “a×b” can be in the range of about 0.2 to about 0.3. In this case, the phase retardation variation can be compensated in the range of about 200 nm to about 300 nm. 
     In the case of using the liquid crystal layer  132  as the refractive index anisotropic layer of the HWP  130 , circular polarization of the external light can occur by the polarizer  101  despite varying incident angles of the external light. Accordingly, the polarizer  101  including the HWP  130  can effectively prevent reflection of the external light, irrespective of the incident angle of the external light. 
     The HWP  130  according to the first exemplary embodiment can include an alignment layer, and the alignment layer can function as the support member  131 . Accordingly, the liquid crystal layer  132  can be formed on the alignment layer. 
     The alignment layer can serve to determine an alignment direction of the discotic compound  133 . 
     The alignment layer can be prepared through a rubbing treatment of a polymer, a rhombic vacuum deposition process, a Langmuir-Brojet (LB) process, or the like. In this instance, the polymer for forming the alignment layer can include ω-trycosanoic acid, dioctadecylmethylammonium chloride, methyl stearic acid, and the like. 
     In detail, the alignment layer can be prepared through a rubbing treatment of a polymer. The polymer for forming the alignment layer can include PVA. In addition, a polymer which is modified through a hydrophobic group being bonded thereto can be used to form the alignment layer. For example, a polymer in which a hydrophobic group is introduced to PVA is used. Due to the affinity of the hydrophobic group with the discotic compound  133 , the hydrophobic group can be suitable for uniformly aligning the discotic compound  133 . 
     The hydrophobic group can be bonded to an end of a backbone of PVA or a side chain of PVA. 
     The hydrophobic group can include an aliphatic group having 6 or more carbon atoms or an aromatic group. For example, an alkyl group or an alkenyl group having 6 or more carbon atoms is used as the hydrophobic group. 
     In a case of bonding the hydrophobic group to the end of the backbone of PVA, a linking group can be introduced between the hydrophobic group and the end of the backbone of PVA. Examples of the linking group can include —S—, —C(CN)R 1 —, —NR 2 —, and the like. As used herein, R 1  and R 2  each denote a hydrogen atom or an alkyl group having one to 6 carbon atoms. 
     To prepare the alignment layer, a commercially available PVA film can be used. 
     The rubbing treatment can be performed by rubbing a surface of a film for forming the alignment layer a few number of times in a predetermined direction. For the rubbing treatment, a cloth in which a fiber has a uniform length and thickness and is uniformly arranged can be used. 
     The alignment layer can have an alignment direction RD, and in a case of forming the alignment layer by the rubbing treatment, the alignment direction RD can be parallel to a direction of the rubbing treatment. The discotic compound  133  can be aligned based on the alignment direction RD of the alignment layer. 
     A transmission axis of the liquid crystal layer  132  can be determined based on the alignment direction of the discotic compound  133 . The transmission axis of the liquid crystal layer  132  can be a transmission axis of the HWP  130 . 
     The alignment layer is not necessarily required in the polarizer  101 , and can be omitted. For example, the alignment layer is omitted in the preparing process of the polarizer  101 , subsequently to using the alignment layer for aligning the discotic compound  133 . In this case, a transparent base film can be used as the support member  131  for supporting the discotic compound  133 , in lieu of the alignment layer. 
     Further, the support member  131  can be omitted. For example, the liquid crystal layer  132 , aligned by the alignment layer, is formed directly on the QWP  120 . 
     Although not illustrated, the polarizer  101  according to the first exemplary embodiment can further include an adhesive layer interposed between the linear polarizer  110  and the QWP  120  and an adhesive layer interposed between the QWP  120  and the HWP  130 . 
     In addition, the linear polarizer  110 , the QWP  120 , and the HWP  130  can be formed to intersect one another at a predetermined angle to achieve effective reflection of external light. 
     For example, the QWP  120  is formed to intersect the linear polarizer  110  at an angle of θ 2  based on a transmission axis, and the HWP  130  is formed to intersect the linear polarizer  110  at an angle of θ 3  based on the transmission axis. As used herein, θ 2  is an angle between the transmission axis A 1  of the linear polarizer  110  and the transmission axis A 2  of the QWP  120 , and can be in a range of about 70 degrees to about 80 degrees. In addition, θ 3  is an angle between the transmission axis A 1  of the linear polarizer  110  and the alignment direction RD of the alignment layer, and can be in the range of about 12 degrees to about 17 degrees. 
     When the polarizer  101  is formed on the display device, and the like, the y axis can be a direction parallel to a direction in which the range of an incident angle of external light is relatively great when viewed from a user. For example, in a case of using the polarizer  101  in a display device having a greater horizontal length than a vertical width, the y axis can be a direction parallel to the horizontal length of the display device. 
       FIGS. 9A and 9B  are graphs illustrating results of experiments on reflectivity of a polarizer.  FIG. 9A  illustrates a result of a simulation experiment on reflectivity of a polarizer including a HWP in which a refractive index anisotropy is absent; and  FIG. 9B  illustrates a result of a simulation experiment on reflectivity of the polarizer  101  according to the first exemplary embodiment. 
     In detail, the HWP used in the experiment of  FIG. 9A  can have a refractive index in which “Nx=Ny=Nz=1.6”, and a HWP used in the experiment of  FIG. 9B  can have a refractive index in which “Nx=Nz=1.77” and “Ny=1.6”. A relatively dark portion of  FIGS. 9A and 9B  indicates a portion in which reflection of external light is effectively prevented, and a relatively bright portion thereof indicates a portion in which relatively low efficiency in preventing reflection of external light is exhibited. As illustrated in  FIGS. 9A  and  9 B, the polarizer  101  according to the first exemplary embodiment can effectively prevent reflection of external light in all directions irrespective of an incident angle of external light and can be excellent in representing a black color. 
     As such, the polarizer  101  according to the first exemplary embodiment can have the HWP  130  having a negative A-plate property, and can prevent reflection of external light irrespective of an incident angle of external light. Accordingly, a product using the polarizer  101  according to the first exemplary embodiment can be excellent in representing a black color and can have an excellent contrast ratio. 
     Hereinafter, a second exemplary embodiment will be described with reference to  FIG. 4 . A repeated description of the aforementioned component will be omitted herein for conciseness. 
       FIG. 4  is an exploded perspective view illustrating a polarizer  102  according to the second exemplary embodiment. 
     The polarizer  102  according to the second exemplary embodiment includes a linear polarizer  110 , a HWP  130  formed on the linear polarizer  110 , and a QWP  120  formed on the HWP  130 . A configuration of the HWP  130  and the QWP  120  according to the second exemplary embodiment can be the same as that of the first exemplary embodiment. 
     Hereinafter, a third exemplary embodiment will be described with reference to  FIG. 5 . 
       FIG. 5  is a cross-sectional view illustrating a polarizer  103  according to the third exemplary embodiment. 
     The polarizer  103  according to the third exemplary embodiment can include a linear polarizer  110 , a QWP  120 , a HWP  130 , and a support layer  150  interposed between the linear polarizer  110  and the QWP  120 . 
     The support layer  150  can serve to support and protect the polarizer  103 . The support layer  150  can use, for example, a tri-acetyl cellulose (TAC) film. The TAC film can have excellent durability and mechanical strength. 
     In addition, the polarizer  103  according to the third exemplary embodiment can further include an adhesive layer  160  formed on the HWP  130 . The adhesive layer  160  can be formed of an adhesive resin. 
     Hereinafter, a fourth exemplary embodiment will be described with reference to  FIGS. 6 and 7 . 
       FIG. 6  is a plan view illustrating an OLED display  104  according to the fourth exemplary embodiment.  FIG. 7  is a cross-sectional view taken along line I-I′ of  FIG. 6 . 
     As illustrated in  FIGS. 6 and 7 , the OLED display  104  according to the fourth exemplary embodiment can include a display panel  301  and a polarizer  101 . 
     The display panel  301  can include a first substrate  311 , a wiring unit  330 , an OLED  210 , and a second substrate  312 . 
     The first substrate  311  can use an insulating substrate formed of one or more of the following materials: glass, quartz, ceramic, and plastic. However, the fourth exemplary embodiment is not limited thereto, and the first substrate  311  can also use a metallic substrate formed of stainless steel, or the like. 
     A buffer layer  321  can be formed on the first substrate  311 . The buffer layer  321  can include one or more of various inorganic and organic layers. The buffer layer  321  can serve to reduce or effectively prevent the infiltration of impure elements such as moisture into the wiring unit  330  or the OLED  210  through the first substrate  311  and can also planarize a surface of the first substrate  311 . However, the buffer layer  321  is not necessarily required, and can be omitted. 
     The wiring unit  330  can be formed on the buffer layer  321 . The wiring unit  330  can include a switching thin film transistor (TFT)  10 , a driving TFT  20 , and a capacitor  80 , and can drive the OLED  210 . The OLED  210  can display an image by emitting light based on a driving signal transmitted from the wiring unit  330 . 
       FIGS. 6 and 7  illustrate the OLED display  104  as an active matrix organic light emitting diode (AMOLED) display having a 2Tr-1Cap structure in which a single pixel includes two thin film transistors, for example, the switching TFT  10  and the driving TFT  20 , and a single capacitor, for example, the capacitor  80 . However, the fourth exemplary embodiment is not limited thereto. For example, the OLED display  104  according to the fourth exemplary embodiment can have various structures in which a single pixel includes three or more thin film transistors and two or more capacitors and an additional wiring can further be included. As used herein, the term “pixel” refers to a minimum unit for displaying an image, and the OLED display  104  can display an image through a plurality of pixels. 
     The switching TFT  10 , the driving TFT  20 , the capacitor  80 , and the OLED  210  can be included in each pixel. In addition, a gate line  351 , a data line  371  insulated from and intersecting the gate line  351 , and a common power line  372  can be formed in the wiring unit  330 . A single pixel can be defined by a boundary among the gate line  351 , the data line  371 , and the common power line  372 . However, the definition of the pixel is not limited thereto. The pixel can be defined by a pixel defining layer (PDL)  391  or a black matrix. 
     The OLED  210  can include a first electrode  211 , an organic light emitting layer  212  formed on the first electrode  211 , and a second electrode  213  formed on the organic light emitting layer  212 . A hole and an electron from the first electrode  211  and the second electrode  213 , respectively, are injected into the organic light emitting layer  212  to be combined with one another to thereby form an exciton. The OLED  210  can emit light by energy generated when the exciton falls from an excited state to a ground state. 
     The capacitor  80  can include a pair of capacitor plates  358  and  378 , which are formed to have an insulating interlayer  345  therebetween. In this instance, the insulating interlayer  345  can be a dielectric material. Capacitance of the capacitor  80  can be determined by an amount of electric charges stored in the capacitor  80  and a level of a voltage between the capacitor plates  358  and  378 . 
     The switching TFT  10  can include a switching semiconductor layer  331 , a switching gate electrode  352 , a switching source electrode  373 , and a switching drain electrode  374 . The driving TFT  20  can include a driving semiconductor layer  332 , a driving gate electrode  355 , a driving source electrode  376 , and a driving drain electrode  377 . The switching semiconductor layer  331  can be insulated from the switching gate electrode  352  by a gate insulating layer  341 , and the driving semiconductor layer  332  can be insulated from the driving gate electrode  355  by the gate insulating layer  341 . 
     The switching TFT  10  can be used as a switching element that selects a pixel to perform light emission. The switching gate electrode  352  can be connected to the gate line  351 . The switching source electrode  373  can be connected to the data line  371 . The switching drain electrode  374  can be formed to be spaced apart from the switching source electrode  373  and can be connected to one of the capacitor plates, for example, the capacitor plate  358 . 
     The driving TFT  20  can apply, to the first electrode  211 , a driving power for allowing the organic light emitting layer  212  of the OLED  210  in the pixel selected by the switching TFT  10 , to perform light emission. The driving gate electrode  355  can be connected to the capacitor plate  358 , which is connected to the switching drain electrode  374 . The driving source electrode  376  and the other capacitor plate  378  can be connected to the common power line  372 . The driving drain electrode  377  can be connected to the first electrode  211  of the OLED  210  through a contact hole. 
     Due to the configuration of the switching TFT  10  and the driving TFT  20  as described above, the switching TFT  10  can be operated by a gate voltage applied to the gate line  351  to thereby transmit a data voltage applied to the data line  371  to the driving TFT  20 . A voltage having a level that is substantially equal to a difference between a level of a data voltage transmitted by (or from) the switching TFT  10  and a level of a common voltage applied from the common power line  372  to the driving TFT  20  can be stored in the capacitor  80 . A current having a level equivalent to the level of the voltage stored in the capacitor  80  can flow into the OLED  210  through the driving TFT  20  such that the OLED  210  can emit light. 
     In the fourth exemplary embodiment, the first electrode  211  can be an anode that injects holes and the second electrode  213  can be a cathode that injects electrons. However, the fourth exemplary embodiment is not limited thereto, and can be modified such that the first electrode  211  is a cathode and the second electrode  213  is an anode. 
     A planarization layer  346  can be formed on the insulating interlayer  345 . The planarization layer  346  can be formed of an insulating material, and can protect the wiring unit  330 . The planarization layer  346  and the insulating interlayer  345  can be formed of the same material. 
     The drain electrode  377  of the driving TFT  20  can be connected to the first electrode  211  of the OLED  210  through the contact hole formed in the planarization layer  346 . 
     In the fourth exemplary embodiment, the first electrode  211  can include a reflective layer and the second electrode  213  can include a transflective layer. Accordingly, a light generated in the organic light emitting layer  212  can be transmitted through the second electrode  213  to be emitted. For example, the OLED display  104  according to the fourth exemplary embodiment is a top-emission-type display device. 
     The reflective layer and the transflective layer can be formed of one or more of the following metals: magnesium (Mg), silver (Ag), gold (Au), calcium (Ca), lithium (Li), chromium (Cr), aluminum (Al), and copper (Cu) or an alloy thereof. In this instance, the type of the layer, that is, whether the reflective layer or the transflective layer, can be determined based on a thickness of the layer. In general, the transflective layer has a thickness of less than or equal to about 200 nm. As the thickness of the transflective layer decreases, a level of light transmittance can increase, and as the thickness of the transflective layer increases, the level of light transmittance can decrease. 
     In detail, the first electrode  211  can include a reflective layer formed of one or more of the following metals: Mg, Ag, Au, Ca, Li, Cr, Al, and Cu, and a transparent conductive layer formed on the reflective layer. In this instance, the transparent conductive layer can be formed of transparent conductive oxide (TCO), for example, one or more of the following materials: indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), aluminum zinc oxide (AZO), and indium oxide (In 2 O 3 ). Since such a transparent conductive layer has a relatively high work function, hole injection through the first electrode  211  can be readily performed. 
     In addition, the first electrode  211  can have a triple-layer structure in which a transparent conductive layer, a reflective layer, and a transparent conductive layer are sequentially stacked. 
     The second electrode  213  can include a transflective layer formed of one or more of the following metals: Mg, Ag, Au, Ca, Li, Cr, Al, and Cu. 
     Although not illustrated, at least one of a hole injection layer (HIL) and a hole transporting layer (HTL) can further be interposed between the first electrode  211  and the organic light emitting layer  212 . In addition, at least one of an electron transporting layer (ETL) and an electron injection layer (EIL) can further be interposed between the organic light emitting layer  212  and the second electrode  213 . 
     The organic light emitting layer  212 , the HIL, the HTL, the ETL, and the EIL can be referred to as an organic layer. Such an organic layer can be formed of a low molecular weight organic material or a polymer organic material. 
     The PDL  391  can have an aperture. The aperture of the PDL  391  can expose a portion of the first electrode  211 . The first electrode  211 , the organic light emitting layer  212 , and the second electrode  213  can be sequentially stacked in the aperture of the PDL  391 . The second electrode  213  can be formed on the organic light emitting layer  212  and on the PDL  391 . The PDL  391  can define a light emission area. 
     Although not illustrated, a capping layer can be formed on the second electrode  213 . The capping layer (not illustrated) can protect the OLED  210 . 
     In order to protect the OLED  210 , the second substrate  312  can be formed on the OLED  210  to be opposite to the first substrate  311 . The second substrate  312  can be formed of the same material as that forming the first substrate  311 . 
     The polarizer  101  can be formed on the display panel  301 . The polarizer  101  according to the fourth exemplary embodiment can have the same configuration as that of the polarizer according to the first exemplary embodiment. In detail, the polarizer  101  can be formed on the second substrate  312  corresponding to a display surface of the display panel  301 . In this instance, the polarizer  101  can be adhered to the second substrate  312  by the adhesive layer  160 . Since a description pertaining to the configuration of the polarizer  101  is provided with reference to the first exemplary embodiment, a detailed description thereof will be omitted herein for conciseness. 
     Hereinafter, a fifth exemplary embodiment will be described with reference to  FIG. 8 .  FIG. 8  is a cross-sectional view illustrating an OLED display  105  according to the fifth exemplary embodiment. 
     The OLED display  105  according to the fifth exemplary embodiment can include a thin film encapsulation layer  250  formed on an OLED  210 . 
     The thin film encapsulation layer  250  can include one or more inorganic layers  251 ,  253 , and  255  and one or more organic layers  252  and  254 . The thin film encapsulation layer  250  can have a structure in which the inorganic layers  251 ,  253 , and  255  and the organic layers  252  and  254  are alternately stacked. In this instance, the inorganic layer  251  can be formed as a lowermost layer of the stacked structure of the thin film encapsulation layer  250 . For example, the inorganic layer  251  is formed to be most adjacently to the OLED  210 . Although  FIG. 8  illustrates the thin film encapsulation layer  250  as including the three inorganic layers  251 ,  253 , and  255  and the two organic layers  252  and  254 , the fifth exemplary embodiment is not limited thereto. 
     The inorganic layers  251 ,  253 , and  255  can be formed of one or more of the following materials: Al 2 O 3 , TiO 2 , ZrO, SiO 2 , AlON, AlN, SiON, Si 3 N 4 , ZnO, and Ta 2 O 5 . The inorganic layers  251 ,  253 , and  255  can be formed through a chemical vapor deposition (CVD) process or an atomic layer deposition (ALD) process. However, the fifth exemplary embodiment is not limited thereto, and the inorganic layers  251 ,  253 , and  255  can be formed through various processes known in the pertinent art. 
     The organic layers  252  and  254  can be formed of a polymer-based material. Such a polymer-based material can include an acrylic resin, an epoxy resin, polyimide, polyethylene, and the like. The organic layers  252  and  254  can be formed through a thermal deposition process. The thermal deposition process for forming the organic layers  252  and  254  can be performed within the range of temperature in which the OLED  210  is not damaged. However, the fifth exemplary embodiment is not limited thereto, and the organic layers  252  and  254  can be formed through various processes known in the pertinent art. 
     The inorganic layers  251 ,  253 , and  255  formed to have a high density of a thin film can serve to reduce or effectively prevent the infiltration of moisture or oxygen thereinto. The infiltration of moisture or oxygen into the OLED  210  can be prevented largely by the inorganic layers  251 ,  253 , and  255 . 
     Moisture or oxygen passing through the inorganic layers  251 ,  253 , and  255  can be blocked further by the organic layers  252  and  254 . The organic layers  252  and  254  can exhibit relatively low efficiency in preventing the moisture infiltration, as compared to the inorganic layers  251 ,  253 , and  255 . However, the organic layers  252  and  254  can also serve as a buffer layer to reduce stress between respective layers of the inorganic layers  251 ,  253 , and  255  and the organic layers  252  and  254 , aside from preventing the moisture infiltration. Further, since the organic layers  252  and  254  have a planarization property, an uppermost surface of the thin film encapsulation layer  250  can be planarized. 
     The thin film encapsulation layer  250  can have a thickness of about 10 μm or less. Accordingly, an overall thickness of the OLED display  102  can be significantly reduced. 
     The polarizer  101  can be formed on the thin film encapsulation layer  250 . The polarizer  101  can use the polarizer according to the first exemplary embodiment. 
     As set forth above, according to at least one of the disclosed embodiments, the polarizer can include the phase difference plate having refractive index anisotropy, and can effectively prevent reflection of an external light irrespective of an incident angle of the external light. In addition, the display device can include the polarizer capable of effectively preventing reflection of external light, can be excellent in representing a black color, and can have a high contrast ratio. 
     From the foregoing, it will be appreciated that various embodiments in accordance with the present disclosure have been described herein for purposes of illustration, and that various modifications can be made without departing from the scope and spirit of the present teachings. Accordingly, the various embodiments disclosed herein are not intended to be limiting of the true scope and spirit of the present teachings. Various features of the above described and other embodiments can be mixed and matched in any manner, to produce further embodiments consistent with the invention.