Abstract:
An organic light emitting diode (OLED) display is disclosed. In one embodiment, the OLED display includes i) a substrate having first and second surfaces opposing each other and ii) an organic light emitting diode (OLED) formed over the substrate, wherein the OLED is closer to the first surface than the second surface of the substrate. The display may also include i) a light scattering layer formed between the first surface of the substrate and the organic light emitting diode and ii) a light absorbing layer formed between the first surface of the substrate and the light scattering layer or on the second surface of the substrate.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0106073 filed in the Korean Intellectual Property Office on Oct. 28, 2010, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The described technology generally relates to an organic light emitting diode (OLED) display, and more particularly, to an organic light emitting diode (OLED) display suppressing reflection by external light. 
         [0004]    2. Description of the Related Technology 
         [0005]    An organic light emitting diode (OLED) display is a self-emitting display device which has an organic light emitting diode emitting light to display an image. Light is emitted when excitons generated by combining electrons and holes in an organic emission layer fall from an excited state to a ground state, and the light from a matrix of OLEDs results in an image. 
         [0006]    However, when an OLED display is used in high intensity lighting conditions, the generation of a black image and an overall image contrast are reduced due to reflection of light received from the environment. 
         [0007]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the described technology 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 
       [0008]    One inventive aspect is an organic light emitting diode (OLED) display having advantages of improving a contrast characteristic by suppressing reflection of external light. 
         [0009]    Another aspect is an organic light emitting diode (OLED) display including: a substrate an organic light emitting diode formed on the substrate; a light scattering layer formed between the substrate and the organic light emitting diode; and a light absorbing layer disposed between the substrate and the light scattering layer or on an opposite surface to a surface of the substrate facing the organic light emitting diode. 
         [0010]    The light scattering layer may include a scattering material. 
         [0011]    At least two layers of the light scattering layers may be disposed. 
         [0012]    The light scattering layer may be adjacent to the substrate and interfaces where the light scattering layers and the substrate are in contact with each other may be formed in uneven structures, respectively. 
         [0013]    The light scattering layer may have a relatively higher refractive index than the substrate. 
         [0014]    The light absorbing layer may have relatively higher transmittance for blue light than for red light or green light by 10% to 30%. 
         [0015]    The light absorbing layer may include at least one light absorbing material of a black inorganic material, a black organic material, and metal. 
         [0016]    The light absorbing layer may be disposed on an opposite surface to a surface of the substrate facing the organic light emitting diode and the light absorbing layer may have a moth-eye structure. 
         [0017]    The light absorbing layer may be disposed on an opposite surface to the surface of the substrate facing the organic light emitting diode and the light absorbing layer may be subjected to anti-reflection coating or low refraction coating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross-sectional view of an organic light emitting diode (OLED) display according to a first embodiment. 
           [0019]      FIG. 2  is a partial exploded cross-sectional view of the organic light emitting diode (OLED) display of  FIG. 1 . 
           [0020]      FIG. 3  is a cross-sectional view of an organic light emitting diode (OLED) display according to a second embodiment. 
           [0021]      FIG. 4  is a cross-sectional view of an organic light emitting diode (OLED) display according to a third embodiment. 
           [0022]      FIG. 5  is a cross-sectional view of an organic light emitting diode (OLED) display according to a fourth embodiment. 
           [0023]      FIG. 6  is a cross-sectional view of an organic light emitting diode (OLED) display according to a fifth embodiment. 
           [0024]      FIG. 7  is a graph illustrating transmittances of Experimental Example and Comparative Example according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Embodiments will be described more fully hereinafter with reference to the accompanying drawings. As those skilled in the art would realize, the described embodiments may be modified in various different ways. 
         [0026]    Further, like reference numerals designate like elements throughout the specification. In addition, in describing various embodiments, components different from the previous embodiments will be mainly described. 
         [0027]    In addition, the size and thickness of each component shown in the drawings are arbitrarily shown for understanding and ease of description, and not considered limiting. 
         [0028]    In the drawings, the thickness of layers, films, panels, regions, etc., may be exaggerated for clarity. In the drawings, for understanding and ease of description, the thicknesses of some layers and areas may be exaggerated. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. 
         [0029]    Hereinafter, an organic light emitting diode (OLED) display  101  according to a first embodiment will be described with reference to  FIGS. 1 and 2 . 
         [0030]    As shown in  FIG. 1 , the organic light emitting diode (OLED) display  101  according to the first embodiment includes a substrate  111 , a light absorbing layer  401 , a light scattering layer  121 , and an organic light emitting diode (OLED)  70 . In addition, the organic light emitting diode (OLED) display  101  further includes a thin film transistor (TFT)  10  driving the organic light emitting diode  70 , at least one insulating layer  468  disposed between the organic light emitting diode  70  and the substrate  111 , a pixel defining layer  190  classifying the organic light emitting diode  70  by the pixel unit, and a sealing member  210  facing the substrate  111  and covering the organic light emitting diode  70 . Herein, the pixel is a minimum unit which the organic light emitting diode (OLED) display  101  displays an image. 
         [0031]    The substrate  111  may be formed by a transparent insulating substrate made of glass, quartz, ceramic, etc., or a transparent flexible substrate made of plastic and the like. 
         [0032]    The organic light emitting diode  70  includes a first electrode  710 , an organic emission layer  720 , and a second electrode  730 . The first electrode  710  is an anode which is a hole injection electrode and the second electrode  730  is a cathode which is an electron injection electrode. However, the first embodiment is not limited thereto. That is, the first electrode  710  may be the cathode and the second electrode  730  may be the anode. 
         [0033]    In the first embodiment, the organic light emitting diode (OLED) display  101  has a bottom emission type structure. That is, light emitted from the organic light emitting diode  70  transmits the substrate  111  and is emitted to the outside, thereby displaying the image. 
         [0034]    Accordingly, the first electrode  710  of the organic light emitting diode  70  is formed by a transparent electrode or a semi-transmissive electrode. Further, the second electrode  720  of the organic light emitting diode  70  is formed by a reflective electrode. 
         [0035]    The transparent electrode includes at least one of Indium Tin Oxide (ITO), Indium Zinc Oxide (IZO), Zinc Indium Tin Oxide (ZITO), Gallium Indium Tin Oxide (GITO), Indium Oxide (In 2 O 3 ), Zinc Oxide (ZnO), Gallium Indium Zinc Oxide (GIZO), Gallium Zinc Oxide (GZO), Fluorine Tin Oxide (FTO), and Aluminum-Doped Zinc Oxide (AZO). 
         [0036]    The reflective electrode and the semi-transmissive electrode may be made of metal. In this case, the reflective electrode and the semi-transmissive electrode are discriminated from each other depending on a thickness. In general, the semi-transmissive electrode has a thickness in the range of about 5 nm to about 100 nm and the reflective electrode has a thickness relatively larger than the semi-transmissive electrode. According to the thickness of the semi-transmissive electrode, transmittance and reflectance of light vary. In one embodiment, as the thickness of the semi-transmissive electrode becomes smaller, the transmittance of light increases and as the thickness of the semi-transmissive electrode becomes larger, the transmittance of light decreases. Transmittance of light according to the thickness may be different for each metal. The above range (about 5 nm to about 100 nm) may provide an optimum balance between the transmittance of light and the electric characteristic of the semi-transmissive electrode. For example, when the thickness of the semi-transmissive electrode is less than or equal to about 100 nm, the transmittance of light is enhanced. Also, when the thickness of the semi-transmissive electrode is greater than or equal to about 5 nm, an electric characteristic is not substantially deteriorated. However, depending on the embodiment, the thickness of the semi-transmissive electrode may be greater than about 100 nm or less than about 5 nm. 
         [0037]    The organic emission layer  720  may be formed of multiple layers including a light emitting layer, a hole-injection layer (HIL), a hole-transporting layer (HTL), an electron-transporting layer (ETL), and an electron-injection layer (EIL). Among above-mentioned layers, the rest layers other than the light emitting layer may be omitted depending on the embodiment. In the case where the organic emission layer  720  includes all the above-mentioned layers, the hole-injection layer is disposed on the first electrode  710  which is the hole injection electrode and the hole-transporting layer, the electron-transporting layer, and the electron-injection layer are sequentially stacked thereon. In addition, the organic emission layer  720  may further include other layers as necessary. 
         [0038]    Further, when the first electrode  710  is formed by the semi-transmissive electrode and the second electrode  730  is formed by the reflective electrode, the organic light emitting diode (OLED) display  101  can improve use efficiency of light, that is, luminance by using a microcavity effect. The microcavity effect may be maximized by adjusting the distance between the first electrode  710  and the second electrode  730  of the organic light emitting diode  70 . In addition, in order to maximize the microcavity effect, the distance between the first electrode  710  and the second electrode  730  varies depending on colors of light emitted by the organic light emitting diode  70 . The distance between the first electrode  710  and the second electrode  730  for maximizing the microcavity effect is relatively largest in the organic light emitting diode  70  emitting red-based light and smallest in the organic light emitting diode  70  emitting blue-based light 
         [0039]    Accordingly, when cavity layers having different thicknesses are disposed between the first electrode  710  and the second electrode  730  depending on colors of emitting light, it is possible to effectively improve luminance efficiency to power. That is, the cavity layer having the relatively largest thickness is disposed in the organic light emitting diode  70  emitting red-based light and the cavity layer having the relatively smallest thickness is disposed or the cavity layer may be omitted in the organic light emitting diode  70  emitting red-based light or the cavity layer emitted. 
         [0040]    The cavity layer may be separately formed between the first electrode  710  and the second electrode  730  and formed by thickening one or more layers of the hole-injection layer (HIL), the hole-transporting layer (HTL), the electron-transporting layer (ETL), and the electron-injection layer (EIL) included in the organic emission layer  720 . 
         [0041]    In the organic light emitting diode (OLED) display  101  according to the first embodiment, as shown in  FIG. 1 , the organic light emitting diode  70  emitting red-based light has the relatively largest thickness and the organic light emitting diode  70  emitting blue-based light has the relatively smallest thickness. 
         [0042]    The pixel defining layer  190  has an opening exposing at least a part of the first electrode  710 . The organic emission layer  720  emits light in the opening of the pixel defining layer  190 . That is, the opening of the pixel defining layer  190  defines a light emitting region actually generating light. 
         [0043]    The sealing member  210  is attached and sealed to the substrate  111  by a sealant (not shown) disposed around the edge. The sealing member  210  may be formed of a transparent insulating substrate made of glass, quartz, ceramic, plastic, etc. or a metallic material. 
         [0044]    However, the sealing member  210  is not limited to the first embodiment. Therefore, the sealing member  210  may be formed by an encapsulation thin-film structure in which at least one of a transparent insulating organic layer and a transparent insulating inorganic layer is stacked. 
         [0045]    As shown in  FIG. 2 , the thin film transistor  10  includes a semiconductor layer  133 , a gate electrode  153 , a source electrode  135 , and a drain electrode  137 . The drain electrode  137  of the thin film transistor  10  may be electrically connected with the first electrode  710  of the organic light emitting diode  70 . 
         [0046]    In addition, the thin film transistor  10  in  FIG. 2  includes a polycrystalline semiconductor layer and has a top gate structure in which the gate electrode  153  is formed on the semiconductor layer  133 . However, the first embodiment is not limited thereto. For example, the thin film transistor  10  may be modified in various structures known to those skilled in the art. 
         [0047]    Further, a plurality of insulating layers  468  such as a gate insulating layer  140 , an interlayer insulating layer  160 , and a planarization layer  180  are disposed between the substrate  111  and the organic light emitting diode  70 . The insulating layers  468  may be made of various inorganic materials or organic materials known to those skilled in the art. 
         [0048]    The light scattering layer  121  is disposed between the substrate  111  and the organic light emitting diode  70 . In  FIGS. 1 and 2 , the light scattering layer  121  is disposed between the thin film transistor and the substrate  111 , but the first embodiment is not limited thereto. That is, the light scattering layer  121  may be any one of the plurality of insulating layers  468 . 
         [0049]    Meanwhile, the light scattering layer  121  may act as a buffer layer at the same time. The buffer layer prevents moisture or impurities generated from the substrate  111  from diffused and infiltrated, flattens the surface, and helps crystallization by controlling a transferring speed of heat in a crystallizing process for forming the semiconductor layer  133 . 
         [0050]    In the first embodiment, the light scattering layer  121  contains a scattering material scattering light. As the scattering material, various materials known to those skilled in the art may be used. 
         [0051]    In the first embodiment, the light absorbing layer  401  is disposed on an opposite surface to a surface of the substrate facing the organic light emitting diode  70 , that is, an outer surface of the substrate  111 . 
         [0052]    The light absorbing layer  401  includes at least one light absorbing material of a black inorganic material, a black organic material, and metal. For example, the light absorbing material may be carbon black, polyene-based pigment, azo-based pigment, azomethine-based pigment, diimmonium-based pigment, phthalocyanine-based pigment, quinone-based pigment, indigo-based pigment, thioindigo-based pigment, dioxadin-based pigment, quinacridone-based pigment, isoindolinone-based pigment, metal oxide, metal complex, and aromatic hydrocarbons, and the like. 
         [0053]    In addition, the light absorbing layer  401  has relatively higher transmittance for blue light than for red light or green light by about 10% to about 30%. Accordingly, luminance of blue light having relatively lower luminous efficiency may be improved. 
         [0054]    In addition, the outer surface of the light absorbing layer  401 , that is, the opposite surface to the surface facing the substrate  111  may be subjected to anti-reflection coating or low refraction coating. When the light absorbing layer  401  is subjected to the anti-reflection coating or low refraction coating, reflection of light may be effectively suppressed. 
         [0055]    According to the configurations described above, the organic light emitting diode (OLED) display  101  according to the first embodiment can improve a contrast characteristic by suppressing reflection by external light. 
         [0056]    Hereinafter, a second embodiment will be described with reference to  FIG. 3 . 
         [0057]    As shown in  FIG. 3 , the organic light emitting diode (OLED) display  102  according to the second embodiment includes a plurality of light scattering layers  121  and  122 . For example, the organic light emitting diode (OLED) display  102  may include a first light scattering layer  121  and a second light scattering layer  122 . In  FIG. 3 , the organic light emitting diode (OLED) display  102  includes two light scattering layers  121  and  122 , but the second embodiment is not limited thereto. 
         [0058]    At least one of the first light scattering layer  121  and the second light scattering layer  122  may be one of several insulating layers  468  disposed between the substrate  111  and the organic light emitting diode  70  and disposed between the substrate  111  and the thin film transistor  10 . 
         [0059]    According to the configurations described above, the organic light emitting diode (OLED) display  102  according to the second embodiment can also improve a contrast characteristic by suppressing reflection by external light. 
         [0060]    Hereinafter, a third embodiment will be described with reference to  FIG. 4 . 
         [0061]    As shown in  FIG. 4 , the organic light emitting diode (OLED) display  103  according to the third embodiment includes a light absorbing layer  402  disposed between the substrate  111  and the light scattering layer  121 . 
         [0062]    As such, although the light absorbing layer  402  is formed in a sealed space between the substrate  111  and the sealing member  210 , it is possible to improve a contrast characteristic by suppressing reflection by external light. 
         [0063]    Hereinafter, a fourth embodiment will be described with reference to  FIG. 5 . 
         [0064]    As shown in  FIG. 5 , the organic light emitting diode (OLED) display  104  according to the fourth embodiment includes a light scattering layer  125  adjacent to a substrate  115 . In addition, an interface where the light scattering layer  125  and the substrate  115  are in contact with each other is formed in an uneven structure. That is, protrusions  1159  and  1259  are formed on one surface of the light scattering layer  125  which is in contact with the substrate  115  and one surface of the substrate  115  which is in contact with the light scattering layer  125 , respectively. In this case, the light scattering layer  125  has a relatively higher refractive index than the substrate  115 . As such, light is scattered on the interface having the uneven structure where the light scattering layer  125  and the substrate  115  are in contact with each other. In addition, light may further be scattered by a difference in refractive index between the light scattering layer  125  and the substrate  115 . The uneven structure is not limited to the structure shown in  FIG. 5  and may be modified into various structures capable of scattering light. 
         [0065]    Meanwhile, in the fourth embodiment, the light scattering layer  125  may include a scattering material as well as the uneven structure 
         [0066]    However, the fourth embodiment is not limited thereto. Accordingly, in the fourth embodiment, the light scattering layer  125  may not include the scattering material. 
         [0067]    According to the configurations described above, the organic light emitting diode (OLED) display  104  according to the fourth embodiment can also improve a contract characteristic by effectively suppressing reflection by external light. 
         [0068]    Hereinafter, a fifth embodiment will be described with reference to  FIG. 6 . 
         [0069]    As shown in  FIG. 6 , the organic light emitting diode (OLED) display  105  according to the fifth embodiment includes a light absorbing layer  403  disposed on an opposite surface to a surface of the substrate  111  facing the organic light emitting diode  70 , that is, an outer surface of the substrate  111 . In addition, an outer surface of the light absorbing layer  403  is formed by a moth-eye structure  4038 . Accordingly, the light absorbing layer  403  also suppresses reflection of light. 
         [0070]    According to the configurations described above, the organic light emitting diode (OLED) display  105  according to the fifth embodiment further effectively suppresses reflection by external light, thereby improving contrast characteristic. 
         [0071]    Hereinafter, referring to  FIG. 7 , Experimental Example according to the first embodiment is compared with Comparative Examples as described below. 
         [0072]    Experimental Example includes both the light scattering layer  121  and the light absorbing layer  401  formed according to the first embodiment. In contrast, Comparative Example 1 does not include the light scattering layer  121  and Comparative Example 2 does not include both the light scattering layer  121  and the light absorbing layer  401  as compared with Experimental Example. 
         [0073]    As shown in  FIG. 7 , Experimental Example has relatively higher transmittance than Comparative Example 1 and Comparative Example 2. That is, Experimental Example effectively suppresses reflection of light as compared with Comparative Example 1 and Comparative Example 2. 
         [0074]    According to at least one of the disclosed embodiments, an organic light emitting diode (OLED) display can improve a contrast characteristic by suppressing reflection of external light. 
         [0075]    While this disclosure has been described in connection with certain embodiments, it is to be understood that the disclosed embodiments are not considered limiting. Accordingly, various modifications and equivalent arrangements are included within the spirit and scope of the appended claims.