BACKGROUNDS OF THE INVENTION In general, the OLED is one of representative flat panel displays along with a liquid crystal display (LCD), a plasma display panel (PDP) and a field emission display (FED). As shown in Fig. 1, the OLED includes the first electrode 12, at least one organic light-emitting layer 14 formed on the first electrode 12, and the second electrode 16 formed on the light-emitting layer 14 while facing the first electrode 12. Conventionally, the first electrode 12 is made of materials having a high work function, for example, Indium Tin Oxide, polyaniline and, Ag, and the second electrode 16 is made of materials having a low work function (generally, less than 4eV), for example, Al, Mg-Ag, Li, and Ca. The organic light-emitting layer 14 is composed of an organic luminescent single compound or a conjugated polymer. For facilitating hole injection and transportation, a hole transporting layer 22 can be provided between the first electrode 12 and the light-emitting layer 14. Also, an electron transporting layer 26 is generally provided between the second electrode 16 and the light-emitting layer 14 for electron injection and transportation.

In operation, the hole and the electron are produced at the first electrode 12 and the second electrode 16 by applying a voltage. The produced hole and the electron are injected into the light-emitting layer 14 via the hole transporting layer 22 and the electron transporting layer 26, respectively. The injected hole and the electron are recombined in the light-emitting layer 14, which induces light radiation, and the radiated light is displayed through the first electrode 12 and a substrate 10 made of optically transparent material. As aforementioned, the OLED is a self light-emitting device, and it has many advantages including a high response speed, a wide viewing angle and a low driving voltage of about 4 V.

However, the cathode 16 of the OLED is made of a metal having a light reflective smooth surface, and the display images produced in the OLED is generally observed through the transparent the anode 12 and substrate 10. Thus, the light from the outside of the OLED can be reflected at the surface of the cathode 16, which deteriorates the contrast ratio of the display images of the OLED.

To prevent this drawback, an anti-reflective film 30 as shown in Fig. 2 is developed. The anti-reflective film 30 includes a linear polarization film 32 and a (R/4) film 34 attached at a surface of the linear polarization film 32. The linear polarization film 32 is for linearly-polarizing a light, and the (A/4) film 34 is for rotating the polarization direction of the linearly-polarized light by 90 degrees.

Also, a protective film 38 can be formed on the other surface of the linear polarization film 32, and the (R/4) film 34 is bonded to the substrate 10 of the OLED with an adhesive film 36.

In operation, the non-polarized light from outside is linearly-polarized in a certain direction when passing the linear polarization film 32, and then the polarization direction of the linearly-polarized light is rotated by 90 degree at (R/4) film 34. The 90 degree-rotated light is then reflected at the surface of the cathode 16, which changes the polarization direction mirror-symmetrically. The reflected light passes through the (? t/4) film 34, and the polarization direction of the reflected light becomes perpendicular to that of the original linearly-polarized light.

Thus, the original linearly-polarized light and the reflected light are offset and eliminated, which prevent the deterioration of the contrast ratio. However, the anti-reflective film 30 increases the cost of OLED, and additional step of bonding the anti-reflective film 30 to the substrate 10 OLED is required for producing the OLED.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an anti-reflective OLED for improving the contrast ratio of an image displayed on the OLED.

It is other object of the present invention to provide an anti-reflective OLED for preventing a reflection of external light on a cathode thereof.

It is another object of the present invention to provide an anti-reflective OLED having a simple structure and not requiring a conventional anti-reflective film.

To accomplish the above objects, the present invention provides an anti- reflective OLED comprising a first electrode formed on a transparent substrate, at

least one organic layer including an organic light-emitting layer, a second electrode and a light-absorbing material. The at least one organic layer is interposed between the first electrode and the second electrode, and includes an electron injecting layer and/or an electron transporting layer between the organic light-emitting layer and the second electrode. The light-absorbing material is disposed in at least one organic layer, or on a boundary of the organic layer.

Preferably, the light-absorbing material is disposed in the electron injecting layer or the electron transporting layer in the amount of about 0.1 to 50 weight %.

Alternatively, the light-absorbing material can form a layer of thickness of about 1 to 500A on the boundary of the organic layer. Examples of the light-absorbing material include a carbon black, an iron oxide, a black dye, a black pigment and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference numerals indicate the same or the similar components, wherein: Fig. 1 is a cross sectional view of a conventional OLED; Fig. 2 is a cross sectional view of a conventional anti-reflective film for preventing the light reflection on OLED; and Fig. 3 is a cross sectional view of an OLED according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION As shown in Fig. 3, the OLED according to an embodiment of the present invention includes the first electrode 12 formed on a transparent substrate 10, a hole injecting layer 21, and a hole transporting layer 22 for injecting and transporting holes into an organic light-emitting layer 14. On the hole transporting layer 22, the organic light-emitting layer 14, an electron transporting layer 26, an electron injecting layer 25 and the second electrode 16 are successively formed.

The OLED according to the embodiment of the present invention further includes a light-absorbing material 40 which is disposed in at least one organic layer between the first electrode 12 and the second electrode 16, for example, in the electron injecting layer 25, in the electron transporting layer 26 or in the organic light-emitting layer 14. In OLED according to the present invention, the light from outside of the OLED is absorbed or blocked by the light-absorbing material 40, and the external light cannot reach to or reflect at the second electrode 16, which enhances the contrast ratio of a display image of the OLED.

Alternatively, the light-absorbing material 40 can form a layer on a boundary of the organic layer, for example, between the electron injecting layer 25 and the electron transporting layer 26, and between the electron transporting layer 26 and the organic light-emitting layer 14.

The light-absorbing material 40 can be an organic material or an inorganic material, and various materials having the light absorption property can be widely used as the light-absorbing material 40. The non-limiting examples of the light- absorbing material 40 includes a carbon black, a iron oxide, a black dye, or a black

pigment such as Fe304, Fe 20 3Mn 20 3, and more preferable material is carbon black or the iron oxide. If properly choosing the light-absorbing material 40 with considering the potentials of the organic layers, the light-absorbing material 40 can also work as the electron injecting layer 25, the electron transporting layer 26 or a hole blocking layer.

The layer including the light-absorbing material 40 can be formed by various conventional film casting methods, for example, by spin-coating or spin- casting the mixture of the light-absorbing material 40 and the materials for forming the organic layer such as electron injecting layer 25 and the electron transporting layer 26. Alternatively, the mixture of the light-absorbing material 40 and the materials for forming the electron injecting layer 25, or the electron transporting layer 26 can be co-deposited by a thermal evaporation, a sputtering or a chemical vapor deposition to form the organic layer including the light-absorbing material 40.

When the light-absorbing material 40 forms a light-absorbing layer on the boundary of the organic layer, the light-absorbing layer can be formed by various conventional film casting methods, for example, spin-coating, spin-casting and sputtering.

The amount of the light-absorbing material 40, which is disposed or doped in the electron injecting layer 25 or the electron transporting layer 26, is preferably about 0.1 to 50 weight%, and more preferably, about 1 to 10 weight% on the basis of the total weight of the organic layer including the light-absorbing material 40.

Meanwhile, if the light-absorbing material 40 forms a separate light-absorbing

layer by deposition, the thickness of the light-absorbing layer is about 1 to 500A, preferably about 5 to 100A. If the amount of the light-absorbing material 40 is less than 1 weight% or the thickness of the light-absorbing layer is less than 1 A, the reflection of external light may not be sufficiently prevented. If the amount of the light-absorbing material 40 is more than 10 weight% or the thickness of the light-absorbing layer is more than 500A, the transportation of electron or hole can be restricted, and the luminescent efficiency of the OLED can be deteriorated.

The organic light-emitting layer 14 can be made of various conventional chemical compound for manufacturing the OLED. For example, an organic luminescent single compound, oligomer or polymer can be used to form the organic light-emitting layer 14. Examples of such organic luminescent compound includes tris (8-quinolinolato) aluminum (Alq3), 10-benzo [h] quinolinol-beryllium complex (BeBq2) or tris (4-methyl-8-quinolinolate) aluminum (Almq), which emits green light (540-550 nm). Examples of the blue luminescent single compound include a metal complex such as Balq (Bis (2-methyl-8-quinolinolato) (para-phenyl- phenolato) aluminum) or an organic compound such as strylarylene-based derivatives DPVBi (1,4-bis (2, 2'-diphenyl-vinyl) biphenyl), oxadiazole-based derivatives, bisstrylanthrancene-based derivatives, bisstrylanthracene-based derivatives such as BczVBi (4,4'-Bis ((2-carbazole) vinylene) biphenyl), or a-NPD (4,4'bis [N- (1-napthyl-N-phenyl-amino) biphenyl]). Examples of the red luminescent organic compound include [2-methyl-6- [2- (2, 3,6, 7-tetrahydro-1H, 5H- benzoM] quinolizin-9-yl) ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2). To enhance the luminance and the life-time of the OLED, a dopant having high

luminescent efficiency can also be added to the organic light-emitting layer 14. In addition, as conventionally known, the organic light-emitting layer 14 can be formed with a luminescent polymer, such as PPP (poly (p-phenylenylene) and PPV (poly (phenylene vinylene).

If necessary, the hole injection layer 21 and the hole transporting layer 22 can be formed to facilitate the injection and transportation of holes and to block electrons. Non-limitedly, tri (phenyidiamine) derivatives, strylamine derivatives or amine derivatives having fused aromatic ring can be used to form the hole injecting layer 21 and the hole transporting layer 22. Also, 4,4', 4"-tris [3- methylphenyl (phenyl) amino] triphenylamine (m-MTDATA) or copper pthalocyanine (CuPc) can be used to form the hole injection layer 21, and N-N'-diphenyl-N- N'bis (3-methylphenyl)- [1-1'-biphenyl]-4-4'-diamine (TPD) or a-NPD (4,4'bis [N- (1- napthyl-N-phenyl-amino) biphenyl]) can be used to form the hole transport layer 22.

The electron injecting layer 25 and the electron transporting layer 26 is to inject or transport electrons to the organic light-emitting layer 14 from the cathode 16, and quinoline derivatives such as Alq3 can be conventionally used to form the electron injecting and transporting layer 25,26. The thickness of the light-emitting layer 14, the hole injection layer 21, the hole transporting layer 22, the electron injection layer 25 or the electron transporting layer 26 can be varied according to the use of OLED, materials for forming the layers, and manufacturing method, but are generally 5-500 nm.

The first electrode 12 can be conventionally formed with ITO, polyaniline or Ag, which has a high work function, and the second electrode 16 for producing

electrons can be conventionally formed with Al, Mg, Li, Ca, or alloy thereof, which has a low work function. The substrate 10 is also well-known in the art, and can be made of transparent material such as glass, flexible polymer film or semiconductor such as silicon or gallium arsenide. As described above, in the present invention, the light-absorbing material is doped in at least one layer between the anode and the cathode of OLED, which prevents or minimizes the external light reflection at the cathode, and improves the contrast ratio of the OLED. The OLED according to the present invention has simple structure and can be produced with low cost compared with the conventional OLED having the anti-reflective film.

While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.