1. Field of the Invention
The present invention relates to a light extraction substrate and an organic light-emitting device having the same, and more particularly, to a light extraction substrate which can realize a superior light extraction efficiency when applied to an organic light-emitting device, and an organic light-emitting device having the same.
2. Description of Related Art
In general, an organic light-emitting diode (OLED) includes an anode, a light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into a hole injection layer and then migrate from the hole injection layer through a hole transport layer to the organic light-emitting layer, and electrons are injected from the cathode into an electron injection layer and then migrate from the electron injection layer through an electron transport layer to the light-emitting layer. Holes and electrons that are injected into the light-emitting layer recombine with each other in the light-emitting layer, thereby generating excitons. When the excitons transit from an excited state to a ground state, light is emitted.
Organic light-emitting displays including an OLED are divided into a passive matrix type and an active matrix type depending on the mechanism that drives the N*M number of pixels which are arranged in the shape of a matrix.
In an active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least two thin-film transistors (TFTs) and one capacitor. Due to this configuration, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform luminance. The active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays.
However, as shown in FIG. 5, only about 20% of light generated from an OLED is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate 10 and an organic light-emitting layer 30 which includes an anode 20, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer and an electron injection layer and by a total internal reflection originating from the difference in the refractive index between the glass substrate 10 and the air. Specifically, the refractive index of the internal organic light-emitting layer 30 ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode 20 ranges from 1.8 to 1.9. Since the two layers have a very small thickness ranging from 200 to 400 nm and the refractive index of glass used for the glass substrate 10 is about 1.5, a planar waveguide is thereby formed inside the organic light-emitting device. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate 10 is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate 10, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate 10. Since the ratio of the trapped light is up to about 35%, only about 20% of the generated light is emitted to the outside.
In addition, as shown in FIG. 6, in order to overcome the foregoing problem, in the related art, a low index grid (LIP) 50 is formed on the ITO anode 20. The grid 50 converts the direction of the light that travels in the waveguide mode to the front surface, thereby improving light extraction efficiency.
FIG. 7 shows simulation results on the organic light-emitting device shown in FIG. 6. The effect of improving the light extraction efficiency is increased when the refractive index of the grid 50 is lower. However, there are problems in that almost no materials have a refractive index of 1.2 or less and that the price of a material is more expensive when the refractive index is lower. In addition, when the grid 50 is formed on the ITO anode 20, as shown in FIG. 6, a stepped portion is formed. Consequently, a leakage current may occur. In addition, the organic light-emitting device shown in FIG. 6 has the problem of difficult processing. For example, in some cases, the surface of the anode 20 which adjoins the organic light-emitting layer 30 is metamorphosed in the process of forming the grid 50 on the ITO anode 20, thereby changing the work function. Furthermore, holes are not injected into the organic light-emitting layer 30 through the portion of the anode 20 on which the grid 50 is formed, and the size of the electric field applied thereto is different from the surroundings, thereby decreasing the uniformity of the light generated.
In addition, as shown in FIG. 8, in the related art, a convex-concave structure 60 is disposed under the anode 20 (with respect to the paper surface), i.e. in the interface between the anode 20 and the glass substrate 10, in order to improve light extraction efficiency.
As described above, the anode 20 and the organic light-emitting layer 30 generally serve as one light waveguide between the cathode 40 and the glass substrate 10. Accordingly, in the state in which the anode 20 and the organic light-emitting layer 30 act in a waveguide mode, when the nanoscale convex-concave structure 60 which causes light scattering is formed on the surface that adjoins the anode 20, the waveguide mode is disturbed, so that the quantity of light that is extracted to the outside is increased. However, when the convex-concave structure 60 is formed below the anode 20, the shape of the anode 20 resembles the shape of the convex-concave structure 60 below the anode 20, thereby increasing the possibility that a sharp portion may be localized. Since the OLED has a stacked structure of very thin films, when the anode 20 has a sharp protruding portion, the current is concentrated in that portion, which is a cause of a large leakage current or decreases power efficiency. Accordingly, in order to prevent such deterioration of the electrical characteristics, a flat film 70 is necessarily added when the convex-concave structure 60 is formed below the anode 20. The flat film 70 is required to be made of a material that anode 20. When the refractive index of the flat film 70 is low, most of the light is reflected at the interface between the anode 20 and the flat film 70 and then is trapped between the anode 20 and the organic light-emitting layer 30, which is referred to as the waveguide mode. In addition, the flat film 70 is required to be as thin as possible. If the flat film 70 is too thick, more light may be unnecessarily absorbed, and the distance between the convex-concave structure 60 and the organic light-emitting layer 30 may become too large, thereby reducing the scattering effect.
The information disclosed in the Background of the Invention section is provided only for enhancement of (or better) understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.