1. Field of the Invention
The present invention relates to a display device, and more particularly, to an organic electroluminescent (EL) device and a method of manufacturing the same, wherein the picture quality of an organic electroluminescent display (OELD) can be improved.
2. Description of the Background Art
Recently, as the size of display devices increases, there is an increasing need for flat panel display devices that occupy a smaller area. Electroluminescent (EL) devices have come into the spotlight as one of these flat panel display devices.
The electroluminescent devices can be mainly classified into inorganic electroluminescent devices and organic electroluminescent devices depending on the type of materials used.
The inorganic electroluminescent devices are adapted to emit light in such a manner that a high electric field is applied to a light-emitting unit, and electrons are accelerated within the high electric field in such a way to collide toward the center of emission, thereby exciting the center of emission.
Further, the organic electroluminescent devices are adapted to emit light when exciton, which is generated through combination of electrons and holes that are injected from an electron injection node (cathode) and a hole injection node (anode) into the light-emitting unit, falls from an excited state to a ground state.
The inorganic electroluminescent devices with the above-described operating principle require a high electric field. Thus, a high voltage of 100 to 200V is required as a driving voltage. On the contrary, the organic electroluminescent devices are advantageous in that they can be driven at a low voltage of about 5 to 20V. Thus, research on the organic electroluminescent devices has been actively performed.
In addition, the organic electroluminescent devices have excellent characteristics such as a wide viewing angle, high response rates and high contrast, and they can be thus used for graphic display pixels, pixels of a television image display surface light source and so on. Also, the organic electroluminescent devices are suitable for a next-generation flat panel display device since they are light and have a good color sense.
The organic electroluminescent devices are usually called organic electroluminescent devices. This will be below described in more detail.
Organic EL refers to a display mode in which characters, images, etc. are displayed using an organic luminescent device that emits light by itself if being applied with current. In the concrete, organic EL refers to a condition in which light is generated through a process where electrons and holes form electron and hole pairs within semiconductors, or carriers are excited to a higher energy state and are then stabilized.
The response speed of the organic electroluminescent device is ten thousand times faster than that of thin film transistor liquid crystal display (TFT-LCD) devices. Thus, the organic electroluminescent device has an advantage in that it can produce a relatively stable motion picture without blurring. Also, the material of the organic electroluminescent device, which constitutes the screen, is able to emit light by itself. Thus, the organic electroluminescent device does not require a backlight as in LCD product groups. Accordingly, the organic electroluminescent device has advantages in that the life span of a battery can be increased since less power is needed, and a thickness of the panel can be reduced commensurate with the thickness of a backlight which would otherwise have to be used. The organic EL display panel is disadvantageous in implementing a large-size screen, but is advantageous in resolution, power consumption, etc., compared with a PDP.
The organic electroluminescent device has been consistently improved in terms of technology. It is thus expected that the organic electroluminescent device will be in its place as next-generation display devices.
The organic EL display can be classified into a passive matrix (PM) type and an active matrix (AM) type depending on its driving mode.
The PM type is a structure in which a driving circuit is provided outside an organic EL panel to light-emit an organic electroluminescent device. This type is advantageous in that the structure of the panel is simple and the cost is low because the driving circuit is located outside the panel. However, this PM type has problems in that the current flowing through respective pixels must be the same so as to make uniform the brightness of the whole pixel, and power consumption is significantly greater since the charge/discharge current applied to a capacitive load is high.
Meanwhile, the AM type is a structure in which a driving circuit such as TFT is provided in each pixel. Thus, this AM type has advantages in that power consumption is low and deviation in the brightness is small, compared with the PM type.
Meanwhile, the organic EL panel has reflectance of over 80%, which results in degraded contrast.
Further, since the cathode of a device is usually made of a metal having good reflectance, external light, which is incident to the device, is reflected from the surface of the cathode, and is then mixed with light generated from a light-emitting layer.
In order to solve this problem, a circular polarizing filter is formed at the bottom of a transparent substrate in most organic electroluminescent devices, thus reducing the reflection of the externally incident light from the cathode.
FIG. 1 is a cross-sectional view illustrating the construction of a conventional organic electroluminescent (EL) device.
The conventional organic electroluminescent device includes a transparent anode 102 formed on a transparent substrate 101, an organic EL layer 103 having a hole injection layer (HIL), a hole transport layer (HTL), an emitting layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL), the organic EL layer 103 being coated on the transparent anode 102 through a vacuum deposition method, and a metal cathode 104 of good reflectance, which is formed on the organic-EL layer 103.
In this structure, however, if outer atmosphere is bright, external light that is incident on the device is reflected by the metal electrode 104, and is then mixed with light that is generated from the emitting layer of the organic EL layer 103. Accordingly, contrast is significantly lowered.
In order to solve this problem, in the prior art, a circular polarizing filter 105 is disposed in front of the organic electroluminescent device so as to reduce the reflection of external light. That is, when external light is incident, half of the light is blocked by the circular polarizing filter 105 and the remaining half of the light is blocked when being reflected back from the cathode 104. It is thus possible to prohibit the lowering of contrast due to the external light.
Accordingly, a filter 105 for lowering reflectance and improving contrast is formed in front of the panel. The filter usually includes a circular polarizer (hereinafter, referred to as “circular polarizing filter”).
FIG. 2 is a conceptual view for explaining the structure of a conventional circular polarizing filter and the principle of blocking external light using such a filter.
A circular polarizing filter 210 has a structure in which a linear polarizer 201 and a λ/4 retarder 205 are overlapped. Natural external light is transformed into light that vibrates in a given direction while passing through the linear polarizer 201 of the circular polarizing filter 210. The light is then transformed into light that rotates in a spiral form while passing through the λ/4 retarder 205. In more detail, the λ/4 retarder 205 uses birefringence crystals. When light of a given wavelength, which is vertically incident to an incident surface, transmits the crystals, the λ/4 retarder 205 makes the light have a phase difference of 90 degrees. Since light reflected from the front of the panel is redirected through the circular polarizing filter 210 having this characteristic, reflectance can be lowered by approximately 30 to 50%.
The circular polarizing filter is effective in reducing the reflectance by some degree. However, the circular polarizing filter has an adverse effect of lowering transmissivity, and has a disadvantage in that the cost is high. Furthermore, the reduction in transmissivity results in lowered luminous efficiency of the organic EL display panel.