Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-06-26
2004-08-03
Vu, David (Department: 2821)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000, C349S138000, C257S059000
Reexamination Certificate
active
06771328
ABSTRACT:
This application claims the benefit of Korean Patent Application Nos. 2001-44928 filed on Jul. 25, 2001, and 2001-61982 filed on Oct. 9, 2001, which are hereby incorporated by reference in their entirety as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to organic electroluminescent devices, and more particularly, to an active matrix electroluminescent device.
2. Discussion of the Related Art
Flat panel display (FPD) devices having small size, lightweight, and low power consumption may have been a subject of recent research in the coming of the information age. FPD devices may be classified into two types depending on whether the device emits or receives light. One type is a light-emitting type display device that emits light to display images, and the other type is a light-receiving type display device that uses an external light source to display images. Plasma display panels (PDPs), field emission display (FED) devices, and electroluminescent (EL) devices are examples of the light-emitting type display devices. Liquid crystal display (LCD) devices are examples of the light-receiving type display device.
Among many kinds of FPD devices, LCD devices are widely used because of their excellent characteristics of resolution, color display and display quality. However, since the LCD device is the light-receiving type display device, it has some disadvantages such as poor contrast ratio, narrow viewing angle, and difficulty in enlarging its size. Therefore, new types of FPD need to be researched and developed to overcome the aforementioned disadvantages.
Recently, organic EL devices have been of the most interest in research and development because they are light-emitting type displays having a wide viewing angle and a good contrast ratio as compared with the LCD devices. Since a backlight is not necessary, the organic EL device can be light weight and thin. Furthermore, the organic EL device has low power consumption. When driving the organic EL device, a low voltage of direct current (DC) can be used, and a rapid response speed can be obtained. As widely known, since the organic EL device is totally solid phase, unlike the LCD device, it is sufficiently strong to withstand external impacts and has a greater operational temperature range. Additionally, the organic EL device can be manufactured at a low cost; particularly since a manufacturing process of the EL device is very simple in contrast with a LCD device or a PDP, only deposition and encapsulation apparatuses are necessary for manufacturing the organic EL device.
As an operating method for the organic EL device, a passive matrix operating method not using additional thin film transistors (TFTs) is conventionally utilized. However, since the passive matrix organic EL device has many limitations in resolution, power consumption and life time, an active matrix organic EL device has been researched and developed as a next generation display device requiring high resolution and large display area. In the passive matrix organic EL device, scanning lines and signal lines perpendicularly cross each other to be arranged in a matrix shape. In the active matrix organic EL device, a TFT is disposed at each pixel as a switch turning on/off a first electrode connected to the TFT and a second electrode facing the first electrode is commonly used.
A scanning voltage is sequentially applied to the scanning lines to operate each pixel in the passive organic EL device. To obtain the required average brightness, an instantaneous brightness of each pixel during the selection period should reach a value resulting from multiplying the average brightness by the number of scanning lines. Accordingly, as the number of the scanning lines increases, applied voltage and current increases. Therefore, the passive matrix organic EL device is not adequate to a display of high resolution and large area because the device is easily deteriorated and the power consumption is high.
However, in the active matrix organic EL device, a voltage applied to the pixel is stored in a storage capacitor, thereby maintaining the voltage and driving the device until a voltage of next frame is applied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an active matrix organic EL device of low power consumption, high resolution and large area may be made.
FIG. 1
is an equivalent circuit diagram showing a basic pixel structure of a conventional active matrix organic EL device.
In
FIG. 1
, a scanning line
10
is arranged in a first direction, and signal line
20
and a power line
30
are arranged in a second direction perpendicular to the first direction, thereby defining a pixel region. The signal line
20
and the power line
30
are spaced apart form each other. A switching TFT “T
S
,” an addressing element, is connected to the scanning line
10
and the signal line
20
, and a storage capacitor “C
ST
” is connected to the switching TFT “T
S
” and the power line
30
. A driving TFT “T
D
,” a current source element, is connected to the storage capacitor “C
ST
” and the power line
30
, and an organic EL diode “D
EL
” is connected to the driving TFT “T
D
.”
The organic EL diode “D
EL
” has a double-layer structure of organic thin films between an anode and a cathode. When a forward current is applied to the organic EL diode “D
EL
,” an electron and a hole are recombined to generate a electron-hole pair through the P(positive)-N(negative) junction between the anode providing the hole and the cathode providing the electron. The electron-hole pair has a lower energy than the separated electron and hole. Therefore, an energy difference occurs between the recombination and the separation of electron-hole pair, and light is emitted due to the energy difference. The switching TFT “T
S
” adjusts the forward current through the driving TFT “T
D
” and stores charges in the storage capacitor “C
ST
”
A principle of driving the active matrix organic EL device will be illustrated.
When a voltage is applied to a selected scanning line
10
according to a selection signal, a gate electrode of the switching TFT “T
S
” is turned on and a data signal of the signal line
20
passes the switching TFT “T
S
.” The data signal is applied to the driving TFT “T
D
” and the storage capacitor “C
ST
.” Accordingly, a gate electrode of the driving TFT “T
D
” is turned on and a current of the power line
30
is supplied to organic EL diode “D
EL
” through the driving TFT “T
D
.” As a result, light is emitted. An open degree of the gate electrode of the driving TFT “T
D
” depends on the data signal so that a gray level can be obtained by adjusting a current flowing through the driving TFT “T
D
.” Moreover, since the data signal stored in the storage capacitor “C
ST
” is applied to the driving TFT “T
D
” during a non-selected period, light is continuously emitted at the organic EL diode “D
EL
” until a voltage of the next frame is applied to the scanning line.
Accordingly, the active matrix organic EL device uses a lower voltage and a lower instantaneous current in contrast with the passive matrix organic EL device. Since the organic EL diode is continuously driven during one frame regardless of the number of the scanning lines, the active matrix organic EL device is adequate for low power consumption, high resolution, large area.
In the active matrix organic EL device, a current flows through a TFT. Therefore, a poly silicon (p-Si) TFT of high field effect mobility is preferable to a conventional amorphous silicon (a-Si) TFT of a low field effect mobility. Since the p-Si TFT has high field effect mobility, a driving circuit may be formed on a substrate with the p-Si TFT so that a cost for driving integrated circuit (IC) is reduced and fabrication is simplified. A low temperature crystallization method through a laser annealing of a-Si is widely used as a fabricating method of p-Si.
FIG. 2
is a schematic cross-sectional view of a conventional active matrix or
Kim Sung-Ki
Park Jae-Yong
Park Yong-In
Yoo Juhn-Suk
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
Vu David
LandOfFree
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