KR100501314B1 - Flat panel display device - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix flat panel display, comprising green, red, and blue pixel regions, including a driving thin film transistor for driving the pixels having the same active channel length and width, and the driving thin film transistor. The number of grain boundaries of the polycrystalline silicon included in the active channel region of the pixel is different for each pixel, thereby providing a flat panel display device for driving the pixels without changing the width or size of the active channel region of the composition thin film transistor. It is possible to achieve white balance even with an active channel region of the same size without changing, and to obtain an appropriate luminance by supplying an appropriate current for each subpixel, and to prevent deterioration of life.

Description

Flat Panel Display {FLAT PANEL DISPLAY DEVICE}

[Industrial use]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix flat panel display, and more particularly, to an active matrix flat panel display having polysilicon as an active channel region and including thin film transistors having different directions for each subpixel. .

[Prior art]

Recently, an EL display device using a self-luminous EL element has attracted attention as a display device replacing CRTs and LCDs.

In addition, a display device having a TFT as a switching element for driving the EL element has also been researched and developed.

In such a flat panel display device, it is used as a switching element or a driving device of a pixel. An active matrix active matrix (AM) type organic light emitting display device includes at least two thin film transistors (sub-pixel) for each sub-pixel. TFT).

The organic electroluminescent device has a light emitting layer made of an organic material between an anode electrode and a cathode electrode. In this organic electroluminescent device, as the anode and cathode voltages are applied to these electrodes, holes injected from the anode are moved to the light emitting layer via the hole transport layer, and electrons are transferred from the cathode electrode via the electron transport layer to the light emitting layer. Electrons and holes recombine in this light emitting layer to generate excitons, and as the excitons change from the excited state to the ground state, the light emitting material of the light emitting layer emits light to form an image. In the case of a full color organic light emitting display device, as the organic light emitting device, a full color is realized by including pixels emitting three colors of red (R), green (G), and blue (B).

However, in the organic light emitting display device as described above, the light emission efficiency (Cd / A) of the red, green, and blue light emitting layers emitting the respective colors is different for each of the colors. In addition, since the luminance of the light emitting layer is approximately proportional to the current value applied to each subpixel, when the same current is applied, some colors have low luminance, and some colors have high luminance, so that the appropriate color balance or white balance is obtained. Difficult to obtain).

For example, since the luminous efficiency of the green light emitting layer is 3 to 6 times higher than that of the red light emitting layer and the blue light emitting layer, more current needs to flow through the red and blue light emitting layers to achieve white balance.

On the other hand, as a conventional method for adjusting the white balance, Japanese Patent Laid-Open No. Hei 5-107561 discloses a method of differently applying a voltage supplied through a drive line, that is, a Vdd value for each pixel.

In addition, Japanese Patent Laid-Open No. 2001-109399 discloses a method of adjusting the white balance by adjusting the size of the driving TFT. That is, when the channel width of the channel region of the driving TFT is W and the channel length is L, the W / L value is designed differently for each pixel of red, green, and blue, and each organic field of red, green, and blue is designed. The amount of current flowing through the light emitting device is controlled.

Japanese Laid-Open Patent Publication No. 2001-290441 discloses a method of matching white balance by forming each pixel in a different size. In other words, the white light emitting area of the green light emitting area having the highest luminous efficiency is formed smaller than the light emitting areas of the red and blue light emitting areas to achieve white balance and long life. This difference in light emitting area can be made possible as the area of the anode electrode.

In addition, a method of controlling luminance by controlling a current amount by varying a voltage range applied through a data line for each pixel of red, green, and blue is known.

By the way, the above-described method does not consider the crystal structure of the TFT of the flat panel display device using polycrystalline silicon. That is, when considering the crystal state of the polycrystalline silicon included in the TFT active channel region, the current mobility may vary according to the crystal state of the polycrystalline silicon, and in this case, a problem in which white balance cannot be achieved by the above methods may occur. Can be.

On the other hand, in the organic electroluminescent element, when the amount of current flowing through the organic electroluminescent element per subpixel exceeds the limit value, the luminance per unit area is greatly increased by the amount of current exceeding the limit value, and thus the life of the organic electroluminescent element is drastically reduced. Done. Therefore, there is a need to supply an optimum amount of current per subpixel for the lifetime of the device.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems described above, and an object of the present invention is to provide a flat panel display that can achieve white balance even under the same driving voltage without changing the size of the active channel of the driving TFT. To provide.

Further, another object of the present invention is to provide a flat panel display device which obtains an appropriate luminance by supplying an optimum current to each subpixel and does not shorten the lifespan.

The present invention to achieve the above object, the present invention

And a driving thin film transistor for driving pixel electrodes having the same channel length and width, wherein the number of primary grain boundaries of polycrystalline silicon included in the active channel region of the driving thin film transistor is different for each pixel. Provided is a display device.

Hereinafter, with reference to the accompanying drawings of the present invention will be described in more detail.

1 is a plan view illustrating a structure of an active region of a thin film transistor of an active matrix organic electroluminescent device according to a preferred embodiment of the flat panel display according to the present invention. As shown in FIG. 1, each pixel of the organic EL device is provided such that subpixels of red (R), green (G), and blue (B) are repeatedly arranged in the longitudinal direction (up and down direction in FIG. 1). . However, the configuration of the pixels is not necessarily limited thereto, and the subpixels of each color may be arranged in various patterns such as a mosaic or a grid to form the pixels.

In such an organic electroluminescent device, a plurality of gate lines 51 are disposed in the lateral direction (left and right directions in FIG. 1), and a plurality of data lines 52 are disposed in the longitudinal direction. In addition, a drive line 53 for supplying electric power is also arranged in the longitudinal direction. These gate lines 51, data lines 52, and drive lines 53 are provided to surround one subpixel.

Meanwhile, in the above configuration, each of the subpixels of the red (R), green (G), and blue (B) pixels includes at least two thin film transistors of the first thin film transistor and the second thin film transistor. The first TFT may be a switching TFT for controlling the operation of the device according to the signal of the gate line 51, and the second TFT may be a driving TFT for driving the device. Of course, the number and arrangement of the TFTs may vary depending on the characteristics of the display, the driving method, and the like, and the arrangement may also exist in various ways.

As described above, in the organic electroluminescent display, the red, green, and blue pixels differ in luminance due to the difference in the luminous efficiency of the light emitting layer, so that white balance can not be adjusted for the same current value. none. Table 1 shows the current values to be applied to each of the red, green, and blue sub-pixels in order to satisfy the efficiency and white balance of the red, green, and blue organic light emitting layers commonly used in the organic light emitting display. .

Red green blue Efficiency (Cd / A) 6.72 23.37 4.21 Display pixel current 0.276 0.079 0.230 Display pixel current ratio 3.5 One 2.9

As can be seen in Table 1 above, the current value that must flow in order to achieve the white balance is the smallest green sub-pixel, the next blue sub-pixel, the most red current should flow. .

In the present invention, polycrystalline silicon is used as the semiconductor layer for forming the transistor. Accordingly, in the present invention, the same driving voltage is achieved by including the number of primary grain boundaries differently in the size of the same active channel region for each of the red, green, and blue display pixels in the active channel region of the second TFT used as the driving thin film transistor. White balance can also be achieved.

2 is a view showing a structure in which a thin film transistor is disposed on polycrystalline silicon.

As can be seen in FIG. 2, the polycrystalline silicon thin film is a crystallized amorphous silicon thin film by a known sequential lateral solidification (SLS), but the above crystal structure is not necessarily limited to the crystal structure formed by the SLS method. If the crystal structure of the polycrystalline silicon thin film is the structure shown in Fig. 2, any crystallization method can be applied, and preferably, a crystallization method by laser can be used.

The SLS method takes advantage of the fact that the grains of silicon grow in a direction perpendicular to the interface at the interface between the liquid and solid phase, and transmit a laser beam using a mask to melt a portion of the amorphous silicon, and the molten silicon Crystals are formed by allowing crystal growth to occur from the boundary between the portion of and the portion of the unmelted silicon to the portion of the molten silicon.

As shown in FIG. 2, the crystal structure formed by the SLS method extends in a direction substantially perpendicular to the primary grain boundary between the plurality of primary grain boundaries spaced apart from each other by a predetermined interval and the primary grain boundary. Secondary grain boundaries. The primary grain boundary is formed in a direction perpendicular to the growth direction of the crystal grains and is formed when the crystal growth meets each other.

In other words, when the grains of polycrystalline silicon are large and ordered for improving TFT characteristics, grain boundaries occur between adjacent grains due to the finite size of the grains.

The term " grain size " in the present invention refers to the distance between grain boundaries that can be identified, and is generally defined as the distance between grain boundaries belonging to an error range.

The grain boundaries are known to act as traps for electric charge carriers. In particular, when the polycrystalline silicon is manufactured by SLS, the number of primary grain boundaries formed almost perpendicular to the grain growth direction can have a direct or indirect fatal effect on the TFT characteristics.

3 is a graph showing a change in current mobility according to the number of primary grain boundaries. Referring to FIG. 3, it can be seen that current mobility linearly decreases as the number of primary grain boundaries increases, whether in a PMOS transistor or an NMOS transistor.

That is, it can be seen that when the "primary" grain boundary is perpendicular to the current direction flowing from the source to the drain of the TFT, the "primary" grain boundary acts as a trap for the movement of the charge carriers.

In addition, the secondary grain boundary, which is generally perpendicular to the primary grain boundary and has a larger number than the primary grain boundary, also affects the current mobility.

4 is a graph showing a change in current mobility according to the number of secondary grain boundaries.

As can be seen in FIG. 4, the current grain mobility decreases as the number of secondary grain boundaries increases.

However, since the number of grain boundaries that can act as traps for current flow is more influenced by the primary grain boundaries and the secondary grain boundaries do not act as a trap, the current mobility characteristic is that the primary grain boundaries are the source. This is relatively better than parallel to the current flowing from to drain.

On the other hand, as discussed above, if charge carriers must move across a large number of grain boundaries (“secondary” grain boundaries) (ie, when the “primary” grain boundaries are parallel to the current direction), The current characteristics are poor due to the increase in the number of grain boundaries, which are traps, for the active channel, whereas the variation in positional variation in the active channel substrate is small (i.e., when the grain boundaries change from one to two and there are 100 grain boundaries). Variation in Variation from 102 to 102) Uniformity of TFTs can be secured.

That is, the "primary" grain boundary has a large variability due to the current movement according to the change of the number of grain boundaries, but the "secondary" grain boundary has a small variability due to the current movement with the change of the grain boundary number.

Therefore, in the present invention, the difference in the current value is determined by the number of primary grain boundaries or the number of secondary grain boundaries of the polycrystalline silicon included in the active channel region of the second TFT of FIG. 1, which is a driving thin film transistor that supplies current to the light emitting device. It is achieved by doing it differently. That is, by varying the number of primary grain boundaries included in the active channel region of the first TFT of each of the red, green, and blue subpixels, the current value supplied to the light emitting device of each subpixel, for example, the organic electroluminescent device, is changed. will be.

In other words, the number of primary grain boundaries included in the active channel of the second TFT is determined by the current value flowing through each subpixel at the same driving voltage. Therefore, in order to achieve the white balance, the primary grain boundary included in the active channel region of the second TFT of the green subpixels is minimized in the direction in which the current value of the green subpixels having the highest luminance is the lowest. Primary grain boundary included in each active channel region of the red second TFT, the blue second TFT, and the green second TFT in a direction in which the current value of each subpixel is lowered in the order of red, blue, and green subpixels. You must adjust the number of.

As a result, the luminance of each subpixel is compensated for, so that the white balance can be adjusted.

Thus, the number of primary grain boundaries included in the active channel region of the second TFT may also be determined by the charge mobility of the active channel region. This is because a greater amount of current in the active channel region may flow, while a smaller amount of current may flow in the channel region.

Therefore, in order to achieve white balance, the number of primary grain boundaries included in the active channel region of the second TFT of the green subpixel should be adjusted in the direction in which the charge mobility of the green subpixel having the highest luminous efficiency is lowest. Preferably, the number of primary grain boundaries included in the active channel region of the second TFT of each subpixel is adjusted in the order of increasing red, blue, and green subpixels, or the number of grain boundaries is adjusted in blue and green. The number of boundaries must be the same.

That is, the smallest number of primary grain boundaries included in the active channel region of the red second TFT so that the charge mobility of the active channel region of the red second TFT of the red subpixel is the largest, and the second of the blue subpixel. The number of primary grain boundaries included in the active channel region of the TFT is next smaller, so that the number of primary grain boundaries included in the active channel region of the second TFT of the green subpixel is adjusted to be the largest.

As a result, the current values in the respective subpixels show the difference as described above, and the luminance of each subpixel is compensated for, thereby achieving a white balance.

In addition, the number of grain boundaries included in the active channel region of the second TFT may vary depending on the light emitting material forming the light emitting layer, and after calculating a current ratio for matching luminance and white balance of each pixel in advance, the green subpixel is referred to. The number of grain boundaries included in the active channel region of the second TFT of each subpixel may be set.

Hereinafter, a structure and a manufacturing method of an organic EL device according to an embodiment of the present invention will be described with reference to FIGS. 5, 6, and 7.

5 is a partially enlarged plan view illustrating a single pixel in FIG. 1, FIG. 6 is a cross-sectional view taken along line II-II in FIG. 5, and FIG. 7 is a cross-sectional view taken along line III-III in FIG. 5. It is sectional drawing to show.

As shown in FIGS. 5, 6, and 7, a buffer layer 2 is formed on an insulating substrate 1, which is a glass material, and the first TFT 10 and the second TFT 20 are formed on the buffer layer 2. ), A capacitor 30 and an organic electroluminescent element (organic EL element) 40 are formed. The buffer layer 2 may be formed of SiO ₂ , and may be deposited by PECVD, APCVD, LPCVD, ECR, or the like. The buffer layer 2 can be deposited to about 3,000 Pa.

An amorphous silicon thin film is deposited on the buffer layer 2, and may be deposited at about 500 μm. The amorphous silicon thin film as described above may be crystallized into a polycrystalline silicon thin film by various methods. In this case, the crystallized polycrystalline silicon thin film includes a primary grain boundary extending in the longitudinal direction as shown in FIG. 2 and a secondary grain boundary perpendicular to the primary grain boundary. In the preferred embodiment of the present invention, the SLS method is used to obtain such a crystal structure as described above, but any crystallization method may be used as long as it is a crystallization method capable of obtaining such a crystal structure.

In this case, in order to control the number of grain boundaries included in each pixel, the crystallization is performed by changing a mask pattern during crystallization for each pixel. That is, in the green pixel region, in order to reduce the current value and the current mobility, the overlapping rate at which the mask pattern is overlapped is reduced so that a large number of grain boundaries are included. On the other hand, in the red region having low luminance efficiency, the laser mask is irradiated and crystallized with the possible mask pattern having a high overlap ratio such that the grain boundaries are small in the number of grain boundaries included in the active channel region of the driving TFT.

The grain boundary includes both a primary grain boundary and a secondary grain boundary.

In this manner, the polycrystalline silicon thin film is formed so that the number of grain boundaries included in the active channel region of the driving TFT is different by varying the overlapping rate of the mask pattern for each pixel.

After forming the polycrystalline silicon thin film, as shown in FIG. 1, the subchannels are patterned so that the active channel regions of the second TFTs are perpendicular to each other in the direction of the grain boundary. At this time, the grain boundary is a primary grain boundary or a secondary grain boundary. At this time, the active channel region of the first TFT is also patterned at the same time.

After patterning the active channel region, a gate insulating film is formed by SiO _{2 or the} like by PECVD, APCVD, LPCVD, ECR, etc., and then a conductive film is formed by MoW, Al / Cu, or the like. Patterning to form a gate electrode. The active channel region, the gate insulating layer, and the gate electrode may be patterned by various procedures and methods.

After patterning of the active channel region, the gate insulating film, and the gate electrode, the source and drain regions are doped with N-type or P-type impurities.

After the doping process is completed, as shown in FIGS. 6 and 7, the interlayer insulating film 4 and the passivation film 5 are formed, and then the source electrodes 14, 24 and the drain electrode 15 are formed through contact holes. 25 is connected to the active channel regions 11 and 21 to form a planarization film 6. Such a film structure can of course employ various structures depending on the device design.

On the other hand, the EL element 40 connected to the second TFT 20 can be formed by various methods. First, the anode electrode 41 connected to the drain electrode 25 of the second TFT 20 by ITO. ) Is patterned and then the organic film 42 is formed thereon. In this case, the organic film 42 may be a low molecular or high molecular organic film. When the low molecular organic film is used, a hole injection layer, a hole transport layer, an organic light emitting layer, an electron transport layer, an electron injection layer, etc. may be formed in a single or complex structure. The organic materials available may also be copper phthalocyanine (CuPc), N, N'-di (naphthalen-1-yl) -N, N'-diphenyl-benzidine (N, N'-ya (napht halene-1 -yl) -N, N'-diphenyl-benzidine (NPB), tris-8-hydroxyquinoline aluminum (Alq3) and the like can be applied in various ways. These low molecular weight organic films are formed by the vacuum vapor deposition method.

The polymer organic film may have a structure including a hole transporting layer (HTL) and a light emitting layer (EML). In this case, PEDOT is used as the hole transporting layer and poly-phenylenevinylene (PRP) and polyfluorene (Polyfluorene) are used as the light emitting layer. Polymer organic materials such as) are used and are formed by screen printing or inkjet printing.

After the organic film is formed in this manner, the cathode electrode 43 may be formed by depositing or patterning the cathode electrode 43 on Al / Ca or the like. The upper portion of the cathode electrode 43 is sealed by a glass or metal cap.

What has been described above is the case where the present invention is applied to the organic electroluminescent element, but the present invention is not limited thereto, and of course, the present invention can be applied to any structure that can use a TFT such as a liquid crystal display device or an inorganic electroluminescent element.

In addition, the layer structure of the organic electroluminescent device according to the preferred embodiment of the present invention is not necessarily limited to the above-described, it is a matter of course that the present invention can be applied to any other structure.

As described above, in the present invention, white balance can be achieved even with an active channel region of the same size without changing the width or size of the active channel region of the composition thin film transistor for driving the pixel and without changing the driving voltage. By supplying an appropriate current for each, proper luminance can be obtained and life degradation can be prevented.

In addition, by controlling only the amount of current flowing through the device without increasing the area occupied by the driving thin film transistor per pixel, it is possible to solve the problem of reducing the aperture ratio and improve reliability.

1 is a plan view illustrating a structure of an active region of a thin film transistor of an active matrix organic electroluminescent device according to a preferred embodiment of the flat panel display according to the present invention.

2 is a view showing a structure in which a thin film transistor is disposed on polycrystalline silicon.

3 is a graph showing a change in current mobility according to the number of primary grain boundaries.

4 is a graph showing a change in current mobility according to the number of secondary grain boundaries.

FIG. 5 is a partially enlarged plan view illustrating a single pixel in FIG. 1.

FIG. 6 is a cross-sectional view taken along the line II-II of FIG. 5.

FIG. 7 is a cross-sectional view taken along line III-III of FIG. 5.

Claims (10)

A driving thin film transistor having green, red, and blue pixel regions, and having a driving thin film transistor having the same active channel length and width, and including a crystal grain boundary of polycrystalline silicon included in the active channel region of the driving thin film transistor. A flat panel display device wherein the number of pixels is different for each pixel. The method of claim 1, The number of primary grain boundaries of the polycrystalline silicon is green most, red and blue are the same. The method of claim 1, And a number of primary grain boundaries of the polycrystalline silicon in order of green, blue, and red regions. The method of claim 1, The number of primary grain boundaries of the polycrystalline silicon is the same as green and blue, and the red is the least. The method of claim 1, And the grain boundaries are perpendicular to a direction in which current flows in an active channel region of each driving thin film transistor. The method of claim 5, And the grain boundary is a primary grain boundary. The method of claim 5, And the grain boundary is a secondary grain boundary. The method of claim 7, wherein And the flat panel display device has the smallest number of primary grain boundaries included in an active channel region of a driving thin film transistor of a green pixel. The method of claim 8, In the flat panel display, the number of primary grain boundaries included in an active channel region of a driving thin film transistor of a blue pixel and a red pixel is the same, or the number of primary grain boundaries included in an active channel region of a driving thin film transistor of a blue pixel is a red pixel. And less than the number of primary grain boundaries included in the active channel region of the driving thin film transistor. The method of claim 1, The flat panel display is any one of a liquid crystal display, an inorganic electroluminescent element, and an organic electroluminescent element.