JPH1114988A - Liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display capable of advantageously attaining a full-color display high in resolution without necessitating a color filter by optimizing the material parameter and the device parameter of a bend cell. SOLUTION: In a display panel 10, a phase compensating plate 3 (phase difference plate) is arranged in front of a bend oriented liquid crystal cell (bend cell) 1 whose the cell gap is <=7 μm and the panel 10 is composed by interposing the bend cell 1 and the phase compensating plate 3 by orthogonal polarizers 5, 5. A surface light source (back light) 7 for successively irradiating three color light beams of red (R), green (G) and blue (B) is provided. In this case, the phase compensating plate 3, the lower surface side of the orthogonal polarizer 5 and its upper surface side are arranged just before or just after a glass substrate, just before or just after three terminal element matrix 19 and just before the phase compensating plate or just before the glass substrate 12, respectively. Consequently, the display panel 10 is divided into the pixels of a matrix arrangement by an active matrix for driving the panel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display generally used for electronic display devices such as televisions and computer monitors, and more particularly to a liquid crystal display suitable for a sequential color illumination system.

[0002]

2. Description of the Related Art Liquid crystal displays (hereinafter abbreviated as LCDs) are basically based on black and white display, but each pixel in a liquid crystal cell is provided with a red, green and blue micro color filter to provide each color. A method of controlling the transmittance of each pixel by using a liquid crystal (micro color filter method) has been developed, and it has become possible to display any color (JSPS 142nd Committee: Liquid Crystal Device Handbook (199)
0), p.492, Nikkan Kogyo Shimbun).

However, the micro color filter system has the following problems. (1) The color filters for each color absorb and waste light having a spectrum of 2/3 or more of the incident light. For example, a red color filter wastes 70-90% of the light energy to absorb the blue and green spectra. (2) The original one is set by the set of three pixels of red, green and blue.
In order to display pixels, there is a three-fold loss in terms of both resolution and driving circuit.

As a technique for solving these problems, there is a sequential color illumination system (field sequential system) that sequentially displays three color images with one pixel without attaching a color filter to each pixel (Shoichi Matsumoto: Liquid Crystal). Display Technology (1996), p.50, Industrial Books).

[0005]

In order to prevent flickering of eyes due to color switching in the sequential color illumination system, three colors are used for one frame time (one set of screen display for three colors). Time) is about 1 /
It is necessary to switch in 60 s, ie, about 1/180 s per color, ie, about 6 ms. In addition, switching of the image of each color,
That is, for example, 2/3 of this time is allocated to the electric writing of the screen and the response of the liquid crystal, and if the backlight is turned on in the remaining 1/3 of the time, 1 ms is allocated to the electric writing of the screen. For example, the response time of the liquid crystal needs to be about 3 ms or less.

However, a sequential color illumination type LCD which has such a high-speed response, can provide a halftone display, and is easy to manufacture has not yet been realized. For example, a TN (twisted nematic) liquid crystal cell (abbreviated as a TN cell) currently used for a high-quality active matrix (AM) LCD has a response time as shown in FIG. 20 to 100 ms
Therefore, it cannot be used for sequential color lighting. When applied to a strong and low-speed sequential color illumination system, flicker occurs and the display quality is significantly reduced. The response of an STN (super twisted nematic) liquid crystal cell used in a simple matrix system is much slower, with a response time of 50 to 300 ms. In addition, although the ferroelectric liquid crystal cell has a high-speed response, it is generally difficult to display halftones, and requires an extremely thin cell gap.
There is a problem that a troublesome alignment treatment is required.

On the other hand, the present inventor has disclosed in Japanese Patent Application Laid-Open No. 7-84254, a bend alignment liquid crystal cell (including a liquid crystal cell having a twisted alignment in the center of the cell. A liquid crystal display device using a phase difference plate was proposed. In this element, a biaxial phase compensator for three-dimensionally compensating the retardation is superimposed on a bend cell to suppress the viewing angle dependency, and the viewing angle is significantly widened. By designing the phase compensator to optically compensate for the orientation of the liquid crystal at a certain low voltage, the bend cell can be set to T
Since the device can be driven by the same voltage as that of the N cell, the device can be realized by a conventional manufacturing process. The response time of this bend cell (cell gap is 8 μm) is 2 to 8 ms as shown in FIG.
10 times or more). However, since the response time greatly exceeds 3 ms depending on the magnitude of the change of the gray level (gray level), it seems that it is still insufficient to constitute a display of a sequential color illumination system.

Accordingly, an object of the present invention is to propose a liquid crystal display which can realize a sequential color illumination system, that is, can advantageously achieve a high-resolution full-color display without a color filter.

[0009]

As a result of intensive studies, the present inventors have found that the response speed of the bend cell can be further increased by optimizing the material parameters and device parameters of the bend cell. As a result, the present invention described below has been made. The present invention provides a display panel in which a phase compensator (a phase difference plate) is arranged in front of a bend-aligned liquid crystal cell (bend cell) having a cell gap of 7 μm or less, and the cell and the phase compensator are sandwiched between orthogonal polarizers; The display panel is provided with a TFT (thin film transistor) active matrix for partitioning the display panel into pixels arranged in a matrix and driving each pixel, and a surface light source for sequentially irradiating red, green and blue light on the back surface of the display panel. It is a liquid crystal display which is a feature.

The term "bend-aligned liquid crystal cell (bend cell)" refers to a literal "liquid crystal cell having a bend-aligned structure", and an "optically optically equivalent" twisted alignment exists at the center of the cell. Liquid crystal cell ". In the present invention, it is preferable to further include a writing device that executes writing (refresh) a plurality of times for each pixel before irradiation of each color light, and it is preferable that a material of the TFT is polysilicon. .

Further, the present invention is a writing method for a liquid crystal display, wherein writing is performed a plurality of times for each pixel before irradiation of each color light using the writing device. It is preferable that the polarity of the write voltage of the writing device be inverted every writing. It is also preferable that the polarity of the writing voltage of the writing device be inverted for each field of each color light.

It is also preferable that the polarity of the writing voltage of the writing device is inverted every frame composed of red, green and blue light.

[0013]

FIG. 1 is a conceptual diagram of a display panel provided in the present invention. As shown in the figure, this display panel
Reference numeral 10 denotes a structure in which a phase compensator (phase difference plate) 3 is disposed in front of a bend alignment liquid crystal cell (bend cell) 1, and the bend cell 1 and the phase compensator 3 are sandwiched between orthogonal polarizers 5, 5.
11 is a bend-aligned liquid crystal, 12 is a glass (substrate) sandwiching the liquid crystal, and the glass 12 is provided with a transparent conductive film for applying a voltage. Reference numeral 7 denotes a surface light source (backlight) for sequentially irradiating three color lights of red (R), green (G), and blue (B) on the back of the display panel 10, and a color switching circuit is not shown. The active matrix will be described later.

The bend cell 1 is one in which liquid crystal molecules on the upper and lower substrates 12 are inclined in opposite directions. When a certain voltage is applied to this cell, the liquid crystal takes a bend alignment,
Alternatively, the orientation is such that a twist orientation exists in the center of the cell. Since any of these orientations is almost equivalent electro-optically, the liquid crystal cell according to the present invention is represented by the term "bend oriented liquid crystal cell (bend cell)".

Japanese Patent Application Laid-Open No. 7-84254 discloses that a liquid crystal cell having such an orientation has a high response speed of an orientation change (ie, a transmittance change) with respect to a voltage change.
In the present invention, in order to further increase the response speed in light of the above object, the cell gap is made smaller and, for example, LIXON TD-6004XX and TD-6001XX manufactured by Chisso Corporation,
We selected a low-viscosity liquid crystal such as the TD-5068XX and optimized the pretilt angle and phase compensator accordingly.

Since the transmittance at the same driving voltage tends to decrease when the cell gap is reduced, the cell gap is conventionally usually set to exceed 7 μm. However, in this case, the response time of about 3 ms or less is achieved. Have difficulty. On the other hand, according to the present invention, even if the cell gap is less than 7 μm, the decrease in transmittance is small, and by doing so, a response time of about 3 ms or less can be achieved.

However, if the cell gap is made smaller than 1.5 μm, the birefringence Δ
Since a liquid crystal in which n exceeds 0.6 is required and it is difficult to manufacture with the current technology, the lower limit of the cell gap is set to 1.5 μm. The viscosity of the liquid crystal is preferably 70 mPa · s or less. If it exceeds 70 mPa · s, the response will be slow.

The pretilt angle is preferably 2 to 30 °. If the angle is less than 2 °, the bias voltage V _CR for causing the bend alignment to be several volts or more, and the driving voltage will increase as a whole. If the angle exceeds 30 °, the retardation will decrease and the transmittance will decrease. Both are not preferred. However, if a method of introducing a monomer into the liquid crystal and irradiating ultraviolet rays while applying a voltage to the liquid crystal to form a polymer network is used, the preferable range of the pretilt angle is 0.1 to 30.0.
°.

In optimizing the phase compensating plate, similarly to the above, the phase compensating plate is designed so as to optically compensate for the orientation of the liquid crystal at a certain low voltage (about 6 V or less). The refractive index is set to n _Y > n _X > n _Z as shown in FIG. However, this condition varies depending on the physical constants of the liquid crystal material,
In some cases, n _Y > n _Z > n _X. FIG. 2 shows the relationship between the transmittance and the voltage of the bend cell having a cell gap of 6 μm.
(B) is a graph showing a halftone ((a) 1-6) switching response characteristic. In FIG. 3B, the change from the low level to the high level of the halftone corresponds to the rise time, and the reverse change corresponds to the fall time. From the figure, the goal is that the response time mainly depends on the final halftone level, that the response time be slower as the voltage is lower, and that the slowest is only slightly slower than 3 ms and within 3 ms overall. You can see what has been achieved.

The orthogonal polarizer needs to be provided in order to visualize the change in the orientation of the liquid crystal due to the electric field, but it is preferable to select an orthogonal polarizer having optical characteristics with small wavelength dependence. By the way, using the display panel according to the present invention, LC
In the case of configuring D, the driving method for exciting liquid crystal is to adopt an active matrix (AM) driving method using a TFT, which is generally used for a TN cell with a color filter and can be applied to a transmission type and can obtain high quality image quality. desirable.

FIG. 3 is an explanatory view of a three-terminal element type AM driving LCD. The basic structure is, as shown in FIG. 3A, a three-terminal element comprising a source, a gate and a drain on a substrate in a matrix. And a liquid crystal layer is arranged on a three-terminal element matrix in which a display electrode and a capacitor are connected to a source. Normally, an unpatterned entire (common) electrode substrate is used as the transparent electrode facing the three-terminal element matrix electrode substrate.

When the display panel according to the present invention is used, the liquid crystal layer of FIG. 3A is replaced with the liquid crystal 11 of FIG. 1, and the display electrode 20 and the transparent full-surface electrode 15 of FIG. Although not shown, the phase compensating plate is immediately before or immediately after the glass substrate, the orthogonal polarizer is on the lower surface immediately before or immediately after the three-terminal element matrix 19, and the upper surface is immediately before the phase compensating plate or on the glass substrate. Placed immediately before 12 respectively. That is, the display panel is partitioned into pixels arranged in rows and columns by an active matrix that drives the display panel.

FIG. 3B shows the operation principle of this type of LCD. Scan row electrodes (gate bus, scan line) X ₁ , X ₂ ,
.., _Xn are sequentially scanned by a line-sequential drive scanning circuit, all three terminal elements on one gate bus are simultaneously turned on simultaneously, and a signal from a hold circuit is synchronized with this scanning. Column electrodes (drain bus, signal line) Y ₁ , Y ₂ ,
..., via the Y _m, (this is called refresh (writing)) for supplying a signal charge in all the capacitor attached to the 3-terminal element of this conductive state. This signal charge continues to excite the liquid crystal of all pixels on the gate bus until the next frame scan.

FIG. 3B shows the "line-sequential drive system".
In addition to this, instead of providing a hold circuit, a circuit corresponding to a scanning circuit connected to a gate bus is provided, and writing is sequentially performed for each pixel. Driving method "can also be adopted. By the way, in the sequential color illumination system, a margin time until the pixel on the scanning line at the bottom of the screen reaches a predetermined halftone is 1/3 of that of the micro color filter system, so that the writing time is inevitable. Be shorter. In such a case, especially in a liquid crystal having a relatively low purity and low resistance, the alignment does not reach a saturated state in a normal writing operation and the orientation becomes unstable during holding, so that the image quality may be deteriorated as a whole. Such a problem can be avoided by performing writing in a plurality of ways.

For this reason, in the present invention, it is preferable to provide a writing device for executing writing to each pixel a plurality of times for each irradiation of each color light. As a result, the restriction on the purity of the liquid crystal is relaxed, so that the manufacturing cost is further reduced. In this writing apparatus, when the number of times of writing is N, the scanning cycle of the scanning circuit in FIG.
Therefore, it can be easily configured.

Further, if the voltage is continuously applied with the same polarity, the liquid crystal may be broken. Therefore, it is desirable to configure the writing device to invert the polarity of the writing voltage in the above-described writing every time. However, the reversal of the polarity is not necessarily performed for each writing, and writing with the same polarity can be performed continuously as long as the liquid crystal is not broken. For example, inversion may be performed for each color light field, red,
The inversion may be performed for each frame composed of three color lights of green and blue. Reducing the number of inversions of the write voltage in this manner is preferable from the viewpoint of the driving capability of the amorphous silicon transistor for driving the liquid crystal.

Further, since the writing time is shortened as described above, the material of the TFT provided in the liquid crystal display of the present invention has a higher operating speed than the amorphous silicon generally used in a TN cell with a normal color filter. It is preferable to use polysilicon having a high speed.

[0028]

[Example] As liquid crystal, LIXON TD-6004XX manufactured by Chisso Corporation
Using a bend cell having a cell gap of 6 μm, a viscosity of 27.8 mPa · s, and a pretilt angle of 5 °, display characteristics were measured by performing an experimental simulation of an active matrix LCD (AM-LCD) using TFTs. In this experiment, as shown in FIG.
Assuming one pixel of the LCD, an FET (field effect transistor) drive circuit was formed, and the storage capacitance of the capacitor Cs was twice as large as that of the liquid crystal cell. In addition, the surface light source 7
As shown in FIG. 5, R, G, and B are sequentially transmitted from each of the cold cathode fluorescent tubes emitting R, G, and B colors to the light guide plate 22 and the diffusion plate 13.
Through the bend cell 1 so as to evenly irradiate it.

FIGS. 6A to 6C are explanatory diagrams of an example of the AM-LCD simulation experiment. FIGS. 6A to 6C show waveforms of a gate voltage Vg, a drain voltage Vd and a source voltage Vs, respectively, and FIG. The transmittance response curve, (e) shows the output of the backlight, respectively. The common voltage Vc was kept at a constant value (about 7 V). Here, the drain voltage Vd is also referred to as a write voltage.

Here, six refreshes (writes) are performed for each color image during the display time for one color (6 ms as described above). At this time, one writing time is 1 ms, but in order to correspond to a pixel in a display having 500 scanning lines, the writing time 2 μs per scanning line in this case is set to Vg pulse width, 0.998. ms is the pulse interval.

The reason why refreshing is performed a plurality of times is as follows. That is, as described above, the response time of the orientation change of the liquid crystal molecules is about 1 to 3 ms, which is much slower than the above-described writing time per scanning line of 2 μs. Therefore, halftone display can be performed more accurately if refreshing is performed twice or more to change the orientation to a required state. Also, to increase the response speed,
In the first stage of the refresh, a method of changing the voltage larger than the voltage corresponding to the desired halftone and then applying the original halftone voltage may be adopted. Alternatively, the refresh may be performed only once by applying a voltage in consideration of a change in electric capacity accompanying a change in alignment of the liquid crystal. Further, if a slight shift of the halftone due to the change of the electric capacity is ignored, it is not necessary to set the voltage in consideration of the change of the halftone as described above. It is desirable to set the writing time to about 1 ms even when such refreshing is performed only once. This is because it takes about 3 ms after the writing of the last scanning line is completed until the liquid crystal responds. If it is necessary to further increase the writing time, the light emitting time of the light source should be shortened, for example, to 1 ms. It can be as long as 4 μs per scanning line.

In the example of FIG. 6, the polarity of Vd is inverted every writing, but the polarity of writing in one frame may be the same and the polarity may be inverted in the next frame. . Further, the polarity may be inverted for each field of each color of RGB. The reason for inverting the polarity of Vd is that the orientation of the liquid crystal, that is, the transmittance is determined by the effective value of Vs−Vc and is not influenced by the polarity. However, if a voltage is continuously applied with the same polarity, the liquid crystal is broken. It is. In addition, since it is difficult for an amorphous silicon driving transistor included in a liquid crystal cell to pass a large current as compared with a polysilicon transistor, a good waveform cannot be obtained when the driving is performed at high speed, and the load is reduced. This is because it may be over-applied, which is a problem in characteristics.

FIGS. 7 and 8 are similar explanatory diagrams when the driving method of the liquid crystal is changed in the experiment of FIG. FIG.
FIG. 4 is an explanatory diagram of an experimental example of a field inversion method in which the polarity of the write voltage Vd is inverted for each field of each of RGB colors. FIG. 8 is an explanatory diagram of an experimental example of a frame inversion method in which three colors of RGB are defined as one frame and the polarity of the write voltage Vd is inverted for each frame. 7 and 8, all conditions are the same as those in FIG. 6 except for the driving method. For this reason, FIG. 6 will be described in detail below as an example, and description of FIGS. 7 and 8 will be omitted.

FIG. 6 shows an example in which a voltage application pattern for displaying only R is taken. That is, the liquid crystal shutter (halftone variable shutter) is opened during the R display period in FIG. 6E and closed during the G and B display periods. Opening of liquid crystal shutter (halftone 6), closing (halftone 1)
| Vs-Vc | corresponding to is 6V and 2V, respectively, as shown in FIG. As a result, as shown in FIG. 6D, the transmittance of the liquid crystal quickly responds to the applied voltage, and a response time of about 3 ms can be achieved for both rise and fall.
A clean monochromatic display of R was obtained on the screen.

Similarly, an experiment was conducted in which the shutter was opened only during each of the G and B display periods, but clear G and B single-color displays were obtained. An experiment was conducted in which the shutter was opened for a combination of two or more colors of R, G, and B to obtain various composite color displays of halftone 6;
Experiments were performed for all combinations of G and B by changing the initial level and final level of the halftone in addition to the set of open and closed (halftones 1 and 6), and various composite color displays of any strength were obtained. It was confirmed that it could be obtained.

Then, it was confirmed that the driving methods shown in FIGS. 7 and 8 were exactly the same as those shown in FIG. That is, from the above-described results of the AM-LCD simulation experiment, it was found that a full-color display without flicker can be realized by devising the drive waveform. Also, it was confirmed that a transistor using amorphous silicon having a low operation speed can be applied as it is by improving the driving method.

[0037]

According to the present invention, compared with the conventional LCD,
A remarkable effect of realizing a full-color liquid crystal display that does not require a color filter, which is remarkably excellent in resolution, viewing angle, productivity, and the like is realized.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a display panel provided in the present invention.

FIGS. 2A and 2B show the relationship between the transmittance and the voltage of a bend cell having a cell gap of 6 μm, and FIG.
6) A graph showing switching response characteristics.

FIG. 3 is an explanatory diagram of a three-terminal element type AM drive LCD.

FIG. 4 is a FET drive circuit diagram simulating an AM-LCD using a TFT.

FIG. 5 is a schematic diagram illustrating a configuration example of a surface light source (backlight).

FIGS. 6A to 6C are explanatory diagrams of an example of an AM-LCD simulation experiment, wherein FIGS. 6A to 6C show waveforms of a gate voltage Vg, a drain voltage Vd, and a source voltage Vs, respectively, and FIG. , (E) show the output of the backlight, respectively.

FIGS. 7A to 7C are explanatory diagrams when the driving method of the experiment shown in FIG. 6 is a field inversion driving method, and FIGS. 7A to 7C show a gate voltage Vg, a drain voltage Vd, and a source voltage Vs, respectively.
(D) shows the transmittance response curve of the liquid crystal cell, and (e) shows the output of the backlight.

FIGS. 8A to 8C are explanatory diagrams when the driving method of the experiment shown in FIG. 6 is a frame inversion driving method, in which waveforms of a gate voltage Vg, a drain voltage Vd, and a source voltage Vs; (d) shows the transmittance response curve of the liquid crystal cell, and (e) shows the output of the backlight.

FIG. 9 is a graph showing a halftone switching response characteristic of a TN cell.

FIG. 10 is a graph showing a halftone switching response characteristic of a bend cell having a cell gap of 8 μm.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 bend alignment liquid crystal cell (bend cell) 3 phase compensator (retarder) 5 orthogonal polarizer 7 plane light source (backlight) 10 display panel 11 liquid crystal 12 glass (substrate) 13 diffusion plate 14 optical fiber 15 transparent whole surface electrode 16 3 Terminal element 17 Capacitor 18 Substrate 19 Three-terminal element matrix 20 Display electrode 21 Cold cathode fluorescent tube 22 Light guide plate

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI G09F 9/35 305 G09F 9/35 305

Claims (6)

[Claims] 1. A display panel comprising: a phase compensator disposed in front of a bend-aligned liquid crystal cell having a cell gap of 7 μm or less; and a cell and a phase compensator sandwiched between orthogonal polarizers; T that partitions each pixel and drives each pixel
FT active matrix and red on the back of the display panel,
A liquid crystal display comprising: a surface light source for sequentially irradiating green and blue light of three colors. 2. The liquid crystal display according to claim 1, further comprising a writing device that executes writing to each pixel a plurality of times before irradiating each color light. 3. The liquid crystal display according to claim 2, wherein the polarity of the writing voltage of the writing device is inverted every writing. 4. The liquid crystal display according to claim 2, wherein the polarity of the writing voltage of the writing device is inverted for each color light field. 5. The writing device of claim 1, wherein the polarity of the writing voltage is red,
3. The liquid crystal display according to claim 2, wherein the inversion is performed for each frame composed of three color lights of green and blue. 6. The liquid crystal display according to claim 1, wherein a material of said TFT is polysilicon.