JP3504480B2 - Liquid crystal display - Google Patents

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 in 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 A liquid crystal display (hereinafter, appropriately abbreviated as LCD) was originally based on monochrome display. However, each pixel in a liquid crystal cell is provided with a red, green, and blue micro color filter for each color. A method (micro color filter method) for controlling the transmittance of pixels in liquid crystal by a liquid crystal has been developed and it has become possible to display arbitrary colors (Japan Society for the Promotion of Science, Committee 142: 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 in the 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 spectrum. (2) The original 1 by the set of three pixels of red, green and blue
In order to display pixels, there is a loss of 3 times in terms of resolution and driving circuit.

As a technique for solving these problems, there is a sequential color illumination system (field sequential system) in which an image of three colors is sequentially displayed in one pixel without attaching a color filter to each pixel (Shouichi Matsumoto: Liquid crystal). Display Technology (1996), p.50, Industrial Books).

[0005]

In order to prevent the occurrence of flicker in the eyes due to color switching in the above-mentioned sequential color illumination system, three colors are displayed in one frame time (one set of screen display with three colors). About 1 /
It is necessary to switch over 60 s, that is, about 1/180 s per color, that is, about 6 ms. In addition, switching of images of each color,
That is, if 2/3 of this time is allocated to the electrical writing of the screen and the response of the liquid crystal, and the backlight is turned on in the remaining 1/3 of the time, 1 ms is allocated to the electrical writing of the screen. For example, the response time of liquid crystal needs to be within about 3 ms.

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

On the other hand, the present inventor has disclosed in JP-A-7-84254 that a bend alignment liquid crystal cell (including a liquid crystal cell having a twist alignment in the center of the cell; hereinafter abbreviated as a bend cell as appropriate) and a phase compensating plate ( A liquid crystal display device using a retardation plate has been proposed. In this element, a biaxial phase compensating plate that three-dimensionally compensates for the retardation is superposed on the bend cell to suppress the viewing angle dependency and significantly widen the viewing angle. By designing the phase compensator to optically compensate the liquid crystal alignment at a certain low voltage, the bend cell is
Since the device can be driven at the same voltage as the N cell, the device can be realized by the 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, which is less than 1/10 of the TN cell (response speed is
10 times or more). However, the response time greatly exceeds 3 ms depending on the magnitude of the change in the gray level, which is still insufficient for constructing a display of the sequential color illumination system.

[0008] Therefore, an object of the present invention is to propose a liquid crystal display which can realize a sequential color illumination system, that is, which can advantageously achieve 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 cell can be further improved by optimizing the material parameters and device parameters of the bend cell. As a result, the following invention was made. The present invention has a cell gap of 7 μm or less, a viscosity of 70 mPa · s or less, and a pretilt angle of 2 ° or more and 10 °.
A phase compensator (retarder) is placed in front of a bend-aligned liquid crystal cell (bend cell) having a length of less than 3 mm, and the cell and the phase compensator are sandwiched by orthogonal polarizers, and the response time between halftones is 3 ms or less. Display panel and TFT (thin film transistor) that divides the display panel into matrix-arranged pixels and drives each pixel
Active matrix and red, green on the back of the display panel,
A liquid crystal display, comprising: a surface light source of a sequential color illumination system that sequentially emits light of three colors of blue.

The term "bend alignment liquid crystal cell (bend cell)" literally means "a liquid crystal cell having a bend alignment structure", and "twist alignment is present in the cell center part, which is almost electro-optically equivalent to this. Also includes a liquid crystal cell. In the present invention, it is preferable to further include a writing device that performs writing (refreshing) on each pixel a plurality of times before irradiation with each color light, and it is preferable that the material of the TFT is polysilicon. .

The present invention is also a writing method for a liquid crystal display, characterized in that writing is performed a plurality of times on each pixel before irradiation of each color light by using the writing device. Then, it is preferable that the polarity of the writing voltage of the writing device is inverted every writing. It is also suitable that the polarity of the writing voltage of the writing device is inverted every 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 three color lights of red, green and blue.

[0013]

1 is a conceptual diagram of a display panel included in the present invention. This display panel as shown in the figure
The reference numeral 10 is constituted by disposing a phase compensating plate (phase difference plate) 3 in front of a bend alignment liquid crystal cell (bend cell) 1 and sandwiching the bend cell 1 and the phase compensating plate 3 between orthogonal polarizers 5, 5.
Reference numeral 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 voltage application. Reference numeral 7 denotes a surface light source (backlight) for sequentially irradiating the back surface of the display panel 10 with three color lights of red (R), green (G), and blue (B), and the 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 surfaces of the upper and lower substrates 12 are tilted in opposite directions. When a certain voltage is applied to this cell, the liquid crystal is in the bend orientation,
Alternatively, the orientation is such that there is a twist orientation in the center of the cell. Since any of these alignments are almost equivalent in terms of electro-optics, the liquid crystal cell according to the present invention is represented by the term “bend alignment liquid crystal cell (bend cell)”.

It has been disclosed in Japanese Patent Laid-Open No. 7-84254 that the liquid crystal cell having such an orientation has a fast response speed of the orientation change (that is, the transmittance change) with respect to the voltage change.
In the present invention, in order to further improve the response speed in view of the above purpose, with a smaller cell gap, for example, Chisso Co., Ltd. LIXON TD-6004XX, TD-6001XX,
A liquid crystal with low viscosity such as TD-5068XX was selected, and the pretilt angle and phase compensator were optimized accordingly.

Since the transmittance at the same drive voltage tends to decrease as the cell gap is made smaller, the cell gap is usually set to exceed 7 μm in the past, but with this, a response time of about 3 ms or less can be 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 it is attempted to make the cell gap smaller than 1.5 μm, a liquid crystal having a birefringence Δn of more than 0.6 is required due to the relationship with the retardation, and it is difficult to manufacture with the current technology. Therefore, the lower limit of the cell gap is tentatively set to 1.5 μm.
And The viscosity of the liquid crystal shall be 70 mPa ・ s or less .
It If it exceeds 70 mPa · s, the response will be slow.

The pretilt angle is preferably 2 to 30 °. When it is less than 2 °, the bias voltage V _CR for taking the bend orientation becomes several V or more and the driving voltage becomes high as a whole, and when it exceeds 30 °, the retardation becomes small and the transmittance becomes low. Neither is preferable. However, if a method of forming a polymer network by introducing a monomer into liquid crystal and applying a voltage to the liquid crystal to form a polymer network, the preferred range of the pretilt angle is 0.1 to 30.0.
It becomes °.

In optimizing the phase compensating plate, it is designed so as to optically compensate the alignment of the liquid crystal at a certain low voltage (about 6 V or less), as described above. The refractive index of this is n _Y > n _X > n _Z as shown in FIG. However, this condition changes depending on the physical property constant 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 in (a) of the bend cell having a cell gap of 6 μm.
(B) is a graph showing halftone (1-6 of (a)) switching response characteristics. In the same figure (b), the change from the low level of the halftone to the high level corresponds to the rise time, and the opposite change corresponds to the fall time. From the figure, it can be said that the response time mainly depends on the final halftone level, that it becomes slower as the voltage is lower, and even if it is the slowest, it is slightly slower than 3 ms and the overall goal is to be within 3 ms. You can see what has been achieved.

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

FIG. 3 is an explanatory view of a three-terminal element type AM driven LCD. The basic structure is, as shown in FIG. 3A, a matrix of three-terminal elements consisting of a source, a gate and a drain on a substrate. The liquid crystal layer is disposed on the three-terminal element matrix which is provided in the above and has the display electrode and the capacitor connected to the source. Usually, as the transparent electrode facing the three-terminal element matrix electrode substrate, an unpatterned whole surface (common) electrode substrate is used.

When the display panel according to the present invention is used, the liquid crystal layer of FIG. 3A is the liquid crystal 11 of FIG. 1, and the display electrode 20 and the transparent entire surface electrode 15 of FIG. (Not shown), the phase compensator is immediately before or after the glass substrate, the orthogonal polarizer is immediately before or immediately after the three-terminal element matrix 19 on the lower surface side, and immediately before the phase compensator or at the glass substrate on the upper surface side. Placed just before 12 respectively. That is, the display panel is divided into pixels arranged in a matrix by the active matrix driving the display panel.

The operating principle of this type of LCD is shown in FIG. Scan row electrodes (gate bus, scan line) X ₁ , X ₂ ,
, X _n are sequentially scanned by the scanning circuit of the line-sequential drive system, all the three-terminal elements on one gate bus are temporarily turned on simultaneously, and signals from the hold circuit are synchronized with this scanning. Column electrodes (drain bus, signal line) Y ₁ , Y ₂ ,
,, Y _m , to supply signal charges to all capacitors coupled to the 3-terminal element in the conductive state (this is called refresh (writing)). This signal charge continues to excite the liquid crystal of all pixels on this gate bus until the next frame is scanned.

Incidentally, FIG. 3 (b) shows a "line sequential drive system".
In addition to this, a circuit corresponding to the scanning circuit connected to the gate bus is provided instead of the hold circuit, and writing is sequentially performed for each pixel. It is also possible to adopt a “drive system”. By the way, in the sequential color lighting system, the writing time is inevitable because the margin time until the pixel on the scanning line at the bottom of the screen reaches a predetermined halftone is 1/3 of that in the micro color filter system. It gets shorter. In this case, particularly in the case of a liquid crystal having a relatively low purity and a low resistance, the saturation state cannot be reached in one normal writing, and the orientation becomes unstable during the hold, and the image quality as a whole may deteriorate. Such a problem can be avoided by performing the writing in plural.

For this reason, in the present invention, it is preferable to provide a writing device that executes writing to each pixel a plurality of times for each irradiation of each color light. As a result, the restrictions on the purity of the liquid crystal are alleviated, and the manufacturing cost is further reduced. In such a writing device, when the number of writings is N, the scanning cycle of the scanning circuit in FIG. 3B is 1 / N of the holding cycle of the holding circuit.
Since it corresponds to such a control circuit, it can be easily configured.

Further, since the liquid crystal is broken if the voltage is continuously applied with the same polarity, it is desirable to configure the writing device so that the polarity of the writing voltage in the above writing is inverted every time. However, it is not always necessary to invert the polarity every time writing is performed, and writing with the same polarity can be continuously performed as long as the liquid crystal is not broken. For example, it may be reversed for each color light field, or red,
It is also possible to invert every frame composed of three colors of green and blue. It is preferable to reduce the number of times of inversion of the write voltage in this way also from the driving ability of the amorphous silicon transistor for driving the liquid crystal.

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

[0028]

[Example] LIXON TD-6004XX manufactured by Chisso Corporation as a liquid crystal
The display characteristics were measured by conducting an experimental simulation of an active matrix LCD (AM-LCD) using a TFT using a bend cell having a cell gap of 6 μm, a viscosity of 27.8 mPa · s and a pretilt angle of 5 °. In this experiment, as shown in FIG.
An FET (Field Effect Transistor) drive circuit was formed by regarding one pixel of the LCD, and the storage capacitance of the capacitor Cs was given a value twice that of the liquid crystal cell. The surface light source 7 is
As shown in FIG. 5, R, G and B are sequentially arranged from the cold cathode fluorescent tubes emitting light of R, G and B to the light guide plate 22 and the diffusion plate 13.
The bend cell 1 was configured to be uniformly irradiated to the back surface thereof.

FIG. 6 is an explanatory view of an example of this AM-LCD simulation experiment. Waveforms of gate voltage Vg, drain voltage Vd and source voltage Vs are respectively shown in (a) to (c), and (d) is a liquid crystal cell. A transmittance response curve, (e) shows the output of the backlight, respectively. The common voltage Vc was maintained at a constant value (about 7V). Here, the drain voltage Vd is also referred to as a write voltage.

In this case, each color image is refreshed (written) six times 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 pixels in a display of 500 scanning lines, the writing time per scanning line of 2 μs is Vg pulse width 0.998. The pulse interval was ms.

The reason why the refreshing is performed a plurality of times is as follows. That is, as described above, the response time of the alignment change of the liquid crystal molecules is about 1 to 3 ms, which is significantly slower than the writing time of 2 μs per scanning line as described above. Therefore, it is possible to more accurately perform halftone display by performing refreshing twice or more to change the orientation to the required state. Also, in order to increase the response speed,
In the initial stage of refreshing, the voltage may be changed to be larger than the voltage corresponding to the desired halftone, and then the original halftone voltage may be applied. Alternatively, the refresh may be performed only once by applying a voltage in consideration of the change in the electric capacity due to the change in the orientation of the liquid crystal. Further, if a slight shift of the halftone due to the change of the electric capacity is neglected, it is not necessary to set the voltage in consideration of the change of the halftone as described above. Even when such refreshing is performed only once, it is desirable to set the writing time to about 1 ms. This is because it takes about 3 ms until the liquid crystal responds after the writing of the last scan line is completed. If it is necessary to further increase the writing time, if the light emission time of the light source is shortened to, for example, 1 ms, and the liquid crystal response is the same 3 ms, the entire writing time is 2 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 polarities may be inverted for each field of each color of RGB. The reason for reversing 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. Is. In addition, since it is more difficult for the amorphous silicon drive transistor configured in the liquid crystal cell to flow a larger current than a polysilicon transistor, a good waveform cannot be obtained when driven at high speed, and the load is reduced. This is because it will be oversprayed and will be a problem in terms of characteristics.

FIG. 7 and FIG. 8 are similar explanatory views when the driving system of the liquid crystal is changed in the experiment of FIG. Figure 7
FIG. 9 is an explanatory diagram of an experimental example in a field inversion method in which the polarity of the write voltage Vd is inverted for each field of RGB colors. FIG. 8 is an explanatory diagram of an experimental example in a frame inversion method in which the RGB three colors are set as one frame and the polarity of the write voltage Vd is inverted for each frame. 7 and 8, the conditions are the same as those in FIG. 6 except for the driving method. Therefore, a detailed description will be given below with reference to FIG. 6 as an example, and description of FIGS. 7 and 8 will be omitted.

FIG. 6 shows an example of a voltage application pattern in which only R is displayed. That is, the liquid crystal shutter (variable halftone shutter) is opened during the R display period of FIG. 6E and closed during each of the G and B display periods. Liquid crystal shutter open (halftone 6), closed (halftone 1)
.Vertline.Vs-Vc.vertline. Corresponds to 6V and 2V, respectively, as shown in FIG. As a result, as shown in FIG. 6 (d), the transmittance of the liquid crystal quickly responds to the applied voltage, and a response time of about 3 ms can be achieved at both rising and falling.
A beautiful R single color display 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 beautiful single color displays of G and B were obtained. It is to be noted that an experiment in which the shutter is opened is performed for a combination of two or more colors of R, G, and B, and various synthetic color displays of halftone 6 are obtained.
For all combinations of G and B, experiments were performed by changing the initial level and final level of the intermediate tone to a combination other than the set of open and closed (halftone 1, 6), and various synthetic color displays of arbitrary strength were displayed. It was confirmed that it was obtained.

It was confirmed that the driving systems shown in FIGS. 7 and 8 were exactly the same as those shown in FIG. That is, it was found from the above AM-LCD simulation experiment results that a flicker-free full-color display can be realized by devising the drive waveform. It was also confirmed that by improving the driving method, a transistor using amorphous silicon, which has a slow operating speed, can be applied as it is.

[0037]

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

[Brief description of drawings]

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

FIG. 2A shows a relationship between transmittance and voltage in 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 3-terminal element type AM drive LCD.

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

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

FIG. 6 is an explanatory diagram of an example of an AM-LCD simulation experiment, in which (a) to (c) are waveforms of a gate voltage Vg, a drain voltage Vd, and a source voltage Vs, respectively, and (d) is a liquid crystal cell transmittance response curve. , (E) show the output of the backlight, respectively.

7A and 7B 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 are gate voltage Vg, drain voltage Vd, and source voltage Vs, respectively.
, (D) shows the liquid crystal cell transmittance response curve, and (e) shows the output of the backlight.

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

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

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

[Explanation of symbols]

1 Bend alignment liquid crystal cell (bend cell) 3 Phase compensation plate (phase difference plate) 5 Orthogonal polarizer 7 Surface light source (backlight) 10 Display panel 11 LCD 12 Glass (substrate) 13 Diffuser 14 optical fiber 15 Transparent full surface electrode 16 3-terminal element 17 Capacitor 18 PCB 19 3-terminal element matrix 20 Display electrode 21 Cold cathode fluorescent tube 22 Light guide plate

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-7-49509 (JP, A) JP-A-6-161381 (JP, A) JP-A-7-199149 (JP, A) JP-A-4- 318595 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1 / 139,1 / 133

Claims (6)

(57) [Claims] 1. A cell gap of 7 μm or less and a viscosity of 70 mPa ·
s or less, a phase compensation plate is arranged in front of a bend-aligned liquid crystal cell having a pretilt angle of 2 ° or more and less than 10 °, and the cell and the phase compensation plate are sandwiched by orthogonal polarizers, and a response time between halftones is 3 ms or less. Display panel, a TFT active matrix that drives each pixel by partitioning the display panel into matrix-arranged pixels, and a sequential color illumination system that sequentially irradiates red, green, and blue light on the back surface of the display panel. A liquid crystal display having a surface light source. 2. The liquid crystal display according to claim 1, further comprising a writing device that performs writing a plurality of times on each pixel 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 field of each color light. 5. The polarity of the writing voltage of the writing device is red,
3. The liquid crystal display according to claim 2, wherein the inversion is performed for each frame composed of three colors of green and blue. 6. The liquid crystal display according to claim 1, wherein the material of the TFT is polysilicon.