JP2004171016A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device which excels in response speed and visual field characteristics, can realize a stable display without being affected by the variation in the characteristics of switching elements and with which the stability of a halftone display can be obtained as well. <P>SOLUTION: The liquid crystal display device performs the display in a period when the light transmittance by a liquid crystal material sealed into a liquid crystal panel and provided with spontaneous polarization is constant without depending on the time when a TFT (Thin Film Transistor)11 is on. Even if the charge implantation time of the liquid crystal material is changed by the variation in the characteristics of the TFT11, the change in transmitted light is averted and the stable display including the halftone is possible. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display using a liquid crystal material having spontaneous polarization.

With the progress of so-called office automation (OA) in recent years, OA devices represented by word processors, personal computers, and the like have been widely used.
Further, with the spread of OA equipment in such offices, demand has arisen for portable OA equipment that can be used both in offices and outdoors, and there has been a demand for smaller and lighter OA equipment. A liquid crystal display device is widely used as one of means for achieving such an object. In particular, a liquid crystal display device is an indispensable technology not only for reducing the size and weight but also for reducing the power consumption of a portable OA device driven by a battery.

  Incidentally, liquid crystal display devices are roughly classified into a reflection type and a transmission type. The reflective type is a configuration in which light rays incident from the front of the liquid crystal panel are reflected on the back side of the liquid crystal panel, and the image is visually recognized with the reflected light. This is a configuration in which an image is visually recognized by transmitted light. The reflection type is inferior in visibility because the amount of reflected light is not constant depending on environmental conditions, but is inexpensive. Therefore, the reflection type is widely used as a single color (for example, white / black display) display device such as a calculator and a clock. It is not suitable for a display device such as a personal computer that performs color or full-color display. For this reason, a transmissive display device is generally used as a display device such as a personal computer for performing multi-color or full-color display.

  On the other hand, current color liquid crystal display devices are generally classified into an STN (Super Twisted Nematic) type and a TFT-TN (Thin Film Transistor-Twisted Nematic) type in view of the liquid crystal material used. The STN type is relatively inexpensive in manufacturing cost, but has a problem that it is not suitable for displaying moving images because crosstalk is likely to occur and the response speed is relatively slow. On the other hand, the display quality of the TFT-TN type is higher than that of the STN type, but a backlight of high luminance is required because the transmittance of the liquid crystal panel is only about 4% at present. For this reason, in the TFT-TN type, the power consumption by the backlight increases, and there is a problem in use when carrying a battery power supply. Further, the TFT-TN type has problems such as a low response speed, particularly a halftone response speed, a narrow viewing angle, and difficulty in adjusting the color balance.

In view of the above, a ferroelectric liquid crystal material or an antiferroelectric liquid crystal material having a high response speed to an applied electric field of the order of several hundreds to several μs is used, and these liquid crystal materials are used as switching elements such as TFTs. A liquid crystal display device driven by using the same has been developed. When a ferroelectric or antiferroelectric liquid crystal material is used as the liquid crystal material, the viewing angle is extremely wide because the liquid crystal molecules are always parallel to the substrate (glass substrate) regardless of the applied voltage. There is no practical problem.
JP-A-6-160812 JP-A-10-254390 JP-A-5-107541

  In a liquid crystal display device that uses a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance and drives the liquid crystal substance using a switching element, a substantial charge is caused due to a variation in characteristics of the switching element. There is a problem that a stable display, in particular, a stable halftone display cannot be obtained due to a variation in the injection time.

  The present invention has been made in view of such circumstances, and it is possible to realize a stable display without being affected by variations in the characteristics of switching elements, as well as being excellent in response speed and viewing angle characteristics. It is another object of the present invention to provide a liquid crystal display device capable of obtaining stability of halftone display.

  A liquid crystal display device according to claim 1, wherein a liquid crystal material having spontaneous polarization is sealed in a gap formed by at least two substrates, and the light transmittance of the liquid crystal material is controlled for each pixel. In a liquid crystal display device provided with a switching element that is turned on / off as much as possible, a time during which the switching element is turned on is set to 4 to 20 μs, and a voltage for gradation display is set to a value corresponding to the pixel within a time of 4 to 20 μs. In which a gray scale display is performed by applying to the liquid crystal material.

  The present inventors have found that a liquid crystal substance having spontaneous polarization has a period in which the light transmittance due thereto is substantially constant without depending on the time when the switching element is turned on. Based on such knowledge, in the liquid crystal display device according to the first aspect, gradation display is performed during a time period of 4 to 20 μs during which the switching element whose light transmittance is substantially constant is turned on. Therefore, even if the charge injection time into the liquid crystal material changes due to variations in the characteristics of switching elements such as TFTs, the transmitted light does not change, and stable gradation display is possible.

  The liquid crystal display device according to claim 2 is characterized in that, in claim 1, the time of 4 to 20 μs is a time during which a response of spontaneous polarization of the liquid crystal material hardly occurs.

  In the liquid crystal display device according to the second aspect, since the response of spontaneous polarization of the liquid crystal substance hardly occurs during the period in which the switching element is turned on, the light generated when the switching element is turned off. The transmittance is substantially constant without depending on the period during which the switching element is on. By performing display during the ON period of the switching element where such spontaneous polarization response hardly occurs, even if the charge injection time to the liquid crystal material changes due to variations in the characteristics of the switching element such as a TFT, etc. Since almost no spontaneous polarization response occurs, the amount of charge injected into the liquid crystal substance can be kept constant, and transmitted light does not change, and stable gradation display can be performed.

  According to a third aspect of the present invention, there is provided a liquid crystal display device according to the first or second aspect, further comprising a backlight that emits three-color light in a time-division manner.

  According to the liquid crystal display device of the third aspect, since a backlight capable of time-division light emission is provided, stable color display without using a color filter is possible.

  In the liquid crystal display device of the present invention, the gradation display is performed within a period in which the light transmittance of the liquid crystal substance does not depend on the time during which the switching element is turned on. Variations in light transmittance due to variations and the like can be suppressed, and uniform and stable gradation display can be performed.

Hereinafter, the present invention will be specifically described with reference to the drawings showing the embodiments.
FIG. 1 is a schematic diagram showing an example of the overall configuration of the liquid crystal display device of the present invention. As shown in FIG. 1, the liquid crystal panel in the liquid crystal display device of the present invention is configured as a structure between two polarizing films 1 and 5. Specifically, the liquid crystal panel is configured by laminating a polarizing film 1, a glass substrate 2, a common electrode 3, a glass substrate 4, and a polarizing film 5 in this order from the upper side to the lower side.

  Pixel electrodes 10 corresponding to individual display pixels arranged in a matrix are formed on the surface of the glass substrate 4 on the side of the common electrode 3. A liquid crystal drive control means 20 having a data driver, a scan driver and the like is connected between the common electrode 3 and the pixel electrode 10. The individual pixel electrodes 10 are on / off controlled by the TFTs 11. The individual TFTs 11 are driven by selectively turning on / off the data lines 12 by the data driver and the scanning lines 13 by the scan driver. Then, the light transmittance of each pixel is controlled according to the data from the data line 12.

  A liquid crystal material is filled in a gap between the two alignment films disposed on the upper surface of the pixel electrode 10 and the lower surface of the common electrode 3 on the glass substrate 4 to form a liquid crystal layer. In addition, on the lower layer (back surface) side of the liquid crystal panel, a backlight including a light source 7 and a light guide plate + light diffusion plate 6 is provided.

  Next, the display operation will be described. In the first writing scan (data writing scan), a voltage signal corresponding to the pixel data is supplied from the data driver of the liquid crystal drive control means 20 to each pixel of the liquid crystal panel. Thereby, the voltage is applied, the light transmittance is adjusted, and an image corresponding to the pixel data is displayed.

  Then, in the second write scan (data erase scan), a voltage signal having a characteristic opposite to that of the first write scan is supplied from the data driver of the liquid crystal drive control means 20 to each pixel of the liquid crystal panel. As a result, a voltage having the same strength and opposite polarity as the voltage applied to each pixel during the first writing scan is applied to each pixel of the liquid crystal panel, and the display of each pixel of the liquid crystal panel is erased.

  In the first scan (data write scan) and the second scan (data erase scan), the voltage of the signal supplied to each pixel of the liquid crystal panel is the same and only the polarity is different. Application of the components is prevented.

(1st Embodiment)
The first embodiment is an example in which a color display is performed by using a backlight of white light and selectively transmitting white light with color filters of three primary colors.

  First, the liquid crystal panel shown in FIG. 1 was manufactured as follows. A TFT substrate in which the individual pixel electrodes 10 were formed in a matrix of 640 × RGB × 480 with a pitch of 0.10 mm × 0.10 mm and the number of pixels was manufactured. After washing such a TFT substrate and a glass substrate 2 having an RGB micro color filter and a common electrode 3, a polyimide is applied by printing and baked at 200 ° C. for 1 hour to form a polyimide film of about 200 ° C. on an alignment film. As a film.

  Further, these alignment films were rubbed with a rayon cloth and overlapped with a gap maintained by a silica spacer having an average particle diameter of 1.6 μm between them, thereby producing an empty panel. A ferroelectric liquid crystal material containing a naphthalene-tolane-based liquid crystal material as a main component was sealed between both alignment films of the empty panel. This naphthalene-tolane-based liquid crystal material has spontaneous polarization. The produced panel was sandwiched between two polarizing films 1 and 5 in a crossed Nicols state such that when the ferroelectric liquid crystal molecules were inclined to one side, the liquid crystal panel became dark.

  A voltage of 5 V was applied to the manufactured liquid crystal panel, and the light transmittance was measured. FIG. 2 is a graph showing the relationship between the time during which the TFT 11 is on (hereinafter referred to as TFT selection time) and the light transmittance near the center of the liquid crystal panel when the applied voltage is 5 V.

  As the TFT selection time lengthened, the light transmittance increased, and when the TFT selection time reached 350 μs, a light transmittance of 100% was obtained. However, when the TFT selection time was in the range of 4 to 20 μs, the light transmittance was almost constant at around 50%. This phenomenon is considered to be caused by the switching of spontaneous polarization.

  That is, when the TFT selection time is short, spontaneous polarization inversion hardly occurs within the TFT selection time. Therefore, in the state where the TFT is off, only the reversal of spontaneous polarization corresponding to the amount of charges injected during the period when the TFT is on occurs. For this reason, it is considered that a period in which the light transmittance is constant occurs. On the other hand, if the TFT selection time is long, spontaneous polarization inversion occurs within the TFT selection time. The rate of this inversion increases as the TFT selection time increases. Due to this phenomenon and the reversal of spontaneous polarization in the TFT off state corresponding to the amount of charge injected during the period when the TFT is on, the light transmittance increases.

  Similarly, the light transmittance near the center of the liquid crystal panel was measured with the applied voltages of 3 V, 7 V, and 10 V. FIG. 3 is a graph showing the relationship between the TFT selection time and the light transmittance near the center of the liquid crystal panel when the applied voltage is 3 V (◇), 5 V (□), 7 V (△), and 10 V (○). It is. Even when the applied voltage is changed, it can be seen that the light transmittance is constant when the TFT selection time is 20 μs or less.

  In the first embodiment, the TFT selection time is set to 15 μs which is 20 μs or less, and the driving display is performed by the driving method shown in FIG. 4 (1/60 s per frame). As a result, there was no change in the light transmittance, and a uniform and stable display was realized over the entire display area.

  Further, in the first embodiment, since a constant light transmittance is obtained according to the magnitude of the applied voltage, halftone display is possible. FIG. 5 is a graph showing the relationship between the applied voltage and the light transmittance when the TFT selection time is set to 5 μs. It is clear that the light transmittance according to the applied voltage is obtained, and halftone display is possible.

(Comparative example)
Using a liquid crystal panel having the same configuration as that of the first embodiment, the TFT selection time was set to 30 μs, and drive display was performed by the drive method shown in FIG. As a result, the light transmittance in the halftone state was not stable, and a uniform and stable display could not be obtained in the entire display area.

  In this comparative example, display was performed in two upper and lower divisions in order to make the frame frequency the same as in the first embodiment. It is known that such an upper and lower split display does not adversely affect display characteristics. Therefore, it can be seen that the difference in the display characteristics between the first embodiment and the comparative example was caused by the set TFT selection time.

  In order to investigate the cause of the difference between the first embodiment and the comparative example, the relationship between the TFT selection time and the light transmittance at five positions (A to E) of the liquid crystal panel as shown in FIG. The measurement was performed at an applied voltage of 5V. FIG. 7 shows the measurement results. In the period where the TFT selection time was 20 μs or less, no difference in light transmittance was observed at each position, but when the time exceeded 20 μs, a difference in light transmittance was observed at each position. It is considered that uniform display could not be obtained in the comparative example due to such factors. This factor is considered to occur because a substantial difference occurs in the charge injection time due to variations in the switching time of the TFT.

(2nd Embodiment)
The second embodiment is an example of performing full-color display by causing a backlight of three color lights to emit light in a time-sharing manner without using a color filter as in the first embodiment. The light source 7 of the second embodiment has three primary colors, namely, red (R), green (G), and blue, on the surface facing the light guide plate + light diffusion plate 6 as shown in FIG. (B) is an LED array in which LEDs emitting each color are sequentially and repeatedly arranged. The light guide plate + light diffusion plate 6 functions as a light emitting area by guiding light emitted from each LED of the light source 7 to the entire surface thereof and diffusing the light to the upper surface.

Then, the LEDs of the backlight are sequentially emitted in the order of red, green, and blue in a predetermined cycle, and the display is performed by switching each pixel of the liquid crystal panel in line units in synchronization with the light emission. During the period in which each color is emitting light, two data scans (first data write scan and second data erase scan) are performed as in the first embodiment, and an image corresponding to pixel data is displayed. You.
However, the timing is adjusted so that the end timing of the second scan (data erase scan) of a certain color coincides with the start timing of the first scan (data write scan) of the next color.

  The liquid crystal panel shown in FIG. 1 was manufactured as follows. A TFT substrate in which the individual pixel electrodes 10 were formed in a matrix of 640 × 480 pixels at a pitch of 0.10 mm × 0.10 mm was manufactured. After such a TFT substrate and the glass substrate 2 having the common electrode 3 were washed, polyimide was applied by printing and baked at 200 ° C. for 1 hour to form a polyimide film of about 200 ° as an alignment film.

  Further, these alignment films were rubbed with a rayon cloth and overlapped with a gap maintained by a silica spacer having an average particle diameter of 1.6 μm between them, thereby producing an empty panel. A ferroelectric liquid crystal material containing a naphthalene-tolane-based liquid crystal material as a main component was sealed between both alignment films of the empty panel. The produced panel was sandwiched between two polarizing films 1 and 5 in a crossed Nicols state such that when the ferroelectric liquid crystal molecules were inclined to one side, the liquid crystal panel became dark.

  This liquid crystal panel was overlapped with a backlight capable of emitting red, green, and blue light in a time-division manner. The light emission timing and color of the backlight are controlled in synchronization with the data writing / erasing scanning of the liquid crystal panel.

  In the second embodiment, the TFT selection time is set to 5 μs at which the light transmittance is constant, and the driving display is performed by the driving method shown in FIG. 9 (1/180 s for each color, 1/60 s for one frame). Was. As a result, there was no change in the light transmittance particularly in the halftone state, and a uniform and stable color display was realized over the entire display area.

  In the above example, a ferroelectric liquid crystal is used as the liquid crystal material, but it is needless to say that a similar effect can be obtained by using an antiferroelectric liquid crystal. By using a ferroelectric or antiferroelectric liquid crystal substance with spontaneous polarization as the liquid crystal substance, the period during which the light transmittance by the liquid crystal substance is almost constant without depending on the time when the switching element is turned on is stable. Is obtained.

1 is a schematic diagram illustrating an example of the entire configuration of a liquid crystal display device according to the present invention. 9 is a graph showing a relationship between a TFT selection time (time during which the TFT is turned on) and light transmittance (applied voltage: 5 V). 5 is a graph showing the relationship between TFT selection time and light transmittance (applied voltage: 3 V, 5 V, 7 V, 10 V). FIG. 7 is a diagram illustrating an example of a driving method of the liquid crystal display device according to the first embodiment and a comparative example. It is a graph which shows the relationship (TFT selection time 5 microseconds) between an applied voltage and light transmittance. It is a schematic diagram which shows the several position which measures the relationship between TFT selection time and light transmittance. It is a graph which shows the relationship between TFT selection time and light transmittance (applied voltage 5V) in a plurality of measurement positions. It is a schematic diagram which shows the example of a structure of the light source (LED array) of 2nd Embodiment. FIG. 11 is a diagram illustrating an example of a driving method of the liquid crystal display device according to the second embodiment.

Explanation of reference numerals

7 Light source
11 TFT

Claims (3)

A switching element in which a liquid crystal material having spontaneous polarization is sealed in a gap formed by at least two substrates, and is turned on / off to control light transmittance of the liquid crystal material corresponding to each pixel. In the liquid crystal display device provided with
The switching element is turned on for 4 to 20 μs, and a voltage for gradation display is applied to the liquid crystal material corresponding to the pixel within the time of 4 to 20 μs to perform gradation display. Liquid crystal display device.   2. The liquid crystal display device according to claim 1, wherein the time of 4 to 20 [mu] s is a time during which a response of the spontaneous polarization of the liquid crystal material hardly occurs.   3. The liquid crystal display device according to claim 1, further comprising a backlight that emits three-color light in a time-division manner.

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