JP3659964B2 - Liquid crystal display - Google Patents

Description

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

With the recent progress of so-called office automation (OA), OA devices represented by word processors, personal computers, and the like are widely used.
Furthermore, the spread of office automation equipment in offices has generated demand for portable office automation equipment that can be used both in offices and outdoors, and there is a demand for reduction in size and weight. 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 technique for reducing the power consumption of a portable OA device driven by a battery as well as reducing the size and weight.

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

  On the other hand, current color liquid crystal display devices are generally classified into STN (Super Twisted Nematic) type and TFT-TN (Thin Film Transistor-Twisted Nematic) type in terms of the liquid crystal material used. Although the STN type is relatively inexpensive to manufacture, there is 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 TFT-TN type has higher display quality than the STN type. However, since the transmittance of the liquid crystal panel is currently only about 4%, a high-brightness backlight is required. For this reason, the TFT-TN type has a problem in use when carrying a battery power supply due to an increase in power consumption by the backlight. The TFT-TN type also has problems such as a low response speed, particularly a halftone response speed, a narrow viewing angle, and difficulty in adjusting the color balance.

From the above viewpoint, a ferroelectric liquid crystal material or an anti-ferroelectric liquid crystal material whose response speed to an applied electric field is as high as several hundreds to several μs is used, and these liquid crystal materials are used as switching elements such as TFTs. Liquid crystal display devices that are driven and used have been developed. When a ferroelectric liquid crystal material or an 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 using a ferroelectric liquid crystal material or an anti-ferroelectric liquid crystal material and driving these liquid crystal materials using a switching element, substantial charge is caused due to variations in characteristics of the switching element. There is a problem in that variations in injection time occur and stable display, particularly halftone display stability cannot be obtained.

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

In the liquid crystal display device according to claim 1, 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 corresponding to each pixel. In a liquid crystal display device provided with a switching element that is turned on / off as much as possible, the light transmittance by the liquid crystal material is substantially constant without depending on the on-time of the switching element. For a certain period of time, 4 to 20 μs is a period in which the response of the spontaneous polarization of the liquid crystal material hardly occurs, 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. In this case, gradation display is performed.

  The present inventors have found that in a liquid crystal substance having spontaneous polarization, there is 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 when such a switching element having a substantially constant light transmittance is turned on. Therefore, even if a change occurs in the charge injection time to the liquid crystal material due to variations in characteristics of switching elements such as TFTs, the transmitted light does not change, so that stable gradation display is possible.

Since the switching element so that the response of the spontaneous polarization of the liquid crystal material within a period that is turned on almost no light transmittance when the switching element is turned off is the switching element is turned on It becomes almost constant without depending on the period. And by performing the display in the ON period of the switching element in which the response of such spontaneous polarization hardly occurs, even if the charge injection time to the liquid crystal material changes due to the variation in characteristics of the switching element such as TFT, Since there is almost no spontaneous polarization response, the amount of charge injected into the liquid crystal substance can be made constant, the transmitted light does not change, and stable gradation display is possible.

The liquid crystal display device according to claim 2, characterized by comprising Oite to claim 1, a backlight for time division emitting three color light.

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

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

Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments thereof.
FIG. 1 is a schematic diagram showing an example of the overall configuration of the liquid crystal display device of the present invention. 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, as shown in FIG. 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 common electrode 3 side. A liquid crystal drive control means 20 having a data driver, a scan driver, etc. is connected between the common electrode 3 and the pixel electrode 10. Each pixel electrode 10 is ON / OFF controlled by a TFT 11, and each TFT 11 is driven by selectively turning on / off a data line 12 by a data driver and a scanning line 13 by a scan driver. Then, according to data from the data line 12, the light transmittance of each pixel is controlled.

  A liquid crystal layer is formed by filling a gap between the alignment films disposed opposite to the upper surface of the pixel electrode 10 on the glass substrate 4 and the lower surface of the common electrode 3 with a liquid crystal material. Note that a backlight composed of the light source 7 and the light guide plate + light diffusion plate 6 is provided on the lower layer (rear) side of the liquid crystal panel.

  Next, the display operation will be described. In the first write scan (data write 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, a voltage is applied, the light transmittance is adjusted, and an image corresponding to the pixel data is displayed.

  In the second write scan (data erasure scan), a signal having a voltage 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 intensity and reverse 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.

  Since the first scanning (data writing scanning) and the second scanning (data erasing scanning) have the same magnitude and only different polarity of the voltage of the signal supplied to each pixel of the liquid crystal panel, the direct current to the liquid crystal Application of components is prevented.

(First embodiment)
The first embodiment is an example in which color display is performed by using a white light backlight and selectively transmitting white light through three primary color filters.

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

  Furthermore, these alignment films were rubbed with a cloth made of rayon and overlapped with a gap held between them by a silica spacer having an average particle diameter of 1.6 μm to produce an empty panel. A ferroelectric liquid crystal material mainly composed of a naphthalene-tolane-based liquid crystal material was sealed between the alignment films of the empty panel. This naphthalene-tolane-based liquid crystal substance has spontaneous polarization. The produced panel was sandwiched between two polarizing films 1 and 5 in a crossed Nicol state so that the ferroelectric liquid crystal molecules would be in a dark state when tilted to one side to obtain a liquid crystal panel.

  A light transmittance was measured by applying a voltage of 5 V to the produced liquid crystal panel. 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 5V.

  The light transmittance increased as the TFT selection time increased, and 100% light transmittance was obtained when the TFT selection time reached 350 μs. 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 the effect of switching of spontaneous polarization.

  That is, when the TFT selection time is short, the spontaneous polarization inversion hardly occurs within the TFT selection time. Therefore, in the state in which the TFT is off, only spontaneous polarization inversion corresponding to the amount of charge injected during the period in which 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, when the TFT selection time is long, the spontaneous polarization is inverted within the TFT selection time. This inversion ratio increases as the TFT selection time increases. The light transmittance increases due to this phenomenon and the reversal of the spontaneous polarization in the TFT off state corresponding to the amount of charge injected during the period in which the TFT is on.

  Similarly, the light transmittance in the vicinity of the center of the liquid crystal panel was measured with applied voltages of 3V, 7V, and 10V. 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 (Δ), and 10 V (10). It is. It can be seen that even when the applied voltage is changed, 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 drive display is performed by the drive method as shown in FIG. 4 (1/60 s in one frame). As a result, there was no change in the light transmittance, and a uniform and stable display could be realized over the entire display area.

  In the first embodiment, since a certain 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)
A liquid crystal panel having the same configuration as in the first embodiment was used, the TFT selection time was set to 30 μs, and drive display was performed by a drive method as shown in FIG. As a result, the light transmittance in the halftone state is not stable, and a uniform and stable display cannot be obtained over the entire display area.

  In this comparative example, in order to make the frame frequency the same as that of the first embodiment, the display was performed in the upper and lower parts. It is well known that such a vertically divided display does not adversely affect the display characteristics. Therefore, it can be seen that the difference in display characteristics between the first embodiment and the comparative example is caused by the length of 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. Measurement was performed at an applied voltage of 5V. The measurement results are shown in FIG. In the period when the TFT selection time was 20 μs or less, no difference in light transmittance was observed at each position, but when it exceeded 20 μs, a difference in light transmittance was observed at each position. Due to such factors, it is considered that a uniform display could not be obtained in the comparative example. This factor is considered to be caused by a difference in substantial charge injection time due to variations in TFT switching time.

(Second Embodiment)
The second embodiment is an example in which full-color display is performed by time-division emission of a three-color light backlight without using a color filter as in the first embodiment. The light source 7 of the second embodiment has three primary colors, that is, red (R), green (G), and blue on the surface facing the light guide plate + light diffusing plate 6, as schematically shown in FIG. (B) is an LED array in which LEDs emitting each color are sequentially and repeatedly arranged. The light guide plate + light diffusing plate 6 functions as a light emitting region by guiding light emitted from each LED of the light source 7 to the entire surface and diffusing it to the upper surface.

Then, the backlight LED emits light sequentially in the order of red, green, and blue in a predetermined cycle, and display is performed by switching each pixel of the liquid crystal panel in units of lines in synchronization therewith. During the period in which each color is emitted, the same two data scans (first data write scan and second data erase scan) as in the first embodiment are performed, and an image corresponding to the pixel data is displayed. The
However, the timing is adjusted so that the end timing of the second scan (data erasure 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 produced as follows. A TFT substrate in which individual pixel electrodes 10 were formed in a matrix of a pitch of 0.10 mm × 0.10 mm and a number of pixels of 640 × 480 was produced. 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 mm as an alignment film.

  Furthermore, these alignment films were rubbed with a cloth made of rayon and overlapped with a gap held between them by a silica spacer having an average particle diameter of 1.6 μm to produce an empty panel. A ferroelectric liquid crystal material mainly composed of a naphthalene-tolane-based liquid crystal material was sealed between the alignment films of the empty panel. The produced panel was sandwiched between two polarizing films 1 and 5 in a crossed Nicol state so that the ferroelectric liquid crystal molecules would be in a dark state when tilted to one side to obtain a liquid crystal panel.

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

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

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

It is a schematic diagram which shows the example of a whole structure of the liquid crystal display device of this invention. It is a graph which shows the relationship (applied voltage 5V) between TFT selection time (time when TFT is turned on) and light transmittance. It is a graph which shows the relationship (applied voltage 3V, 5V, 7V, 10V) between TFT selection time and light transmittance. It is a figure which shows an example of the drive method of the liquid crystal display device in 1st 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 (applied voltage 5V) of TFT selection time and the light transmittance in a several measurement position. It is a schematic diagram which shows the structural example of the light source (LED array) of 2nd Embodiment. It is a figure which shows an example of the drive method of the liquid crystal display device in 2nd Embodiment.

Explanation of symbols

7 Light source
11 TFT

Claims (2)

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 driven on / off to control the light transmittance of the liquid crystal material corresponding to each pixel In the liquid crystal display device provided with
The time during which the switching element is turned on does not depend on the on-time of the switching element, and the light transmittance of the liquid crystal material is substantially constant, and the spontaneous polarization response of the liquid crystal material hardly occurs. A liquid crystal display device characterized in that a gradation display is performed by applying a voltage for gradation display to the liquid crystal material corresponding to the pixel within a period of 4 to 20 μs, which is a time of 4 to 20 μs.   The liquid crystal display device according to claim 1, further comprising a backlight that emits light of three colors in a time-sharing manner.

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