JP3033257B2 - 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 display device, and more particularly to a display device using a liquid crystal.

[0002]

2. Description of the Related Art In recent years, with the realization of thin liquid crystal display devices, laptop computers and the like have become widespread. Currently, a transmissive liquid crystal display device using a backlight is mainly used as the display device. However, a transmissive liquid crystal display device consumes a large amount of power due to a backlight, and is not suitable for long-term use (portable type) using a battery. Therefore, a demand for a thin and lightweight reflective liquid crystal display device that does not require a backlight is increasing. Further, it is expected that a large-sized high-definition liquid crystal display device having a diagonal of 14 inches to 20 inches / 1000 lines or more will be realized.

As a conventional liquid crystal display device, a DSTN in which a super-twisted nematic liquid crystal element driven by a simple matrix is combined with an optical compensation cell and an FTN in which an optical compensation film is combined have been put to practical use. Also,
A display device using an antiferroelectric liquid crystal capable of displaying a higher definition than DSTN or FTN is also being developed.

[0004] "Y. Yamada et al .: Jpn. J. Appl. Phys.
29 (1990) 1757 ", a display method of the antiferroelectric liquid crystal display device will be described with reference to FIGS. 4, 5 and 6. FIG. FIG.
FIG. 3 is a diagram showing a relationship between an optical axis of a liquid crystal and a polarizing plate. The substrate is X
The voltage is in the Y plane and is applied in the ± Z axis directions. OA
Is the optical axis when no voltage is applied, that is, in the antiferroelectric phase, and OF (+) and OF (-) are transferred to the ferroelectric phase by applying sufficiently high positive and negative voltages, respectively. It is the optical axis when it is. The ferroelectric phase when a positive voltage is applied and the ferroelectric phase when a negative voltage is applied are called a ferroelectric phase (+) and a ferroelectric phase (-), respectively. OF (+) or OF (-) and OA
Is called a tilt angle, and is represented by θ. Here, the light absorption axis P1 of one polarizing plate is set parallel to OA, and the light absorption axis P2 of the other polarizing plate is orthogonal to P1.
When no voltage is applied, the optical axis is OA which is parallel to P1 and is in an off state where light is not transmitted. When a voltage is applied, the optical axis is OF (+) or OF (-) which is not parallel to P1. Therefore, the light is in an on state where light is transmitted. If the light transmittance in the antiferroelectric phase and the ferroelectric phase is represented by IA and IF, respectively,
IA <IF. Further, since no voltage is applied between adjacent pixels, the pixels are always off.

FIG. 5 is a diagram showing a hysteresis characteristic in an electric field induced antiferroelectric-ferroelectric phase transition. OA and P1
Are parallel to each other, so that when the applied voltage is 0 as described above, it is in the off state. When the positive voltage is increased, when the voltage becomes higher than the threshold value V (AF) t, the phase shifts from the antiferroelectric phase to the ferroelectric phase.
The phase transition to (+) starts, and the light transmittance increases as indicated by arrow 1. When the voltage reaches the saturation value V (AF) s, the phase transition is completed, and the change in light transmittance is saturated. next,
If the voltage is reduced from the ON state, the voltage will be V (FA) t
Until the ferroelectric phase (+) is held, the light transmittance does not change and remains on, and when the voltage becomes lower than V (FA) t, the phase transition to the antiferroelectric phase starts. , The light transmittance changes as shown by the arrow 2. Then, when the voltage becomes 0, it completely returns to the antiferroelectric phase (off state). Similarly, when a negative voltage is applied, the light transmittance changes according to arrows 3 and 4.

FIG. 6 shows an example of a driving method for driving the antiferroelectric liquid crystal having the hysteresis characteristic shown in FIG. 5 in a simple matrix. Vt and Vd in FIG. 6A are voltage waveforms applied to the scanning electrode and the signal electrode, respectively, and FIG. 6B is a composite waveform thereof, and the composite waveform is applied to the liquid crystal layer. One frame includes a selection period and a non-selection period indicated by S and NS, and the selection period S includes a reset period and a writing period indicated by R and W. During the reset period, the liquid crystal layer is reset to an off state by applying 0 volt to the liquid crystal layer. And when selecting the ON state,
After a voltage greater than V (AF) s is applied in the latter half of the writing period to switch to the ON state, a unipolar sustain voltage waveform (V0-V2 to V0 + V2) is applied to maintain the ON state. When the off state is selected, a voltage of V (AF) t or less is applied in the latter half of the writing period, and then a unipolar sustain voltage waveform is applied to maintain the off state.

[0007] If a DC voltage is applied to the liquid crystal layer or the spontaneous polarization of the liquid crystal is always kept in the same direction, an electric charge is biased inside the liquid crystal layer, which adversely affects the hysteresis characteristics. Therefore, if the polarity of the voltage waveform applied to the liquid crystal layer is inverted for each frame, the applied voltage can be converted to an alternating current. Furthermore, in order to prevent the direction of spontaneous polarization from being biased on average, two consecutive frames are set, and when a ferroelectric phase is selected, the ferroelectric phase (+)
And the ferroelectric phase (-) are selected. Ferroelectric phase (+)
Is selected and when the ferroelectric phase (-) is selected,
The angle between the optical axes OF (+), OF (-) and P is θ
Therefore, their light transmittances are equal to each other and optically equivalent.

[0008]

However, the conventional method has the following problems.

DSTN and FTN are not suitable for high-definition display because the contrast ratio decreases and the display quality when viewed obliquely decreases as the number of scanning lines increases. Further, although an optical compensation cell or an optical compensation film is used to reduce coloring, a perfect black and white display has not yet been realized, and the appearance is not very good.

Next, the antiferroelectric liquid crystal display device according to the prior art has various problems as described below.

(1) When the backlight is used in a reflection type which requires no backlight and consumes low power, sufficient brightness is rarely obtained because ambient light is used as illumination light. If a conventional antiferroelectric liquid crystal display is used in a reflection type, the entire screen is dark and difficult to see because pixels between which no voltage is applied are always in an off state as described above.

(2) When used in a reflection type, importance is placed on the brightness in the ON state rather than the contrast ratio in order to make the display easier to see. The light transmittance of the contrast ratio and ON state of a reflection type, respectively CR _r, I _r, they each CR _t of a transmission type, if indicated by I _t, the following equation holds.

CR _r = (CR _t ) ² > 1 (1) I _r = (I _t ) ² <1 (2) That is, even if the contrast ratio CR _t in the transmissive type is relatively low, the reflective type may be used. Thus, a sufficient contrast ratio can be obtained. Further, since the light transmittance is smaller than 1, in order to obtain a sufficient brightness in reflection type must close as possible to 1 the light transmittance I _t in the transmission type.
Since the light transmittance in the ON state is proportional to sin ² (2θ), I _t
Is preferably set to θ = 45 ° in order to make close to 1. However, as can be seen from FIG. 4, the ferroelectric phase (+) and the ferroelectric phase
To make the brightness equal when (-) is selected,
OA and P1 must be parallel. Further, the tilt angle θ is a value determined by the properties of the liquid crystal material, and is not a property that can be freely set. Therefore, the prior art is disadvantageous for the reflection type.

(3) Since θ changes with temperature, the brightness of the screen changes with temperature. This is not preferable for the reflection type in which brightness is emphasized.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a large-sized high-definition screen which is bright and has a good appearance even when used in a reflection type, and whose brightness does not change with temperature. A liquid crystal display device is provided.

[0016]

According to the present invention, there is provided a liquid crystal display device comprising a polarizing plate and a liquid crystal element having an antiferroelectric liquid crystal sandwiched between a pair of substrates. The polarizing plate is arranged so that the light transmittance of the liquid crystal in the antiferroelectric phase is higher than the light transmittance of the ferroelectric phase.

[0017]

The present invention will be described below in detail with reference to examples.

FIG. 2 is a schematic diagram of the liquid crystal display device used in this embodiment. Form a transparent electrode 22 on a glass substrate 21,
Further, polyimide was applied as a liquid crystal alignment film 23. Further, a uniaxial orientation treatment was performed by a rubbing method. However, it is optional whether or not the two glass substrates are coated with a liquid crystal alignment film and subjected to a uniaxial alignment treatment. Further, an insulating layer may be provided between the liquid crystal alignment film and the transparent electrode. An antiferroelectric liquid crystal 24 is sealed between such two substrates to form a liquid crystal element. Then, this liquid crystal element is sandwiched between two polarizing plates 25, and a reflecting plate 26 is further provided.

FIG. 1 is a diagram showing a display method according to the present invention. The angle between P1 and OA is different from the conventional method,
°. If a voltage is not applied, the phase is an antiferroelectric phase and the optical axis is OA forming an angle of 45 ° with P1, so that the optical axis is turned on. When a voltage is applied, the ferroelectric phase (+) or the ferroelectric phase (-) is obtained, and the optical axis is 45 ° -θ with P1 or P2.
OF (+) or OF (−) forming an angle of the above, so that the light is in an off state with low light transmittance. Then, between pixels to which no voltage can be applied is always on. In the prior art, IA
<IF, but in the present invention, IA> IF.

When the temperature changes, OF (+) or OF (-)
Although the angle between P1 and P1 changes, the angle between OA and P1 does not change, so that the brightness in the ON state (non-display portion) is always constant. However, the contrast ratio changes with temperature. Generally, the higher the temperature, the smaller θ becomes,
The light transmittance in the off state increases, and the contrast ratio decreases. However, as described above, since the contrast ratio is apparently higher in the reflection type, the merit of the brightness of the screen including pixels is greater than the decrease in the contrast ratio.

As a method of injecting the liquid crystal, a method is used in which after the inside is evacuated, the liquid crystal is dropped into an injection port and the liquid crystal is injected by a capillary phenomenon. The smaller the inter-substrate distance (liquid crystal layer thickness: d), the smaller the total amount of liquid crystal to be injected, but the lower the injection speed. In addition, there is a high possibility that the liquid crystal does not completely enter and air bubbles remain, or dust is mixed in, resulting in unevenness in the distance between the substrates, thereby lowering the yield. Therefore, in manufacturing, the larger the distance between the substrates, the better. However, the antiferroelectric liquid crystal layer is merely a birefringent medium, and the transmitted light spectrum when the antiferroelectric liquid crystal is sandwiched between two polarizing plates has a retardation d · Δn
(Δn is the birefringence of the liquid crystal layer) as shown in FIG. Therefore, in order to perform monochrome display, d
D must be set so that the value of Δn is about 0.25 μm, and if d is increased, the color will be colored. Since Δn varies depending on the phase, Δn in the antiferroelectric phase and the ferroelectric phase of the antiferroelectric liquid crystal are represented by Δn (A) and Δn (F), respectively. Although the values differ depending on the liquid crystal material, for example, Δn (A) = 0.088, Δn (F) = 0.113.
Δn (A) <Δn (F). Therefore, the optimum value of d is different between the case where the ON state is given by the ferroelectric phase (prior art) and the case where the ON state is given by the antiferroelectric phase (the present invention). The optimal values in the prior art and the present invention are d
When expressed by (F) and d (A), d (A) = d (F) × 0.113 / 0.088, and the optimum value in the present invention is larger. As described above, according to the present invention, the thickness of the liquid crystal layer (distance between the substrates) can be made larger than that of the related art, so that the time for injecting the liquid crystal between the substrates is reduced, and the liquid crystal does not enter completely. The possibility that bubbles remain or unevenness in the distance between the substrates is reduced, and the yield increases. In the following examples, the thickness of the liquid crystal layer was set so that monochrome display was possible.

Example 1 In this example, (S) -4- (1-trifluoromethylheptyloxycarbonyl) -phe was used as a liquid crystal material.
nyl 4- (5-dodecyloxypyrimidin-2-yl) benzoate (TF
Using MHPDOPB), the ambient temperature was set to be an antiferroelectric phase. Table 1 shows the contrast ratio (CR) and the brightness at each temperature. However, each value in the reflection type is a value obtained from Expressions (1) and (2). For comparison, Table 2 shows the characteristics of the liquid crystal display device manufactured by the conventional technique. Comparing the two, the comparative example has a much higher contrast ratio. However, if the contrast ratio is 1:15 or more, the difference can hardly be distinguished in practical use. On the other hand, the brightness in the ON state is brighter in the present invention, and since the pixels are always in the ON state, the entire screen is bright and easy to see. Further, in the conventional example, the brightness in the ON state changes depending on the temperature, but according to the present invention, the brightness is constant regardless of the temperature.

DSTN and FTN have a high contrast ratio when the display screen is viewed from the front, but the contrast ratio sharply decreases as the viewing angle is inclined. However, when the antiferroelectric liquid crystal is used, the contrast ratio hardly changes even when viewed from a direction inclined by 50 ° from the normal line of the screen, and a very easy-to-view display is possible.

[0024]

[Table 1]

[0025]

[Table 2]

(Example 2) Here, a liquid crystal material was
(1-methylheptyloxycarbonyl) -phenyl 4'-octyl-oxybip
Using henyl-4-carboxylate, the environmental temperature was set to be an antiferroelectric phase. Table 3 shows the contrast ratio and brightness at each temperature. Table 4 shows the characteristics of the liquid crystal display device manufactured by the conventional technique. The contrast ratio of the liquid crystal display according to the present invention is a very low value. But,
When used as a reflection type in which only ambient light can be used as illumination light, the brightness of the on state of the liquid crystal display device according to the prior art is substantially the same level as the brightness of the off state of the liquid crystal display device according to the present invention. It should be noted. The liquid crystal display device according to the prior art has a high contrast ratio because the liquid crystal material used here has a small θ, but is very dark and cannot be used. On the other hand, if the present invention is applied to this liquid crystal material, it can be used because the contrast ratio is low but the screen is bright.

[0027]

[Table 3]

[0028]

[Table 4]

(Embodiment 3) The liquid crystal display device prepared in Embodiment 1 was obtained by removing the polarizing plate 25 adjacent to the reflection plate. The liquid crystal material used is TFMHPDOB. However, in order to realize the same monochrome display as in the first embodiment, the thickness of the liquid crystal layer was set to １ ／ of that in the first embodiment. In this case, since only one polarizing plate is provided, the display characteristics are substantially equivalent to the transmission type. As can be seen from Table 1, the contrast ratio in the transmission type is lower than that in the reflection type, but the screen is brighter.
In this embodiment, the reflection plate is independently provided using two transparent substrates. Alternatively, a light-reflective substrate or electrode on the side where no polarizing plate is provided may be used.

In each of the above embodiments, the reflection type is used, but the transmission type can be used.

[0031]

As described above, according to the present invention, the screen is bright and has a good appearance even when used in a reflection type, the brightness does not change with temperature, and the distance between the substrates is increased. The liquid crystal injection time is shortened,
This has the effect of improving the yield.

[Brief description of the drawings]

FIG. 1 is a diagram showing a display method according to the present invention.

FIG. 2 is a schematic view of a liquid crystal display device used in an example.

FIG. 3 is a diagram showing a transmitted light spectrum.

FIG. 4 is a diagram showing a relationship between an optical axis of a liquid crystal and a polarizing plate according to a conventional method.

FIG. 5 is a diagram showing hysteresis characteristics in an electric field induced antiferroelectric-ferroelectric phase transition.

FIG. 6 is a diagram showing an example of a driving method for driving the antiferroelectric liquid crystal having the hysteresis characteristic shown in FIG. 5 in a simple matrix.

[Explanation of symbols]

 OA: Optical axis in antiferroelectric phase OF (+): Optical axis in ferroelectric phase (+) OF (-): Optical axis in ferroelectric phase (-) P1, P2, polarization Plate light absorption axis 21 Glass substrate 22 Transparent electrode 23 Liquid crystal alignment film 24 Antiferroelectric liquid crystal layer 25 Polarizing plate 26 ... Reflector

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/141 G02F 1/1337 G02F 1/1335 G02F 1/133 G09F 9/30 G09G 3/36

Claims (1)

(57) [Claims] 1. A liquid crystal display device comprising a polarizing plate and a liquid crystal element having an antiferroelectric liquid crystal sandwiched between a pair of substrates, wherein the antiferroelectric liquid crystal has a high light transmittance in an antiferroelectric phase. A liquid crystal display device, wherein the polarizing plate is arranged so as to have a higher light transmittance in a dielectric phase.