JPH05264956A - Double layer stn type liquid crystal display device - Google Patents

Abstract

(57) [Summary] [Object] To provide a two-layer STN type liquid crystal display device capable of always displaying an image of good quality regardless of temperature change. [Structure] A pair of substrates 21 and 22 of a color compensating liquid crystal cell 2 are provided with retardation adjusting electrodes 26 and 27, respectively, and a driving circuit 34 for applying a retardation adjusting voltage having a value according to temperature to the electrodes 26 and 27. Is connected to the color compensating liquid crystal cell 2 and the retardation of the color compensating liquid crystal cell 2 is adjusted according to the temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-layer STN type liquid crystal display device.

[0002]

2. Description of the Related Art Recently, as a dot matrix liquid crystal display device for displaying a television image or the like, in order to improve the multiplex driveability, liquid crystal molecules have a twist angle (200 ° to 270 °) larger than that of a normal TN type liquid crystal cell. STN type liquid crystal display devices using STN (Super Twisted Nematic) type liquid crystal cells that are twist-aligned with each other have been developed.

In this STN type liquid crystal display device, a pair of polarizing plates are arranged so as to sandwich an STN type liquid crystal cell, and one polarizing plate has its transmission axis direction on the one polarizing plate side of the liquid crystal cell. Is arranged with a certain angle of deviation (35 ° to 50 °) with respect to the liquid crystal molecule orientation direction on the substrate surface of, and linearly polarized light incident on the liquid crystal cell through one polarizing plate (incident side polarizing plate) The birefringence of the liquid crystal layer causes elliptically polarized light, and the other polarizing plate (exiting side polarizing plate) controls transmission of light emitted from the liquid crystal cell to display an image.

However, the STN type liquid crystal display device has a problem that the display screen is colored. This is because the liquid crystal layer has different anisotropy of refractive index with respect to light of each wavelength, and thus the transmittance of light in a specific wavelength range increases.

Therefore, a two-layer STN type liquid crystal display device in which an STN type color compensating liquid crystal cell is arranged on the light incident side or the light emitting side of the STN type liquid crystal display cell has been conventionally proposed. FIG. 7 is a cross-sectional view of a conventional double-layer STN type liquid crystal display device, in which a color compensating liquid crystal cell is arranged on the emission side of a display liquid crystal cell.

This two-layer STN type liquid crystal display device is formed by stacking two STN type liquid crystal cells 1 and 2 and providing a pair of polarizing plates 3 and 4 with the two liquid crystal cells 1 and 2 sandwiched therebetween. Of the two STN type liquid crystal cells 1 and 2, the liquid crystal cell 1 on the upper side (light incident side) in the figure is for display,
The liquid crystal cell 2 on the lower side (light emission side) is used for color compensation. Hereinafter, the liquid crystal cell 1 for display will be referred to as a display cell, and the liquid crystal cell 2 for color compensation will be referred to as a compensation cell.

The display cell 1 has a liquid crystal 13 enclosed between a pair of transparent substrates 11 and 12 which are polymer-bonded through a frame-shaped sealing material 10, and the surfaces of the substrates 11 and 12 facing each other are opposite to each other. Transparent display electrodes 14 and 15 are formed, and alignment films 16 and 17 are further formed thereon. The display cell 1 is of a simple matrix type, and one of the display electrodes 14 and 15 is a scanning electrode and the other is a signal electrode.

On the other hand, the compensating cell 2 has a liquid crystal 23 sealed between a pair of transparent substrates 21 and 22 which are polymer-bonded via a frame-shaped sealing material 20. There is no display electrode, and only the alignment films 24 and 25 are formed.

The alignment films 16, 17 and 24, 25 of the display cell 1 and the compensation cell 2 are obtained by rubbing the film surface of a horizontal alignment film made of a polyimide resin or the like. The alignment treatment directions (rubbing directions) of the films 16 and 17 are deviated from each other by 200 ° to 270 °,
The alignment treatment directions of the alignment films 24 and 25 on both substrate surfaces of the compensation cell 2 are also deviated by the same angle.

The same nematic liquid crystal is used for the liquid crystals 13 and 23 of the display cell 1 and the compensation cell 2, and the liquid crystal of one of the display cell 1 and the compensation cell 2 is levorotatory. An optically active substance (chiral liquid crystal or the like) is mixed, and a dextrorotatory optically active substance is mixed in the liquid crystal of the other cell. The liquid crystal molecules of the display cell 1 and the compensation cell 2 have the same twist angle (200 ° to 270 °) in opposite directions.
The display cell 1 and the compensation cell 2 have almost the same retardation Δn · d (the product of the refractive index anisotropy Δn of the liquid crystal and the layer thickness d of the liquid crystal layer). Designed to be equal.

The orientation directions of the liquid crystal molecules on the surfaces of the substrates 12 and 21 on the sides of the display cell 1 and the compensation cell 2 which are adjacent to each other are substantially orthogonal to each other. Further, of the pair of polarizing plates 3 and 4, the transmission axis direction of the lower polarizing plate (emission side polarizing plate) 4 is the liquid crystal cell (compensation cell) 2 on the lower polarizing plate 4 side.
The direction of the transmission axis of the upper polarizing plate (incident side polarizing plate) 3 is
It is substantially orthogonal to the transmission axis direction of the lower polarizing plate 4.

In FIG. 7, reference numeral 30 denotes a display drive circuit for applying a display drive voltage between the display electrodes 14 and 15 of the display cell 1, and the two-layer STN type liquid crystal display device has the display cell 1 Is driven by multiplex driving with high duty.

This two-layer STN type liquid crystal display device is
The above-described display coloring in the display cell 1 of the mold is canceled by the compensation cell 2. As described above, the liquid crystal molecule twist directions of the display cell 1 and the compensation cell 2 are opposite to each other, and both cells are If the retardation values Δn · d of 1 and 2 are set to be substantially equal to each other, the axes of the ellipses of the elliptically polarized light for each wavelength caused by the optical rotatory dispersion phenomenon of the liquid crystal layers of the display cell 1 and the compensation cell 2 are substantially the same. Coloring of the display due to leakage of light in a specific wavelength range can be eliminated.

[0014]

However, the above-mentioned conventional two-layer STN type liquid crystal display device has a problem that the display quality changes depending on the ambient temperature thereof, and particularly the display quality at low temperature deteriorates.

This is because the response (response speed) of the display-driven display cell 1 changes according to the change in viscosity with temperature (the viscosity of the liquid crystal is low at high temperature and the response of the liquid crystal cell is fast, but at low temperature). This is because the viscosity of the liquid crystal is high and the response of the liquid crystal cell becomes slower.) When the temperature becomes low and the response of the display cell 1 becomes slower, the display quality of the liquid crystal display device becomes worse. An object of the present invention is to provide a two-layer STN type liquid crystal display device which can always display an image of good quality regardless of temperature change.

[0016]

According to the present invention, liquid crystal molecules are provided on the incident side or the exit side of an STN type liquid crystal display cell having a display electrode in a direction opposite to the liquid crystal molecule twist direction of the display liquid crystal cell. In a two-layer STN type liquid crystal display device in which STN type color compensating liquid crystal cells in a twisted arrangement are laminated, and a pair of polarizing plates are arranged between the liquid crystal cells,
A pair of substrates for the color compensating liquid crystal cell is provided with a retardation adjusting electrode respectively, and a retardation adjusting voltage having a value depending on temperature is applied to the retardation adjusting electrode of the color compensating liquid crystal cell, and according to the temperature. A drive circuit for applying a voltage drive signal to the display electrodes of the display liquid crystal cells is connected to each of the liquid crystal cells.

[0017]

That is, in the present invention, the retardation of the color compensating liquid crystal cell is adjusted according to the temperature by applying a retardation adjusting voltage having a value depending on the temperature to the liquid crystal of the color compensating liquid crystal cell. That is, the retardation of the liquid crystal cell for color compensation is made smaller than the retardation of the liquid crystal cell for display as the temperature becomes lower, and the difference between the retardation of the liquid crystal cell for display and the retardation of the liquid crystal cell for color compensation is When the display liquid crystal cell is adjusted to be larger, the threshold voltage of the display liquid crystal cell shifts in the positive direction, so if the display liquid crystal cell is driven by a high voltage drive signal according to the shift amount, Even when the liquid crystal cell of the display liquid crystal cell has a high viscosity and the liquid crystal cell has a high viscosity, the response of the liquid crystal display cell does not slow down, and the liquid crystal display device displays It is possible to prevent deterioration of quality, thus always be a good display quality.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a sectional view of a two-layer STN type liquid crystal display device. In FIG. 1, components corresponding to those of the conventional liquid crystal display device shown in FIG. 7 are designated by the same reference numerals and the description thereof will be omitted.

In this embodiment, a color compensating liquid crystal cell (hereinafter, referred to as compensation cell) 2 laminated on an incident side or an outgoing side of a display liquid crystal cell (hereinafter, referred to as display cell) 1 is surrounded by a sealing material 20. Further, the retardation adjusting electrodes (transparent electrodes) 26 and 27 having an area covering almost the entire liquid crystal enclosing region are provided, and the alignment films 24 and 25 of the compensation cell 2 are provided.
Is formed on the retardation adjusting electrodes 26 and 27.

The retardation adjusting electrodes 26, 27
Is for adjusting the retardation Δn ₂ · d ₂ of the compensation cell 2 by applying a voltage to the liquid crystal 23 of the compensation cell 2. When a voltage is applied to the liquid crystal 23 of the compensation cell 2,
The tilt angle formed by the long axis of the liquid crystal molecules and the substrate surface changes according to the applied voltage, and the refractive index anisotropy Δn ₂ of the liquid crystal 23 changes (the liquid crystal layer thickness d ₂ is constant), and the retardation Δn of the compensation cell 2 changes. ₂ · d ₂ changes.

The molecules of the liquid crystals 13 and 23 of the display cell 1 and the compensation cell 2 are twist-arranged in opposite directions at the same twist angle (200 ° to 270 °) as in the conventional liquid crystal display device. And the retardation Δn ₁ · d _{1 of the} display cell 1 and the retardation Δn ₂ · d of the compensation cell 2
It is almost equal to ₂ . Further, the orientations of liquid crystal molecules on the surfaces of the substrates 12 and 21 of the display cell 1 and the compensation cell 2 which are adjacent to each other are substantially orthogonal to each other, and the transmission axis directions of the pair of polarizing plates 3 and 4 are the same as those of the conventional liquid crystal display. It is in the same direction as the device.

On the other hand, in FIG. 1, reference numeral 31 is a temperature sensor for detecting the temperature around the liquid crystal display device. This temperature sensor 31 is provided on both sides of the facing portion between the display cell 1 and the compensation cell 2. 1 and 2 are arranged close to each other. The temperature sensor 31 is connected to the temperature detection circuit 32. The temperature detection circuit 32 outputs a temperature signal according to the temperature detected by the temperature sensor 31, and the temperature signal is sent to the control unit 33.

In addition, the compensation cell 2 includes both substrates 2
A compensation cell drive circuit 34 for applying a retardation adjustment voltage (AC voltage) having a value according to temperature is connected between the retardation adjustment electrodes 26 and 27 of Nos. 1 and 22. The compensation cell drive circuit 34 is controlled by the control unit 33.

That is, the control unit 33 controls the output voltage of the compensation cell drive circuit 34 according to the temperature signal from the temperature detection circuit 32. For example, the operating temperature range of the liquid crystal display device is 0 ° C. to 50 ° C. If it is, the temperature sensor 31
When the detection temperature is 50 ° C., the output voltage of the compensation cell drive circuit 34 is set to 0 V, and when the detection temperature is lower than 50 ° C., the compensation cell drive circuit 34 outputs the retardation adjustment voltage and the detection temperature is low. The output voltage of the compensation cell drive circuit 34 is increased with the increase (approaching 0 ° C.).

Further, the control unit 33 controls the display drive circuit 3
The voltage of the drive signal applied between 0 and the display electrodes 14 and 15 of the display cell 1 is also controlled according to the temperature (the temperature signal from the temperature detection circuit 32).

Next, the display operation of the above-mentioned double-layer STN type liquid crystal display device will be described. This liquid crystal display device is driven for display by multiplex driving the display cell 1 at high duty, and the display in the display cell 1 is performed. Is colored by the compensation cell 2.

When the ambient temperature of the liquid crystal display device (the temperature detected by the temperature sensor 31) is as high as 50 ° C., the retardation adjusting voltage is not applied between the retardation adjusting electrodes 26 and 27 of the compensation cell 2. The liquid crystal 23 of the compensation cell 2 is in the initial alignment state, and therefore the retardation Δn ₂ · d ₂ of the compensation cell 2 is almost equal to the retardation Δn ₁ · d ₁ of the display cell 1. When the temperature is 50 ° C, the viscosity of the liquid crystal is low,
Since the liquid crystal molecules of the display cell 1 start up with good response to the application of the driving voltage, the response of the display cell 1 is fast and therefore the quality of the displayed image is good.

On the other hand, as shown in FIG. 2, the response of the display cell 1 is delayed because the response time is increased as the temperature is lowered, and the response of the display cell 1 is delayed.
The display quality of the liquid crystal display device deteriorates.

Therefore, in this embodiment, when the temperature becomes lower than 50 ° C., a retardation adjusting voltage having a value corresponding to the temperature is applied between the compensation cell driving circuit 34 and the retardation adjusting electrodes 26 and 27 of the compensation cell 2. applied to adjust the retardation [Delta] n ₂ · d ₂ of the compensation cell 2, the retardation [Delta] n ₂ · d ₂ of the compensation cell 2, the display cell 1 along with the temperature becomes lower retardation [Delta] n ₁ · d
And less than _1, the retardation Δn ₁ · of the display cell 1
The difference between d ₁ and the retardation Δn ₂ · d ₂ of the compensation cell 2 is controlled to be large.

That is, FIG. 3 shows that the refractive index anisotropy Δ of the liquid crystals 13 and 23 of the display cell 1 and the compensation cell 2 with respect to the temperature change.
The change of n is shown, and the solid line shows the change of Δn ₁ (value in the state where the display drive voltage is not applied) of the liquid crystal 13 of the display cell 1, and the broken line shows the change of Δn ₂ of the liquid crystal 23 of the compensation cell 2. Shows.

As shown in FIG. 3, the liquid crystal 13 of the display cell 1
Δn ₁ of becomes larger as the temperature becomes lower. On the other hand, since the same liquid crystal material as the liquid crystal 13 of the display cell 1 is used for the liquid crystal 23 of the compensation cell 2, if the retardation adjustment voltage is not applied to the compensation cell 2, the compensation cell 2
Δn ₂ of the liquid crystal 23 also changes as shown by the solid line,
Since the retardation adjustment voltage is applied to the liquid crystal 23 of the compensation cell 2 from the compensation cell drive circuit 34, Δn ₂ of the liquid crystal 23 of the compensation cell ₂ is equal to the change in the tilt angle of the liquid crystal molecules due to the application of the voltage. It becomes smaller than Δn 1 of the liquid crystal 13 of ₁ .

Further, the retardation adjustment voltage applied from the compensation cell driving circuit 34 to the compensation cell 2 becomes higher as the temperature becomes lower as described above, and therefore Δn ₁ of the liquid crystal 13 of the display cell 1 is increased. and the difference between [Delta] n ₂ of the liquid crystal 23 of the compensation cell 2 increases as the temperature decreases. In this embodiment, the retardation adjustment voltage is applied from the compensation cell drive circuit 34 to the compensation cell 2 when the temperature becomes lower than 50 ° C.
In this case, Δn ₂ of the liquid crystal 23 of the compensation cell ₂ is
This is the same as Δn ₁ of the liquid crystal 13.

Since the liquid crystal layer thicknesses d ₁ and d ₂ of the display cell 1 and the compensation cell 2 are constant (d ₁ = d ₂ ), as described above, Δn ₂ of the liquid crystal 23 of the compensation cell 2 depends on the temperature. If the change in the display cell is adjusted by applying a retardation adjustment voltage, the retardation Δn ₂ · d ₂ of the compensation cell 2 is made smaller than the retardation Δn ₁ · d _{1 of} the display cell 1 as the temperature becomes lower, 1 retardation Δn ₁ · d ₁ and compensation cell 2 retardation Δn
The difference from ₂ · d ₂ can be increased.

FIG. 4 shows the retardation difference (Δn ₁ · d ₁ −Δn ₂ ···) between the display cell 1 and the compensation cell 2 at each temperature.
d ₂ ), and this retardation difference is increased as the temperature decreases.

FIGS. 5 and 6 show the electro-optical characteristics of the double-layer STN type display device at low temperature. In FIG. 5, the retardation Δn ₂ · d ₂ of the compensation cell 2 is the retardation Δn ₁ ·. Characteristics when equal to d ₁ , FIG.
Shows the characteristics when the retardation Δn ₂ · d ₂ of the compensation cell 2 is made smaller than the retardation Δn ₁ · d ₁ of the display cell 1.

[0037] FIG. 5 and characteristics as can be seen by comparing the Figure 6, at low temperatures, the retardation [Delta] n ₂ · d ₂ of the compensation cell 2 of the display cell 1 retardation [Delta] n ₁
The threshold voltage Vth of the display cell 1 when it is smaller than d ₁ (FIG. 6) is the retardation Δn _{2 of the} compensation cell _2.
· D ₂ is shifted in the positive direction with respect to the threshold voltage Vth of the display cell 1 at time equal to the retardation [Delta] n ₁ · d ₁ of the display cell 1 (Fig. 5).

As described above, when the retardation Δn ₂ · d ₂ of the compensation cell 2 is adjusted, the threshold voltage of the display liquid crystal cell 1 is shifted accordingly. The voltage value of the drive signal applied between the display electrodes 14 and 15 is also controlled according to the temperature,
When the threshold voltage of the display liquid crystal cell 1 is shifted in the positive direction, if the display liquid crystal cell 1 is driven by a drive signal of a high voltage corresponding to the shift amount, the temperature becomes low and the display liquid crystal cell 1 Even if the liquid crystal 13 has a high viscosity, the response of the display liquid crystal cell 1 is not delayed, and it is possible to prevent the deterioration of the display quality of the liquid crystal display device. Therefore, according to the above embodiment, the liquid crystal display device can always have good display quality regardless of the temperature change.

The value of the retardation adjusting voltage applied to the compensation cell 2 may be determined according to the change of Δn ₁ of the liquid crystal 13 of the display cell 1 with respect to the temperature change. For example, Δn ₁ of the liquid crystal 13 of the display cell 1 may be determined. Is changed as shown by the solid line in FIG. 3, the Δn of the liquid crystal 23 is added to the compensation cell 2.
₂ by applying a retardation adjustment voltage that changes as indicated by a broken line in FIG. 3, the retardation difference of the compensation cell 2 with respect to the display cell _{1 (Δn 1 · d 1 -Δn} 2 ·
d ₂ ) may be adjusted as shown by the solid line in FIG.

The following [Table 1] shows 0 ° C, 20 ° C, 40 ° C,
At each temperature of 50 ° C., Δn ₁ and Δn ₂ between the liquid crystal 13 of the display cell 1 and the liquid crystal 23 of the compensation cell 2 and the Δn difference between the liquid crystal 13 of the display cell 1 and the liquid crystal 23 of the compensation cell 2 shown in FIG. (Δn ₁ −Δn ₂ ) and the retardation difference (Δn ₁ · d ₁ −Δn ₂ · d ₂ ) shown in FIG. 4 are shown. The retardation difference in [Table 1] is that the liquid crystal layer thicknesses d ₁ and d ₂ of the display cell 1 and the compensation cell ₂ are d ₁ = d ₂ = 5 μm.
It is a value when m.

[0041]

[Table 1]

In the above liquid crystal display device, the lower the temperature, the larger the retardation difference between the display cell 1 and the compensation cell 2, but this retardation difference is 0.100 μm at the maximum (when the temperature is 0 ° C.), With such a retardation difference, the display cell 1 can be colored by the compensation cell 2 for display coloration.

The retardation difference does not necessarily have to be adjusted strictly, but may be adjusted in a straight line connecting the value at 50 ° C. and the value at 0 ° C. as shown by the chain line in FIG. 4, for example. Good.

[0044]

The two-layer STN type liquid crystal display device of the present invention is
By applying a retardation adjustment voltage having a value depending on temperature to the liquid crystal of the liquid crystal cell for color compensation, the retardation of the liquid crystal cell for color compensation is adjusted according to the temperature. Of the retardation of the display liquid crystal cell as the temperature becomes lower, and if the difference between the retardation of the display liquid crystal cell and the retardation of the color compensation liquid crystal cell is increased, Since the threshold voltage of the display liquid crystal cell shifts in the positive direction, if the display liquid crystal cell is driven by a drive signal of a high voltage according to the shift amount, the temperature becomes low and Even if the viscosity of the liquid crystal becomes high, the response of the display liquid crystal cell is not delayed, and it is possible to prevent the deterioration of the display quality of the liquid crystal display device. , And therefore it is always possible to improve display quality.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a two-layer STN type liquid crystal display device showing an embodiment of the present invention.

FIG. 2 is a diagram showing a change in response of liquid crystal depending on temperature.

FIG. 3 is a diagram showing changes in refractive index anisotropy Δn of liquid crystals of a display liquid crystal cell and a compensating liquid crystal cell with respect to temperature change.

FIG. 4 shows the retardation difference (Δn ₁ · d ₁ −Δn ₂ ···) between the display liquid crystal cell and the compensating liquid crystal cell at each temperature.
shows the d _2).

FIG. 5: Retardation Δn ₂ · d _{2 of the} compensating liquid crystal cell
Is an electro-optical characteristic diagram when is equal to the retardation Δn ₁ · d _{1 of the} display liquid crystal cell.

FIG. 6 is a retardation Δn ₂ · d _{2 of the} compensating liquid crystal cell.
FIG. 6 is an electro-optical characteristic diagram in which is smaller than the retardation Δn ₁ · d _{1 of the} liquid crystal cell for display.

FIG. 7 is a sectional view of a conventional double-layer STN type liquid crystal display device.

[Explanation of symbols]

1 ... Display liquid crystal cell, 11, 12 ... Substrate, 13 ... Liquid crystal,
14, 15 ... Display electrodes, 16, 17 ... Alignment film, 2 ... Color compensation liquid crystal cells 21, 22 ... Substrate, 23 ... Liquid crystal, 2
4, 25 ... Alignment film, 26, 27 ... Retardation adjusting electrode, 34 ... Compensation cell drive circuit.

Claims (1)

[Claims] 1. An STN type liquid crystal cell having a display electrode, wherein liquid crystal molecules are twist-arranged in a direction opposite to a twist direction of liquid crystal molecules of the liquid crystal cell for display on an incident side or an emitting side of the liquid crystal cell for display. A two-layer STN in which liquid crystal cells for color compensation are laminated, and a pair of polarizing plates are arranged between the liquid crystal cells.
Type liquid crystal display device, a retardation adjusting electrode is provided on each of the pair of substrates of the color compensating liquid crystal cell, and a retardation adjusting voltage having a value according to temperature is applied to the retardation adjusting electrode of the color compensating liquid crystal cell. A two-layer STN type liquid crystal display device, wherein a drive circuit for applying a drive signal of a voltage according to temperature to a display electrode of the display liquid crystal cell is connected to each of the liquid crystal cells.