JP4725871B2 - Liquid crystal display - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a D-STN liquid crystal display device in which a decrease in contrast due to a temperature change is improved by utilizing characteristics of a liquid crystal material.
[0002]
[Prior art]
2. Description of the Related Art A D (double layered) -STN type liquid crystal display device, which is an improvement of an STN (super twisted nematic) type liquid crystal display device (STN-LCD), is widely known as a liquid crystal display for personal computers, televisions, digital cameras, and the like.
[0003]
This liquid crystal display device (D-STN-LCD) can display black and white while preventing coloring of the back shadow, which was a drawback of STN-LCD.
FIG. 6 is a schematic diagram showing a D-STN-LCD. As shown in the figure, a two-layer liquid crystal display device having a driving STN (driving cell) 10 and a compensating STN (compensation cell) 20 is obtained. ing.
[0004]
In this figure, 11 and 12 indicate the glass substrate of the drive cell 10, 13 indicates the liquid crystal of the drive cell 10, 21 and 22 indicate the glass substrate of the compensation cell 20, and 23 indicates the liquid crystal of the compensation cell 20.
Reference numeral 14 denotes an incident light side polarizing plate, and reference numeral 24 denotes an outgoing light side polarizing plate.
[0005]
The drive cell 10 and the compensation cell 20 of this D-STN-LCD have the twist directions of the liquid crystal molecules reversed, but other configurations and conditions (twist angle, pretilt angle, liquid crystal material, liquid crystal birefringence) The ratio (Δn), cell thickness (d), etc.) are all the same.
The twist direction of the liquid crystal molecules is defined by a chiral material added to the liquid crystal, and the twist angle is about 160 to 270 degrees.
[0006]
The D-STN-LCD configured as described above compensates for an interference phenomenon in which incident polarized light is caused by the birefringence effect of the drive cell 10 (colored is caused by this interference) by the compensation cell 20 having a reverse twist direction. Resolves the problem of coloring the back shadow.
[0007]
As described above, the D-STN-LCD can display black and white without color, but the contrast ratio is also lowered because the OFF voltage is lowered as the temperature rises.
[0008]
FIG. 7 shows the temperature dependence of the OFF voltage at which the maximum contrast ratio can be obtained in the D-STN-LCD. Thus, the OFF voltage decreases as the temperature increases.
This is due to the physical properties of the liquid crystal material because the anisotropy of the elastic constant and dielectric constant of the liquid crystal decreases with increasing temperature.
[0009]
Therefore, a temperature compensation circuit is added to the drive circuit of the D-STN-LCD, and this temperature compensation circuit changes the drive voltage according to the temperature to prevent the contrast ratio from being lowered due to the decrease of the OFF voltage. ing.
The temperature compensation circuit is generally configured using a thermistor.
[0010]
[Problems to be solved by the invention]
As described above, the conventional D-STN-LCD is provided with a temperature compensation circuit in the drive circuit, so that the cost is increased accordingly.
Also, due to the temperature sensitivity variation of the thermistor and the temperature difference between the LCD cell part and the thermistor part, the drive voltage cannot be compensated with high precision, resulting in a decrease in contrast ratio. was there.
[0011]
In view of the above, an object of the present invention is to provide a D-STN-LCD that can obtain a high contrast using characteristics of a liquid crystal material without providing a temperature compensation circuit or the like.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, as a first invention, in a D-STN liquid crystal display device having a drive cell and a compensation cell , the temperature of the liquid crystal of the drive cell and the liquid crystal of the compensation cell is increased. As a result, the liquid crystal material in which the birefringence of the liquid crystal of the compensation cell is lower than the birefringence of the liquid crystal of the driving cell is formed to obtain a high contrast with respect to the temperature change, and the TV generated when the temperature rises. A liquid crystal display device is proposed in which an OFF voltage is set in the vicinity of the minimum value of the bounce on the characteristic curve.
[0013]
As a second invention, in a D-STN type liquid crystal display device having a drive cell and a compensation cell, the liquid crystal of the compensation cell has the same birefringence near room temperature as the liquid crystal of the drive cell. As the temperature rises, it is made of a liquid crystal material whose birefringence decreases with respect to the liquid crystal of the driving cell , and is configured to obtain a high contrast with respect to a temperature change, and on the TV characteristic curve generated when the temperature rises. A liquid crystal display device is proposed in which an OFF voltage is set in the vicinity of the minimum value of the bounce.
[0014]
As a third invention, in a D-STN type liquid crystal display device having a drive cell and a compensation cell, the liquid crystal of the compensation cell has the same birefringence at room temperature as the liquid crystal of the drive cell, and has a nematic phase. The phase transition temperature from the isotropic phase to the isotropic phase is made of a liquid crystal material that is low in comparison with the liquid crystal of the driving cell , and a high contrast is obtained with respect to the temperature change. A liquid crystal display device is proposed in which an OFF voltage is set in the vicinity of the local minimum value.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of a liquid crystal display device (D-STN-LCD) showing an embodiment of the present invention.
[0016]
In this figure, 30 is a driving cell, 31 and 32 are glass substrates of the driving cell, 33 is a liquid crystal of the driving cell, 40 is a compensation cell, 41 and 42 are glass substrates of the compensation cell, and 43 is compensation. The liquid crystal of the cell is shown.
Further, 34 is a polarizing plate on the incident light side, and 44 is a polarizing plate on the outgoing light side.
[0017]
The D-STN-LCD of this embodiment is characterized in that the liquid crystal 33 of the drive cell 30 and the liquid crystal 43 of the compensation cell 40 are formed of different liquid crystal materials. The conditions are the same as in the conventional example.
[0018]
As described above, the liquid crystal 33 of the drive cell 30 and the liquid crystal 43 of the compensation cell 40 are formed of different liquid crystal materials.
The liquid crystals 33 and 43 have the same birefringence near room temperature, and are formed of a liquid crystal material in which the birefringence of the liquid crystal 43 is lower than the birefringence of the liquid crystal 33 as the temperature rises.
[0019]
With this configuration, the OFF voltage for maximum contrast can be set similarly to room temperature without changing the drive voltage even in an environment where the temperature has risen.
As a result, even when the temperature rises, the OFF voltage does not decrease, and high contrast monochrome display is possible.
[0020]
In order to form the liquid crystals 33 and 43 as described above, for example, the liquid crystals 33 and 43 have the same birefringence near room temperature, and the phase transition temperature (T _NI ) from the nematic phase to the isotropic phase. Can be realized by using a liquid crystal material that makes the liquid crystal 43 lower than the liquid crystal 33.
[0021]
Next, an example in which the present invention is specifically implemented will be described in comparison with a conventional example.
"Conventional example"
Drive cell 1) Substrate: Glass substrate with transparent electrode 2) Alignment film: Alignment film made by Hitachi Chemical LQ-1800 Pretilt angle 6 degrees 3) Rubbing: Rayon rubbed cloth used 4) Twist angle: 240 ° Left twist 5 ) Cell thickness: 6.0 μm
6) Liquid crystal: Merck liquid crystal Δn = 0.15 (25 ° C.) T _NI = 105 ° C. 0.8 wt% of Merck S-811 added as a chiral material (left twist)
・ Compensation cell 1) Substrate: Glass substrate (no transparent electrode)
2) Alignment film: Alignment film made by Hitachi Chemical LQ-1800 Pretilt angle 6 degrees 3) Rubbing: Rayon rubbed cloth used 4) Twist angle: 240 ° right twist 5) Cell thickness: 6.0 μm
6) Liquid crystal: Merck liquid crystal Δn = 0.15 (25 ° C.) T _NI = 105 ° C. Add 0.8 wt% of Merck S-811 as a chiral material (right twist)
-Superposition of both cells Liquid crystal molecules facing each other are orthogonal-Polarizing plate 1) Polarizing plate: G-1220 made by Nitto Denko
2) Polarization angle of polarizing plate: Arranged so that the angle formed by the rubbing direction outside the driving cell and the polarizing plate transmission axis on the driving cell side is 45 degrees in a crossed Nicol arrangement.
"Example"
Drive cell 1) Substrate: Glass substrate with transparent electrode 2) Alignment film: Alignment film made by Hitachi Chemical LQ-1800 Pretilt angle 6 degrees 3) Rubbing: Rayon rubbed cloth used 4) Twist angle: 240 ° Left twist 5 ) Cell thickness: 6.0 μm
6) Liquid crystal: Merck liquid crystal Δn = 0.15 (25 ° C.) T _NI = 105 ° C. 0.8 wt% of Merck S-811 added as a chiral material (left twist)
・ Compensation cell 1) Substrate: Glass substrate (no transparent electrode)
2) Alignment film: Alignment film made by Hitachi Chemical LQ-1800 Pretilt angle 6 degrees 3) Rubbing: Rayon rubbed cloth used 4) Twist angle: 240 ° right twist 5) Cell thickness: 6.0 μm
6) Liquid crystal: Merck liquid crystal Δn = 0.15 (25 ° C.) T _NI = 94 ° C.
Add 0.8 wt% of Merck R-811 as chiral material (right twist)
-Superposition of both cells Liquid crystal molecules facing each other are orthogonal-Polarizing plate 1) Polarizing plate: G-1220 made by Nitto Denko
2) Polarization angle of polarizing plate: Arranged so that the angle formed by the rubbing direction outside the driving cell and the polarizing plate transmission axis on the driving cell side is 45 degrees in a crossed Nicol arrangement.
FIG. 2 is a characteristic diagram showing the temperature dependence of the contrast ratio between the conventional example and the example described above.
This characteristic diagram is measured by fixing the driving voltage of the D-STN-LCD to a voltage that obtains the maximum contrast ratio at room temperature.
[0024]
As can be seen from this characteristic diagram, the contrast ratio of the conventional D-STN-LCD greatly decreases as the temperature rises, but the contrast ratio of the D-STN-LCD of the present embodiment hardly decreases. .
[0025]
Further, the transmittance vs. voltage characteristics (TV characteristics) at room temperature (25 ° C.) and 80 ° C. were measured for the conventional example and the D-STN-LCD of this example.
The measurement result of the conventional D-STN-LCD is shown in FIG. 3, and the measurement result of the D-STN-LCD of this example is shown in FIG.
[0026]
As can be seen from the TV characteristics in FIG. 3, the D-STN-LCD of the conventional example starts with substantially the same curve at 25 ° C. and 80 ° C., but at 80 ° C., the drive is lower than 25 ° C. Since the change in transmittance starts with voltage, driving at the same drive voltage as that for obtaining the maximum contrast ratio at room temperature (25 ° C.) increases the transmittance at the OFF voltage and lowers the contrast ratio.
[0027]
Like the TV characteristic of FIG. 4, in the D-STN-LCD of this embodiment, a so-called bounce occurs in which the TV characteristic curve at 80 ° C. decreases once as the voltage rises. Since the OFF voltage is set, the contrast ratio does not decrease even when the temperature rises to 80 ° C.
[0028]
FIG. 5 is a characteristic diagram showing the temperature dependence of the birefringence Δn of the liquid crystal material forming the liquid crystals 33 and 43 of the drive cell 30 and the compensation cell 40 provided in the D-STN-LCD of this embodiment. is there.
[0029]
As can be seen from this characteristic diagram, the birefringence of the liquid crystals 33 and 43 of both cells is approximately equal at room temperature and below room temperature, but the birefringence of the liquid crystal 43 of the compensation cell 40 increases as the temperature increases. Smaller than the birefringence of the liquid crystal 33.
[0030]
As described above, the embodiments and examples of the present invention have been described. In order to cause a bounce in the TV characteristic of the D-STN-LCD, the birefringence Δn and cell thickness d of the compensation cell are set to Δ of the driving cell. What is necessary is just to make it smaller than nd.
[0031]
Therefore, when implementing the present invention, it is not limited to using liquid crystal materials having different phase transition temperatures (T _NI ) as in the above-described embodiments.
It can be implemented using other liquid crystal materials that can realize the relationship between the driving cell and the compensation cell as described above regardless of the phase transition temperature (T _NI ).
[0032]
【The invention's effect】
As described above, the liquid crystal display device of the present invention uses the characteristics of the liquid crystal material forming the liquid crystal of the driving cell and the compensation cell, and is configured to suppress the OFF voltage drop due to the temperature rise. Without providing a circuit or the like, it is possible to prevent a decrease in contrast due to a temperature rise.
[0033]
As a result, temperature compensation can be reliably performed, and a high-contrast liquid crystal display device suitable for reducing the cost of the product can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a D-STN-LCD showing an embodiment of the present invention.
FIG. 2 is a characteristic diagram showing the temperature dependence of the contrast ratio between a conventional D-STN-LCD and a D-STN-LCD embodying the present invention.
FIG. 3 is a characteristic diagram showing transmittance versus voltage characteristics of a conventional D-STN-LCD at room temperature (25 ° C.) and 80 ° C.
FIG. 4 is a characteristic diagram similar to FIG. 3 showing transmittance versus voltage characteristics of a D-STN-LCD embodying the present invention.
FIG. 5 is a characteristic diagram showing the temperature dependence of the birefringence of the liquid crystal provided in the driving cell and the compensation cell of the D-STN-LCD embodying the present invention.
FIG. 6 is a schematic configuration diagram of a D-STN-LCD shown as a conventional example.
FIG. 7 is a characteristic diagram showing temperature dependence of an OFF voltage at which a maximum contrast ratio can be obtained in a conventional D-STN-LCD.
[Explanation of symbols]
30 Driving cell 31, 32 Glass substrate 33 Liquid crystal 34 Polarizing plate 40 Compensation cell 41, 42 Glass substrate 43 Liquid crystal 44 Polarizing plate

Claims (3)

In a D-STN liquid crystal display device having a drive cell and a compensation cell,
The liquid crystal of the driving cell and the liquid crystal of the compensation cell are formed of a liquid crystal material in which the birefringence of the liquid crystal of the compensation cell is lower than the birefringence of the liquid crystal of the driving cell as the temperature rises .
A liquid crystal display device characterized in that a high contrast is obtained with respect to a temperature change, and an OFF voltage is set in the vicinity of a minimum value of a bounce on a TV characteristic curve generated when the temperature rises. In a D-STN liquid crystal display device having a drive cell and a compensation cell,
The liquid crystal of the compensation cell is made of a liquid crystal material that has the same birefringence near room temperature as the liquid crystal of the driving cell and lowers the birefringence of the liquid crystal of the driving cell as the temperature rises above room temperature. ,
A liquid crystal display device characterized in that a high contrast is obtained with respect to a temperature change, and an OFF voltage is set in the vicinity of a minimum value of a bounce on a TV characteristic curve generated when the temperature rises. In a D-STN liquid crystal display device having a drive cell and a compensation cell,
The liquid crystal of the compensation cell has the same birefringence near room temperature as the liquid crystal of the driving cell, and is formed of a liquid crystal material whose phase transition temperature from the nematic phase to the isotropic phase is lower than that of the liquid crystal of the driving cell .
A liquid crystal display device characterized in that a high contrast is obtained with respect to a temperature change, and an OFF voltage is set in the vicinity of a minimum value of a bounce on a TV characteristic curve generated when the temperature rises.

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