JPH11242200A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve display which is stable and has a high reliability by providing chiral nematic liquid crystal whose dielectric anisotropy is <= a specific value between a scanning electrode member and a signal electrode member to enlarge an operating margin with respect to the fluctuation of thicknesses of a liquid crystal layer. SOLUTION: A scanning electrode member 4 is composed of a transparent substrate 1 consisting of glass or the like, a scanning electrode 2 consisting of ITO or the like and an oriented film 3. On the other hand, relating to a signal electrode member 5, signal electrodes 7 consisting of ITO or the like and an oriented film 8 are formed on a transparent substrate 6 consisting of glass or the like. The scanning electrode member 4 and the signal electrode member 7 are disposed to face oppositely with chiral nematic liquid crystal 10. The chiral nematic liquid crystal 10 has an initial twisted structure and has two quasi-stable states different from the initial twisted state by the voltage difference to be impressed after a voltage is impressed on the liquid crystal. Then, components are adjusted so that the dielectric anisotropy &Delta;&epsi; of the liquid crystal 10 is <= 12.0 and preferably it satisfies 6.0<&Delta;&epsi; <9.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a memory-type bistable liquid crystal display device comprising a chiral nematic liquid crystal capable of gradation display.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open Nos. Hei 6-230751 and Hei 6-235920 propose a memory-type bistable liquid crystal display device using a chiral nematic liquid crystal. It describes two metastable states and a practical driving method for switching between the two.

The above-mentioned memory-type bistable liquid crystal display device has two
ON (white) display and O
This is a method for performing FF (black) display.

That is, switching between black display and white display is performed by a difference between selection voltages applied after a reset voltage, and with respect to a twist angle φ ₀ (180 °) in an initial state.
360 ° state (φ ₀ + π) when the selection voltage is set small
On the other hand, when the selection voltage is set large, the 0 ° state (φ ₀ −π) is realized.

In addition, various nematic liquid crystal materials have been proposed for such a memory-type bistable liquid crystal display device so as to have a uniform display characteristic over a large area (Japanese Patent Application Laid-Open Nos. 8-69017 and 8-69017). -69018, JP-A-8-69019).

[0006]

According to the memory-type bistable liquid crystal display device having the above structure, an unstable state (a state in which the 0 ° state and the 360 ° state are mixed) between the two metastable states. And switching is performed with this unstable state in between. However, when the display device is driven by a simple matrix, parameters that can be driven under optimal conditions have not yet been found, and therefore a problem that stable display cannot be performed. There is.

[0007] Even if the liquid crystal material is improved, it is difficult to achieve uniform display characteristics over the entire display region due to the non-uniform thickness of the liquid crystal layer. The reason will be described below in detail.

In the memory-type bistable liquid crystal display device, the ratio (d / p) between the liquid crystal layer thickness d and the twist pitch p is adjusted so that the liquid crystal has a structure twisted at 180 ° in the initial state. Uses chiral nematic liquid crystal material.

The above d / p is a very important parameter, and if d / p is too small, 36 of the two metastable states
The 0 ° state does not appear, while if d / p is too large, the 0 ° state of the two metastable states does not appear,
At d / p in the meantime, both the 360 ° state and the 0 ° state are realized. Usually, 360 ° in the initial state
For the d / p region where both states of / 0 ° appear, d / p is adjusted based on the center value.

However, since the twist pitch p of d / P is constant and depends on the thickness d of the liquid crystal layer, variations occur when the thickness of the liquid crystal layer is controlled, and color filters, insulating films, alignment films, etc. Due to the variation of each layer thickness of
It is considerably difficult to set the liquid crystal layer thickness d uniformly.

Therefore, when the thickness of the liquid crystal layer changes, d / p
May deviate from the d / p region where both states of 360 ° / 0 ° are exhibited, and switching is not performed between the two metastable states, that is, the operation margin with respect to the fluctuation of the liquid crystal layer thickness d is narrow. There was a problem.

Therefore, in the memory-type bistable liquid crystal display device having the above structure, the range of the d / p region in which both the 360 ° and 0 ° states are exhibited, that is, the difference between the maximum value and the minimum value of d / p. It can be said that the larger Δd / p (operation margin) is, the better.

Therefore, the inventor of the present invention has conducted intensive studies in view of the above circumstances, and conducted various repetitive experiments with various parameters such as various elastic constants. And found that it can be defined by the dielectric anisotropy Δε.

The present invention has been completed on the basis of this finding. The purpose of the present invention is to specify only the dielectric anisotropy Δ の of the chiral nematic liquid crystal and to increase the operation margin with respect to the fluctuation of the liquid crystal layer thickness d. Another object of the present invention is to provide a memory-type bistable liquid crystal display device which achieves stable and highly reliable display.

[0015]

According to the present invention, there is provided a liquid crystal display device comprising: a scanning electrode member in which a scanning electrode pattern in which a large number of scanning electrodes are arranged on a transparent substrate;
The dielectric anisotropy Δε is 12.0 or less between a signal electrode pattern in which a number of signal electrodes are arranged on a transparent substrate and a signal electrode member in which an alignment film is sequentially formed. It has a structure, in which a voltage that causes a Freedericksz transition in an initial state is applied, and thereafter, a chiral nematic liquid crystal that exhibits two metastable states different from the initial state due to a difference from the applied voltage is characterized. .

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A bistable liquid crystal display device of the present invention will be described with reference to FIGS. FIG. 1 is a schematic cross-sectional view of a liquid crystal display device A of the present invention, FIG. 2 shows a relationship between two metastable states and an unstable state (unstable region), and FIG. The relationship between the voltage and the unstable state (unstable region) is shown. FIG. 4 shows the driving voltage waveform used in the example of the present invention. FIGS. 5 to 8 show the evaluation results of d / p in the present invention, and FIG. 9 shows the d / p in the comparative example.
It is an evaluation result.

In the liquid crystal display device A of the present invention, 1 is a transparent substrate made of glass or the like, 2 is a scanning electrode made of ITO or the like, 3 is an alignment film, and these constitute a scanning electrode member 4. In the signal electrode member 5, a signal electrode 7 made of ITO or the like and an alignment film 8 are formed on a transparent substrate 6 made of glass or the like. Each of the alignment films 3 and 8 is made of polyimide and has been rubbed. Note that an insulating film made of SiO ₂ or the like may be interposed between the scanning electrode 2 and the alignment film 3 or between the signal electrode 7 and the alignment film 8.

The scanning electrode 2 and the signal electrode 7 are patterned so as to intersect with each other, and each intersection is defined as a pixel area (for example, 300 μm × 300 μm). Further, the scanning electrode member 4 and the signal electrode member 5 Chiral nematic liquid crystal 10 (addition of an optically active additive to a liquid crystal composition exhibiting a nematic phase at room temperature [manufactured by E. Merck: S
−811]). A seal member for enclosing the chiral nematic liquid crystal 10 is provided between the members 4 and 5 around the display area, and a spacer is arranged in the liquid crystal 10 to adjust the film thickness. Polarizing plates 13 and 14 are provided outside the transparent substrates 1 and 6.

According to the liquid crystal display device A having the above-described structure, the chiral nematic liquid crystal 10 has a twisted structure in an initial state, and the initial state is changed by a voltage difference applied after a voltage causing a Freedericks transition is applied in the initial state. Memory type bistable type having two different metastable states. For example, the twist angle φ in the initial state
_When the twisted state of φ ₀ + π (= 360 °) with respect to ₀ (= 180 °) is in a dark state, the polarizing plate 13, 14 has a positional relationship (crossed Nicols), and the bright state has a twist angle φ. ₀₋ π (= 0 °).

Next, as shown in FIG. 2, the liquid crystal display device A using the bistable type is switched between 0 ° and 360 ° across an unstable state (unstable region).

When such switching is performed, the selection voltage after applying a voltage that causes the Freedericksz transition in the initial state is lower than V1, the 360 ° state is set, if the selection voltage is increased to V2, the 0 ° state, and Vl to V
If it is between 2, an unstable state in which 0 ° and 360 ° are mixed occurs. For example, when the polarizing plates 13 and 14 are crossed Nicols, the twisted state of φ ₀ + π (360 °) with respect to the twist angle φ ₀ (180 °) of the initial state is a dark state, and the twist state is φ _0.
Assuming that the state of −π (0 °) is the bright state, the unstable state is a mixture of two states (bright state and black state), indicating a state between white and black. Further, the switching using the two metastable states is performed in the same manner by using the relationship in which the selection voltages are set to V3 and V4 as shown in FIG.

The matrix driving of the liquid crystal display device A will be described with reference to FIG. Assuming that the scanning potential is VC and the signal potential is Vns, a selection pulse value (selection voltage) VS1 (= ± | VC + Vns |) for expressing the 0 ° state and 360 °
Selection pulse value (selection voltage) VS for expressing state
2 (= ± | VC−Vns |), VS2−VS1
= 2Vns. And the widths of the unstable regions △ 1, △ 2
When the condition of 2Vns> △ 1 and △ 2 is satisfied, a 360 ° / 0 ° state appears, and matrix driving becomes possible.

Therefore, if the widths △ 1 and △ 2 of the unstable region are larger than 2 Vns, matrix driving cannot be performed. Conversely, the smaller (narrower) △ 1 and △ 2 with respect to 2Vns, the larger the operation margin can be taken, so that even when the values of △ 1 and △ 2 change, the two metastable states can be switched. Can be switched and displayed.

In the liquid crystal display device A of the present invention, the component is adjusted so that the dielectric anisotropy Δε of the chiral nematic liquid crystal 10 satisfies 12.0 or less, preferably 6.0 <Δε <9.0. I do.

Next, examples of the present invention will be described with reference to (Examples 1) to (Example 4) and comparative examples will be described with reference to (Example 5).

Example 1 The chiral nematic liquid crystal 10 used in the present example has a dielectric anisotropy ΔΔ at 25 ° C.
The components were adjusted so that ε was 6.70.

As for Δd / p (operation margin), the twist pitch (P) of the liquid crystal display device A was kept constant at 2.7 μm, and the thickness of the liquid crystal layer was changed in various ways. When a test drive waveform as shown in FIG. 5 was applied and evaluated, a result as shown in FIG. 5 was obtained.
However, regarding the driving conditions, VR = 30 V, TR = 3 m
s, Td = 200 μs, Ts = 100 μs, Vns =
When the voltage is set to 1.5 V and Vs (selection voltage) is varied, and the unstable region width satisfies the condition of 2 Vns (= 3.0 V) or less, a 360 ° / 0 ° state appears and two metastable states Switching between states can be performed.

The circles in the figure indicate the case where the 360 ° / 0 ° state is developed and switching between the two metastable states is completed. The crosses indicate only one of the 360 ° state and the 0 ° state. And the switching between the two metastable states could not be performed. Then, as is apparent from FIG. 5, the operation margin Δd / p became 0.1.

(Example 2) In this example, the components were adjusted in the same manner as in (Example 1). That is, the components were adjusted such that the dielectric anisotropy Δε at 25 ° C. became 8.70. The twist pitch of the chiral nematic liquid crystal 10 is set to 2.9 μm.
I made it. When Δd / p (operation margin) was measured in the same manner, Δd / p = 0.08 as shown in FIG.

Example 3 In this example, the components were adjusted in the same manner as in Example 1. That is, the components were adjusted so that the dielectric anisotropy Δε at 25 ° C. became 3.11. The twist pitch of the chiral nematic liquid crystal 10 is set to 2.7 μm.
I made it. Then, when Δd / p (operation margin) was measured in the same manner, Δd / p = 0.03 as shown in FIG.

Example 4 In this example, the components were adjusted in the same manner as in Example 1. That is, the components were adjusted so that the dielectric anisotropy Δε at 25 ° C. became 11.64. Also,
The twist pitch of the chiral nematic liquid crystal 10 is 2.7 μ
m. Then, when Δd / p (operation margin) was similarly measured, Δd / p = 0.05 as shown in FIG.
Became.

Example 5 In this example, the components were adjusted so that the dielectric anisotropy Δε at 25 ° C. became 14.36. The twist pitch of the liquid crystal was set to 2.5 μm. Then, when Δd / p (operation margin) was measured under the same conditions, a 360 ° / 0 ° state did not appear as shown in FIG. 9 and Δd / p was not obtained.

It should be noted that the present invention is not limited to the above embodiment, and various changes and improvements may be made without departing from the scope of the present invention. For example, a color liquid crystal display device provided with a color filter has a similar effect.

[0034]

As described above, according to the liquid crystal display device of the present invention, based on the finding that it is more advantageous to make the region width of the unstable state as narrow as possible, it is found that the dielectric constant of the chiral nematic liquid crystal is different. By simply limiting the anisotropy Δε to 12.0 or less, it is possible to perform a flickerless high-duty matrix drive by enlarging the operation margin with respect to the fluctuation of the liquid crystal layer thickness d, thereby achieving a stable and highly reliable display. And could be applied to high definition displays with many scanning lines.

[Brief description of the drawings]

FIG. 1 is an enlarged longitudinal sectional view of a main part of a liquid crystal display device of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between two metastable states and an unstable state in the liquid crystal display device of the present invention.

FIG. 3 shows two metastable states and an unstable state (unstable region)
FIG. 4 is an explanatory diagram showing a relationship between the power and the voltage.

FIG. 4 is a waveform diagram of a driving voltage used in an example.

FIG. 5 is an explanatory diagram showing evaluation of d / p in the present invention.

FIG. 6 is an explanatory diagram showing the evaluation of d / p in the present invention.

FIG. 7 is an explanatory diagram showing the evaluation of d / p in the present invention.

FIG. 8 is an explanatory diagram showing the evaluation of d / p in the present invention.

FIG. 9 is an explanatory diagram illustrating d / p evaluation of a comparative example.

[Explanation of symbols]

 A Liquid crystal display device 1, 6 Transparent substrate 2 Scan electrode 3, 8 Alignment film 4 Scan electrode member 5 Signal electrode member 7 Signal electrode 10 Chiral nematic liquid crystal 13, 14 Polarizing plate Vl, V2, V3, V4, VS1, VS2 selection voltage VC scanning potential Vns signal potential Δε Dielectric anisotropy of liquid crystal

Claims (1)

[Claims] A scanning electrode member in which a plurality of scanning electrodes are sequentially arranged on a transparent substrate; and a scanning electrode member in which a plurality of signal electrodes are arranged on a transparent substrate. A dielectric anisotropy Δε of 12.0 or less between the alignment film and the signal electrode member in which the alignment film is sequentially formed;
In addition, a chiral nematic liquid crystal that has a twisted structure in the initial state, applies a voltage that causes the Freedericksz transition in the initial state, and exhibits two metastable states different from the initial state due to a difference from the applied voltage thereafter. Liquid crystal display.