JPH0730321B2 - Liquid crystal display element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a field effect liquid crystal display device having excellent time-division driving characteristics.

[Prior Art] A conventional liquid crystal display element called a twisted nematic type has a 90 degree twisted spiral structure made of a nematic liquid crystal having a positive dielectric anisotropy between two electrode substrates and has both electrodes. A polarizing plate was arranged outside the substrate so that its polarizing axis (or absorption axis) was orthogonal or parallel to the liquid crystal molecules adjacent to the electrode substrate (Japanese Patent Publication No.
51-13666).

In order to orient the liquid crystal molecules so as to form a spiral structure twisted by 90 degrees between the two electrode substrates, for example, a method of rubbing the surface of the electrode substrate in contact with the liquid crystal in one direction with a cloth, a so-called rubbing method (tilt method). Angle 1-3 degrees). The rubbing direction at this time, that is, the rubbing direction is the alignment direction of the liquid crystal molecules. The two electrode substrates thus oriented are made to face each other with a gap so that the rubbing directions intersect each other at about 90 degrees, and the two electrode substrates are bonded with a sealant, and in the gap. When a nematic liquid crystal having a positive dielectric anisotropy is enclosed, the liquid crystal molecules have a helical molecular arrangement rotated by about 90 degrees between these electrode substrates. Polarizing plates are provided on the upper and lower sides of the liquid crystal cell thus constructed, and the polarization axis or absorption axis thereof is made substantially parallel to the alignment direction of the liquid crystal molecules adjacent to the respective electrode substrates. Here, the definition of the amount representing the time-division drive characteristic required for the following description will be briefly described.

FIG. 2 shows a typical voltage-luminance characteristic of a conventional liquid crystal display device having a spiral structure of liquid crystal molecules twisted by 90 degrees.
This is a relative value of the reflected brightness with respect to the applied voltage, and the initial value of the brightness is 100% and the final value (value when the applied voltage is sufficiently large) is 0%. Generally, the voltage at which the relative brightness is 90% is the threshold value V _th , and the voltage at which the relative brightness is 10% is the saturation value V _{sat, which is used} as a guideline for the liquid crystal characteristics. However, in practical use, if 90% or more, the pixel is sufficiently bright and the liquid crystal is not lit, and if it is 50% or less, the pixel is sufficiently dark, and the liquid crystal may be in the lit state. % And 50% are the threshold voltage V
Let _{th be} the saturation voltage V _sat .

Further, the electro-optical characteristics of the liquid crystal display element also change depending on the viewing direction, and this characteristic limits the visual field in which good display quality can be obtained.

Here, the definition of the viewing angle φ will be described with reference to FIG. In the figure, the rubbing direction of the upper electrode substrate 11 of the liquid crystal display element 1 is 2, the rubbing direction of the lower electrode substrate 12 is 3, and the twist angle of the liquid crystal molecules is 4. Further, the orthogonal coordinate XY axis is set on the surface of the liquid crystal display element 1, the X axis direction is defined as a direction that bisects the twist angle 4 of the liquid crystal molecule, the Z axis is defined as the normal direction of the XY plane, and the observation direction 5 Is defined as the viewing angle φ. In this case, for the sake of simplicity, the observation direction 5 is in the XZ plane. Further, φ shown in FIG. 3 is positive,
When viewed from such a direction, the contrast is high, so such a direction is referred to as a viewing direction.

In FIG. 2, the voltage at which the luminance at an angle φ = 10 degrees is 90% is V _th1 , the voltage at which the luminance is 50% is V _sat1, and the voltage at which the luminance at an angle φ = 40 degrees is 90% is V _th2 . At this time, the rising characteristic γ, the angle characteristic Δφ, and the time division ability m are defined by the following equations.

The time-division driving characteristics of the conventional liquid crystal display element are Δn when the refractive index anisotropy of the liquid crystal is Δn and the gap between the upper and lower electrode substrates is d.
・ Depends on d, and when Δn ・ d is large (for example, 0.
Γ is good (small) and Δφ is bad (small) for 8 μm or more). On the other hand, when Δn · d is small (for example, 0.8 μm
In the following, γ is bad (large) and Δφ is good (large). However, when comparing the time-sharing ability m, Δn
The smaller d is better. The specific examples described above are shown in Table 1 on the next page.

Here, the time division driving will be briefly described by taking a dot matrix display as an example. As shown in FIG. 4, stripe-shaped Y electrodes (signal electrodes) 13 are formed on the lower electrode substrate 12.
Forming an X electrode (scanning electrode) 14 on the upper electrode substrate 11,
The characters are displayed by turning on or off the liquid crystal at the intersection of the X and Y electrodes. In the figure, n scan electrodes are
_{X 1, X 2 ...... X n} , X 1, X 2 ...... division driving time by repeating the _{X n} and line sequential scanning. When a scan electrode is selected,
For all pixels on that electrode, the signal electrodes 13 Y ₁ , Y ₂
...... by Y _m, providing a display signal selection or non-selection time in accordance with signals to be displayed. In this way, lighting or non-lighting of the intersection is selected by the combination of the voltage pulses applied to the scanning electrodes and the signal electrodes. The number of scan electrodes X in this case corresponds to the number of time divisions.

In the conventional liquid crystal display device, the time division number 32 or 64 is practically limited because only the time division driving characteristics as exemplified in Table 1 can be obtained. However, in recent years, demands for improving the image quality of liquid crystal display elements and increasing the amount of display information have become strict, and the required specifications cannot be satisfied.

[Problems to be solved by the invention]

In order to provide a liquid crystal display device having a good image quality even when the number of time divisions is 100 or more, the present inventors have previously proposed to change the cell structure of the combination of the twist angle and the polarizing plate to a completely new method (Japanese Patent Laid-Open No. 2003-242242). (Sho 60-50511, Japanese Patent Application No. 60-50454, Japanese Patent Application No. 60-70950). A top view of the cell is shown in FIG. 5, and a perspective view thereof is shown in FIG. It is an object of the present invention to provide a liquid crystal display device having a better time division characteristic and display image quality by selecting a liquid crystal material particularly suitable for such a structure.

[Means for solving problems]

As described above, the present inventor first set the twist angle of the helical structure of liquid crystal molecules to 160 to 260 degrees, and the polarization axis or the absorption axis of the polarizing plate provided with the helical structure sandwiched between adjacent electrode substrates. The liquid crystal molecule alignment direction is shifted by 20 to 70 degrees.
The present inventors have proposed a structure of a liquid crystal display device in which the product Δn · d of the thickness d of the liquid crystal layer and the refractive index anisotropy Δn of the liquid crystal expressed in μm is 0.7 to 1.2 μm. for,
A more suitable liquid crystal material than the conventional one was found, and a liquid crystal display element having more excellent time division characteristics and display image quality was obtained by using this material.

[Action]

As described above, in the case of an element having a cell structure twisted by about 200 degrees, the effect of the physical properties tends to be completely different from that of the conventional 90 degree twist when selecting a liquid crystal material, as described below.

First, the characteristics of a liquid crystal display device having such a large twist angle and the viewing angle dependence of the voltage-transmittance characteristic are shown in FIG. ) Shows the case of 180 degree twist and compares. As can be seen from these figures, the element having a 180-degree twisted structure has a sharp fall characteristic (γ characteristic) due to the voltage of the transmittance, and the characteristic is clearly improved compared to the case of the conventional 90 degree twist. is doing. This tendency becomes remarkable as the twist angle is increased. In this way, an element with an element structure twisted by 160 to 260 degrees has a steep falling characteristic due to voltage, so the difference in transmittance due to the application or non-application of voltage during time-division driving becomes large, and the time-division driving is higher than before. Will be able to. No matter what kind of liquid crystal is used, by increasing the twist angle, the falling characteristics (γ characteristics) are greatly improved as compared with the 90 degree twist. However, depending on the liquid crystal material, the relationship between the γ characteristic in the case of 90 degree twist and the γ characteristic in the case of 160 to 260 degree twist does not show the same tendency, and in the case of 90 degree twist, materials with good γ characteristic are generally The conventional theory that the smaller the elastic modulus ratio (k ₃₃ / k ₁₁ ) is, the better (Eur
odisplay 84, “Liquid crystal properties in relatio
n to Multiplex-ing requirements ": Gunter Bauer) did not apply. This may be due to the relationship between the elastic modulus of twist and other factors due to the increased twist angle, but theoretically Is not yet clear.

So far, the material with the best characteristics in 90 degree twist is the pyrimidine-based material with a small elastic modulus ratio (k ₃₃ / k ₁₁ ). It was said that the material containing such as was good. In fact, in the 90 degree twist, such a tendency is clearly shown as shown in FIG. here Means a γ characteristic (voltage rising characteristic) obtained by taking the ratio of the transmittance of 80% and the voltage of 20% shown in FIG. 1 at an angle of 10 degrees from the normal line.

FIG. 8 shows the relationship between the ratio of elastic coefficients and the γ characteristic in the case of 180 ° twist and 230 ° twist. As is clear from the figure, in this case, the pyrimidine-based compound with a smaller elastic modulus
The γ characteristic is worse than that of H (phenylcyclohexane) system. In this way, when twisted about 200 degrees, the conventional idea does not hold, and the PCH system shows good results, and the liquid crystal material is selected with a different idea from the case of the cell structure with a twist of 90 degrees. Clearly there is a need. Also, as is clear from FIG.
When 230 degrees twisted from 0 degrees twisted, the γ characteristic was improved. Therefore, it is clarified that the γ characteristic is improved as the twist angle is increased.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to Examples and Reference Examples. In both examples, Chiral LCD S811 (Merck)
Respectively, with the appropriate twist values (18
In case of 0 degree twist, d / p = 0.40 to 0.50, where p: pitch of chiral liquid crystal, d: cell thickness of liquid crystal cell).

The element used for measuring each of the following examples was one in which an organic alignment film such as polyimide was applied or printed and the interface was rubbed in a specified direction with gauze or the like. The angle (tilt angle) between the device interface and the liquid crystal molecules in contact with the device interface was about 3 degrees.

All the interfaces used to evaluate the liquid crystal material were oriented in one direction by rubbing, but the tilt angle due to rubbing is generally 3 degrees or less, and is considered to be 5 degrees or less at most. The tilt angle is measured by K. Suzuki e
t al.:Appl.Phys.Lett., 33 (7), 561 (1978).

Example I In the following liquid crystal material composition system (1), the material of A (ester cyclohexane ECH liquid crystal, structural formula Experiments were performed by selecting four mixed systems of EC material of this structure
It is expressed as Hnom, and it depends on the average value of carbon number n + m, the refractive index anisotropy Δn, and the CP unit in the composition of (1) containing A, respectively. Viscosity, nematic liquid crystal → liquid transition point T _N-1 . When the average value of carbon number n + m exceeds 6, it was not possible to adopt as an example because of excessive viscosity.

The above-mentioned A-1 and A-2 have an average value of carbon number n + m of 6 or less, and the viscosity is within a practical range, but when the average value of carbon number n + m exceeds 6, the viscosity becomes excessive and it is used as a display device. could not. Compositions of the ECH liquid crystal materials A-3 and A-4, which have a large sum of carbon numbers and thus have an excessive viscosity, will be described below as reference examples.

Materials with different molecular weights from A-1 to A-4 are 30 wt.
% Of material system FIG. 1 (c) shows the data obtained by twisting at 180 degrees and when twisting at 90 degrees. The 180 ° twist and 90 ° twist show opposite tendencies. In the conventional 90 degree twisted element, if the value of n + m in the ECH system increases, the ratio of the elastic coefficients of k ₃₃ / k ₁₁ decreases, so γ
Although it has been explained that the characteristics are improved and the contrast is increased, it is shown that this concept does not apply at all in the region of 180 degree twist as described above, and a different concept is required.

Example II In the following liquid crystal material mixture system, a material with a high transition point and ECH
It is possible to obtain a liquid crystal having extremely excellent characteristics by combining the liquid crystals. The codes, structural formulas, and transition points of the high transition point materials used here are shown below.

Of these, L-1, L-2, and L-3 are also included in Example I.

In addition, a tran system liquid crystal Is abbreviated as LVFnom.

Compositions of Examples II-1, II-2, and II-3, transition points T _N-I , Δ
n, viscosity of 25 ℃, (180 degree twist), V by V unit (The voltage that is recognized as being substantially lit at a viewing angle of 10 degrees)
Below.

Example II-1 has the above high T_NIMaterial and ECH liquid crystal to Np liquid crystal
(Δε ≧ ε −ε_⊥) A simple system containing 30% PCH3
It This system has the highest γ characteristics due to the large amount of ECH content.
It is an excellent material system.

Examples II-2 and II-3 are systems in which Δn is increased by adding LVF liquid crystal to Example II-1. Although the γ characteristic deteriorates as the mixing ratio of the LVF system increases, Example II-1
Because of the good characteristic of, the γ characteristic is maintained at a good level of 1.054 even in the system II-3 whose Δn is increased to 0.166.

Example III The γ-characteristics were further improved by using the following materials as the high T _N-I point material.

Compared with the case of Example II, L-3 and L-4, which are high _NI point materials, are L-
In this example, 5 and L-6 were changed. Both L-5 and L-6 having three cyclohexane rings have improved viscosity and γ characteristics as compared with those in Example II, suggesting that material selection is important.

Next, as a reference example, the data obtained when the ECH system in Example III was replaced with the PCH system was as follows.

PCH302 and PCH304 have lower viscosities than ECH because they have lower viscosities, but the T _N-I points are lower, and in particular, the γ characteristic is significantly lower than that of ECH. PCH liquid crystal is an important material for lowering the viscosity, but ECH liquid crystal is particularly excellent in terms of improving the γ characteristic, and in particular, the material in which the sum of the carbon numbers n and m in the alkyl groups is 6 or less. It is clear from that shown in Example I.

〔The invention's effect〕

As described above, according to the present invention, a liquid crystal display device in which the twist angle of the helical structure of liquid crystal molecules is increased to 160 to 260 degrees, the liquid crystal material according to the present invention (the γ characteristic is improved by the ECH low molecular weight liquid crystal , PCH3 secures positive dielectric anisotropy as a whole, and the transition point as a whole is increased by adding a high-transition-point liquid crystal material). It is easy to see and use.

[Brief description of drawings]

FIG. 1 (a) is a diagram showing the viewing angle dependence of the voltage-transmittance characteristic of a conventional device having a twist angle of 90 degrees, and FIG. 1 (b) is the viewing angle of the voltage-transmittance characteristic of a device having a twist angle of 180 degrees. FIG. 1 (c) is a diagram showing the dependence, which is the sum of carbon numbers in the liquid crystal material (m +
n) and γ characteristics are shown in comparison between the case of a twist angle of 90 ° and the case of a twist angle of 180 °. Fig. 2 shows a typical liquid crystal display device having a spiral structure of liquid crystal molecules twisted by 90 °. Showing the voltage-luminance characteristic, FIG. 3 is an explanatory view of the definition of the viewing angle φ, FIG. 4 is an explanatory view of the time division driving, and FIG. 5 is a cell in which the twist angle is increased and the combination of the polarizing plates is changed. 6 is a top view of the device having the structure, FIG. 6 is a perspective view of the device, and FIG. 7 is a twist angle of 90.
FIG. 8 is a graph showing the relationship between the elastic coefficient ratio of the element and the γ characteristic, and FIG. 8 is a graph showing the relationship between the elastic coefficient ratio of the element with the twist angle of 180 ° and the twist angle of 230 ° and the γ characteristic according to the present invention. Is. 1 ... Liquid crystal display element, 2, 6 ... Rubbing direction of upper electrode substrate, 3, 7 ... Rubbing direction of lower electrode substrate, 4,
10 ... twist direction of liquid crystal molecules, 8 ... absorption axis or polarization axis direction of upper polarizing plate, 9 ... absorption axis or polarization axis direction of lower polarizing plate, 17 ... liquid crystal molecule.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 62-143990 (JP, A) JP 60-50511 (JP, A) JP 59-115377 (JP, A)

Claims (1)

[Claims] 1. A nematic liquid crystal having a positive dielectric anisotropy and to which an optical rotatory substance is added is sealed between a pair of upper and lower electrode substrates facing each other, and liquid crystal molecules having a thickness of 160.about. 2
Forming a spiral structure twisted by 60 degrees, and the polarization axis or the absorption axis of the polarizing plate provided with the spiral structure sandwiched therebetween is shifted by 20 to 70 degrees from the liquid crystal molecule alignment direction of the adjacent electrode substrate. An ester having a sum of carbon numbers m and n of 6 or less in a liquid crystal display device having a product Δn · d of the liquid crystal layer thickness d expressed in μm units and the liquid crystal refractive index anisotropy Δn of 0.7 to 1.2 μm. Cyclohexane liquid crystal material Phenylcyclohexane liquid crystal material with 5 to 60% by weight and n of 2 to 7 5 to 50% by weight and containing A liquid crystal display device comprising a liquid crystal having a composition in which a total of 3 to 4 liquid crystal liquids having a transition temperature of 200 ° C. or higher and 5 to 35% by weight are mixed.