Twisted Nematic Liquid Crystal Display Device Having a Particular Pitch-to-Thickness Ratio

1. Field of the Invention

This invention relates to a twisted nematic (TN) liquid crystal display device having an enhanced multiplex driving characteristic.

2. Prior Art

A schematic sectional diagram of a conventional twisted nematic (TN) liquid crystal display device is shown in FIG. 5. In this diagram, in order to orient so that nematic liquid crystal molecules may be formed in a twisted spiral structure between two electrode forming substrates a, b, for example, the oblique evaporation method or the so-called rubbing method of rubbing the surface of electrode substrates contacting with the liquid crystal in one direction by using a cloth may be employed. When the two electrode substrates a, b thus oriented are opposedly placed so that the orientation directions may be orthogonal to each other as shown in FIG. 6, and nematic liquid crystal g having a positive dielectric antisotrophy is injected into their gap, the liquid crystal molecules are arranged in a spiral structure rotated 90.degree. between the electrode substrates a, b. In FIG. 6, "j" denotes the rubbing direction of upper electrode substrate a, and "k" indicates the rubbing direction of lower electrode substrate b. Polarizing plates h, i are disposed above and beneath the liquid crystal cell composed in this way, in which the direction of axis of polarization l of the polarizing plate h and the direction of axis of polarization m of the polarizing plate i are arranged so as to coincide nearly with the polarizing directions of the upper and lower electrode substrates a, b respectively (see FIG. 6). Meanwhile, in FIG. 5, c, d are transparent electrodes, and e, f are deflecting films.

Incidentally, when such conventional matrix type liquid crystal display device is driven by the optimum voltage averaging method of count number N, the ratio .alpha. of the effective voltage at lit picture element Vrms(ON) to the effective voltage at unlit picture element Vrms(OFF) is expressed as follows: ##EQU1## In this equation, if N.fwdarw..infin., then .alpha..fwdarw.1. That is, as the count number N increases, the difference between Vrms(OFF) and Vrms(ON) decreases. Therefore, to realize a matrix type liquid crystal display device with a huge quantity of display information, in order to obtain a favorable contrast ratio if the voltage difference of Vrms(ON) and Vrms(OFF) is small, it is required to develop a liquid crystal material possessing a sharp threshold characteristic or improve the orientation film. Recently, in particular, in the field of liquid crystal display device, as the display information increases, there is a mounting demand for an expansion of the quantity of display information. However, in such conventional TN liquid crystal display devices, it is not flexible enough to cope with increase of the count number N.

Accordingly, as a method of improving the above point, an attempt to improve the multplex driving characteristic by setting the twisting angle .phi. of the liquid crystal molecules larger than 90.degree. has been proposed. That is, in order to twist the liquid crystal molecules more than 90.degree., an optical active substance to induce the twisted spiral structure of liquid crystal molecules is added more to the nematic liquid crystal, and the content of the optical active substance is adjusted so that the ratio d/p of the resultant spiral pitch p of nematic liquid crystal to the liquid crystal layer thickness d may nearly satisfy the relation of d/p.apprxeq..phi./360.degree..

However, in such a liquid crystal display device having a high content of an optical active substance, the orientation of the liquid crystal molecules was disturbed when a voltage was applied, and the light scattered about as a result to lower the contrast ratio. To solve this problem, it was also attempted to form an orientation film by oblique evaporation and extend the angle formed by liquid crystal molecules and electrode substrates, that is, the pretilt angle, but in this oblique evaporation method, it is not easy to form a uniform orientation film over a wide area and it was difficult to obtain a liquid crystal display device of a wide area. Besides, in this method, since it was difficult to orient uniformly and simultaneously on plural electrode substrates, it was difficult to apply in a mass production system.

Hence to solve the above-discussed problems, it is a primary object of this invention to present a twisted nematic liquid crystal display device excelling in the multiplex driving characteristic and which is applicable as a liquid crystal display device of high duty multiplex driving.

It is another object of this invention to present a twisted nematic liquid crystal display device which can be oriented by the rubbing method and is notably improved in the mass production efficiency.

In order to achieve the above objects, the twisted nematic liquid crystal display device of this invention relates to a liquid crystal cell having a twisted spiral structure by disposing a pair of substrates opposedly to each other with the rubbed orientation films formed on the electrodes, placing a nematic liquid crystal having a positive dielectric anisotrophy containing an optical active substance between this pair of substrates, and shifting the orientation direction of said liquid crystal molecules by a specified angle of .phi. on both substrates, wherein said angle is set in a range of 210.degree..ltoreq..phi..ltoreq.300.degree., and the ratio d/p of the spiral pitch p of said liquid crystal molecules to layer thickness d of said liquid crystal cell is set in a range of (.phi./360-1/4)<d/p<.phi./360.

FIG. 1 is a schematic sectional diagram of a twisted nematic liquid crystal display device of this invention;

FIG 2 is a diagram to show the rubbing angle of the liquid crystal display device of this invention, the twisting angle of its liquid crystal molecules, and the setting angle of polarizing plates;

FIG. 3 is a diagram to indicate the dependency of the contrast ratio on the twisting angle .phi.;

FIG. 4 is a curve to show the voltage-transmission characteristic for explaining the multiplex driving characteristic;

FIG. 5 is a schematic sectional view of a conventional TN liquid crystal display device; and

FIG. 6 is a diagram to show the rubbing angle of a conventional TN liquid crystal display device and setting angle of polarizing plates.

Referring now to the drawings, some of the embodiments of this invention are described below.

(1) First embodiment:

A schematic sectional view of a liquid crystal cell of this invention is shown in FIG. 1. In this diagram, transparent electrodes 3, 4 made of indium oxide are patterned and formed on the lower face of an upper glass substrate 1 and on the upper face of a lower glass substrate 2, and orientation filsm 5, 6 made of polyimidesilane high molecular film are formed on their surfaces respectively, and their surfaces were rubbed in one direction by using a cloth, and a liquid crystal cell was prepared. Numeral 7 refers to a liquid crystal layer, 8 is an upper polarizing plate, and 9 is a lower polarizing plate.

The orientation films 5, 6 were manufactured by applying, by using a spinner, a 30 wt.% solution of N-methyl pyrrolidone of polyamide acid which is a precursor of a polyimidesilane high molecule obtained by condensing 0.1 mol of 1,3-bis (aminopropyl)-tetramethyldisiloxane, 0.9 mol of 4,4'-diaminodephenyl ether, and 1.0 mol of 3,3', 4,4'-benzophenon tetracarboxylic anhydride in N-methyl pyrrolidone, and then heating at 350.degree. C. for an hour to close the ring.

FIG. 2 is a diagram to show the relation between rubbing angle and twisting angle of liquid crystal molecule in the liquid crystal display device of the above constitution. In this diagram, .psi. refers to the angle formed by the rubbing direction 10 of the upper glass substrate 1 and the rubbing direction 11 of the lower glass substrate 2, and .phi. indicates the twisting angle of liquid crystal molecule, and it is adjusted to satisfy the relation of .psi.=360.degree.-.phi.. Incidentally, it is known already that the twisting angle of liquid crystal molecule depends on the value of the ratio d/p of the intrinsic spiral pitch p induced by adding an optically active substance to the nematic liquid crystal to the layer thickness d of the liquid crystal layer, and that it is necessary to adjust in a range of (.phi./360.degree.-1/4) <d/p<(.phi./360.degree.+1/4) in order to set the twisting angle of liquid crystal molecule to .phi.. However, much has not been investigated about the generation of domain as mentioned above in consideration of the liquid crystal material and the value of d/p.

Accordingly, the present inventors searched for domain-free d/p region in representative nematic liquid crystals (Np) possessing positive dielectric anisotrophy, that is, biphenyl, pyrimidine, dioxane, and ester type liquid crystals. The results are shown in Table 1.

From this table it is known that domain will not be generated, in any liquid crystal, if d/p is adjusted to satisfy the relation of (.phi./360.degree.-1/4)<d/p<.phi./360.degree. in a range of .phi..ltoreq.300.degree.. As shown in FIG. 3, meanwhile, it is preferable to set in a range of 210.ltoreq..phi..ltoreq.300.degree. in order to obtain a favorable multiplex driving characteristic.

Furthermore, it is also known that the findings about the d/p region to restrict the generation of domain can be applied, not only to the liquid crystals shown in Table 1, but also to other nematic liquid crystals of which general formula is expressed as: ##STR21## Besides, regarding orientation films 5, 6, it has been confirmed that similar results may be obtained by using the following polyimide type high molecular film which is obtained by dehydrating and closing the rings of inorganic matter such as SiO.sub.2 and SiO or polyamic acid. ##STR22## wherein R.sub.1 is ##STR23## R.sub.2 is ##STR24## R.sub.3 is ##STR25## and X.sub.1 to X.sub.5 denote (CH.sub.2).sub.n --, --O--(CH.sub.2 CH.sub.2 O).sub.n --, --O--, --S--, --SO.sub.2 --, etc.; and R.sub.4, R.sub.5, R.sub.6, R.sub.7 represent hydrogen atoms and lower alkyl groups, R.sub.8, R.sub.9 are hydrogen atoms, methyl group, ethyl group, or trifluoromethyl group, R.sub.10 is a bivalent hydrocarbon group, and R.sub.11 is a monovalent hydrocarbon group.

The above results about the range of d/p to inhibit the generation of domain, however, were not applicable to such liquid crystal materials as cyanocylohexane type liquid crystal ##STR26## and cyanocyclohexyl cyclohexane type liquid crystal ##STR27## That is, in the mixed liquid crystal materials composed of these liquid crystals, when the twisting angle .phi. of liquid crystal is nearly over 250.degree., the domain inhibiting d/p range becomes very narrow, and it was found that the practical value is lost in this condition. Therefore, in the rubbed liquid crystal cell, as the condition to inhibit generation of domain, selection of the material of liquid crystal to be used and setting of d/p seem to be of the first importance.

Meanwhile, when other liquid crystal material is mixed into these Np type liquid crystals, more practical liquid crystal materials with respect to liquid crystal temperature range and response characteristics may be presented. The inventors, therefore, further investigated materials to be mixed into Np type liquid crystals, and discovered that adverse effects on the action to inhibit the generation of domain are few in the compounds included in a compound group (II) or compound group (III), as the material to be mixed into said Np type liquid crystal, wherein the compound group (II) means any compound expressed in a general formula ##STR28## wherein R.sub.8 to R.sub.14 are straight-chain alkyl groups with 1 to 7 carbon atoms, and R.sub.9 O and R.sub.12 O are straight-chain alkoxy group with 1 to 6 carbon atoms, and the compound group (III) means any compound expressed in a general formula ##STR29## wherein R.sub.15 to R.sub.18 denote straight-chain alkyl groups with 1 to 7 carbon atoms. In this case, in order to present practical materials, it is preferable to mix at least one of the compounds included in the compound group (II) or compound group (III) in the Np type liquid crystal, and also to add at least one optical 14 active substance by a proper content. As the optical 14 active substance to be added, a preferable example is a chiral nematic liquid crystal expressed in ##STR30## (wherein 2MB* is ##STR31## C* is a symmetric carbon), or a cholesteric liquid crystal having a cholesterol ring as expressed in ##STR32## (wherein X is ##STR33##

In addition, the present inventors also confirmed by investigations that the domain generation inhibiting effect is lowered if the content of Np type liquid crystal is less than about 15 wt.%. Hence, the content of Np type liquid crystal must be over 15 wt.%. As for the orientation films, too, it has been ascertained that the domain d/p region is wider in the organic material than in the inorganic material such as SiO.sub.2, in the practical materials mentioned above, and that the range is widest when using, among the orientation films belonging to the organic materials, polyimide silane type high molecular film obtained from the mixture of 1,3-bis(aminopropyl)tetramethyl disiloxane, 4,4'-diaminodiphenylether and 3,3', 4,4'-benzophenon tetracarboxylic anhydride.

Therefore, by the combination of the above liquid crystal materials and orientation films, it is now possible to manufacture super-twisted nematic liquid crystal devices by rubbing treatment, which had been considered to be extremely difficult conventionally, easily and at high reproducibility.

The electro-optical characteristic of this embodiment is compared below with that of a conventional twisted nematic liquid crystal display device. In this case, the structure of liquid crystal cell is the same as shown in FIG. 1. The rubbing angle .psi. is 90.degree., and the twisting angle of liquid crystal molecule .phi. is 270.degree.. The layer thickness d of liquid crystal layer 4 is 4.5 .mu.m, and the orientation films 5, 6 are the above-mentioned polyimidesilane type high molecular films. The composition of nematic liquid crystal used in this comparison is shown in Table 2.

Furthermore, this nematic liquid crystal contains 2.11 wt.% of CB-15 (dextrorotatory) as an optically active substance so that the twisting angle .phi. of liquid crystal molecule may be 270.degree.. At this time, the spiral pitch p of liquid crystal is 6.9 .mu.m, and d/p about 0.65, which is within said range of (.phi./360-1/4)<d/p<.phi./360 of the above condition. Meanwhile, since the dielectric anisotrophy .DELTA.n of the nematic liquid crystal shown in Table 2 is 0.20, the value of .DELTA.n.multidot.d is 0.9 .mu.m. Besides, from the viewpoint of coloring of display area in the absence of electric field, it is preferable to adjust so that the value of .DELTA.n.multidot.d may settle within a range of 0.5 .mu.m.ltoreq..DELTA.n.multidot.d.ltoreq.1.1 .mu.m. The polarizing plates are set at angles of .beta.=70.degree. and .gamma.=20.degree., where .beta. is the angle formed by the direction of axis of polarization 12 of the upper polarizing plate 8 shown in FIG. 2 and the rubbing direction 10 of the upper glass substrate 1, and .gamma. is the angle formed by the direction of axis of polarization 13 of the lower polarizing plate 9 and rubbing direction 11 of the lower glass substrate 2. Besides, from the viewpoint of the multiplex driving characteristic and coloring of the display surface in the presence of an electric field, it is preferable to set, in the case of positive type display, at .beta.+.gamma.=60.degree. and in a range of 60.degree.<.beta.<80.degree., and 10.degree.<.gamma.<30.degree., and in the case of negative type display, at .beta.+.gamma.=180.degree., and in a range of 60.degree.<.beta.<80.degree., and 100.degree.<.gamma.<120.degree..

In the thus composed liquid crystal display device, it has been confirmed that the display is excellent being free from disturbance of orientation of liquid crystal molecules, that is, generation of domain, which may likely appear at the time of the application of voltage as stated above.

Next, the quantity of .alpha. explained below is defined in order to compare the multiplex driving characteristic of the liquid crystal display device based on this embodiment with that of the conventional TN liquid crystal display device. FIG. 4 shows the typical voltage-transmission characteristic including the case of this embodiment. The direction of observation is the normal direction of the liquid crystal display plane.

Supposing the voltage when the transmission is 90% to be V.sub.90 % and the voltage when it is 10% to be V.sub.10%, .alpha. is defined as

This value of .alpha. expresses the steepness of voltage-transmission characteristic, and as the value of .alpha. approaches 1, the multiplex driving characteristic improves. Accordingly, when the nematic liquid crystal in the same composition as in Table 2 was used and the value of .alpha. of the conventional TN type liquid crystal display device was measured, it was 1.50, while that of the liquid crystal display device of this invention was 1.08.

Thus, in the liquid crystal display device of this invention, the multiplex driving characteristic is notably improved. In addition, the liquid crystal display device of this invention has been confirmed to show a high contrast ratio in a wide viewing angle range as compared with the conventional liquid crystal display device.

(2) Second embodiment

Table 3 shows the composition of nematic liquid crystal used in this embodiment.

The structure of the liquid crystal cell is the same as in the first embodiment. The concentration of the added CB-15 was 2.36 wt.%, and the spiral pitch p of the liquid crystal induced by this is 6.2 .mu.m. Since the layer thickness d of liquid crystal layer is 4.0 .mu.m, the ratio d/p is about 0.65. The dielectric anisotrophy of this liquid crystal .DELTA.n is 0.23, and the .DELTA.n.multidot.d is 0.92 .mu.m. Therefore, the values of d/p and .DELTA.n.multidot.d are within the range of the condition in the first embodiment, that is, (.phi./360-1/4)<d/p<.phi./360, 0.5 .mu.m<.DELTA.n.multidot.d<1.1 .mu.m. The value of .alpha. to express the multiplex driving characteristic of thus composed liquid crystal display device was 1.09.

Hence, this embodiment has been also confirmed to show a high contrast ratio in a wide viewing angle range as compared with the conventional liquid crystal display device.

Incidentally, when the nematic liquid crystal shown in Table 4 was used, similar effects as experienced in the first and second embodiments were observed.