JP2555390C - - Google Patents

Description

The present invention relates to an electro-optical display device having a very small viewing angle dependence of the contrast and, in particular, a steep electro-optical characteristic curve. For liquid crystal display elements, the properties of nematic or nematic-cholesteric liquid crystal materials whose optical properties, such as light transmission, light scattering, birefringence, reflection or color change significantly under the influence of an electric field, are exploited. You. The operation of such display elements is based, for example, on the phenomenon of dynamic scattering, deformation of the alignment phase, the Schadt-Helfrich effect or the SBE effect in twisted cells. Among these conventional types of liquid crystal display elements, display elements which exhibit a large surface area, a thin structure and low power consumption, and which can present a large amount of information, have recently become particularly important. Electronic data processing, office automation (this includes in particular electronic word processing), the video industry and, in the not-so-distant future, the automotive industry will require this type of intelligent display device.
Due to their physical properties, liquid crystal displays are destined for these display systems. Highly intelligent liquid crystal displays having a large surface area and using nematic liquid crystals are operated by a so-called time division method. This control results in a display having good contrast independent of the viewing angle only if the electro-optical characteristic curve of the display is very steep. Ratio of elastic constants related to bending and spreading (K ₃ / K ₁ )
Until the use of a liquid crystal having an extremely small value, the steepness of the electro-optical characteristic curve of the TN cell was limited. As is known, in TN cells the contrast can be improved by increasing the twist angle (twist angle) of 90 ° [EP-A-0,098,070; Waters, EP. Raynes and V.W. Mol. Cryst. Liq. Cryst. 123 (1985) p. 303; West German Published Patent Application DE 3,423,993 A1; Sheffer and J.M. Nehri
J. ng. Appl. Phys. 58 (1985) p. 3022; TJ. Scheffer and J.
Appl. Phys. Lett. 45 (1984) 1021]. The effect of the various liquid crystal parameters on the steepness of the electro-optical characteristic curve, and thus on the contrast, is known for TN cells, but some important liquid crystal properties in the case of display elements with a twist angle> 90 ° are important. Sex has not yet been substantially defined. The effect of the elastic constant on the contrast of a display element having a high twist angle is described in TJS
It has not been discussed at all by Cheffer et al. M. Waters et al. Make a very, very simplified estimation. Therefore, there is still a particular problem in the practical use of supertwist cells (TN cells with a twist angle of> 90 °, particularly preferably in the range of 150 to 250 ° according to the above-mentioned references). These problems include, inter alia, the realization of a steep electro-optical characteristic curve and the associated good contrast, and this contrast should also be as insensitive to viewing angle as possible. The present invention is based on the object of providing an electro-optical display element with a high twist angle (twist angle), which is accompanied by the highest contrast and the minimum viewing angle dependence and guarantees the steepest electro-optical characteristic curve. . It has now been found that the ratio K ₃ / K ₁ for large surface tilt angles must always be as large as possible in order to achieve the steepest characteristic curve and thus the optimum contrast. However, large surface tilt angles are difficult to achieve or can only be achieved using prohibitive and therefore expensive materials. The only method known from the literature consists in obliquely sputtering the substrate surface with SiO ₂ . In contrast, rubbing of organic coatings currently used in production lines only leads to relatively small surface tilt angles. C. M. Water
s et al. and TJ. For a twist angle of 3 / 2π, which is preferred by Scheffer et al., The surface tilt angle obtained by rubbing the organic coating leads to domain creation and scattering effects. A combination of a twist angle of> 90 ° and a surface tilt angle of at most 10 ° appears to be possible in practice. Surprisingly, it has a suitable ratio of the elastic constants for bending and torsion K ₃ / K ₂ or a suitable ratio of the elastic constants for bending and spreading K ₃ / K ₁ with a suitable combination of material parameters. It has been found that the use of a liquid crystal mixture provides good contrast independent of the steep electro-optic characteristic curve or viewing angle for small surface tilt angles and twist angles from 160 ° to 220 °. Accordingly, the present invention provides a nematic liquid crystal material having a positive dielectric anisotropy and two support plates forming a cell containing at least one chiral additive together with an edge sealant, at least one polarizer, and at least one polarizer. Improving the contrast effect of an electro-optic display element utilizing the super-twisted birefringence effect (SBE) with multiplexing drive that has an analyzer and has a small surface tilt angle and a numerical twist angle of 160 ° to 220 ° Regarding the method, the method of the present invention is a method comprising the characteristic requirements as described below. A liquid crystal material formed of two support plates, at least one polarizer, at least one analyzer, and an edge sealing material, and having at least one of a nematic liquid crystal material having a positive dielectric anisotropy and a chiral additive. And a contrast effect of an electro-optical display element using a super twisted birefringence effect (SBE) that can be multiplexed and driven using a cell having a surface tilt angle of 1 ° to 3 ° and a twist angle of 160 ° to 220 ° The voltage-transmittance characteristic curve is kept monostable while the ratio K ₃ / K _{1 of the} elastic constants relating to the bending and spreading of the liquid crystal material has the smallest possible value, and Ratio of elastic constants related to bending and twisting of liquid crystal material K ₃ /
By adjusting the K _2, a method of improving the contrast effect of the electro-optical display element, characterized in that to improve the steepness of the characteristic curve. The invention furthermore relates to a corresponding method for improving the contrast effect of a supertwist cell. The structure of the liquid crystal display device according to the present invention comprises at least one polarizer, at least one analyzer, an electrode substrate and an electrode, each of which has a preferred orientation of liquid crystal molecules adjacent thereto. The surface treatment between one electrode and the other is usually twisted by 160 ° to 220 ° with respect to each other, and such a structure corresponds to a conventional design for this type of display element. I do. Here, the term conventional design has a broad meaning and is a variant and modification of super twist cells known from the literature, in particular display elements and matrix display elements including additional magnets according to DE-OS 2,748,738. Encompasses all of the design. The surface tilt angles on the two support plates can be the same or different. The same tilt angle is preferred. However, in comparison with the conventional display elements based on twisted nematic cells, the basic difference of the display elements according to the invention lies in the choice of the liquid crystal parameters of the liquid crystal layer. In the display device according to the invention, the liquid crystal parameters K ₃ / K ₁ and K ₃ / K ₂ in the liquid crystal phase are selected in such a way that the steepest electro-optical characteristic curve as well as maximum contrast and minimum viewing angle dependence are guaranteed. Use the liquid crystal phase. Figure 1 is K ₃ where K ₃ / K ₁ has a decisive influence on the contrast of the display device
It is an illustrative formula illustrates the range of / K _1. The ratio K ₃ / K ₂ is plotted as a function of the cell torsion angle (β) for a surface tilt angle αo = 1 °. The continuous line separates region A and region B. On the continuous line and in the immediate vicinity, the ratio K ₃ / K ₁ has only a small effect on the slope of the characteristic curve, whereas in order to obtain a steep characteristic curve, the region A In this case, the ratio K ₃ / K ₁ must be as large as possible, while in region B the ratio K ₃ / K ₁ must be as small as possible. Furthermore, the characteristic curve must be bistable in region A but monostable in region B. In terms of avoiding hysteresis, the steepest monostable characteristic curve is an ideal solution. The ideal combination of the liquid crystal parameters and the display parameters is indicated by the hatched area in FIG. A new finding that K ₃ / K ₁ affects, together with the K ₃ / K ₂ constant and well-defined cell torsion and surface tilt angles, was not found in the prior art. Furthermore, it is not found in published scientific literature on supertwist cells that bistability generally occurs only over certain deformation regions. This is explained in more detail in FIG. The tilt angle αm at the center of the liquid crystal layer is expressed as a function of a numerical value obtained by dividing the applied effective voltage by the threshold voltage. This data is for a twist angle of 160 °, a surface tilt angle of 1 °, and K ₃ / K ₂ = 2. From FIG. 2, it can be seen that for K ₃ / K ₁ = 1.6, bistability occurs in the relatively small deformation region. Bistability is defined as the fact that different values of αm are found for one and the same V / Vo, whereas for a monostable curve with K ₃ / K ₁ = 0.4, the value between αm and V / Vo There is always a clear correlation. However, for display elements having a twist angle of> 90 °, αm is between 60 ° and 85 °.
The ° range is of decisive importance. Thus, it is not the curve according to K ₃ / K ₁ = 1.6 that has a small bistable range that leads to a better light transmission characteristic curve, which is overall steeper, and thus better contrast, but to K ₃ is a curve related to ₃ / K ₁ = 0.4.
This is illustrated in the following figures. FIG. 3 shows the definition of the polarizer position. FIG. 4 shows the light transmission data as a function of voltage for various wavelengths at a twist angle of 220 ° and for a given polarizer arrangement. From FIG. 4 it can be seen that the display already switches well between dark and light for 550 nm light. At other wavelengths, there is always residual light transmission between the on and off states, so the display appears colored under white light illumination. FIG. 4 attempts to show that a very steep electro-optical characteristic curve can be achieved by appropriate selection of the material and indicator parameters. This curve is for normal incidence light. However, the contrast as a function of the viewing angle is decisive for the quality of the display. This is the fifth
It is shown in the figure. The light transmission is shown as a function of the viewing angle φ for various V / Vo ratios for a measurement angle of 45 ° in a polar diagram with a twist angle of 220 °, a surface tilt angle of 1 ° and a wavelength of 550 nm, respectively. I have. 4 and 5, the position of the polarizer and the material and cell parameters are the same. The contrast is obtained by dividing the light transmittance at V / Vo = 0.95 for the corresponding φ angle by the light transmittance at another V / Vo state. The measurement angles θ and φ are specified in FIG. It has been quite clearly demonstrated that good contrast values require relatively high V / Vo values or, in other words, require αm values between 60 ° and 85 ° (compare FIG. 7). FIG. 7 shows α of 60 ° to 85 °.
This was obtained by specifically considering only the m range. However, to be precise, this range is fundamentally important for the quality of display elements having a twist angle> 90 °. Liquid crystal dielectrics having material parameters in the range according to the invention can be prepared from conventional liquid crystal substrates. Many such materials are known from the literature. First
The dielectric used in the display element according to the invention, which is included in the range B of the figure, advantageously comprises at least 15% by weight, preferably at least 15% by weight, preferably of a mixture of at least 2, preferably 3 to 15, especially 4 of the liquid crystal compounds. Contains at least 30% by weight, especially at least 95% by weight,
Each of the mixtures contains at least one structural element selected from the following I to XI. Optionally these structural elements I~IV, for example F, Cl, CH ₃ and (or) may be substituted by CN, or structural elements I~IV may be in the form of corresponding N- oxide . These liquid crystal compounds preferably further contain a 1,4-phenylene group. The proportion of associative compound should be as small as possible, generally <30%, preferably <20%.
, And especially at <15%. This is best achieved by using a terminal polar component having structural elements X and / or XI. The dielectric used in the display element according to the invention preferably contains the highest possible proportion of the associative compound. These dielectrics advantageously contain at least 15% by weight, preferably at least 30% by weight, in particular at least 60% by weight, of a mixture of at least two, preferably 3 to 15, liquid crystal compounds. The dielectrics each contain at least one structural element selected from the following series of structural elements XII to XIV: These liquid crystal compounds preferably further contain a 1,4-phenylene group. Such dielectrics may further contain customary amounts of dyes and / or doping substances, provided that their liquid crystal parameters do not depart from the scope according to the invention. The following examples are intended to illustrate the invention without limiting it. Percentages are weight percentages, unless otherwise stated. All temperature data are in ° C. Example 1 p-trans-4-propylcyclohexylbenzonitrile 16%, trans-1-p-ethylphenyl-4-propylcyclohexane 21%, trans-1-p-methoxyphenyl-4-propylcyclohexane 20%, 4-ethyl -4 '-(trans-4-propylcyclohexyl) -biphenyl
11%, 4-ethyl-4 '-(trans-4-pentylcyclohexyl) -biphenyl
10%, 4,4'-bis- (trans-4-propylcyclohexyl) -biphenyl 4%
4,4'-bis- (trans-4-pentylcyclohexyl) -biphenyl 5%
4,4'-bis- (trans-4-propylcyclohexyl) -2-fluorobiphenyl 3%, 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -biphenyl 6% And a supertwisted cell containing a dielectric consisting of 4% of 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -fluorobiphenyl has a torsion angle of 220 ° and a steep electric field. It shows a very low dependence of the optical characteristic curve and the contrast on the viewing angle. Example 2 15% p-trans-4-propylcyclohexylbenzonitrile, 4% 3-fluoro-4-cyanophenyl p-ethylbenzoate, 4% 3-fluoro-4-cyanophenyl p-propylbenzoate, trans, trans-4 -Propyloxy-4'-propylcyclohexylcyclohexane 17%, trans, trans-4-methoxy-4'-pentylcyclohexylcyclohexane 14%, trans, trans-4-ethoxy-4'-pentylcyclohexylcyclohexane 13%, trans-4 -Propylcyclohexyltrans, trans-4-propylcyclohexylcyclohexane-4-carboxylate 5%, trans-4-pentylcyclohexyltrans, trans-4-propylcyclohexylcyclohexane-4-carbo 5% of sylate, 5% of trans-4-propylcyclohexyltrans, trans-4-butylcyclohexylcyclohexane-4-carboxylate, 5% of trans-4-pentylcyclohexyltrans, trans-4-butylcyclohexylcyclohexane-4-carboxylate, 4% 4,4'-bis- (trans-4-propylcyclohexyl) -2-fluorobiphenyl, 4% 4,4'-bis- (trans-4-pentylcyclohexyl) -2-fluorobiphenyl and 4- (trans Supertwisted cells containing a dielectric consisting of 5% of -4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -2-fluorobiphenyl have a twist angle of 220 ° and a steep electro-optical characteristic curve. And contrast for viewing angles A very low dependency. Example 3 3-fluoro-4-cyanophenyl p-ethylbenzoate 5%, 3-fluoro-4-cyanophenyl p-propylbenzoate 5%, trans-1-p-propylphenyl-4-pentylcyclohexane 9%, trans- 1-p-methoxyphenyl-4-propylcyclohexane 21%, trans-1-p-ethoxyphenyl-4-propylcyclohexane 14%, 4-ethyl-4 '-(trans-4-propylcyclohexyl) -biphenyl
12%, 4-ethyl-4 '-(trans-4-pentylcyclohexyl) -biphenyl
11%, 4,4'-bis- (trans-4-propylcyclohexyl) -biphenyl 4%
4,4'-bis- (trans-4-propylcyclohexyl) -2-fluorobiphenyl 4%, 4,4'-bis- (trans-4-pentylcyclohexyl) -2-fluorobiphenyl 5%, 4- ( Trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -biphenyl 5% and 4- (trans-4-pentylcyclohexyl) -4'-(trans-4-propylcyclohexyl) -2-fluoro Supertwist cells containing a dielectric of 5% biphenyl exhibit a steep electro-optical characteristic curve and a low dependence of contrast on viewing angle at a twist angle of 220 °. Example 4 p-trans-4-propylcyclohexylbenzonitrile 8%, 4-ethyl-4'-cyanobiphenyl 4%, trans-1-p-ethylphenyl-4-propylcyclohexane 13%, trans-1-p-methoxy Phenyl-4-propylcyclohexane 18%, trans-1-p-ethoxyphenyl-4-propylcyclohexane 9%, 4-ethyl-4 '-(trans-4-propylcyclohexyl) -biphenyl
10%, 4-ethyl-4 '-(trans-4-pentylcyclohexyl) -biphenyl
10%, 4,4'-bis (trans-4-propylcyclohexyl) -biphenyl 4%, 4,4'-bis (trans-4-pentylcyclohexyl) -biphenyl 4%, 4,4'-bis (trans- 4-pentylcyclohexyl) -2-fluorobiphenyl 4%, 4,4'-bis (trans-4-propylcyclohexyl) -2-fluorobiphenyl 4%, 4- (trans-4-pentylcyclohexyl) -4 '-( Supertwist containing a dielectric consisting of 6% trans-4-propylcyclohexyl) -biphenyl and 6% 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -2-fluorobiphenyl The cell has a 220 ° twist angle and low dependence of steep electro-optic characteristic curves and contrast on viewing angle. Indicate the sex. Example 5 10% p-trans-4-ethylcyclohexylbenzonitrile, 17% p-trans-4-propylcyclohexylbenzonitrile, 9% 2-p-cyanophenyl-5-propyl-1,3-dioxane, 2-p 6% cyanophenyl-5-butyl-1,3-dioxane, 5% 3-fluoro-4-cyanophenyl p-ethylbenzoate, 7% 3-fluoro-4-cyanophenyl p-propylbenzoate, 4-cyanophenyl p-trans-4-ethylcyclohexyl benzoate 4
%, 4-cyanophenyl p-trans-4-pentylcyclohexyl benzoate 4%, trans, trans-4'-propyloxy-4-propylcyclohexylcyclohexane 14%, 4-ethyl-4 '-(trans-4-propylcyclohexyl) ) -Biphenyl 4%, 4-ethyl-4 '-(trans-4-pentylcyclohexyl) -biphenyl 3%, 4,4'-bis- (trans-4-propylcyclohexyl) -biphenyl 4%
4,4'-bis- (trans-4-propylcyclohexyl) -2-fluorobiphenyl 4%, 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -biphenyl 5% And a supertwist cell containing a dielectric consisting of 4% of 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -2-fluorobiphenyl has a twist angle of 160 ° and a steep slope. 1 shows a low dependence of the electro-optic characteristic curve and contrast on viewing angle. Example 6 p-trans-4-ethylcyclohexyl-benzonitrile 5%, p-trans-4-propylcyclohexyl-benzonitrile 12%, p-trans-4-butylcyclohexyl-benzonitrile 8%, p-trans-4- 10% pentylcyclohexyl-benzonitrile, 3% 4-ethyl-4'-cyanobiphenyl, 6% 3-fluoro-4-cyanophenyl p-ethylbenzoate, 8% 3-fluoro-4-cyanophenyl p-propylbenzoate, 4-cyano-4 '-(trans-4-pentylcyclohexyl) -biphenyl 5%, 4-ethyl-4'-(trans-4-propylcyclohexyl) -biphenyl
11%, 4-ethyl-4 '-(trans-4-pentylcyclohexyl) -biphenyl 9%, trans-1-p-ethoxyphenyl-4-propylcyclohexane 9%, 4,4'-bis- (trans-4 -Propylcyclohexyl) -biphenyl 3%
3,4,4'-bis- (trans-4-propylcyclohexyl) -2-fluorobiphenyl, 4% 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -biphenyl And a supertwist cell containing a dielectric consisting of 4% of 4- (trans-4-pentylcyclohexyl) -4 '-(trans-4-propylcyclohexyl) -2-fluorobiphenyl has a twist angle of 160 ° and a steep slope. 2 shows a very low dependence of the electro-optic characteristic curve and the contrast of

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1: The ratio K ₃ / K _{2 of the} elastic constants relating to bending (K ₃ ) and torsion (K ₂ ) as a function of the total torsion (β) of the cell. FIG. 2: Tilt angle αm at the center of the cell as a function of reduced voltage V / Vo. Total torsion: β = 160 ° Alignment angle of molecules on the substrate surface: αo = 1 ° Ratio of elastic constants related to bending (K ₃ ) and torsion (K ₂ ): K ₃ / K ₂ = 2.67 Dielectric anisotropy ( Δε) and the dielectric constant perpendicular to the director (ε⊥): Δε / ε
⊥ = 0.33 Cell thickness d = 8 μm FIG. 3: Schematic drawing of a twisted liquid crystal cell with polarizer and analyzer. β = total twist of the liquid crystal layer Ψ = angle between the director on the first substrate surface and the direction passing through the polarizer Ψ '= angle between the director on the first substrate surface and the direction passing through the analyzer a) β = 160 ° b) β = 220 ° FIG. 4: Light transmittance as a function of reduction of voltage V / Vo. Cell data: Total torsion: β = 220 ° Polarizer: − = − 45 ° Analyzer: Ψ ′ =-15 ° Alignment angle of molecules on substrate surface: αo = 1 ° Bend (K ₃ ) and twist (K ₂ ) ): K ₃ / K ₂ = 2.67 Ratio of elastic constants related to bending (K ₃ ) and spreading (K ₁ ): K ₃ / K ₁ = 0.4 Dielectric anisotropy (△ ε) and director To the dielectric constant (ε⊥) perpendicular to
/Ε⊥=0.33 Cell thickness: d = 8 μm Parameter: wavelength of incident light λ ₁ = 450 nm λ ₂ = 550 nm λ ₃ = 650 nm FIG. 5: Angle dependence of light transmittance. Cell data: Total torsion: β = 220 ° Polarizer: Ψ = −45 ° Analyzer: Ψ ′ =-15 ° Alignment angle of molecules on substrate surface: αo = 1 ° Bending (K ₃ ) and twisting (K ₂ ) ): K ₃ / K ₂ = 2.67 Ratio of elastic constants related to bending (K ₃ ) and spreading (K ₁ ): K ₃ / K ₁ = 0.4 Dielectric anisotropy (△ ε) and director To the dielectric constant (ε⊥) perpendicular to
ε / ε⊥ = 0.33 Cell thickness: d = 8 μm Parameter: Conversion voltage: V / Vo = 0.95─ V / Vo = 1.05− − − V / Vo = 1.15− · −−− V / Vo = 1.4 · · · Fig. 6: Schematic drawing illustrating measurement angles θ and φ. x = direction of director on first substrate surface β = total twist of cell θ = angle with respect to cell perpendicular φ = azimuth FIG. 7 = tilt angle αm at center of cell as a function of reduced voltage V / Vo. Total torsion: β = 220 ° Alignment angle of molecules on substrate surface: αo = 1 ° Ratio of elastic constants related to bending (K ₃ ) and torsion (K ₂ ): K ₃ / K ₂ = 2.67 Bending (K ₃ ) And the ratio of the elastic constants related to the spread (K ₁ ): K ₃ / K ₁ = 0.4 The ratio between the dielectric anisotropy (△ ε) and the dielectric constant perpendicular to the director (⊥∥): △ ε
/Ε⊥=0.33 Cell thickness: d = 8 μm Converted voltage V / Vo in which the angle dependence of the light transmittance shown in FIG. 5 was calculated.
Is plotted on the graph.

Claims (1)

Claims: (1) A nematic liquid crystal material having a positive dielectric anisotropy, formed of two support plates, at least one polarizer, at least one analyzer, and an edge sealing material, and a chiral additive. A twist angle of 160 ° to 220 ° and both of the two support plates have a surface tilt angle of 1 ° to 3 °, and Super twisted birefringence effect (SBE) capable of multiplex driving using cells in which both surface tilt angles of two support plates are substantially identical to each other
This is a method for improving the contrast effect of an electro-optical display element utilizing the bending and twisting of a liquid crystal material so that the voltage-transmittance characteristic curve is as steep as possible while maintaining monostable. The characteristic curve can be adjusted by adjusting the ratio K ₃ / K ₂ of the elastic constants and adjusting the ratio K ₃ / K _{1 of the} elastic constants relating to the bending and spreading of the liquid crystal material to have the smallest possible value. A method for improving the contrast effect of an electro-optical display device, characterized by improving the steepness of the image. (2) Formed with two support plates, at least one polarizer, at least one analyzer, and an edge sealing material, and contains at least one of a nematic liquid crystal material having a positive dielectric anisotropy and a chiral additive. Liquid crystal material, the twist angle of 160 ° to 220 ° and both of the two support plates have a surface tilt angle of 1 ° to 3 °, and both of the two support plates Super-twisted birefringence effect (SBE) that can be driven by multiplexing using cells whose surface tilt angles are substantially the same as each other
An electro-optical display element using the liquid crystal material according to the present invention, wherein the voltage-transmittance characteristic curve is as steep as possible while maintaining monostability, and the ratio K _{3 of the} elastic constant relating to the bending and twisting of the liquid crystal material. By adjusting / K ₂ and adjusting the ratio K ₃ / K _{1 of the} elastic constants relating to the bending and spreading of the liquid crystal material to have the smallest possible value, the steepness of the characteristic curve is improved. An electro-optical display device, comprising: