JP2717731B2 - Liquid crystal display device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a field effect type liquid crystal display device having excellent time division driving characteristics. 2. Description of the Related Art A conventional liquid crystal display element called a twisted nematic type has a 90-degree twisted helical structure formed by a nematic liquid crystal having a positive dielectric anisotropy between two electrode substrates. Outside the electrode substrate, a polarizing plate is arranged so that its polarization axis (or absorption axis) is substantially parallel to 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 helical structure twisted by 90 degrees between 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 or a so-called rubbing method is used. Done. 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 opposed to each other with a gap so that their rubbing directions cross each other at substantially 90 degrees, and the two electrode substrates are bonded with a sealant. When a nematic liquid crystal having a positive dielectric anisotropy is enclosed, the liquid crystal molecules have a helical molecular arrangement rotated by approximately 90 degrees between the electrode substrates. Polarizing plates are provided above and below the liquid crystal cell thus configured, and their polarization axes or absorption axes are made substantially parallel to the arrangement direction of liquid crystal molecules adjacent to each electrode substrate. here,
The definition of the amount representing the time-division driving characteristic required for the following description will be briefly described. FIG. 6 shows typical voltage-luminance characteristics of a conventional liquid crystal display device having a helical structure of liquid crystal molecules twisted by 90 degrees. This is the relative value of the reflection luminance with respect to the applied voltage. The initial value of the luminance (when no voltage is applied) is set to 100%, and the final value (when no light is reflected or transmitted) is set to 0%. ing. In practice, if it is 80% or more, the pixels are sufficiently bright and the liquid crystal is not lit,
If it is 20% or less, the pixel may be in a sufficiently dark lighting state. Hereinafter, in this specification, the relative luminance is 80%, 2
The voltages that become 0% are referred to as a threshold voltage Vth and a saturation voltage Vsat, respectively. Furthermore, the electro-optical characteristics of the liquid crystal display element vary depending on the viewing direction, and this characteristic limits the field of view from 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 liquid crystal molecules is 4. The surface of the liquid crystal display element 1 has rectangular coordinates XY.
Axis, the X-axis direction is defined as the direction that bisects the twist angle 4 of the liquid crystal molecules, the Z-axis is defined as the normal direction of the XY plane, and the angle at which the observation direction 5 is the Z-axis is defined as the viewing angle φ. . Further, when φ shown in FIG. 7 is positive and the contrast is high when viewed from such a direction, such a direction is referred to as a viewing direction. In FIG. 6, the luminance at diopter φ = 10 degrees is 8
When the voltage at which the voltage becomes 0% is Vth ₁ , the voltage at which the voltage becomes 20% is Vsat _1, and the voltage at which the luminance at a diopter φ = 40 degrees becomes 80% is Vth ₂ , the rising characteristic γ, the angle characteristic Δφ, and time division The function m is defined as follows. γ = Vsat ₁ / Vth ₁ Δφ = Vth ₂ / Vth ₁ m = Vth ₂ / Vsat ₁ The time-division driving characteristic of the conventional liquid crystal display element is as follows: the refractive index anisotropy of the liquid crystal is Δn, and the gap between the upper and lower electrode substrates is d. Δn
Ｄ is good (small) and Δφ is bad (small) when Δn · d is large (for example, 0.8 μm or more). On the other hand, when Δn · d is small (for example, 0.
(8 μm or less), γ is poor (large) and Δφ is good (large). However, when compared with time division capability m, Δn
-Shows almost the same value regardless of the magnitude of d. Table 1 shows the above specific examples. [Table 1] Here, the time division driving will be briefly described by taking a dot matrix display as an example. As shown in FIG. 8, stripe-shaped Y electrodes (signal electrodes) 13 are formed on the lower electrode substrate 12 of FIG. 7, and X electrodes (scanning electrodes) 14 are formed on the upper electrode substrate 11 in the same manner. The liquid crystal at the intersection of the X, Y and Y electrodes is turned on or off. In the figure, n scanning electrodes are represented by X ₁ , X ₂ ,... Xn, X ₁ ,
X ₂ · · · · · Xn and repeated repeated line sequential scanning division driving time to. When a certain scanning electrode is selected, all the pixels on that electrode are applied to the signal electrodes 13 as Y ₁ , Y ₂ ,
········································································· Selected or non-selected display signals are added simultaneously according to the signal to be displayed. In this way, lighting or non-lighting of the intersection is selected by the combination of the voltage pulse applied to the scanning electrode and the signal electrode. The number n of the scanning electrodes X in this case corresponds to the number of time divisions. In a conventional liquid crystal display device, only the time-division driving characteristics as shown in Table 1 can be obtained, so that the number of time-division 32 or 64 is practically limited. However, in recent years, demands for improving the image quality of liquid crystal display elements and increasing the amount of display information have become severe, and the required specifications have not been satisfied. An object of the present invention is to achieve a very excellent time-division driving characteristic by taking a cell structure completely different from that of the conventional liquid crystal display element and controlling the twist pitch of the liquid crystal and the thickness of the liquid crystal layer to specific ranges. Another object of the present invention is to provide a liquid crystal display device having good image quality even when the number of time divisions is 100 or more. In order to achieve the above object, a liquid crystal display device according to the present invention has a twisted helical pitch of a liquid crystal material and a thickness of a liquid crystal layer defined in a specific relationship. The helix angle of the helical structure of the liquid crystal molecules is in the range of 140 to 250 degrees, a pair of polarizing plates is provided before and after the helical structure of the liquid crystal molecules, and the absorption axes (or polarization axes) of the polarizing plates are attached to the electrode substrate. The liquid crystal molecules are arranged so as to be shifted by a certain angle with respect to the alignment direction of adjacent liquid crystal molecules. In the liquid crystal display device of the present invention, the helix angle of the helix structure of the liquid crystal molecules is set in the range of 140 to 250 degrees, and the ratio between the helix helix pitch of the liquid crystal material and the thickness of the liquid crystal layer has a specific relationship. And the absorption axis (or polarization axis) of the polarizing plate and the orientation direction of the liquid crystal molecules are defined in a specific angle relationship, the applied voltage-light transmittance characteristic curve becomes steep, and the light scattering phenomenon occurs. Can be prevented, so that the time-division driving characteristics can be greatly improved and a high-contrast display can be achieved. Next, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an arrangement direction (rubbing direction) of liquid crystal molecules, a twist direction of liquid crystal molecules, and an absorption axis (or polarization axis) direction of a polarizing plate when the liquid crystal display element according to the present invention is viewed from above. In this case, the liquid crystal molecules are twisted counterclockwise by an angle α from the rubbing direction 6 of the upper electrode substrate to the rubbing direction 7 of the lower electrode substrate. The angle β ₁ between the rubbing direction 6 of the upper electrode substrate and the absorption axis (or polarization axis) 8 of the upper polarizer is defined by the twist direction 10 of the liquid crystal molecules (in this case, the rubbing direction 6 of the upper electrode substrate). taken same counterclockwise direction), similarly defined as the angle beta ₂ also beta ₁ and the absorption axis (or polarization axis) 9 of the lower rubbing direction 7 of the electrode substrate and the lower polarizing plate. Here, plus an angle of an integral multiple of beta ₁ or beta ₂ to 180 degrees, it goes without saying that equivalent to beta ₁ and beta _2. Hereinafter, the values of β ₁ and β ₂ are represented by the minimum value of the equivalent angle group. Further, the twist direction 10 and the twist angle α of the liquid crystal molecules are defined by the rubbing direction 6 of the upper electrode substrate, the rubbing direction 7 of the lower electrode substrate, and the type and amount of the light-emitting substance added to the nematic liquid crystal. Hereinafter, in order to simplify the description, as the light-emitting substance to be added, for example, Merck S81 represented by the following formula:
1 As shown in FIG. 1, a case having the same intrinsic twist direction as the twist direction 10 in FIG. 1 is considered. Generally, a nematic liquid crystal to which a light-emitting substance is added has a helical pitch P (μm) according to the type and the amount of the light-emitting substance. That is, the liquid crystal molecules are naturally twisted by 360 degrees while traveling the distance P in the helical axis direction. Assuming that the concentration of the light-emitting substance added to the nematic liquid crystal is C (%), there is a relationship of PC = constant. Therefore, when an arbitrary distance d (μm) is taken, the liquid crystal molecules are twisted by 360 d / P degrees between them. However, in an actual liquid crystal display device, since the alignment direction of the liquid crystal molecules at the boundary is forcibly restricted by the rubbing direction, only the twist determined by the upper and lower rubbing directions is allowed. This will be described with reference to FIG. In the figure, when the target twist is 10 and the twist angle is α, and when the thickness of the liquid crystal layer is d (μm), the helical pitch P of the liquid crystal is obtained.
Satisfying the relationship of α ≒ 360 d / P, it can be easily understood that the liquid crystal molecules have the desired twist 10. But d
The torsion when / P becomes very small becomes torsion 20 in the figure, and the twist when d / P becomes very large becomes torsion 30 in the figure. As described above, in an actual liquid crystal display element, even if d / P continuously changes, the twist can only take a discrete state. Here, the case of the twist 10 is called a normal twist, the case of the twist 20 is called an under twist, and the case of the twist 30 is called an over twist. Here, the condition that the twist becomes 20 under twist is d / P <(α−90) / 360, and the condition that the twist becomes 30 over twist is d / P> (α + 90) / 360. Therefore, the condition that the twist becomes 10 normal twists is (α−90) / 360 ≦ d / P ≦ (α + 90) / 360. By keeping d / P within this range, the desired twist of the liquid crystal molecules can be obtained. However, during various experiments, it was found that, even with a normal twist (normal twist), when a voltage is applied, there is an alignment state that scatters light when a voltage is applied. When the display is in an alignment state that scatters light, the contrast of the display is significantly deteriorated, so that the display is not practical. As a result of more detailed experimental studies, the conditions under which the liquid crystal is defined by the rubbing directions of the upper and lower electrode substrates are determined to have a normal twisted structure and not produce an alignment state that scatters light when voltage is applied. It has been found that the relationship between the torsion angle α of the molecule and d / P needs to satisfy the range shown by the shaded region in FIG. Specifically, the lower limit of d / P is given by (α-90) / 360, and the upper limit slightly changes with α, but is at least 0.6 or less. As is apparent from FIG. 3, the maximum value of the twist angle α of the liquid crystal molecules is also limited, and is limited to 250 degrees, and the upper limit that can be realized more practically stably is 240 degrees. The lower limit of the twist angle α of the liquid crystal molecules is limited by the twist angle dependence of the display color of the liquid crystal display element and the time-sharing drive characteristic, and the lower limit is 140 degrees. Therefore, the lower limit of d / P is naturally determined to be 0.14. In the figure, area A,
B and C indicate under and normal overtwist areas, respectively. The experimental results shown in FIG. 3 indicate that the alignment method of the liquid crystal molecules is polyimide or the like (for example, polyimide isoindoloquinazolinedione (JP-B-58-23610)) or polyimide benzimidazopyrololone (JP-A-54-133358). JP-A-2002-209686), which is obtained by rubbing an organic film, in which the tilt angle becomes 6 degrees or less, and can be applied to a liquid crystal display element using a liquid crystal molecule alignment method similar to at least this method. In FIG. 1, the angle β ₁ between the absorption axis (or polarization axis) 8 of the upper polarizer and the rubbing direction 6 of the upper electrode substrate, the absorption axis (or polarization axis) 9 of the lower polarizer and the rubbing of the lower electrode substrate. The angle β ₂ formed with the direction 7 is preferably set in the range of 20 to 70 degrees, more preferably in the range of 30 to 60 degrees, in consideration of the time-sharing drive characteristics, display brightness, color, and the like. Is desirable. In FIG. 1, β ₁ and β ₂ are defined with the twist direction 10 of the liquid crystal molecules being counterclockwise. However, even when the twist direction of the liquid crystal molecules is clockwise, how to take β ₁ and β ₂ is clockwise. It goes without saying that it is the same if it is adjusted in the turning direction. Further, the liquid crystal display device according to the present invention exhibits a remarkable Δn · d dependency, and preferably 0.8 μm ≦ Δn · d ≦ 1.2 μm in terms of time-division driving characteristics, contrast, brightness and color. It is particularly preferable to satisfy the condition of 0.9 μm ≦ Δn · d ≦ 1.1 μm. Here, the value of Δn generally has wavelength dependence, and tends to be large on the short wavelength side and small on the long wavelength side. The value of Δn used in this specification is obtained by using a He—Ne laser beam (wavelength: 6328 °).
Since it was measured at 25 ° C., the value of Δn · d in the present specification slightly changed when measured at another wavelength. Here, the structure and measurement results of a specific example of the liquid crystal display device according to the present invention will be described. FIG. 4 shows the relationship between the structure, that is, the rubbing direction of the electrode substrate, the twist direction and angle of the helical structure of the liquid crystal molecules, and the absorption axis of the polarizing plate. The liquid crystal used is a nematic liquid crystal containing a phenylcyclohexane (PCH) liquid crystal as a main component, and has a d / P of about 0.4 depending on the thickness of the liquid crystal layer as a light-transmitting substance.
In this case, 0.4 to 0.9% by weight is added. Δn of this mixed liquid crystal is 0.143.
In FIG. 4, the angle between the rubbing directions 6 and 7 of the upper and lower electrode substrates is 180 degrees, and the light-emitting substance S81 is formed.
By 1, the twist direction is 10, and the twist angle α is 180 degrees. The rubbing directions 6 and 7 and the absorption axes 8 and 9 of the polarizing plate
Angle beta ₁ with, beta ₂ is 45 degrees either. With the above cell structure, a liquid crystal cell was formed in which Δn · d was changed by changing the thickness d of the liquid crystal layer, and the color and brightness were observed.
Table 2 shows the representative results. [Table 2] From this result, it was found that both the brightness and the color were at a level that did not cause a problem as a display element when Δn · d was around 1 μm. From a more detailed study of Δn · d, it was found that there was no practical problem when Δn · d was in the range of 0.8 μm to 1.2 μm. Next, Table 3 shows the results of measuring the time-division driving characteristics of the liquid crystal cell having Δn · d = 1 μm. It can be seen that all of γ, Δφ, and m are significantly improved as compared with the conventional liquid crystal display element shown in Table 1. [Table 3] Although the absorption axis was used as the axis of the polarizing plate in FIG. 4, almost the same result was obtained even when the polarization axis was used. Further, in the embodiment, a mixed liquid crystal containing PCH as a main component was used, but it is needless to say that the same effect can be obtained with another kind of nematic liquid crystal having a positive dielectric anisotropy. It is also possible to add a dye to the liquid crystal in order to improve the contrast and adjust the color, but it goes without saying that the basic effects of the present invention are not changed. In the above example, the twist direction of the helical structure is described as counterclockwise. However, as shown in FIG. 5, it is needless to say that the same effect can be obtained in the case of a clockwise twist direction. Needless to say, the type of the light-emitting substance is not limited as long as the relationship between the rubbing direction and the twisting direction is maintained as shown in FIGS. 1, 4 and 5. As described above, according to the present invention, it is possible to realize a high time-division driving characteristic which has never been possible in the past, and to achieve high quality even in high-time-division driving with 100 or more time divisions. A liquid crystal display element having display characteristics can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing the relationship among the alignment direction of liquid crystal molecules, the twist direction of liquid crystal, and the axial direction of a polarizing plate of a liquid crystal display element according to the present invention. FIG. 2 is an explanatory diagram showing a possible twist angle of a liquid crystal molecule. FIG. 3 is an explanatory diagram showing an allowable range of a relationship between a twist angle α and d / P in the present invention. FIG. 4 is an explanatory diagram showing the relationship among the alignment direction of liquid crystal molecules, the twist direction of liquid crystal, and the axial direction of a polarizing plate in the first embodiment of the liquid crystal display device according to the present invention. FIG. 5 is an explanatory diagram showing the relationship among the arrangement direction of liquid crystal molecules, the twist direction of liquid crystal, and the axial direction of a polarizing plate in a second embodiment of the liquid crystal display device according to the present invention. FIG. 6 is an explanatory diagram showing voltage-luminance characteristics of liquid crystal display electrons used for defining time-division driving characteristics. FIG. 7 is an explanatory diagram that defines a measurement direction of a time-division driving characteristic. FIG. 8 is a diagram illustrating time-division driving. [Description of Signs] 1 ... Liquid crystal display element, 2,6 ... Rubbing direction of upper electrode substrate, 3,7 ... Rubbing direction of lower electrode substrate, 4,10 ... Twist of liquid crystal molecules Direction (normal twist), 8: direction of absorption axis or polarization axis of upper polarizing plate, 9: direction of absorption axis or polarization axis of lower polarizing plate, 11: upper electrode substrate, 12: lower Twisting when the side electrode substrate, 20... D / P is small (under twist), and twisting when 30... D / P is large (over twist).

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Shinji Hasegawa               3300 Hayano Mobara-shi, Chiba Sun               Inside the Mobara factory (72) Inventor Yoshio Hanada               3300 Hayano Mobara-shi, Chiba Sun               Inside the Mobara factory                (56) References European Publication 131216 (EP, A1)

Claims (1)

(57) [Claims] An upper electrode substrate and a lower electrode substrate that are arranged to face each other via a liquid crystal layer, an upper polarizer disposed outside the upper electrode substrate, and a lower polarizer disposed outside the lower electrode substrate And a striped scanning electrode formed on the liquid crystal layer side of the upper electrode substrate and a striped signal electrode formed on the liquid crystal layer side of the lower electrode substrate. When a display signal for selection is applied to the scanning electrodes which are arranged opposite to each other so as to intersect and are repeatedly line-sequentially scanned by time division driving of 100 or more time divisions, a display signal corresponding to display contents Is applied to the corresponding signal electrode, the liquid crystal of the liquid crystal layer has a positive dielectric anisotropy, and a phenylcyclohexane-based mixed nematic liquid crystal to which a light-emitting substance is added has a lower limit in the thickness direction. As one It forms a spiral structure twisted at an angle α in the vicinity of 240 degrees as an upper limit for stable driving from 0 degrees, and furthermore, a liquid crystal molecule arrangement of the liquid crystal at the upper electrode substrate interface and the liquid crystal at the lower electrode substrate interface. All the molecular arrangements are defined by the rubbing alignment treatment so as to be oriented at a tilt angle of 6 degrees or less, and d / is the ratio of the liquid crystal layer thickness d (μm) to the liquid crystal helical pitch P (μm). When the value of P is (α-9)
0) /360≦d/P≦0.6, and the direction of the polarization axis or absorption axis of the upper polarizing plate and the liquid crystal molecule alignment direction at the interface of the upper electrode substrate adjacent to the upper polarizing plate are set. The angle formed is in the range of 20 to 70 degrees, and the angle at which the direction of the polarization axis or the absorption axis of the lower polarizing plate deviates from the liquid crystal molecule alignment direction at the interface of the lower electrode substrate adjacent to the lower polarizing plate, The direction of the polarization axis or the absorption axis of the upper polarizer is 20 to 70 degrees in the same direction when viewed in a plane with the direction of the angle formed by the liquid crystal molecule alignment direction at the interface of the upper electrode substrate adjacent to the upper polarizer, A liquid crystal display device characterized in that the product Δnd · d of the thickness d (μm) of the liquid crystal layer and the refractive index anisotropy Δn of the liquid crystal is in the range of 0.8 to 1.2 μm. 2. The angle between the direction of the polarization axis or absorption axis of the upper polarizing plate and the liquid crystal molecule alignment direction at the interface of the upper electrode substrate adjacent to the upper polarizing plate is shifted within a range of 30 to 60 degrees, and
The angle between the direction of the polarization axis or absorption axis of the lower polarizing plate and the liquid crystal molecule alignment direction at the interface of the lower electrode substrate adjacent to the lower polarizing plate is shifted within a range of 30 to 60 degrees. The liquid crystal display device according to claim 1.