JPS6353529B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a twisted nematic field effect liquid crystal display device, and more particularly, to a twisted nematic liquid crystal display device, a coloring phenomenon caused by interference color caused by elliptically polarized light can be solved by optical color correction. This invention relates to a two-layer liquid crystal display device in which the weight is reduced by stacking liquid crystal layers as plates.

In recent years, there has been a demand for an expanded amount of display information in the field of liquid crystal display devices, and demand trends are shifting from conventional segment-type displays to matrix-type displays. However, in order to expand the amount of displayed information with a matrix display, it is necessary to increase the so-called multiplex drive frequency (duty ratio), which results in a decrease in display contrast and a narrowing of the viewing angle range. A problem arises. One way to solve this problem is to calculate the liquid crystal layer thickness d and the liquid crystal birefringence Δn (= _ne −n _p ;
It has been proposed that n _e ; refractive index for extraordinary light, n _p : refractive index for normal light) should be decreased respectively {D. Meyerhofer: J. Appl. Phys. 48
1179 (1977)}. However, in a twisted nematic field effect liquid crystal display device, when d and Δn are decreased, the mode of light propagating in the liquid crystal is no longer linearly polarized, but becomes elliptically polarized. As a result, a phenomenon occurs in which interference colors occur when combined with a polarizer. This effect is also called the Mauguin effect, and is d・Δn
Significant at 2 μm. For display devices, this phenomenon means that inactive areas of the display are colored, resulting in a decrease in display contrast with active areas, leading to deterioration in display quality.

FIG. 1 is a schematic cross-sectional view of a conventionally used twisted nematic field effect liquid crystal display device. Here, 1a and 1b are transparent substrates, 2
3a and 2b are transparent electrodes, 3a and 3b are liquid crystal molecular alignment layers, 4 is a twisted nematic liquid crystal layer, 5 is a sealing material that also serves as a spacer, 6a and 6b are polarizing filters, and 7 is a drive circuit. In order to make the display device with this structure suitable for multiplex driving, the product of the layer thickness d of the liquid crystal layer 4 and the birefringence Δn of the liquid crystal material constituting the liquid crystal layer 4 in the visible wavelength region must be kept small. As a value, for example 0.36μm
It has been experimentally confirmed that good multiplex drive can be established by setting d·Δn to 2.0 μm. Incidentally, this lower limit value corresponds to the minimum d.Δn value at which linearly polarized light in the shortest wavelength range of visible light is rotated by 90° through the twisted nematic liquid crystal layer as linearly polarized light. These are theoretically based on C.H.Gooch and H.
J.Phys.D by A.Tarry: Appl.Phys. 8 1575
(1975). By the way, on the other hand,
When d·Δn becomes a value smaller than 2 μm, the light beam transmitted through the liquid crystal layer becomes elliptically polarized light due to the optical rotation dispersion phenomenon, and when it passes through a pair of polarizers, an interference color effect is produced.

The present invention solves the above-mentioned problems by making full use of technical means.The present invention solves the above-mentioned problems by making full use of technical means. In order to significantly reduce the coloring phenomenon, we developed a two-layer structure in which a conventional single-layer twisted nematic field effect liquid crystal display cell is superimposed with a twisted nematic liquid crystal layer that does not have a power supply means. In order to obtain the effect of a liquid crystal display device that combines a conventional single-layer cell with an optical compensator to reduce the coloring phenomenon, the compensator is replaced with a twisted nematic liquid crystal layer. The object of the present invention is to provide a new and useful multilayer liquid crystal display device.

According to the present invention, in a display liquid crystal cell including a twisted nematic liquid crystal layer having a twisted helical axis in a direction substantially perpendicular to a substrate, liquid crystal molecules are twisted and oriented. The transmitted light, which has become elliptically polarized light, undergoes optical rotation in the opposite direction due to the liquid crystal molecules twisted in the opposite direction during the process of transmitting through the liquid crystal layer that acts as a correction plate. The extraordinary light component and the normal light component of the incident polarized light are exchanged with each other because the liquid crystal molecules that initially come into contact with each other are approximately orthogonal to each other, and the elliptically polarized light is canceled out to form the original linearly polarized light. trying to return to. Therefore, the polarized light obtained by passing through the pair of polarizers becomes linearly polarized light without an elliptically polarized component, and high contrast without coloring due to light interference can be obtained.

FIG. 2 is a schematic cross-sectional view of a two-layer liquid crystal display device showing one embodiment of the present invention. Here 1a, 1
b, 1c are transparent substrates, 2a, 2b are transparent electrodes, 3
a, 3b, 3c, 3d are liquid crystal molecule alignment layers; 4a,
4b is a twisted nematic liquid crystal layer, 5 is a sealing material which also serves as a spacer, and 6a and 6b are polarizing filters. Further, 8 is a reflecting plate installed as necessary. In the figure, the upper liquid crystal cell acts as a color correction plate, and the lower liquid crystal cell is connected to a drive circuit 7 and acts as a display liquid crystal cell.

FIG. 3 is an explanatory diagram showing the long axis orientation γ (referred to as director) of liquid crystal molecules on the surface of each substrate in the embodiment of FIG. 2. The direction of the arrow in the figure is defined as the direction of the long axis of the liquid crystal molecules having a tilt angle Δθ with respect to the substrate as shown in FIG.

Next, specific materials of each component used in this example are shown below.

For transparent substrates 1a, 1b, 1c, 0.1mm~
Use 3mm thick soda glass. transparent electrode 2a,
2b is formed on a transparent substrate by appropriately patterning by photolithography or the like using In ₂ O ₃ (SnO ₂ added) having a thickness of 300 Å to 4000 Å. Liquid crystal molecule alignment layer 3
The elements a, 3b, 3c, and 3d are formed by oblique vapor deposition of SiO or by rubbing the insulating layer of SiO ₂ or polyimide directly or with a polishing cloth after treatment with a silane coupling agent. The twisted nematic liquid crystal layers 4a, 4b are selected from materials having a nematic liquid crystal phase such as biphenyl liquid crystal, ester liquid crystal, cyclohexane liquid crystal, azoxy liquid crystal, etc. A small amount of optically active substance is added to prevent this. Specifically, for example, the following are used. When giving a right-handed twist direction to Merck's ZLI-1646 liquid crystal, which is a cyclohexane-based liquid crystal,
When approximately 0.1 wt % of -15 is added to provide a left-handed twist direction, a mixed liquid crystal containing approximately 0.1 wt % of cholesteryl nonanoate from Eastman Kodak is suitable. The Δn of this mixed liquid crystal is at room temperature of 20°C in both left-handed and right-handed twist directions.
It is 0.08 for light of 589 nm. In this embodiment, the film thickness d is 6.2μ for each of the liquid crystal layers 4a and 4b.
It was set as m. Note that, in the case of liquid crystal materials having the same Δn, it is desirable that the layer thicknesses of the liquid crystal layers 4a and 4b be approximately equal, and a relative difference within about 30% is an allowable range. Furthermore, when the liquid crystal materials of the liquid crystal layers 4a and 4b have different Δn, it is desirable that the relative values of d·Δn of each layer are approximately equal;
A difference of approximately 30% or less is acceptable. Furthermore, this d・
The value of Δn is 0.4~0.6μm, 0.8~1.2μm or 1.5~
The range is selected to be 1.8 μm. It is preferable that the orientation directions of liquid crystal molecules in each of the liquid crystal layers 4a and 4b on the centrally located transparent substrate 1a are orthogonal to each other. In addition, each liquid crystal layer 4a, 4b
The molecular orientation is set such that the twist directions of the liquid crystal layer 4a are opposite to each other, for example, the liquid crystal layer 4a is twisted in the right-handed direction, and the liquid crystal layer 4b is twisted in the left-handed direction. The sealing material is an epoxy resin mixed with glass fibers with a diameter of 6.2 μm, which also serves as a spacer.The polarizing plate is a film-like material made of iodine, dye, or polyene, such as Sanritsu Electric's L- 82-18 type is suitable. On the reflective plate,
Sandblasted aluminum or a scattering plate made of acrylic plate and aluminum vapor-deposited are used.

FIG. 5 is a spectrum diagram showing the effect of this example. The vertical axis in FIG. 5 shows the intensity of transmitted light, and the horizontal axis shows the wavelength of light. Curve (A) represents the transmitted light spectrum when the polarization directions of the pair of polarizing filters are parallel to each other in the conventional single-layer twisted nematic field effect liquid crystal display device shown in FIG.
B shows the transmitted light spectrum when the polarization directions of a pair of polarizing filters of a two-layer liquid crystal display device according to an embodiment of the present invention shown in FIG. 2 are orthogonal to each other. The most desirable curve in the spectrum shown in Figure 5 is Ts
= 0 holds true for all wavelengths, and in this case, the above-mentioned coloring phenomenon due to interference color will not be observed. The curve (B) of the present invention is substantially equal to this ideal value, and the coloring phenomenon observed in conventional devices is virtually eliminated.

As described above, by using the present invention, even in a twisted nematic field effect liquid crystal display device with a d/Δn value smaller than 2.0 μm, which is suitable for multiplex driving, there is no interference color and the display quality can be improved. A high quality display device is realized.

Incidentally, even in a two-layer twisted nematic field effect liquid crystal display device, the above-mentioned third
A sufficient effect cannot be obtained in any direction other than the alignment direction of the liquid crystal molecules shown in the figure (the twist direction of the liquid crystal molecules in the two layers and the relative long axis direction of the liquid crystal molecules in the intermediate substrate 1a). An example is shown in FIG. In FIG. 6, curve A is for the case where the liquid crystal molecules have the orientation direction shown in FIG. 3, curve B is for the liquid crystal layers 4a and 4b in the twist direction shown in FIG. 7, and curve C is for the liquid crystal layers 4a and 4b. The twist direction of the substrate 1 is reversed as shown in FIG.
This is a case where the long axis directions of the nearest liquid crystal molecules on a are parallel. As is clear from Figure 6, when the liquid crystal molecules have the orientation shown in Figure 3, the condition of Ts = 0 is satisfied at almost all wavelengths, but this condition is not satisfied with the orientation of the liquid crystal molecules shown in Figures 7 and 8. It has not been. Therefore, it is desirable to set the alignment direction of the liquid crystal molecules as shown in FIG. In addition, when no voltage is applied between the electrodes 2a and 2b, the interference color due to elliptically polarized light is corrected, but when a sufficient voltage is applied between the electrodes 2a and 2b, the interference color of a single layer of the liquid crystal layer 4b, which serves as a compensator, occurs. . The conditions for minimizing this interference color for light with a wavelength of 550 nm, which has the highest visible sensitivity, are d.
The value of Δn is set to 0.5 μm, 1.0 μm or 1.65 μm. Each of these values actually has a certain tolerance range, so in practice the value of d・Δn can be set to 0.4 μm to 0.6 μm, 0.8 to 1.2 μm, or 1.5 to 1.8 μm.
It is necessary to set it within the range of . Figure 9 shows the amount of transmitted light of the measurement light obtained by setting the polarization directions parallel to each other in the configurations of Figures 2 and 3 to form a dark state when voltage is applied and a bright state when no voltage is applied. FIG. 3 is an explanatory diagram showing the relationship between applied voltage and light transmittance when the value of d·Δn is changed to 0.7 μm and 0.5 μm for light having a measurement wavelength of 550 nm using a 0.08 μm liquid crystal. The curve l ₁ is d・Δn=0.7μm, and the curve l ₂ is d・Δn=0.7μm.
The data is Δn=0.5 μm. As is clear from FIG. 9, setting d·Δn=0.5 μm further improves display quality.

As described above, the two-layer liquid crystal display device incorporating the correction plate of the liquid crystal layer of the present invention can drive a high-quality display without interference colors without deteriorating the characteristics for multiplexing. Therefore, by applying the present invention, low power consumption information displays with a large amount of information, such as terminal displays of small computers, character and graphic displays over telephone lines, text broadcasting, and small televisions, become possible. .

In the above embodiment, three glass substrates were laminated, but as shown in FIG.
It is also possible to use a structure in which cells b, 1c, and 1d are stacked as independent cells. Other symbols in FIG. 10 correspond to those in FIG.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a conventional twisted nematic field effect liquid crystal display device. FIG. 2 is a schematic cross-sectional view of a two-layer liquid crystal display device showing one embodiment of the present invention. FIG. 3 is an explanatory diagram showing the long axis direction of liquid crystal molecules on the surface of each substrate in the embodiment shown in FIG. FIG. 4 is an explanatory diagram defining the long axis direction of liquid crystal molecules having a tail angle θ. FIG. 5 is a spectrum diagram illustrating the effect of the embodiment shown in FIG. 2. FIG. 6 is an explanatory diagram of a spectrum that changes depending on the alignment direction of liquid crystal molecules. FIGS. 7 and 8 are explanatory diagrams showing liquid crystal molecule orientation directions corresponding to the spectra in FIG. 6. FIG. 9 is an explanatory diagram showing the relationship between applied voltage and light transmittance when the value of d·Δn is changed. FIG. 10 is a schematic cross-sectional view of a two-layer liquid crystal display showing another embodiment of the present invention. 1a, 1b, 1c, 1d...transparent substrate, 2a,
2b...transparent electrode, 3a, 3b, 3c, 3d...
Liquid crystal molecular alignment layer, 4a, 4b... Twisted nematic liquid crystal layer, 5... Sealing material, 6a, 6
b...Polarizing filter, 7...Drive circuit, 8...
a reflector.

Claims (1)

[Claims] 1. In a two-layer liquid crystal display device in which two liquid crystal layers in which the long axis direction of liquid crystal molecules is helically aligned are stacked in the helical axis direction and are interposed between a pair of upper and lower polarizers,
The liquid crystal layer has liquid crystal molecules stacked such that the orientation directions of the liquid crystal molecules closest to each other are substantially orthogonal, and the respective rotation directions are opposite to each other, and the layer thickness of each of the liquid crystal layers is d ₁ , d ₂ and birefringence values Δn ₁ , Δn ₂ are 0.7×d ₁・Δn ₁ <d ₂・Δn ₂ <1.3
×d ₁・Δn ₁ , and their values are 0.4 μm
～0.6μm, 0.8μm～1.2μm or 1.5μm～1.8μm
An electrode to which a driving voltage for changing the orientation of liquid crystal molecules is applied is arranged in only one of the liquid crystal layers, and the other liquid crystal layer is used as an optical color correction plate. A two-layer liquid crystal display device.