JPH0572543A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To improve the display contrast of the liquid crystal display device of a matrix type. CONSTITUTION:Plural pieces of signal electrodes 2 arrayed in parallel with each other apart prescribed intervals (g) are formed on one substrate 1. The other substrate 3 is disposed to face this substrate. Plural pieces of scanning electrodes 4 which are arrayed in parallel with each other and are arranged to intersect with the signal electrodes 2 are formed on the inner surface of this substrate. A liquid crystal layer 5 having a prescribed thickness (d) is crimped between the two substrates 1 and 3. Oriented films 6 are formed on the boundaries between at least one substrate and the liquid crystal layer 5. The spacings(g) between the adjacent signal electrodes 2 are set at half the thickness (d) of the liquid crystal layer 5 or below. The space parts between the signal electrodes are operated in the same manner as picture element parts and the display contrast is improved by such constitution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a matrix type liquid crystal display device, and more particularly to improvement of a cell structure including electrode arrangement.

[0002]

2. Description of the Related Art FIG. 6 shows a typical sectional cell structure of a conventional simple matrix type liquid crystal display device. One substrate 1
On the inner surface of 01, a plurality of signal electrodes 102 arranged in parallel with each other at a predetermined interval are formed. The other substrate 10 is separated from the substrate 101 by a predetermined gap.
3 are arranged facing each other. On the inner surface of the counter substrate 103, a plurality of scanning electrodes 104 arranged to intersect the signal electrodes 102 are formed. A liquid crystal layer 105 having a predetermined thickness is sandwiched between both substrates 101 and 103. An alignment film 106 is formed at the interface between the liquid crystal layer 105 and each of the substrates 101 and 103 to control the alignment state of liquid crystal molecules contained in the liquid crystal layer 105.

FIG. 7 shows a signal electrode group 102 and a scanning electrode group 10.
4 shows the planar arrangement relationship of No. 4. Pixels 107 are formed at the intersections of the signal electrodes 102 and the scanning electrodes 104, as indicated by hatching. When a voltage is applied to the pixel 107, the alignment state of liquid crystal molecules changes according to the magnitude of the voltage and the transmittance of the pixel changes. In this way, the matrix display is performed by controlling the turning on and off of the pixels. In practice, multiplex driving is performed, and a lighting signal or a non-lighting signal is applied to the signal electrodes while applying a scanning signal to the scanning electrodes 104 line-sequentially. Therefore, the lighting voltage VON and the extinguishing voltage VOFF applied to each pixel are effective values.
The display device is driven by setting the effective lighting voltage above the threshold voltage of the liquid crystal and setting the turn-off voltage below the threshold voltage. As is well known, the effective value ratio of the lighting voltage and the extinction voltage is set by the optimum bias method.

On the other hand, the drive voltage is not applied to at least one portion of the inner surface of the substrate where no electrode exists, that is, the space 108 indicated by the dotted line hatching. Therefore, the alignment state of the liquid crystal molecules located in the electrode space 108 remains in the initial state, and the transmittance does not change.

[0005]

Generally, there are a positive mode and a negative mode as a display mode of a matrix liquid crystal display device. In the positive mode, the display background is set to a white state with high transmittance and the pixels are selectively turned on to reduce the transmittance and display black. On the contrary, in the negative display, the display background is set to a black state with low transmittance, and the pixels are selectively turned on to locally increase the transmittance to perform white display. Now, consider a negative display as an example. Generally, the transmittance T of a liquid crystal cell is determined by a predetermined cell constant, that is, retardation Δn · d. In addition,
Δn represents the refractive index anisotropy of the liquid crystal, and d represents the thickness of the liquid crystal layer. FIG. 8 shows the relationship between the transmittance T and the retardation in the negative display. In case of negative display,
The minimum value of the transmittance of the unlit pixels forming the background is 0.
The cell constant is set to be 2%. By the way, the inter-electrode space 108 shown in FIG. 7 also constitutes a display background.
However, at least one electrode does not exist in this space 108, and the thickness d of the liquid crystal layer is different from the thickness of the liquid crystal layer of the pixel 107. Therefore, the retardation Δn · d also changes and the minimum transmittance cannot be obtained. For example,
As shown in the graph of FIG. 8, the transmittance is about 5%. Therefore, there is a problem that light leakage occurs in the inter-electrode space 108 and the display contrast is deteriorated. This relationship is the same in the positive display, and the transmittance of the inter-electrode space portion is slightly lower than the transmittance of the unlit pixel portion.
Conventionally, measures have been taken to apply a so-called black stripe mask in order to prevent a reduction in display contrast due to the inter-electrode space portion. However, if the black stripe mask is formed so as to be aligned with the space between the electrodes, another problem arises that the number of manufacturing steps of the matrix liquid crystal display device increases and the cost increases.

Although the above-mentioned problems of the conventional technique have been described with respect to the simple matrix type liquid crystal display device, the same problems are recognized in the active matrix type liquid crystal display device. In this case, the problem of contrast reduction becomes significant especially when positive display is performed.

[0007]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention provides a cell structure capable of effectively preventing a decrease in display contrast due to light leakage in the inter-electrode space portion. With the goal. The means adopted to achieve such an object is that in the matrix type liquid crystal display device, the interval between adjacent signal electrodes, that is, the space between the electrodes is set to half or less of the thickness of the liquid crystal layer.

[0008]

When the space between the signal electrodes is reduced, a voltage of a predetermined level is applied not only to the pixel portion but also to the liquid crystal existing in the space between the signal electrodes as a result of the sneak of the electric field applied to the liquid crystal layer. At this time, if the space between the signal electrodes is set to be less than half the thickness of the liquid crystal layer, the liquid crystal existing in the space portion also changes its alignment state and operates. Therefore, when the adjacent pixels are maintained in the same state, the liquid crystal present in the space between them is also affected and maintains the same state. For example, when the pixel electrodes adjacent to each other in the scanning electrode direction are in the black display state, the space between them is also displayed in black, and light leakage can be effectively prevented. On the contrary, when the adjacent pixels are in the white display state, the transmittance of the space between them is also increased and the display contrast is improved.
In addition, the electrode aperture ratio is improved by reducing the space between the electrodes.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic partial sectional view showing an embodiment in which the present invention is applied to a simple matrix type liquid crystal display device. As shown in the figure, a plurality of signal electrodes 2 are formed on the inner surface of one of the substrates 1 and arranged in parallel with each other at a predetermined interval g. The substrate 1 is made of transparent glass, for example, and the signal electrode 2 is made of a transparent conductive material such as ITO. The other substrate 3 is arranged to face the substrate 1 with a predetermined gap therebetween. This substrate 3 is also made of a glass material. On the inner surface of the substrate 3, a plurality of scanning electrodes 4 are formed which are aligned in parallel with each other and arranged to intersect the signal electrodes 2. This scanning electrode 4 is also made of a transparent conductive material such as ITO. A liquid crystal layer 5 having a predetermined thickness d is sandwiched between both substrates 1 and 3. The liquid crystal layer 5 constitutes an electro-optical material layer. An alignment film 6 is provided at the interface between each of the substrates 1 and 3 and the liquid crystal layer 5. The alignment film 6 acts on the liquid crystal molecules contained in the liquid crystal layer 5 to control the alignment state. Therefore,
Depending on the kind of alignment state required, it may be formed only on at least one interface. The alignment film 6 is obtained by applying a polymer film such as polyimide and then rubbing in a predetermined direction. Alternatively, it may be formed by oblique vapor deposition of an inorganic material such as silicon monoxide.
Various alignment states such as homogeneous alignment, homeotropic alignment, twisted nematic alignment or super twisted nematic alignment can be realized depending on the type of alignment film. In particular, the super twist nematic alignment is suitable for simple matrix driving because the threshold characteristic of the liquid crystal layer is extremely steep. Finally, if necessary, a polarizing plate 7 is attached to the surfaces of both substrates 1 and 3, and the change in the orientation of the liquid crystal molecules in the liquid crystal layer 5 is taken out as a change in the transmittance.

In this embodiment, the distance g between adjacent signal electrodes, that is, the space between the signal electrodes is set to be half the thickness d of the liquid crystal layer 5 or less. For example, in the case of super twist nematic alignment, the thickness d of the liquid crystal layer 5 is set to about 6 μm. In this case, the space g between the signal electrodes is 3μ.
It is set to m or less. Generally, these cell constants are determined based on the refractive index anisotropy Δn of the liquid crystal material used so that a predetermined retardation Δn · d can be obtained in optical design. As described above, the simple matrix type liquid crystal display device is subjected to multiplex driving, and while the scanning signal is applied to the scanning electrode 4 line-sequentially, the lighting signal or the extinguishing signal is selectively applied to each signal electrode 2. As shown,
When attention is paid to the pixels lined up along the same scanning electrode 4, by reducing the space g between the signal electrodes, the electric field applied in the thickness direction of the cell is sneak up, and the liquid crystal existing in the space portion is also affected. The voltage is applied.

FIG. 2 is a plan view of the liquid crystal cell shown in FIG. 1, and particularly shows only the signal electrode 2 and the scanning electrode 4.
In this embodiment, the black stripe mask 8 is applied only to the space between the scanning electrodes 4. The mask 8 is formed by printing black ink or the like. This black stripe mask is for improving display contrast. On the other hand, no black stripe mask is provided between the signal electrodes 2. Each signal electrode 2
Pixels 9 are formed at the intersections of the scanning electrodes 4 with each other. or,
An inter-signal electrode space 10 is left between the pixels 9 adjacent to each other along the scanning electrodes 4. No black stripe mask is applied to this space 10. This space portion 10 is influenced by the adjacent pixel portion 9 and its transmittance changes.

Next, the operation of the simple matrix type liquid crystal display device shown in FIGS. 1 and 2 will be described. Figure 3 shows the transmittance T
6 is a graph showing the relationship between the drive voltage V and the drive voltage V. This graph is measured with respect to a cell that performs negative display in the super twist nematic mode. The solid curve represents the invention sample and the dotted curve represents the conventional sample. The electrode aperture ratio is set to 81% in all the samples. In the sample of the invention, the space g between the signal electrodes is set to ½ of the thickness d of the liquid crystal layer. Further, in the conventional sample, the space g between the signal electrodes is set to be considerably larger than the thickness d of the liquid crystal layer. As is clear from the graph of FIG. 3, in the invention sample, the effective lighting voltage VO is applied to all the pixels.
When N is applied, the average transmittance T reaches 18%. That is, the space portion 10 between the lighting pixels 9 adjacent to each other along the scanning electrode 4 also changes to the lighting state. On the other hand, in the conventional sample, the effective lighting voltage V is applied to all the pixels.
When ON is applied, the average transmittance T remains at 17%.
This is because the space between adjacent pixels does not operate without being affected by the voltage application and remains in the off state.
Next, when the effective extinguishing voltage VOFF is applied to all the pixels in the invention sample, the transmittance in the pixel portion is lowered to the minimum of 0.2%. At this time, the space portion sandwiched between the adjacent extinguished pixels is also driven to the extinguished state and the transmittance thereof is 0.
It drops to 6%. Therefore, the average transmittance in the completely extinguished state is reduced to about 0.2% or more. The display contrast obtained in this case is 18% / 0.2% = 90. On the other hand, in the conventional sample, when the effective extinguishing voltage VOFF is applied to all the pixels, the transmittance in the extinguished pixel is lowered to the minimum of 0.2% as in the invention sample. However, since the transmittance of the space portion sandwiched between the adjacent unlit pixels does not change, it remains at a relatively large value of 5%. As a result, in the conventional sample, the average transmittance at the time of all extinguishing is about 0.6%. Therefore, the contrast is 17% / 0.6% = 28. Like this
A significant difference appears in the average contrast between the invention sample and the conventional sample.

Next, FIG. 4 is a graph showing the relationship between the transmittance T of the space portion sandwiched between adjacent light-off pixels and the ratio of the space dimension g to the thickness d of the liquid crystal layer. As is clear from the figure, when the ratio g / d is set to 0.5 or less, the space portion is substantially completely operated by the influence of the adjacent extinguished pixels to be in the extinguished state, so that the transmittance is about 0.6%. Can be achieved. On the other hand, when the ratio g / d becomes 0.5 or more, the operation of the liquid crystal located in the space becomes incomplete and becomes 2.0.
Almost no operation occurs with the above ratio. Therefore, the transmittance of the space portion remains at a high level of about 5%.

Finally, another embodiment in which the present invention is applied to an active matrix type liquid crystal display device will be described with reference to FIG. A plurality of pixel electrodes 12 arranged in a matrix are formed on the inner surface of one substrate 11. Further, a plurality of switching elements 13 such as an insulated gate field effect transistor for operating the individual pixel electrodes 12 are also formed. The switching element 13 is a thin film transistor element formed on the insulating substrate 11. Further, a selection wiring 14 for selecting the switching element 13 is provided for each row. The selection wiring 14 is connected to the gate electrode of the transistor switching element 13. Further, the drive wiring 15 is provided between each column of the pixel electrodes 12 at a predetermined distance from the end of each pixel electrode. The drive wiring 15 is connected to the drain electrode of each transistor switching element 13,
It is for driving the selected switching element. The source electrode of each transistor switching element 13 is connected to the corresponding pixel electrode 12.
The other substrate 16 is arranged opposite to the substrate 11. A counter electrode is formed on the inner surface of the counter substrate 16. A liquid crystal layer having a predetermined thickness is sandwiched between both substrates 11 and 16. In addition, an alignment film is provided at the interface between at least one substrate and the liquid crystal layer.

In such a structure, each pixel electrode 12
The distance g between the vertical end of the liquid crystal layer and the drive wiring 15 is the thickness d of the liquid crystal layer
Is set to less than half. Similar to the embodiment shown in FIGS. 1 and 2, also in this example, the electric field wraps around the space between the pixel electrode 12 and the adjacent drive wiring 15, and operates similarly to the pixel portion. That is, the drive wiring 15
This is because the electric potential of the pixel 12 is often at the same level. On the other hand, since the selection pulse is only momentarily applied to the selection wiring 14, such an effect is not recognized.

[0016]

As described above, according to the present invention, in the matrix type liquid crystal display device, the distance between the space between the signal electrodes or the space between the end of the pixel electrode and the drive wiring is determined by the thickness of the liquid crystal layer. By setting it to half or less, the space portion can be operated in accordance with the pixel, so that there is an effect that light leakage in the space portion can be suppressed and the display contrast can be improved. In addition, by reducing the space interval, it is possible to improve the electrode aperture ratio and further improve the display contrast.

[Brief description of drawings]

FIG. 1 is a schematic partial sectional view showing an embodiment in which the present invention is applied to a simple matrix type liquid crystal display device.

FIG. 2 is a schematic partial plan view of the liquid crystal display device shown in FIG.

FIG. 3 is a graph showing a relationship between a transmittance and a driving voltage of the liquid crystal display device shown in FIGS. 1 and 2.

FIG. 4 is a graph showing the relationship between the transmittance of a liquid crystal display device and the ratio of space spacing to liquid crystal layer thickness.

FIG. 5 is a schematic partial plan view showing another embodiment in which the present invention is applied to an active matrix type liquid crystal display device.

FIG. 6 is a partial cross-sectional view showing a conventional simple matrix type liquid crystal display device.

7 is a schematic partial plan view of the liquid crystal display device shown in FIG.

FIG. 8 is a graph showing a relationship between transmittance and retardation in a conventional liquid crystal display device.

[Explanation of symbols]

 1 substrate 2 signal electrode 3 substrate 4 scanning electrode 5 liquid crystal layer 6 alignment film d thickness of liquid crystal layer g space between signal electrodes

Claims (2)

[Claims] 1. A substrate having a plurality of signal electrodes aligned in parallel with each other at a predetermined interval, and a plurality of scans aligned in parallel with each other and arranged to intersect the signal electrodes. The other substrate having electrodes and opposed to the one substrate, a liquid crystal layer having a predetermined thickness sandwiched between the two substrates, and provided at the interface between at least one substrate and the liquid crystal layer. In the liquid crystal display device including an alignment film, the distance between the signal electrodes is set to half or less of the thickness of the liquid crystal layer. 2. A plurality of pixel electrodes arranged in a matrix, a plurality of switching elements for operating the individual pixel electrodes, a selection wiring for selecting the switching elements for each row, and individual pixel electrode ends. One substrate having a drive wiring for driving a selected switching element, which is provided between columns of pixel electrodes at a predetermined interval, and the other substrate having a counter electrode and arranged to face the one substrate. In a liquid crystal display device comprising a substrate, a liquid crystal layer having a predetermined thickness sandwiched between both substrates, and an alignment film provided at the interface between at least one of the substrates and the liquid crystal layer. A liquid crystal display device, characterized in that a distance between an electrode end and the drive wiring is set to be half or less of a thickness of the liquid crystal layer.