WO1999018475A1

WO1999018475A1 - Liquid crystal cell

Info

Publication number: WO1999018475A1
Application number: PCT/DE1998/002806
Authority: WO
Inventors: Hagen Klausmann; Gunther Haas
Original assignee: Robert Bosch Gmbh
Priority date: 1997-10-07
Filing date: 1998-09-22
Publication date: 1999-04-15
Also published as: EP1019781A1; JP2001519547A; KR20010024349A; DE19744249A1

Abstract

The invention relates to a liquid crystal cell, comprising a first and a second layered structure (21, 23), a liquid crystal (43) which is positioned between the two layered structures, two electrodes which are allocated to the liquid crystal, at least one of said electrodes being provided in the first layered structure (21) as an electrode layer (47), at least one polarisation layer (49) which is provided in the first layered structure (21), and an illumination means (5). Said illumination means (5) is mounted on the side of the first layered structure (21) which faces away from the liquid crystal. The invention is characterised in that the polarisation layer (49) in the first layered structure (21) is located on the side of the electrode layer (47) which faces away from the illumination means (5).

Description

Liquid crystal cell

State of the art

The invention relates to a liquid crystal cell with a first and a second layer, a liquid crystal which is arranged between the two layers, two electrodes which are associated with the liquid crystal, and an illumination means which is arranged on the side of the second layer which faces away from the liquid crystal .

Liquid crystal cells of this type are generally known. They are used for the construction of so-called LCD display units in which the individual liquid crystal cells are arranged in a matrix. Generally, a liquid crystal cell consists of a liquid crystal and two plates that sandwich the liquid crystal between them. Each of the two plates comprises a translucent glass substrate as the carrier layer, on each of which a polarization layer and an electrode layer are applied, the polarization layer being on the side of the carrier layer facing away from the liquid crystal. at A liquid crystal cell in the so-called active matrix technology contains a matrix of row and column lines on the so-called active plate, which drive the transparent electrodes. At each intersection between the row and column there is a thin film transistor which is connected to the associated pixel electrode. This means that all pixels of the matrix can be controlled individually. By applying a control voltage to the electrodes, the liquid crystal is aligned and thus changes its light transmission behavior. With the help of the two polarization layers, a cell can then be built up which, depending on the voltage applied, transmits light which a backlighting unit supplies.

The front plate opposite the active plate bears a shadow mask, which is also known as the so-called Black Maxtrix. The Black Matrix has the task of covering the unaddressed areas of the liquid crystal cell, in particular the areas between the pixel electrodes, in order to prevent unwanted transmission through these areas and thus to ensure good contrast. The disadvantage of the black matrix, however, is that the total transmission of the cell is reduced. Depending on the design of the matrix of row and column lines, the opening, that is the ratio of translucent to opaque areas, of the shadow mask is 50% to 80%.

Especially for larger liquid crystal display units, for example for use as a com- puter onitor, liquid crystal cells are increasingly being used which work according to the so-called "in-plane switching" (IPS) method, as is disclosed in the European patent specification 0 509 025 B1. In contrast to the liquid crystal cell construction explained above, in the IPS liquid crystal cell the two electrodes are arranged on the active plate, the liquid crystal being aligned by a lateral electric field. The electrodes of such a liquid crystal display unit form an interdigital electrode structure. The advantage of the IPS cell lies in its very wide viewing angle, as it is required for computer monitors, for example. However, the IPS cell has the disadvantage that the transmission value of a cell is relatively low. On the one hand, the addressed areas must be covered with a black matrix, on the other hand, each pixel itself has low transmission, due to the interdigital electrodes. The design of the interdigital electrodes made of transparent material increases the transmission only slightly, since areas above the electrodes do not switch and the transparent state, in the normal case, corresponds to the switched state in the IPS cell. Typically, an opening (taking into account the black matrix and the interdigital electrodes) of approx. 30% results for an IPS cell.

Advantages of the invention

The liquid crystal cell with the features of claim 1 has the advantage that the light yield is significantly increased. this will achieved in that the polarization layer, that is, the polarization filter, is provided on the liquid crystal side of the black matrix and / or the electrode layer. As a result, the light reflected at the black matrix or the electrodes is not attenuated by absorption caused by the polarization filter. The light reflected on the second layer then returns to the liquid crystal cell by being reflected again on the illuminant.

In an advantageous development of the invention, the liquid crystal cell is designed as an IPS cell, in which the two electrodes lie in one plane. Especially with this type of liquid crystal cell, in which the opening is in the region of 30%, the luminous efficacy can be increased particularly significantly.

In an advantageous development of the invention, the lighting means has additional light reflection means which are used for back reflection. This leads to a further increase in the luminous efficacy, since the reflection means can be specifically adapted to the given situation. Due to the optimization of the reflection means, this has a positive effect with regard to the light yield that can be achieved compared to the use of the lighting means itself as a reflector.

In an advantageous development of the invention, the electrodes or the black matrix have a highly reflective surface, at least on the side facing the illuminant. The proportion of the absorbed by this surface This allows reduced light and thus significantly increases the light output.

In an advantageous development of the invention, the side of the electrodes facing the liquid crystal is anti-reflective, so that reflections are prevented here. This has the advantage that the contrast under ambient light is not impaired.

In an advantageous development of the invention, the polarization layer is designed as a combined LPP / LCP layer, as published for example in Nature 381 from May 16, 1996, with embedded dichroic dye molecules.

In an advantageous development of the invention, the polarization layer has an orientation layer for aligning the adjacent liquid crystal. The advantage of this arrangement is, in particular, the saving of a separate orientation layer.

In an advantageous development of the invention, the polarization layer is designed as a liquid-crystalline polymer layer with embedded dichroic dye molecules.

In an advantageous development of the invention, the second layer has a color filter which is arranged between the electrode layer and the polarization layer.

In an advantageous development of the invention, the liquid crystal is designed as a guest-host liquid crystal or on the basis of liquid crystal polymer compounds. The liquid crystal cell according to the invention can be used particularly advantageously in an LCD display device, preferably in a color LCD display device (LCD = liquid crystal display), such as are used as monitors in portable computers or in the motor vehicle sector for navigation systems.

drawing

The invention will now be explained in more detail using an exemplary embodiment with reference to the drawing. Show:

FIG. 1 shows a schematic illustration to explain the principle of the invention,

FIG. 2 shows a schematic representation of the structure of a liquid crystal cell according to a first exemplary embodiment,

FIG. 3 shows a schematic representation of the structure of a liquid crystal cell according to a second exemplary embodiment,

Figure 4 shows a schematic representation of the structure of a liquid crystal cell according to a third embodiment, and

Figure 5 shows the basic structure of an LCD module. Embodiments

5 shows the basic structure of a known LCD module (LCD = liquid crystal display). Such modules are used, for example, in the automotive field, for example in navigation systems, or in mobile communication and as computer monitors. The LCD module 1, which can be designed as a black and white or as a color display, comprises a housing 3 in which an illumination unit 5, a control unit 7 and a liquid crystal cell unit 9 are accommodated.

The lighting unit 5 comprises a lamp 11 mounted on the side, a light guide 13 and a reflector 15. The lighting unit 5 is designed such that the emitted light essentially runs in the direction of the arrow P and irradiates the liquid crystal cell unit 9 uniformly.

The liquid crystal cell unit 9, which - viewed in the beam direction P - follows the illumination unit 5, comprises, in addition to two foils 17, a first polarization filter 19, two layers 21, 23 which enclose the liquid crystal between them, and a second polarization filter 25.

In addition to control electronics 27, the control unit 7 comprises driver modules 29 which control liquid crystal cells of the liquid crystal cell unit 9 via data lines 31.

The liquid crystal cell unit 9 consists of a multiplicity of individual liquid crystal cells which are arranged in a matrix and separately from one another are controllable. The control of the individual cells takes place in so-called TFT liquid crystal cells with the aid of thin film transistors, which are addressed via the data lines 31 and the electrodes apply a voltage to a cell.

The schematic structure of a liquid crystal cell according to a first exemplary embodiment will now be explained in more detail with reference to FIG. 2.

The liquid crystal cell 41 has a layered structure which comprises the first layer 21, the second layer 23 and a liquid crystal 43 lying between the two layers.

The first layer 21 has a glass substrate 45 serving as a carrier, followed by an electrode layer 47, a polarization layer 49 and an orientation layer 51 adjoining the liquid crystal 43.

The electrode layer 47 has an integrated black matrix, a control transistor and the lines necessary for controlling the transistor, these elements being shown as control units 53 for the sake of clarity. Furthermore, the electrodes 55 necessary for aligning the liquid crystal are provided in the electrode layer. In the present exemplary embodiment, the two electrodes required to control a liquid crystal cell are arranged in one plane. It is a so-called IPS cell, in which the individual electrodes intermesh like a comb (interdigital electrode structure). A detailed presentation Position of this IPS cell is disclosed in European patent specification 0 509 025 B1, the disclosure content of which is hereby fully incorporated. Figure 2 shows that the individual electrodes 55 are arranged parallel to each other and span a space 57 between them. The free space 57 is translucent, while the electrodes 55 and the control unit 53 are opaque.

The second layer 23 has a similar structure, but no actively controllable elements are provided. For this reason, the first layer 21 is also referred to as the active layer. The second layer 23 likewise has a glass substrate 59, on which — in the case of color LCD modules — a color filter layer 61 and an orientation layer 63 are applied. The second layer 23 further comprises a polarization layer, which is not shown for reasons of clarity. It can preferably be implemented as a component of the color filter layer 61.

The operation of this liquid crystal cell unit 41 is now as follows:

In an activated state of a liquid crystal cell, light emitted by the illumination unit 5, which is indicated by a light beam 65, is transmitted through the second layer 23. This is due to the fact that the direction of polarization is rotated in a direction corresponding to the direction of polarization of the second polarization layer in the second layer 23 during the passage of the liquid crystal. The light emitted by the lighting unit 5 can dig through only those areas in the first layer 21 which are translucent. In the present exemplary embodiment, these are only the free spaces 57. On the undersides, that is to say the sides of the electrodes 55 facing the lighting unit 5, the black matrix integrated in this layer and the control unit 53, the light is reflected back in the direction of the lighting unit 5, such as indicated by an arrow 67. This beam 67 reflected on the electrode surface comes back to the illumination unit 5 and is then reflected back via the reflector 15 in the direction of the first layer 21. This provides the advantage that the reflected light can ultimately be used almost completely to illuminate a liquid crystal cell. This reflected light component is also not at least partially absorbed by a polarization filter, since the polarization layer 49 is arranged beyond the electrode layer 47 with respect to the illumination unit 5.

The principle according to the invention therefore consists, on the one hand, in designing the electrodes in such a way that they reflect light and, on the other hand, in that the reflected light is not at least partially absorbed by a polarization layer. A corresponding representation of this principle can be seen in FIG. 1. It is thus shown in this figure that the light emitted by the lighting unit 5 on the one hand penetrates the liquid crystal cell unit 9 and on the other hand is reflected there back to the lighting unit 5. This reflected portion, which is identified by 69, is in turn reflected on the lighting unit 5 back to the liquid crystal cell unit 9. animals. This process can continue several times, as shown in FIG. 1, since the beam 69 reflected back by the liquid crystal cell unit 9 experiences essentially no attenuation.

FIG. 3 shows a schematic illustration of a liquid crystal cell unit 71 according to a second exemplary embodiment. For the sake of simplicity, the same layers as in the structure according to FIG. 2 are identified by the same reference numerals, so that they are not described again.

In contrast to the exemplary embodiment according to FIG. 2, the second layer 23 has the electrode layer 47, in which transparent electrodes (not shown in the figure) and the control unit 53 are provided. In this case, the active layer is the layer 23 which faces an observer B of the liquid crystal cell unit.

Instead of the electrode layer 47, the first layer 21 has a layer 73 which lies between the polarization layer 49 and the glass substrate 45. In addition to transparent electrodes, this layer 73 has a shadow mask 75 (black matrix), which makes the control unit 53 opaque to opposite areas. In the space between the opaque areas of the shadow mask 75, translucent areas 77 are provided, which can be used as a color filter, for example, by introducing an appropriate optical material. The side of the shadow mask 75 facing the lighting unit 5 is designed in such a way that very good light reflection takes place. Thus, as already described in connection with the exemplary embodiment according to FIG. 2, it is also possible here for the light to be reflected back to the illumination unit 5 without being attenuated by the polarization filter and from there to be radiated back to the liquid crystal cell via appropriate reflection means.

Another exemplary embodiment of a liquid crystal cell unit 81 is shown schematically in FIG. The structure of this liquid crystal cell unit 81 corresponds essentially to that of the exemplary embodiment according to FIG. 3, so that the repeated description of the elements and layers identified by the same reference numerals is dispensed with.

In contrast to the previous exemplary embodiment, the electrode layer 47 is provided in the first layer 21 between the polarization layer 49 and the glass substrate 45. In addition, this layer 47 contains the shadow mask 75 described above. In return, the color filter layer has been included in the second layer 23 as layer 83. This color filter layer 83 lies between the glass substrate 59 and the orientation layer 63.

In the present exemplary embodiment, both the side of the electrodes 55 and the control unit 53 facing the lighting unit 5 and that of the shadow mask 75 are designed to be light-reflecting. In this case too, the flexion of the light in the direction of the illumination unit 5 and from there back to the liquid crystal cell unit 81 an increase in the light yield is achieved.

Of course, other layer structures are also conceivable. According to the invention, it is only necessary to ensure that the light is well reflected on a side facing the lighting unit 5 and that the reflected beam does not have to pass through a polarization filter, which would result in a weakening of light.

The polarization layer is, for example, a liquid-crystalline polymer layer in which dichroic dye molecules are embedded. In addition, the polarization layer can also be formed from a combined LPP / LCP layer with embedded dichroic dye molecules. The polarization layer can simultaneously serve as an orientation layer for the liquid crystal, so that a separate orientation layer is not required.

The shadow mask is preferably made of a metal that has very good reflective properties. In the case of an IPS cell, the electrodes should also be made of a highly reflective material, but the side facing the viewer is non-reflective via one or more dielectric intermediate layers.

2 to 4, the second polarization layer, which forms the second polarization filter 25, is not shown for the sake of clarity. It is possible to apply this, for example, on the side of the glass substrate of the second layer 23 facing away from the liquid crystal. Of course, it is also conceivable to provide the second polarization layer on the side of the glass substrate facing the liquid crystal.

If the lighting unit 5 functions according to the light guide principle, as shown in FIG. 5, it should be noted that the light reflected back does not pass through the light guide and is absorbed. It is therefore necessary to provide a reflector attached under the light guide, which is generally not in optical contact with the light guide.

Claims

Expectations

1. Liquid crystal cell with a first and a second. Layer, a liquid crystal, which is arranged between the two layers, two electrodes assigned to the liquid crystal, with one of the electrodes being provided as an electrode layer in the second layer, at least one in the second. Layer provided polarization layer, and an Eleuchtungs ~ means, which is arranged on the side of the liquid crystal away from the second layer, characterized in that the polarization layer (4S) in the second layer (21) on the Eleuchtung≤ means (5) facing side of the electrode layer (47) is arranged.

2. liquid crystal cell according to claim 1, characterized in that the Eleuchtmittel (5) serves as a light reflection means for reflecting the light rays reflected by the second layer.

3. liquid crystal cell according to claim 1 or 2, characterized in that the second layer (21) has both electrodes (55) as the electrode layer (47), the electrodes forming an ir.terdigitale electrode structure.

4. liquid crystal cell according to one of the preceding claims, characterized in that the second layer (21) comprises a carrier layer (45) (substrate) on which the electrode layer (47) and the polarization filter layer (49) are applied , and that the second layer (21) further comprises a mask layer (75) which is provided between the carrier layer (45) and the electrode layer (47) and makes certain oaks opaque.

5- liquid crystal cell according to claim ^h ; characterized in that the mask layer (75) is formed from a reflective material, preferably metal.

6- liquid crystal cell according to one of claims 1 to 5, characterized in that the side of the electrodes (55) facing the illuminating means (5) is light-reflecting and that the side of the electrodes facing away from the illuminating means (5) is anti-reflective, preferably by means of dielectric interlayers.

7. liquid crystal cell according to one of the preceding claims, characterized in that the polarization layer (59) is formed from a combined LPP / LCP layer with embedded dichroic dye molecules.

⁸ - liquid crystal cell according to claim ^? , characterized in that the polarization layer (49) serves as an orientation layer for aligning the adjacent liquid crystal.

^Q. Liquid crystal cell according to one of claims 1 to c, characterized in that the polarization layer (49) is designed as a liquid crystalline polymer layer with embedded dichroic dye olecules.

₁₀ • Liquid crystal cell according to one of the previous ones. Sayings, characterized in that the second layer (21) comprises a color filter, which is arranged between the electrode layer and polarization layer (49).

π. Liquid crystal cell according to one of the previous ones. Claims, characterized in that the liquid crystal (43) is designed as liquid crystal liquid or on the liquid crystal film of Cc-pounds.

12. Use of a liquid crystal cell according to one of claims 1 to ιι in an LCD display device, preferably in a color LCD Ar.zεicevcrrichtunσ.