WO1998035266A1

WO1998035266A1 - Liquid crystal display with a plurality of picture elements

Info

Publication number: WO1998035266A1
Application number: PCT/DE1998/000192
Authority: WO
Inventors: Christina Heinmueller; Gunther Haas
Original assignee: Robert Bosch Gmbh
Priority date: 1997-02-07
Filing date: 1998-01-22
Publication date: 1998-08-13
Also published as: JP2001510586A; EP0958524A1; DE19704717A1

Abstract

The present invention relates to a liquid crystal display with a plurality of picture elements, comprised of a liquid crystal cell and polarizers (P1, P2) mounted on the upper and lower faces of said cell, which includes two substrates (DS, GS) with transparent electrodes between which is inserted the liquid crystal. In order to compensate for the fact that the optical features of the liquid crystal display, such as transmittance and contrast, are dependent upon an angular field, each picture element is divided into at least two under-picture element in which the liquid crystals have various orientations and, inserted between a polarizer and said liquid crystals, at least one biaxial delay layer (V1, V2) with different refractive indexes ne, no, nz. The refractive index nz appears in the delay layer, in an axis substantially parallel to the normal of said liquid crystal cell.

Description

Multi-pixel liquid crystal display

The invention relates to a liquid crystal display with a plurality of pixels consisting of a liquid crystal cell and polarizers according to the preamble of claim 1.

State of the art:

Liquid crystal displays have become known in a wide variety of embodiments. However, STN and TN liquid crystal cells are used most frequently. STN stands for Super Twisted Nematic and TN for Twisted Nematic.

Twisted Nemetic is intended to express the fact that a twisted nematic liquid crystal phase is used.

The structure of a conventional TN display is shown in Figure 3 a and 3 b. A liquid crystal is enclosed between two glass plates, each of which is coated with a transparent electrode. The liquid crystal molecules are indicated by elongated rods. With the help of an orientation layer, the liquid crystal molecules are oriented in such a way that they are arranged in a helical manner, twisted by 90 °, between the substrates. Polarizing filters are attached to the outside of the glass plates, the transmissive preferred direction of which is shown in FIG. 3 by a double arrow. Located on the side of the light the polarizer, on the side of the light exit the analyzer. In the voltage-free state of the cell shown in FIG. 3a, polarized light occurs, is also rotated by 90 ° due to the 90 ° liquid crystal olekülhelix and can exit through the analyzer. Liquid crystal displays that are transparent when de-energized are referred to as "normally white displays". If there is sufficient voltage between the electrodes, the align

Liquid crystal molecules parallel to the electric field, so that incident light remains essentially unaffected and is therefore absorbed by the analyzer. In this state, the liquid crystal display is dark. With a "normally white display", the transmission of the light steadily decreases with increasing voltage. In this way, a defined transmission and thus a gray value (gray level) can be uniquely assigned to a specific voltage. In conventional liquid crystal displays, however, the transmission and thus the gray levels depend on the viewing angle. As can be seen in FIG. 4, the viewing angle can be clearly represented with the aid of polar coordinates. The angle φ in the display area takes on values from 0 to 360 °.

The angle θ has a value range from 0 to ax. 90 ° and shows how far the viewing angle is inclined out of the normal of the liquid crystal display. 6 a shows the transmission as a function of the angle of inclination θ for φ = 0 ° and four different gray levels gl to g4. Even at φ = +/- 27 ° T (g4)> T (g3). This means that there is an unwanted grayscale inversion at this point. 6b shows the transmission

T as a function of the angle of inclination θ for φ = 90 °. Here a grayscale inversion already results for an angle θ> 12 °. In the same way as the grayscale Contrast of the liquid crystal display is a function of θ and φ. This is defined as the ratio of the largest to the smallest transmission. Analogous to the evaluation of the grayscale inversion, a range of θ can be found for each φ in which the contrast is greater than a minimum contrast.

The conoscope diagram provides a clear representation of the functions of θ and φ (cf. Fig. 5). Here the

Inclination angle θ and the angle φ are shown on a circular disk, where θ corresponds to the radial component and φ to the azimuth. Both the results for the grayscale inversion and the contrast values can now be entered in this diagram. A region is generally defined as a suitable viewing angle region (cross-hatched and longitudinally hatched area in FIG. 5) in which a minimum contrast is not undershot (see. Cross-hatched area in FIG. 5) and additionally no grayscale inversion occurs (see FIG. 5, hatched area). The boundary line of the contrast surface is referred to as the ISO contrast curve. As can be seen in FIG. 5, a conventional TN liquid crystal display without special measures only has a very narrow suitable viewing angle range (cross-hatched and longitudinally hatched area).

In order to enlarge this, various solution principles are currently used.

In the European patent application EP 646 829 AI the use of a compensation layer is proposed to enlarge the viewing angle range. As an essential component, this comprises a discotic retardation film which is produced from a discotic liquid crystal. Such a film is optically uniaxial, the ordinary refractive index n _{0 being} greater than the extraordinary one

Refractive index n _e . Therefore, the delay V _R = d _R • (n _e - n ₀₎ negative (d _R is the thickness of the film). The retardation value of these films is equal in magnitude to the retardation value of the liquid crystal layer, but has the opposite sign. The delay layer described in the application, in addition to compensating for the birefringence by the beam material in the liquid crystal, compensates in particular for a birefringence caused by the liquid crystal edge molecules on the substrate surface of the liquid crystal cell, which have a certain angle of attack (pre-tilt) to the substrate surface. The discotic retardation film must always be adapted to the orientation of the liquid crystal for a desired compensation effect. If such a film is applied between the cell and the polarizing filter, the suitable viewing angle range can be significantly improved.

Another possibility of improving the suitable viewing angle range is to compensate for the asymmetry in the viewing angle dependency of the contrast and with regard to the viewing angle dependency of the grayscale inversion (cf. FIG. 5). This asymmetry is caused by the non-symmetrical alignment of the liquid crystal molecules. One method of obtaining a symmetrical viewing angle range for the liquid crystal cell is to subdivide each pixel into two subpixels, each with an opposite orientation of the liquid crystal molecules, so that the viewing angle range of the first subpixel is rotated by 180 ° with respect to the viewing angle range of the second subpixel ( Figure 7, 2-domain method, KH Yang: Record 1991 Int. Display Res. Conf., San Diago, California). This is because the asymmetrical viewing angle areas of the subpictures then add up to an overall relatively small but essentially point-symmetrical viewing angle area (cf. FIG. 8). A further enlargement of this viewing angle range with a compensation film, as disclosed in European patent application EP 646 829 AI, is in principle ruled out, since the discotic layer is adapted to a specific orientation of the liquid crystal and thus only one sub-pixel could be compensated for in each case.

As an alternative, there is the possibility of dividing a pixel into four subpixels (4-domain method). In this case, each sub-pixel is given a different orientation, in which e.g. B. on a substrate side with diagonally adjacent sub-pixel regions, the liquid crystal edge molecules are oriented rotated by 180 ° relative to one another (cf. FIG. 10). The technological effort with the 4-domain method is decidedly higher than with the 2-domain method, but a comparatively larger suitable viewing angle range can be achieved (see FIG. 11).

With this method too, an improvement of the suitable viewing angle range by the compensation layer described above is ruled out for the reasons already mentioned.

The object of the invention is to provide a liquid crystal display in which the largest possible viewing angle range is available, in which there is no gray level inversion and the contrast is above a minimum value (for example 10). This object is solved by the features of claim 1.

Advantageous and expedient developments of the liquid crystal display according to the invention are specified in the subclaims.

The invention is based on a liquid crystal display with several pixels, which consists of a liquid crystal cell and polarizers, which are arranged on the top and bottom of the liquid crystal cell, the liquid crystal cell comprising two substrates with transparent electrodes and a liquid crystal enclosed between them. The essence of the invention lies in the fact that each pixel is divided into at least two subpixels in which the liquid crystal has different orientations and that at least between one polarizer and the liquid crystal there is an optically biaxial retardation layer with different refractive indices n _e , n ₀ , n ₂ Compensation of viewing angle dependencies of optical properties of the liquid crystal display, such as. B. the transmission and contrast, is provided, the refractive index n ₂ in a normal to

Liquid crystal cell substantially parallel axis occurs in the retardation layer. In this way, the advantages of a "multi-domain method" can be combined with the advantages of a layer that compensates for birefringence in the liquid crystal cell. Due to the optical biaxial nature, the refractive behavior of the liquid crystal edge molecules set up in the liquid crystal cell relative to the substrate surface can be compensated in the desired manner, whereby a particularly effective compensation can be achieved. The basic orientation of the liquid crystal molecules must be observed, but an opposite 180 ° orientation of the liquid crystal edge molecules is permissible, which is the last In the end, the possibility of using a delay layer for differently oriented subpixels is permitted. In addition, the delay layer according to the invention, in combination with the “multi-domain method”, in particular when the liquid crystal molecules are oriented by exposing a photosensitive layer, is an inexpensive alternative to comparatively similarly effective compensation layers (eg discotic delay films).

In order to obtain a particularly effective compensation of the viewing angle dependencies, it is further proposed that the refractive indices n _e , n ₀ , n _{z of} the biaxial retardation layer be dimensioned such that n _e - n ₀ > 0 and n _B - n ₀ <0.

In order to obtain a comparatively simple structure of the liquid crystal display, it is also proposed that the delay layer be arranged on the liquid crystal cell between the polarizer and the liquid crystal cell.

For a particularly compact construction, it is advantageous if the delay layer is arranged in the liquid crystal cell between the liquid crystal and the substrate.

A particularly simple and compact structure results if the substrate itself forms a retardation layer (plastic substrate with a suitable retardation).

In order to ensure a high degree of symmetry, it is also proposed to arrange the delay layer on both sides between the liquid crystal and polarizers. In the case of a delay layer applied on both sides, it is advantageous if the layer fulfills the following dimensioning regulations:

70 nm <(n _e - n _Q ) • d <200 nm and - 200 nm <(n _z - n ₀ ), where d is the thickness of the retardation layer. If the delay layer is only present on one side, essentially the same values specified in nm can be used without a significant deterioration in the compensation effect.

In a particularly preferred embodiment, one or more plastic films are used as the retardation layer. For example, a retardation layer made of a unidirectional plastic film, but not stretched to saturation, can be used. It is also possible to use a retardation layer made of a plastic film that is stretched bidirectionally. Finally, a combination of one or more uniaxial positive and one or more uniaxial negative films is possible. Retardation films have the advantage that they can be applied in a simple manner, for example laminated.

The delay layer can also be realized by a holographic element or in the form of a liquid crystal polymer.

It is particularly preferred that the liquid crystal is oriented in the sub-pixels of a pixel in such a way that the viewing angle areas of the individual sub-pixels complement one another to form a more symmetrical viewing angle area. In the case of a TN liquid crystal cell with pixels that are divided into two subpixels, this can be done by that the angles of attack of the liquid crystal edge molecules of the subpixels on the base substrate are opposite, that the angles of attack of the liquid crystal edge molecules of the subpixels on the cover substrate are opposite, that the angles of attack of the liquid crystal edge molecules of the subpixels on both substrates are opposite, that the angles of attack of the liquid crystal edge molecules of the subpixels are different on the base substrate are large that the angles of attack of the liquid crystal edge molecules of the sub-pixels on the cover substrate are of different sizes, or that the angles of attack of the liquid crystal molecules of the sub-pixels are different due to the cover substrate.

The twisting of the TN cell can be between 80 and 100 o.

Liquid metal molecules can be oriented on the substrate surfaces by mechanical rubbing of an orientation layer.

In a particularly preferred embodiment, a photosensitive layer is used to orient the liquid crystal molecules on the substrate surfaces, in which the preferred direction of the liquid crystal molecules is determined by an exposure process.

In order to achieve a high contrast, it is also proposed that border areas between the sub-pixels are covered by masking.

In an advantageous embodiment, each pixel has a switching element such as a thin film transistor, a thin film diode or a metal-insulator-metal diode.

In a further advantageous embodiment, a thin-film transistor and a storage capacitor are arranged in each pixel, at least one electrode of the storage capacitor being realized by a conductive opaque layer which covers the boundary region between the sub-pixels. In this way, with the aperture of the liquid crystal display remaining the same, a cover for the border area between the sub-pixels is realized.

Drawings:

Various exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, giving further advantages and details. Show it

1 shows the schematic structure of a liquid crystal display according to the invention,

2 shows the alignment of the refractive indices in an optically biaxial retardation film according to the invention,

Fig. 3 a u. b the schematic structure of a conventional twisted nematic liquid crystal display,

4 shows a model to illustrate the coordinates for determining the viewing direction, 5 shows the depiction of the viewing angle dependence for contrast and grayscale inversion of a conventional nematic liquid crystal cell,

Fig. 6 a u. b. the course of the transmission as a function of the angle of inclination θ for different grayscale settings,

Fig. 7 shows the basic orientation of the

Liquid crystal edge molecules for a pixel divided into two sub-pixels,

8 shows the viewing angle dependence of the contrast and the gray scale inversion for an alignment of the liquid crystal molecules according to FIG. 7 without a delay layer,

9 shows the viewing angle dependence of contrast and

Greyscale inversion for an arrangement according to FIG. 7 with a biaxial retardation layer according to the invention,

10 shows the orientation of the liquid crystal edge molecules on the base and cover substrates when the pixel is divided into four subpixels,

11 shows the diagram belonging to FIG. 10 for the viewing angle dependence of contrast and grayscale inversion without delay film and

FIG. 12 shows the diagram belonging to FIG. 10 for the viewing angle dependence of contrast and grayscale inversion with an optically biaxial retardation layer according to the invention. Description of the exemplary embodiments:

The starting point for the description of the exemplary embodiments, as already mentioned in the introduction to the description, is a conventional, twisted nematic (twisted nematic) liquid crystal display according to FIGS. 3a and b. Between two glass substrates 1, 2 there is a liquid crystal twisted by 90 °, the elongated liquid crystal molecules of which are indicated by rods 3. A polarizer 4, 5 is attached to each of the glass substrates. The transmissive direction of the polarizers (indicated by a double arrow) coincides with the orientation of the longitudinal axis of the liquid crystal edge molecules, but it can also be oriented at 90 ° to it. 3a shows the voltage-free state in which penetrating light (symbolized by the broad arrow) is rotated by 90 ° due to the liquid crystal helix and can then leave the polarizer 5 unhindered. The cell is translucent and appears bright. When the voltage is applied, the liquid crystal molecules align themselves in the electric field due to their dielectric anisotropy, as a result of which the light penetrating from below (also shown in FIG. 3b by a broad arrow) passes unhindered through the liquid crystal cell and therefore at the polarizer 5 is absorbed. In this state, the liquid crystal cell appears dark.

An undesirable effect with liquid crystal cells is the viewing angle dependency, especially of gray levels and contrast. The coordinates shown in FIG. 4 are introduced to determine the viewing direction. The Indian

The angle φ lying on the substrate plane S should run from 0 to 360 °, whereas the angle θ indicates the angle of inclination from the liquid crystal normal (z). In terms of Figures 5, 6a and 6b should be referred to the introduction to the description. In Fig. 5, the longitudinally and transversely hatched area of a conventional TN cell, which can be regarded as a suitable viewing angle area, is in need of improvement.

For this purpose, an inventive structure of a liquid crystal display according to FIG. 1 is used. A liquid crystal is located between a base substrate (GS) and a cover substrate (DS), the longitudinal axes of the liquid crystal edge molecules are aligned on the base or cover substrate in accordance with the arrow shown. A delay layer VI, V2 is arranged on the top and bottom of the liquid crystal cell formed by base and cover substrates. The extraordinary axis (n _e ) in the retardation layer is symbolized by arrows, whereby it runs perpendicular to the alignment of the liquid crystal molecules on the correspondingly adjacent substrate (for the position of the further optical axes in the retardation layer, see FIG. 2). Finally, a polarizing filter P1 or P2 is applied to the delay layers VI and V2, the absorbing axis of which runs parallel to the alignment of the liquid crystal molecules in the correspondingly adjacent substrate (dashed double arrow). In addition to a strictly vertical or parallel orientation between the specified directions, the projection of the extraordinary axis of the liquid crystal edge molecules onto the substrate surface with the absorbing axis of the polarizer can include an angle of 0 ° to 5 ° for the base and top substrates and the extraordinary axis of the retardation layer form an angle of 85 ° to 90 ° with the absorbing axis of the polarizer.

According to the invention, a picture element according to FIG. 7 is to be divided into sub-picture elements, with neighboring ones Subpixels 6, 7 are oriented opposite to one another in accordance with the symbolically indicated molecular longitudinal axes 8 by 180 °. Such an orientation results in an essentially point-symmetrical course of the grayscale inversion or the boundary line for an area in which a minimum contrast is not undershot (cf. FIG. 8). The area in which no grayscale inversion occurs is shown in FIG. 8 by the line provided with crosses, whereas the isocontrast curve is defined for a minimum contrast of 10 by square dots.

Through the additional use of a retardation film according to the invention, the individual areas, but in particular the cutting area, in which neither the contrast falls below a minimum contrast nor a grayscale inversion occurs, are significantly enlarged (cf. FIG. 9).

An even better starting situation with regard to the viewing angle dependence of contrast and grayscale inversion is achieved in that a picture element according to FIG. 10 is divided into four sub picture elements. The alignment of the molecular longitudinal axes 8 shown in FIG. 10 results in different orientations of the liquid crystal for all four subpixels. As a result, the symmetry of the viewing angle dependency of contrast and grayscale inversion is improved somewhat, but in particular the cut surface, in which the contrast does not fall below a minimum contrast or no grayscale inversion occurs, is greatly increased. If the optically biaxial retardation film according to the invention is now additionally used, the behavior with respect to contrast and grayscale inversion is again significantly improved, as is shown in the diagram in FIG. Up to one

The inclination angle θ of approximately 65 ° does not fall anywhere Contrast below a minimum contrast, in the exemplary embodiment a contrast of 10. Likewise, up to one

Inclination angle θ of approximately 65 ° regardless of the viewing angle φ no gray scale inversion takes place.

The biaxial retardation film according to the invention thus offers the possibility of producing liquid crystal displays which, with the additional use of the “multi-domain method”, makes excellent viewing-angle dependencies possible.

Claims

Claims:

1. Liquid crystal display with several pixels consisting of a liquid crystal cell and polarizers (4, 5) which are arranged on the top and bottom of the liquid crystal cell, the liquid crystal cell having two substrates (1, 2) with transparent electrodes and a liquid crystal (3 ), characterized in that each pixel is subdivided into at least two subpixels (6, 7) in which the liquid crystal has different orientations and that at least between one polarizer (4, 5) and the liquid crystal (3) there is an optically biaxial retardation layer ( VI, V2) with different refractive indices n _e , n ₀ , n _z to compensate for viewing angle dependencies of optical properties of the liquid crystal display, such as the transmission and the contrast, the refractive index n _{z being} in an axis essentially parallel to the normal of the liquid crystal cell in the delay layer occurs.

2. Liquid crystal display according to claim 1, characterized in that at the refractive indices n _β , n ₀ , n _{z of} the biaxial retardation layer, n _e - n ₀ > 0 and n _z - n ₀ <0.

3. Liquid crystal display according to one of the preceding claims, characterized in that the delay layer on the liquid crystal cell between the polarizer (4, 5) and the liquid crystal cell is arranged.

4. Liquid crystal cell according to one of the preceding claims, characterized in that the delay layer in the liquid crystal cell between the liquid crystal (3) and the substrate (1, 2) is arranged.

5. Liquid crystal cell according to one of the preceding claims, characterized in that the

Retardation layer is arranged on both sides between liquid crystal (3) and polarizers (4, 5).

6. Liquid crystal display according to one of the preceding claims, characterized in that in the case of a delay layer applied on two sides, the delay layer has the following properties: 70 nm <(n _e -n ₀ ) * d <200 nm and -200 nm <(n _z - n ₀ ) * d <0, where d is the thickness of the retardation layer.

7. Liquid crystal display according to claim 6, characterized in that when the delay layer is applied on one side, the values given in nanometers are essentially unchanged.

8. Liquid crystal display according to one of the preceding claims, characterized in that one or more plastic films can be used as the delay layer.

9. Liquid crystal display according to claim 8, characterized in that a unidirectional, but not stretched to saturation plastic film is provided as the delay layer.

10. Liquid crystal display according to claim 8, characterized in that the delay layer is a bidirectionally stretched plastic film.

11. Liquid crystal display according to one of claims 8 to 10, characterized in that the delay layer through a combination of one or more uniaxial positive and one or more uniaxial negative foils is realized.

12. Liquid crystal display according to one of the preceding claims, characterized in that the delay layer is realized by a holographic element.

13. Liquid crystal display according to one of the preceding claims, characterized in that the delay layer is realized in the form of a liquid crystal polymer.

14. Liquid crystal display according to one of the preceding claims, characterized in that in the

Subpixels of a pixel of the liquid crystal is oriented so differently that the

Supplement the viewing angle ranges of the individual sub-pixels to form a substantially point-symmetrical viewing angle range.

15. Liquid crystal display according to one of the preceding claims, characterized in that the liquid crystal cell is a TN cell in which the twist is 80 ° to 100 °.

16. Liquid crystal display according to one of the preceding claims, characterized in that a photosensitive layer can be used for the orientation of liquid crystal molecules on the substrate surface, in which the preferred direction of attached to it

Liquid crystal molecules can be determined by an exposure process.

17. Liquid crystal display according to one of the preceding claims, characterized in that border regions between the sub-pixels are covered by a mask.

18. Liquid crystal display according to one of the preceding claims, characterized in that each pixel has a switching element.

19. Liquid crystal display according to one of the preceding claims, characterized in that each pixel has a thin-film transistor and a storage capacitor, at least one electrode of the storage capacitor being realized by a conductive opaque layer which covers the boundary region between the sub-pixels.