1. Field of the Invention
The present invention relates to a liquid crystal display unit and a method for its production.
In recent years, the display areas of liquid crystal display units of such devices as computer terminals, personal computers, word processors, TVs and the like have become increasingly larger, and their uses are expected to become further expanded in the future, accompanied by greater demand to realize liquid crystal display units with visual angle characteristics equaling those of CRTs (cathode ray tubes).
2. Description of the Related Art
Some techniques have already been proposed for improving viewing angle characteristics, including a PDN type (Polymer Dispersed Liquid Crystal Display with Crossed Nicols; see Japanese Unexamined Patent Publication No. 4-212928), an amorphous TN type (Amorphous Twisted Nematic Liquid Crystal Display; see SID '94 Digest, p. 915-918) and a PTN type (Polymer Twisted Nematic Liquid Crystal Display; see Japanese Unexamined Patent Publication No. 5-27242).
FIGS. 7A and 7B are illustrations of PDN-type liquid crystal display units of the prior art, FIG. 7A showing a state without application of voltage and FIG. 7B showing a state with application of voltage.
In these drawings, 21 is a transparent base plate, 22 is a transparent conductive film, 23 is a transparent base plate, 24 is a transparent conductive film, 25 is a polymer, 26 is a liquid crystal molecule, 27 is a liquid crystal droplet, 28 is a liquid crystal layer, 29 and 30 are polarizer, 31 is a round cushion-shaped film with optical anisotropy, 32 is an alternating current power source and 33 is a switch.
In this conventional PDN-type liquid crystal display unit, the opposing transparent base plate 21 with the transparent conductive film 22 and transparent base plate 23 with the transparent conductive film 24 form a structure sandwiching the liquid crystal layer 28 containing the polymer 25 and liquid crystal droplets 27. Also, the pair of polarizers 29 and 30 whose directions of polarization are orthogonal are laid over the outside of the transparent base plates 21 and 23.
A round cushion-shaped film 31 with optical anisotropy, such as a polycarbonate polyaxially stretched film, is laid between the transparent base plate 23 and the polarizer 30 on the display side.
The alternating current power source 32 and the switch 33 are connected in series between the transparent conductive film 22 and the transparent conductive film 24.
In this liquid crystal display unit, when no voltage is applied from the alternating current power source 32 between the transparent conductive films 22 and 24, the liquid crystal molecules 26 in each of the liquid crystal droplets 27 are controlled so as to line up at the interface with the polymer 25, but since the orientation of the liquid crystal molecules 26 on the liquid crystal droplets 27 is unordered, incident light 34 from the back side is scattered by the liquid crystal molecules 26 and passes through the polarizing plate 30 appearing milky white, and bright (see FIG. 7A).
When a voltage is applied from the alternating current power source 32 between the transparent conductive films 22 and 24, the liquid crystal molecules 26 in the liquid crystal droplets 27 align in the direction of the electric field, and as a result light scattering is reduced and incident light 34 from the back side may pass through the liquid crystal layer 28 unhindered.
However, since the polarizers 29 and 30 are arranged in crossed Nicols, the light cannot pass through the polarizer 30, giving a dark appearance.
When the liquid crystal panel of this liquid crystal display unit is viewed at an oblique inclination angle, the liquid crystal molecules 26 and the incident light 34 both are viewed from an oblique inclined angle and the optical effect of the liquid crystal layer 28 appears to some degree causing light leakage, and therefore the round cushion-shaped film 31 with optical anisotropy is inserted to block the resulting leaking light which passes through the polarizing plate 30.
Generally speaking, when the refractive index of a liquid crystal molecule is expressed in three-dimensional terms it may be compared to a rugby ball-shaped index ellipsoid, and the refractive index when viewed at an oblique inclination angle corresponds to the ellipse section resulting from cutting the rugby ball-shaped index ellipsoid at an oblique inclination angle.
On the other hand, when the refractive index of a molecule of the round cushion-shaped film with optical anisotropy is expressed in three-dimensional terms, it exhibits the characteristics of a round cushion-shaped index ellipsoid, and the refractive index thereof when viewed at an oblique inclination angle corresponds to the ellipse section resulting from cutting the round cushion at an oblique inclination angle.
Also, the ellipse section resulting from cutting the rugby ball-shaped index ellipsoid of the liquid crystal molecule at an oblique inclination angle and the ellipse section resulting from cutting the index ellipsoid of the round cushion-shaped film with optical anisotropy at an oblique inclination angle have their long axes and short axes rotated 90.degree. to each other.
Consequently, when the ellipse section resulting from cutting the rugby ball-shaped index ellipsoid of the liquid crystal molecule at an oblique inclination angle and the ellipse section resulting from cutting the index ellipsoid of the round cushion-shaped film with optical anisotropy at an oblique inclination angle are superimposed a circle results, and the optical effect is canceled out.
Therefore, as mentioned above, the superimposition of the round cushion-shaped film 31 with optical anisotropy cancels out the effect of the inclined incident light, and therefore the black display produced when a voltage is applied between the electrodes has a consistently dark appearance even when viewed from an oblique inclination angle, giving a constant display when viewed from any direction, and producing the effect of a wider visual angle.
FIGS. 8-A-1 to 8-A-4 and 8-B-1 to 8-B-4 are illustrations of amorphous TN-type liquid crystal display units of the prior art, FIGS. 8-A-1 to 8-A-4 showing the prior art and FIGS. 8-B-1 to 8-B-4 showing amorphous TN-type liquid crystal display units.
In these drawings, 41 is a first transparent conductive base plate, 42 is a second transparent conductive base plate, 43 and 44 are alignment films, 45 is a liquid crystal molecule, 46 is a liquid crystal layer and 47 and 48 are non-rubbed polymer films.
A prior art TN (Twisted-nematic) liquid crystal display unit with rubbed alignment films will now be explained with reference to FIGS. 8-A-1 to 8-A-4.
In this TN liquid crystal display unit of the prior art, the liquid crystal layer 46 comprised of liquid crystal molecules 45 is sandwiched between the first transparent conductive base plate 41 with an alignment film 43 which is rubbed in the direction perpendicular to the plane of the paper, and the second transparent conductive base plate 42 with an alignment film 44 which is rubbed left to right on the plane of the paper.
In this TN liquid crystal display unit, even at clearing temperatures at which the orientation of the liquid crystal molecules 45 is random (isotropic), the liquid crystal molecules 45 contacting with the alignment films 43 and 44 of the first transparent conductive base plate 41 and the second transparent conductive base plate 42 which are rubbed in orthogonal directions are aligned in these respective directions of alignment by the restraint of the alignment films 43 and 44 (see FIG. 8-A-1).
When the temperature is lowered below the clearing temperature, the effect of the liquid crystal molecules 45 contacting with the upper and lower alignment films 43 and 44 reaches inside the liquid crystal layer 46, and a column of liquid crystal molecules 45 begins to grow (see FIG. 8-A-2).
When the temperature is further lowered, the effect of the liquid crystal molecules 45 contacting with the upper and lower alignment films 43 and 44 reaches further inside the liquid crystal layer 46, and the column of liquid crystal molecules 45 grows larger (see FIG. 8-A-3).
When the temperature is even further lowered, the effect of the liquid crystal molecules 45 contacting with the upper and lower alignment films 43 and 44 reaches even further inside the liquid crystal layer 46, and the column of liquid crystal molecules 45 is completely formed between the upper and lower alignment films 43 and 44. Since the twist origins of the liquid crystal molecules 45 are the same in state, the viewing angle characteristics are narrower (see FIG. 8-A-4).
An amorphous TN-type liquid crystal display unit will now be explained with reference to FIGS. 8-B-1 to 8-B-4.
In this amorphous TN-type liquid crystal display unit, the liquid crystal layer 46 comprising liquid crystal molecules 45 is sandwiched between a first transparent conductive base plate 41 and a second transparent base plate 42 with non-rubbed polymer (polyimide PI) films 47 and 48.
In this amorphous TN-type liquid crystal display unit, at clearing temperatures at which the orientation of the liquid crystal molecules 45 is random, the surfaces of the non-rubbed polymer films 47 and 48 in contact with the liquid crystal molecules 45 are rough, and therefore the liquid crystal molecules 45 are not aligned, being oriented in random directions (see FIG. 8-B-1).
When the temperature is further lowered, a column of aligned liquid crystal molecules 45 does not grow from the surface of the liquid crystal layer 46 in contact with the non-rubbed polymer films 47 and 48, but rather grows from the interior (bulk region) (see FIG. 8-B-2).
When the temperature is even further lowered, the alignment of the liquid crystal molecules 45 occurring in the bulk region is promoted, causing the column of liquid crystal molecules 45 to grow larger (see FIG. 8-B-3).
When the temperature is still further lowered and a temperature of 30.degree. C. below the clearing temperature is maintained, the alignment of the liquid crystal molecules occurring in the bulk region is further promoted, and the column of liquid crystal molecules 45 in each region is completed between the non-rubbed polymer films 47 and 48, with the liquid crystal molecules 45 becoming rigidly fixed in that state (memory effect).
In this state, the twist origins of the liquid crystal molecules 45 in contact with the surfaces of the non-rubbed polymer films 47 and 48 are random, and therefore the viewing angle characteristics of each of the columns of liquid crystal molecules 45 are averaged to provide improved viewing angle characteristics and to obtain a gray scale (see FIG. 8-B-4).
FIG. 9 is an illustration of a PTN-type liquid crystal display unit of the prior art.
In this drawing, 51 and 52 are transparent conductive base plates, 53 denotes liquid crystal molecules, and 54 denotes a material which disturbs the twisted arrangement of the liquid crystal molecules.
In this conventional PTN-type liquid crystal display unit, liquid crystals such as cholesteric liquid crystals or chiral nematic liquid crystals are injected between the pair of transparent conductive base plates 51 and 52 with transparent electrodes and alignment films on their opposing inner sides, and there is also injected a material 54 to disturb the twisted arrangement of the liquid crystal molecules, for example an organic polymer such as polymethyl methacrylate, polystyrene melamine resin, urea resin, phenol resin or polydiisopropyl fumarate, or a photopolymerized polymer such as polyurethane acrylate, polyester acrylate, epoxy acrylate or polyether acrylate, in order to change the arrangement of the liquid crystal molecules 53.
As a result, the twisted orientations of the liquid crystal molecules along the lines A, B and C vary slightly depending on the position of the liquid crystal layer, even with the same viewing angle direction.
Since the twisted orientations of the liquid crystal molecules 53 vary slightly along the lines A, B and C in this drawing, application of a voltage causes the liquid crystal molecules 53 to be aligned differently, and the orientations of the liquid crystal molecules 53 are averaged with respect to the viewing angle direction, thus resulting in averaging of the apparent transmittance/applied voltage characteristics, improved viewing angle characteristics, and suppression of the abnormal display inversion phenomenon.
In the PDN-type liquid crystal display unit described above it has not been possible to impart optical activity to the liquid crystal molecules, and consequently the display brightness of displayed white has been only 1/3 that of TN-types.
In addition, the sizes of the domains lack uniformity in amorphous TN-type liquid crystal display units and are large, and when the display surface is viewed from an oblique inclination angle a patchy black and white pattern is produced.
When the display surface is viewed from an oblique inclination angle, the black portions and white portions are of an extremely small identical size and there are few problems so long as they are dispersed, but since the domains are large, they appear as patches.
This condition is the same in the case of PTN-type liquid crystal display units, which have had the problem of patchiness when viewed from an oblique inclination angle.
It is an object of the present invention to provide a liquid crystal display unit with satisfactory viewing angle characteristics and which eliminates such patches when viewed from an oblique inclination angle.